We report the development of AlGaN based deep ultraviolet light emitting diodes (UV-LEDs) by molecular beam epitaxy. By growing the AlGaN well layer under Ga-rich conditions to produce strong potential fluctuations, internal quantum efficiency of a quantum well structure emitting at 300 nm was found to be 32%. By combining such Ga-rich growth condition in the active region with polarization field enhanced carrier injection layers, deep UV-LEDs emitting at 273 nm were obtained with output power of 0.35 mW and 1.3 mW at 20 mA continuous wave and 100 mA pulsed drive current, respectively. The maximum external quantum efficiency was 0.4%.

A fiber-optic pump-probe setup is used to demonstrate all-optical switching based on intersubband cross-absorption modulation in GaN/AlN quantum-well waveguides, with record low values of the required control pulse energy. In particular, a signal modulation depth of 10 dB is obtained with control pulse energies as small as 38 pJ. Such low power requirements for this class of materials are mainly ascribed to an optimized design of the waveguide structure. At the same time, the intersubband absorption fully recovers from the control-pulse-induced saturation on a picosecond time scale, so that these nonlinear waveguide devices are suitable for all-optical switching at bit rates of several hundred Gb/s.

Near‐ultraviolet light emitting diodes (LEDs), based on one monolayer thick InN/GaN multiple quantum wells, were grown by radio‐frequency plasma‐assisted molecular beam epitaxy. The active region was grown at 685 °C, a temperature where the growth of thick InN layers is not possible. High resolution transmission electron microscopy revealed that the InN well layers were grown pseudomorphically in between the GaN barriers and were characterized by well defined interfaces and uniform thickness. The LED structures exhibited electroluminescence emission at 384 nm, with a FWHM of about 14 nm. The peak emission wavelength was independent of the injection current, indicating the absence of quantum confined Stark effect in these structures. Output optical power of 0.84 mW was measured at 200 mA in a bare die configuration. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An ultraviolet electroabsorption modulator based on AlGaN∕GaN quantum wells is demonstrated. Enhanced excitonic absorption in the quantum wells at around 3.48eV was achieved using a Schottky contact to partially cancel the polarization-induced electric fields in the quantum well layers. A change in the absorption coefficient greater than 4×104cm−1 was obtained for a modest reverse bias of 10V. The observed blueshift in the exciton energy was smaller than that predicted by theoretical calculations. This is accounted for by variations in the background carrier concentration in the wells with reverse bias.

Ultraviolet electroabsorption modulators based on bulk GaN films and on GaN/AlGaN multiple quantum wells were developed and characterized. In both types of devices, the absorption edge at room temperature is dominated by excitonic effects and can be strongly modified through the application of an external electric field. In the bulk devices, the applied voltage causes a broadening and quenching of the excitonic absorption, leading to enhanced transmission. In the quantum-well devices, the external field partially cancels the built-in polarization-induced electric fields in the well layers, thereby increasing the absorption. Unlike optical modulators based on smaller-bandgap zinc blende semiconductors, the bulk devices here are shown to provide similar performance levels as the quantum well devices, which is mainly a consequence of the uniquely large exciton binding energies of nitride semiconductors.