Current Spreading Length and Injection Efficiency in ZnO/GaN-Based Light-Emitting Diodes

Abstract

Abstract-We report on carrier injection features in lightemitting diodes (LEDs) based on non-intentionally doped-ZnO/p-GaN heterostructures. These LEDs consist of a ZnO layer grown by chemical bath deposition (CBD) onto a p-GaN template without using any seed layer. The ZnO layer (~1 m thickness) consists of a dense collection of partially-coalesced ZnO nanorods, organized in wurtzite phase with marked vertical orientation, whose density depends on the concentration of the solution during the CBD process. Due to the limited conductivity of the p-GaN layer, the recombination in the n-region is strongly dependent on the spreading length of the holes, Lh, coming from the p-contact. Moreover, the evaluation of Lh is not easy and generally requires the design and the fabrication of several LED test-patterns. We propose a simple and effective method to calculate Lh, just based on simple considerations on I-V characteristics, and a way to improve the injection efficiency in the n region based on a noncircular electrode geometry. In particular, an interdigitated electrode structure is proved to be more efficient in terms of hole injection from n-to p-region. Index Terms-ZnO/GaN heterostructures, ZnO/GaN-based light-emitting diodes, current spreading length, contact injection, chemical bath deposition of ZnO nanorods

Progress in Violet Light-Emitting Diodes Based on ZnO/GaN Heterojunction

“…This result confirms that the CBD technique leads to wurtzite nanostructures with a pronounced vertical orientation. As reported in reference [29], the conductivity of the p-GaN layer is of crucial importance to have an efficient hole injection in the n-region. A low current injection results in both a poor light emission performance and a high turn-on voltage.…”

“…It is worth pointing out that the optical power poorly depends on LED size, given that the recombination region is limited by the hole spreading length which is lower than the LED size [29]. As reported in reference [29], the conductivity of the p-GaN layer is of crucial importance to have an efficient hole injection in the n-region. A low current injection results in both a poor light emission performance and a high turn-on voltage.…”

“…This result confirms that the CBD technique leads to wurtzite nanostructures with a pronounced vertical orientation. ZnO nanorods were fabricated by CBD, which has been extensively illustrated in our previous papers [29,35,36]. The nutrient solution was prepared with zinc nitrate hexahydrate (Sigma-Aldrich, St. Louis, MO, USA, reagent grade 98%) and hexamethylenetetramine (Alfa Aesar, Ward Hill, MA, USA, ACS 99+%) in deionized water.…”

“…Therefore, in ZnO/GaN heterojunction LEDs, particular care should be taken to use p-GaN templates of high-crystalline quality, not only to improve the hole injection in the n-region, but also to guarantee an efficient light emission. It is worth pointing out that the optical power poorly depends on LED size, given that the recombination region is limited by the hole spreading length which is lower than the LED size [29]. Figure 4a presents the PL spectra of p-GaN and nid-ZnO (wafer A).…”

“…In particular, the distance between the pand the n-contacts plays a key role in the efficiency of ZnO/GaN heterojunction LEDs. In a previous work [29], we proved that the recombination in the n-region depends on the spreading length of the holes coming from the p-contact; due to the poor conductivity of both the p-GaN and the nid-ZnO, the recombination occurs in the proximity of the edge of the n-contact. In particular, since the p-GaN is the base layer, hole injection would not occur along the vertical direction; the low conductivity of the ZnO does not allow the holes to spread for the full length of the n-pad.…”