Effect of hydrogen plasma treatment on the electrical properties of sputtered N-doped cuprous oxide films

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Materials Science and Engineering B 118 (2005) 179–182

Effect of hydrogen plasma treatment on the electrical properties of

sputtered N-doped cuprous oxide films

Yang-Ming Lu

a,∗

, Chun-Yuan Chen

a

, Ming Hong Lin

b

a_{Department of Electronics Engineering and Nano Research and Development Center, Kun Shan University of Technology, Taiwan, ROC} b_{Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, ROC}

Abstract

Cuprous oxide (Cu2O) is a direct-gap semiconductor with band-gap energy of 2.0 eV and is regarded as one of the most promising materials for application in photovoltaic cells. Practical application has not been achieved to date because of the difficulty of controlling its electrical properties such as reducing the resistivity. It is known that nitrogen doping is one of the effective methods to reduce the resistivity of cuprous oxide films. In this study, N-doped cuprous oxide films have been deposited onto Corning 1737 at a constant substrate temperature of 350◦C using a magnetron co-sputtering process in a mixture of oxygen and argon gases. It was found that the hole carrier concentration of nitrogen-doped cuprous oxide films increased from 9.0× 1017_cm−3_{to 4.0}_{× 10}18_cm−3_{as the nitrogen flow rate increased from 4 to 12 ml min}−1_{. The} lowest resistivity of Cu2O film doped with nitrogen obtained in this study was 14.8 cm and further downed to a value of 9.1 cm after 1 min of hydrogen plasma treatment. Because the mobility of the carrier is almost constant, it is believed that the hydrogen plasma treatment may terminate the dangling bonds of Cu and result in increasing the carrier concentration which leads to decrease in the resistivity of the Cu2O films.

Keywords: N-doped cuprous oxide; Hydrogen plasma treatment; Magnetron co-sputtering process

1. Introduction

Cu2O has long been considered a promising candidate for possible application in photovoltaic devices. It is a reddish, non-stoichiometric p-type semiconductor with both ionic and covalent character and a direct forbidden band gap of 2.17 eV, an acceptor level 0.4 eV above the valence band, and donor levels 1.1 eV and 1.3 eV below the conduction band[1]. Many attractive advantages have been pointed out for cuprous oxide

[2]:

(1) The starting material (Cu) is relatively cheap, non-toxic and abundantly available on Earth.

(2) The production cost of cuprous oxide is relatively low. (3) The Hall mobility of cuprous oxide is usually larger than

10 cm2/V s in spite of the low formation temperature[3]. (4) Production of large area devices with cuprous oxide is

possible.

∗_{Corresponding author.}

(5) Cu2O has a high absorption coefficient in visible regions and is a non-stoichiometric p-type semiconductor with an estimated theoretical efficiency of solar energy con-version approaching 12% for a top cell in a two (or three) solar cell stack[4].

Several techniques have been used to prepare thin films of Cu2O, such as: electrodeposition[5], spraying[6], chemical vapor deposition[7], thermal oxidation[8], molecular beam epitaxy [9], and reactive sputtering [10]. In most of these studies, a mixture of Cu2O and CuO is generally obtained. Hence, the essential problem is to find out the specific reaction parameters for producing pure Cu2O.

2. Experimental

Cuprous oxide films were deposited on to Corning 1737 glass substrates by a DC magnetron sputtering system. The sputtering was carried out with a 75 mm diameter copper 0921-5107/$ – see front matter © 2005 Published by Elsevier B.V.

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182 Y.-M. Lu et al. / Materials Science and Engineering B 118 (2005) 179–182

Fig. 7. Resistivity variation of Cu2O films with hydrogen plasma treatment time.

4. Conclusions

The lowest resistivity of Cu2O film doped only with ni-trogen obtained in this study was 14.8 cm. The hydrogen plasma treatment provides an effective method to passivate the carrier traps. The resistivity of a Cu2O film was low-ered to 9.5 cm after nitrogen doping followed by hydrogen plasma treatment. Doping nitrogen into a Cu2O film, not only reduces its resistivity but also decreases its grain size.

Acknowledgement

The authors are grateful to the Nation Science Council (NSC) in Taiwan for providing the financial support for this work. The project number is NSC 92-2216-E-168-005.

References

[1] A.E. Rakshani, Solid State Electron. 29 (1986) 7.

[2] N. Tabuchi, H. Matsumura, Jpn. J. Appl. Phys. 41 (2002) 5060. [3] H. Matsumura, A. Hujii, T. Kitatani, Jpn. J. Appl. Phys. 35 (1996)

5631.

[4] E.Y. Wang, D. Trivich, H. Sawalha, G. Thomas, in: Proceedings of the COMPILES INT. Conference, Dhahran, Saudi Arabia, 1975, Heliotech and Dev 1 (1976) 643.

[5] Y.C. Zhou, J.A. Switzer, Mater. Res. Innovat. 2 (1998) 22. [6] M. Ottosson, J.O. Carlsson, Surf. Coat. Technol. 78 (1996) 263. [7] T. Maruyama, Jpn. J. Appl. Phys. 37 (1998) 4099.

[8] Y.S. Gong, C. Lee, C.K. Yang, J. Appl. Phys. 77 (1995) 5422. [9] R. Kita, K. Kawaguchi, T. Hase, T. Koga, R. Itti, T. Morishita, J.

Mater. Res. 9 (1994) 1280.

[10] D. Trivih, E.Y. Wang, R.J. Komp, A.S. Kakar, in: Proceedings of the 13th IEEE Photovoltaic Specialists Conference, Washington, DC, IEEE, New York, 1978, p. 174.

[11] S. Ishizuka, S. Kato, T. Maruyama, K. Akimoto, J. Appl. Phys. 40 (2001) 2765.

[12] S. Ishizuka, S. Kato, Y. Okamoto, T. Sakurai, K. Akimoto, N. Fuji-wara, H. Kobayashi, Appl. Surf. Sci. 216 (2003) 94.

[13] S. Ishizuka, S. Kato, Y. Okamoto, K. Akimoto, J. Cryst. Growth 237–239 (2002) 616.