Erzsébet Tóth-Czifra

Tóth-Czifra Truan

Dr Naomi Truan
email: naomi.truan@uni-leipzig.de

Management Work ows Volume 1: Creating and analyzing multilingual parliamentary corpora Document initiated

Keywords: corpus linguistics, contrastive analysis, parliamentary debates, Text Encoding Initiative (TEI), linguistic annotation, discourse analysis

Overview of the data workflow 'Creating and analyzing multilingual parliamentary corpora'

[Download the schema as a PDF on HAL-SHS to make it machine-readable]

In this resource, you can follow a step-by-step description of a research data workflow involving the annotation of multilingual parliamentary corpora (French, German, British) according to the guidelines of the Text Encoding Initiative (TEI). Read further if you are interested in working with the TEI, analyzing parliamentary corpora, or simply would like to see a validated example of how FAIR and open data is implemented in the context of a PhD dissertation in Corpus Linguistics.

Work ow

Overview of the data workflow 'Creating and analyzing multilingual parliamentary corpora'

[Download the schema as a PDF on HAL-SHS to make it machine-readable]

In the following pages, we detail the six steps by including excerpts of an interview of Naomi Truan conducted by Erzsébet Tóth-Czifra and Marta Błaszczyńska on 07.10.2020 as well as references and useful resources in boxes.

Step 1: Selection of the data relevant to the research question To enable the cross-linguistic analysis of third-person reference based on empirical data, British, French, and German parliamentary debates are gathered for a contrastive corpus study. To ensure the comparability of the three national contexts, the selection was restricted to national debates revolving around the European Council meetings.

The analysis was limited to the o cial transcripts of parliamentary records; audio and video resources were consulted on a qualitative basis only.

"I did not engage with audio and video data extensively, which I could have, but chose not to because I did not need it for my aims. I was indeed working mostly in semantics and pragmatics and did not include multimodal elements into the analysis. If it had to be included, it would have been a whole different story because then you would have to add the time stamp to the XML-TEI annotation, etc."

All the texts have been retrieved from the o cial websites of the respective parliaments where the transcripts are shared as public data.

Resources:

http://hansard.parliament.uk/ for the House of Commons (HTML) http://pdok.bundestag.de/ for the German Bundestag (PDF) http://archives.assemblee-nationale.fr/ for the Assemblée nationale (HTML)

Step 2: Harmonization of the sub-corpora "[At the time of my first investigations, in 2014-2015, both the British House of Commons and the French Assemblée nationale displayed the parliamentary proceedings in HTML, which allows for a quick, easy, and accurate retrieval of the content. The German corpus, on the other hand, was based on PDF files. PDF files are noticeably less adequate for further encoding and tagging. In this case, the files have sometimes suffered from inadequate word breaks, thus necessitating minor corrections. I carried out the encoding process into the TEI Guidelines by combining manual and automatic processing workflows, with the idea of keeping both the content and the metadata of the sources."

Source materials have been converted into a TEI-XML format with the help of the GROBID suite:

• TEI stands for Text Encoding Initiative, a community of digital humanists who collectively de ne a type of XML format. • XML stands for Extensible Markup Language, a markup language that de nes encoding standards for documents in formats that are both human-readable and machine-readable.

"In particular, we used the GROBID software suite, which provides a relatively efficient transformation process from PDFs to a decent TEI format, although not fully compliant with the target encoding scheme. Attention was given to unifying the final format across the three languages and parliamentary settings so that the same phenomena and features would be encoded exactly in the same way for each sub-corpus."

Resources:

GROBID software suite: https://github.com/kermitt2/grobid. Romary, Laurent & Patrice Lopez. 2015. GROBID -Information Extraction from Scienti c Publications.

ERCIM, Data Sharing and Re-Use. https://hal.inria.fr/hal-01673305/document.

Step 3.1: Setting the scene

The TEI header places emphasis on identifying speakers in a way that is most appropriate and exible for the sake of analysis (e.g. enabling the stable identi cation of speakers even if their political a liation changes over the years). See p. 15 here.

Step 3.2: Encoding the content

The body of the XML-TEI corpora went through an automated, language-speci c part-of-speech tagging with the help of Tree Tagger part-of-speech annotation tool directly integrated into the external open-source software TXM. You can follow the documentation of the import process and visualize the corpus through an XSL style sheet and an XML stylesheet here.

Output: .csv tables that were exported from the software TXM according to some speci c queries / CQP requests (e.g. concordance lists and collocations with personal pronouns or nouns and nominal expressions).

One of the challenges was that the XML-TEI annotation was not as easily imported into the TXM software as we thought, which required email exchanges with the TXM team.

"

We actually needed at least half a year, if not a whole year, to get TXM to recognize the TEI annotation. To make it work through the software, so to say. And for me, this was the most important thing, because I had had annotated my data set for quite a long time and knew that I wanted to work with the software TXM. But it didn't match. This is also why I worked a lot qualitatively-in the meantime, before we figured out how the corpus could be imported. I actually began working with the data by extracting examples directly from the TEI-XML files. But it's not the real workflow. I was waiting for someone to build the bridge."

Another challenge was that we wanted to optimize heterogeneous data for contrastive reuse. The three sub-corpora were available in di erent formats and o ered di erent possibilities for coding. Naomi had to nd a common denominator, a uni ed annotation framework that works equally well for all three of them but is also suitable for contrastive analysis. This comes with limitations in terms of richness of the annotation for those who wish to reuse only one sub-corpus, for instance, working only on the French one.

Resources:

Open-source software TXM: http://textometrie.ens-lyon.fr/?lang=en. Step 4: Quality assurance and import into TXM Due to the relatively small size of the corpus and the qualitative nature of the analysis, quality assurance consisted of manual checks of the encoding performed in an iterative fashion. Also, since the analysis had been carried out as Naomi's PhD, there was no possibility of employing an assistant as co-annotator. As a next step, the TXM team provided support during the import of manual codes to TXM.

"And since the corpus was rather small and I felt like engaging with the data was part of my job anyways, I actually did everything by myself. I listened to a lot of podcasts [during the manual coding and the successive imports into TXM]. It was a great time, but it was not automatic at all."

Step 5: Data sharing and publication

Parallel to the development of the annotation framework, and before the publication of any paper making use of the annotated corpora, Naomi opted for the publication of the annotated corpora in Open Access. Her main motivations were scholarly transparency and reproducibility. In order to allow other scholars to trace and eventually reproduce the analysis, or parts of it, as it unfolded, Naomi shared a rich documentation of the outputs and throughput.

The corpora and the annotation framework are deposited in the ORTOLANG repository because:

• ORTOLANG is speci cally designed for linguistic data with the possibility to attach several linguistic descriptors (language, genre, source type, etc.). • As a complementary service to Huma-Num (very large research infrastructure), ORTOLANG exhibits standards of a trustable repository: long-term archiving, permanent identi ers (PIDs), version control, Creative Commons open licensing framework, precise identi cation of the various contributors to a resource.

The deposits contain:

1. The XML les annotated according to the guidelines of the Text Encoding Initiative (TEI).

2. The German corpus also includes the original PDF les of the transcriptions. For the other two corpora, a link is provided to the transcripts openly available in HTML format. "And for instance, the fact that my corpus or my corpora have been downloaded 100 times and have been used for several different projects, although the corpus annotation did not go through a peer-review process at the time [it has in the meantime], shows me that this is valuable and that my standards actually are used and useful to the community."

A reuse challenge: proper citation is sometimes missing from reuse cases even if there is a 'Cite as' snippet prominently displayed on the landing pages of the deposits. In order to cite data properly, have a look at these recommendations:

Resources:

Andreassen, Helene N., Andrea Still, what is also clear from Naomi's words is that sharing your data is feasible within the timeframe of a PhD project. Each of these windows, each of these open documentation components can create rich ways of engagement with the work and can thus advance one's career and position in the eld. We hope that this document has shown you in a practical way the possible impacts of working on your project with sharing in mind.

Tóth

  providing 1) an overview of the debates being part of the corpora; 2) a generic and speci c description of the annotation framework. License: Creative Commons CC-BY license requiring proper attribution to the author (CC-BY 4.0). For more information on licenses, check this page: https://creativecommons.org/licenses/. Screenshot of the file structure of the German corpus Truan, Naomi. 2016. Parliamentary Debates on Europe at the Deutscher Bundestag (1998-2015) [Corpus]. ORTOLANG (Open Resources and TOols for LANGuage). https://hdl.handle.net/11403/de-parl. The publication of the corpus is interlinked with research papers and complemented with additional documentation in the form of blog posts (see "Resources". 2019. How to make the most of your publications in the humanities?" in the hands on session "Do-it-yourself Self-archiving". [Presentation slides]. https://www.fosteropenscience.eu/node/2605 Tóth-Czifra & Truan 2021 | CC BY 4history is already available and presumably, the reuse cases Naomi is aware of are only fractions of the real reuse instances. Typical reusers are fellow scholars working on parliamentary debates, with disciplines ranging from linguistics to political sciences and sociology. Citations of the corpora in research projects reusing them Source: Truan, Naomi. "Outils numériques pour les SHS", in Ici et là, 13/02/2020, https://icietla.hypotheses.org/1280. Slide 18 translated into English.

  Heiden, Serge. 2010. The TXM Platform: Building Open-Source Textual Analysis Software Compatible with the TEI Encoding Scheme. In Ryo Otoguro, Kiyoshi Ishikawa, Hiroshi Umemoto, Kei Yoshimoto & Yasunari Harada (eds.), 24th Paci c Asia Conference on Language, Information and Computation, 389-398. Institute for Digital Enhancement of Cognitive Development, Waseda University. tags.hypotheses.org/81. Truan, Naomi & Laurent Romary. forthcoming. Building, Encoding, and Annotating a Corpus of Parliamentary Debates in XML-TEI: A Cross-Linguistic Account. Journal of the Text Encoding Initiative (14). Preprint: ⟨halshs-03097333v2⟩.

https://halshs.archives-ouvertes.fr/halshs-00549764/document

[START_REF] Heiden | The TXM Platform: Building Open-Source Textual Analysis Software Compatible with the TEI Encoding Scheme[END_REF]

. https://

ethically and even emotionally.

  To conclude, and also because in this data story, we aimed to initiate an honest discussion about data work ows and openness, it is worth highlighting that opening a bigger window on one's scholarly processes means a signi cant amount of investment, not only in terms of time and e ort but also Working with sharing in mind from the beginning involves being ready to be exposed. As Naomi puts it:"You have to feel ready to be exposed and to feel like vulnerability is part of your research. And for me, it has been amazing because I think sharing my data has opened a lot of new things. Now I am working more qualitatively as well, practicing autoethnography, and basically involving what research does to you. So I think that [opening my workflow and publishing my annotated corpora] has been very helpful. But it is hard. It is really hard because people do not always value it the way you would want them to."

L. Berez-Kroeker, Laura Collister, Philipp Conzett, Christopher Cox, To conclude

Tóth-Czifra & Truan 2021 | CC BY 4.0

See http://cwb.sourceforge.net/, accessed on 31.05.2021. Tóth-Czifra & Truan

| CC BY 4.0 6/15

Truan, Naomi. "From TEI-XML to TXM, HTML and back" in Bag of tags, 26/12/2016,

See http://cwb.sourceforge.net/, accessed on

31.05.2021. Tóth-Czifra & Truan 2021 | CC BY 4.0 14/15

Tóth-Czifra & Truan 2021 | CC BY 4.0 15/15

carried out by a single person as part of a PhD project.

Step 3: Creation of the TEI annotation framework Text Encoding Initiative annotation framework enables two levels: the TEI Header (metadata) and the text level (utterances, quotes). The part-of-speech tagging allowing for CQP queries was automated and directly integrated into the open-source TXM software.

CQP stands for Corpus Query Processor, a project developed within the IMS Open Corpus Workbench (CWB). As indicated on the website, "The IMS Open CorpusWorkbench (CWB) is a collection of open-source tools for managing and querying large text corpora (ranging from 10 million to 2 billion words) with linguistic annotations. Its central component is the exible and e cient query processor CQP". 1 The integration of CQP for speci c linguistic queries and TEI as a general annotation framework for digital texts is speci c to linguistics, but the questions it raises are central to all Digital Humanities elds.

"I carried out the encoding process into the TEI Guidelines by combining manual and automatic processing workflows, with the idea of keeping both the content and the metadata of the sources."

Why use TEI standards?

• The TEI builds on and fosters established, well-documented community standards that are widely acknowledged in the author's eld of study and above: corpus-based linguistic modelling and analysis. • The TEI is exible enough to allow for customized and easy-to-enrich annotations that are tailored to a speci c research question (by contrast to ad hoc solutions such as a speci c metadata schema for parliamentary debates). • The thorough documentation of the decisions made to construct the annotation framework and its publication comes with the degree of scholarly transparency that enables reproducibility and reusability of the framework for other research settings and/or purposes.

Resources


Thomas Poulard

Martin Dres

Marie-Cécile Nierat

Isabelle Rivals

Jean-Yves Hogrel

Thomas Similowski

Jean-Luc Gennisson

PhD. Damien Bachasson
email: d.bachasson@institut-myologie.org

Marie-Cécile Niérat

Is maximal diaphragm tissue velocity suited for the assessment of diaphragm contractility?

the work; Acquisition or analysis or interpretation of data for the work; Drafting the work or revising it critically for important intellectual content; Final approval of the version to be published; Agreement to be accountable for all aspects of the work Jean-Yves Hogrel: Conception or design of the work; Acquisition or analysis or interpretation of data for the work; Drafting the work or revising it critically for important intellectual content; Final approval of the version to be published; Agreement to be accountable for all aspects of the work Thomas Similowski: Conception or design of the work; Acquisition or analysis or interpretation of data for the work; Drafting the work or revising it critically for important intellectual content; Final approval of the version to be published; Agreement to be accountable for all aspects of the work Jean-Luc Gennisson: To the Editor: We appreciate the opportunity given by the Editor to respond to Dr. Beltrami's Letter regarding our recent publication [START_REF] Poulard | Ultrafast ultrasound coupled with cervical magnetic stimulation for noninvasive and non-volitional assessment of diaphragm contractility[END_REF] in the Journal of Physiology.

Below, we have paraphrased the main criticisms, each followed by our response.

Dr. Beltrami questioned the design of our study:

"Participants underwent a single trial, where a minimum of 3 stimulations were performed at every intensity, and 2 to 3 of these were used for subsequent analysis. In our opinion this does not constitute an adequate setting to evaluate inter-session reliability"

One must note that inter-session reliability was not assessed in the current work. However, we assessed intra-session reliability of P di,tw , V di,max , and TF di,tw by comparing values obtained at any given stimulation intensity. As clearly mentioned in the Discussion: "between-day reliability of V di,max remains to be investigated". A different design will be chosen to investigate this question.

Dr. Beltrami expresses concerns regarding the interpretation we made of standard error of measurements (SEM):

"In our opinion the authors misinterpreted the message given by their standard error of measurement (SEM) […] the relative SEM for V di,max is 2.5 bigger than that of Pdi tw […] As previously discussed with Dr Beltrami in several email exchanges, the message we tried to convey was that SEM and ICC for V di,max appeared to be acceptable, and importantly, much more reliable than TF di,tw . It must also be noted that one must be careful when inferring SEM values expressed as a percentage from SEM and average values. To achieve this, analysis of logtransformed variables is required to estimate errors when the standard deviations expressed as percentages (coefficients of variation) apply more accurately to a full population given the between-individual variability [START_REF] Hopkins | Measures of reliability in sports medicine and science[END_REF]. Moreover, one should not lose sight of the fact that these analyses involve all responses at all intensities and are likely to be less reliable than what can be expected when considering supramaximal stimulation only. ICC for V di,max was > 0.85, which can be considered high enough to be considered as a reproducible index [START_REF] Koo | A Guideline of Selecting and Reporting Intraclass Correlation Coefficients for Reliability Research[END_REF]. Dr Beltrami's statement regarding the SEM of TF di,tw is also incorrect as percentage and percentage points were not confused. All SEM values were presented in the corresponding unit for each variable (i.e. cmH 2 O for P di,tw , mm.s -1 for V di,max , and % for TF di,tw ) . Although the ICC for V di,max was outside the 95% confidence interval of that for P di,tw , the ICC was > 0.85 which supports V di,max as a reproducible index when it comes to relative reliability [START_REF] Koo | A Guideline of Selecting and Reporting Intraclass Correlation Coefficients for Reliability Research[END_REF].

Dr. Beltrami also writes the following:

"P di,tw values are typically only used from supra-maximal stimulation, so although it is interesting that there was a correlation between the two variables when low stimulation intensities are considered, it is unclear from the data whether there is a relationship between P di,tw and V di,max ."

In the current work, change in cervical magnetic stimulation intensity was used as an experimental paradigm for eliciting various levels of P di,tw . Therefore, obtaining supramaximal P di,tw was not essential for our work. We used a broad range of cervical magnetic stimulation intensities to investigate the sensitivity of indices derived from ultrafast ultrasound imaging to changes in diaphragm contraction levels. Within our publication, Figure 7.A clearly illustrates the relationship between P di,tw and V di,max . In the text, we clearly state that a significant relationship was found between P di,tw and V di,max within all participants (ρ = 0.64 -1.00, all p<0.05) as well as at the group level, as assessed using repeated measures correlation coefficient (R = 0.75 (95%CI: 0.65, 0.83), p<0.0001).

Dr Beltrami also stated:

"We recalculated the data from Poulard, and found no relationship between P di,tw and V di,max (R 2 = 0.01, F (1,11) = 0.157, P = 0.699)"

Dr. Beltrami did not find the same results as we did because he did not perform the same analysis as us (although it remains unclear which exact analysis Dr Beltrami performed, since we found a coefficient of determination of 0.39 when considering all data points sent to Dr. Beltrami). In the publication, we did not calculate a coefficient of determination (R 2 ) but a repeated measure correlation coefficient (R) [START_REF] Bakdash | Repeated Measures Correlation[END_REF]. As mentioned in the Statistical Analysis section of our paper, this technique "considers the independence of repeated measures between individuals, so that potential confounding factors, such as between-participant variability, do not interfere".

Dr. Beltrami also raises some concerns about the fact that V di,max may be independent of P di,tw , because several participants reached supramaximality for P di,tw and not for V di,max , or vice-versa.

The key concept behind this part of the Discussion is that the recruitment of accessory inspiratory muscles may also contribute to Pdi tw , particularly at high stimulation intensities [START_REF] Wragg | Comparison of cervical magnetic stimulation and bilateral percutaneous electrical stimulation of the phrenic nerves in normal subjects[END_REF][START_REF] Laghi | Comparison of magnetic and electrical phrenic nerve stimulation in assessment of diaphragmatic contractility[END_REF][START_REF] Attali | Influence of neck muscles on mouth pressure response to cervical magnetic stimulation[END_REF]. In some participants, V di,max reached a plateau at ~90 % of maximal stimulation intensity while Pdi tw increased up to 100 %. Because we hypothesized V di,max to be highly specific of diaphragm contraction and, unlike P di,tw , potentially unaffected by the recruitment of neck inspiratory muscles, we suggested that V di,max may be an interesting index to detect supramaximality of evoked diaphragm contraction.

Dr. Beltrami concludes with the following statement:

"While V di,max might prove itself better at identifying contractility/weakness of the diaphragm, in our opinion inferences in this direction cannot be made from the available data."

We believe that Dr Beltrami's concerns regarding the potential power of Vdi,max for the detection of diaphragm dysfunction is beyond the scope of this paper of which we must recall the main objective: capturing a 300-ms lasting evoked contraction of the diaphragm using ultrafast ultrasound and investigating potential relationships between metrics derived from ultrafast ultrasound with the reference method i.e. Pdi,tw. Accordingly, we can only concur with this statement, as it was never an objective of the current work to determine if V di,max could identify diaphragm weakness as the participants recruited were all healthy and with normal respiratory function. For these reasons, we explicitly stated that "Further studies will focus on this specific point (i.e. V di,max ability to determine diaphragm dysfunction), with the perspective that V di,max may be one parameter, among others, guiding clinicians through the assessment of diaphragm contractility". We are currently investigating the diagnostic power of our approach in patients with suspicion of diaphragm dysfunction. In due time, we will be pleased to share our findings with the scientific community.

The PhD fellowship of TP is funded by the Fondation EDF that is supporting the RespiMyo project, which includes the current work. This study was also supported by the Association Française Contre Les Myopathies (AFM).

  Conception or design of the work; Acquisition or analysis or interpretation of data for the work; Drafting the work or revising it critically for important intellectual content;Final approval of the version to be published; Agreement to be accountable for all aspects of the work Damien Bachasson: Conception or design of the work; Acquisition or analysis or interpretation of data for the work; Drafting the work or revising it critically for important intellectual content; Final approval of the version to be published;Agreement to be accountable for all aspects of the work

	Running Title:
	Dual Publication: N/a
	Funding: Fondation EDF: Thomas Poulard, Jean-Yves Hogrel, Jean-Luc Gennisson,
	Damien Bachasson, N/A; Association Francaise contre les Myopathies (Association
	Française contre les Myopathies): Thomas Poulard, Jean-Yves Hogrel, Damien
	Bachasson, N/A

Disclaimer: This is a confidential document.

the work; Acquisition or analysis or interpretation of data for the work; Drafting the work or revising it critically for important intellectual content;

Fundings

The PhD fellowship of TP is funded by the Fondation EDF that is supporting the RespiMyo project, which includes the current work. This study was also supported by the Association Française Contre Les Myopathies (AFM).

Additional information

Competing interests

None.

Author contributions

All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.


Mathieu De Lafontaine

Erwine Pargon

Guillaume Gay

Camille Petit-Etienne

Sylvain David

Jean-Paul Barnes

Névine Rochat

Abdelatif Jaouad

Maïté Volatier

Simon Fafard

Vincent Aimez

Maxime Darnon

Anisotropic and low

Keywords: Plasma etching, III-V semiconductors, SiCl 4 /H 2 plasma, Through cell via contacts, Multijunction solar cells, Concentrated photovoltaics

Introduction

Over the past few years, through cell via contacts for multijunction solar cells have been the object of a growing interest to replace standard grid line and busbar front contact. [START_REF] Zhao | Via-hole fabrication for iii-v triple-junction solar cells[END_REF]; [START_REF] Richard | Simulation of a through cell via contacts architecture for hcpv multi-junction solar cells[END_REF]; [START_REF] Salvetat | Iii-v multi-junction solar cell using metal wrap through contacts[END_REF]; [START_REF] Oliva | Iii-v multi-junction metal-wrap-through (mwt) concentrator solar cells[END_REF]; [START_REF] Richard | Simulation of through-cell vias contacts under non-uniform concentrated light profiles[END_REF]; [START_REF] Lafontaine | Via sidewall insulation for through cell via contacts[END_REF]] A schematic of the new contact architecture is presented in figure 1.

[ [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] The goal of this new interconnection design is to reduce the shading and the resistive losses by transferring the front side contact towards the backside by using electrically insulated and metallized vias. [START_REF] Richard | Simulation of a through cell via contacts architecture for hcpv multi-junction solar cells[END_REF]] From simulation results, an absolute cell effi- ciency increase of 3 % could be expected by using through cell via contact instead of standard contacts. [START_REF] Richard | Simulation of a through cell via contacts architecture for hcpv multi-junction solar cells[END_REF]] The busbar suppression could also increase the power yield per wafer by 20%. [START_REF] Richard | Simulation of a through cell via contacts architecture for hcpv multi-junction solar cells[END_REF]]

Another study has shown that this architecture could reduce the losses from the light non-uniformities. [START_REF] Richard | Simulation of through-cell vias contacts under non-uniform concentrated light profiles[END_REF]] Finally, this new architecture has been successfully fabricated on InGaP/AlGaAs dual junction heterostructure. [START_REF] Salvetat | Iii-v multi-junction solar cell using metal wrap through contacts[END_REF]; [START_REF] Oliva | Iii-v multi-junction metal-wrap-through (mwt) concentrator solar cells[END_REF]] To the best of our knowledge, through cell via contacts were never fabricated on III-V/Ge triple junction heterostructures.

Plasma etching anisotropic via-holes with low sidewall damage and high aspect ratio (AR) still represents a challenge in fabricating through cell via contacts on III-V/Ge triple junction heterostructures. Indeed, an heterostructure consisting of several III-V layers (InGaP, (In)GaAs, GaAs, Al-InP, AlGaInP, AlGaAs) and quantum dot materials grown on germanium must be etched with a high aspect ratio (AR>3), a good anisotropy and with limited sidewall damage. Only a few studies have been published on mul-tijunction solar cells plasma etching and different plasma chemistries were used. Cl 2 /Ar and BCl 3 /Cl 2 have been used to etch shallow vias (AR 1).

[ [START_REF] Zhao | Via-hole fabrication for iii-v triple-junction solar cells[END_REF]; [START_REF] De Lafontaine | Impact of via hole integration on multijunction solar cells for through cell via contacts and associated passivation treatment[END_REF]] However, these chemistries might not be suited to perform high aspect ratio anisotropic etching since a chlorine-rich chemistry may generate significant sidewall erosion. [de [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] It was previously shown that using SiCl 4 -based plasma can reduce the sidewall erosion thanks to the SiCl x inhibiting layer. [START_REF] Zhao | Inductively coupled plasma etching of through-cell vias in iiiv multijunction solar cells using sicl4/ar[END_REF]; [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] Furthermore, hydrogen addition to the plasma chemistry further reduces the sidewall erosion. [ [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] In addition, hydrogen plasma has also been used previously for its passivating properties. [START_REF] Wang | A detailed study of h2 plasma passivation effects on gaas/si solar cell[END_REF]; [START_REF] Soga | Hydrogen plasma passivation of gaas on si substrates for so-51 lar cell fabrication[END_REF]; [START_REF] Lagowski | Passivation of the dominant deep level (el2) in gaas by hydrogen[END_REF]] Finally, we have shown that a time-multiplexed plasma process may be used for anisotropic Ge etching with a high aspect ratio. [START_REF] Darnon | Deep germanium etching using time multiplexed plasma etching[END_REF]] Solar cell performance is expected to be lowered after plasma etching due to surface recombination on the sidewall by a larger perimeter-to-area ratio.

[ [START_REF] Espinet-Gonzlez | Analysis of perimeter recombination in the subcells of gainp/gaas/ge triple-junction solar cells[END_REF]; [START_REF] Belghachi | Modelling of the perimeter recombination effect in gaas-based micro-solar cell[END_REF]; [START_REF] De Lafontaine | Impact of via hole integration on multijunction solar cells for through cell via contacts and associated passivation treatment[END_REF]] Furthermore, it is possible that the process creates plasmainduced damage on the sidewalls. [de [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] It was demonstrated that by choosing the proper plasma chemistry, it may be possible to reduce the plasma damage and the associated photovoltaic performance loss.

[ [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] Plasma damage on the etched sidewall must therefore be characterized and limited.

In this study, a complete plasma etching process is proposed and characterized to pattern high aspect ratio via-holes on III-V/Ge multijunction heterostructures. The etch morphology was characterized by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM).

The chemical composition of the etched sidewall was analyzed by energydispersive spectroscopy (EDX) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Cathodoluminescence measurements were also performed in order to evaluate the plasma damage on the sidewalls. Furthermore, multijunction solar cells with standard contacts and via-holes etched with this process have been fabricated in order to evaluate the possible photovoltaic performance loss induced by the plasma process.

Experiments

Etching process

The epiwafers used in this study are quantum dot-enhanced III-V/Ge triple junction heterostructures that consist of an In-rich (∼25%) InGaP top cell, an (In)GaAs middle cell (with low In content ∼1%) and a Ge bottom cell. [START_REF] Fafard | Solar cell with epitaxially grown quantum dot material[END_REF]] A cross-section scanning electron microscope (SEM) image of the ∼7µm-thick III-V epitaxy is presented in fig. 2 magnetically-enhanced reactive ion etching plasma process (MERIE). The patterns consist in via-holes with a diameter of 5µm, up to 50µm and trenches with a width of 5µm, up to 100µm. These patterns will lead to an aspect ratio ranging between 0.7 and 8 after the complete etching process. Then the active region of the III-V/Ge heterostructure is etched as presented in figure 3 b. The plasma etching experiments are carried out in an inductively coupled plasma (ICP) reactor, the Plasmalab100 ICP etcher from Oxford.

Both the bias power and the inductive source power are operated at 13.56

MHz. The 1cm x 1cm samples were mounted on a 4 silicon wafer covered with silicon oxide deposited by PECVD. Fomblin oil was used to ensure a good thermal conductivity between the sample and the carrier wafer. The main etching gas used was SiCl 4 , to which, hydrogen could be added as a secondary gas with different ratio to the total gas flow (0%, 33% or 67%). The etching experiments were performed at three different temperatures: 20 • C, 140 • C and 200 • C. The total gas flow was set to 18sccm and the pressure was controlled at 4mTorr. The inductive source and bias power were set to 440W and 260W respectively for the results presented in sections 3.1.1, 3.1.2 and 3.1.3. Unfortunately, due to a chamber condition drift, the inductive source and bias power had to be updated to 550W and 470W respectively for the results presented in sections 3.1.4, 3.1.5 and 3.2. This update had to be made to keep the same etch morphology (for each condition sets) throughout the study. After this first etching step, the active region sidewalls are protected from further damage with a ∼100nm-thick silicon oxide liner deposited by PECVD, as shown in figure 3 c. The liner is etched at the bottom of the patterns (fig. 3 d) with an anisotropic CF 4 plasma in the same Oxford ICP reactor. Then, the via etching is completed by using a time-multiplexed plasma etching process to etch the Ge substrate, as presented in fig. 3 e.

The hard mask and the silicon oxide liner can then be removed by using a 49% hydrofluoric acid immersion for 5 minutes. (fig. 3 f).

Etching process characterization

After the first etching step (fig 3 b), several characterizations were performed. The samples were cleaved in order to study the etched sidewalls. The etch morphology was characterized by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). For samples with severe erosion over all the layers, the hard mask was used as a reference to extract the sidewall erosion values. After etching, the chemical composition of the inhibiting layer was analyzed by energy-dispersive spectroscopy (EDX). The SEM-EDX detector used was an XFlash detector 5030 from Bruker whereas the TEM-EDX detector was from Oxford.

Then, the inhibiting layers are removed with a 1% HF dip selectively over the III-V/Ge sidewalls. After the inhibiting layer removal, the III-V/Ge sidewalls were studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and cathodoluminescence. The ToF-SIMS analyses were performed with an Ion-TOF TOF-SIMS5 instrument with a detection of the negative ions only. A Bi 2+ depth generated by the sputtering is measured by AFM to convert the time scale into a sputtered depth.

The plasma damage on the sidewalls was also studied by cathodoluminescence. The measurements were performed in a Rosa instrument from Attolight and the samples were cooled with liquid He to 4 K. Figure 4 b) shows the area probed during the measurements. The samples were cleaved in order to expose the sidewalls of a 50µm diameter via. This large via size was chosen to minimize the possible luminescence contribution from the cleaved sidewall. The via sidewalls were probed on a centered 8*8µm raster. Furthermore, a 3 keV beam energy was chosen to limit the bulk luminescence as the beam only probed the first ∼50nm. The cathodoluminescence data are analyzed with the Labspec software that enables to extract the cathodoluminescence coming from the InGaP top cell and the (In)GaAs middle cell only.

Multijunction solar cell fabrication

Furthermore, the impact of the etching process was also studied on the photovoltaic performance. Multijunction solar cells with standard contacts (busbar and grid line front contact and back contact) were fabricated. Using the aforementioned plasma process (up to step b of fig. 3), shallow (∼10µm) via holes were etched on the solar cells in order to assess the photovoltaic performance loss associated to via-hole etching. The complete solar cell fabrication process is detailed in previous publications. [ [START_REF] De Lafontaine | Impact of via hole integration on multijunction solar cells for through cell via contacts and associated passivation treatment[END_REF][START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] The back contact is formed by a Ni/Au evaporation.

The grid line and busbar front contact is patterned by photolithography and Ni/Ge/Au/Ni/Au evaporation. A second photolithography and a plasma etching process detailed above is performed to pattern the 10 µm-diameter vias on the cells and trenches to electrically isolate one cell from another. On those samples, photoresist was used as a mask instead of PECVD SiO 2 . One row of vias was patterned between every grid line and the pitch between the vias of a row was 60, 80 or 100µm. The latest characteristic allowed to vary the perimeter-to-area ratio to study the total plasma damage on the device, which scales with the sidewall surface area. Some devices did not have any vias patterned and were used as reference cells in order to assess the plasma damage associated specifically to via-hole etching. Those reference samples also need plasma patterning to electrically isolate the solar cells, meaning that for each plasma chemistries investigated, there is a reference sample.

The comparison of the photovoltaic performance of the different reference samples (with no vias) also gives an insight on the plasma chemistry impact, which is especially relevant for solar cell isolation applications. The photovoltaic performance was studied as a function of the hydrogen ratio to the total gas flow during the plasma process. For the solar cell fabrication, the cathode temperature was fixed at 20 • C during the plasma process. The last step consists in etching the contact layer in a NH 4 OH/H 2 O 2 /H 2 O solution.

Despite the fact that these solar cells aim to evaluate the loss associated to via-hole etching and plasma damage, the (In)GaAs middle cell sidewalls and Ge may be slightly etched during this chemical etching step.[ [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]]

Multijunction solar cell characterization

To assess the plasma damage, the via-hole etching and the impact of hydrogen addition on the solar cell performance, one-sun current-voltage (IV) measurements were performed. The open-circuit voltage (V oc ) was specifically studied since it is the parameter that is the most sensitive to the plasma damage on the sidewalls. The Newport Oriel Sol1A solar simulator with an AM1.5D spectrum was used as the light source for the characterizations and the device temperature was kept at 25 • C. After the first measurement, the solar cells were kept in a cleanroom environment for 5 months and were then measured again in order to study the impact the plasma damage and hydrogen addition over time. Since, the V oc changes measured are usually quite small, [ [START_REF] De Lafontaine | Impact of via hole integration on multijunction solar cells for through cell via contacts and associated passivation treatment[END_REF][START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] at least three solar cells for each combination of pitches and hydrogen fractions were fabricated and measured in order to ensure that the small changes measured were reproductible. The V oc values presented are the average of the V oc of all the cells of a specific pitch and hydrogen fraction combination.

Results and discussion

3.1. III-V/Ge etching process characterization 3.1.1. Impact of temperature with a pure SiCl 4 plasma First, the impact of the cathode temperature on the etch morphology for a pure SiCl 4 plasma was studied. Oxygen can be added to the inhibiting layer during the etching process, whether it was coming from the reactor walls, [Bouchoule et al. (2008a)] or from the SiO 2 cover plate etching. The released oxygen can oxidize the deposited SiCl x on the sidewalls, forming a SiO x Cl y -like layer. It can then turn into a more SiO x -like layer after air exposure since the reaction with ambient air is not controlled. This reaction can be explained by a substitution of chlorine by oxygen. [Bouchoule et al. (2008b); [START_REF] Desvoivres | X-ray photoelectron spectroscopy investigation of sidewall passivation films formed during gate etch processes[END_REF]]

The SiO 2 cover plate etching (fig. 7) creates an atomic oxygen flux (Φ), which can be estimated by using the following equation:

Φ = ρSE (f Si m Si + f O m O ) V m F O (1)
In this equation, Φ is the atomic oxygen flux (in sccm), ρ is the SiO 2 density (2.2 g cm 3 ), S is the cover plate area exposed to the plasma (78.5 cm 2 ), E is the etch rate of the SiO 2 cover plate (70-120 nm min ), f Si and f O are the atomic percentage of silicon and oxygen in the cover plate respectively ( 1 3 and 2 3 ), m Si and m O are the silicon and oxygen molar masses respectively (28.09 g mol and 16 g mol ), V m is the molar volume for an ideal gas (22400 cm 3 mol ) and F O is the fraction of the etched byproducts contributing towards the atomic oxygen flow ( 23 ). From this estimation, an oxygen equivalent atomic flux of 1.5 sccm is probably added to the plasma at 20 • C and it decreases to 0.9 sccm at 200 • C since the SiO 2 etch rate decreases from 120 to 70 nm min , which confirms that oxygen can be incorporated into the inhibiting layer in these conditions. These gas flows represent less than 8% of the total gas flow, which is clearly not negligible nor controlled during the process. The oxygen content could be controlled by adding a small oxygen flow, which could improve the control over the process. However, no oxygen flow was intentionally added as both oxygen and hydrogen can not be used simultaneously.

As for the specific deposition mechanism, the inhibiting layer species can come from the sputtering of the bottom of the etched features in a direct line of sight or come from the plasma gas phase (both the etch byproducts or gas phase condensation). [START_REF] Luere | Etch mechanisms of silicon gate structures patterned in sf6/ch2f2/ar inductively coupled plasmas[END_REF]; [START_REF] Hubner | Calculations on deposition and redeposition in plasma etch processes[END_REF]] Species coming from the sputtering of the etched features tend to deposit a conformal layer on the sidewalls. This deposition mechanism could explain the homogeneous thickness of the inhibiting layer obtained at 20 • C. On the other hand, species coming from the plasma gas phase tend to form thicker layers on the top of the sidewalls than on the bottom. Such mechanism is mainly responsible for the formation of the inhibiting layer at 200 • C since no layer was formed on the Ge, at the bottom of the sidewalls. The species coming from the SiO 2 hard mask etching do not seem to drive the inhibiting layer deposition rate since the SiO 2 etch rate reaches a minimal value at 200 • C (fig. 7). Consequently, the gas phase species from the SiCl 4 precursor drive the inhibiting layer formation at high temperature.

The temperature alters the deposition mechanisms since the thin layer with an homogeneous thickness (100nm) obtained at 20 • C becomes thicker (4µm) spatially localised on the (In)GaAs layers at 200 • C. This behaviour indicates that there is a competition between the sidewall erosion, driven by the atomic chlorine, and the SiCl x species deposition. At 20 • C, the inhibiting layer is porous (fig. 8 b), which explains why there is a small sidewall erosion on walls: (1) the deposition rate is increased at high temperature, as previously reported in SiCl 4 -based plasma-enhanced chemical vapor deposition [START_REF] Huang | Fast growth of polycrystalline film in sicl4/h2 plasma[END_REF]] and (2) the isotropic morphology shields the inhibiting layer from the plasma ion bombardment.

Impact of hydrogen addition at 200 • C

The hydrogen addition to the gas flow was also investigated. Figure 10 presents cross-section SEM images of the etched patterns and figure 11 presents the average etch rate, the average selectivity and the sidewall erosion of the InGaP, the (In)GaAs and Ge layers for a SiCl 4 /H 2 plasma at 200 • C with different hydrogen fraction to the total gas flow (0%, 33% and 67%). When hydrogen is added to the plasma mixture, the sidewall erosion is reduced below <1µm for all the III-V materials and it is completely eliminated in the vicinity of the Ge substrate. As shown in fig 11, the average etch is also signficantly reduced by increasing the H 2 flow (from 1.7 µm min to 450 nm min ). However, H 2 addition at 200 • C does not alter the SiO 2 hard mask etch rate ((76±4) nm min , fig. 7). Therefore, the III-V/Ge over SiO 2 selectivity drops from 24 to 6 by adding hydrogen to the plasma mixture (fig 11).

Hydrogen addition has two major impacts on the plasma process: (1) it decreases the chlorine radical flux by scavenging atomic chlorine and (2) it enhances the SiCl x species deposition [de [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF][START_REF] Huang | Fast growth of polycrystalline film in sicl4/h2 plasma[END_REF]; [START_REF] Gatilova | Investiga-tion of inp etching mechanisms in a cl2/h2 inductively coupled plasma by optical emission spectroscopy[END_REF]; [START_REF] Bruno | Mechanism of silicon film deposition in the rf plasma reduction of silicon tetrachloride[END_REF]] As a matter of fact, it was reported that a high hydrogen fraction can completely swap the process regime from etching to deposition. [START_REF] Bruno | Mechanism of silicon film deposition in the rf plasma reduction of silicon tetrachloride[END_REF]] Both of these mechanisms can be understood with the following equations:

SiCl x → SiCl x-1 + Cl (2) H + Cl → HCl (3)
At high hydrogen fraction, the hydrogen eliminates the atomic chlorine within the plasma. [START_REF] Bruno | Mechanism of silicon film deposition in the rf plasma reduction of silicon tetrachloride[END_REF]] A smaller chlorine radical flux reduces the chemical etching of the process, which explains the reduction in both the sidewall erosion and the average etch rate (fig. 11). Such mechanism was previously observed on InP/InGaAs and InP/InGaAlAs heterostructure plasma etching. [START_REF] Guilet | Optimization of a cl2h2 inductively coupled plasma etching process adapted to nonthermalized inp wafers for the realization of deep ridge heterostructures[END_REF]] The ratio of the sidewall erosion over the etched depth is reduced with increasing hydrogen flow, which leads to an etching process that is, overall, more anisotropic.

The inhibiting layer thickness and morphology are significantly altered by increasing the hydrogen fraction. The inhibiting layer thickness is first reduced (33% H 2 ), to finally disappear from the (In)GaAs sidewalls at 67%.

In the latter conditions, it is now spatially localised on the InGaP sidewalls and its thickness does not exceed 200nm (not shown here). This thickness reduction can be explained by the fact that the precursor flow has been reduced threefold. This is consistent with a previous article that showed that the deposition rate in SiCl 4 /H 2 PECVD is first increased with hydrogen flow but then falls off after surpassing a critical hydrogen flow. [START_REF] Huang | Fast growth of polycrystalline film in sicl4/h2 plasma[END_REF]] This is further confirmed by another study that has shown that the deposition rate is linearly related to the product of H and SiCl x in the gas phase and that there is an optimal H 2 /SiCl 4 ratio, for which, the deposition rate is maximized. [START_REF] Bruno | Mechanism of silicon film deposition in the rf plasma reduction of silicon tetrachloride[END_REF]] The hydrogen addition changes severely the deposition mechanism since the hydrogen atoms can induce the free-bond silicon species, which are involved in the growth process. [START_REF] Bruno | Mechanism of silicon film deposition in the rf plasma reduction of silicon tetrachloride[END_REF]] In this present study, the optimal deposition rate may have been surpassed, which can explain the inhibiting layer thickness reduction.

Furthermore, the SiO 2 hard mask etching can not explain the changes in the inhibiting layer thickness. Indeed, the SiO 2 hard mask etch rate, and its associated oxygen atomic flow remains about the same with H 2 addition at 200 • C ((76±4) nm min , fig. 7).

Impact of hydrogen addition at 20 • C

Hydrogen has also been added to the gas flow of the processes performed at 20 • C. Figure 12 presents cross-section SEM images of the etched patterns and the sidewalls are all steep, which confirms that a temperature of 20 • C limits the chemical etching. Figure 13 presents the average etch rate, the average selectivity and the sidewall erosion of the (In)GaAs and the Al-rich layers layers for a SiCl 4 /H 2 plasma at 20 • C with different hydrogen fractions to the total gas flow. By adding hydrogen, the average vertical etch rate is decreased by 57% to reach ∼450 nm min and it can be explained by the same mechanism presented above: the hydrogen scavenges the atomic chlorine and it limits the chemical etching. Furthermore, since all experiments are performed at a fixed pressure, adding hydrogen implies that the SiCl 4 flow must be reduced. This will reduce the partial pressure associated to Cl-based species and therefore, the heterostructure etch rate as well. This characteristic provides a second explanation on the origin of the reduced etch rate. This etch rate reduction is less significant at 20 • C than 200 • C (75%) since chemical etching is less dominant without thermal activation. Interest- ingly, the etch rates at 200 • C and 20 • C are identical when hydrogen consists in 67% of the total gas flow. It demonstrates that the thermal activation no longer drives the average etch rate at a high hydrogen gas flow. Hydrogen addition at 20 • C reduces the SiO 2 hard mask etch rate (from 125 nm min at 0% H 2 to 85 nm min at H 2 , fig. 7). However, the III-V/Ge over SiO 2 selectivity is mainly driven by the heterostructure etch rate reduction since it drops from 8.5 to 5.3 by adding hydrogen to the plasma mixture (fig 11). As presented in fig. 14, only the (In)GaAs and several Al-rich layers were slightly eroded, whereas both the InGaP and the Ge do not present sidewall erosion at 20 • C.

The erosion is reduced to 0 and 15nm for the (In)GaAs and the Al-rich layers respectively by increasing the hydrogen flow up to 67% (fig. 13) and it is yet another outcome of chlorine scavenging with hydrogen.

Figure 15 presents a TEM-EDX cartography of the (In)GaAs sidewall and its inhibiting layer after an etching process performed at 20 • C and with 67% H 2 . As the hydrogen fraction is increased, the inhibiting layer is still conformal and protects all the layers but it becomes less porous, as shown in fig. 8 c). This characteristic occurs because the reduced chemical etching creates less competition with the deposition. The hydrogen addition also creates a denser layer by scavenging some of the chlorine directly from the inhibiting layer [de [START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]; [START_REF] Gatilova | Investiga-tion of inp etching mechanisms in a cl2/h2 inductively coupled plasma by optical emission spectroscopy[END_REF]] with the following reaction:

SiCl x + H → SiCl x-1 + HCl (4)
This inhibiting layer has a 50nm thickness, which is thinner than its 200 • C counterpart (∼200nm). However, it is impossible to comment on the

III-V/Ge sidewall composition

The atomic composition of the sidewall was further investigated by timeof-flight secondary ion mass spectrometry (ToF-SIMS). Before the measurement, the inhibiting layer was stripped with diluted HF. This way, it is possible to study the elements that may have been incorporated into the III-V/Ge sidewall during the plasma process. Figure 16 presents the average chlorine ion intensity taken from the sidewalls as a function of the depth into the etched III-V/Ge sidewall for all three processes performed at 20 • C. The samples etched at both 140 • C and 200 • C were not studied as the sidewall erosion is not suitable for the final application. For a pure SiCl 4 plasma, a high amount of chlorine is incorporated into the III-V/Ge sidewall. As expected, the chlorine concentration decreases with the depth into the etched sidewall. Chlorine concentration decreases with hydrogen addition and it is mainly located within the first 1nm. At 67%, it seems that chlorine penetration depth is more spread. The reduced chlorine concentration can be explained by the chlorine atom scavenging by the hydrogen. Furthermore, since adding hydrogen requires reducing the SiCl 4 flow in order to keep both a fixed pressure and a fixed total gas flow, there will be less Cl-based species available in the plasma. Both of these characteristics reduce the amount of atomic chlorine atoms in the plasma that are available to be incorporated into the sidewall. It was previously shown that chlorine diffusion in III-V materials may create Cl-related defects. [START_REF] Landesman | Evidence of chlorine ion penetration in inp/inasp quantum well structures during dry etching processes and effects of induced-defects on the electronic and structural behaviour[END_REF]] Therefore, this mechanism could also reduce the Cl-related defect density.

Figure 17 presents the average hydrogen ion intensity taken from the sidewalls as a function of the depth into the etched III-V/Ge sidewall for all three processes performed at 20 • C. For a pure SiCl 4 plasma, a low amount of hydrogen is incorporated in the III-V/Ge sidewall. Since no hydrogen was intentionally added for this process, this low amount can be attributed to residual hydrogen within the ToF-SIMS chamber. A larger amount is incorporated with the plasma etching process with 33% H 2 . The hydrogen concentration within the III-V/Ge sidewall is even larger when the hydrogen fraction is raised from 33% to 67%. In both the later cases, the maximum concentration is located 2nm beneath the sidewall surface.

Sidewall plasma damage

To further study these properties, cathodoluminescence measurements were performed on the III-V semiconductor plasma-etched sidewalls after the inhibiting layer removal. Figure 18 presents the cathodoluminescence intensity of the InGaP top cell as a function of the photon energy for the pure SiCl 4 process (0%), the process with 67% H 2 performed at 20 • C and a cleaved sidewall used as a reference. The maximum peak intensity is reached at ∼1.84eV which is consistent with the InGaP band gap energy. It is possible to notice that the intensity is at least twice as large with any plasma process instead of the cleaved sample. This is a first indication that the plasma process does not damage much the heterostrucure in the vicinity of the InGaP top cell. However, the cathodoluminescence intensity is increased by 700% when a high hydrogen fraction is added when compared to the cleaved sidewall. Such behaviour can be explained by a reduction of the nonradiative defect on the InGaP sidewall. This characteristic is in agreement with the ToF-SIMS results. The high hydrogen fraction within the plasma helps scavenging the chlorine which reduces the Cl-induced non-radiative defects. Furthermore, the hydrogen incorporation into the sidewalls (fig. 17) can passivate impurities or vacancy-related recombination centers. Hydrogen can passivate impurities in semiconductors as previously shown on Si. [START_REF] Chang | Theory of hydrogen passivation of shallowlevel dopants in crystalline silicon[END_REF]] and GaAs [START_REF] Johnson | Hydrogen passivation of shallow-acceptor impurities in p-type gaas[END_REF]] In addition, it has been observed that, after GaN plasma etching, a hydrogen plasma passivates the nitrogen vancancies, which also leads to a luminescence increase. [START_REF] Chen | Individual roles of atoms and ions during hydrogen plasma passivation of surface defects on GaN created by plasma etching[END_REF]] The authors also observed that a high hydrogen-radical-to-ion flux ratio is preferable to maximize the passivating properties on GaN. In the case of our study, since the sidewalls are investigated, the geometry naturally promotes a high neutral-to-ion flux ratio in this vicinity. Despite the fact that InGaP and (In)GaAs were studied in this work, it is possible that a similar passivation process occurred. This property provides a second explanation on the 700% luminescence increase shown in fig. 18. Both the previous publications and the results of this study suggest a specific passivation mechanism.

The hydrogen radicals passivate the impurities or vacancy-related recombination centers, whereas the geometry enables an ion flux at grazing angle on the sidewalls, which limits the dissociation of the newly-formed bonds with hydrogen.

Figure 19 presents the cathodoluminescence intensity of the (In)GaAs middle cell as a function of the photon energy for the pure SiCl 4 process (0%), the process with 67% H 2 performed at 20 • C and a cleaved sidewall used as a reference. The maximum peak intensity is reached at ∼1.4eV which is consistent with the (In)GaAs band gap energy. The peak intensity variation associated to the (In)GaAs middle cell does not vary more than 30% between all three samples. Therefore, the luminescence peak does not change much whether the sample was cleaved, plasma etched or even when hydrogen was added. In the vicinity of the (In)GaAs layers, no benefits from the hydrogen addition could be observed. This result is not consistent with a previous publication indicating that hydrogen plasmas have passivating properties by removing an arsenic excess and its associated native oxide (As 2 O 3 ). [START_REF] Gottscho | Realtime monitoring of lowtemperature hydrogen plasma passivation of gaas[END_REF]] A similar mechanism was also reported for GaAs passivation with silicon nitride by PECVD. [START_REF] Richard | Mechanisms of gaas surface passivation by a one-step dry process using low-frequency plasma enhanced chemical deposition of silicon nitride[END_REF] In the case of this study, the etched sidewall does not have a native oxide since it was still considered bulk before the process. Therefore, the hydrogen had no native oxide to react with. Hydrogen could still remove an arsenic excess on the etched sidewall. [START_REF] Gottscho | Realtime monitoring of lowtemperature hydrogen plasma passivation of gaas[END_REF]] However, the cathodoluminescence results indicate that this passivation mechanism was poorly efficient (fig. 19). An explanation for this could be that the As-excess was not completely removed during the plasma etching process. This explanation is consistent with the etching sequence of the heterostructure (fig. 2). The InGaP top cell is the first subcell to be etched and it has the slowest etch rate. During this part of the process, the (In)GaAs sidewalls can not be passivated since this layer is not etched yet. The (In)GaAs middle cell is the second subcell to be etched, during which, the top cell sidewalls are still exposed to the plasma and its passivating properties. This means that the InGaP sidewalls are exposed to the plasma for a greater period of time than the (In)GaAs middle cell.

Consequently, the InGaP top cell received a complete passivation resulting in a 700% luminescence increase (fig. 18), whereas the middle cell had an incomplete passivation because a large fraction of the process was required to etch the top cell and it leads to no significant changes in cathodoluminescence (fig. 19).

Secondary peaks between 1.45eV and 1.5eV can also be observed in fig.

19. They may come from the interactions of different III-V materials both above and below the (In)GaAs layers. Because of the intricacy of the heterostructure studied, no specific identification or explanation for those secondary peaks can be provided in the scope of this study. A separate study on isotype substrates will be required to rigorously identify these secondary peaks and the associated etching mechanisms. or with a high hydrogen fraction (67%). A first measurement was performed right after the fabrication cycle (T 0 ) and the via-hole etching process induces a relative V oc loss between 2% and 3%. These values are in agreement with previous studies. [ [START_REF] De Lafontaine | Impact of via hole integration on multijunction solar cells for through cell via contacts and associated passivation treatment[END_REF][START_REF] De Lafontaine | Influence of plasma process on iii-v/ge multijunction solar cell via etching[END_REF]] However, a slightly smaller loss (∼2%) is obtained when a high hydrogen flow (67%) is used instead of a pure SiCl 4 plasma (∼3%). These measurements are below the precision of the measurement setup. In order to confirm that the hydrogen addition has also a positive effect on the open-circuit voltage, a good level of reproductibility was obtained. At least three solar cells for each pitch and hydrogen fraction combination, for a total of 24 solar cells, were fabricated and measured. All the solar cells of each subset present the same V oc within 0.1% and 0.8%. These values are all below the V oc changes associated to hydrogen addition and for all the via pitches studied. Therefore, the smaller V oc loss associated to hydrogen addition is real and not simply a statistical artifact.

Solar cell performance characterization

Furthermore, it is possible to notice that, for both the cells with vias and the reference cells, when a high hydrogen flow is used, a higher opencircuit voltage is obtained. The passivating properties of this process is also observable on the standard reference cells since the mesa etching step used to A slight V oc increase can be observed as the pitch is increased from 60µm to 100µm on the cells with vias etched with a pure SiCl 4 plasma. This is related to the reduction of the perimeter-to-area-ratio with increasing via pitch. As the perimeter-to-area of the device decreases, the total sidewall recombination is less dominant and the solar cell presents a higher V oc . However, this behaviour is not noticeable on the cells with via-holes etched with a high hydrogen fraction. The V oc does not seem to change much by increasing the via pitch and this may also be explained by an efficient sidewall passivation. With the defect passivation, the complete device is less sensitive to perimeter-to-area effect, hence the steady V oc over all the studied via pitches.

One of the challenges with passivation is to ensure that it has a good stability over time. Therefore, these characterizations were also performed five months after the initial measurement in order to assess the stability of the hydrogen passivation. The results are also presented in fig. 20 (T 0 +5 months). For all the samples studied, the influence of the hydrogen addition to the plasma mixture is still noticeable after five months. Indeed, all the solar cells etched with a high hydrogen fraction plasma still exhibit a higher open-circuit voltage when compared to their solar cell counterpart etched with the pure SiCl 4 process. The impact of via-hole etching remains the same after five months as the relative V oc loss is still between 2% and 3%. Furthermore, the relative loss associated to via-hole etching is still lower when hydrogen is added to the plasma mixture (2.1-2.7% depending on the via pitch) compared to the pure SiCl 4 plasma (2.3-2.9% depending on the via pitch). Since solely the hydrogen addition to the plasma mixture can explain this behaviour, this result indicates that the hydrogen plasma passivation persists over time for at least 5 months. The long-lasting stability of hydrogen-based passivation was also previously observed in another study. [START_REF] Gottscho | Realtime monitoring of lowtemperature hydrogen plasma passivation of gaas[END_REF]] For through cell via contact multijunction solar cells, the via sidewalls will be covered by a dielectric barrier that is expected to further increase the stability of the semiconductor sidewalls.

Etching process completion

The SiCl 4 /H 2 process detailed above is only one step (fig. 3 b) of the full etching process (fig. 3). The SiCl 4 /H 2 plasma is solely used to etch the active layers of the III-V/Ge heterostructure without sidewall erosion and with limited plasma damage. To reach those goals, this step was performed with the 67% H 2 process performed at a cathode temperature of 20 • C. However, additional steps are required to obtain deep and anisotropic vias through the III-V/Ge heterostructures as shown in fig. 3. To protect the sidewalls from further damage, a silicon oxide liner is deposited by PECVD (step c in fig. 3).

Then, a CF 4 -based plasma process is used to etch the liner at the bottom of the patterns (step d in fig. 3) by using the same SiO 2 hard mask. Figure 21 a) presents cross-section SEM images after both the liner deposition and the anisotropic liner etching. can be obtained with an aspect ratio of 5, but aspect ratios larger than 10 After this last step, the liner still protects the sidewall of the active region from plasma damage. In summary, these last steps enables deep and high aspect ratio etching through the III-V/Ge heterostructure while protecting the active region from further damage.

Conclusion

We propose a complete plasma etching process to etch high aspect ratio patterns in III-V/Ge triple junction solar cell heterostructure with low damage for the fabrication of multijunction solar cells. By investigating the influence of H 2 flow and cathode temperature, we demonstrated that the lateral erosion is strongly reduced at low temperature and high H 2 flow.

Chlorine radicals scavenging, lower volatility of InCl x etch by-products and denser inhibiting layer deposition at the pattern sidewall enable a minimal to also persist over time since the voltage increase was still noticeable after 5 months, which makes it a promising process for complete device fabrication.

A SiO 2 liner and a time-multiplexed Ge etching process are also presented in order to etched high aspect ratio patterns while protecting the active region sidewalls. In summary, the best plasma conditions enabled to etch patterns anisotropically through a complex III-V/Ge heterostructure with minimal damage conditions that persist over time. Therefore, this plasma etching process is promising for the fabrication of a through cell via contact architecture on multijunction solar cell but also for conventional triple junction solar cells isolated by plasma etching or for other III-V based photonic applications.

Acknowledgement

Figure 1 :

 1 Figure 1: Schematic of a multijunction solar cell with a through cell via contact architecture. [de Lafontaine et al. (2019)]

  . A complete etching process has been developed to pattern deep (>50µm) and anisotropic vias on III-V/Ge heterostructures and its schematics are outlined in fig 3. The first fabrication step consists in patterning a 5µm-thick silicon oxide hard mask on the epitaxial heterostructure (fig 3 a). Silicon oxide is deposited by plasma-enhanced chemical vapor deposition (PECVD).It is then patterned by using contact photolithography and a Ar/C 4 F 8 /O 2

Figure 2 :

 2 Figure 2: Cross-section scanning electron image of the III-V/Ge triple junction heterostructure.

Figure 3 :

 3 Figure 3: Schematics of the complete via-hole etching process.

Figure 4 :

 4 Figure 4: Schematic of the a) ToF-SIMS measurement setup of the etched sidewall and b) the cathodoluminescence measurement setup of the etched sidewall.

  Figure 5 presents cross-section SEM images as a function of the cathode temperature of the etched patterns with a pure SiCl 4 plasma. It shows a degradation of the heterostructure profile with increased temperature. As shown in fig 6, the lateral erosion of Ge and (In)GaAs layers gradually increases with temperature (from 100nm to 3µm). The In-rich InGaP erosion is absent below 150 • C and it skyrockets to 10µm at 200 • C. Isotropic etching results from the chemical action of chlorine rad-icals, which is known to be activated by temperature, explaining the lateral etch rate increase. [Ohori et al. (2019); Pearton et al. (1994)] As for the In-rich InGaP, InCl x are supposed to be the etch byproducts, which volatility limits InGaP etching at low temperature and those compounds become volatile above 150 • C. [Chen et al. (2000); Asakawa et al. (1998)] This explains why the InGaP lateral etching is considerably enhanced above 150 • C.

Figure 6

 6 Figure6also shows an increase of the vertical average etch rate of the heterostructure with temperature, which is mainly attributed to the activation of the vertical etching of the InGaP layer. In combination with the SiO 2 etch rate reduction at high temperature, as shown in fig.7, the III-V/Ge over SiO 2 etch selectivity increases with temperature. The SiO 2 etch rate reduction at high temperature can be explained by the enhanced deposition of SiCl x species, as previously reported in SiCl 4 -based plasma-enhanced chemical vapor deposition.[START_REF] Huang | Fast growth of polycrystalline film in sicl4/h2 plasma[END_REF]] Despite the isotropic behaviour observed at higher temperature, there is a deposition of etch inhibitors on the pattern sidewalls (or etch-inhibiting layer) that can be observed. At 20 • C, an inhibiting layer with an homogeneous thickness (100nm) along the sidewalls is observed on the STEM image presented in fig 8 b). By increasing the temperature, the inhibiting layer becomes spatially localized on the (In)GaAs region and can be several micrometers thick, as shown on the SEM-EDX image in fig. 9. This layer seems porous and ex-situ EDX reveals a SiO-like chemical composition although no oxygen is voluntary introduced. A similar chemical composition is detected at all temperatures. Despite the fact that no chlorine is detected, it is likely that a SiCl x layer is deposited while the etch is performed.

Figure 5 :

 5 Figure 5: Cross-section SEM images of the patterns etched with a pure SiCl 4 plasma at a) 20 • C, b) 140 • C and c) 200 • C. Fig 5 d), e) and f) are close-ups of the sidewalls etched at 20 • C, 140 • C and 200 • C respectively. It can be noted that, at 140 • C, the (In)GaAs layers with low indium concentration exhibit a larger sidewall erosion than the In-rich InGaP layers. This trend is reversed at 200 • C, as the In-rich InGaP layers exhibit a larger sidewall erosion than the (In)GaAs layers.

Figure 6 :

 6 Figure 6: Average etch rate, selectivity and sidewall erosion of the main semiconductors in the heterostructure obtained with a pure SiCl 4 plasma as a function of the cathode temperature.

Figure 7 :

 7 Figure 7: SiO 2 etch rate in SiCl 4 /H 2 as a function of the cathode temperature

Figure 9 :

 9 Figure 9: EDX cartography of the patterned sidewall and its inhibiting layer in the vicinity of an (In)GaAs layer etched with a pure SiCl 4 plasma at 200 • C. Fig 9 a) is a SEM image presenting a close-up of fig 5 c) and it presents the area sampled by EDX whereas fig 9 b), c), d), e) present the EDX intensities associated to Si, Ga, O and As respectively.

Figure 10 :

 10 Figure 10: Cross-section SEM images of the patterns etched at 200 • C with SiCl 4 and a) 0%, b) 33% and c) 67% H 2 fraction of the total gas flow. Fig 10 d), e) and f) are close-ups of the sidewalls etched at 200 • C with SiCl 4 and a) 0%, b) 33% and c) 67% H 2 fraction of the total gas flow.

Figure 11 :

 11 Figure 11: Average etch rate, selectivity and sidewall erosion of the main semiconductors in the heterostructure, obtained at 200 • C with a SiCl 4 /H 2 plasma as a function of the percentage of the hydrogen flow to the total flow.

Figure 12 :

 12 Figure 12: Cross-section SEM images of the patterns etched at 20 • C with SiCl 4 and a) 0%, b) 33% and c) 67% H 2 fraction of the total gas flow.

Figure 13 :

 13 Figure 13: Average etch rate, selectivity and sidewall erosion of the main materials obtained at 20 • C with a SiCl 4 /H 2 plasma as a function of the percentage of the hydrogen flow to the total flow.

Figure 14 :

 14 Figure 14: Cross-section STEM images performed on the etched sidewall. Fig 14 a) is a SEM image presenting the areas analyzed by STEM. The STEM images were performed in the vicinity of the (In)GaAs middle cell of the patterns etched at 20 • C with SiCl 4 and b) 0% c) 33% and d) 67% H 2 fraction of the total gas flow as well as in the vicinity of the Al/In-rich layers of the patterns etched at 20 • C with SiCl 4 and e) 0%, f) 33% and g) 67% H 2 fraction of the total gas flow.

Figure 15 :

 15 Figure 15: TEM-EDX cartography of the sidewall and its inhibiting layer in the vicinity of an (In)GaAs layer etched with a SiCl 4 and H 2 (67% of the total gas flow) plasma at 20 • C. The SEM image on fig 15 a) presents the area analyzed by TEM-EDX whereas fig 15 b), c), d), e) f) and g) present the EDX intensities associated to Si, O, Cl, In, Ga and As respectively.

Figure 16 :

 16 Figure 16: ToF-SIMS measurement of the average chlorine ion intensity taken from the sidewalls measured as a function of the depth into the etched III-V/Ge sidewall for different hydrogen fraction of the total gas flow. The measurement was performed after the inhibiting layer removal.

Figure 17 :

 17 Figure 17: ToF-SIMS measurement of the average hydrogen ion intensity taken from the sidewalls measured as a function of the depth into the etched III-V/Ge sidewall for different hydrogen fraction of the total gas flow. The measurement was performed after the inhibiting layer removal.

Figure 18 :

 18 Figure 18: Cathodoluminescence intensity of the InGaP top cell as a function of the photon energy. The measurement was performed after the inhibiting layer removal.

Figure 19 :

 19 Figure 19: Cathodoluminescence intensity of the (In)GaAs middle cell as a function of the photon energy. The measurement was performed after the inhibiting layer removal.

Figure 20

 20 Figure 20 presents the one-sun AM1.5D open-circuit voltage of multijunction solar cells with vias with different via pitches (60µm, 80µm and 100µm) compared to multijunction solar cells without via-holes (Ref). Both the vias and the isolating trenches were etched with either a pure SiCl 4 plasma (0%)

  electrically isolate one cell to another is also performed by the same plasma etching process. This result is of interest even beyond the fabrication of the through cell via contact architecture. It provides an appealing pathway to increase the conversion efficiency of III-V based solar cells with standard contacts since this step is usually performed by wet chemical etching on state-of-the-art multijunction solar cells.[START_REF] Dimroth | Wafer bonded four-junction gainp/gaas//gainasp/gainas concentrator solar cells with 44.7% efficiency[END_REF]; Geisz et al.(2020)]The enhanced open-circuit voltage with hydrogen can be explained by the quality of the etched sidewalls. A reduction of the non-radiative defect density reduces the recombination at the edge of the device and it results in a higher open-circuit voltage. This result provides a solid confirmation for the mechanism presented by the ToF-SIMS and cathodoluminescence measurements in the previous sections. Indeed, it was previously observed that hydrogen passivate defect-related recombination centers on solar cells resulting in a higher conversion efficiency.[START_REF] Wang | A detailed study of h2 plasma passivation effects on gaas/si solar cell[END_REF];[START_REF] Soga | Hydrogen plasma passivation of gaas on si substrates for so-51 lar cell fabrication[END_REF]

Figure 20 :

 20 Figure 20: Open-circuit voltage of multijunction solar cells with vias with different via pitches (60µm, 80µm and 100µm) compared to multijunction solar cells without via-holes (Ref). Both the vias and the isolating trenches were etched with either a pure SiCl 4 plasma (0%) or with a high hydrogen fraction (67%). The measurements were performed right after the fabrication cycle (T 0 ) and after 5 months (T 0 + 5 months).

  Figure 21 b), c) and d) present close ups of fig. 21 a)on the sidewall. It is possible to observe that a continuous liner covers all the III-V/Ge sidewall. This demonstrates that the PECVD process is sufficiently conformal to enable deposition on the active layer sidewall, which provides protection against further damage. Moreover, the CF 4 plasma process is sufficiently anisotropic to etch the liner at the bottom of the patterns while keeping the sidewall undamaged. The last step is a time-multiplexed plasma process to etch the Ge substrate. It consists of a SF 6 /O 2 isotropic etching step multiplexed with a C 4 F 8 passivation step in order to obtain anisotropic etching. Figure22presents cross-section SEM images of the etched patterns after the Ge etching process (step e in fig.3) In figure22 a), it is possible to notice that a depth of 50µm

Figure 21 :

 21 Figure 21: Cross-section SEM images of the etched patterns after liner deposition and liner etching (step d of fig. 3).

Figure 22 :

 22 Figure 22: Cross-section SEM images of the etched patterns after the time-multiplexed Ge etching (step e of fig. 3).

  LN2 is a joint International Research Laboratory (IRL 3463) funded and co-operated by Université de Sherbrooke (Canada) and CNRS (France) as well as INSA Lyon, ECL, UCBL, Université Grenoble Alpes (UGA) as well as the French national nanofabrication network RENATECH. The TOF-SIMS and cathodoluminescence measurements were supported by the French "Recherches Technologiques de Base" program and were performed on the Nano Characterization Platform (PFNC) of the CEA Grenoble. The support from NSERC, Prompt and STACE through the MARS-CPV project is acknowledged. M. de Lafontaine acknowledges FRQNT and Université Grenoble Alpes for financial support. The authors would like to thank T. Chevolleau, T. Charvolin, S. Litaudon and D. Drouin for fruitful discussions.

beam at 30 kV was used for the analysis with a 21*21µm field of view, a 70µs cycle time, and a 512 pixels resolution. The sample was tilted to perform the mapping on the etched sidewall of a 100µm-wide trench, as shown in fig

a). The mass of the ions sputerred from the sample surface is recorded over the experiment duration, allowing to reconstruct an ion profile with the sputtering time. At the end of the process, the pattern