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Highly insulating layers are a prerequisite for gallium nitride (GaN)‐based power electronic devices. For this purpose, carbon doping is one of the currently pursued approaches. However, its impact on the optical and electrical properties of GaN has been widely debated in the scientific community. For further improvement of device performance, a better understanding of the role of related defects is essential. To study optically active point defects, photoluminescence is one of the most frequently used experimental characterization techniques. Herein, the main recent advances in the attribution of carbon‐related photoluminescence bands are reviewed, which were enabled by the interplay of a refinement of growth and characterization techniques and state‐of‐the‐art first‐principles calculations developed during the last decade. The predicted electronic structures of isolated carbon defects and selected carbon‐impurity complexes are compared to experimental results. Taking into account both of these, a comprehensive overview on the present state of interpretation of carbon‐related broad luminescence bands in bulk GaN is presented. [ABSTRACT FROM AUTHOR]

AlGaN growth by hydride vapor phase epitaxy on patterned sapphire substrates has been investigated. Growth on honeycomb-shaped holes is disturbed by parasitic growth of c-plane oriented AlGaN crystallites on n-plane sapphire facets. Triangular hole-like structures allow for complete suppression of parasitic c-plane oriented AlGaN crystallites and coalescence of c-plane AlGaN at very low layer thickness. Additionally, triangular columnar sapphire patterns allow for biaxial strain relaxation and higher crystalline perfection. A total pressure of 400 hPa is favored for lateral overgrowth of ternary AlGaN by hydride vapor phase epitaxy. Lower pressures lead to strong composition inhomogeneity and higher pressure to lower crystal perfection. Additionally, very low V/III ratios of less than 10 yield best results. Lower ammonia supply support the growth of beneficial {11-22}-AlGaN crystallites on sapphire m-sidewalls which supresses undesired AlGaN growth on sapphire n-facets. [ABSTRACT FROM AUTHOR]

To the end of improvement of layer morphology and crystalline perfection of thick AlGaN layers grown by hydride vapor phase epitaxy (HVPE) as well as for the suppression of crystallite formation during growth, the impact of AlN buffer layer properties on AlGaN growth was investigated. While the surface morphology of 500 nm thick AlN layer improves with higher V/III ratio toward 50 and lower growth temperatures of 1020 °C, its use as a buffer layer for thick AlGaN layers leads to strong degradation of AlGaN surface and even to formation of different crystallites during growth. Best surface morphology and crystalline perfection of thick AlGaN layers was observed for the growth on an AlN buffer layer with a higher crystal quality disregarding its 3D morphology and high surface roughness. Best results were achieved at medium V/III ratio of about 10-20 and AlN buffer growth temperature of 1060 °C. [ABSTRACT FROM AUTHOR]

C-plane-oriented sapphire substrates that were patterned on the nanoscale were overgrown by AlN using metal-organic vapor phase epitaxy. The occurrence of undesired misaligned AlN growth was detected. We found that this misaligned growth can be overcome by a proper choice of growth temperature and V/III ratio. Up to 8 μm thick c-plane-oriented AlN with a coalesced surface was obtained. An effective dislocation reduction was found due to bending of threading dislocation lines toward free surfaces during lateral growth. The distribution of crystal defects suggests that step bunching in AlN is accompanied by dislocation accumulation. Furthermore, nearly defect-free AlN crystallites with a hexagonal shape and a size of about 2 μm were observed. Schematic cross-section representation of AlN grown on nano-patterned sapphire. Different AlN crystal orientations (arrows) and dislocations (solid black lines) are indicated. [ABSTRACT FROM AUTHOR]

The exacting standards of quality and reliability of structured substrates in the semiconductor industry require in-depth knowledge concerning the interior structure of the devices. In this application note we present [...]

In this work results on the influence of the total pressure, V/III ratio and substrate miscut direction on the growth of Al0.45Ga0.55N on trench patterned sapphire by hydride vapour phase epitaxy are presented. It was found that for trenches running along the [11-20]sapphire direction a substrate miscut towards m-plane is favourable to obtain a coalesced c-plane oriented AlGaN layer. With an optimized total pressure and V/III ratio coalesced flat c-oriented AlGaN layers of up to 40 µm thickness were grown. The layers show transparency for wavelengths above 280 nm. An abrupt change in the luminescence spectra from band edge dominated to yellow luminescence dominated emission was found at the change over from uncoalesced to coalesced regions. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Carbon‐doping of GaN layers with thickness in the mm‐range is performed by hydride vapor phase epitaxy. Characterization by optical and electrical measurements reveals semi‐insulating behavior with a maximum of specific resistivity of 2 × 1010 Ω cm at room temperature found for a carbon concentration of 8.8 × 1018 cm−3. For higher carbon levels up to 3.5 × 1019 cm−3, a slight increase of the conductivity is observed and related to self‐compensation and passivation of the acceptor. The acceptor can be identified as CN with an electrical activation energy of 0.94 eV and partial passivation by interstitial hydrogen. In addition, two differently oriented tri‐carbon defects, CN‐a‐CGa‐a‐CN and CN‐a‐CGa‐c‐CN, are identified which probably compensate about two‐thirds of the carbon which is incorporated in excess of 2 × 1018 cm−3. [ABSTRACT FROM AUTHOR]

Thick Al x Ga 1− x N layers were grown by hydride vapor phase epitaxy on hexagonally patterned sapphire substrates. Non-c-planar growth is found inside the etched honeycombs which in part hinders coalescence of the c-plane AlGaN layer growing on top of the ridges. From X-ray diffraction, electron backscatter diffraction and scanning electron microscopy, the orientations of the parasitic crystallites were identified as {11–22} and {1–103} AlGaN growing on m-plane sapphire sidewalls as well as c-plane oriented AlGaN growing on n-plane sidewall facets which are located in the corners of the combs. According to the geometry of parasitic crystallites, it is further observed, that the semipolar growth occurring on sapphire m-plane sidewalls does not hinder the coalescence of c-plane AlGaN growing on top of the ridges, whereas fast propagation of parasitic crystallites nucleating on n-plane sidewall facets leads to delayed layer coalescence. [ABSTRACT FROM AUTHOR]

The interplay between band gap renormalization and band filling (Burstein-Moss effect) in n-type wurtzite GaN is investigated. For a wide range of electron concentrations up to 1.6 × 1020 cm-3 spectroscopic ellipsometry and photoluminescence were used to determine the dependence of the band gap energy and the Fermi edge on electron density. The band gap renormalization is the dominating effect up to an electron density of about 9 × 1018 cm-3; at higher values the Burstein-Moss effect is stronger. Exciton screening, the Mott transition, and formation of Mahan excitons are discussed. A quantitative understanding of the near gap transition energies on electron density is obtained. Higher energy features in the dielectric functions up to 10 eV are not influenced by band gap renormalization. [ABSTRACT FROM AUTHOR]