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APPLIED PHYSICS LETTERS VOLUME 78, NUMBER 11 12 MARCH 2001an amorphous silica layer. This can be understood inview of the fact that the formation enthalpy of SiO2 is muchlarger in magnitude than that of ZnO ~i.e., DHSiO252910.7 kJ/mol vs DHZnO52350.5 kJ/mol!,6 and there-fore the formation of amorphous oxide is favored over theZnO formation on Si surface. This oxidation problem mightbe alleviated or avoided if there exist any proper ways ofcontrolling the amount of oxygen species around the Si sur-face during the initial stage of ZnO growth. This, however, isconsidered to be difficult in most growth techniques. In thecase of sputtering technique, for example, Si surface is ex-posed to the oxygen species coming from the ambient gasused ~in the reactive sputtering case! or coming from theoxide target itself ~in the nonreactive sputtering case!. Therehas been no report on direct epitaxial growth of ZnO films onSi.

In this letter, we report epitaxial growth of ZnO films onSi substrates using an epitaxial GaN buffer in conjunction

in terms of obtaining single-phase epitaxial films with goodsurface morphology. Direct growth usually results in amor-phous, polycrystalline, or multiphase films, mainly due to thedifficulty in controlling the nitridation of Si surface duringthe nucleation stage of GaN growth. The difficulty level de-pends on details of the growth method used, such as the typeof a nitrogen source and the mechanism of generating activenitrogen, and the temperature and time of the pregrowth andnucleation stages of a growth process used.

In this work we have investigated a rf magnetron sput-tering technique in growing epitaxial GaN films on Si andthen ZnO films on the GaN-buffered Si substrates. The sput-ter target used for epitaxial GaN films was 6N-purity galliumcontained in a 2 in. diameter stainless-steel cup. The sub-strates used were Si~111! wafers ~p-type: 0.11.0 V cm re-sistivity!. The Si wafers were cleaned using a standard RadioCorporation of America ~RCA! cleaning procedure followedby a treatment in HF solution ~i.e., immersion in 1% HFsolution for 20 s!.10 This HF treatment is to provide hydro-Epitaxial growth of ZnO films on Si suan epitaxial GaN buffer

Ahmed Nahhas and Hong Koo Kima)Department of Electrical Engineering, University of PittsbJean BlachereDepartment of Materials Science and Engineering, Univer~Received 30 October 2000; accepted for publication

We report on epitaxial growth of ZnO films on Si~1layer. A rf magnetron sputtering process has been devbuffers on Si, and then ZnO films on the GaN-buffshows that both the ZnO and GaN films are of a monorelationship of ZnO@0001#//GaN@0001#//Si@111#ZnO@112I01#//GaN@112I0#//Si@11I 0# along the in-planeZnO/GaN films on Si demonstrates the feasibility adevices on the same substrate. 2001 American In

ZnO is a versatile material, and has been extensivelystudied for various applications such as varistors, transduc-ers, transparent conducting electrodes, sensors, and catalysts.While polycrystalline ZnO is commonly used in these con-ventional applications, there has been a growing interest inobtaining single-crystalline ZnO films on varioussubstrates.14 ZnO is a IIVI wide band gap semiconductorwith a relatively large exciton binding energy, and holds apotential for light emitting/detecting or nonlinear optical de-vices in the UV range.

ZnO on Si offers an interesting opportunity that the vari-ous functional properties of ZnO can be combined with theadvanced Si electronics on the same substrate. Direct growthof epitaxial ZnO films on Si, however, is known to be anextremely difficult task. It usually results in amorphous orpolycrystalline films.5 The difficulty basically stems from thefact that Si surface gets easily oxidized during the nucleationstage of a ZnO growth process, which results in formation ofa!Electronic mail: kim@ee.pitt.edu

1510003-6951/2001/78(11)/1511/3/$18.00Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tstrates using

gh, Pittsburgh, Pennsylvania 15261

y of Pittsburgh, Pittsburgh, Pennsylvania 15261January 2001!

! substrates using an epitaxial GaN bufferoped and utilized in growing epitaxial GaNed Si substrates. X-ray diffraction analysisystalline wurtzite structure with an epitaxial

along the growth direction andirection. The successful growth of epitaxial

promise of integrating various functionaltute of Physics. @DOI: 10.1063/1.1355296#

with a rf magnetron sputtering technique. The idea of using aGaN buffer is based on the following reasoning. First, theformation enthalpy of Si3N4 is smaller in magnitude than thatof SiO2 ~i.e., DHSi3N452743.5 kJ/mol vs DHSiO252910.7 kJ/mol!.6 This implies that the nitridation of Sisurface ~i.e., formation of an amorphous SiN layer on Siduring the initial growth period of GaN! would be morecontrollable than the oxidation problem of Si in the growthof ZnO directly on Si. This further suggests greater feasibil-ity of growing epitaxial films of GaN directly on Si, com-pared with the ZnO-on-Si case.7 Second, epitaxial growth ofZnO on GaN is also expected to be highly feasible, sinceGaN is known to be relatively stable in oxygen ambient.8 Infact, epitaxial growth of both GaN-on-Si9 and ZnO-on-GaN-buffered sapphire9 has been reported by various groups, andthis supports the earlier reasoning.

Despite the successful reports, direct epitaxial growth ofGaN on Si is still known to be a challenging task, especiallygen termination on Si surface atoms. This hydrogen termina-

1 2001 American Institute of Physicso AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

tion is known to have the effect of protecting the siliconsurface from oxidation for some time. After the cleaning thewafer was immediately loaded to a growth chamber. Thesputter system was pumped to a base pressure of 1026 Torrby a turbomolecular pump backed with a rotary-vane pump.The substrate temperature was raised to 800 C and was heldat that temperature for 3060 min. This high temperaturetreatment is expected to help remove residual oxide on Sisurface. Residual oxide remaining on Si surface prior to thestart of GaN growth is believed to have a detrimental effecton obtaining epitaxial GaN films. Held at 800 C, the targetwas presputtered for 30 min in Ar ambient with a shutterclosed in order to remove any contamination. The substratetemperature was then lowered to 650 C, and the target waspresputtered for 15 min in Ar/N2~30/70! ambient in order toform a nitrided surface on the Ga target. A 1020-nm-thickGaN buffer layer was grown at 650 C. The substrate tem-perature was raised to 700 C and then a GaN main layerwas grown. The following conditions were used for othersputter parameters: target-to-substrate distance of 2.25 in., rfpower of 110 W, and ambient gas pressure of 10 mTorr. Thegrowth rate was measured 1.5 mm/h.

The structural properties of the grown films were char-acterized by x-ray diffraction ~XRD!. Figure 1 shows anXRD pattern ~u/2u scan! of a 1-mm-thick GaN film grown ona Si~111! substrate. The XRD pattern shows two strongpeaks, one at 2u528.4 from Si~111! and the very strongpeak at 2u534.5 which corresponds to the GaN~0002!planes. Figure 2 shows a phi scan for the (101I 1) planes of aGaN film grown on a Si~111! substrate. The phi scan for the~002! planes of the Si substrate is also superposed on the phiscan for the GaN film to show the in-plane orientation be-tween film and substrate. It is clear from Fig. 2 that the GaNfilm shows the six-fold azimuthal symmetry consistent withthe wurtzite crystal structure. The three-fold symmetry of theSi pattern is expected from the ~111! orientation of silicon.The normal of the GaN film has a @0001# orientation parallelto the @111# of silicon. The orientation of the Si peaks in Fig.2 with those of GaN indicates that the in-plane orientation ofGaN on Si is GaN@112I0#//Si@11I 0# . It should be mentionedhere that the GaN films grown on a basal plane of sapphireshow 30 rotation in their in-plane relation to substrate.11 Incontrast to the case of GaN on sapphire, the GaN films

FIG. 1. X-ray diffraction u/2u-scan profile of a 1.0-mm-thick GaN layergrown on Si~111! by a sputtering technique.

1512 Appl. Phys. Lett., Vol. 78, No. 11, 12 March 2001grown on Si~111! do not show any rotation, indicating thatthe triangular network of the wurtzite ~0002! plane of GaN isin registry with that of the Si~111! substrate.12 It has beenDownloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject treported that direct growth of GaN on Si~111! usually resultsin phase-mixtured films and therefore a proper buffer layer~such as AlN or SiC! has been used to obtain single-phasefilms.13,14 It should be noted that our GaN films, directlygrown on Si, are of a single-phase monocrystalline hexago-nal structure.

ZnO films were then grown on top of the GaN-bufferedSi. A 5N-purity ZnO target was used in conjunction with a rfmagnetron sputter source. Prior to deposition, the target waspresputtered for 5 min in order to remove any contamination.ZnO films were then deposited in Ar or Ar/O2 ambient. Thesubstrate temperature was held at 700 C during the deposi-tion. The following deposition parameters were optimized toobtain highly epitaxial ZnO films: target-to-substrate dis-tance ~1.752.25 in.!, rf power ~5080 W!, and gas pressure~1020 mTorr!.

Figure 3 shows XRD phi scans of a 0.5-mm-thick ZnOfilm grown on a 0.5-mm-thick-GaN-buffered Si~111! sub-strate. The (101I 1) planes are chosen for the phi scans ofboth ZnO and GaN films. The phi scans are very character-istic of the orientation of the films, i.e., both have the samesix-fold symmetry with an exact match in orientation. Basedon the XRD analysis results, it is concluded that the ZnOfilm is monocrystalline with its surface normal as @0001# andits in-plane orientation ZnO@112I0#//GaN@112I0#//Si@11I 0# . Inthis work we have also investigated direct growth of ZnOfilms on Si~111! substrates that are prepared in the sameprocedure as the GaN growth case. The ZnO films grown onSi without a GaN buffer are highly c-axis orientated as char-acterized by XRD u/2u scan. XRD pole figure analysis, how-ever, reveals that the ZnO films grown on Si are polycrystal-line in their in-plane orientation. This comparative studyclearly confirms the importance of using an epitaxial GaNbuffer in growing monocrystalline ZnO films on Si sub-strates.

FIG. 2. XRD phi scan profiles for the (101I 1) plane of GaN ~top! and the~002! plane of Si~111! ~bottom!.

Nahhas, Kim, and BlachereFigure 4 shows surface morphology of a ZnO filmgrown on GaN-buffered Si. The film shows a crack-freesmooth surface. The GaN buffer layer grown on Si was also

o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

expected to be less serious. Second, a sputter-deposition pro-cess itself usually induces a significant amount of compres-sive stress to a film.18 This intrinsic stress is expected to

1513Appl. Phys. Lett., Vol. 78, No. 11, 12 March 2001 Nahhas, Kim, and Blachereconfirmed to be crack-free, although it is not shown in thisletter. It has been reported that GaN growth on Si often re-sults in cracked films.15 This indicates that the films are un-der a significant amount of tensile stress during post-growthcooling. This stress is attributed to a relatively large differ-ence in their thermal expansion coefficients ~i.e., 3.631026 K21 of Si vs 5.631026 K21 of GaN16! and also to ahigh growth temperature ~over 1000 C in the case of metal-organic chemical vapor deposition!. In contrast, a sputteringtechnique offers the following advantages in growing crack-free GaN films on Si as demonstrated in this work. First, thegrowth temperature is relatively low, in the range of650 700 C,17 therefore the thermal mismatch problem is

FIG. 3. X-ray diffraction phi-scan profiles of a ZnO/GaN/Si~111! structure:the (101I 1) planes of ZnO ~top! and GaN ~bottom!.

FIG. 4. Surface morphology of a ZnO film grown on a GaN-buffered Sisubstrate.Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tprovide a certain amount of compensation on the tensilestress and thus to alleviate the cracking problem. The use ofa GaN buffer also offers unique advantages over other buff-ers reported so far ~such as AlN and SiC!: good lattice matchto ZnO ~less than 2% mismatch!, and the controllability ofelectrical conductivity in a broad range, which will allow forgreater flexibility in device design.

In summary, we have demonstrated that the use of aGaN buffer in conjunction with the rf sputtering technique isa viable way of growing crack-free, epitaxial ZnO films onSi substrates. The successful growth of epitaxial ZnO/GaNfilms on Si demonstrates the feasibility and promise of inte-grating various functional devices on the same substrate.

This work was supported by the Office of Naval Re-search under Grant No. 00014-99-0663.1 M. A. L. Johnson, S. Fujita, W. H. Rowland, W. C. Hughes, J. W. Cook,and J. F. Schetzina, J. Electron. Mater. 25, 855 ~1996!.

2 Y. F. Chen, D. M. Bagnall, Z. Zhu, T. Sekiuchi, K. Park, K. Hiraga, T.Yao, S. Koyama, M. Y. Shen, and T. Goto, J. Cryst. Growth 181, 165~1997!.

3 A. Ohtomo, K. Tamura, K. Saikusa, K. Takahashi, T. Makino, Y. Segawa,H. Koinuma, and M. Kawasaki, Appl. Phys. Lett. 75, 2635 ~1999!.

4 R. D. Vispute, V. Talyansky, S. Choopun, R. P. Sharma, T. Venkatesan,M. He, X. Tang, J. B. Halpern, M. G. Spencer, Y. X. Li, L. G. Salamanca-Riba, A. A. Iliadis, and K. A. Jones, Appl. Phys. Lett. 73, 348 ~1998!.

5 See, for example, T. Shirasawa, T. Honda, F. Koyama, and K. Iga, Mater.Res. Soc. Symp. Proc. 449, 373 ~1997!.

6 D. R. Lide, CRC Handbook of Chemistry and Physics, 74th ed. ~ChemicalRubber, Boca Raton, FL, 1994!, p. 5-4.

7 It should be noted here that the feasibility of epitaxial growth usuallydepends not only on the thermodynamic properties but also on the kineticissues involved and other process details. In the magnetron sputteringcase, for example, the plasma is highly confined to the sputter source side,i.e., near the target surface. The active nitrogen species are then generatedand distribute mostly near the target surface ~a Ga target is used in thiswork!. This nonuniform distribution of active nitrogen would enhanceproduction of GaN sputtered particle fluxes but not formation of SiN onthe substrate side.

8 N. J. Watkins, G. W. Wicks, and Y. Gao, Appl. Phys. Lett. 75, 2602~1999!.

9 T. Lei, M. Fanciulli, R. J. Molnar, T. D. Moustakas, R. J. Graham, and J.Scanlon, Appl. Phys. Lett. 59, 944 ~1991!.

10 G. A. Martin, B. N. Sverdlov, A. Botchkarev, H. Morkoc, D. J. Smith,S.-C. Y. Chen, W. H. Thompson, and M. H. Nayfeh, Mater. Res. Soc.Symp. Proc. 395, 67 ~1996!.

11 See, for example, R. D. Vispute, V. Talyansky, R. P. Sharma, S. Choopun,M. Downes, T. Venkatesan, K. A. Jones, A. A. Iliadis, M. Asif Khan, andJ. W. Yang, Appl. Phys. Lett. 71, 102 ~1997!.

12 K. S. Stevens, A. Ohtani, A. F. Schwartzman, and R. Beresford, J. Vac.Sci. Technol. B 12, 1186 ~1994!.

13 W. J. Meng and J. Heremans, J. Vac. Sci. Technol. A 10, 1610 ~1992!.14 D. Wang, Y. Hiroyama, M. Tamura, M. Ichikawa, and S. Yoshida, Appl.

Phys. Lett. 77, 1846 ~2000!.15 C. A. Tran, A. Osinski, R. F. Karlicek, and I. Berishev, Appl. Phys. Lett.

75, 1494 ~1999!.16 S. Strite and H. Morkoc, J. Vac. Sci. Technol. B 10, 1237 ~1992!.17 J. Ross and M. Rubin, Mater. Lett. 12, 215 ~1991!; E. Kim, B. Lee, A.

Nahhas, and H. K. Kim, Appl. Phys. Lett. 77, 1747 ~2000!.18 W. J. Meng and T. A. Perry, J. Appl. Phys. 76, 7824 ~1994!.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp