Optimization of GaN Channel Conductivity in AlGaN/GaN HFET Structures Grown by MOVPE
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E11.11.1
Optimization of GaN Channel Conductivity in AlGaN/GaN HFET Structures Grown by MOVPE S.M. Hubbard, G. Zhao, D. Pavlidis, E. Cho, W. Sutton EECS Department, The University of Michigan Ann Arbor, MI 48109-2122, USA and Darmstadt University of Technology, Department of High Frequency Electronics Merckstrasse 25, 64283 Darmstadt, Germany E-mail: [email protected], [email protected] or [email protected] ABSTRACT Optimization of GaN channel conductivity in AlGaN/GaN Heterojunction Field Effect Transistor (HFET) structures was performed using High Resistivity (HR) GaN templates grown by Metal-organic Vapor Phase Epitaxy (MOVPE). The GaN sheet resistance was tuned using final nucleation layer (NL) annealing temperature. Using an annealing temperature of 1033°C, GaN with sheet resistance of 1010 Ω/sq was achieved, comparable to that of Fe-doped GaN. XRay Diffraction (XRD) and Photoluminescence (PL) analysis show that the high resistance GaN is achieved due to compensating acceptor levels introduced through edge-type threading dislocations. XRD analysis also shows optimization of annealing temperature provided a means to maximize GaN sheet resistance without significantly degrading material quality. AlGaN/GaN HFET layers grown using HR GaN templates gave surface and interface roughness of 14 and 7 Å, respectively. The 2DEG Hall mobility and sheet charge of HFETs grown using HR GaN templates was comparable to similar layers grown using unintentionally doped (UID) GaN templates. INTRODUCTION HR GaN templates are important for electronic devices in order to ensure proper drainsource current saturation, complete channel pinch-off, low loss at high frequencies, and low cross-talk between adjacent devices. The use of such templates allows better optimization of IIINitride based HFET devices for both high-frequency and high-power applications. However, it is well known that GaN exhibits high unintentional n-type doping levels due to residual oxygen donors introduced during heteroepitaxy. This high level of background n-doping is detrimental to both electronic device properties and effective doping control in optical devices. Due to the high density of residual donors, semi-insulating GaN is typically grown by compensating the donors with acceptor states. One method of compensation involves tuning the growth conditions in order to self-compensate the material with either higher threading dislocation density or other defects[1,2]. Tuning the growth conditions does not rely on any additional MO sources (such as Fe or other heave metals) and therefore does not suffer from reactor contamination effects[3]. We have found that by adjusting only the final nucleation layer (NL) annealing temperature, we can achieve high levels of GaN thin film resistivity without significantly degrading the material quality. Both XRD and PL were used in this work to further understand the relation of the compensation mechanisms to growth parameters such as NL layer annealing. The use of in situ laser reflectance allowed us to correlate
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