Melvin McLaurin

Basal Plane Stacking-Fault Related Anisotropy in X-ray Rocking Curve Widths of m-Plane GaN

A model of basal plane stacking faults as boundaries between incoherently scattering domains in m-plane GaN films is reviewed. m-Plane GaN films are analyzed with a modified version of the Williamson–Hall analysis in order to determine the length-scale of coherent scattering and tilt-mosaic contribution to X-ray rocking curve widths for the primary in-plane directions. This analysis shows that basal plane stacking faults are the predominant source of rocking-curve width anisotropy in the m-plane films, and indicate that the modified Williamson–Hall analysis can be used as a non-destructive technique for measuring basal plane stacking fault densities in m-GaN films.

Molecular-beam epitaxy of p-type m-plane GaN

We report on the plasma-assisted molecular-beam epitaxy of Mg-doped (101¯0) GaN on (101¯0) 6H–SiC. Secondary ion mass spectroscopy measurements show the incorporation of Mg into the GaN films with an enhanced Mg incorporation under N-rich conditions relative to Ga-rich growth. Transport measurements of Mg-doped layers grown under Ga-rich conditions show hole concentrations in the range of p=1×1018 to p=7×1018cm−3 and a dependence between hole concentration and Mg beam equivalent pressure. An anisotropy in in-plane hole mobilities was observed, with the hole mobility parallel to [112¯0] being higher than that parallel to [0001] for the same hole concentration. Mobilities parallel to [112¯0] were as high as ∼11.5cm2∕Vs (at p∼1.8×1018cm−3).

Growth of p-type and n-type m-plane GaN by molecular beam epitaxy

Plasma-assisted molecular beam epitaxial growth of Mg-doped, p-type and Si-doped, n-type m-plane GaN on 6H m-plane SiC is demonstrated. Phase-pure, m-plane GaN films exhibiting a large anisotropy in film mosaic (similar to 0.2 degrees full width at half maximum, x-ray rocking curve scan taken parallel to [112 (

High Quality, Low Cost Ammonothermal Bulk GaN Substrates

) over bar0] versus similar to 2 degrees parallel to [0001]) were grown on m-plane SiC substrates. Maximum hole concentrations of similar to 7x10(18) cm(-3) were achieved with p-type conductivities as high as similar to 5 Omega(-1) cm(-1) without the presence of Mg-rich inclusions or inversion domains as viewed by cross-section transmission electron microscopy. Temperature dependent Hall effect measurements indicate that the Mg-related acceptor state in m-plane GaN is the same as that exhibited in c-plane GaN. Free electron concentrations as high as similar to 4x10(18) cm(-3) were measured in the Si-doped m-plane GaN with corresponding mobilities of similar to 500 cm(2)/V s measured parallel to the [112 (

Blue GaN-based light-emitting diodes grown by molecular-beam epitaxy with external quantum efficiency greater than 1.5%

) over bar0] direction. (c) 2006 American Institute of Physics.

Heteroepitaxial Growth of GaN on 6H-SiC(0001) by Plasma-Assisted Molecular Beam Epitaxy

Ammonothermal GaN growth using a novel apparatus has been performed on c-plane, m-plane, and semipolar seed crystals with diameters between 5 mm and 2 in. to thicknesses of 0.5–3 mm. The highest growth rates are greater than 40 µm/h and rates in the 10–30 µm/h range are routinely observed for all orientations. These values are 5–100× larger than those achieved by conventional ammonothermal GaN growth. The crystals have been characterized by X-ray diffraction rocking-curve (XRC) analysis, optical and scanning electron microscopy (SEM), cathodoluminescence (CL), optical spectroscopy, and capacitance–voltage measurements. The crystallinity of the grown crystals is similar to or better than that of the seed crystals, with FWHM values of about 20–100 arcsec and dislocation densities of 1 ×105–5 ×106 cm-2. Dislocation densities below 104 cm-2 are observed in laterally-grown crystals. Epitaxial InGaN quantum well structures have been successfully grown on ammonothermal wafers.

Defect-mediated surface morphology of nonpolar m-plane GaN

We have grown blue (480 nm) nitride semiconductor light emitting diodes (LEDs) by plasma-assisted molecular beam epitaxy (MBE) on GaN templates. Packaged devices exhibited output powers up to 0.87 mW at 20 mA forward current. The corresponding external quantum efficiency was 1.68%. Utilizing a combination of direct current (dc) and pulsed electroluminescence measurements it has been demonstrated that at low (<20 mA) dc conditions the emission from these devices is governed by the combined effects of bandfilling and screening of electrostatic fields. However, at larger currents device heating dominates the emission properties.

Gallium adsorption onto (112̄0) gallium nitride surfaces

We report on the correlation between growth conditions and defect densities, as well as the defect generation mechanism, for GaN directly deposited on H-etched SiC(0001) by plasma-assisted molecular-beam epitaxy. 10 nm thin GaN nucleation layers were deposited at different conditions (600 to 800 °C, Ga-lean to Ga-rich). The island size increased from about 50 nm (600 °C, Ga-lean) to about 250 nm (800 °C, Ga-rich). Additionally, we examine epilayers nucleated under these conditions but overgrown with 300 nm GaN (720 °C, Ga-stable). These samples clearly show atomic steps, however, they exhibit rather different defect densities for different nucleation conditions (10 11 cm -2 for 600 °C and Ga-lean to 10 10 cm -2 for 800 °C and Ga-rich) as determined by plan-view transmission electron microscopy. Furthermore, we performed growth-stop experiments using GaN-thicknesses ranging 2 to 2000 nm for studying the defect generation. It appears that island coalescence is the main source of threading dislocations.

Low extended defect density nonpolar m-plane GaN by sidewall lateral epitaxial overgrowth

The role of extended defects in determining the atomic scale surface morphology of nonpolar {11¯00} m-plane gallium nitride has been elucidated. The heteroepitaxially grown m-GaN films are commonly reported to yield striated surface morphologies (slate morphology) correlated with their high densities of basal plane stacking faults. Here, the growth window was explored to allow nonslate morphologies for hydride vapor phase epitaxy. Lateral epitaxial overgrowth was then utilized to produce m-GaN films with three regimes of different extended defect contents. Elimination of stacking faults from the m-GaN yielded step-flow features with an average step height of 4–7 ML even for slate morphology growth conditions.

p-type conduction in stacking-fault-free m -plane GaN

We report on a systematic study of transients in reflection high energy electron diffraction specular intensities due to adsorption and subsequent desorption of gallium onto (1120) GaN surfaces both during the gallium adsorption by itself or during the growth of GaN by plasma-assisted molecular beam epitaxy. We determined the boundaries between N-rich growth, Ga-rich growth with only a two-dimensional phase of adsorbed Ga, and Ga-rich growth with droplets of liquid Ga. The thermal dependence of the boundary between the Ga-rich regimes with and without droplets was found to be approximately 3.2 eV for both the cases of gallium adsorption and GaN growth. This temperature dependence is explained in terms of a quasiequilibrium model for the interaction between the surface phases. Evidence is presented indicating that the two-dimensional phase saturates at submonolayer coverage for temperatures below 700 °C.