Ge incorporated epitaxy of (110) rutile TiO2 on (100) Ge single crystal at low temperature by pulsed laser deposition

    Ge incorporated epitaxy of (110) rutile TiO 2 on (100) Ge single crystal at low temperature by pulsed laser deposition Takahiro Nagata, Kazuyoshi Kobashi, Yoshiyuki Yamashita, Hideki Yoshikawa, Chinnamuthu Paulsamy, Yoshihisa Suzuki, Toshihide Nabatame, Atsushi Ogura, Toyohiro Chikyow PII: DOI: Reference:

S0040-6090(15)00795-6 doi: 10.1016/j.tsf.2015.08.031 TSF 34579

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

20 May 2015 14 August 2015 18 August 2015

Please cite this article as: Takahiro Nagata, Kazuyoshi Kobashi, Yoshiyuki Yamashita, Hideki Yoshikawa, Chinnamuthu Paulsamy, Yoshihisa Suzuki, Toshihide Nabatame, Atsushi Ogura, Toyohiro Chikyow, Ge incorporated epitaxy of (110) rutile TiO2 on (100) Ge single crystal at low temperature by pulsed laser deposition, Thin Solid Films (2015), doi: 10.1016/j.tsf.2015.08.031

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Ge incorporated epitaxy of (110) rutile TiO2 on (100) Ge single crystal at low temperature by pulsed laser

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deposition

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Takahiro Nagataa.b.*, Kazuyoshi Kobashia,c, Yoshiyuki Yamashitaa,d, Hideki Yoshikawaa,d, Chinnamuthu

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Paulsamya, Yoshihisa Suzukia,c, Toshihide Nabatamea, Atsushi Ogurac and Toyohiro Chikyowa Nano-Electronics Materials Unit, International Center for Materials Nanoarchitectonics, National Institute

for Materials Science (WPI-MANA), 1–1 Namiki Tsukuba, Ibaraki 305–0044, Japan JST, PREST, 4-1-8 Honcho, Kawaguchi, Saitama, 332–0012, Japan

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Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa 214–8571, Japan

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Synchrotron X-ray Station at SPring-8, NIMS, 1-1-1 Koto, Sayo-cho, Sayo, Hyogo 679–5148, Japan.

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* Corresponding author; Tel.: +81-29-860-4546, Fax:+81-29-860-4916

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E-mail address: [email protected]

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Abstract

The thin film growth of (110) rutile TiO2 on a (100) Ge substrate at a substrate temperature of 450°C, which is generally the growth temperature of anatase TiO2, was demonstrated by using pulsed laser deposition. X-ray diffraction and x-ray photoelectron spectroscopy revealed that the incorporation of Ge into TiO2 enhances

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the rutile phase formation, and the ambient oxygen condition enhances the Ge oxide diffusion. Photoelectron spectroscopy also revealed that the valence band offset of rutile TiO2 and p-type Ge is approximately 2.5±0.1 eV with a type II band alignment. Keywords: rutile TiO2, gate oxide, germanium, Pulsed laser deposition. 1. Introduction A Ge channel has been attracting a lot of attention as a replacement for the Si channel used in current Si-based metal-oxide-semiconductor (CMOS) devices. This is because a Ge channel has high electron and hole mobility, which lead to a higher drive current [1,2], and Ge has a narrower band gap than Si thus allowing supply voltage scaling [3,4]. However, Ge has the same issue as Si, namely an unintentionally oxidized layer with a low dielectric constant (~5.6) can form at an oxide/Ge interface. Furthermore, in contrast with SiO2,

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which is a good insulating layer for MOS devices, GeOx is thermodynamically unstable [5,6] and water soluble [7]. These properties cause high defect densities at the interface between high-k and Ge and a large hysteresis in

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the capacitance-voltage (C-V) characteristic [8,9]. To overcome these problems, some groups have

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demonstrated the formation of stable GeO2 [10,11] and the surface passivation of Ge such as GeN (GeON)

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passivation [12,13], Si passivation [14], and F passivation [15]. These approaches achieved the improvements of electrical properties such as reduced C-V characteristic hysteresis and reduced leakage current. However, the dielectric constant of these passivation layers is still low and is insufficient for further equivalent oxide thickness (EOT) scaling for future logic devices. Therefore, we have proposed the direct growth of rutile TiO 2

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on Ge. The formation energy of the TiO2 (ΔG= −887.625 kJ/mol) is low compared with that for GeO2 (ΔG= −518.5 kJ/mol) [16,17], which makes it possible to suppress the formation of GeOx. In addition, rutile TiO2 has a much higher dielectric constant (k = 90~170, depending on lattice orientation) than GeOx and other common

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dielectric materials such as HfO2 [18,19], which enables further EOT scaling. According to the bulk phase diagram of TiO2 [20], rutile TiO2 is a high temperature phase that generally requires a high growth temperature of above 600 °C although rutile TiO2 is more thermally stable and has higher dielectric constant than other TiO2

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polymorphs. However, with thin film growth, the crystalline phase of TiO2 depends on the deposition conditions

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such as substrate orientation, substrate temperature, and oxygen partial pressure [21,22]. In this study, we have demonstrated the low temperature growth of rutile TiO2 on Ge by pulsed laser deposition conditions.

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deposition (PLD), and investigated the crystal structure dependence of TiO2 and the TiO2/interface on the

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2. Experimental apparatus and method A p-type (100) Ge substrate with a resistivity of 0.01-0.05 Ω cm was used as a substrate. A native oxide (GeOx) was completely removed by annealing at 420 °C for 10 min under an ultra-high vacuum (UHV) condition (<6.7×10−6 Pa) before the TiO2 deposition, which was confirmed by the Ge 2×1 surface reconstruction pattern of reflection high-energy electron diffraction (RHEED) as shown in Fig. 1(a). The TiO2 film was deposited on the Ge substrate by PLD with a KrF excimer laser (λ = 248 nm) at a substrate temperature of 450 to 600 °C under various ambient gas conditions ranging from UHV to an oxygen partial pressure of 1.3×10 −3 Pa. The laser energy density and frequency were set at 0.7 J/cm2 and 5 Hz, respectively, and a sintered TiO2 ceramics target was used for the PLD. The crystallization was observed in-situ by using RHEED. The crystal structure was identified by the X-ray diffraction (XRD) method with a 5 kW rotating anode Cu target and a high-resolution 2D detector (Bruker AXS, D8 Discover Super Speed with GADDS). Part of the Debye-Scherrer

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ring was two-dimensionally detected with the 2D-detector system. In this system, the 2θ and ψ angles can be

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detected simultaneously. Cross-sectional transmission electron spectroscopy (TEM; JEOL, JEM-2100F) was

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employed for the direct imaging of the TiO2/Ge interface. The chemical bonding states were determined by hard x-ray photoelectron spectroscopy (HAXPES) at the SPring-8 BL15XU undulator beamline with a hard x-ray of

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5950.2 eV monochromatized using a Si (111) double monochromator, a Si (333) channel-cut post-monochromator and Gammadata Scienta R4000 spectrometer [23] , and conventional x-ray photoelectron spectroscopy (SXPES) using a monochromated Al Kα x-ray source (Thermo Scientific, Theta Probe: hν= 1486.6 eV). HAXPES is a suitable tool for investigating a heterostructure interface because it has a longer mean

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free path than conventional x-ray photoelectron spectroscopy (SXPES). With Ge, the inelastic mean free path (IMFP) of a photoelectron from the Ge 2p core level for HAXPES and SXPES calculated using the

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Tanuma-Powell-Penn (TPP-2M) equation are 7.8 and 0.9 nm, respectively. [24] The total energy resolutions of HAXPES and SXPES were 240 and 700 meV, respectively. To determine the absolute binding energy, the XPS data were calibrated against the Au 4f7/2 peak (84.0 eV) and the Fermi level position of Au set at the same

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3. Results and Discussion

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ground level as the sample.

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Fig. 1(b) shows the RHEED pattern for TiO2 deposited on Ge under various deposition conditions. The RHEED patterns for TiO2 at a substrate temperature of 300 °C in UHV and for TiO2 at a substrate temperature of 300 °C in oxygen (oxygen partial pressure of 1.3×10−3 Pa) exhibit halo and very weak broad spot patterns, respectively, meaning that the crystal structures of TiO2 deposited at a substrate temperature of 300 °C are still

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amorphous and/or microcrystalline. At substrate temperatures above 450 °C, RHEED patterns are observed for both conditions. The TiO2 at a substrate temperature of 450 °C in UHV (denoted as UHV-TiO2) exhibits a mixture of weak and strong streak patterns, suggesting the existence of a secondary phase and/or rotated domain structure. TiO2 at a substrate temperature of 450 °C in oxygen (oxygen partial pressure of 1.3×10−3 Pa) (denoted as O2-TiO2) shows spot patterns, and the spacing between the planes in the atomic lattice is the same as with the weak streaked pattern of UHV-TiO2. However, we could not observe the RHEED pattern from different azimuth angles due to the structural problem of the PLD system. For this reason, XRD measurements were performed on UHV- and O2-TiO2 to identify the crystal structure of crystallized TiO2 film in detail. Fig. 2(a) and (b) respectively show 2-dimensional XRD images of UHV- and O2-TiO2 with a thickness of 20 nm. UHV- and O2-TiO2 have spot shaped diffraction peaks at 18.9 and 27.3 °, respectively, indicating that they are highly oriented structures. With O2-TiO2, the peak can be assigned as the (110) reflection of the rutile

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TiO2. In contrast, with UHV-TiO2, according to the standard x-ray diffraction powder pattern [25] , there are two candidates for the γ-phase TiO2 structures; γ-Ti3O5 (200) with a monoclinic structure and γ-TiO2 (020) with an

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orthorhombic structure. To confirm the crystal structure and infer whether the film grew epitaxially on the (001)

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Ge, x-ray pole figure measurements were performed as shown in Fig. 2(c)-(f). With O2-TiO2, there is also a

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four-fold symmetry structure with an 8-spot pattern (ψ= 67.5°) as shown in Fig. 2(c). A theoretical pole figure image of the {101} plane for rutile TiO2 is shown in Fig. 2(d). If the [001] direction of rutile TiO2 (110) is grown perpendicularly in the [100] direction of Ge (001), the pole figure pattern is a rectangular 4-spot pattern. These results indicate that the crystal structure of TiO2 is rutile and includes a 90° degree rotated domain. With

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UHV-TiO2, two patterns originating from two different crystal structures were confirmed as shown in Fig. 2(c) and (d). In Fig. 2(c), peaks with four-fold symmetry with 90° in the ϕ scan can be seen at ψ = 45.7° for the

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{−312} plane of γ-Ti3O5. In Fig. 2(d), peaks with four-fold symmetry with 90° in the ϕ scan can be seen at ψ = 45° for the {111} plane of γ-TiO2. Consequently, with O2-TiO2, rutile TiO2 was obtained, whose

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crystallographic relationships with Ge were (110) TiO2 || (001) Ge and [100] TiO2 || [100] Ge with the 90° rotated domain. There are three possible candidates for the origin of the rutile TiO2 crystallization; mismatch,

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Ge surface structure, and Ge incorporation. The lattice mismatch between [100] Ge // 2 × [001]TiO2 is −4 %. In addion, the lattice mismatch between 4 ×[010]Ge // 5 × [10–1]TiO2 is −1 %. A 90° rotated domain should

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effectively compensate for the anisotropic mismatch. A similar type of epitaxial growth has been observed for ZnO on a c-plane sapphire substrate, which have large mismatch and show a 30° rotated domain structured epitaxial thin film [26,27]. Furthermore, according to a previous report, Ge and Si surfaces have oriented

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dangling bonds, which rotate by 90° in an anteroposterior step structure at the surface [28,29]. However, rutile TiO2 is a high temperature phase. With (100) SrTiO3, which is used as a substrate for the epitaxial growth of anatase TiO2 [30], a TiO2 film deposited under the same conditions as O2-TiO2 was amorphous (not shown). So we also considered Ge incorporation. According to a previous report, some dopants promote the crystallization of the rutile phase [31]. TEM and PES measurements were performed to investigate the interface structure between TiO2/Ge and the Ge bonding state. Fig. 3(a) shows cross-sectional TEM images of UHV- and O2-TiO2. There is no interfacial oxidization layer at either interface. However, with UHV-TiO2, an intermixing layer was observed at the TiO2/Ge interface with a thickness of less 1 nm. In contrast, O2-TiO2 showed a strained and/or defective region at the interface. Furthermore, Fig. 3(b) shows the composition depth profiles measured by energy dispersive x-ray spectrometry (EDX). At both interfaces, the diffusion of Ge into the TiO2 layer was confirmed. The diffusion lengths of UHV-

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and O2-TiO2 were approximately estimated to be 3 ±1 and 6 ±1 nm, respectively. This Ge diffusion into the TiO2 layer and the difference in interface structure were observed in the XPES measurements as differences in the

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chemical bonding states as shown in Fig. 4. Fig. 4(a) shows HAXPES and SXPES of Ti 2p2/3 spectra for UHV-

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and O2-TiO2 at various take-off angels (TOAs); here the TOA of 72.5° is surface sensitive, while that of 27.5° is

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bulk sensitive (near the interface). With UHV- TiO2, there are shoulder peaks on the lower binding energy side (457.6 eV) in addition to the TiO2 bonding state at 459.3 eV, which is attributed to the defective TiO2 bonding state [32]. With HAXPES, this peak was pronounced. As the TOAs increased, the intensity ratio of defective TiO2 to TiO2 decreased. In terms of the IMFP of Ti 2p2/3 (HAXPES; 8.1 nm and SXPES; 2.1 nm), most of the

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HAXPES and SXPES detection regions are at the TiO2/Ge interface and the TiO2 layer, respectively, meaning that there is a defective TiO2 region near the interface. Furthermore, Fig. 4(b) shows Ge 2p spectra obtained with HAXPES and SXPES for UHV- and O2-TiO2. As a reference HAXPES and SXPES results are shown for a

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Ge substrate with a naturally oxidized layer. A GeO2 bonding state was observed for both structures at around 1221 eV with SXPES, meaning that the valence number of the Ge diffused into the TiO2 layer is +4. According to the previous report, GeO2 also has the rutile structure with the lattice constants of a= 0.44 and 0.29 nm [33].

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By following the Vegard's law, when the diffused Ge substitutes the Ti site in TiO2 and reaches 30 mol% of Ge

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in the TixGe1-xO2 system, the lattice mismatches between [100]Ge // 2 × [001]TiO2 and 4 ×[010]Ge // 5 × [10–1]TiO2 become below ±1 %, meaning that the Ge incorporation can enhance the epitaxial growth of rutile TiO2 from

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the point of view of the crystallographic match. However, the maximum solid solubility of GeO2 in TixGe1-xO2 solid solution system is approximately 25 mol% fabricated under the high temperature and high pressure conditions [34,35], suggesting that there may be mixed structures of rutile GeO2 and TiO2 at the interface. Furthermore, the intensity ratio of Ge 2p to Ti 2p2/3 obtained by the SXPES of O2-TiO2 (1.51) is higher than that

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of UHV- TiO2 (0.13). Note that the probing depth of Ge 2p SXPES is approximately 2.7 nm (probing depth: 3×IMFP [24]), meaning that the Ge reached at near the TiO2 surface. These results suggests that the ambient oxygen condition oxidizes the Ge surface and enhances the GeOx diffusion into the TiO2 layer, which is consistent with the EDX results. As regards MOS application, band alignment is also important. The band alignment of Ge and rutile TiO2 was investigated using the PES spectra as shown in Fig. 5. The valence band maximum (VBM) of TiO2 and Ge was estimated by a linear extrapolation of the onset energy of SX and HAXPES in the valence band region as 2.7±0.1 and 0.2±0.1 eV as indicated in Fig. 5(a), respectively. According to a previous report, the band gaps of Ge and rutile TiO2 are 0.80 and 3.0 eV, respectively [36] The estimated valence band offset between Ge and rutile TiO2 is 2.5±0.1 eV, which is a type II band alignment and a p-n junction structure, revealing that the intrinsic TiO2 properties are unsuitable for a Ge based MOS structure. However, there is still room to improve

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these properties by TiO2 acceptor doping. Additionally, according to a previous report, some TiO2 acceptors

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such as Mg and Mn promote the crystallization of the rutile phase [31].

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4. Conclusions

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We have proposed the crystallization of rutile TiO2 on Ge for the next generation Ge based MOS structure. In this study, we attempted to fabricate (110) oriented rutile TiO2 film on a Ge substrate by PLD and investigate the crystal structure of TiO2 and the orientation relationship between TiO2 and Ge by RHEED and XRD. The RHEED and XRD results show that rutile TiO2 (110) is grown on Ge (001) epitaxially. In addition, XPS

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analysis revealed the incorporation of Ge diffusion in the rutile TiO2 formation and the suppression of GeO2

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formation. These results suggest that rutile TiO2 has potential for use in future logic devices. Acknowledgements

We are grateful to HiSOR Hiroshima University and JAEA/SPring-8 for the development of HAXPES at

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BL15XU in SPring-8, and Dr. S. Ueda and Mr. T. Ishimaru for technical support with the HAXPES

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measurements at BL15XU in SPring-8. The HAXPES measurements were performed with the approval of the NIMS Beamline Station (Proposal No. 2011B4611 and 2012A4613). We are also grateful to Mr. T. Takei for

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technical support with the TEM observation. WPI-MANA was established by the World Premier International Research Center Initiative (WPI), the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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Fig. 1. RHEED patterns of (a) Ge (100) substrate and (b) TiO2 films on Ge (100) substrates at substrate temperatures of 300, 450, and 600°C under a UHV (UHV-TiO2) and oxygen partial pressure of 1.3×10−3 Pa (O2-TiO2). The incidence azimuth of the electron beam is the [110] direction of the Ge substrate. Fig. 2. 2D-XRD images of (a) O2- and (b) UHV-TiO2. XRD pole figure images of (c) O2-TiO2, (d) theoretical patterns of the {101} plane for rutile TiO2, and (e), (f) UHV-TiO2.

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Fig. 3. (a) Cross-sectional TEM images and (b) a composition depth profile measured by EDX of

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UHV- and O2-TiO2.

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Fig. 4. (a) PES for Ge 2p for UHV- and O2-TiO2. Dashed and solid lines show spectra obtained by SXPES and HAXPES, respectively. (b) SXPES for Ti 2p2/3 for UHV- and O2-TiO2 at

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various take-off angles (TOAs). Dashed and dashed-dotted lines indicate the fitting spectra and background at a TOA of 27.5°. The inset shows the measurement setup. Fig. 5. (a) Valence band spectra for O2-TiO2 measured by SXPES and HAXPES. (b)

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Experimentally obtained band diagram of O2-TiO2/Ge heterostructure.

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Research highlights

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▶ We grew (110) rutile TiO2 film on (100) Ge substrate.

▶ Ge diffusion enhances the crystallization of rutile TiO2.

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▶ Rutile TiO2 crystallizes at the crystallization temperature of anatase phase.

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▶ Band alignment between rutile TiO2 and p-type Ge is type II band alignment.