InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0 1 0) and on semipolar (1 0 1 2), (1 1 2 2), (1 0 1 1) as well as (2 0 2 1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0 1 1) < (1 1 2 2) = (0 0 0 1) < (2 0 2 1) < (1 0 1 0) = (1 0 1 2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0 1 0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0 1 0) and the semipolar (1 0 1 2) and (2 0 2 1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1 2 2) and (1 0 1 1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

The design and Mg-doping profile of AlN/Al0.7Ga0.3N electron blocking heterostructures (EBH) for AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) emitting below 250 nm was investigated. By inserting an AlN electron blocking layer (EBL) into the EBH, we were able to increase the quantum well emission power and significantly reduce long wavelength parasitic luminescence. Furthermore, electron leakage was suppressed by optimizing the thickness of the AlN EBL while still maintaining sufficient hole injection. Ultraviolet (UV)-C LEDs with very low parasitic luminescence (7% of total emission power) and external quantum efficiencies of 0.19% at 246 nm have been realized. This concept was applied to AlGaN MQW LEDs emitting between 235 nm and 263 nm with external quantum efficiencies ranging from 0.002% to 0.93%. After processing, we were able to demonstrate an UV-C LED emitting at 234 nm with 14.5 μW integrated optical output power and an external quantum efficiency of 0.012% at 18.2 A/cm2.

No abstract

We report on the performance of AlGaN-based deep ultraviolet light-emitting diodes (UV-LEDs) emitting at 265 nm grown on stripe-patterned high-temperature annealed (HTA) epitaxially laterally overgrown (ELO) aluminium nitride (AlN)/sapphire templates. For this purpose, the structural and electro-optical properties of ultraviolet-c light-emitting diodes (UVC-LEDs) on as-grown and on HTA planar AlN/sapphire as well as ELO AlN/sapphire with and without HTA are investigated and compared. Cathodoluminescence measurements reveal dark spot densities of 3.5 × 10 9    cm − 2 , 1.1 × 10 9    cm − 2 , 1.4 × 10 9    cm − 2 , and 0.9 × 10 9    cm − 2 in multiple quantum well samples on as-grown planar AlN/sapphire, HTA planar AlN/sapphire, ELO AlN/sapphire, and HTA ELO AlN/sapphire, respectively, and are consistent with the threading dislocation densities determined by transmission electron microscopy (TEM) and high-resolution X-ray diffraction rocking curve. The UVC-LED performance improves with the reduction of the threading dislocation densities (TDDs). The output powers (measured on-wafer in cw operation at 20 mA) of the UV-LEDs emitting at 265 nm were 0.03 mW (planar AlN/sapphire), 0.8 mW (planar HTA AlN/sapphire), 0.9 mW (ELO AlN/sapphire), and 1.1 mW (HTA ELO AlN/sapphire), respectively. Furthermore, Monte Carlo ray-tracing simulations showed a 15% increase in light-extraction efficiency due to the voids formed in the ELO process. These results demonstrate that HTA ELO AlN/sapphire templates provide a viable approach to increase the efficiency of UV-LEDs, improving both the internal quantum efficiency and the light-extraction efficiency.

The optical polarization of emission from ultraviolet (UV) light emitting diodes (LEDs) based on (0001)-oriented AlxGa1−xN multiple quantum wells (MQWs) has been studied by simulations and electroluminescence measurements. With increasing aluminum mole fraction in the quantum well x, the in-plane intensity of transverse-electric (TE) polarized light decreases relative to that of the transverse-magnetic polarized light, attributed to a reordering of the valence bands in AlxGa1−xN. Using k ⋅ p theoretical model calculations, the AlGaN MQW active region design has been optimized, yielding increased TE polarization and thus higher extraction efficiency for bottom-emitting LEDs in the deep UV spectral range. Using (i) narrow quantum wells, (ii) barriers with high aluminum mole fractions, and (iii) compressive growth on patterned aluminum nitride sapphire templates, strongly TE-polarized emission was observed at wavelengths as short as 239 nm.

No abstract

No abstract

Polarized photoluminescence of strained quantum wells grown on c-plane, semipolar (1012), (1122), (1011), (2021) planes, and nonpolar GaN substrates was studied experimentally and in theory. The observed optical polarization switching between the substrate orientations (1012) and (1122) is in accordance with our general model of polarization switching, based on a k x p model of arbitrary substrate orientation. Spectrally resolved measurements of the polarization degree stemming from (1012) samples show that the maximum of the polarization degree is red-shifted with respect to the maximum of the photoluminescence intensity. We ascribe this effect to an increased polarization of the transitions for higher indium content