Semi-insulating Gallium Nitride Research Articles

Low Specific Contact Resistivity of 10−3Ω·cm2 for Ti/Al/Ni/Au Multilayer Metals on SI-GaN:Fe Substrate

For the application of third-generation compound semiconductors based on high-voltage, high-power, and high-frequency devices, particularly in photoconductive semiconductor switches (PCSSs), the ohmic contact preparation technology on semi-insulating substrates is crucial. However, iron-doped semi-insulating gallium nitride (SI-GaN:Fe) with low carrier concentration and high bulk resistivity has a very difficult time achieving good ohmic contact characteristics. Here, we have demonstrated preliminary results on an interesting class of good ohmic contact of Ti/Al/Ni/Au on Ga-face of SI-GaN:Fe. The results indicated that the Ti and Al thickness ratio is a critical parameter when determining the optimum annealing temperature and time for ohmic contact. With alloyed Ti/Al/Ni/Au stacks of 20 nm/120 nm/55 nm/45 nm on SI-GaN:Fe after annealing at 850 °C for 35 s in N2 atmosphere, a low specific contact resistivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-3}\,\,\Omega \cdot $ </tex-math></inline-formula> cm2 is achieved. At a future date, such excellent ohmic contact characteristics can be applied to the SI-GaN:Fe-based high-power PCSS.

Free-standing carbon-doped semi-insulating GaN wafer grown by HVPE

Four-inch semi-insulating free-standing gallium nitride (GaN) wafers grown by hydride vapor phase epitaxy (HVPE) with carbon doping were obtained by a self-separated process. The as-grown wafer thickness can reach 900 µm without cracks. The dopant gas was methane (CH4) with concentration of 5% CH4 in N2. The FWHMs of (002) and (102) x-ray diffraction rocking curves were lower than 100 arcsec and typically were 40 – 60 arcsec. The resistivity was measured to be greater than 1010 Ω-cm.

Blocking Characteristics of Photoconductive Switches Based on Semi-Insulating GaP and GaN

This article presents results of research work aimed at manufacturing photoconductive semiconductor switches (PCSSs) based on semi-insulating (SI) gallium phosphide (GaP) and gallium nitride (GaN). Currently, the work is in progress to determine the optimal values of PCSS parameters. In this article, the parameters of the selected semiconductor materials used for making PCSSs, the device operation principle, and possible areas of use are presented. The paper demonstrates the construction of test PCSSs based on SI GaP and SI GaN and results of blocking characteristics measurements without the illumination, as well as with illumination with a small photon flux. Further research directions are presented also.

Migration of carbon from Ga sites to N sites in GaN: a combined PAS and hybrid DFT study

Carbon (C) is of great importance to realize semi-insulating gallium nitride (GaN) for power electronic devices. We demonstrate that C can migrate from Ga sites to N sites after high temperature annealing of C doped GaN. The migration process is revealed through the observation of the generated Ga vacancies-related defects after annealing by positron annihilation spectroscopy. The activation energy of this migration process is estimated to be 2.5–2.8 eV from the temperature dependent annealing experiments, which is well consistent with the theoretical results from first-principles calculations.

Insulating GaN Epilayers Co-Doped with Iron and Carbon

The morphology and electrical properties of doped semi-insulating gallium nitride (GaN) epilayers have been studied. It was found that doping-induced improvement of the insulating properties of GaN epilayers with increased level of doping with carbon or iron is limited by the accompanying deterioration of surface morphology. The character of impurity-related morphology development is different for the two dopants. It is established that the co-doping with carbon and iron allows planarity of the GaN surface to be retained along with a significant improvement of insulating properties of epilayers.

(Invited) Process Development for GaN-Based Photoconductive Semiconductor Switches (PCSS)

High critical field strength and saturation electron velocity of gallium nitride (GaN) makes it a promising material for future compact, high voltage, and high-speed photoconductive switches. Photoconductive semiconductor switches (PCSSs) have advantages for pulsed power applications since it is a device that turns on when illuminated with fast rise time, low jitter, and allows for high repetition rate. Semi-insulating GaN is necessary to sustain the high blocking voltages required for such devices. This is accomplished by compensation doping with doped with iron, carbon, or magnesium. However, a strong photoresponse is also necessary to enable optical triggering of the switch. We have found that carbon-doped GaN (GaN:C) simultaneously reduces the off-state leakage current and extends the blocking voltage, while still allowing for high on-state photocurrent compared to unintentionally doped GaN. However, forming a low resistance contact to highly insulating GaN:C is challenging. Our approach is to study both Si ion implantation and Si-doped confined epitaxial growth as techniques to achieve low resistance N+ contacts. Implantation and activation is a planar process that is promising for formation of contact regions while mitigating high electric field points within the device. In this work, we implant Si with a box profile to a depth of 0.4 μm into GaN:C with carbon concentrations of 5x1017 and 1x1018 cm-3 grown on SiC substrates. Then, a sputtered AlN capping layer is introduced before annealing at various temperatures to preserve the surface from decomposition during annealing. Activation anneals are performed using rapid thermal annealing as well as the previously reported multicycle rapid thermal annealing processes. The AlN capping layer is then selectively wet etched and Ti/Al/Ni/Au contacts are deposited and alloyed at 850 °C. As a comparison, Si-doped low temperature and high temperature GaN layers were grown by MOCVD using a LPCVD SiO2 mask. Top-to-top lateral PCSSs are fabricated and characterized to identify the impact of the contact processing on the ON-resistance, off-state leakage, blocking voltage, and photocurrent. Finally, as the device is a top-side illuminated lateral device, the electric fields at the surface must be properly managed to realize reliable high voltage operation. The appropriate surface passivation and encapsulation processes to realize field plate structures while simultaneously enabling top-side UV illumination will be discussed.

Dielectric properties of highly resistive GaN crystals grown by ammonothermal method at microwave frequencies

Permittivity, the dielectric loss tangent and conductivity of semi-insulating Gallium Nitride crystals have been measured as functions of frequency from 10 GHz to 50 GHz and temperature from 295 to 560 K employing quasi TE0np mode dielectric resonator technique. Crystals were grown using ammonothermal method. Two kinds of doping were used to obtain high resistivity crystals; one with deep acceptors in form of transition metal ions, and the other with shallow Mg acceptors. The sample compensated with transition metal ions exhibited semi-insulating behavior in the whole temperature range. The sample doped with Mg acceptors remained semi-insulating up to 390 K. At temperatures exceeding 390 K the conductivity term in the total dielectric loss tangent of Mg compensated sample becomes dominant and it increases exponentially with activation energy of 1.14 eV. It has been proved that ammonothermal method with appropriate doping allows growth of high quality, temperature stable semi-insulating GaN crystals.

The effect of thermal reactor neutron irradiation on semi-insulating GaN

Semi-insulating Gallium nitride was irradiated by fast and thermal neutrons with fluences from 1014 to 1016 n/cm2. Depth-resolved cathodoluminescence spectroscopy was used to determine defects changes before and after irradiation. The results revealed two kinds of defects affected near-band emission recombination from two opposite directions. One was attributed to irradiation-induced N vacancies that contribute to near-band emission recombination. Another was attributed to irradiation-induced deep level defects that contribute to sub-band gap recombination and thus decrease the near-band emission recombination.

Accurate dependence of gallium nitride thermal conductivity on dislocation density

The authors experimentally find that the thermal conductivity of gallium nitride depends critically on dislocation density using the 3-omega technique. For GaN with dislocation densities lower than 106cm−2, the thermal conductivity is independent with dislocation density. The thermal conductivity decreases with a logarithmic dependence for material with dislocation densities in the range of 107–1010cm−2. These results are in agreement with theoretical predictions. This study indicates that the hydride vapor phase epitaxy method offers an attractive route for the formation of semi-insulating gallium nitride with optimal thermal conductivity values around 230W∕mK and very low dislocation density near 5×104cm−2.

Wide bandgap semiconductor detectors for harsh radiation environments

In this work two wide bandgap materials, silicon carbide (SiC) and gallium nitride (GaN), were investigated for their performance in harsh radiation environments. Schottky devices were fabricated on vanadium doped SiC (V-SiC), Okmetic semi insulating (SI) non-vanadium doped SiC, SI GaN grown by MOCVD (metal organic chemical vapour deposition) and bulk GaN. Completed devices were electrically characterised and the CCE (charge collection efficiency) calculated from pulse height spectra of 241Am α particles. SI GaN samples were irradiated with estimated neutron fluences of up to 10 16 n / cm 2 (Ljubljana), proton fluences of 10 16 p / cm 2 (CERN), and a dose of 600 Mrad of 10 keV X-rays (ICSTM, London). V-SiC samples were irradiated up to 5 × 10 14 π / cm 2 using 300 MeV/c pions (PSI). Electrical characterisation and CCE calculations were repeated after irradiation to observe changes in properties caused by radiation induced damage.

Ion beam induced charge imaging of epitaxial GaN detectors

We report the use of ion beam induced charge imaging to characterise the charge signal uniformity of epitaxial gallium nitride radiation detectors. The detectors were fabricated from 2μm thick semi-insulating gallium nitride, grown by MOCVD on a sapphire substrate. A carrier concentration of 1.4×1015cm−3 was measured using capacitance–voltage measurements. Ion beam induced charge imaging was carried out with a 2MeV alpha particle beam focussed to a 3μm diameter and raster scanned across the device. The resulting ion beam images show excellent charge signal uniformity in this material with no evidence of material defects or polycrystalline structure on the micrometer length scale. No evidence of charge signal trapping was observed in these devices.