Author name used in this publication: X. Y. ZhouAuthor name used in this publication: J. MiaoAuthor name used in this publication: J. Y. DaiAuthor name used in this publication: H. L. W. ChanAuthor name used in this publication: C. L. ChoyAuthor name used in this publication: Y. Wang2006-2007 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Choy, CL

Dai, J

Li, Q

Miao, J

Wang, Y

Zhou, X

Applied Physics Letters

PolyU Institutional Repository

Epitaxial growth of SrTiO3 thin film on Si by laser molecular beam epitaxyX. Y. Zhou, J. Miao, J. Y. Dai, H. L. W. Chan, C. L. Choy, and Y. WangaDepartment of Applied Physics, The Hong Kong Polytechnic University, Hong Kong SAR,China and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong SAR, ChinaQ. LiDepartment of Physics, The Chinese University of Hong Kong, Hong Kong SAR, ChinaReceived 3 August 2006; accepted 8 December 2006; published online 3 January 2007SrTiO3 thin films have been deposited on Si 001 wafers by laser molecular beam epitaxy using anultrathin Sr layer as the template. X-ray diffraction measurements indicated that SrTiO3 was wellcrystallized and epitaxially aligned with Si. Cross-sectional observations in a transmission electronmicroscope revealed that the SrTiO3/Si interface was sharp, smooth, and fully crystallized. Thethickness of the Sr template was found to be a critical factor that influenced the quality of SrTiO3and the interfacial structure. Electrical measurements revealed that the SrTiO3 film was highlyresistive. © 2007 American Institute of Physics. DOI: 10.1063/1.2430407The epitaxial growth of SrTiO3 STO thin film on sili-con has attracted extensive interest in recent years.1,2 TheSTO/Si heterostructure not only provides an ideal modelstructure for the study of growth dynamics and kinetics ofpervoskite-structured oxides on silicon3–8 but is also a usefulsubstrate for various microelectronic devices, such as siliconcomplementary metal oxide semiconductor devices, randomaccess memory, and microelectromechanical systems.9–11Good crystallinity, epitaxial alignment, and clean interfaceare the major requirements of STO and the STO/Si hetero-structure. Several thin film deposition techniques, includingmolecular beam epitaxy MBE and metal-organic chemicalvapor deposition MOCVD, have been adopted for the syn-thesis of STO.2,12 While the processing conditions and thinfilm growth mechanisms are significantly different in thesedeposition processes, it is noted that the use of strontium orstrontium oxide template has become a common and—to acertain extent—“standard” procedure in the latest literaturereports. The major purpose of introducing such a template isto suppress the formation of an amorphous silicon oxidelayer on the Si surface and assist the growth of STO.5,8,9,12–14Under optimal conditions, it has been reported that highquality STO films have been deposited by MBE andMOCVD on Si.2,12In this letter, we report on the epitaxial growth of STOon Si by laser molecular beam epitaxy laser MBE using aSrTiO3 ceramic target. Compared with many other thin filmdeposition methods such as magnetron sputtering andMBE, the laser-MBE technique has the advantage of precisecomposition control. The possible disadvantage of this tech-nique, however, is that the use of a ceramic target may makeit more difficult to obtain a clean interface between STO andSi. This is because the plume generated by the irradiation ofthe ceramic surface by a pulsed laser beam may containquite a number of oxygen atoms and ions which can reactwith silicon, leading to the formation of an amorphous sili-con oxide layer on the substrate surface. Thus how to sup-press or prevent the oxidizing reactions is the major problemto be tackled in this work.The deposition was conducted via a three-step proce-dure: 1 Wet cleaning, a 001 Si wafer was treated by stan-dard chemical cleaning procedures followed by HF etching.The freshly cleaned wafer, together with two targets high-purity Sr and SrTiO3, was then installed in the vacuumchamber of the laser-MBE system. After the target installa-tion, the chamber was immediately evacuated by using amechanical pump and a turbomolecular pump. 2 Dry clean-ing, when the pressure in the chamber was lower than 110−6 Torr, the Si wafer was heated up to 850 °C and keptat this temperature for 15 min, with the aim of removing theresidual hydrogen on the Si surface. Then the temperaturewas lowered to a temperature Td and a very thin layer ofstrontium was deposited by irradiating the Sr target with apulsed KrF excimer laser beam. This step aimed to removethe SiO2 layer that had been freshly formed during theprocessing.15,16 3 Deposition, following the deposition ofthe Sr layer a STO layer was deposited using the same laser.Initially a relatively low substrate temperature 650 °Cand low oxygen pressure 0.02 Pa were used, while at alater stage the temperature and oxygen pressure were in-creased to 800 °C and 50 Pa, respectively.Crystallographic characterizations of the samples wereperformed in a Bruker AXS D8 Discover x-ray diffracto-meter. The surface morphology of the STO thin film wasobserved in an atomic force microscope AFM Digital In-strument Nanoscope IV working in the tapping mode. AJEOL JEM-2011 transmission electron microscopy TEMoperating at 200 kV was used to observe the crystal structureand interfaces at the atomic level. The high resolution scan-ning transmission electron microscopy image was taken us-ing a high angle annular dark field detector in a scanningtransmission electron microscope STEM Tecnai G2 FEG.The electron energy loss EEL spectroscopy was performedin the STEM line scan mode, using a Gatan imaging filteringsystem attached to the same microscope. The electron probesize was maintained at 0.3 nm. Gold dot electrodes 200 min diameter were deposited on top of the film by magnetronsputtering, and the current-voltage I-V characteristicsof the Au/STO/Si capacitor metal oxide semiconductorstructure were measured using an Advantest TR8652 digitalelectrometer.aAuthor to whom correspondence should be addressed; electronic mail:apywang@inet.polyu.edu.hkAPPLIED PHYSICS LETTERS 90, 012902 20070003-6951/2007/901/012902/3/$23.00 © 2007 American Institute of Physics90, 012902-1The structures of the STO-film and STO/Si interfacewere found to be very sensitive to the processing parameters,including the deposition temperature, atmosphere and pres-sure, and laser energy and pulse rate. This observation isconsistent with literature reports.2,4,17 Among these param-eters, however, the parameters that have the most criticalinfluence are the thickness of the Sr template tSr and thedeposition temperature for the Sr template Td. Samplesprepared under optimized processing conditions Td650±50 °C and tSr1.2±0.4 nm tend to be of high qual-ity and have a sharp and clean free of amorphous materialSTO/Si interface, while samples prepared under conditionsoutside the above range either have a rough and sawlikeinterface when the deposition temperature is too high and/orthe Sr template is too thick or a thick amorphous layerwhen the deposition temperature is too low and/or the Srtemplate is too thin. This processing-structure relationship,we believe, is closely related to the reaction SiO2→Si+2Oand the role that Sr plays in the reaction. SiO2 in the abovechemical equation corresponds to the amorphous SiO2 layerthat forms on the fresh Si surface during its contact with airi.e., in the time interval after the chemical cleaning andbefore the achievement of high vacuum in the chamber andexposure to the plume in the first few seconds of STO depo-sition. The rate of the decomposition of SiO2 at high tem-peratures could be enhanced by the catalyst Sr, as suggestedin the literature. It is reasonable to assume that, under opti-mized conditions, the SiO2 layer could be completely re-moved, so the surface of the Si substrate would become veryclean.In the following we will show the typical structure ofthe sample prepared under optimized conditions. Figure 1shows the x-ray diffraction XRD  /2 scan of the sample.The STO thin film has a pure perovskite phase with latticeshighly oriented along the 00l direction. The in-planealignment of the STO thin film with respect to the majoraxes of the 001 Si substrate was also investigated by XRDoff-axis  scans and the results are shown in the inset ofFig. 1. Based on the XRD data, it is concluded that thecrystal structures of STO and Si have the relationship of100STO 100Si and 100	STO 110	Si, which is consistentwith results reported in the literature.10,18 The surface mor-phology of the STO thin film was observed in an AFM pic-ture not shown. The average grain size was in the range of40–50 nm and the root-mean-square roughness was about1.0 nm over a 11 m2 area, which suggests that STO hasa fairly flat film surface.The cross-sectional structures of the STO/Si observed ina TEM are shown in Figs. 2a and 2b high resolution andconventional resolution images, respectively. The STO layerwas found to be well crystallized and epitaxially aligned. Thecrystallographic relationship of STO and Si, determined bymeans of selected area electron diffraction patterns notshown, was found to be consistent with the XRD observa-tions. Between the STO and Si layers, a clean, sharp, andfully crystallized interface layer was observed. The thicknessof the interface layer is about 1.2 nm. No misfit dislocationswere found at the interface, implying that there is only aslight elastic distortion in the STO lattice along the in-planedirection due to the lattice mismatch between the STO and Silayers the lattice spacings along STO 100 and Si 110 are0.390 and 0.385 nm, respectively, and the mismatch betweenthem is 1.4%. As a comparison, the cross-sectional viewof a STO/Si sample with a rough and sawlike interface struc-ture prepared under conditions of Td750 °C and tSr3.6 nm is shown in Fig. 2c.A more detailed microstructure of the STO-film/Si-substrate interface was disclosed by the high resolutionSTEM, as shown in Fig. 3. A Sr-deficient layer with a thick-ness of 1–2 ML was found in-between the Si and the STOfilm. This observation suggests that strontium deposited inthe step of dry etching of the fabrication process had actuallyevaporated from the Si surface after the reaction of SiO2FIG. 1. X-ray diffraction patterns of STO/Si. The inset shows the offset-scan results of the sample.FIG. 2. Cross-sectional observation of STO/Si samples under TEM. aHigh resolution image and b conventional resolution image of a sampleprepared under optimized conditions and thus having a clean and sharpinterface. c Conventional resolution image of a sample with a poor inter-facial structure. The sample was prepared with processing conditions ofTd750 °C and tSr3.6 nm.FIG. 3. Electron energy loss spectra taken at the Ti L edge from locations A,B, and C, as indicated in the high resolution scanning transmission electronmicroscopy image.012902-2 Zhou et al. Appl. Phys. Lett. 90, 012902 2007→Si+2O. The EEL spectra taken at the Ti L edge fromlocations A, B, and C as marked in the STEM image reveala valence decrease of the Ti, as the electron probe movesfrom the film interior to the interface. One can see that theEEL spectrum taken from location A has a double peak split-ting feature, which is characteristic of the Ti4+. The peaksplitting disappears with peak center redshifting in the EELspectrum taken from location B. This is the signature of Ti3+.The double peak further shifts to lower energy at location Cbefore the Ti signal completely vanishes, suggesting a furtherdecrease in the Ti valence. The variation of Ti valence isobviously the result of the change of the atmosphere in thestep of STO deposition. The results exclude firstly the pos-sible formation of interfacial SiO2 layer when one should notobtain any signal from the Ti, and secondly the existence ofinterfacial TiSi2, when one should expect valence increase inTi instead of decrease. Such interfacial configuration is dif-ferent from the previous literature reports, and interestingenough, it also leads to epitaxial growth of STO on Si. Fur-ther study on the interface structure is still in progress.The electrical properties of the STO thin film were alsodetermined. Figure 4 shows the I-V measurement results.The film was found to be electrically highly resistive. Whenthe test voltage=1 V, for example, the leakage current den-sity of the STO film 200 nm thick is 510−6 A/cm2.Apart from the structural and electrical characterizations, wehave developed a heterostructure of NBCO/STO/Si NBCOstands for the superconducting oxide with a nominal compo-sition of NdBa2Cu3O7 to demonstrate the application ofSTO/Si as a substrate. The deposition conditions for theNBCO film were very much similar to that for STO. X-raydiffraction measurements revealed that the NBCO was wellcrystallized and epitaxially aligned with STO picture notshown. The electric measurements indicated that the electri-cal resistivity of NBCO decreased as the temperature de-creased and dropped very quickly when the temperature wasbelow 90 K, a behavior very much analog to the NBCOfilms grown on SrTiO3 single crystal substrates.19In summary, high quality SrTiO3 thin films have beendeposited on Si wafer through laser molecular beam epitaxy.Prepared under optimal conditions, STO thin films werefound to be well crystallized and epitaxially aligned. Theinterface between STO and Si was clean and sharp. The elec-trical measurements indicated that the STO film was highlyresistive. Future work will include the further optimization ofthe laser-MBE processing and investigation of the interfacestructure.This study was financed by the Hong Kong ResearchGrants Council CERG 5284/05, the Hong Kong Polytech-nic University ICRG A-PG18, and the Hong Kong Innova-tion and Technology Fund ITS/009/06. The authors aregrateful to R. K. Zheng, X. B. Lu, and P. F. Lee of the HongKong Polytechnic University for their kind help in the re-search and D. Tang of the Chinese University of Hong Kongfor the STEM work.1J. Robertson, J. Vac. Sci. Technol. B 18, 1785 2000.2Z. Yu, Y. Liang, C. Overgaard, X. Hu, J. Curless, H. Li, Y. Wei, B. Craigo,D. Jordan, R. Droopad, J. Finder, K. Eisenbeiser, D. Marshall, K. Moore,J. Kulik, and P. Fejes, Thin Solid Films 462/63, 51 2004.3X. Zhang, A. A. Demkov, H. Li, X. Hu, Y. Wei, and J. Kulik, Phys. Rev.B 68, 125323 2005.4J. Lettieri, J. H. Haeni, and D. G. Schlom, J. Vac. Sci. Technol. A 20, 13322002.5R. A. McKee, F. J. Walker, and M. F. Chisholm, Phys. Rev. Lett. 81, 30141998.6C. H. Ahn, K. M. Rabe, and J. M. Triscone, Science 303, 488 2004.7R. A. McKee, F. J. Walker, and M. F. Chisholm, Science 293, 468 2001.8H. Mori and H. Ishiwara, Jpn. J. Appl. Phys., Part 2 30, L1415 1991.9K. Eisenbeiser, J. M. Finder, Z. Yu, J. Ramdani, J. A. Curless, J. A.Hallmark, R. Droopad, W. J. Ooms, L. Salem, S. Bradshaw, and C. D.Overgaard, Appl. Phys. Lett. 76, 1324 2000.10Y. Wang, C. Ganpule, B. T. Liu, H. Li, K. Mori, B. Hill, M. Wuttig, R.Ramesh, J. Finder, Z. Yu, R. Droopad, and K. Eisenbeiser, Appl. Phys.Lett. 80, 97 2002.11T. Zhao, S. B. Ogale, S. R. Shinde, R. Ramesh, R. Droopad, J. Yu, K.Eisenbeiser, and J. Misewich, Appl. Phys. Lett. 84, 750 2004.12M. N. K. Bhuiyan, A. Matsuda, T. Yasumura, T. Tambo, and C.Tatsuyama, Appl. Surf. Sci. 216, 590 2003.13G. J. Norga, C. Marchiori, A. Guiller, J. P. Locquet, Ch. Rossel, H.Siegwart, D. Caimi, J. Fompeyrine, and T. Conard, Appl. Phys. Lett. 87,262905 2005.14R. A. McKee, F. J. Walker, M. Buongiorno Nardelli, W. A. Shelton, andG. M. Stocks, Science 300, 1726 2003.15M. N. K. Bhuiyan, H. Kimura, T. Tambo, and C. Tatsuyama, Jpn. J. Appl.Phys., Part 1 44, 677 2005.16B. K. Moon and H. Ishiwara, Jpn. J. Appl. Phys., Part 1 33, 1472 1994.17M. N. K. Bhuiyan, H. Kimura, T. Tambo, and C. Tatsuyama, Appl. Surf.Sci. 249, 419 2005.18G. Y. Yang J. M. Finder, J. Wang, Z. 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Epitaxial growth of SrTiO₃ thin film on Si by laser molecular beam epitaxy

Epitaxial growth of SrTiO₃ thin film on Si by laser molecular beam epitaxy

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