Review of III-nitride optoelectronic materials for light emission and detection

Abstract

We will review the significant achievements relating to optoelectronic devices based on III-nitrides at the center for quantum devices (CQD). Based on GaN/InGaN multiple-quantum well structures, we demonstrated blue laser diodes at a wavelength of ∼405 nm. This achievement was particularly significant at the time, because while no defect reduction technique was used, a fairly low threshold current density was achieved (3.8 kA/cm2). In the past few years, however, the interest has shifted towards shorter wavelength light emitters, i.e. ultraviolet LEDs and LDs. Lower crystalline quality and unsatisfactory doping levels of AlGaN compound semiconductors posed serious challenges en route to the realization of UV light emitters. However, steady progress in the growth of AlGaN and AlN epilayers made it possible to overcome some of the difficulties. To date, we have been able to demonstrate UV LEDs at wavelengths as short as 265 nm (corresponding to ∼45% Al in AlxGa1–xN) with optical output powers of over 5 mW. We have addressed the n-type AlGaN doping problem by using a Si–In co-doped scheme. We also employed high-quality AlGaN/AlN superlattice templates for the reduction of defects. We have also demonstrated 280 nm UV LEDs with output powers of over 6 mW and external quantum efficiencies of over 0.25%. Despite all the success in the realization of short-wavelength UV LEDs, UV laser diodes at these short wavelengths are yet to be realized. The main difficulties are the low material quality, high device resistance leading to excessive heating of the device, realization of smooth cavity mirrors, and issues related to the cracking of the material. We have also demonstrated different types of photodetectors in the UV range of the spectrum: photoconductors, MSM photodetector, Schottky barrier photodetectors, and p–i–n photodiodes to name a few. The most promising type of photodetector for realization of UV imaging focal plane arrays is the p–i–n photodiode. Realization of high-efficiency AlGaN-based p–i–n photodiodes becomes more difficult when considering the need for the collection of the light from the backside of the substrate. However, similar to our back-emission UV LED structure, we have demonstrated back-illuminated p–i–n solar-blind photodiodes with external quantum efficiencies as high as 68% under no applied bias and 74% under –5 V of bias. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)