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Band Discontinuities at Si/Transparent Conducting Oxide Heterostructures from ab-initio Quasiparticle Calculations. B. Höffling , A. Schleife, F. Fuchs, C. Rödl, and F. Bechstedt Institut für Festkörpertheorie und –optik Friedrich-Schiller-Universität Jena and
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Band Discontinuities atSi/Transparent Conducting Oxide Heterostructures from ab-initio Quasiparticle Calculations B. Höffling, A. Schleife, F. Fuchs, C. Rödl, and F. Bechstedt Institut für Festkörpertheorie und –optik Friedrich-Schiller-Universität Jena and European Theoretical Spectroscopy Facility (ETSF) School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
Outline • Motivation • Electronic StructureCalculations • MesoscopicMethods • Vacuum Level Alignment • Branch Point EnergyAlignment • ComparisonofResults • Si/In2O3: Interface Model Alignment • Summary School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
1. Motivation: Why Si/TCO Interfaces? • Transparent Conducting Oxides like ITO and ZnO are used as transparent electrodes in photovoltaic and optoelectric devices. • Key properties such as ionization energy, electron affinity, charge neutrality level and work function are poorly known. • Electronic properties of Si/TCO heterojunctions determine the efficiency of Si-based solar cells School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
1. Motivation: Electronic Properties of Interfaces School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
1. Motivation: Electronic Properties of Interfaces Type I Type III Type II School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
2. Electronic StructureCalculations • Spatially non-local XC-potential HSE03 used for zeroth approximation of XC self-energy • QP wave functions used to compute QP shifts using many-body pertubation theory in the G0W0 approach. -> QP band structure of bulk materials F. Fuchs et al., Phys. Rev. B 76, 115109 (2007) School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
3. Methods: Electronic Properties of Interfaces • Si and TCO have • Different bond types • Different lattice constants • Different lattice structures -> Construction of structural interface model highly non-trivial • Mesoscopic methods that don‘t require detailed knowledge of interface geometries can help. Si lattice ZnO lattice School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
3.1 The VacuumAlignmentMethod • requires: ionization energy I=Evac-Ev electron affinity A=Evac-Ec with QP bandgap Eg=I-A R.L. Anderson, Solid State Electron. 5, 341 (1962) School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
3.1 The VacuumAlignmentMethod • Electrostatic potential at surface obtained by DFT-LDA repeated-slab supercell calculations • Plane averaged electrostatic potential with bulk oscilations and vacuum plateau • QP-CBM and VBM relative to electrostatic bulk oscillations known • Alignment yields ionization energy and electron affinity CBM VBM I A School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
3.1. The VacuumAlignmentMethod • ΔEv=I1-I2 • ΔEc=A1-A2 • We obtain Type II and Type III heterostructures (exception: SnO2) -> good charge carrier separation School on Nanophotonics and Photovoltaics 2010
3.2 Branch Point AlignmentMethod: Fundamentals Basic concept: Virtual gap states (ViGS) V. Heine, SS 2, 1 (1964); PR A 138, 1689 (1965) • EBP is the energy at which the character changes from donor- to acceptor-like behavior • We use QP energies to approximate the BZ-average of the midgap energy A. Schleife et al., APL 94, 012104 (2009) School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
3.2 Branch Point AlignmentMethod: Consequences • Surface/Interface induces ViGS and pinns Fermi level at EBP • We use QP energies to approximate the BZ-average of the midgap energy • EBP >CBM creates creates charge accumulation layer near oxide surface • confirmed for ZnO: M. W. Allen et al., Phys. Rev. B 81, 075211 (2010) School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
3.2 Branch Point AlignmentMethod • Type II and Type III heterostructures • Branch point in good agreement with experiments • EBP> Eg in all TCOs • SnO2 now Type II heterostructure • Similar values for ΔEv: Common anion rule School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
4. ComparisonofResults: Band Lineup • Good agreement between the two methods • Exception: SnO2 • Possible reason: no surface states at this orientation School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
5. Si/In2O3: Interface Model Alignment • Band offsets via averaged electrostatic potential: ΔEc= -1.07 eV ΔEv= 2.95 eV • Shift due to charge transfer-induced dipole moment? • Integration shows a transfer of 3 electrons into the oxide. But: only about 0.5 electrons into the slab. -> ionic component in Si-O bonding School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
6. Summary • We calculated branch point levels, ionization energies and electron affinities for Si, In2O3, SnO2, and ZnO. • Band offsets for Si/TCO interfaces determined by two different alignment methods in good agreement with each other (exception: SnO2) • Branch Point Energy Alignment and Vacuum Energy Alignment are usefull tools for the efficient calculation of band discontinuities that don‘t require detailed structural interface models • Interface Model Alignment confirms predictions. • For Si/TCO heterostructures a tendency for Type II or misaligned Type III heterostructures is observed -> Good charge carrier separation School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
B. Höffling et al., APL 97, 032116 (2010) a P.D.C. King et al., Phys. Rev. Lett. 101, 116808 (2008), P.D.C. King et al., Phys. Rev. B 79, 205211 (2009) b W. Martienssen and H. Warlimont eds., Handbook of Condensed Matter and Materials Data, (Springer, Berlin, 2005) c K. Reimann and M. Steube, Solid State Commun. 105, 649 (1998) d W. Walukiewicz, Physica B 302-303, 123 (2001) e P.D.C.King et al., Phys. Rev. B 80, 081201 (2009) f A. Klein, Appl. Phys. Lett. 77, 2009 (2000) g K. Jacobi et al. Surf. Sci. 141, 109 (1984) h W. Mönch, Semiconductor Surfaces and Interfaces, (Springer, Berlin, 2001) i C. Kiliç and A. Zunger, Appl. Phys. Lett. 81, 73 (2002) Thank you for your attention! School on Nanophotonics and Photovoltaics 2010 Benjamin Höffling et al.
