Graded InGaN Buffers for Strain Relaxation in GaN/InGaN Epliayers Grown on Sapphire

Graded InGaN buffers are employed to relax the strain arising from the lattice and thermal mismatches between GaN/InGaN epilayers grown on sapphire. The formation of V-pits in linearly graded InGaN/GaN bulk epilayers is illustrated. The V-pits were sampled using Atomic Force Microscopy and Scanning Electron Microscopy to examine their variation from the theoretical geometry shape. We discovered that the size of the V-pit opening in linearly graded InGaN, with and without GaN cap layer, has a Gaussian distribution. As such, we deduce that the V-pits are produced at different rates, as the growth of the InGaN layer progresses. In Stage I, the V-pits form at a slow rate at the beginning and then accelerate in Stage II when a critical thickness is reached before decelerating in Stage III after arriving at a mean size. It is possible to fill the V-pits by growing a GaN cap layer. It turns out that the filling of the V-pits is more effective at lower growth temperature of the GaN cap layer and the size of the V-pits opening, which is continued in to GaN cap layer, is not dependent on the GaN cap layer thickness. Furthermore, graded InGaN/GaN layers display better strain relaxation as compared to conventionally grown bulk GaN. By employing a specially design configuration, the V-pits can be eliminated from the InGaN epilayer.Singapore-MIT Alliance (SMA

Graded InGaN Buffers for Strain Relaxation in GaN/InGaN Epilayers Grown on sapphire

Graded InGaN buffers were employed to relax the strain arising from the lattice and thermal mismatch in GaN/InGaN epilayers grown on sapphire. An enhanced strain relaxation was observed in GaN grown on a stack of five InGaN layers, each 200 nm thick with the In content increased in each layer, and with an intermediate thin GaN layer, 10 nm thick inserted between the InGaN layers, as compared to the conventional two-step growth of GaN epilayer on sapphire. The function of the intermediate layer is to progressively relax the strain and to annihilate the dislocations that build up in the InGaN layer. If the InGaN layers were graded too rapidly, more dislocations will be generated. This increases the probability of the dislocations getting entangled and thereby impeding the motion of the dislocations to relax the strain in the InGaN layer. The optimum growth conditions of the intermediate layer play a major role in promoting the suppression and filling of the V-pits in the GaN cap layer, and were empirically found to be a thin 10 nm GaN grown at 750 0°C and annealed at 1000 0°C.Singapore-MIT Alliance (SMA

The Effect of Periodic Silane Burst on the Properties of GaN on Si (111) Substrates

The periodic silane burst technique was employed during metalorganic chemical vapor deposition of epitaxial GaN on AlN buffer layers grown on Si (111). Periodic silicon delta doping during growth of both the AlN and GaN layers led to growth of GaN films with decreased tensile stresses and decreased threading dislocation densities, as well as films with improved quality as indicated by x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. The possible mechanism of the reduction of tensile stress and the dislocation density is discussed in the paper.Singapore-MIT Alliance (SMA

Laser Fabrication by Using Photonic Crystal

This paper involves the calculation for composition of different layer used in laser structure and the simulation of cavity, formed by creating air columns in the InGaAsP medium, for square lattice. The aim of this project is to fabricate approximately zero threshold current lasers. This project involves FDTD simulation for optimizing dimension of the device, fabrication of laser structure and finally characterization of the device structure.Singapore-MIT Alliance (SMA

Fabrication and I-V Characterization of ZnO Nanorod Based Metal-Insulator-Semiconductor Junction

We report on the characteristics of a ZnO based metal insulator semiconductor (MIS) diode comprised of a heterostructure of n-ZnO nanorods/n-GaN. The MIS structure consisted of unintentional - doped n type ZnO nanorods grown on n-GaN sample using hydrothermal synthesis at low temperature (100°). The ZnO nanorod layer was vertically grown from the GaN sample, having the diameter 100nm and length 2µm. Then, an insulator layer for electrical isolation was deposited on the top of ZnO nanorod layer by using spin coating method. A metal layer (gold) was finally deposited on the top. The I-V dependences show a rectifying diode like behavior with a leakage current of 2.10⁻⁵ A and a threshold voltage of about 3V. Depend on the thickness of the insulator, the I-V dependences of the n-ZnO/n-GaN heterostructure was varied from rectifying behavior to Ohmic and nearly linear.Singapore-MIT Alliance (SMA

Enhancement in Indium Incorporation for InGaN Grown on InN Intermediate Layer

InN has been grown on GaN with a thin intermediate layer of InGaN by metalorganic chemical vapor deposition (MOCVD) to further enhance indium incorporation in subsequent InGaN layer. Trimethylindium (TMI) and ammonia (NH₃) were used as the source for InN growth and transmission electron microscopy (TEM) confirmed the presence of pyramid-like structure of InN. A layer of InGaN subsequently grown on top of these InN pyramids shows a red-shift of ~20 nm relative to InGaN layer grown directly on GaN using the same growth condition. However, there is no significant pits reduction. An alternative method to enhance indium incorporation is to grow the InN by adding a small amount of trimethygallium (TMG) into the TMI and NH₃ flow. This method provides a seed layer for the InN growth and it gives a higher density of InN pyramids. X-ray diffraction (XRD) measurement of this sample shows a high indium incorporation to give InGaN with x~0.26 as compared to x~0.22 for sample grown without TMG flow in the InN layer.Singapore-MIT Alliance (SMA

Structural analysis of metalorganic chemical vapor deposited AlN nucleation layers on Si (111)

AlN nucleation layers are being investigated for growth of GaN on Si. The microstructures of high-temperature AlN nucleation layers grown by MOCVD on Si (111) substrates with trimethylaluminium pre-treatments have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM results show that with TMA pre-treatments, AlN grows in a pseudo-2-dimensional mode because the lateral growth rate of AlN is increased, and the wetting property of the AlN on silicon is improved. Also, no amorphous SiNx layer was observed at the interface with TMA pre-treatments and AlN films with good epitaxial crystalline quality were obtained. Transmission electron diffraction patterns revealed that the AlN and Si have the crystallographic orientation relationship AlN [0001]âSi[111] and AlN[11 2 0] âSi[110]. High resolution transmission electron microscopy indicates a 5:4 lattice matching relationship for AlN and Si along the Si [110] direction. Based on this observation, a lattice matching model is proposed.Singapore-MIT Alliance (SMA

Evolution of AlN buffer layers on Silicon and the effect on the property of the expitaxial GaN film

The morphology evolution of high-temperature grown AlN nucleation layers on (111) silicon has been studied using atomic force microscopy (AFM). The structure and morphology of subsequently grown GaN film were characterized by optical microscopy, scanning electron microscopy, x-ray diffraction, and photoluminescence measurement. It was found that a thicker AlN buffer layer resulted in a higher crystalline quality of subsequently grown GaN films. The GaN with a thicker buffer layer has a narrower PL peak. Cracks were found in the GaN film which might be due to the formation of amorphous SiNx at the AlN and Si interface.Singapore-MIT Alliance (SMA

Fabrication of Two-Dimensional Photonic Crystals in AlGaInP/GaInP Membranes by Inductively Coupled Plasma Etching

The fabrication process of two-dimensional photonic crystals in an AlGaInP/GaInP multi-quantum-well membrane structure is developed. The process includes high resolution electron-beam lithography, pattern transfer into SiO₂ etch mask by reactive ion etching, pattern transfer through AlGaInP/GaInP layer by inductively coupled plasma (ICP) etching and a selective undercut wet etch to create the freestanding membrane. The chlorine-based ICP etching conditions are optimized to achieve a vertical sidewall. The photonic crystal structures with periods of a=160-480nm are produced.Singapore-MIT Alliance (SMA