High efficiency n-ZnO/p-SiC heterostructure photodiodes grown by plasma-assisted molecular-beam epitaxy

Abstract

Heteroepitaxial n-ZnO films have been grown on commercial p-type 6H-SiC substrates by plasma-assisted molecular-beam epitaxy, and n-ZnO/p-SiC heterojunction mesa structures have been fabricated and their photoresponse properties have been studied. Current–voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than 2×10⁻⁴ A/cm² at −10 V, a breakdown voltage greater than 20 V, a forward turn-on voltage of ∼5 V, and a forward current of ∼2 A/cm² at 8 V. Photosensitivity of the diodes was studied at room temperature and a photoresponsivity of as high as 0.045 A/W at −7.5 V reverse bias was observed for photon energies higher than 3.0 eV.

Introduction

The semiconductor ZnO has a direct wide band gap $(E_g\sim 3.3\mathrm{eV})$ and is attractive for optoelectronics applications due to advantages over GaN such as the availability of ZnO bulk single crystals and a large exciton binding energy (∼60 meV ) [1]. Because growth of reproducible high quality p-type ZnO films has not yet been achieved [2], fabrication of ZnO p–n homojunction-based LEDs remains to be accomplished. For this reason, growth of n-type ZnO on other p-type materials could provide an alternative way for realizing ZnO based p–n heterojunctions. This approach has received considerable attention, and many hetero-p–n junctions have been realized using various p-type materials with n-ZnO [2]. ZnO-based heterostructures have been considered as a candidate not only for light emitting devices but also for photodetectors [3], [4], [5], [6]. Among the available transparent conductive oxide materials, ZnO films have promising properties for photodetectors due to their good electrical and optical properties, relatively low deposition temperatures, simplicity of fabrication processes, and therefore, low costs. Moreover, ZnO-based photodetectors have superior resistance to ionizing radiation and high-energy particles, and do not require an antireflection layer. The main factor influencing the properties of heterostructures is the close lattice match of the components. In this respect 6H-SiC ($E_g\sim 3.05\mathrm{eV}$ [7]) is a good candidate since it has wurtzite crystal structure and relatively good lattice matching to ZnO with lattice mismatch of ∼4%, and p-6H-SiC substrates are commercially available. Previously, 6H-SiC substrates have been used for heteroepitaxial growth of ZnO and high quality of the grown ZnO films has been demonstrated [8], [9]. But despite this, there have been only a few reports on the growth of n-type ZnO on p-type 6H-SiC [10], [11], [12]. Recently we reported on the growth of n-ZnO/p-6H-SiC heterostructure diodes by plasma-assisted molecular-beam epitaxy (MBE) and on their photoresponse properties [11], [12]. Since then further research was done in this area, more samples were grown at different conditions. Here we report on new results obtained in such n-ZnO/p-6H-SiC heterostructures. In this vein and for the present work, n-ZnO films were grown on p-6H-SiC substrates by plasma-assisted MBE and n-ZnO/p-6H-SiC heterojunction diodes were fabricated, and their photoresponse properties were studied.