Ultraviolet light emitting diode with n-ZnO:Ga/i-ZnO/p-GaN:Mg heterojunction

doi:10.1016/j.tsf.2009.03.149

Thin Solid Films

Volume 517, Issue 17, 1 July 2009, Pages 5106-5109

https://doi.org/10.1016/j.tsf.2009.03.149 Get rights and content

Abstract

n-ZnO:Ga/p-GaN:Mg heterojunction light emitting diodes (LEDs) were fabricated at different growth temperatures and carrier concentrations in the n-type region. The effects of growth temperature and carrier concentration on the electrical and emission properties were investigated. The I–V and EL results showed that the improved device performances such as lower turn-on voltage and true ultraviolet emission were achieved with the insertion of a thin intrinsic layer between n-ZnO:Ga and p-GaN:Mg. This observation was attributed to a lowering of energy barriers for the supply of electrons and holes into intrinsic ZnO and recombination in the intrinsic ZnO with the absence of deep-level emission.

Introduction

ZnO based materials are potential semiconductors for high efficiency light emitting diodes (LEDs) in the blue and ultraviolet wavelength regions. The excellent physical and chemical properties of ZnO and oxide alloys, which include high exciton binding energy (60 meV) and possibility of wet etching processes, are expected to make them suitable alternative materials to nitrides in the next-generation illumination systems [1], [2]. The UV detecting devices with high responsivity have also been reported using ZnO based Schottky and p–n diodes [2], [3]. However, the difficulty in obtaining p-type ZnO films with high reproducibility and stability has limited the development of ZnO based optoelectronic devices, even though many groups have reported the fabrication of homojunction p–n diodes using p-type ZnO films [4], [5], [6], [7], [8].

The alternative approaches to the fabrication of ZnO based LEDs using p-type nitride epilayers grown on sapphire substrates have been suggested to overcome the p-type related problems in device applications [9]. The p-type GaN has the advantage for n-type ZnO epitaxy on account of the smaller lattice mismatch, same crystal structure, domain matching epitaxy growth, and technical stability. The n-type ZnO can be obtained conveniently using intrinsic and extrinsic methods due to the formation of native oxygen vacancies and incorporation of metal impurities such as Al, Ga, and In, respectively [10], [11], [12]. Among them, Ga is expected to be the most effective n-type dopant in ZnO because the covalent bond length of Ga–O (1.92 Å) is similar to that of Zn–O (1.97 Å) and Ga₂O₃ formed is less reactive than Al₂O₃ in Al doped ZnO [13]. However, the general LED structures have p–i–n stacking, where the intrinsic semiconductors are the emissive layer. In this study, various heterostructures were grown, using Mg doped GaN (GaN:Mg) epilayer as a p-layer and Ga doped ZnO (ZnO:Ga) as a n-layer, for fabrication of LED devices. The effects of the electrical properties of the ZnO grown by sputtering on the device performance were examined.

Section snippets

Experiments

ZnO films were grown on p-GaN:Mg epilayers by magnetron sputtering, and used to fabricate the n-ZnO/p-GaN heterostructure based LEDs. The p-GaN was deposited on a c-plane sapphire substrate using metalorganic chemical vapor deposition (MOCVD). A commercial ZnO (99.999%, 4 in) and 1 wt.% Ga doped ZnO targets were used as ZnO sources, and a mixture of high purity argon and oxygen at a ratio of 1:2 was used as ambient gas. The ZnO films were grown at 750 °C (high temperature, HT) and 450 °C (low

Results and discussions

In this study, various ZnO based materials such as HT n-ZnO:Ga (750 °C), LT n-ZnO:Ga (450 °C), and undoped i-ZnO (growth temperature, 450 °C) with different electrical properties were grown on p-GaN:Mg for the fabrication of p–n or p–i–n LEDs. Their electrical properties were examined using Hall measurements with the films grown directly on sapphire substrates, as shown in Table 1. When 1 wt.% Ga doped ZnO was used as a sputtering target, the ZnO:Ga film exhibited n-type semiconducting

Summary

ZnO thin films were grown on p-GaN epilayers by magnetron sputtering for fabrication of p–n heterostructure LEDs. The LEDs with HT n-ZnO:Ga/p-GaN:Mg showed non rectifying behavior due to high conductivity of n-ZnO:Ga, while the deposition of LT n-ZnO:Ga prior to the growth of HT n-ZnO:Ga led to diode characteristics. The EL emissions from these n-ZnO:Ga/p-GaN:Mg LEDs (samples 1 and 2) were mainly in the visible region, and coincided with the deep-level position of the PL results of n-ZnO:Ga

Acknowledgements

This work was supported by the KRF Grant (KRF-2008-314-D00153) and the Energy and Resources Technology Development Project from the MOCIE. This research was also financially supported by the Ministry of Knowledge Economy (MKE) and the Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology.

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Ultraviolet light emitting diode with n-ZnO:Ga/i-ZnO/p-GaN:Mg heterojunction

Abstract

Introduction

Section snippets

Experiments

Results and discussions

Summary

Acknowledgements

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