Materials for Heterojunction Devices

Abstract

Heterojunction devices are playing an increasingly important role in opto­ electronics. They are produced by combining semiconductors that have differing bandgap energies but closely matching lattice parameters. These devices are epitaxially formed on single-crystal substrates. Laser diodes ( 1) and photodetectors are the most important optoelectronic hetero­ structures, although other types of devices, such as transistors, can also be fabricated. The use of heterostructures has made possible dramatic improvements in 111-V compound laser diode performance. However, the addition of heterojunctions to optical detectors has had a lesser impact on their performance. Therefore, this review concentrates mostly on material considerations bearing on laser diode fabrication. The 111-V compound heterojunction lasers have been extensively developed in the past decade starting with the single-heterojunction AIGaAs/GaAs close confinement laser diode (2,3). Since then, structures have been developed with up to four heterojunctions and other III-V and IV-VI materials have been used for heterojunction laser fabrication. The addition of heterojunctions to lasers of the GaAs alloy family has led to a large reduction in the threshold current density at room tempera­ ture, thus making continuous wave operation possible. This development led to practical light sources for high data rate, optical fiber communication. Semiconductor lasers can emit in a spectral range extending from the far infrared (A � 33 11m) to the visible (A � 0.5 11m). However, because some interesting direct bandgap semiconductors cannot be doped both nand p-type, only part of the range is attainable with p-n junction injection devices. For example, the wide-bandgap II-VI compounds CdS and ZnO cannot be doped p-type, and therefore the production of the