n-ZnO

Thin Solid Films 445 (2003) 317–321

UV-detector based on pn-heterojunction diode composed of transparent oxide semiconductors, p-NiOyn-ZnO Hiromichi Ohtaa,*, Masao Kamiyab, Toshio Kamiyaa,b, Masahiro Hiranoa, Hideo Hosonoa,b a

Hosono Transparent ElectroActive Materials Project, ERATO, JST, KSP C-1232, 3-2-1 Sakado, Takatsu, Kawasaki 213-0012, Japan b Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan

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 trilayered oxide films of ZnOyNiOyITO 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-NiOyn-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 photoresponsivity was fairly weak at the zero bias voltage, it was enhanced up to ;0.3 A Wy1 by applying a reverse bias of y6 V under an irradiation of 360-nm light, which is comparable to that of commercial devices. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Nickel oxide; Optoelectronic devices; Photoconductivity; Solid-phase-epitaxy; Zinc oxide

1. Introduction Ultraviolet (UV) radiation that reaches the Earth’s surface is 280–400 nm in wavelengths (UV-A and UVB), 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 Schottkyjunction diodes of wide-gap semiconductors such as GaN w1–4x, ZnSe w5,6x, ZnS w7x and diamond w8x 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 ptype NiO and n-type ZnO, which represents a combi*Corresponding author. Tel.: q81-44-850-9787; fax: q81-44-8192205. E-mail address: [email protected] (H. Ohta).

nation of TOSs with simple crystal structures, to fabricate transparent UV-detector. ZnO is a typical ntype 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 w9–11x. Liq 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 w12x). Here we report fabrication and performance of a UVdetector based on the pn-heterojunction diode composed of TOSs, p-NiOyn-ZnO. We have successfully fabricated pn-heterojunction of n-ZnO and p-NiO films with highcrystalline qualities and an abrupt hetero-interface by a pulsed-laser-deposition (PLD) combined with a solidphase-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.

0040-6090/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090(03)01178-7

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2. Experimental 2.1. 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 (ls248 nm) excimer laser (;1 J cmy2 per pulse, 20 ns and 10 Hz) at an oxygen pressure of 3=10y3 Pa. A Li-doped NiO (NiO:Li) polycrystalline layer was grown on the ITO film w13x, which was epitaxially grown on the (1 1 1) YSZ substrate, at room temperature using a 10 at.% Lidoped NiO disk as a target. The resulting bi-layer film was annealed at 1300 8C in a furnace for 30 min in air to convert the NiO layer from polycrystalline to singlecrystalline (SPE). The film surface was fully capped with an YSZ plate to suppress Li2O vaporization during the annealing w14x. Then, an n-type ZnO (electron carrier concentration: ;1=1018 cmy3, Hall mobility: ;60 cm2 (V s)y1) layer was grown on the NiO:Li film at 700 8C w15x. Film thicknesses of ZnO, NiO:Li and ITO were 500, 300 and 450 nm, respectively. The crystalline quality and mutual orientations of the tri-layer film were analyzed by a high-resolution X-ray diffraction (HRXRD, ATX-G, Cu Ka1, Rigaku Co.) and a highresolution electron microscope (HREM, JEM-4000EX, VACC.s400 kV, JEOL Co.). 2.2. Device fabrication and characterization Optical absorption spectra of the tri-layered films of n-ZnOyp-NiO:LiyITO was measured at room temperature. The tri-layer sample was dry-etched to form a mesa structure of 300=300 mm2 as shown in Fig. 1. A gold metal (Au) contact was formed on top of the nZnO layer. Although Coppa et al. w16x have reported that Au metal deposited on oxygen plasma-treated ntype ZnO surfaces functioned as a Schottky contact, a good ohmic contact was formed between our n-type ZnO layer and the Au metal film. Current (I)–voltage (V) characteristics of the resultant pn-heterojunction diode were measured using a semiconductor parameter analyzer (4200-SCS, Keithley) at room temperature. UV-spectral response was measured under photo-irradiation from the substrate (schematically illustrated in the inset of Fig. 6) using a monochromated Xe lamp light at varied reverse biased voltages. 3. Results and discussion 3.1. Crystal quality of tri-layer film Fig. 2 shows AFM images of (a) as-deposited and (b) 1300 8C-annealed bi-layer film of NiO:LiyITO on YSZ substrate. Although grain structures were observed in Fig. 2a, atomically flat terraces with steps of an

Fig. 1. Photograph of the pn-heterojunction UV-detector composed of AuNn-ZnOyp-NiO:LiNITOyAu. Surface area of the MESA is 300=300 mm2.

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 cmy1, 6=1018 cmy3, 0.1 cm2 (V s)y1 and q620 mV Ky1, respectively, confirming that the resulting NiO:Li layer is a p-type semiconductor. We confirmed the ohmic contact between the ITO and p-NiO:Li layers (a combination of ITO and NiO films has been reported to form a pn-junction w17x). Intense diffraction peaks of 0 0 0 2ZnO, 1 1 1NiO:Li, 2 2 2ITO were observed, together with 1 1 1YSZ in the out-of-plane (synchronous scan of 2u and v in the horizontal plane) HR-XRD pattern as shown in Fig. 3a, which indicated that all the layers were highly oriented on the YSZ substrate. On the other hand, intense diffraction peaks of 1 1 2 0ZnO, 2 2 0NiO:Li, 4 4 0ITO were observed together with 2 2 0YSZ in the in-plane (synchronous scan of 2ux and f in the azimuth plane) HRXRD pattern (Fig. 3b) of the tri-layer samples, indicating that all the layers were heteroepitaxially grown on the YSZ (1 1 1) substrate with epitaxial relationships of (0 0 0 1)w1 1 2¯ 0xZnONN(1 1 1)w1 1 0xNiO NN (1 1 1)w1 1 0xITONN(1 1 1)w1 1 0xYSZ . This structure is supported visually by a cross-sectional HREM image in Fig. 2c. Although several misfits due to a large mismatch (;9%) between the O–O spacings of ZnO (0.162 nm) and NiO (0.148 nm) were observed at the interface, the atomic configurations at the interface are connected

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comparison with calculations based on a ligand-field theory w11x. In addition, two broad absorption bands, probably attributable to neutral acceptor, were also seen at ;1 and ;2.4 eV in the differential spectrum between NiO:Li and pure NiO. Fig. 5 shows an I–V characteristics of the p-NiOynZnO heterojunction diode measured in dark (UV-OFF)

Fig. 2. AFM images of a NiO layer grown on an ITO (1 1 1) film by PLD method (a) as-deposited state; (b) 1300 8C-annealed SPE. Atomically flat terrace with atomic order steps are seen in (b).

smoothly, confirming that the heteroepitaxial growth took place. 3.2. Device evaluation Fig. 4 shows an optical absorption spectrum of the tri-layer n-ZnOyp-NiO:LiyITO films. For comparison, each absorption spectrum of the ZnO and NiO:Li single layer is also plotted. The fundamental absorption edge of the tri-layer film is located in the UV region (;3.3 eV), which corresponds to the tail of ZnO absorption. The film is almost transparent in the visible region, although several weak absorption bands appear from 0.4 to 3.3 eV. These bands are attributable to intra-3d transitions of Ni2q ions, which were assigned as transitions from the 3A2 ground state to 3T1, 3T1 or 1E, 1T2 or 1A1, and 3T1 excited states shown in the inset, by

Fig. 3. (a) Out-of-plane and (b) in-plane HR-XRD patterns of ZnOyNiOyITO tri-layer grown on YSZ (1 1 1) substrate. All the layers were heteroepitaxially grown on the substrate with a relationship of (0 0 0 1)w1 1 2¯ 0xZnONN(1 1 1)w1 1 0xNiO NN(1 1 1)w1 1 0xITO NN(1 1 1) w1 1 0x YSZ. (c) Cross-sectional HREM image of the interface region of ZnO (upper) and NiO (lower) heterojunction.

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Fig. 4. Optical absorption spectrum of a ZnOyNiO:LiyITO tri-layer grown on an YSZ (1 1 1) surface. Each absorption spectrum of ZnO or NiO:Li layer is also shown. Magnified absorption spectra of NiO:Li, pure NiO and their differential spectrum are shown in the inset. Intra-ionic d–d transition energy of Ni2q (3d8 electronic configuration) energy calculated using a ligand-field theory w11x is shown in the inset.

and under UV (300–400 nm in wavelength)-illuminations (UV-ON, total power density: ;0.33 W cmy2) at room temperature. Clear rectifying I–V characteristics were obtained with a forward threshold voltage of ;1 V. The ideality factor h of the diode obtained from the dark J (current density)–V characteristic was ;2, indicating that carrier recombination in the depletion layer dominates the carrier transport. In other words, when a forward bias greater than ;1 V was applied to the junction, electrons in the ZnO layer may be transported to the NiO:Li layer, followed by efficient recombination between the positive hole in the NiO:Li layer and the injected electrons. Although the dark reverse current of the diode was very small, a UV-illumination generated a rather large photocurrent. The open circuit voltage (VOC), short circuit current (ISC ) and fill factor (FF) of the diode were ;0.2 V, ;0.5 mA and 0.26, respectively. Photovoltaic energy conversion efficiency of the diode was very low (;0.01%) compared to conventional solar cell materials such as Cu(InAl)Se2 (;17% w18x). Fig. 6 shows UV-spectral response of the p-NiOynZnO heterojunction diode. The figure also plots a transmission spectrum of the tri-layer film. Although the photo-responsivity was very small under zero bias condition, it was increased drastically by the UV-illumination when the reverse bias was applied and it reached ;0.3 A Wy1 at 360 nm. The responsivity is comparable to those of commercial GaN UV-detectors (;0.1 A Wy1) w4x. An efficient photoluminescence associated with the exciton in ZnO peaking at 377 nm was observed

Fig. 5. Typical I–V characteristic of a pn-heterojunction diode under dark (UV-OFF) and UV-illumination (UV-ON, total power density: 0.33 W cmy2) at room temperature. The forward threshold voltage was ;1 V. The ideality factor was ;2. Inset shows the magnified curve.

at room temperature under He–Cd (325 nm) laser irradiation of the tri-layer film. The formation of excitons prevents photovoltaic current generation under zero bias voltage. On the other hand, applying a reverse bias to the junction may decompose excitons into mobile carriers or suppress exciton formation. Therefore, efficient UV-responsivity is obtained near the fundamental absorption edge of ZnO under the reverse bias condition. 4. Conclusion We have demonstrated that a transparent p-NiOynZnO heterojunction diode is suitable to a portable UVdetector. A single-crystalline transparent pn-hetero-

Fig. 6. Spectral response of the diode at several reverse biases voltages. The device structure is also shown in the inset.

H. Ohta et al. / Thin Solid Films 445 (2003) 317–321

junction diode composed of TOSs, p-type NiO and ntype 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 h of the diode was ;2. Efficient UV-response was observed up to ;0.3 A Wy1 at 360 nm (y6 V biased), a value comparable to those of commercial GaN UV-detectors (;0.1 A Wy1) w4x. References w1x M. Asif Khan, J.N. Kuznia, D.T. Olson, J.M. Van Hove, M. Blasingame, L.F. Reitz, Appl. Phys. Lett. 60 (1992) 2917. w2x E. Monroy, F. Calle, E. Munoz, F. Omnes, Appl. Phys. Lett. 74 (1999) 3401. w3x G. Parish, S. Keller, P. Kozodoy, J.P. Ibbetson, H. Marchand, P.T. Fini, S.B. Fleischer, S.P. DenBaars, U.K. Mishra, Appl. Phys. Lett. 75 (1999) 247. w4x S. Yagi, Appl. Phys. Lett. 76 (2000) 345, http:yy www.fujixerox.co.jpyreleasey2001y1129_uv.html (in Japanese).

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