# Non-linear decomposition of gallium x-ray induced Auger transitions: A new path to quantify gallium-based III–V materials

Kirène Gaffar, Solène Béchu, Romain Jouanneaud, Guillaume Monier, Christine Robert-Goumet, Arnaud Etcheberry, Muriel Bouttemy

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## ► To cite this version:

Kirène Gaffar, Solène Béchu, Romain Jouanneaud, Guillaume Monier, Christine Robert-Goumet, et al. Non-linear decomposition of gallium x-ray induced Auger transitions: A new path to quantify gallium-based III–V materials. Journal of Vacuum Science & Technology A, 2025, 43 (4), 10.1116/6.0004551. hal-05143160

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Distributed under a Creative Commons Attribution 4.0 International License
Non-linear decomposition of Gallium X-Ray
induced Auger transitions: a new path to quantify
Gallium-based III-V materials

Kirène Gaffar ¹, a), Solène Béchu¹, Romain Jouanneaud², Guillaume Monier²,
Christine Robert-Goumet², Arnaud Etcheberry¹, Muriel Bouttemy¹

¹Institut Lavoisier de Versailles, UVSQ, Université Paris-Saclay, CNRS, UMR 8180, 45 avenue des Etats-Unis, 78035 Versailles CEDEX, France

² Université Clermont Auvergne, Clermont Auvergne INP, CNRS, Institut Pascal, F-63000 Clermont-Ferrand, France

a) Electronic mail: kirene.gaffar@cnrs.fr

Interest in Ga-based III-V semiconductors has grown in many applicative domains such
as solar applications or power electronics. However, using the conventional Al Kα XPS
source (most commonly used irradiation) on GaN materials leads to an overlap between
photopeaks and Auger lines, complicating the interpretation, especially the quantification
which requires to separate the Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> Auger line and the N 1s photopeak signals.
In this paper, we propose an upstream study to developed a meticulous decomposition
procedure to address this issue,. This procedure relies on the joint decomposition of the
twin Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> and Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub> X-AES lines based on the non-linear least square
decomposition model with realistic decomposition parameters adjusted on a reference
Ga-As binary alloy exempted from N and oxide. In addition, a similar procedure has been
developed on a Ga<sub>2</sub>O<sub>3</sub> reference to consider any oxide presence, Ga based alloys being
hardly prone to rapid surface oxidation. A complete decomposition has been evaluated on
three Ga binary alloys namely GaAs, GaP, GaSb, all presenting a small native oxide on
the surface. A coherent separation between the alloy and oxide environments is
demonstrated, even for ultrathin oxide layers and a remarkable reproducibility of the
decomposition pattern for the alloy phase, whatever the nature of the III-V binary alloy.
Quantification is made using either photopeaks or Auger transitions and a good agreement of the alloy ratio Ga/V element is found in any case. These preliminary results are auspicious for a further implementation of this methodology on N-containing Ga compounds.

# I. INTRODUCTION

The III-V semiconductors are widely used in microelectronic and optoelectronic fields because of their outstanding electronic and optical properties. Among the most used characterization methods adopted for III-V materials such as X-ray diffraction (XRD), X-ray fluorescence (XRF), Reflection High Energy Electron Diffraction (RHEED), Secondary Ion Mass Spectrometry (SIMS) or Scanning Transmission Electron Microscopy (STEM) ¹⁻⁴, X-ray photoemission spectroscopy (XPS) appeared to be the optimum balance between sample preparation process, high resolution, and surface sensitivity. Contrary to STEM measurements, largely used for its atomic resolution, XPS doesn’t require specific sample preparation or any calibration as SIMS. It is not only non-destructive but also quantitative, providing key information on chemical environments thanks to a direct quantification with reliable sensitivity factors to evaluate epitaxial thin films quality used for HEMT fabrication. However, in particular in Gallium Nitride (GaN) based compounds case, when using standard XPS Al Kα source (excitation energy hν = 1486.6 eV) the existence of overlaps between Ga L₂M₄.₅M₄.₅ Auger line and the N 1s photopeak makes the decomposition challenging, resulting in difficult access to the exact composition and, thus possibly leading to misinterpretation. Al X-ray source is the
most commonly used in the photoemission community, but GaN based materials could
also be analysed using different X-ray excitation sources to shift the kinetic energy
position of the X-ray induced Gallium lines (X-AES) and prevent its interference with the
N 1s region. Alternative sources that can be found in laboratory are Magnesium (1253.6
eV) or even harder X-ray sources such as Silver (2984.3 eV), Chromium (5417.0 eV) or
Gallium (9251.7 eV), but these are not as widely available as Al or Mg. Moreover, the
use of Mg source does not completely solve the problem. Indeed, this source being not
monochromatic, broader spectra are measured and other Auger transitions, namely the Ga
L<sub>2</sub>M<sub>2.3</sub>M<sub>2.3</sub> (Binding energy, BE = 356 eV) and the Ga L<sub>3</sub>M<sub>2.3</sub>M<sub>2.3</sub> (BE = 364 eV) lines
that extend from 356 eV to 434 eV, will interfere with the N 1s core level, complicating
the background subtraction <sup>5</sup>.

Numerous papers were already published on GaN surfaces analyses by XPS, mainly for
the microelectronic field, where different steps of devices fabrication such as surface
preparation (physical or chemical) or ohmic contact annealing impact <sup>6-8</sup> were
investigated. This showcased that the interest for GaN-based materials is undeniable and,
while XPS had proven to be essential to understand their chemistry, a proper
quantification is still missing. Some decompositions were sometimes proposed but
limited to comparative studies. In this paper we propose a realistic modelling of the Ga
L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> X-Auger transitions (X-AES) to separate the N contribution with accuracy.
The innovative aspect of the methodology elaborated resides on a decomposition
procedure considering the extended Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub> and Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> X-AES spectral
region. The similarity of the two transitions is inherent to the Auger de-excitation process
itself and the decomposition realized on the Ga L₃M₄,₅M₄,₅ transition helps fixing the
decomposition parameters of the Ga L₂M₄,₅M₄,₅ transition.

This paper presents an upstream study which was necessary to develop the decomposition
procedure of the Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub> and Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> Auger transitions doublet alone, in the
absence of N 1s photopeak. This requires to first consider Auger transitions acquired on a
reference Ga alloy, not containing N neither superficial oxide layer, to set the
decomposition parameters. This was realized on an oxide-free GaAs alloy reference
specifically prepared to this end. We have pushed this study to the point of taking into
consideration the presence of an oxide environment in the decomposition procedure.
Indeed, Ga alloys are renowned for their affinity to oxygen and gallium oxide are
difficult to remove <sup>9-12</sup>. A Ga<sub>2</sub>O<sub>3</sub> reference, the expected oxide phase of most Ga-alloys<sup>13</sup>
was thus also considered and the measured Auger transitions modelled using the same
methodology.

The decomposition of Ga L₃M₄,₅M₄,₅ and Ga L₂M₄,₅M₄,₅ Auger transitions doublet has
been performed using the non-linear least square (NLLS) method ¹⁴⁻¹⁶. The choose of
non-linear method over the linear is based on the observation of the background, which
will not exactly be identical from a sample to another, and on the anticipation of possible
charging effects. The NLLS method is more flexible as it allows a better adjustment to fit
the experimental envelop by authorizing small variations of the decomposition
parameters, such as the background subtraction, kinetic energy shifts (position), peak
broadening (full width at half maximum - FWHM) or area ratio. The linear method
doesn't allow any modification of the fixed contributions shapes to reproduce the
experimental envelop, preliminary recorded on references samples and being
representative of the different environments considered. This difference, but in a larger extent the pros and cons of both models have been highlighted in a comparative study of Indium X-AES transitions¹⁶.

The methodology to set up the NLLS decomposition procedure of Ga based III-V compounds we just briefly introduced will be presented step by step. The decomposition of the alloy and oxide environments will be detailed and the capability to separate these two environments will be illustrated on 3 different alloys, GaAs, GaSb and GaP, presenting different thicknesses of native oxide on the surface. The reliability of the quantification will be assessed by calculating the ratio between Ga (III element) and As, Sb or P (element V), accordingly noted as III/V ratio, using photopeaks and Auger transitions interchangeably¹⁷.

# I. EXPERIMENTAL

## a. Ga-based binary alloys and Gallium oxide samples preparation

The GaAs reference alloy sample (noted as GaAs(T) here) was prepared at Pascal Institute from Clermont-Auvergne University. To totally eradicate oxide, this sample is firstly ex *situ* chemically cleaned by soaking for 2'30'' in an HCl/IPA solution, as proposed by Tereshchenko *et al*¹⁸. It is then quickly introduced in an Ultra High Vacuum (UHV) assembly. An *in situ* annealing at 530°C for 45 minutes is carried out to improve the deoxidation and leads to a crystalline surface. For the gallium oxide reference, a Ga₂O₃ powder from Thermo Scientific, (99.99%) was used. A small amount of powder was diluted in a few drops of isopropanol solution and then deposited on a piece of silicon wafer. Once dried, the mixture formed a layer of gallium oxide thick enough for XPS analysis. For the final test study on oxidized III-V binary Ga compounds, commercial
substrates of GaAs (n doped from El Cat Inc.), Gallium Antimony (GaSb, p doped from Wafer Technology) and Gallium Phosphide (GaP, undoped from El Cat Inc.) are employed. A surface preparation consisting in a wet chemical deoxidation procedure was operated on these Ga-based compounds. This was realized by immersing them in a 3M HCl solution for 3 minutes, completed by an extensive rinsing step in deionized water (18.2 m Ω).¹⁹

The samples were mounted in air on the sample holder and introduced quickly inside the XPS spectrometer, leading to the formation of a native oxide whose thickness depend on the oxidizability of the Ga alloy.

b. XPS analyses

XPS spectra acquisition was operated with a ThermoFisher© NEXSA spectrometer equipped with a conventional monochromatic Al Kα X-Ray source (1486.6 eV). High energy resolution spectra were recorded using a Constant Analyzer Energy (CAE) mode at 20 eV Pass Energy (PE), a step size of 0.1 eV, a dwell time set at 100 ms, these conditions resulting in an Ag 3d$_{5/2}$ FWHM value of 0.62 eV. Charge compensation was necessary to analyse the GaP isolating sample and was consequently used for all samples to keep comparable peaks shapes leading to a working pressure inside the analysis chamber was 2.8 x 10$^{-7}$ mbar. Not only the main photopeaks of the constitutive elements of the different alloys studied, namely C 1s, O 1s, P 2p, Ga 3d, Ga 2p, As 3d, As 2p, Sb 4d, Sb 3d were measured but also their principal Auger transitions Ga L$_{2,3}$M$_{4,5}$M$_{4,5}$, As L$_{2}$M$_{2,3}$M$_{4,5}$ and Sb L$_{4}$M$_{4,5}$M$_{4,5}$. This wide panel of peaks situated all along the energy scale enabled us to cleverly associate peaks in the same energy range for quantification, ensuring similar electron escape depths. Importantly, a large spectral window covering a
binding energy range from 380 eV to 420 eV (i.e Kinetic Energy, KE from 1106 to
1046 eV) was set to collect Ga Auger electrons of both the Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> and Ga
L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub> lines to take advantage of the similar shape of these two transitions.

Concerning the GaAs<sub>(T)</sub> sample prepared at Pascal Institute from Clermont-Auvergne
University, spectra were acquired onsite with a SPECS Proven XPS system. It is
equipped with a monochromatic Al Kα X-Ray source running at 400W and oriented at
55° to the axis of the analyser. The PHOIBOS-150 hemispherical analyser is running in a
CEA mode, using high magnification lens mode, a 10 eV pass energy, a 0.1 eV energy
step and a 100 ms dwell time, without charge compensation. Under such conditions, the
FWHM of Ag3d<sub>5/2</sub> is 0.5 eV, obtained on a reference silver sample cleaned by Ar<sup>+</sup>sputtering. The pressure during the analysis is below 1 x 10<sup>-9</sup> mbar. These acquisition
conditions were chosen in order to obtain equivalent photopeaks and Auger transitions
shapes compared to the one acquired with ThermoFisher© NEXSA spectrometer.

All data treatments were processed with the CasaXPS software²⁰ using the Relative
Sensitivity Factors (RSF) values available in the libraries of each constructor.

No calibration was performed on all sets of data and the energy positions of the C 1s C-C
chemical environment can be found in Table S1. in the supplemental material section.

For XPS and AES spectroscopies, the most commonly adopted backgrounds are linear, Shirley or Tougaard ones²¹. Due to the complexity of Auger lines shape but the necessity to conserve the “twin” shape of Ga L₂M₄.₅M₄.₅ and Ga L₃M₄.₅M₄.₅ lines an offset has to be visually laid out in order to satisfy this condition and conserve the same ratio for all the decompositions. Shirley backgrounds are more difficult to adjust, even while applying an offset, whether using the “Shirley” or “L Shirley” in CasaXPS. In the first
case, the offset enables to vertically move the background and a gap is created between
the experimental data and the background as the extremities of the background are not
linked to experimental curve anymore, leading to errors when performing the
decomposition. In the second case, the offset enables a slight distortion of the Shirley "S"
shape but it is not easy to obtain an equivalent shape and keep fixed the area ratio
between both transitions. The FIG. S1. in the Supplemental Material describes the visual
difference between the Shirley, L Shirley and U 2 Tougaard backgrounds. All the
decompositions shown in this paper are performed using the U 2 Tougaard <sup>21-23</sup>
background model which considers the modeling of inelastic scattering experienced by
electrons. The value of this offset is arbitrarily chosen and enables a slight distortion of
the Tougaard background line on the left side which enables a small adjustment of the
shape. The C parameter values are reported in Table S2 of the Supplemental Material. A
GL (50) lineshape is chosen for all the modeled peaks but one could also use an LA (50)
lineshape less susceptible to noise level than GL. This choice doesn't affect significantly
the final quantification (± 0.5%) and is described in the FIG. S2. in the Supplemental
Material.

**II. Results and discussion**

a. *Methodology*

Performing an accurate decomposition of the Ga L₂M₄,₅M₄,₅ / N 1s spectral region requires, as explained in the introduction, to reliably separate the two features. The methodology we present here aims to precisely model the Ga L₂M₄,₅M₄,₅ Auger transition by means of a robust decomposition procedure which has to be developed on a Ga L₂M₄,₅M₄,₅ totally isolated. To do so, a perfectly clean GaAs (GaAs(T)) sample prepared
according to the Tereschenko method¹⁸ (for more detail see experimental section) was used as reference for the acquisition of the experimental Ga L₂M₄,₅M₄,₅ envelop of the Ga alloy environment. This decomposition, is far from being straightforward and we propose an innovative procedure to improve it when N 1s photopeak overlaps by using the Ga L₃M₄,₅M₄,₅ Auger transition to decompose the Ga L₂M₄,₅M₄,₅ one.

In fact, the Auger process – also known as the Auger-Meitner process²⁴ – involves three energy levels, leading to a final state with two vacancies, one created during the excitation process by X-rays (photoelectron emission) and the other by the non-radiative relaxation process to release the energy excess (Auger electron emission) and this leads to families of broad X-AES lines such as the Ga L₃M₄,₅M₄,₅ and Ga L₂M₄,₅M₄,₅ transitions. These two transitions are situated at 420 eV (BE, KE = 1067 eV) and 395 eV (BE, KE = 1091 eV) respectively when using Al source and this energy separation is inherent to the energy difference between the L₂ and L₃ orbitals, i.e. 2p₁/₂ and 2p₃/₂ using the spectroscopic notation, from which the first vacancy is generated. Consequently, these two X-AES lines can be assimilated to a doublet spaced with an energy equivalent to that which separate the Ga 2p photopeaks, i.e. the spin-orbit coupling of 26.9 eV. The mimetic shape of the envelops of these two transitions results from the Auger process and from the absence of L₂L₃M₄,₅ Coster-Kronig process for Ga element. But they are not exact replicas, as coulomb interaction energy between the two vacancies and the relaxations energy intra/interatomic must be considered. This comes to our decomposition procedure by working on both X-AES transitions, Ga L₃M₄,₅M₄,₅ and Ga L₂M₄,₅M₄,₅ (both noted Ga LMM), as we would naturally do on Gallium 2p photopeaks for example. The NLLS method is employed and, as we have just justified, we can take
advantage using the Auger transition Ga L₃M₄,₅M₄,₅ (KE = 1067 eV) situated at higher
kinetic energy, (without the N 1s peak overlap in GaN also), to define accurate
decomposition parameters (background, lineshapes, energy positions, area, FWHM) and
transpose this set of parameters on the higher (KE) Auger transition. With the NLLS
decomposition method, small adjustment of parameters is authorized, to take account for
possible background change in these two regions due to sample nature and the natural
asymmetry of Auger process, as mentioned before (unperfect “twin” GaLMM Auger
peaks). To set a representative background, having a trend with a physical sense, two
separated U 2 Tougaard backgrounds are positioned around the two transitions with the
same energy range, despite their slightly different backgrounds’ appearances. This
ensures that distortion of the nominal shape fixed on the Ga L₃M₄,₅M₄,₅ is minimized and
that the areas of the different contributions employed can be used for quantification and
comparative studies. Beyond the continuous background shape, the crucial point lies in
choosing the two Auger transition ends for the decomposition procedure that will be
discussed in the results. An assessment of the mismatch between both X-AES lines can
be calculated allowing repeatability and consistency of the decomposition methodology
to be checked, as uncertainties coming from the Tougaard background model limitations.

For further studies, presence of oxides can constitute possible sources of errors.
Disposing of a perfectly deoxidized Ga-based compound surface requires ultimate care to
prevent oxygen contact. As the native oxide present on Ga material surface is mainly in
the form of Ga₂O₃¹³, the first step of the methodology will be the decomposition of two
reference samples, one representative of a gallium alloy perfectly deoxidized (GaAs(T))
and another of the oxide phase, Ga₂O₃. Finally, these two patterns will be used to
decompose and determine the proportion of each environment in the case of three Ga based binary alloys (namely GaAs, GaP and GaSb) presenting an ultrathin superficial oxide layer to evaluate the consistency of the methodology and of the decomposition procedure.

### A.1 Cleaned GaAs reference decomposition

In order to unveil the characteristic shape of Ga L₃M₄,₅M₄,₅ and Ga L₂M₄,₅M₄,₅ X-AES lines, an oxide-free GaAs surface (GaAs<sub>(T)</sub> sample) is obtained thanks to the specific cleaning process described in section II. The use of a clean GaAs reference is a crucial step to come out with the characteristic alloy shape. The decomposition parameters are first set for the Auger transition Ga L₃M₄,₅M₄,₅ situated at lower kinetic energy, before transposing them to the Auger transition positioned at higher kinetic energy. As explained in the experimental section, an offset on the background is laid out. The value of this offset is optimized for each decomposition and enables a slight distortion of the Tougaard background line on the left side of the background shape. It is necessary to find an offset value with a good compromise between the two extremities points defined for the background (around each transitions) and its shape (to avoid cutting off one part of the data and preserve equivalent shapes for each Auger line). Another option would be to fix a unique background from side to side of the two transitions. This method appeared to be more difficult to execute due to the great increase of background intensity with the kinetic energy and a more important inelastic contribution for the lower Auger transition background. Also, an in-depth study could be necessary to better understand loss peaks that contribute to the background in between the two transitions, such as plasmons. The
main contribution (noted A on FIG. 1.), positioned at the maximum intensity of
Ga L$_{3}$M$_{4.5}$M$_{4.5}$ transition is fixed and 4 other contributions are introduced. A total of 5
contributions are required to satisfactorily reproduce the experimental envelop.
The number of contributions introduced to model the X-AES lines is arbitrary and more
peaks could be added but this will also generate more possibility of errors. In this case,
this number is chosen in view of having the lowest number of contributions for the
modeling to match the experimental data but also theoretical studies on Auger transitions
probabilities²⁵. The background range is preserved for both lines. Decomposition parameters (i.e. FWHM and areas) of the added contributions B, C, D and E are fixed

<figure><img src="image_3.png" /></figure>

with reference to the main contribution A. Then, this set of contributions is replicated to the Ga L₂M₄.₅M₄.₅ transition. All the corresponding decomposition parameters are shown in Table I. By convention, positions will be given in kinetic energy for Auger transitions.

Likewise, the main contribution F of the Ga L₂M₄.₅M₄.₅ transition is also constrained in relation to the A peak and defines the decomposition parameters of the 4 other contributions G, H, I and J. Small adjustments of the constraints are permitted, controlling that the peak widths change in the same trend for all the other contributions and that the energy shift is also coherent (± 0.3 eV). When propagating the decomposition on the Ga L₂M₄.₅M₄.₅ Auger transition, we observe that the homothetic relationship with the Ga L₃M₄.₅M₄.₅ Auger transition is not perfectly verified. This is a not-so-surprising result given the complexity of the Auger process and of complex background modeling. Nevertheless, real coherence can be seen. The FWHM for the main peak of Ga L₃M₄.₅M₄.₅ and Ga L₂M₄.₅M₄.₅ (respectively peak A and F) are identical and so is the number of contributions needed to model each transition.
<table><caption>FIG. 1. Decomposition of Ga L$_{3}$M$_{4.5}$M$_{4.5}$ and Ga L$_{2}$M$_{4.5}$M$_{4.5}$ Auger transition of the fully deoxidized GaAs(T) reference sample.</caption><thead><tr><th></th><th>Species</th><th>Peak identifier</th><th>Position (eV)</th><th>FWHM (eV)</th><th>Area</th></tr></thead><tbody><tr><td rowspan="5">Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub></td><td rowspan="5">Ga-As</td><td>A</td><td>1065.8</td><td>1.30</td><td>12418</td></tr><tr><td>B</td><td>A + 3.8</td><td>A*1.28</td><td>A*0.47</td></tr><tr><td>C</td><td>A + 1.4</td><td>A*1.30</td><td>A*0.43</td></tr><tr><td>D</td><td>A - 1.3</td><td>A*1.39</td><td>A*0.15</td></tr><tr><td>E</td><td>A - 5.4</td><td>A*1.57</td><td>A*0.07</td></tr><tr><td rowspan="5">Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub></td><td rowspan="5">Ga-As</td><td>F</td><td>A + 26.9<br>(± 0.3)</td><td>1.30</td><td>6521</td></tr><tr><td>G</td><td>F + 3.5</td><td>F*1.84</td><td>F*0.5</td></tr><tr><td>H</td><td>F + 1.4</td><td>F*0.98</td><td>F*0.31</td></tr><tr><td>I</td><td>F - 0.9</td><td>F*1.89</td><td>F*0.31</td></tr><tr><td>J</td><td>F - 5.2</td><td>F*1.48</td><td>F*0.13</td></tr></tbody></table>

<caption>Table I. Peak decomposition table for Ga L$_{3}$M$_{4.5}$M$_{4.5}$ and Ga L$_{2}$M$_{4.5}$M$_{4.5}$ transitions of the Ga$_{2}$O$_{3}$ reference sample.</caption>

## A.2 Ga$_{2}$O$_{3}$ reference decomposition

In addition to the GaAs(T) sample used to model the Ga alloy environment, a Ga$_{2}$O$_{3}$ sample is needed to model the Ga oxide environment. As performed on the GaAs(T) Auger transition, the decomposition parameters of the Ga LMM region of the Ga$_{2}$O$_{3}$ reference are determined following the same procedure. The higher intensity contribution A is set for the Ga L$_{3}$M$_{4.5}$M$_{4.5}$ transition and two additional contributions B and C are introduced in this case to fit the experimental curve. The whole set of contributions is transposed to the Auger transition at higher kinetic energy. The decomposition
parameters are gathered in Table II. and the resulting decomposition is illustrated FIG. 2.

Comparing the FWHM parameters between both X-AES lines (Ga L$_{3}$M$_{4.5}$M$_{4.5}$ and Ga L$_{2}$M$_{4.5}$M$_{4.5}$) confirms the good coherency between both transitions. We can notice that the FWHM (of the major contribution, main peak A) is higher for the Ga$_{2}$O$_{3}$ reference.

This can be explained by two different reasons: the first one is the number of contributions set up for the decomposition. Only 3 contributions are used to decompose each transition of the Ga$_{2}$O$_{3}$ sample while 5 contributions are needed for each transition of the GaAs$_{(T)}$ sample. The second reason is related to the chemical environment. It is known that oxides FWHM are generally larger than metals or semi-conductors and this is what is observed here.

<figure><img src="image_4.png" /><figcaption>FIG. 2. Decomposition of Ga L$_{3}$M$_{4.5}$M$_{4.5}$ and Ga L$_{2}$M$_{4.5}$M$_{4.5}$ Auger transition of the Ga$_{2}$O$_{3}$ reference sample.</figcaption></figure>

<table><thead><tr><th></th><th>Peak<br>identifie<br>r</th><th>Position<br>(KE, eV)</th><th>FWHM<br>(eV)</th><th>Area<br>(eV)</th></tr></thead><tbody><tr><td rowspan="3">Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub></td><td>A</td><td>1062.2</td><td>2.67</td><td>137352</td></tr><tr><td>B</td><td>A + 3.4</td><td>A*1.27</td><td>A*0.52</td></tr><tr><td>C</td><td>A - 3.8</td><td>A*1.65</td><td>A*0.11</td></tr></tbody></table>
<table><tr><td rowspan="3">Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub></td><td>D</td><td>A+ 26.9 (± 0.3)</td><td>2.65</td><td>69930</td></tr><tr><td>E</td><td>D + 2.8</td><td>D*1.48</td><td>D*0.49</td></tr><tr><td>F</td><td>D - 3.7</td><td>D*1.79</td><td>D*0.18</td></tr></table>

Table II. Peak decomposition table for Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub> and Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> transitions of the Ga<sub>2</sub>O<sub>3</sub> reference sample.

b. Implementation of the methodology to Ga-based compounds (GaAs, GaSb and GaP)

Firstly, the decomposition of the X-AES lines is applied into a GaAs sample with this time, a small oxide layer on the surface, as confirmed by the C 1s and O 1s spectra presented in the FIG. S3. in the supplemental material. The decomposition requires employing for each Auger transition both the set of 5 contributions of the GaAs(T) and of the set of 3 contributions of Ga<sub>2</sub>O<sub>3</sub> environment. The 16 contributions are successfully propagated into the GaAs experimental spectral envelop. The position and FWHM of the main peak for the alloy and the oxide environment is sensibly the same for each transition. The values are reported in Table III. The same decomposition procedure is implemented on Ga LMM Auger transition measured on two additional III-V gallium alloys, GaSb and GaP. They have undergone the same deoxidation treatment and transfer procedure than the GaAs sample. Similarly, to the GaAs sample, these two reference samples also present an oxide layer on the surface, as shown in FIG. S4 and S5 in the
Supplemental Material section, but the thickness of the native oxide formed depends on the element associated to Ga.

FIG. 3. Decomposition of Ga L₃M₄,₅M₄,₅ and Ga L₂M₄,₅M₄,₅ Auger transition for a) GaAs, b) GaSb and c) GaP samples presenting native oxides at the surface.

The decomposition used for the GaAs sample, combining the alloy and the oxide environment patterns, is propagated into the Ga LMM transitions of GaSb and GaP samples. FIG. 3. shows the results of the three decompositions and evidence the higher surface reactivity of Sb for which a remarkable higher proportion of oxide is observed. On contrary, only ultrathin native oxide layers are present at the surface of GaAs and GaP samples. Importantly, the good agreement with the patterns generated by the decomposition procedure and the experimental envelop for these three alloys

<figure><img src="image_5.png" /></figure>

demonstrates that ultrathin oxide layer can be detected and that the decomposition procedure is robust as it works equally for a thin or thicker oxide film with coherent KE and FWHM evolution.

Considering the pattern of the alloy environment alone, an apparent and remarkable similarity between the Auger signature of the 3 binary alloys is observed. This is well illustrated by the constancy of the FWHM of the main peak (Table III), which constrains
all the others. This suggests similar Auger processes for the three Ga based binary compounds. The energy position of the main peak depends on the nature of the alloyed element to Ga and the shifts observed are consistent with the different electronegativities of the elements: higher is the electronegativity (χ<sub>Sb</sub>= 2.05, χ<sub>As</sub>= 2.18, χ<sub>P</sub>= 2.19) lower is the kinetic energy (KE<sub>Sb</sub>= 1093.3 eV, KE<sub>As</sub>= 1093.2 eV, KE<sub>P</sub>= 1092.7 eV).

<table><caption>Table III. Position and FWHM evolution of the main contribution (A) of the alloy and oxide environments for Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub> and Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> transitions for three different Ga based binary alloys (GaAs, GaSb, GaP).</caption><thead><tr><th rowspan="3">Sample</th><th rowspan="3">Transitions</th><th colspan="4">GaAs</th></tr><tr><th colspan="2">Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub></th><th colspan="2">Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub></th></tr><tr><th>Ga alloy</th><th>Ga oxide</th><th>Ga alloy</th><th>Ga oxide</th></tr></thead><tbody><tr><td rowspan="2">Environment</td><td>Position (KE, eV)</td><td>1066.3</td><td>1062.1</td><td>1093.2</td><td>1089.2</td></tr><tr><td>FWHM (eV)</td><td>1.40</td><td>2.68</td><td>1.40</td><td>2.70</td></tr><tr><th rowspan="3">Sample</th><th colspan="4">GaSb</th></tr><tr><td>Position (KE, eV)</td><td>1066.4</td><td>1061.9</td><td>1093.3</td><td>1088.6</td></tr><tr><td>FWHM (eV)</td><td>1.41</td><td>2.65</td><td>1.40</td><td>2.66</td></tr><tr><th rowspan="3">Sample</th><th colspan="4">GaP</th></tr><tr><td>Position (KE, eV)</td><td>1065.9</td><td>1062.6</td><td>1092.7</td><td>1089.4</td></tr><tr><td>FWHM (eV)</td><td>1.41</td><td>2.70</td><td>1.41</td><td>2.75</td></tr></tbody></table>

### c. Validation of the methodology

To complete the previous qualitative examination of the mismatch between the modelled envelop and the experimental envelop the overall are ratio between the oxide environment and the alloy environment was calculated for each sample, using either the Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub> or the Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> transition. The results gathered in the Table IV. show that the difference between the two ratios obtained leads to an error percentage of less than 1% for the three alloys studied, which is extremely satisfactory since for the complete decomposition, 16 contributions are required. This demonstrates that the small adjustments realized to adapt the decomposition of each transition leads to equivalent
composition values, meaning that both transitions can be indifferently used for quantification. At the same time, this result validates the consistency of the developed methodology using the Ga L$_{3}$M$_{4.5}$M$_{4.5}$ line to decompose the Ga L$_{2}$M$_{4.5}$M$_{4.5}$ one.

<table><caption>Table IV. Evolution of Ga oxide / Ga alloy ratios for Ga L$_{3}$M$_{4.5}$M$_{4.5}$ and Ga L$_{2}$M$_{4.5}$M$_{4.5}$ X-AES lines of the GaAs, GaSb and GaP samples modeled in the section B. The error represents the difference between the ratios Ga oxide/Ga alloy calculated for each transition.</caption><thead><tr><th rowspan="2">Sample</th><th>Ga oxide (Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub>)</th><th>Ga oxide (Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub>)</th><th rowspan="2">Error</th></tr><tr><th>Ga alloy (Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub>)</th><th>Ga alloy (Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub>)</th></tr></thead><tbody><tr><td><b>GaAs</b></td><td>0.06</td><td>0.06</td><td>&lt; 1 %</td></tr><tr><td><b>GaSb</b></td><td>0.74</td><td>0.73</td><td>&lt; 1 %</td></tr><tr><td><b>GaP</b></td><td>0.03</td><td>0.03</td><td>&lt; 1 %</td></tr></tbody></table>

To go further, the different ratios of the alloying elements (III/V ratios) are calculated using different couples of peaks, either photopeaks or Auger transitions, but corresponding to electrons emitted from similar escape depths. Indeed, due to the presence of a superficial oxide layer the sample has to be considered as a bi-layer system. To do so, the Inelastic Mean Free Path (IMFP) and corresponding probed depth for Ga L$_{2}$M$_{4.5}$M$_{4.5}$ transition, Ga 2p and Ga 3d photopeaks are calculated for each alloy using the QUASES-IMFP-TPP2M version 3.0 software 26 with the non-relativistic TPP-2 M formula. The KE, IMFP and probed depth for each alloy are gathered in Table S3 in the supplemental material. As an example, according to this table S3, for the GaAs alloy, the different duets of photopeaks and Auger transitions will be the following: Ga 2p$_{3/2}$/As 2p$_{3/2}$, Ga L$_{2}$M$_{4.5}$M$_{4.5}$/As L$_{2}$M$_{2.3}$M$_{4.5}$ and Ga 3d/As 3d. For the GaAs alloy, the presence of an oxide contribution is clearly identified on the Ga 3d and Ga 2p$_{3/2}$ peaks as illustrated in Fig. S4 and S5 of the supplemental material. In the present case, the oxide ratio determined with the Ga 2p$_{3/2}$ photopeak, most sensitive to the surface state, leads to
$Ga_{oxide}/Ga_{alloy} = 0.09$ and agrees with the one measured with the Auger transitions (0.06).

In the case of As, only $As^0$ is detected on the As 3d and As $2p_{3/2}$ photopeaks ($As_{elemental}/As_{alloy} = 0.1$). This is not surprising at short timescale of the oxidation process since $As^0$ is known to be the corrosion intermediate formed prior As oxide formation.

Consequently, As LMM transition would also require a decomposition to extract the individual contributions of alloy and elemental environments and refine the calculation of the $Ga_{alloy}/As_{alloy}$ ratio. An approximation is thus made by taking the whole As LMM shape to perform the quantification and a value of $\frac{Ga_{alloy}}{As} = 1.1$ is obtained. For the GaP sample, only P 2p is considered, the P 2s being not far enough to generate any significant evolution of the $\frac{Ga}{P}$ ratio and less intense. Here also, oxide is only detected for Ga element with $Ga_{oxide}/Ga_{alloy} = 0.04$ (Figure S5) in line with the value obtained with the Auger transitions (0.03). In the case of GaSb decomposition, the film is so thick that the oxide presence is no more negligible. The Ga $2p_{3/2}$ leads to $Ga_{oxide}/Ga_{alloy} = 1.1$ when 0.73 was found with the Ga LMM ratios, Ga LMM probing deeper. Concerning Sb 3d a ratio of 0.3 is obtained for Sb$_{oxide}$/Sb$_{alloy}$ indicating that without Sb MNN transition decomposition the Gaalloy/Sballoy ratio will remain approximative as the separation of the oxide and alloy environments are mandatory to get a good stoichiometry value for the alloy phase. As a consequence, the graph FIG. 4. presents a summary of the $\frac{III}{V}$ ratios obtained for GaAs and GaP compounds only and the different duets of peaks used along with the associated probed thicknesses determined using TPP2M model implemented in the Quases software. The small deviation of the ratios obtained for each sample between the three probed depths demonstrates that a reliable quantification is achievable when
using indifferently Auger transitions or photopeaks, provided that the alloy environment
can be properly separated from the oxide one or oxide phase negligible.

FIG. 4. III-V alloy ratio obtained for two GaAs and GaP samples at different probed depths using selected duets of peaks on the base of their similar IMPF (oxide superficial film negligible). The As content from the Auger line is a little bit overestimated as oxide environment could not be separated.

<figure><img src="image_6.png" /></figure>

**III. Summary and conclusion**

An innovative methodology has been elaborated to perform accurate quantification of Ga
based alloys using conventional XPS (Al source) by developing a rigorous decomposition
procedure to model the Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> and Ga L<sub>3</sub>M<sub>4.5</sub>M<sub>4.5</sub> Auger transition using three Ga
binary alloys (GaAs, GaP and GaSb). An enlarged spectral region recording of transitions was realized to take advantage of the similar shapes of these peaks, inherent to the Auger deexcitation process, and permitting to reinforce the robustness of the decomposition procedure by locking the decomposition of the Ga L₂M₄,₅M₄,₅ with respect to the decomposition pattern of the Ga L₃M₄,₅M₄,₅ transition. The NLLS method is employed to keep possible some acceptable adjustments of the background subtraction and of the set of contributions used for the decomposition. The spectral signatures of the alloy and oxide environments could be determined on a perfectly oxygen-free GaAs(T) and a Ga₂O₃ reference samples, helping us to fix decomposition parameters such as the background model, the peak shapes, energy positions and FWHM and area ratios and unveil characteristic patterns for the alloy and the oxide environment. The number of contributions used was minimized to limit errors while retaining coherent FWHM with a physical meaning, i.e. lifetime of excited state, by fixing FWHM in the range of those measured for photopeaks acquired in the same experimental conditions. Interestingly, this number of contributions, at first sight arbitrary, agrees with modelling results found in the literature for Ga LMM Auger transitions confirming that realistic parameters are used.

This set of contributions of the two chemical environments was then implemented on a GaAs, GaSb and GaP samples presenting native oxide films whose thicknesses depends on their air reactivity. We show that a quasi-identical pattern of the alloy environment is obtained for the three alloys whatever the nature of the element associated to Ga, showcasing that the same Auger process are involved. The decomposition procedure is validated by the good match of the representative oxide/alloy contributions ratios calculated from each Auger transition. The determination of constant III-V alloy ratios
using indifferently Auger transitions and photopeaks (provided they are in the same energy range) highlight the possibility to use Auger transitions for quantification purpose.

To conclude, the procedure detailed in this paper relies on a robust decomposition procedure and represents a real asset to address the Ga L<sub>2</sub>M<sub>4.5</sub>M<sub>4.5</sub> / N 1s overlapping issue for GaN based compounds quantification.

## SUPPLEMENTAL MATERIAL

See supplementary material at [URL will be inserted by AIP Publishing] for C 1s photopeak position for each reference sample as well as C 1s and O 1s spectra of each reference sample: GaAs<sub>(T)</sub>, GaAs, GaP and Ga 2p<sub>3/2</sub> and Ga 3d decomposition for each Ga alloy reference sample as well as the corresponding associated element peaks decomposition namely P 2p, As 2p, As L<sub>4</sub>M<sub>4.5</sub>M<sub>4.5</sub>, As 3d, Sb L<sub>4</sub>M<sub>4.5</sub>M<sub>4.5</sub>, Sb 4d and Sb 3d - O 1s region is also illustrated. The evolution of Ga 2p<sub>3/2</sub> and Ga 3d FWHM is also reported for each reference sample. Comparison between GL and LA lineshapes as well as the background choice and the U 2 Tougaard background details used for all the decompositions are also shown. IMFP and probed thicknesses are calculated using QUASES-IMFP-TPP2M version 3.0 software for GaAs and GaP samples.

## AUTHOR DECLARATIONS

### Conflicts of Interest

The authors have no conflicts to disclose.

## DATA AVAILABILITY
The authors thank DGA for the funding of the GREAT project.

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