Fabrication of AlGaN MQWs for electron-beam target for deep-ultraviolet light sources
In an attempt to satisfy the technical requirements of deep-UV LEDs, we focus on a UV light source using AlGaN as a target for electron-beam (EB) pumping, because the target does not require p-type AlGaN. Such UV light sources with a simple structure are also suitable for use as large-scale light sources. Furthermore, the emission wavelength can be designed without using a filter by changing the AlN mole fraction in the AlGaN. Recently, Watanabe et al. have fabricated a device using BN powder with an emission wavelength of 225 nm, a maximum output power of 1 mW, and a power efficiency of 0.6% . However, it was difficult to control the emission wavelength. Oto et al. obtained 240-nm emission with an output power of 100 mW by EB pumping using 128-nm-thick AlxGai_xN/AlN quantum wells, and a power efficiency of 40% was achieved with no metal back layer on the EB input surface , though a detailed investigation of the characteristics has not been reported yet. In this study we have grown Si-doped AlGaN MQWs on AlN/sapphire templates by LP-MOVPE, and investigated their characteristics in detail for the purpose of developing AlGaN MQW targets for EB-pumped deep-UV light sources for industrial applications. With the optimized well and barrier thicknesses, we next evaluated the deep- UV light output power. Figure 7.17 shows the deep-UV light output power and conversion efficiency of an AlGaN MQW target, with optimized well and barrier thicknesses, pumped by a 10-kV, 5-mm-diameter EB. The AlGaN MQW with an Al metal back layer (thickness 20 nm) was pumped by the EB, and the deep- UV light emitted from the target that passed through the sapphire substrate to the outside was measured using a calibrated silicon photodetector. The spectral sensitivity of the detector was checked before and after the evaluation, and it was confirmed that there was no change in sensitivity. As shown in Fig. 7.16, the deep-UV light output power gradually increases with EB input power. The AlGaN MQW target exhibits a deep-UV light output power of more than 15 mW at a wavelength of 256 nm, and the estimated conversion efficiency is more than
Fig. 7.17. Deep-UV light output power and conversion efficiency of optimized AlGaN MQW target pumped by a 10-kV, 5-mm-diameter EB.
0.75% at an EB input power of 2.0 W. In the CL analysis for the structure optimization described previously, the peak intensity of the optimized structure was about ten times higher than that of an AlGaN single-layer structure . However, in the deep-UV light output power evaluation here, the light output power of the optimized structure for an EB input power of 1.0 W was only about five times higher than that of an AlGaN single-layer structure. It seems to be caused by the fact that the target temperature depends on EB input power, and light extraction efficiency depends on the buffer layer structure.