Adsorption-Controlled Epitaxy and Twin Control of $γ$-GaSe on GaAs (111)B

Abstract

The III-Se layered semiconductors, including InSe and GaSe, are promising optoelectronic materials due to their relatively high electron mobilities at room temperature, nonlinear optical responses, ferroelectricity, self-passivated van der Waals surfaces, and ability to alloy and synthesize heterostructures for bandgap engineering. Adsorption control is a widely utilized strategy for controlling the stoichiometry and phase formation of these materials; however, the bounds of the adsorption-controlled growth window for GaSe have not been systematically established. Additionally, challenges with control over polytype and twinning remain. Here, we use molecular beam epitaxy to experimentally map the adsorption-controlled growth window of GaSe films on vicinal GaAs (111)B substrates. The observed phase boundaries show qualitative agreement with Ellingham diagram predictions. All films crystallize in the $γ$ ($R3m$) polytype. Increasing growth and annealing temperature leads to decreased mosaicity measured by x-ray rocking curve and smoother surfaces measured by atomic force microscopy, at the expense of a transition from singly oriented $γ$ to twinned $γ$ with $60\degree$ rotated domains.

Figures (10)

• Figure 1: (a) Ellingham diagram for the Ga-Se calculated using Se$_6$ as the gaseous species. (b) Symmetric $2\theta-\omega$ x-ray diffraction scans for films grown at a Se/Ga atomic flux ratio of 30 for varying growth temperatures. (c) Experimentally observed phases for MBE growth of GaSe on GaAs (111)B. Inserts show Reflection High Energy Electron Diffraction (RHEED) patterns for GaSe and mixed GaSe + Ga$_2$Se$_3$. Filled marker size corresponds to the inverse of the GaSe 004 $\omega$ rocking curve full width at half maximum ($1/\Delta\omega_\mathrm{fwhm}$), which scales from 1/1.172$\degree$ to 1/4.506$\degree$. Open marker indicates no rocking curve measured.
• Figure 2: (a) X-ray rocking curves of the GaSe 004 reflection for fixed Se/Ga flux ratio 20/1. (b) Summarized rocking curve width and root mean square (RMS) roughness versus growth or anneal temperature.(c-f) Atomic Force Microscopy (AFM) topography images of GaSe samples grown at 325 $\degree$C, 400 $\degree$C, 450 $\degree$C, and 400 $\degree$C plus a post growth anneal at 520 $\degree$C. Scale bars are 400 nm.
• Figure 3: HAADF-STEM image of the GaSe film grown at 400 $\degree$C and annealed at 520 $\degree$C, measured along a $\langle100\rangle_\mathrm{GaSe} \parallel \langle110\rangle_\mathrm{GaAs}$ zone axis. Ga atoms are in red, and the Se atoms are in light green.
• Figure 4: (a-c) Selected area electron diffraction (SAED) of the $20L$-type reflections for films grown at 400 $\degree$C and 450 $\degree$C, and the film annealed at 520 $\degree$C. Full SAED patterns in Appendix Figure \ref{['fig:diffraction']}. (d) Simulated electron diffraction pattern. (e) Crystal structure of $\gamma$-GaSe for the R0 and R30 twin orientations, viewed along $\langle 100 \rangle_\mathrm{GaSe}$ and $\langle 110 \rangle_\mathrm{GaSe}$, respectively. (f) Azimuthal $\varphi$ x-ray diffraction scans of the GaSe $1 \, 0 \, \underline{10}$ film reflection, referenced to the GaAs substrate $311$ reflection.
• Figure 5: Common GaSe polytypes.