Keywords: XPS, InP, surface evolution

We are pleased to submit our research article entitled "Surface evolution of InP substrates at the frontier between deoxidation and dissolution in HCl solutions" by Solène Béchu, Damien Aureau and Arnaud Etcheberry for publication in the special ECASIA issue of the Surface and Interface Analysis journal.

We confirm that this work is original and has not been published elsewhere nor is currently under consideration for publication elsewhere. In this paper, we explore the surface reactivity of InP substrate when facing different concentrations of HCl. For this purpose, we study by XPS, SEM and AFM the reactivity of (100), (111) and polycrystalline InP substrates when immersed into HCl solutions. We proved that after exceeding a concentration of 6 M, the morphological properties of the InP substrates evolve drastically, with a destructuration of the InP network. However, this destructuration is only morphological since all surfaces conserve a perfect stoichiometry, except the (111) face B, which is initially phosphorus-rich and presents an important release of PH 3 when immersed into HCl 12 M. After the HCl immersion, a small enrichment in indium can be noticed, but with a really low value. This paper highlights the importance of controlling the concentration of HCl during the deoxidation stage, to avoid the morphological destructuration even if the chemical properties are conserved.

We believe that this work is an important tool for the surface and the semiconductor communities regarding the importance of deoxidation for processing semiconductors. Consequently, we think that is of interest to readers in the areas your journal's publications.

In case you accept our manuscript, please note that we are not interested to use the colour versions of our illustrations in the print version.

Please address all correspondence concerning this manuscript to me at solene.bechu@uvsq.fr. 24 A strength of III-V materials chemistry is to propose well-defined quaternary, ternary and 25 binary alloys and to combine them by epitaxy in multi-structures, with micrometric or 26 nanometric layers to design energy band diagrams of hetero-structures, with tuneable optical 27 and/or electronical responses for the whole component. [START_REF] Zavala-Moran | Structural, Optical and Electrical 241 Characterizations of Midwave Infrared Ga-Free Type-II InAs/InAsSb Superlattice 242 Barrier Photodetector[END_REF] The control of these particular objects 28 is achieved by mastering chemical engineering of the interfaces between the layers, the 29 substrate surfaces supporting epitaxy, the final encapsulations, and the etchings. In such a 32 To understand the fundamentals of the III-V chemistry, the researchers have at their disposal 33 binary alloys (InP, InAs, InSb, GaN, GaP, GaAs, GaSb) perfectly mastered. Their comparative 34 study in the face of the same source of interaction is extremely valuable. Here again, XPS plays 35 a central role. Among all III-V binary alloys, InP has a prominent place since it is present in 36 many devices as active device materials, capping layers and even substrates for epitaxial 37 growths, for which surface control is essential.

38 Indeed, achieving surface chemical controls of InP substrates is a necessity for the growth of 39 high-quality epitaxial films or the device's performance. The key to obtaining such surfaces is 40 the control of intentional oxidation and deoxidation stages. Regarding the deoxidation process, 41 each laboratory possesses its own expertise: thermal desorption of the oxide, 6-8 ion sputtering 42 followed by annealing, 9 H 2 plasma, [START_REF] Losurdo | Study of the H2 remote plasma cleaning of InP 256 substrate for epitaxial growth[END_REF] wet chemical treatments… [START_REF] Sun | Optimized cleaning method for 259 producing device quality InP(100) surfaces[END_REF][START_REF] Pluchery | Wet chemical cleaning of InP surfaces investigated 262 by in situ and ex situ infrared spectroscopy[END_REF] Within the literature, the 43 wet chemical deoxidation of InP is widely described with the importance of the acid choice and 44 how it affects oxide removal. [START_REF] Sun | Optimized cleaning method for 259 producing device quality InP(100) surfaces[END_REF][START_REF] Pluchery | Wet chemical cleaning of InP surfaces investigated 262 by in situ and ex situ infrared spectroscopy[END_REF] HCl is an acid of choice to perform the deoxidation, however, 45 the interactions existing between HCl and InP are very specific, with potential dissolution.

46 Indeed, so lone or in association with other chemicals, HCl concentrated solutions can generate 47 more or less efficient dissolution, governed by an initial interaction between the undissociated 48 HCl molecules and the InP surface lattice. The detailed dissolution mechanisms and associated 49 kinetic rates are available in the literature. [START_REF] Notten | The Etching of InP in HCl Solutions: A Chemical Mechanism[END_REF][START_REF] Clawson | Guide to references on III-V semiconductor chemical etching[END_REF][START_REF] Cuypers | Wet Chemical Etching 269 of InP for Cleaning Applications . I. An Oxide Formation/Oxide Dissolution Model[END_REF][START_REF] Van Dorp | Wet 272 Chemical Etching of InP for Cleaning Applications. II. Oxide Removal[END_REF][START_REF] Aytac | Thinning of InP by chemical etching[END_REF] 92 The immersion of (100) InP substrates within HCl solutions with different concentrations led 93 to a large range of morphological evolutions as observed in Figure 1. Two different regimes 94 can be distinguished: the low concentration range (from 0 to 6 M), where no gas emission was 95 observed and the high concentration range (above 6 M), with a spontaneous and strong emission 96 from all the InP surface in contact with the solution of an ultra-dense and characteristic fog of 97 micro bubbles, due to the continuous release of PH 3 . [START_REF] Kafalas | Hydrolysis of A III B V Intermetallic Compounds[END_REF] Within the first regime, the integrity of 98 the morphology is preserved (Figure 1 a. to f.) while surface degradation is noticeable in the 99 second regime (Figure 1 124 One could expect that the modification of the morphology would induce changes within the 125 photopeak answer, with an enlargement of the full-width half at maximum (FWHM) for 126 example. [START_REF] Béchu | A challenge 286 for x-ray photoelectron spectroscopy characterization of Cu(In,Ga)Se2 absorbers: The 287 accurate quantification of Ga/(Ga + In) ratio[END_REF] However, as observed in Table 1, the FWHM values of the In

  the position of the XPS is obviously a quantitative tool for chemical or even electronic 31 (band-off set) characterizations.

120

  g. to j.). According to the AFM images and the increase of the surface 100 roughness, the degradation is evolving progressively with the HCl concentration, with the InP 101 substrate immersed in the 9 M HCl solution representing the beginning of the degradation 102 observed on the InP substrate immersed in the 12 M HCl. InP (100) substrates immersed in HCl 103 12 M showing a drastic topographic modification was already observed by Tuck and Baker, 20104 but the progressivity of this morphological degradation at HCl concentrations higher than 6 M 105 is shown here for the first time. As soon as the (100) InP substrate is immersed in HCl solution, the In-O contribution disappears 121 while a small reminiscence (inferior to 0.6 at. %) of P-O is observed. No modification of the 122 initial position of the photopeak is noticed and the modified Auger parameters remain in the 123 associated range of the InP (852.6 eV).[START_REF] Naumkin | NIST X-ray Photoelectron 284 Spectroscopy Database[END_REF]

  3d 5/2 and P 2p 3/2 127 photopeaks don't evolve, reaching 0.79 ± 0.01 eV and 0.60 ± 0.02 eV, respectively. The only 128 noticeable difference is related to the phosphate contribution, with a larger range of value, 129 which is attributed to the really low amount of oxide detected, inducing variabilities during the 130 fitting procedure. 131 132 Figure 2: P 2p and In 3d 5/2 XPS spectra for raw InP (100) substrate (a. and b.), InP (100) 133 substrate immersed for 5 min in HCl 3M (c. and d.), InP (100) substrate immersed for 5 min in 134 HCl 6M (e. and f.), InP (100) substrate immersed for 5 min in HCl 9 M (g. and h.) and InP 135 (100) substrate immersed for 5 min in HCl 12 M (i. and j.). 136 Another expectation related to morphology evolution observed in Figure 1 could be an 137 enrichment in In or P during the strong dissolution regimes (9 M and 12 M). The In/P ratios 138 calculated in

146

  If the HCl concentration doesn't influence the chemical behaviour of (100) InP substrate, 147 interrogations can remain about the influence of HCl when facing different crystallographic 148 orientations of InP substrate. To elucidate it, (111) InP substrates, on face A (In-rich) and on 149 face B (P-rich), as well as polycrystalline InP substrates, were immersed in HCl 12 M for 5 150 minutes. 151 152 Figure 3: Cross-section SEM and AFM images for raw InP (111) substrate (a. and b.), InP (111) 153 substrate face A (In-terminated) immersed for 5 min in HCl 12 M (c. and d.), InP (111) substrate

evolution of InP substrates at the frontier between deoxidation and dissolution

	1 Surface 2	in HCl solutions	To:
			Editors of Surface and
	3		Solène Béchu * , Damien Aureau, Arnaud Etcheberry	Interface Analysis
	4	Institut Lavoisier de Versailles (ILV), Université de Versailles Saint-Quentin-en-Yvelines,
	5	Université Paris-Saclay, CNRS, UMR 8180, 45 avenue des Etats-Unis, CEDEX, 78035
	6		Versailles, France
	7	Keywords: X-ray photoemission spectroscopy, InP, surface evolution
	8
	9 Abstract
	F 10 Surfaces of InP substrates with different crystallographic orientations are investigated when o r 11 facing HCl solutions. The immersion of (100) InP in a wide range of HCl solutions with
	12 different concentrations showed that the frontier between deoxidation and dissolution is around P 13 6 M, with a morphological destructuration. However, no chemical evolutions are noticed even e 14 at high concentrations. Similar observations were performed with other crystallographic e r 15 orientations (polycrystalline and (111) InP substrates), with topographic surface modifications
	16 but no chemical ones. 17 1. Introduction 18 III-V semiconductors hold a well-established and specific place in the electronic devices R e v i e 19 industry. Because of the exceptional adaptability of their optical and electrical properties, III-V w 20 semiconductors offer the possibility of designing reliable components, targeted as to their use
	21 and performing in application fields as diverse as light-emitting diodes, lasers, large wavelength
	22 range photodetectors, solar cells, optical switches, quantum well high electron mobility
	23 transistors… 1-4
		Sincerely,
				Solène Béchu

Thank you for your consideration of this manuscript.

  (111) and polycrystalline n-type InP substrates were purchased from InPact Electronic 63 Materials Ltd. Surfaces were mirror-like finished and denoted as epiready quality by the 64 supplier for the (100) and (111) substrates. Samples were directly immersed for 5 minutes (min)

	62 (100), 65 in HCl solutions (37 %, ACS quality, from SUPELCO) at different concentrations
	66 (respectively, 3 mol L -1 (M), 6 M, 9 M and 12 M). Then, the samples were rinsed for 1 minute
	67 in deionised water, dried under N 2 flow and directly transferred within the spectrometer. Air
	68 interactions last less than 5 to 10 min.
	69	2.2. XPS
	70 XPS surface chemical analyses were conducted on a Thermo Electron Nexsa spectrometer
	71 equipped with a monochromatic Al-Kα X-Ray source (1486.6 eV). The Thermo Electron
	F 72 procedure was used to calibrate the Nexsa spectrometer by using metallic Cu and Au samples F
	o r 73 intern references (Cu 2p 3/2 at 932.6 eV and Au 4f 7/2 at 84.0 eV). High energy resolution spectra o r
		P P
	e e r 78 paths from Avantage library associated with the spectrometer and the corresponding e e r
	79 transmission function. 80 2.3. SEM	R e v R e v
	i e w 81 Secondary electron micrographs (SEM) analyses were performed using a JEOL JSM 7001F 82 microscope with a patented "in-lens" Schottky field emission gun (FEG). Imaging analyses 83 were realized at 15 kV accelerating voltage and 10 mm working distance. i e w
	84	2.4. AFM
	54 This work aims at unveiling the evolution of InP surfaces, either when facing a deoxidation or 85 A Dimension ICON AFM (Bruker) was used to observe the morphology of the InP substrates.
	55 a dissolution. Indeed, very few works deal with the modifications of InP substrates according 86 The AFM topography measurements were performed in air with the Peak Force tapping mode.
	56 to the concentration of HCl solutions or the crystallographic orientations, especially at high 87 For this purpose, a silicon tip on a nitride lever (ScanAsyst Air model, Bruker), with a 0.4 N/m
	57 concentrations. 18 In order to, XPS, Atomic Force Microscope (AFM) and Secondary Electrons 88 spring constant and a nominal tip radius of 2 nm was used with a resolution of 256 pixels × 256
	58 Microscope (SEM) analysis are performed to unveil those evolutions and to understand the 89 pixels.
	59 mechanisms lying behind.
	90 3. Results and Discussion
	60 2. Materials and Methods
	91	3.1. Influence of the HCl concentration for (100) InP substrates
	61	2.1. Materials

However if the InP thinning using HCl 50 concentrated solution is established, the chemical surface evolution of InP substrates is barely 51 investigated at high HCl concentrations not saying if, during the dissolution or even the 52 deoxidation processes in HCl, the surface stoichiometry of InP substrates evolves, with a 53 preferential etching of either indium or phosphorus. 74 were acquired with an X-ray spot size of 400 μm and using a constant analyzer energy (CAE) 75 mode of 20 eV and 0.1 eV as energy step size. Regarding the nature of the sample, charge 76 compensation wasn't applied. XPS spectra were treated using a Shirley background subtraction, 77 and XPS compositions were deduced using the sensitivity factors and the inelastic mean free

Table 1 :

 1 Table1refute this hypothesis since the In InP /P InP ratios remain within a range of 139 1.03 ± 0.05, the raw value being at 1.04. Thus, the surface chemistry of (100) InP substrate is 140 not modified when exposed to a large range of concentrations of HCl, even if some 141 morphological modifications appear. Parameters fits for the XPS spectra displayed in Figure2. The ratios are calculated 143 from the In 3d 5/2 and P 2p photopeaks.

	142 Position (eV)	FWHM (eV)	Ratios	Modified
			Attribution	In 3d 5/2	P 2p 3/2	In 3d 5/2	P 2p 3/2	In InP / P InP	In tot / P tot	Auger parameter (eV)
		Raw substrate	InP oxide		445.0 445.9		129.2 133.5	0.79 1.03	0.59 1.12	1.04	1.05	852.7
			InP		445.0		129.2	0.80	0.62
		HCl 3 M								0.99	0.97	852.8
			oxide		/		132.9	/	0.90
			InP		445.0		129.2	0.78	0.60
		HCl 6 M	oxide	/ F		133.2	/	1.09	1.04	1.02	852.7
		HCl 9 M	InP		o 444.9 r	129.2	0.78	0.60	1.02	1.04	852.7
	144	HCl 12 M	oxide InP oxide		/ 445.0 P 133.3 129.2 e / 133.1 e r / 0.79 /	0.94 0.60 1.07	1.04	1.06	852.7
	145	R v e 3.2. Influence of the crystallographic orientation of the InP substrate
									i e w

Table 2 :

 2 Parameters fits for the XPS spectra displayed in Figure 4. The ratios are calculated 210 from the In 3d 5/2 and P 2p photopeaks. To understand the interactions between HCl solutions and InP surfaces during deoxidation and 214 dissolution, the roles of the HCl concentration and the crystallographic orientation of the InP 215 substrates were investigating. If a concentration frontier at 6 M is noticeable for the 216 morphological evolution of the (100) InP substrates, with the destructuration of the InP network 217 due to dissolution, no specific modification of the surface chemistry information is impacted 218 by the concentration of HCl. Indeed, no enrichment in indium or phosphorus is observed, and 219 this, within the first 10 nm of the substrate. 220 Regarding the crystallographic orientation, again, at high HCl concentrations, morphological 221 modifications are noticed, with an important differentiation between the face A and the face B 222 of the (111) InP substrate. However, those topographical evolutions don't influence the

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