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JOURNAL OF APPLIED PHYSICS VOLUME 84, NUMBER 5 1 SEPTEMBER 1998diodes ~LEDs!, emitter and laser applications.1 TheseZnO/a-Al2O3 heterostructures can also be used to grow IIInitrides where the band gap can be tailored between 3.39 eV~GaN! and 6.2 eV ~AlN!, which exhibit a complete solidsolubility. The ZnO films have been grown using a variety ofthin film growth techniques such as chemical vapor deposi-tion ~CVD!,2 sputtering,3 and pulsed laser deposition.48More recently, high-quality ZnO films were grown by pulsedlaser deposition by Vispute et al.9 with Rutherford channel-ing yield, xmin;2%3%, on sapphire and AlN films onZnO/a-Al2O3 with considerably higher xmin ~60%70%!. Amuch higher minimum channeling yield clearly indicates thefilm to be highly defective and textured only in @0001# ori-entation. The objective of this work is to optimize pulsedlaser deposition conditions to produce epitaxial layers with alow defect content, and investigate in detail the nature ofepitaxial growth, and characteristics of defects and interfacesusing transmission electron microscopy ~TEM! and x-raydiffraction ~XRD! techniques.

II. EXPERIMENT

The ZnO films with the thickness of ;250 nm weresynthesized by ablating a sintered zinc oxide target with apulsed excimer laser ~l5248 nm, pulse width525 ns! with

using a bulk AlN target. The base vacuum during the depo-sition of the AlN films was ;1 331026 Torr. The slowcooling of the ZnO and AlN films were done sequentially in1023 Torr of oxygen and nitrogen, respectively. The charac-terization of the films was carried out using TEM ~Topcon002 B electron microscope operating at 200 kV! and XRDtechniques. The electrical resistivity measurements were per-formed at room temperature using the four-point probemethod. The samples for TEM were prepared using conven-tional sample preparation techniques including mechanicalpolishing and dimpling, Ar1 ion milling at 6.5 and 2 kV atthe final stage.

III. RESULTS AND DISCUSSION

Figures 1~a! and 1~b! show typical XRD scans from anoptimized ZnO film deposited at the substrate temperature of790 C and of the AlN films deposited on the ZnO film at atemperature of ;770 C, respectively. The rocking curvewidth of the ~0002! reflection of ZnO in as-deposited filmswas measured to be 0.16 whereas the rocking curve widthof the ~0006! peak of sapphire was 0.09. The ZnO filmdeposited at the substrate temperature of 750 C showed therocking curve width of 0.17. These are some of the bestresults in the literature, as shown by XRD. The consistentDefects and interfaces in epitaxial ZnOheterostructures

J. Narayan, K. Dovidenko, A. K. Sharma, and S.Department of Materials Science and Engineering, North CCarolina, 27695-7916

~Received 23 February 1998; accepted for publicatio

We have investigated the nature of epitaxy, defectsdomains!, and heterointerfaces in zinc oxide filmspossibility of using it as a buffer layer for growing gfilms were grown on sapphire using pulsed laser depThe epitaxial relationship of the film with resp(0001)ZnOi~0001!sap , with in-plane orientation relatioorientation relationship corresponds to a 30 rotatiosapphire substrate, which is similar to the epitaxialsapphire. The threading dislocations in ZnO were fouThe planar defects ~mostly I1 stacking faults! were fabout 105 cm21. We have grown epitaxial AlN filmsapphire heterostructure as a substrate and observedAlN/ZnO interface. The implications of low defect coand the role of ZnO films as a buffer layer for IIIInstitute of Physics. @S0021-8979~98!05917-9#

I. INTRODUCTIONThe majority of the applications based on zinc oxide,

such as varistors, transparent conducting electrodes, andsurface-acoustic-wave devices, use polycrystalline material.However, there is a considerable interest in growing highquality crystalline ZnO ~bandgap53.3 eV! films on practicalsubstrates such as sapphire (a-Al O ) for UV light emitting2590021-8979/98/84(5)/2597/5/$15.00

Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject t/a-Al2O3 and AlN/ZnO/a-Al2O3

ktyabrskyrolina State University, Raleigh, North

1 June 1998!

dislocations, stacking faults, and inversionown on ~0001! sapphire and explored theup III nitrides. High quality epitaxial ZnO

ition in the temperature range 750800 C.t to ~0001! sapphire was found to beship of @0110#ZnOi@1210#sap . This in-planeof ZnO basal planes with respect to the

rowth characteristics of AlN and GaN onto have mostly 1/3^1120& Burgers vectors.nd to lie in the basal plane with density ofat temperatures around 770 C using ZnO/he formation of a thin reacted layer at theent in ZnO films compared to IIIV nitridesnitrides are discussed. 1998 American

an energy density in the range of 3 to 4 J/cm2. The oxygenpartial pressure and the substrate temperatures for the growthof these films were in the range 1024 1025 Torr and 750800 C, respectively. The base vacuum prior to depositionwas in the range of 131027 to 531028 Torr. The AlNfilms were deposited on ZnO films at the substrate tempera-tures between 700800 C without nitrogen partial pressure7 1998 American Institute of Physics

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results were obtained from several optimized films. The AlNfilm @Fig. 1~b!# was found to be epitaxial c-axis oriented. Ascan be seen in the diffractogram, there are no other orienta-tions except the (000l) reflections of ZnO and AlN films.However, the AlN films deposited on ZnO/a-Al2O3 hetero-structures at the substrate temperatures of 700, 740, and800 C were found to be highly textured polycrystalline.

The ZnO/a-Al2O3 and AlN/ZnO/a-Al2O3 thin film het-erostructures were investigated to determine the nature ofdefects and interfaces and assess their potential for deviceapplications. Figure 2~a! shows a plan-view TEM micro-graph of a ZnO/a-Al2O3 film under two-beam diffractionconditions with vector g5(1210). The selected area diffrac-tion ~SAD! pattern from an area about 0.5 mm in diametercontaining both the film and the substrate is shown in Fig.2~b!. The SAD pattern was taken along the film and substratenormal @0001# where the arrows indicate @0110#ZnO and@1210#sap reflections. Additional weaker spots visible in theSAD pattern arise from the double diffraction of the sub-strate and the film. From diffraction patterns, we determined

FIG. 1. ~a! X-ray diffractogram from ZnO film deposited at the substratetemperature of ;790 C; ~b! XRD scan of the AlN/ZnO/a-Al2O3 hetero-structure with the AlN film deposited at the optimized temperature;770 C.

2598 J. Appl. Phys., Vol. 84, No. 5, 1 September 1998the following epitaxial relationship between the film and thesubstrate: (0001)ZnOi~0001!sap , @0110#ZnOi@1210#sap , and

Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject t@1210#ZnOi@0110#sap . This relationship corresponds to a 30in-plane rotation of the film with respect to the substrate.This 30 rotation strongly suggests that the epitaxial relation-ship is dictated by AlO bonding at the interface. The epi-taxial relationship is the same as that for group III-nitridesgrown on ~0001! sapphire substrate.10 This rotation leads toan alignment of (0110) ~spacing 2.814 ! planes of ZnOwith (1210) planes ~spacing 2.379 ! of sapphire. Underthese conditions seven planes of sapphire match with sixplanes of ZnO. Thus, the epitaxial growth of ZnO ona-Al2O3 is controlled by domain matching epitaxy11,12~similar to the growth of group III nitrides on sapphire10!where integral multiples of planes or lattice constants matchacross the interface.

FIG. 2. ~a! Plan-view ~0001! TEM image of ZnO film taken under g5(1210) two-beam conditions. ~b! Selected area diffraction pattern of ZnOepilayer and sapphire substrate taken in the @0001# direction of both the filmand the substrate, (0110)ZnO and (1210)sap reflections are indicated by ar-rows.

Narayan et al.Figures 3~a! and 3~b! show cross-section TEM imagesusing diffraction vectors g5(0110)ZnO and g5(0002)ZnO


where defects and the interface between the ZnO film and thesubstrate are clearly delineated. Most of the defects in thefilm are dislocations and planar defects ~stacking faults indi-cated by white arrows! which align along the basal ~0001!planes. The threading dislocation density decreases rapidlywith the distance from the interface. Their density near theinterface ~first 100 nm! can be estimated as 2.031010 cm22 whereas at the top of the ZnO film the densitydrops to about 107 cm22. This value is several orders ofmagnitude lower than the typical values (109 1010 cm22) ingroup III-nitride films.13,14

The density of planar defects (;105 cm21) is quite highin the ZnO film and decreases only by a factor of 3 in the topregion of the film. The origin of these defects is likely due tosome nonstoichiometry of the film, which results in the faultnucleation at the ~0001! terraces. These defects were foundto be mostly low-energy I1 stacking faults having a singlestack of fcc sequence. The average width of the stackingfaults is 100 nm giving rise to a partial dislocation density ofapproximately 1010 cm22. The value of 1010 cm22 can beobtained using a plan-view micrograph, Fig. 2~a!, whichshows the projection of the entire ZnO film.

A remarkable feature of the observed dislocation con-figuration is that most of the dislocations lie in the basalplane. This is opposite to the case of group III-nitride filmswhere the majority of the dislocation lines are normal to the~0001! plane and constitute low-angle sub-boundaries andcolumnar film morphology.14 However, as in the nitridefilms, the contrast from most of the dislocations in ZnOcross-sectional specimens disappear when the scattering vec-tor is g5(0002) @Fig. 3~b!#, indicating that the Burgers vec-

FIG. 3. Cross-section TEM images of AlN/ZnO/a-Al2O3 heterostructurenear the @2110#ZnO zone. ~a! Image was taken under g5(0110)ZnO two-beam conditions. Arrows indicate stacking faults. ~b! Image was taken underg5(0002)ZnO . Arrows point the inversion domains in ZnO.

J. Appl. Phys., Vol. 84, No. 5, 1 September 1998tors are parallel to the basal plane. Therefore, perfect dislo-cations have a 1/3@1120# Burgers vector, as the majority of

Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tdislocations in the group III-nitride films. The micrographsin Figs. 2 and 3 reveal numerous dislocation reactions in thedislocation network. The main result of these reactions is thedecrease of the threading dislocation density with the filmthickness.

We have also observed the formation of small inversiondomains at the ZnO/sapphire interface. These domains areindicated by arrows in the micrograph taken with g5(0002)ZnO @Fig. 3~b!#. The inversion domains are domainshaving the crystal polarity opposite to that of the matrixmaterial.15 ZnO is a polar crystal and, therefore, the direc-tions 0001 and 0001 are not equivalent ~the direction of theatomic bond is from Zn to O or from O to Zn, respectively!.To prove the inversion nature of these domains we used amultiple beam dark-field technique16 in the @1120# zoneaxis. The contrast from the domains was inverted in the im-ages taken using opposite reflections ~0002 and 0002 in thiscase!, which is a characteristic feature of the inversion do-mains. The domains nucleated at the interface and had thethickness of about 15 nm and the lateral dimensions of ;30nm. The polarity of the growing film is an important issuesince, for example, in group III nitrides the characteristicfeatures are columnar-looking inversion domains sometimesgoing through the entire film thickness.14,17 Also, the surfaceroughness seems to be dependent on the crystal polarity inthe case of GaN crystals18 and thin films.19 If the ZnO filmsare to be used for the subsequent IIIV nitride growth, thepolarity of the ZnO can influence significantly the polarityand final quality of the growing nitride layer. Using conver-gent beam electron diffraction, we determined the polarity ofour ZnO films to be Zn terminated ~the direction of the bondalong c axis is from Zn to O!. Further implications of theseon defect characteristics are being studied.

High-resolution TEM studies were carried out to inves-tigate the nature of the interfaces between ZnO/a-Al2O3 .The micrograph taken in @2110#ZnOi@1010#sap zone axes@Fig. 4~a!# clearly shows that the ZnO/a-Al2O3 interface isatomically sharp with no mixed layer near the interface. Themisfit dislocations are also visible at the interface and con-stitute the array when every 7th (2110) plane of sapphire ~onaverage! terminates at the interface. The nature of domainepitaxy is clearly illuminated in Fig. 4~b!, where there is amissing plane in the film after every six to seven planes. ThisFourier-filtered image also shows that most of the disloca-tions during domain epitaxy are formed during the initialstages of film growth and are confined to the interface re-gion.

In the second part of this study, we investigated thegrowth characteristics of IIIV nitrides on ZnO/a-Al2O3substrates. Since AlN and ZnO lattice constants are closelymatched within 4.0% ~as shown in the Table I!, we expect alattice matching epitaxy between the AlN film and ZnO sub-strate as opposed to a domain matching epitaxy between theZnO film and a-Al2O3 substrate. Figure 5~a! shows a cross-section TEM micrograph of the AlN/ZnO/a-Al2O3 hetero-structure. From the diffraction pattern containing AlN/ZnO

2599Narayan et al.interface @Fig. 5~b!#, we determined that AlN grows on ZnOwithout any rotation. We also observed a 3060--wide


mixed layer at AlN/ZnO interface. This reacted layer is poly-crystalline. From the diffraction pattern analysis and the cal-culation of power spectra of high-resolution images, we de-termined the structure of the layer to be spinel ZnAl2O4 (a58.0848 ). The polycrystallinity of the layer and the factthat AlN maintains expected epitaxial relationships with theZnO substrate indicate that the spinel layer was most prob-ably formed as a result of a reaction after AlN has alreadygrown to a certain thickness. Therefore, initial epitaxialgrowth of AlN is not affected by this reaction. The surface ofthe ZnO layer is rough which can be seen in Fig. 5~b!. De-spite that, the AlN film showed good crystallinity and epi-taxial growth near 770 C. However, the films grown at700740 C and at 800 C were polycrystalline. We envis-age that below this temperature the crystallinity of the film

FIG. 4. ~a! High resolution TEM cross-sectional image of the ZnO film nearthe film/substrate interface ~shown by black arrows!. The terminating planescorresponding to the misfit dislocations are indicated by white arrows. ~b!Fourier-filtered image using opposite $1010%ZnOi$1120%sap reflexes showingthe match of the corresponding planes. Misfit dislocations at the interfaceare indicated. Numbers in the bottom of the picture correspond to the num-ber of planes between the misfit dislocations. Note that every 7th or 6th(2110) plane of sapphire terminates at the interface.

TABLE I. Lattice constants and thermal expansion coefficients of sapphire,ZnO, and AlN.

MaterialLattice

constant, Thermal expansioncoefficient, 1026/K

a-Al2O3 a54.758 7.5~R-3c , hexagonal! c512.991 8.5ZnO a53.249 2.9~P63mc , hexagonal! c55.206 4.75AlN a53.111 4.2

2600 J. Appl. Phys., Vol. 84, No. 5, 1 September 1998~P63mc , hexagonal! c54.979 5.3

Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tbecomes poor due to insufficient adatom mobility causingthe random nucleation whereas at higher temperatures inter-facial reaction between AlN and ZnO creates a polycrystal-line mixed layer which leads to formation of polycrystallineAlN films. The predominant defects in the AlN films arethreading dislocations with the 1/3@1120# Burgers vectoraligned normal to the basal plane with a density of;1010 cm22. These dislocations result from the columnarmorphology of the film. Further studies are needed to com-pletely understand the role of the interfacial reaction and tooptimize the growth conditions ~including the elimination ofZnO surface roughness! to obtain higher quality AlN films.

The electrical resistivity measured at room temperaturefor the ZnO films is in the range of 58 V cm for as-grownfilms which is two orders of magnitude higher than that ob-tained by other researchers9 for their high quality films. Thispoints towards better stoichiometry of the films grown in ourwork. Zinc oxide is known20 to grow with nonstoichiometryhaving oxygen vacancies. However, it may be possible toimprove stoichiometry and also reduce defect density bypostannealing the films in O2 . Our recent experiments beforeand after annealing show a substantial improvement in theoptical properties of ZnO films21 as well as an increase of

2

FIG. 5. ~a! Cross-sectional TEM micrograph of the AlN/ZnO/a-Al2O3 het-erostructure taken under g50002 two-beam conditions. Columnar morphol-ogy of the AlN layer is clearly visible. ~b! SAD patterns ofAlN/ZnO/a-Al2O3 heterostructure taken in the @2110#ZnO zone.

Narayan et al.resistivityup to 5310 V cm. We were able to resolvethree exciton resonances in the optical transmission spectra


of the annealed films. The determination of valence bandsplitting, exciton binding energies, and band gaps at 77 and300 K clearly show these films to be comparable to high-quality bulk single crystals.

IV. SUMMARY

We have grown high-quality epitaxial ZnO films on~0001! sapphire. A 30 rotation of the ZnO film with respectto the a-Al2O3 substrate is similar to the growth of group IIInitrides on a a-Al2O3 substrate. This rotation is dictated bythe oxygen sublattice in sapphire which matches with the Znsublattice in the ZnO film. The predominant defects in theZnO film were found to be dislocations, planar defects~stacking faults!, and inversion domains. Most of the dislo-cations lie in the basal plane giving a layered growth mor-phology contrary to the group III-nitride films having colum-nar morphology. The density of threading dislocations inZnO ~Burgers vector 1/3@1120#! decreases with the filmthickness giving a value of about 107 cm2 for the 250 nmthickness. This is considerably lower than encountered inthin films of group III nitrides which are around109 1010 cm22. Epitaxial AlN films were grown around

Smart Structures. We acknowledge useful discussions withDr. R. D. Vispute.1 R. F. Service, Science 276, 895 ~1997!.2 J. Nishino, S. Ohshio, and K. Kamata, Mater. Res. Soc. Symp. Proc. 363,219 ~1995!.

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11 J. Narayan, P. Tiwari, X. Chen, R. Chowdhary, and T. Zheleva, Appl.Phys. Lett. 61, 1290 ~1992!; US Patent No. 5,406,123 ~11 April 1995!.

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2601J. Appl. Phys., Vol. 84, No. 5, 1 September 1998 Narayan et al.770 C on a ZnO/a-Al2O3 substrate showing no rotation ofthe crystal lattice of AlN with respect to ZnO. At thesegrowth temperatures, an interfacial reaction creates a poly-crystalline layer whose effect has to be minimized to obtainhigh-quality epitaxial AlN films.

ACKNOWLEDGMENTS

This research was supported by the National ScienceFoundation under the Center for Advanced Materials andDownloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject t16 R. Serneels, M. Snykers, P. Delavignette, R. Gevers, and S. Amelinckx,Phys. Status Solidi B 58, 277 ~1973!.

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Narayan, Appl. Phys. Lett. ~to be published!.o AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

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