Hybrid green LEDs with n-type ZnO substituted for n-type GaN in an inverted p-n junction

Abstract

Recently, the GaN and ZnO materials systems have attracted considerable attention because of their use in a broad range of emerging applications including light-emitting diodes (LEDs) and solar cells. GaN and ZnO are similar materials with direct wide bandgaps, wurtzite crystal structure, high thermal stability and comparable thermal expansion coefficients, which makes them well suited for heterojunction fabrication. Two important advantages of GaN over ZnO are the reliable p-type doping and the mature know-how for bandgap engineering. Thus GaN-based LEDs can be made to emit from the deep UV right into the green through alloying with Al and In, respectively. The performance is not identical at all wavelengths, however, and the performance of InGaN-based green LEDs is still relatively poor [1,2]. This is largelt due to the relatively high In content in the active layers, which makes them unstable at the elevated substrate temperatures (Ts) typically used for the growth of the p-type GaN top layer and thus causes In to leak out of the active layers in the InGaN/GaN Multi-Quantum Wells (MQW) [2,3]. This significantly reduces the performance & lifetime of the LEDs. In this work, an n-type ZnO was substituted for the n-type GaN layer in an (In)GaN-based green LED with an inverted p-n structure (see inset of Figure 1). The top layer was thus the n-type ZnO rather than p-type GaN. Through use of Pulsed Laser Deposition (PLD) for this ZnO growth, the ultimate growth step could thus be performed at significantly lower Ts than is typically required for GaN growth in Metal-Organic Chemical Vapor Deposition (MOCVD). High crystallographic quality of the final hybrid LED structure & the integrity of the MQWs were confirmed by X-Ray Diffraction. The devices showed rectifying I/V characteristics with a turn-on voltage of 2.5 V (Figure 1) & a discrete green Electroluminescence (EL) emission peaked at around 510 nm, which was readily visible to the naked eye (Figure 2). These results indicate that PLD-grown ZnO could be a good alternative to MOCVD-grown GaN for the n-type layer & that such inverted hybrid structures could hold the prospect for the development of green LEDs with superior performance.