M. A. Stroscio

POLAR OPTICAL-PHONON SCATTERING IN THREE- AND TWO-DIMENSIONAL ELECTRON GASES

The concept of the polar optical momentum relaxation time for phonon energies larger than the thermal energy is applied to the analytical calculations of the electron mobility in bulk GaAs and in a two‐dimensional (2D) electron gas. The theory also accounts for nonparabolicity. For the bulk material, the analytical formula is in good agreement with experimental data even at elevated temperatures where it can be used as an extrapolation formula. The comparison with numerical simulations for the 2D gas shows that the analytical formula for the 2D case applies when intersubband scattering is unimportant. When many subbands are involved in scattering processes, the polar optical mobility can be estimated using the analytical formula for the bulk case. Hence the results provide convenient analytical equations for polar optical mobility which can be used in device simulators.

On the detectivity of quantum-dot infrared photodetectors

We report on the analysis of thermally-limited operation of quantum-dot infrared photodetectors (QDIPs). A device model is developed and used to calculate the QDIP detectivity as a function of the structural parameters, temperature, and applied voltage, as well as to determine the conditions for the detectivity maximum. The QDIP detectivity is compared with that of quantum-well infrared photodetectors (QWIPs). This work clarifies why the existing QDIPs are still inferior to QWIPs and shows that a significant improvement in the QDIP performance can be accomplished by the utilization of dense QD arrays with small QDs.

Electron‐optical‐phonon scattering rates in a rectangular semiconductor quantum wire

One‐dimensional electron‐optical‐phonon interaction Hamiltonians in a rectangular quantum wire consisting of diatomic polar semiconductors are derived under the macroscopic dielectric continuum model. The scattering rates calculated in a GaAs square quantum wire show that when the quantum wire is free‐standing in vacuum, the interaction by the surface‐optical phonon modes is very strong and may dominate over other scattering processes, especially with dimensions of about 100 A or less. When the wire is embedded in a polar semiconductor (AlAs to be specific), the scattering rates by the surface‐optical phonon modes become generally smaller, but yet comparable to those by the confined longitudinal‐optical modes as the wire dimension shrinks. A considerable decrease in the total scattering rate for optical phonons as a result of simple reduction in dimensionality is not observed in this study.

Quantized vibrational modes of nanospheres and nanotubes in the elastic continuum model

The properties of nanoscale spheres and tubes are of recent interest due to the discovery of the fullerene molecule and the carbon nanotube. These carbon structures can be modeled as nanoscale spherical or cylindrical shells. In this article, these nanostructures are treated in the thin shell approximation with the elastic properties taken to be those of the graphene sheet. A quantization prescription is applied to the classical elastic modes to facilitate the first calculations of the quantum-mechanical normalizations of selected modes. These modes are shown to be amenable to the study of electron-phonon interactions. Indeed, electron-phonon interaction Hamiltonians are derived. Moreover, it is shown for such a tube of finite length that the electron-phonon interaction strength depends on the axial position. As a special case it is shown that the dispersion relation for the clamped tube depends on the length of the tube. In this article we consider both the vibrational frequencies and the mode quantizati...

Transfer matrix method for interface optical-phonon modes in multiple-interface heterostructure systems

Interactions of carriers with interface optical phonons dominate over other carrier–phonon scatterings in narrow quantum-well structures. Herein, a transfer matrix method is used to establish a formalism for determining the dispersion relations, electrostatic potentials, and Frohlich interaction Hamiltonians of the interface optical phonons for multiple-interface heterostructure systems within the framework of the macroscopic dielectric continuum model. This method facilitates systematic calculations for complex structures where the conventional method is very difficult to implement. Several specific cases are treated to illustrate the advantages of the general formalism.

Electron-optical-phonon interaction in binary/ternary heterostructures

The macroscopic dielectric continuum model is used to derive electron‐optical‐phonon interaction Hamiltonians for binary/ternary heterostructures containing both single and double heterointerfaces. The formulation presented in this work leads to a general prescription for the calculation of mode‐strength coefficients in ternary‐containing heterostructures. An illustration of these results is provided by exhibiting the mode strengths of the interaction Hamiltonians for the interface and the half‐space longitudinal optical modes in a GaAs/AlyGa1−yAs single heterostructure.

Transition from longitudinal‐optical phonon scattering to surface‐optical phonon scattering in polar semiconductor superlattices

Dielectric continuum models of optical‐phonon modes predict an enhancement in the strength of the surface‐optical (SO) modes in double‐barrier heterostructures as the heterojunction‐to‐heterojunction separation is reduced. There is currently no consensus on the nature of the electron‐SO‐phonon coupling interaction. In this work, the ratio of electron scattering by the SO‐phonon modes to that by the confined longitudinal‐optical (LO) phonon modes is calculated for a GaAs/AlAs short‐period superlattice based on the assumption that the electron‐SO‐phonon interaction may be described by a scalar potential. The scaling of the ratio of electron‐SO‐phonon scattering to electron‐LO‐phonon scattering as a function of the superlattice period provides a sensitive test of the appropriateness of the scalar‐potential model.

Interface‐phonon‐assisted transitions in quantum‐well lasers

Interface‐phonon‐assisted transitions are shown to be important in determining the rates for bound–bound transitions as well as the optimum quantum‐well parameters for recently proposed quantum‐well lasers including the tunneling injection laser and the quantum cascade laser. In particular, it is demonstrated through the use of the dielectric continuum model for interface and confined phonons that the exploitation of interface‐phonon‐assisted transitions offers a means of maximizing key transition rates and thus of optimizing the performance of both the tunnel injection laser and the quantum cascade laser.

Transport via the Liouville equation and moments of quantum distribution functions

Abstract This paper (i) examines through numerical solutions of the coupled coordinate representation Liouville and Poisson equations, the use of the Bohm quantum potential to represent the equilibrium distribution of density and energy in quantum feature size structures; (ii) discusses the development of the nonequilibrium quantum hydrodynamic (QHD) equations with dissipation through the truncation of the quantum distribution function; and (iii) compares select results of the QHD equations incorporating the Bohm potential to the exact Liouville equation solutions. The broad conclusion of the study is that for structures of current interest such as HEMTs, only quantum mechanical solutions, or the incorporation of the quantum potential as a modification of the classical equations will permit representative solutions of such critical features as the sheet charge density.

Theoretical calculation of longitudinal‐optical‐phonon lifetime in GaAs

The anharmonic decay of longitudinal‐optical (LO) phonons in zinc‐blende semiconductors has been studied. Based on an approach in which the anharmonic crystal potential is estimated using the theory of elasticity, the lifetime of LO phonons via emission of two acoustic phonons is calculated as a function of lattice temperature and phonon wave vector. Application of this model to bulk GaAs shows an excellent agreement with available experimental data. Since the parameters employed in the model can be obtained experimentally, the approach provides a useful tool to investigate LO‐phonon lifetimes in semiconductors.