Novel devices employing epitaxial wide bandgap semiconductors : physics, electronics and materials characterization

Citation

Bandić, Zvonimir Z. (2000) Novel devices employing epitaxial wide bandgap semiconductors : physics, electronics and materials characterization. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/cnyb-cq58. https://resolver.caltech.edu/CaltechTHESIS:10052010-115649444

Abstract

This thesis describes the developments of novel semiconductor devices based on epitaxial wide bandgap semiconductors GaN and ZnS. The number of interesting and exciting results in physics, electronics and materials science of these systems were found in studies motivated by these devices. This thesis consists of three major topics, structural characterization and kinetic growth modeling of the GaNAs/GaAs superlattices, structural and optical characterization and solid phase recrystallization of ZnS thin films grown on GaN and sapphire substrates, and design and fabrication of GaN high power devices as well as measurement of fundamental electronic properties of GaN, such as minority carrier diffusion lengths and lifetimes and critical field for electric breakdown. The set of GaNAs/GaAs superlattices grown by molecular beam epitaxy was analyzed by high resolution X-ray diffraction and cross—sectional transmission electron microscopy. The nitrogen incorporation and GaNAs/GaAs interface sharpness were experimentally found to strongly depend on growth temperature. The activation energies for nitrogen desorption and nitrogen to arsenic segregation were found through simple kinetic model, which is in fine agreement with experimentally obtained results. These fundamental studies provide important insights into growth of GaN on GaAs substrates, which is of significant practical importance for all electronic GaN devices. Zinc sulfide/Gallium nitride heterostructures are potentially interesting system for light emitters in blue and green part of visible spectrum, with DC low power consumption electroluminescent displays being one attractive application of these diodes. Zinc sulfide thin films grown on GaN (0001), GaAs (001) and sapphire (0001) substrates by MBE were characterized by variable temperature photoluminescence and high resolution X-ray diffraction. The structural properties of the films suffered from the large lattice mismatch between ZnS and various substrates which were used. The optical properties of the ZnS films were found to be in direct correlation with structural properties of the films. The ZnS films doped with Al and Ag grown on n and p-type GaN, and sapphire were characterized by low temperature photoluminescence and displayed bright blue luminescence. Fabricated N-ZnS/p-GaN heterostructures were characterized by current-voltage and electroluminescence. Electroluminescence was found to be centered around 390 nm, corresponding to high energy silver band, and it shifted to higher energies with increase in device voltage. Since as grown films suffered from crystalline imperfections, the ZnS thin films on sapphire were recrystallized, by annealing at temperatures above 900 °C at sulfur overpressure of 10 atm. The structural properties of samples significantly improved, indicating more than 10-fold reduction in tilting and excellent crystallinity. The role of sulfur was discussed, and it was found that sulfur is important in preventing film evaporation, increasing boundary migration and providing compliancy to sapphire substrate. The minority carrier diffusion lengths and lifetimes were measured for electrons and holes in unintentionally doped, n and p-type GaN samples grown by several different growth techniques. The experimentally observed diffusion lengths were in the 0.2 — 0.3 µm range for Metal-Organic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxy (MBE) grown samples, and 1 — 2 µm in the case of Halide Vapor Phase Epitaxy (HVPE) grown sample. In the case of MOCVD grown samples, the hole lifetime was estimated to approximately 7 ns, and electron lifetime to approximately 0.1 ns. The same samples were structurally characterized by AFM, and the size of the defect-free regions surrounded by linear dislocations is found to be of the order of measured diffusion length, in qualitative agreement with minority carrier recombination at linear dislocations. A simple model is presented which explains an increase in minority carrier lifetime and diffusion length with a decrease in the dislocation density or increase in the size of defect-free grains. A model which explains why linear dislocations might act as recombination sites is also presented. The important advantage of nitrides and other wide band gap materials for high power devices is a smaller standoff layer thickness for the same standoff voltage, giving smaller ON-state voltage and resistance, smaller power dissipation and larger maximum current density, allowing physically smaller devices for the same power rating. The design rules for nitride based Schottky rectifiers and thyristors are presented. The critical field for electric breakdown and minority carrier recombination lifetimes are found to be important design parameters. Using modeling parameters which are well in the range currently available with GaN, and measured from fabricated devices, design results indicate the possibility of 18 µm thick GaN Schottky rectifiers and 12 µm thick A1GaN thyristors supporting 5 kV standoff voltage. The critical field for electric breakdown was found to be 5 MV/cm from the theoretical studies. The maximum current density for 5 kV thyristors is in the 200 — 400 A/cm^2 range depending on the hole lifetime, and is limited by thermal breakdown. The maximum operating frequency of 5 kV thyristors is in the 1-2 MHz range and also depends on the hole lifetime. Two-terminal GaN Schottky rectifiers were fabricated. The Schottky rectifiers were fabricated on thick GaN layers grown by HVPE and had a standoff voltages in the 450 V to 750 V range, depending on the thickness of the GaN film and contact geometry. Best devices were characterized with reverse current density of 10^(-5)A/cm^2 at reverse bias of 100 V, and 4.2 V ON-state voltage at a forward current density of 100 A/cm^2. Various contact geometries were investigated. It was found that mesa geometry improves ON-state voltage, but causes increase in reverse current density, while that metal field plate geometry significantly reduces reverse current density. The measured critical field for electric breakdown in GaN was found to be (2.5 ± 0.5) MV/cm and it approaches the theoretical estimate of 5 MV/cm. The measured values of critical field are only a lower limit since the reverse breakdown voltage was limited by premature corner and edge breakdown.

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