Charles S. Tsai

Application of selective epitaxy to fabrication of nanometer scale wire and dot structures

The selective growth of nanometer scale GaAs wire and dot structures using metalorganic vapor phase epitaxy is demonstrated. Spectrally resolved cathodoluminescence images as well as spectra from single dots and wires are presented. A blue shifting of the GaAs peak is observed as the size scale of the wires and dots decreases.

Selective epitaxy of GaAs, AlxGa1−xAs, and InxGa1−xAs

Abstract Many device structures benefit from the ability to selectively deposit epitaxial materials. Through the use of a masking material, such as Si3N4 or SiO2, on the substrate surface, patterns generated through standard lithographic procedures can be used to define regions for selective deposition. Highly selective growth can be achieved through the use of growth precursors which contain halogens, such as (C2H5)2GaCl and (C2H5)2AlCl. These compounds decompose, most probably, to the volatile mono-halogen species, e.g. GaCl, and also generate HCl in the gas phase as a reaction by-product. We present experimental results on the morphology and growth behavior of GaAs, AlxGa1−xAs, and InxGa1−xAs using this selective epitaxy technique. Electri cal and optical characterization has been carried out on these materials and selectively grown structures produced by this technique. The interface between the selectively grown material and the underlying substrate was investigated and the conditions for achieving high quality electrical interfaces were determined. A thermodynamic model of this growth chemistry predicts the trends in composition and growth rate. The thermodynamic model, based on the quasi-equilibrium of the halogen-based compounds with the substrate surface, indicates that the growth behavior is very similar to the inorganic-based growth of these compounds. Experimental applications of this technique to high speed digital device structures and sub-micron dimensioned optical structures are presented.

Facet modulation selective epitaxy–a technique for quantum-well wire doublet fabrication

The technique of facet modulation selective epitaxy and its application to quantum-well wire doublet fabrication are described. Successful fabrication of wire doublets in the AlxGa1–xAs material system is achieved. The smallest wire fabricated has a crescent cross section less than 140 A thick and less than 1400 A wide. Backscattered electron images, transmission electron micrographs, cathodoluminescence spectra, and spectrally resolved cathodoluminescence images of the wire doublets are presented.

Lower-dimensional quantum structures by selective growth and gas-phase nucleation

There is increasing interest in the potential application of quantum dots and quantum wires to various solid state devices. In this article, the physics of lower-dimensional quantum structures will be reviewed with emphasis on applications. In addition, two fabrication approaches under investigation at Caltech will be discussed. The first is based on selective nucleation of III–V semiconductors on a patterned host substrate and the second is gas-phase nucleation of III–V clusters. Recent results on formation of nanocrystal GaAs and silicon clusters in the gas phase are presented.

Formation of Highly-Uniform and Densely-Packed Arrays of GaAs Dots by Selective Epitaxy

Formation of highly-uniform and densely-packed arrays of GaAs dots by selective epitaxy using diethylgallium-chloride and arsine is reported. The arrays of GaAs dots are imaged using atomic force microscopy (AFM). Accounting for the AFM tip radius of curvature, the smallest GaAs dots formed are 15-20 nm in base diameter and 8-10 nm in height with slow-growth crystal planes limiting individual dot growth. Completely selective GaAs growth within dielectric-mask openings at these small size-scales is also demonstrated. The uniformity of the dots within each array ranged from 6% for the larger dots to 16% for the smallest dots (normalized standard deviations of the areas of individual dots within each array).

Group III–V and Group IV Quantum Dot Synthesis

Semiconductor structures that exhibit quantum confinement effects in three dimensions have attracted considerable attention owing to their potential as tools for exploration of conceptually simple mesoscopic systems, and also because of their potential for new optoelectronic devices. In order to observe unique quantum dot transport and optical properties at room temperature, the characteristic dimensions of the carrier confining potentials and structures should be less than 10–20 nm. Although the electronic structure issues are quite different for group III–V semiconductors (prototypically GaAs/AlGaAs) than for group IV semiconductors (prototypically Si and Ge), growth of dense arrays of small (≤ 10 nm), uniformly-sized structures are important goals for both materials systems. In particular, there is a compelling need for development of synthesis techniques capable of making denselypacked,uniformly-sized structures which are less than 10–15 nm in size, over large areas.

Quantum wire and quantum dot semiconductor lasers

There is currently great interest in fabrication of structures that are two and three dimensional analogs of the conventional quantum well. We review here the physics behind the use of arrays of such lower dimensional structures in semiconductor laser active layers. Methods which are currently under investigation for producing such structures will be discussed.