N. Kuznetsov

Electric breakdown in GaN p‐n junctions

Electric breakdown in GaN p‐n junctions was investigated. GaN p+‐p‐n+ structures were grown on 6H–SiC substrates by metalorganic chemical vapor deposition. Mg and Si were used as dopants. Mesa structures were fabricated by reactive ion etching. Capacitance–voltage measurements showed that the p‐n junctions were linearly graded. The impurity gradient in the p‐n junctions ranged from 2×1022 to 2×1023 cm−4. Reverse current–voltage characteristics of the p‐n junctions were studied in the temperature range from 200 to 600 K. The diodes exhibited abrupt breakdown at a reverse voltage of 40–150 V. The breakdown had a microplasmic nature. The strength of the electric breakdown field in the p‐n junctions depended on the impurity gradient and was measured to be from 1.5 to 3 MV/cm. It was found that the breakdown voltage increases with temperature. The temperature coefficient of the breakdown voltage was ∼2×10−2 V/K.

Insulating GaN:Zn layers grown by hydride vapor phase epitaxy on SiC substrates

Fabrication of high-performance GaN microwave devices, which are the excellent candidates for new generation of high-power solid-state components, requires insulating GaN substrate materials. Due to lack of bulk GaN crystals and particularly lack of semi-insulating GaN substrates, we propose insulating GaN layers on silicon carbide as substrates for the fabrication of GaN-based microwave devices. In this work, we demonstrate insulating GaN layers on silicon carbide substrates. Insulating GaN layers doped with zinc were grown on silicon carbide substrates by hydride vapor phase epitaxy. High crystal quality of the grown material was proved by x-ray diffraction measurements showing the full width at a half maximum of ω-scan rocking curve of about 100 arcsec. Temperature dependence of specific resistivity of the GaN:Zn layers was measured in the temperature range from 200 to 500 K. The value of the specific resistivity was found to be 1012 Ω cm at 300 K and 109 Ω cm at 500 K.

Chemical vapor deposition of 4H–SiC epitaxial layers on porous SiC substrates

Epitaxial 4H–SiC layers were grown by chemical vapor deposition (CVD) on porous silicon carbide. Porous SiC substrates were fabricated by the formation of a 2 to 15 μm thick porous SiC layer on commercial off-axis 4H–SiC substrates. The thickness of CVD grown layers was about 2.5 μm. The concentration Nd–Na in the layers was about 7×1015 cm−3. The layers were investigated for their surface roughness, crystal structure, deep level concentration, and minority carrier diffusion length. It was found that the characteristics of SiC epitaxial layers grown on porous SiC substrates were significantly improved compared to those of SiC layers grown on standard SiC substrates.

Characterization of AlN/SiC Epitaxial Wafers Fabricated by Hydride Vapour Phase Epitaxy

AlN/SiC epitaxial wafers were fabricated using hydride vapor phase epitaxy. The thickness of AlN layer was varied from 0.1 to 0.5 μm. Surface of AlN layers was characterized by high-energy electron diffraction and atomic force microscopy. The crystal structure of AlN layers was studied using X-ray diffraction. The minimum value of the full width at half maximum of ω-scan (0002) reflection rocking curve was about 120 arcsec. The specific resistivity of AlN was measured in the temperature range from 300 to 700 K.

Electrical characteristics of GaN/6H-SiC n-p heterojunctions

Abstract n-Type GaN layers of high crystalline quality have been grown on p-type 6H-SiC substrates by HVPE. The electronic properties of the n-p heterojunctions indicate a typical diode behaviour. The turn-on voltage of the forward I-V characteristic is about 2 V while the reverse characteristic showing an abrupt breakdown at voltage close to 30 V. Using C-V measurements, the density of interface traps has been estimated to be 7 × 1012 cm−2. These interface traps limit the current transport in the p-6H-SiC/n-GaN heterojunction.

GaN PN-Structures Grown by Hydride Vapor Phase Epitaxy

For the first time, GaN pn-junctions were fabricated by hydride vapor phase epitaxy. GaN pn-structures were grown directly on 6H-SiC substrates without any buffer layer. Undoped GaN layers were n-type with N d -N a concentration ranged from 1×10 17 to 5×10 18 cm −3 . Magnesium was used as an acceptor to grow p-type GaN layers. Mg atomic concentration determined by secondary ion mass spectroscopy ranged from 5×10 19 to 5×10 20 cm −3 . As-grown GaN layers doped with Mg were p-type, and p-type conductivity was improved by post-growth anneal. Mesa diodes with a vertical current flow geometry were formed by reactive ion etching. The position of the GaN pn-junction was determined by the electron beam induced current method. The electrical characteristics of the pn diodes were studied. Electroluminescence from the pn diodes was measured.