Transport and absorption in strain-compensated Si/Si 1−xGe x multiple quantum well and cascade structures deposited on Si 0.5Ge 0.5 pseudosubstrates

Abstract

Various strain-compensated Si/SiGe quantum well structures with high Ge concentrations in the SiGe wells (70–85% Ge) have been successfully deposited by very low-temperature molecular beam epitaxy on thick strain-relaxed Si 0.5Ge 0.5 buffer layers. The samples have been subject to intensive and careful characterization of their structural, electronic and optical properties. X-ray diffraction and reflectivity measurements using synchrotron radiation allowed accurate determination of layer thicknesses and composition of the quantum well structures. The r.m.s. roughness at Si/SiGe interfaces is less than 0.4 nm I– V characteristics of resonant tunneling devices reveal strong negative differential resistance peaks for heavy hole tunneling but no trace for tunneling via light hole states. Two-dimensional hole gases with a mobility of up to 0.49 cm 2/Vs in 7 nm wide, modulation-doped Si 0.8Ge 0.2 quantum wells with Si barriers and relaxed Si 0.5Ge 0.5 spacer layers underline the high structural quality. The dependence of hole mobility on well width can be explained by an interface roughness of 0.4 nm, in accordance with the X-ray reflectivity measurements, and the associated fluctuation of the strain at the interfaces. Using the structural parameters determined by X-ray analysis as input parameters, the intersubband absorption spectra of strain-compensated quantum well structures have been calculated by the 6 band k· p method. The theoretical predicted spectra are in excellent agreement with those obtained by experiments. Our comprehensive study results in a set of very congruent data enabling to custom design complex Si/SiGe structures such as p-type quantum cascade emitters.