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A heteroepitaxial GaN-on-sapphire template growth strategy is presented that eliminates wafer bow in the final structure at both room temperature and at high crystal growth temperatures. The technology is demonstrated here on 2″ and 4″ substrate sizes with GaN epitaxy thicknesses up to 150 µm with a sub-nm, epi-ready, final surface roughness. Wafer bow vs. temperature measurements for (a) 15 µm of GaN on 4″ 650 µm thick sapphire showing 280 µm bow shift, (b) 10 µm GaN film on 2″ 430µm thick sapphire showing 120 µm bow shift, and (c) and (d), 15 µm of FLAAT GaN on 2″ and 4″ sapphire, respectively, each showing less than 30 µm of bow shift. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the N- and Ga-polar faces of bulk GaN substrates with photoluminescence (PL) and the surface photovoltage (SPV) technique using a Kelvin probe attached to an optical cryostat. Experiments were conducted in vacuum. Some of the surfaces were mechanically polished (MP), while others were epi-ready after a chemical-mechanical polish (CMP). From the SPV measurements, the band bending in a sample having both surfaces treated with the CMP method was calculated to be about 0.83 and 0.70 eV for the Ga- and N-polar surfaces, respectively. The restoration of the SPV after ceasing the UV illumination showed that the SPV from CMP-treated surfaces behaved as predicted by a thermionic model, whereas the SPV from MP-treated surfaces restored with a much faster-than-predicted rate. This result can be interpreted by the hopping of charge carriers in the highly-defective near-surface layer of the MP-treated samples. Remarkably, removing the top 700 nm defective layer by dry etching restored the quality of the electrical and optical properties of GaN. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical properties, Fe concentration, and deep centers in semi-insulating Fe-doped GaN substrates grown by hydride vapor phase epitaxy (HVPE) were characterized by temperature-dependent Hall-effect measurements, secondary ion mass spectroscopy, and thermally stimulated current (TSC) spectroscopy. Five adjacent samples from a low-[Fe] wafer displayed very high resistivity, dominated by a center at 0.94 eV. At least six traps were observed in the samples by TSC, with trap B (0.56-0.60 eV) being dominant. A metastable trap A1 at ∼0.82 eV appeared after white-light illumination at 300 K. A sample from a high-[Fe] wafer displayed a lower resistivity, dominated by a center at 0.58 eV. The largest TSC peak in this sample was trap A1, although trap B also appeared. These TSC traps are compared with deep-level-transient-spectroscopy traps reported in conductive epitaxial and bulk HVPE-GaN. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonpolar a- and m-plane bulk GaN sliced from boules : structural and optical characteristics

Vertical Schottky rectifiers were fabricated on two GaN wafers synthesised by hydride vapor phase epitaxy (HVPE) process: n- epilayer on n+ bulk and n- bulk. Full backside ohmic contact and circular Schottky contacts were deposited on the N face and Ga face of the substrate, respectively. The devices showed excellent electrical characteristics. The device fabricated on n- epilayer on n+ bulk substrate showed a breakdown voltage of 348 V, a specific on-state resistance of 4.2 mΩ·cm2 and a leakage current density of 9.2×10-6 A/cm2 at -90 V. The device fabricated on n- bulk substrate showed a breakdown voltage of 600 V, a specific on-state resistance of 1.3 mΩ·cm2 and a leakage current density of 8.1×10-8 A/cm2 at -100 V. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the development of non-polar light emitting diodes (LEDs) covering the emission spectra from 480 to 520 nm (dominant wavelength), i.e. from cyan to the green region. The devices are obtained via GaInN-based homoepitaxy on non-polar m -plane GaN bulk substrate. Owing to the absence of piezoelectric polarization, these LEDs exhibit stable emission color with a wavelength shift less than 3 nm when the drive current density is changed from 0.1–30 A/cm2. However, we observe a decreasing electroluminescence efficiency as the emission wavelengths increases from cyan to green. We tentatively attribute this to a higher density of defects in the longer wavelength structures. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

M-plane non-polar bulk GaN substrates were regrown by hydride vapour phase epitaxy (HVPE), aiming to enlarge their lateral size. The non-polar GaN substrates were synthesized by growing thick GaN boules by HVPE along the c-axis, and wafering transversely to expose non-polar (m- or a-plane) surfaces. Non-polar GaN substrates obtained in this manner contains lower defect density comparing to those produced by heteroepitaxial growth. After the regrowth, substantial lateral expansion in [0001], [1120] and [1120] was observed. Threading dislocation densities were estimated by cathodoluminescence measured at the cross-section areas. The dislocation densities in the substrate and the regrown areas were approximately 5×106 cm–2 and 1×106 cm–2, respectively, indicating a comparable structural quality achieved by the regrowth. Non-polar GaN substrates prepared by this process provide a promising method to fabricate devices for next generation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the growth of Al0.25Ga0.75N/GaN heterostructures on low dislocation density semi-insulating c-axis GaN substrates. Room temperature Hall mobilities up to 1805 cm2/Vs at sheet carrier densities of 0.77x1013 cm–2 have been measured. By varying the GaN buffer layer thickness in these homoepitaxially-grown Al0.25Ga0.75N/GaN heterostructures, we observed a buffer-induced modulation of the room temperature 2DEG sheet carrier densities and Hall mobilities. The increase in sheet carrier density and corresponding decrease in mobility as the GaN buffer layer thickness is reduced below 0.75 μm is related to the presence of Si impurities at the bulk GaN substrate/epitaxial interface. Capacitance-voltage measurements and SIMS analysis confirm the presence of Si impurities at the surface prior to and after epitaxial growth. The factor of 2 reduction in the room temperature mobility is consistent with a predicted theoretical mobility reduction based on intersubband scattering. We have also been able to separate the contributions to the 2DEG carrier density from the ionized donors and the polarization field; the magnitude of each is ∼5x1012 cm–2. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polar and nonpolar bulk GaN substrates with low defect density and high structural and optical quality are demonstrated. The effect of doping by silicon, oxygen and iron within moderate doping levels on the properties of the polar GaN substrates was found uncompromised, as confirmed by high resolution X-ray diffraction and low temperature photoluminescence spectroscopy. In contrast, the lattice parameters were affected significantly, which has to be considered in the subsequent homoepitaxial device growth. The boule growth and respectively the nonpolar substrate homogeneity were found to be hampered by the doping, due to surface microcracking and higher impurity incorporation, while n-type undoped nonpolar substrates were demonstrated of superior quality.