Reduced defect densities in the ZnO epilayer grown on Si substratesby laser-assisted molecular-beam epitaxy using a ZnSepitaxial buffer layer

T. Onuma, S. F. Chichibu,a) and A. UedonoInstitute of Applied Physics and Graduate School of Pure and Applied Sciences, University of Tsukuba,1-1-1 Tennodai, Tsukuba 305-8573, Japan

Y.-Z. Yoo and T. ChikyowCOMET-NIMS, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japanand Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,Midori-ku, Yokohama 226-8503, Japan

T. SotaDepartment of Electrical Engineering and Bioscience, Waseda University, 3-4-1 Ohkubo,Shinjuku 169-8555, JapanM. KawasakiInstitute for Materials Research, Tohoku University, Sendai 980-8755, Japanand Combinatorial Materials Exploration and Technology (COMET), Tsukuba 305-0044, JapanH. KoinumaMaterials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,Midori-ku, Yokohama 226-8503, Japan and Combinatorial Materials Exploration and Technology(COMET), Tsukuba 305-0044, Japan(Received 6 July 2004; accepted 13 October 2004)

Nonradiative photoluminescence (PL) lifetime stnrd and point defect density in the s0001d ZnOepilayer grown on s111d Si substrates by laser-assisted molecular-beam epitaxy (L-MBE) using as0001d ZnS epitaxial buffer layer were compared with those in the ZnO films on s111d and s001d Sisubstrates prepared by direct transformation of ZnS epilayers on Si by thermal oxidation [Yoo et al.,Appl. Phys. Lett. 78, 616 (2001)]. Both the ZnO films exhibited excitonic reflectance anomalies andcorresponding PL peaks at low temperature, and the density or size of vacancy-type point defects(Zn vacancies), which were measured by the monoenergetic positron annihilation measurement, inthe L-MBE epilayer was lower than that in the films prepared by the oxidation transformation. TheZnO epilayer grown on a s0001d ZnS epitaxial buffer on s111d Si exhibited longer tnr of 105 ps atroom temperature. 2004 American Institute of Physics. [DOI: 10.1063/1.1832734]

ZnO is an excellent candidate for use in visible and ul-traviolet light emitters, due to its large band gap energys3.37 eVd, and large exciton binding energy s59 meVd.1 Theuse of Si substrate may enable one to integrate ZnO-basedoptoelectronic devices with Si technologies. However, thegrowth of ZnO on Si is difficult due to the formation of anamorphous SiO2 at the ZnO/Si interface. Therefore, there areonly a few reports on the growth of single crystalline ZnOfilms on (111) Si using electron-beam (EB) evaporation,2,3laser-assisted molecular beam epitaxy (L-MBE),4 metalor-ganic vapor phase epitaxy,5 and molecular beam epitaxy.6Miyake et al.2 have grown ZnO films on Si substrates by EBevaporation using ZnS buffer layers, and also prepared ZnOfilms by direct transformation of epitaxial ZnS films on s111dSi by thermal oxidation.3 Yoo et al.4 expanded these tech-niques growing ZnS epilayers on s001d and s111d Si sub-strates by L-MBE followed by the oxidation transformationinto ZnO.

According to the advantages of using a combination oftime-resolved photoluminescence (TRPL) and positron-annihilation7 measurements, one can discuss the correlationbetween the photoluminescence (PL) lifetime stPLd, which isdominated by the nonradiative lifetime stnrd at elevated tem-peratures, and density or size of vacancy-type defects insemiconductors. The authors have investigated bulk and ep-itaxial ZnO using this complementary method8 and con-cluded that nonradiative recombination processes are notsolely governed by single point defects, but by certain defectspecies introduced by the presence of Zn vacancies sVZndsuch as vacancy complexes.

In this letter, results of optical reflectance (OR), PL,TRPL, and monoenergetic positron annihilation measure-ments on the ZnO films on Si substrates are shown to visu-alize the reduction of nonradiative defect and point defectdensities in the L-MBE films using a ZnS epitaxial bufferlayer. The results are compared with those obtained for theZnO films transformed from ZnS by thermal oxidation, bulkZnO single crystal,9 and ZnO epilayers grown by L-MBE onthe nearly lattice matched ScAlMgO4 (SCAM) substrates.8

The ZnO films investigated were (i) a 300-nm-thicks0001d ZnO epilayer grown on a 50-nm-thick s0001d ZnSepitaxial buffer layer prepared on a s111d Si substrate, (ii) a

a)Author to whom correspondence should be addressed; also at:Photodynamics Research Center, Institute of Physical and ChemicalResearch (RIKEN), Sendai, 980-0868, Japan; electronic mail:optoelec@bk.tsukuba.ac.jp

APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 23 6 DECEMBER 2004

0003-6951/2004/85(23)/5586/3/$22.00 2004 American Institute of Physics5586Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

100-nm-thick s0001d ZnO film on a s111d Si substrate pre-pared by the transformation of a 350-nm-thick ZnS epilayerby thermal oxidation, and (iii) a 90-nm-thick s1011d ZnOfilm on a s001d Si substrate prepared by the transformation ofa 300-nm-thick s001d ZnS epilayer by thermal oxidation.The ZnO and ZnS epilayers were grown by L-MBE using aKrF excimer laser,4 and the thermal oxidation for the lattertwo samples was carried out in an O2 ambient at 900C for1 h. It should be noted that the thickness of the two trans-formed ZnO films s90100 nmd was decreased from the ini-tial ZnS film thickness s300350 nmd; this sublimationproblem is one of the characteristics of this method. Theformer two s0001d ZnO films on s111d Si exhibited thewurtzite structure with sixfold symmetry having in-plane ep-itaxial relation ZnO f1100g iSi f110g. The s001d ZnS epil-ayer on s001d Si was transformed to f1011g-oriented wurtziteZnO film, in which the c axis is 56 tilted from the substratenormal. The growth details will be found elsewhere.4

PL and OR spectra of the ZnO/Si films at 8 K are shownin Fig. 1. The PL was excited using the 325.0 nm line of acw HeCd laser s2 mWd. Considering the large absorptioncoefficient of ZnO (.53105 cm1 at 325 nm), the penetra-tion depth of the laser is estimated to be 20 nm. For com-parison, polarized OR spectra of the bulk ZnO single crystal9are also shown [upper two traces (i) and (ii)]. Due to the C6vsymmetry, ZnO exhibits an optical anisotropy911 for transi-tions from three separate valence bands to the conductionband. Three excitons are referred to as A, B, and C excitonsin order of increasing the transition energy.10 The A and Bexciton transitions are allowed for light polarization E per-pendicular to the c axis sEcd, where E is the electric fieldcomponent, and the C transition is allowed for E parallel tothe c axis sE icd. All ZnO/Si films exhibited excitonicanomalies in their OR spectra at around 3.373.39 eV inaddition to the internal multiple reflection fringes. Althoughthe s0001d ZnO epilayers suffer from certain biaxial tensile

strain due to the in-plane lattice-mismatch s0.15%d, the en-ergies of the anomalies nearly agreed with those of free Aand B excitons at 3.377 and 3.386 eV,911 reflecting a weakstrain dependency of A and B exciton energies in ZnO.12 Thespectral broadening in the excitonic resonances is due to thepartial relaxation of the residual lattice strain and crystal im-perfections. Indeed, the full width at half maximum(FWHM) value of the x-ray s0002d ZnO diffraction peak,which reflect the distribution of out-plane lattice constant,were as large as 0.16 for ZnO/ZnS/ s111d Si and 0.19 forZnO/ s111d Si. The OR spectrum of the f1011g-orientedZnO/ s001d Si film exhibited an additional reflectanceanomaly due to C-exciton resonance at 3.42 eV,911 asshown in Fig. 1(c). It should be noted that the sample suffersfrom triaxial anisotropic strains, and the crystal symmetry isno longer C6v.

PL spectra of the samples exhibited a peak at 3.356 eVand noticeable higher energy shoulders at around 3.37 and3.39 eV. They are assigned as being due to the radiativerecombination of excitons bound to neutral In donor13 and aconvolution of free A and B exciton emissions, respectively.The results indicate that the qualities of present ZnO filmsare high enough to observe excitons. However, a dominantdeep emission band at 2.4 eV (green luminescence: GL) wasobserved, and the FWHM values of the near-band-edge(NBE) emission peak were as large as 130150 meV at293 K (data not shown). The NBE emission peak energies at293 K were 3.28 eV for ZnO/ZnS/ s111d Si and 3.25 eV forZnO/ s111d Si and ZnO/ s001d Si. They were 2050 meVlower than the free A and B exciton emission energys3.3 eVd at 300 K,8 implying the presence of certain boundstates originating from impurities or defects.

To compare the nonradiative defect density in the films,TRPL signals of the NBE peak were measured at 293 K,since tPL at room temperature is in general governed by tnr.The PL was excited by a frequency-tripled mode-lockedTi:sapphire laser (276 nm, 100 fs), and the signal was col-lected using a standard streak-camera acquisition system.The TRPL signals exhibited a biexponential behavior, asshown in Fig. 2. Both the fast and slow decay components (t1 and t2, respectively) may represent the nonradiative freecarrier lifetimes in ZnO. As summarized in Table I, t1 and t2values of the ZnO/Si films s23105 psd were much shorterthan those of the bulk ZnO single crystal (t1=970 ps andt2=14 ns).

8 The results indicate the presence of high density

FIG. 1. PL and OR spectra measured at 8 K of (a)s0001d ZnO/ s0001dZnS/ s111d Si grown by L-MBE, and (b)s0001d ZnO/ s111d Si and (c) s1011d ZnO/ s001d Si films prepared by trans-formation of ZnS by the thermal oxidation. For comparison, polarized ORspectra of the bulk ZnO single crystal are also shown in the insets (i) and (ii)(after Ref. 9).

FIG. 2. TRPL signals measured at 293 K of the near-band-edge emission at3.28 eV for the s0001d ZnO/ s0001dZnS/ s111d Si epilayer and that at3.25 eV for the s0001d ZnO/ s111d Si and s1011d ZnO/ s001d Si films pre-pared by the oxidation transformation. The curve denoted by system rep-resents the gross system response.

Appl. Phys. Lett., Vol. 85, No. 23, 6 December 2004 Onuma et al. 5587

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nonradiative defects. However, as shown in Fig. 2 and TableI, t1 and t2 values for the s0001d ZnO/ZnS/ s111d Si epil-ayer were increased by a factor of approximately 2, indicat-ing the reduced nonradiative defect density. Note that t2 ofZnO/ZnS/ s111d Si s105 psd was comparable to tPL of thehigh-quality ZnO epilayer grown by L-MBE on the SCAMsubstrate s110 psd.8

The monoenergetic positron beam line8 was used to de-tect point defects in the ZnO films as a function of depth. Apositron implanted into a condensed matter annihilates withan electron and emits two 511 keV g rays, which are broad-ened due to the momentum component of the annihilatingelectronpositron pair. Because the momentum distributionof electrons in neutral or negatively charged vacancy-typedefects (positron trapping centers) differs from that in defect-free regions, these defects can be detected by measuring theDoppler broadening spectra of annihilation radiation. The re-sulting change in the spectra is characterized by the Sparameter,7,8 which reflects the change due to the annihila-tion of electronpositron pairs with low momentum distribu-tion. Since VZn is one of the most probable positron trappingcenters in ZnO,8 S parameter can be used as a measure ofsize/concentration of VZn. On the other hand, the positrondiffusion length sLdd decreases with increasing the gross den-sity of positron trapping and scattering centers such as inter-stitials and neutral or positively charged defects [Zn intersti-tials sZnid, oxygen interstitials sOid, and oxygen vacanciessVOd]. The physical backgrounds and experimental condi-tions have been given in Refs. 7 and 8. The S parameterswere measured as a function of incident positron energy E,which define the mean positron implantation depth. The Svalue of a particular film and Ld were determined from theSE curve (the data are not shown, but are similar to those inRef. 8), and are summarized in Table I. The S parameters ofthe films prepared by the oxidation transformation (0.45 and0.46) were larger than those of the bulk ZnO s0.4196d8 andthe ZnO epilayer grown on SCAM (0.4259).8 Moreover, theLd values (3.6 and 3.8 nm) were an order of magnitudeshorter than those in the bulk ZnO s52 nmd8 andZnO/SCAM s21 nmd.8 The results indicate that oxidizedfilms contain high density (large size) VZn and positivelycharged point defects, respectively. Conversely, Sof the s0001d ZnO epilayer grown on ZnS/ s111d Si was0.43, which was comparable to that ofZnO/SCAM s0.4259d.8 The decrease in S indicates the de-crease in size/density of VZn-related defects, and the resultwas consistent with the TRPL results that the nonradiativedefect density in ZnO/ZnS/ s111d Si was the lowest among

the present ZnO/Si films. Note that Ld in ZnO/ZnS/ s111d Sis5.9 nmd was also improved by a factor of 1.6. However, itwas still far shorter than those in the bulk ZnO s52 nmd8 orZnO/SCAM s21 nmd,8 implying the presence of extensiveneutral and positively charged defects (Zni, Oi, and VO) anddefect complexes. As a matter of fact, PL spectra at 293 K ofthe ZnO/Si films were dominated by the deep GL band (datanot shown), of which origin has been proposed to be VO orZni-related donor-type defect complexes.14 Further purifica-tion processes to reduce In donors and optimization pro-cesses to reduce the positron scattering centers are requiredto obtain improved qualities of ZnO films on Si substrates.

In summary, reductions in the nonradiative defect den-sity and point defect density in the ZnO epilayer grown ons111d Si by L-MBE using the ZnS epitaxial buffer layer wereshown. Both the ZnO/ZnS/ s111d Si epilayer and theZnO/Si films prepared by the oxidation transformation ofZnS/Si exhibited excitonic features in their low-temperatureOR and PL spectra. The ZnO/ZnS/ s111d Si exhibited thelongest nonradiative PL lifetime s105 psd and the smallest Sparameter (0.43) at 293 K.

This work was supported in part by the inter-universitycooperative program of the Institute for Materials Research,Tohoku University and MEXT, Japan (21st Century COEprogram Promotion of Creative Interdisciplinary MaterialsScience for Novel Functions and Grant-in-Aid for ScientificResearch Nos. 15656080, 16360146, and 14GS0204).

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TABLE I. Free exciton emission lifetimes (t1 and t2), S parameters for theannihilation g ray, and diffusion lengths of positrons sLdd at 293 K in ZnOfilms on Si substrates.

Sample t1 spsd t2 spsd S Ld snmd

s0001d ZnO/ZnS/ s111d Si 40 105 0.43 5.9s0001d ZnO/ s111d Sia 23 40 0.46 3.8s1011d ZnO/ s001d Sia 35 80 0.45 3.6ZnO single crystalb 970 14 000 0.4196 52ZnO/SCAM s800Cdb 110 fl 0.4259 21aTransformed from ZnS epilayers by the direct thermal oxidation.bReference 8.

5588 Appl. Phys. Lett., Vol. 85, No. 23, 6 December 2004 Onuma et al.

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