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Band alignment of nonpolar (101¯ 0) ZnO on (112) LaAlO3
Accepted Manuscript Band alignment of nonpolar (10 Image 10) ZnO on (112) LaAlO3
Jianli Wang, Xinfeng Chen, Aixia Zhang, Dongmei Bai, Gang Tang, Junting Zhang, C. Stampfl PII:
S0038-1098(18)30284-9
DOI:
10.1016/j.ssc.2018.10.002
Reference:
SSC 13511
To appear in:
Solid State Communications
Received Date:
20 May 2018
Accepted Date:
02 October 2018
Please cite this article as: Jianli Wang, Xinfeng Chen, Aixia Zhang, Dongmei Bai, Gang Tang, Junting Zhang, C. Stampfl, Band alignment of nonpolar (10 Image 10) ZnO on (112) LaAlO3, Solid
State Communications (2018), doi: 10.1016/j.ssc.2018.10.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Band alignment of nonpolar (10𝟏0) ZnO on (112) LaAlO3
Jianli Wang1,2,, Xinfeng Chen1, Aixia Zhang1, Dongmei Bai3, Gang Tang1, Junting Zhang1, C. Stampfl2,* 1
School of physical science and technology, China University of Mining and Technology, Xuzhou 221116, China
2 3
School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia
School of Mathematics, China University of Mining and Technology, Xuzhou 221116, China
Abstract The stability and band alignments of ZnO on (112) LaAlO3 substrates are studied by firstprinciples calculations. We have created the ZnO/LaAlO3 heterostructure by consideration of the initial adsorption of the first ZnO layers on (112) LaAlO3 surfaces. The atomic charges, electronic density of states, and band alignment are systematically analyzed for the optimized ZnO/LaAlO3 heterojunction. The ZnO/LaAlO-terminated LaAlO3 heterostructure is a potential candidate for the design of ZnO-based metal oxide semiconductor devices because the band offsets are larger than 1 eV and the interface does not produce any gap states.
PACS: 68.35.bg; 68.35.-p; 68.43.Bc; 73.20.At Keywords: A. ZnO; A. LaAlO3; D. Band offsets
Corresponding author.
E-mail address: [email protected] (Jianli Wang), [email protected] (Catherine Stampfl). 1
ACCEPTED MANUSCRIPT I. Introduction The group II-VI compound semiconductor zinc oxide (ZnO) is a promising candidate for potential optoelectronic applications [1] due to its wide direct band gap 3.44 eV in the near ultraviolet region [2], and a high exciton binding energy 60 meV at room temperature [2]. ZnObased metal-oxide-semiconductor field-effect transistors (MOSFETs) with a high dielectric constant gate oxide (high-κ) have high potential for very large scale integrated circuits [3] and logic circuits [4]. A high-quality heterojunction between a high-κ gate dielectric material and the ZnO semiconductor is desirable for the continued miniaturization of highly integrated MOSFETs. The perovskite lanthanum aluminate (LaAlO3 denoted as LAO) with a dielectric constant of 25.1 [5] and a large bandgap of 5.7 eV [6], is a promising candidate for future gate dielectric applications of ZnO-based MOSFETs. It is therefore desirable to integrate the ZnO semiconductor onto a LAO film. As wurtzite ZnO is non-centrosymmetric, the electrostatic field along the common growth c-direction for pristine or unreconstructed ZnO polar surfaces [7,8] caused by the alternate stacking of zinc ion (Zn2+) layers with oxygen ion (O2-) layers, limits its applications for optoelectronic devices. Meanwhile, the reconstructed polar ZnO surfaces are difficult to model due to the amount of possible reconstructed configurations. By comparison, the nonpolar m-plane ZnO (1010) is easier to model and represent the largest surface area in ZnO crystals. Heteroepitaxial growth of nonpolar ZnO in the m-plane has been reported on LAO (112) single crystal substrates in experiments [9-13]. The deposition of m-plane ZnO epitaxial films was demonstrated on LAO (112) single crystal substrates using pulsed laser deposition due to the similar atomic structures and small lattice mismatch. [9-13] The epitaxial orientation relationships between ZnO and LAO are (1010)ZnO||(112)LAO, [1210]ZnO||[111]LAO, and [0001]ZnO||[110]LAO [1012]. A promising candidate material should exhibit larger band offsets to effectively prevent the tunneling behavior of carriers and a band offset of over 1 eV is required for Si-based MOSFETs [14]. For ZnO-based MOSFET devices, knowledge of the energy band alignment and interfacial microstructure is of particular importance, which dictates many of the characteristics of optoelectronic devices. In the present work we perform first-principles investigations of the stability and band offsets of ZnO/LAO heterostructures. II. Methodology The calculations of the structural and electronic properties of ZnO on LAO (112) are carried 1
ACCEPTED MANUSCRIPT out through density functional theory (DFT), with projector augmented-wave pseudopotentials [15] as implemented in the Vienna Ab-initio Simulation Package [16]. The generalized gradient approximation (GGA) as parametrized by the Perdew–Burke–Ernzerhof (PBE) functional [17] is used to approximate the quantum mechanical electron-electron exchange and correlation potential. We employ the PBE-GGA to optimize the atomic structure in the surface reconstructions. The electronic structure of the resulting energetically favorable reconstructions is then predicted by using the Heyd-Scuseria-Ernzerhof hybrid functional [18] with a fraction α of Fock exchange of 0.4 [19-22] and a range separation parameter of 0.2 Å−1 allowing for a precise prediction of the band alignment of ZnO/LAO heterostructure. We employ a plane wave cutoff energy of 600 eV. Atomic positions are relaxed until residual forces are smaller than 0.02 eV/Å. The criterion to stop the relaxation of the electronic degrees of freedom is set by both the total energy and the band structure energy differences between two consecutive steps to be less than 10−4 eV. The Brillouinzone integrations are carried out with a Gaussian smearing [23] of sigma = 0.05 eV, and Monkhorst-Pack [24] meshes of 10×10×8 for bulk ZnO and 20×20×20 for bulk LAO. Within the above computational scheme, the optimized lattice parameters for ZnO bulk in the wurtzite structure are aZnO=3.278 Å and cZnO=5.330 Å and for LAO bulk in the cubic perovskite structure is aLAO=3.812 Å. [21,22], which are in good agreement with the experimental values [25,26]. The DFT calculation predicts an energy gap of 3.47 eV for bulk ZnO and an indirect band gap of LAO is 5.73 eV [21,22], which is consistent with the previous theoretical values [27-29]. The theoretical lattice mismatches between ZnO (1010) and LAO (112) are 1.13 % and 0.71 % along the directions of ZnO [0001] and [1210], respectively, which is small and makes it possible to grow high quality ZnO films on the LAO substrate. A (1×1) supercell with an atomic slab of twenty-five LAO layers and a vacuum region equivalent to 80.68 Å is used to model the LAO surfaces (see Fig. 1). The z-axis is perpendicular to the surface. All the atomic layers of the substrate, and any adsorbate, are allowed to relax in all of the geometry optimization calculations. The surface and interface calculations are carried out with a 4×4×1 Monkhorst-Pack mesh. The total energy differs by less than 0.04 eV after increasing the mesh to (6×6×1).
2
ACCEPTED MANUSCRIPT
Fig. 1 Perspective view ((a) left and (b) right) and top view of the LaAlO-terminated LAO (112) (middle (c)) and the O2-terminated LAO (112) (middle (d)) surface with the different considered ZnO layer stacking plane positions (color rectangle) indicated. The first ZnO layer schematic (e) describes the first OZn layer, but with the elemental identity of the Zn and oxygen atoms switched. The layer numbers of LAO substrate are also indicated. III. Results and Discussion The adsorption of the initial ZnO layer is now studied on ideal LAO (112) surfaces. We focus on the high symmetric arrangements (see Fig. 1) based on the periodicity and symmetry of the LAO (112) surfaces. The total energies for the first ZnO adsorbed layer on the LAO (112) surfaces are calculated and the results are summarized in Table 1. The results show that the first ZnO adsorbed layer preferentially adsorbs on the LaAlO-terminated surface with X=X1 and Y= Y0=0. For the O2-terminated surface, the first OZn adsorbed layer can form on the substrate without translation (i.e. X=X0=0, Y=Y0=0). Table 1. The total energy (in eV) of the first ZnO or OZn layer on the LaAlO-terminated and the O2-terminated LAO (112) surfaces. The minimum total energy is taken as the zero energy reference. 3
ACCEPTED MANUSCRIPT LaAlO-termination
ZnO
OZn
O2-termination
Y0
Y1
Y2
Y3
Y0
Y4
Y5
Y6
X0
1.848
1.928
2.534
2.642
X0
0
2.139
3.382
0.819
X1
2.118
2.328
0.004
2.767
X2
0.696
1.560
2.688
1.739
X0
2.558
1.876
4.206
2.586
X0
1.564
1.937
2.991
1.592
X1
0
2.237
5.026
3.094
X2
0.198
0.666
3.369
1.620
From the results of the energy of the initial ZnO adsorbed layer, we thus design the interface atomic arrangements of ZnO (1010)/LAO (112) heterostructures (Figs. 2). The ZnO/LAO heterostructures are modeled by a slab thickness of thirteen ZnO atomic layers and twenty-five LAO atomic bilayers. [21, 22, 30] We use lattice constants a=5.391, b=6.603 Å and c=100.856 Å for the ZnO/LAO heterostructures. All atoms in the heterostructures are allowed to relax in the calculations. For the relaxed ZnO/LaAlO-terminated LAO interface (ZnO/LaAlO) and the relaxed ZnO/O2-terminated LAO interface (ZnO/O2), the interplanar distances are 0.754 Å and 2.828 Å, respectively. It can be seen that bonds are formed at ZnO/LAO interfaces from the isosurfaces of the interface charge densities (i.e. the difference in total electron density relative to sum of separated ZnO and LAO slabs) in Figs. 2.
Fig. 2 The relaxed atomic structure with isosurfaces of the interface charge densities of (a) ZnO/LaAlO and (b) ZnO/O2 interfaces (the yellow region represents charge accumulation, and the 4
ACCEPTED MANUSCRIPT cyan region indicates charge depletion), the change of the atomic charges of (c) ZnO/LaAlO and (d) ZnO/O2 interfaces compared to the bulk charges (the cation (anion) that gains electron density becomes more positive or less negative; the less positive or more negative value is associated with the more electrons in the cation (anion)), and the planar average of the electrostatic potential for (e) ZnO/LaAlO and (f) ZnO/O2 interfaces. Also shown as insets in the latter figures are the energy position and the value of the top edge of the valence band, the bottom edge of the conduction band, and the average of the electrostatic potential in the bulk-like region of each material. The atomic charges in the various surface or interface structures are determined using the Bader topological charge analysis [31]. The difference between the atomic charges in relaxed structures and those in the bulk can be used to analyze the change in valence electrons (see Figs. 3 and 4) [32] because the electron numbers do not give the exact value of valence electrons. The largest change occurs at the surface or interface of the ZnO/LAO heterostructures. The interface Zn atoms increase their electron density (0.20 e) at the ZnO/LaAlO interface, making them less positive. The largest change in the Zn charge occurs in the second layer of the ZnO/LaAlO interface. The interface oxygen atoms in LAO decrease their electron density (0.14 e), while the interface oxygen atoms in ZnO acquire more electron density (0.11 e) at the ZnO/LaAlO interface. The interface oxygen atoms in LAO decrease their electron density (0.85 e) at the ZnO/O2 interface, making them less negative. The band offsets of the ZnO/LAO heterojunctions are determined by the electrostatic potential. The planar average of the electrostatic potential [33] V(x,y,z) in Cartesian coordinates can be expressed wtih the following equation:
V (z)
1 A xy
A xy
V( x , y, z)dxdy , where the z-
axis is perpendicular to the interface. Thus, the electrostatic potential V(z) is z-dependent and periodic in the xy-plane. Figures 2 show the variation of the planar-averaged electrostatic potential for ZnO/LAO heterostructures. We evaluate the conduction band offset (CBO) and valence band offset
(VBO)
[34]
by
means
ZnO E CBO/VBO E Average E LAO Average ZnO
LAO
ZnO/LAO
ZnO
of
the
planar-averaged
electrostatic
potential,
ZnO LAO ZnO E CBM/VBM E CBM/VBM E Average E LAO Average
Bulk
,
LAO
where E CBM/VBM , E CBM/VBM , E Average , and E Average are the energy of the bottom edge of the conduction band, the top edge of the valence band, and the average of the electrostatic potential in 5
ACCEPTED MANUSCRIPT the bulk ZnO and LAO or the bulk-like region in the ZnO/LAO heterojunction, respectively. The CBO and VBO are 1.12 eV and 1.13 eV for the ZnO/LaAlO heterostructure, respectively, which are nearly symmetric. For the ZnO/O2 heterostructure the CBO and VBO are 0.48 eV and 1.78 eV, respectively, and the former value is less than the desired 1 eV [14]. Thus, the ZnO/LaAlO interfaces are predicted to be suitable for the design of MOSFET devices because the valence band offset and conduction band offset are both larger than 1 eV. The layer density of states (LDOS) of ZnO/LAO heterostructures are displayed in Figs. 3 and 4. The orbital nature of the interface in-gap states can be identified from the partial density of states. For the ZnO/LaAlO heterostructures, gap states are observed in Fig. 3 and are mainly derived from the La 5d6s, Al 3s3p, and O 2s2p states in LAO. There is an obvious band gap of about 0.95 eV, which shows that this interface maintains semiconductor properties. For the ZnO/O2 heterostructure, some gap states of Zn 3d and O 2p state character appear near the top of the valence band at the ZnO surface adjacent to the vacuum. The band gap states of ZnO clearly decrease near the interface. There is a band gap of about 1.43 eV for the ZnO/O2 heterostructure.
Fig. 3 The layer resolved and total density of states (TDOS) of the ZnO/LAO heterostructure. The vertical dotted line gives the location of the Fermi level.
6
ACCEPTED MANUSCRIPT
Fig. 4 The density of states of the ZnO/O2 heterostructure. The vertical dotted line gives the location of the Fermi level. IV. SUMMARY The stability and band offsets of ZnO on LaAlO3 (112) substrates are studied using DFT. The initial adsorption structure of the first ZnO layers is determined by a layer-by-layer mode. The electronic properties are calculated and analyzed for the ZnO/LAO heterostructures. Based on the planar-averaged electrostatic potential, the CBO and the VBO are 1.12 eV and 1.13 eV for the ZnO/LaAlO heterostructure, respectively, indicating it is a promising candidate for ZnO-based metal oxide semiconductor devices.
Acknowledgment This work has been supported by the Fundamental Research Funds for the Central Universities under Grant no. 2017XKQY092. We are grateful to the High Performance Computing Center of China University of Mining and Technology for the award of CPU hours to accomplish this work.
References 7
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ACCEPTED MANUSCRIPT
Research Highlights
Band alignment for the ZnO/LaAlO3 heterostructure.
The initial adsorption structure of the first ZnO layers on (112) LaAlO3 surfaces.
The ZnO/LaAlO interface is suitable for metal insulator semiconductor devices.
1
Jianli Wang, Xinfeng Chen, Aixia Zhang, Dongmei Bai, Gang Tang, Junting Zhang, C. Stampfl PII:
S0038-1098(18)30284-9
DOI:
10.1016/j.ssc.2018.10.002
Reference:
SSC 13511
To appear in:
Solid State Communications
Received Date:
20 May 2018
Accepted Date:
02 October 2018
Please cite this article as: Jianli Wang, Xinfeng Chen, Aixia Zhang, Dongmei Bai, Gang Tang, Junting Zhang, C. Stampfl, Band alignment of nonpolar (10 Image 10) ZnO on (112) LaAlO3, Solid
State Communications (2018), doi: 10.1016/j.ssc.2018.10.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Band alignment of nonpolar (10𝟏0) ZnO on (112) LaAlO3
Jianli Wang1,2,, Xinfeng Chen1, Aixia Zhang1, Dongmei Bai3, Gang Tang1, Junting Zhang1, C. Stampfl2,* 1
School of physical science and technology, China University of Mining and Technology, Xuzhou 221116, China
2 3
School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia
School of Mathematics, China University of Mining and Technology, Xuzhou 221116, China
Abstract The stability and band alignments of ZnO on (112) LaAlO3 substrates are studied by firstprinciples calculations. We have created the ZnO/LaAlO3 heterostructure by consideration of the initial adsorption of the first ZnO layers on (112) LaAlO3 surfaces. The atomic charges, electronic density of states, and band alignment are systematically analyzed for the optimized ZnO/LaAlO3 heterojunction. The ZnO/LaAlO-terminated LaAlO3 heterostructure is a potential candidate for the design of ZnO-based metal oxide semiconductor devices because the band offsets are larger than 1 eV and the interface does not produce any gap states.
PACS: 68.35.bg; 68.35.-p; 68.43.Bc; 73.20.At Keywords: A. ZnO; A. LaAlO3; D. Band offsets
Corresponding author.
E-mail address: [email protected] (Jianli Wang), [email protected] (Catherine Stampfl). 1
ACCEPTED MANUSCRIPT I. Introduction The group II-VI compound semiconductor zinc oxide (ZnO) is a promising candidate for potential optoelectronic applications [1] due to its wide direct band gap 3.44 eV in the near ultraviolet region [2], and a high exciton binding energy 60 meV at room temperature [2]. ZnObased metal-oxide-semiconductor field-effect transistors (MOSFETs) with a high dielectric constant gate oxide (high-κ) have high potential for very large scale integrated circuits [3] and logic circuits [4]. A high-quality heterojunction between a high-κ gate dielectric material and the ZnO semiconductor is desirable for the continued miniaturization of highly integrated MOSFETs. The perovskite lanthanum aluminate (LaAlO3 denoted as LAO) with a dielectric constant of 25.1 [5] and a large bandgap of 5.7 eV [6], is a promising candidate for future gate dielectric applications of ZnO-based MOSFETs. It is therefore desirable to integrate the ZnO semiconductor onto a LAO film. As wurtzite ZnO is non-centrosymmetric, the electrostatic field along the common growth c-direction for pristine or unreconstructed ZnO polar surfaces [7,8] caused by the alternate stacking of zinc ion (Zn2+) layers with oxygen ion (O2-) layers, limits its applications for optoelectronic devices. Meanwhile, the reconstructed polar ZnO surfaces are difficult to model due to the amount of possible reconstructed configurations. By comparison, the nonpolar m-plane ZnO (1010) is easier to model and represent the largest surface area in ZnO crystals. Heteroepitaxial growth of nonpolar ZnO in the m-plane has been reported on LAO (112) single crystal substrates in experiments [9-13]. The deposition of m-plane ZnO epitaxial films was demonstrated on LAO (112) single crystal substrates using pulsed laser deposition due to the similar atomic structures and small lattice mismatch. [9-13] The epitaxial orientation relationships between ZnO and LAO are (1010)ZnO||(112)LAO, [1210]ZnO||[111]LAO, and [0001]ZnO||[110]LAO [1012]. A promising candidate material should exhibit larger band offsets to effectively prevent the tunneling behavior of carriers and a band offset of over 1 eV is required for Si-based MOSFETs [14]. For ZnO-based MOSFET devices, knowledge of the energy band alignment and interfacial microstructure is of particular importance, which dictates many of the characteristics of optoelectronic devices. In the present work we perform first-principles investigations of the stability and band offsets of ZnO/LAO heterostructures. II. Methodology The calculations of the structural and electronic properties of ZnO on LAO (112) are carried 1
ACCEPTED MANUSCRIPT out through density functional theory (DFT), with projector augmented-wave pseudopotentials [15] as implemented in the Vienna Ab-initio Simulation Package [16]. The generalized gradient approximation (GGA) as parametrized by the Perdew–Burke–Ernzerhof (PBE) functional [17] is used to approximate the quantum mechanical electron-electron exchange and correlation potential. We employ the PBE-GGA to optimize the atomic structure in the surface reconstructions. The electronic structure of the resulting energetically favorable reconstructions is then predicted by using the Heyd-Scuseria-Ernzerhof hybrid functional [18] with a fraction α of Fock exchange of 0.4 [19-22] and a range separation parameter of 0.2 Å−1 allowing for a precise prediction of the band alignment of ZnO/LAO heterostructure. We employ a plane wave cutoff energy of 600 eV. Atomic positions are relaxed until residual forces are smaller than 0.02 eV/Å. The criterion to stop the relaxation of the electronic degrees of freedom is set by both the total energy and the band structure energy differences between two consecutive steps to be less than 10−4 eV. The Brillouinzone integrations are carried out with a Gaussian smearing [23] of sigma = 0.05 eV, and Monkhorst-Pack [24] meshes of 10×10×8 for bulk ZnO and 20×20×20 for bulk LAO. Within the above computational scheme, the optimized lattice parameters for ZnO bulk in the wurtzite structure are aZnO=3.278 Å and cZnO=5.330 Å and for LAO bulk in the cubic perovskite structure is aLAO=3.812 Å. [21,22], which are in good agreement with the experimental values [25,26]. The DFT calculation predicts an energy gap of 3.47 eV for bulk ZnO and an indirect band gap of LAO is 5.73 eV [21,22], which is consistent with the previous theoretical values [27-29]. The theoretical lattice mismatches between ZnO (1010) and LAO (112) are 1.13 % and 0.71 % along the directions of ZnO [0001] and [1210], respectively, which is small and makes it possible to grow high quality ZnO films on the LAO substrate. A (1×1) supercell with an atomic slab of twenty-five LAO layers and a vacuum region equivalent to 80.68 Å is used to model the LAO surfaces (see Fig. 1). The z-axis is perpendicular to the surface. All the atomic layers of the substrate, and any adsorbate, are allowed to relax in all of the geometry optimization calculations. The surface and interface calculations are carried out with a 4×4×1 Monkhorst-Pack mesh. The total energy differs by less than 0.04 eV after increasing the mesh to (6×6×1).
2
ACCEPTED MANUSCRIPT
Fig. 1 Perspective view ((a) left and (b) right) and top view of the LaAlO-terminated LAO (112) (middle (c)) and the O2-terminated LAO (112) (middle (d)) surface with the different considered ZnO layer stacking plane positions (color rectangle) indicated. The first ZnO layer schematic (e) describes the first OZn layer, but with the elemental identity of the Zn and oxygen atoms switched. The layer numbers of LAO substrate are also indicated. III. Results and Discussion The adsorption of the initial ZnO layer is now studied on ideal LAO (112) surfaces. We focus on the high symmetric arrangements (see Fig. 1) based on the periodicity and symmetry of the LAO (112) surfaces. The total energies for the first ZnO adsorbed layer on the LAO (112) surfaces are calculated and the results are summarized in Table 1. The results show that the first ZnO adsorbed layer preferentially adsorbs on the LaAlO-terminated surface with X=X1 and Y= Y0=0. For the O2-terminated surface, the first OZn adsorbed layer can form on the substrate without translation (i.e. X=X0=0, Y=Y0=0). Table 1. The total energy (in eV) of the first ZnO or OZn layer on the LaAlO-terminated and the O2-terminated LAO (112) surfaces. The minimum total energy is taken as the zero energy reference. 3
ACCEPTED MANUSCRIPT LaAlO-termination
ZnO
OZn
O2-termination
Y0
Y1
Y2
Y3
Y0
Y4
Y5
Y6
X0
1.848
1.928
2.534
2.642
X0
0
2.139
3.382
0.819
X1
2.118
2.328
0.004
2.767
X2
0.696
1.560
2.688
1.739
X0
2.558
1.876
4.206
2.586
X0
1.564
1.937
2.991
1.592
X1
0
2.237
5.026
3.094
X2
0.198
0.666
3.369
1.620
From the results of the energy of the initial ZnO adsorbed layer, we thus design the interface atomic arrangements of ZnO (1010)/LAO (112) heterostructures (Figs. 2). The ZnO/LAO heterostructures are modeled by a slab thickness of thirteen ZnO atomic layers and twenty-five LAO atomic bilayers. [21, 22, 30] We use lattice constants a=5.391, b=6.603 Å and c=100.856 Å for the ZnO/LAO heterostructures. All atoms in the heterostructures are allowed to relax in the calculations. For the relaxed ZnO/LaAlO-terminated LAO interface (ZnO/LaAlO) and the relaxed ZnO/O2-terminated LAO interface (ZnO/O2), the interplanar distances are 0.754 Å and 2.828 Å, respectively. It can be seen that bonds are formed at ZnO/LAO interfaces from the isosurfaces of the interface charge densities (i.e. the difference in total electron density relative to sum of separated ZnO and LAO slabs) in Figs. 2.
Fig. 2 The relaxed atomic structure with isosurfaces of the interface charge densities of (a) ZnO/LaAlO and (b) ZnO/O2 interfaces (the yellow region represents charge accumulation, and the 4
ACCEPTED MANUSCRIPT cyan region indicates charge depletion), the change of the atomic charges of (c) ZnO/LaAlO and (d) ZnO/O2 interfaces compared to the bulk charges (the cation (anion) that gains electron density becomes more positive or less negative; the less positive or more negative value is associated with the more electrons in the cation (anion)), and the planar average of the electrostatic potential for (e) ZnO/LaAlO and (f) ZnO/O2 interfaces. Also shown as insets in the latter figures are the energy position and the value of the top edge of the valence band, the bottom edge of the conduction band, and the average of the electrostatic potential in the bulk-like region of each material. The atomic charges in the various surface or interface structures are determined using the Bader topological charge analysis [31]. The difference between the atomic charges in relaxed structures and those in the bulk can be used to analyze the change in valence electrons (see Figs. 3 and 4) [32] because the electron numbers do not give the exact value of valence electrons. The largest change occurs at the surface or interface of the ZnO/LAO heterostructures. The interface Zn atoms increase their electron density (0.20 e) at the ZnO/LaAlO interface, making them less positive. The largest change in the Zn charge occurs in the second layer of the ZnO/LaAlO interface. The interface oxygen atoms in LAO decrease their electron density (0.14 e), while the interface oxygen atoms in ZnO acquire more electron density (0.11 e) at the ZnO/LaAlO interface. The interface oxygen atoms in LAO decrease their electron density (0.85 e) at the ZnO/O2 interface, making them less negative. The band offsets of the ZnO/LAO heterojunctions are determined by the electrostatic potential. The planar average of the electrostatic potential [33] V(x,y,z) in Cartesian coordinates can be expressed wtih the following equation:
V (z)
1 A xy
A xy
V( x , y, z)dxdy , where the z-
axis is perpendicular to the interface. Thus, the electrostatic potential V(z) is z-dependent and periodic in the xy-plane. Figures 2 show the variation of the planar-averaged electrostatic potential for ZnO/LAO heterostructures. We evaluate the conduction band offset (CBO) and valence band offset
(VBO)
[34]
by
means
ZnO E CBO/VBO E Average E LAO Average ZnO
LAO
ZnO/LAO
ZnO
of
the
planar-averaged
electrostatic
potential,
ZnO LAO ZnO E CBM/VBM E CBM/VBM E Average E LAO Average
Bulk
,
LAO
where E CBM/VBM , E CBM/VBM , E Average , and E Average are the energy of the bottom edge of the conduction band, the top edge of the valence band, and the average of the electrostatic potential in 5
ACCEPTED MANUSCRIPT the bulk ZnO and LAO or the bulk-like region in the ZnO/LAO heterojunction, respectively. The CBO and VBO are 1.12 eV and 1.13 eV for the ZnO/LaAlO heterostructure, respectively, which are nearly symmetric. For the ZnO/O2 heterostructure the CBO and VBO are 0.48 eV and 1.78 eV, respectively, and the former value is less than the desired 1 eV [14]. Thus, the ZnO/LaAlO interfaces are predicted to be suitable for the design of MOSFET devices because the valence band offset and conduction band offset are both larger than 1 eV. The layer density of states (LDOS) of ZnO/LAO heterostructures are displayed in Figs. 3 and 4. The orbital nature of the interface in-gap states can be identified from the partial density of states. For the ZnO/LaAlO heterostructures, gap states are observed in Fig. 3 and are mainly derived from the La 5d6s, Al 3s3p, and O 2s2p states in LAO. There is an obvious band gap of about 0.95 eV, which shows that this interface maintains semiconductor properties. For the ZnO/O2 heterostructure, some gap states of Zn 3d and O 2p state character appear near the top of the valence band at the ZnO surface adjacent to the vacuum. The band gap states of ZnO clearly decrease near the interface. There is a band gap of about 1.43 eV for the ZnO/O2 heterostructure.
Fig. 3 The layer resolved and total density of states (TDOS) of the ZnO/LAO heterostructure. The vertical dotted line gives the location of the Fermi level.
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Fig. 4 The density of states of the ZnO/O2 heterostructure. The vertical dotted line gives the location of the Fermi level. IV. SUMMARY The stability and band offsets of ZnO on LaAlO3 (112) substrates are studied using DFT. The initial adsorption structure of the first ZnO layers is determined by a layer-by-layer mode. The electronic properties are calculated and analyzed for the ZnO/LAO heterostructures. Based on the planar-averaged electrostatic potential, the CBO and the VBO are 1.12 eV and 1.13 eV for the ZnO/LaAlO heterostructure, respectively, indicating it is a promising candidate for ZnO-based metal oxide semiconductor devices.
Acknowledgment This work has been supported by the Fundamental Research Funds for the Central Universities under Grant no. 2017XKQY092. We are grateful to the High Performance Computing Center of China University of Mining and Technology for the award of CPU hours to accomplish this work.
References 7
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Research Highlights
Band alignment for the ZnO/LaAlO3 heterostructure.
The initial adsorption structure of the first ZnO layers on (112) LaAlO3 surfaces.
The ZnO/LaAlO interface is suitable for metal insulator semiconductor devices.
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