Electronic structure and band alignment at the NiO and SrTiO3 p-n

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Electronic structure and band alignment at the NiO and SrTiO3 p-n heterojunctions Kelvin H L Zhang, Rui Wu, Fengzai Tang, Weiwei Li, Freddy Oropeza, Liang Qiao, Vlado Lazarov, Yingge Du, David J Payne, Judith L. MacManus-Driscoll, and Mark Blamire ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06025 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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ACS Applied Materials & Interfaces

Electronic structure and band alignment at the NiO and SrTiO3 p-n heterojunctions

Kelvin H. L. Zhang1*, Rui Wu1, Fengzai Tang1*, Weiwei Li1, Freddy E. Oropeza2, Liang Qiao3*, Vlado K. Lazarov4, Yingge Du5, David J. Payne2, Judith L. MacManus-Driscoll1, Mark G. Blamire1 1

Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage

Road, Cambridge, CB3 0FS, United Kingdom 2

Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, UK

3

School of Materials, the University of Manchester, Manchester M13 9PL, UK

4

Department of Physics, University of York, Heslington, York YO10 5DD, UK

5

Physical Sciences Division, Physical & Computational Sciences Directorate, Pacific Northwest

National Laboratory, Richland, Washington 99352, USA Email: [email protected], [email protected] and [email protected]

Abstract: Understanding the energetics at the interface including the alignment of valence and conduction bands, built-in potentials, and ionic and electronic reconstructions, is an important challenge in designing oxide interfaces that have controllable multi-functionalities for novel (opto-)electronic devices. In this work, we report detailed investigations on the hetero-interface of wide bandgap p-type NiO and n-type SrTiO3 (STO). We show that despite a large lattice mismatch (~7%) and dissimilar crystal structure, high-quality NiO and Li doped NiO (LNO) thin films can be epitaxially grown on STO(001) substrates through a domain matching epitaxy (DME) mechanism. X-ray photoelectron spectroscopy (XPS) studies indicate that NiO/STO heterojunctions form a type II “staggered” band alignment. In addition, a large built-in potential of up to 0.97 eV was observed at the interface of LNO and Nb doped STO (NbSTO). The LNO/NbSTO p-n heterojunctions exhibit a large rectification ratio of 2×103, but also a large ideality factor of 4.3. The NiO/STO p-n heterojunctions have important implication for applications in photocatalysis and photodetector as the interface provides favourable energetics for facile separation and transport of photogenerated electrons and holes. Keywords: Transparent Conducting Oxides; Oxide Heterojunctions; Electronic Structure; Band Offset; Photocatalysis; NiO; Perovskite Oxide. 1

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Introduction The hetero-interfaces of functional metal oxides have been attracting extensive interest, because they display an exceedingly wide range of intriguing multi-functionalities for design of novel (opto-)electronic and spintronic devices.1, 2 In addition, oxide hetero-interfaces are also being actively pursued in designing material systems for enhanced photocatalytic activity for solar water splitting,3-5 fast ionic conductivity and enhanced electrocatalytic activity for electrochemical energy conversions.6-8 However, many fundamental questions still exist, ranging from the growth of dissimilar oxide materials, interfacial electronic states, to ionic and electronic reconstruction, and their effect on electron/spin/magnetic transport and scattering.9-13 Therefore, similar to the case of traditional semiconductors, an in-depth understanding of the interfacial atomic and electronic structure for oxide heterointerfaces is of vital importance for a better control of their intriguing properties for the advancement of their future applications.14 In this work, we report investigations on the hetero-interface of wide bandgap p-type NiO and n-type SrTiO3 (STO). STO, the prototypical perovskite oxide, is a very important substrate for the growth of oxide thin films and heterointerfaces.15 It has a large bandgap of 3.25 eV and becomes an n-type semiconductor by alio-valent doping or by the formation of oxygen vacancies. It supports the formation of a two-dimensional electron gas on its surface and at the interface with LaAlO3.2, 16 NiO is a classic transition metal oxide that has been investigated extensively because of its correlated electron behaviour.17 The bandgap values between 3.4 eV and 4 eV have been reported in the literature. It becomes the well-known p-type transparent semiconductor by formation of Ni vacancies or Li+ doping.18-20 These properties make NiO itself a very useful material as hole transport layer in thin film photovoltaics,21 as an electrocatalyst for water splitting,22 and as an active layer in resistive switching memory devices.23 The integration of NiO

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with STO will essentially form an all-oxide transparent p-n heterojunction, and is of particular interest for “transparent” electronics, ultraviolet LEDs and detectors, and spintronic applications.24-26 Moreover, recent reports have demonstrated many enhanced functionalities originating from the interfacial effect of NiO-STO. Domen et al. reported NiO loaded STO show high activity toward photocatalytic water splitting,22 although an understanding of the role of NiO/STO interface in this process has remained elusive.27 Very recently, interesting bipolar resistive switching behaviour was reported for both NiO thin films and nanocrystals on Nb doped STO heterojunctions.28-30 The switching mechanism was speculated to be due to electric field modulation of oxygen vacancies and built-in potential at the interface. Despite these studies, there is still limited work devoted to examination of the interfacial properties of this system. These open questions have motivated us to perform a detailed study on the atomic and electronic structures of the NiO/STO interfaces using combined XRD, scanning transmission electron microscopy (STEM) and x-ray photoemission spectroscopy (XPS) for epitaxial NiO and Li doped NiO films grown on STO substrates. Experimental details NiO and 6% Li doped NiO (LNO) films were epitaxially grown on (001)-oriented STO, 0.1%Nb doped STO (NbSTO) and MgO substrates by pulsed laser deposition (PLD) from respective stoichiometric targets. The LNO target was prepared by mixing and grinding the appropriate proportions of Li2CO3 and NiO. The powder was heated at 650 oC for 8 hours, and then pelletized and heated again at 850oC for 12 hours. Laser ablation was performed at a repetition rate of 5 Hz and an energy density of 1.0 J/cm2 with a 248 nm KrF excimer. TiO2-terminated STO and NbSTO substrates were used as the substrates. Films with thicknesses ranging from 2 to 50 nm were grown in 0.12 Torr oxygen, at a substrate temperature of 500°C, and cooled to

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room temperature in 1 Torr O2. The crystal structure and epitaxial relationship were determined by high-resolution XRD using a PANalytical four-circle diffractometer in θ-2θ scans and reciprocal space mapping modes. Cross-sectional scanning transmission electron microscopy (STEM) specimens were prepared with an FEI Helios dual-beam focused ion beam/scanning electron microscope (FIB/SEM) using a standard lift-out approach. A FEI Titan3 80-300 kV TEM with a spherical aberration corrector for the probe-forming lens operating at 300 kV was used for high resolution high-angle annular dark-field (HAADF) STEM imaging and electron energy loss spectrum (EELS) was acquired using a Gatan Tridiem spectrometer fitted with the microscope. Optical absorption measurements were performed at room temperature using a Cary 5000 spectrophotometer in the photon energy range of 0.5–3.5 eV. The electronic structure, band offsets, and bending were measured by high-resolution XPS using monochromatic Al Kα1 x-ray (hγ= 1486.6 eV) with a SPECS PHOIBOS 150 electron energy analyzer. The total energy resolution was 0.50 eV. The binding energy was calibrated using a polycrystalline Au foil placed in electrical contact with the film surfaces after deposition. This also helped avoid charging effects during XPS measurements. For band-offset measurement, thin epitaxial NiO/STO and LNO/STO heterojunctions with thicknesses of a few nanometers were used because of the relatively smaller probe depth for soft XPS of ∼5 nm. The currentvoltage (I-V) characteristics of the p-n heterojunctions were measured in the Pt/LNO/NbSTO/Ag configuration with a Keithley 2400 multi-functional digital source meter at room temperature with. Pt top electrodes were deposited by DC magnetron sputtering through muti-hole shadow mask with a 500 µm diameter. The Ag bottom electrodes were prepared on the bottom of NbSTO using Ag paste. Good ohmic contact was confirmed by fabricating Ag/NbSTO/Ag and

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Pt/LNO/Pt structures; both show linear I-V curves and the current is much larger than that of the LNO/NbSTO. Result and Discussions NiO adopts a rocksalt crystal structure with a lattice constant of 4.177 Å, while the perovskite STO has a lattice constant of 3.905 Å (Figure 1a). Assuming cube-on-cube epitaxy, this results in a large lattice mismatch of 7% between NiO and STO. We firstly demonstrate that NiO thin films can be epitaxially grown on STO(001) through a domain matching epitaxy (DME) mechanism. Figure 1b shows typical θ-2θ XRD out-of-plane scans for a 18 nm thick NiO and 12 nm LNO film grown on STO(001) substrates. The (002) reflections for both NiO and LNO are observed close to the STO(002), while the (001) and (003) reflections are forbidden for the facecentered cubic structure. Detailed scans around the (002) reﬂection reveal both the NiO and LNO films show Kiessig fringes, confirming the high quality of the epitaxial films, as also confirmed by AFM (Figure 1c) and high-resolution STEM images (Figure 2). The inset of Figure 1b shows the increase of the film (002) Bragg angle, i.e. reduction of out-of-plane lattice constant with Li doping. Since Li+ cations with octahedral coordination have a larger ionic radius of Li+ (0.90 Å) than that of Ni2+ (0.83 Å), the reduction of the lattice constant for LNO is most likely due to the smaller size of Ni3+ cations (0.70 Å) associated with hole doping.31 Figure 1d shows a typical reciprocal space map (RSM) around the (113) reflection of STO for the 18 nm NiO film on STO, from which we can extract the in-plane and out-of-plane lattice parameters of the film. Figure 1e show the in-plane and out-of-plane lattice parameters of the films as function of thickness extracted from their corresponding RSMs. The in-plane lattice strain is nearly relaxed for the film as thin as 2 nm (only -0.6% remains). The lattice parameters attain bulk values when the film thickness is more than10 nm. The fast strain relaxation is expected with the large lattice 5



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mismatch (7%) between NiO and STO. According to the Matthews-Blakeslee theory32, the critical thickness for strain relaxation in a system of 7% mismatch is ~ 1.5 nm. Cross-sectional STEM measurements were performed to further examine the interfacial structure and epitaxial relationship. A smooth, uniform film over a large lateral length scale is evident from a low-magnification HAADF image (Figure 2a). Figure 2b shows an atomicresolution HAADF image at the interface region of NiO/STO viewed down to [100] zone axis direction. Since the intensity of an atomic column in HAADF imaging mode is proportional to Zn of the elements (where Z is atomic mass and n=1.5 to 2.0), the brightest spots in the image represent the Sr atomic columns, whereas the less bright spots at the centers of the Sr square lattice are the Ti atomic columns. The atomic structure of the interface is determined by NiO columns directly bonded with TiO2 atomic planes. Furthermore, we also observed periodic misfit dislocations network in the direction. The dislocation network is outlined by the corresponding Fourier filtered images shown in Figure 2c. The average dislocation spacing is ~ 6.2 nm, corresponding to 16 lattice planes of STO matched with 15 planes of NiO. This phenomenon is well consistent with the feature of domain matching epitaxy (DME) for systems with a large lattice mismatch proposed by Narayan & Larson.33, 34 In DME, integral multiples of lattice planes containing densely packed rows are matched across the interface. In such a way, the large mismatch can be effectively accommodated by the periodic dislocations localized at the interface and thereby the epitaxial relationship can be maintained. It should be noted that such a mismatch accommodation pathway by dislocations in DME contrasts with the conventional lattice match epitaxy where an initial coherent-strained growth mode is followed by strain relaxation process, leading to a conversion from two-dimension (2D) to 3D island growth and/or formation of a high density of threading dislocations. In DME, dislocations form as soon as the

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epi-materials start to grow on the substrate. The dislocations are mostly localized at interface, while the film above the interface becomes nearly strain free and grows in 2D mode. The DME in our NiO/STO is similar to the well-studied MgO/STO system where a high quality 2D STO thin films can be grown despite a 7% mismatch between them.35 The electronic and optical properties of NiO and LNO films were characterized using XPS and O-K edge EELS, and optical absorption spectroscopy. The XPS valence band (VB) spectrum of NiO shown in Figure 3a consists of hybridized O 2p and Ni 3d electronic states, with the O 2p character dominating in the region of 4–12 eV, and the Ni 3d-derived feature in 0-4 eV. The valence band maximum (VBM) for NiO was determined to be at 0.8 eV below the Fermi level (Ef) by linear extrapolation of the leading edge of the VB region to the extended baseline of the VB spectra. This linear method has already been proven to consistently yield correct VBMs for semiconductors with an accuracy of about ±0.1 eV.36 The VBM position confirms the p-type character of the undoped NiO, likely due to the formation of Ni vacancies, although whether the hole state is of Ni3+ or O- character is a matter of much debate because of the strong electron correlation in this system.17, 37, 38 The O K-edge EELS measures transitions from the O 1s core level to unoccupied states with partial O 2p character arising from O 2p-Ni 3d hybridization, and thus can be qualitatively related to the unoccupied density of states above EF.39, 40 Therefore the feature at 532 eV shown in Figure 3b corresponds to the unoccupied Ni 3d eg state hybridized with O 2p, forming the bottom of CB. The other higher energy features at 534-543 eV correspond to transitions to the Ni 4s and 4p states. Li doping effectively introduces hole states into the top of VB and moves the VBM 0.35 eV below the EF (Figure 3a). An additional unoccupied state appears at 529 eV in O K-edge EELS (Figure 3b), which can be assigned to be the hole state (acceptor state) induced by Li doping. The overall trend in our spectra is consistent

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with the XPS VB and x-ray absorption spectroscopy for Li doped NiO bulk powders reported by J. Vanelp et al.17 The optical absorption measurements were performed on NiO and LNO films (~30 nm thick) grown on double-side polished MgO(001) substrates. Here the purpose for using MgO was to avoid the strong absorption onset at 3.2 eV from STO substrates. Figure 3c show plots of the optical absorption spectra of both films, along with Tauc bandgap plots of (αhν)2 in the inset. The NiO on MgO is highly transparent, because of the large optical bandgap of NiO (3.65 eV as determined from our measurement). On the other hand, Li doping induces two broad weak absorption features centered at ~ 1.0 eV and 2.2 eV. We assign the two features to the excitations from the VB to the empty hole state observed in O K edge EELS (see Figure 3b and schematic in Figure 3d). We determined the valence band offsets (∆EV) between NiO (LNO) and STO (NbSTO) by XPS using the method of Kraut et al.41, 42 In this method, the binding energies (BE) of some core levels and VBM for both the individual pure materials and their thin heterojunctions (THJ) are needed. Here we used the 30 nm NiO films (Figure 4a) and bare STO substrate (Figure 4e) as references for pure materials, from which Sr 3d5/2, Ni 3p core levels and VBM are measured. Thin NiO and LNO films with thicknesses of 2 nm and 4 nm on STO and NSTO are used as THJs. As an example, the ∆EV for NiO on STO is given by: ∆Ev = (ENi3p – Ev)NiO – (ESr3d5/2 – Ev)STO + (ESr3d5/2 – ENi3p)THJ

where (ENi3p – Ev)NiO is the BE difference between Ni 3p and VBM for NiO, (ESr3d5/2 – Ev)STO is the BE difference between Sr 3d5/2 and VBM for STO, and (ESr3d5/2 – ENi3p)THJ is the BE difference between Sr 3d5/2 and Ni 3p for THJ. The VBM values for thick films, STO and NSTO

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are determined by linear extrapolation of the leading edge of the VB to the baseline, as the method mentioned above. Furthermore, once the ∆EV is known, the conduction band offset (∆EC) can be readily determined by ∆EC=∆EV + (EgNiO-EgSTO), where EgNiO is the bandgap of NiO (LNO) as determined to be ~3.65 eV, and EgSTO is the bandgap of STO (3.25 eV). Table I summarizes values of the BE difference and the resulting ∆EV and ∆EC values for the thin-film heterojunctions. The ∆EV and ∆EC for NiO/STO are 1.60 eV and 2.0 eV respectively, indicating the band alignment is staggered (i.e., a type II heterostructure). Figure 5a depicts the deduced band diagram for NiO/STO, assuming that no band bending occurs at the interface because of the low carrier concentration in the undoped NiO and STO. There is negligible ∆EV difference for 2 nm and 4 nm thick films. The slightly larger ∆EV for LNO/STO (1.68 eV) compared to that for NiO/STO is likely due to the creation of more holes by Li doping which pins the Fermi level closer to the top of VB. The ∆EV for 4 nm thick LNO film on NbSTO are 1.88 eV. Here the use of conductive NbSTO allows us to determine the EF position of the substrate on an absolute scale. From the XPS VB spectrum the BE of VBM of NbSTO was determined to be 3.2 eV. Given STO bandgap is 3.25 eV, this value implies the EF is at 0.05 eV below the CBM, which is consistent with the EF position estimated from the Nb doping concentration. Figure 5b shows the equilibrium band diagram (i.e., Fermi level aligned) of the LNO/NbSTO heterojunction. Knowing the position of the VBM relative to EF of NbSTO and LNO and their respective bandgaps, the net barrier heights for CBM and VBM at the interface can be estimated to 3.25 eV and 2.85 eV, respectively. A comparison of these values with the ∆EV at the interface of the LNO/NbSTO system indicates that there should be an overall 0.97 eV built-in potential (Vbi) at the interface region of the LNO/NbSTO due to the charge transfer across the p-n interfaces.

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The large VB offset and band bending observed at the NiO-STO interface has important implications for NiO loaded STO 22, 27 and other NiO loaded oxide materials used as highly active photocatalysts for water splitting.43 It has been shown NiO-STO is one of the few materials that can achieve overall water splitting without the need for expensive precious metals. However, the role of NiO is still not clear in the literature; it has been interpreted as a protonreduction catalyst, as a water oxidation catalyst, or as a photocathode.27 Our result is in support of NiO as hole trap sites for water oxidization; the band bending at the NiO-STO interface provides a built-in potential to facilitate the separation of photogenerated excitons, driving electrons toward the bulk of STO and holes to the NiO surface for water oxidation. In addition to the built-in potential, the large ∆EC creates a large barrier for electrons, and thus effectively reduces electron-hole recombination at the surface. We suggest that increasing the depth and magnitude of band bending potential by optimizing the doping of each material would be an effective way to further improve the water splitting efficiency for this system. Figure 6 shows the current-voltage (I-V) characteristics for a 20 nm LNO on NbSTO p-n junction. The diode shows distinct rectifying behaviour by applying both reverse and forward bias. The rectifying ratio is about 2×103 at ±2.2 V, which is a high value in comparison with other oxide-based p-n diodes (see inset in Figure 6).44-46 According to the energy band diagram shown in Figure 5b, the value of ∆EV is 0.4 eV less than that of ∆EC, and thus holes should be the dominant carriers under forward bias. In principle a forward voltage of Va= (Vbi+∆Ev)/e= 2.85 V is necessary to overcome the barriers for injecting holes from the LNO into NbSTO. However, the experimental turn-on voltage for our diodes is ~1.8 V. The much lower turn-on voltage indicates the participation of other transport mechanisms. We further fit the I–V characteristics using the Shockley equation: I=Is(exp(qV/nkT)-1), where q is the electron charge,

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n the ideality factor, k the Boltzmann constant, and T the absolute temperature. The ideality factor of our diode is determined to be 4.3 at the forward voltage range of V=1.0-1.8 eV. A large ideality factor (n>2.0) has been observed in several other wide bandgap p-n junctions, and have been attributed to the presence of additional interface states, coupled defect-level recombination, and space-charge-limited conduction.46, 47 Grundmann et al. have recently theoretically derived an ideality factor of 2 for wide-bandgap type-II heterostructures (e.g., p-NiO/n-ZnO and p-CuI/nZnO), assuming the current is entirely due to interface recombination.48 We suggest that interface electronic states induced by the periodic dislocations at the interface are likely to be the recombination centers for enhanced current flow at a lower forward voltages as well as a large ideality factor for our p-n diode. Our discussions on the interface band alignment and transport characteristics of LNO/NbSTO are based on the framework of one electron rigid-band picture. However, hole doped NiO is a well-known strongly correlated electron material. Hence, it is also likely that the charge modulation by applying an external field invokes new electron reconstructions in both the LNO film and the LNO/NbSTO interface which modifies the interface band alignment in a dynamic way.

Conclusions In summary, we have investigated the atomic and band energy alignments at the interface between p-type NiO and n-type STO. We have shown that thin films of rocksalt NiO can be epitaxially grown on perovskite STO(001) through a domain matching epitaxy mechanism. Detailed XPS and optical spectroscopic studies reveal NiO/STO and LNO/NbSTO heterointerfaces form a type II band alignment. A large built-in potential of up to 0.97 eV was also observed at the interface of LNO/NbSTO. The type II band alignment together with a large 11



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built-in potential provides favourable energetics for facile separation and transport of photogenerated electrons and holes. The pn diode of LNO/NbSTO show good rectifying behaviour, although the fitting of I-V characteristics showed a large ideality factor of 4.3. The deviation from an ideal p-n diode behaviour is attributed to interface recombination due to the interfacial electronic states induced by the periodic dislocations and/or the strong electron correlation effect in NiO.

Acknowledgements

K.H.L. Zhang acknowledges the funding support from Herchel Smith Postdoctoral Fellowship by University of Cambridge. J. L. MacManus-Driscoll and W. Li acknowledge support from EPSRC grant EP/N004272/1. D. J. Payne acknowledges support from the Royal Society (UF100105) and (UF150693).

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Figure 1. (a) Schematic model of NiO (rocksalt) on TiO2-terminated STO; (b) XRD θ-2θ scans of the scan of 18 nm NiO and 12 nm LNO films on STO. Inset show the fringes in the vicinity of the STO(002) reflection; (c) AFM image of the 12 nm LNO on STO; (d) a typical reciprocal space map (RSM) of a 18 nm NiO film around the (113) reflection. (e) Change of the in-plane and out-of-plane lattice parameters for NiO and LNO films as function of thickness extracted from RSMs.

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Figure 2 (a) Large area STEM image of NiO film on STO(001); (b) Atomic resolution STEMHAADF image of the interface region; (c) Fourier filtered image, obtained by [020] diffraction reflection from the STO substrate and NiO epilayer, outlining the common (020) planes between the film and substrate; Five dislocations localised at the interface are highlighted, which corresponds to average 16 lattice planes of STO matched with 15 planes of NiO.

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XPS valence band NiO Ni3d8 LNO

(b) O K-edge EELS

Ef

Intensity (a.u.)

Intensity (a.u.)

(a)

O2p

10

8 6 4 2 Binding Energy (eV)

(c)

NiO LNO

3

Hole

528

0

(d)

532 536 540 Electron Energy (eV) LNO

NiO

(αhγ)2

12

Absorption Coeff. (x105 cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CB

CB

2 4.0

3.5

3.0

Ef Ef Ni 3d8

1

O 2p6

Hole Ni 3d8

VB

O 2p6

VB

0 5

4

3 2 1 Photon energy (eV)

0

Figure 3. (a) X-ray photoemission VB spectra (b) oxygen K-edge EELS of the 20 nm thick NiO and LNO films; (c) optical absorption coefficient as a function of photon energy; inset shows a plot of (αhγ)2 vs hγ for the extrapolation of bandgaps of NiO and LNO. (d) Schematic energy diagram for NiO and LNO.

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Sr 3d

Ni 3p

VB

30 nm NiO (a) 4 nm NiO/STO

Intensity (a.u.)

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(b) 4 nm LNO/STO (c) 4 nm LNO/NbSTO (d)

STO

(e)

134 132 130 74 72 70 68 66 64 Binding Energy (eV)

6

4

2

0

-2

Figure 4. XPS VB and Sr 3d, Ni 3p spectra for (a) 30 nm thick NiO film, (b) 4 nm NiO/STO, (c) 4 nm LNO/STO and (d) 4 nm LNO/NbSTO heterojunctions, and (e) bulk STO; the VBM for the 30 nm NiO and bulk STO as determined by linearly extrapolating the leading edge of the VB to the extended baseline of the VB spectra and are shifted to be at zero binding energy. All the spectra for heterojunctions were shifted to align Sr 3d5/2 peaks at 130.50 eV in order to identify the trend for band offsets.

Table 1. The relative binding energies and band offsets for thin film heterojunctions of NiO and LNO on STO and NbSTO. The binding energies were shifted to align the Sr 3d5/2 peaks at 130.50 eV. ESr3d5/2

ENi 3p

∆ESr3d-Ni3p

∆Ev

∆Ec

STO bulk

130.5

̶

̶

̶

̶

NiO bulk

̶

66.50

̶

̶

̶

4 nm NiO/STO

130.5

64.90

65.60

1.60

2.0

4 nm LNO/STO

130.5

64.82

65.68

1.68

2.08

4 nm LNO/NbSTO

130.5

64.62

65.88

1.88

2.28

2 nm NiO/STO

130.5

64.87

65.63

1.63

2.03

2 nm LNO/NbSTO

130.5

64.67

65.83

1.83

2.23

∆Ev = (ENi3p – Ev)NiO – (ESr3d5/2 – Ev)STO + ∆(ESr3d5/2 – ENi3p)THJ

∆‫ ܸܧ∆ = ܥܧ‬+ (EgNiO - EgSTO), where EgNiO and EgSTO are the bandgaps of NiO and STO, taken to be 3.65 eV and 3.25 eV, respectively. 16


Page 17 of 23

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(a)

(b) STO

NbSTO

NiO

LNO CB

CB CB

e- ∆E = 2.28 eV C

CB

∆EC=2.0 eV

Ef ∆Ev=1.60 eV

∆EV= 1.88 eV

VB

VB h+ VB

VB

Figure 5. Band diagrams of (a) NiO/STO and (b) LNO/NbSTO heterojuNctions deduced from XPS measurements; ∆‫ ܸܧ‬and ∆‫ ܥܧ‬are the respective band offsets for VB and CB; the bandgap of STO (NbSTO) and NiO (LNO) are taken to be 3.25 and 3.65 eV, respectively.

Figure 6. The semi-logarithmic current vs voltage characteristics of the LNO/NbSTO p-n heterojunction measured at room temperature; The inset show the corresponding I-V in linear scale and schematic diagram for the device measurement.

17



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Table of Contents graphic

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reference 1.

Hwang, H. Y.; Iwasa, Y.; Kawasaki, M.; Keimer, B.; Nagaosa, N.; Tokura, Y. Emergent

Phenomena at Oxide Interfaces. Nat. Mater. 2012, 11, 103-113. 2.

Ohtomo, A.; Hwang, H. Y. A High-mobility Electron Gas at the LaAlO3/SrTiO3

Heterointerface. Nature 2004, 427, 423-426. 3.

Mayer, M. T.; Lin, Y. J.; Yuan, G. B.; Wang, D. W. Forming Heterojunctions at the

Nanoscale for Improved Photoelectrochemical Water Splitting by Semiconductor Materials: Case Studies on Hematite. Accounts of Chemical Research 2013, 46, 1558-1566. 4.

Moniz, S. J. A.; Shevlin, S. A.; Martin, D. J.; Guo, Z. X.; Tang, J. W. Visible-light

Driven Heterojunction Photocatalysts for Water Splitting - a Critical Review. Energy & Environmental Science 2015, 8, 731-759. 5.

Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting.

Chemical Society Reviews 2009, 38, 253-278. 6.

Seitz, L. C.; Dickens, C. F.; Nishio, K.; Hikita, Y.; Montoya, J.; Doyle, A.; Kirk, C.;

Vojvodic, A.; Hwang, H. Y.; Norskov, J. K.; Jaramillo, T. F. A Highly Active and Stable IrOx/SrIrO3 Catalyst for the Oxygen Evolution Reaction. Science 2016, 353, 1011-1014. 7.

Lee, S.; Zhang, W. R.; Khatkhatay, F.; Wang, H. Y.; Jia, Q. X.; MacManus-Driscoll, J. L.

Ionic Conductivity Increased by Two Orders of Magnitude in Micrometer-Thick Vertical YttriaStabilized ZrO2 Nanocomposite Films. Nano Letters 2015, 15, 7362-7369. 8.

Risch, M.; Stoerzinger, K. A.; Maruyama, S.; Hong, W. T.; Takeuchi, I.; Shao-Horn, Y.

La0.8Sr0.2MnO3-delta Decorated with Ba0.5Sr0.5Co0.3Fe0.2O3-delta: A Bifunctional Surface

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

for Oxygen Electrocatalysis with Enhanced Stability and Activity. Journal of the American Chemical Society 2014, 136, 5229-5232. 9.

Nakagawa, N.; Hwang, H. Y.; Muller, D. A. Why Some Interfaces Cannot be Sharp. Nat.

Mater. 2006, 5, 204-209. 10.

Chakhalian, J.; Millis, A. J.; Rondinelli, J. Whither the Oxide Interface. Nat. Mater. 2012,

11, 92-94. 11.

Chambers, S. A. Epitaxial Growth and Properties of Thin Film Oxides. Surface Science

Reports 2000, 39, 105-180. 12.

Chambers, S. A. Epitaxial Growth and Properties of Doped Transition Metal and

Complex Oxide Films. Advanced Materials 2010, 22, 219-248. 13.

Qiao, L.; Zhang, K. H. L.; Bowden, M. E.; Varga, T.; Shutthanandan, V.; Colby, R.; Du,

Y.; Kabius, B.; Sushko, P. V.; Biegalski, M. D.; Chambers, S. A. The Impacts of Cation Stoichiometry and Substrate Surface Quality on Nucleation, Structure, Defect Formation, and Intermixing in Complex Oxide Heteroepitaxy-LaCrO3 on SrTiO3(001). Advanced Functional Materials 2013, 23, 2953-2963. 14.

Kroemer, H. Nobel Lecture: Quasielectric Fields and Band Offsets: Teaching Electrons

New Tricks. Reviews of Modern Physics 2001, 73, 783-793. 15.

Koster, G.; Kropman, B. L.; Rijnders, G.; Blank, D. H. A.; Rogalla, H. Quasi-ideal

Strontium Titanate Crystal Surfaces Through Formation of Strontium Hydroxide. Applied Physics Letters 1998, 73, 2920-2922. 16.

Meevasana, W.; King, P. D. C.; He, R. H.; Mo, S. K.; Hashimoto, M.; Tamai, A.;

Songsiriritthigul, P.; Baumberger, F.; Shen, Z. X. Creation and Control of a Two-dimensional Electron Liquid at the Bare SrTiO3 Surface. Nat. Mater. 2011, 10, 114-118. 17.

Vanelp, J.; Eskes, H.; Kuiper, P.; Sawatzky, G. A. Electronic-structure of Li-doped NiO.

Phys. Rev. B 1992, 45, 1612-1622. 18.

Sato, H.; Minami, T.; Takata, S.; Yamada, T. Transparent Conducting P-type NiO Thin

Films Prepared by Magnetron Sputting. Thin Solid Films 1993, 236, 27-31. 19.

Gupta, R. K.; Ghosh, K.; Kahol, P. K. Fabrication and Characterization of NiO/ZnO p-n

Junctions by Pulsed Laser Deposition. Physica E-Low-Dimensional Systems & Nanostructures 2009, 41, 617-620.

20


Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20.

Zhang, K. H. L.; Xi, K.; Blamire, M. G.; Egdell, R. G. P-type Transparent Conducting

Oxides. Journal of Physics-Condensed Matter 2016, 28, 383002. 21.

Irwin, M. D.; Buchholz, B.; Hains, A. W.; Chang, R. P. H.; Marks, T. J. p-Type

Semiconducting Nickel Oxide as an Efficiency-enhancing Anode Interfacial Layer in Polymer Bulk-heterojunction Solar Cells. Proceedings Of the National Academy Of Sciences Of the United States Of America 2008, 105, 2783-2787. 22.

Domen, K.; Kudo, A.; Onishi, T. Mechanism of Photocatalytic Decomposition of Water

into H2 and O2 over NiO-SrTiO3. Journal of Catalysis 1986, 102, 92-98. 23.

Son, J. Y.; Shin, Y. H. Direct Observation of Conducting Filaments on Resistive

Switching of NiO Thin Films. Applied Physics Letters 2008, 92 (22), 3. 24.

Ohta, H.; Kamiya, M.; Kamaiya, T.; Hirano, M.; Hosono, H. UV-detector Based on pn-

Heterojunction Diode Composed of Transparent Oxide Semiconductors, p-NiO/n-ZnO. Thin Solid Films 2003, 445, 317-321. 25.

Zhang, K. H. L.; Du, Y. G.; Papadogianni, A.; Bierwagen, O.; Sallis, S.; Piper, L. F. J.;

Bowden, M. E.; Shutthanandan, V.; Sushko, P. V.; Chambers, S. A. Perovskite Sr-Doped LaCrO3 as a New p-Type Transparent Conducting Oxide. Advanced Materials 2015, 27, 51915195. 26.

Yu, X. G.; Marks, T. J.; Facchetti, A. Metal oxides for Optoelectronic Applications. Nat.

Mater. 2016, 15, 383-396. 27.

Townsend, T. K.; Browning, N. D.; Osterloh, F. E. Overall Photocatalytic Water

Splitting with NiOx-SrTiO3 - a Revised Mechanism. Energy & Environmental Science 2012, 5, 9543-9550. 28.

Sullaphen, J.; Bogle, K.; Cheng, X.; Gregg, J. M.; Valanoor, N., Interface Mediated

Resistive Switching in Epitaxial NiO Nanostructures. Applied Physics Letters 2012, 100, 203115. 29.

Zhao, M.; Zhu, Y. D.; Wang, Q. W.; Wei, M. C.; Liu, X. L.; Zhang, F.; Hu, C.; Zhang, T.

T.; Qiu, D.; Li, M. Y.; Xiong, R., Electric Field-induced Coexistence of Nonvolatile Resistive and Magnetization Switching in Pt/NiO/Nb:SrTiO3 Heterostructure. Applied Physics Letters 2016, 109, 013504. 30.

Chen, S. C.; We, C. K.; Kuo, T. Y.; Peng, W. C.; Lin, H. C. Characterization and

Properties of NiO Films Produced by rf Magnetron Sputtering with Oxygen Ion Source Assistance. Thin Solid Films 2014, 572, 51-55. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

31.

Page 22 of 23

Goodenough, J. B.; Wickham, D. G.; Croft, W. J. Some Magnetic and Crystallographic

Properties of the System Lix+Ni1-2x++Nix+++O. Journal of Physics and Chemistry of Solids 1958, 5, 107-116. 32.

Matthews, J. W.; Blakeslee, A. E. Defects in Epitaxial Multilayers: 1. Misfit

Dislocations. Journal of Crystal Growth 1974, 27, 118-125. 33.

Narayan, J.; Larson, B. C. Domain epitaxy: A Unified Paradigm for Thin Tilm Growth.

Journal of Applied Physics 2003, 93, 278-285. 34.

Zhang, K. H. L.; Lazarov, V. K.; Galindo, P. L.; Oropeza, F. E.; Payne, D. J.; Lai, H. H.

C.; Egdell, R. G. Domain Matching Epitaxial Growth of In2O3 Thin Films on alphaAl2O3(0001). Crystal Growth & Design 2012, 12, 1000-1007. 35.

Zhu, Y. Y.; Song, C. Y.; Minor, A. M.; Wang, H. Y. Cs-Corrected Scanning

Transmission Electron Microscopy Investigation of Dislocation Core Configurations at a SrTiO3/MgO Heterogeneous Interface. Microsc. microanal. 2013, 19, 706-715. 36.

Chambers, S. A.; Droubay, T.; Kaspar, T. C.; Gutowski, M. Experimental Determination

of Valence Band Maxima for SrTiO3, TiO2, and SrO and the Associated Valence Band Offsets with Si(001). Journal of Vacuum Science & Technology B 2004, 22, 2205-2215. 37.

Dawson, J. A.; Guo, Y.; Robertson, J. Energetics of Intrinsic Defects in NiO and the

Consequences for its Resistive Random Access Memory Performance. Applied Physics Letters 2015, 107, 122110. 38.

Chen, H. R.; Harding, J. H. Nature of the Hole States in Li-doped NiO. Phys. Rev. B

2012, 85, 115127. 39.

Degroot, F. M. F.; Grioni, M.; Fuggle, J. C.; Ghijsen, J.; Sawatzky, G. A.; Petersen, H.

Oxygen 1s X-ray-absorption Edges of Transition-metal oxides. Phys. Rev. B 1989, 40, 57155723. 40.

Zhang, K. H. L.; Du, Y.; Sushko, P. V.; Bowden, M. E.; Shutthanandan, V.; Sallis, S.;

Piper, L. F. J.; Chambers, S. A. Hole-induced Insulator-to-metal Transition in La1-xSrxCrO3 Epitaxial Films. Phys. Rev. B 2015, 91, 155129. 41.

Kraut, E. A.; Grant, R. W.; Waldrop, J. R.; Kowalczyk, S. P. Precise Determination of

the Valence-band Edge in X-ray Photoemission Spectra - application to measurement of semiconductor interface potentials. Physical Review Letters 1980, 44, 1620-1623.

22


Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


42.

Kraut, E. A.; Grant, R. W.; Waldrop, J. R.; Kowalczyk, S. P. Semiconductor Core-level

to Valence-band Maximum Binding-energy Difference - Rrecise Determination by X-ray Photoelectron-spectroscopy. Phys. Rev. B 1983, 28, 1965-1977. 43.

Kato, H.; Asakura, K.; Kudo, A. Highly Efficient Water Splitting into H-2 and O-2 over

Lanthanum-doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure. Journal of the American Chemical Society 2003, 125, 3082-3089. 44.

Yang, H.; Luo, H. M.; Wang, H.; Usov, I. O.; Suvorova, N. A.; Jain, M.; Feldmann, D.

M.; Dowden, P. C.; DePaula, R. F.; Jia, Q. X. Rectifying Current-voltage Characteristics of BiFeO3/Nb-doped SrTiO3 Heterojunction. Applied Physics Letters 2008, 92, 112508. 45.

Sawa, A.; Fujii, T.; Kawasaki, M.; Tokura, Y., Highly Rectifying

Pr0.7Ca0.3MnO3/SrTi0.9998Nb0.0002O3 p-n junction. Applied Physics Letters 2005, 86, 213001. 46.

Grundmann, M.; Klupfel, F.; Karsthof, R.; Schlupp, P.; Schein, F. L.; Splith, D.; Yang,

C.; Bitter, S.; von Wenckstern, H. Oxide Bipolar Electronics: Materials, Devices and Circuits. Journal of Physics D-Applied Physics 2016, 49, 092109. 47.

Schein, F. L.; von Wenckstern, H.; Grundmann, M. Transparent p-CuI/n-ZnO

Heterojunction Diodes. Applied Physics Letters 2013, 102, 092109. 48.

Grundmann, M.; Karsthof, R.; von Wenckstern, H. Interface Recombination Current in

Type II Heterostructure Bipolar Diodes. Acs Applied Materials & Interfaces 2014, 6, 1478514789.

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Electronic structure and band alignment at the NiO and SrTiO3 p-n

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