Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

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Journal of Crystal Growth 300 (2007) 509–518 www.elsevier.com/locate/jcrysgro

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy F. Niu, B.W. Wessels Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Cook Hall, 2220 Campus Dr., Evanston, IL 60208, USA Received 26 September 2006; accepted 7 December 2006 Communicated by D.P. Norton Available online 30 December 2006

Abstract Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO3 (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr4Ti3O10 second phase at the STO/Si interface in a Sr-rich film. r 2007 Elsevier B.V. All rights reserved. PACS: 68.03.Cd; 68.37.Lp; 68.55.Jk; 81.15.Hi Keywords: A1. Interfaces; A1. Surface structure; A3. Molecular beam epitaxy; B1. Oxide thin film; B2. Semiconducting silicon

1. Introduction The realization of perovskite oxide epitaxial layers on Si by molecular beam epitaxy (MBE) through precise atomic level control of the oxide/Si interface opens a new path for integrating multifunctional materials and devices with Sibased microelectronic and photonic circuits [1,2]. For example, using the SrTiO3 (STO) as the buffer layer, epitaxial growth of the GaAs layer on Si has been reported [3]. Integration of ferroelectric [4,5], colossal magnetoresistive [6], magnetic [7], conducting oxide thin films [8] on STO-buffered Si has been demonstrated. However, the effects of strain relaxation on nucleation and growth mechanisms and effects of the non-stoichiometric STO layers on surface morphology and surface and interfacial structures during early-stage growth are still not well Corresponding author. Tel.: +1847 491 7797; fax: +1847 491 7820.

E-mail address: [email protected] (F. Niu). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.12.024

understood. A better understanding of these issues is key to obtaining high-quality heteroepitaxial thin films on Si. Previous research has shown that STO layer growth on Si (0 0 1) proceeds via a two-dimensional (2D) growth (Frank–van der Merwe) mode under stoichiometric and slightly Sr-rich conditions [1,9,10]. This has been supported by theoretical predictions [11]. Furthermore, McKee et al. [1] reported that the STO/Si interface showed perfect registry as indicated by Z-contrast transmission electron microscopy (TEM). This is supported by a recent X-ray diffraction (XRD) study of thin 5–10 ML STO films [12]. In contrast, Aguirre-Tostado et al. [13] observed that the interface was incoherent for the STO thin films on Si as thin as 4 nm by glancing angle in-plane XRD y2y scans. Furthermore, He et al. [14] recently have observed by cross-section high-resolution TEM (XTEM) that the STO and Si interface is semi-coherent with a well-defined misfit dislocation network. The measured dislocation density agreed with that calculated from the lattice mismatch

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between the STO and Si. Thus formation of a misfit dislocation network can fully accommodate the strain due to lattice mismatch. Film stoichiometry has also been shown to have important effects on the microstructure and surface morphology of STO [15]. However, it is difficult to achieve precise control of the STO stoichiometry during deposition [16]. There are few detailed studies on effect of Sr/Ti ratio on crystalline and epitaxial quality, interfacial reaction, defects and surface morphology of epitaxial STO thin film on Si [14–17]. He et al. [14] reported that STO growth by MBE shows a 2D growth mode under stoichiometric conditions, no interface interactions between STO and Si were observed. However, if excess Ti atoms are present an amorphous TiO2 phase forms that resides in grain boundary area that subsequently diffuses to interface and promote interfacial reactions to form Ti silicide phases such as TiSi2. These precipitates nucleate misorientated STO grains and promote columnar growth. To better understand the role of relaxation of misfit strain, interfacial coherency and dominant growth mechanisms, we used an in situ reflection high-energy electron diffraction (RHEED) technique in combination with XRD, atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and plan view TEM, and found that the STO/Si interface is incoherent. Furthermore the interfacial strain is dominated by a large tensile strain due to thermal mismatch rather than a small compressive strain due to lattice misfit. Furthermore, we used both AFM and plan view TEM to analyze surface structure and morphology of STO under different Sr/Ti ratios from Sr rich, stoichiometric to Ti rich. Our AFM results indicate that the stoichiometric STO show dense steps on the surface whereas the Sr-rich STO shows many terraces. The Ti-rich STO however shows large precipitates on the surface. A secondary phase Sr4Ti3O10 was identified in an Sr-rich film at STO and Si interface by plan view TEM and electron energy loss spectrometer (EELS). 2. Experimental procedure 2.1. Growth procedure Epitaxial STO epitaxial films (5–10 nm thick) were grown in an SVT Associates SN35 MBE system equipped with a low-temperature Sr effusion cell, a high temperature, Ti effusion cell and a molecular oxygen source. The base pressure is 4  1010 Torr. The MBE system is capable of depositing on 3-in Si wafers. Before deposition, the Si (0 0 1) wafer was cleaned ex situ in ozone for 15 min to remove surface contamination, and immediately loaded into the MBE system. The wafer was then heated up to 850 1C in vacuum and held for 5–15 min to thermally desorb any native SiO2. The clean surface shows a clear Si (2  1) reconstruction. Subsequently, the Si surface was passivated by depositing 0.5 ML of Sr to form a chemisorbed Sr silicide layer at 700 1C. This layer prevents formation of an amorphous SiO2 film on the clean Si

surface during initial STO growth. An SrSi (2  1) surface superstructure was observed by RHEED [2]. Epitaxial STO layers were grown by solid-state epitaxy [9]. An amorphous layer consisting of Sr, Ti and O was first grown by evaporating metallic sources in the presence of molecular oxygen at a partial pressure of 8  108 Torr and a Si substrate temperature of 250 1C. The temperature of the amorphous film was subsequently ramped up to and held at 550 1C for 15–60 min without oxygen present. The amorphous film crystallized and a clear and streaky singlecrystalline STO RHEED pattern was formed. One to two monolayers of STO were deposited for the first and second crystallization cycles with the STO growth rate of 1.7 A˚/ min. It was found that the crystallization temperature is very sensitive to Sr/Ti flux ratio. Either Sr or Ti-rich condition results in recrystallization at higher temperatures or even lack of crystallization. 2.2. Characterization For thin film characterization, an in situ KSA 400 RHEED system with an accelerating voltage of 10 keV was employed during MBE deposition. XRD y2y scans from the fully integrated stack were measured using a four-circle X-ray diffractometer with a graphite monochromator and 12 kW rotating anode CuKa source. A Digital Instrument multimode scanning probe microscope in the contact mode was employed for analysis of the surface morphology. The film thickness is measured ex situ by a Tencor P10 surface profiler and confirmed by both XRD K-fringes and XTEM. The Sr and Ti molar fluxes were calibrated as a function of their cell temperatures by depositing on a Si wafer and measuring the individual growth rate and converted to their fluxes taking into account of their bulk molar volumes. All the calibrations are conducted at the fixed substrate temperature of 250 1C. The STO stoichiometry is controlled by maintaining the same Ti cell temperature and adjusting the Sr cell temperatures during co-deposition and confirmed by ex situ XPS quantitative analysis. XPS was conducted ex situ using an Omicron ESCA probe equipped with an electron flood gun to neutralize surface charges and a scanning ion gun to remove surface contamination and perform depth profiling. The STO thin film was normally pre-cleaned with methylene chloride, hexane, acetone and methanol subsequently in an ultrasonic bath. All binding energies of various peak positions were calibrated with respect to the energy of C1s peak (284.5 eV). To quantify compositional analysis, a stoichiometric STO single-crystal sample was used for calibration of the sensitivity factors of Sr and Ti. A Hitachi H2000 TEM with a field emission gun operated at an accelerating voltage of 200 keV, and equipped with an electron energy dispersive (EDX) spectrometer, a Gatan energy filter (GIF) and an EELS was used for structural and chemical analysis. The plan view TEM sample was prepared by first protecting the film side from reaction with water and

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mechanical damage during handling with Crystal Bond. The sample was then mechanically thinned and polished from the Si substrate side to 100 mm, followed by further dimpling to 5–10 mm. Finally, ion milling was used to thin the sample to electron transparency by a Model 691 precision ion polishing system. 3. Results 3.1. Strain relaxation during initial epitaxial growth 3.1.1. Relaxation of interfacial strain vs. the STO thickness by in situ RHEED To analyze the surface structure and morphology during the initial STO growth, in situ RHEED patterns were taken after each recrystallization cycle. Fig. 1(a) reveals the SrSi (2  1) superstructure on the Si substrate along Si [1 1 0] azimuth after deposition of 12 ML Sr at 700 1C and cooled down to 250 1C. Figs. 1(b–d) show evolution of RHEED patterns for a slightly Sr-rich STO thin film as a function of

511

the STO thickness (2, 4–15 MLs). All the RHEED patterns consist of dominant streaks overlapped with weak threedimensional (3D) transmission spots indicating a mainly very smooth STO matrix (terraces, contributing to the sharp streaks) and few large islands (contributing to the spots). Coincidence of the RHEED spots with the streaks indicates that these islands are crystalline STO having the same lattice constant with that of the smooth STO matrix. To extract quantitative information in situ regarding strain relaxation, a series of RHEED line scans were taken as a function of STO layer thickness as shown in Fig. 2(a). The relaxation of the in-plane misfit strain e defined by e ¼ (dSTOdSi)/dSTO ¼ (gSigSTO)/gSi, is measured as a function of the STO thickness as seen in Fig. 2(b). The term dSTO corresponds to the measured STO lattice spacing, and gSTO corresponds to the spacing between the ¯ or (0 2) rods in reciprocal lattice space. STO (0 0) and ð0 2Þ The dSi corresponds to measured Si surface lattice spacing and gSi is the measured spacing between the Si (0 0) and (0 2) rods on the Si (2  1) reconstruction pattern in the

Fig. 1. Evolution of RHEED pattern as a function of STO layer thickness: (a) SrSi (2  1) superstructure; (b) STO layer at 2 ML thickness; (c) STO layer at 4 ML thickness; (d) STO layer at 14 ML thickness. All STO RHEED patterns consist of both dominant diffraction streaks overlapped with 3D transmission spots.

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512

2800

80 70

(002) STO

2400

(004) Si

bulk STO

60

2000 Intensity

50 5 MLs

40

1600

3 MLs

30

1200 1 ML

20

(001) STO

800 10

Si

0 100 150 200 250 300 350 400 450 500 550 600 Reciprocal spacing (pixels)

400

10

Relaxation of Misfit Strain (%)

9% 8%

15

20

25 θ scan (°)

30

35

40

Fig. 3. XRD y2y scan of a 14 ML thick epitaxial STO layer on (0 0 1) Si. The measured STO lattice constant is 3.930 A˚.

STO[100]

7% 6%

#121305 #111705

5%

#062805

STO[110]

4% 3% 2% 1%

Bulk STO with lattice misfit strain of 1.67%

0% 0

1

2

3

4

5

6

7

8

9 10 11 12 13 14

STO thickness (MLs) Fig. 2. (a) RHEED line scans as a function of the STO layer thickness indicating an incoherent interface; (b) measured misfit strain relaxation vs. the STO layer thickness.

14 ML thick STO thin film was measured. The diffraction pattern in Fig. 3 reveals only STO (0 0 2) and weak (0 0 1) peaks together with the Si (0 0 4) and weak Si (0 0 2) peaks which indicates a single-phase, epitaxial STO film. The measured out of plane lattice constant of STO is a0 ¼ 3.930 A˚, compared to the bulk value a0 ¼ 3.905 A˚. This result is consistent with Aguirre-Tostado et al. [13] for the similar thickness STO film on Si . The fact that both the measured out of plane lattice spacing by XRD and in-plane lattice spacing by RHEED show expansion is attributed to an elastic anomaly associated with the out of plane polarization of STO. 3.2. Effects of non-stoichiometry

reciprocal space. The plot indicates that (1) the initial interface between STO and Si is incommensurate, since the misfit strain is relaxed from the first ML; (2) the strain relaxation does not saturate at the calculated lattice misfit strain (1.7%) between bulk STO and Si. This indicates the interfacial strain is not dominated by the lattice misfit strain but most likely by thermal mismatch strain especially upon considering a large difference in thermal expansion coefficient (a) between STO (a ¼ 9  106 K1) and Si (a ¼ 2.5  106 K1). This finding is consistent with the result of Aguirre-Tostado et al. [13] on the measured interfacial strain; (3) magnitude of strain relaxation depends on crystallographic direction, i.e. 7% along Si /1 0 0S and 5% along Si [1 1 0]. It is not yet clear why. 3.1.2. XRD results To verify the crystalline structure and phase purity of the deposited STO thin film, an XRD y2y scan from the

3.2.1. Effect of the non-stoichiometry on STO surface structure measured by in situ RHEED To study effects of stoichiometry on surface morphology, structure, composition and interfacial structure, STO thin films with different Sr/Ti ratios were deposited. The ratio is controlled by varying Sr cell temperature while keeping the Ti cell temperature constant. Table 1 lists Sr cell temperature, summaries of AFM morphology and RHEED feature and observed starting crystallization temperatures of each STO film. The Ti cell temperature was 1675 1C. The RHEED images for films grown with different Sr/Ti flux ratio are shown in Fig. 4. Fig. 4(a) (as well as Fig. 1(b)) reveals a typical RHEED pattern under slightly Sr-rich condition, the strong extra 12 order rods along STO [1 1 0] azimuth were always observed for the first recrystallization cycle and sometime for other cycles under this condition. Note also the RHEED pattern shows both a higher diffuse background and

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Table 1 Sr cell temperature for each STO layer growth and corresponding AFM morphology, RHEED feature and starting crystallization temperatures Sr cell temperature (1C)

RHEED

AFM

Crystallization temperature (1C)

Composition

375 373 372.4 371.2 370

Strong 12 order streaks in STO [1 1 0] azumth order streak, spotty Very weak or no 12 order but streaky No 12 order streaks, extra spots No 12 order streaks, extra spots

Terraces and few islands Terraces and few islands Dense steps and few island Dense steps and few island Islands and big precipitates

4600 4550 4420 4450 4520

Sr rich Sr rich Close to stoichiometry Ti rich Ti rich

1 2

The Ti cell temperature was 1675 1C.

Fig. 4. RHEED patterns under (a) Sr-rich condition reveal extra

1 2

order streaks; (b) stoichiometric condition; (c) Ti-rich condition reveal extra spots.

broader streaks compared to the pattern from the stoichiometric STO film as seen in Fig. 4(b). This indicates lower crystallinity. The real composition is 8% Sr rich as measured by XPS. These 12 order rods were also observed by Yu et al. [10] on STO grown under slightly Sr-rich conditions. Fig. 4(b) presents the RHEED pattern of a STO film grown under the stoichiometric condition, it is sharp and very streaky with very low background indicating 2D growth and high degree of crystallinity. Weak or no 12 order rods without any extra spotty patterns are observed. The RHEED taken from the Ti-rich film, however, not only shows a highly diffuse background, broader streaks but also extra spots in the STO [1 1 0] azimuth indicating lower crystallinity, a rough surface and presence of secondary phases on the surface as shown in Fig. 4(c).

3.2.2. Effect of the non-stoichiometry on surface morphology measured by AFM To investigate the effect of stoichiometry of STO on the surface morphology and the origin as to why RHEED patterns show both streaks and spots, AFM analysis was conducted. The image from a 5 nm thick Sr-rich STO indicates large regimes of atomically flat terraces with few localized islands as shown in Fig. 5(a). The terrace area has an average RMS roughness 3 A˚ with terrace height 3 nm. The islands with a size ranging from 100 to 300 nm and height ranging from 5 to 9 nm as shown in Fig. 5(a) inset have a volume fraction of 5%. The islands are rectangular in shape with both sides either parallel or perpendicular to the Si primary flat /1 1 0S. The RHEED from the Ti-rich STO film as seen in Fig. 5(c) shows not only the islands similar to the Sr-rich film, but extra

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8000 STO thin film

7000

STO crystal

O1s1/2

CTs (counts)

6000 Ti 2P3/2

5000 4000

Sr3p3/2 Sr3d5/2

Ti 2P1/2 Sr3p1/2

3000 C31s1/2 Sr3s1/2

2000 1000 0 100

200

300

400

500

600

Binding Energy (eV) Fig. 6. XPS spectra from a MBE grown STO thin film and a STO singlecrystal substrate.

No other impurity elements except C are present. The small C peak at 284.4 eV is attributed to contamination caused by specimen handling or open exposure to air. Figs. 7(a and b) show detailed scans of the Sr 3d peaks and Ti 2p peaks from both the STO film and the STO crystal. The peaks from the STO film match those from the STO standard very well; again no other peaks from such as SrO, partially oxidized Ti 3+, Ti 2+, Sr or Ti silicides or silicates were detected. The Sr/Ti molar ratio was determined by dividing the elemental peak area by the relative sensitivity factor. The Sr and Ti sensitivity factor ratio was calibrated using the spectrum of the standard STO crystal substrate. An Sr/Ti molar ratio of 1.08 is obtained, which indicates the film is Sr rich (8%). Fig. 5. AFM images from (a) Sr-rich sample show STO islands; (b) stoichiometric condition shows steps; (c) Ti-rich condition show surface precipitates.

precipitates, presumably related to Ti-rich phases such as TiSi2. The stoichiometric STO film shows, however, dense steps parallel to Si /1 1 0S in addition to hillocks as seen in Fig. 5(b) that are similar to those on the Sr-rich STO film. In addition, all three films show the common feature of the faceted hillocks, independent of the Sr/Ti ratio. 3.2.3. Effect of the non-stoichiometry on surface composition by XPS In order to determine surface composition of the STO film, a surface sensitive technique XPS was employed. Fig. 6 shows the XPS spectra of the Sr slightly rich STO thin film and the STO standard near their surface in the binding-energy range of 0–1200 eV. The two scans match very well. The STO film surface contains only Sr, Ti and O.

3.2.4. Effect of non-stoichiometry on interfacial structure by plan view TEM To analyze the detailed surface/interfacial microstructure, a plan view TEM sample from the Sr-rich STO film was prepared. A bright field TEM image together with a dark field image was taken along close to the Si [0 0 1] zone axis where the best contrast was obtained as shown in Figs. 8(a and b). The STO surface consists of light contrast terraces and dark contrast circular regions. This morphology is consistent with the AFM analysis. The light and dark contrast most likely arises from either differences in mass contrast or crystal orientation. Parallel moire´ fringes were observed inside the circular regions, which extend continuously across the whole area and terminate at the boundaries. These continuous fringes are due to the single crystal-like STO thin film overlapping with the underlying Si substrate. The selected area diffraction pattern (SADP) from the circular region shown in Fig. 8(c) consists of two sets of spotty patterns. One is from the Si [0 0 1] zone axis (labeled

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4000 STO thin film

3500

STO crystal

Intensity (cts)

3000 2500 2000 1500 1000 500 0 450

455

460

465

470

Binding Energy (eV) 4000 3500

STO thin film STO crystal

Intensity (cts)

3000 2500 2000

515

the terrace areas as shown in Fig. 8(a). Fig. 8(b) is a dark field image using a portion of the polycrystalline rings in Fig. 8(d) showing the needle-shaped phase is mainly located in the terrace area and are randomly orientated. Most of the circular regions, however, are free of the needles except for only a few underneath the islands. It is believed that these needles reside at the STO/ Si interface or inside STO rather than on the surface. In order to identify these precipitates, we measured the radius of each polycrystalline ring and converted to lattice spacing using the calibrated camera constant based on the known Si lattice parameters. Table 2 lists the measured lattice spacing and normalized intensity from the STO film together with JCPDS data of several possible Sr-rich phases [18]. The measured data match both the STO as well as the Sr-rich Sr4Ti3O10 phase. Sr4Ti3O10 has a tetragonal structure with a ¼ 3.9 A˚, c ¼ 28.1 A˚ [18], the structure is very close to STO. To further identify which of the two phases is dominant, an elemental mapping of Sr, Ti, Si and O by EELS was conducted. The analysis indicates that the precipitates are definitely richer in Sr compared to the STO matrix as shown in Fig. 9(b). Based on the combined SADP and EELS results, we attribute the precipitates to Sr4Ti3O10 nanocrystallites. 4. Discussion

` 1500 1000 500 0 125

130

135

140

145

Binding Energy (eV) Fig. 7. (a) XPS spectra of Ti 2p core-level peaks from the STO film and the single-crystal substrate; (b) XPS spectra of Sr 3d core-level peaks from the STO film and the single-crystal substrate. The XPS spectrum indicates that the STO film by MBE has chemical composition similar to that of a single-crystal STO; no other peaks from such as SrO, partially oxidized Ti 3+ , Ti 2+, Sr or Ti silicides or silicates were detected.

by a dotted large square) and the other one from the STO [0 0 1] zone axis (labeled by a small square) with weak STO {1 0 0} spots in each square center. There is a 451 rotation in-plane between the two sets. This confirms that this circular region consists mainly of epitaxial STO. However, Moire´ fringes were not observed in the terrace area suggesting lower crystallinity or columnar growth. This is supported also by the SADP from the terrace areas, which consists of polycrystalline rings overlapped with a set of diffraction spots due to the Si [0 0 1] zone axis as shown in Fig. 8(d). Note also the STO {1 0 0} spots are absent, probably due to lower crystalline quality in the terrace areas. In addition to the absence of Moire´ fringes and STO {1 0 0} spots, there are many needle shaped precipitates in

Our RHEED observations confirm the interface between STO and Si is incoherent from the first ML as STO immediately relaxes during and after deposition of the first ML. No coherent wetting layer is observed. The STO thin film up to 14 MLs is subject to an in-plane tensile strain rather than a compressive strain. We therefore exclude the possibility of the Stranski–Krastanov (SK) transition mode. Nucleation of the STO thin film by a 3D island growth mode (Volmer–Weber mode) is also not supported by our RHEED observations indicating a mainly atomically smooth STO surface. We propose that the STO growth is mainly by a 2D growth (Frank–van der Merwe) mixed with localized island growth (5%) up to 14 MLs under stoichiometric or slightly Sr-rich condition. Based on the RHEED observations and surface XPS analysis we believe the islands are large coherent STO crystals. However, we attribute formation of these islands to heterogeneous nucleation at isolated impurity phases such as SiC nuclei that form during SiO2 desorption step at 800–850 1C after removal of the native SiO2 layer. Clean Si surface is very susceptible to C contamination and readily forms SiC islands. Therefore to obtain high crystalline quality and atomically smooth STO layers on Si it is crucial to minimize C-related species in the MBE vacuum chamber. Finally, our plan view TEM images show some precipitates that are most likely Sr4Ti3O10 present at interface as determined by elemental mapping by EELS and SADP. We suggest the formation of the precipitates is driven by local composition fluctuations. The SrO–TiO2

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Fig. 8. Plan view TEM images and corresponding SADPs. (a) Bright field image; (b) dark field image from the same area as (a); (c) SADP from A area indicating polycrystalline precipitates and lower STO crystalline quality; (d) SADP from B circular region indicating higher crystalline quality STO, an STO island is located in the center of B circle.

Table 2 Measured lattice parameters of the precipitates by TEM and JCPDS files from STO and Sr4Ti3O10 phases Measured values

STO JCPDS files

S4T3O10 JCPDS files

Ring diameters (mm)

Measured lattice spacing (nm)

Measured intensity (%)

D spacing (nm)

Intensity

Index

D spacing (nm)

Intensity

Index

19.3 23.8 27.3 33.5 39.0 44.0

0.28 0.23 0.2 0.16 0.14 0.12

100 10 80 50 40 30

0.2759 0.2253 0.1952 0.1594 0.1381 0.1235

100 30 50 40 25 15

110 200 211 111 220 310

0.27973 0.21691 0.20071 0.15997 0.13986 0.12368

100 62 21 14 37 28 12 12 8 58

107 110 115 0014 200 217 1114 2014 220 307 310

phase diagram [19] indicates that the only thermodynamically equilibrium phases on the Sr-rich side are tetragonal Sr4Ti3O10, Sr3Ti2O7 and Sr2TiO5 if no interaction with Si is involved; or Sr /Ti silicates such as Sr2TiSi2O8, Ti2Si4O14 or silicides such as SrTi, Sr2Ti, SrSi2 are formed if interfacial reactions with Si substrates are involved. However, it is believed that SrO and silicon are thermodynamically stable in contact at 1000 K based on experimental observations [20]. Suzuki et al. [17] who studied defects in homoepitaxial non-stoichiometric perovskite STO (ABO3) film found that for the A-site-excess STO thin film excess SrO was accommodated as Ruddlesden-Popper planar faults without forming secondary phases even though this type of

0.27597 0.195 0.16228 0.13789 0.12333

defects is meta-stable due to associated large compressive strain. The microstructure of the B-site-excess STO thin film on the other hand consists of STO and amorphous TiO2-rich phase. Ota et al. [16] observed surface roughening of homoepitaxial STO thin film grown by MBE under Sr-rich condition (up to molar 15%) compared to much flat surface of stoichiometric STO and flat surface of Ti-rich STO with shallow concavities. They suggested that the excess Ti is incorporated in the STO film while the extra Sr is precipitated onto the surface of the film. However no proof of surface SrO precipitates supported by either RHEED or XPS was provided. Our TEM analysis identified only Sr4Ti3O10 phase, neither silicates nor

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Fig. 9. TEM image and elemental mapping by EELS: (a) TEM image; (b) Sr elemental mapping indicating Sr-rich area; (c) Ti elemental mapping; (d) Si elemental mapping; (e) oxygen elemental mapping.

silicides has been observed under the excess Sr conditions, in agreement with Ref. [20]. These precipitates could promote columnar growth and formation of misorientated STO grains in the terrace area as neither Moire´ fringes nor sharp STO{1 0 0} diffraction spots were observed. 5. Conclusions We have systematically studied interfacial strain relaxation as its dependence on the STO thickness during initial epitaxial growth by MBE on (0 0 1) Si. In situ RHEED analysis indicates that STO layer is incoherent even for the first ML. The interfacial strain is dominated by a large tensile strain due to thermal mismatch. Stoichiometric STO

grows by a 2D growth mode, since AFM observation indicates that the STO surface consists mainly of 2D terraces. XPS compositional analysis indicates that no other Sr- or Ti-rich phases are present on the surface. Nonstoichiometry strongly affects surface morphology and surface/interfacial structure. A nanoscale secondary phase Sr4Ti3O10 identified by TEM was observed at the STO/Si interface when the STO film is Sr rich. Acknowledgments We gratefully acknowledge the help of Dr. L.X. Cao for the high-resolution XRD analysis. This work was supported by the United Sates Air Force under Contract No.

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AFRL-33615-02-C-5053, the National Science Foundation under Contract Nos. MRSEC DMR-0076077 and ECS 0123469 and by SVTA Inc. through an MDA-STTR phase II contract.

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