Thin Solid Films 420–421(2002) 112–116

0040-6090/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž02.00742-3

Characterization of films and interfaces in n-ZnOyp-Si photodiodes

J.Y. Lee, Y.S. Choi, W.H. Choi, H.W. Yeom, Y.K. Yoon, J.H. Kim, S. Im*

Institute of Physics and Applied Physics, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, South Korea

Abstract

We report on the fabrication of n-ZnOyp-Si heterojunction photodiodes. RF sputtering was performed to deposit ZnO films onp-Si substrates at various substrate temperatures of 300, 480 and 5508C using Ar:O ratios of 6:1. Typical rectifying behaviors2

were observed from most of the diodes as characterized by the current–voltage(I–V) measurement. Some of the diodes exhibitphotoelectric effects under illumination using monochromatic red light with a wavelength of 670 nm. Maximum quantumefficiency of 53% was obtained under a reverse bias condition from a diode with ZnO film deposited at 4808C. Measuringphotoluminescence, transmittance, sheet resistance from the ZnO films, and characterizing the n-ZnOyp-Si interface with X-rayphotoelectron spectroscopy, it is concluded that the diodes with n-ZnO deposited at 4808C conserve relatively a high film qualityand good interface junction to exhibit the best photoelectric property.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: n-ZnOyp-Si photodiode; Current–voltage; Photoelectric; Leakage current; Interface junction

1. Introduction

As one of the multifunctional thin oxide films withgood optical transmission, electrical conductivity, andcathodoluminescence for various electronic devices ordisplays, ZnO has been widely used and studiedw1,2x.It belongs to the class of transparent conductive oxidestogether with indium tin oxide(ITO) and tin oxide. Thecomponents of ITO, namely, In and Sn, are limited insupply and are more expensive than Znw3,4x. The ZnOhas another advantage over the ITO because it is ableto tolerate reducing chemical environmentsw5x. It canmatch, even if not exceed, the optical and electricalproperties of ITO. In addition, thin film ZnO is a directand wide band gap(E s3.3 eV) semiconductor wheng

the film is deposited at a moderate temperaturew6,7x.Transmittance of visible light through the thin ZnO ismore than 80%w8,9x.The present paper is based on a motivation to use a

polycrystalline n-ZnO film deposited on a p-Si as apractical antireflecting photon-window for Si photodi-odes and simultaneously as a semiconducting layer

*Corresponding author. Tel.:q82-2-2123-2842; fax:q82-2-392-1592.

E-mail address: semicon@yonsei.ac.kr(S. Im).

which causes a carrier-depletion plus a built-in barrierpotential in the n-ZnOyp-Si junction. Due to the widebandgap of ZnO, low energy photons in the visiblerange may be collected mainly at the depletion regionof the p-Si after they are transmitted through the ZnO.It is interesting that such a simple photodiode structureas the n-ZnOyp-Si has not been widely reported exceptby a few studies to datew10,11x. It is probably becausethe semiconducting ZnO film can be achieved only at amoderately elevated temperature at which a thin SiOlayer may be grown at the n-ZnOyp-Si interface. Pho-toelectric effects from the photodiode will be wellobserved only if two important qualities in the photo-diode are satisfied: a good interface(or n–p junction)quality for a low leakage current and a good film qualityfor the light transmission and carrier transport. In thepresent work, we fabricated the n-ZnOyp-Si heterojunc-tion photodiode to observe the photoelectric effects andto find optimum process temperature conditions to max-imize the effects.

2. Experimental

p-Type (5 V cm) Si (1 0 0) wafers were used assubstrates for the n-ZnOyp-Si diodes. Si(1 0 0) wafers

113J.Y. Lee et al. / Thin Solid Films 420 –421 (2002) 112–116

Fig. 1. PL spectra of the ZnO films deposited on p-Si at differenttemperatures. UV emission is observed from the ZnO films depositedat 480 and 5508C.

were cut into 1.5 cm by 1.5 cm pieces. Prior to thedeposition, the wafers were dipped for 1 min intobuffered HF(HF:H Os1:7) to remove native oxides.2

Then the samples were ultrasonically cleaned with ace-tone, methanol, and de-ionized water for 10 min. Finally,the wafers were blown dry with nitrogen. Undoped ZnOfilms were deposited in a RF magnetron sputteringsystem using a 0.6 cm-thick sintered zinc oxide targetwith 2 in. diameter(99.9% purity, Kurt J. Lesker Co.).The substrate holder was placed 10 cm away from thetarget. The chamber was evacuated to a base pressureof 1=10 Torr. The ZnO films were deposited on Siy6

substrates at different substrate temperatures of 300, 480and 5508C with a working pressure of 10 mTorr. Gasflow ratio of Ar and O was fixed mainly at 6:12

according to our early resultsw10x. The chosen RFpower and the deposition period were 120 W and 1 h,respectively.After deposition, the samples were again cut into

small 3=3 mm pieces for current–voltage(I–V) meas-2

urements. To provide a low contact resistance of elec-trode, a 0.5 mm-diameter indium dot was deposited ontoZnO surface at a corner. In the same manner, a largearea ohmic-contact of indium was made onto the back-side of the p-Si wafer. For a reverse bias, a positive biaswas applied to the n-ZnO film on p-Si. The measure-ments were performed with and without irradiation of ared laser beam(670 nm wavelength, 1.85 eV) deliveringa power density of approximately 20 mWycm onto the2

ZnO surface. The red diode laser was 3 cm away fromthe sample, and illuminated it at a normal incidence.The laser beam size covered the entire ZnO surface. Allmeasurements were taken in a dark room. Based onprinciples using the optical power and measured photo-currents, we estimated responsitivity and quantum effi-ciency of a n-ZnOyp-Si heterojunction diodew12x.The thickness of ZnO films was measured by 2.01

MeV He Rutherford backscattering spectrometry and4 2q

was approximately 100 nm for all the samples. Photo-luminescence(PL) was performed using 351 nm exci-tation of an Ar ion laser at a power of 80 mW toinvestigate the formation of stoichiometric semiconduct-ing ZnO films,. Transmittance of the films was alsomeasured on films deposited on corning glass. X-rayphotoelectron spectroscopy(XPS) using monochromaticAl Ka was used to investigate the chemical states ofthe ZnOySi interface immediately after ultra-thin ZnOfilms had been deposited on p-Si for 3 min at 300, 480and 5508C.Electrical properties of the ZnO films were investi-

gated using transmission line method(TLM) pattern ofAu–Al (1:1) alloy and Van der Pauw method. Theelectrical conduction of the samples was usually n-type.

3. Results and discussion

According to the PL spectra shown in Fig. 1, anevident ultra-violet (UV) emission peak is observednear 382 nm from the ZnO films deposited at 5508Calong with a broad low intensity band centered near 430nm. The band was reported as being caused by thedefects in the grain boundaries of ZnOw13x. The filmdeposited at 4808C shows a small UV peak. The sampledeposited at 3008C does not show any emission signalnear the UV range. The intensity of the UV emissioncan be used as a standard for the crystalline andstoichiometric quality of the ZnO semiconductor filmw14x. It means that the samples prepared at 5508C havethe best quality among all the samples.The I–V characteristics of three different heterojunc-

tion diodes are shown in Fig. 2a and b. Typical rectifyingbehaviors are observed from all the n-ZnOyp-Si diodes.Dark leakage current shown in Fig. 2a gradually increas-es with the deposition temperature. These phenomenamay presumably be explained by the formation of adefective interlayer between n-ZnO and p-Si and willbe discussed later. The best photoelectric effects areobserved from the diode with a ZnO film deposited at480 8C as shown in Fig. 2b. Photo-currents obtainedfrom other samples are observed to be very low com-pared to the diode with the ZnO prepared at 4808C.This implies that there is an optimum deposition tem-

114 J.Y. Lee et al. / Thin Solid Films 420 –421 (2002) 112–116

Fig. 2. TheI–V curves obtained from the n-ZnOyp-Si diodes.(a) Darkcurrent behaviors(dark) were plotted to be compared with(b) photo-current behaviors(light) obtained from the illuminated diodes.

Fig. 3. The optical transmittance vs. wavelength for two ZnO filmsdeposited at 300 and 5508C on Corning glasses. Both of them showalmost the same transmittance level beyond the wavelength of 375nm.

Fig. 4. TheI–V curves obtained by probing two ohmic-contact elec-trodes(�1 and�2 in the TLM pattern). Those Au–Al alloy elec-trodes were deposited on ZnO surface. The film deposited at 3008Chas a five order higher sheet resistance(361 MVyh) than the oneprepared at 5508C (5.3 KVyh).

perature, which should not be too high, to obtain thebest photoelectric performance. The quantum efficiencyis estimated from theI–V results(at 5 V reverse bias)for the optimum diode and is approximately 53%. It isgenerally understood that the photoelectric effects resultfrom the light-induced electron–hole generation at then–p junction and particularly at the depletion region ofthe p-Si. Because of the limited penetration depth of thelight into the p-Si, the photo-currents are normallysaturated in a few volts of reverse bias as shown in theI–V curve of the best n-ZnOyp-Si photodiode(with n-ZnO prepared at 4808C).The diodes with n-ZnO deposited at 3008C in Fig.

2b exhibit the lowest photoelectric effect under the redillumination. It is probably due to the poor quality ofthe film. Although, some amount of incident lighttransmitted through the film reaches to the depletedregion of the p-Si, the excited electrons and holes maynot be efficiently collected in the ZnO film. The twospectra of Fig. 3 exhibit the transmittance resultsobtained from the ZnO films deposited on corningglasses at 300 and 5508C. Both of them show the

normal transmittance of more than 75% and only a littlevariation of transmittance was observed beyond the ZnOband edge(375 nm,;3.3 eV) between the two whilethey exhibited a significant difference in the PL emissionof Fig. 1. This means that the film deposited at 3008Cis quite transparent regardless of its poor stoichiometricquality.The electrical conductance of the film, however, again

reflects the effects of the poor quality as expected andshown in theI–V curves of Fig. 4. The curves wereobtained by probing the TLM Au–Al alloy pattern(electrode�1 and�2 in the inset) that was thermallydeposited on the films as ohmic-contact electrodes. Theresistance was higher for the film deposited at lowertemperatures. In Particular, the ZnO film prepared at

115J.Y. Lee et al. / Thin Solid Films 420 –421 (2002) 112–116

Fig. 5. Si 2p XPS spectra show the presence of SiO at the ZnOySiinterface. Note the shadow areas at the Si 2p levels in SiO .2

300 8C shows 361 MVyh as its sheet resistance, whichis four to five orders higher than other films. It may beacting as a semi-insulating layer to inhibit the photo-generated carriers in p-Si from being transported throughthe ZnO film.Then, why does the diode with the film deposited at

480 8C show much higher quantum efficiency than theone prepared at 5508C? Even though, the highest filmquality is obtained at 5508C as shown in Fig. 1, thediode made with this ZnO film is inferior to the diodemade with a ZnO film deposited at 4808C in thephotoelectric response. In the present work, the reasonwas investigated through probing the n–p junctions inthe n-ZnOyp-Si interfaces of all the diodes by XPS rightafter ultra-thin ZnO films were deposited. Fig. 5 is a setof XPS spectra showing the Si 2p levels. These spectraclearly show the presence of SiO at the ZnOySi2

interface(the peak at 103.3 eV) together with the Si 2psignal from the pure Si substrate beneath the SiO(the2

peak at 99.2 eV). At a temperature of 3008C, a verythin SiO layer is formed on the Si substrate. The peak2

shift is due to charging effects by the thickness ofSiO layer that increases with the deposition temperature2

w15x. It is qualitatively inferred from the spectra that theSiO formed at the interface at 5508C has the largest2

layer thickness(please compare the amount of shadowareas in the Si 2p level of SiO). The thick insulating2

layer formed at the interface would block the collectionof the carriers generated at the p-Si by photons, and itwould also cause the large current leakage, located atthe junction interface of the photodiode.

Therefore, it can now be explained why the photodi-ode prepared at 4808C has exhibited the best photoe-lectric performance. The diode has sufficiently satisfiedtwo important conditions for the performance: relativelygood ZnO film quality for carrier transport and the goodinterface junction with a moderately thin SiO layer.2

4. Conclusion

The n-ZnOyp-Si heterojunction photodiodes were fab-ricated using RF-sputter deposition of n-ZnO films onp-Si substrates. Under the monochromatic red lightillumination with a wavelength of 670 nm, the maximumamount of photo-current is obtained under reverse biasconditions from a n-ZnOyp-Si heterojunction with theZnO film deposited at 4808C. ZnO films deposited onp-Si at 3008C are so poor in terms of carrier conductionquality and stoichiometry that the n-ZnOyp-Si diodesusing these films do not exhibit good photoelectricperformance. Although, the ZnO films deed at 5508Cshow the best stoichiometric quality, a thick SiO layer2

was found at the n-ZnOyp-Si junction interface by XPSanalysis and it acts as an insulating and a leakage-causing source. It is thus concluded that depositing n-ZnO at 480 8C for the photodiode may achieve arelatively high film quality and good interface junctionthat exhibits good photoelectric performance.

Acknowledgments

The work has been supported by the Brain Korea 21project and partly supported by Korea Research Foun-dation KISTEP fund(�M2-0204-25-0033-02-A09-03-004).

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