Sub-300 nm AlGaN lasers on bulk AlN substrates

We describe recent progress on the development of sub-300 nm lasers at the Palo Alto Research Center (PARC). We successfully explored the feasibility of using high quality bulk AlN substrates for the epitaxial growth of UV laser structures. The substrates were grown by physical vapor transport. They feature excellent crystalline quality with typical dislocation densities < 10-3 cm-2, x-ray diffraction rocking curve values of about 20 arc sec for the (002)-reflection and a rms surface roughness of 0.1 nm. The AlxGa1xN/AlyGa1yN hetero-structures were grown by metal-organic vapor phase epitaxy. The laser structures included AlN/AlGaN transition layers, AlGaN cladding layers and AlGaN waveguides. The whole layer stack below the MQW region was designed towards laser diodes emitting at 250 nm. Various active zones with emission wavelengths between 267 and 291 nm were tested by time-resolved photoluminescence (PL) studies and optically pumping laser experiments. Long PL decay times at room temperature of 900 ps from the multiple quantum well emission were determined. This is a direct confirmation the high structural quality of the devices. Lasing was achieved for all test structures by optically pumping with a KrF excimer laser at an emission wavelength of 248 nm. The lasing threshold pump power density for the shortest emission wavelength (267 nm) was as low as 126 kW/cm2, which is comparable to blue-violet devices. The polarization of the emitted laser light was found to be TE polarized for all test structures. Concepts for realizing the p-side of the UV laser diodes will also be discussed.