Study of In0.53Ga0.47As/InP interface in III–V heterostructures by coupling monoatomic argon sputtering and high-energy x-ray photoelectron spectroscopy

G. Tsamo Tagougue, E. Martinez, M. Veillerot, V. Thoréton, M. Martin, T.

Baron, G. Lefevre, S. Cavalaglio, F. Bassani

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G. Tsamo Tagougue, E. Martinez, M. Veillerot, V. Thoréton, M. Martin, et al. Study of In0.53Ga0.47As/InP interface in III–V heterostructures by coupling monoatomic argon sputtering and high-energy x-ray photoelectron spectroscopy. Journal of Applied Physics, 2025, 138 (2), pp.025301. 10.1063/5.0270597 . hal-05351110

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Study of In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface in III-V heterostructures by coupling monoatomic argon sputtering and high-energy x-ray photoelectron

Cite as: J. Appl. Phys. 138, 000000 (2025); doi: 10.1063/5.0270597

Submitted: 12 March 2025 · Accepted: 16 June 2025 ·

Published Online: ■■■ 2025

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G. Tsamo Tagougue,<sup>1,2,a)</sup> E. Martinez,<sup>1,a)</sup> M. Veillerot,<sup>1)</sup> V. Thoréton,<sup>1)</sup> M. Martin,<sup>2)</sup> T. Baron,<sup>2)</sup>
G. Lefevre,<sup>3)</sup> S. Cavalaglio,<sup>2)</sup> and F. Bassani<sup>2)</sup>

Q1

9 AFFILIATIONS

10 Université Grenoble Alpes, CEA, Leti, F-38000 Grenoble, France

11 ²Université Grenoble Alpes, CNRS, CEA/LETI Minatec, Grenoble INP, LTM, 38000 Grenoble, France

12 Université Grenoble Alpes, CEA, Liten, F-38000 Grenoble, France

14 a) Authors to whom correspondence should be addressed: guy-vano.tsamotagougue@cea.fr and eugenie.martinez@cea.fr

16 ABSTRACT

17 The non-destructive characterization of the buried InGaAs/InP interface is implemented using a combination of argon (Ar+) sputtering and
18 high-energy x-ray photoelectron spectroscopy (HAXPES). HAXPES is not surface sensitive but rather volume sensitive, so we can model the
19 HAXPES peak intensities by neglecting the surface damage to InGaAs caused by argon sputtering. This modeling allowed a precise location
20 of the HAXPES in-depth analysis after sputtering, in terms of InGaAs and InP probed thicknesses, as well as an estimation of the relative
21 elemental concentrations. An experimental study of the HAXPES peaks showed the abruptness of the InGaAs/InP interface, free of element
22 interdiffusion, and enabled the quantification of the composition of the InGaAs layer. The abruptness of this interface is also confirmed by
23 time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning transmission electron microscopy-energy dispersive x-ray
24 (STEM-EDX) analyses. ToF-SIMS profiles acquired with a depth resolution of 1.5 nm showed rather abrupt profiles, and STEM-EDX con-
25 firmed homogeneous compositions and flat interface.

26 © 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(https://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0270597

28 I. INTRODUCTION

The direct bandgap of III-V semiconductors makes them
promising for optoelectronic applications such as LEDs, lasers,
and photodetectors. In addition to their direct bandgap, the high
carrier mobility in the particular case of In<sub>0.53</sub>Ga<sub>0.47</sub>As and InP
also makes them interesting for heterojunction bipolar transistors
(HBTs).<sup>1,2</sup> The choice of concentrations of 53 at. % of In and
47 at. % of Ga in the alloy originates in that it gives the same
lattice parameter as InP and, therefore, in an InGaAs/InP inter-
face free from dislocations. Both layers are p-doped with Zn, and
a buried InAlAs layer is used to prevent Zn diffusion into the InP
substrate. During the epitaxy of this InGaAs/InP/InAlAs/InP het-
erostructure, the growth conditions can affect the interdiffusion of
the main elements (In, Ga, As, and P) between the layers. This is
one of the causes of the decrease of device performance.<sup>3,4</sup> It is

then very important to understand and minimize this phenome-
non, by adjusting growth parameters in order to have the most
abrupt possible interfaces.
43
44

A way to obtain chemical information about buried interfaces is
to use high-energy x-ray photoelectron spectroscopy (HAXPES).<sup>3</sup> In
particular, the chromium source Cr Kα (hν = 5415 eV) can be used
to probe deeper than standard XPS.<sup>6</sup> The intent here is to character-
ize the quality of the top InGaAs/InP interface in terms of possible
intermixing of the main elements. Using one of its major strengths,
i.e., being able to probe deeper than the sputtered damaged layer, we
will use argon ion sputtering and HAXPES to characterize non-
destructively the InGaAs/InP interface. Several sputtering steps will
be done to probe the bulk InGaAs layer as well as the buried
InGaAs/InP interface. Furthermore, we want to verify the compo-
sition of In<sub>0.5-x</sub>Ga<sub>0.5+x</sub>As in the In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP/InAlAs/InP
57
heterostructures. However, HAXPES with a chromium source is rather new and knowledge about relative sensitivity factors (RSFs) is needed for quantification. We will estimate them both theoretically, using calculated photoionization cross sections at high energy, as well as experimentally, from the peak intensity ratios measured on a reference sample.

HAXPES data modeling will also be done in order to precisely locate the layers being analyzed after sputtering. A two-layer model will be used to estimate the residual InGaAs thickness, from the calculation of the peak intensity ratios. Since the top InGaAs layer is rather thick (~80 nm), the sputtering time will be quite long with possible modifications of the surface composition and morphology. The impact of the surface damaged layer on the total HAXPES peak intensities will be estimated, from the ion penetration depth. Atomic Force Microscopy (AFM) will also be implemented the check the evolution of the surface roughness before and after sputtering.

Because of its greater sensitivity and possibly its high depth resolution, time-of-flight secondary ion mass spectrometry (TOF-SIMS) will also be used as a complementary technique. ToF-SIMS will be used to probe deeper into the stack in order to check the quality of all the interfaces of the InGaAs/InP/InAlAs/InP heterostructure. The crystallographic quality of the stack will also be examined through Scanning Transmission Electron Microscopy (STEM) observations, using energy dispersive x-ray (EDX) detection for qualification of the elemental composition of the different layers.

## II. EXPERIMENTAL DETAILS

The heterostructure was grown using applied materials 300 mm III-V Metal Organic Chemical Vapor Deposition (MOCVD). It consists of the following stack, with the thicknesses expected from growth parameters: In<sub>0.53</sub>Ga<sub>0.47</sub>As-p (80 nm)/InP-p (80 nm)/InAlAs-nid (50 nm)/InP-n (350 µm). The first and second layers are zinc (Zn)-doped. The third layer is non-intentionally doped, and the fourth is the sulfur (S)-doped substrate. The schematization of this heterostructure is shown in Fig. 1. After growth, the sample is removed from the MOCVD chamber for characterization.

<figure><img src="image_5.png" /><figcaption>FIG. 1. Schematic description of the studied heterostructure grown by MOCVD.</figcaption></figure>

A thin lamella for cross-sectional STEM was prepared using a dual focused ion beam-scanning electron microscope (FIB-SEM, FEI Helios 450S) equipped with an energetic Ga ion beam. First, the sample was coated with an e-beam induced SiO<sub>2</sub>/Pt capping layer to protect the heterostructure during the FIB milling process. Next, thanks to a micromanipulator, the thin lamella was moved and stuck to a grid and thinned down to 50 nm. STEM-EDX measurements were then performed using an Osiris TEM operated at 200 keV equipped with four EDX detectors to map the total chemical concentrations of In, Ga, Al, As, P, and O.

HAXPES measurements were made with Quantes from ULVAC-PHI equipped with a monochromatized Cr Kα (hν = 5.415 keV) x-ray source. All the measurements were carried out with a beam size of 100 µm and a pass energy of 112 eV, leading to an energy resolution of 0.9 eV. It has been shown that when using a Cr Kα (hν = 5.415 keV) x-ray source, the deeper the core level, the greater the effective ionization cross section.⁶ We, therefore, decided to study the core levels In3d<sub>5/2</sub>, Ga2p<sub>3/2</sub>, As2p<sub>3/2</sub>, and P1 s to characterize the In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface. HAXPES analyses were carried out with a takeoff angle (TOA) θ of 45° to be more sensitive to the InGaAs/InP interface, through the residual InGaAs layer left after sputtering. The HAXPES spectra were decomposed using CasaXPS software, the Lorentzian asymmetric (LA) line shape function and the Shirley method for the removal of background. In the absence of the C1 s carbon peak when using high energy x rays, the calibration was made with respect to the In3d<sub>5/2</sub> peak at an energy of 444.5 eV, referenced with respect to the C1 s peak at 285 eV when using an Al Kα source (1486.6 eV).⁷ Prior to HAXPES measurements, an initial ion sputtering with argon (Ar+) at an incident angle of 45°, with an energy of 1 keV over an area of 3 mm² was carried out. The idea is to reduce the thickness of the InGaAs layer by argon sputtering to probe the interface.

ToF-SIMS analyses were performed using a dual beam TOF SIMS5 instrument by IONTOF GmbH. Depth profiles were measured using a Cs<sup>+</sup> sputter beam at 1 keV with a sputtered area of 250 × 250 µm<sup>2</sup> and a Bi<sup>4+</sup> analysis beam at 25 keV with an analyzed area of 80 × 80 µm<sup>2</sup>. Analyses were performed with the positive polarity of the analyzer. The primary current for both ion beams was checked for each measurement to always operate under the same experimental conditions. Both ion guns were operated with an incidence angle of 45° with respect to the sample surface normal. The pressure in the ultrahigh vacuum chamber was around 2 × 10<sup>-10</sup> mbar.

AFM surface analyses were carried out on a Bruker Dimension Icon in the tapping mode with VTESPA-300 Si tips to evaluate the evolution of surface roughness before and after argon sputtering.

## III. DETAILS OF HAXPES DATA MODELING

To fully characterize the In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface, in particular to calculate the thickness of the remaining In<sub>0.53</sub>Ga<sub>0.47</sub>As layer after sputtering, we modeled the theoretical HAXPES core levels intensities (Fig. 2). This two-layer model consists of an In<sub>0.53</sub>Ga<sub>0.47</sub>As layer with thickness d on top of a semi-infinite layer of InP.

The formula giving the HAXPES intensity for a given photoelectron i crossing a lattice of a compound j is⁸
<figure><img src="image_6.png" /><figcaption>FIG. 2. Two layers modeling of the HAXPES core level intensities from the In0.53Ga0.47As/InP interface.</figcaption></figure>

$$I_i = \Phi N_i(z) \cdot (\text{RSF})_i \cdot \exp \frac{-z}{\lambda_i^j \sin(\theta)} \text{dz,} \quad (1)$$

where $\Phi$ is the photon flux, $N(z)$ is the volume density of element i in matrix j at depth z, $\lambda_i^j$ is the inelastic mean free path (IMFP) of element i in matrix j, and $\theta$ is the analysis angle between the surface of the sample and the detector, also called takeoff angle.

(RSF)$_i$ is the relative sensitivity factor defined by

$$(\text{RSF})_i = \sigma_i \cdot \lambda_i^j \cdot T_i, \quad (2)$$

where $\sigma_i$ is the photoelectron cross section and $T_i$ the transmission function of the analyzer.

Using Eq. (1), we can calculate different HAXPES core-level intensities as

$$\begin{aligned} I_{\text{InP}}^{\text{InP}} &= \Phi \cdot N_{\text{In}}^{\text{InP}} \cdot (\text{RSF})_{\text{InP}}^{\text{InP}} \cdot \lambda_{\text{InP}}^{\text{InP}} \\ & \quad \cdot \sin(\theta) \cdot \exp \frac{-d}{\lambda_{\text{InP}}^{\text{InP}}} \cdot \sin(\theta), \end{aligned} \quad (3)$$

$$I_{\text{P1s}}^{\text{InP}} = \Phi \cdot N_{\text{P1s}}^{\text{InP}} \cdot (\text{RSF}_{\text{P1s}}^{\text{InP}} \cdot \lambda_{\text{P1s}}^{\text{InP}} \cdot \sin(\theta) \cdot \exp \frac{-d}{\lambda_{\text{P1s}}^{\text{InP}} \cdot \sin(\theta)}, \quad (4)$$

$$\begin{aligned} I_{\text{InP1s}}^{\text{InP}} = \Phi \cdot N_{\text{InP1s}}^{\text{InP}} \cdot (\text{RSF}_{\text{InP1s}}^{\text{InP}} \cdot \lambda_{\text{InP1s}}^{\text{InP}} \cdot \sin(\theta) \cdot \exp \frac{-d}{\lambda_{\text{InP1s}}^{\text{InP}} \cdot \sin(\theta)} \cdot \sin(\theta) \cdot \exp \frac{-d}{\lambda_{\text{InP1s}}^{\text{InP}} \cdot \sin(\theta)}, \end{aligned} \quad (5)$$

$$\begin{aligned} I_{\text{Ga2P3/2}}^{\text{In0.53Ga0.47As}} &= \Phi \cdot N_{\text{Ga2P3/2}}^{\text{In0.53Ga0.47As}} \cdot (\text{RSF})_{\text{Ga2P3/2}}^{\text{In0.53Ga0.47As}} \cdot \lambda_{\text{Ga2P3/2}}^{\text{In0.53Ga0.47As}} \\ & \quad \cdot \sin(\theta) \cdot 1 - \exp \frac{-d}{\lambda_{\text{Ga2P3/2}}^{\text{In0.53Ga0.47As}} \cdot \sin(\theta)}, \end{aligned} \quad (6)$$

$$\begin{aligned} I_{\text{As2P3/2}}^{\text{In0.53Ga0.47As}} &= \Phi \cdot N_{\text{As2P3/2}}^{\text{In0.53Ga0.47As}} \cdot (\text{RSF})_{\text{As2P3/2}}^{\text{In0.53Ga0.47As}} \cdot \lambda_{\text{As2P3/2}}^{\text{In0.53Ga0.47As}} \\ & \quad \cdot \sin(\theta) \cdot 1 - \exp \frac{-d}{\lambda_{\text{As2P3/2}}^{\text{In0.53Ga0.47As}} \cdot \sin(\theta)}, \end{aligned} \quad (7)$$

The convergence of the theoretical and experimental HAXPES intensity ratio $(I_{\text{Ga2P3/2}} + I_{\text{As2P3/2}})/I_{\text{P1s}}$ allows us to deduce the thickness d of the In0.53Ga0.47As layer. Knowing that the depth probed in HAXPES is $d_p = 3 \cdot \lambda \cdot \sin(\theta)$, the thickness passed through after the interface can also be calculated as $d_p-d$ as shown in Fig. 2.

From these HAXPES intensities, the concentration of a chemical element i in matrix j is calculated using the following relationship:

$$C_i^j = \frac{\overline{\text{RSF}}_i}{\overline{\text{P}}_i \cdot \frac{\overline{\text{RSF}}_k}{\overline{\text{k}}}} \quad (8)$$

To estimate the theoretical RSFs, IMFPs have been calculated using the Quases IMFP-TPP2 M software with an error of 10%. Using the effective photoionization cross sections at 5000 and 6000 eV from Refs. 10 and 11, we deduced the cross sections at 5415 eV of the different core levels by linear interpolation. The latter values were compared with the cross sections calculated using the formula developed by Cant et al. and show a good match between the results, as shown in the supplementary material (see Sec. I). For a pass energy of 112 eV, we calculated the transmission factor of the analyzer for core level i (Ti), which is the ratio between the peak area and the pass energy and defined as follows:

$$T_i = \frac{A}{E_p} = \frac{B}{a^2 + \frac{E_c}{E_p}} \frac{C}{A}, \quad (9)$$

with A being the peak area, $E_p$ being the pass energy, and $E_c$ being the kinetic energy of the core level. The coefficients a and b depend on the analyzer. They are obtained by the linear least square fit of the data applying Eq. (9) using the spectral regions of sputter cleaned Cu, Ag, and Au. We then calculated the theoretical relative sensitivity factors (RSFs) of the different core levels for each matrix crossed.

To determine the experimental RSFs, we used a bulk In0.53Ga0.47As reference material with a thickness of 1.5 µm where concentration values are confirmed by x-ray diffraction (XRD) analysis. We assumed that the theoretical and experimental RSFs of gallium are equal. This may be justified by low gallium sputter after argon sputtering of InGaAs. We have also confirmed this observation on a bulk In0.53Ga0.47As reference material using Al Kα XPS.
<table><caption>TABLE I. Theoretical values used for estimating HAXPES core level intensities.</caption><thead><tr><th>Lattice crossed j</th><th>Element i</th><th>Volume density N<sub>i</sub> of i in j</th><th>Cross section at 5415 eV (σ<sub>i</sub>) (kb)</th><th>Transmission factor (T<sub>i</sub>)</th><th>Inelastic mean free paths λ<sup>i</sup> (nm)</th><th>(RSF)<sup>i</sup>Th</th><th>(RSF)<sup>i</sup>Ex</th></tr></thead><tbody><tr><td rowspan="2">InP</td><td>In3d<sub>5/2</sub></td><td>1</td><td>2.71</td><td>0.86</td><td>8.5 ± 0.85</td><td>~19.8</td><td>19.8</td></tr><tr><td>P1 s</td><td>1</td><td>10.85</td><td>0.93</td><td>6.0 ± 0.6</td><td>~60.5</td><td>60.5</td></tr><tr><td rowspan="3">In<sub>0.53</sub>Ga<sub>0.47</sub>As</td><td>In3d<sub>5/2</sub></td><td>0.53</td><td>2.71</td><td>0.86</td><td>8.6 ± 0.86</td><td>~20.0</td><td>18.5</td></tr><tr><td>Ga2p<sub>3/2</sub></td><td>0.47</td><td>6.98</td><td>0.89</td><td>7.6 ± 0.76</td><td>~47.2</td><td>47.2</td></tr><tr><td>As2p<sub>3/2</sub></td><td>1</td><td>9.40</td><td>0.90</td><td>7.3 ± 0.73</td><td>~61.7</td><td>43.7</td></tr></tbody></table>

analysis. The results from this observation are shown in the supplementary material (see Sec. II). The experimental RSFs of indium and arsenic were, therefore, recalculated using the experimental HAXPES intensities and the concentration ratios of the elements in the reference In<sub>0.53</sub>Ga<sub>0.47</sub>As material as follows:

$$ \text{RSF}_{\text{As}}^{\text{In0.53Ga0.47As}}(\text{ex}) = \frac{C_{\text{Ga}}^{\text{In0.53Ga0.47As}}}{C_{\text{As}}^{\text{In0.53Ga0.47As}}} \cdot \frac{I_{\text{Ga}}^{\text{In0.53Ga0.47As}}(\text{ex})}{I_{\text{As}}^{\text{In0.53Ga0.47As}}(\text{ex})} \cdot \text{RSF}_{\text{Ga}}^{\text{In0.53Ga0.47As}}(\text{th}), \quad (10) $$

$$ \text{RSF}_{\text{In}}^{\text{In0.53Ga0.47As}}(\text{ex}) = \frac{C_{\text{Ga}}^{\text{In0.53Ga0.47As}}}{C_{\text{In}}^{\text{In0.53Ga0.47As}}} \cdot \frac{I_{\text{Ga}}^{\text{In0.53Ga0.47As}}(\text{ex})}{I_{\text{In}}^{\text{In0.53Ga0.47As}}(\text{ex})} \cdot \text{RSF}_{\text{Ga}}^{\text{In0.53Ga0.47As}}(\text{th}). \quad (11) $$

Here, the concentrations of indium, gallium, and arsenic in the InGaAs reference material are 23.5%, 26.5%, and 50%, respectively, as deduced from the targeted composition. The abbreviations ex and th mean experimental and theoretical, respectively. The theoretical and experimental RSFs of indium and phosphorus in InP

were considered equal because this matrix is not sputtered before HAXPES analysis.

The parameters used to calculate the theoretical HAXPES intensities are summarized in Table I.

HAXPES modeling of the In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface is made after argon sputtering, so we used the Stopping and Range of Ions in Matter (SRIM) software<sup>14</sup> to simulate the implantation of argon atoms in In<sub>0.53</sub>Ga<sub>0.47</sub>As. This model can give the implantation depth range of argon in In<sub>0.53</sub>Ga<sub>0.47</sub>As. The idea is to ensure that sputtering damages to the In<sub>0.53</sub>Ga<sub>0.47</sub>As surface are limited so that HAXPES modeling can be carried out safely with a good reliability. In our study, InGaAs was then defined with a chemical composition of 26.5 at. % indium, 23.5 at. % gallium, and 50 at. % arsenic and with a density of 5.5 g/cm<sup>3</sup> corresponding to a zinc-blende crystal structure like GaAs, where some Ga atoms are substituted by In atoms.

To obtain information on the chemical composition of the InGaAs layer, a HAXPES analysis was carried out before crossing the interface (InGaAs probed) after argon sputtering for low time as described in Fig. 3(a). To non-destructively characterize the InGaAs/InP interface, a HAXPES analysis was conducted at the

<figure><img src="image_7.png" /><figcaption>FIG. 3. Schematic description of different steps of coupled argon sputtering and HAXPES characterization: (a) before crossing the interface and (b) after crossing the interface.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>FIG. 4. STEM 2D cross sections of the InGaAs/InP interface: (a) HAADF, (b) BF, respectively.</figcaption></figure>

same measurement point after crossing the interface (InGaAs/InP probed). This was achieved by extending the argon sputtering time, allowing the detection of photoelectrons coming from the interface, as shown in Fig. 3(b).

## IV. RESULTS AND DISCUSSION

After the growth, the high-angle annular dark-field (HAADF) [Fig. 4(a)] and the bright field (BF) [Fig. 4(b)] images acquired by STEM were used to determine the quality of the interface between InGaAs and InP. The standard BF mode provides an image where the contrast is related to electron absorption; thus, it is lower for heavier materials. The HAADF mode is another method allowing us to discriminate atoms according to their atomic number: the higher this atomic number, the higher is the intensity. These images show that the layers are well-defined and crystalline and that the InGaAs/InP interface is sharp. The thicknesses of the InGaAs and InP layers are around 82 and 79 nm, respectively. The depth profiles and mappings obtained by energy dispersive x ray (EDX) for the complete InGaAs/InP/InAlAs/InP heterostructures are shown in Fig. 5. The value of the top oxide layer (not shown in Fig. 4) thickness was estimated to be around 4 nm. The thickness of InGaAs including the native oxide is, therefore, around 86 nm. Note that the EDX oxygen profile also shows, in the first 15 nm, the SiO₂/Pt capping layer used to protect the heterostructure during ion milling. Using the Cliff-Lorimer method in Bruker's ESPRIT software, EDX measurements estimate the concentrations in the InGaAs layer as 26 ± 3 at. % for In, 21 ± 2 at. % for Ga, 46 ± 5 at. % for As, and 5 ± 0.5 at. % for O. These values are semi-quantitative, with a 10% margin of error due to uncertainties in the sample thickness (related to FIB preparation) and possible contamination from the TEM column. The oxygen concentration is likely background noise level, as confirmed by HAXPES and ToF-SIMS, indicating the actual negligible oxygen content.

<figure><img src="image_9.png" /><figcaption>FIG. 5. EDX depth profiles and elemental mappings of InGaAs/InP/InAlAs/InP heterostructures.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>FIG. 6. 5 × 5 µm² AFM topography images: (a) as grown sample and (b) after Ar⁺ sputtering for 40 min.</figcaption></figure>

Atomic Force Microscopy (AFM) was carried out on the as-grown sample. The roughness (Root Mean Square, RMS) of the In<sub>0.53</sub>Ga<sub>0.47</sub>As surface is 0.3 nm, and the surface displays atomic steps [Fig. 6(a)]. AFM has been also made in the crater after Ar⁺ sputtering for 40 min. The roughness remains the same at around 0.3 nm, and the surface remains smooth, but there are no more atomic steps [Fig. 6(b)]. The low variation in InGaAs surface roughness before and after ion sputtering has also been observed in Ref. 15. Since HAXPES (Cr kα) which is more bulk sensitive, is used here, surface damages will not influence too much the spectra and the chemical composition should be, thus, weakly modified compared to the as-grown material.<sup>16</sup>

SRIM simulations were run to get the penetration depth profile of Ar⁺ ions with an energy of 1 keV and incident angle of 45° in a 10 nm-thick In<sub>0.53</sub>Ga<sub>0.47</sub>As film. The mean depth of Ar⁺ ion distribution, also called projected ion range, is 2.1 nm [Fig. 7(a)]. The standard deviation of the ion distribution, also called straggling, is 1.3 nm [Fig. 7(a)]. Using amorphized In<sub>0.53</sub>Ga<sub>0.47</sub>As thicknesses of 2, 1, and 0.5 nm, we calculated the ratio of this surface contribution with respect to the total HAXPES peak intensity, by using HAXPES signal attenuation as a function of In<sub>0.53</sub>Ga<sub>0.47</sub>As depth probe [Fig. 7(b)] and a sampling depth of 18 nm (for instance, in the first case, by integrating the signal over the [0, 2 nm] and [0, 18 nm] intervals). The HAXPES signal from the low damaged surface layer, therefore, represents around 30%, 20%, and 15% of the total signal, respectively, for these three thicknesses. HAXPES is, therefore, very bulk sensitive in our case, i.e., the contribution of the sputter-induced damaged layer on the measured intensities can be considered as rather secondary.

In view of these results: (a) low roughening and amorphization of the surface and (b) low weight of surface contribution on

the total HAXPES intensity, the HAXPES model was used without further taking into account the effect of Ar⁺ sputtering.

After thinning the top In<sub>0.53</sub>Ga<sub>0.47</sub>As layer by Ar⁺ sputtering for 26 min, several HAXPES peaks can be measured (Fig. 8): (a) the In3d<sub>5/2</sub> peak with In-(GaAs) bonds at a binding energy of 444.5 eV and a full width at half maximum (FWHM) of 1.5 eV [Fig. 8(a)], the Ga2p<sub>3/2</sub> peak with Ga-(InAs) bonds at a binding energy of 1117.62 eV and a FWHM of 1.8 eV [Fig. 8(b)], and the As2p<sub>3/2</sub> peak with As-(InGa) bonds at a binding energy of 1323.17 eV and a FWHM of 1.9 eV [Fig. 8(c)]. The absence of the P1 s peak shows that the HAXPES source only probes In<sub>0.53</sub>Ga<sub>0.47</sub>As. The absence of the O1 s peak (not shown here) shows that there is no oxygen pollution at the In<sub>0.53</sub>Ga<sub>0.47</sub>As sputtered surface nor inside the bulk layer. Using the experimental relative sensitivity factors of Table I and peak fitting with CasaXPS, we were able to determine the relative atomic concentrations of the various elements in the In<sub>0.53</sub>Ga<sub>0.47</sub>As layer. The concentrations are 26 ± 3 at. % of indium (In), 23 ± 2 at. % of gallium (Ga), and 52 ± 5 at. % of arsenic (As) (see Table III). These concentrations, estimated with an error of 10%,<sup>17</sup> are in good agreement with the concentrations expected from MOCVD growth. They are also consistent with the values that were obtained by theoretical modeling using Eqs. (3)–(8) as shown in Fig. 9(b), in particular for In<sub>0.53</sub>Ga<sub>0.47</sub>As bulk [for thicknesses > d<sub>p</sub> = 3·λ·sin(45°) ~ 18 nm].

After going on sputtering to reach the In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface (a second ion sputtering of 14 min for a total time of 40 min was carried out on the same area as before), the same HAXPES peaks were measured (Fig. 8). The In3d<sub>5/2</sub> peak is unchanged in terms of position and width, but is now representative of both In-(GaAs) and In-P bonds, as the P signal is now clearly seen. The In-O bonds, which are shifted by an energy of +0.8 ± 0.1 eV with respect to the In-P bonds, are not present.<sup>18</sup>
<figure><img src="image_11.png" /><figcaption>FIG. 7. (a) Penetration depth profile of Ar⁺ ions with 1 keV energy into In<sub>0.53</sub>Ga<sub>0.47</sub>As simulated by SRIM, (b) HAXPES signal attenuation as a function of In<sub>0.53</sub>Ga<sub>0.47</sub>As depth probe.</figcaption></figure>

The Ga2p<sub>3/2</sub> and As2p<sub>3/2</sub> peaks also remain unchanged (same positions and widths). The Ga–O bonds, which are shifted by an energy of +0.8 ± 0.1 eV with respect to the Ga–(InAs) bonds, are not present either.<sup>19</sup> Similarly, As<sub>2</sub>O<sub>3</sub> and As<sub>2</sub>O<sub>5</sub> shifted, respectively, by an energy of +2.7 and +3.7 eV with respect to the As–(InGa) bonds are not present.<sup>19</sup> We note the presence of the P1 s peak with P–In bonds at a binding energy of 2142.29 eV and a FWHM of 1.5 eV. This shows that after sputtering for 40 min, the In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface is probed. The P–P bonds, which are shifted by an energy of +1.2–1.6 eV with respect to the P–In bonds, are not present,<sup>18</sup> as well as the P–O bonds, which are shifted by an energy of +4.5–5.5 eV with respect to the P–In bonds.<sup>18</sup> The absence of the O1 s peak (see Sec. IV in the *supplementary material*) shows that there is no oxygen pollution at this In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface and in the layers. This is confirmed by the absence of oxidized components in all the spectra. The In3d<sub>5/2</sub>, Ga2p<sub>3/2</sub>, and As2p<sub>3/2</sub> peaks measured before (26 min) and after (40 min) crossing the interface overlap very well. This shows that there is no interdiffusion of major elements between the In<sub>0.53</sub>Ga<sub>0.47</sub>As and InP layers. Indeed, this phenomenon would have induced some widening of these spectra due to the subsequent disorder and/or the formation of new chemical bonds such as P–As or P–Ga. Similarly, this superposition of peaks also shows that HAXPES is not surface sensitive. Again, if that was the case, some widening of HAXPES peaks would have been observed as a result of surface amorphization. The relative atomic concentrations of the various elements were again estimated (see Table II). The concentration of indium (In) has risen from 26 ± 3 to 35 ± 4 at. %, which is normal because we are adding the concentration of indium in InP. As a result, the concentration of gallium (Ga) has decreased from 23 ± 2 to 16 ± 2 at. % and the concentration of arsenic (As) from 52 ± 4 to 40 ± 4 at. %. The concentration of phosphorus (P) is 9 ± 1 at. % (Table II).

Note that the comparison of HAXPES core levels before and after crossing the InGaAs/InP interface is not sufficient to conclude about the presence or absence of interdiffusion in our system. Indeed, the spectra of Fig. 8(a) show that no detectable changes would be seen in the case of P atoms substituting As sites into InGaAs, as the In3d spectrum does n't change when moving from In–(Ga,As) bonds to a mixture of In–(Ga,As) and In–P bonds. Further data must be considered to really conclude about an abrupt InGaAs/InP interface. First, the HAXPES intensity ratio of gallium to arsenic is constant and equal to 0.5 ± 0.05 before and after crossing the interface. This is consistent with the expected value for the top InGaAs layer, and this result confirms the absence of a real change in the chemical composition of this layer near the buried InGaAs/InP interface. Second, the quality of this interface is confirmed by the STEM-EDX results. As discussed before, the STEM images as well as the EDX profiles and elemental mappings show that the interface is abrupt with no interdiffusion of the major elements. To be even more convincing, additional ToF-SIMS profiles are shown in Fig. 9, confirming that all the interfaces of the heterostructure are rather abrupt.

Using the ToF-SIMS depth profiles in Fig. 9, the upward depth resolutions with 16%–84% methodology<sup>20</sup> are 1.5 nm for the first interface using PC<sub>S</sub><sup>+</sup>, 2.5 nm for the second interface using AsC<sub>S</sub><sup>+</sup>, and 1.5 nm for the third interface using PC<sub>S</sub><sup>+</sup> (Table III). This value does not change between the first and third interfaces. This shows that the depth resolution is not degraded with increasing sputtering times. ToF-SIMS profiles, thus, confirm that all the interfaces of the heterostructure are very sharp, in particular the top InGaAs/InP one, in agreement with the STEM-EDX and HAXPES results.

The theoretical curve, used for estimating the thickness of In<sub>0.53</sub>Ga<sub>0.47</sub>As, of the HAXPES intensity ratio (I<sub>Ga2p3/2</sub> + I<sub>As2p3/2</sub>)/I<sub>P1s</sub>,
<figure><img src="image_12.png" /><figcaption>FIG. 8. (a) In3d<sub>5/2</sub>, (b) Ga2p<sub>3/2</sub>, (c) As2p<sub>3/2</sub>, and (d) P1 s HAXPES peak decompositions with Ar<sup>+</sup> sputtering time.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>FIG. 9. ToF-SIMS profiles of the InGaAs/InP/InAlAs/InP heterostructure.</figcaption></figure>

calculated from Eqs. (4), (6), and (7), is shown in Fig. 10(a). We observed the convergence of the theoretical and experimental (I<sub>Ga2p3/2</sub> + I<sub>As2p3/2</sub>)/I<sub>P1s</sub> HAXPES intensity ratios at a value of 4.4 as shown in Fig. 10(a). This corresponds to an In<sub>0.53</sub>Ga<sub>0.47</sub>As thickness of 6.5±0.7 nm. After 40 min of sputtering, approximately 74±7 nm of In<sub>0.53</sub>Ga<sub>0.47</sub>As was removed. We can, therefore, deduce a sputtering speed of 1.8 nm/min, indicating that after sputtering for 26 min, 48±5 nm of In<sub>0.53</sub>Ga<sub>0.47</sub>As are removed, leaving 32±3 nm (Table II).

After 40 min argon sputtering, we can, therefore, determine the depth of InP probed below the interface from d<sub>p</sub>-d, which is equal to 11.5 ± 1 nm, while approximately 6.5 ± 0.7 nm of In<sub>0.53</sub>Ga<sub>0.47</sub>As is left at the surface. This justifies the decrease in Ga and As concentrations after crossing the In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface. Theoretical absolute concentrations estimated for the InGaAs/InP system with a residual 6.5 nm-thick InGaAs layer using Eq. (8) as shown in Fig. 10(b) give the following values: 35.5 at. % of In, 17 at. % of Ga, 36.5 at. % of As, and 11 at. % of P (Table II). The latter values correspond to separate relative concentrations of 25 at. % of In, 24 at. % of Ga, and 51 at. % of As for

<table><caption>TABLE III. Summary of upward depth resolutions of the different interfaces using PC<sub>s</sub><sup>+</sup> and AsC<sub>s</sub><sup>+</sup>.</caption><thead><tr><th colspan="3">Upward depth resolution</th></tr><tr><th>First interface<br>(PCs<sup>+</sup>)</th><th>Second interface<br>(AsCs<sup>+</sup>)</th><th>Third interface<br>(PCs<sup>+</sup>)</th></tr></thead><tbody><tr><td>~1.5 nm</td><td>~2.5 nm</td><td>~1.5 nm</td></tr></tbody></table>

the InGaAs layer and 61 at. % of In and 39 at. % of P for the InP layer. These results show a good match with the expected concentration values in the InGaAs layer (see Table II) and a poor match with the expected concentrations in the InP layer (50 and 50 at. %). The concentration mismatch in the InP layer can be explained by the difference in depths probed [3·λ·sin(45°)] for the In3d<sub>5/2</sub> (binding energy 444.5 eV) and P1s (binding energy 2142.29 eV) core levels, which are around 18 ± 2 and 12.6 ± 1 nm, respectively (due to the large kinetic energy of ~1700 eV difference between these core levels). Note that other reasons could also explain this difference such as the photoionization cross-sectional estimation at high photon energy.

Even after ion sputtering with argon for 40 min, the theoretical chemical composition of the In<sub>0.53</sub>Ga<sub>0.47</sub>As layer remained consistent with the experimental one. This confirms that HAXPES is not sensitive to sputtered-induced surface damages, as mentioned in Ref. 16. This is, thus, one of the major advantages of HAXPES, i.e., being compatible with preliminary Ar<sup>+</sup> sputtering because not sensitive to the damaged surface layer. The proportion of the damaged surface layer was estimated on materials exhibiting preferential sputtering such as GaN and Ta<sub>2</sub>O<sub>5</sub> as well as for a metal alloy such as Au/Cu. SRIM calculations were carried out in order to estimate the argon implantation depths (see Sec. III in the supplementary material). Photoelectrons emitted from the surface damaged layer represent an average of 35%, 32%, and 34% of the total signal for GaN, Ta<sub>2</sub>O<sub>5</sub>, and Au/Cu, respectively. The proportion of the damaged layer is, thus, rather small such as for InGaAs. However, this is for the moment early to conclude about the versatility of HAXPES combined with Ar<sup>+</sup> sputtering as a comprehensive non-destructive interface evaluation method. When considering such sputtering sensitive materials, more experimental data are needed here to track potential alterations of the HAXPES core levels due to preferential sputtering (such as oxygen for oxides).

<table><caption>TABLE II. Theoretical and experimental elemental concentrations after 26 and 40 min of argon (Ar<sup>+</sup>) sputtering.</caption><thead><tr><th>Ar<sup>+</sup> sputtering time (min)</th><th>Depth probe d<sub>p</sub> = 3·λ·sin(45°) (nm)</th><th>Matrix probed</th><th>In<sub>0.53</sub>Ga<sub>0.47</sub>As thickness d (nm)</th><th>InP probed d<sub>p</sub>-d (nm)</th><th>Elements</th><th>Experimental concentration (at. %)</th><th>Theoretical concentration (at. %)</th></tr></thead><tbody><tr><td rowspan="3">26</td><td rowspan="3">~18</td><td rowspan="3">InGaAs</td><td rowspan="3">32 ± 3</td><td rowspan="3">No</td><td>In</td><td>26 ± 3</td><td>26.5</td></tr><tr><td>Ga</td><td>23 ± 2</td><td>23.5</td></tr><tr><td>As</td><td>52 ± 5</td><td>50</td></tr><tr><td rowspan="4">40</td><td rowspan="4">~18</td><td rowspan="4">InGaAs and InP</td><td rowspan="4">6.5 ± 0.7</td><td rowspan="4">11.5 ± 1</td><td>In</td><td>35 ± 4</td><td>35.5</td></tr><tr><td>Ga</td><td>16 ± 2</td><td>17</td></tr><tr><td>As</td><td>40 ± 4</td><td>36.5</td></tr><tr><td>P</td><td>9 ± 1</td><td>11</td></tr></tbody></table>
<figure><img src="image_14.png" /><figcaption>FIG. 10. Theoretical modeling HAXPES curves of the In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP interface as a function of In<sub>0.53</sub>Ga<sub>0.47</sub>As thickness: (a) (I<sub>Ga2p3/2</sub> + I<sub>As2p3/2</sub>)/I<sub>P1s</sub> HAXPES theoretical intensity ratio and (b) concentration of different chemical elements.</figcaption></figure>

V. CONCLUSION

The aim of this study was to characterize the quality of the top
buried InGaAs/InP interface in In<sub>0.53</sub>Ga<sub>0.47</sub>As/InP/InAlAs/InP het-
erostructures and determine the concentrations of indium, gallium,
and arsenic in InGaAs using a combination of argon sputtering and

HAXPES. The superposition of HAXPES peaks before and after crossing the interface showed first that HAXPES is bulk sensitive (not sensitive to surface amorphization induced by argon sputtering) and second that the interface InGaAs/InP is abrupt. HAXPES signals modeling enabled us to determine the indium, gallium, and arsenic concentrations in InGaAs bulk, which are close to the values

predicted after MOCVD growth. It also provided us with values for
the residual InGaAs thickness remaining after sputtering as well as
the depth of InP probed. The concentration of indium, gallium,
arsenic, and phosphorus was also determined when probing the
InGaAs/InP interface. They are consistent with flat and homoge-
neous layers without the interdiffusion or segregation of the main
elements. ToF-SIMS also confirmed the high chemical abruptness
of the InGaAs/InP interface, with a depth resolution of 1.5 nm. This
study shows the abruptness of the InGaAs/InP interface and, conse-
quently, the optimized MOCVD growth parameters, which are very
important for the manufacture of HBT transistors. HAXPES
appears here to be an interesting technique to investigate buried
inter interfaces when combined with Ar sputtering because not sensitive
to the sputter-induced surface damaged layer.

SUPPLEMENTARY MATERIAL

The supplementary material is divided into four parts:

1. Comparison of ionization efficiency cross sections: This section shows the agreement between the ionization efficiency cross sections reported in Refs. 10–12.

2. Ga-rich surface after argon sputtering: This part demonstrates that argon sputtering results in a gallium-rich surface.

3. Effect of argon sputtering on other materials: This section presents the impact of argon sputtering on various materials such as GaN, Ta<sub>2</sub>O<sub>3</sub>, and Au/Cu.

4. Disappearance of the oxygen peak after argon sputtering: The final part shows the absence of the oxygen peak after 26 and 40 min of argon sputtering.

ACKNOWLEDGMENTS 479

This work, carried out on the Platform for Nanocharacterisation (PFNC), was supported by the "Recherche Technologique de Base" 480 and "France 2030-ANR-22-PEEL-0014" programs of the French 481 National Research Agency (ANR). This work was partially supported 482 by the LabEx MINOS under Contract No. ANR-10-LABX-0055. 483

AUTHOR DECLARATIONS 485

Conflict of Interest 486

The authors have no conflicts to disclose. 487

Author Contributions 488

G. Tsamo Tagougue: Conceptualization (equal); Data curation 489 (equal); Methodology (equal); Resources (equal); Writing – original 490 draft (equal). E. Martinez: Supervision (equal); Writing – review & 491 editing (equal). M. Veillerot: Supervision (equal); Writing – review 492 & editing (equal). V. Thoréton: Supervision (equal); Writing – 493 review & editing (equal). M. Martin: Supervision (equal); 494 Writing – review & editing (equal). T. Baron: Supervision (equal); 495 Writing – review & editing (equal). G. Lefevre: Supervision 496 (equal); Writing – review & editing (equal). S. Cavalaglio: Supervision 497
498 (equal); Writing – review & editing (equal). F. Bassani: Supervision

499 (equal); Writing – review & editing (equal).

500 DATA AVAILABILITY

501 The data that support the findings of this study are available
502 from the corresponding authors upon reasonable request.

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