No abstract

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.

For AlGaN-based multi-quantum-well light emitters grown on c-plane substrates there is a tendency for the polarization of the emitted light to switch from transverse electric (TE) polarization to transverse magnetic (TM) polarization as the wavelength decreases. This transition depends on various factors that include the strain in the quantum well. Experimental results are presented that illustrate the phenomenon for nitride light emitting diodes (LEDs) grown on sapphire and on bulk AlN. Model calculations are presented which quantify the dependence of the TE/TM switch on the quantum well strain and the Al composition in the barriers surrounding the well.

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.

Optically pumped ultraviolet lasers were fabricated on low-defect-density bulk (0001) AlN substrates. AlxGa1-xN/AlyGa1-yN heterostructures were grown by metal–organic vapor phase epitaxy near atmospheric pressure. Time-resolved photoluminescence studies of the multiple quantum well emission show long decay times of 900 ps at room temperature and confirm the high structural quality of the epitaxial layers. Laser resonators with a length of about 1 mm were formed by cleaving the AlN crystal to obtain m-plane mirror facets. Lasing is demonstrated at a wavelength of 267 nm with a threshold power density as low as 126 kW/cm2 at room temperature. The laser emission was transverse electrically polarized.