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Materials Letters 62 (20

Deposition of K-doped p type ZnO thin films on (0001) Al2O3 substrates

Wu Jun ⁎, Yang Yintang

Key laboratory of Ministry of Education for Wide Band Gap Semiconductor Materials and Devices, Xidian University, Xi'an 710071, PR China

Received 28 September 2007; accepted 17 October 2007Available online 24 October 2007

Abstract

To study the possibility of group-I elements as p type dopants, potassium (K) has been used to prepare p-ZnO thin films deposited on (0001)Al2O3 substrates by radio frequency magnetron sputtering technique. The electrical and structure properties of as-grown films have been mainlyinvestigated using Hall measurement, X-ray diffraction (XRD), atom force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). TheK-doped ZnO films exhibit p type conductivity, and the best sample is obtained at 500 °C substrate temperature and 30% oxygen partial pressurewith a hole carrier concentration of 8.92×1017 cm−3, resistivity of 1.8Ω cm, and hole mobility of 3.89 cm2/V s. One dense and uniform surface isalso observed from the K-doped films with an average grain size of 8.3 nm and surface roughness of 98.3 nm. The ratio of Zn, O and K atoms infilms is 1:1.75:0.06.© 2007 Elsevier B.V. All rights reserved.

Keywords: Zinc oxide; Thin films; Deposition; Potassium; Radio frequency magnetron sputtering

1. Introduction

In recent years intensive attention has been paid to zinc oxide(ZnO) due to its many excellent properties, such as a direct wideband gap (∼3.37 eV), large exciton binding energy (∼60 meV),strong emission, large saturation velocity (∼3.2×107 cm/s) andhigh breakdown voltage [1]. However, the preparation of p-typeZnO, a crucial issue for utilizing its advantages in optoelec-tronic devices, is proven to be difficult owing to its self-compensation of native point defects (oxygen vacancy or zincinterstitial, for instance) [2,3]. Despite years of investigations,the most suitable acceptor dopant remains in controversy. Asone of the group-V candidates, nitrogen (N) [4–6] is ofparticular interest because of its smaller atomic radius. Whilerecent reports about other group-V acceptors, phosphorus (P)[7,8] or arsenic (As) [9], also present some better results thanthat of N. Alternatively, p-ZnO can also be fabricated by dopinggroup-I species, although there still exists a debate on thecompensation mechanism. Lee et al. [10,11] suggested that Li(Na)-H complexes could be helpful to the availability of low

⁎ Corresponding author. Tel.: +86 29 88201539; fax: +86 29 88201562.E-mail addresses: wujun1203@163.com (W. Jun), ytyang@xidian.edu.cn

(Y. Yintang).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.10.035

resistivity p-ZnO, while Wardle et al. [12] argued that p-typedoping would be passivated by the information of Li–Hcomplexes. A comparison between group-I and -V species [13]revealed that substitutional group-I elements were shallowacceptors and easier to be compensated by donors. A newprogress is reported in [14] as regards the Li:p-ZnO films with acarrier concentration of up to ∼1017 cm−3, which suggested apromising prospect in laboratory for group-I species used as p-type dopants. In this paper, we search for another group-Ielements, potassium (K), and propose a method for fabricatingp-ZnO thin films deposited on (0001) Al2O3 substrates by radiofrequency (rf) magnetron sputtering technique. The electricalproperties, crystalline structure and composition of the as-growfilms have been investigated.

2. Experimental details

ZnO films were deposited on polished (0001) Al2O3 substratesin a FJL520 vacuum rf magnetron sputtering system. K was dopedinto films by using a ZnO target (99.99% purity) mixed with 1 wt.% K2O (99.99% purity). The vacuum chamber was evacuated to abase pressure of 6×10−4 Pa, and then sputtering gases, high purityargon (Ar: 99.99%) and oxygen (O2:99.995%) gas, wereintroduced with a constant total pressure about 3–5 Pa. Thesubstrate temperature (Ts) and oxygen partial pressure (PO) were

Fig. 1. XRD profiles of p-K:ZnO films deposited on (0001) Al2O3 substratesunder different growth conditions (s.1: TS=300 °C, PO=20%; s.3: TS=500 °C,PO=20%; s.5: TS=700 °C, PO=20%; s.7: TS=500 °C, PO=30%).

1900 W. Jun, Y. Yintang / Materials Letters 62 (2008) 1899–1901

kept at 300–700 °C and 20–40%, respectively. All the films weredeposited for 60 min with a sputtering power of 150 W, andannealed for 30 s at 500 °C after deposition. The thickness of allprepared ZnO films was about 200–220 nm.

Hall measurements were carried out in the van der Pauwconfiguration using a commercial system (BIO-RADHL5500PC)at room temperature. All ZnO films were characterized by X-raydiffraction (XRD-7000, Shimadzu) with a Cu Ka radiation source(0.1541 nm) and an operating power of 40 kV×40mA. The depthprofile of filmswas investigated by atom forcemicroscopy (AFM)(NanoScope III a MultiMode AFM, Veeco, USA), operating intapping mode with single tip. The composition of the films wasobtained by means of X-ray photoelectron spectroscopy (XPS)(PHI-5400, Mg Ka source, operating power-400 W).

3. Results and discussion

The Hall measurement results at room temperature are shown inTable 1. All the as-grown K-doped samples show p type conductivitywith a higher hole carrier concentration (∼1016–1017 cm−3) and alower resistivity (the lowest is to ∼10°Ω cm). It is seen that thesubstrate temperature and oxygen partial pressure play an importantrole in the electrical properties. 500 °C is the optimal growthtemperature. Higher temperature than 500 °C will lead to moreincident atoms reflected from the deposition surface, resulting in aconcentration decrease of doping atoms. As PO is 30%, the best resultis achieved with a carrier concentration of 8.92×1017 cm−3, resistivityof 1.8 Ω cm and mobility of 3.89 cm2/V s (s.7). Excessive oxygencontent (over 30%, for example s. 8) in sputtering chamber willdecrease the sputtering efficiency with lower argon gas concentration,which makes it possible to increase the resistivity of films.

The crystalline structure of samples 1, 3, 5 and 7 from Table 1 hasbeen investigated by XRD, as shown in Fig. 1. The (0002) plane, apreferential orientation reported in Li:ZnO [13] and V-group dopedZnO [8], becomes weak. While the typical (1000), (1001) and (1100)planes of ZnO (PDF 36-1451) appear, which indicates that the filmspossess a polycrystalline hexagonal wurtzite structure [15]. Further-more, no peak from other compounds is detected besides those of ZnO,indicating that the films consist of ZnO single phase and have a highpurity. As PO is kept 20% (s1, 3, 5), s.3 shows the strongest peaks,demonstrating again that 500 °C is the optimal growth temperature.Sharper peaks observed in s.7 imply that the crystalline structure can beimproved when PO is increased.

The surface morphology of K-doped ZnO films is illustrated inFig. 2. The typical 3D AFM image shows that the films consist of

Table 1Electrical properties of ZnO films under different growth conditions

Sample Ts PO Carrier concentration Resistivity Mobility Conductivity

No. °C % cm−3 Ω cm cm2/V s Type

1 300 20 3.20×1016 256.4 0.75 P2 400 20 8.90×1016 68.1 1.02 P3 500 20 6.47×1017 4.3 2.27 P4 600 20 2.38×1017 12.2 2.15 P5 700 20 6.00×1016 95.7 1.10 P6(3) 500 20 6.47×1017 4.3 2.27 P7 500 30 8.92×1017 1.8 3.89 P8 500 40 3.19×1017 10.7 1.83 P

densely packed columns perpendicular to the substrate. The root meansquare (rms) roughness of an 878.4×878.4 nm2 area is 98.3 nm and theaverage grain size of the same area is 8.3 nm, which means a highsmoothness of the surface and a super smallness of the crystal.

XPS analysis was carried out to confirm whether the films containK or not and estimate the composition of as-grown p:K-ZnO films. Thetypical element composition is Zn 30.5%, O 67.8%, and K 1.7%.Typical XPS spectra of K-doped ZnO films are illustrated in Fig. 3. Asbe seen in Fig. 3a, the K2p core-level peak at a binding energy of293.1 eV is believed to be associated with the incorporation of Kelement into the films. The O1s peaks (Fig. 3b) are located at 530.4 eVand 532.2 eV, indicating two chemical states of oxygen correspondingto Zn–O bond and H–O bond, respectively. The relative concentrationratio of both bonds is 1:3.7. After subtracting the part of H–O state, theratio of Zn, O and K atoms in films can be expressed as 1:1.75:0.06.From this result we can find that the content of O is higher, deviatingfrom the stoichiometry of ZnO, which may be caused by the O-richatmosphere in deposition process. Additionally, some dissociative Oatoms on the surface of films are also included in the calculationprocess, which may also result in higher result of O content.

In fact, the excess O atoms in sputtering atmosphere not onlydeceases greatly the concentration of VO, one of main donor defects inZnO, but increases that of Oi and VZn, both of them being acceptors and

Fig. 2. 3D AFM image of p-K:ZnO films grown on (0001) Al2O3 substrates(TS=500 °C, PO=30%).

Fig. 3. XPS spectra of typical K-doped ZnO films (TS=500 °C, PO=30%): (a). K2p peak at 293.1 eV clearly indicating that K is incorporated in the films; (b) O1s peakand its fitted curve that is divided into two parts peaked at 530.4 eV, corresponding to Zn–O bond, and at 532.2 eV, corresponding to H–O bond.

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contributing in a certain degree to the p-type conductivity. Thus, p typeZnO films are easier to be obtained with the help of rich oxygen.

4. Conclusions

K-dopedp-ZnO thin films have been deposited on (0001)Al2O3

substrates using rf magnetron sputtering technique under richoxygen atmosphere. The analysis results indicate that K-dopedZnO films possess the best electrical properties and crystallinestructure at 500 °C substrate temperature and 30% oxygen partialpressure. The optimal p type K-doped ZnO films possess a higherhole carrier concentration of 8.92×1017 cm−3 and a lowerresistivity of 1.8 Ω cm. Although O-rich growth condition canlead to a higherO content in films, it would also be beneficial for theapparition of p type conductivity simultaneously. Much furtherwork still needs to be done, but the results reveal thatK can also be apromising candidate for preparing p-ZnO films.

Acknowledgments

We greatly thank Profs. Chai Changchun, Li Yuejin and JiaHujun for their helpful discussion on results, and highlyappreciate Prof. Lu Zhengxin (Xi'an University of Technology)for his warmhearted help in experimental device.

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