High-temperature p-type polarization doped AlGaN cladding for sub-250 nm deep-UV quantum well LEDs by MBE

Abstract

Deep UV (DUV) light-emitting diodes (LEDs) are finding increased application in many areas including water purification and sterilization. Sub-270 nm emission is ideal for these applications since bacterial DNA absorbs strongly in this wavelength regime. To extract high energy photons (∼5 eV), the LED cladding regions must be transparent and therefore consist of high Aluminum content (>60%) n- and p-AlGaN. Ensuring high electrical conductivity in such cladding regions becomes increasingly difficult with increasing Al content due to the large dopant activation energies, especially for the acceptor dopant, Magnesium (Mg) [1]. To maximize Mg incorporation by MBE, a commonly utilized approach is to grow the p-cladding region at a low temperature (∼630 °C [2]). A low growth temperature causes defects and undesired compositional inhomogeneity, leading to inferior vertical hole transport (Fig. 1). To address these issues, a combination of high temperature MBE growth of the p-AlGaN cladding and polarization-induced doping is employed. While high growth temperature ensures good crystal quality and compositional uniformity for the Al-rich p-AlGaN, the polarization-induced doping compensates for the reduced Mg incorporation at high temperature. A sub-250 nm DUV LED is demonstrated for the first time using MBE-grown p-AlGaN layers at a high temperature of 730 °C. SIMS analysis (Fig. 2) showed that Mg incorporation decreases by ∼5 times over the temperature range from 650–730°C for 75% AlGaN, and that Mg incorporation decreases at high Al content, consistent with thermodynamic principles [3]. Based on the observations from Fig. 1 and 2, a substrate temperature of 730 °C was chosen for the p-cladding layer for a sub-250 nm DUV LED structure.