Doan Nhat Quang

A Semiclassical Approach to the Electron Gas in Two-Dimensional Semiconductor Structures

A theory is developed of the density of states (DOS) of the two-dimensional electron gas (2D EG) in semiconductor heterostructures, taking into account the effect of disorder caused by some random field existing in the sample. For a smooth random field, the calculation is carried out within its Gaussian statistics and a semiclassical approach. A simple closed expression thus obtained includes the classical DOS and its quantum correction as well, which describe the DOS of the 2D EG in explicit dependence on the rms of the potential and of the force of the random field. The disorder effect is found to smear out the step-like singularity of the DOS at an unperturbed band edge of the ideal 2D EG into a tail deep below the band edge. A detailed treatment is given of the case when the disorder is due to remote ionized impurities, which are distributed randomly or correlated in the sample.

Deep Tail in the Density of States of a Disordered Two-Dimensional Electron Gas

A theory is given of the density of states (DOS) in the low-energy tail for the disordered two-dimensional electron gas (2D EG) in a semiconductor heterostructure subjected to Gaussian random fields of any origin (especially of short range). The calculation is carried out by means of a path-integral technique within the approximation based on a non-local harmonic modelled action and that of the ground-state contribution. A simple analytic expression for the 2D DOS and different variational equations for the curvature of the trial well are then obtained, which describe the DOS tail in explicit dependence on the potential correlator, i.e., on the disorder origin as well as the geometry of the realistic 2D EG. An interpolation scheme for obtaining the 2D DOS over the whole energy region is proposed. The DOS in the deep tail is found to roughly go as a decreasing exponential function whose exponent is proportional to |E| n with n varying from 1 to 2 for very short- and long-range potential correlations, respectively. The case of a Gaussian potential correlator is thoroughly examined.

Key scattering mechanisms for holes in strained SiGe/Ge/SiGe square quantum wells

We present a theory of the low-temperature transport of holes confined in the Ge strained channel of single-side modulation-doped SiGe/Ge/SiGe square quantum wells (QWs). Besides the well-known scattering mechanisms such as remote impurities and surface roughness, the theory includes misfit deformation potential. We prove that due to the effect from doping-induced band bending, the surface roughness and misfit deformation potential scatterings are considerably strengthened. Accordingly, these are found to be the key scattering mechanisms in the SiGe/Ge/SiGe system, which are still a subject under debate. Our theory can explain all recent experimental data about the transport properties of interest, namely, the carrier-density dependences of the hole mobility and the ratio of the transport to quantum lifetimes. Further, the calculated hole mobility in Ge strained QWs exhibits a special channel-width dependence with a sharp peak, which was observed but has not been explained so far.

Two-Side Doping Effects on the Mobility of Carriers in Square Quantum Wells

We presented a theoretical study of the effects from two-side (2S) doing on low-temperature lateral transport in square quantum wells (QWs). Within a variational approach, we obtained analytic expressions for the carrier distribution, screening function, and autocorrelation functions for various scattering mechanisms. We found that the mobility of a 2S-doped square QW is larger than that of the one-side (1S) doped counter part for scattering from both interfaces or from the top interface. However, the former is smaller than the latter for scattering from the bottom (substrate-side) interface. The mobility of a 2S-doped square QW exhibits a well-width evolution slower than the power-of-six law characteristic of the undoped QW. The mobility may be enhanced by 2S doping. We examine the dependence of the enhancement factor on QW parameters for optimization of the structure. This factor may achieve an order of magnitude, which is much larger than that provided by earlier methods. Our theory is able to reproduc...

Correlation-length dependence of lifetime ratios: Individual estimation of interface profile parameters

We show that the ratio between relaxation lifetimes dominated by roughness-related scatterings in heterostructures is a well-defined function of the correlation length. Thus, we propose an efficient method for individual estimation of the two size parameters of interface profiles from transport data. Instead of the normal simultaneous fitting of both parameters to lifetimes, we adopt a two-step procedure of (i) inferring the correlation length from some lifetime ratio and then (ii) fitting the roughness amplitude to some lifetime. Similarly, the ratio of roughness-induced linewidths in intersubband absorption may give such an estimation from optical data.

Analytic Solution for the Density of States of the Disordered Quasi-One-Dimensional Electron Gas in a Quantum Wire

A theory is presented of the density of states (DOS) over the entire energy spectrum of the disordered quasi-one-dimensional electron gas (1DEG) in a quantum wire. The disorder is caused by Gaussian random fields of any origin, especially those of short range. The solution is derived by means of a 1D version of the path-integral technique within the approximation based on a non-local harmonic modeled action. A simple analytic expression for the 1D DOS and different variational equations for the curvature of the trial well are then obtained, where the autocorrelation function of the random field plays the key role as the input function for disorder interaction. This enables us, for the first time, to examine in detail the effect from disorder of various origins on the DOS of 1DEGs in quantum wires of arbitrary geometry as well as to incorporate the many-body screening by 1DEGs. The theory is verified by reproducing the well-known asymptotics of the white-noise energy spectrum given by earlier theories. N...

The Channel-width Dependence of the Low-temperature Hole Mobility in Ge-rich Narrow Square Quantum Well Studied by the Band-bending Method

We employ the theory of band-bending effects to explain the channel-width dependence of the mobility of a two-dimentional hole gas (2DHG) in narrow square Si/Si\(_{1-x}\)Ge\(_x\)/Si quantum well at high Ge content. The numerical calculation of scattering mechanisms is shown in comparison with the ones from the previous computations. Our method enables a better quantitative description of recently measured data about the dependence of the 8 K mobility of holes in a Si/Si\(_{0.2}\)Ge\(_{0.8}\)/Si quantum well on the channel width varying from 25 - 70A

Effect of Single-Side Modulation Doping on Low-Temperature Transport Properties in Square Infinite Quantum Wells

A variational approach is given for the effect from single-side modulation doping on low-temperature transport properties of the charge carriers confined in a square infinite quantum well (QW). We obtained analytic expressions which describe the doping effects on the carrier distribution in the well, their roughness-induced scattering in the in-plane and screening by them. The calculation of the transport lifetimes is performed for holes in a SiGe/Ge/SiGe square QW, and the result is found in quantitative agreement with recently measured dependence on experimental conditions such as channel width and carrier density.