Alexander Usikov

Diffusion-limited nonradiative recombination at extended defects in hydride vapor phase epitaxy GaN layers

Time-resolved free-carrier absorption and transient grating techniques were applied to determine carrier lifetimes and diffusion coefficients in a set of hydride vapor phase epitaxy GaN layers of various thickness (from 10 to 145 μm). A linear increase in nonradiative carrier lifetime in 80–800 K range found to be in a correlation with decrease of the bipolar carrier diffusion coefficient. This correlation confirmed that recombination rate is governed by carrier diffusive flow to the grain boundaries of columnar defects. A model of diffusion-governed nonradiative lifetime was proposed for fitting the measured lifetime values in the layers of different thickness as well as lifetime dependence on temperature or threading dislocation density.

Accumulation of Background Impurities in Hydride Vapor Phase Epitaxy Grown GaN Layers

We report on accumulation of background Si and O impurities measured by secondary ion mass spectrometry (SIMS) at the sub-interfaces in undoped, Zn- and Mg-doped multi-layer GaN structures grown by hydride vapor phase epitaxy (HVPE) on sapphire substrates with growth interruptions. The impurities accumulation is attributed to reaction of ammonia with the rector quartz ware during the growth interruptions. Because of this effect, HVPE-grown GaN layers had excessive Si and O concentration on the surface that may hamper forming of ohmic contacts especially in the case of p-type layers and may complicate homo-epitaxial growth of a device structure.

Structural defects responsible for excessive leakage current in Schottky diodes prepared on undoped n-GaN films grown by hydride vapor phase epitaxy

Schottky diodes fabricated on undoped n-GaN films grown by hydride vapor phase epitaxy showed more than two orders of magnitude higher reverse current if the films contained open core defects. The open core defects were revealed by scanning electron microscope observation in secondary electrons, microcathodoluminescence (MCL), and electron beam induced current (EBIC) modes. Plan-view EBIC imaging showed that such films contained a relatively high density of large (∼10 μm in diameter) dark defects that were absent in good films with low leakage current. In plan-view scanning electron microscope images, pits with the density similar to the density of dark defects were observed. Cross-sectional MCL observation showed that the pits terminated the vertical micropipes starting near the interface with the substrate. Some of the micropipes closed approximately halfway through the grown thickness. The regions of micropipes, either closed or not, showed a higher intensity of bandedge and defect MCL bands. Possible ...

Ultraviolet photodetectors based on GaN and AlxGa1-xN epitaxial layers

The effect of epilayer structural peculiarities on electro- physical properties of the epilayers and on parameters of the UV photodetectors has been investigated. Random distribution of charged centers, which is associated with boundaries of mosaic structure domains being typical of III- nitrides, has been shown to result in a low Schottky barrier height, a high leakage current, and persistent photoconductivity in an undoped GaN. The introduction of low Si concentration minimized the effect as well as allows the Schottky barrier height to be obtained close to the difference between work functions of GaN and metal. The characteristics of Al0,1Ga0.9N Schottky barrier photodetectors have been given. MSM photodetectors and Schottky barrier photodetectors over spectral range 200-365 nm with characteristics close to those of the best analogs, in particular, leakage current density of about 10-8A cm-2, have been obtained using domestic GaN grown by MOCVD on sapphire substrates of (0001) orientation.

Graphene/SiC Functionalization for Blood Type Sensing Applications

Functionalization of graphene/SiC dies by nitro-phenyl and its reduction to phenyl-amine is discussed. The graphene films were formed on a SiC substrate by the substrate surface thermal decomposition at 1800-2000°C. The functionalizing procedure included a two-step electrochemical process monitored by cyclic voltammetry and the die resistance. Functionalized graphene/SiC dies with applied antibody were blood sensitive and can be potentially applied to identify promptly types of the blood.

HVPE GaN Growth on 4H SiC and Die Dicing

Hydride Vapor Phase Epitaxy (HVPE) was used to grow 1-4 μm thick undoped GaN layers on 4H-SiC and sapphire substrates. To adjust mechanical strain and crack formation in the GaN/SiC samples, the AlGaN-based buffer layer was grown at low temperature (920-980°C) and the GaN layer was grown at a higher temperature (1000-1040°C). Laser scribing through the GaN layer or the SiC substrate was applied to fabricate dies from the GaN/SiC and GaN/sapphire samples. The laser irradiation passing through the GaN layer to the sapphire substrate or through the SiC substrate to the GaN layer, along two orthogonal directions created a net of micro-cavities in sapphire and melted grooves in SiC that promote easy breakage of the sample into rectangular dies.

Investigation of direct water photoelectrolysis process using III-N structures

GaN, GaN/AlGaN and GaN/InGaN-based structures were used to study water photoelectrolysis in KOH-based electrolyte, measurement of current-potential characteristics, investigation of electrode corrosion and for hydrogen generation. The corrosion process of p-n AlGaN/GaN structure starts in the p-layers, spreads via vertical channels associated with threading defects, and continues laterally along the n-layers, where large local hollows and voids were observed. The H2 production rate of 0.3-0.6 ml/cm2×h was measured for n-GaN structure.

Fabrication and Characterization of Ultraviolet Light Emitting Diodes with Nanometer Scale Compositionally Inhomogeneous Active Regions

Fabrication and Characterization of Ultraviolet Light Emitting Diodes with Nanometer Scale Compositionally Inhomogeneous Active Regions A.V. Sampath, M.L Reed, G.A. Garret, E.D. Readinger, H. Shen, M. Wraback US Army Research Laboratory, Adelphi MD 20783 C. Chua, N.M. Johnson Palo Alto Research Center, Palo Alto, CA 94304 A.S. Usikov, O.V. Kovalenkov, L.V. Shapovalova, V.A. Dimitriev Technologies and Devices International, Silver Spring MD, Silver Spring MD, 20904

InN Nano Rods and Epitaxial Layers Grown by HVPE on Sapphire Substrates and GaN, AlGaN, AlN Templates

ABSTRACT This paper contains results on InN growth by Hydride Vapor Phase Epitaxy (HVPE) on various substrates including sapphire, GaN/sapphire, AlGaN/sapphire, and AlN/sapphire templates. The growth processes were carried out at atmospheric pressure in a hot wall reactor in the temperature range from 500 to 650oC. Arrays of nano-crystalline InN rods with various shapes were grown directly on sapphire substrates. Continuous InN layers were grown on GaN/sapphire, AlN/sapphire and AlGaN/sapphire template substrates. X-ray diffraction rocking curves for the (00.2) InN reflection exhibit the full width at half maximum (FWHM) as narrow as 0.075 deg. for the nano-rods and 0.128 deg. for the continuous layers grown on GaN/sapphire templates. INTRODUCTION Group III nitride compounds using InGaN-based active regions have attracted much attention as structures for short wavelength emitters operated in visible spectral range. Ability to grow high-quality thick InN-based active layers thought to be a way for further increasing the light emitters efficiency. Hydride vapor phase epitaxy (HVPE) is well-known method to produce both thick low-defect GaN, AlGaN, AlN templates and free-standing GaN substrates for nitride device fabrication. One of the major technical issues in the development of InN materials is low dissociation temperature and high dissociation pressure of InN. Results of InN and InGaN growth by HVPE are limited. First experiments have been reported more than 25 years ago when InN layers were grown using reaction between ammonia and InCl

Thick AlN layers grown by HVPE on sapphire substrates

ABSTRACT Thick low defect AlN and AlGaN layers grown on ultra violet (UV) transparent substrates are considered as promising substrate materials for the UV light emitters and detectors. Electrically insulating thick AlN layers may serve as the substrates for AlGaN/GaN-based high power high electron mobility transistors (HEMTs). In this paper we report on crack-free up to 20 µm thick AlN layers grown by stress control HVPE on 2-inch sapphire substrates. As-grown surface had a characteristic pyramidal morphology. Being t hick enough, AlN layers can be polished to improve surface roughness. The minimum full width at half maximum (FWHM) values of AlN ω-scan x-ray (00.2) and (10.2) rocking curves was about 500 and 1000 arcsec, respectively. The XRD analysis was applied for the threading dislocation density evaluation in grown AlN layer. Screw dislocation density was found to be (3-7)x10 8 cm -2 for the layers from 3 to 12 µm thick. INTRODUCTION Hydride vapour phase epitaxy (HVPE) is known to produce low defect GaN substrate materials for electronic and optoelectronic GaN-based devices. High quality HVPE grown GaN layers and free-standing material have been recently demonstrated by several research teams as the substrates for GaN-based power devices [1], high-frequency transistors [2], and blue lasers [3]. However, GaN is not optically transparent in a UV spectral region for wavelengths shorter than 360 nm and wider band gap material are needed for deep UV applications. Due to large band gap (6.2 eV), high thermal conductivity (3.3 W/cm-K) and close thermal and lattice match to AlGaN layers, AlN is promising substrate material for advanced high power UV emitting devices (LEDs and LDs), UV detectors, ultra high power high frequency electronic devices (AlGaN/GaN HEMTs). However, bulk AlN substrates of required size (2-inch diameter and larger) are not available. One way to get over this issue is to fabricate AlN template substrates comprising thick low defect AlN layer grown on a foreign substrate. The main technical challenge here is to minimize stresses in such template substrate and to avoid cracking of the AlN layer. Recently we have reported on stress control HVPE technology to grow thick (>50µm) crack-free AlN layers on SiC substrates [4]. In this paper we report on crack-free AlN layers up to 20 µm thick grown by HVPE on 2-inch sapphire substrates. Results of 3-15 µm thick layers characterization are discussed.