In this thesis we propose a new device structure-the graded injector-for short-wavelengthLEDs and laser diodes, and describe the growth, fabrication and characterizationof green LEDs based on this structure. We also discuss the first growthby MBE of Mg_xCd_(1-x)Se alloys, which are used for the graded injection region ofthe LEDs.
In spite of the important technological applications for short-wavelength LEDsand diode lasers, and in spite of over three decades of work on fabrication of lightemitters from II-VI compounds, no II-VI LEDs or laser diodes are commerciallyavailable, 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 aII-VI with a bandgap large enough to emit green or blue light.
In recent years modern crystal growth techniques have contributed to substantial progress on- and a revival of interest in- II-VI compounds. This progresshas occurred mainly on two fronts: metastable doping during low-temperaturegrowth; and heterojunction approaches made possible by the increased controlover the growth process afforded by new techniques such as MBE. We have pursuedthe latter approach, developing a novel heterojunction structure to avoiddifficult doping.
In Chapter 1 we discuss applications of short-wavelength light emitters and thehistory of II-VI light emitter research, including current work by other groups. Weexamine the role of heterojunctions in LEDs and diode lasers in Chapter 2, andgraphically display theoretical and experimental values for band offsets among avariety of semiconductors using "McCaldin Diagrams." These diagrams help toclarify the problem, allow us to draw general conclusions about what we can andcannot 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 widebandgaplight emitters, then discuss the growth, fabrication and characterizationof 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 theunfavorable band offset between CdSe and ZnTe.
II-VI MBE has been studied much less than III-V MBE, and many aspects ofthe growth are different due to differences in the material systems. Furthermore,there had been little previous research on II-VI compounds containing Mg, andnone using modern crystal growth techniques. Thus in Chapter 4 we discuss IIVIMBE and some of the difficulties encountered in growing graded-Mg_xCd_(1_x)Sedevices.
