Urbach tails in semiconductors are often associated to effects of compositional disorder. The Urbach tail observed in InGaN alloy quantum wells of solar cells and LEDs by biased photocurrent spectroscopy is shown to be characteristic of the ternary alloy disorder. The broadening of the absorption edge observed for quantum wells emitting from violet to green (indium content ranging from 0 to 28%) corresponds to a typical Urbach energy of 20 meV. A 3D absorption model is developed based on a recent theory of disorder-induced localization which provides the effective potential seen by the localized carriers without having to resort to the solution of the Schrödinger equation in a disordered potential. This model incorporating compositional disorder accounts well for the experimental broadening of the Urbach tail of the absorption edge. For energies below the Urbach tail of the InGaN quantum wells, type-II well-to-barrier transitions are observed and modeled. This contribution to the below bandgap absorption is particularly efficient in near-UV emitting quantum wells. When reverse biasing the device, the well-to-barrier below bandgap absorption exhibits a red shift, while the Urbach tail corresponding to the absorption within the quantum wells is blue shifted, due to the partial compensation of the internal piezoelectric fields by the external bias. The good agreement between the measured Urbach tail and its modeling by the new localization theory demonstrates the applicability of the latter to compositional disorder effects in nitride semiconductors.

We demonstrate a marked increase in the possible growth domain and growth rate of the O plasma-assisted molecular beam epitaxy of β-(AlxGa1−x)2O3, by adding the element In during growth. We explain these enhancement results from a metal-exchange catalytic effect. This mechanism allows us to synthesize β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures at growth conditions that are not accessible in the absence of In, stabilizing the monoclinic β-phase. We demonstrate the growth of β-(AlxGa1−x)2O3 at growth temperatures up to 900 °C. Moreover, we illustrate how additional In on the β-(AlxGa1−x)2O3 surface acts as a surface active agent, improving the crystal quality of the synthesized β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures. These structures are shown to be of the highest crystal quality up to an Al concentration of x = 0.2. We predict the novel growth mode introduced for ternary III–O thin film synthesis — shown by the example of β-(AlxGa1−x)2O3 — to be applicable for a wide range of thin film materials, whose individual constituents possess material properties similar to those discussed for the constituents contributing to β-(AlxGa1−x)2O3.

The disinfection industry would greatly benefit from efficient, robust, high-power deep-ultraviolet light-emitting diodes (UV–C LEDs). However, the performance of UV–C AlGaN LEDs is limited by poor light-extraction efficiency (LEE) and the presence of a large density of threading dislocations. We demonstrate high power AlGaN LEDs grown on SiC with high LEE and low threading dislocation density. We employ a crack-free AlN buffer layer with low threading dislocation density and a technique to fabricate thin-film UV LEDs by removing the SiC substrate, with a highly selective SF6 etch. The LEDs (278 nm) have a turn-on voltage of 4.3 V and a CW power of 8 mW (82 mW/mm2) and external quantum efficiency (EQE) of 1.8% at 95 mA. KOH submicron roughening of the AlN surface (nitrogen-polar) and improved p-contact reflectivity are found to be effective in improving the LEE of UV light. We estimate the improved LEE by semiempirical calculations to be 33% (without encapsulation). This work establishes UV LEDs grown on SiC substrates as a viable architecture to large-area, high-brightness, and high-power UV LEDs.

We demonstrate efficient semipolar (11-22) 550 nm yellow/green InGaN light-emitting diodes (LEDs) with InGaN barriers on low defect density (11-22) GaN/patterned sapphire templates. The InGaN barriers were clearly identified, and no InGaN clusters were observed by atom probe tomography measurements. The semipolar (11-22) 550 nm InGaN LEDs (0.1 mm size) show an output power of 2.4 mW at 100 mA and a peak external quantum efficiency of 1.3% with a low efficiency drop. In addition, the LEDs exhibit a small blue-shift of only 11 nm as injection current increases from 5 to 100 mA. These results suggest the potential to produce high efficiency semipolar InGaN LEDs with long emission wavelength on large-area sapphire substrates with economical feasibility.

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Using atom probe tomography, it is demonstrated that Mg doping of GaN nanowires grown by Molecular Beam Epitaxy results in a marked radial inhomogeneity, namely a higher Mg content in the periphery of the nanowires. This spatial inhomogeneity is attributed to a preferential incorporation of Mg through the m-plane sidewalls of nanowires and is related to the formation of a Mg-rich surface which is stabilized by hydrogen. This is further supported by Raman spectroscopy experiments which give evidence of Mg-H complexes in the doped nanowires. A Mg doping mechanism such as this, specific to nanowires, may lead to higher levels of Mg doping than in layers, boosting the potential interest of nanowires for light emitting diode applications.

We report direct growth of 1550-nm InGaAsP multi-quantum-well (MQW) structures in densely packed, smooth, highly crystalline, and millimeter-long InP nanoridges grown by metalorganic chemical vapor deposition on silicon-on-insulator (SOI) substrates. Aspect-ratio-trapping and selective area growth techniques were combined with a two-step growth process to obtain good material quality as revealed by photoluminescence, scanning electronic microscopy, and high-resolution X-ray diffraction characterization. Transmission electron microscopy images revealed sharp MQW/InP interfaces as well as thickness variation of the MQW layer. This was confirmed by atom probe tomography analysis, which also suggests homogenous incorporation of the various III-V elements of the MQW structure. This approach is suitable for the integration of InP-based nanoridges in the SOI platform for new classes of ultra-compact, low-power, nano-electronic, and photonic devices for future tele- and data-communications applications.