Chemical Physics Letters 469 (2009) 318–320

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Chemical Physics Letters

journal homepage: www.elsevier .com/locate /cplet t

Extended X-ray absorption fine structure study of p-type nitrogen doped ZnO

Wei Mu, Lei L. Kerr *, Nadia LeyarovskaDepartment of Paper and Chemical Engineering, Miami University, OH 45056, USAArgonne National Lab, Argonne, IL 60439, USA

a r t i c l e i n f o

Article history:Received 21 November 2008In final form 7 January 2009Available online 13 January 2009

0009-2614/$ - see front matter � 2009 Published bydoi:10.1016/j.cplett.2009.01.011

* Corresponding author. Fax: +1 513 529 0761.E-mail address: kerrll@muohio.edu (L.L. Kerr).

a b s t r a c t

p-Type nitrogen doped ZnO was studied by using extended X-ray absorption fine structure (EXAFS) at theZn K edge. The p-type ZnO was fabricated on glass substrates by a low cost catalyst-free thermal evap-oration process. The EXAFS measurement showed that the bonding length of Zn–O and Zn–Zn wasincreased after converting to p-type due to the incorporation of nitrogen atoms. The EXAFS analysis indi-cated that N atoms might exist as diatom form of N–N in ZnO film.

� 2009 Published by Elsevier B.V.

1. Introduction

A full realization of the optoelectronic potential of ZnO in highefficiency tandem solar cells [1–3] and blue/UV light emittingdiodes [4–7] requires high quality p-type ZnO and a robust controlof the p-type doping. Nitrogen is the most promising candidate forp-type doping in ZnO [8]. Various thin film growth techniques suchas metal organic chemical vapor deposition (MOCVD) [9–13], sput-tering [14–17], Molecular Beam Expitaxy (MBE) [18] and pulsed la-ser deposition (PLD) [19] are used to grow p-type ZnO. Amongthese, MOCVD and sputtering are the most popular methods.Although the success of p-type conductivity has been demon-strated, these growth methods require expensive ultra high vac-uum equipments and the film p-type conductivity is unstable.For example, our previous Raman study suggested [20] that inMOCVD process, the growth conditions should be precisely con-trolled in order to avoid the incorporation of carbon defects be-cause the nitrogen–carbon-related defect complexes change thefilm conductivity from p-type to n-type and causes the instabilityof the film. Sputtering process usually yields non-stoichiometricfilm with excess oxygen vacancy which contributes to the n-typeconductivity of ZnO:N [21,22].

In this Letter, a low cost thermal evaporation process using atube furnace was developed to grow ZnO film in order to avoidthe carbon contamination from the precursor as it was used inMOCVD process. Even though tube furnace has been used to grownanostructured ZnO [23–26], no literature was found to use thismethod to make p-type ZnO:N.

The doping mechanism and the crystal structure parameters arevery important but are unknown in the development of p-typeZnO:N. Extended x-ray absorption fine structure (EXAFS) has beenaccredited as a precise method for molecular structural character-

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ization. It could provide the information on the atomic structurearound the specific atom (in this experiment, it is Zn atom), suchas near-neighbor atom distance and the fluctuation in bond length.The precision of distance can be about 0.01 angstrom, or even bet-ter [27]. In this study, the EXAFS study was performed on p-typeZnO:N grown on glass substrates using NO or NO/N2 as dopantsource which are the commonly used growth conditions. The ef-fects of two gas dopant sources, pure NO and NO/N2 mixture onZnO:N local structure and subsequently on the resulting film elec-trical properties were investigated.

2. Experiment

ZnO:N films were deposited on glass substrates by a catalyst-free, low cost thermal evaporation process. Zn powder was vapor-ized in a tube furnace under 20 KPa vacuum pressure and depos-ited on the glass substrate at 500 �C for 2 h. Oxygen and Argonwas introduced during the whole process as reactive gas and car-rier gas, respectively. The saturated O2 growth condition dimin-ishes the possibility of producing non-stoichiometric film asencountered during the sputtering process [21,22]. Then, the filmwas annealed under dopant gas of pure NO or 5% NO/N2 mixtureat 350 �C under atmosphere pressure for 1 h to achieve p-typeZnO:N. EXAFS experiment was done on 12 BM in Advanced PhotonSource Division at Argonne National Lab.

3. Results and discussion

EXAFS data were collected at the Zn K edge (9659 eV). The X-rayabsorption coefficient (l) shown in Fig. 1 was acquired from thepure ZnO (without annealing in dopant gas) near the Zn K edgemeasured at room temperature. At Zn K edge, the X-ray self-absorption of the ZnO with less than 500 lm length is negligible.

The structure around a probe atom can be described bystudying the fine structure above the absorption edge [28]. The

Fig. 2. EXAFS (v2j) from ZnO film (top), ZnO film annealed in pure NO (middle) andZnO film annealed in 5% NO/N2 mixture (bottom) as a function of the photoelectronwave vector k.

W. Mu et al. / Chemical Physics Letters 469 (2009) 318–320 319

absorption coefficient (l) was analyzed by the Athena V0.8.053,Artemis V0.8.010 with the IFEFFIT package version 1.2.10. Fig. 2shows the calculated EXAFS of pure ZnO and p-type ZnO at theZn K edge as a function of the photoelectron wave vector k. To min-imize the error, only the data between the k range of 2–13.958 Å�1

were used in further analysis.Processed by Fourier transformation and the theoretical calcu-

lation [29], the EXAFS data were converted to R space (radius fromZn atom) as shown in Fig. 3. The peaks in Fig. 3 correspond to theposition of atoms which contribute to the scattering wave con-struction. The fits including single- and multi-scattering pathswere taken into account in the data analysis. The solid lines inFig. 3 show the Fourier transformed experiment EXAFS data of pureZnO and p-type ZnO:N. The dashed line is Artemis fitting. Twokinds of dopant gases were used to make p-type ZnO:N. One ispure NO and the other is 5% NO/N2. The peaks are shifted by about0.4 Å on the r axis from their true bond lengths. The data discussedin Table 1 are after the 0.4 Å shift correction.

As seen from Fig. 3, the Zn–Zn bonding is located at peak posi-tion of 3.22 Å. The broad peak around 1.9 Å is due to Zn–O. Nitro-gen doping seems to affect the two shoulder peaks of Zn–O peak.The shoulder peaks at 1.00 Å (marked �) and 1.15 Å (marked D)were less significant for undoped ZnO (Fig. 3a) than these for theZnO:N (Fig. 3b and c). These two shoulder peaks are more promi-nent in the film with better p-type performance (ZnO:N using pureNO as dopant gas, Fig. 3b). The increase of Zn–Zn bonding due tonitrogen incorporation is in agreement with EXAFS study by Linet al. [22]. However, the shoulder peak was not discussed eventhough it was also observed in their study. In their work, the de-tailed doping mechanism as its relation to the film conductivitywas not the focus.

Table 1 shows the bonding length of Zn–O and Zn–Zn as derivedfrom EXAFS. The pure ZnO film has Zn–Zn and Zn–O bondinglength of 3.213 Å and 1.927 Å, respectively. This is in good agree-ment with the bonding length as obtained from X-ray diffraction(XRD) study (Zn–O 3.254 Å and Zn–Zn 1.979 Å) shown in Fig. 4and the results of previous studies of bulk ZnO [30,31]. The XRDpattern showed the wurtzite structure for pure ZnO film, ZnO filmannealed in pure NO and ZnO film annealed in 5% NO/N2. Thebonding length can be then calculated from the lattice constantsusing [002] and [101] peaks and the geometry of wurtzite struc-ture. The resistivity, hole mobility, hole concentration and conduc-tivity type listed in the last four columns of Table 1 were measuredby Hall Effects. As shown in Table 1, the bonding length of ZnO ob-tained from EXAFS experiment fits well to the values calculated byArtemis. The bonding lengths of Zn–O and Zn–Zn were increasedafter annealing for both nitrogen dopant gases. The increase ofZn–Zn bonding length might be due to the steric effect. In a

Fig. 1. Normalized X-ray absorption coefficient of pure ZnO as a function ofincident X-ray energy at the Zn K edge.

Fig. 3. Fourier transformed EXAFS data. (a) ZnO film & Artemis fitting, (b) ZnO filmannealed under pure NO and (c) ZnO annealed under 5% NO/N2 mixture. The errorin the bond distances is less than ±1%.

wurtzite structured ZnO unit cell, one oxygen atom is neighboringwith four Zn atoms forming the geometry of triangular pyramidwith oxygen located in the pyramid center. The substitute of N–N diatom occupies larger space than a single oxygen atom. TheZn atoms will then be pushed away from their original sites andthus enlarge the distance between Zn–Zn. The high electron den-sity of the N–N will also repulse the electron pair between Zn–Nto be shifted to the Zn atom side, which causes the Zn atoms tobe withdrew outwards. The increase of Zn–O bonding length in this

Table 1ZnO and ZnO:N Bonding length and their electrical property.

Zn–O d(Å)

Zn–Zn d(Å)

Resistivity q(X cm)

Mobilitycm2/V–S

Hole Conc./cm3

Conductivity type

ZnO model calculated by Artemis 2.04 3.229 – – – –ZnO film 1.927 3.213 3.23 5.30 3.65E+17 nZnO:N using dopant gas of pure NO 1.96 3.236 25.6 0.614 3.96E+17 pZnO:N using dopant gas of 5% NO/N2 mixture 1.964 3.243 324 7.45 2.58E+15 p

Fig. 4. XRD of ZnO film (bottom), ZnO film annealed in pure NO (middle) and ZnOfilm annealed in 5% NO/N2 mixture (top).

320 W. Mu et al. / Chemical Physics Letters 469 (2009) 318–320

work is not in agreement with the EXAFS study by Lin et al. [22]. Intheir work, the Zn–O bonding length remained constant and Zn–Znunderwent slightly increase after N+ implantation and no clearexplanation was given for such an observation. Since the Zn–Nbonding (1.77 Å) [32] is actually shorter than Zn–O bonding(1.939 Å) [33]. The increase of Zn–O bonding length from EXAFSdata might indicate that during the annealing process, nitrogenatom can exist in the form of diatom N–N in ZnO film. The extraN2 molecules in the 5% NO/N2 gas lowered electrical performanceas revealed by the higher resistivity and lower hole concentration.It has been well reported in the literature that N–N acts as a donorin ZnO:N film [34,35].

4. Conclusions

In conclusion, p-type ZnO:N has been obtained via a catalyst-free, low cost thermal evaporation process. ZnO:N crystal finestructure was analyzed by EXAFS. It has been shown from the EX-AFS data that the bonding length of Zn–Zn and Zn–O of p-typeZnO:N is longer than these of pure ZnO. It indicates that N atomis not simply substituting the O atom. There could be the existenceof N–N bonding. A reduction of the p-type performance was ob-served for ZnO:N using 5% NO/N2 which is probably due to thepresence of more diatoms of N–N in the film.

Acknowledgements

The authors would like to thank the funding support from DOEBES Grant DE-FG02-07ER46389 administrated by Dr. Refik Kortan.

The authors also feel grateful for Dr. David Look and Mr. Tim Coo-per at Wright State University for Hall Effect measurements. Use ofthe Advanced Photon Source was supported by the U.S. Depart-ment of Energy, Office of Science, Office of Basic Energy Sciences,under Contract No. DE-AC02-06CH11357.

References

[1] M.Q. Wang, X.G. Wang, Sol. Eng. Mater. Sol. Cell 92 (2008) 357.[2] J.C. Lee, K.H. Kang, S.K. Kim, K.H. Yoon, I.J. Park, J. Song. Sol. Eng. Mater. Sol. Cell

64 (2000) 185.[3] Z.G. Chen, Y.W. Tang, L.S. Zhang, L.J. Luo, Electrochim. Acta 51 (2006) 5870.[4] V. Khranovskyy, G.R. Yazdi, G. Lashkarev, A. Ulyashin, R. Yakimova, Phys. Stat.

Sol. (a) 1 (2008) 144.[5] H. Ohta, M. Orita, M. Hira, H. Hosono, J. Appl. Phys. 10 (2001) 5720.[6] W. Liu et al., Appl. Phys. Lett. 9 (2006) 092101.[7] A. Tsukazaki et al., Jpn. J. Appl. Phys. 44 (2005) L643.[8] C.H. Park, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202.[9] X. Li et al., J. Vac. Sci. Technol. A 4 (2003) 1342.

[10] Z.Q. Fang, B. Claflin, D.C. Look, L.L. Kerr, X. Li, J. Appl. Phys. 102 (2007) 023714.[11] A. Dadgar et al., Superlattice Microstruct. 38 (2005) 245.[12] W. Liu et al., J. Cryst. Growth 310 (2008) 3448.[13] C.L. Perkins, S.H. Lee, X. Li, S.E. Asher, T.J. Coutts, J. Appl. Phys. 3 (2005) 034907.[14] M. Tu, Y. Su, C. Ma, J. Appl. Phys. 100 (2006) 053705.[15] J. Wang, V. Sallet, F. Jomard, A.M. Botelho do Rego, E. Elamurugu, R. Martins, E.

Fortunato, Thin Solid Films 24 (2007) 8785.[16] V. Tvarozek, K. Shtereva, I. Novotny, J. Kovac, P. Sutta, R. Srnanek, A. Vincze,

Vac. 2 (2007) 166.[17] K.S. Ahn, Y.F. Yan, M. Al-Jassim, J. Vac. Sci. Technol. B 4 (2007) L23.[18] H.W. Liang et al., Phys. Status Solidi. A 6 (2005) 1060.[19] J.G. Lu, Y.Z. Zhang, Z.Z. Ye, L.P. Zhu, L. Wang, B.H. Zhao, Q.L. Liang, Appl. Phys.

Lett. 88 (2006) 222114.[20] L.L. Kerr, X. Li, M. Canepa, A.J. Sommer, Thin Solid Films 13 (2007) 5282.[21] I.H. Choi, J. Korean Phys. Soc. 4 (2005) 696.[22] C.C. Lin, S.Y. Chen, S.Y. Cheng, H.Y. Lee, Appl. Phys. Lett. 84 (2004) 5040.[23] J. Grabowska, K.K. Nanda, E. McGlynn, J.P. Mosnier, M.O. Henry, Surf. Coat

Technol. 200 (2005) 1093.[24] X.H. Kong, X.M. Sun, X.L. Li, Y.D. Li, Mater. Chem. Phys. 82 (2003) 997.[25] A. Umar, S.H. Kim, Y.S. Lee, K.S. Nahm, Y.B. Hahn, J. Cryst. Growth 282 (2005)

131.[26] Q.X. Zhao, P. Klason, M. Willander, Appl. Phys. A 88 (2007) 27.[27] S.Q. Wei, Z.H. Sun, Z.Y. Pan, X.Y. Zhang, W.S. Yan, W.J. Zhong, Nucl. Sci. Technol.

6 (2006) 370.[28] E.A. Stern, Phys. Rev. B 10 (1974) 3027.[29] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 58 (1998) 7565.[30] N.H. Tran, A.J. Hartmann, R.N. Lamb, J. Phys. Chem. B 103 (1999) 4264.[31] Z. Wu, Y. Zhou, X. Zhang, Appl. Phys. Lett. 84 (2004) 4442.[32] D.E. Partin, D.J. Williams, M. O’Keeffe, Solid State Chem. 132 (1997) 56.[33] S.H. Park, S.Y. Seo, S.H. Kim, S.W. Han, Appl. Phys. Lett. 88 (2006) 251903.[34] L.G. Wang, A. Zunger, Phys. Rev. Lett. 90 (2003) 256401.[35] E.C. Lee, Y.S. Jin, K.J. Chang, Physica B 308–310 (2001) 912.