ZnO Schottky ultraviolet photodetectors

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Abstract

We present the results of Schottky UV photodetectors fabricated on n-type ZnO epitaxial films. The ZnO films were grown on R-plane sapphire substrates by metalorganic chemical vapor deposition. The metal–semiconductor–metal (MSM) photodetectors were fabricated by using Ag as Schottky contact metal. For comparison, ZnO photoconductive detectors were also fabricated by using Al as ohmic contact metal. IV characteristics of these devices were analyzed. At a reverse bias of 1 V, the circular Schottky photodiode exhibits a leakage current approximately 5 orders of magnitude smaller than that of its photoconductive counterpart. The photoresponsivity of the ZnO Schottky type MSM UV detector is 1.5 A/W and the leakage current is about 1 nA at 5 V bias. The detector shows a fast photoresponse component with a rise time of 12 ns and a fall time of 50 ns.

Introduction

Recently, there has been increasing interest in high quality ZnO films. Optical pumped lasing in the UV range [1], and an optically addressed UV light modulator [2] have been reported. Epitaxial ZnO films are promising for UV photodetector applications due to their wide and direct band gap and large photoresponse. We previously reported the epitaxial ZnO UV photoconductive type detector, which was fabricated using aluminum as the ohmic metal for a metal–semiconductor–metal (MSM) device structure [3]. The device showed fast photoresponse time (∼1 μs) due to high material quality of a ZnO epilayer. However, Schottky type photodetectors are more attractive due to their high speed and low noise performance. Till today, there has been no Schottky type of photodetector fabricated on ZnO epitaxial films. Fabricius et al. made Schottky barrier type UV sensitive photodiodes using Au on a thin sputtered layer of polycrystalline ZnO [4]. These photodiodes exhibited slow operating speed (rise time ∼20 μs and decay time ∼30 μs) and low quantum efficiency (1%) due to a large amount of recombination centers in the polycrystalline ZnO layers. In this paper, we report on Schottky type photodetectors fabricated on high quality ZnO epilayers, which are grown on the R-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD).

Section snippets

Experimental procedure

(112̄0) ZnO epitaxial films were grown on R-plane (011̄2) sapphire substrates in a low-pressure MOCVD reactor. Diethyl zinc (DEZn) and O2 were used as the source materials. The reactants were introduced into the reactor by two separate injectors to reduce gas phase reaction. DEZn flow rate was 50–100 sccm, and O2 flow rate was 1000–2000 sccm. Growth temperature range was from 380°C to 420°C. Typical growth rate was 1–2 μm/h. The details of the system and growth conditions were reported in Ref. [5]

Results and discussion

Shown in Fig. 1 are the IV characteristics of ZnO MSM circular devices, including both Schottky and ohmic types. The linear IV relation from the Al–ZnO–Al structure clearly indicates the ohmic behavior of the Al on n-type ZnO contact. On the other hand, the rectified IV relation from the Ag–ZnO–Al confirms Schottky junction formation between Ag and n-type ZnO. In a Schottky diode, the general IV characteristics are represented by [7]

J=JsexpqVnkT−1,where Js=AT2exp(−ΦB/kT) is the saturation

Conclusion

We have demonstrated Schottky type UV photodetectors based on ZnO epitaxial films grown on R-plane Al2O3 by MOCVD. Ag was used to form Schottky contact on n-type ZnO with a barrier height about 0.84 eV. A low frequency photoresponsivity of 1.5 A/W at a bias of 5 V was obtained. The leakage current of the device at 5 V bias was in the order of 1 nA. The photoresponse of the detector showed a fast component with a rise time of 12 ns, and a fall time of 50 ns.

Acknowledgments

The work has been supported by Rutgers University under SROA grant.

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