G. N. Maracas

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.

Experimental evaluation of low‐frequency oscillations in undoped GaAs to probe deep level parameters

A new method for characterizing undoped, semi‐insulating GaAs is presented. The low‐frequency current oscillations on undoped liquid encapsulated Czochralski GaAs substrates are used to determine deep level energies, cross sections, and concentrations. A Fourier transform is used to resolve the various frequency components of the signal. The frequency is seen to vary almost four orders of magnitude in a 50‐K temperature range. Plotting the temperature dependence of each frequency component in an Arrhenius plot gives activation energies and cross sections corresponding to the deep levels responsible for the oscillations. It is seen that two closely spaced levels cause the low‐frequency oscillations. In addition, this technique allows the determination of trap parameters as a function of electric field at values much less than the critical field for intervalley transfer in GaAs.

A GaAs/AlGaAs asymmetric Fabry-Perot reflection modulator with very high contrast ratio

Performance results for a normally on, electroabsorptive, surface-normal Fabry-Perot reflection modulator are presented. The device employs a cavity with a 100 AA GaAs/100 AA Al/sub 0.3/Ga/sub 0.7/As multiple quantum well and top and bottom quarter-wave mirrors with 4 and 19.5 periods, respectively. Very low values of off-state reflectance were measured, giving a maximum contrast ratio >1000 (30 dB) and a maximum reflectance difference of 64.3%. The contrast ratio is, to the authors knowledge, the largest reported to date.<<ETX>>

Electron mobility enhancement by confining optical phonons in GaAs/AlAs multiple quantum wells

We report experimental evidence for an enhanced electron mobility in a multiple quantum well structure in which the optical phonon modes are confined. By inserting n ultrathin layers of AlAs into a host GaAs well, thus dividing the bulklike host well into n+1 miniwells, we observe a substantial enhancement in electron mobility for temperatures T≳100 K. We have also studied the dependence of the electron mobility on the miniwell width. The electron mobility is found to first increase with decreasing miniwell width and then decrease after reaching a maximum value at the width around 45 A.

Electrical and optical characterization of gas source and solid source MBE low temperature buffers

MBE GaAs buffer layers grown at low substrate temperatures (200–300°C) have been shown to significantly reduce backgating and sidegating in GaAs integrated circuits. The isolation provided by these buffers is attributed to a high level of compensating traps in the layers induced by excess arsenic and arsenic antisite defects. Structures were grown by both gas source and solid source MBE in a VG Semicon V80H dual chamber system. The structures allow us to study characteristics of the LTB itself as well as the quality of active layers grown upon these buffer layers. The insulating characteristics of the gas and solid source LTBs are comparable. However, in contrast to control layers grown on semi-insulating GaAs, we observe considerably higher trap concentrations in FET active layers grown on LTBs. Deep level transient spectroscopy (DLTS) measurements show several resolvable electron and hole traps, plus a band of shallow hole traps. Due to the differences in growth kinetics for gas and solid source growth such a close similarity was not expected. The resistivity of the LTBs and the traps incorporated into the active layers appear to be similar for solid source (As4) and gas source (As2) growth. Furthermore, the characteristics of proximity annealed layers continue to change for varied length low temperature anneals. This indicates that the excess arsenic continues to diffuse into the active device layers degrading device stability.

Application of the digital alloy composition grading technique to strained InGaAs/GaAs/AlGaAs diode laser active regions

Molecular‐beam epitaxy (MBE) growth, lasing performance, and transmission electron microscopy (TEM) of In0.25Ga0.75As asymmetric triangular quantum well (ATQW) active regions is reported. The digital alloy technique was utilized to form three active regions of widths 200, 300, and 400 A. Each structure lased with a threshold current density of 313, 152, and 241 A/cm2, respectively. The lasing wavelength range was 979–1030 nm. This is the first demonstration that controlled, digitally graded quantum wells can be incorporated into laser diode active regions.

Temperature‐dependent pseudodielectric functions of GaAs determined by spectroscopic ellipsometry

We present the pseudodielectric functions of GaAs in the temperature range between 30 and 650u2009°C. Data in the spectral range of 1.24<E<5.00 eV was obtained by spectroscopic ellipsometry in a molecular beam epitaxy system specially designed for this purpose. All measurements were performed in situ to avoid the presence of surface adsorbates, oxides, roughness and to ensure a group V stabilized surface is maintained. A simple two‐phase model could thus be used to extract the dielectric functions. Temperature dependence of the critical point energies E1, E1+Δ1, E0’ and E2 are also determined.

In situ spectroscopic ellipsometry in molecular beam epitaxy for photonic devices

Abstract In situ spectroscopic ellipsometry (SE) has been shown to be a versatile technique for monitoring growth in ultrahigh vacuum epitaxial growth systems. For instance, typical MBE parameters of substrate temperature, growth rate, alloy composition and thickness of growing layers have been measured during the growth of heterostructures in solid-source and gas-source MBE. The growth of AlAs/GaAs quantum wells has also been investigated in studies where the growth was monitored in real time with and without growth interruption. The difference in interfacial abruptness of the heterojunction was then determined. This paper first discusses some practical considerations of implementing an SE onto a gas-source MBE. Examples of monitoring substrate temperature, oxide desorption, surface smoothing and heterojunction growth of a test structure will then be presented followed by a demonstration of quantum well growth and growth interruption. The first use of high-temperature GaAs and AlAs optical constants for thickness and alloy composition determination at the MBE growth temperature is presented which enabled the growth and calibration of distributed Bragg reflectors for use in vertical cavity lasers and modulators.

Low-frequency oscillations and routes to chaos in semi-insulating GaAs

Abstract We investigate the characteristics of low-frequency oscillations in semi-insulating GaAs as a function of bias voltage. We find a change in character from simple periodic to irregular, chaotic behavior. A theoretical model is outlined which emphasizes the role of diffusion. The oscillations in SI GaAs are compared to similar phenomena in other semiconductors. A universality of behavior among the various materials is observed and explained.

Contactless electroreflectance characterization of three InGaAs quantum wells placed in a GaAs/AlGaAs resonant cavity

Abstract Contactless electroreflectance at 300 K has been used to characterize an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser test structure, i.e. three InGaAs quantum wells (QWs) placed in a GaAs/AlGaAs resonant cavity. The three observed resonances from the InGaAs QWs have made it possible to evaluate the well width and In composition. The 1020 nm lasing spectrum correlates very well with the energy of the lowest lying QW feature. Also, the built-in electric field in the structure has been determined from the Franz-Keldysh oscillations in the GaAs signal. Furthermore, there is some evidence for Be interdiffusion into the QW region.