Gerald J. Iafrate

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.

10 μm infrared hot‐electron transistors

A new hot‐electron transistor for 10 μm infrared radiation detection is presented and discussed. The device utilizes an infrared sensitive GaAs/AlGaAs multiple quantum well structure as emitter, a wide quantum well as base, and a thick quantum barrier placed in front of the collector as an electron energy high pass filter. The energy filter selectively permits the higher energy photocurrent to pass to the collector; the lower energy dark current is rejected by the filter, and is drained through the base. The device detectivity, as noted by the collector photocurrent measurements, is much enhanced in comparison with companion infrared photoconductive devices.

Size dependence of the thermal broadening of the exciton linewidth in GaAs/Ga0.7Al0.3As single quantum wells

We have studied the temperature dependence of the linewidth, Γ(T), of the fundamental absorption edge in bulk GaAs and four GaAs/Ga0.7Al0.3As single quantum wells of different well width using photoreflectance. As a result of the size dependence of the exciton‐longitudinal optical phonon interaction, the thermal broadening of the linewidth diminishes as the dimensionality and size of the system are reduced.

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.

High detectivity InGaAs base infrared hot‐electron transistor

An infrared hot‐electron transistor with a thin (300 A)InGaAs base layer is constructed. By adopting a thin base material with a large Γ‐L valley separation, the photocurrent transfer ratio is improved by a factor of four in comparison with the GaAs base transistor. As a result, the detectivity of the transistor is increased to 1.4×1010 cm√Hz/W at 77 K with a cutoff wavelength of 9.5 μm, two times as large as the companion state‐of‐the‐art GaAs quantum well photoconductor. Combined with the lower dark current, the voltage responsivity and the noise equivalent temperature difference of a detector array can be improved by more than an order of magnitude.

Infrared absorption and photoconductive gain of quantum well infrared photodetectors

We have conducted a detailed study on the properties of a multiple quantum well infrared photodetector (QWIP) with a single bound state. From the optical absorption experiment, we found that the peak absorption energy is determined by the resonant states associated with each individual well and not by the global miniband structure. From the transport experiment, we observed that the photoelectron distribution over the QWIP is extremely narrow and close to the top of the quantum well barriers, indicative of the diffusive nature of the hot‐electron transport in this structure. At large bias, the photoelectron distribution begins to shift up in energy, and is found strongly correlated to the observed photoconductive gain of the detector.

Application of quantum-based devices: trends and challenges

Revolutionary nanofabrication techniques and trends have opened the way to fabricating quantum wells, quantum wires and quantum dots that may provide the basic building blocks for future nanoelectronic and mesoscopic (quantum) device technologies. Furthermore, these trends lead to new opportunities for realizing quantum-based information processing devices but many challenges must be addressed and intensive international basic research is essential for the full exploitation of these revolutionary devices.

Dramatic reduction in the longitudinal-optical phonon emission rate in polar-semiconductor quantum wires

Abstract Novel quantum-effect polar-semiconductor structures underlie technologies portending dramatic enhancements in the capability to process information orders of magnitude faster than is possible currently. In many embodiments of these quantum-effect structures, charges are transported in quasi-one-dimensional quantum wires which must support the transport of charges at high mobilities. However, it has recently been demonstrated that the longitudinal-optical (LO) phonons established at quantum-wire interfaces lead to dramatic enhancements in carrier-phonon interactions and concomitant degradation in carrier mobility. This letter demonstrates that phonon modes may be tailored through the judicious use of metal-semiconductor interfaces in such a way as to dramatically reduce unwanted emission of interface LO phonons and, consequently, to lead to the achievement of high quantum-wire mobility.

Electron interaction with confined acoustic phonons in cylindrical quantum wires via deformation potential

The effects of phonon confinement on electron–acoustic‐phonon scattering is studied in cylindrical semiconductor quantum wires. In the macroscopic elastic continuum model, the confined‐phonon dispersion relations are obtained for several crystallographic directions with the two cardinal boundary conditions: free‐surface and clamped‐surface boundary conditions. The scattering rates due to the deformation potential interaction are obtained for these confined phonons and are compared with those of bulk‐like phonons for a number of quantum wire materials. The results show that the inclusion of acoustic phonon confinement effects may be crucial for calculating accurate low‐energy electron scattering rates in nanostructures. It is also demonstrated that the scattering rates may be significantly influenced by the direction of phonon propagation, especially for low‐energy electrons. Furthermore, it has been found that there is a scaling rule governing the directional dependence of the scattering rates: the direct...

Application of superlattice bandpass filters in 10 μm infrared detection

Recently, experimental evidence has revealed that the energy distribution of the dark current in a typical multiple quantum well GaAs infrared detector is extremely broad, in contrast to the narrowly distributed photocurrent. In this letter, we present the current transfer ratio of an infrared hot‐electron transistor with a superlattice collector filter. From the current transfer characteristics, we demonstrate that the superlattice is able to collect electrons with specific energy against a broad background. The energy filtering characteristics can be attributed to the underlying band structure of the superlattice. When the filter is applied to infrared radiation detection, the detectivity of the transistor is improved.