Structural and optical properties of high-quality ZnTe homoepitaxial layers
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Structural and optical properties of high-quality ZnTe homoepitaxial layersJ. H. Chang, M. W. Cho, H. M. Wang, H. Wenisch, T. Hanada, T. Yao, K. Sato, and O. Oda Citation: Applied Physics Letters 77, 1256 (2000); doi: 10.1063/1.1290155 View online: http://dx.doi.org/10.1063/1.1290155 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/77/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effects of substrate treatment and growth conditions on structure, morphology, and luminescence ofhomoepitaxial ZnTe deposited by metalorganic vapor phase epitaxy J. Appl. Phys. 96, 1230 (2004); 10.1063/1.1762711 Structural and luminescent properties of ZnTe film grown on silicon by metalorganic chemical vapor deposition J. Vac. Sci. Technol. A 20, 1886 (2002); 10.1116/1.1507344 Structural and optical properties of epitaxial and bulk ZnO Appl. Phys. Lett. 80, 2078 (2002); 10.1063/1.1464218 Aluminum-doped n -type ZnTe layers grown by molecular-beam epitaxy Appl. Phys. Lett. 79, 785 (2001); 10.1063/1.1390481 The structural and optical properties of high quality ZnTe grown on GaAs using ZnSe/ZnTe strained superlatticesbuffer layer J. Appl. Phys. 84, 2866 (1998); 10.1063/1.368429
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Structural and optical properties of high-quality ZnTe homoepitaxial layersJ. H. Chang,a) M. W. Cho, H. M. Wang, H. Wenisch, T. Hanada, and T. YaoInstitute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980, Japan
K. Sato and O. OdaCentral R&D Laboratory, Japan Energy Corporation, 3-17-35 Niizo-Minami, Toda, Saitama 335, Japan
~Received 4 January 2000; accepted for publication 5 July 2000!
The structural and optical properties of high-quality homoepitaxial ZnTe films are investigated. Asubstrate surface treatment using diluted HF solution plays a key role in growing device-qualityZnTe layers. X-ray diffraction analysis of ZnTe epilayers based on the crystal-truncation-rodmethod suggests that a homoepitaxial ZnTe film grown on a HF-treated substrate can be regardedas an ideal truncated crystal without an interfacial layer, while a ZnTe layer grown on a substratewithout HF treatment suggests the presence of an interfacial layer which may lead to degradedcrystallinity of ZnTe overlayers. The crystal quality of the homoepitaxial ZnTe layers with HFtreatments are characterized by an extremely narrow x-ray diffraction linewidth of 15.6 arcsec anddominant very sharp excitonic emission lines with dramatically reduced deep-level emissionintensity in the photoluminescence~PL! spectrum. Three bound excitonic emission lines at neutralacceptors are observed in the PL from the high-quality ZnTe homoepitaxial layers in addition to thefree-exciton emission line, suggesting the presence of three different kinds of residual acceptorimpurities. © 2000 American Institute of Physics.@S0003-6951~00!02635-8#
Although ZnTe is an ideal light-emitting material for thegreen-wavelength region, owing to its direct band gap of2.27 eV~546 nm! at room temperature, no successful growthof high-quality ZnTe has been reported yet. This is partlybecause most of the ZnTe epitaxy has been performed onIII–V substrates with various degrees of lattice misfit.1–4 Thepoor quality of heteroepitaxial ZnTe layers arises from lat-tice misfit,2 thermal misfit,3 and impurity diffusion from thesubstrate.4 In this letter, we report on high-quality ZnTe ho-moepitaxy layers grown by molecular-beam epitaxy~MBE!.The optical and structural properties of the homoepitaxialZnTe layers disclose a bright prospect for application to ef-ficient green light emitters.
The growth technique of high-qualityp-type ZnTe~001!substrates by the vertical Bridgman method has been alreadyestablished by some of the present authors.6 Even if the bulkproperties of the substrate were excellent, which is in thepresent case, special precaution should be paid to the surfacetreatment of the substrate for high-quality epitaxial growth.In order to remove polishing damages and flatten the sub-strate surface, we performed chemical etching with Br~3%!methanol for 30 s at room temperature, which is optimized interms of the surface roughness and x-ray diffraction~XRD!linewidth. This etching did improve the crystallinity of thesubstrate surface, as evidenced by the remarkable narrowingin the linewidth of the~004! XRD scan from 30–40 to 17arcsec. The substrate surface morphology, measured byatomic-force microscopy, changed from scratched featureswith a root-mean-square~rms! roughness of 2 nm to asmooth surface with a rms roughness of 1 nm. This processis followed by etching with a HF~3%!-distilled ionized wa-ter solution for 30 s at room temperature just prior to install-ing a substrate into the load-lock chamber of the MBE sys-
tem. The whole process takes just about 60 s and isperformed in air. Extensive Auger-electron-spectroscopymeasurements indicate that surface oxide layers and surfacecontamination, including C and S, are almost completely re-moved by the HF etching. The remaining surface contami-nation, if any, was removed by heating the substrate in ultra-high vacuum up to 350 °C, as evidenced by the appearanceof the (231) reconstruction pattern in the reflection high-energy electron diffraction~RHEED! @Fig. 1~a!#. Note thatthis temperature is low enough to prevent strong thermaldesorption of Zn or Te atoms from the surface, which isquite important to preserve the surface smoothness. Al-
a!Electronic mail: [email protected]
FIG. 1. RHEED patterns of HF-treated ZnTe observed~a! during the ther-mal treatment of a substrate at 350 °C and~b! at the end of buffer layergrowth at 240 °C.
APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 9 28 AUGUST 2000
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though the (231) reconstruction shows up through the ther-mal cleaning process, the RHEED pattern still shows anelongated streaky pattern indicative of the presence of tinyprotrusions on the surface. We found that the deposition of athin ZnTe buffer layer at lower temperature on such a surfaceprovides an atomically flat surface, as indicated by a sharp(231) streaky RHEED pattern@Fig. 1~b!#, which eventuallyleads to the growth of high-quality ZnTe epilayers as will bediscussed in the following. The cleanliness and the atomicalsmoothness of the ZnTe surface was further demonstrated byRHEED intensity oscillations from the very beginning ofZnTe growth on such low-temperature buffer layers~Fig. 2!.It should be mentioned that RHEED intensity oscillationswere not observed on ZnTe surfaces without HF etching.
To highlight the effect of HF treatment on the crystalquality of ZnTe epilayers, we prepared two sets of sampleswith the same thickness~0.5 mm!: sample A, grown on aHF-treated substrate, and sample B, grown on a substratewithout HF treatment. All other growth conditions remainunchanged.
The excellent structural properties of the ZnTe epilayerscan be demonstrated by the narrow linewidths in the XRDcurves for different scan modes. Figure 3~a! shows a typicaldouble-crystal~004! v–2u scan of sample A. The line shapeis symmetric with a full width at half maximum~FWHM! of15.6 arcsec, which is very close to the instrumentation limitof the XRD system and is remarkably narrower than theFWHM value of sample B~20.4 arcsec! shown in Fig. 3~b!and that of a ZnTe substrate~17.0 arcsec!. In addition to thebroader linewidth, the~004! diffraction peak of the ZnTelayer grown on a substrate without HF treatment shows anasymmetric line shape with an extended long tail, as shownin Fig. 3~b!, which suggests the presence of a degraded in-terface layer.
In order to study in more detail the implication of theobserved difference in XRD, we have analyzed the observedXRD line shape based on the crystal-truncation-rod~CTR!method.7 As can be seen in Fig. 3~a! ~solid curve!, the lineshape of the~004! scan of sample A can be well simulated bythe CTR analysis with good agreement, while the asymmet-ric line shape of sample B cannot be fitted by a simple CTR
analysis. It is likely that the discrepancy from the simulatedline shape could be reduced, if the presence of a strainedinterfacial layer is assumed.
The optical properties of ZnTe grown by either ho-moepitaxy or heteroepitaxy have been studied by manyauthors.1–5 However, the optical properties are still far fromdevice application level, since those photoluminescence~PL!spectra of ZnTe epilayers showed considerable participationof deep levels and luminescence bands related to structuraldefects. For investigation of the optical properties, we used aHe–Cd laser~325 nm! as an excitation source with an exci-tation density of 1.2 W cm22. The PL spectrum of ap-ZnTesubstrate at 10 K shows bright luminescence bands consist-ing of a neutral acceptor bound exciton at 2.356 eV, domi-nant donor–acceptor~DA! pair emission at 2.322 eV, and theLO phonon replicas of DA pair emission with negligibledeep-level emission.6
The PL spectra of the homoepitaxy ZnTe films~a! withand ~b! without HF treatment are shown in Fig. 4. We notethat the PL spectrum of the ZnTe film without HF treatmentis very similar to those previously reported. We could ob-serve a sharp and strong line at 2.376 eV due to (A0 , X)@Fig. 4~b!#, as was observed in a homoepitaxy ZnTe layer.1
However, the PL properties of the ZnTe epilayers grown onthe ZnTe substrates with and without HF treatment show aconsiderable difference both in spectrum and luminescenceintensity. The inset of Fig. 4 shows PL spectra at 10 K ofZnTe epilayers grown on ZnTe substrates~a! with the HFtreatment~sample A! and ~b! without HF treatment~sampleB!. Although, both samples A and B show dominant near-band-edge~NBE! emission with a very small deep-levelemission intensity, the NBE emission intensity of sample Ais by approximately five times larger than that of sample B,while the detailed investigation showed a larger deep-levelemission intensity for sample B than sample A.
Details of the NBE emission are shown in Fig. 4 to-gether with the reflectance spectrum of sample A. Sample A
FIG. 2. RHEED intensity oscillations observed at the very beginning ofgrowth on a low-temperature buffer layer. The substrate temperature is300 °C. FIG. 3. The~004! v–2u rocking curves of ZnTe homoepitaxy layers:~a!
sample A grown on a ZnTe substrate with HF treatment and~b! sample Bgrown without HF treatment. Solid lines represent the results of crystal-truncation-rod analysis. The line shape of sample A can be well simulatedby CTR analysis, but the line shape of sample B can not be reproduced dueto its asymmetric line broadening.
1257Appl. Phys. Lett., Vol. 77, No. 9, 28 August 2000 Chang et al.
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shows a free-exciton emission line~X! at 2.3819 eV andthree distinct bound exciton emission lines at shallow accep-tors (A0 , X) at 2.3770, 2.3692, and 2.3646 eV.8–13 One LOphonon replica of the (A0 , X) emission at 2.3646 eV is ob-served at 2.3381 eV. A very weak DA pair emission ataround 2.322 eV and its LO phonon replica are also ob-served. Since the PL spectrum of ap-ZnTe substrate at 10 Kshowed bright near-band-edge luminescence bands, whichconsisted of excitonic emission bound to a neutral acceptorat 2.356 eV, DA pair emission at 2.322 eV, and LO phononreplicas of the DA pair emission with negligible deep-levelemission,6 most of the observed luminescence bands in Fig.4 originate from the ZnTe epilayers except the weak DAemission band.
It would be interesting to compare the observed excitonemission photon energies with those reported for bulk ZnTe.The free-exciton emission energy of 2.3819 eV from theZnTe layer is to be compared with the bulk value of 2.3809eV. We observed three bound exciton emission lines to neu-tral acceptors at 2.377, 2.369, and 2.365 eV with the 2.365eV emission being dominant from sample A. Correspondingemission lines from bulk ZnTe are 2.375, 2.368, and 2.362
eV,11–13 which have been assigned to acceptor impurities ofLi, Ag, and Au, respectively. The binding energy of excitonsat those impurities are estimated to be 4.9, 12.7, and 17.3meV, which lead to the associated impurity binding energiesof 60, 152.4, and 207.6 meV based on Haynes’ rule,13 whichare very similar to reported values.12 In the case of sample B,we observe an excitonic emission line at 2.376 eV bound toa neutral acceptor and the DA pair emission band. The 2.376eV emission has been assigned to the (A0 , X) emission line,which has an exciton binding energy of 4.6 meV, where thefree-exciton~X! position is indicated by the reflectance spec-trum at 2.3806 eV in Fig. 4. The line widths of the free-exciton emission line are 1.2 meV for sample A and 3.3 meVfor sample B. Such a difference in linewidth would reflect instrain in epitaxial layers, which is consistent with the XRDcharacterization.
In conclusion, we have grown device-quality homoepit-axy ZnTe films employing a surface treatment method usingHF, which will greatly contribute to the development oflight-emitting devices in the pure-green region fabricated onZnTe substrates. Homoepitaxial ZnTe films show high crys-tal quality with a narrow XRD linewidth of 15.6 arcsec alongwith the dominant excitonic emission with a very narrowlinewidth of 1.2 meV. The deep-level luminescence, which isalways present in heteroepitaxial ZnTe layers and homoepi-taxial layers so far reported, is negligibly small in intensity.
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FIG. 4. Low-temperature~10 K! PL spectra of homoepitaxy ZnTe layers~a!with HF treatment~sample A! and~b! without HF treatment~sample B!. Theobserved lines at 2.3819 and 2.3806 eV~indicated by the dip of reflectance!is assigned to free-exciton lines~X! of samples A and B, respectively. TheFWHMs of the free-exciton lines are estimated as 1.2 meV~sample A! and3.3 meV~sample B!.
1258 Appl. Phys. Lett., Vol. 77, No. 9, 28 August 2000 Chang et al.
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