Tuning the formation of p-type defects by peroxidation of CuAlO2 filmsJie Luo, Yow-Jon Lin, Hao-Che Hung, Chia-Jyi Liu, and Yao-Wei Yang Citation: Journal of Applied Physics 114, 033712 (2013); doi: 10.1063/1.4816044 View online: http://dx.doi.org/10.1063/1.4816044 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/114/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Quantum size effect in the photoluminescence properties of p-type semiconducting transparent CuAlO2nanoparticles J. Appl. Phys. 112, 114329 (2012); 10.1063/1.4768933 Effect of high-energy electron beam irradiation on the properties of ZnO thin films prepared by magnetronsputtering J. Appl. Phys. 105, 123509 (2009); 10.1063/1.3149783 Characterization and optoelectronic properties of p -type N-doped Cu Al O 2 films Appl. Phys. Lett. 90, 191117 (2007); 10.1063/1.2679233 The influence of Cu ∕ Al ratio on properties of chemical-vapor-deposition-grown p -type Cu–Al–O transparentsemiconducting films J. Appl. Phys. 98, 033707 (2005); 10.1063/1.1997293 Mn-doped Cu 2 O thin films grown by rf magnetron sputtering J. Appl. Phys. 97, 10D318 (2005); 10.1063/1.1852319

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 18:28:19

Tuning the formation of p-type defects by peroxidation of CuAlO2 films

Jie Luo (羅傑),1 Yow-Jon Lin (林祐仲),1,a) Hao-Che Hung (洪浩哲),2 Chia-Jyi Liu (劉嘉吉),2

and Yao-Wei Yang (楊曜瑋)11Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan2Department of Physics, National Changhua University of Education, Changhua 500, Taiwan

(Received 12 April 2013; accepted 2 July 2013; published online 19 July 2013)

p-type conduction of CuAlO2 thin films was realized by the rf sputtering method. Combining

with Hall, X-ray photoelectron spectroscopy, energy dispersive spectrometer, and X-ray

diffraction results, a direct link between the hole concentration, Cu vacancy (VCu), and

interstitial oxygen (Oi) was established. It is shown that peroxidation of CuAlO2 films may lead

to the increased formation probability of acceptors (VCu and Oi), thus, increasing the hole

concentration. The dependence of the VCu density on growth conditions was identified for

providing a guide to tune the formation of p-type defects in CuAlO2. Understanding the defect-

related p-type conductivity of CuAlO2 is essential for designing optoelectronic devices and

improving their performance.VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4816044]

I. INTRODUCTION

Transparent conducting oxides (TCOs) are widely used

in the fabrication of optoelectronic and electronic devices.

Many TCOs have a wide band gap because of the significant

contribution of the ionic character to the chemical bonds

between metallic cation and oxide ions. Wide-gap semicon-

ductors are difficult to doped p-type. Several mechanisms

leading to doping difficulty are known: low solubility, com-

pensation by low-energy native defects, and deep impurity

level. CuAlO2 thin films with a delafossite structure have

attracted much attention because they were found to be

p-type TCOs.1–7 Although CuAlO2 is by far the most studied

of the delafossite materials, a critical question about the con-

ductivity of this material remains unanswered. The source of

hole carriers in CuAlO2, and indeed the conductivity mecha-

nism itself, is also a matter of much debate. Many theoretical

studies have been undertaken to understand the defects and

conductivity of delafossite materials.8,9 However, no

researcher has been able to accurately model p-type defects

in Cu-based materials.10 Nolan used first principles density

functional theory to investigate the defect chemistry of

CuAlO2.5 The defect calculations suggest that the p-type

conductivity of CuAlO2 is due to the easy formation of Cu

vacancies (VCu).5 Scanlon and Watson utilized the screened

hybrid exchange functional to investigate defects in CuAlO2

and found that VCu and copper on aluminum antisites will

dominate under Cu-deficient/Al-deficient conditions.11 The

mechanism of the defect-related conduction type in CuAlO2

has been studied and remains a controversial subject.3,5,6,8–12

Experimental work is needed to clarify what defects contrib-

ute to the widely observed p-type conductivity in CuAlO2.

Knowledge of the defect type in CuAlO2 is crucial for the

physical understanding of p-type conductivity. Of more in-

terest is the dependence of the VCu density on growth

conditions. This allows the determination of conditions

under which a formation of a VCu is favored, providing a

guide to tune the p-type defect formation in CuAlO2. A suit-

able choice of conditions will promote VCu formation in

CuAlO2. Here, we introduce the sputter technique to tune the

conductivity of CuAlO2. We found that p-type conductivity

is closely related to the acceptors in CuAlO2. Tuning the for-

mation of p-type defects by peroxidation of CuAlO2 films

has never been reported until now.

II. EXPERIMENTAL DETAILS

CuAlO2 films were grown on glass substrates by a rf

magnetron sputtering system. Prior to film deposition, the

glass substrates were ultrasonically cleaned in acetone, etha-

nol, and deionized water. The target size was 2 in. and the

target-substrate distance was 65mm. Three deposition condi-

tions were employed to identify the dependence of the VCu

density on growth conditions for providing a guide to tune

the formation of p-type defects in CuAlO2.

A. Film deposition for CuAlO2 samples from group A

The CuAlO2 films were prepared on glass substrates by

rf magnetron sputtering using a high-purity CuAlO2 target

(rf power was fixed at 60W). The target was presputtered in

an Ar atmosphere for 10min to clean the surface and remove

any possible contamination. The sputtering pressure was

fixed at 5� 10�3 Torr. The substrate temperature was fixed

at 500 �C. Ar was used as an ambient gas for sputtering. The

flow of Ar was 70 SCCM (SCCM denotes standard cubic

centimeter per minute). The sputtering time was 60min.

B. Film deposition for CuAlO2 samples from group B

The CuAlO2 films were prepared on glass substrates by

rf magnetron sputtering using a high-purity CuAlO2 target

(rf power was fixed at 60W). The sputtering pressure was

fixed at 5� 10�3 Torr. The substrate temperature was fixed

at 500 �C. The target was used in conjunction with Ar/O2 as

a)Author to whom correspondence should be addressed. Electronic mail:

rzr2390@yahoo.com.tw. Tel.: 886-4-7232105 ext. 3379. Fax: 886-4-

7211153

0021-8979/2013/114(3)/033712/5/$30.00 VC 2013 AIP Publishing LLC114, 033712-1

JOURNAL OF APPLIED PHYSICS 114, 033712 (2013)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 18:28:19

an ambient gas for sputtering. The flow of Ar and O2 was 70

and 5 SCCM, respectively. The sputtering time was 60min.

Recently, it was reported the properties of CuAlO2 films are

closely related to oxygen.5,13–16 It is expected that such an

oxygen effect occurs equally in our case, since p-type con-

ductivity is sensitive to oxygen during growth.

C. Film deposition for CuAlO2 samples from group C

The CuAlO2 films were prepared on glass substrates by

rf magnetron sputtering. A high-purity CuAlO2 target (rf

power was fixed at 60W) and an Al target (rf power was

fixed at 30W) were used for deposition of CuAlO2 films.

The sputtering pressure was fixed at 5� 10�3 Torr. The sub-

strate temperature was fixed at 500 �C. The target was used

in conjunction with Ar/O2 as an ambient gas for sputtering.

The flow of Ar and O2 was 70 and 5 SCCM, respectively.

The sputtering time was 60min. Mraz and Schneider meas-

ured the energy distribution function of O� ions for sputtered

Al in an Ar/O2 atmosphere.17 Therefore, an Al target was

used for deposition of CuAlO2 films. It is expected that the

incorporation of peroxo species into CuAlO2 during growth

will occur, since p-type conductivity is sensitive to peroxida-

tion of CuAlO2 films.

D. Characterization

The film thickness was 656 5 nm for CuAlO2 samples

from groups A, B, or C. The structural properties were exam-

ined by X-ray diffraction (XRD). The carrier concentration,

mobility, resistivity, and conduction type were obtained from

the Hall measurements in the van der Pauw configuration for

all samples. The electrodes were fabricated by depositing Au

metal on the CuAlO2 layer through a shadow mask. X-ray

photoelectron spectroscopy (XPS) was employed to identify

the atomic concentrations of the elements and the chemical

bonding state of the CuAlO2 layers. XPS measurements were

performed using a monochromatic Al Ka X-ray source. To

enhance surface sensitivity, the XPS data were taken at a

take-off angle of 45� from the surface. They were calibrated

by taking the C 1s peak as a reference. The XPS core-level

peaks were deconvolved into their various components using

an interactive least-squares computer program; the curves

were taken as 80% Gaussian and 20% Lorentzian mixed func-

tions. The atomic concentrations of the elements were deter-

mined using the peak area and the atomic sensitivity factors of

elements. The chemical composition of the film was also esti-

mated by energy dispersive spectrometer (EDS). Two meth-

ods were tried to quantify the film composition.

III. RESULTS AND DISCUSSIONS

Figure 1 shows the XRD spectra pertaining to materials

that corresponded to various processing conditions. All these

films were deposited onto substrates of glass. In the XRD

spectra shown in Fig. 1, peaks of the CuAlO2 phases are,

respectively, marked. Three peaks [corresponding to the

CuAlO2 (006), CuAlO2 (101), and CuAlO2 (018) planes]

were observed.12,18–20 There were no observable changes in

the XRD spectra of the CuAlO2 samples. For the CuAlO2

samples, the Al 2p region consists of the Al 2p (Al3þ) peakand Cu 3p peak.21 Thus, it was deconvolved into their vari-

ous components using an interactive least-squares computer

program. Figure 2 shows the Al 2p (Al3þ) XPS spectra of

CuAlO2 samples from groups A, B, and C, respectively. The

binding energy corresponding to Al 2p (Al3þ) is about

74 eV, which is similar to earlier reports.16,22,23 There were

no observable changes in the Al 2p XPS spectra of samples

from groups A, B, and C. For samples from groups A, B, or

C, the atomic concentration of Al ([Al]) was determined

using the Al 2p peak area and the atomic sensitivity factor of

Al. Note, the different grown conditions did not lead to re-

markable changes in Al content. Thus, the Al mole fraction

in CuAlO2 films was set at a value of 1. [Al] was used as a

reference in calculating of the Cu/Al and O/Al atomic con-

centration ratios ([Cu/Al] and [O/Al]). EDS showed that the

atomic ratio of Cu to Al was close to [Cu/Al], as determined

from XPS measurements. The Cu0.93AlO1.96 (group A),

Cu0.83AlO2.16 (group B), and Cu0.78AlO2.55 (group C) films

were grown using the sputter method.

FIG. 1. XRD patterns of CuAlO2 samples from groups (a) A, (b) B, and (c) C.

FIG. 2. Al 2p (Al3þ) core-level spectra of CuAlO2 samples from groups (a)

A, (b) B, and (c) C.

033712-2 Luo et al. J. Appl. Phys. 114, 033712 (2013)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 18:28:19

Table I shows the carrier concentration, mobility, resis-

tivity and conduction types of Cu0.93AlO1.96, Cu0.83AlO2.16,

and Cu0.78AlO2.55 films, respectively. The Cu0.93AlO1.96,

Cu0.83AlO2.16, and Cu0.78AlO2.55 films show p-type behav-

ior. We found that the mobility of Cu0.78AlO2.55 samples is

lower than that of Cu0.83AlO2.16 (Cu0.93AlO1.96) samples.

We suggest that ionized defect scattering dominates in the

high hole-density Cu0.78AlO2.55 film and affects carrier mo-

bility. In addition, the hole concentration of Cu0.78AlO2.55

samples is higher than that of Cu0.83AlO2.16 samples and the

hole concentration of Cu0.83AlO2.16 samples is higher than

that of Cu0.93AlO1.96 samples, indicating that different [Cu/

Al] ([O/Al]) affect the hole concentration. Note, the hole

concentration increases with decreasing [Cu/Al] and increas-

ing [O/Al], implying that the increased hole concentration is

considered to come from an increased number of acceptors

[VCu and interstitial oxygen (Oi)]. For Cu0.93AlO1.96,

Cu0.83AlO2.16, or Cu0.78AlO2.55 films, [Cu/Al]< 1 implied

the existence of VCu. The Cu0.83AlO2.16 and Cu0.78AlO2.55

films show [O/Al]> 2, suggesting the existence of Oi.

However, the Cu0.93AlO1.96 film exhibits [O/Al]< 2, sug-

gesting the existence of oxygen vacancies. For Cu0.93AlO1.96

films, we found the hole compensating oxygen vacancies

limit the formation of VCu-related defects. Defect formation

can be tuned, so under oxidizing conditions formation of Cu

vacancies is promoted. Fang et al. identified two types of

point defects (that is, VCu and Oi), which can be present in

CuAlO2 and generate trap levels inside the energy gap.3

They suggested that VCu is the origin of the p-type conduc-

tivity, while Oi, as the deep level defect.3 It forms a localized

state and does not contribute to the p-type conductivity.3

Tate et al. suggested that the conduction mechanism for

p-type carriers in CuAlO2 is the charge transport in the va-

lence band, and the holes are thermally activated from

copper-vacancy acceptor states located about 0.7 eV above

the valence-band maximum.6 However, we found that the

increased oxygen content in CuAlO2 films may lead to an

increased formation probability of Oi, thus, increasing the

formation probability of VCu. Based on the XPS, Hall, and

EDS results, the increased hole concentration is considered

TABLE I. The carrier concentration, mobility, conduction type and resistivity of the delafossite films determined by Hall measurements.

Groups Conduction type Carrier concentration (cm�3) Mobility (cm2 V�1 s�1) Resistivity (X cm) [Cu/Al] [O/Al]

Cu0.93AlO1.96 A p 8.88� 1013 0.60 11 7304 0.93 1.96

Cu0.83AlO2.16 B p 1.45� 1015 0.82 5256 0.83 2.16

Cu0.78AlO2.55 C p 8.21� 1016 0.10 761 0.78 2.55

FIG. 3. Cu 2p core-level spectra of

CuAlO2 samples from groups (a) A,

(b) B, and (c) C.

033712-3 Luo et al. J. Appl. Phys. 114, 033712 (2013)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 18:28:19

to come from the increased number of acceptors (VCu and

Oi). Note, the CuAlO2 performance may be affected by Oi

and the formation of p-type defects may be tuned by the con-

trolling oxidation of CuAlO2 films. Zhang et al. found that

the excess oxygen caused by high oxygen partial pressure

during growth may provide hole carriers for CuAlO2 films.16

Nolan used first principles density functional theory to inves-

tigate the defect chemistry of CuAlO2 and found the depend-

ence of the formation energy on growth conditions (i.e.,

oxygen content).5 According to Nolan’s computational

result,5 we found that CuAlO2 shows strongly reduced VCu

(Oi) formation energies under oxidizing conditions, provid-

ing insight into enhancing the stability of acceptors and

increasing the number of acceptors. Consequently, the for-

mations of VCu and Oi are favored under peroxidizing

conditions.

Figure 3 shows the Cu 2p XPS spectra of CuAlO2 sam-

ples from groups A, B, and C, respectively. Figure 3 shows

two strong peaks appear at 932.8 and 952.8 eV, which are in

agreement with the binding energies of Cu 2p3/2 and Cu 2p1/

2, respectively. The peaks could be attributed to the existence

Cuþ.3,24 Figure 3 also shows two weak peaks appear at 934.0

and 954.0 eV, which are in agreement with the binding ener-

gies of Cu 2p3/2 and Cu 2p1/2, respectively. The peaks could

be attributed to the existence Cu2þ.25 The shake-up satellites

peaks usually appearing in CuO and at a binding energy of

940–945 eV are also seen in these spectra.26,27 Quantitative

analysis of the XPS spectra gives the atomic ratios of Cu2þ

to [Cu1þþCu2þ] ([Cu2þ/(Cu1þþCu2þ)]) of 34.7%, 27.9%,

and 20.6% for CuAlO2 samples from groups A, B, and C,

respectively. Based on the absence of CuO XRD peaks

(Fig. 1), we suggest Cu2þ existing in the forms of Cu-O

dimers hybridized in CuAlO2 lattice. [Cu2þ/Al] is a product

of both [Cu2þ/(Cu1þþCu2þ)] and [Cu/Al]. It is shown that

[Cu2þ/Al] decreases with decreasing [Cu/Al]. However,

there were no significant changes in [Cu1þ/Al]. We suggest

that transformation from Cu2þ to VCu introduces holes,

increasing the hole concentration.

Figure 4 shows O 1s core-level spectra of the CuAlO2

films from groups A, B, and C, respectively. The peak posi-

tioned at about 530.5 eV is attributed to O2�. Figure 4(c)

shows the difference in the O 1s XPS spectra of the films de-

posited with and without an Al target. The peak located at

�532 eV could be attributed to the incorporation of (O2)2�

peroxo species into Cu0.78AlO2.55.28 Cho et al. suggested that

the increased peak intensity could be due to incorporation of

the (O2)2� peroxo species into AlCdO deposited with an Al

target.29 Thus, we suggested that the occurrence of the reac-

tion may lead to peroxidation, increasing the formation proba-

bility of Oi. The increased oxygen content in the film may

result in an increased number of VCu, increasing the hole

concentration. This can explain why the hole concentration of

the Cu0.78AlO2.55 film is higher than the Cu0.83AlO2.16

(Cu0.93AlO1.96) film. This paper suggests that the incorpora-

tion of (O2)2� into the delafossite film induced a peroxidic

film, increasing the acceptor density. Lan et al. found the

introduction of hole carriers ionized by VCu and Oi in the

excess oxygen condition.12 Time domain measurements pro-

vided evidence of the existence of electron traps in CuAlO2.30

IV. CONCLUSIONS

Knowledge of the defect type and of its variation with

changing [Cu/Al] and [O/Al] in the delafossite film is crucial

for the physical understanding of p-type conductivity. The

dependence of the VCu density on growth conditions was

found. Combining with the Hall, XPS, EDS, and XRD

results, a direct link between the hole concentration, VCu and

Oi was established. It is shown that the hole concentration

increases with decreasing [Cu/Al] and increasing [O/Al].

This could be explained by the number of holes produced by

ionized VCu. The increased hole concentration was also

attributed to the increased number of Oi. Peroxidation of

CuAlO2 would increase the formation probability of Oi, thus,

increasing the number of VCu. Understanding the defect-

related p-type conductivity of CuAlO2 is essential for design-

ing optoelectronic devices and improving their performance.

ACKNOWLEDGMENTS

The authors acknowledge the support of the National

Science Council of Taiwan (Contract No. 100-2112-M-018-

003-MY3) in the form of grants.

1Z. Deng, X. Fang, R. Tao, W. Dong, D. Li, and X. Zhu, J. Alloys Compd.

466, 408 (2008).2V. Jayalakshmi, R. Murugan, and B. Palanivel, J. Alloys Compd. 388, 19(2005).

3M. Fang, H. He, B. Lu, W. Zhang, B. Zhao, Z. Ye, and J. Huang, Appl.

Surf. Sci. 257, 8330 (2011).4Y. Yang, L. Wang, H. Yan, S. Jin, T. J. Marks, and S. Li, Appl. Phys. Lett.

89, 051116 (2006).5M. Nolan, Thin Solid Films 516, 8130 (2008).6J. Tate, H. L. Ju, J. C. Moon, A. Zakutayev, A. P. Richard, J. Russell, and

D. H. McIntyre, Phys. Rev. B 80, 165206 (2009).7J. Pellicer-Porres, A. Segura, A. S. Gilliland, A. Munoz, and P. Rodrıguez-

Hernandez, Appl. Phys. Lett. 88, 181904 (2006).8I. Hamada and H. Katayama-Yoshida, Physica B 376–377, 808 (2006).9H. Katayama-Yoshida, T. Koyanagi, H. Funashima, H. Harima, and A.

Yanase, Solid State Commun. 126, 135 (2003).

FIG. 4. O 1s core-level spectra of CuAlO2 samples from groups (a) A, (b)

B, and (c) C.

033712-4 Luo et al. J. Appl. Phys. 114, 033712 (2013)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 18:28:19

10D. O. Scanlon, B. J. Morgan, and G. W. Watson, J. Chem. Phys. 131,124703 (2009).

11D. O. Scanlon and G. W. Watson, J. Phys. Chem. Lett. 1, 3195

(2010).12W. Lan, W. L. Cao, M. Zhang, X. Q. Liu, Y. Y. Wang, E. Q. Xie, and H.

Yan, J. Mater Sci. 44, 1594 (2009).13B. L. Stevens, C. A. Hoel, C. Swanborg, Y. Tang, C. Zhou, M. Grayson,

K. R. Poeppelmeier, and S. A. Barnett, J. Vac. Sci. Technol. A 29, 011018(2011).

14S. Takahata, K. Saiki, T. Imao, H. Nakanishi, M. Sugiyama, and S. F.

Chichibu, Phys. Status Solidi C 6, 1105 (2009).15K. Hayashi, H. Hayashida, and Y. Nakano, J. Metastable Nanocryst.

Mater. 20, 563 (2004).16Y. Zhang, Z. Liua, L. Feng, and D. Zang, Appl. Surf. Sci. 258, 5354(2012).

17S. Mraz and J. M. Schneider, Appl. Phys. Lett. 89, 051502 (2006).18G. Li, X. Zhu, H. Lei, H. Jiang, W. Song, Z. Yang, J. Dai, Y. Sun, X. Pan,

and S. Dai, J. Sol-Gel Sci. Technol. 53, 641 (2010).19A. N. Banerjee and K. K. Chattopadhyay, J. Appl. Phys. 97, 084308

(2005).

20J. Li, X. Wang, S. Shi, X. Song, J. Lv, J. Cui, and Z. Sun, J. Am. Ceram.

Soc. 95, 431 (2012).21R. S. Yu, C. J. Lu, D. C. Tasi, S. C. Liang, and F. S. Shieu,

J. Electrochem. Soc. 154, H838 (2007).22D. Briggs and M. P. Seah, Practical Surface Analysis: Auger and X-rayPhotoelectron Spectroscopy, 2nd ed. (Wiley, Weinheim, 1990).

23V. I. Nefedov, X-ray Photoelectron Spectroscopy of Solid Surface (VSP

BV, Utrecht, 1988).24H. Gong, Y. Wang, and Y. Luo, Appl. Phys. Lett. 76, 3959 (2000).25S. Velu, K. Suzuki, C. S. Gopinath, H. Yoshida, and T. Hattori, Phys.

Chem. Chem. Phys. 4, 1990 (2002).26P. E. Larson, J. Electron Spectrosc. Relat. Phenom. 4, 213 (1974).27Z. Q. Yao, B. He, L. Zhang, C. Q. Zhuang, T. W. Ng, S. L. Liu, M. Vogel,

A. Kumar, W. J. Zhang, C. S. Lee, S. T. Lee, and X. Jiang, Appl. Phys.

Lett. 100, 062102 (2012).28K. H. Lee, H. W. Jang, K. B. Kim, Y. H. Tak, and J. L. Lee, J. Appl. Phys.

95, 586 (2004).29W. M. Cho, G. R. He, T. H. Su, and Y. J. Lin, Appl. Surf. Sci. 258, 4632(2012).

30Y. J. Lin, J. Luo, and H. C. Hung, Appl. Phys. Lett. 102, 193511 (2013).

033712-5 Luo et al. J. Appl. Phys. 114, 033712 (2013)

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 18:28:19