The structure and properties of metal-semiconductor interfaces

Following the past seventeen-year developmental path in the research of semiconductor superlattices and quantum wells, significant milestones are presented with emphasis on experimental investigations in the device physics of reduced dimensionality performed in cooperation with the materials science of heteroepitaxial growth.

A bird's-eye view on the evolution of semiconductor superlattices and quantum wells

The dynamics of photogenerated carriers in semiconductor structures with reduced dimensionality has been the subject of intensive investigations in recent years [1,2]. State-of-the-art band-gap engineering technologies enable us to tailor low-dimensional semiconductor systems with desirable optoelectronic properties and study the fundamental aspects of carrier dynamics. This has increased tremendously our fundamental understanding of the dynamic properties of artificial semiconductor structures and has also resulted in a wide range of novel devices such as quantum well lasers, modulators, and detectors, as well as all-optical switches. Nevertheless, the bulk band structure of semiconductors seems to dominate optoelectronic properties since the strength of interband transitions is largely governed by the atomiclike Bloch parts of the wave function [3]. Thus it appears at first glance unavoidable that strong interband optical transitions are linked to direct band-gap semiconductors with short radiative lifetimes such as GaAs, whereas long radiative lifetimes of photogenerated carriers imply utilization of semiconductors with indirect band gaps such as Si and correspondingly reduced interband absorption. Initial attempts to employ band-gap engineering in order to combine strong interband absorption with long radiative lifetimes have focused on so-called doping superlattices [4]. There, alternate n and p doping along the growth direction is utilized to combine a direct gap in momentum space with an indirect gap in real space which causes a spatial separation of photogenerated electron-hole se-hd pairs and hence considerably prolonged lifetimes. Here, we introduce a new way of band-gap engineering in which we expose a semiconductor quantum well of a direct gap material to a moving potential superlattice modulated in the plane of the well. We show that the confinement of photogenerated e-h pairs to two dimensions, together with the moving lateral superlattice, allows reversible charge separation [5]. We demonstrate that the combination of both the advantages of strong interband absorption and extremely long lifetimes of the optical excitations is achieved without affecting the superior optical quality of the quantum well material. The spatial separation of the electron-hole pairs is achieved via the piezoelectric potential of acoustic waves propagating along the surface of a semiconductor quantum well system. On a piezoelectric substrate, the elliptically polarized surface acoustic waves (SAWs) are accompanied by both lateral and vertical piezoelectric fields which propagate at the speed of sound. Those fields can be strong enough to field ionize optically generated excitons and to confine the resulting electrons and holes in the moving lateral potential wells separated by one-half wavelength of the SAW. The spatial separation dramatically reduces the recombination probability and increases the radiative lifetime by several orders of magnitude as compared to the unperturbed case. We further demonstrate that the dynamically trapped electron-hole pairs can be transported over macroscopic distances at the speed of sound and that deliberate screening of the lateral piezoelectric fields of the SAW leads to an induced radiative recombination after long storage times at a location remote from the one of e-h generation. This conversion of photons into a long lived e-h polarization which is efficiently reconverted into photons can serve as an optical delay line operating at sound velocities. The undoped quantum well samples used in our experiments are grown by molecular beam epitaxy on a (100)GaAs substrate. The quantum well consists of 10 nm pseudomorphic In0.15Ga0.85As grown on a 1 mm thick GaAs buffer and is covered by a 20 nm thick GaAs cap layer. The active area of the sample is etched into a 2.5 mm long and 0.3 mm wide mesa (see inset of Fig. 1) with two interdigital transducers (IDTs) at its ends. The IDTs are designed to operate at a center frequency fSAW › 840 MHz. They are partially impedance matched to the 50 V radio frequency (rf) circuitry using an on-chip matching network, thus reducing the insertion

https://arxiv.org/pdf/cond-mat/9704029.pdf

Acoustically Driven Storage of Light in a Quantum Well

The unusual electrical and optical properties of three important types of semiconductor superlattices are reviewed. The artificial structures which were grown by molecular beam epitaxy (MBE) consist of a periodic sequence of ultrathin crystalline layers of alternating composition (Al x Ga1-x As/GaAs or Ga x In1-x As/GaAs y Sb1-y ) or of alternating doping (n-GaAs/p-GaAs). The electronic energy bands in these superlattives are split into quasi-two-dimensional subbands whose spacing and width can be tailored by appropriate choice of the design parameters of the structure. After a brief outline of the electronic properties of compositional superlattices, we discuss the unique properties of GaAs doping superlattices in detail. The space charge induced band edge modulation in doping superlattices leads to an indirect energy gap in real space. As a consequence, the electrons and holes are spatially separated and their recombination lifetimes are strongly enhanced, i.e. non-equilibrium free-carrier dist...

Compositional and doping superlattices in III-V semiconductors

The effect of surfaces on the optical properties of GaAs nanowires is evidenced by comparing nanowires with or without an AlGaAs capping shell as a function of the diameter. We find that the optical properties of unpassivated nanowires are governed by Fermi-level pinning, whereas, the optical properties of passivated nanowires are mainly governed by surface recombinations. Finally, we measure a surface recombination velocity of 3 x 10(3) cm s(-1) one order of magnitude lower than values previously reported for (110) GaAs surfaces. These results will serve as guidance for the application of nanowires in solar cell and light emitting devices.

Impact of surfaces on the optical properties of GaAs nanowires

Luminescence and Raman measurements on a new type of superlattice consisting of $n$- and $p$-type doped GaAs layers grown by molecular-beam epitaxy confirm crucial predictions of theory. A strongly tunable energy gap is found in luminescence. Raman experiments provide the first observation of electronic subbands in purely space-charge-induced quantum wells. A combined analysis of the luminescence and Raman data yields excellent agreement with self-consistent subband calculations based only on the design parameters of the sample.

H.J. Stolz

Papers

Observation of Tunable Band Gap and Two-Dimensional Subbands in a Novel GaAs Superlattice

Photoluminescence study of electron-hole recombination across the tunable effective gap in GaAs n-i-p-i superlattices

Surface band bending on clean and oxidized (110)-GaAs studied by Raman spectroscopy

Electrical resistance of epitaxial crystalline films of (SN)x

Quantization of photoexcited electrons in GaAs nipi crystals