n - Zn O ∕ p - Ga N : Mg heterojunction light emitting diode (LED) mesas were fabricated on c-Al2O3 substrates using pulsed laser deposition for the ZnO and metal organic chemical vapor deposition for the GaN:Mg. High crystal quality and good surface morphology were confirmed by x-ray diffraction and scanning electron microscopy. Room temperature (RT) photoluminescence (PL) showed an intense main peak at 375nm and a negligibly low green emission indicative of a near band edge excitonic emission from a ZnO layer with low dislocation/defect density. The LEDs showed I-V characteristics confirming a rectifying diode behavior and a RT electroluminescence (EL) peaked at about 375nm. A good correlation between the wavelength maxima for the EL and PL suggests that recombination occurs in the ZnO layer and that it may be excitonic in origin. This also indicates that there is significant hole injection from the GaN:Mg into the ZnO.
Avalanche p-i-n photodiodes were fabricated on AlN templates for back illumination. Structures with different intrinsic layer thicknesses were tested. A critical electric field of 2.73MV∕cm was estimated from the variation of the breakdown voltage with thickness. From the device response under back and front illumination and the consequent selective injection of holes and electrons in the junction, ionization coefficients were obtained for GaN. The hole ionization coefficient was found to be higher than the electron ionization coefficient as predicted by theory. Excess multiplication noise factors were also calculated for back and front illumination, and indicated a higher noise contribution for electron injection.
Avalanche multiplication has been observed in solar-blind AlGaN-based p-i-n photodiodes. Upon ultraviolet illumination, the optical gain shows a soft breakdown starting at relatively low electric fields, eventually saturating without showing a Geiger mode breakdown. The devices achieve a maximum optical gain of 700 at a reverse bias of 60 V. By modeling the device, it is found that this corresponds to an electric-field strength of 1.7MV∕cm.
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