Thin Solid Films 403–404(2002) 553–557

0040-6090/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž01.01550-4

Optimizing n-ZnOyp-Si heterojunctions for photodiode applications

J.Y. Lee, Y.S. Choi, J.H. Kim, M.O. Park, S. Im*

Institute of Physics and Applied Physics, Yonsei University, Seoul, 120-749, South Korea

Abstract

N-ZnOyp-Si heterojunction photodiodes have been fabricated by sputter deposition of n-ZnO films on p-Si substrates. Thesubstrate temperatures of 300, 400, 480 and 5508C were taken for the n-ZnO film deposition using an AryO ratio of 6:1. All2

the diodes show typical rectifying behaviors as characterized by the current–voltage(I–V) measurement in a dark room and theirphotoelectric effects from the diodes have been observed under illumination using monochromatic red light with a wavelength of670 nm. Maximum amount of photo-current or responsivity is obtained under reverse bias conditions from a n-ZnOyp-Siheterojunction when the ZnO film was deposited at 4808C while the ZnO films deposited at 5508C show the best stoichiometricand crystalline quality. Junction leakage or dark current is much higher in the diode with n-ZnO deposited at 5508C than in theother diodes. It is thus concluded that for a photodiode the quality of the diode junction is as important as that of the n-ZnO filmdeposited on p-Si.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: n-ZnOyp-Si; Photodiode; Current–votage; Photocurrent; Stoichiometry; Crystalline quality

1. Introduction

Zinc-oxide(ZnO) films have been extensively studiedfor practical applications including bulk acoustic reso-natorsw1x, grating-coupled waveguard filtersw2x, acous-tic-electric devicesw3x, transparent conducting electrodematerials for various electronic devices such as solarcells, electroluminescence displays, etc.w4,5x. It belongsto the class of transparent conductive oxides(TCOs)together with indium tin oxide(ITO) and tin oxide(TO). Among the TCO materials available, zinc oxidefilms have promising properties due to their electricaland optical properties in combination with low costs,non-toxicity, and relatively low deposition temperaturew6,7x. Presently ITO layers are used as energy efficientwindows for solar cells and liquid crystal displays. Thecomponents of ITO, namely In and Sn, are limited insupply and are more expensive than zinc. Moreover, theZnO has another advantage over the ITO because it isable to tolerate reducing chemical environmentw8x. For

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

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

instance ZnO films are more stable than ITO basedfilms in the presence of H plasma. It can match, even2

if not exceed, the optical and electrical properties ofITO. In addition, ZnO is a direct and wide band gap(E s3.3 eV) semiconductor when the film is depositedg

at a moderate temperature.The present paper is based on a study to use a

polycrystalline n-ZnO film deposited on a p-Si as apractical photon-window for Si photodiode as well as asemiconducting layer which causes a depletion regionand a built-in barrier potential in the n-p junction.Because the ZnO has a wide bandgap of approximately3.3 eV w9x, low energy photons in a visible range maymainly be collected at the depletion region of the p-Siif they are transmitted through the ZnO. However, thephotoelectric effects from the photodiode can be wellobserved only if two important qualities in the photo-diode are satisfied: the good quality junction and thefilm quality for the light transmission. In the presentwork, we have investigated the ZnOyp-Si heterostruc-tures and have reported an optimum process conditionto achieve both for the good junction and the highquality film.

554 J.Y. Lee et al. / Thin Solid Films 403 –404 (2002) 553–557

Fig. 1. X-Ray diffraction spectra of the ZnO films deposited on p-Si.All the samples mainly show(002) diffraction but the FWHMdecreases with the deposition temperature.

2. Experimental

P-type (10 Vcm) Si (100) wafers were used assubstrates for the heterojunction ZnOySi diodes. Si(100) wafers were cut into pieces of 1.5 cm by 1.5 cm.Prior to the deposition, the wafers were dipped for 1min into buffered oxide etchant(HFyH Os1:7) to2

remove native oxide. Then the samples were ultrasoni-cally cleaned with boiling acetone, ethanol, and de-ionized water for 10 min. Finally the wafers were rinsedwith de-ionized water and then blown dry with nitrogengun. ZnO films were deposited with a RF magnetronsputtering system using a 0.6-cm-thick pressed zincoxide target with 5 cm diameter(99.9% purity, Kurt J.Lesker Co.). The substrate holder was placed 80 mmaway from the target. The chamber was evacuated to abase pressure of 1=10 torr before substrate heating.y6

The ZnO films were deposited on Si substrates atdifferent substrate temperatures of 300, 400, 480 and5508C at a working pressure of 10 mtorr. The Ar andO flow ratio was fixed at 6:1 according to our previous2

research resultsw10x. The chosen RF power and thedeposition period were 80 W and 1 h, respectively. Afterdeposition, the samples were again cut into small piecesof 3=3-mm size forI–V measurements. To provide alow contact resistance of electrode, 0.5-mm-diameterindium dot of a circle form was deposited onto ZnOsurface at a corner. In the same manner, a large areaohmic contact of indium was made onto the backsideof the p-Si wafer. TheI–V characteristics of fabricateddiodes were measured using a Hewlett–Packard semi-conductor parameter analyzer(model HP4145B). For areverse bias, positive bias was applied to the n-ZnOfilm on p-Si. The measurements were performed withor without a red laser beam(670 nm wavelength)delivering a maximum power density of 56 mWycm2

onto ZnO surface. The red diode laser was 3 cm awayfrom the sample, and illuminated it with a normalincident direction. The laser beam size was big enoughto cover the ZnO surface. All measurements were takenin a dark room. Based on the optical power andmeasured photocurrent, we estimated responsitivity andquantum efficiency of an n-ZnOyp-Si heterojunctiondiode w11,12x. The thickness of ZnO films were meas-ured by 2.01 MeV He Rutherford backscattering4 2q

spectrometry(RBS). The resulting films were in athickness range from 100 to 110 nm. The crystalorientation and quality of the films were investigated bythe u–2u method of X-ray diffraction(XRD) where aNi filtered Cu-Ka (ls1.54 A) source was used. Optical˚reflectance measurements were also performed toobserve the reflectance of deposited ZnO films on p-Si.Electrical properties of the ZnO films were investigatedusing van der Pauw Hall measurements. The electricalconduction of all the samples was n-type. The resistivityranged from 0.0004 to 0.43Vcm and electron concen-

tration was from;10 to 10 cm , respectively.19 16 y3

Finally, photoluminescence(PL) was performed toinvestigate the formation of semiconducting stoichio-metric ZnO films, using 351 nm excitation of an Ar ionlaser.

3. Results and discussion

Fig. 1 shows X-ray diffraction (XRD) spectraobtained from the ZnO films deposited in an AryO2ratio of 6:1. As the substrate temperature increases, the(002) diffraction in the polycrystal ZnO becomesincreasingly sharp. According to the XRD spectra,FWHM (the Full Width of Half Maximum) of the(002)peak decreases with the deposition temperature, that is,the grain ofc-axis oriented texture increases in size withthe temperature.According to PL spectra shown in Fig. 2, UV emis-

sion is only observed from the ZnO film deposited at5508C and the samples prepared at 400 and 4808C showa broad peak near 430 nm which is caused by defectsrelated to the grain boundary of ZnOw11x. The appear-ance of the UV emission simply means that the sampleprepared at 5508C has the best stoichiometric qualityamong all the samplesw9x.

555J.Y. Lee et al. / Thin Solid Films 403 –404 (2002) 553–557

Fig. 2. PL spectra of the ZnO films deposited on p-Si at differenttemperatures. UV emission is only observed from the ZnO depositedat 5508C.

Fig. 3. Plots of resistivity and electron concentration vs. temperature.

The results of Hall measurements plotted in Fig. 3support those of PL experiments in a degree. As thedeposition temperature increases, the electrical resistivity

of the ZnO goes up to 0.43Vcm while the electronconcentration decreases with the temperature. Becausethe high resistivity means that the ZnO sample containssmall amount of oxygen vacancies causing n-type con-duction w9,11x, it is also likely that the film prepared at5508C may be the best quality sample in stoichiometry.The measured current–voltage(I–V) characteristics of

the four heterojunction diodes are shown in Fig. 4.Typical rectifying and photoelectric behaviors areobserved from the n-ZnOyp-Si diode with the ZnOprepared at 4808C. The samples dark leakage currentshown as a reference is relatively small whereas itsphoto-current generated under red illumination is muchhigh. Photocurrents obtained from other samples areobserved low compared to the diode with the ZnOprepared at 4808C. Even though the best quality of thefilm has been obtained at 5508C as observed in Figs.1–3, the diode with the ZnO film was quite inferior tothe diode of the film deposited at 4808C in the photoe-lectric aspect. Regardless of the film quality, there maybe a defective oxide layer inserted between n-ZnO andp-Si. In fact the p-Si substrate may react with oxygengases under a partial pressure of 1–2 mtorr at an elevatedtemperature at the initial stage of ZnO deposition. Thereaction is more probable at the temperature of 5508Cthan at lower temperatures. Then, the defective SiOx

oxide layer located in the depletion region of the n–pjunction may become a large leakage source. Further-more, according to the poor photoelectric effectsobserved from the diode with the ZnO prepared at5508C, it is presumed that the interface or junction mayhave SiO -ZnO mixed layer working as light-absorbingx

media.In Fig. 4, it is also noted that the diodes with n-ZnO

obtained at temperatures lower than 4808C producemuch less amounts of photo-currents. It is generallyunderstood that the photoelectric effects result from the

556 J.Y. Lee et al. / Thin Solid Films 403 –404 (2002) 553–557

Fig. 4. The current–voltage curves obtained from the n-ZnOyp-Si diodes. Photo-currents under illumination(light) are shown from the diodes.Dark current behavior(dark) is plotted as a reference after measured from the n-ZnOyp-Si diode with the ZnO deposited at 4808C.

Table 1Properties of the n-ZnOyp-Si heterostructures

Temp. Dark current density Optical reflectance Responsivity Quantum efficiency(Ar yO )2 (A ycm at 5 V)2 at 670 nm (min. value at 5 V, AyW) (min. value at 5 V)

3008C 5.05E-4 15% 3.32E-3 0.60%(6:1)4008C 4.23E-5 17% 0.062 11.47%(6:1)4808C 3.16E-4 18% 0.219 40.53%(6:1)5008C 1.16E-3 15% 0.019 3.52%(6:1)

light-induced electron-hole generation at the n–p junc-tion and particularly at the depletion area of the p-Si.Because of the limited penetration depth of the lightinto the p-Si, the photocurrents are normally saturatedin a few volts of reverse bias as shown in theI–V curveof the good n-ZnOyp-Si photodiode. Based on thegeneral understanding mentioned above, the diodes withn-ZnO obtained at 300 and 4008C are not effective tohave the electron-hole pairs generated under the redillumination w10x. Particularly the diode with an n-ZnOprepared at 3008C shows almost no sign of the photoe-lectric effects. It is probably because the poor qualityfilms are so defective that large amount of the incidentlight may not be transmitted through the film to reachthe depleted region of the p-Si.Other important properties of the n-ZnOyp-Si heter-

ostructures were measured and summarized in Table 1.Thickness and optical reflectance of all the ZnO filmsare similar. The quantum efficiency and responsivitycalculated from theI–V results(5 V reverse bias) areshown for all the diodes. In the present work themaximum quantum efficiency of approximately 40%was obtained from the photodiode with n-ZnO deposited

at 4808C. As expected and mentioned, a high darkleakage current was obtained from the junction with n-ZnO prepared at 5508C. The current leakage of thediode is one order higher than those of other diodes.

4. Conclusion

N-ZnOyp-Si heterojunction photodiodes have beenfabricated by sputter deposition of n-ZnO films on p-Sisubstrates. All the diodes show rectifying behaviors ascharacterized by the current–voltage(I–V) measurementin a dark room. Photoelectric effects have been exhibitedunder illumination using monochromatic red light witha wavelength of 670 nm. Maximum amount of photo-current or responsivity is obtained under reverse biasconditions from a n-ZnOyp-Si heterojunction when theZnO film was deposited at an optimum temperature of4808C. Even though the ZnO films deposited at 5508Cshow the best stoichiometric and crystalline qualityamong other films deposited at lower temperatures, thejunction leakage or dark current of the diode is muchhigher than those of other diodes. It is thus concludedthat for a photodiode the quality of the diode junction

557J.Y. Lee et al. / Thin Solid Films 403 –404 (2002) 553–557

is as important as that of the n-ZnO film deposited onp-Si.

Acknowledgements

The work has been supported by the Brain Korea 21project and partly supported by Korea Research Foun-dation(KRF) fund (2000-2-0816).

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