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

1Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage

Road, Cambridge, CB3 0FS, United Kingdom 2Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, UK3School of Materials, the University of Manchester, Manchester M13 9PL, UK4 Department of Physics, University of York, Heslington, York YO10 5DD, UK5Physical Sciences Division, Physical & Computational Sciences Directorate, Pacific Northwest

National Laboratory, Richland, Washington 99352, USA

Email: ft274@cam.ac.uk and liang.qiao@manchester.ac.uk

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

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

2

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

3

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

current-voltage (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

4

Ag/NbSTO/Ag and 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 face-centered 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

5

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 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 atomic-resolution 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 <100> 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

6

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 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

7

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

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

8

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 (ΔEV) 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 ΔEV for NiO on STO is given by:

ΔEv = (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

are determined by linear extrapolation of the leading edge of the VB to the baseline, as the

method mentioned above. Furthermore, once the ΔEV is known, the conduction band offset (ΔEC)

can be readily determined by ΔEC=ΔEV + (EgNiO-Eg

STO), 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 ΔEV and ΔEC values for the thin-film

heterojunctions. The ΔEV and ΔEC 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 ΔEV difference for 2

nm and 4 nm thick films. The slightly larger ΔEV for LNO/STO (1.68 eV) compared to that for

9

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 ΔEV 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 ΔEV 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.

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 proton-reduction

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 ΔEC creates a large barrier for electrons, and thus effectively reduces

10

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 ΔEV is 0.4 eV less than that of ΔEC, and thus holes should be the

dominant carriers under forward bias. In principle a forward voltage of Va= (Vbi+ΔEv)/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, 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/n-ZnO),

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

11

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

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

12

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).

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-

13

0 10 20 30 40 50

4.15

4.16

4.17

4.18

4.19

4.20

NiO on STO

6% Li:NiO on STO

in-plane out-of plane

Latti

ce c

onst

ant (

Å)

Film thickness (nm)

20 25 30 35 40 45 50 55 60 65 70 75

42 44 46 48

STO

(003)

STO

(002)

STO

(001)

NiO

6%Li:NiO

Log

Inte

nsity

(a.u

.)

(degree)

NiO

6%Li:NiO

(a) (d)

(b)

3.2 3.4 3.6 3.87.0

7.2

7.4

7.6

7.8

(113)

q z II [0

01] (

Å-1

) STO

NiO

qxyII [110] (Å-1)

(e)

(c)

1 μm

plane lattice parameters for NiO and LNO films as function of thickness extracted from RSMs.

14

Figure 2 (a) Large area STEM image of NiO film on STO(001); (b) Atomic resolution STEM-HAADF 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.

15

(a) (c)

SrTi

SrTiNiNi

Sr

Sr

(b)

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 hγfor the extrapolation of bandgaps of NiO and LNO. (d) Schematic energy diagram for NiO and LNO.

16

528 532 536 540

Inte

nsity

(a.u

.)

O K-edge EELS

Electron Energy (eV)12 10 8 6 4 2 0

Ni3d8

Ef

O2p

XPS valence band NiO LNO

Inte

nsity

(a.u

.)

Binding Energy (eV)

Hole

(b)

O 2p6

Ni 3d8

CB

Ef

O 2p6

Ni 3d8

CB

VB

Ef Hole

VB

(a)

5 4 3 2 1 00

1

2

3

Abs

orpt

ion

Coe

ff. (x

105

cm-1

)

Photon energy (eV)

NiO LNO

4.0 3.5 3.0

(h

)2

(c) (d) NiO LNO

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 3pΔESr3d-

Ni3pΔEv ΔEc

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

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

17

134 132 130 74 72 70 68 66 64 6 4 2 0 -2

STO

Sr 3d VBNi 3p

Inte

nsity

(a.u

.)

(a)

(b)

(c)

(d)

(e)

4 nm LNO/NbSTO

Binding Energy (eV)

4 nm NiO/STO

4 nm LNO/STO

30 nm NiO

∆𝐸𝐶 = ∆𝐸𝑉 + (EgNiO - Eg

STO), where EgNiO and Eg

STO are the bandgaps of NiO and STO, taken to be 3.65 eV and 3.25 eV, respectively.

Figure 5. Band diagrams of (a) NiO/STO and (b) LNO/NbSTO heterojuNctions deduced from XPS measurements; ∆𝐸𝑉 and ∆𝐸𝐶 are 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.

18

3 2 1 0 -1 -2 -310-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Cur

rent

(mA

)

Applied Voltage (V)

3 2 1 0 -1 -20

5

10

Cur

rent

(A

)

LaCoO3p-NiOn-SrTiO3

V

Ef

NbSTO LNO

ΔEv=1.60 eV

STO NiO

e-

h+

(a) (b)

ΔEC= 2.28 eVCB

VB

CB

VB

ΔEV= 1.88 eV

CB

VB

CB

VB

ΔEC=2.0 eV

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.

Table of Contents graphic

19

Ef

SrTiO3 NiO

e-

h+

ΔEC= 2.28 eV

ΔEV= 1.88 eV

CB

VB

CB

VBLaCoO3

p-NiOn-SrTiO3

V

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