Krishnan Balakrishnan

Growth and characterization of cubic GaN

We have grown high-quality cubic GaN epilayers on GaAs and 3C-SiC substrates by molecular beam epitaxy technique using dimethylhydrazine or ammonia/nitrogen plasma as a nitrogen source. An X-ray diffraction peak width of 16 min and a low-temperature photoluminescence peak width of 19 meV were achieved. Various surface reconstruction transitions have been observed for cubic GaN(0 0 1) surfaces, recently. These results, along with previously published studies on cubic nitrides, are summarized, and the current status of the growth and characterization of cubic nitrides including AlN and InN is discussed.

The origin of persistent photoconductivity and its relationship with yellow luminescence in molecular beam epitaxy grown undoped GaN

The results of persistent photoconductivity (PPC) and photoluminescence measurements made on radio-frequency plasma assisted molecular beam epitaxy grown, undoped, GaN are reported in this work. Hexagonal GaN (h-GaN) epilayers grown on sapphire and cubic GaN (c-GaN) epilayers grown on GaAs and cubic SiC substrates, are employed in this study. Three clear experimental evidences are reported to claim that the commonly seen persistent photoconductivity and yellow luminescence (YL) are related to each other through the same defect. First, PPC is observed only in those samples which show YL. Second, the threshold (the minimum photon energy required) to observe PPC is determined as 1.6±0.2 eV, which is almost at the same energy at which the YL band starts raising. Third, the photocurrent increases monotonically from 1.8 to 2.2 eV, which is consistent with the broad nature of YL band.

Microstructure of epitaxial lateral overgrown AlN on trench-patterned AlN template by high-temperature metal-organic vapor phase epitaxy

A high-quality thick AlN layer without cracks was grown on sapphire by the combination of the high-temperature metal-organic vapor phase epitaxial growth and epitaxial lateral overgrowth methods. The dislocation behavior and growth mode of AlN are investigated in detail. The dislocation density of the AlN layer thus grown was less than 107cm−2.

High-Temperature Metal-Organic Vapor Phase Epitaxial Growth of AlN on Sapphire by Multi Transition Growth Mode Method Varying V/III Ratio

High-quality AlN layers were grown on c-plane sapphire substrates by high-temperature metal-organic vapor phase epitaxy. AlN layers of about 9 µm in thickness with an atomically flat surface were obtained without cracks. Multiple modulation of the V/III ratio during growth led to a reduction in the number of dislocations during the growth transition period. The dislocation density of the AlN layers was found to be less than 3×108 cm-2.

Dislocations in AlN Epilayers Grown on Sapphire Substrate by High-Temperature Metal-Organic Vapor Phase Epitaxy

The growth temperature of AlN layers is one of the most important factors in metal-organic vapor phase epitaxy (MOVPE) growth. AlN layers were grown using our customized high-temperature MOVPE system. The crystalline quality was discussed on the basis of X-ray diffraction, atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements. The samples grown at a temperature of 1400 °C had much improved crystalline quality in terms of the X-ray rocking curve full width at half maximum values and AFM root-mean-square roughness. In addition, according to TEM analysis, edge type dislocations caused by small-angle grain boundaries were predominant under a low growth temperature, whereas these dislocations became much fewer with increasing growth temperature.

First Demonstration of Semipolar Deep Ultraviolet Light Emitting Diode on m-Plane Sapphire with AlGaN Multiple Quantum Wells

We report on the first demonstration of a semipolar AlGaN based deep ultraviolet (UV) light emitting diode (LED), with a peak emission wavelength of 307 nm. The LED structure was grown on an m-plane sapphire substrate using metal organic chemical vapor deposition (MOCVD). A combination of pulsed MOCVD (PMOCVD) grown AlN and a short period AlN/AlGaN superlattice structure was found to be instrumental in achieving singular semipolar structural phase of (1122) with a defect density low enough to fabricate the light emitting device. The on-wafer optical output power of the fabricated LED was ~20 µW at a dc pump current of 20 mA.

Influence of High Temperature in the Growth of Low Dislocation Content AlN Bridge Layers on Patterned 6H-SiC Substrates by Metalorganic Vapor Phase Epitaxy

A novel high temperature metalorganic vapor phase epitaxy (MOVPE) growth of AlN bridge layer is reported. Positive influence of high temperature on the growth rate and reduction of dislocation content in the AlN bridge layer has been observed. Transmission electron microscopy, X-ray diffraction, and atomic force microscopy analyses confirmed that the layer had high structural quality and smooth morphology.

Structural Analysis of Cubic GaN through X-Ray Pole Figure Generation

Epilayers of cubic GaN have been grown on (001) GaAs substrates by molecular beam epitaxy using an electron cyclotron resonance (ECR) plasma nitrogen source. In order to study the existence of secondary crystallographic phases in the GaN epilayers, X-ray pole figures were generated on the (001) surfaces of the samples. The exact locations of misoriented cubic and secondary hexagonal phases have been identified and their relative intensities were estimated. The X-ray pole figure technique has been proven to be extremely useful for the structural characterization of cubic GaN epilayers.

Computer Simulation for Analysis of Lattice Polarity of Wurtzite GaN{0001} Film by Coaxial Impact Collision Ion Scattering Spectroscopy

Signal intensities of coaxial impact collision ion scattering spectra have been computed for c-axis-oriented GaN films at various incident angles in order to analyse the lattice polarity based on the three-dimensional two-atom triple-scattering model. It was found that Ga and N signal intensities show specific incident angle dependences on (0001) and (000) surfaces. The physical image of each characteristic feature in the spectra was also clarified.

Vertical Injection Thin Film Deep Ultraviolet Light Emitting Diodes with AlGaN Multiple-Quantum Wells Active Region

Vertically conducting thin film high power deep ultraviolet (DUV) light emitting diodes with peak emission wavelength of 280 nm are reported. The light emitting diodes (LEDs) were fabricated using a laser assisted lift-off process. Single chip devices exhibited a record high output power of 5.5 mW at a continuous-wave (cw) current density of only 25 A/cm2. Their lifetime is estimated to be well over 2000 h. We attribute the superior performance of the devices to reduced thermal impedance and the vertical current conduction device geometry.