K. I. Kolokolov

Temperature dependence of intersubband transitions in InAs/AlSb quantum wells

We have carried out a systematic temperature-dependent study of intersubband absorption in InAs/AlSb quantum wells from 5 to 10 nm well width. The resonance energy redshifts with increasing temperature from 10 to 300 K, and the amount of redshift increases with decreasing well width. We have modeled the transitions using eight-band k⋅p theory combined with semiconductor Bloch equations, including the main many-body effects. Temperature is incorporated via band filling and nonparabolicity, and good agreement with experiment is achieved for the temperature dependence of the resonance.

Intersubband Transitions in InAs/AlSb Quantum Wells

We have studied intersubband transitions in InAs/AlSb quantum wells experimentally and theoretically. Experimentally, we performed polarization-resolved infrared absorption spectroscopy to measure intersubband absorption peak frequencies and linewidths as functions of temperature (from 4 K to room temperature) and quantum well width (from a few nm to 10 nm). To understand experimental results, we performed a self-consistent 8-band k⋅p band-structure calculation including spatial charge separation. Based on the calculated band structure, we developed a set of density matrix equations to compute TE and TM optical transitions self-consistently, including both interband and intersubband channels. This density matrix formalism is also ideal for the inclusion of various many-body effects, which are known to be important for intersubband transitions. Detailed comparison between experimental data and theoretical simulations is presented.

Doping-induced type-II to type-I transition and interband optical gain in InAs/AlSb quantum wells

We show that proper doping of the barrier regions can convert the well-known type-II InAs/AlSb quantum wells (QWs) to type I, producing strong interband transitions comparable to regular type-I QWs. The interband gain for TM mode is as high as 4000 1/cm, thus providing an important alternative material system in the midinfrared wavelength range. We also study the TE and TM gain as functions of doping level and intrinsic electron–hole density.

Microscopic modeling of intersubband optical processes in Type II semiconductor quantum wells: linear absorption

Intersubband absorption spectra are analyzed using the density matrix theory under the second Born approximation. The intersubband semiconductor Bloch equations are derived from the first principles including electron-electron and electron-longitudinal optical phonon interactions, whereas electron-interface roughness scattering is considered using Andos theory. A spurious-states-free 8-band k•p Hamiltonian is used, in conjunction with the envelope function approximation to calculate the electronic band structure self-consistently for type II InAs/AlSb multiple quantum well structures. We demonstrate the interplay of various physical processes in the absorption spectra in the mid-infrared frequency range.

Intersubband transitions in narrow InAs/AlSb quantum wells

We have investigated intersubband transitions (ISBTs) in InAs/AlSb multiple quantum wells. In wells from 7 to 10 nm wide, the ISBT energy increases with decreasing well width and temperature. We do not observe photoluminescence (PL) from these wells. In wells from 2.4 to 6 nm wide, we observe PL but not ISBTs. We have calculated the band structure of these samples using an 8 band k.p theory including strain and many-body effects. We have modelled the dependence of the ISBT energy on well width and temperature. In addition, we have observed the effects on ISBTs of QW interface type and Si doping in the well.

Evaluation of interfaces in narrow InAs/AlSb quantum wells

InAs/AlSb quantum wells may be grown with two types of interfaces: InSb-like and AlAs-like. The interface type refers to the half-monolayer of the well material and half monolayer of barrier which are in contact. The type and quality of the quantum well interface is critical to the ISBT intensity and lineshape and, to a lesser extent, position. In addition to FTIR spectroscopy of the ISBT, we have performed transmission electron microscopy (TEM) to directly evaluate the quality of the interfaces at the atomic level. In order to evaluate the effects of interface type and quality on ISBT intensity, lineshape, and linewidth, we studied the TEM of a 10 nm QW sample with InSb-InSb interfaces and a 3 nm QW sample with InSb-AlAs interfaces.

Spurious states free solutions of the k·p Hamiltonian for heterostructures

We developed a method to eliminate the spurious solutions of the k•p Hamiltonian in the envelope function approximation applied to the quantized states of heterostructures by introducing an off-diagonal k2 term. This results in a modification in the fourth and higher order terms in k of the band dispersion, which keeps the dispersion at the Γ point but modifies it at large k so that converts spurious states to the harmless evanescent ones. We show that the modification to the Hamiltonian leads to the monotonic behavior of the conduction band as a function of k and thus removes the spurious solutions in the calculations of confined states for all popular III-V compounds and their alloys.