Molecular beam epitaxy of single-crystalline bixbyite (In$_{1-x}$Ga$_x$)$_2$O$_3$ films ($x \leq 0.18$): Structural properties and consequences of compositional inhomogeneity

Abstract

In this work, we show the heteroepitaxial growth of single-crystalline bixbyite (In 1-x Ga x ) 2 O 3 films on (111)-oriented yttria-stabilized zirconia substrates using plasma-assisted molecular beam epitaxy under various growth conditions. A pure In 2 O 3 buffer layer between the substrate and (In 1-x Ga x ) 2 O 3 alloy is shown to result in smoother film surfaces and significantly improved crystallinity. Symmetric out-of-plane 2θ -ω x-ray diffraction scans show a single (111) crystal orientation and transmission electron microscopy confirms the single-crystallinity up to x = 0.18 and only slight film quality deterioration with increasing Ga content. Partially relaxed layers are demonstrated via reciprocal space mapping with lattice parameters fitting well to Vegard's law. However, the Ga cations are not evenly distributed within the films containing nominally x > 0.11: inclusions with high Ga density up to x = 0.50 are observed within a "matrix" with x ≈ 0.08. The cubic bixbyite phase is preserved, in both the "matrix" and the inclusions. Moreover, for x ≥ 0.11, both the Raman phonon lines as well as the optical absorption onset remain nearly constant. Hard x-ray photoelectron spectroscopy measurements also indicate a widening of the band gap and exhibit similar saturation of the Ga 2p core level position for high Ga contents. This saturation behavior of the spectroscopic properties further supports the limited Ga incorporation into the "matrix" of the film.

The electrical conductivity of cubic (In$_{1-x}$Ga$_x$)$_2$O$_3$ films ($x\le0.18$): Native point defects, Sn-doping, and the surface electron accumulation layer

“…Detailed information on the growth of these single-crystalline samples and the determination of the Ga content therein are reported in our recent work in Ref. 40.…”

“…The presence of the SEAL of the films was identified by a combination of conventional soft x-ray photoelectron spectroscopy (SXPES: hv = 1.49 keV) using an Al Kα light source and hard x-ray photoelectron spectroscopy (HAXPES: hv = 5.95 keV), as also documented in our recent works. 40,41 HAX-PES measurements were performed at room temperature at the revolver undulator beamline at BL15XU of SPring-8. 44 A detailed description of the experimental setup of HAX-PES at the beamline was described elsewhere.…”

“…36,37 IGTO has been further proposed as an alternative transparent conducting electrode for light-emitting diodes (LEDs), with the intention to increase their external quantum efficiency in the near-ultraviolet spectral range, due to its increased absorption edge, compared to ITO. 38 Recently, we have studied well-defined, high-quality, single-crystalline (In 1-x Ga x ) 2 O 3 layers at the low-x end grown by plasma-assisted molecular beam epitaxy (PA-MBE) in terms of surface and film morphology, crystalline quality, and homogeneity 40 as well as increasing band gap and optical absorption edge with x. 41 In this work we investigate the effect of Ga on the unintentional conductivity of the bulk and the SEAL and demonstrate intentional Sn-doping.…”