M. D. Craven

Structural characterization of nonpolar (112̄0) a-plane GaN thin films grown on (11̄02) r-plane sapphire

In this letter we describe the structural characteristics of nonpolar (1120) a-plane GaN thin films grown on (1102) r-plane sapphire substrates via metalorganic chemical vapor deposition. Planar growth surfaces have been achieved and the potential for device-quality layers realized by depositing a low temperature nucleation layer prior to high temperature epitaxial growth. The in-plane orientation of the GaN with respect to the r-plane sapphire substrate was confirmed to be [0001]GaN‖[1101]sapphire and [1100]GaN‖[1120]sapphire. This relationship is explicitly defined since the polarity of the a-GaN films was determined using convergent beam electron diffraction. Threading dislocations and stacking faults, observed in plan-view and cross-sectional transmission electron microscope images, dominated the a-GaN microstructure with densities of 2.6×1010u2009cm−2 and 3.8×105u2009cm−1, respectively. Submicron pits and crystallographic terraces were observed on the optically specular a-GaN surface with atomic force m...

Defect reduction in (11¯00) m-plane gallium nitride via lateral epitaxial overgrowth by hydride vapor phase epitaxy

This letter reports on extended defect density reduction in m-plane (11¯00) GaN films achieved via lateral epitaxial overgrowth (LEO) by hydride vapor phase epitaxy. Several dielectric mask patterns were used to produce 10 to 100 μm-thick, partially and fully coalesced nonpolar GaN films. X-ray rocking curves indicated the films were free of wing tilt. Transmission electron microscopy showed that basal plane stacking fault (SF) and threading dislocation (TD) densities decreased from 105cm−1 and 109cm−2, respectively, less than 3×103cm−1 and ∼5×106cm−2, respectively, in the Ga-face (0001) wing of the LEO films. SFs persisted in ⟨0001⟩-oriented stripe LEO films, though TD reduction was observed in the windows and wings. Band-edge cathodoluminescence intensity increased 2 to 5 times in the wings compared to the windows depending on the stripe orientation. SFs in the low TD density wings of ⟨0001⟩-stripe films did not appear to act as nonradiative recombination centers.

Threading dislocation reduction via laterally overgrown nonpolar (112̄0) a-plane GaN

Threading dislocation density reduction of nonpolar (1120) a-plane GaN films was achieved by lateral epitaxial overgrowth (LEO). We report on the dependence of morphology and defect reduction on crystallographic stripe orientation. Stripes aligned along [0001] and [1100], the most favorable a-plane GaN LEO stripe orientations, possessed well-behaved, symmetric morphologies. Threading dislocation reduction via mask blocking was observed by transmission electron microscopy for [1100] stripes which had optimal rectangular cross-sections. Cathodoluminescence studies showed increased light emission for the overgrown regions in comparison to the window regions. The extent of lateral overgrowth of these stripes was asymmetric due to the opposing polarities of the vertical c-plane sidewalls. Conversely, threading dislocations propagated into the symmetric overgrown regions of [0001] stripes which possessed coexisting inclined and vertical {1010} facets.

Structural and morphological characteristics of planar (112̄0) a-plane gallium nitride grown by hydride vapor phase epitaxy

This letter discusses the structural and morphological characteristics of planar, nonpolar (1120) a-plane GaN films grown on (1102) r-plane sapphire by hydride vapor phase epitaxy. Specular films with thicknesses over 50 μm were grown, eliminating the severely faceted surfaces that have previously been observed for hydride vapor phase epitaxy-grown a-plane films. Internal cracks and crack healing, similar to that in c-plane GaN films, were observed. Atomic force microscopy revealed nanometer-scale pitting and steps on the film surfaces, with rms roughness of ∼2 nm. X-ray diffraction confirmed the films are solely a-plane oriented with on-axis (1120) and 30° off-axis (1010) rocking curve peak widths of 1040 and 3000 arcsec, respectively. Transmission electron microscopy revealed a typical basal plane stacking fault density of 4×105u2009cm−1. The dislocation content of the films consisted of predominately edge component (bedge=±[0001]) threading dislocations with a density of 2×1010u2009cm−2, and mixed-characte...

Microstructural evolution of a-plane GaN grown on a-plane SiC by metalorganic chemical vapor deposition

This letter describes the relationship between the morphological evolution of heteroepitaxial a-plane GaN films and the formation of the extended defect structure. The initial a-plane GaN growth on a-plane SiC substrates (via a high temperature AlN buffer layer) follows a Volmer–Weber growth mode. Consequently, the coalescence of three-dimensional (3D) islands generates threading dislocations which dominate the nonpolar GaN film’s microstructure (3×1010u2009cm−2). Exposed nitrogen-face surfaces, identified using x-ray diffraction measurements and convergent beam electron diffraction analysis, are present throughout the 3D growth and are the likely source of basal plane faulting (7×105u2009cm−1) within the film. Atomic force microscopy and scanning electron microscopy were used to image the morphological transition, which was correlated to changes in the a-GaN crystal tilt mosaic measured by x-ray rocking curves.

Bulk GaN flip-chip violet light-emitting diodes with optimized efficiency for high-power operation

We report on violet-emitting III-nitride light-emitting diodes (LEDs) grown on bulk GaN substrates employing a flip-chip architecture. Device performance is optimized for operation at high current density and high temperature, by specific design consideration for the epitaxial layers, extraction efficiency, and electrical injection. The power conversion efficiency reaches a peak value of 84% at 85u2009°C and remains high at high current density, owing to low current-induced droop and low series resistance.

Improved quantum efficiency in nonpolar (112̄0) AlGaN/GaN quantum wells grown on GaN prepared by lateral epitaxial overgrowth

Radiative and nonradiative excitonic transitions in nonpolar (1120)u200aAlxGa1−xN/GaN multiple quantum wells (MQWs) grown on the GaN template prepared by lateral epitaxial overgrowth (LEO-GaN) were investigated. The structural advantages of using nonpolar orientations were confirmed by a moderate shift of the photoluminescence (PL) peak energy and negligible change in low-temperature PL lifetime with decreasing GaN well width, both of which are the results of eliminating quantum-confined Stark effects due to the polarization fields that exist in polar (0001) MQWs. Appearance of the correct in-plane light polarization and improved internal quantum efficiency for the PL peak in the MQWs on LEO-GaN were attributed to the reduction in densities of nonradiative defects and bound states.

Optical evidence for lack of polarization in (112¯0) oriented GaN∕(AlGa)N quantum structures

We apply continuous and time resolved photoluminescence spectroscopy for studying GaN∕AlGaN multiquantum wells structures grown on nonpolar a-plane GaN templates. We found that (a) the energy of the emission from the nonpolar samples decreases slightly with the quantum well width, in a manner explained by the quantum size effect only; (b) the energy differences between the absorption and the emission peaks are independent of the well width; and (c) the decay time of the photoluminescence is only slightly dependent on the quantum well width and is quite similar to that of bulk GaN. These observations are markedly different from measurements obtained from conventional polar [0001] oriented quantum well samples. They clearly demonstrate the absence of an electric field in the nonpolar samples. Our observations are favorably compared with an eight bands k∙P model calculations.

Nonpolar a-plane p-type GaN and p-n Junction Diodes

Growth and electrical characteristics of Mg-doped p-type nonpolar (112¯0) a-plane GaN films, grown on (11¯02) r-plane sapphire substrates via metalorganic chemical vapor deposition, were investigated as a function of growth rate, the ammonia to trimethylgallium flow ratio (V/III ratio), and the growth temperature. The electrical conductivity of the films exhibited a strong dependence on the growth parameters. Secondary-ion-mass-spectroscopy measurements indicated that more Mg was incorporated at higher growth rate and at lower growth temperatures. The Mg concentration in the films increased linearly with the Mg flow. A maximum hole concentration of 6.8×1017cm−3 was achieved at room temperature for a Mg concentration of 7.6×1019cm−3, corresponding to 0.9% ionization. Further increase in the Mg concentration resulted in increased surface roughness as well as a significant decrease in the hole concentration. p-n junction diodes were fabricated using nonpolar a-plane GaN, and the current-voltage characteristi...

Nonpolar (11&1macr;0) a-Plane Gallium Nitride Thin Films Grown on (1&1macr;02) r-Plane Sapphire: Heteroepitaxy and Lateral Overgrowth

Nonpolar (1120) a-plane GaN films with planar surfaces were grown on (1102) r-plane sapphire substrates via metalorganic chemical vapor deposition by depositing a low temperature nucleation layer prior to the high temperature epitaxial growth. The film/substrate epitaxial relationship was defined by X-ray diffraction measurements to be [0001] GaN ∥ [1101] sapphire and [1100] GaN ∥ [1120] sapphire while the polarity of the GaN c-axis was confirmed using convergent beam electron diffraction. The film morphology and microstructure were studied using atomic force microscopy and transmission electron microscopy (TEM), respectively. Threading dislocations (TDs), which dominated the heteroepitaxial a-plane GaN microstructure (density ∼2.6 x 10 10 cm -2 ), were reduced via lateral epitaxial overgrowth from mask stripe openings. The overgrowth from stripes aligned along [1100] was dislocation-free, as observed using TEM and cathodoluminescence, and dependent upon crystal polarity. For all other stripe orientations, TDs were observed to propagate into the overgrown regions.