Towards Sub-300 nm laser diodes on bulk AlN substrates

Abstract

In this presentation, we report recent progress on development towards sub-300 nm lasers diodes by using high-quality bulk AlN substrates. The single crystal AlN substrates were fabricated from AlN boules, which were grown by physical vapor transport (Fig. 1). 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 rms surface roughness of 0.1 nm. The AlxGa1xN/AlyGa1yN hetero-structures were grown by metal-organic vapor phase epitaxy. The test laser structures included AlN/AlGaN transition layers, AlGaN cladding layers and AlGaN waveguides. The whole layer stack below the multiple quantum well (MQW) region was designed towards laser diodes with emission wavelength near 250 nm. Various active zones with design wavelengths between about 240 and 290 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 for a device emitting at 267 nm. This is a direct confirmation of the high structural quality of the devices. Lasing was achieved for all test structures by optically pumping with a KrF or ArF excimer laser with an emission wavelength of 248 nm and 193 nm, respectively (Fig. 2). The lasing threshold pump power density for the device with emission wavelength at 267 nm was as low as 126 kW/cm2 [3], which is comparable to blue-violet devices. The shortest lasing wavelength attempted and demonstrated to date is 237 nm. The polarization of the emitted light is strongly dependent on the actual structural design of the active zone [4]. Our experimental results on the polarization of the laser emission will be compared with theoretical model calculations. Additionally, concepts for realizing the p-side of the UV laser diodes will also be discussed.