Applied Surface Science 254 (2008) 7972–7975

Lattice deformation of ZnO films with high nitrogen concentration

S.H. Park a,*, J.H. Chang c, H.J. Ko d, T. Minegishi a, J.S. Park a, I.H. Im a, M. Ito e, D.C. Oh f,M.W. Cho a,b, T. Yao a,b

a Center for Interdisciplinary Research, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japanb Institute for Materials Research, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japanc Major of Nano Semiconductor, Korea Maritime University, Youngdo-ku, Pusan 606-791, Republic of Koread Korea Photonics Technology Institute, Kwangju 500-210, Republic of Koreae Institute of Multidisciplinary Research for Advanced Material, Tohoku University, Aoba-ku, Sendai 980-8577, Japanf Department of Defense Science & Technology, Hoseo University, Sechul-ri, Baebang-myun, Asan 336-795, Republic of Korea

A R T I C L E I N F O

Article history:

Available online 20 April 2008

PACS:

34.35.+a

Keywords:

Nitrogen incorporation

ZnO

Lattice strain

A B S T R A C T

Nitrogen-doped ZnO/Al2O3 films grown at elevating temperatures (300–800 8C) have been investigated

by plasma-assisted molecular beam epitaxy (P-MBE). High nitrogen concentration (�1022 cm�3) is

achieved in the films grown at relatively low growth temperature (<500 8C) range. High nitrogen

concentration accompanies considerable degradation of crystallinity and residual tensile strain, which

was evaluated by high resolution X-ray diffraction (HRXRD). The structural evolution is discussed in

terms of the increase of complex defect density in the films. The ionization energy of acceptor was

estimated as �140 meV from the excitation power dependence of donor–acceptor pair emission line by

using low temperature (10 K) photoluminescence spectroscopy.

� 2008 Published by Elsevier B.V.

Contents lists available at ScienceDirect

Applied Surface Science

journal homepage: www.elsevier.com/locate/apsusc

1. Introduction

Zinc oxide (ZnO) has attracted much attention because of wideapplications for various devices such as piezoelectric transducer,varistor, optical waveguide, and solar cells [1]. Since ZnO has awide band gap of 3.3 eV and a large exciton binding energy of60 meV, this semiconductor has been considered as a promisingmaterial for short wavelength optoelectronic devices [2].

However, ZnO has the asymmetric doping limitation thatpredominantly favors n-type conductivity due to the formation ofnative defects such as oxygen vacancy, zinc interstitutials, whichhampers obtaining high-quality p-type ZnO layers required foroptoelectronic devices. Acceptor impurities such as N [3], P [4], andAs [5] has been examined to obtain p-type ZnO with high carrierconcentration. Among them, the most promising candidate for p-type doping is regarded as N atoms with the smallest ionizationenergy and most similar ionic radius with oxygen, however, holeconcentration is still limited to the order of �1017 cm�2 [6]. Toincrease hole concentration in ZnO films, growth temperatureshould be determined first, since it affects both kinetics of impurityincorporation and crystallinity of host crystal. Optimum growthtemperature for the growth of high-quality layers is reported to be

* Corresponding author. Tel.: +81 22 795 4404.

E-mail address: shpark@imr.tohoku.ac.jp (S.H. Park).

0169-4332/$ – see front matter � 2008 Published by Elsevier B.V.

doi:10.1016/j.apsusc.2008.04.047

as high as 800 8C or even much higher substrate temperature [7].As the substrate temperature increase, the crystalline quality ofZnO layers is greatly improved due to enhanced surface migrationof adatoms. However, since the sticking coefficient of nitrogenatom is quite low due to its high vapor pressure, substratetemperature should be kept low enough (for example <400 8C) forthe efficient incorporation of acceptor impurities. Such lowtemperature growth will degrade the crystallinity of p-dopedZnO layers. Therefore, systematic investigation on the nitrogenincorporation into ZnO as a function of growth temperature isobviously required. Moreover, the studies of nitrogen incorpora-tion into Zn-polar ZnO will help to realized more efficient p-typedoping and to fabricate the optoelectronic devices such as LED, UVdetector.

In this paper, nitrogen-doped ZnO (ZnO:N) films were grown onAl2O3 substrates at various growth temperatures (300–800 8C) byplasma-assisted molecular beam epitaxy (P-MBE). Nitrogenconcentration, crystallinity, lattice strain and emission propertiesof ZnO films are systematically investigated and the structuralevolution related with nitrogen doping is discussed.

2. Experimental

ZnO:N films were grown on Al2O3 substrates by plasma-assisted molecular beam epitaxy (P-MBE), in which oxygen and

Fig. 1. (a) c-axis and (b) a-axis lattice constants of ZnO:N films grown at elevating

temperatures(300–800 8C), which are evaluated from (0 0 0 2) and (1 0 1 1) 2u–vscans, respectively. Each inset indicates (0 0 0 2) and (1 0 1 1) 2u–v X-ray rocking

curves.

S.H. Park et al. / Applied Surface Science 254 (2008) 7972–7975 7973

nitrogen were supplied by using a RF plasma source with a powerof 350 W. A Knudsen cell with dual heating zones was used for Znsupplying. The Zn flux was 0.9 A/s and N2 flow rate was 0.5 sccm,which is four times lower than O2 flow rate (2 sccm). Prior to thegrowth, substrates were degreased by conventional organicsolvents, followed by etch in using a H2SO4:H3PO4 = 3:1 solutionat 160 8C for 1 min.

We have carried out the nitrogen doping on Zn-polar ZnO, sinceZn-polar face incorporates nitrogen atoms into ZnO moreefficiently than O-polar face due to bond configuration [8]. Inorder to successfully achieve the Zn-polarity, thick MgO film(12 nm) as polarity conversion layer was grown on Al2O3 substrate.It is noted that Zn-polarity can be realized by inserting an MgObuffer layer of specific thickness [9]. And then we grew Zn-polarZnO buffer (30 nm) on a MgO layer at low temperature (450 8C),The Zn-polar ZnO buffer layer was annealed at 850 8C for 15 min toimprove surface roughness. Undoped Zn-polar ZnO layers(200 nm) were grown at high growth temperature (750 8C).Oxygen rich growth conditions were employed for the growthof ZnO films [10]. The growth rate of Zn-polar ZnO film wereestimated to 1.7 times faster than O-polar ZnO film grown on thinMgO layer (�1 nm) [9].

Eventually, nitrogen doping was performed at various growthtemperatures (300–800 8C) on undoped ZnO films. The thickness ofthe doped films was controlled to 250 nm and the growth rate wasestimated to be 0.2 A/s by using a surface profiler.

Both c- and a-axis lattice constants were measured by highresolution X-ray diffraction (HRXRD). The (0 0 0 2) and (1 0 1 1)reflections were investigated to evaluate the c- and a-axis latticeconstants, respectively. Nitrogen concentration was estimated bysecondary ionization mass spectroscopy (SIMS). Photolumines-cence (PL) spectra of ZnO films were measured at 10 K using the325 nm line from a He–Cd laser as an excitation source and amonochromator with a focal length of 32 cm. Excitation powerdependence of PL was used to estimate the acceptor bindingenergy.

3. Results and discussions

Fig. 1 shows the c-axis (a) and a-axis (b) lattice constant ofZnO:N with various growth temperatures of 300–800 8C, which areobtained from X-ray 2u–v scans by using the (0 0 0 2) and (1 0 1 1)reflections, respectively. The insets indicate corresponding 2u–vscans. When the growth temperature is higher than 500 8C, ZnO:Nreveals almost equivalent lattice constant (both c- and a-axis) tothat of undoped ZnO (u-ZnO) film with the same film thickness.However, considerable contraction of a-axis lattice and expansionof c-axis lattice constant is observed at the growth temperaturelower than 500 8C, which indicates that ZnO:N film suffer fromhighly tensile strain.

There are at least two components which may cause theresidual tensile strain.

One is the effect of film thickness, the other is nitrogenincorporation. Heteroepitaxial ZnO films grown on Al2O3 substratesundergo serious residual strain induced by the lattice mismatch(½aAl2O3

� aZnO�=aAl2O3¼ 18:3%) [11] and the difference of coefficient

of thermal expansion (CTE) (½CTEAl2O3� CTEZnO�=CTEAl2O3

¼ �34%)[12,13]. Lattice mismatch will be relaxed with increasing the filmthickness up to a certain thickness, while thermal stress is stillimportant for thick films [14]. We have prepared undoped ZnO (u-ZnO) films under the same growth conditions as ZnO:N films. Notethat at relatively high growth temperature (�500 8C) region, a-/c-axis lattice constants of ZnO:N (thickness�250 nm) are very similarto those of u-ZnO with the same thickness, while ZnO:N films grownat lower temperatures exhibit considerable contraction of c-axis

lattice constant and expansion of a-axis lattice constant, whichindicate that the tensile strain still remains in the ZnO:N films.Therefore, we can conclude that at the low growth temperaturerange (300 8C < Tgrowth < 500 8C) tensile strain in ZnO:N occurs as aconsequence of nitrogen incorporation, however, at the high growthtemperature region (500 8C < Tgrowth < 800 8C) obvious effect ofnitrogen incorporation is not observed.

Now we should consider the effect of nitrogen incorporation onthe structural deformation of ZnO films mainly observed in the lowgrowth temperature range. Note that the bond length of the Zn–Nbond (2.04 A) [15] is longer than that of the Zn–O bond (1.93 A)[16], therefore, lattice expansion is expected when nitrogen isincorporated into ZnO.

However, as shown in Fig. 1, the residual tensile strain increasesas the growth temperature decreases. We suggest that complexdefects composed of intrinsic point defect and nitrogen areresponsible for the tensile strain in ZnO:N grown at lowtemperature region. When a nitrogen atom is incorporated intoZnO, N atoms and O atoms become unstable to form defect such asoxygen vacancy (Vo), zinc interstitial (Zni), Zn antisite (Zno) and soon because of low defect formation energy of those complexdefects [17]. Poykko et al. have claimed that when Se lattice site aresubstituted by nitrogen atoms in ZnSe films, the relaxation energythrough compensating process of impurity–interstitial pair islower than that of the isolated acceptor impurity and the other

Fig. 3. Lattice mismatch and XRD FWHM (inset of this figure) of ZnO:N films grown

at the elevating temperatures.

S.H. Park et al. / Applied Surface Science 254 (2008) 7972–79757974

defects, as a result impurity related complex will be easily formed[18]. Such complexes will make large influence on lattice deforma-tion. Lee et al. have reported on various complexes of nitrogen andintrinsic defects which are formed in ZnO:N films with high nitrogenconcentration (>1019 cm�3) [18]. Among various combinations,nitrogen substitution (No) and Zn antisite (Zno) complex, so-called‘‘No–Zno complex’’, can be easily formed because it has a strongerbinding energy of 1.52 eV compared with the complexes such as No

and Vo (No–Vo complex) and No and Zni (No–Zni complex) with lowbinding energy of 0.31 and 1.06 eV, respectively. Moreover, the No–Zno complex is known to induce a large lattice contraction of 0.75 A[19]. Based on the above discussion, the increase of residual tensilestrain in ZnO:N grown at low growth temperature range istentatively assigned due to the formation of No–Zno complex.

Fig. 2 shows the nitrogen concentration ([N]) in ZnO films withvarious growth temperatures of 300–800 8C, which is quantita-tively determined by SIMS measurements using Cs positive ions.We have used homoepitaxial ZnO:N film as a reference for theevaluation of [N] in the samples. The inset of Fig. 2 shows depthprofile for nitrogen atom for the growth temperatures of 300, 500,700 and 800 8C. When the growth temperature is lower than 500 8C[N] is saturated at �1022 cm�3. However, it abruptly decreases asthe growth temperature increases; 3 � 1020 cm�3 at 800 8C. Lowgrowth temperature is favorable to incorporate nitrogen atomsinto ZnO films due to higher sticking coefficient of nitrogen [20].

Fig. 3 shows the evolution of lattice mismatch and crystallinityof ZnO:N films as a function of [N]. The lattice mismatch seldomchanged up to [N] � 8 � 1021 cm�3, which corresponds to that ofZnO:N grown at 500 8C. Lattice mismatch rapidly increases as [N]increases. The inset of Fig. 3 shows the dependence of XRD fullwidth at half maximum (FWHM) of ZnO on growth temperature.Considerable degradation of crystallinity is observed below thegrowth temperature of 500 8C in terms of XRD FWHM. This factstrongly indicates that incorporated nitrogen atoms have stronginfluence on the crystallinity of ZnO:N. Note that XRD FWHMreflects inhomogeneous strain in the film due to imperfectionssuch as point defects and dislocations [21].

It is well known that high growth temperature is preferable togrow the high crystalline quality ZnO [7]. However, latticemismatch is hardly affected by growth temperature. Based onthe above discussion on residual strain, the abrupt increase oflattice mismatch and XRD FWHM below 500 8C is interpreted interms of nitrogen incorporation.

Fig. 2. Nitrogen concentration in ZnO films grown at various growth temperatures

(300–800 8C) determined by SIMS measurement. The inset shows SIMS spectra of

nitrogen atoms at the growth temperature of 300, 500, 700 and 800 8C.

Fig. 4 shows the low temperature (10 K) near band edgeemission (NBE) spectra of u-ZnO and ZnO:N films. The u-ZnO film(Fig. 4(a)) shows dominant emission lines such as free exciton (FX,3.374 eV) and neutral donor bound exciton emission (DoX,3.361 eV) [22], indicating relatively high optical quality of u-ZnO (Zn-polar ZnO). However, the FX and D8X emission intensitiesdecrease fast as a consequence of nitrogen incorporation.

Fig. 4(b) shows PL spectrum of ZnO:N (Tgrowth = 500 8C).Luminescence intensity of D8X line decreases compared with u-ZnO and unknown emission lines emerges at 3.343 and 3.287 eV.Those peaks are tentatively assigned to acceptor (3.343 eV) andstructural defect (3.287 eV) related emission lines, respectively,based on the previous reports [23,24]. Also, we have performedexcitation power dependence of donor–acceptor pair (DAP)emission positioned at 3.248 eV [22,25]. The DAP lines exhibitsblue shift of 5 meV. It corresponds to an acceptor ionization energy

Fig. 4. 10 K PL spectra for u-ZnO and ZnO:N film grown at 300 and 500 8C.

S.H. Park et al. / Applied Surface Science 254 (2008) 7972–7975 7975

of �140 meV by using following equation [26], which is in goodagreement with previous results (142 meV) [25,27]:

EDAP ¼ Eg � ED � EA þq2

er (1)

where EDAP is PL peak position of DAP emission, ED and EA are therespective ionization energies of donor and acceptor as isolatedimpurities. We used ED of 20 meV [27]. The shifting value (�5 meV)of DAP line was used for Coulomb term in the above mentionedequation. Further increase of nitrogen dose makes the DAP linedominant (Fig. 4(c) ZnO:N at 300 8C). Although we can still identify aweak D8X line at 3.361 eV, the sample reveals very weak emissionintensity. The PL spectra shown in Fig. 4 quantitatively supports theevaluated nitrogen concentration in the ZnO:N films.

4. Conclusion

ZnO:N films were grown on Al2O3 substrate at elevating growthtemperatures (300–800 8C). At the low growth temperature range(300 8C < Tgrowth < 500 8C), high nitrogen concentration, residualtensile strain and considerable degradation of crystalline qualityare observed. Ionization energy of acceptor in ZnO:N film (500 8C)is estimated to be �140 meV, however, luminescence intensitydecreased fast as increasing nitrogen concentration in the ZnO:Nfilms. The residual tensile strain in ZnO:N films is tentativelydiscussed in terms of nitrogen related defect complex such as No–Zno complex.

Acknowledgement

We appreciate valuable comments from Dr. H. Kato of StanleyCorp.

References

[1] F.C.M. Van de Pol, Ceram. Bull. 69 (1990) 1959.[2] D.M. Bagnall, Y.F. Chen, Z. Zhu, M.Y. Shen, T. Goto, T. Yao, Appl. Phys. Lett. 73

(1998) 1038.[3] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys.

Lett. 81 (2002) 1830.[4] K.K. Kim, H.S. Kim, D.K. Hwang, J.H. Lim, S.J. Park, Appl. Phys. Lett. 83 (2003)

63.[5] S. Limpijumnong, M.F. Smith, S.B. Zhang, Appl. Phys. Lett. 89 (2006) 222113.[6] Y.R. Ryu, T.S. Lee, H.W. White, Appl. Phys. Lett. 83 (2003) 87.[7] H.J. Ko, Y. Chen, S.K. Hong, T. Yao, J. Appl. Phys. 92 (2002) 4354.[8] H. Maki, I. Sakaguchi, N. Ohashi, S. Sekiguchi, H. Haneda, J. Tanaka, N. Ichinose, Jpn.

J. Appl. Phys. 42 (2003) 75.[9] H. Kato, K. Miyamoto, M. Sano, T. Yao, Appl. Phys. Lett. 84 (2004) 4562.

[10] H. Kato, M. Sano, K. Miyamoto, T. Yao, Jpn. J. Appl. Phys. 42 (2003) 1002.[11] A. Setiawan, H.J. Ko, S.K. Hong, Y. Chen, T. Yao, Thin Solid Films 445 (2003)

213.[12] Y. Kagamitani, A. Yoshikawa, T. Fukuda, T. Ono (private communication).[13] http://global.kyocera.com/prdct/fc/product/pdf/s_c_sapphire.pdf.[14] S.H. Park, T. Hanada, D.C. Oh, T. Minegishi, H. Goto, J.S. Park, I.H. Im, J.H. Chang, G.

Fujimoto, M.W. Cho, T. Yao, Appl. Phys. Lett. 91 (2007) 31904.[15] X. Zhihau, J. Fengyi, W. Qixin, R. Jianping, Trans. Nonferrous Met. Soc. China 16

(2006) 854s.[16] C.H. Park, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202.[17] A.F. Kohan, G. Ceder, D. Morgan, Phys. Rev. B 61 (11) (2000) 15019.[18] S. Poykko, M.J. Puska, R.M. Nieminen, Phys. Rev. B 57 (1998) 12174.[19] E.C. Lee, Y.S. Kim, Y.G. Jin, K.J. Chang, Phys. Rev. B 64 (2001) 85120.[20] C.J.Y. Qiu, S.A. Nikishin, H. Temkin, Appl. Phys. Lett. 74 (1999) 516.[21] X.L. Sun, Y.T. Wang, H. Yang, L.X. Zheng, D.P. Xu, J.B. Li, Z.G. Wang, J. Appl. Phys. 87

(2000) 3711.[22] H.W. Liang, Y.M. Lu, D.Z. Shen, Y.C. Liu, J.F. Yan, C.X. Shan, B.H. Li, Z.Z. Zhang, J.Y.

Zhang, X.W. Fan, Phys. Stat. Sol. 202 (2005) 1060.[23] D.C. Look, B. Claflin, Phys. Stat. Sol. 241 (2004) 624.[24] B.K. Meyer, H. Alves, D.M. Hofmann, W. Kriegseis, D. Forster, F. Bertram, J.

Christen, A. Hoffmann, M. Strabburg, M. Dworzak, U. Haboeck, A.V. Rodina, Phys.Stat. Sol. 241 (2004) 231.

[25] A. Zeuner, H. Alves, D.M. Hofmann, B.K. Meyer, A. Hoffmann, U. Haboeck, M.Strassburg, M. Dworzak, Phys. Sol. 234 (2002) R7.

[26] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971.[27] S.J. Jiao, Y.M. Lu, D.Z. Shen, Z.Z. Zhang, B.H. Li, Z.H. Zheng, B. Yao, J.Y. Zhang, D.X.

Zhao, X.W. Fan, J. Lumin. 122 (2007) 368.