Underlying GaN Layer Research Articles

Growth and Characterization of GaN Underlying Layer Used in Blue-Violet GaN-Based Laser Diodes on Sapphire

ABSTRACTThe underlying GaN layers on which laser diodes are fabricated have been improved through two steps. In the first step, GaN single layer on sapphire was investigated. The residual strain and etch pit density were measured. We found that they reflect the optical quality. We found that the threading dislocation can be reduced to 4 × 108 cm−2. The optical quality depends on the residual strain and dislocation density. In the next step, we have utilized epitaxial lateral overgrowth ELO) technique. The optimized GaN layer on sapphire with the smallest dislocation density was used as seed layer. In the wing region of ELO-GaN, the threading dislocation density was reduced to 1 × 106 cm−2. On the other hand, in the seed region, dislocation density remained 4 × 108 cm−2. Photoluminescence intensity in the wing region was three times as large as that in the seed. The laser diodes were fabricated on the ELO-GaN layer, so that the ridge stripe was fabricated over the wing region, and its properties were compared with those of laser diodes on sapphire. It was found that the lifetime can be increased by using the ELO-GaN layer as the underlying layer.

Indium Incorporation and Droplet Formation during InGaN Molecular Beam Epitaxy

General trends in In incorporation into InGaN ternary compounds during plasma-enhanced MBE are studied theoretically. The theoretical predictions agree well with the experimental data obtained by Böttcher et al. [3]. Growth conditions favorable for indium droplet appearance on the growing InGaN surface are revealed. It is shown that there exists a narrow growth window where efficient In incorporation into InGaN can be reached without droplet formation. Elastic strain caused by the lattice constant mismatch between the InGaN and the underlying GaN layer affects significantly the critical reactive nitrogen flux necessary to avoid the droplet appearance.

Metalorganic chemical vapor deposition selective growth and characterization of InGaN quantum dots

InGaN quantum dots (QDs) have been formed by metalorganic chemical vapor deposition selective growth on Si-patterned GaN/sapphire substrates. The QDs were fabricated on GaN plinths grown within nanoscale circular windows opened in the Si mask by both focused ion-beam irradiation and photoassisted wet chemical etching. Using this process, the epitaxial growth of GaN plinths could be achieved on an underlying GaN layer spared from process-induced damage. The optical properties of the InGaN QDs have been investigated by low-temperature cathodoluminescence measurements. Compared to the band-gap emission from a reference sample of InGaN/GaN multiple quantum wells, the evident blueshift of the emission peak from the InGaN QDs was demonstrated. This result suggests that the carriers in the InGaN QDs were three-dimensionally confined.

Growth of Crack-Free Thick AlGaN Layer and its Application to GaN-Based Laser Diode

AbstractIn the field of group-III nitrides, hetero-epitaxial growth has been one of the most important key technologies. A thick layer of AlGaN alloy with higher AlN molar fraction is difficult to grow on sapphire substrate, because the alloy layer is easily cracked. It is thought that one cause of generating cracks is a large lattice mismatch between an AlGaN and a GaN, when AlGaN is grown on the underlying GaN layer. We have achieved crack-free Al0.07Ga0.93N layer with the thickness of more than 1mm using underlying Al0.05Ga0.95N layer. The underlying Al0.05Ga0.95N layer was grown directly on sapphire by using the lowtemperature-deposited buffer layer (LT-buffer layer). Since a lattice mismatch between the underlying Al0.05Ga0.95N layer and upper Al0.07Ga0.93N layer is relatively small, the generation of cracks is thought to be suppressed. This technology is applied to a GaN-based laser diode structure, in which thick n-Al0.07Ga0.93N cladding layer grown on the Al0.05Ga0.95N layer, improves optical confinement and single-robe far field pattern in vertical direction.

Pendeo-Epitaxy - A New Approach for Lateral Growth of Gallium Nitride Structures

A new process route for lateral growth of nearly defect free GaN structures via Pendeo-epitaxy is discussed. Lateral growth of GaN films suspended from {110} side walls of [0001] oriented GaN columns into and over adjacent etched wells has been achieved via MOVPE technique without the use of, or contact with, a supporting mask or substrate. Pendeo-epitaxy is proposed as the descriptive term for this growth technique. Selective growth was achieved using process parameters that promote lateral growth of the {110} planes of GaN and disallow nucleation of this phase on the exposed SiC substrate. Thus, the selectivity is provided by tailoring the shape of the underlying GaN layer itself consisting of a sequence of alternating trenches and columns, instead of selective growth through openings in SiO2 or SiNx mask, as in the conventional lateral epitaxial overgrowth (LEO).Two modes of initiation of the pendeo-epitaxial GaN growth via MOVPE were observed: Mode A - promoting the lateral growth of the {110} side facets into the wells faster than the vertical growth of the (0001) top facets; and Mode B - enabling the top (0001) faces to grow initially faster followed by the pendeo-epitaxial growth over the wells from the newly formed {110} side facets. Four-to-five order decrease in the dislocation density was observed via transmission electron microscopy (TEM) in the pendeo-epitaxial GaN relative to that in the GaN columns. TEM observations revealed that in pendeo-epitaxial GaN films the dislocations do not propagate laterally from the GaN columns when the structure grows laterally from the sidewalls into and over the trenches. Scanning electron microscopy (SEM) studies revealed that the coalesced regions are either defect-free or sometimes exhibit voids. Above these voids the PEGaN layer is usually defect free.

Biaxial strain in Al xGa 1− xN/GaN layers deposited on 6H-SiC

Al x Ga 1− x N/GaN multilayers ( x≤0.12) deposited on AlN(0001) buffer layer/6H-SiC(0001) substrate combinations have been investigated using high resolution X-ray diffraction. Rocking curves ( ω) showed that the Al x Ga 1− x N layers in the main contained a reduced dislocation density relative to the underlying GaN layers. The line widths of the radial scans (2 θ− ω) of the Al x Ga 1− x N layers increased as the Al mole fraction increased due to alloy broadening. The in-plane lattice constant ( a) of the Al x Ga 1− x N layer for each sample was equivalent to that of the GaN layer, indicating the layers were coherently strained. The strain in the AlGaN layer was tensile for each sample. This was determined using high-resolution reciprocal space maps of the (015) and (024) planes which revealed that the Al x Ga 1− x N and GaN lattice points had the same value of the in-plane components of the reciprocal lattice vector ( S̄ ‖). For higher Al mole fractions, the samples were severely cracked, indicating the strain in the Al x Ga 1− x N layers exceeded critical values.

Pendeo-Epitaxy - A New Approach for Lateral Growth of Gallium Nitride Structures

AbstractA new process route for lateral growth of nearly defect free GaN structures via Pendeoepitaxy is discussed. Lateral growth of GaN films suspended from {1120} side walls of [0001] oriented GaN columns into and over adjacent etched wells has been achieved via MOVPE technique without the use of, or contact with, a supporting mask or substrate. Pendeo-epitaxy is proposed as the descriptive term for this growth technique. Selective growth was achieved using process parameters that promote lateral growth of the { 1120) planes of GaN and disallow nucleation of this phase on the exposed SiC substrate. Thus, the selectivity is provided by tailoring the shape of the underlying GaN layer itself consisting of a sequence of alternating trenches and columns, instead of selective growth through openings in SiO2 or SiNx mask, as in the conventional lateral epitaxial overgrowth (LEO).Two modes of initiation of the pendeo-epitaxial GaN growth via MOVPE were observed: Mode A - promoting the lateral growth of the {1120} side facets into the wells faster than the vertical growth of the (0001) top facets; and Mode B - enabling the top (0001) faces to grow initially faster followed by the pendeo-epitaxial growth over the wells from the newly formed {1120} side facets. Four-to-five order decrease in the dislocation density was observed via transmission electron microscopy (TEM) in the pendeo-epitaxial GaN relative to that in the GaN columns. TEM observations revealed that in pendeo-epitaxial GaN films the dislocations do not propagate laterally from the GaN columns when the structure grows laterally from the sidewalls into and over the trenches. Scanning electron microscopy (SEM) studies revealed that the coalesced regions are either defect-free or sometimes exhibit voids. Above these voids the PEGaN layer is usually defect free.

Magnetic field dependent Hall data analysis of electron transport in modulation-doped AlGaN/GaN heterostructures

Experimental magnetic field dependent Hall and resistivity data is presented for two modulation-doped AlGaN/GaN heterostructures in the temperature range from 6 to 300 K and for a magnetic field up to 12 T. The mobility and concentration of electrons within the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface and within the underlying GaN layer are readily separated and characterized using quantitative mobility spectrum analysis. The observed transport parameters of the 2DEG are explained using the classical band theory for a degenerate electron gas. Analysis of the temperature dependencies of mobility and electron concentration in the GaN layer and 2DEG indicates that electron transport in the GaN layer is dominated by carriers in the conduction band for the case of low-doping (<1017 cm−3), and by conduction via an impurity band for highly doped material (>1018 cm−3). The simultaneous analysis of the multilayer AlGaN/GaN structure applied in this work renders the results applicable directly to modulation-doped field-effect transistors based on similar structures.

Structural Properties of Nitrides Grown by Omvpe on Sapphire Substrate

AbstractCrystalline quality of nitrides on sapphire by OMVPE has been investigated. First, in-situ observations of the crystallization process of the low temperature deposited AIN buffer layer or GaN buffer layer on sapphire substrate have been performed. Small hexagonal mesas were formed from the sapphire to the surface and finally they formed a stacked structure. Secondly, a low temperature deposited buffer layer located between the high temperature grown GaN was found to reduce the etch pit density of GaN films. Thirdly, structural properties of Ga1−xInxN (0 ≤x≤0.21, and x= l) on GaN and GaInN/GaN MQWs on GaN have been characterized by Xray diffraction. Coherently grown GaInN showed almost the same twisting as the underlying GaN layer, while free standing InN showed large twisting. Thickness of the well layers in MQWs has been controlled within one monolayer preciseness, and the fluctuation of alloy composition has been controlled to within 2%

Structural and Optical Properties of Nitride-Based Heterostructure and Quantum-Well Structure

ABSTRACTStructural and optical properties of nitride based heterostructure and quantum well structure were investigated. Both AIGaN and GaInN ternary alloys are found to grow coherently on the underlying GaN layer. Compressive strain of GaInN is found to cause quantum confined Stark effect, thus affects the luminescence properties of nitride-based quantum wells.