Structural and optical characterization of GaS1−xSex layered mixed crystals grown by chemical vapor transport

Abstract

Alloying is a prominent strategy for tailoring the properties of semiconductors and expanding their usability. Accordingly, an entire series of GaS1–xSex (0 ≤ x ≤ 1) alloys were grown by chemical vapor transport and their structural and optical properties were studied. It was found that, for optimal crystalline quality, the growth temperatures should decrease as x increases. The scanning electron microscopy images of the specimens showed GaS1-xSex are layered crystals. The transmission electron microscopy (TEM) images showed the GaS1−xSex crystals have hexagonal structures with high crystalline quality. Their Raman and X-ray diffraction (XRD) results indicated GaS and GaS0.8Se0.2 are in the 2 H β-phase, GaSe is in the 2 H ε-phase, while those with intermediate x are in the 2 H β-ε mixed phase. The frequencies of the Raman modes decrease continuously as x increases. This single-mode behavior differs from the dual-mode behavior reported by the literature. The lattice constants a and c of the GaS1−xSex crystals, estimated from the TEM and XRD results, increase as x increases. The variation in a with x follows linear Vegard’s law. However, the variation in c with x is quadratic. The indirect and direct bandgaps of GaS1–xSex, determined from the absorption spectra, decrease as x increases. The difference between the indirect and direct bandgaps shrinks as x increases. The minimum difference is only 20 meV, which makes GaS1−xSex with high x a pseudo-direct bandgap semiconductor. The piezoreflectance spectra of the samples showed excitonic signals, which may be attributed to the direct interband transitions. The wavelength of the samples’ photoluminescence peaks increases with x and covers the visible range. Thus, GaS1–xSex shows great promise for optoelectronic applications.