Structure and Properties of Epitaxial Dielectrics on GaN

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Title: Structure and Properties of Epitaxial Dielectrics on GaN
Author: Wheeler, Virginia D.
Advisors: Dr. Douglas Irving, Committee Member
Dr. Doug Barlage, Committee Member
Dr. Mark Johnson, Committee Chair
Dr. Jon-Paul Maria, Committee Member
Dr. Zlatko Sitar, Committee Member
Abstract: GaN is recognized as a possible material for metal oxide semiconductor field effect transistors (MOSFETs) used in high temperature, high power and high speed electronic applications. However, high gate leakage and low device breakdown voltages limit their use in these applications. The use of high-κ dielectrics, which have both a high permittivity (ε) and high band gap energy (Eg), can reduce the leakage current density that adversely affects MOS devices. La2O3 and Sc2O3 are rare earth oxides with a large Eg (6.18 eV and 6.3 eV respectively) and a relatively high ε (27 and 14.1 respectively), which make them good candidates for enhancing MOSFET performance. Epitaxial growth of oxides is a possible approach to reducing leakage current and Fermi level pinning related to a high density of interface states for dielectrics on compound semiconductors. In this work, La2O3 and Sc2O3 were characterized structurally and electronically as potential epitaxial gate dielectrics for use in GaN based MOSFETs. GaN surface treatments were examined as a means for additional interface passivation and influencing subsequent oxide formation. Potassium persulfate (K2(SO4)2) and potassium hydroxide (KOH) were explored as a way to achieve improved passivation and desired surface termination for GaN films deposited on sapphire substrates by metal organic chemical vapor deposition (MOCVD). X-ray photoelectron spectroscopy (XPS) showed that KOH left a nitrogen-rich interface, while K2(SO4)2 left a gallium-rich interface, which provides a way to control surface oxide formation. K2(SO4)2 exhibited a shift in the O1s peak indicating the formation of a gallium-rich GaOx at the surface with decreased carbon contaminants. GaOx acts as a passivating layer prior to dielectric deposition, which resulted in an order of magnitude reduction in leakage current, a reduced hysteresis window, and an overall improvement in device performance. Furthermore, K2(SO4)2 resulted in an additional 0.4 eV of upward band bending at the surface, which should be considered when determining heterojunction band offsets with GaN. Epitaxial La2O3 and Sc2O3 were successfully deposited on GaN by molecular beam epitaxy (MBE). Sc2O3 exhibited a cubic bixbyite crystal structure, while La2O3 had a mix of both cubic and hexagonal crystal structures. A highly defective structure was observed for La2O3, compared to Sc2O3, resulting from its larger mismatch with GaN (14.5% and 8.9%, respectively). TEM images indicated an abrupt atomic interface for Sc2O3 films, but an interfacial layer was observed for La2O3 on GaN. Additionally, La2O3 was shown to be extremely reactive with water and carbon dioxide in air, forming both hydroxides and carbonates within 15 minutes of exposure. Therefore, tantalum and silicon were investigated as in-situ capping metals to prevent these deleterious effects. XPS was utilized to determine a valence band offset (VBO) and conduction band offset of 1.9 ± 0.1 eV and 0.9 ± 0.1 eV for La2O3 on GaN. Similarly, Sc2O3 had a VBO and CBO of 0.8 ± 0.1 eV and 2.1 ± 0.1 eV, respectively. Both oxides exhibited sufficient band offsets to prevent thermionic emission of carriers, even at high operation temperatures, making them good candidates for insulator layers in high temperature, high power applications. Preliminary C-V curves, for La2O3 and Sc2O3 MOS capacitors, showed large charge accumulation layers, extremely high permittivity values, and low hysteresis windows indicative of low density of interface traps and fixed oxide charges. I-V curves showed a reduction in leakage current density for both oxides compared to Si3N4, a readily used gate dielectric for GaN devices. The larger reduction achieved with La2O3 films is attributed to a passivating interfacial layer that minimizes the amount of dislocations propagating into the oxide. These preliminary results point to the viability of these gate oxides in GaN MOSFET devices.
Date: 2010-08-03
Degree: PhD
Discipline: Materials Science and Engineering

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