Column III nitride growth, characterization and devices

Abstract

This thesis describes a number of advancements in the III-V nitride material system achieved at Caltech. Major improvements in optimized growth of III-V nitride materials by radio frequency molecular beam epitaxy are presented. The first GaN based MEMs devices are reported, and their fabrication and characterization are detailed. The growth of InN is discussed, and the morphology and crystal quality are presented. In addition, the switching behavior of the first GaN thyristor is reported. The surface chemistry and band offsets of the nitrides are also investigated using x-ray photoelectron spectroscopy. These investigations are presented in two parts. Part I describes the growth and characterization of III-V nitride materials and includes a discussion of piezoelectric effects in this material system. Part II describes the device related successes mentioned above. The focus of the second chapter is the description and calculation of piezoelectric phenomena in nitride heterojunctions. The magnitude of the piezoelectric effects in nitride materials is such that entirely new device concepts can be developed. On the other hand, traditional devices are impacted by these polarization fields, and proper accounting for them must be made for device design. Some of these effects are calculated and new devices employing these fields proposed. The major issues pertaining to MBE growth are discussed in the third chapter. Substrates, nitridation, film nucleation, and the dependence of film quality on growth parameters are investigated by means of AFM, cathodoluminescence, x-ray diffraction, and reflection high energy electron diffraction. It is found that the buffer layer deposition parameters and V/III ratio during subsequent growth are the major factors in determining the resulting films crystal quality. In addition to growth, x-ray photoelectron spectroscopy has been used to investigate the surface chemistry of column-III nitrides. It was found that the oxidation of GaN was self limiting and did not proceed significantly farther than the first mono layer. Many of the core level and valence band maximum positions were determined, as well as the valence band offsets for the AIN/AlGaN system. Part II describes characterization of nitride devices. The switching behavior of a GaN thyristor is described and possible explanations for its behavior investigated. GaN pri and pin diodes are characterized electrically and via electroluminescence. It is found that less than ideal electrical properties of these devices may be related to a high level of deep trap states that produce high levels of leakage current as well as suppress the light emission of these diodes. Finally, a new method of preferentially etching n type GaN is presented. This technique is extended to the fabrication of GaN MEMs devices, arid piezoelectric strain sensors and micropumps fabricated with this technique are characterized.