Materials Science and Engineering B56 (1998) 256–262

Coaxial impact-collision ion scattering spectroscopy analysis ofZnO thin films and single crystals

T. Ohnishi a,*, A. Ohtomo b, I. Ohkuboa, M. Kawasaki b, M. Yoshimoto a,H. Koinuma 1,a

a Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku Yokohama 226-8503, Japanb Department of Inno6ati6e and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku Yokohama 226-8502, Japan

Abstract

Coaxial impact-collision ion scattering spectroscopy (CAICISS) was employed to analyze the surface structure of ZnO singlecrystals and thin films with various thickness. Epitaxial ZnO films grown on sapphire (a-Al2O3) (0001) by laser molecular beamepitaxy was found to choose the [0001( ] growth direction (the (0001( ) O face termination) among asymmetric c-axis directions ofwurtzite structure by comparing the spectra and their incident angle dependences of the films with those of well-defined (0001) Znface and (0001( ) O face surfaces of bulk single crystals. The incident angle dependences of Zn signal intensity for single crystalsand thick (\10 nm) films can be well fitted with the simulated curves without taking reconstruction of the surface into account.However, the CAICISS results for the very thin film (2 nm) showed considerable deviation from the data of single crystals,suggesting the existence of the distorted region during the initial growth stage induced by the large lattice mismatch between ZnOand sapphire (18%). The growth mechanism and preferential [0001( ] growth direction of ZnO are discussed based on preferentialtetrahedral bonding between Zn and O atoms on the sapphire surface. © 1998 Elsevier Science S.A. All rights reserved.

Keywords: CAICISS; ZnO thin film; Polarity; Laser MBE

1. Introduction

ZnO can be a promising candidate for future opticalapplications of ultraviolet light emittig devices. It has awide bandgap of 3.37 eV and a larger exciton bindingenergy (60 meV) compared with those of GaN (28meV) and ZnSe (19 meV). We have shown that laseraction due to exciton recombination at room tempera-ture could be achieved at a low threshold (24 kWcm−2) of optical pumping [1]. These films were de-posited on sapphire (0001) substrates by laser molecularbeam epitaxy (laser MBE) resulting in c-axis orientedheteroepitaxy. The films were composed of hexagonallyshaped nanocrystals arranged into a honeycomb-likestructure. The optical properties of the films were verysensitive to the crystal quality and the size of nanocrys-tals [1]. To control these factors, it is crucially impor-tant to understand the heteroepitaxy mechanisminvolved in such a large lattice mismatch (18%) system.

ZnO with wurtzite structure has a polarity in c-axisorientation because the stacking sequence of atomiclayers along the c-axis is asymmetric. An epitaxial thinfilm with its c-axis normal to the surface, as determinedby X-ray diffraction, can be either [0001] or [0001( ]oriented. This ambiguity can be cleared either by ana-lyzing the topmost termination of the films [2] or de-tailed cross sectional transmission electron microscopyanalyses as was reported for GaN films [3]. We haverecently shown that a thick (200 nm) ZnO film has atermination of (0001( ) O face by using coaxial impact-collision ion scattering spectroscopy (CAICISS) sug-gesting that the film grows as [0001( ] orientation. In thistechnique, a low energy He ion beam is impinged on asurface, and He ions and neutrals backscattered by ahead-on collision with target atoms on the topmostsurface are detected as a time of flight (TOF) spectrum.Because of the large scattering cross section of lowenergy ions, TOF spectrum and its incident angle de-pendence are very sensitive to the composition andstructure of topmost atomic layers [4,5]. This techniqueenables us to determine the surface polarity of ZnO

* Corresponding author. E-mail: ohnishi1@oxide.rlem.titech.ac.jp1 Also a member of CREST, Japan science & Technology Corpora-

tion.

0921-5107/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved.

PII S0921-5107(98)00241-4

T. Ohnishi et al. / Materials Science and Engineering B56 (1998) 256–262 257

films from the comparison of TOF spectra and theirincident and azimuthal angle dependences for the thinfilms and single crystal surfaces.

In this paper, we report on the incident angle depen-dences of Zn signal intensity for single crystal surfacesand thin films with various thickness to discuss theinitial growth mode, crystal growth direction, and sur-face structure.

2. Experimental

Commercial ZnO (0001) single crystals grown by ahydrothermal technique (Litton Airtron) were used asreference samples. A single crystal was annealed at1000°C in air for 2 h to obtain clean and atomicallywell-defined surfaces. High quality c-axis oriented ZnOfilms were grown on sapphire (a-Al2O3) (0001) sub-strates at a temperature of 550°C by ablating a ceramicZnO target (99.999%) with KrF excimer laser pulses (1J cm−2) in 1×10−6 Torr of O2 gas. Surface morphol-ogy of the specimens was observed with an atomic forcemicroscope (AFM, Seiko SPI-3700). Film crystallinityand the epitaxial relationship to the substrate weredetermined by a Philips MRD four-circle X-ray diffrac-tometer (XRD) using a Cu Ka source.

The specimens were introduced into an ultra highvacuum chamber equipped with a CAICISS (ShimadzuCAICISS-I) analyzer, a reflection high-energy electrondiffraction (RHEED), and a two-cycle computer-con-trolled goniometer. The schematic diagram of the sys-tem is shown in Fig. 1. In-plane crystallographicorientation of the films was determined by 25 keV-RHEED. The CAICISS measurements were carried outat about 200°C at a background pressure of less than2×10−9 Torr. The He ion beam (2 keV) was choppedinto pulses of 150 ns duration at a 100 kHz repetition

rate. The specimen was set on the goniometer and theion beam was focused by the Einzel lens into a 3 mmdiameter spot on the sample surface 836 mm away fromthe microchannel plate to detect the backscatteredcharged and neutral He particles for obtaining TOFspectra. The analyzer system was further optimizedcompared with the previous report [2] by inserting theEinzel lens and adjusting the amplifying circuit to resultin better resolution and sensitivity. The time-averagedcurrent of the incident ion beam was about 100 pA.

3. Results and discussions

AFM image and RHEED pattern taken on the(0001) and (0001( ) surfaces (backside each other) of anannealed single crystal and those for 200 nm-thick ZnOfilm are shown in Fig. 2(a–c), respectively. The [0001]and [0001( ] directions of the single crystals were distin-guished by measuring the piezoelectric voltage uponcompression. Atomically flat terraces and steps can beseen in all AFM images. Hexagonal faceting of unit cellhigh (0.52 nm) steps is clearly visible in the image of thefilm (c) [1]. The (0001( ) surface of the single crystal (b)showed unit cell high steps regularly arranged in onedirection whereas that for (0001) surface showedbunched steps with about 2 nm high (a). In theRHEED pattern obtained at [112( 0] azimuth on the(0001) surface of the single crystal (a), spotty streakscan be seen due to the transmission of the electronbeam through the bunched steps. Sharp streaks andKikuchi-lines can be observed on both the single crystal(0001( ) and the film surfaces, indicating their ideal flat-ness and high crystallinity (Fig. 2b, c).

Fig. 3 shows XRD patterns of the 200 nm-thick ZnOfilm grown on a sapphire (000l) substrate. An XRDu–2u scan (a) clearly shows only the {000l} peaks ofwurtzite-type ZnO and sapphire, indicating that thefilm is c-axis oriented. The full width at half-maximum(FWHM) of the XRD rocking curve (v) for (0002)ZnO peak was as narrow as 0.06°. In-plane alignmentwas determined using f scans of {101( 1} peaks of ZnOand {112( 3} peaks of the sapphire substrate (Fig. 3b).The in-plane epitaxial relationship was found to beZnO [101( 0]//sapphire[112( 0], indicating a 30° rotation ofthe ZnO unit cell with respect to that of sapphire. Thelattice parameter of 200 nm-thick ZnO film was foundto be a=0.325 nm and c=0.520 nm (bulk: a=0.324nm, c=0.520 nm), indicating that the crystal was wellrelaxed.

Fig. 4 shows the TOF spectrum taken at normalincidence for the (0001) Zn face of the single crystal.The peak at 6680 ns corresponds to head-on collisionbetween the incident He ions and topmost Zn atoms.From the view of normal incidence, the Zn atoms in thesecond layer below the surface are behind the O atoms

Fig. 1. A schematic diagram of the CAICISS-RHEED system. Thesystem is composed of ultra high vacuum chamber, the CAICISSanalyzer, RHEED, and computer-controlled two-cycle goniometer.The goniometer can precisely control the incident angle (a) andazimuthal angle (f).

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Fig. 2. AFM images (500×500 nm) in left panels and RHEED patterns in right panels. ZnO single crystals annealed at 1000°C in air: (a) (0001)surface, (b) (0001( ) surface. (c) 200 nm-thick film on sapphire (0001) grown by laser MBE. RHEED patterns were observed with [112( 0] azimuth,25 keV incident electron beam energy, and a glancing angle of 1°.

in the first layer, i.e. they are inside of the shadow conewhere He ion trajectory is prevented from entering.Therefore they do not contribute to the signal. Thispeak can thus be assigned to the He particles whichcollided with Zn atoms located in the topmost atomiclayer. The tail observed for longer TOF comes frommultiple scattering and inelastic interaction of He parti-cles in the surface region. Unlike the results previouslyreported [2], the peak for the O atoms can be now

detected at the TOF of 8080 ns, in spite of the muchsmaller scattering cross section of O atom compared tothat of Zn atom. This is because of the fine tuning ofthe preamplifying circuit just after the microchannelplate. The other two TOF spectra for the (0001( ) O faceof single crystal and 200 nm-thick film surfaces weresimilar to that of the (0001) Zn face because both Znand O atoms are evenly visible by He ions impingedfrom normal incidence for such wurtzite c-plane sur-

T. Ohnishi et al. / Materials Science and Engineering B56 (1998) 256–262 259

Fig. 3. XRD u–2u scan (a), and f scans of {101( 1} peaks of ZnO and{112( 3} peaks of sapphire substrate (b). These results clearly showthat epitaxial film is c-axis oriented in out of plane and ZnO film has30° in-plane rotation with respect to the sapphire substrate.

Fig. 5. Incident angular dependences of the Zn signal intensity whenthe specimen was tilted along the �112( 0� azimuth. (a) single crystal(0001) Zn face, (b) simulated curve of (a), (c) single crystal (0001( ) Oface, (d) simulated curve of (c), and (e) 200 nm-thick film. Simulationwas based on a three-dimensional two-atom model [7] for virtualsurface cut from ideal bulk structures without any reconstruction.The variation of the film (e) is similar to that of (0001( ) O face (c).

[6]. In our ZnO case, no significant difference wasobserved between the (0001) Zn face and (0001( ) O faceof ZnO. Therefore the angle dependences of Zn signalintensity is discussed in this paper.

The Zn signal intensity is plotted in Fig. 5 as afunction of the incident angle a when the crystal wastilted in the [112( 0] direction. As can be clearly seen inFig. 5(a, c), the (0001) Zn face and (0001( ) O facesurfaces gave quite different angle dependences due tothe different atomic arrangement in the surface region.The spectrum obtained on the 200 nm-thick film (Fig.5e) coincides with that of the single crystal (0001( ) Oface shown in Fig. 5(c). We can therefore conclude thatthe film surface has the [0001( ] polarity, i.e. the (0001( ) Oface termination.

A qualitative interpretation for the incident angledependence on the Zn signal intensity was given in ourprevious paper [2]. The angle region with low signalintensity corresponds to the situation where Zn atomsof the second layer from the topmost surface are hiddenby the shadow cones made by atoms of the first layer.The peaks correspond to the focusing effect which takesplace when the edge of the shadow cone, having con-centrated He particle flux, hits the Zn atoms in thesecond layer. In order to correlate the dependencebetter to the structure of the surface region, we made asimulation based on a three-dimensional two-atommodel [7]. The simulated curves are shown in Fig. 5(b,d). The surface structures subjected to the simulation

faces. Ishiyama et al. concluded that the terminatingatomic layer for c-planes of 6H-SiC (0001) substratewas a Si-layer for both the (0001) and (0001( ) surfacesmainly from the TOF spectra at the normal incidence

Fig. 4. Time of flight spectra of the single crystal (0001) Zn facesurface. The incident angle was normal to the surface. The oppositeside of ZnO crystal, i.e. (0001( ) O face and 200 nm-thick film surfacealso showed similar spectra.

T. Ohnishi et al. / Materials Science and Engineering B56 (1998) 256–262260

were (0001) surface terminated by Zn atoms (b) and(0001( ) surface terminated by O atoms (d) without anysurface reconstructions, i.e. virtual surfaces made bystructural parameters of bulk ZnO crystal reported inRef. [8]. These simulated data well reproduce most ofthe features seen in the experimental results. We havealso simulated the incident angle dependences for the(0001) surface terminated by O atoms and (0001( ) sur-face terminated by Zn atoms, which correspond to thesurfaces having three dangling bonds for each atomsitting at the topmost surface. The simulated resultswere far from the experimental results, indicating suchconfigurations having high surface energy are not for-ward to terminate the surface. The deviation betweenexperimental and simulated curve on the (0001) Zn faceis much lager than that of the (0001( ) O face. It suggeststhat the larger degree of surface reconstruction takesplace on the (0001) Zn face than on the (0001( ) O face.We also simulated by taking account of the surfacereconstruction determined by low energy electron dif-fraction [9], where (0001) Zn face shows contraction ofthe top Zn layer by about 0.02 nm and the (0001( ) Oface does not show any contraction. However, theresulting curve of the (0001) Zn face shows little changefrom that without any reconstruction and could notcompletely reproduce the experimental result. Muchlager reconstruction should be taken into account forthe (0001) Zn face surface. A quantum chemical calcu-lation of ZnO surfaces are in progress to give betterunderstanding of the surface reconstruction.

As was reported in our previous paper and above, itis clear that high crystallinity ZnO film has (0001( ) Oface. However, it does not necessarily means that thefilm grows [0001( ] direction normal to the substratesurface. Here we report the surface termination of ZnOfilms with various thickness to make sure the crystalgrowth direction. Fig. 6 shows the incident angle de-pendences of Zn signal intensity for 2, 10 and 200 nmthick films deposited under the same conditions. Similarcurves were obtained for the 200 nm (a), 25 nm (notshown) and 10 nm (b) films. It indicates that the surfaceatomic arrangement for the films thicker than 10 nm issimilar to that of the single crystal (0001) O face surfaceand the films grow in a homoepitaxy fashion. However,2 nm-thick film showed considerably different angledependence, relatively high intensity and less variationwith angle. We have pointed out a selective desorptionof [0001] growth domain at the initial stage because ofhigh vapor pressure as one of the reasons for the [0001( ]growth of ZnO film. Therefore, we thought it might bepossible to have mixed orientation of [0001] and [0001( ]at the initial growth stage. However, we have verifiedthis is not the case by comparing the experimentalresults for the 2 nm-thick film with calculated curvessynthesized with adding those of (0001) Zn and (0001( )O faces of ZnO single crystal (Fig. 5a, c) with various

ratio. The best fitted curve were obtained for a combi-nation ratio of (0001) Zn face:(0001( ) O face=55:45 asshown in Fig. 6(d). The resultant curve could ratherreproduced the experiment for larger incident angleregion, but the lower angle region deviated. Therefore,the mixed orientation at initial stage is concluded to beinsufficient to explain the angle dependence of 2 nm-thick film. The existence of residual strain near theinterface should be taken into account.

The mismatch between ZnO and sapphire is so large,as high as 18.4% by compering the 30° rotation of ZnOunit cell in a–b plane to that of sapphire (ZnO [101( 0]//sapphire [112( 0] and ZnO (0001( )//sapphire (0001);aZnO=0.3250nm, asapphire/3=0.2750 nm) that theatomic arrangement should be considerably distortednear the interface. It is easy to imagine that distinctshadowing and focusing effect can not be applied toultra thin films. If such a distortion is introduced at theinterface, more remarkable effects for the low incidenceangle (small a) region should arise because the projec-tion of shadow cone in a–b plane has cos a component.

Now, we will discuss the atomic arrangement at theZnO/sapphire interface. We have reported that if aminimum repeating layer of sapphire satisfies a chargeneutrality, a stoichiometry, and a non-dipole moment,the surface of (0001) plane of sapphire should be termi-nated by Al atoms as shown in Fig. 7(a) [10]. However,the surface is still dominated by O atoms because all Oatoms of the second layer are exposed in the topmostsurface due to the much larger ionic radius of O (0.14nm) than that of Al atoms (0.053 nm). When ZnO

Fig. 6. Incident angular dependences of the Zn signal intensity. (a)200 nm-thick, (b) 10 nm-thick, and (c) 2 nm-thick ZnO films. Thecurve in (c) was best fitted as (d) calculated from the combination ofexperimental curves for the (0001) Zn face (Fig. 5 (a)) and the (0001( )O face (Fig. 5 (c)) with a rate of 55/45.

T. Ohnishi et al. / Materials Science and Engineering B56 (1998) 256–262 261

Fig. 7. Schematic drawings of atomic arrangement both from the top of view, �0001�, and side view, [2( 110] for (a) sapphire substrate and (b) ZnOfilm. In the top of view, only two atomic layers are shown because they are responsible to form the interfacial bonding. In the top view of (a),it is clearly shown that O atoms on sapphire dominates the surface structure comparing with Al.

crystal grows, there can be two possibilities, whether Oatoms of ZnO make bonding to Al atoms of sapphireor Zn atoms of ZnO do with O atoms of sapphire. Ifthe ZnO molecules migrating on the sapphire feel top-most O atoms, the (0001) Zn face (opposite side of thesurface (0001( ) O face appeared on the topmost surface)should come into contact with the substrate. The bot-

tom Zn atoms right at the interface should make bondswith the O atoms of the sapphire surface but have tocompromise with high degree of strain. As illustrated inFig. 7 (a, b), the six-fold symmetric Zn arrangement ofthe film is rotating by 30° in a–b plane from the Oarrangement of the sapphire which has also six-foldsymmetry. Therefore, the ZnO [0001( ] growth is consid-

T. Ohnishi et al. / Materials Science and Engineering B56 (1998) 256–262262

ered to occur with having 30° rotation in a–b plane,which was revealed in the result of XRD measurements(Fig. 3 (b)).

On the other hand, the preferential [0001] growth wasreported for GaN films grown on sapphire [3,11,12].The lattice constants of wurtzite GaN are close to thoseof ZnO (GaN; a=0.319 nm, c=0.519 nm). The oppo-site growth direction of GaN compared with ZnO canbe attributed to taking a similar scenario into account,where reductive atmosphere during metal organicchemical vapor deposition with pretty rich hydrogenatmosphere reduces oxygen content of the sapphiresurface and N atoms of GaN prefer to make bonds toAl atoms of the sapphire, resulting in [0001] growth.The oxidizing atmosphere during ZnO growth shouldproduce a O dominant surfaces resulting in ZnO [0001( ]growth in our case, whereas the Al dominant surfaceshould result in the preferential GaN [0001] growth aswas proposed by Wu et al. [12].

4. Conclusion

We have identified the growth direction of c-axisoriented ZnO film on sapphire (0001) substrates to be[0001( ] by means of CAICISS analysis on well-definedsurfaces of ZnO single crystals and thin films withvarious thickness deposited by laser MBE. We dis-cussed the origin of the preferential [0001( ] growth with30° rotation in a–b plane based on the consideration ofatomic arrangement at the interface between sapphireand ZnO.

Acknowledgements

Two of the authors (T. O. and A. O.) are supportedby JSPS Research Fellowships for Young Scientists.This work was supported in part by RFTF program ofJSPS (96P00205) and Special Coordination Funds ofSPA of Japan Government.

References

[1] Z.K. Tang, P. Yu, G.K.L. Wong, M. Kawasaki, H. Koinuma,Y. Segawa, Solid stae Commun. 103 (1996) 459; Y. Segawa,A.Ohtoma, M. Kawasaki, Z.K. Tang, P. Yu, G.K.L. Wong,Phys. Stat. Sol. 202 (1997) 669.

[2] T. Ohnishi, A. Ohtomo, M. Kawasaki, K. Takahashi, M. Yoshi-moto, H. Koinuma, Appl. Phys. Lett. 72 (1998) 824.

[3] L.T. Romana, J.E. Northrup, M.A. O’Keefe, Appl. Phys. Lett.69 (1996) 2394.

[4] T. Buck, in: A.W. Czanderna (Ed.), Methods of Surface Analy-sis, Elsevier, Amsterdam, 1975, p. 75.

[5] J.-M. Beuken, P. Bertrand, Surf. Sci. 162 (1985) 329.[6] O. Ishiyama, M. Shinohara, T. Nishihara, F. Ohtani, S. Nishino,

J. Saraie, Inst. Phys. Conf. Ser. No. 142 (1995) 485.[7] M. Shinohara, O. Ishiyama, T. Nishihara, F. Ohtani, M. Yoshi-

moto, T. Maeda, H. Koinuma, Proc. Of the 2nd NIRIM Int.Symp. On Advanced Materials (1995) 203

[8] H. Schulz, K.H. Thiemann, Solid State Comm. 32 (1979) 783.[9] C.B. Burke, A.R. Luinsky, Surf. Sci. 50 (1975) 605.

[10] T. Maeda, M. Yoshimoto, T. Ohnishi, G.H. Lee, H. Koinuma,J. Cryst. Growth 17 (1997) 95.

[11] B. Daudin, J.L. Rouviere, M. Arlery, Appl. Phys. Lett. 69 (1996)2480.

[12] X.H. Wu, L.M. Brown, D. Kapolnek, S. Keller, S.P. DenDaars,J.S. Speck, J. Appl. Phys. 80 (1996) 3228.

.