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Materials Science and Engineering B 176 (2011) 846–849

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

ptical and structural properties of Cu-doped �-Ga2O3 films

ijun Zhang, Jinliang Yan ∗, Qingshan Li, Chong Qu, Liying Zhang, Wanfeng Xiechool of Physics, Ludong University, Hongqi Road, No. 186, Yantai 264025, PR China

r t i c l e i n f o

rticle history:eceived 7 January 2011eceived in revised form 21 March 2011ccepted 10 April 2011

a b s t r a c t

The intrinsic and Cu-doped �-Ga2O3 films were grown on Si and quartz substrates by RF magnetronsputtering in an argon and oxygen mixture ambient. The effects of the Cu doping and the post ther-mal annealing on the optical and structural properties of the �-Ga2O3 films were studied. The surfacemorphology, microstructure, optical transmittance, optical absorption, optical energy gap and pho-

ey words:u-doped �-Ga2O3

ptical propertyptical band gaphermal annealing

toluminescence of the �-Ga2O3 films were significantly changed after Cu-doping. After post thermalannealing, Polycrystalline �-Ga2O3 films were obtained, the transmittance decreased. After Cu-doping,the grain size decreased, the crystal quality deteriorated and the optical band gap shrunk. The UV, blueand green emission bands were observed and discussed. The UV and blue emission were enhanced anda new blue emission peak centred at 475 nm appeared by Cu-doping.

F magnetron sputtering

. Introduction

A need for deep-ultraviolet (UV) transparent conductive oxideTCO) films has recently emerged for use as antistatic electric lay-rs of phase shift masks for photolithography and as transparentlectrodes for UV optoelectronic devices [1–6]. Conventional trans-arent conductive oxides such as ITO and ZnO are opaque in theeep-UV region (<300 nm) due to a small energy band gap [7].ew TCO materials need to be explored for this purpose. Creat-

ng deep-UV transparent thin films presents several difficulties.ince the position (scaled from the vacuum level) of the conduc-ion band bottom is relatively high, the donor levels tend to becomeeep levels. Introduction of the shallow donor levels into the com-ounds for efficient release of electrons into the conduction band isore difficult than in narrow gap oxides. This significantly reduces

he number of candidate materials for deep-UV TCO. �-Ga2O3 is aide band (4.8 eV) intrinsic insulator [8–11] and can be changed

o n-type semiconductor by doping [12]. It is suitable for the nexteneration optoelectronic devices operating at shorter wavebandecause it has the widest band gap among TCOs.

Masahiro Orita [13] used the floating zone method to grow Sn-oped �-Ga2O3 single crystals with (1 0 0) plane whose electricalonductivity is as high as 8.2 S cm−1. Kiyoshi [14] reported that Si-oping can help to improve the photoluminescence of the �-Ga2O3.

he Si-doped �-Ga2O3 and Sn-doped �-Ga2O3 behave as the n-typeemiconductor because of donor impurity and oxygen deficiency15]. Therefore, with a view to developing the short wavelength

∗ Corresponding author. Tel.: +86 535 6672730.E-mail addresses: yanjinliang@yahoo.cn, zhang882384@yahoo.com.cn (J. Yan).

921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2011.04.014

© 2011 Elsevier B.V. All rights reserved.

optoelectronic devices based on �-Ga2O3, it is urgently needed toprepare p-type �-Ga2O3 films. As one of group 1B elements, Cu isa potential P-type dopant for �-Ga2O3. In this article, Cu-doped �-Ga2O3 films were grown by sputtering, and their optical propertiesand structural properties were studied.

2. Experimental

The �-Ga2O3 films were grown on Si (1 1 1) and UV transparencyquartz (JGS1) substrates by RF magnetron sputtering. The used tar-gets were pure �-Ga2O3 (99.99%) ceramic target and Cu-doped�-Ga2O3 ceramic target. After substrate cleaning, sputtering wasperformed in an O2/Ar (1:1) mixture ambient for 20 min at 1 Pa.Un-doped and Cu-doped (5 wt%) �-Ga2O3 thin films were preparedto investigate influence of the copper impurity in the �-Ga2O3 thinfilms. The thickness of the films was about 100 nm. The substratetemperature was fixed at room temperature during the deposi-tion. The samples were post-annealed at 800 ◦C for 1 h in nitrogen(99.99%) ambient. The crystal orientation, photoluminescence,optical transmittance, optical absorption and surface morphol-ogy of the samples were characterized using X-ray diffraction(D/MAX 2500V), fluorescence spectrometer (RF-5301), ultravioletspectrophotometer (UV2550) and scanning electron microscopy(JSM-5160V).

3. Results and discussion

3.1. Crystal structure and surface morphology

Fig. 1 shows the XRD patterns of un-doped and Cu-doped�-Ga2O3 films deposited onto Si substrates post-annealed in

Y. Zhang et al. / Materials Science and Engineering B 176 (2011) 846–849 847

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Fig. 3. Transmittance spectra of the as-deposited and post-annealed �-Ga2O3 films.

ig. 1. XRD patterns of un-doped and Cu-doped �-Ga2O3 films deposited on Siubstrate and post-annealed in N2 ambient at 800 ◦C for 1 h.

2 ambient at 800 ◦C for 1 h. Polycrystalline �-Ga2O3 films arebtained after post-annealing as shown in Fig. 1. The intensity ofhe (1 1 1) peak decreases after Cu-doping. Diffraction peaks fromther phases are not observed. It indicates that the Ga ions are sub-tituted by the Cu ions without changing the monoclinic structurefter thermal annealing at 800 ◦C. The (1 1 1) peak (2� = 35.59◦) ofhe diffraction pattern of Cu-doped sample is slightly shifted toarger degree (2� = 35.62◦) as compared to the un-doped one. It

eans that small variation in the lattice parameters occurs afteru doping, which results from the ion radius difference betweenhe Ga and Cu in �-Ga2O3 films.

Fig. 2 shows the surface morphology of un-doped �-Ga2O3 andu-doped �-Ga2O3 films post-annealed in N2 ambient at 800 ◦Cor 1 h. The grain size of un-doped �-Ga2O3 film is bigger than therain size of Cu-doped �-Ga2O3 film. The grain growth of �-Ga2O3lms is hindered by Cu-doping. An obvious deterioration of therystallization quality is observed after Cu-doping. The results aren accordance with the XRD results.

.2. Optical properties

Fig. 3 shows the optical transmission spectra of the �-Ga2O3lms before and after annealing. The sample thickness is about00 nm. As shown in Fig. 3, the transmittance spectra of the un-oped films have a high transmittance more than 83% (includingubstrates) in the visible region and an excellent transmittanceore than 70% in the UV region (285–400 nm). Before annealing,

he transmittance slightly decreases after doping. After post-nnealing, the transmittance of the Cu-doped sample obviouslyeteriorates in the whole optical region. This indicates that the

optant Cu is activated by post-annealing.

Fig. 4 shows that the average absorption of the Cu-doped �-a2O3 films is higher than the un-doped one before annealing.fter annealing, the absorption was greatly decreased for the un-

Fig. 2. SEM images of un-doped (a) and Cu-doped (b) �-Ga2

Fig. 4. Optical absorption spectra of the as-deposited and post-annealed �-Ga2O3

films.

doped sample and increased for the Cu-doped sample. All the filmshave a sharp absorption edge in the deep ultraviolet region. Thereis a near-edge absorption in the post-annealed Cu-doped �-Ga2O3film, which indicates that the Cu atoms are activated and serve aseffective acceptors in the post-annealed film. Thus, the origin of thenear-edge absorption is related to the Cu acceptor states in the bandgap of Cu-doped �-Ga2O3 film. The near-edge absorption repre-sents transitions from singly ionized Cu acceptors to shallow donorsand the conduction band bottom or from singly ionized Cu accep-tors to deep level donors such as oxygen vacancy (Vo) or intrinsicdefects. The absence of near-edge absorption in the as-depositedCu-doped �-Ga2O3 film presumably suggests that the Cu impuritiesare almost not activated in this case. Therefore, the deteriorationof the transmittance is mainly caused by the near-edge absorption

in the post-annealed Cu-doped �-Ga2O3 film.

The optical band gaps of �-Ga2O3 films are estimated by extrap-olating the linear portion of the square of absorption coefficient

O3 films post-annealed in N2 ambient at 800 ◦C for 1 h.

848 Y. Zhang et al. / Materials Science and En

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ig. 5. (˛hv)2 versus photon energy plots of the �-Ga2O3 films as-deposited (a) andost-annealed in N2 ambient at 800 ◦C for 1 h (b).

gainst photon energy using the equation.

˛h�)2 = B(hv − Eg) (1)

Here B is a constant. Fig. 5(a) shows the (˛hv)2 versus pho-on energy plots of the as-deposited �-Ga2O3 films. The un-doped-Ga2O3 film has an optical band gap of 4.83 eV. This value is ingreement with the reported values 4.9 eV for un-doped �-Ga2O316]. The optical band gap of the �-Ga2O3 films decreases to 4.73 eVfter Cu-doping. The decrement of the band gap is 0.1 eV, which isrobably caused by impurity energy level of Cu in the �-Ga2O3lms.

Fig. 5(b) shows (˛hv)2 versus photon energy plots of the �-a2O3 films post-annealed in N2 atmosphere at 800 ◦C for 1 h. Aramatic change occurs after the post annealing treatment. Theptical band gap of the Cu-doped �-Ga2O3 shrinks to 4.65 eV. How-ver, the optical band gap of the un-doped �-Ga2O3 increases to.88 eV. The appearance of the (1 1 1) peak in the XRD patterns

ndicates that the crystal quality of the un-doped �-Ga2O3 films isreatly improved after post-annealing. So the defect state in the for-idden band is reduced and the optical band gap is expanded. Thebvious shrinkage of the optical band gap of the post-annealed Cu-oped �-Ga2O3 film indicates that the impurity Cu atoms have been

argely activated as acceptors in the �-Ga2O3 film and introduce thecceptor energy level on the top of the valence band.

.3. Photoluminescence spectra

Fig. 6 shows the room temperature PL spectra of the post-nnealed �-Ga2O3 films excited at 280 nm. As illustrated, the

ig. 6. Room temperature PL spectra of the �-Ga2O3 films post-annealed at 800 ◦Cor 1 h in N2 ambient excited at 280 nm.

gineering B 176 (2011) 846–849

un-doped �-Ga2O3 film shows a broad UV–Blue emission, very sim-ilar to that in Cu-doped �-Ga2O3 film. However, the Cu-dopingenhances the PL emission intensity. This emission band can bedivided into three Gaussian bands as shown in Fig. 6(a). Besidesthe common UV and blue emission, the green emission can be dis-cerned. In the un-doped sample, both the UV and blue emissionsare equally excited. Three major emissions are centred at 376 nm,406 nm and 425 nm respectively, while one green emission is cen-tred at 498 nm. These peak positions undergo a slight change for theCu-doping sample. It is worth noticing that there is an additionalpeak centred at 475 nm for the Cu-doping sample.

The emission peak in UV region is associated with the recom-bination of the self-trapped excitons, which is an intrinsic process[17]. The broad green band emission centred at 498 nm is gener-ated by the radial recombination of a photo generated hole withthe electron in ionized VO, VGa or VGa:VO and CuGa. Moreover, theblue luminescence is excited with photon energy slightly below theband gap of �-Ga2O3, which suggests that the acceptor defects withground state close to the valence band are involved. The blue emis-sion would originate from the recombination of an electron on adonor and a hole on an acceptor. An electron in a donor band is cap-tured via a tunnel transfer by a hole on an acceptor to form a trappedexciton which can emit the blue photon. These defects includethe donor and the acceptor, the acceptor contains gallium–oxygenvacancy pair (VO,VGa)′ and copper substituting gallium (CuGa

′). So

the blue luminescence is greatly enhanced when the CuGa′ concen-

tration increases after Cu-doping. A lot of doped defects occur inthe crystallization process and a large quantity of Cu atoms residesin the original Ga site of the Cu-doped �-Ga2O3 films in the thermalannealing process. These impurity defects generally serve as deepdefect acceptors in semiconductors and would induce new energylevels in the band gap [18]. As a result, a new emission band with apeak at about 475 nm appears.

4. Conclusions

Polycrystalline �-Ga2O3 films are obtained by post-annealing.The crystallization quality deteriorates after Cu-doping. The grainsize of intrinsic �-Ga2O3 film is bigger than the grain size of Cu-doped �-Ga2O3 film. The average absorption of the as-depositedCu-doped �-Ga2O3 films is higher than that of the un-doped one.After post-annealing, the transmittance and optical band gap sig-nificantly decrease for the Cu-doped �-Ga2O3 films. The averageabsorption decreases for the un-doped �-Ga2O3 films and increasesgreatly for the Cu-doped �-Ga2O3 films. The characteristic lumines-cence bands appear in UV, blue and green spectral regions. The UVand blue emissions are enhanced and a new blue emission peakcentred at 475 nm appears for the Cu-doped �-Ga2O3 films.

Acknowledgements

This work is supported by the National Natural Science Foun-dation of China (Grant No. 10974077), Natural Science Foundationof Shandong Province, China (Grant No. 2009ZRB01702), A Projectof Shandong Province Higher Educational Science and TechnologyProgram (J10LA08).

References

[1] J. Hao, M. Cocivera, J. Phys. D 35 (2002) 433–439.[2] M. Ogita, K. Higo, Y. Nakanishi, Y. Hatanaka, Appl. Surf. Sci. 721 (2001) 175–178.[3] H.W. Kim, K.N. Kim, Mater. Sci. Eng. B 110 (2004) 34–37.[4] M. Rebien, W. Henrion, M. Hong, J.P. Mannaerts, M. Fleischer, Appl. Phys. Lett.

81 (2002) 250–256.[5] M. Passlack, E.F. Schubert, W.S. Hobson, M. Hong, N. Moriya, S.N.G. Chu, J. Appl.

Phys. 77 (1995) 686–690.[6] E.G. Víllora, K. Shimamura, K. Aoki, K. Kitamura, Thin Solid Films 500 (2006)

209–213.

and En

[[[

[

[

[

[

Y. Zhang et al. / Materials Science

[7] K.-S. Kim, H.W. Kim, C.M. Lee, Mater. Sci. Eng. B 98 (2003) 135–139.[8] Z. Hajnal, J. Miro, G. Kiss, F. Reti, R.C. Herndon, J.M. Kuperberg, J. Appl. Phys. 86

(1999) 3792–3797.[9] N. Ueda, H.R. Hosono, H. Kawazoe, Appl. Phys. Lett. 71 (1997) 933–938.10] H.G. Kim, W.T. Kim, J. Appl. Phys. 62 (1987) 2000–2006.

11] H.H. Tippins, Phys. Rev. A316 (1965) 140–145.12] J.G. Zhang, C.G. Xia, Q. Deng, J. Phys. Chem. Solids 67 (2006)

1656–1659.13] O. Shigeo, S.Z.K. Norihito, A. Naoki, T. Masahiko, S. Takamasa, N. Kazuo, S. Toetsu,

Thin solid films 516 (2008) 5763–5767.

[

[

gineering B 176 (2011) 846–849 849

14] S. Kiyoshi, G. Encarnación, Víllora, Takekazu, K.A. Ujiie, Appl. Phys. Lett. 92(2008) 201914–201918.

15] N. Suzuki, S. Ohira, M. Tanaka, T. Sugawara, K. Nakajima, T. Shishido, Phys. StatusSolidi 4 (2007) 2310–2315.

16] K. Takakura, D. Koga, H. Ohyama, J.M. Rafi, Y. Kayamoto, M. Shibuya, H.

Yamamoto, J. Vanhellemont, Physica B 404 (2009) 4854–4857.

17] G. Pinaki, C. Supriya, C. Subhadra, Low-dimens. Syst. Nanostruct. 23 (2004)81–85.

18] L. Fu, Y.Q. Liu, P.A. Hu, K. Xiao, G. Yu, D.B. Zhu, Chem. Mater. 15 (2003)4287–4292.