Power-dependent photoluminescence in strained In_xGa_1-xN/GaN multiple-quantum wells: Simulations of alloying and interface-specific effects

Combined experimental and theoretical efforts are focused to study hexagonal In_xGa_1-xN/GaN[0001] multiple-quantum wells (MQWs). Plasma-assisted molecular-beam epitaxy (PA-MBE) is used to grow high-quality MQWs with multiplicity of 1, 3 and 5. Characterizations methods based on scanning tunneling electron microscopy (STEM) and photoluminescence (PL) indicated that each period is composed of 10 nm GaN barrier and 2.5 nm In_xGa_1-xN well with x ≤ 0.12. Usually, these MQWs have radiations with the blue region. However, in power (from 0.008 mW to 8 mW) dependent micro-photoluminescence (PL), measured at room temperature, blue shifts of about 11.11 nm, 11.94 nm and 14.94 nm were observed corresponding to the single-quantum well (1-QW), 3-MQW, and 5-MQW, respectively. While in literature such shift is speculated to be attributed to so-called "quantum-confined stark effect" (QCSE) in localized luminescent centers with the InGaN wells caused by inhomogeneity of alloy distribution, we extended a rigorous theoretical investigation based on 3D tight-binding method using simple sp³-basis set. The theoretical results show that the wells are isolated and the experimentally observed blue-shifts should be caused by alloying and interface fluctuations. Specifically, the larger blue-shift in 5-MQW sample should be attributed to higher bi-axial strain and interface-specific effects that cause further increase of the hole well's (h-Well) depth, V₀ ^J, and increase in number of localized hole states within the h-Well. Furthermore, this study reveals the role of bi-axial strain, well composition, and interface specific effects on the peculiar behaviors of valence-band offset (VBO) in In_xGa_1-xN/GaN MQWs.