Deep UV AlGaN lasers
Optoelectronic devices such as light-emitting diodes (LEDs) and lasers emitting in the ultraviolet (UV) spectral range are of great interest for various applications. These include water purification, bioagent detection systems, medical diagnosis devices, UV curing, and material processing. However, the realization of high performance devices within the AlGaInN material system remains challenging due to physical limitations and the complexity of synthesizing material of high crystalline quality. Typically, the performance of such devices significantly degrades as the wavelength becomes shorter with increasing Al fraction in the epitaxial layers. This reduction in performance can be ascribed in part to reduced electron and hole concentrations as well as to a reduction in injection efficiency. Another major factor underlying the lower performance is the degradation in material quality, for example, when using foreign substrates such as sapphire. For these reasons, the shortest emission wavelength for a laser diode reported to date is 336 nm (Yoshida et al., APL 93, 241106).
In this presentation, we report recent progress on the development towards sub-300 nm laser diodes by using high-quality bulk AlN substrates. The single-crystal AlN substrates feature excellent crystalline quality with typical dislocation densities < 103 cm-2, x-ray diffraction rocking curve values of about 20 arc sec for the (002)-reflection, and rms surface roughness of 0.1 nm. The AlxGa1-xN/AlyGa1-yN hetero-structures were grown by metal-organic vapor phase epitaxy. The test laser structures include AlN/AlGaN transition layers, AlGaN cladding layers, and AlGaN waveguides. The whole layer stack and the multiple quantum well active regions have been designed for wavelengths between about 240 and 290 nm and evaluated by time-resolved photoluminescence (PL) spectroscopy and optically pumping laser experiments. Long PL decay times at room temperature of 900 ps from the multiple quantum well emission were determined for a device emitting at 267 nm. This is a direct confirmation of the high structural quality of the devices. For diode heterostructures the p-layer utilizes polarization-assisted p-doping, and STEM reveals high structural integrity. For Al-compositions of about 60% within the p-cladding, the electrical performance on complete test diodes exceeds 20 kA/cm2.
Lasing was achieved for all test structures by optically pumping with either a KrF or ArF excimer laser with emission wavelength of 248 nm and 193 nm, respectively. Optically pumped lasing has been demonstrated over a wide spectral range from 325 nm to 237 nm. A new record for the threshold power density was recently achieved: 40kW/cm2 at ? = 265 nm (vs. previous published record of 126 kW/cm2). This is a sensitive metric for high optical gain in the active layer. The 237 nm emission is one of the shortest wavelengths yet reported for a photo-pumped AlGaN heterostructure laser. In agreement with computational theory, while the 253 nm emission is TE polarized, the 237nm emission is TM polarized, for the selected MQW parameters. Bulk AlN is an excellent platform for developing deep UV lasers and LEDs. * The work was supported by the Defense Advanced Research Projects Agency CMUVT Program (PM: Dr. John Albrecht) under U.S. Army Contract No. W911NF-10-02-0102.
Johnson, N. M.; Cheng, B.; Choi, S.; Chua, C. L.; Knollenberg, C.; Northrup, J. E.; Teepe, M. R.; Wunderer, T.; Yang, Z. Deep UV AlGaN lasers. Invited talk at the 9th International Symposium on Semiconductor Light Emitting Devices; 2012 July 23; Berlin, Germany.