Influence of electrodes layout on hydrothermally-grown GaN/ZnO LEDs

Light-emitting diodes (LEDs) based on zinc oxide, with wide direct band gap, have drawn much attention in the last years. ZnO, which is natural n-type, has excellent physical and chemical properties, is inexpensive, abundant and nontoxic. Unfortunately, the preparation of reproducible and high quality p-type ZnO film is comparatively difficult, due to the low solubility of acceptor dopants, self-compensating effects, and acceptor level energy height. Although many groups have reported on ZnO-based homostructure LEDs, the results are controversial as recently reviewed in [1]. Hence, heterojunction LEDs based on ZnO as the nside and GaN as the p-side of the junction have been put forward [2]. Among the several approaches reported in literature for the fabrication of these devices, the hydrothermal growth of crystalline n-ZnO films on p-GaN is particularly attractive because it is a large-area compatible, low-temperature, simple and low-cost method [3]. As far as it concerns the metal contacts, usually a simple layout consisting of a bottom and annular p-pad on GaN surrounding a thin film of ZnO with a circular n-pad above it, is used. In this case, in order to recombine across the full area of the device, the holes injected from the annular pcontact should travel through p-GaN for a maximum distance equal to the radius of the circular n-pad covering the n-ZnO. Although the p-GaN could be highly doped, it has not a very high conductivity. Thus, if the device has a very large radius, the holes may not travel for the full length of the n-pad. As a consequence, the active area where the radiative recombination takes place may be small and confined to the edge of the n-contact. Furthermore, light emission is annular and non-uniform through the device, with an associated low output power; this problem has a greater incidence as the device size increases. In this work we propose a solution to improve the carrier transport and the injection efficiency of ZnO/p-GaN LEDs grown by hydrothermal method and emitting at 390 nm. As contacts layout an interdigitated structure, as shown in Fig. 1, is used instead of a circular one. The width, the length, the number of the fingers and the distance between them, are designed to have the same area of circular LEDs fabricated on the same substrate and used as a comparison. The measured I-V characteristics in the two cases are reported in Fig. 2. The distance covered by the carriers is lower in the interdigitated devices when compared to the circular ones. In this way, the efficiency results to be higher and light emission more uniform.