n-Si/SiGe quantum cascade structures for THz emission

Abstract

In this work we report on modelling the electron transport in n-Si/SiGe structures. The electronic structure is calculated within the effective-mass complex-energy framework, separately for perpendicular (X z ) and in-plane (X xy ) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is described via scattering between quantized states, using the rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon, optical phonon, alloy and interface roughness scattering are taken in the model. The calculated U/I dependence and gain profiles are presented for a couple of QC structures.

An n-type SiGe/Ge QC structure utilizing the deep Ge quantum well for electron at the Γ point

“…The quantum well for electrons at the point is about 370 meV deep, which is consistent with the result in [10]. (The small difference is caused by the differences between the referenced parameters.)…”

“…The effective mass of the electrons in the band is much smaller than that of the heavy hole. The effective masses of the band electron and heavy hole in Si 1−x Ge x alloy can be expressed as 0.041m o + 0.115(1−x)m o and 0.28m o + 0.21(1−x)m o , respectively [10]. The small effective mass of the carrier can help to increase the wavefunction's coupling and carrier tunnelling rate, as well as to enhance the operating current.…”

“…The second is the presence of the heavy, light, splitoff valence bands, which induces strong nonparabolicity and anisotropy of the in-plane dispersion of subbands, resulting in a complicated consideration of the energy band design and a broadening of the transition line. Recently, n-type Si-based QC structures utilizing the band [10] and L band [11] have been proposed, but they still have the problems of a larger effective electron mass and small conduction band offset.…”