Deep-level transient spectroscopy study of narrow SiGe quantum wells with high Ge content

We present a detailed theoretical and experimental study of hole emission processes in p-type Si/SiGe/Si structures under the nonequilibrium conditions found in deep-level transient spectroscopy (DLTS) investigations. We clarify the possibilities and limitations of DLTS applied to quantum-well (QW) structures. We report an observation of the effect of thermally activated tunneling induced by local high electric field on the emission rate of confined holes. In the limit of high external electric field F, the hole emission rate ${\mathit{e}}_{\mathit{T}}$ increases with F according to ${\mathit{e}}_{\mathit{T}}$=${\mathit{e}}_{\mathit{T}}$(0)exp(${\mathit{F}}^2$/${\mathit{F}}_{\mathit{c}}^2$), where the characteristic field ${\mathit{F}}_{\mathit{c}}$ agrees with theoretical calculations. The effect of nonequilibrium carrier diffusion is determined from the dependence of the DLTS signal on the pulse frequency, allowing one to estimate the effective hole diffusion coefficient. Interface roughness scattering, which controls the carrier mobility in the narrow QW, is investigated. The observed diffusion coefficient depends only slightly on the width for the investigated narrow SiGe QW’s (2–3 nm), but depends strongly on the Ge content (x=0.3–0.5) in agreement with theory. A strong increase of the hole emission activation energy with decreasing nonequilibrium hole concentration gives evidence for considerable fluctuations of lateral potential, inducing lateral localization of confined holes at low temperatures. We show that interface roughness is responsible for these lateral potential fluctuations. © 1996 The American Physical Society.