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APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 4 24 JULY 2000temperature excitonic stimulated emission, and high charac-teristic temperature for optical threshold power.4 Such non-linear optical effects would be much more enhanced ifbiexcitons were involved in the optical processes because ofthe giant oscillator strength effect.5 In fact, low-thresholdlasing based on optical processes associated with biexcitonshas been theoretically predicted.6

The formation of biexcitons in bulk ZnO has been al-ready reported.7 However, the formation of biexcitons in epi-taxial layers has not been reported yet, although the epitaxialgrowth of ZnO has been performed on various substratesincluding Al2O3,1 spinel,8 CaF2,9 and epitaxial GaN~epi-GaN!.10 In order to realize the formation of biexcitons,the crystal quality should be high enough to avoid excessscatterings by impurities and crystalline defects before asso-ciation of excitons. We have developed the P-MBE tech-nique employing low-temperature buffer layers to minimizelattice strain in ZnO epitaxial layers. This letter will reportthe formation of biexcitons in high-quality ZnO epitaxiallayers grown on epi-GaN.

ZnO films were grown by P-MBE equipped with an oxy-gen rf-plasma source ~SVT, RF 4.5! and a Zn solid source on4-mm-thick epi-GaN predeposited by metalorganic

Figure 1 shows a reciprocal-space mapping of the ~0002!diffraction spots of a ZnO epilayer and an epi-GaN substratemeasured by high-resolution x-ray diffractometry. The thick-nesses of the ZnO film is about 2 mm. The x-ray diffraction~XRD! intensity ratio of ZnO to epi-GaN is 0.8. Since bothdiffraction peaks of the ZnO layer and epi-GaN are located atthe same v value, the c axes of both layers align well. The

a!Author to whom correspondence should be addressed; electronic mail: FIG. 1. Reciprocal-space mapping of the ~0002! diffraction spots of a ZnOBiexciton emission from high-qualityby plasma-assisted molecular-beam e

H. J. Ko,a) Y. F. Chen, and T. YaoInstitute for Materials Research, Tohoku University, KatahK. Miyajima, A. Yamamoto, and T. GotoGraduate School of Science, Tohoku University, Aramaki,~Received 4 January 2000; accepted for publication 1

We have investigated the optical and structural proepitaxial GaN ~epi-GaN! by plasma-assisted molecubuffer layers. High-resolution x-ray diffraction for bothat crystalline defects in ZnO films have a similarityZnO epilayers grown on epi-GaN is basically determispectrum at 10 K exhibits very sharp exciton emdeep-level emission is negligible, indicative of smalla free-exciton emission line in the low-excitation regband due to biexcitons at its low-energy side as theemission band emerges even under the intermediate etimes smaller than the previously reported threshold festimated to be 15 meV, in agreement with previousemission line due to excitonexciton scattering domInstitute of Physics. @S0003-6951~00!01830-1#

Recent progress in the growth technique for ZnO, inparticular, by plasma-assisted molecular-beam epitaxy ~P-MBE! using an oxygen plasma source1 has enabled thegrowth of high-quality epitaxial layers enough to demon-strate excitonic optical properties in the ultraviolet range,including room-temperature excitonic lasing,2 high-koko@imr.tohoku.ac.jp

5370003-6951/2000/77(4)/537/3/$17.00Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tnO films grown on epitaxial GaNitaxy

a, Aoba-Ku, Sendai 980-8577, Japan

ba-Ku, Sendai 980-8578, Japan

une 2000!

rties of high-quality ZnO films grown on-beam epitaxy employing low-temperaturesymmetric and asymmetric reflexes showsepi-GaN used as a substrate. The quality ofd by epi-GaN. The photoluminescence ~PL!ion with a linewidth of 1.5 meV, whilesidual strain. At 77 K, PL is dominated bye, while it is overtaken by a new emissioncitation intensity increases. This biexcitonitation regime of 100 W/cm2, which is 100bulk ZnO. The biexciton binding energy is

esults. At the higher excitation regime, theates the PL spectrum. 2000 American

chemical-vapor deposition on (0001)Al2O3. A low-temperature ZnO buffer layer was deposited at 300 C fol-lowed by high-temperature annealing at 800 C. ZnO filmswere grown at 700 C. Details of the P-MBE of ZnO withlow-temperature buffer layers have been discussedelsewhere.11epilayer and epi-GaN substrate. The intensities are plotted logarithmically.

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

full width at half maximum ~FWHM! of the ~0002! diffrac-tion spots for the v/2u scan is nearly the same for the twolayers: 19 arcsec for the ZnO film and 13 arcsec for epi-GaN.The FWHM value for ZnO is even narrower than the bestvalues so far reported on ZnO layers (32 arcsec).12 Further-more, the diffraction curves were found to be symmetric inline shape, which suggests a uniform distribution of latticestrain. This is in contrast to the observed asymmetric lineshapes of diffraction peaks from ZnO layers grown onsapphire1 and (111)CaF2 ~Ref. 12! substrates.

The FWHM values along the v direction ~Dv! of the~0002! reflexes for the ZnO film and epi-GaN are 5.08 arc-min and 5.12 arcmin, respectively. Asymmetric ~1011! re-flexes were measured in a f scan, which is sensitive to thein-plane distribution of the lattice parameter. The measuredFWHM values of the ~1011! reflexes of the ZnO film andepi-GaN are 9 and 7.2 arcmin, respectively. It is noted thatthe linewidth of the ~1011! diffraction peak of the ZnO filmis narrower than the corresponding linewidth of ZnO epilay-ers so far reported.10,13

The linewidth of the asymmetric ~1011! diffraction issensitive to pure edge-threading dislocations, while thesedislocations have only secondary affects on the linewidth ofthe symmetric ~0002! diffraction.14 As has been reported,15pure edge-threading dislocations were dominant in GaNfilms grown on c sapphire, in which the threading disloca-tions run along the @0001# direction. This may partly explainthe observed narrower linewidth for the ~0002! diffractioncompared to the ~1011! diffraction. It is noted that the line-widths of the ~1011! peaks are about two times larger thanthe ~0002! peaks, which suggests that crystalline defects inZnO and GaN have similar features. Hence, the quality of theZnO films grown on epi-GaN is basically limited by thequality of epi-GaN used as a substrate.

Figure 2 shows the low-temperature photoluminescence~PL! spectrum of the ZnO layer at 10 K excited by the 325

FIG. 2. Low-temperature PL at 10 K from a ZnO film grown on epi-GaNwith a low-temperature buffer layer. The inset shows details of the PL andreflectance spectrum in the near-band-edge region.

538 Appl. Phys. Lett., Vol. 77, No. 4, 24 July 2000nm line of a HeCd laser. The monochromator used for low-excitation PL measurements has a grating of 1200grooves/mm with a blaze wavelength of 400 nm. AlthoughDownloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tthe spectral response was not calibrated, the sensitivity ataround 540 nm was 0.67 referring that at 400 nm as unity.The inset shows details of the near-band-edge emission andreflectance spectrum. It is remarkable that the deep-levelemission band at around 2.3 eV is hardly observed even inthe logarithmic plot. The linewidth of the dominant excitonicemission lines at 3.367 and 3.360 eV are as narrow as 1.5and 2.2 meV, respectively. Both emission lines can be attrib-uted to exciton emission bound to neutral donors, I2 ~Ref.16! and I4 ,17 respectively. As the linewidth of the excitonicemission in low-temperature PL is sensitive to local strain inthe layers, such a narrow linewidth is a result of small re-sidual strain. We note that the observed linewidth is narrow-est among the ZnO epitaxial layers so far reported. A smallhump observed at around 3.373 eV is due to the A excitontransition, as confirmed by comparison with the reflectancespectrum. The reflectance spectrum shows the sharp featuresof the optical transitions associated with A (XA) and B (XB)excitons located at 3.376 and 3.390 eV, respectively. Thebinding energies of the excitons at neutral donors are esti-mated to be 8.3 and 14.1 meV for the I2 and I4 emissionlines, respectively. These values agree well with the reportedvalues.16,17

Figure 3 shows PL spectra under various excitation in-tensities at 77 K. The high-excitation PL spectra were mea-sured using the frequency-tripled line ~355 nm! of aNd:YAG laser ~10 Hz, 7 ns!. Emission from the sample isdispersed by a 150 mm single monochromator and detectedby a charge-coupled-device camera. Figure 3~a! shows thelow-excitation PL spectrum of the ZnO excited by theHeCd laser. The dominant free-exciton emission is ob-served at 3.370 eV with a low-energy tail due to overlappingof bound exciton emissions (I2 :3.362 eV, I4 :3.357 eV!. Theemission peaks at 3.309 and 3.235 eV originate from the

FIG. 3. Normalized PL spectra of a ZnO epilayer for various excitationintensities at 77 K.

Ko et al.radiative recombination of free excitons associated with1-LO and 2-LO phonons, respectively. As the excitation in-tensity increases above 150 W/cm2, a new emission peak

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denoted by M appears at 3.35 eV in the low-energy side ofthe free-exciton emission line @Fig. 3~b!#. With further in-crease in excitation intensity above 470 W/cm2, a secondpeak denoted by P emerges at around 3.32 eV @Fig. 3~c!#.The P emission band shifts to lower energy as the excitationintensity increases @Figs. 3~c! and 3~d!#. According to theprevious assignments,2,18 the P band originates from inelasticscattering between free excitons. As a result of suchexcitonexciton scattering, one exciton is excited into ahigher state (n52,3,4,...,), while the other exciton annihi-lates with emitting a photon whose energy is roughly locatedin between 3.328 eV (n52) and 3.308 eV (n5). The

the well-resolved sharp exciton emission of I2 and I4 withlinewidths of 1.5 and 2.2 meV are dominant and that thedeep-level emission at around 2.3 eV was negligible. Biex-citon emission in high-excitation PL at 77 K has been ob-served from high-quality ZnO epilayers. The biexciton bind-ing energy is estimated to be 15 meV in agreement with theprevious results. The biexciton emission band emerges at anexcitation intensity as low as 100 W/cm2, which is 100 timessmaller than the reported optical threshold for biexcitonemission from bulk ZnO.

The authors wish to thank Dr. H. Weinisch for discus-

539Appl. Phys. Lett., Vol. 77, No. 4, 24 July 2000 Ko et al.integrated PL intensity of the M band increases superlinearlywith increasing excitation density. This result indicates thatthe M band is associated with a biexciton state. The bindingenergy of the biexciton is estimated to be 15 meV, whichagrees well with the reported value, 14.7 meV.7 We note thatthe M band emerges at an excitation intensity as low as 100W/cm2, which is 100 times smaller than the reported opticalthreshold of 10 kW/cm2 for biexciton emission from bulkZnO.19 This difference is tentatively ascribed to the differ-ence in quality of the materials. The kinetics of excitons withincrease in excitation intensity would be qualitatively under-stood as follows. As the excitation intensity increases, theprobability of association of excitons becomes frequent,which leads to the formation of biexcitons. With further in-crease in excitation intensity, the kinetic energy of some ofthe excitons becomes higher than the biexciton binding en-ergy, which would enhance inelastic excitonexciton scatter-ing. Hence, the M band is gradually taken over by the Pband, as the excitation intensity increases further. Those ob-served features support the conjecture that the M band origi-nates from the biexciton state. We stress again that the ob-servation of the M band is as a result of the high quality ofthe ZnO epilayer.

In conclusion, the quality of the ZnO films grown onepi-GaN was assessed by high-resolution XRD and low-temperature PL. The linewidths of the XRD for symmetricand asymmetric reflection planes showed that the crystallinedefects both in the ZnO films and in the epi-GaN used as asubstrate have similar features. The quality of the ZnO epil-ayers grown on epi-GaN is basically limited by the epi-GaNquality. The low-temperature PL spectrum of the ZnO filmswas composed of exciton emission at 3.367 eV (I2), 3.360eV (I4), and 3.373 eV (XA). The PL spectrum shows thatDownloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tsions on the x-ray results. The authors thank Dr. TakayoshiMaeda in Sumitomo Chemicals Co., Ltd., for supporting thisresearch.

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