AlGaN Buffer Research Articles

Spatial variation of luminescence of InGaN alloys measured by highly-spatially-resolved scanning cathodoluminescence

Cathodoluminescence (CL) measurements were performed on thick InGaN samples over a wide range of indium concentrations ([ln]=0.03–0.20). The layers were grown on c-plane sapphire using an AlGaN buffer layer, followed by 4 μm GaN, and 100 nm InGaN films. These thick InGaN layers were exceptionally specular, especially at low indium concentrations. Some degree of microscopic roughness was observed for x>0.15. With the objective of correlating the microstructure to the spatial dependence of the luminescence, average CL spectra over large areas were compared with high-spatial-resolution CL measurements. The variation of the indium-related peaks over large areas was observed under low magnification scanned CL. The full width at half-maximum in the spot mode is typically smaller than wide-area scans by a factor of 2 to 3. The size of the areas with constant emission wavelength changes significantly with indium content. It has also been observed that the distribution of the local CL peak wavelength changes from single-mode Gaussian to a multimodal distribution for x>0.1. A shoulder of the main peak at the low energy side appears at higher indium content. Spot-mode spectra show that this low energy shoulder consists of single lines at specific wavelengths. In highly resolved monochromatic images we correlate the low energy side shoulder to two structural defects: pinholes and grooves. Particularly, bright luminescence occurs at the center of the grooves.

Electrical Properties of GaN/Si Grown by MOCVD

ABSTRACTGaN growth on Si substrate is very attractive for realizing low cost electronic devices. We grew a thin GaN film on p-type Si(111) substrate using AlGaN high temperature buffer without using a conventional low temperature buffer. A homogeneous buffer layer was obtained at 1093 K and a homogenous 500 nm thick GaN layer was also obtained without any crack. Using a transmission electron microscopy (TEM), we observed that the cross-section of GaN and AlGaN buffer was very smooth and also the surface of GaN was flat although the threading dislocations were observed. Furthermore, we directly fabricated a metal semiconductor field effect transistor (MESFET) using a 500 nm-thick GaN/Si without any high resistive GaN layer. A Schottky electrode was Pt/Au and an ohmic electrode was Al/Ti/Au. A Schottky breakdown voltage was over 100 V. Also, we confirmed a high temperature operation of the MESFET using a thin GaN film on Si substrate at 573 K.

A GaN-free LED Structure for High UV-light Extraction

ABSTRACTWe improved the extraction of ultraviolet light from AlGaN-based light emitting diodes (LEDs) at the wavelength of about 350 nm, by introducing a transparent structure that is free from binary GaN. We used an AlN-template layer on a sapphire substrate as starting medium of the metal organic vapor phase epitaxial growth. The buffer layer is an Al0.2Ga00.8N alloy. We also used a short period alloy superlattice as transparent and conductive p-type cladding and p-type contact layers. The resultant epitaxial structure is confirmed to be transparent with the transmittance of about 90% within the wavelength range of 340 – 400 nm. The crystal quality of the Al0.2Ga00.8N buffer layer was estimated by the transmission electron microscope (TEM) observation. The dislocation density of AlGaN buffer layer was highly reduced on the AlN template layer, both of which have a low density of screw and mixed-type dislocations, of the order of 10−8cm−2. We also found that light extraction is improved by a factor of 6 by introducing the transparent LED structure and a p-contact mirror. The resultant LED shows high performance under RT-CW operation, where 1 mW output power at 348 nm with 20 mA injection corresponds to the external quantum efficiency of 1.4 %. The maximum power was 7 mW at 220 mA. The emission spectrum is highly monochromatic with the UV-to-visible intensity ratio of about 1000. We also demonstrate the application of this transparent UV-LED to white light source in a bottom-emission geometry.

Laser-induced reactive epitaxy of binary and ternary group III nitride heterostructures

Group III nitride films are deposited on sapphire (0 0 0 1) substrates using the laser-induced reactive epitaxy (LIRE) method. Liquid group III metal targets are ablated by a focused nanosecond Nd:YAG laser beam in a nitrogen atmosphere of 0.1 mbar. In the plasma plume, the molecular nitrogen is cracked to atomic nitrogen. The cracking process is investigated by optical plasma spectroscopy. By varying the laser pulse energy, the III/V ratio can be controlled. The investigated heterostructures consist of a AlGaN buffer layer, a low temperature (LT) GaN layer and finally a GaN epilayer. By optimising the III/V ratio and the growth temperature of both LT-GaN buffer and epilayer, the luminescent and electronic properties are strongly enhanced. As best results, we achieved residual carrier concentrations of 2×10 17 cm −3 and mobilities of 70 cm 2/Vs. The FWHM value for the near band-edge luminescence was 76 meV and for the rocking curve at the (0 0 0 2) reflex it was 720 arcsec. InN and InGaN films with In contents up to 13% are grown using molten In and InGa alloy as target materials. The InN films commonly have high electron mobilities. We found values up to 350 cm 2/Vs. For the Mg-doping of GaN, a liquid GaMg alloy is used. The results show that we are able to incorporate the required amount of Mg into the nitride.

Photoluminescence Study of Piezoelectric Polarization in Strained AlxGa1?xN/GaN Single Quantum Wells

We report a low temperature photoluminescence study on two identical Al 0.13 Ga 0.87 N/GaN single quantum wells (QWs), which are pseudomorphically grown on either a GaN or an AlGaN buffer layer. The red shift of the QW emission due to the quantum confined Stark effect, is found to be strongest in the QW deposited on AlGaN, in contrast to what is to be expected form the estimated built-in electric fields due to spontaneous and piezoelectric polarization fields. Screening of the built-in electric field by a relatively high sheet charge is one of the possible reasons for the observed discrepancy.

Removal of 6H-SiC substrate influence when evaluating GaN thin film properties via x-ray

AbstractNon-uniformity in GaN thin films deposited on 6H-SiC can make determining the effects of growth variables difficult. Results presented in this work show the effects of the SiC substrates on the GaN films, and how to correct for these effects to obtain meaningful data about the properties of the thin film rather than the substrate underneath. Rocking curve values of GaN thin films are found to track almost 1:1 with the values of the underlying SiC. Plotting rocking curves with respect to the substrate, as well as a variable of importance can therefore yield more meaningful and reliable comparisons instead of plotting the data for the variable alone. This procedure is used to demonstrate the effects of thickness and AlN and AlGaN buffer layers on GaN thin films.

Growth and characterization of graded AlGaN conducting buffer layers on n + SiC substrates

GaN films on top of doped, graded AlGaN conducting buffer layers were grown by metal-organic chemical vapor deposition on n + SiC substrates. The effect of initial AlGaN composition and buffer layer doping level on the structural and morphological properties of these films and the conduction between these films and the substrate was investigated. A minimum resistance of 2 Ω was measured for vertical test structures.

Drastic Reduction of Threading Dislocation Density of AlGaN on SiC Wafer by Using Highly-Si-Incorporated AlGaN Superlattice

ABSTRACTA new in-situ technique to reduce threading dislocation density (TDD) within sub- micron growth is demonstrated by using metal-organic vapor-phase epitaxy (MOVPE). We achieved drastic reduction of TDD of AlGaN buffer on SiC substrate by inserting highly-Si- incorporated AlGaN/undoped AlGaN superlattice (SL). TDD of AlGaN was decreased from 2×1010 to 7×107 cm−2 by inserting the SL with the total growth thickness of 0.8νm. Si incorporation in AlGaN SL was estimated to be 1.2×1020 cm−3. This technique is exactly in- situ process without complicated fabrication processes, and the surface is kept flat throughout the total growth. This method is especially useful on SiC wafer in order to prevent cracks with thin growth layer. We confirmed the similar effects for GaN and AlGaN buffer on sapphire substrates.

Heterostructure for UV LEDs Based on Thick AlGaN Layers

Thick AlGaN layers and GaN/AlGaN heterostructures were grown by low pressure MOCVD on (0001) sapphire substrates utilizing a low temperature AlGaN buffer layer. The distribution of Al in the thick AlGaN layers was observed to be non-uniform as a function of depth. The Al content gradually increases from the substrate towards the epilayer surface. Moreover, fluctuations of Al content are also noticeable. The saturation of impurity-related emission with increasing current density was observed in EL spectra of LEDs consisting of AlGaN/GaN/AlGaN DH sandwiched by a 2 μm-thick bottom layer of GaN:Si and 0.5 μm-thick layer of GaN:Mg. The dominant near-band edge emission of the GaN active layer was found to be strongly absorbed in the thick bottom layer. Utilizing a 2 μm-thick AlGaN bottom layer instead of the GaN one allowed the absorption edge to be shifted towards higher energies. A single peak at 362 nm with FWHM of 14 nm was observed in this type of LED. Luminescence properties of various types of heterostructures are also discussed.

Residual strains in GaN grown on 6H-SiC

Abstract The triple crystal modification of Bond method was used for lattice constant measurements and for the study of residual strains in GaN layers grown on 6H-SiC (0001) substrates. GaN layers grown by MOCVD employing AlN and AlGaN buffer layers and GaN layers grown by HVPE without buffer layer were investigated. It was found that the residual strains in GaN were considerably reduced by use of the AlGaN buffer layer. The dependence of residual strains on thickness and composition of buffer layer could be explained by the different degree of relaxation mismatch stresses and change of thermal stresses in GaN layers, grown on SiC with different buffer layers.

Microwave performance of AlGaN/GaN inverted MODFET's

A continuous wave output power of 1.5 W/mm with a power added efficiency of 17.5% has been achieved at 4 GHz in inverted AlGaN/GaN MODFET's (IMODFET's) with 2 μm gate lengths and 78 μm gate widths. The current gain and available power gain cutoff frequencies were 6 and 11 GHz, respectively. We suggest that the input characteristics of GaN-based FET's play an important role in the output power that can be obtained. In the present devices, high transconductance, 100 mS/mm, retained over a 5 V input swing is thought to alleviate the limitations imposed by the input characteristics. Moreover, the buried AlGaN buffer layer is suggested as having assisted in the reduction of the output conductance which aids the power gain.

Metalorganic Vapor Phase Epitaxy of Thick InGaN on Sapphire Substrate

Growth of thick InGaN films was performed on single crystal films of GaN or AlGaN and buffer layers of AlN, GaN or AlGaN by metalorganic vapor phase epitaxy (MOVPE). In the growth of InGaN on GaN or AlGaN films, smooth InGaN films with small indium mole fractions were grown in the initial growth stage caused by strong stress due to lattice mismatch. However, with an increase in thickness of the InGaN film surface became rough due to the generation of InGaN grains with large indium mole fractions. Smooth and thick (∼2 µ m) InGaN films with large indium mole fractions were grown on AlN, GaN or AlGaN buffer layers.

Status of Nitride Based Light Emitting and Laser Diodes on SiC

AbstractSingle crystal thin films with compositions from the A1N-InN-GaN system were grown via metal-organic chemical vapor deposition (MOCVD) on single crystal 6H-SiC substrates. Blue light emitting (LED) and laser diode (LD) structures were fabricated. The conducting buffer layer LEDs employed an AlGaN buffer layer which provides a conduction path between SiC and the active device region. The external quantum efficiency of the LEDs was 3% at 20 mA- 3.6V and peak emission wavelength of 430 nm. Violet and blue LDs were fabricated and consisted of an 8-well InGaN/GaN multiple quantum well (MQW) active region in a separate confinement heterostructure (SCH) design. Lasing was obtained both on structures using an insulating buffer layer, and also on structures using a conducting buffer layer. The resulting lasers operated at room temperature using pulsed and continuous wave operation with an emission wavelength of 404-435 rim. The lowest threshold current density obtained for lasing was 11 kA/cm2.