Graded Injection: A New Approach to Wide-Bandgap Light Emitters

Citation

Phillips, Mark C. (1993) Graded Injection: A New Approach to Wide-Bandgap Light Emitters. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/nsaj-9v42. https://resolver.caltech.edu/CaltechTHESIS:01022013-131949945

Abstract

In this thesis we propose a new device structure-the graded injector-for short-wavelength LEDs and laser diodes, and describe the growth, fabrication and characterization of green LEDs based on this structure. We also discuss the first growth by MBE of Mg_xCd_(1-x)Se alloys, which are used for the graded injection region of the LEDs.

In spite of the important technological applications for short-wavelength LEDs and diode lasers, and in spite of over three decades of work on fabrication of light emitters from II-VI compounds, no II-VI LEDs or laser diodes are commercially available, and the demand for short-wavelength devices remains largely unfilled. This is due primarily to the difficulty of obtaining both p- and n-type doping in a II-VI with a bandgap large enough to emit green or blue light.

In recent years modern crystal growth techniques have contributed to substant ial progress on- and a revival of interest in- II-VI compounds. This progress has occurred mainly on two fronts: metastable doping during low-temperature growth; and heterojunction approaches made possible by the increased control over the growth process afforded by new techniques such as MBE. We have pursued the latter approach, developing a novel heterojunction structure to avoid difficult doping.

In Chapter 1 we discuss applications of short-wavelength light emitters and the history of II-VI light emitter research, including current work by other groups. We examine the role of heterojunctions in LEDs and diode lasers in Chapter 2, and graphically display theoretical and experimental values for band offsets among a variety of semiconductors using "McCaldin Diagrams." These diagrams help to clarify the problem, allow us to draw general conclusions about what we can and cannot hope to do, and lead to the device proposal of Chapter 3.

In Chapter 3 we first propose a new kind of heterojunction structure for widebandgap light emitters, then discuss the growth, fabrication and characterization of devices based on this proposal. The experiments demonstrate that the graded Mg_xCd_(1_ x)Se structure allows injection of electrons into p-type ZnTe in spite of the unfavorable band offset between CdSe and ZnTe.

II-VI MBE has been studied much less than III-V MBE, and many aspects of the growth are different due to differences in the material systems. Furthermore, there had been little previous research on II-VI compounds containing Mg, and none using modern crystal growth techniques. Thus in Chapter 4 we discuss IIVI MBE and some of the difficulties encountered in growing graded-Mg_xCd_(1_x)Se devices.

Thesis Files