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20wurtzitic-GaN have been demonstrated on ZnO~0001!,7106H-SiC,6,11 MgAl2O4,12 Si~111!,13 GaAs,14 and GaN.15 Thedifficulty in using new substrates lies primarily in the surfacepreparation prior to the GaN growth, as well as the stabilityof the surface during the first stage of growth.

We shall emphasize that the ZnO substrates present nu-merous advantages compared to sapphire such as: smallerlattice and thermal expansion mismatch compared to GaN,alignment of the cleavage planes, same staking order ~2H!,possibility of substrate doping suitable for vertical devices,possibility of the selective removal of the entire substrate,and lattice match with InxGa12xN/InxAlyGa12x2yN hetero-structures.

trast, the O-terminated surface becomes rough after etchingin the same solution, and the etching rate is an order ofmagnitude faster compared to the Zn-terminated surface. Weused the same conditions for growth on both surface polari-ties, i.e., the same III/V flux ratio and the growth tempera-ture, in order to delineate the effect of only the surface po-larity on the quality of GaN. The first problem encounteredon the growth of GaN on ZnO using reactive ammonia mo-lecular beam epitaxy ~RMBE! is the reactivity of ZnO withammonia. This reactivity is dependent on both the growthtemperature ~Tg! and the polarity of ZnO surface.

Figure 1 depicts the photoluminescence ~PL! spectra atroom temperature ~RT! of GaN/ZnO grown on oxygen facefor different substrate temperatures. We noticed that the fullwidth at half maximum ~FWHM! of the band edge peakdecreases with increasing the growth temperature. FWHM of36 meV at RT has been obtained with growth temperaturearound 750 C. At higher growth temperatures, ammonia in-

a!Electronic mail: hamdani@ux1.cso.uiuc.edub!On sabbatical leave at Wright Laboratories under a University Resident

Research Professor program funded by the Air Force Office of ScientificResearch.Effect of buffer layer and substrate suby molecular beam epitaxy of GaN on

F. Hamdani,a) A. E. Botchkarev, H. Tang, W. KimMaterials Research Laboratory and Coordinated Science LUrbana-Champaign, 104 S. Goodwin Avenue, Urbana, Illi

~Received 23 April 1997; accepted for publication 26

We present results on the effect of substrate surface pof GaN epitaxial layers grown on ZnO~0001! subsepitaxy. The possible effects dealing with the disparitbeen eliminated. Photoluminescence and reflectivityface leads to higher quality GaN on ZnO comparedobtained by using different low-temperature AlN, Gresult has been obtained with lattice-matched In0.Institute of Physics. @S0003-6951~97!02647-8#

Gallium nitride and its alloy compounds with aluminumnitride and indium nitride form a direct wide-band-gap semi-conductor family, which is promising for optoelectronic andhigh-temperature or high-power device applications.14 Al-though some 16% lattice mismatch exists, between thesecompounds and sapphire substrate, reasonably good qualityGaN has been obtained by both metalorganic vapor phaseepitaxy ~MOVPE!2,3 and molecular beam epitaxy ~MBE!5techniques. Many group have reported controllable n- andp-type doping by using Si and Mg, respectively, as dopingatoms with AlN or GaN buffer layers. This breakthrough haspermitted the realization of light-emitting diodes ~LED!2 aswell as pulsed and continuous laser devices3 grown onAl2O3.

Despite the progress made, the density of defects is stillconsidered too high2,6 (1010 cm22) to achieve the desiredviolet and blue lasers operating continuously at room tem-perature with the required longevity for digital informationread and write operation, as well as high-power electronicdevices.4 Moreover, the cleavage planes of GaN and sap-phire substrates do not align, and films on sapphire withthickness required for lasers crack. Consequently, growth onalternative substrates which could overcome these difficul-ties is being explored. MBE and MOVPE growth ofAppl. Phys. Lett. 71 (21), 24 November 1997 0003-6951/97/71(21Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tface polarity on the growthZnOand H. Morkocb)

boratory, University of Illinois atis 61801

eptember 1997!

larity, oxygen and zinc faces, on the qualitytes by reactive ammonia molecular beamin surface preparation of the two faces haveeasurements demonstrate that the oxygenthe zinc face. We also present optical data

N, and InxGa12xN buffer layers. The bestGa0.80N buffer layer. 1997 American

We reported in a previous paper ~Ref. 7! a successfulgrowth of GaN epilayers on ZnO substrate without using anybuffer layer. In this letter, we report on an investigation ofthe effect of the buffer layer as well as the surface polarity onthe optical quality of GaN epilayers grown on ZnO. TheGaN epilayer thickness is around 2 mm for all the samplesreported in this study. The ~0001! ZnO substrates weregrown by hydrothermal method. The surface termination oneach sides consist of either group-II ~Zn! atoms; named zincface or group-VI ~oxygen! atoms; named oxygen face. Al-though the zinc face does not have unpaired electrons, theoxygen face does making it more chemically active.

The growth of GaN on polar substrates is strongly de-pendent on the interface polarity between the GaN epilayerand the substrate surface. It has been demonstrated that Si-terminated 6H-SiC~0001! surface is more suitable for theGaN growth compared to the C-terminated surface.11 Al-though the latter can be made smoother. The effect of thesurface polarity of sapphire substrate on the quality of GaNlayers has also been investigated.16 In this work a chemicaletching in nitric acid of ZnO was employed to determine thesurface polarity prior to growth. The Zn-terminated surfacebecomes smoother after a 2 min etch in nitric acid. In con-3111)/3111/3/$10.00 1997 American Institute of Physicso AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

teracts more strongly with ZnO surface. This reactivity leadsto the degradation of the GaN quality as well as peeling offof the GaN layer from the substrate. Moreover, the FWHMof RT-PL increases to 60 meV at Tg5780 C. At evenhigher temperatures, Tg.800 C, ammonia etches the ZnOsurface and no growth of GaN occurs.

The comparison of the low-temperature PL and reflec-tivity data obtained on GaN layers grown on both surfacepolarities indicates that the O surface appears more suitablefor the growth of GaN, since it provides more danglingbonds compared to the Zn-terminated surface. The PL resultsshow that the ratio between the free exciton peak and thedonor bound excitons is two orders of magnitudes higher inthe O-terminated surface case compared to the Zn-terminatedcase. The PL results suggest that the samples grown on theZn-terminated surface of ZnO contain more impurities com-pared to those grown on the O-terminated surface of ZnO.

Figure 2 shows the 4.2 K photoluminescence ~PL! andthe reflectivity data obtained on oxygen-face ZnO~0001! sub-strate at Tg5750 C. The positions of the free excitons areas follows: A exciton ~3476 meV!, B exciton ~3489.4 meV!,and C exciton ~3511.2 meV!. The oscillations observed in

FIG. 1. Room-temperature photoluminescence spectra of GaN/ZnO epilay-ers grown at different substrate temperatures.

FIG. 2. ~a! Differential reflectivity and ~b! photoluminescence spectra at 4.2K of GaN grown on the oxygen face of ZnO. Labels A, B, C, and A0Xdesignate free and bound excitons.3112 Appl. Phys. Lett., Vol. 71, No. 21, 24 November 1997Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tthe reflectivity spectrum at lower energies are due to theinterference fringes between the reflections from the surfaceof GaN and the GaN/ZnO interface. The discontinuity in thefringes occurring in the range between 3.38 and 3.45 eV, isdue to the spectral dispersion of the refractive index of ZnOclose to the energy gap. The PL spectrum shows a peak atthe same energy position as the A exciton in the reflectivityspectra, which allowed us to attribute this peak to the A freeexciton. The free and shallow bound excitons are the onlytransitions observed in these samples. Within the detectionlimit of the measurement setup, no yellow PL signal wasobserved in the low-temperature PL spectra in any of thesamples grown on ZnO~0001! substrates. The room-temperature PL spectra of GaN/ZnO show a very small yel-low with an intensity level three to four orders of magnitudelower compared to the band edge PL peak. The intensity ofthe yellow PL signal measured on our GaN samples grownby RMBE on sapphire substrates is two to three orders ofmagnitude lower compared to the band edge PL peak. It isworth to notice that other groups have already reported eitherthe absence or the low intensity level of the yellow PL signalmeasured on GaN samples grown on sapphire substrates byRMBE.1719

Figure 3 shows the 4.2 K photoluminescence ~PL! andthe reflectivity data obtained on zinc-face ZnO~0001! sub-strate at Tg5750 C. The reflectivity spectrum shows thatonly the A ~3481 meV! and B ~3496 meV! excitons can beresolved. The broadening of the free exciton line is attributedto the lower crystalline quality of GaN grown on zinc-faceZnO, such as structural disorder, defects, and impurities. Incontrast to the O face case, the PL peak position does notmatch the position of A exciton and it is therefore attributedto donor-bound exciton DX with binding energy of 12 meV.The position of free excitons observed in reflectivity spectra,indicates that GaN grown on zinc face is under a more com-pressive stress compared to GaN grown on oxygen face. Webelieve that this strain state is mainly controlled by the inter-face bonding between ZnO and GaN. Two different bondingconfigurations may be possible for each surface polarity, i.e.,for oxygen-face ON or OGa and for zinc-face ZnGa or

FIG. 3. ~a! Reflectivity and ~b! photoluminescence spectra at 4.2 K of GaNgrown on zinc-face ZnO. Labels A, B, C, and D0X , and A0X are given asidentifications of free and bound excitons.Hamdani et al.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

ZnO substrates. Low-temperature thin buffer layers of GaNor In0.20Ga0.80N have been found imperative for a better qual-ity of GaN. Owing to its softness and lattice match to ZnO,In0.20Ga0.80N buffer layers led to higher quality GaN epilay-ers.

This work was funded by grants from the Office of Na-val Research, with Dr. R. G. Brandt, Dr. C. E. Wood, Dr. Y.S. Park, and Mr. M. Yoder monitoring, and the Air ForceOffice of Scientific Research with Dr. G. L. Witt monitoring.One of the authors ~H.M.! acknowledges many insightfuldiscussions on the topic with and encouragement by Dr. C.ZnN. Due to its smallest covalent radii, the ON interfacialbonding is expected to be the stronger among other possibleinterfacial bonding configurations.

We have investigated the effect of various buffer layersin order to optimize the quality of GaN by paying attentionto the reactivity of ammonia with ZnO. Low-temperature(Tg;650 C) buffer layers such as AlN, GaN, andIn0.20Ga0.80N have been used in order to initiate the growthprocess. PL and reflectivity data clearly indicate that the low-temperature buffers grown with radio-frequency-assistedMBE give improved optical quality of GaN compared to thesame buffer layers grown using RMBE. We present in Fig. 4,the reflectivity derivative spectra of GaN grown with differ-ent buffer layers on the oxygen-face of ZnO substrates. Thereflectivity of GaN grown on AlN buffer layer does not showany free exciton indicative of the fact that AlN buffer layersare detrimental for the GaN crystal quality. The smallestFWHM of the three free excitons ~A, B, and C! of GaNepilayer is obtained through the use of a lattice-matchedIn0.20Ga0.80N buffer layer. The positions of these three exci-tons are shifted to lower energies as compared to the resultsobtained so far. This may be an indication that the InGaNbuffer layer relaxes the strain more efficiently by confiningthe dislocations. A transmission electron microscopy study isunder way to investigate the propagation of dislocationsfrom the buffer layer to the GaN.

In conclusion, the investigation of the effect of the sur-face polarity indicates that better quality GaN epilayers canbe obtained on the oxygen face, compared to the zinc face of

FIG. 4. ~a! Reflectivity derivative spectra at 4.2 K of GaN grown withdifferent buffer layers on oxygen-face ZnO: ~a! AlN, ~b! GaN, and ~c!In0.20Ga0.80N buffer layers used. Labels A, B, and C are given as identifica-tions of free excitons.Appl. Phys. Lett., Vol. 71, No. 21, 24 November 1997Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tW. Litton, Dr. D. C. Reynolds, Dr. D. C. Look, and Dr. W.Harsch, and Mr. Gene Cantwell.

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