Resistive Switching Mechanism of SingleCrystalline Oxide Schottky Junctions: Macroscopic and Nanoscopic Characterizations Haeri Kim, Eunsongyi Lee, Minji Gwon, Ahrum Sohn, El Mostafa Bourim, and DongWook Kim Department of Physics Ewha Womans University Seoul 120750, KOREA Email) dwkim@ewha.ac.kr EPIC012 Buenos Aires, Argentina Dec. 13, 2012 URL) http://edpl.ewha.ac.kr Seungbum Hong Materials Science Division Argonne National Laboratory Lemont, Illinois 60439, USA
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Agenda 4 Motivation Why Single Crystal Schottky Junctions? Approach I: Macroscopic Characterization Conventional IV and CV analyses Internal photoemission spectroscopy Ambient effects AC admittance spectroscopy Approach II: Nanoscopic Characterization Planar metalti metal junctions Ambient effects surface potential measurements Conclusion
Resistive Switching in Metal Oxides 5 Metal/insulator/metal (MIM) structures Sawa, Materials Today 11, 28 (2008). Metal Oxides Oxygen vacancy Mobile dopant bulk resistivity and interface potential profile change Compositional variation suboxides and/or metallic phase formation Local electrical conduction paths Joule heating
Why Single Crystals? 6 Metal/insulator/metal (MIM) structures Park et al., Appl. Phys. Lett. 93, 042102 (2008). Phark et al., Appl. Phys. Lett. 94, 022906 (2009). 11 pads/12 pads 7 pads/16 pads Metal Oxides Oxygen vacancy Mobile dopant bulk resistivity and interface potential profile change Compositional variation suboxides and/or metallic phase formation Single Crystal Junctions Model System for Mechanism Studies Local electrical conduction paths Joule heating
Why Metal/Oxide Contacts? 7 Voltagecurrent characteristics of twoterminal system Three components: series connection of the two contact regions and the bulk layer Exemplary case for rectifying contacts contact 1 Contact 1 Metal 1 Insulator V A bulk Contact 2 Metal 2 contact 2 I Contact 1 Ohmic bulk contribution I Resultant characteristic V V Contact 2
Pt/SrTiO 3 Schottky Junctions 8 Retention Resistance( J J S J Endurance F Resistance ( 10 8 10 7 10 6 10 5 10 0 10 1 10 2 10 3 10 4 10 5 Time (sec.) 10 7 10 6 HRS LRS S * 2 A T HRS exp( qv / nk T) exp( q / k B B B T) J (A/cm 2 ) 10 5 J F : Forward current J S :Saturation current * 10 4 ALRS : Richardson constant n : Ideality factor 10 3 kb : Boltzman constant 0 200 400 600 800 1000 B : Barrier Height # of pulses 10 3 10 1 10 1 10 3 10 5 10 7 Start Ni 1.0 0.5 0.0 0.5 1.0 V (V) Ti Electrode Work function IV Ti 4.33 ev Ohmic Ni Au Pd Pt Park et al., J. Appl. Phys. 103, 054106 (2008). 5.15 ev 5.1 ev 5.12 ev 5.65 ev Rectifying Pt Pd Au
Switching Mechanism? 9 Oxygen vacancy migration Carrier trapping/detrapping Pt STO E C E F E V Fujii et al., Phys. Rev. B 75, 165101 (2007). Fujii et al., Appl. Phys. Lett. 86, 012107 (2005). Jeon et al., Appl. Phys.Lett. 89, 042904 (2006).
Internal Photoemission Spectroscopy (IPES) 10 Photoresponse R, Lee et al., Appl. Phys. Lett. 98, 132905 (2011). hν E Fm Isc (A) 1240(m V) R P (W) (nm) in R( h q ) where IPE qф Bn e q B 2 h B E g Bandtoband excitation e qψ bi R 1/2 (A.U.) E c 0.03 0.02 0.01 0.003 0.002 0.001 0.000 qφ B 1.2 1.3 1.4 1.5 1.6 1.7 E g 0.00 1.0 1.5 2.0 2.5 3.0 3.5 Energy (ev) Internal photoemission 1 V sweep HRS HRS LRS LRS 2 V sweep HRS HRS LRS LRS Bandtoband excitation E g h + E v
Internal Photoemission Spectroscopy (IPES) 11 Lee et al., Appl. Phys. Lett. 98, 132905 (2011). Conventional IV vs. IPES 1.5 q B (ev) 1.2 1.0 0.8 HRS LRS HRS LRS IPES IV IPES IV 1.0 0.5 0.0 E (A.U.) 0.6 1 2 qφ B from IV < qφ B from IPES Sweep Voltage (V) lower barrier dominant in IV mean barrier dominant in IPES
Transport Mechanism 12 Lee et al., Appl. Phys. Lett. 98, 132905 (2011). Inverse slope (=kt) (mev) 70 60 50 40 30 LRS HRS 20 22 24 26 28 30 32 34 36 38 kt (mev) FE TE (=1) HRS qф BM E Fm Pt e e STO Eg E c E v LRS qф BM E Fm Pt e e STO Eg E c E v Thermionic emission (TE) : kt >> E 00 J TE F J TE 0 qv exp kt & ~ 1 Field emisssion near E F (FE) : kt << E 00 Thermionicfield emission (TFE) : combination of TE & FE J J FE F TEF F J FE 0 J TEF 0 exp exp qv E 00 qv E, 0, where E 00 where E 0 q 2 E 00 N * m s E coth kt 00
Air vs. Vacuum 13 Bourim and Kim, Curr. Appl. Phys. (2013). Bipolar switching observed in both air and vacuum (a) Current (A) 10 0 10 2 10 4 10 6 10 8 10 10 10 12 Current (A) 10 1 Air Vacuum Air after Vacuum 10 3 10 5 10 7 10 9 10 11 10 13 Air Vacuum 2 1 2 0 11 02 2 1 1 2 0 1 2 Voltage (V) Voltage (V) (b) Air Vacuum Voltage (V)
Admittance Spectra of Pt/SrTiO 3 14 Interface trap states AC conductance Bourim and Kim, Curr. Appl. Phys. (2013). Nicollian and Goetzberger, The Bell System Technical Journal (1967). [Ref.] Suzuki et al., J. Appl. Phys. 81, 6830 (1997).
Admittance Spectra of Pt/SrTiO 3 15 Bourim and Kim, Curr. Appl. Phys. (2013). G P / (F) 2.0x10 9 1.5x10 9 1.0x10 9, air, vacuum HRS HRS LRS LRS 5.0x10 10 0.0 10 2 10 3 10 4 10 5 10 6 10 7 Frequency, (Hz) Peaks in the conductance spectra interface trap states! G P vs. : 1 exp us ub peak v 1 n th n i
Admittance Spectra of Pt/SrTiO 3 16 1 exp us ub v n th n i Bourim and Kim, Curr. Appl. Phys. (2013). Pt STO Pt STO E C E C u S u B E F E i u S u B E F E i E V E V Less band bending Smaller u S Smaller (higher peak ) LRS and Vacuum More band bending Larger u S Larger (lower peak ) HRS and Air
Planar Junctions for Transport and SPM Studies 17? V V! E F E C m n ev 0 W B Oxide Oxide Ti Pt E V Ohmic contact Schottky contact Issues 1)Contact and bulk contribution 2)Roles of Schottky vs. Ohmic contacts asymmetric Pt/Ti /Ti structures: separation of contact and bulk contribution to the transport 3)Real space observation simultaneous transport and SPM (scanning probe microscopy) measurements V tip A Ti Ti Pt 2 m V R Total = R B + R C,Pt
ElectricFieldInduced Resistance Change 18 Current vs. time during the electrical stress Kim et al., J. Phys. D: Appl. Phys. 50, 505305 (2010). 0.4 Current (na) 0.2 0.0 0.2 0.4 +40 V 40 V 0.6 0.8 0 10 20 30 Time (min.) Planar junction fabrication Ti (100) annealed in N 2 /H 2 (95:5): 450 o C, 1 hr. Photolithography and liftoff patterning of electrodes
Surface Potential Maps KPFM Measurements 19 Kim et al., J. Phys. D: Appl. Phys. 50, 505305 (2010). Ti Ti Pt (Potential)/(Current) (10 12 ) 0.3 0.2 Ti initial R C,Pt Ti +40 V 0.1 initial 40 V R B 0.0 0 2 4 6 8 10 Position (m) Pt initial R Total R Total = R B + R C,Pt 40 V electrical stress R C,Pt < 0 & R B < 0 +40 V electrical stress R C,Pt ~ 0 but R B < 0
Work Function Maps KPFM Measurements 20 H. Kim and D.W. Kim, Appl. Phys. A 102, 949 (2011). Topography 120 nm Ti Work function (W sample ) initial 1 um Pt 0 nm W ( E E ) ev Sample C F S S E l S O 2 O 2 ev S W Sample 40 V for 30 min. 5.1 ev E C (E C E F ) E F E V +40 V for 30 min. 4.7 ev Oxygen chemisorption + e
Scenario for Planar Junctions vacuum 10 5 21 H. Kim and D.W. Kim, Appl. Phys. A 102, 949 (2011). air Current (A) 10 6 10 7 10 8 10 9 vacuum air 5 0 5 Voltage (V) W sample 5.1 ev 4.7 ev
AFM Charge Writing of Ti : Ambient Dependence 22 H. Kim et al., Appl. Phys. Lett. 100, 022091 (2012). Charge writing EFM imaging H 2 /Ar : scanned by the SPM tip with a DC bias of +10 V in contact mode : scanned by the SPM tip with an AC modulation voltage to the tip while vibrating the cantilever at its mechanical resonance. freq. H 2 /Ar Ar 2 V Amplitude Phase 1 m 0 V +20 o 220 o
AFM Charge Writing of Ti : Ambient Dependence 23 H. Kim et al., Appl. Phys. Lett. 100, 022091 (2012). Atomic Force Microscopy (MFP3D, Asylum Research) 1 Control the ambient H 2 /Ar Ar 2 Tip induced stress V DC Closed fluid cell 3 Check result V AC
AFM Charge Writing of Ti : Ambient Dependence 24 H. Kim et al., Appl. Phys. Lett. 100, 022091 (2012). Amplitude (V) 1.5 1.0 0.5 0.0 0 Ar H 2 /Ar Amplitude Phase & Ar H 2 / Ar Phase ( o ) 90 180 0 1 2 3 Position (m) C F1 ( Vdc Vsurf ) V z ac sint Vdc V surf (V) 0 Amplitude = C Vsurf z 0 if 0Vsurf C Phase 0 180 if 0 Vsurf z
Work Function of Ti : Ambient Dependence 25 H. Kim et al., Appl. Phys. Lett. 100, 022091 (2012). W sample (ev) 6.5 6.0 5.5 5.0 4.5 H 2 /Ar Ar E l E F,tip ev surf W tip φ S evs W S Tip Ti Х E l E C E F E V Ned sin s e 34.3 (ev) Δ S = W S [ ] W S [H 2 /Ar] = 1.3 ev 3.8 % coverage in ambient W E E ev S C F S S Oxygen adsorption ambient effects on W sample Surface oxygen vacancy sites are preferential adsorption sites Diebold, Surf. Sci. Rep. 48, 53 (2003) Oxygen vacancy density of 4.3 ~ 9.6 % ML estimated Yim et al., Phys. Rev. Lett. 104, 036806 (2010).
AFM Charge Writing of Ti : Scenario 26 H. Kim et al., Appl. Phys. Lett. 100, 022091 (2012). +V H 2 /Ar V O Ar h + h + h + h + h + h + h + O 2 O 2 O 2 O 2 O 2 O 2 O 2 O 2 h + h + h + h + h + h + h + h + h + h + h + h + h + h + Oxygen ( ) chemisorption formation of oxygen ions ( ) and holes (h + ) SPMtipinduced generation an oxygen vacancy ( ) in the lattice desorption of the oxygen ions at the surface V O
Resistive Switching of Pt/SrTiO 3 Nanocontacts 27 (under preparation). 60 40 Current (na) 20 0 20 40 60 1.5 1.0 0.5 0.0 0.5 1.0 1.5 Sample bias (V)
Summary 28 28 Electrical characterization suggested that the charging/discharging of the interface trap states dominated the resistive switching behaviors of the Pt/SrTiO 3 single crystal Schottky junctions. The ambient dependence indicated that the surface trap states were influenced by the oxygen adsorption/desorption. Simultaneous transport and SPM measurements showed that the surface oxygen distribution determined the resistance of the Ti single crystal planar junctions. The SPM tip could modify local surface charge distribution. Pt STO E C u S u B E F E i E V
EDPL Members E. M. Bourim Research Professor Research Funding E. Lee (Ph.D.) H. Kim (Ph.D.) M. Gwon (Ph.D.) A. Sohn (Ph.D.) Pioneer Research Center Program Y. Cho (Ph.D.) Y. Kim (MS) J. Kim (MS) New & Renewable Energy Technology Development Program
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