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APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 5 3 FEBRUARY 2003x 12xjusted from 3.3 to 4.0 eV. The current research onMgxZn12xO thin film growth is concentrated on the growthon ~0001! C-plane sapphire and ~0001! ScAlMgO4 substratesusing pulsed-laser deposition and laser molecular-beam epi-taxy technologies.58 Only limited information is availableon the growth of MgxZn12xO thin film on C-plane sapphireusing metalorganic vapor-phase epitaxy.9 Metalorganicchemical-vapor deposition ~MOCVD! technique has the ad-vantages of large-area deposition and offers high throughputdue to higher growth rates and hence is promising for indus-trial applications.

In single crystal bulk ZnO, strong optical anisotropy ex-ists at near band gap photon energies due to the differentselection rules for light polarized parallel and perpendicularto the c axis.10 In a ZnO film epitaxially grown on a R-planesapphire, the lattice mismatch along the c axis @0001# of ZnOis only 1.53%, while the mismatch perpendicular to the caxis and in the plane of the film ~i.e., along the @1100#

sensitive devices operating in the UV range.MgxZn12xO films were grown on R-plane sapphire sub-

strates in a vertical, rotating disk MOCVD reactor. The de-tails of the MOCVD growth system were reportedpreviously.11 Diethyl zinc, bis-methylcyclopentadienyl mag-nesium, and O2 were used as precursor materials in a streamof argon carrier gas. Substrate temperatures ranged from 380to 400 C. Typical growth rates for ZnO films were around1.21.5 mm/h, whereas the low vapor pressure of Mg re-duces the overall growth rate for MgxZn12xO films to around0.60.7 mm/h. The sapphire substrates are nominal R-plane(0112) wafers obtained from a commercial vendor. Thecrystallographic orientation and structural quality of the as-grown films were determined using a Bruker D8-Discoverfour-circle x-ray diffractometer with four bounce Ge ~220!-monochromated Cu Ka1 radiation and either 2 or 0.2 mmslits on the detector arm. The surface morphology of thefilms was characterized using a Leo-Zeiss field-emissionscanning electron microscopy ~FESEM! and Rutherfordbackscattering ~RBS! analysis was used to determine the Mgcomposition in the films.a!Author to whom correspondence should be addressed; electronic mail:Growth and structural analysis of metdeposited 112 0 MgxZn1xO 0xR-plane Al2O3 substrates

S. Muthukumar, J. Zhong, Y. Chen, and Y. Lua)School of Engineering, Rutgers University, Piscataway, NeT. SiegristBell Laboratories, Lucent Technologies, 600 Mountain Ave

~Received 3 October 2002; accepted 10 December 20

MgxZn12xO (0,x,0.33) thin films were grownmetalorganic chemical vapor deposition. It was foundthickness of ;50 is needed to achieve wurtzite-typx-ray Dv(1120) rocking curve and D2u(1120) full wwere measured to be 0.275 and 0.18, respectively,films. In-plane reflections show the lower lattice mismon R-plane sapphire. Optical transmission spectra indAmerican Institute of Physics. @DOI: 10.1063/1.1541

ZnO has a direct energy band gap of ;3.3 eV at roomtemperature. In comparison to GaN, ZnO has a higher freeexciton binding energy of 60 meV. It is more resistant toradiation damage.1 Furthermore, epitaxial films of ZnO canbe grown at much lower temperatures, around 400 C. Opti-cally pumped lasing in the UV range at room temperaturehas been reported in ZnO thin films.2,3 ZnO films are thuspromising for UV photodetectors due to their wide and directband gap and large photoresponse, and it has been shownthat such a detector can achieve high-speed operation.4 Inapplications for UV optoelectronics, one of the key issuesthat needs to be addressed is the growth of heterostructuresand quantum wells for energy band engineering. This can beachieved by growth of wurtzite-type MgxZn12xO solid solu-tion thin films. By varying the Mg (0,x,0.33) composi-ylu@ece.rutgers.edu

7420003-6951/2003/82(5)/742/3/$20.00Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tlorganic chemical vapor33 films on 011 2

Jersey 08854-8058

e, Murray Hill, New Jersey 07974

2!

n R-plane (0112) sapphire substrate byat a thin ZnO buffer layer with a minimum

MgxZn12xO films on R-plane sapphire. Thedth at half maximum for Mg0.18Zn0.82O filmdicating strong mosaicity and strain in thech along the c axis of the MgxZn12xO filmsate the good quality of the films. 200350#

direction! is about 18.3%. Table I lists the structural andlattice parameters parallel and perpendicular to the c axis ofZnO film on R-plane sapphire. The resulting in-plane aniso-tropic strain in the film is expected to influence its physicalproperties. Furthermore, the in-plane optical anisotropy inthis system11 can be used to design polarization sensitivedevices, such as an optically addressed high-contrast UVlight modulator.12 MgxZn12xO (0,x,0.33) films and het-erostructures with ZnO would be suitable for producing de-vices that operate at even shorter wavelengths.

In this letter we report the growth and characterization ofMgxZn12xO (0,x,0.33) thin films on R-plane (0112)sapphire substrate using MOCVD techniques. The structuralanalysis of MgxZn12xO films on R-plane sapphire indicatesthat this system would be useful for designing polarizationThe MgxZn12xO films studied here were between 0.6

2003 American Institute of Physicso AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

FIG. 1. RutherfordMg0.3Zn0.7O film. Thelation with experimen

i!

Downloaded 19 Deand 0.7 mm thick. RBS analysis of the samples was per-formed using a probe beam of 1.99 MeV He1 ions and astandard surface detector positioned at 165. Figure 1 showsthe RBS spectra of a randomly aligned Mg0.3Zn0.7O film.The Mg composition is determined from fitting the simulatedprofile13 to the experimental data. The MgxZn12xO filmsgrown on R-plane sapphire are smooth and dense as seen inthe FESEM image shown in Fig. 2.

The room temperature transmission spectra were mea-sured with a UV-visible spectrometer. The absorption coeffi-cient a is calculated after excluding the substrate reflectionloss. A simplified energy band edge model a2}(hv2Eg) isadopted to determine the energy band gap (Eg) ofMgxZn12xO for various Mg compositions. Figure 3 shows aplot of a2 versus photon energy (hv) for MgxZn12xO filmswith x50, 0.18, 0.25, 0.3, respectively. The inset of Fig. 3shows the deduced energy band gap (Eg) of MgxZn12xO asa function of Mg composition. The sharp absorption edge inFig. 3 confirms the good optical property of the MgxZn12xOfilms. The slight deviation from the linear relationship resultsprimarily from the errors of Mg compositions determined byRBS. With the increase of the Mg composition, the changesin slopes of the absorption edges also show alloy-broadeningeffect.

The MgxZn12xO (x

Mg0.3Zn0.7O film (a53.2537 ) compared to that of theZnO film (a53.2480 ) grown on R-sapphire substrate. Thefull width at half maximum ~FWHM! of the D2u (1120)and D (1120) for the Mg0.18Zn0.82O film was measured tobe 0.18 and 0.275, respectively while for the Mg0.3Zn0.7Ofilm the values were measured to be 0.18 and 0.38, respec-tively. The broadening in D (1120) rocking curve FWHMindicates increased mosaicity of the films with higher Mgconcentration. These results are comparable to those mea-sured for ZnO films of similar thickness grown onR-sapphire substrates.11,15

For a better evaluation of the crystal quality of the films,in-plane reflections were also characterized. Table II lists the

FIG. 3. A plot of a2 vs photon energy for MgxZn12xO films with differentMg compositions (x50, 0.18, 0.25, 0.3!. The inset shows the deduced en-ergy band gap as a function of Mg composition.

FIG. 4. X-ray measurements of ~a! 2u-v scan of Mg0.3Zn0.7O film grown onR-sapphire substrate, and ~b! w scan of the $1011% and $1010% family of

744 Appl. Phys. Lett., Vol. 82, No. 5, 3 February 2003planes from Mg0.3Zn0.7O film. The zero of the w axis is aligned along the@0111# direction of the sapphire substrate.

Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject trocking curve ~D! FWHM measured of various reflectionsof Mg0.18Zn0.82O film. The FWHM values were averagedover both asymmetric positions ~6 configurations! to reduceerrors due to absorption and geometric alignment. The rock-ing curve FWHM decreased slightly for reflections that arecloser to the c axis, where the lattice mismatch is lowest.Similarly, for the asymmetric reflections (3030) and (3032)which have nearly the same angle of inclination with respectto the symmetric surface plane (1120), the FWHM de-creased with increasing c-axis component in the reflection.These findings are similar to the in-plane x-ray characteris-tics observed for pure ZnO films grown on R-sapphire sub-strates.

In summary, MgxZn12xO(0,x,0.33) films have beengrown on (0112) R-sapphire substrates using MOCVD. AZnO buffer layer with a minimum thickness of 50 isneeded to prevent the loss of wurtzite-type crystal structurein MgxZn12xO films epitaxially grown on R-sapphire sub-strates. The results of the x-ray diffraction and transmissionspectra show the films thus obtained are of good quality.

The authors acknowledge Dr. H. Schultze of the Depart-ment of Physics and Astronomy, Rutgers University, for helpwith RBS measurement and analysis. This work was sup-ported by the National Science Foundation through GrantNo. ECS-0088549.

1 D. C. Look, Mater. Sci. Eng., B 80, 383 ~2001!.2 D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, M. Y. Shen, and T. Goto, Appl.Phys. Lett. 73, 1038 ~1998!.

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

4 S. Liang, H. Sheng, Z. Huo, Y. Liu, Y. Lu, and H. Shen, J. Cryst. Growth2252-4, 110 ~2001!.

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

6 T. Makino, C. H. Chia, N. T. Tuan, H. D. Sun, Y. Segawa, M. Kawasaki,A. Ohtomo, K. Tamura, and H. Koinuma, Appl. Phys. Lett. 77, 975~2000!.

7 T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura,T. Yasuda, and H. Koinuma, Appl. Phys. Lett. 78, 1237 ~2001!.

8 A. Ohtomo, M. Kawasaki, I. Ohkubo, H. Koinuma, T. Yasuda, and Y.Segawa, Appl. Phys. Lett. 75, 980 ~1999!.

9 W. I. Park, G. C. Yi, and H. M. Jang, Appl. Phys. Lett. 79, 2022 ~2001!.10 W. Y. Liang and A. D. Yoffe, Phys. Rev. Lett. 20, 59 ~1968!.11 C. R. Gorla, N. W. Emanetoglu, S. Liang, W. E. Mayo, M. Wraback, H.

Shen, and Y. Lu, J. Appl. Phys. 85, 2595 ~1999!.12 M. Wraback, H. Shen, S. Liang, and Y. Lu, Appl. Phys. Lett. 74, 507

~1999!.13 W. N. Lennard and C. P. McNorgan, QUARK RBS program version 1.3,

www.quarksimulation.com14 P. A. Stampe, M. Bullock, W. P. Tucker, and R. J. Kennedy, J. Phys. D 32,

1778 ~1999!.

TABLE II. X-ray rocking curve ~D! FWHM measured along the variousreflections of the Mg0.18Zn0.82O film.

Reflection Dv FWHM ~degrees!Angle to the

(1120) plane ~degrees!(1120) 0.275 0(2131) 0.301 15.82(3030) 0.305 30.00(2133) 0.221 33.15(3032) 0.255 35.44

Muthukumar et al.15 B. P. Zhang, Y. Segawa, K. Wakatsuki, Y. Kashiwaba, and K. Haga, Appl.Phys. Lett. 79, 3953 ~2001!.

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