D Cheynis

P Barge

M Maury

S Juhel

Pierre Lagrasta

Müller

M Labrot

F Cheynis

D Barge

P Maury

M Juhel

S Lagrasta

P Müller

Improvement of etching and cleaning methods for integration of raised source and drain in FD-SOI technologies

Keywords: SiBCN, SiGe, SiCoNi TM Preclean, FD-SOI, Raised Source/Drain, WET clean

Improvement of etching and cleaning methods for integration of raised source and drain in FD-SOI technologies

Introduction

The scaling of CMOS devices down to nanometric sizes has lead to various integrations strategies to control short channel effects. Two distinct integrations are now clearly identified for technological nodes below 20 nm: FINFET integration [1,2] and FD-SOI [3].

FDSOI integration requires the use of very thin channel materials, typically below 7 nm to obtain an accurate control of the channel [4,5]. Such dimensions are inadequate for junction engineering in the source and drains, which thus needs to be raised by Epitaxy. This technique is commonly referred to as Raised Source Drain (RSD) in the literature [6].

Figure 1 schematizes the principal fabrication steps of RSD devices. The basic brick for designing and building a FDSOIbased device consists in a SOI substrate formed by an ultra-thin Si or SiGe film, defining the device channel, on a thick insulator (BOX for buried oxide) bonded to a Si substrate. In the first steps of the process, the SOI substrate is topped by a 3D gate insulated from the channel by a High-K dielectric layer (HfO 2 ). The whole surface is covered with thin low-k dielectric layers (0p) of silicon nitride (typically Si 3 N 4 or SiBCN). This step is followed by a photoresist coating which, after photolithography, exposes denuded zones for further etching of the low-k dielectric layers (step 2). The etching leaves resist residues that are removed by successive dry (plasma) and wet strips but leads to the formation of an oxide layer (step 3). The step 4 consists in removing the so-formed oxide layer (while avoiding any silicon nitride

A C C E P T E D M A N U S C R I P T 2 consumption) followed by a surface cleaning. The last step (step 5) is the subsequent epitaxial growth of the SiGe:B raised source and drain. Notice that conventional cleaning processes of SOI samples are usually based on high temperature treatments but such thermal treatments cannot be used for ultra-thin SOI films that, when heated, agglomerate in 3D islands [7,8,9,10,11,12]. In Ref. [12] we proposed a new plasma-based cleaning methods (SiCoNi process) for SOI substrates that avoids any high temperature treatment. In this study, our goal is to go beyond this first approach and to integrate these etching and cleaning methods in the principal fabrication steps of SOI-based devices. From a methological point of view, we focus in this study on the ultimate criterion to optimize the different technological steps involved in the RSD-epitaxy brick that consists in a careful comparison of the electrical performances of the devices fabricated with or without the optimizated processes. Details regarding the targets for the different electrical performances and parameters can be found in Ref. [START_REF] Labrot | Low temperature raised source and drain epitaxy for Fully Depleted Silicon on Insulator (FD-SOI) technology[END_REF]. In this paper, we will simply use, as an ultimate test, the increase of the number of active dies per cm2 issued from the optimized process.

The paper, is divided in four parts: after a short presentation of the methodology, we describe the silicon nitride deposition (low-k dielectric layer), its etching properties and the queue time effect after etching (steps 1 and 2). In the third part we show how SiCoNi process may be modified to increase the SiO 2 /silicon nitride etching selectivity (step 4). In the fourth part we show that a combination of wet and siconibased methods enables to obtain a channel clean surface whose state (roughness, contamination, defectivity) enables a subsequent RSD epitaxy of good quality.

I/ Methodology: I.1/ Simulation of the process

For the sake of simplicity we will not study real devices (characterized by complex 3D geometries) but will focus on planar surfaces that reproduce the successive technological processes including thin-film deposition, etch, clean... The so-studied samples (300 mm wafers) are described in Fig. 2. They simulate the initial SOI surface state (Fig. 2.2) exposing the channel surface,

A C C E P T E D

M A N U S C R I P T 3 the surface state after silicon nitride deposition (low-k dielectric layer deposition, Fig. 2.3), after the silicon nitride etching (Fig. 2.4), after the surface cleaning including the formation and the removal of a wet oxide layer (Fig. 2.5) and after the subsequent epitaxy of Si 1-x Ge x :B epitaxy defining the RSD layer.

I.2/ Methods:

We will use various dry and wet cleaning processes reported in table 1 and detailed in appendix A: 1: HF-rinse-and dry (called ''HF last''), 2: SiCoNi Preclean, 3: HF-RCA, 4: HF-last + SiCoNi, 5: HF-RCA+SiCoNi. The cleaned and uncleaned surfaces are characterized by measuring their roughness, defectivity and chemical composition by using classical tools described in table 2. (for more details the reader is referred to [12]).

Process

II/ Optimization of step 2: silicon nitride deposition and etching II.1/ Silicon nitride choice

For CMOS technologies, oxide or nitride dielectrics are used to encapsulate and isolate the gate laterally (Fig. 1 step 1, see appendix B for details regarding the dielectric layer deposition). These dielectric layer also has a spacer function to precisely localize dopant implantations and subsequent RSD epitaxial growth close to the gate. Since these doped regions must be very close to the channel, the spacer has to be as thin as possible. The lower limit of the thickness can be reduced by using low permittivity material. Other criteria have to be taken into consideration to choose the dielectric material. It should resist to HF etching and should not oxidize. Si 3 N 4 silicon nitride is standardly used in CMOS technology as a low-k dielectectric. However, boron decreases the silicon nitride permittivity and carbon increases its resistance to HF etching [13]. Consequently SiBCN coating has been considered in the present work as it decreases the effective capacitance of the spacer (permittivity of 5.2 instead of 7.5 for Si 3 N 4 ) and improves by 10% the performance and speed of the final transistor compared to standard nitride spacer [14].

II.2/ Silicon nitride characterization after deposit

A C C E P T E D M A N U S C R I P T

4

Figure 3 shows AFM images and RMS measurements of surfaces after nitride deposition. The measured roughness is smaller for Si 3 N 4 than for SiBCN by almost a factor 2. Such a difference is essentially due to the grain size. Indeed despite a polycrystalline configuration for both materials, SiBCN forms larger grains than Si 3 N 4 (see AFM pictures) [13,14].

The surface defectivity measured by SIR is greater for SiBCN than for Si 3 N 4 (~30 defects per wafer). Despite its greater defectivity, SiBCN material shows better performance (permittivity and etching selectivity -see section III). As a result, we will use SiBCN as a dielectric spacer in the following.

II.3/ Silicon nitride etching and queue time impact

In CMOS technology, the removal of the silicon nitride film (step 2) is a critical step as it represents a possible source of device damages. Indeed a possible over-etch of the nitride may result in damages of a thin oxide and the underlying silicon substrate [START_REF] Nojiri | Dry Etching Technology for Semiconductors[END_REF]. For this purpose, in industrial conditions, the etching is generally realized in two stages: a quick etching stage followed by a slow selective etching stage. This procedure avoids corner effects at the vertical-spacer/channel junction and thus helps to obtain a vertical spacer with a constant thickness. In the case under study (simulation process) we only use the quick etching stage. The silicon nitride etching (Fig. 1 step 2) consists in dry/etch based on N 2 /O 2 plasma.

For practical reasons, there is always a queue time between the etching (step 2) and the surface preparation before the RSD deposition (Fig. 1 step 5). During this queue time the surface may degrade by surface contamination and/or oxidation. This queue-time cannot be reduced to zero. We thus study the influence of the queue time on the surface defects (Fig. 4) and on the surface roughness (Fig. 5). As shown in Fig 4, the etching may leave polymers defects (typically CF x H, CF x N, CF x O) and resin residues (CH-CH) on the surface. Due to their high volatility these residues may partially disappear during the queue time. It is for instance the case for fluoride components which reduce by a factor 2 for queue times of 14 days (see table 3). However, at smaller scale, polymers tracks remain, so the surface has to be cleaned by wet and dry strips [START_REF] Gabriel | [END_REF]. In Figure 5 Finally, notice that, the surface contamination has a huge effect on the subsequent epitaxy. For instance we report in Fig. 7 SEM images obtained after subsequent epitaxy of SiGe:B on these contaminated surfaces. Since the epitaxy enlarges the surface defects [12] the queue time drastically decreases the quality of the epitaxial layer. Such large defects correspond to bright spots in post-clean AFM images in Fig. 5. The results show that the selectivity decreases with a reduction of the NF 3 gas flow with respect to the NH 3 (compare for instance processes 2 and 3 or 9 and 10). The selectivity rate increases with the temperature. The heating time does not have any impact on the selectivity, but a minimum time must be kept for salt desorption. The results, synthesized in Fig. 7, unambiguously show that the best selectivity is achieved for process 9 of Table 4, i.e. with low temperature and high gas flow. Moreover for a given process (reference process) a better selectivity is achieved with SiBCN than with Si 3 N 4 . Beyond the etching selectivity, the different processes, described in Table 4, lead to different surface states that have been characterized by SIR and are described in Table 5. We artificially divide the defects in two populations: "Small defects" (40 nm <x ≤ 150 nm) and "large defects" (150nm <x ≤ 1 µm). The number of defects increases for lower temperatures and higher NF 3 /NH 3 flow ratio but still remains fully compatible with device working conditions for all reported samples.

IV/ Optimization of step 5: combination of wet and dry cleanings to improve the channel surface quality

Throughout the successive technological steps, the devices to-be are

A C C E P T E D M A N U S C R I P T 7
regularly cleaned. It is especially the case of the channel surface on which any defect may be detrimental for subsequent source and drain epitaxy. Based on our previous work [12] on the optimization of the surface cleaning of a SOI wafer (zero level in Fig 1 ) we compare the states (roughness, contamination, defectivity) of the channel surface that result from various combinations of wet and SiCoNi-based dry cleanings.

All the experimental results are compared to a reference process that corresponds to a surface simply exposed to HF-RCA + SiCoNi treatment (best cleaning treatment for the initial stage depicted in Fig. 2 [12]).

IV.1/ Comparison of surface defectivity before and after cleaning

Figure 8 shows the total number of defects (measured by SIR) after nitride deposition, after nitride etching and after different cleaning processes. After silicon nitride deposition and etching, the number of defects remains low for both Si and SiGe surfaces. For most of the tested clean combinations, the surface defectivity remains comparable with the reference process. Only the single SiCoNi clean produces a significant larger number of defects. However when combined with the HF-last process (HF-last+SiCoNi), the resulting surface exhibits a number of defects per wafer lowered by a factor 2 with respect to the reference process (HF RCA+SiCoNi).

The benefit of SiCoNi is thus clearly evidenced: without SiCoNi cleaning the defectivity is above 4 defects.cm -2 and with the SiCoNi process, this number is reduced to less than 0.1 defect.cm -2 .

IV.2/ Surface contamination induced by the cleaning processes

A quantitative analysis of the surface residual contamination due to cleaning processes is not easy to perform since in our experimental conditions we transfer the sample from the cleaning chamber to another chamber equipped with analytical tools. This transfer may contribute to surface contamination. The surface contamination has thus been analyzed by SIMS on capped films that means after subsequent epitaxy of a protective layer (SiGeB in our case).

The SIMS concentration profiles I(z), where z is the analysis depth normalized to the thickness of the SiGe:B epitaxial deposit, obtained after the different cleaning processes are reported in Fig 9 . The surface of the epitaxially grown film corresponds to z=0 while the location of the initial cleaned surface (before being further embedded by protective epitaxy) corresponds now to the interface located at z=1. For the sake of clarity, the SIMS profiles corresponding to different cleaning processes have been vertically shifted.

According to the cleaning process various residual contaminants have been detected. They are reported in Table 6 and essentially correspond to C, O, Cl and F. We can see that most of the cleaning methods, excepted the HF-RCA+SiCoNi and HF-last process, lead to a surface contamination greater than the one measured on the reference sample cleaned by HF-RCA+SiCoNi [12]. Notice that the F contamination, and the residual associated roughness, may be reduced by a decrease of the queue time between the formation of the (NH 4 ) 2 SiF 6 salt and its desorption during the SiCoNi process (see ref [12]).

IV.3/ Synthesis

Table 7 synthesizes the main results. In terms of defectivity, contamination at the SiGe/SiGe:B interface (channel/RSD interface)

A C C E P T E D

M A N U S C R I P T 9 and morphology, the best surface state is reached with HF RCA + SiCoNi. Even if the SIMS profiles are slightly wider in this study than those reported in [12], the final surface state is comparable to the one obtained by the standard low temperature treatment used for SiGe in its bulk configuration [12].

Process Defectivity Contamin ation

Morpho logy

HF-last ☺ ☺  SiCoNi Preclean    HF-last + SiCoNi ☺  ☺ HF-RCA + SiCoNi ☺ ☺ ☺

VII/ Improvement of electrical devices

Electrical performances have been measured on an ensemble of MOSFET transistors. More precisely characteristics in current and tension, leakage currents, carrier mobility, access resistance and capacity values have been measured. They are reported in Ref. [START_REF] Labrot | Low temperature raised source and drain epitaxy for Fully Depleted Silicon on Insulator (FD-SOI) technology[END_REF]. The characteristics of the optimized devices all belong to the targeted values which have been standardly defined as a prerequisite for being used in electronic industry (ITRS roadmap). The ultimate test is the number of active dies per wafer, that means the number of dies that perfectly fit the functioning zone. This analysis show a huge enhancement of this percentage since the number of electrical active devices increases from 40 % to 90 %.

VIII/ Conclusion

Studying planar surfaces that simulate the principal steps of fabrication of FD-SOI based devices, we have been able to optimize the conditions to epitaxially grow Raised Source and Drain. More precisely we show that:

(1) The best silicide to be used is SiBCN.

(2) The SiCoNi process may be adapted to control the SiO 2 /Silicon nitride etching selectivity.

(3) The queue time between nitride etching and surface cleaning has to be reduced to avoid surface contamination, (4) The best surface preparation for raised source and drain integration in FD-SOI devices is the HF-RCA + SiCoNi (process 9 in table 3). Such a low thermal budget process corresponds to an optimized version of the method proposed in Ref. [12] and avoids any dewetting of the channel in the experimental conditions used.

(5) Analysis of electrical devices show that all these improvements lead to a doubling of the percentage of electrical active dies per wafer.

The so proposed recipes developed for blanket wafers may be easily transferred to real patterned substrate to reach high quality CMOS devices as shown in Fig. 10.  FD-SOI technology requires the use of very thin channel and thus the use of raised sources and drains.

 The quality of the epitaxially grown RSD depends on the surface quality of the channel and thus depends on many technological steps preliminar to the RSD subsequent growth.

 The present report describes how etching and cleaning methods may be improved to reach high quality FD-SOI based CMOS devices.

Figure 1 :

 1 Figure 1: Initial state before gate fabrication and principal steps of fabrication of a FD-SOI based device from a basic gate-brick. Step 1/ Silicon nitride spacer coating, step 2/ Silicon nitride etching, step 3/ wet and dry strips with oxide formation, step 4/ surface cleaning (oxide removal without spacer consumption), step 5/ subsequent epitaxial growth of SiGe:B source and drain.

Figure 2 :

 2 Figure 2 : Description of different SiGe samples (300 mm SOI wafer) under study: 1) initial silicon bulk used as a reference material, 2) SiGe layer grown by epitaxy (channel layer), 3) nitride silicon deposition (low-k dielectric layer), 4) silicon nitride etching, 5) surfaces cleaned by various processes, 6) SiGe:B layer grown by epitaxy (Raised-Source-Drain epitaxy). The steps in bracket correspond to figure 1.

Figure 3 :

 3 Figure 3: 1 µm x 1 µm atomic force microscopy images associated with different types of silicon nitride surface.

5 Figure 4 :

 54 Figure 4: Top view SEM images after post nitride etching of surface evolution for different day (queuetimes impact) (a) on SiGe epitaxy, (b) on Si (100). In this last case (b) a polymer residue observed just after etching spontaneously desorbs during queue-times and thus disappears on the SEM images.

Figure 5 : 3 :

 53 Figure 5: (a) 1 µm x 1 µm atomic force microscopy images associated with post nitride etching on Si(0 0 1) and SiGe epitaxy. (b) RMS value on different Si/Oxide and SiGe/oxide interfaces for different treatments (Post Etching, Post clean with queue-times (day +14) and no queue-time).

Figure 6 :

 6 Figure 6: Top view SEM images after SiGe:B epitaxy. (a) Raised source and drain reference epitaxy (SiGe:B) with no queue-time, (b) Epitaxy (SiGe:B) raised source and drain on Si with queue-times (Day +14), (c) Epitaxy (SiGe:B) raised source and drain on SiGe with queuetimes (Day +14).

Figure 7 :

 7 Figure 7: Study of selectivity (SiO 2 /Silicon nitride) for different process SiCoNi. In the right part we compare the selectivity for SiBCN and Si 3 N 4 materials (error bars correspond to statistic errors).

Figure 8 :

 8 Figure 8: Total number of defects on a 300 mm wafer measured in different conditions that means after nitride deposition (step 1) (black), after nitride etching (step2) (red), and after surface cleaning (step 4) by wet cleans, plasma cleans and combination of both for SiGe.

A

  C C E P T E D M A N U S C R I P T 8

Figure 9 :

 9 Figure 9: SIMS depth profiles of the F, C, Cl and O atoms in SiGe/SiGe:B interface.

Figure 10 -

 10 Figure 10 -CMOS FD-SOI devices obtained from the optimized processes described in this paper.

Table 1 :

 1 Description of the different processes used in this work to prepare the surface in step 5 in Fig.2.

		Description	Passivation	Chemical Oxide
	1	HF last	-H	No
	2	SiCoNi Preclean (NH4F)	-H	No
	3	HF-RCA	-OH	Yes
	4	HF-last + SiCoNi	-H	No
	5	HF-RCA + SiCoNi	-H	No

Table 2 :

 2 Methods of analysis used in this work.

Table 4 :

 4 Description of the various SiCoNi processes checked in this study with the used parameters. The last column shows the selectivity obtained for each process.

	6

Table 5 :

 5 Total number of defects on a 300 mm wafer measured for various processes.

	Process	T etch =35°C	T etch =32,5°C	T etch =30°C
		small	big	small	big	small	big
		defect	defect	defect	defect	defect	defect
	1, 5, 8	11	2	13	4	24	3
	2, 6, 9	12	4	8	1	8	1
	3, 7, 10	5	3	5	1	25	27
	4, 11	7	2			15	3

Table 6 :

 6 Chemical

contaminations of the SiGe/SiGe:B interface for different surface preparations as determined by SIMS.

Table 7 :

 7 Synthesis of experimental results for different surface preparations of SiGe surface.

Acknowledgements

This work was supported by ANRT N°1030/2013.

A C C E P T E D M A N U S C R I P T

Removing particles SC1 oxide and desoxidation simultaneously. This has the effect of oxidize the silicon as the basis particles, and then "reject" the particles by electrostatic repulsion during the desoxydation. The effectiveness of SC1 may be increased with the addition of an action Physics: (megasonic or Spray: not used in your case)

SC2

(or dHCl for diluted HCl)

HCl (Hydrochloric acid) then DI water

Removing of metal contamination SC2 dissolved metals. In the case, we will in fact dilute HCL (H 2 O 2 not used but single wafer tool).

HF (HF-Last

A C C E P T E D M A N U S C R I P T

12

Appendix B: Atomic Layer Deposition of SiN or SiBCN for spacer

The process to realize a dielectric film having Si-N, Si-C or Si-B bonds on a semi-conductor substrate is based on ALD (Atomic Layer Deposition) [22]. The majority of ALD reactions use two precursors [START_REF] Tsuji | [END_REF]. These precursors react with the surface in a sequential, self-limiting, manner. Through repeated exposures to the different precursors (cycle process), a thin film is deposited. ALD deposition at 450°C-650°C allows a good uniformity, a minimal incorporation of impurities and adequate stoichiometry. The so-obtained silicon nitride is polycrystalline as reported in Ref. 13 and 14.

Appendix C: Design of experiments (DOE) and statistic approach

Our main goals are: For this purpose we use a design of experiments based on 2 4+1 Fractional factorial design [START_REF] Pukelsheim | Optimal Design of experiments[END_REF]. The sodefined matrix consists in the 11 experiments reported in table 4. Processes 5, 6 and 7 are identical and thus enable to estimate the statistical errors. The other processes of table 4 correspond to nine variants of the initial SiCoNi process. Linear regression models have thus been used to infer the response associated to the screening of the 4 experimental parameters. These fits are obviously relevant only if the variation of the response to a screened-factor is greater than the statistical error bars defined at 1σ. This is expressed numerically by a quality factor R 2 close to 1 (R 2 = 0.998). Within this scheme (i) the experimental parameters have been optimized, (ii) the statistical errors have been measured. The differences in selectivity (Table 4) or in the total number of defects (Table 5) are thus found to be statistically relevant. For instance the selectivity recorded for processes 9 and 11 table 4 is better than the others when considering the error bars reported in Fig. 7.


Marta K Schütz

Matthieu Petit
email: matthieu.petit@univ-amu.fr

Lisa Michez

Alain Ranguis

Guillaume Monier

Christine Robert- Goumet

Jean-Manuel Raimundo
email: jean-manuel.raimundo@univ-amu.fr

Thiol-functionalization of Mn 5 Ge 3 thin films

Keywords: thiol, passivation, self-assembled monolayers, manganese germanide, Mn 5 Ge 3, wettability, spintronics, surface free energy

The surface state of Mn 5 Ge 3 exposed to air and common colvents, is analyzed by XPS.

The surface free energy of Mn 5 Ge 3 is calculated with the OWRK method.

The grafting of Mn 5 Ge 3 films by octane-and perfluorodecane-thiol SAMs is proposed.

INTRODUCTION

Over the last decade the manganese germanide Mn 5 Ge 3 has appeared to be a potential candidate for spin injection into group-IV semiconductors due to its room-temperature ferromagnetism properties (Curie temperature T C = 297 K), high spin polarization, metallic character, ability to epitaxially grow on Ge(111) substrates as well as compatibility with the already-existing Si-Ge-based technology [START_REF] Zeng | Epitaxial ferromagnetic Mn5Ge3 on Ge(111)[END_REF][START_REF] Le Thanh | Epitaxial growth and magnetic properties of Mn5Ge3/Ge and Mn5Ge3Cx/Ge heterostructures for spintronic applications[END_REF][START_REF] Olive-Mendez | Epitaxial growth of Mn5Ge3/Ge(111) heterostructures for spin injection[END_REF].

Moreover, several studies have shown that its T C can be enhanced by carbon doping into the interstitial sites of the Mn 5 Ge 3 lattice and that the Schottky contact between Ge and Mn 5 Ge 3 may be suitable for electrical spin injection [START_REF] Gajdzik | Strongly enhanced Curie temperature in carbon-doped Mn5Ge3 films[END_REF][START_REF] Slipukhina | Simulation of the enhanced Curie temperature in Mn5Ge3Cx compounds[END_REF][START_REF] Spiesser | Control of magnetic properties of epitaxial Mn5Ge3Cx films induced by carbon doping[END_REF][START_REF] Petit | Mn5Ge3C0.6 /Ge(111) Schottky contacts tuned by an n-type ultra-shallow doping layer[END_REF]. Furthermore, hybrid structures of organic molecules and inorganic materials have been recently in the spintronic community's crossroad. The first purpose was to use the molecules as spin transport media because of the rather weak coupling between the spin and orbital momentum of the electron that allows keeping the spin orientation over long distances. It has been rapidly anticipated that molecules could also have strong impacts on the magnetic properties, for instance, several teams dealing with spin valves reported negative magnetoresistance [START_REF] Ding | Inverse Magnetoresistance in Polymer Spin Valves[END_REF][START_REF] Xu | Tunneling magnetoresistance observed in La0.67 Sr0.33 Mn O3 /organic molecule/Co junctions[END_REF]. These phenomena confirmed the growing interest on the developing molecular "spinterface" which could lead to single multi-functional devices thanks to the "LEGO" capability of molecular engineering [START_REF] Galbiati | Spinterface: Crafting spintronics at the molecular scale[END_REF][START_REF] Cinchetti | Activating the molecular spinterface[END_REF][START_REF] Moodera | On the path toward organic spintronics[END_REF][START_REF] Tran | Magnetic properties of bcc-Fe(001)/C60 interfaces for organic spintronics[END_REF].

Molecules and biomolecules possessing a magnetic core are potential candidates to be grafted on the magnetic substrate Mn 5 Ge 3 . One of the most promising approaches to modify surface involves the deposition of organic self-assembled monolayers (SAMs). SAMs constitute a method of choice to achieve highly dense and ordered thin films, robustly anchored to large surface areas either on clean metallic (e.g. Au, Ag), oxidized (e.g. SiO 2 , ITO) or semiconducting (e.g. Si, Ge) substrates. This method does not require either sophisticated and expensive instrumentation, or intricate sample preparations, expert manpower and allows the opportunity to vary both the length of the chains and functional groups to manipulate macroscopic properties of the surface, such as wettability, biocompatibility and protein/cellular adhesion [START_REF] Ulman | Formation and Structure of Self-Assembled Monolayers[END_REF][START_REF] Schreiber | Structure and growth of self-assembling monolayers[END_REF][START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF][START_REF] Whitesides | Molecular engineering of surfaces using selfassembled monolayers[END_REF]. Furthermore, magnetic properties of metals and diluted magnetic semiconductors (DMS) such as (Ga, Mn)As have been tailored by SAMs surface modifications leading to induced magnetic moments or modification of ferromagnetic properties [START_REF] Wang | Robust Manipulation of Magnetism in Dilute Magnetic Semiconductor (Ga,Mn)As by Organic Molecules[END_REF][START_REF] Kreutz | Modification of ferromagnetism in semiconductors by molecular monolayers[END_REF][START_REF] Raman | Interface-engineered templates for molecular spin memory devices[END_REF].

Alkanethiols are so far the most studied self-assembled molecules because of the covalent polarized bond formed between thiol headgroups and appropriate substrates (e.g. Au, Ge, Ag, Cu, Pt, Hg, GaAs).

Regarding Ge surface, HBr and HCl are often used to halogenate the surface of this semiconductor prior to the SAMs formation, mainly to avoid the process of reoxidation of the surface. Indeed, the classical hydrogenation, which is used for silicon surface passivation, is less stable in the case of the Ge surface [START_REF] Cai | 1-Dodecanethiol based highly stable self-Assembled monolayers for germanium passivation[END_REF][START_REF] Ardalan | Formation of Alkanethiolate Self-Assembled Monolayers at Halide-Terminated Ge Surfaces[END_REF][START_REF] Ardalan | Reaction mechanism, bonding, and thermal stability of 1-alkanethiols self-assembled on halogenated Ge surfaces[END_REF][START_REF] Hohman | Simple, robust molecular self-assembly on germanium[END_REF][START_REF] Lefevre | Towards organic film passivation of germanium wafers using diazonium salts: Mechanism and ambient stability[END_REF]. The inherent ability of adsorbed monolayers to modify physicochemical properties of surfaces makes this approach attractive to implement the organic functionalization of the surface of the Mn 5 Ge 3 /Ge system. To the best of our knowledge, functionalization of Mn 5 Ge 3 /Ge with alkylthiols has never been done before. The Mn 5 Ge 3 thin films being elaborated under ultra high vacuum conditions by molecular beam epitaxy (MBE), we present herein a study of the surface state of the Mn 5 Ge 3 thin films under air and common solvents exposure and secondly the first insights on the functionalization of Mn 5 Ge 3 with thiol molecules.

MATERIALS AND METHODS

Substrates.

Sb-doped Ge(111) wafers (n-type, 600 µm thickness, 12-16 Ω cm) were purchased from Umicore (Belgium). Mn 5 Ge 3 /Ge(111) heterostructures were grown by molecular beam epitaxy (MBE) technique according to the method described by Petit & al [START_REF] Petit | Very lowtemperature epitaxial growth of Mn5Ge3 and Mn5Ge3C0.2 films on Ge(111) using molecular beam epitaxy[END_REF]. This method is based on the co-deposition at room temperature of Mn and Ge. It allows growing high crystalline quality films of Mn 5 Ge 3 with an abrupt interface on Ge(111) and with a small RMS roughness (ca. 2 nm). The Mn 5 Ge 3 thin films were 30 nm thick.

Grafting procedures.

2.2.1 Cleaning. Mn 5 Ge 3 /Ge(111) substrates were first successively sonicated in acetone, deionized water and ethanol. Each cleaning step was repeated three times with a fresh solvent for 5 min, followed by blow-drying with argon gas.

Thiol-functionalization.

Cleaned Mn 5 Ge 3 /Ge(111) substrates were immersed into octanethiol (OT) (CH 3 (CH 2 ) 7 SH, Alfa Aesar) or perfluorodecanethiol (PFDT) (CF 3 (CF 2 ) 7 CH 2 CH 2 SH, Sigma-Aldrich) solutions for surface functionalization. The experiments were carried out at room temperature to avoid any interdiffusion of Ge and Mn at the Mn 5 Ge 3 /Ge interface that could be detrimental for further spintronic applications. Freshly thiol solutions were prepared in ethanol prior to be used. The vials were backfilled with argon gas in order to minimize O 2 in the headspace. After experiments, the substrates were rinsed three times with ethanol to remove physisorbed molecules on the surface and dried over 24 h in a desiccator at room temperature. We have mainly investigated two parameters: i) the thiol concentration (from 1 to 5 mM) and ii) the immersion reaction time (from 12 to 24, 36, 48 and 72 h). First, a range of concentration of thiol was tested, at fixed immersion time of 24h, in order to determine the optimal concentration needed. Optimum condition of concentration was selected (4 mM, see the Results and Discussion) and experiments were conducted by varying the immersion times.

Characterizations.

Foremost, a chemical analysis of the untreated Mn 5 Ge 3 substrates have been done by X-ray photoelectrons spectroscopy (XPS) in an ultrahigh vacuum system equipped with an Omicron DAR 400 X-ray source. The Mg anode was used to perform the analysis. The hemispherical analyser (HSA) is an Omicron EA 125 (normal detection was used). The angle between the X-ray source and the HSA was equal to 55°. The topographies of the substrates were imaged by atomic force microscopy (AFM) with a Nanoscope IIIA Multimode (Digital instruments) equipped with a 10×10×2.5 µm scanner. The images were recorded in the tapping mode at room temperature using silicon nitride probes (HQ: NSC15/AL BS, Mikromasch). The curvature radius of the silicon tips was about 8 nm (from the supplier specifications). Contact angle measurements were performed for the assessment of the hydrophobic-hydrophilic character of the substrates. Static contact angles (CA) were measured with an OCA 15 apparatus (DataPhysics) at room temperature using the sessile drop method and image analysis of the drop profile (SCA20 software). Deionized water droplet volume was 5 μL, and the contact angle was measured 10 s after the drop was deposited onto the surface.

RESULTS AND DISCUSSION

3.1 Characterization of the Mn 5 Ge 3 /Ge(111) substrates before the grafting process.

XPS:

The Mn 5 Ge 3 samples were grown in a MBE chamber and taken out from the UHV chamber for the grafting process. The different samples were exposed successively to atmosphere and chemical cleaning prior undergoing the SAMs grafting procedures. To our knowledge this is the first report describing both aging changes of the Mn 5 Ge 3 surface under air exposure, stability under chemical cleaning/etching with polar aprotic or protic solvents such as acetone and ethanol respectively.

After air exposure, Mn 5 Ge 3 surface has been analyzed by XPS in order to determine the chemical state of the surface atoms and then the surface free energy has been measured. The Ge 3d , O 1s and Mn 2p core levels were recorded and the parameters for the XPS fits are given in the electronic supplementary information (ESI). The Figure 1 shows the electronic state of the Mn 5 Ge 3 surface after air exposure.

Figures 1a and1b exhibit the evolution of the Ge 3d core level before (cleaned) and after air exposure whereas fig. 1c concerns the evolution of the Mn 2p core level. The "cleaned" Ge 3d core level exhibits one peak at 29.1 eV, related to Ge atoms in the bulk state in Mn 5 Ge 3 (blue contributions in Figure 1a and 1b respectively). On Fig. 1b, the peak around 31.5 eV is typical of germanium oxide. It can be deconvoluted into two main contributions corresponding to the Ge atoms in the oxidized states Ge 2+ and Ge 3+ [START_REF] Schmeisser | Surface oxidation states of germanium[END_REF]. Thus, the oxidized state of the Ge atoms differs from a cleaned (non oxidized) Mn 5 Ge 3 surface and to a Ge(111) substrate in which the Ge 4+ state, corresponding to the GeO 2 oxide (see ESI), is prevailing. This difference is also confirmed by the analysis of the O 1s core levels (see ESI).

Regarding the Mn 2p core level (Fig. 1c), the oxidation by air leads to a strong shift of about 2.5 eV at higher energy that could be attributed to the Mn atoms in oxidized states around 2-2.7 + similar to those found in MnO or Mn 3 O 4 [START_REF] Carver | Use of XRay Photoelectron Spectroscopy to Study Bonding in Cr, Mn, Fe, and Co Compounds[END_REF]. These features confirm a clear difference for the native oxide structures of Ge(111) and Mn 5 Ge 3 suggesting that the latter may correspond to a ternary oxide of Mn-Ge-O. Moreover the Fig. 1d exhibits the evolution of the Ge 0 as well as the oxide contributions Ge 2+ and Ge 3+ for the Ge 3d core level along a depth profile in the oxidized Mn 5 Ge 3 layer. The depth profile has been made using an Ar + ionic sputtering. We have evidenced that the oxidation of the Mn 5 Ge 3 thin film took place in the first 4 nm, after one week of air exposure. This indicates that the oxidation of the Mn 5 Ge 3 film is not limited to the very surface and oxygen diffuses into the layer. Prior the SAMs formation, the Mn 5 Ge 3 film surface is successively cleaned with acetone then ethanol and carefully analysed each step by XPS (Figure 2). The Ge3d core level is followed after each solvent rinse highlighting specific behaviours (Fig. 2a,2b). For instance, after acetone cleaning, the surface exhibits a Ge 4+ contribution unlike the ethanol case. The deconvolution profile after ethanol treatment is very similar to the one of the air exposed sample and also the intensity of the overall oxide component is smaller than in that case. This last observation could be explained by the fact that GeO 2 is soluble in water and that we used a 96% ethanol. This is consistent with several studies regarding the influence of wet surface treatments and SAM deposition processes which have demonstrated that water or a mixture of water and ethanol can dissolve germanium oxide [START_REF] Hohman | Simple, robust molecular self-assembly on germanium[END_REF][START_REF] Onsia | A study of the influence of typical wet chemical treatments on the germanium wafer surface[END_REF][START_REF] Prabhakaran | Oxidation of Ge(100) and Ge(111) surfaces: an UPS and XPS study[END_REF]. Surprisingly, concerning the Mn 2p3/2 and O 1s core levels (Fig. 2c and2d), no significant discrepancy can be noticed. The O 1s shapes slightly differ due to the stoichiometry changes of the oxide accordingly to the solvent used and to the water sensitivity of the germanium oxide component of the overall oxide. These XPS results clearly show that the oxide layer does not consist only on a germanium oxide layer but contains also a manganide part. Consequently the nature of the oxide layer may be close to that of a ternary oxide Mn-O-Ge. The oxidation state of the Mn 5 Ge 3 oxide layer slightly changes depending on environmental conditions. This observation was also carried out on Ge surfaces, which are known to exhibit Ge 2+ and/or Ge 4+ oxidation states depending on the surface treatments [START_REF] Hanrath | Chemical Surface Passivation of Ge Nanowires[END_REF][START_REF] Collins | Chemical functionalisation of silicon and germanium nanowires[END_REF].

Surface free energy (SFE):

SFE determinations have been achieved using the Owens, Wendt, Rabel and Kaelble (OWRK) method with the measure of the contact angles (CAs), with different liquids (deionized water, formamide, glycerol and dichloromethane), on the air exposed Mn 5 Ge 3 surface [START_REF] Rabel | Einige Aspekte der Benetzungstheorie und ihre Anwendung auf die Untersuchung und Veränderung der Oberflächeneigenschaften von Polymeren[END_REF][START_REF] Owens | Estimation of the surface free energy of polymers[END_REF][START_REF] Kaelble | Dispersion-Polar Surface Tension Properties of Organic Solids[END_REF]. Based on the Fowkes' method, the surface free energy is divided into a polar part (non-dispersive, P) and a dispersive part (D) (Eq.1) [START_REF] Fowkes | Attractives forces at interfaces[END_REF]:

σ sl =σ s +σ l -2 ( √ ( σ s D σ l D ) + √ ( σ s P σ l P )) (Eq.1)
The interfacial tension σ sl between liquid and solid is calculated with the two surface tensions σ s and σ l of the solid and the liquid and the similar interactions between the phases. These interactions are interpreted as the geometric mean of a dispersive component σ D and a polar one σ P of the surface tension. The Figure 3 displays the plot of the OWRK method. The corresponding linear fit affords a dispersive contribution of σ D Mn5Ge3 = 14.7 mJ.m -2 and a polar part of σ P Mn5Ge3 = 31.8 mJ.m -2 , which corresponds to a total SFE of σ Mn5Ge3 = 46.5 mJ.m -2 . As for comparison, the SFE of SiO 2 , an extensively studied surface for SAM deposition, is within the range of 40-77 mJ.m - 2 with a predominance of the polar component fostering the interactions with polar head-groups such as thiol derivatives [START_REF] Chibowski | Surface free energy and wettability of silyl layers on silicon determined from contact angle hysteresis[END_REF][START_REF] Hejda | Surface Free Energy Determination by Contact Angle Measurements -A Comparison of Various Approaches[END_REF][START_REF] Miskiewicz | Influence of SiO2 surface energy on the performance of organic field effect transistors based on highly oriented, zone-cast layers of a tetrathiafulvalene derivative[END_REF]. In addition, it is known that under vacuum conditions, the surface of Mn 5 Ge 3 is terminated by Mn atoms therefore the SFE value of an unchanged surface might be close to that of the Mn metal, around 1350-1600 mJ.m -2 [START_REF] Dau | An unusual phenomenon of surface reaction observed during Ge overgrowth on Mn5Ge3/Ge(111) heterostructures[END_REF][START_REF] Aqra | Surface energies of metals in both liquid and solid states[END_REF]. Moreover, the reported SFE value for a pure manganese oxide surface (Mn 3 O 4 , 72 mJ.m -2 ) supports the XPS data, evidencing that the surface of a Mn 5 Ge 3 thin film is strongly affected both by air exposition and solvents treatments [START_REF] Kulkarni | Hausmannite Manganese oxide cathodes for supercapacitors: Surface Wettability and Electrochemical Properties[END_REF]. It was observed that OT molecules could easily chemisorb onto the Mn 5 Ge 3 surface without any significant changes in the surface roughness, except for concentrations ≥ 5 mM. Increasing the solutions concentration (≥ 5 mM) ensue the formation of multilayers that affect dramatically the roughness (fig. 4f). Noteworthy, if the concentration is lower than 4 mM (Figures 4d: 3mM, 4e: 4mM) small separated aggregates of about 30-50 nm in diameter and 2-2,5 nm in height are present within the monolayer. This height is in the range of the length of the grafted molecules (~2,5 nm). A different behaviour was obtained when PFDT was used to passivate the surface of the substrate. Results obtained with PFDT molecules exhibited higher RMS roughness on average but with a greater dispersion of the values. Water CA measurements for each experiment have revealed that the hydrophobicity of the Mn 5 Ge 3 /Ge SAM-modified substrate does not significantly change between both thiol molecules regardless the used concentration for an immersion reaction time of 24 hours, reaching values around 85°. However, PFDT molecules provided the highest values of CA at lower concentrations than OT grafting which is normally expected due to higher hydrophobic character of the perfluoroalkyl chains compared to the alkyl chains.

In order to evaluate the effect of the immersion time on the Mn 5 Ge 3 /Ge functionalization, we have No significant difference on the hydrophobicity nor the roughness of the surface was observed by varying the immersion time. The maximum value of the CA was obtained for 36h of immersion and reached 95°, before decreasing slightly. However, the grafting process occurs before 12h of immersion.

We observed by AFM (not shown here) that a more homogeneously ordered and dense layer is formed after 36 hours of immersion whereas prolonged immersion time (> 48 hours) conducts to the formation of aggregates and less ordered layers. This behaviour is similar to the kinetic observed for nalkylthiolates in 1:1 water-ethanol solvents mixture deposited on Ge surfaces [START_REF] Cai | 1-Dodecanethiol based highly stable self-Assembled monolayers for germanium passivation[END_REF][START_REF] Hohman | Simple, robust molecular self-assembly on germanium[END_REF]. Nevertheless, the measured CAs remain lower than the ones reported on Ge surfaces, around 100-110°. This feature could be ascribed to an uncompleted coverage of the Mn 5 Ge 3 surface by the OT SAM or a deterioration of the SAM upon extended immersion time that could facilitate the diffusion of ethanol till the film surface and leading to a possible dissolution of the oxide layer (attested by a decrease of the CA after 36h).

CONCLUSIONS

We presented herein a simple and effective method to deposit SAMs on Mn 5 Ge 3 thin films. We have evidenced that thiol derivatives (i.e. OT and PFDT) can easily chemisorb onto Mn 5 Ge 3 surface as selfassembled monolayers. Our results showed that changes in surface roughness are more pronounced with PFDT than with OT molecules. The optimal conditions, based on the concentrations and immersion reaction time, to ensure well-ordered, dense and homogeneous SAMs were found to be a thiol concentration of [C] = 4 mM and an immersion time of 36 hours. Higher values of these parameters resulted mainly in an increase of the surface roughness due to the formation of aggregates on the surface and less ordered SAM layers. Comparing the SAMs from OT and PFDT we did not observe any significant changes in the wettability properties of the film surface even it was expected that perfluorinated alkylthiols could render the Mn 5 Ge 3 surfaces more hydrophobic (C-F bonds are more hydrophobic than the C-H bonds). Further improvements on the deposition methodologies still need to be matured for Mn 5 Ge 3 substrate and could lean for instance towards the use of a solvent mixture of 1:1 ethanol-water and a process temperature around 60-80°C which has been proved to be efficient for the deposition of SAMs onto Ge substrates [START_REF] Cai | 1-Dodecanethiol based highly stable self-Assembled monolayers for germanium passivation[END_REF][START_REF] Hohman | Simple, robust molecular self-assembly on germanium[END_REF]. Moreover, further XPS analysis should be performed on the grafted surfaces so as to assess the formation of the covalent polarized bonds between the molecules and the Mn 5 Ge 3 substrate. Nevertheless this seminal report on functionalization of Mn 5 Ge 3 paves the way to new routes and new opportunities for modifications to access to novel hybrid materials in combination with the fascinating magnetic properties of Mn 5 Ge 3 .

Figure 1 :

 1 Figure 1: XPS spectra related to the effect of air exposure on the Mn 5 Ge 3 surfaces. a) Reference: Ge 3d core level of a cleaned (non oxidized) Ge(111) substrate. b) Deconvolution of the Ge 3d core levels from an oxidized Mn 5 Ge 3 surfaces. c) Superposition of Mn 2p3/2 core levels of a cleaned and oxidized Mn 5 Ge 3 layers. d) Evolution of the contributions of the Ge 3d core level according to a depth profile of oxidized Mn 5 Ge 3 thin films done by ionic Ar + bombardment.

Figure 2 :

 2 Figure 2: XPS analysis of the Mn 5 Ge 3 surface after exposure to acetone and ethanol. a) and b) Ge 3d core level. c) Mn 2p3/2 and d) O 1s core levels. (For c) and d), the air exposed case is included as a comparison).

Figure 3 :

 3 Figure 3: Plot of the Owens, Wendt, Rabel and Kaelble method for calculating the surface free energy of the Mn 5 Ge 3 thin film exposed to air.

3. 2

 2 Grafting of the Mn 5 Ge 3 /Ge(111) substrates. The Mn 5 Ge 3 /Ge(111) substrates have been subjected to SAMs surface modifications with two different alkyl thiols namely OT and PFDT (1-octanethiol and perfluorodecanethiol, respectively) by varying the thiol concentrations and then the immersion reaction times. The morphology and wettability of the newly formed SAMs were investigated with AFM and water CA measurements, respectively, for a given immersion time of 24h. The surfaces roughness obtained from the AFM images and the CA values are presented on the Figures 4a and 4b (the AFM images are available in the ESI).

Figure 4 :

 4 Figure 4: a)-b) RMS Roughness (from 2×2 μm 2 AFM images) and water CA versus thiol molecule concentrations for Mn 5 Ge 3 surfaces grafted with a) OT and b) PFDT SAMs. (Immersion time = 24h). c)-f) AFM images 500×500nm 2 of OT SAMs for concentration of 0 (image c) (i.e. Mn 5 Ge 3 bare

  performed the experiments with OT solutions at a concentration of [C] = 4 mM as, both the dispersion of the RMS roughness and the CA values are lower than those measured with the PFDT. The immersion reaction time was varied from 12 to 72 hours. RMS roughness and the water CA are displayed in Figure 5.

Figure 5 :

 5 Figure 5: RMS roughness and water CA of Mn 5 Ge 3 /Ge substrates after functionalization with 4 mM OT as a function of immersion time.

5 -S5 6 -

 56 2x2 μm 2 AFM topography images of Mn 5 Ge 3 /Ge substrate after functionalization with octanethiol (OT) for 24 hours as a function of thiol concentration: a) Mn 5 Ge 3 surface prior to grafting, b)-f) OT concentrations 1, 2, 3, 4, 5 mM. 2x2 μm 2 AFM topography images of Mn 5 Ge 3 /Ge substrate after functionalization with perfluorodecanethiol (PFDT) for 24 hours as a function of thiol concentration: a)-e) PFDT concentrations 1, 2, 3, 4, 5 mM. S6 80

ACKNOWLEDGEMENT M.K.S. acknowledges the National Council of Scientific and Technologic Development (CNPq) of the Brazilian Ministry of Science, Technology and Innovation (MCTI) for the postdoctoral fellowship. All the authors are thankful to C Gérard from the Chemical-biochemical service of the CINaM, for the surface free energy measurements.

Electronic Supplementary Informations (ESI)

Thiol-functionalization of Mn 5 Ge 3 thin films Marta K. Schütz, 1 Matthieu Petit, 1 * Lisa Michez, 1 Alain Ranguis, 1 Guillaume Monier, 2 Christine Robert-Goumet, 2 Jean-Manuel Raimundo 1 * 1 Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), Aix Marseille Université, CNRS, CINaM UMR CNRS 7325, 13288 Marseille, France 2 Clermont Université, Université Blaise Pascal, Institut Pascal, BP 10448, F-63000, Clermont-Fd, France * Corresponding authors: jean-manuel.raimundo@univ-amu.fr, matthieu.petit@univ-amu.fr 1-XPS : Survey spectrum of the Mn 5 Ge 3 surface after air exposure