# Composition analysis of III-nitrides at the nanometer scale: Comparison of energy dispersive X-ray spectroscopy and atom probe tomography

Bastien Bonef, Miguel Lopez-Haro, Lynda Amichi, Mark Beeler, Adeline

Grenier, Eric Robin, Pierre-Henri Jouneau, Nicolas Mollard, Isabelle Mouton, Catherine Bougerol, et al.

► To cite this version:

Bastien Bonef, Miguel Lopez-Haro, Lynda Amichi, Mark Beeler, Adeline Grenier, et al. Composition analysis of III-nitrides at the nanometer scale: Comparison of energy dispersive X-ray spectroscopy and atom probe tomography. Nanoscale Research Letters, 2016, 11, pp.461. 10.1186/s11671-016-1668-2. hal-01385434

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# Composition Analysis of III-Nitrides at the Nanometer Scale: Comparison of Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography

Bastien Bonef¹,<fnref n="2" />*, Miguel Lopez-Haro¹,<fnref n="2" />, Lynda Amichi¹,<fnref n="2" />, Mark Beeler¹,<fnref n="2" />, Adeline Grenier¹,³, Eric Robin¹,<fnref n="2" />, Pierre-Henri Jouneau¹,<fnref n="2" />, Nicolas Mollard¹,<fnref n="2" />, Isabelle Mouton¹,³, Benedikt Haas¹,<fnref n="2" />, Eva Monroy¹,<fnref n="2" /> and Catherine Bougerol¹,⁴

## Abstract

The enhancement of the performance of advanced nitride-based optoelectronic devices requires the fine tuning of their composition, which has to be determined with a high accuracy and at the nanometer scale. For that purpose, we have evaluated and compared energy dispersive X-ray spectroscopy (EDX) in a scanning transmission electron microscope (STEM) and atom probe tomography (APT) in terms of composition analysis of AlGaN/GaN multilayers. Both techniques give comparable results with a composition accuracy better than 0.6% even for layers as thin as 3 nm. In case of EDX, we show the relevance of correcting the X-ray absorption by simultaneous determination of the mass thickness and chemical composition at each point of the analysis. Limitations of both techniques are discussed when applied to specimens with different geometries or compositions.

**Keywords:** Energy dispersive X-ray spectroscopy, Atom probe tomography, Nanoscale composition analysis, Quantitative composition analysis, III-Nitride nanostructures

## Background

As optoelectronic devices push towards higher performance, engineering the quantum confinement at the nanoscale become the key for device design. However, such a fine tuning of the physical properties faces one major bottleneck, namely the accurate determination of alloy compositions at the nanometer scale. For ternary or quaternary alloys, methods based on the analysis of the lattice parameters, such as X-ray diffraction, nano-beam electron diffraction [1], or geometrical phase analysis of high-resolution transmission electron microscopy images [2], cannot discriminate between strain and composition effects. Chemical analysis techniques such as energy dispersive X-ray spectroscopy (EDX) or atom probe tomography (APT) are better adapted to address this problem. However, their accuracy and spatial resolution need to be assessed in the case of III-nitrides. For

instance, APT has been used to study in clustering in InGaN alloys [3–7] or AlN interlayers in AlGaN/AlN/GaN heterostructures [8]. However, getting quantitative composition information of nanometer-size nitride heterostructures from APT remains a challenge that requires careful optimization of the experimental conditions (temperature, laser wavelength and power, evaporation field) which are known to influence the ratio of the detected atomic species [9–11]. Combining data obtained by complementary approaches is a way to overcome these difficulties. Very recently, Griffiths et al. have reported the composition analysis of InGaN quantum wells from APT and quantitative scanning transmission electron microscopy (STEM) [12] which is an alternative approach to the TEM-based method first proposed by Rosenauer et al. [13]. Combining two chemical analysis techniques, namely EDX and APT, appears as an interesting alternative, the main problem to get accurate EDX quantitative data being linked to the absorption of the X-ray emission, which depends on the

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

<footnote>* Correspondence: bbonef.prof@gmail.com
¹Université Grenoble Alpes, F-38000 Grenoble, France</footnote>

<footnote n="2">²INAC, CEA-Grenoble, 17 av. des Martyrs, F-38000 Grenoble, France
Full list of author information is available at the end of the article</footnote>
energy of the considered X-ray line, and the mass thickness of the sample.

## Methods

The goal of this work was to evaluate and compare EDX and APT analysis of III-nitride alloys, in terms of spatial resolution and composition accuracy. For that purpose, we have chosen to investigate an AlGaN/GaN multilayer stack grown by plasma-assisted molecular beam epitaxy (PAMBE) consisting of a repetition (40 periods) of four AlGaN layers with nominal Al concentrations of 12, 0, 5, and 7% and nominal thicknesses of 3, 3, 11, and 6 nm, respectively. A high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image of a period of the structure is shown in Fig. 1. The stack was deposited on a 30-nm-wide Al<sub>0.05</sub>Ga<sub>0.95</sub>N layer on a GaN-on-Si (111) template. Growth details and optical characterization can be found in ref. [14]. We will show that, after optimization of the acquisition and analysis parameters, both EDX and APT give comparable results with a composition accuracy better than ±0.6% even for layers as thin as 3 nm. The combination of both techniques paves the way to precise quantitative analysis of three-dimensional (3D) features at the nanoscale in nitride semiconductors.

## Results and Discussion

### Energy Dispersive X-ray Spectroscopy (EDX)

EDX data have been collected on an FEI-Osiris microscope operated at 200 kV and equipped with a Brücker EDX system consisting of four silicon drift detectors which ensure a high signal-to-noise ratio even at a probe size <1 nm. The sample has been prepared as wedge-shaped by focused ion beam (FIB) to study the influence of the thickness on the composition measurements. The quantitative EDX analysis has been carried out by two methods, namely the Cliff-Lorimer method (or *k*-factor method) [15] implemented in commercial EDX software and the *ζ*-factor method, first proposed by Watanabe et al. [16]. The Cliff-Lorimer method relates the concentration ratio of two elements to the ratio of the intensity peaks in the spectrum through the so-called *k*-factors. Its most serious limitation is the difficulty to correct the X-ray absorption, since it requires prior knowledge of the specimen thickness and density at the point of analysis, which are usually unknown. Such a correction is crucial for precisely quantifying light atoms like N and Al in AlGaN. On the other hand, the *ζ*-factor method takes into account X-ray absorption, thanks to the simultaneous determination of the mass thickness and composition of the sample at each point of analysis. Basically, the method uses an iterative procedure to compute the mass thickness, composition, and absorption correction terms. More details for thin lamella

analysis can be found in the original paper of Watanabe et al. [16]. Although this new quantification procedure is still unavailable on commercial systems, our group is developing this new method using reference samples of known thickness and composition [17]. In the present case, the *ζ*-factors for N, Al, and Ga K-lines as well as Ga L-lines have been directly measured on the same equipment, at the same operating conditions, using the following as reference samples: GaN, Si<sub>3</sub>N<sub>4</sub>, and AlN for N; Al<sub>2</sub>O<sub>3</sub>, MgAl<sub>2</sub>O<sub>4</sub>, albite, orthoclase, and AlN for Al; and GaP, GaN, and GaAs for Ga.

Figure 2a shows Al chemical maps recorded at different positions along the growth axis: close to the GaN buffer layer (left), an enlargement of a period at an intermediate depth (middle), and at the top part (right). The variation of the sample thickness over these different

<figure><img src="image_4.png" /><figcaption>**Fig. 1** HAADF-STEM image taken along the [11-20] zone axis showing one period of the multilayer stack with a scheme of the stack given in the *inset*</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2** EDX analysis at different positions along the growth axis: close to the GaN buffer (**left**), zoom on four periods at an intermediate position (**middle**), and at the top part (**right**). **a** Al chemical maps. **b** Specimen thickness. **c** N/(Ga+Al) ratio. **d** Al/(Ga+Al) ratio. **e** Probe size</figcaption></figure>

regions, retrieved from the ζ-factor analysis, is displayed in Fig. 2b. The variation of the N/(Ga+Al) ratio integrated along the height of Fig. 2a images is presented in Fig. 2c. In the case of the ζ-factor method (red curve), thanks to the point-by-point absorption correction, the N/(Ga+Al) ratio remains equal to one, as expected from the (Al,Ga)N stoichiometry. This is markedly different from the *k*-factor results (blue curve), for which the ratio is far too low (around 0.3) except for top of the sample (on the right) where the specimen thickness is below 100 nm. In this area, as the specimen thickness decreases, the measured N/(Ga+Al) ratio increases up to 0.8, but it never reaches 1.

Concerning the Al concentration, Fig. 2d shows the variation of the Al/(Ga+Al) ratio calculated using the ζ-factor (red curve) and *k*-factor (blue curve) methods. The spatial integration procedure was as described for the N/(Ga+Al) ratio, and the dotted line marks the ratio calculated from the nominal composition of each layer. All along the specimen, good agreement exists between the nominal (dotted) and ζ-factor curves (red), and the

four different layers integrating the stack period can be clearly identified. The Al concentrations extracted from these measurements are 11.9, 0.4, 5.6, and 7.3%, to be compared to the 12, 0, 5, and 7% nominal values. The agreement is not so good for the *k*-factor results (blue). Moreover, in this case, the Al concentration decreases as the thickness of the specimen increases. This behavior outlines the relevance of the absorption correction, and thus the benefit of the ζ-factor method for light element analysis. The correction is even more crucial for thick specimens (>100 nm).

If we focus on the ζ-factor results, the composition measurement of the thinnest GaN layer (3 nm) gives an idea of the spatial resolution limit due to the probe broadening effect. A value of 1% is obtained in the thicker area of the specimen, whereas it was 0.4% in the thinnest region (note that 0% of Al is indeed measured in the GaN 150-nm-wide GaN buffer layer). Figure 2e depicts an estimation of the probe size, which is 1–2 nm in the thin region of the specimen (graphs on the right side), but reaches 5 nm as the specimen gets thicker
(graphs on the left side). Consequently, the measure-
ment of the 3-nm-wide layers is affected by the chemical
composition of the adjacent layers, leading to an artifi-
cial decrease of the Al content in the 12% layer and the
apparent presence of Al in the pure GaN layer. In turn,
the concentration obtained in wider layers is reliable and
accurate all along the specimen.

Atom Probe Tomography (APT)

The same AlGaN sample has been studied by atom
probe tomography (APT) using a CAMECA FlexTAP
equipment with an ultraviolet laser (λ = 343 nm) oper-
ated in pulsed mode and at 20 K to improve both the
spatial resolution and the quantification [18]. A FEI
Strata 400S FIB system was employed to needle shape
the sample [19]. A backside lift-out was used to reverse
the analysis direction in order to evaporate first the 30-
nm-wide Al<sub>0.05</sub>Ga<sub>0.95</sub>N layer grown below the multilayer
stack. The evaporation of such a layer of known com-
position (Al/(Ga+Al) = 5.6% as determined from EDX)
allows fine tuning of the analysis parameters (the laser
energy is varied to obtain the correct Al/(Al+Ga) ratio)
before reaching the stack. In our experiment, the best
conditions corresponded to a laser energy of 1 nJ, and
we measured a charge state ratio Ga++/Ga+ around 0.15
[11]. The atom positions and their time of flight were re-
corded, and the 3D sample volume was reconstructed
using the IVAS software, which enables an iterative
process in which the initial geometrical characteristics of
the tip as well as the detector efficiency are varied until

the thicknesses of the reconstructed layers correspond
to the experimental values determined from STEM im-
ages with straight interfaces.

A 3D reconstruction of the sample tip, starting from
the Al₀.₀₅Ga₀.₉₅N buffer layer and including seven pe-
riods of the stack, is presented in Fig. 3a. A zoom corre-
sponding to the region marked as a black rectangle is
shown in Fig. 3b, whereas Fig. 3c reports the variation of
the Ga/(Ga+Al) and Al/(Ga+Al) ratios along the growth
axis. These ratios have been obtained by counting the
number of Al and Ga atoms in sampling boxes of 17 x
17 × 0.5 nm³ with a moving step of 0.5 nm. The values
of the Al/(Ga+Al) ratio for the four layers of a period
are 12.6 ± 0.7, 0.6 ± 0.2, 5.4 ± 0.3, and 7.5 ± 0.4%, in good
agreement with the values obtained from EDX. The
slight deviation between the measured and nominal layer
thicknesses can be associated to a decrease of the detec-
tion efficiency (40% instead of 62% expected for our
APT equipment). This effect has been previously re-
ported in the case of III-nitrides [9, 11] and indicates
that some atoms are preferentially detected without cor-
relation with the laser pulses or can be eventually evapo-
rated as neutral species.

Concerning the spatial resolution, we notice that some
Al (0.6 ± 0.2%) is detected in the 3-nm-wide GaN layers,
whereas there are no Al counts emerging from the noise
in the 150-nm-wide GaN layer. This is likely due to tra-
jectory aberrations at the interfaces between GaN and
adjacent AlGaN layers, linked to the difference in the
evaporation fields of these layers. Figure 4a shows that

<figure><img src="image_6.png" /><figcaption>**Fig. 3** APT analysis. **a** 3D reconstruction of the sample tip evaporated including seven periods of the stack. **b** Zoom over the black rectangle marked in **a**. **c** Variation along the growth axis of the Ga/(Ga+Al) and Al/(Ga+Al) ratios for the volume shown in **b**</figcaption></figure>
<figure><img src="image_7.png" /></figure>

the evolution of the Ga⁺⁺/Ga⁺ ratio (indicating the evap-
oration field strength) during the tip evaporation pre-
sents marked discontinuities at the heterointerfaces.
The red dotted line, which gives a trend of the aver-
age field, slowly increases during the evaporation and
is associated to a slow increase of the N/(Al+Ga) ra-
tio, as show in Fig. 4b. Increasing the Al composition
of a layer increases the average III-N bond strength
and therefore the field required for its evaporation.
This field variation induces a variation in the N de-
tection [19]. This implies that the composition ana-
lysis will bring reliable results only close to the
region where the evaporation parameters have been
optimized, i.e., a few periods in our case. This
phenomenon will be further aggravated in case of
layers having very different and higher Al content.

Conclusions

This study shows that the correlative use of EDX in
STEM and APT can provide reliable composition mea-
surements in thin nitride layers, which are structural key
parameters to understand electrical and optical proper-
ties of future devices. In particular, the Al content in
AlGaN multilayers can be obtained in a reliable and
comparable manner by both EDX and APT with a
precision better than 0.6% even for layers of a few nano-
meters in width. It should be mentioned that the same
sample has been investigated by X-ray diffraction ((Θ-
2Θ) scans). The experimental diffractogram was per-
fectly fitted with the simulation performed considering
the layers' compositions given by EDX/APT and the

layers' thicknesses measured on STEM images, which
confirms the reliability of our composition analyses. In
case of EDX analysis, we have shown the necessity to
use the ζ-factor approach which corrects precisely the
X-ray absorption, thanks to the simultaneous determin-
ation of the mass thickness and chemical composition of
the sample at each point of the analysis. This correction
is particularly relevant for a reliable quantification of
light atoms like N and Al, even for specimens thinner
than 100 nm. The spatial resolution limit is imposed by
the probe broadening effect, which is directly related to
the specimen mass thickness and composition. This has
to be taken into account especially when the width of
the layers and the probe size are in the same range.
Typically, the thinner the sample, the smaller the beam
broadening; however, in very thin samples, the signal-to-
noise ratio degrades, and hence the precision of the
measurement. Regarding APT, the results are not af-
fected by the specimen thickness, but precise quantifica-
tion requires reference data provided by independent
techniques, namely a layer of well-known composition
to tune the laser energy at the first steps of the evapora-
tion, and a STEM image of the sample to calibrate the
3D reconstruction. The challenge to quantify simultan-
eously Al, Ga, and N stands in the difference in the III-
N bond strengths and consequently the field variations
at the heterointerfaces.

Abbreviations

3D: Three-dimensional; APT: Atom probe tomography; EDX: Energy dispersive X-ray spectroscopy; FIB: Focused ion beam; HAADF: High-angle annular dark-field; PAMBE: Plasma-assisted molecular beam epitaxy; QSTEM: Quantitative scanning transmission electron microscope; STEM: Scanning transmission electron microscopy

Acknowledgements

Thanks are due to Y. Genuist and Y. Curé for the technical support and to E. Bellet-Amalric for the X-ray diffraction experiments.

Funding

This work is supported by the EU ERC-StG "TeraGaN" (#278428) project.

Authors' Contributions

CB, ER, and BB designed the concept and the experiments. BB prepared the APT tips and performed the APT analysis. MLH and ER designed and performed the EDX analysis. NM prepared the FIB lamella. BH took the TEM images. MB and EM designed and performed the growth of the sample. LA, AG, IM, and PHJ contributed to the data analysis. All authors wrote and revised the paper. All authors read and approved the final manuscript.

Competing Interests

The authors declare that they have no competing interests.

Author details

¹Université Grenoble Alpes, F-38000 Grenoble, France. ²INAC, CEA-Grenoble, 17 av. des Martyrs, F-38000 Grenoble, France. ³LETI, CEA-Grenoble, MINATEC Campus, 17 av. des Martyrs, F-38000 Grenoble, France. ⁴Institut NEEL, CNRS, 24 av. des Martyrs, F-38000 Grenoble, France.
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