Epitaxy: The key to high-performance semiconductor devices

Abstract

Epitaxy is defined as "the overgrowth in layers of a crystalline substance deposited in a definite orientation on a crystalline substratum." The application of this process to the controlled fabrication of semiconductor materials has led to a wide variety of important applications, from the ubiquitous light-emitting diode (LED) to ultrahigh-speed transistors (e.g., pseudomorphic high electron mobility transistors-pHEMTs) to high-density, compact semiconductor laser arrays (e.g., vertical cavity surface emitting lasers-VCSELs). The power of epitaxy brings new applications to the forefront of electronics and photonics technology each year. In the two articles presented here, we address in some detail the growth of compound semiconductor materials by metal-organic chemical vapor deposition (MOCVD) and by molecular-beam epitaxy (MBE). These two approaches represent the state-of-the-art in fabricating highly complex, high-performance homoand heterostructure materials. While each technique has their respective proponents, these methods should be viewed as complementary, wherein the advantages of a given approach should be weighed in view of the demands of the application. Bandgap engineering, a phrase coined several years ago by Federico Capasso of the AT&T Bell Laboratories, is appropriate for the application of epitaxial crystal-growth techniques. The tremendous flexibilityintheMOCVDandMBEmethods permits control of chemical composition, impurity doping, or layer growth down to a single atomic layer. Thus, the semiconductor bandgap may be engineered for optical response (absorption or transmission), to achieve quantum confinement with heterostructures, to create graded compositions, and so on. These capabilities provide the device designer with unprecedented flexibility in selecting and optimizing device properties and performance. Many applications of compound semiconductor heterostructures are very demanding of the physical, optical, and electronic properties of the host materi-