# Amorphous GaN matrix embedded nanocrystals exhibiting bulk band-gap luminescence

Lakshman Srinivasan, Jean-Luc Maurice, Stefano Pirotta, Stéphane Collin, Erik Johnson, Pere Roca i Cabarrocas, Karim Ouaras

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Lakshman Srinivasan, Jean-Luc Maurice, Stefano Pirotta, Stéphane Collin, Erik Johnson, et al. Amorphous GaN matrix embedded nanocrystals exhibiting bulk band-gap luminescence. APL Materials, 2025, 13 (6), 10.1063/5.0253319. [hal-05388096](https://hal.science/hal-05388096v1)

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# Amorphous GaN matrix embedded nanocrystals exhibiting bulk band-gap luminescence

Lakshman Srinivasan<fnref n="1" />,<fnref n="2" />, Jean-Luc Maurice<fnref n="1" />, Stefano Pirotta<fnref n="3" />, Stéphane Collin<fnref n="3" />, Erik V. Johnson<fnref n="1" />, Pere Roca i Cabarrocas<fnref n="1" />,<fnref n="2" />, Karim Ouaras<fnref n="1" />,<fnref n="4" />

## AFFILIATIONS

Keywords - Gallium Nitride (GaN), Plasma, Reactive Sputtering, Cathodoluminescence (CL), semiconductor defects.

## ABSTRACT

This work reports on room temperature sputtering growth of GaN thin films, exhibiting band-edge luminescence without thermal annealing or post-deposition processing. Particularly, we investigate and correlate their luminescence spectra with their structural properties (amorphous or polycrystalline phases), the presence of defects, and the grain features by combining cathodoluminescence and HR-TEM characterizations. The working pressure and Ar/N₂ flow ratio are found to have a profound effect on both the structural and optical properties of the films. Notably, the only film with band-edge luminescence is grown at the highest investigated pressure (13.3 Pa) and 40%N₂ concentration and exhibits single-grained crystallites of small sizes (~5nm). Moreover, this polycrystalline film has an almost equal fraction of amorphous and crystalline phases. This suggests that the presence of smooth and nanometer-sized crystals in an amorphous matrix (inducing grain surface passivation) could be a synergetic combination to achieve luminescent films grown at room temperature.

<figure><img src="image_2.png" /></figure>

Gallium nitride (GaN) has emerged as a pivotal material in various optoelectronic and electronic applications owing to its wide bandgap (3.4 eV), high electron mobility, and excellent thermal stability<fnref n="1" />. Its unique properties have spurred extensive research aiming at optimizing its growth and enhancing its performance for diverse technological applications<fnref n="2" />. Traditional deposition methods of GaN films are Metal Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE), and Molecular Beam epitaxy (MBE), which either require complex and costly equipment or a high substrate temperature (~800-1100°C)<fnref n="3" />. Growth performed at high temperature leads to several issues such as thermal mismatch strain between GaN and foreign substrates such as silicon, which results in film cracking or wafer bowing<fnref n="4" />. Reactive sputtering has emerged as a promising process option for GaN growth, attracting significant attention in recent years owing to: (i) its low-temperature operation (down to room temperature), which mitigates thermal damage⁵ and is compatible with the use of flexible substrates; (ii) the possibility of scaling up to industrial scale; and (iii) its sparing use of gases that are both inexpensive and non-toxic, e.g. a plasma made of a mixture of Ar/N₂ facing a Ga target suffices to produce GaN thin films. Sputtering has already been used to prepare epitaxial-grade GaN films that have been exploited to produce optoelectronic devices⁶. However, in that work, growth temperatures as high as 800° C were required to achieve high-crystalline quality films, a temperature close to that of conventional processes. Therefore, to promote and spread the utilization of sputtering of III-V materials at an industrial level, it is necessary to demonstrate a clear benefit that would make it competitive with the standard processes, either from an economical aspect or from a technical/ecological point of view (i.e. lower consumption of gas, low growth temperature). In a recent study, we showed that polycrystalline GaN (pc-GaN) films can be grown on Si substrates at a growth temperature close to room temperature, i.e. without intentionally heating the substrate⁷. Even though the optoelectronic properties of those films would not be sufficient for targeting applications where GaN is both an optically and electronically active material, other applications might

benefit from pc-GaN such as electron transport layers (ETL) in perovskite solar cells⁸ or low-temperature grown pc-GaN (250 °C) for thin film transistors (TFTs)⁹. To go further on the utilization of pc-GaN grown at low temperature to fulfill applications that do not require epi-grade films, requires understanding and optimizing sputtering processes in order to have a better control of GaN growth and to end up with the best possible quality. Following this goal, we present the results of an experimental campaign that aimed at optimizing said growth, relying on the luminescence characterization of GaN films as it provides valuable insights into both their structural and optical characteristics.

This study was performed using steady-state Cathodoluminescence (CL) spectroscopy, which provides insights into defects and crystal quality¹⁰, corroborated with XRD and TEM analysis. The GaN samples were deposited by reactive sputtering using a pure gallium target in liquid state (made of 99.9999% purity melted pellets - Alfa Aesar) and a mixture of Ar/N₂ (both 99.9999% purity) under an operating pressure range of a few to tens of Pa. Details of the reactor system depicted in Figure 1.a can be found elsewhere⁷. In the system, an unheated 2" n-type c-Si (100) substrate is separated from the RF-powered magnetron by a gap of ~5 cm. The typical growth rates span from 1 to 3 Å·s⁻<fnref n="1" />, mostly depending on the working pressure and RF power¹¹. In Figure 1.b&c, an overview of the cross-sectional composition and morphology of a typical pc-GaN film grown at room temperature is shown. These images were obtained using scanning transmission electron microscopy (STEM) coupled to energy dispersive x-ray spectroscopy (EDX). In Figure 1.b, an example of a STEM-EDX image shows a quite homogeneous distribution of Ga and N within the film while the TEM image reveals a uniform growth of GaN with a sharp interface with the Si substrate. The high-resolution cross-sectional TEM image (Figure 1.c) shows the microstructure of an individual grain, which includes the equilibrium wurtzite hexagonal (2H) phase with a stacking fault and a cubic (3C) nano-domain. The occurrence of stacking faults and 3C domains might be related to the low mobility of atoms during growth at room temperature, which can make incorrect stacking kinetically favorable.

<footnote n="1">¹LPICM - CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France.</footnote>

<footnote n="2">²IPVF, Institut Photovoltaïque d'Ile-de-France, 18 Bd Thomas Gobert, 91120 Palaiseau, France.</footnote>

<footnote n="3">³C2N - CNRS, Université Paris-Saclay, 10 boulevard Thomas Gobert, 91120 Palaiseau, France</footnote>

<footnote n="4">⁴Author to whom correspondence should be addressed: karim.ouaras@polytechnique.edu</footnote>
<figure><img src="image_3.png" /><figcaption>Figure 1: a. Schematic of sputtering plasma reactor used for GaN films growth on unheated c-Si substrates; b. Bright-field TEM images corresponding to the cross-section of a GaN film on Si substrate along with its EDX mapping; c. High-resolution TEM images of a pc-GaN film grown at 75mTorr, recorded in the [11'-20] zone exhibiting the stacking order in an individual grain. It reveals a 2H wurtzite structure with stacking faults and the presence of a nano-domain with the metastable cubic 3C polytype. The dotted lines indicate stacking faults; the continuous zigzag line is provided as a guide to the eye.</figcaption></figure>

From these initial observations, two questions can be raised: (i) to what extent do these defects and structural changes affect the luminescence properties of the films? and (ii) what are the plasma conditions that play the most important role in dictating the occurrence of such defects? To address these questions, four GaN samples (S1 to S4) grown at different conditions were investigated. The Ar/N₂ flow ratio effect was assessed through the analysis of S1 (10%N₂), S2 (40%N₂) and S3 (100%N₂) that were all grown at a fixed pressure of 6.6 Pa. The pressure was then doubled to 13.3 Pa at a fixed ratio of 40% N₂ (S4), enabling one to assess the pressure effect by comparison with S2. All the samples were analyzed using Grazing-Incidence X-ray diffraction (GIXRD) using a Panalytical Empyrean Diffractometer (CuKα radiation (λ=1.5406 Å)).

The GIXRD patterns were employed to measure the crystal parameters and to extract the crystalline fraction Fc by assessing the area of the 0002 peak (~34°) and the amorphous area surrounding it, using the method described in ¹²⁻¹⁴. To complete our analysis, room-temperature CL was performed using an Attolight Chronos/Allalin equipment with a 6 keV electrons beam (current@sample ~8.6nA) by scanning a 260x260µm² region of each sample surface and collecting the GaN luminescence spectra¹⁵ at each electronic excitation point. The experimental details can be found in reference¹⁶,¹⁷. CL spectra were corrected for the spectral response of the detection system, and the measurement conditions allowed a quantitative comparison of the luminescence intensity of different samples. To verify whether the peaks appearing in CL spectra were due to eventual interference effects¹⁸, we also analyzed the samples using ellipsometry (Supplementary figure 1).

Figure 2.a-d present the CL average spectra. Each CL spectrum has been de-convoluted to discern the different peaks. Apart from a narrow band-edge luminescence (BE-L) peak at 364 nm, and the defect-related yellow luminescence (YL) band at ~540-580nm¹⁹, all the other features that can be seen in most of the spectra have been discarded and will not be used for the discussion here. Indeed, they coincide with the thin film interference peaks appearing in the respective ellipsometric spectra (plotted in grey dotted lines) and likely do not represent true emission peaks. It is important

to note that defect bands can be attributed to either vacancies or intrinsic point defects related to transitions from a shallow donor to deep acceptors¹⁹. Given the prevalence of shallow donors such as oxygen contaminants in our GaN films – as evidenced by XPS spectra (Figure 2.h) where they are indicated in bold – the YL defect band in GaN is most likely due to highly stable gallium-oxygen complexes ($V_{GaO}$N) which are even more favorable than isolated Ga vacancies ($V_{Ga}$)²⁰⁻²³. All four of the investigated films have been found to exhibit uniform luminescence across the measured surface portion as illustrated in Figure 2.d for S4. The mappings reveal that the CL peak intensity is uniform throughout the sample across a range of at least ~260µm. Figure 2.a1-d1 shows the GIXRD patterns for the four samples revealing that S1 and S3 are essentially amorphous while S2 and S4 are polycrystalline: this trend is even clearer in Figure 2.e where the crystalline fractions Fc are compared between the four samples.

From our analysis, three major observations can be made concerning the effect of N₂ flow on GaN growth at 6.6 Pa (Figure 2. a(n)-b(n)-c(n)):

i) Disregarding the features due to interference effects, all three samples show only the YL peak. This is surprising, as the XRD spectra of the three films show significant differences (Figure 2.a1-d1) as quantified by their crystalline fraction (Figure 2.e). The mostly amorphous samples, S1 and S3, that respectively exhibit a Fc of ~6% and ~10%, are not likely to present band edge luminescence, and do not. As S2 is the film that exhibits the highest Fc (~60%), one would expect efficient electron-hole recombination near the band edge and a narrow peak around 364 nm, but this is not observed. The global similarity of the CL spectra of all three samples is therefore unexpected.

ii) If one focuses on the overall amplitude changes in CL spectra of S1, S2 and S3, it can be clearly seen that S1 (Figure 2.a) shows the weakest luminescence, while S3 shows the strongest. However, again, the changes in CL amplitude do not seem to correlate to the changes in Fc (Figure 2.e) that is occurring when changing the N₂ flow fraction from 10% (S1) to 40% (S2) and 100% (S3).
<figure><img src="image_4.png" /><figcaption>**Figure 2.a-d:** RT Cathodoluminescence (CL) spectra of S1, S2, S3 and S4 respectively in light blue thick solid line; complete fit is in dotted black, fit components are in solid colored lines (red, yellow, light green and violet). Insets **a-i-d** depict the GIXRD spectra of the corresponding GaN films. The inset **d** corresponds to the emission map of sample 4 at a photon energy of 3.41 eV. The scale bar represents the average CL integral normalized to span width (counts/s/eV). Panels **e, f & g:** crystalline fraction ($F_c$) of S1-S4 and lattice parameters 'a' and 'c' of S2-S4 obtained from the XRD spectra. Panel **h:** high-resolution XPS spectra.</figcaption></figure>

To evaluate the effect of the working pressure, S2 and S4 at 6.6 Pa and 13.3 Pa, respectively, (Figure 2.b&d) are now compared. Both samples were deposited using 40%N₂. An important observation in the case of S4 (Figure 2.d) is the appearance of the BE-L peak (~364 nm) which is located at the theoretical energy band-gap value of crystalline GaN. This occurs, even though the crystalline fraction decreases compared to S3. What is also noticeable here is the evolution of the lattice parameters *a* and *c* (Figure 2.f&g). Indeed, while S4 exhibits the BE-L peak of standard GaN, it also possesses lattice parameters (a = 3.19 Å & c = 5.19 Å) which are equal to the theoretical values of GaN²⁴. It is noteworthy that while the value of the lattice parameter *c* of S2 is the same as the theoretical one (Figure 2.g), its parameter *a* deviates from 3.19 Å to 3.21 Å (+0.6%). This would indicate that S2 has a more distorted or strained crystal structure than S4. Hence, in terms of global luminescence features, S4 exhibits the most promising CL spectrum. However, if one were to rely on the XRD pattern and the *F*<sub>c</sub> value as indicators of the film quality, S2 would have been considered as a 'better' sample (Figure 2.b&e). Why then does S4 exhibit better luminescent features despite it exhibiting lower *F*<sub>c</sub> than S2? To answer this question, High Resolution-TEM was employed to identify the differences in the nanostructures of samples S2 and S4.

Figure 3 shows a set of TEM images of S2 (a) and S4 (b) along with that of quasi-amorphous sample S3 (c) (the sample that exhibits the strongest CL amplitude) for comparison purposes. For all the samples, the same set of images were captured so only S2 (Figure 3.a) is detailed hereafter. As shown in Figure 3.a, a large-scale image (Figure 3.a₁) and a HR-TEM image (Figure 3.a₂) were acquired. The Fast Fourier transform (FFT) of the latter (Figure 3.a₃) gives access to the nanostructure of the grains, while the inverse FFT (IFFT) of selected spots (pictures 1 and 2) gives the size of the corresponding crystallite in the GaN thin film. The comparison of the

crystallite images obtained by IFFT is particularly interesting; although the apparent size of a crystallite is larger for S2 (Figure 3.a₃, 10-20 nm) than for S4 (Figure 3.c₅, 2-10 nm), the contrast is uniform in the latter while it is broken up into multiple grains in the former. This observation serves to explain the CL and XRD results presented before, by considering the potential effect of the grain boundaries in polycrystalline films²⁵. The crystallites of S2 (Figure 3.a₃) contain internal grain boundaries, that may contain efficient non-radiative recombination centers. However, the smaller crystallites observed in S4 are single grains, and thus are not detrimental to CL, as there are no internal grain boundaries. The external surfaces of the grains can easily be passivated by an amorphous phase²⁶. This may allow more charge carriers to undergo radiative recombination and emit photons, leading to higher luminescence intensity²⁷, a well-known effect in porous silicon²⁸. The internal grain boundaries of S2 cannot be passivated, non-radiative recombination dominates, and no BE-L is observed. The images of Figure 3 also serve to understand the lattice parameters (*a*,*c*) of the GaN as extracted from the XRD (Figure 2.f&g). While only sample S4 shows values of *a* and *c* close to the expected values, the other polycrystalline film, S2 (Figure 3) has larger crystallites composed of many grains, and also shows an increased lattice strain within the material²⁹ as evidenced by the deviation of its lattice parameter *a* (Figure 2.f). We can also comment on the coherence of the full set of measurements for sample S3. Figure 3.c₅ shows no visible crystalline areas for S3, which means that the peaks in the XRD spectrum of Figure 2.c₁ represent a very small volumetric fraction of the film, or crystals too small to be observed. Furthermore, as the lattice parameter values for sample S3 (Figure 2.f&g) also exhibit a relatively significant deviation from the theoretical GaN values (as for S2), the crystallites that do exist are defective and do not luminesce.
<figure><img src="image_5.png" /><figcaption>Figure 3. S2, S3 and S4 (HR) TEM comparison:</figcaption></figure>

a. TEM of S2: a₁) general view; a₂) HRTEM of the squared area shown in a₁ - the tilted contrast corresponds to a high density of stacking faults; a₃) fast Fourier transform (FFT) of the blue squared area in a₂ - the presence of streaks confirms the stacking faults; images (1) and (2) are digital dark field images from spots 1 and 2 in the FFT, which confirm the defective structure and give a crystallite size of about 20 nm; b. TEM of S4: b₁) general view; b₂) HRTEM of the squared area in b₁; b₃) fast Fourier transform (FFT) of the blue squared area in b₂, exhibiting no streaks in this case, while the digital dark field images (1) and (2) exhibit single-grain crystallites of smaller size (~5 nm); c. TEM of S3: c₁) general view; c₂) enlargement of the squared region in c₁; c₃) HRTEM view of the rectangular area in c₂.

In conclusion, comparing films of different structures sheds light on the interplay between crystalline fraction, lattice parameters, and luminescence characteristics. The variations in CL spectra are particularly impacted by the changes in N₂ fraction in the plasma and working pressure. For samples deposited at lower pressure (6.6 Pa), the samples only displayed broad CL bands at 650 nm (YL). Increasing the N₂ fraction from 10% to 40% and 100% shows an increase in CL intensity, mostly due to an increase in the YL band associated with oxygen complexes. As well, the sample using a N₂ fraction of 40% leads to an elevated crystallinity. However, no band-edge luminescence was observed for this sample. Surprisingly, utilizing a deposition pressure of 13.3 Pa at an N₂ fraction of 40% resulted in a sample with *both* a high crystalline fraction and the characteristic BE-L peak for GaN. HRTEM analyses clarify these results, showing that the larger crystallites (~20 nm) for the sample deposited at 6.6 Pa are polycrystalline with internal grain boundaries, while the smaller and smoother crystallites (~5nm) for the sample deposited at 13.3 Pa are single grains. Surface passivation of these smaller crystals by amorphous material is suggested as the origin of the BE-L observed. Investigations are in progress to improve the luminescence intensity and to minimize the band emissions related to defects by reducing oxygen contamination *inter alia*.

### Acknowledgements

This work was funded by the French National Research Agency trough contract No. ANR-22-CE51-0011-01 (GASPE project) and No. ANR-IEED-002-01. The authors acknowledge the support of the French RENATECH network. This work has also benefited from a grant managed by the Agence Nationale de la Recherche (ANR), as part of the program 'Investissements d'Avenir' under the reference (ANR-18-EURE-0014).

## AUTHOR DECLARATIONS

### Conflict of Interest

The authors have no conflicts to disclose.

### Data availability

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

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