GAAs High-Speed Devices: Physics, Technology, and Circuit Applications

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GALLIUM ARSENIDE CRYSTAL STRUCTURE AND GROWTH

2.1 INTRODUCTION

The rapidly growing use of gallium arsenide—in discrete devices and integrated circuits for microwave, millimeter-wave, optoelectronic, and digital applications—is creating a huge demand for single-crystal substrates of this material. The attainment of low-cost, high-speed GaAs digital IC technology is critically dependent on the availability of large area (4-in.) wafers that exhibit high resistivity (> 10⁸ ohm-cm) and low crystal defects and etch-pit densities. In this chapter we first describe the crystal and band structure, the transport properties of the crystal, and then address some of the different methods used to synthesize and grow bulk crystals, focusing on growth from stoichiometric melts in vertical and horizontal configurations. Crystal defects, impurities, and etch-pit densities will also be discussed.

2.2 CRYSTAL STRUCTURE

A crystal is an object of high symmetry. This symmetry can be used to obtain general information about the properties of crystals without complicated algebra. An ideally perfect single crystal consists in an infinite three-dimensional repetition of an identical building block. This building block, called a basis, can be an atom, a molecule, or a cluster of atoms or molecules [Ash76]. The basis is the quantity of matter contained in the unit cell, whose spatial volume in the shape of a three-dimensional parallelepiped may be translated discrete distances in three dimensions to fill ...