Intersubband relaxation of hot carriers in quantum well systems

We use an ensemble Monte Carlo simulation of coupled electrons, holes and nonequilibrium polar optical phonons in multiple quantum well systems to model the intersubband relaxation of hot carriers measured in ultra-fast optical experiments. We have investigated the effect of various models of confined photon modes on the energy relaxation and intersubband transition rate in single quantum well and coupled well systems. In particular, the symmetry of the atomic displacement with respect to the quantum well has a marked effect on the relative intersubband versus intrasubband scattering rates, depending on whether one considers electrostatic boundary conditions(slab modes) or mechanical boundary conditions(guided modes). In single quantum wells systems, the overall intersubband relaxation time is not found to be strongly dependent on the confined mode model used due to competing effects of hot phonons and the relative intrasubband scattering rates. For coupled well systems, the relaxation rate is much more dependent on the exact nature of the phonon amplitude. Large effects are found associated with localized AlAs interface modes which dominate the intersubband relaxation time.     

Instabilities in quantum point contact structures

We discuss the appearance of strong nonlinearities including S-type negative differential conductance in theI–Vcharacteristics of quantum point contact (QPC) structures. Time-dependent measurements demonstrate that the highly nonlinear d.c.I–Vfeatures are associated with a temporal average of random telegraph switching (RTS) between different current levels. The RTS is only observed when the voltage across the device is such that the chemical potential on one side of the QPC is aligned with the bottom of a one-dimensional subband in the QPC. As the chemical potential is moved further into the subband, the switching behavior disappears until the next subband minimum is reached.     

Two-dimensional electrical characterization of ultrashallow source/drain extensions for nanoscale MOSFETs

State-of-the-art semiconductor devices require accurate control of the full two-dimensional dopant distribution. In this work, we report results obtained on 2D electrical characterization of ultra shallow junctions in Si using off axis electron holography to study two-dimensional effects on diffusion. In particular, the effect of a nitride diffusion mask on lateral diffusion of phosphorous is discussed. Retardation of lateral diffusion of P under the nitride diffusion mask is observed and compared to the lateral diffusion of P under an oxide diffusion mask. The ultra shallow junctions for the study were fabricated by a rapid thermal diffusion process from heavily P doped spin-on-dopants into a heavily B doped Si substrate. These shallow junctions are needed for fabricating source/drain extensions in nanoscale MOSFETs. One-dimensional electrical characterization of the junction was carried out to determine the electrical junction depth and compared to the metallurgical junction depth from SIMS analysis.     

Single-electron quantum dots in silicon MOS structures

We present experimental results for two types of quantum dots, which are embedded within a silicon metal-oxide-semiconductor structure. Evidence is found for single-electron charging at low temperature, and for an asymmetric shape of the dot. First results of simulations of these dots are presented. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

3D modeling of discrete impurity effects in silicon quantum dots: energy level spacing and scarring effects

We present simulation results obtained using a 3D coupled Schrödinger–Poisson equation solver. Of special interest in this work were the effects that discrete impurities have on the energy spectra in the dot and how these effects can be used to better explain conductance peaks observed in experimental measurements. We also explored the behavior of the energy level separations in the closed quantum dot system, observing indications of the onset of chaos.