Investigation of surface roughness of molecular beam epitaxy Ga1−xAlxAs layers and its consequences on GaAs/Ga1−xAlxAs heterostructures

Abstract

We report a detailed investigation of the surface morphology of Ga1−xAlxAs layers grown by molecular beam epitaxy (MBE) as a function of the layer parameters and the growth conditions. Under some conditions Ga1−xAlxAs layers exhibit a surface roughness characterized by a degradation of the in situ electron diffraction pattern and a very broad photoluminescence peak. The amplitude of the roughness, as observed by optical microscopy and measured by alphastep profiler, increases with the layer thickness, the growth rate r, and goes through a maximum for a substrate temperature TS≂650 °C, a flux ratio R=JV/JIII≂6 and an Al composition x≂0.5. However, smooth layers can be obtained using growth conditions like high TS (≥700 °C), low flux ratio (R≤2.5), or low growth rate (<0.5 μm/h). No surface roughness was observed for GaAs and AlAs layers. Moreover, a high concentration of foreign atoms at the surface of the growing layer (Sn dopant for example) yields to a quite perfect surface morphology. This paper also contains data on the consequences of Ga1−xAlxAs surface morphology on the crystalline and optical quality as characterized by x-ray diffraction and 300 K photoluminescence, of single and multiquantum well structures grown on a Ga0.7Al0.3As buffer layer. This quality which decreases with increasing thickness of the Ga0.7Al0.3As buffer layer can be restored using optimized growth conditions or a tin doping flux. Some explanations of the mechanism responsible for the surface morphology of ternary layers are discussed.