AFM micrographs of (a) an N-polar HVPE GaN substrate wafer surface after an organic solvent degrease and etch in 1:1 HF:DI water; N-polar homoepitaxial layer surface of samples C (b) and D (c); (d) surface of a 100-nm-thick UID N-polar GaN layer grown on a 1 -μm-thick, 3 × 1019 cm−3 GaN:Be layer similar to sample D. Note the different lateral scale. The vertical ranges are 38 nm (a), 12 nm (b), 96 nm (c), and 1.4 nm (d). The root-mean-square roughnesses of the surfaces in (a) and (d) are 3.7 and 0.2 nm, respectively. Remnants of Ga droplets after etching with HF are visible in (c) as circular regions of ∼5 μm diameter.
(Color online) (a) SIMS analysis of homoepitaxial N-polar GaN sample C. Silicon, carbon, and oxygen profiles exhibit peaks at the regrowth interface. Si and C concentrations drop rapidly to detection limits in the epitaxial layer, while [O] exhibits a gradual exponential decay. H is at the detection limit in the epitaxial layer and the substrate. The oxygen detection limit is 2 × 1016 cm−3. (b) SIMS analysis of a Ga-polar epitaxial layer showing qualitatively similar impurity profiles, with approximately one order of magnitude lower [O] in the epitaxial layer. Si, C, and H are at or near the detection limits.
Two-beam bright-field cross-sectional transmission electron micrographs of N-polar GaN homoepitaxial layers A (a), (b) and D (c), (d). The diffraction vectors are as shown. Both micrographs in each set are of the same region of the respective sample. The majority of threading dislocations observed, e.g. in (a) and (d), were of the edge or mixed type. Dislocation density tended to diminish as growth temperature increased.
Symmetric (00.4) ω–2θ XRD spectrum of a GaN/AlGaN heterostructure on N-polar GaN. The AlGaN peak FWHM is 530 arc sec. The Al content in the AlGaN layer is determined to be 36%.
(a) Cross-sectional electron micrograph of sample G showing the ultrathin (15 Å) AlN layer and near-surface features (arrowed). (b) High angle annular dark field image of the interface between the GaN channel and AlGaN barrier of sample G. Quantitative analysis reveals 1:1 periodic compositional modulation in the AlGaN barrier consistent with vertical segregation of AlN and GaN. The interface is smooth and abrupt.
Sample ID, substrate temperature, Ga/N flux ratio, and doping (species and concentration) for homoepitaxial Ga layers.
GaN/AlGaN heterostructure sample ID, presence (Y) or absence (N) of Ga droplets on as-grown sample surfaces, UID GaN layer thickness, Hall mobility and sheet density, sheet resistance, and buffer leakage current across a 16 μm annulus at 100 V bias. Values and uncertainties given are averages and standard deviations of five measurements at different locations on each sample.
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