Polarity control of ZnO films grown on nitrided c-sapphireby molecular-beam epitaxy

Xinqiang Wang, Yosuke Tomita, Ok-Hwan Roh, Masayuki Ohsugi, Song-Bek Che,Yoshihiro Ishitani, and Akihiko Yoshikawaa)Center for Frontier Electronics and Photonics, Department of Electronics and Mechanical Engineering,Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

(Received 28 June 2004; accepted 8 November 2004; published online 27 December 2004)

The polarity of molecular-beam epitaxy grown ZnO films was controlled on nitrided c-sapphiresubstrate by modifying the interface between the ZnO buffer layer and the nitrided sapphire. TheZnO film grown on nitrided sapphire was proven to be Zn-polar while the O-polar one was obtainedby using gallium predeposition on nitrided sapphire, which was confirmed by coaxial impactcollision ion scattering spectroscopy and chemical etching effect. The Zn-polar ZnO film showedhigher growth rate, slightly better quality, and different surface morphology in comparison to theO-polar one. 2005 American Institute of Physics. [DOI: 10.1063/1.1846951]

ZnO is now receiving much attention due to its potentialuse in short wavelength optoelectronic devices.13 Like GaN,ZnO has wurtzite crystal structure, which has different polarsurfaces along the c axis, i.e., Zn-polarity and O-polarity.These two faces have different surface configuration, com-position, and chemical structure.4,5 In addition, the differentpolar films also differ in their electronic properties.6 As fre-quently demonstrated in the research of nitrides, the polarityhas large effect on the growth process, material property, andeven in devices fabrication.710 Furthermore, the polarity in-fluences the impurity doping efficiency, for example, Mgdoping has been reported to be much easier on Ga-polar GaNthan on N-polar one.11 Although there is still controversy asto which polarity is better for p-type doping of ZnO, there isno question that the polarity influences the doping efficiencybecause the number of dangling bonds of each Zn or O atomon growth surface is different for different polarities. This isimportant in the research of ZnO because reproduciblep-type doping is the key point and has not been solved suc-cessfully. Therefore, it is very helpful to control the polaritybefore we do p-type doping of ZnO.

Sapphire nitridation is a very effective method to im-prove crystalline quality in the epitaxy of nitrides. We re-ported previously that it was also very effective in the elimi-nation of the rotation domains and the improvement of thequality of ZnO epitaxial film.12 Since the AlN thin layerformed by deep nitridation is N-polar, it provides the unipo-lar surface for the subsequent epitaxy of ZnO. Hence, it be-comes easier to control the polarity of ZnO on this unipolarsurface in comparison to the nonpolar sapphire surface. Thepolarity control of ZnO has been studied and realized onGa-polar GaN template.13 However, there is scarce reportabout the polarity control of ZnO on N-polar nitrides.

In this letter, we will report the polarity control of theZnO film grown on nitrided sapphire. Since the formed AlNlayer by nitridation is N-polar, it is thought that the O-polarZnO should be grown because they are both anion-polar,which is similar to that the N-polar GaN is usually obtainedon nitrided sapphire by molecular-beam epitaxy (MBE).However, Zn-polar ZnO film was obtained in our case. We

found that single monolayer (ML) deposition of Ga on thenitrided sapphire was effective to get the O-polar ZnO film.

The ZnO film was grown by rf-plasma-assisted MBE.Both the nitrogen and oxygen plasma cells are present in thesame chamber. The as-polished c-sapphire substrate was ni-trided for 2 h with a rf power of 520 W and a nitrogen flowrate of 1.6 sccm at 400 C after being thermally cleaned at880 C for 30 min. Then, two kinds of samples were grown,which would be called ZnO/N* and ZnO/Ga/N*. In thecase of ZnO/N*, a ZnO buffer layer was grown directly afternitridation at 400 C with a thickness of 15 nm, where theZn beam flux was 2.43107 Torr while the oxygen flow ratewas 0.5 sccm with a chamber pressure of about 1.23105 Torr and the rf power was 200 W. Then, the tempera-ture was raised to 680 C to grow a ZnO epilayer understoichiometric condition. The Zn beam flux was 5.03107 Torr. The oxygen flow rate was 1.2 sccm with thechamber pressure of about 2.83105 Torr and the rf powerwas 300 W. For the ZnO/Ga/N*, the Ga layer depositionwas performed at 400 C just before the growth of the ZnObuffer layer. Other growth conditions were the same as thoseof ZnO/N*.

The polarity of ZnO films was determined by the coaxialimpact collision ion scattering spectroscopy (CAICISS)14,15and confirmed by chemical etching in 0.05% HCl solutionfor 1 min. In brief, in CAICISS measurement, a low energy(about 2 keV) pulsed He+ ion beam stroke onto ZnO filmsurface and the scattered He+ ions from the surface weredetected at a backscattering angle of 180 by a time-of-flight(TOF) energy analyzer. Since the TOF of He+ ions scatteredfrom Zn atoms was different from that from O atoms, wecould easily get the intensity of signals from Zn or O.CAICISS spectrum in this letter is the dependence of Znsignal intensities on the incident polar angles along f1120gazimuth of the ZnO. The polarities of ZnO films were deter-mined by the comparison of the experimental CAICISSspectra and the simulated ones. Surface morphologies werecharacterized by atomic force microscope (AFM) while highresolution x-ray diffraction (Philips Xpert-MRD) measure-ments were performed to investigate the crystal quality.

Figure 1 shows the experimental CAICISS results of theZnO/N* and the ZnO/Ga/N*, together with the simulateda)Electronic mail: yoshi@faculty.chiba-u.jp

APPLIED PHYSICS LETTERS 86, 011921 (2005)

0003-6951/2005/86(1)/011921/3/$22.50 2005 American Institute of Physics86, 011921-1Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

results of ZnO bulk in different polarities. The simulatedCAICISS spectrum of Zn-polar ZnO is quite different fromthat of O-polar one. The former is characterized by threedominant peaks at u=22, 48, and 72 while six peaks canbe found in the latter at u=16, 22, 32, 52, 68, and 74.These simulated results agree well with experimental onesand have been used by many researchers to determine thepolarity.16 As shown in Fig. 1, the CAICISS spectrum of theZnO/N* is dominant by three peaks at u=22, 48, and 72,which coincides with that of simulated Zn-polar one well,indicating that the ZnO/N* is Zn-polar. The CAICISS spec-trum of the ZnO/Ga/N* is characterized by six peaks at u=16, 22, 32, 50, 64, and 74, which agrees with that ofthe simulated O-polar one, indicating the O-polar film.Hence, we can know that the ZnO/N* is Zn-polar while theZnO/Ga/N* is O-polar.

In our study, we found that the polarity of ZnO film wasdetermined by the interface between the nitrided sapphireand the ZnO buffer layer while the epilayer growth conditionhad no influence. Figure 2(a) shows the schematics of atomicarrangement of the ZnO/N*. The thin AlN layer formed bydeep sapphire nitridation has been proven to be N-polar andis terminated by the N atom plane. The ZnO layer should beO-polar if it also follows the polarity of the AlN layer be-cause they are both anion-polar. However, the ZnO wasproven to be Zn-polar. This indicates that the polarity is in-verted at the interface between the nitrided sapphire and theZnO buffer layer as shown in Fig. 2(a). So how does thisinterface serve as the inversion center? At the initial growthof the ZnO buffer layer, it seems that the Zn atom shouldbond with N atom first because the cationanion bond wouldbe easily formed. In this case, if the ZnN bond belongs to

hexagonal structure, the polarity should not be inverted.However, if the ZnN bond has the same structure as that ofZn3N2, it is possible to invert the polarity because the Zn3N2may play the same role as the Mg3N2, which has been foundto invert the polarity during the GaN epitaxy.17 If the Zn3N2could serve as the inversion center, it should also invert theO-polar ZnO film to the Zn-polar one. However, we couldnot find this phenomenon in our experiment. Furthermore,the formation of the Zn3N2 would be difficult from the view-point of the bonding configuration. Each N atom on the AlNsurface has only one dangling bond but the formation of theZn3N2 requires more. Therefore, the Zn3N2 cannot beformed.

In our in situ RHEED monitoring of the ZnO bufferlayer growth, a halo pattern was observed shortly before theappearance of a ZnO RHEED pattern. This indicated that anamorphous layer was formed. This is reasonable because Oatom may replace the N atom partly at the interface due tothe stronger bond of AlO in comparison to that of AlN.Hence, a very thin amorphous layer including Al, N, O, andZn is probably formed at the interface. This amorphous layeris so thin that it does not change the growth epitaxial rela-tionship between the ZnO and the nitrided sapphire, i.e.,f1010gZnO i f1010gAlN i f1120gAl2O3. On this amorphouslayer, the Zn-polar ZnO is more thermodynamically stabledue to the low growth temperature, resulting in Zn-polarfilm. This coincides with previous report about the tempera-ture dependence of the polarity of ZnO on sapphire.18 Ac-cording to their results, although the misfit between ZnO andAl2O3 was as large as 31.8%, the Zn-polar ZnO was obtainedat 450 C. This indicates that the Zn-polar ZnO is more ther-modynamically stable at low temperature. The growth tem-perature of 400 C in our case is lower than 450 C, whichleads to the growth of Zn-polar ZnO. Here, we notice that theformation of the amorphous layer was due to the fact that theAlN layer under it was terminated by the N atom plane.Therefore, if we deposit only 1 ML Ga, the top layer is not Nbut Ga. In this case, the amorphous layer should not beformed because the GaO bond will form at the interface asshown in Fig. 2(b). Actually, we did not find the halo patternfrom the RHEED in situ investigation during the ZnO bufferlayer growth. Hence, the polarity should not be inverted, i.e.,O-polar ZnO should be obtained. In fact, our CAICISS resultconfirmed that the ZnO/Ga/N* was O-polar as shown inFig. 1.

The thickness of both samples grown in 3 h was 700 nmfor the ZnO/N* and 500 nm for the ZnO/Ga/N*, corre-sponding to the growth rate of 0.65 and 0.46 /s, respec-tively. This indicates that the growth rate of Zn-polar ZnO isabout 1.4 times that of the O-polar one. The reason for thedifferent growth rates is the different dangling bond configu-ration of the growing surfaces. Since each O atom on anO-polar ZnO surface has only one dangling bond along caxis while each O atom has three dangling bonds on theZn-polar one, the Zn sticking coefficient on the O atom planeof the Zn-polar ZnO is higher than that of the O-polar one.19Therefore, the growth rate of the Zn-polar ZnO is higher thanthat of the O-polar one. Both samples show high crystallinequality as shown in Fig. 3. The full width at half maximum(FWHM) values of (002) and (102) v scans for Zn-polarZnO were 119 and 486 arcsec while those for O-polar onewere 73 and 673 arcsec, respectively. The Zn-polar sampleshows better (102) value and worse (002) one in comparison

FIG. 1. Incident angular dependencies of the Zn signal intensity alongf1120g azimuth for the ZnO/N* and ZnO/Ga/N*. The simulated results arealso shown inset.

FIG. 2. Schematic of atomic arrangement of the ZnO/N* (left) and theZnO/Ga/N* (right). The ZnO/N* shows Zn-polar while the ZnO/Ga/N* isO-polar.

011921-2 Wang et al. Appl. Phys. Lett. 86, 011921 (2005)

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to the O-polar one. In general, the FWHM value of (002) vscan is related to screw-type threading dislocations (TDs)while that of (102) v scan is responsible for the edge-typeTDs, which is usually dominant in the epitaxial growth suchas GaN and ZnO.20 Therefore, the Zn-polar film generallyshows better crystalline quality than the O-polar one.

The surface morphology can be a possible way to checkthe polarity because ZnO films in opposite polarities showquite different surface morphologies under the same growthcondition as shown in Figs. 4(a) and 4(c). The surface con-tains hexagonal pits in the Zn-polar ZnO while the hexagonalhillock can be observed in the O-polar one. The smooth sur-face of Zn-polar ZnO can be obtained under O-rich conditionwhile hexagonal pits are usually observed under stoichio-metric and Zn-rich condition. On the other hand, the hexago-nal hillock is usually found in the O-polar ZnO, even underZn-rich condition. Both samples were chemically etched toconfirm the polarity. The surface morphologies after beingetched are shown in Figs. 4(b) and 4(d) because they haveseldom been reported by AFM. Large hexagonal pits can beobserved from the Zn-polar surface while hexagonal islandsare clearly found from the O-polar one. The Zn-polar ZnOkept mirror-like while the O-polar one changed drasticallyduring the etching. The etching rate of O-polar sample ismuch higher than that of the Zn-polar one. This coincides

with previous reports21 and confirms our CAICISS results.In summary, we controlled the polarity of the ZnO films

on nitrided sapphire substrate. The ZnO/N* was Zn-polardue to the low growth temperature and the formation of thinamorphous layer at the interface. One ML Ga is effective toprevent the formation of this amorphous layer, resulting inO-polar ZnO film. The Zn-polar ZnO shows better crystal-line quality and higher growth rate than the O-polar one. TheZnO in opposite polarities show quite different morpholo-gies, which could be a possible way to check the polarity.

This work was partly supported by the grant-in-aid forScientific Research (B) #13450121, Japan Society for thePromotion of Science, and the CREST, Japan Science andTechnology Agency.

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FIG. 3. The comparison of the highresolution XRD symmetric and asym-metric v scans of the ZnO/N* and theZnO/Ga/N*.

FIG. 4. AFM images of the ZnO/N* and the ZnO/Ga/N* before and afteretching. (a) As-grown surface of the ZnO/N*. (b) The surface of theZnO/N* after etching. (c) As-grown surface of the ZnO/Ga/N*. (d) Thesurface of the ZnO/Ga/N* after etching. The image sizes are all 3 mm33 mm.

011921-3 Wang et al. Appl. Phys. Lett. 86, 011921 (2005)

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