Unveiling the growth mode diagram of GaSe on sapphire

TL;DR

This study establishes a wafer-scale growth-mode phase diagram for GaSe on c-plane sapphire by molecular beam epitaxy, integrating in-situ RHEED, Raman spectroscopy, SEM, AFM, EDX, and HR-XRD to connect substrate temperature and Se/Ga flux ratio with crystal phase and surface morphology. The authors identify distinct growth regimes, including Ga droplets, 3D-nanoflakes, 2D GaSe, and Ga2Se3, and reveal a high-temperature 2D GaSe window with partial epitaxy limited by an amorphous interfacial layer that decouples the film from the substrate. In-situ diagnostics enable real-time tracking of transitions between growth modes, with logistic and Arrhenius-like analyses quantifying onset temperatures and activation energies for regime boundaries. The work demonstrates a RHEED-based pathway to synthesize GaSe with controlled phase and morphology and provides a general framework for optimizing growth of other 2D PTMCs on industry-relevant substrates, advancing scalable fabrication of GaSe-based optoelectronics.

Abstract

The growth of two-dimensional epitaxial materials on industrially relevant substrates is critical for enabling their scalable synthesis and integration into next-generation technologies. Here we present a comprehensive study of the molecular beam epitaxial growth of gallium selenide on 2-inch c-plane sapphire substrates. Using in-situ reflection high-energy electron diffraction (RHEED), in-situ Raman spectroscopy, optical and scanning electron microscopies, we construct a diagram of the gallium selenide growth modes as a function of substrate temperature (530-650 °C) and Se/Ga flux ratio (5-110). The growth mode diagram reveals distinct regimes, including the growth of layered post-transition metal monochalcogenide GaSe with an unstrained in-plane lattice constant of 0.371$\pm$0.001 nm and a partial epitaxial alignment on sapphire. This work demonstrates a RHEED-based pathway for synthesizing gallium selenide of specific phase and morphology, and the construction of a phase diagram for high vapor pressure III-VI compounds that can be applied to a wide range of other metal chalcogenide materials.

Figures (6)

• Figure 1: Surface morphology and crystal phase as a function of $\Phi_{\text{Se/Ga}}$ and $T_{sub}$. (a) SEM images showing the surface morphology of gallium selenide films grown at varying $\Phi_{\text{Se/Ga}}$ (rows) and $T_{sub}$ (columns), with selected RHEED patterns (inset) acquired perpendicular to the [10.0] direction. Scale bar: 2 µm. Crosses mark the positions where Raman measurements were performed. (b-c) EDX spectra confirming the elemental composition of clusters on samples grown at $\Phi_{\text{Se/Ga}}$ of 7 (b) and 50 (c). The red lines are Gaussian fits to the experimental data. (d) Raman spectra from the marked regions in (a), identifying the predominant material phases (GaSe, Ga$_2$Se$_3$) under different growth conditions. Spectra are color-coded by growth temperature (cyan to magenta for low to high temperatures, as indicated in (a)). The Raman peaks were identified according to literature values Allakhverdiev_Raman2006Molas_Raman2021Yamada_Ga2Se31992.
• Figure 2: Area and density of clusters. Cluster footprint area (top row) and density (bottom row) extracted from SEM images are shown as a function of $T_{sub}$ for three $\Phi_{\text{Se/Ga}}$ (from left to right: 7, 50, and 100). In (a), clusters primarily consist of Ga droplets and coalesce into larger sizes at high densities (light blue marker). In (b-c), clusters are highly selenidized and coexist with the 3D-nanoflakes morphology at lower $T_{sub}$ (light green region). To determine the onset temperature of cluster formation, the data is fitted (continuous red lines) with logistic functions (exponential in the bottom graph in (a)). Note that the actual onset values are extracted at $\Phi_{\text{Se/Ga}}$ of 8.3, 34.6, and 78.9 due to the additional Ga flux gradient on the wafer.
• Figure 3: In-situ Raman and RHEED of gallium selenide samples. (a) Scatter plot of Raman measurement positions in parameter space, with the marker color indicating the Ga$_2$Se$_3$ to GaSe ratio. The filled contour plot bounded by the white-dashed line also shows the Ga$_2$Se$_3$ to GaSe ratio, calculated only for samples where gallium selenide grows in a 3D mode. The background heatmap provides a qualitative extrapolation of the contour plot (refer to Supplementary Information Note S1 for the employed equation). The colorbar is omitted for both the contour plot and background heatmap as these are intended to illustrate trends, with quantitative values indicated by the marker colors. (b) Boxplot of Ga$_2$Se$_3$ to GaSe ratios derived from Raman data, categorized by three distinct morphologies. The red line indicates the sample's median, while the box represents the interquartile range (IQR). The whiskers extend up to 1.5 times the IQR, with outlier points shown as white dots. (c) Scatter plot of the grown samples as a function of $T_{sub}$ and $\Phi_{\text{Se/Ga}}$, with markers color-coded by RHEED pattern type (e.g. streaky, spotty+rings). The classification of RHEED patterns was performed algorithmically using a Histogram of Oriented Gradients (HOG)-based method (see main text). The accompanying dendrogram illustrates the clustering of similar RHEED patterns based on shape features: streaky patterns (light blue, indicative of 2D planar growth) and spotty/ring patterns (light yellow, associated with mixed or 3D growth). (d) Scatter plot of lattice constant as a function of $T_{sub}$ and $\Phi_{\text{Se/Ga}}$, extracted from RHEED after 3 minutes of growth for all runs. The red line represents the literature lattice constant of GaSe, with the gray box showing a $\pm$0.5% deviation. The dashed blue line indicates the mean lattice constant across all samples.
• Figure 4: Growth mode phase diagram of gallium selenide growth as a function of $T_{sub}$ and $\Phi_{\text{Se/Ga}}$. The predominant growth modes and crystalline phases are represented by color-coded growth regimes: yellow (3D-nanoflakes gallium selenide, predominantly 1:1 stoichiometry), red (Ga$_2$Se$_3$, 2:3 stoichiometry), light blue (Ga droplets), green with blue checks (clusters with GaSe underlayer), green (clusters with no underlayer), blue (2D GaSe growth), and white/gray (desorption or no growth). Data points with error bars represent the experimentally measured boundaries between growth modes. Solid lines are Arrhenius fits corresponding to the experimentally derived boundaries. The boundaries defined by the red and dark gray markers are based on correlations between SEM and optical images. The transition extracted from the blue triangles is derived from SEM-based cluster density and footprint area measurements (see Figure \ref{['fig:4']}).
• Figure 5: AFM maps of the grown films. (a-b) Surface morphology of the GaSe films grown with $\Phi_{\text{Se/Ga}}$=87 at $T_{sub}$=635 $^\circ$C (a) and $T_{sub}$=645 $^\circ$C (b). (c) Surface morphology of GaSe grown with $\Phi_{\text{Se/Ga}}$=50 at 615 $^\circ$C. (d) Root-mean-square (RMS) roughness extracted from 4 $\mu m^2$ AFM micrographs as a function of $T_{sub}$.