Polarity control of ZnO on sapphire by varying the MgO bufferlayer thickness

Hiroyuki Kato,a) Kazuhiro Miyamoto, and Michihiro SanoResearch & Development Center, Stanley Electric Co., Ltd., 1-3-1 Eda-Nishi, Aoba-ku,Yokohama 225-0014, Japan

Takafumi YaoCenter for Interdisciplinary Research, Tohoku University, Aza Aoba, Aramaki, Aoba-ku,Sendai 980-8578, Japan

~Received 30 January 2004; accepted 10 April 2004; published online 14 May 2004!

Polarity-controlled ZnO films with an MgO buffer layer were grown on c-plane sapphire byplasma-assisted molecular-beam epitaxy. Convergent beam electron diffraction results showed thatZn-polarity (1c) growth occurred when the MgO layer was thicker than 3 nm, whereas O-polarity(2c) growth occurred when the layer was less than 2 nm. Reflection high-energy electrondiffraction results revealed that MgO growth was StranskiKrastanov mode, and that the growthmode transition from two- to three-dimensional occurred when the layer was thicker than 1 nm. Inconclusion, polarity conversion apparently occurs due to the different atomic structure between thewetting layer and islands of MgO. 2004 American Institute of Physics.@DOI: 10.1063/1.1759377#

ZnO is an attractive material for high-efficiency ultravio-let optical devices due to its wide band gap of 3.37 eV atroom temperature ~RT! and its large excitonic binding energyof about 60 meV. In fact, optically pumped, excitonic lasingfrom ZnO at RT has been reported,1,2 and excitonic stimu-lated emissions have been observed at temperatures as highas 550 K.3

Crystal structure of ZnO is wurtzite, and c-axis ZnO hastwo distinct polar faces: ~0001! Zn- and ~000-1! O faces.During crystal growth, the difference in polarity betweenthese two faces affects the growth mode, impurity incorpo-ration, and dislocation formation. Both Zn- and O-polar ZnOfilms can be grown by using unipolar substrates such as ZnOand GaN.4,5 In the growth of GaN, which has the same crys-tal structure as ZnO, two different polarity GaN films onnonpolar sapphire substrates were obtained by using differ-ent initial processes before film growth, such as surfacenitridation,6 buffer-layer growth,7 and insertion of additionallayers.8 Controlling the polarity of ZnO epilayers on nonpo-lar sapphire substrates is difficult, however, because ZnOfilms usually have O polarity.912 Because crystal polarityarises from lack of inversion symmetry, inverting the crystalpolarity of an epilayer is possible by inserting a layer withinversion symmetry between the epilayer and the substrate.MgO buffer layers have been used to drastically improvecrystal quality of ZnO epilayers on sapphire substrates.13 Be-cause MgO has rock-salt crystal structure, the crystal polarityof a ZnO overlayer will be inverted if the MgO buffer has awell-defined rock-salt structure.

In this study, control of the polarity of ZnO films onc-plane sapphire grown by plasma-assisted molecular-beamepitaxy ~MBE! was achieved by inserting an MgO bufferlayer. The results showed that the polarity changed from 2c~O polarity! to 1c ~Zn polarity!, depending on the thickness

of the MgO buffer layer between the ZnO film and the sap-phire substrate, and revealed that the polarity conversion iscaused by the difference in atomic structure between the wet-ting layer and islands of MgO.

Substrates used in this study were c-plane sapphire. Eachsubstrate was degreased using ultrasonic cleaning in dichlo-romethane and acetone, and was then etched in a chemicalsolution of H3PO4 :H2SO451:3 at 110 C for 30 min. ZnOepilayers were grown on the substrates with ZnO/MgOdouble-buffer layers by plasma-assisted MBE. Elementalzinc ~7N grade!, magnesium ~6N grade!, and oxygen radiofrequency ~rf! plasma (O2 gas with 6N grade! were used asmolecular-beam sources. Prior to film growth, the substratewas cleaned by heating at 800 C for 30 min under ultrahighvacuum (

surface has a single dangling bond. Therefore, the Zn stick-ing coefficient on O-terminated Zn-polar ZnO is higher thanthat on O-polar ZnO, and thus the growth rate of Zn-polarZnO is higher than that of O-polar ZnO. The growth rates ofZnO films that have an MgO buffer thicker than 3 nm werethe same as that of ZnO on Zn-face ZnO. Therefore, thischange in the growth rate apparently caused the conversionfrom O- to Zn-polarity.

The polarity of the ZnO films on c-sapphire substrateswith MgO buffer layers was determined by comparing theconvergent beam electron diffraction ~CBED! patterns mea-sured along the @1-100# axis of ZnO and the simulatedCBED patterns for transmission electron microscopy speci-mens whose thicknesses were 110160 nm. The measuredCBED pattern of the ZnO film with a 1-nm-thick MgObuffer layer @left image in Fig. 2~a!# agreed well with thesimulated pattern for a specimen thickness of 140 nm @rightimage in Fig. 2~a!#. The simulated pattern reveals that epi-taxial growth occurred in the ~000-1! O-terminated orienta-

tion, consistent with growth previously observed by usingcoaxial impact collision ion scattering spectroscopy.12 On theother hand, the measured CBED pattern of the ZnO film witha 6-nm-thick MgO layer showed vertical reversal comparedwith the pattern of the ZnO film with a 1-nm-thick MgObuffer layer, and agreed well with the simulated pattern for athickness of 158 nm @Fig. 2~b!#. This result indicates thatepitaxial growth of ZnO on a 6-nm-thick MgO layer wasclearly evident in the ~0001! Zn-terminated orientation.

To clarify the polarity conversion mechanism, the evo-lution of reflection high-energy electron diffraction~RHEED! patterns during MgO growth are shown in detail inFig. 3. The sapphire substrate showed a sharp streaky pattern@Fig. 3~a!#. Up to 1-nm-thick, MgO showed a streaky pattern@Fig. 3~b!#, indicating two-dimensional ~2D! nucleation andgrowth as a wetting layer. The rod spacing of MgO in theRHEED pattern was about 11.5% smaller than that of sap-phire. When the thickness exceeded 1 nm, the MgO showeda pattern with elongated spots superimposed on streaks @Fig.3~c!#. When the thickness increased further, small spots ap-peared between the elongated spots and then the streaks dis-appeared, indicating three-dimensional ~3D! growth. Whenthe thickness exceeded 6 nm, the spots became dominant,indicating 3D growth as islands @Fig. 3~d!#. The spacing be-tween spots in this spotty RHEED pattern was about 8.5%smaller than the rod spacing of sapphire, consistent with thein-plane lattice mismatch ~8.4%! between rock-saltMgO~111! and sapphire~0001!. The 31-nm-thick MgO film,namely, MgO islands, had ~111! oriented rock-salt structureand the in-plane orientation relationship between MgO andsapphire was MgO@-110#//sapphire@1-100# as confirmed by2uu and f scans of x-ray diffraction measurements. TheRHEED observations in Fig. 3 indicate that MgO growthwas StranskiKrastanov ~SK! mode, and that the growthmode transition from 2D to 3D occurred when the layerthickness exceeded 1 nm. If the MgO wetting layer has rock-salt structure, then the rod spacing of MgO should be lessthan 8.4% smaller than that of sapphire due to the compres-

FIG. 1. Growth rate of ZnO films as a function of MgO buffer layerthickness.

FIG. 2. Measured and simulated CBED patterns of ZnO films grown onc-plane sapphire with ~a! 1-nm-thick and ~b! 6-nm-thick MgO buffer layers.Simulated patterns correspond to ~a! 140-nm and ~b! 158-nm-specimenthickness.

FIG. 3. Evolution of RHEED patterns during MgO growth. ~a! Bare sap-phire, and a ~b! 0.9-nm-thick, ~c! 1.1-nm-thick, and ~d! 6.2-nm-thick MgObuffer layers.

4563Appl. Phys. Lett., Vol. 84, No. 22, 31 May 2004 Kato et al.

Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

sive strain. However, the rod spacing was 11.5% smaller thanthat of sapphire. The distance between surface atoms calcu-lated based on the rod spacing of the RHEED pattern in theMgO wetting layer @Fig. 3~b!# did not show rock-salt struc-ture when the thickness was less than 1 nm. Therefore, theobserved conversion in the polarity of ZnO was caused bythe difference in atomic structure between the wetting layerand the islands of MgO as discussed next.

One possible mechanism for this polarity conversion isthe formation of MgAl2O4 ~spinel! at the interface. MgAl2O4spinel is well known to form at the MgO/Al2O3 interface bya solid-state reaction under electron-beam irradiation15 or athigh temperature above 850 C.16 The in-plane lattice mis-match between ~0001! sapphire and ~111! spinel is about 4%,inconsistent with the 11.5% revealed by our RHEED obser-vation ~Fig. 3! obtained for films grown at lower temperature~600 C!. Therefore, spinal formation at the interface cannotbe the mechanism for the polarity conversion. Another pos-sible mechanism is the formation of wurtzite MgO at theinterface. Because the theoretical lattice parameters of wurtz-ite MgO are a50.3199 nm and c50.5224 nm, 17 the in-plane lattice mismatch between sapphire and wurtzite MgOis about 16%. Because an MgO layer has compressive straindue to this in-plane lattice mismatch, the rod spacing ofwurtzite MgO should be less than 16% smaller than that ofsapphire. As mentioned above, the observed rod spacing ofthe MgO wetting layer was 11.5% smaller than that of sap-phire. Therefore, this mechanism is the most probable.

Figure 4 shows a schematic of the atomic arrangementsof ZnO on c sapphire with two types of MgO buffer layers.Because MgO growth occurs under O-rich flux conditions,growth mainly proceeds at the O-terminated surface. In theinitial growth stage up to 1 nm, wurtzite MgO as a wetting

layer grows on O-terminated sapphire. Because the topmostO atoms in wurtzite MgO have a single dangling bond, eachZn atom in contact with O atoms has three dangling bondsalong the c axis. As a result, the ZnO film has O polarity. Onthe other hand, because the MgO wetting layer has compres-sive strain, the structure changes from wurtzite to rock-saltdue to relaxation as the layer thickness increases. Eventually,when the layer thickness exceeds 3 nm, MgO islands withrock-salt structure covers the entire surface. Because the top-most O atoms in rock-salt MgO have three-dangling bonds,each Zn atom in contact with O atoms has a single danglingbond along the c axis. As a result, the ZnO film on MgO~111! has Zn polarity.

In conclusion, control of the polarity of ZnO films grownon c-plane sapphire substrates by plasma-assisted MBE wasachieved by inserting an MgO buffer layer between the ZnOfilm and the substrate. The thickness of the MgO buffer iskey for achieving polarity-controlled ZnO films on nonpolarsubstrates. The critical thickness in polarity conversion wasabout 2 nm. MgO growth was SK mode, and the growthmode transition from 2D to 3D occurred when the layer wasthicker than 1 nm. The polarity conversion of ZnO is appar-ently caused by the difference in atomic structure betweenthe wetting layer and the islands of MgO.1 D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and T.Goto, Appl. Phys. Lett. 70, 2230 ~1997!.

2 Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohmoto, H. Koinuma,and Y. Segawa, Appl. Phys. Lett. 72, 3270 ~1998!.

3 D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, M. Y. Shen, and T. Goto, Appl.Phys. Lett. 73, 1038 ~1998!.

4 E. S. Hellman, D. N. E. Buchanan, D. Wiesmann, and I. Brener, MRSInternet J. Nitride Semicond. Res. 1, 16 ~1996!.

5 S. K. Hong, Y. Chen, H. J. Ko, H. Wenisch, T. Hanada, and T. Yao, J.Electron. Mater. 30, 647 ~2001!.

6 M. Sumiya, K. Yoshimura, K. Ohtsuka, and S. Fuke, Appl. Phys. Lett. 76,2098 ~2000!.

7 X.-Q. Shen, T. Ide, S.-H. Cho, M. Shimizu, S. Hara, H. Okumura, S.Sonoda, and S. Shimizu, Jpn. J. Appl. Phys., Part 2 39, L16 ~2000!.

8 K. Xu, N. Yano, A. W. Jia, A. Yoshikawa, and K. Takahashi, J. Cryst.Growth 237, 1003 ~2002!.

9 T. Ohnishi, A. Ohtomo, M. Kawaski, K. Takahashi, and M. Toshimoto,Appl. Phys. Lett. 72, 824 ~1998!.

10 F. Vigue, P. Vennegue`s, S. Vezian, M. Laugt, and J.-P. Faurie, Appl. Phys.Lett. 79, 194 ~2001!.

11 Y. Chen, H.-J. Ko, S.-K. Hong, T. Yao, and Y. Segawa, Appl. Phys. Lett.80, 1358 ~2002!.

12 S.-K. Hong, T. Hanada, Y. Chen, H.-J. Ko, T. Yao, D. Imai, K. Araki, andM. Shinohara, Appl. Surf. Sci. 190, 491 ~2002!.

13 Y. Chen, S.-K. Hong, H.-J. Ko, V. Kirshner, H. Wenisch, T. Yao, K. Inaba,and Y. Segawa, Appl. Phys. Lett. 78, 3352 ~2001!.

14 H. Kato, M. Sano, K. Miyamoto, and T. Yao, J. Cryst. Growth 265, 375~2004!.

15 D. X. Li, P. Pirouz, A. H. Heuer, S. Yadavalli, and C. P. Flynn, Philos.Mag. A 65, 403 ~1992!.

16 C. H. Lei, G. V. Tendeloo, J. G. Lisoni, M. Siegert, and J. Schubert, J.Cryst. Growth 226, 419 ~2001!.

17 W. R. L. Lambrecht, S. Limpijunmnong, and B. Segall, MRS Internet J.Nitride Semicond. Res. 4S1, G6.8 ~1999!.

FIG. 4. Schematic of atomic arrangement of ZnO on c-plane sapphire with~a! a 1-nm-thick MgO buffer layer, and with ~b! an MgO buffer layer thickerthan 3 nm.

4564 Appl. Phys. Lett., Vol. 84, No. 22, 31 May 2004 Kato et al.

Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp