Asako Hirai

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.

Formation and reduction of pyramidal hillocks on m-plane {11¯00} GaN

Surface morphology and hillock reduction were studied on m-plane {11¯00} n-type GaN films and light emitting diode structures grown by metal organic chemical vapor deposition on low defect-density m-plane GaN substrates. For nominally on-axis m-plane films, predominantly pyramidal hillocks were observed, which were composed of two faces symmetrically inclined by 0.1°–0.25° to the ±[112¯0] a direction and two faces inclined by 0.5°–0.95° to the [0001¯] c− and the [0001] c+ directions, respectively. All faces of the pyramidal hillocks for the nominally on-axis GaN films had clearly defined step-terrace structures. Gradual changes in nominal miscut angles from 0° to 10° along the a and the c− directions succeeded in a continuous hillock reduction yielding atomically flat surfaces.

Molecular beam epitaxy and structural anisotropy of m-plane InN grown on free-standing GaN

This study reports on the growth of high-quality nonpolar m-plane [11¯00] InN films on free-standing m-plane GaN substrates by plasma-assisted molecular beam epitaxy. Optimized growth conditions (In/N ratio ∼1 and T=390–430 °C) yielded very smooth InN films with undulated features elongated along the [112¯0] orientation. This directionality is associated with the underlying defect structure shown by the anisotropy of x-ray rocking curve widths parallel to the [112¯0] (i.e., 0.24°–0.34°) and [0001] (i.e., 1.2°–2.7°) orientations. Williamson–Hall analysis and transmission electron microscopy identified the mosaic tilt and lateral coherence length and their associations with different densities of dislocations and basal-plane stacking faults. Ultimately, very low band gap energies of ∼0.67 eV were measured by optical absorption similar to the best c-plane InN.

Defect-mediated surface morphology of nonpolar m-plane GaN

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.

Enhanced terahertz radiation from high stacking fault density nonpolar GaN

Terahertz emission from high stacking fault density m-GaN has been observed using ultrafast pulse excitation. The terahertz signal exhibits a 360° periodicity with sample rotation and a polarity flip at 180°, characteristic of real carrier transport in an in-plane electric field parallel to the c axis induced by stacking fault (SF)-terminated internal polarization at wurtzite domain boundaries. The terahertz emission can be enhanced by several times relative to that from a SF-free m-GaN sample, for which the terahertz signal emanates from surface surge currents and diffusion-driven carrier transport normal to the surface and is independent of the c-axis orientation.

Anisotropy of tensile stresses and cracking in nonbasal plane AlxGa1−xN/GaN heterostructures

AlxGa1−xN films grown on nonpolar m {11¯00} and {112¯2} semipolar orientations of freestanding GaN substrates were investigated over a range of stress states (x≤0.17). Cracking on the (0001) plane was observed beyond a critical thickness in the {11¯00} oriented films, while no cracking was observed for {112¯2} films. Theoretical analysis of tensile stresses in AlxGa1−xN for the relevant planes revealed that anisotropy of in-plane biaxial stress for the nonpolar {11¯00} planes results in the highest normal stresses on the c-planes, consistent with experimental observations. Shear stresses are significant in the semipolar case, suggesting that misfit dislocation formation provides an alternative mechanism for stress relief.

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

Sidewall lateral epitaxial overgrowth (SLEO) is demonstrated by metal organic chemical vapor deposition for nonpolar {11¯00} m-plane GaN films. m-plane GaN films were grown by metal organic chemical vapor deposition on m-plane 6H SiC substrates with an AlN initiation layer. Subsequently, an SiO2 stripe dielectric pattern was formed with 2μm window openings parallel to the [112¯0] a-direction and an 8μm mask and then the m-plane GaN was etched through the window openings to reveal Ga-face (0001) and N-face (0001¯) sidewalls. The SLEO growth was achieved in two growth steps—lateral growth from the sidewalls and subsequent growth through and then over the dielectric mask openings. In comparison to planar m-plane GaN films grown on 6H SiC, the threading dislocation density was reduced from low 1010to3×108cm−2 and the stacking fault density was reduced by one order of magnitude.

Photoelectrochemical etching of p-type GaN heterostructures

We have developed a method for photoelectrochemical etching of p-type semiconductors, including GaN, that relies on the built-in bandbending already inherent to optical devices. Electron-hole pairs are generated by filtered light in a buried small bandgap layer, and a pn junction separates the charge. Electrons are sent into the n-type layer, where they are extracted, and holes to the surface, where they participate in etching reactions. This technique is rapid and inexpensive, and it requires no applied bias or elevated temperatures. This technique has widespread applications to GaN optical devices where ion-damage-free etching or wide tunability of etch parameters is desired.

Photoelectrochemical Undercut Etching of m-Plane GaN for Microdisk Applications

Undercut etching is a necessary technique for a variety of device applications, including microdisk lasers. We have explored bandgap-selective photoelectrochemical etching of nonpolar m-plane GaN for undercut etching applications, including microdisks. These nonpolar optical devices are not limited by the quantum-confined Stark effect that hampers the performance of polar c-plane GaN devices. We discuss the dependence of undercut quality on etchant concentration, illumination intensity, masking material, and epitaxial structure and use this technique to fabricate m-plane microdisks. In these nonpolar microdisks, the in-plane polarization fields have a dramatic effect on the symmetry of the etching in both the undercut etching and in the unwanted etching of the GaN disk layer. With a careful balance of etchant concentration and illumination intensity and a well-designed epitaxial structure, we have achieved smoother optical cavities than were possible in c-plane GaN.

Observation of whispering gallery modes in nonpolar m -plane GaN microdisks

We have fabricated nonpolar GaN/InGaN microdisks using band-gap selective photoelectrochemical etching. These microdisks have a smoother optical cavity than our previous c-plane microdisks, and they support whispering gallery modes with quality factors as high as 2000 after a focused ion beam treatment to the quantum wells. Because of the lack of a Stokes shift in the quantum wells of these m-plane disks, absorption losses play a much more significant role than in our earlier c-plane microdisks, and the light which couples into the modes is emission from the InGaN post rather than the quantum wells within the cavity.