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Non-polar surfaces of HVPE grown GaN were characterized by cathodoluminescence (CL), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). Both of a- and m-plane GaN were prepared by growing thick GaN along the c-axis, and cutting in transverse orientations. The exposed non-polar surfaces were prepared by mechanical polishing (MP) and chemically mechanical polishing (CMP). Non-uniform luminescent characteristics on a- and m-plane GaN were observed in CL images, indicating a higher concentration of impurities in the area of more luminescence. CL spectra from the bulk samples revealed two peaks: 364 nm and 510 nm, related to band edge and impurity defects respectively. The detection by SIMS confirmed that oxygen was inhomogeneously incorporated during the growth of thick GaN layers. Surface qualities of a- and m-plane GaN were also investigated. The lower optical intensities from a-plane GaN at low acceleration voltages indicated more surface damages were introduced during polish. The optical intensity difference from the two samples was reduced at higher acceleration voltages. Similar CL intensities at low acceleration voltages from a- and m-plane GaN substrates prepared by CMP indicated improved surface qualities.
GaN has a wide band gap energy, high electron mobility, high saturation velocity, and excellent thermal properties making it a promising material for high power and high frequency electronic devices. The development of enhancement mode GaN metal oxide semiconductor (MOS) transistors has been elusive due to the non-availability of a good insulating gate dielectric and the difficulty in forming of ohmic source/drain regions. Ion-implantation of dopants causes severe lattice damage requiring a high temperature post-implant anneal and has not been a successful method to obtain acceptable low-resistance source/drain regions. At the same time, gate dielectrics for most compound semiconductors, in addition to difficulties in minimizing the density of interface states which pin the Fermi level by inducing trap levels in the midgap, are degraded by instabilities as a result of high temperature annealing. The paper presents the development ohmic source/drain contacts for GaN MOSFETs by selected area epitaxial regrowth. Re-growth of GaN on patterned substrates by metal-organic chemical vapor deposition (MOCVD) employs a growth regime to decrease the enhanced growth rates and island formation that result from the diffusion of precursors to the selected area. The enhanced growth rate is 4.5μm/hr compared to 0.5μm/hr of the as-grown GaN on the unpatterned substrate. The enhanced growth rate also results in heavily porous GaN. Selected area growth, device processing, the material and device characterization results will be presented. In particular the selected area growth of doped contacts in the 800°C temperature range leads to superior morphology and contact resistance as compared to similar contacts grown at 1060°C. The contact resistivity of the n+ re-growth region measured was ∼2×10-4 Ω-m and the morphology of the re-grown region was comparable to the as-grown GaN with an RMS roughness ∼1.4nm. The success of fabricating low temperature contacts for GaN enhancement mode MOS transistors is a critical step in fabricating these devices opening new applications.
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