Optical Properties of AlN/AlGa(In)N Short Period Superlattices – Deep UV Light Emitting Diodes

ABSTRACTWe report optical properties of deep UV light emitting diodes (LEDs). Devices are based on short period superlattices of AlN/AlxGa1-x(In)N (x ∼ 0.08) grown by gas source molecular beam epitaxy with ammonia. Structures consist of a 50-nm thick AlN nucleation/buffer layer deposited on sapphire. This is followed by a 1-micron thick Si-doped buffer layer of AlGaN or AlN/AlGa(In)N designed to be transparent for wavelengths longer than 240 nm. The design thickness of the superlattice well layers is systematically varied from 0.50 nm to 1.25 nm and the thickness of the barrier is varied from 0.75 nm to 2.00 nm. The n- and p-type SPSLs were doped with Si derived from silane and Mg evaporated from an effusion cell, respectively. We investigate device structures as well as superlattices which are nominally undoped, p-type, and n-type. Optical properties are investigated using reflectance, cathodoluminescence, and, in the case of LEDs, using electroluminescence. By controlling the properties of the superlattice, we obtain optical gaps ranging from 4.5 eV (276 nm) and 5.3 eV (234 nm). A systematic shift between the optical gap and the CL peak emission energy is discussed. Electrical properties are studied using I-V, C-V, and Hall effect. LEDs based on these superlattices and operating in the range of 260 to 280 nm exhibit turn-on voltages in the range of 4 to 6 V and support dc current densities in excess of 500 A/cm2 at room temperature. We present results on the electrical and optical properties of our LEDs designed using these studies.