Robert Bicknell-Tassius

ANALYSIS OF IN0,07GA0,93AS LAYERS ON GAAS COMPLIANT SUBSTRATES BY DOUBLE CRYSTAL X-RAY DIFFRACTION

Double crystal x-ray diffraction data is presented from the most extensive compliant substrate experiment to date. Five consecutive InGaAs–GaAs growths were performed simultaneously on GaAs-based thin film compliant substrates and thick reference substrates. The In0.07Ga0.93As layers were grown to thicknesses below and above the conventional critical thickness. It was found that InGaAs films grown on the compliant substrates have a larger critical thickness and slower strain relief than InGaAs grown on conventional GaAs substrates.

Using neural networks to construct models of the molecular beam epitaxy process

This paper presents the systematic characterization of the molecular beam epitaxy (MBE) process to quantitatively model the effects of process conditions on film qualities. A five-layer, undoped AlGaAs and InGaAs single quantum well structure grown on a GaAs substrate is designed and fabricated. Six input factors (time and temperature for oxide removal, substrate temperatures for AlGaAs and InGaAs layer growth, beam equivalent pressure of the As source and quantum well interrupt time) are examined by means of a fractional factorial experiment. Defect density, X-ray diffraction, and photoluminescence are characterized by a static response model developed by training back-propagation neural networks. In addition, two novel approaches for characterized reflection high-energy electron diffraction (RHEED) signals used in the real-time monitoring of MBE are developed. In the first technique, principal component analysis is used to reduce the dimensionality of the RHEED data set, and the reduced RHEED data set is used to train neural nets to model the process responses. A second technique uses neural nets to model RHEED intensity signals as time series, and matches specific RHEED patterns to ambient process conditions. In each case, the neural process models exhibit good agreement with experimental results.

Strain accommodation in mismatched layers by molecular beam epitaxy: introduction of a new compliant substrate technology

Compliant substrates allow a new approach to the growth of strained epitaxial layers, in which part of the strain is accommodated in the substrate. In this article, compliant substrates are discussed and a new compliant substrate technology based on bonded thin film substrates is introduced. This technology has several advantages over previously published methods, including the ability to pattern both the top and bottom of the material. A new concept enabled by this compliant substrate technology,strain-modulated epitaxy, will be introduced. Using this technique, the properties of the semiconductor material can be controlled laterally across a substrate. Results of two experiments are presented in which low composition InxGa1−xAs was grown by molecular beam epitaxy on GaAs compliant substrates at thicknesses both greater than and less than the conventional critical thickness. It was found that for t > tc, there was an inhibition of defect production in the epitaxial films grown on the compliant substrates as compared to those grown on conventional reference substrates. For t < tc, photoluminescence and x-ray diffraction show the compliant substrates to be of excellent quality and uniformity as compared to conventional substrates.

Strain‐modulated epitaxy: A flexible approach to 3‐D band structure engineering without surface patterning

Thin compliant growth substrates have been used to reduce the strain in lattice‐mismatched overlayers during epitaxial growth. This letter reports a new thin compliant substrate technology which allows these thin substrates to be patterned on the bottom, bonded surface. This lateral strain variation (inverted stressor) in the growing film can be combined with the additional effects of strain‐dependent growth kinetics to realize the lateral control of composition and thickness without any surface topography on the substrate. Initial demonstrations of the growth of InGaAs on GaAs bottom‐patterned thin substrates are presented herein.

Analysis of GaAs Substrate Removal Etching with Citric Acid : H 2 O 2 and NH 4 OH : H 2 O 2 for Application to Compliant Substrates

New properties associated with selective substrate removal have been observed in the application of this technique to GaAs thin film compliant substrates. Citric acid- and NH 4 OH-based etches are used to selectively etch the GaAs substrate and stop on an AlAs layer. The AlAs stop-etch layer is transformed into a layer that is almost twice as thick as the original layer, mismatched to the remaining GaAs epilayer, and has a refractive index around 2.0. Replacement of the single AlAs stop etch layer with multiple thin AlGaAs stop etch layers is proposed to alleviate this problem.

Metastability modeling of compliant substrate critical thickness using experimental strain relief data

A metastability model for GaAs compliant substrates is developed using the compliant substrate partitioning formula and experimental strain relief data. The developed model agrees with compliant substrate strain relief data deduced from double crystal x-ray diffraction and indicates that, for a set of growth conditions and compliant substrate thicknesses, layers of InGaAs of any thickness can be grown free of dislocations. The model developed in this letter is also compared to other compliant substrate critical thickness models, and the authors discuss the mechanisms of partitioning in mismatched layers grown on compliant substrates.

Properties of Hg0.7Cd0.3TeCdTe superlattices with semiconducting wells

We present a study of the electro-optical properties of semiconducting Hg1 − xCdxTeCdTe (0.28 ≤ x ≤ 0.30) superlattice (SL) structures by photo- and magneto-luminescence in the Voigt and Faraday configurations. The motivation for this study was to determine and assess the influence of coupling between SL wells on the electrical and emission properties of these structures. The energy bandgap of the SLs was observed to depend very strongly on barrier width. Both transport and magneto-luminescence measurements confirmed the anisotropy of the electron effective mass and identified an excitonic contribution to the PL emission. Studies of the PL integrated intensity on excitation intensity showed that excitonic corrections were required to adequately fit the luminescence data. Optical gains of 80 cm−1 were obtained for an excitation intensity of 100 kW/cm2 indicating that these SLs have the electro-optical properties required for making efficient mid-infrared detectors and laser diodes.

The growth of AlGaAs–InGaAs quantum-well structures by molecular beam epitaxy: Observation of critical interdependent effects

Critical interdependent effects have been observed in the growth of AlGaAs/InGaAs quantum-well structures by molecular beam epitaxy. It is shown that statistical experimental design is an effective method for quickly optimizing complex device structures. This technique is very useful for the optimization of processes with a large number of interdependent parameters, and allows for the clear visualization and separation of complex interwoven effects. In the present work, we show the importance of the oxide desorption process for the optimal growth of AlGaAs-containing structures.

Strain‐modulated epitaxy: Modification of growth kinetics via patterned, compliant substrates

Bonded, thin film compliant substrates can be used to reduce the strain in a lattice‐mismatched overlayer during epitaxial growth. We have presented an initial demonstration of the use of thin film GaAs compliant substrates fabricated by epitaxial liftoff or substrates removal and bonded to a mechanical host. This processing approach can be coupled with the patterning of the bonded surface to realize a lateral thickness variation. This thickness variation, in turn, can be used to realize a lateral strain variation in the growing mismatched overlayer. The strain can then be used to modify molecular beam epitaxy (MBE) growth kinetics, such as cation desorption and migration. With this technique the lateral control of composition and thickness can be realized without any surface topography. In this article, we discuss the growth of InGaAs films on compliant substrates produced by epitaxial lift‐off and substrate removal. In addition, we discuss the various extrinsic effects associated with the compliant subs...

Statistical experimental design for MBE process characterization

This paper presents a statistically designed experiment for systematic characterization of the molecular beam epitaxy (MBE) process to quantitatively describe the effects of process conditions on the qualities of grown films. This methodology is applied to a five-layer, undoped AlGaAs and InGaAs single quantum well structure grown on a GaAs substrate. Six input factors (time and temperature for oxide removal, substrate temperatures for AlGaAs and InGaAs layer growth, beam equivalent pressure of the As source and quantum well interrupt time) are examined by means of a Resolution IV, 2/sup 6-2/ fractional factorial design requiring sixteen trials. Several responses are characterized, including defect density, X-ray diffraction, and photoluminescence. Results indicate that the manipulation of each of the six factors over the ranges examined are statistically significant and lead to considerable variation in the responses. Following characterization, backpropagation neural networks are trained to model the process responses. The neural process models exhibit very good agreement with experimental results.