Rational growth of semi-polar ZnO texture on a glass substrate for optoelectronic applications

This article has been downloaded from IOPscience. Please scroll down to see the full text article.

2013 J. Phys. D: Appl. Phys. 46 055105

(http://iopscience.iop.org/0022-3727/46/5/055105)

Download details:

IP Address: 137.149.3.15

The article was downloaded on 14/03/2013 at 09:26

Please note that terms and conditions apply.

View the table of contents for this issue, or go to the journal homepage for more

Home Search Collections Journals About Contact us My IOPscience

IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 46 (2013) 055105 (7pp) doi:10.1088/0022-3727/46/5/055105

Rational growth of semi-polar ZnOtexture on a glass substrate foroptoelectronic applicationsB Lu, M J Ma, Y H Ye, J G Lu, H P He and Z Z Ye

State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering,Zhejiang University, Hangzhou 310027, People’s Republic of China

E-mail: binlu@zju.edu.cn (B Lu)

Received 26 September 2012, in final form 13 November 2012Published 28 December 2012Online at stacks.iop.org/JPhysD/46/055105

AbstractSemi-polar ZnO films with surface texture were grown on glass substrates via pulsed-laserdeposition (PLD) through Co–Ga co-doping. Oxygen pressure (P O2) was found to havesignificant effects on the structural and optical properties of the Zn(Co, Ga)O (ZCGO) films.A self-textured film with (1 0 1̄ 1) preferred orientation (PO) was achieved by varying thegrowth conditions including a crucial narrow P O2 window and growth time. A possiblemechanism underlying the PO evolution and the final texture of the films was proposed, whichcan be attributed to the collaboration of the doping effect and the P O2-dependent evolutionaryselection process, in which certain grains can have increased vertical growth rate with respectto the substrate surface through interplane diffusion. Moreover, the growth of undoped pureZnO films proceeded by using the (1 0 1̄ 1)ZCGO film as a buffer layer. The ZnO layersretained a semi-polar characteristic with improved crystallinity and better optical quality. Theepitaxy-like orientation of ZnO layers grown on (1 0 1̄ 1)ZCGO films has applications in thedevelopment of semi-polar ZnO-based light-emitting diodes.

(Some figures may appear in colour only in the online journal)

1. Introduction

In polycrystalline film growth, microstructure and crystalorientations strongly influence the properties, performance andreliability of the films. Preferred orientation (PO) or texture ofpolycrystalline films has been a central topic in the crystalgrowth community. PO is related to the structure, wheregrains have a specific crystallographic orientation normal tothe substrate. Controlling the PO is necessary to obtaindesirable properties because physical properties are oftenanisotropic with respect to the crystal direction. For instance,zinc oxide (ZnO) is considered as the best potential materialfor UV–blue light-emitting diodes (LEDs) and laser diodes(LDs) due to its wide direct bandgap of 3.37 eV at roomtemperature (RT) and a large exciton binding energy of 60 meV[1–3]. ZnO has a hexagonal wurtzite structure and favoursa c-axis PO. As a result, a large internal electric field iscreated along the c-axis due to crystal inversion asymmetry,thereby separating the wave functions of electron and hole

[4]. Thus, the quantum-confined Stark effect redshifts theoptical transition, reduces the 1s exciton binding energy andreduces the exciton oscillator strength, which causes a decreasein the inner quantum efficiency of the emitting devices [5].To overcome the detrimental effects of the internal electricfield, ZnO growth with a nonpolar-axis PO is proposed[4, 6]. The polarization effects can also be minimized bygrowing wurtzite materials on semi-polar planes (parallel tothe plane normals of {1 0 1̄ 1}, {1 0 1̄ 2} and {1 0 1̄ 3} planes,for instance) [7, 8]. These planes have reduced polarizationdiscontinuity in heterostructures compared with the c-planematerials. Specifically, for semi-polar planes oriented ∼45◦

from the c-plane, there is no polarization discontinuity for ZnOhexagonal wurtzite heterostructures [9]. That is, in ZnO-basedLED devices, films are assumed to have nonpolar/semipolar-axis PO to achieve good quantum efficiency.

Consequently, a number of studies have been carriedout to investigate the relation between operating conditionsand nonpolar/semipolar growth [8, 10, 11]. However, the

0022-3727/13/055105+07$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 46 (2013) 055105 B Lu et al

control and development of nonpolar/semipolar-axis POin ZnO community are mainly performed on expensivecrystal substrates, such as m-plane ZnO, a-plane GaN,or (La, Sr)(Al, Ta)O3, which behave with a substrate-orientation-dependent epitaxial growth [10–13]. Therefore,the development of a nonpolar/semipolar-axis PO and texturein a convenient and economical way is favourable, e.g. onamorphous surfaces, which is sometimes referred to as self-texture caused by the film itself without the influence of thesubstrate [14, 15].

Many reports on PO operating conditions exist. However,little is known thus far on the specific growth mechanisms ofthe nonpolar/semipolar-axis PO through doping effect. In thispaper, the growth of semi-polar (1 0 1̄ 1) ZnO films on glasssubstrates through Co–Ga co-doping via careful control of thedeposition conditions of oxygen pressure (P O2) is presented.It was found that with increasing pressure of O2, PO changesfrom the c-axis to the (1 0 1̄ 1) axis. Semipolar-oriented ZCGOfilms with texture were obtained at P O2 around 40 Pa. Furtherdemonstrated in this paper is the inheritance of the PO bygrowth of pure ZnO films with (1 0 1̄ 1) texture (which is notachieved when grown on glass substrates directly under thesame conditions) using the self-textured ZCGO thin film as abuffer layer. That is to say, an epitaxy-like growth of semi-polar pure ZnO layers is achieved. The dominant mechanismsfor the semi-polar self-texture and for the PO change in thisstudy are addressed. The electrical and optical properties ofthe as-prepared films are also systematically studied.

2. Experimental details

A series of Co–Ga co-doped ZnO (ZCGO) films were grownvia pulsed-laser deposition (PLD) on glass substrates froma ceramic oxide Zn1−x−yCoxGayO target. The Co and Gacontents in the target were 5 at% and 1 at%, respectively. Forthe film growth, a KrF excimer laser (Compex102, 248 nm,5 Hz) was used as the ablation source. The base pressure of thedeposition chamber was 1 × 10−4 Pa, and the films (∼500 nmthick) were grown in an O2 (99.999% purity) atmosphere(P O2 = 0.02–45 Pa) at a deposition rate of ∼0.3 nm s−1.The deposition of pure ZnO layers on the ZCGO films wasaccomplished at an oxygen pressure of 40 Pa at a growth rate of∼0.25 nm s−1. The substrate temperature (Ts) was maintainedat 500 ◦C during the deposition for all the samples.

The crystallographic structure and morphologies ofthe films were characterized via x-ray diffraction (XRD),field emission scanning electron microscopy (FE-SEM) andhigh-resolution transmission electron microcopy (HRTEM)imaging. These methods were complemented by fast Fouriertransform (FFT) for structural analysis. The elements presentand the valence state of Co and Ga in the as-deposited filmswere determined via x-ray photoelectron spectroscopy (XPS)using a monochromatized Al Kα radiation of 1486.6 eV. Thecarrier concentration and mobility were measured using thevan der Pauw configuration at RT under a magnetic fieldof 0.32 T. The UV–vis transmission spectra were examinedwithin the wavelength range 300–800 nm using a U-4100spectrophotometer. RT photoluminescence (PL) spectroscopywas excited using a 325 nm Xe lamp.

Figure 1. XRD patterns of the ZCGO films grown on glasssubstrates under different oxygen pressures with the substratemaintained at 500 ◦C during growth.

3. Results and discussion

In this study, the evolution of the crystallographic orientationwith oxygen pressure grown at 500 ◦C was first investigated.As expected, ZCGO films grown under low P O2 (0.02–20 Pa) were self-textured with a c-axis PO, as shown by XRDmeasurements (figure 1). The (0 0 0 2)-oriented, self-texturedfilms were easily obtained. All films were single phased withno evidence of secondary phases within the sensitivity of XRD.In contrast, with a further increase in the oxygen pressure toa higher range (35–45 Pa), in addition to the (0 0 0 2) peak,(1 0 1̄ 1) and other several lower intensity non c-axis-orientedpeaks can also be observed in the spectra. A comparison ofthe peak intensities of films grown under high P O2 shows asignificant increase in the (1 0 1̄ 1) peak intensity for the filmgrown at 40 Pa. The (1 0 1̄ 1)PO, with improved crystallinity,was predominant over the PO along the c-axis within a verynarrow P O2 window around 40 Pa. A further increase in P O2

beyond 45 Pa led to a sharp decrease in crystalline quality. Theresults indicate that a strictly narrow P O2 window is neededto grow a ZCGO semi-polar (1 0 1̄ 1) film on glass substrates.Similar phenomena on P O2 window effect in preparation ofZnO thin films have been reported previously [13].

The surface morphology evolution with respect to theoxygen pressure of these films can be seen in the FE-SEMimages presented in figures 2(a)–(f ). A large variation insurface morphologies between the species grown in low-and high-pressure ranges can be observed. In general,lower pressures lead to grains without any distinct surfacetexture, whereas higher pressures tend to result in theappearance of grains with a beanpod-like texture, as shownin figures 2(d)–(f ). A closer inspection reveals a void surfacestructure for the 40 Pa-correlated films (figure 2(e)) that hasgrains with characteristic dimension of 90–160 nm length and20–45 nm width. It is also observed that the grains obtainedat 40 Pa have improved crystalline quality among the species

2

J. Phys. D: Appl. Phys. 46 (2013) 055105 B Lu et al

Figure 2. FE-SEM images of the surface morphology evolution ofZCGO films grown at different oxygen pressures: (a) 0.02 Pa,(b) 2 Pa, (c) 20 Pa, (d) 35 Pa, (e) 40 Pa and (f ) 45 Pa.

Figure 3. (a) Cross-sectional TEM image of a 40 Pa-depositedZCGO film. A transition layer of several tens of nanometresthickness between the substrate and the well-developed columnarstructures is marked, and one grain with non-equiaxed columnarstructure is outlined as well to highlight its shape. (b) HRTEMimage of the interface between the film and the glass substrate. FFTpattern is shown in the inset. (c) Zoom-in image showingwell-resolved lattice planes taken from the middle portion of thecolumnar structures.

grown at high pressures. Moreover, the SEM images indicatethat the ZCGO films associated with the narrow P O2 windowaround 40 Pa can give rise to a growth parallel to the substratesurface, which is consistent with the XRD results presentedabove.

A cross-sectional transmission electron microcopy (TEM)image of the interface between the film and the substrate wasrecorded to further clarify the PO and structure of the ZCGOfilms obtained at 40 Pa, as shown in figure 3. Figure 3(a)

Figure 4. X-ray photoelectron spectra of 40 Pa-deposited ZCGOfilm. (a) Co 2p and (b) Ga 2p core-level spectrum of the as-preparedfilm, respectively.

presents a typical cross-sectional TEM image of the ZCGOfilm on a large scale. It can be seen that, at several tensof nanometres, either a poorly crystallized or a random POlayer is formed first on the substrate acting as the transitionlayer. This observation is typical in the development of self-textured polycrystalline films on amorphous surfaces. Then,with further deposition, non-equiaxed columnar grains arewell developed with PO, while an intergranular spacing ispresent between the columns. A zoom-in image taken fromthe initial layer at the interface is shown in figure 3(b),exhibiting coexistence of crystallites with PO along differentaxes. Looking through the sample, most crystallites at theinterface are determined as having (1 0 1̄ 1)PO and c-axisPO, which is consistent with its corresponding FFT patternshown in the inset of figure 3(b). With the well-resolvedlattice planes taken from the middle portion of the columnarstructures (figure 3(c)), the measured interplane spacing isabout 0.245 nm, which matches well with the literature valuefor (1 0 1̄ 1) planes in wurtzite ZnO. These results clarify thatthe (1 0 1̄ 1)PO in the polycrystalline film of ZCGO obtainedat 40 Pa gradually appeared from a randomly oriented initiallayer as the film thickness increased and finally dominated thesurface texture. The TEM results are consistent with thoseindicated in the XRD patterns.

The elements present and the valence state of the Coand Ga dopants in the ZCGO film prepared at 40 Pa weredetermined via XPS measurements, as shown in figure 4. Thecharge-shifted spectra were corrected using the adventitiousC 1s photoelectron signal at 285 eV. Figure 4(a) highlightsthe Co 2p core-level spectrum of the as-grown ZCGO film.A comparison of the shapes and positions of the primaryand satellite peaks shows that there is a good fit between theZCGO film and the Co2+ standard [16]. Thus, Co is in a 2+valence state in the ZCGO film. However, the Co2+ state inthe XPS charge state is difficult to confirm; this Co2+ statemay come from the CoO clusters and Co2+ ions incorporatedinto the wurtzite lattice at the Zn2+ ion sites. Although most

3

J. Phys. D: Appl. Phys. 46 (2013) 055105 B Lu et al

Figure 5. RT optical transmittance spectra obtained for the ZCGOthin films grown at different oxygen pressures. Inset shows the XRDspectra of the ZnCoO and ZCGO films obtained under the sameconditions (40 Pa, 500 ◦C) for comparison.

previous studies claimed that the doped Co in ZnCoO is inthe substituted lattice, further experimental support is needed.From figure 4(b), Ga was clearly doped into the ZnCoOthin film.

Figure 5 shows the RT optical transmittance spectra ofthe ZCGO thin films grown at different oxygen pressures.For all the films, transmittance was of the order of 90%in the wavelength range 300–800 nm for the film thicknessbetween 450 and 550 nm. In the spectra, three absorptionbands, located at approximately 566 nm, 613 nm and 661 nm,respectively, were found in all the films, regardless of thegrowth pressures. The three peaks are characteristic of ZnCoOthin films, attributed to the typical d–d transitions: 4A2(F) →2E(G), 4A2(F) → 4Tl(P) and 4A2(F) → 2Al(G), respectively[17, 18]. The d–d transitions originated from Co2+ in place ofZn2+. Thus, it can be inferred that Co ions were successfullysubstituted into the ZnO lattice as Co2+.

The Hall-effect measurements show that all the ZCGOfilms are n-type. As the growth pressure was increased from0.02 to 40 Pa, the carrier concentration sharply decreased from1020 to 1016 cm−3. These results suggest that a relatively highP O2 is beneficial to a PO along a semi-polar direction, whereasit is detrimental to the electrical properties of the film. Thiscan be tentatively explained by the association with oxygenvacancies, which were suppressed in high P O2 atmosphere.Considering the electrical properties, Ga co-doping is clearlynecessary for improving the n-type conduction of the ZnCoOfilms, similar to that previously reported in Ga-doped ZnO [19].Another merit of Ga is that Ga co-doping promotes the changein PO from the c-axis to semi-polar (1 0 1̄ 1), as shown in theinset of figure 5, which exhibits the XRD spectra of ZnCoOand the ZCGO film grown under the same conditions.

The mechanisms that control PO and PO changes werealso examined in this study. Many processes are known tohave the ability to influence the orientation distribution andtexture of polycrystalline thin films [20–24]. Minimization ofsurface energy is most frequently used to explain the originof PO and texture. However, the mechanism of self-texture

Figure 6. Schematic growth models: (a) substrate prior todeposition, (b) initial nuclei without PO, (c) non-equiaxed columnarwithout surface texture, and (d) non-equiaxed columnar withsurface texture. If the process contributing to PO in the growth stageis intergrain surface diffusion related, then (b) → (c), otherwiseinterplane (interfacet) surface diffusion leads to the process(b) → (d). The arrows indicated on the models represent PO ofcrystallites.

cannot be simply attributed to surface energy. In fact, variousself-textures observed during ZnO deposition were dependenton the deposition conditions, as shown in this study. Plausiblemechanisms of PO can be explained in two stages: initial andgrowth stages [21, 24]. A typical mechanism that determinesthe PO during the initial stage is preferential nucleation. In theself-texture frame, preferential nucleation is apparently a lesslikely possibility in determining the texture and PO of films,and grains nucleated on the substrate often exhibit randomorientation, as schematically depicted in figure 6(b). Thistheory is also confirmed experimentally, as indicated in theTEM section. Therefore, focus is given on factors that controlthe PO during the growth stage, which mainly influence thePO in polycrystalline deposition. In the growth stage, themain elementary processes that determine the PO are sticking,surface diffusion and grain growth. Sticking and surfacediffusion are processes at the interface between the growthspecies and the substrate, whereas grain growth occurs in thesolid phase. When surface growth determines the PO, grainswith the fastest growth with a direction normal to the substratesurvive through the growth, envelope the other grains, andfinally dominate the film. This process is called evolutionaryselection [25].

Based on this theory, the PO and morphology evolutionwith the pressure observed on films grown at 500 ◦C may beattributed to the pressure-associated growth rate anisotropy ofthe different crystallographic plane orientations. The growthconditions were far from thermodynamic equilibrium, whichled to random nucleation and grain growth in the earliergrowth stage. In low oxygen pressure atmospheres, the meanfree path of the ablated species is longer than the target–substrate distance, and the deposition species thus have lesspossibility to collide and scatter before they arrive at thesubstrate. Moreover, if the relatively high growth temperature(Ts = 500 ◦C) is taken into account, the energetic species withhigh kinetic energy will have sufficient mobility to overcomethe diffusion barrier of all the planes and freely diffuse across

4

J. Phys. D: Appl. Phys. 46 (2013) 055105 B Lu et al

the grain boundary attached on the edge of the grains with thelowest surface energy to decrease the free energy of the wholesystem. That is, when intergrain diffusion occurs, the grainswith higher surface energy shrink, and the PO should have theminimum surface energy planes parallel to the substrate. Inthis case, the texture becomes non-equiaxed columnar withoutsurface texture (figure 6(c)), which coincides with the SEMresults in this study.

The increase in oxygen pressure from 0.02 to 40 Pa(at which the mean free path of the ablated speciesis considerably shorter than the target–substrate distance)resulted in (i) the kinetic energy variation of the species,which are slowed down by the background pressure due tocollision with each other and have a strongly reduced kineticenergy, (ii) the reduced loss of cations in the films andincreased complexes/molecules (such as Zn–O, Zn–O–Co, etc)or clusters in the vapour phase and at the substrate surface,leading to conserved stoichiometric growth, as currentlysuggested in a study on PLD-grown CuO films [26]. Speciesarriving on the grains with planes of higher surface energiesparallel to the substrate tend to stick to their arrival locationbecause of their relatively low mobility and the higher diffusionbarrier. In contrast, species arriving on the facets of minimumsurface energy (characterized by a low diffusion barrier)possibly have time to diffuse to the edge of the grains due to thelow deposition rate (∼0.3 nm s−1) and jump on the plane withhigher surface energies. As a consequence of this interfacetdiffusion, i.e. surface diffusion among planes, the plane witha higher surface energy becomes the PO because surfacediffusion conceals the plane with higher surface energy. Inthis regime, the film texture becomes non-equiaxed columnarwith surface texture (figure 6(d)). Such a grain growthcompetition led to the peculiar TEM structures observed infigure 3(a), where some grains emerged at the initial layerwhile being enclosed by others as the film thickness increasedbeyond a certain value. It is worth pointing out that our filmswere obtained at Ts/Tm = 0.34, where Ts is the substratetemperature and Tm ∼ 2250 K is the melting temperatureof the deposited material. The processes controlling themicrostructure in this study have some similarity with thestructural mode established for sputtered films, in whichsurface diffusion dominates the growth in zone-2 (0.3 <

Ts/Tm < 0.5) [27].The doping effect has not been taken into account thus

far. The effect of impurity additions on the microstructurehas previously been reported [28]. However, the doping effectis not addressed clearly in the literature. In this study, whenthe doping effect was not considered, the discussion aboveon PO and PO change is not the case, as indicated by theXRD and SEM results (figure 7, sample 3). These resultsshow that pure ZnO grown at 40 Pa on a glass substrateexhibits a c-axis PO. The doping effect of Co can be partlyassociated with the regulation of surface energy among variousplanes. The introduction of Ga promotes the formation ofspecific coordination and complex in the vapour phase andat the substrate surface [19]. Ishihara et al considered thatthe change in PO of AlN films is determined by the growingspecies, i.e. the deposition of AlN species leads to an a-axis

Figure 7. XRD patterns and the corresponding SEM imagesobtained for three different ZnO species grown at 40 Pa and 500 ◦C.Sample 1 (S1: 30′ZCGO + 30′ZnO) and sample 2 (S2:30′ZCGO + 60′ZnO) are ZnO films grown on a semi-polar ZCGObuffer layer (30′ZCGO) with 30 min and 1 h deposition,respectively. Sample 3 (S3: 60′ZnO) had a 1 h deposition of ZnOfilm on a glass substrate without any buffer layer.

PO, whereas the deposition of the Al and N species leadsto a c-axis PO [29]. This supposition exactly considers thesticking process as the dominant factor in dealing with thePO growth. When the sticking process determines the PO,the plane with the highest sticking probability becomes thePO, and the film texture also becomes non-equiaxed columnarwith surface texture (figure 6(d)). Thus, Ga doping may favoursemi-polar growth due to the formation of a specific complex,the sticking coefficient (η) of which is higher on the (1 0 1̄ 1)

plane. However, the validity of this supposition is open toquestion due to the difficulty in determining η. In brief, theimpurity dopants are supposed to be helpful in enhancing thegrowth velocity anisotropies on different planes. A calculationelucidation is needed to further understand the doping effect.

As has been discussed in the introduction, the preparedsemi-polar self-textured ZCGO film can act as a buffer layer.In order to validate whether the PO and texture can be inherited,two undoped pure ZnO films with different thicknesses wereprepared using the as-prepared semi-polar self-textured film asbuffer under the same growth conditions. The XRD results andthe corresponding SEM images of these samples are presentedin figure 7. The deposition time was 30 min and 1 h for samples1 and 2, respectively. As mentioned above, sample 3 withoutany buffer is shown for comparison. Based on the SEMimages, the surface textures in samples 1 and 2 are clearlysimilar to each other, whereas they are different from thatobserved in sample 3, which shows coalescent strip islands.The SEM results are consistent with the XRD patterns, inwhich samples 1 and 2 are in a c-axis PO coexisting with a(1 0 1̄ 1)PO, whereas they are only in a c-axis PO for sample 3.The results indicate that the semi-polar PO and texture of theZCGO film were inherited by the ZnO films. Moreover, thediffracted intensity ratio of semi-polar to polar orientation ofsample 2 was much higher than that of sample 1, implying that

5

J. Phys. D: Appl. Phys. 46 (2013) 055105 B Lu et al

Figure 8. RT PL spectra of four ZnO species (samples 1–4).Sample 4 (S4: 30′ZnO) is the ZnO film grown on a glass substrate at40 Pa and 500 ◦C with a deposition duration of 30 min.

the thicker the film, the higher the tendency of a (1 0 1̄ 1)PO.This finding is consistent with the growth model proposed bythe current authors.

Figure 8 shows the PL measured for the above threesamples taken at RT. For comparison, a thinner c-axis-orientedpure ZnO film with 30 min deposition (sample 4) is includedin figure 8. The films with the ZCGO buffer layer (samples 1and 2) showed a significantly higher intensity ratio of thenear band edge to deep-level emission, indicating a greaterimprovement in crystallinity. Considering the detection depthof PL, the signals were assumed to have originated from thepure ZnO layers. Furthermore, samples 3 and 4 showed a muchclearer peak in the visible region, attributing to defects causedby oxygen vacancies and Zn interstitials [30]. These resultsindicate that the defects in the (1 0 1̄ 1)PO ZnO films were oflow concentration and high optical quality. Moreover, Hallmeasurements showed that all the samples exhibited p-typeconduction with a hole concentration of ∼1017 cm−3. Forp-type conductivity in undoped ZnO thin films obtained underoxygen-rich conditions reported previously, Zn vacancies areprobably the reason for the defect-induced nonintrinsic p-typepolarity [31]. Based on the above results, it can be concludedthat a semi-polar ZnO film with improved crystalline qualitywas prepared via deposition of the undoped ZnO layers on theZCGO self-textured buffer layers.

4. Conclusions

In summary, the orientation and surface texture evolution ofPLD-grown ZCGO films at different oxygen pressures werestudied. The films deposited at low oxygen pressure have ac-axis PO, which evolves to (1 0 1̄ 1)PO when subjected to anarrow P O2 window (40–45 Pa) with higher pressures. Basedon these observations, a mechanism explaining the PO andtexture of polycrystalline ZCGO thin film is proposed. Thismechanism is based on the assumption that different POs arepresent in the initial layer with the existing impurity dopants.Grains with certain PO, which have increased vertical growth

rate depending on their surface adatom mobility, can eventuallysurvive through the growth via evolutionary selection. Finally,undoped ZnO films with inherited PO were prepared using thesemi-polar ZCGO films as a buffer layer. The films showedstrong ultraviolet emission, and the emission related to deep-level defects was limited, which proved that the ZnO filmswere nearly stoichiometric and with improved optical quality.This result is the first step towards a possible improvement ofthese layers for photoelectric device applications.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China under Grant No 51002134,Qianjiang Talent Project of Zhejiang Province under GrantNo 2011R10044 and the Fundamental Research Funds for theCentral Universities under Grant No 2010QNA4002.

References

[1] Bagnall D M, Chen Y F, Zhu Z, Yao T, Koyama S, Shen M Yand Goto T 1997 Appl. Phys. Lett. 70 2230

[2] Tsukazaki A et al 2005 Nature Mater. 4 42[3] Dong J J, Zhang X W, You J B, Cai P F, Yin Z G, An Q,

Ma X B, Jin P, Wang Z G and Chu P K 2010 ACS Appl.Mater. Interfaces 2 1780

[4] Pant P, Budai J D and Narayan J 2010 Acta Mater. 58 1097[5] Waltereit P, Brandt O, Trampert A, Grahn H T, Menniger J,

Ramsteiner M, Reiche M and Ploog K H 2000 Nature406 865

[6] Cagin E, Yang J, Wang W, Phillips J D, Hong S K, Lee J Wand Lee J Y 2008 Appl. Phys. Lett. 92 233505

[7] Sharma R, Pattison P M, Masui H, Farrell R M, Baker T J,Haskell B A, Wu F, DenBaars S P, Speck J S andNakamura S 2005 Appl. Phys. Lett. 87 231110

[8] Wernicke T, Ploch S, Hoffmann V, Knauer A, Weyers M andKneissl M 2011 Phys. Status Solidi b 248 574

[9] Romanov A E, Baker T J, Nakamura S and Speck J S 2006J. Appl. Phys. 100 023522

[10] Taı̈noff D, Al-Khalfioui M, Deparis C, Vinter B, Teisseire M,Morhain C and Chauveau J-M 2011 Appl. Phys. Lett.98 131915

[11] Chou M M C, Chang L W, Hang D-R, Chang D-S and Li C A2009 Cryst. Growth Des. 9 2073

[12] Han X, Dai J, Yu C, Wu Z, Chen C and Gao Y 2010 Appl.Surf. Sci. 256 4682

[13] Chou M M C, Wang S C, Chen C L and Lee C-Y 2010J. Cryst. Growth 312 1170

[14] Myonga S Y and Lim K S 2003 Appl. Phys. Lett. 82 3026[15] Yanagitani T, Matsukawa M, Watanabe Y and Otani T 2005

J. Cryst. Growth 276 424[16] Tuan A C et al 2004 Phys. Rev. B 70 054424[17] Kim K J and Park Y R 2002 Appl. Phys. Lett. 81 1420[18] Srikant V and Clarke D R 1997 J. Appl. Phys. 81 6357[19] Demchenko D O, Earles B, Liu H Y, Avrutin V, Izyumskaya N,

Ozgur U and Morkoc H 2011 Phys. Rev. B 84 075201[20] Kajikawa Y 2006 J. Cryst. Growth 289 387[21] Kajikawa Y, Noda S and Komiyama H 2003 J. Vac. Technol. A

21 1943[22] Nicolay S, Fay S and Ballif C 2009 Cryst. Growth Des. 9 4957[23] Rodriguez-Navarro A B 2001 Thin Solid Films 389 288[24] Thompson C V and Carel R 1995 Mater. Sci. Eng. B 32 211[25] Van der Drift A 1967 Phillips. Res. Rep. 22 267[26] Kawwam M, Alharbi F, Aldwayyan A and Lebbou K 2012

Appl. Surf. Sci. 258 9949

6

J. Phys. D: Appl. Phys. 46 (2013) 055105 B Lu et al

[27] Thornton J A 1997 Annu. Rev. Mater. Sci. 7 239[28] Morinaga Y, Sakuragi K, Fujimura N and Ito T 1997 J. Cryst.

Growth 174 691[29] Ishihara M, Li S J, Yumoto H, Akashi K and Ide Y 1998 Thin

Solid Films 316 152

[30] Lin C-C, Chen S-Y and Cheng S-Y 2004 Electrochem. SolidState Lett. 7 J20

[31] Zeng Y J, Ye Z Z, Xu W Z, Lu J G, He H P, Zhu L P,Zhao B H, Che Y and Zhang S B 2006 Appl. Phys. Lett.88 262103

7