The field of AlGaInN ultraviolet UV light-emitting diodes (LEDs) is reviewed, with a summary of the state-of-the-art in device performance and enumeration of applications. Performance-limiting factors for high-efficiency UV LEDs are identified and recent advances in the development of deep UV emitters are presented.

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.

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The polarization of the in-plane electroluminescence of (0001) orientated (In)(Al)GaN multiple quantum well light emitting diodes in the ultraviolet-A and ultraviolet-B spectral range has been investigated. The intensity for transverse-electric polarized light relative to the transverse-magnetic polarized light decreases with decreasing emission wavelength. This effect is attributed to rearrangement of the valence bands at the Γ-point of the Brillouin zone with changing aluminum and indium mole fractions in the (In)(Al)GaN quantum wells. For shorter wavelength the crystal-field split-off hole band moves closer to the conduction band relative to the heavy and light hole bands and as a consequence the transverse-magnetic polarized emission becomes more dominant for deep ultraviolet light emitting diodes.

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.

Silicon doping of AlxGa1−xN layers with high aluminum mole fractions (0.8 &lt; x &lt; 1) was studied. The AlGaN:Si layers were pseudomorphically grown by metalorganic vapor phase epitaxy on low defect density epitaxially laterally overgrown AlN/sapphire templates. The effects of SiH4/III ratio and aluminum content on the resistivity, the carrier concentration, and the mobility have been investigated. By variation of the SiH4/III ratio during the growth of AlxGa1−xN:Si layers, a recorded low resistivity of Al0.81Ga0.19N:Si was obtained with 0.026 Ω cm. The resistivity increases exponentially with increasing aluminum content to 3.35 Ω cm for Al0.96Ga0.04N, and the optimum SiH4/III ratio is shifted towards lower values. Hall effect measurements show that the increase of the resistivity with increasing aluminum mole fraction is mainly caused by a decrease of the carrier density. The optimized Al0.81Ga0.19N:Si exhibits a carrier concentration of 1.5 × 1019 cm−3 and a mobility of the carriers of 16.5 cm2 V−1 s−1.

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