1 - Heterostructure materials

Summary

It is as easy to count atomies, as to resolve the propositions of a lover.

As You Like It, William Shakespeare

Introduction

Modern growth technologies have made possible the growth of new semiconductor devices with unprecedented control on the atomic level. In this chapter we shall briefly introduce the molecular beam epitaxy (MBE) growth technique and discuss its application to the growth of materials (alloys, pseudomorphic, modulation doped) for new device structures. The chapter will conclude with a review of the cubic crystal structure and its reciprocal lattice, as these concepts are used extensively in Chapters 2 and 3.

MBE technology

One of the most versatile growth techniques available for research is the MBE. In this growth technique a semiconductor substrate is placed in a high-vacuum chamber (see Figure 1.1). Different components such as Ga, Al, As, In, P, and Si are heated in separate closed cylindrical cells. These components escape through an opening in the cylindrical cell and form a molecular beam. These beams are directed toward the substrate. A shutter positioned in front of each cell is used to select the desired molecular beams. By selecting a low temperature for the substrate growth and a slow growth rate (a few micrometers per hour), it is possible to grow high-quality crystals, while making abrupt changes in doping and crystal composition.

This growth technique can be used to grow semiconductor alloys such as Al_xGa_1–xAs, In_xGa_1–xAs, In_xAl_1–xAs, and Si_1–xGe_x, where x, the mole fraction, specifies the composition of the alloy.