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APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 5 31 JANUARY 2000might not be an issue. The most interesting feature of ZnO isthe exciton binding energy, which is 2.4 times larger than thethermal energy at RT and offers the prospect of excitonicemission being utilized at practical device temperatures.With the high oscillator strength associated with excitons,this feature could lead to lower-threshold lasers, higher effi-ciencies, and faster optical device switching.

A number of studies on ZnO film have been performedon sapphire substrates by pulsed-laser deposition2 and oxy-gen plasma-assisted molecular-beam epitaxy3,4 and wurtziteZnO has been dominantly observed. The wurtzite structureinduces piezoelectric field effects by residual strain and thistends to quench the excitonic effects. Although zinc-blendeZnO structures are still hypothetical,5 the ^001& directions inthe zinc-blende structure are free from the piezoelectric field,which offers a plain and attractive platform for exploring thefeatures of excitonic systems without the perturbation field.

In this letter, the possibility of growing zinc-blende ZnOfilms is studied on GaAs~001! substrates and the use of ZnSbuffer layers is proposed to obtain stable ZnO films on the

perature was decreased to 300 C and the surface was moni-tored with reflection high-energy electron diffraction~RHEED!. A ZnS buffer layer with the thickness of 0.14 mmwas grown at 400 C using diethyl zinc ~DEZn! and diter-tiary butyl sulfide sources. The substrate was then heated upto the growth temperature of 550 or 600 C after the ZnSgrowth, and DEZn was supplied again together with oxygenplasma to grow ZnO films on the ZnS buffer layer. Thedetails of the substrate temperature dependence will be re-ported elsewhere, but the main features discussed below arequalitatively common between 550 and 600 C. An electroncyclotron resonance plasma source was used to excite a high-density oxygen plasma with low-ion energies of 1020 eV.4High-purity oxygen gas flow of 2.5 or 3.5 sccm and 2.45GHz microwave power up to 200 W was introduced into theplasma chamber with the magnetic field set to 875 G.

Figure 1~a! shows the RHEED pattern of the ZnO filmwhich was grown directly on the GaAs substrate but wasirradiated with the DEZn flux during the temperature rise tothe growth temperature of 550 C. The pattern reveals ill-defined diffuse streaks, which are characteristic of polycrys-talline structures. It is noted that no irradiation of the DEZna!Electronic mail: mamun@es.hokudai.ac.jpGrowth and characterization of hypoton GaAs001 substrates with ZnS bu

A. B. M. Almamun Ashrafi,a) Akio Ueta, Adrian Aand Ikuo Suemuneb)Research Institute for Electronic Science, Hokkaido UniveJapan

Young-Woo Ok and Tae-Yeon SeongDepartment of Materials Science and Engineering, KwangKwangju 500-712, Korea~Received 2 November 1999; accepted for publicatio

A stable wurtzite phase of ZnO is commonly obsercharacterization of zinc-blende ZnO on GaAs~001! susubstrates using microwave-plasma-assisted mecharacterized by reflection high-energy electron difframicroscope, and atomic force microscope measuremeto lead to the growth of the zinc-blende ZnO filmspolycrystalline with columnar structures, they showtemperature. 2000 American Institute of Physics.

A study of ultrafast nonlinear optical devices and short-wavelength semiconductor lasers has motivated research intolow-dimensional structures of wide band-gap semiconduc-tors. The excitonic nature of these materials has proved to beof particular interest because of the strong nonlinear opticaleffects that can be observed.1 For any of these effects to beof practical use, the excitons must survive at elevated tem-peratures. In this regard, ZnO is the most promising materialbecause of its large band-gap energy of 3.37 eV at roomtemperature ~RT! and the large exciton binding energy of 60meV. Meanwhile, the high cohesive energy and the highmelting point indicate the high bond strength of ZnO, sug-gesting that the degradation of devices during operationb!Electronic mail: isuemune@es.hokudai.ac.jp

5500003-6951/2000/76(5)/550/3/$17.00Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tetical zinc-blende ZnO filmser layersamescu, Hidekazu Kumano,

ty, Kita 12 Nishi 6, Kita-ku, Sapporo 060-0812,

Institute of Science and Technology, (K-JIST),

30 November 1999!

d. In this letter, we report the growth andtrates. The ZnO films grown on GaAs~001!organic molecular-beam epitaxy wereion, x-ray diffraction, transmission electrons. The use of a ZnS buffer layer was foundAlthough the zinc-blende ZnO films wered bright band-edge luminescence at roomS0003-6951~00!01005-6#

GaAs substrates. In the initial growth of the ZnO films onGaAs substrates, oxidation of GaAs surfaces produces amor-phous oxide layers, which prevent the growth of crystallinestructures. The ZnS buffer layer can prevent the formation ofthe amorphous oxide layers and the growth of zinc-blendeZnO will be demonstrated on the ZnS/GaAs substrates.

ZnO films were grown using metalorganic molecular-beam epitaxy. In this study, semi-insulating GaAs~001! sub-strates were chemically cleaned with an etchant ofH2SO4:H2O2:H2O54:1:1 at 60 C. After inserting the sub-strate into the growth chamber, the GaAs surface wascleaned thermally at 550 C under a trisdimethylamino-arsenic flux. After the thermal cleaning, the substrate tem-flux during the substrate temperature rise resulted in no

2000 American Institute of Physicso AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

growth of the ZnO films. On the other hand, the ZnO filmswere successfully grown after the deposition of the ZnSbuffer layer on a GaAs substrate. Figure 1~b! shows theRHEED pattern of the ZnO film grown at the substrate tem-perature of 600 C, which was observed in the @110# direc-tion. The pattern shows that the film is epitaxially grown onthe ZnS buffer layer. Assuming the GaAs lattice constant of5.6533 , that of the ZnO was measured to be 4.46360.015 from the spacing of the spotty pattern.

The ZnO film grown on the ZnS/GaAs was studied byu 2u x-ray diffraction ~XRD! measurement as shown inFig. 2. The XRD plot shows a diffraction peak at 44.70,which is identified to be the ZnO~004!. The lattice constantin the growth direction was estimated to be 4.37 , which isclose to ~but slightly smaller than! the one measured from

FIG. 1. RHEED patterns: ZnO films grown directly ~a! on a GaAs~001!substrate at 550 C and ~b! on a ZnS/GaAs layer at 600 C. The latticeconstant of ZnO was measured to be 4.46360.015 from the spacing ofthe spotty pattern.

Appl. Phys. Lett., Vol. 76, No. 5, 31 January 2000FIG. 2. X-ray diffraction of the ZnO film grown on a ZnS/GaAs layer at600 C. The flow rates of DEZn and oxygen were 2.5631023 Torr and 3.50sccm, respectively. The typical growth rate was 0.32 mm/h. The ZnO filmthickness was 0.46 mm.Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tthe RHEED pattern. This is, however, far different from thatof wurtzite ZnO with 5.21 along the c-axis and 3.25 along the a-axis, but is closer to the calculated zinc-blendeZnO lattice constant of 4.60 .6

Transmission electron microscopy ~TEM! was also per-formed to investigate structural properties of the ZnO grownon the ZnS layer at 550 C. Figure 3 shows a high-resolutionTEM ~HRTEM! lattice image obtained from an interface re-gion including the ZnO and the ZnS with the electron beamaligned along the @1 10#ZnS direction. The image clearlyillustrates the epitaxial relation between the two layers, al-though there exist lattice misfit dislocations at the interfaceas indicated by the arrows. Consistent with the HRTEM re-sults, the transmission electron diffraction ~TED! result ~notshown! showed that both the ZnS buffer layer and ZnO filmhave the same crystal structure ~i.e., the zinc-blende!. TheTED examination showed that the ZnO film is tetragonallydistorted with the in-plane lattice parameter of 4.58 andthe out-of-plane lattice parameter of 4.36 . This gives arelaxed lattice constant of 4.47 , and this also indicates thatthe ZnO film has the zinc-blende structure.

The TEM measurement also showed that the ZnO filmconsists of columnar grains being rotated ;5 regarding theZnS ~and, hence the GaAs substrate!. The surface morphol-ogy assessed by atomic force microscopy ~AFM! indicated

FIG. 3. High-resolution TEM lattice image of an interface region includingthe ZnO and ZnS layers. The film growth temperature was 550 C and therelaxed lattice constant was estimated to be 4.47 .

551Ashrafi et al.FIG. 4. Atomic force microscope image of the ZnO surface and the rmssurface roughness was 5.7 nm. It is noted that the grain boundaries arealigned along the ^110& direction.

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

the presence of such columnar grains. Figure 4 shows anAFM image of the ZnO surface. The measured root-mean-square roughness in the 500 nm3500 nm scan was 5.7 nm.The image reveals the surface structure with grain bound-aries aligned along the @110# and @110# directions. Suchalignment is different from that observed in the hexagonalsurface structure of wurtzite islands,7 indicating the forma-

wavelength of 325 nm was used as an excitation source inthe PL measurements. Ultraviolet emission with peak energyat 3.27 eV and the full width at half maximum of 120 meVwas observed, which is close to the band-gap energy at RT.A RT reflectance measurement showed that the PL peak ex-periences a small stokes shift of ;3 meV. The deep-levelemission at around 2.4 eV was suppressed to below 1/60 ofthe main band-edge peak, which indicates the high opticalquality of the ZnO film grown on the ZnS/GaAs~001! sub-strates.

In conclusion, the growth of a type of zinc-blende ZnOis reported. The formation of the zinc-blende ZnO films wasdiscussed with RHEED, XRD, TEM, and AFM measure-ments. The lattice constant of the zinc-blende ZnO was esti-mated to be 4.47 . The prominent PL peak at 3.27 eV wasobserved and the deep-level emission was suppressed to be-low 1/60 of the main peak at room temperature.

1 E. Hanamura, Phys. Rev. B 37, 1273 ~1988!.2 A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y.Sakurai, Y. Yoshida, T. Yasuda, and Y. Segawa, Appl. Phys. Lett. 72,2466 ~1998!.

3 H.-B. Kang, K. Nakamura, S.-H. Lim, and D. Shindo, Jpn. J. Appl. Phys.,Part 1 37, 781 ~1998!.

4 Y. Chen, D. M. Bagnall, H. Koh, K. Park, K. Hiraga, Z. Zhu, and T. Yao,J. Appl. Phys. 84, 3912 ~1998!.

5 Formation of ZnO nanocrystals in the cubic phase was reported by

FIG. 5. Photoluminescence spectrum of the ZnO film grown at 550 C. TheZnO film thickness was 0.56 mm. Ultraviolet emission at 3.27 eV was domi-nantly observed at room temperature.

552 Appl. Phys. Lett., Vol. 76, No. 5, 31 January 2000 Ashrafi et al.tion of zinc-blende ZnO.Photoluminescence ~PL! measurements were performed

at room temperature on the ZnO/ZnS film grown at 550 Cand the spectrum is shown in Fig. 5. A HeCd laser with aDownloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject telectron-beam-induced oxidation of ZnS surfaces during TEM observa-tions; T. Kogure and Y. Bando, J. Electron Microsc. 47, 7903 ~1993!.

6 E. Jaffe and A. C. Hess, Phys. Rev. B 48, 7903 ~1993!.7 Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koi-numa, and Y. Segawa, Appl. Phys. Lett. 72, 3270 ~1998!.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp