Bijan Tadayon

Extremely low specific contact resistivities for p‐type GaSb, grown by molecular beam epitaxy

We have investigated different metal contacts (Cr/Au, Ti/Pt/Au, and Au) on p‐type GaSb, grown by the molecular beam epitaxy. For Au contacts, specific contact resistivities in the range of 1.4×10−8–7.8×10−8 Ω cm2 have been obtained. These are the lowest values ever reported for p‐type GaSb. A simple procedure for surface preparation is also reported.

Growth of GaAs‐Al‐GaAs by migration‐enhanced epitaxy

The GaAs‐Al‐GaAs structure is grown using migration‐enhanced epitaxy (MEE) method at low temperature on a molecular beam epitaxy machine. With MEE the interdiffusion between Al and GaAs is reduced by a large amount, and the morphology is improved by a large degree. Still, Raman spectrum indicates poor crystallinity for the GaAs of the top layer. The effect of different annealing temperatures on the interdiffusion is also studied.

Electrical characterization of low temperature GaAs layers, and observation of the extremely large carrier concentrations in undoped material

We present here the results of a study on the effect of substrate temperature Ts on the electrical and physical characteristics of low temperature molecular‐beam epitaxy GaAs layers. Based on the x‐ray results, three temperature ranges have been defined for Ts: 1) high range (Ts≥ 460 °C), 2) intermediate range (260 ≤ Ts ≤ 450 °C), and 3) low range (the amorphous range) (Ts ≤ 250 °C). Hall measurements have been performed on the as‐grown samples and on samples annealed at 610 and 850 °C. Si implantation into these layers has also been investigated. From an electrical stand point, the most striking difference between the low range and the intermediate range is the fact that after annealing at 850 °C, the undoped layers grown below or at 250 °C have a low resistivity (net electron concentrations as high as 1.5×1018 cm−3 and mobilities as high as 920 cm2 V−1 s−1), while after anneal the undoped layers grown in the intermediate range have extremely high resistivity (about 8×105 Ω cm).

Reduction of Be diffusion in GaAs by migration‐enhanced epitaxy

Be‐doped GaAs layers were grown by the migration‐enhanced epitaxy (MEE) method at 300 °C. The MEE layers showed practically no electrical activation. Rapid thermal annealing on the MEE layers resulted in mobility and hole concentration comparable to those of conventional molecular beam epitaxy (MBE) layers grown at 600 °C. Secondary‐ion mass spectroscopy showed that the Be diffusion in annealed MEE layers was much smaller than that in conventional MBE layers, especially for highly doped layers. Raman spectroscopy and 4 K photoluminescence were also performed. The MEE method can replace the conventional MBE method for device applications which require high hole concentration with small diffusion.

Increase of electrical activation and mobility of Si‐doped GaAs, grown at low substrate temperatures, by the migration‐enhanced epitaxy method

Si‐doped GaAs layers were grown by the migration‐enhanced epitaxy (MEE) method and by the conventional molecular‐beam epitaxy (MBE) method, for the substrate temperatures between 220 and 670 °C. For the layers grown below 400 °C, the Si activation and mobility of the MEE layers are significantly higher than those of the MBE layers. For substrate temperatures above 400 °C, the MEE and MBE layers have roughly similar Si activation and mobility. The Raman and 4‐K photoluminescence spectra of the layers are consistent with the measured electron concentrations. This work suggests that for Si doping in GaAs at low substrate temperatures (below 400 °C), the MEE method is a very desirable alternative to the conventional MBE method.

Approximate Z-number Evaluation Based on Categorical Sets of Probability Distributions

In this chapter, we present a method for approximate evaluation of Zadeh’s Z-numbers using category sets of probability distributions corresponding to similar certainty measures.

Extremely high peak specific transconductance AlGaAs/GaAs heterojunction bipolar transistors

Self-aligned AlGaAs/GAs heterojunction bipolar transistors with peak specific transconductances as high as 25 mS/ mu m/sup 2/ of emitter area are discussed. These are the highest specific transconductances ever reported for a bipolar transistor. These devices, which contain no indium in the emitter, display specific parasitic emitter resistances of less than 1*10/sup -7/ Omega -cm/sup 2/. This low parasitic resistance is attributed to an improved n-type contact technology, in which a molybdenum diffusion barrier and a plasma-enhanced chemical vapor deposition SiO/sub 2/ overlayer are used to achieve low specific contact resistivities.<<ETX>>

A novel method for the growth of good quality GaAs at extremely low substrate temperatures (as low as 120 °C)

In this work, a novel method for the growth of good quality GaAs at extremely low substrate temperatures (as low as 120 °C) is introduced. This novel method is based on three different methods: The migration‐enhanced epitaxy method, the indium doping method, and the low growth rate method. The good quality of the GaAs layers are confirmed by Raman spectroscopy, 4 K photoluminescence, and transmission electron microscopy. This novel growth method results in the best GaAs material ever grown at 120 °C by any growth method using a molecular beam epitaxy machine.

Preprocessing Method for Support Vector Machines Based on Center of Mass

We present an iterative preprocessing approach for training a support vector machine for a large dataset, based on balancing the center of mass of input data within a variable margin about the hyperplane. At each iteration, the input data is projected on the hyperplane, and the imbalance of the center of mass for different classes within a variable margin is used to update the direction of the hyperplane within the feature space. The approach provides an estimate for the margin and the regularization constant. In the case of fuzzy membership of the data, the membership function of the input data is used to determine center of mass and to count data points which violate the margin. An extension of this approach to non-linear SVM is suggested based on dimension estimation of the feature space represented via a set of orthonormal feature vectors.

Electrical Characterization of Low Temperature GaAs Layers, and Observation of the Extremely Large Carrier Concentrations in Undoped Material

We present here the results of a study on the effect of substrate temperature, T s , on the electrical and physical characteristics of low temperature (LT) molecular beam epitaxy GaAs layers. Hall measurements have been performed on the asgrown samples and on samples annealed at 610 °C and 850 °C. Si implantation into these layers has also been investigated.