UV-detector based on pn-heterojunction diode composed of transparent oxide semiconductors, p-NiO/n-ZnO

doi:10.1016/S0040-6090(03)01178-7

Thin Solid Films

Volume 445, Issue 2, 15 December 2003, Pages 317-321

https://doi.org/10.1016/S0040-6090(03)01178-7 Get rights and content

Abstract

A transparent ultraviolet (UV)-detector was fabricated using a high-quality pn-heterojunction diode composed of transparent oxide semiconductors, p-type NiO and n-type ZnO, and its UV-response was measured at room temperature. Transparent tri-layered oxide films of ZnO/NiO/ITO were heteroepitaxially grown on an YSZ (1 1 1) substrate by a pulsed-laser-deposition combined with a solid-phase-epitaxy technique and they were processed to fabricate a p-NiO/n-ZnO diode. The diodes exhibited a clear rectifying I–V characteristic with an ideality factor of ∼2 and a forward threshold voltage of ∼1 V. Although the photo-responsivity was fairly weak at the zero bias voltage, it was enhanced up to ∼0.3 A W⁻¹ by applying a reverse bias of −6 V under an irradiation of 360-nm light, which is comparable to that of commercial devices.

Introduction

Ultraviolet (UV) radiation that reaches the Earth's surface is 280–400 nm in wavelengths (UV-A and UV-B), and plays a harmful role that may cause skin cancer. To prevent skin cancer due to the UV-radiation, development of portable UV-detector that is mainly composed of pn-junction of wide-gap semiconductor has been required to date. Although several UV-detectors have recently been developed using pn-junction and Schottky-junction diodes of wide-gap semiconductors such as GaN [1], [2], [3], [4], ZnSe [5], [6], ZnS [7] and diamond [8] systems, transparent oxide semiconductors (TOSs) are much more preferable for the fabrication of UV-detectors, because TOSs are optically transparent in visible and near UV-light region, environmental friendly and thermally and chemically stable. In addition, TOSs include a variety of compounds with different crystal structures, which are composed of various combinations of constituent ions.

In this study, we selected a pn-heterojunction of p-type NiO and n-type ZnO, which represents a combination of TOSs with simple crystal structures, to fabricate transparent UV-detector. ZnO is a typical n-type TOS with a band gap of 3.3 eV. On the other hand, NiO is a semitransparent p-type semiconductor having a direct energy gap of ∼3.7 eV, with weak absorption bands due to d–d transitions of 3d⁸ electron configuration in the visible region [9], [10], [11]. Li⁺ doping is known to significantly boost p-type conductivity. Despite the different crystal structures of ZnO (Wurtzite, hexagonal) and NiO (Rock salt, Cubic), a high-quality ZnO epitaxial layer may be grown on a single-crystalline NiO, due to the similar oxygen atomic configurations (sixfold symmetry) of (0 0 0 2) ZnO and (1 1 1) NiO (domain matched epitaxy [12]).

Here we report fabrication and performance of a UV-detector based on the pn-heterojunction diode composed of TOSs, p-NiO/n-ZnO. We have successfully fabricated pn-heterojunction of n-ZnO and p-NiO films with high-crystalline qualities and an abrupt hetero-interface by a pulsed-laser-deposition (PLD) combined with a solid-phase-epitaxy (SPE) technique. The pn-heterojunction diode exhibits good I–V rectifying characteristics, and an excellent UV-response is obtained, comparable to those of wide-gap semiconductors when a reverse bias voltage is applied.

Section snippets

Film growth

Three layers of ITO, p-type NiO and n-type ZnO were deposited sequentially in this order on a (1 1 1) YSZ substrate by PLD using a KrF (λ=248 nm) excimer laser (∼1 J cm⁻² per pulse, 20 ns and 10 Hz) at an oxygen pressure of 3×10⁻³ Pa. A Li-doped NiO (NiO:Li) polycrystalline layer was grown on the ITO film [13], which was epitaxially grown on the (1 1 1) YSZ substrate, at room temperature using a 10 at.% Li-doped NiO disk as a target. The resulting bi-layer film was annealed at 1300 °C in a furnace

Crystal quality of tri-layer film

Fig. 2 shows AFM images of (a) as-deposited and (b) 1300 °C-annealed bi-layer film of NiO:Li/ITO on YSZ substrate. Although grain structures were observed in Fig. 2a, atomically flat terraces with steps of an atomic order height were observed in Fig. 2b, indicating that a polycrystalline NiO:Li film was converted to single-crystalline via SPE. Electrical conductivity, hole concentration, Hall mobility and Seebeck coefficient were 0.1 S cm⁻¹, 6×10¹⁸ cm⁻³, 0.1 cm² (V s)⁻¹ and +620 μV K⁻¹,

Conclusion

We have demonstrated that a transparent p-NiO/n-ZnO heterojunction diode is suitable to a portable UV-detector. A single-crystalline transparent pn-heterojunction diode composed of TOSs, p-type NiO and n-type ZnO, was fabricated, and its photo-responses were measured at room temperature. The diode exhibited clear rectifying I–V characteristics with a forward threshold voltage of ∼1 V, which was significantly lower than the direct band gap energies of ZnO and NiO. The ideality factor η of the

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UV-detector based on pn-heterojunction diode composed of transparent oxide semiconductors, p-NiO/n-ZnO

Abstract

Introduction

Section snippets

Film growth

Crystal quality of tri-layer film

Conclusion

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Phys. Rev. B