Andrey Nikolaev

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.

Properties of Free-Standing GaN Bulk Crystals Grown by HVPE

GaN wafers 200 μm thick and 30 mm diameter were fabricated. GaN was grown by hydride vapor phase epitaxy on SiC substrates and removed from the substrate by reactive ion etching. Lateral size of the GaN wafers was equal to the size of the initial SiC substrates. GaN wafers were cleaved in pieces and these pieces were characterised. It was found that after the fabrication, GaN crystals were slightly deformed and strained. An anneal at 830°C in nitrogen ambient eliminated the residual strains. The FWHM of ω-scan (0002) x-ray rocking curve for annealed crystals was less than 140 arcsec for both sides of the best GaN crystals. The lattice constants measured from both sides of the crystals were c =5.1853±0.0003 A and a = 3.1889±0.0001 A. The N d – N a concentration determined by a mercury probe was about 2×10 17 cm −3 for as-grown GaN surface and about 2×10 19 cm −3 for former interface surface. Photoluminescence spectrum taken at 17 K revealed an edge peak at 3.472 eV with the FWHM value of 2.3 meV. A ratio of the edge peak intensity to the intensity of yellow band was higher than 1000. Initial TEM experiments were performed.

Fabrication and characterization of heterojunction diodes with HVPE-grown GaN on 4H-SiC

GaN/SiC heterojunctions can improve the performance considerably for BJTs and FETs. In this work, heterojunction diodes have been manufactured and characterized. The fabricated diodes have a GaN n-type cathode region on top of a 4H-SiC p-type epi layer. The GaN layer was grown with HVPE directly on off-axis SIC without a buffer layer. Mesa structures were formed and a Ti metallization was used as cathode contact to GaN, and the anode contact was deposited on the backside using sputtered Al. Both current-voltage (I-V) and capacitance-voltage (C-V) measurements were performed on the diode structures. The ideality factor of the measured diodes was 1.1 and was constant with temperature. A built in potential of 2.06 V was extracted from I-V measurements and agrees well with the built in potential from C-V measurements. The conduction band offset was extracted to 1.1 eV and the heterojunction was of type II. The turn on voltage for the diodes is about 1 V lower than expected and a suggested mechanism for this effect is discussed.

Gallium-induced surface reconstruction patterns of GaN grown by molecular beam epitaxy

Abstract We report on an investigation of the mechanisms giving rise to surface reconstruction for GaN grown by molecular beam epitaxy (MBE) on a range of different substrates. We have studied the effects of surface contamination by oxygen or arsenic and demonstrate that both can influence the surface reconstruction. We show that surface reconstruction measured by reflection high-energy electron diffraction (RHEED) is associated with excess Ga on the surface, which undergoes an order–disorder transition at about 400–500°C. For MBE, growth on hydride vapour phase epitaxy (HVPE) GaN/SiC composite substrates having the Ga polarity the 2×2 reconstruction is intrinsic. This intrinsic reconstruction can be destroyed by heating to a high temperature or by oxidation. The intrinsic 2×2 reconstruction can also be destroyed by adding an additional monolayer of more weakly bound Ga, which can be removed by desorption at high temperature. For growth by MBE on sapphire with nitridation, a 3×3 reconstruction is observed on cooling the sample to

High-Temperature Hardness of Bulk Single-Crystal AlN

The hardness of single-crystal aluminum nitride (AlN) 0.5-mm-thick wafers was measured at elevated temperatures and compared with that of other semiconductors. A Vickers indentation method was used to determine the hardness under an applied load of 0.5–5 N in the temperature range 20–1400°C. The average hardness was measured as 17.7 GPa at room temperature. AlN exhibits a hardness higher than that of GaN in the entire temperature range investigated. A high mechanical stability for AlN at high temperatures is deduced.

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.

AlN Wafers Fabricated by Hydride Vapor Phase Epitaxy

The authors report on AlN wafers fabricated by hydride vapor phase epitaxy (HVPE). AlN thick layers were grown on Si substrates by HVPE. Growth rate was up to 60 microns per hour. After the growth of AlN layers, initial substrates were removed resulting in free-standing AlN wafers. The maximum thickness of AlN layer was about 1 mm. AlN free-standing single crystal wafers with a thickness ranging from 0.05 to 0.8 mm were studied by x-ray diffraction, transmission electron microscopy, optical absorption, and cathodoluminescence.

Effects of substrate type on the characteristics of GaN epitaxial films grown by molecular beam epitaxy

Films of GaN have been grown simultaneously on three different substrates, sapphire, silicon carbide and GaN, by molecular beam epitaxy. The structural and optical properties of the films have been studied using x-ray diffraction techniques and low-temperature photoluminescence spectroscopy respectively. There is a clear correlation between the structural and optical properties of the film and the nature of substrate. The lattice parameter of the film is shown to depend on the type of substrate. The incorporation of shallow donors is also strongly influenced. The effect of lattice parameter, mismatch and differential thermal expansion on film properties is discussed.

Dislocation structure of GaN bulk crystals grown on SiC substrates by HVPE

Abstract The dislocation structure of GaN wafers was studied by transmission electron microscopy. These wafers were fabricated using hydride vapor phase epitaxy of thick GaN layers on 6H–SiC substrates and subsequent removal of the substrate by reactive ion etching. Three major types of dislocations were observed in these GaN crystals: (1) threading dislocations (TD), which are mainly parallel to the [0001] direction and extend from the former interface to the crystal surface. They were found to have edge, screw and mixed type; (2) dislocations lying on basal (0001) planes and located near TD through all the thickness of the crystal; (3) dislocations lying preferentially on prismatic planes in the top region of the crystals. Based on experimental results, the influence of initial nucleation stage of heteroepitaxial growth and post-growth cooling on the dislocation distribution in bulk GaN crystals is discussed.