Surfaces and Interfaces of Transparent Conducting Oxides Andreas Klein

Transcription

1 Surfaces and Interfaces of Transparent Conducting Oxides Andreas Klein Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division

2 Transparent conducting oxides d R T high electrical conductivity (>10 3 /Ωcm) high optical transparency (>80%) high infrared reflectivity Realization: Energy E F CB E g >3eV Degenerately doped oxides (d 10 -systems) (ZnO, In 2 O 3, SnO 2, CdO and mixed oxides) VB Andreas Klein TCO Surfaces and Interfaces 2

3 Thin film solar cells ZnO CdS CIGS Mo glass metal CdTe CdS TCO glass Cu(In,Ga)(S,Se) 2 CdS ZnO ITO SnO 2 CdS CdTe Te Met i n + energy E F at interface E CB E CB E VB SEM courtesy ZSW E VB Andreas Klein TCO Surfaces and Interfaces 3

4 The TCO electrode in OLEDs cathode Hole injection determined by work function Work function determined by surface orientation and termination Oxygen exchange determined by surface properties grain orientation determined by surface energy +??? Defect concentration (e.g. O i ) and mobility determined by material properties and doping Andreas Klein TCO Surfaces and Interfaces 4

5 Chemical gas sensors sensitizer Pt contact SnO 2 R E CB E F semicond. surface /metal change of surface potential by adsorption Sensor response explained by surface potential changes (ionosorption model) Changes of SnO 2 bulk doping, E VB (e.g. changes in [V O ]) neglected Andreas Klein TCO Surfaces and Interfaces 5

6 Issues of TCO surfaces and interfaces Surface properties Workfunction of TCOs Oxygen Exchange at TCO surfaces Interfaces Properties Reactivity of interfaces Energy level alignment (barrier heights) TCO oxide surfaces and interfaces are considerably more complex than those of conventional semiconductors Andreas Klein TCO Surfaces and Interfaces 6

7 Andreas Klein TCO Surfaces and Interfaces 7 defect properties

8 Defects in semiconductors Vacancies (V cation (acceptor), V anion (donor) ) Interstitials (Cat i (donor), An i (acceptor) ) Antisite defects (e.g. Ga As and As Ga in GaAs ) Schottky defect pair (V cat + V an ), (Cat i + An i ) Frenkel defect pair (V cat + Cat i ), (V an + An i ) F-centers (vacancy + e, h) Polarons Electrons and holes Impurities (substitutional, interstitial) not stoichiometric, charged stoichiometric, uncharged defect concentration N d N exp hd k T B oxygen interstitial in ZnO Andreas Klein TCO Surfaces and Interfaces 8

9 ZnO Defects Akzeptor Donator H def pinning position E A E D E F compensation of donors by Zinc-vacancies (V Zn ) under oxygen rich conditions Andreas Klein TCO Surfaces and Interfaces 9

10 ZnO Fermi level E F -E VB [ev] ZnO:Al i-zno E CB CB VB E F pinning position low O 2 content Highly doped film high O 2 content Low doped film Oxygen content in sputter gas [%] 50 Change of doping with po 2 Pinning by Zn-vacancies Andreas Klein TCO Surfaces and Interfaces 10

11 Intrinsic defects of TCOs material ZnO ZnO:Al In 2 O 3 In 2 O 3 :Sn defects V Zn, Zn i, V O Al Zn, V Zn V O Sn In, O i p(o 2 ) exchange SnO 2 V O SnO 2 :Sb Sb Sn,?? diffusion Conductivity determined by doping and intrinsic defects (Stoichiometry) Changes of conductivity by changes of stoichiometry require oxygen exchange and diffusion Andreas Klein TCO Surfaces and Interfaces 11

12 Assessment of surface and interface properties Andreas Klein TCO Surfaces and Interfaces 12

13 Thin film deposition gas magnetron sputtering RF/DC power cooling N S N target magnetic field plasma substrate radiative heater (halogen lamp) Wide range of materials low substrate temperatures epitaxial growth possible wide range of parameter variation high deposition rates large area deposition Andreas Klein TCO Surfaces and Interfaces 13

14 Integrated System DAISY-MAT electron analyser UV-source Ion-source monochromated X-ray source load lock magnetron sputtering sample handler MBE organics, metals Analysis Preparation MOCVD ALD magnetron sputtering magnetron sputtering Andreas Klein TCO Surfaces and Interfaces 14

15 Photoemission Barrier Heights core level valence band energy diagram E F -E VB E CB E VB,CL counts hν-workfunction E VB E F -E VB BE (CL) E VB,CL binding energy 0=E F E CL Interface experiment Schottky barrier 1: substrate hν e - 2-N: thin film ( nm) N+1: thick film (>10 nm) E g E F -E VB Φ B,n Φ B,p 0 0 thickness Andreas Klein TCO Surfaces and Interfaces 15

16 work functions of TCOs Andreas Klein TCO Surfaces and Interfaces 16

17 SnO 2 ionization potential reduced (110) surface 9.0 E vac φ I P E CB E F E VB E vac -E VB [ev] SnO 2 :Sb SnO 2 oxidized stoichiometric (110) surface Oxygen content in sputter gas [%] Andreas Klein TCO Surfaces and Interfaces 17

18 SnO 2 work function po 2 SnO 2 E vac E CB E F E VB work function [ev] E CB SnO 2 :Sb SnO 2 :F E F -E VB [ev] Identification of surface termination by ionization potential Andreas Klein TCO Surfaces and Interfaces 18 Thin Solid Films 518, 1197 (2009)

19 SnO 2 preferred orientation XRD 25% O 2 (110) normierte normalized Intensität intensity [willk. Einh.] 10% O 2 5% O 2 2% O 2 1% O 2 100% Ar (101) xo 2 [%] Change of stable surface orientation with oxygen pressure Θ [ ] Andreas Klein TCO Surfaces and Interfaces 19 J. Phys. D 43, (2010)

20 Work function of In 2 O 3 and ITO DFT (100) (111) Almost no change of surface termination with oxygen Work function depends on surface orientation Differences between In 2 O 3 and ITO explained by texture of films Surface oxidation (e.g. via ozone) only possible for (100) orientation Andreas Klein TCO Surfaces and Interfaces 20

21 TCO work functions work function [ev] ZnO ZnO:Al SnO 2 :Sb SnO 2 SnO 2 :F ITO E F -E VB [ev] E F -E VB [ev] E F -E VB [ev] Large variation of work function but E VB does not depend on work function Andreas Klein TCO Surfaces and Interfaces 21 Thin Solid Films 518, 1197 (2009)

22 Oxygen Exchange Andreas Klein TCO Surfaces and Interfaces 22

23 ITO/organic interface O1s C1s UPS O1s C1s UPS C-O 0.8Å 0.4Å 0Å binding energy [ev] binding energy [ev] O O O O O O O O ITO ZnPC ITO + O 2 E F O O energy E CB HOMO Φ B E VB Andreas Klein TCO Surfaces and Interfaces 23 J. Phys. Chem. B 110, 4793 (2006)

24 Conductivity relaxation of ITO (1 bar) 6 ppm 16 ppm 1010 ppm T=590 C d=400nm 0.97 % 9.94 % 97.6 % Log (conductivity) -1/8 Log (po 2 ) Conductivity depends on oxygen pressure Slope related to dominant defect species Andreas Klein TCO Surfaces and Interfaces 24

25 Conductivity relaxation of SnO 2 (1 bar) conductivity [S/cm] C oxygen pressure [Pa] conductivity [S/cm] C oxygen pressure [Pa] time [h] time [h] Almost no change of σ with po 2 at 400 C Equilibrium carrier concentration not achieved Andreas Klein TCO Surfaces and Interfaces 25

26 Surface modification 10 SnO 2 SnO 2 /In 2 O 3 P = 1 bar conductivity [S/cm] σ T temperature [ C] Pa O 2 0.6Pa O time [min] time [min] Exchange at SnO 2 possible with 1nm In 2 O 3 on surface Andreas Klein TCO Surfaces and Interfaces 26

27 Relaxation at low pressure Relaxation observed when starting from reduced surface Oxidation of surface faster than oxidation of bulk Andreas Klein TCO Surfaces and Interfaces 27

28 Oxygen exchange of SnO 2 reduced-sno 2 oxidized-sno 2 SnO 2 /In 2 O 3 Surface termination is important for oxygen exchange Andreas Klein TCO Surfaces and Interfaces 28

29 Interface Formation Andreas Klein TCO Surfaces and Interfaces 29

30 CdS/ZnO substrate oxidation CdS/ZnO CIGS/ZnO In 2 S 3 /ZnO S 2p Cd MNN Ga LMM In MNN intensity [arb. units] ZnO film thickness binding energy [ev] binding energy [ev] no oxidation of substrate oxidation of substrate Andreas Klein TCO Surfaces and Interfaces 30

31 CdS/ZnO initial growth CdS/ZnO CIGS/ZnO surface species O 1s Zn O 2 intensity [arb. units] binding energy [ev] ZnO film thickness ZnO or CdS dissociation of O 2 not possible non-reactive CIGS or In 2 S 3 dissociation of O 2 possible reactive catalytic activity of surface responsible for substrate oxidation Andreas Klein TCO Surfaces and Interfaces 31

32 CdS/ZnO interface properties band alignment microstructure E F - E VBM [ev] ,2 01 ev ~2 nm ZnO polycrystalline, preferred orientation χ = 3,6 ev polycrystalline, statistical orientation χ = 4,5 ev deposition time [s] amorphous CdS amorphous polycrystalline χ = 4,45 ev Venkata Rao et al., Appl. Phys. Lett. 87 (2005), Andreas Klein TCO Surfaces and Interfaces 32

33 CdS/ZnO band alignment Large variation of band alignment E VB depends on Deposition sequence ZnO dep. temperature ZnO doping details explained in: Ellmer, Klein, Rech Transparent Conductive Zinc Oxide (Springer, 2008), chap Andreas Klein TCO Surfaces and Interfaces 33

34 CdS/ZnO Fermi level pinning 1.5 CdS/ZnO:Al ZnO:Al/CdS E F -E VB [ev] eV 1.6eV 1.8eV 2.2eV 1.4eV high doping ZnO:Al CdS 4.0 low doping deposition time [sec] deposition time [min] Fermi level in CdS substrate restricted to 2.0 ± 0.2 ev Ellmer, Klein, Rech Transparent Conductive Zinc Oxide (Springer, 2008), chap Andreas Klein TCO Surfaces and Interfaces 34

35 In 2 S 3 /ZnO Fermi level pinning E F -E VB [ev] (a) 2.8 ev (b) 1.9 ev In 3d S 2p Zn 2p O 1s E CB E VB (a) EF (b) In ZnO deposition time [sec] 2 S 3 In ZnO:Al ZnO:Al 2 S C 250 C+O 2 No band bending in In 2 S 3 and ZnO Band alignment defined by doping levels Andreas Klein TCO Surfaces and Interfaces 35 Klein et al., J. Mater. Sci. 42 (2007), 1890.

36 TCO / CdS band alignment ZnO CdS SnO 2 CdS ITO CdS In 2 O 3 CdS In 2 O 3 CdS E F CB VB Fermi level not within acceptable range Valence band maxima of TCOs at comparable energy but: Band alignment influenced by TCO gap and doping due to Fermi level pinning in CdS Andreas Klein TCO Surfaces and Interfaces 36

37 SnO 2 /Pt interface formation O 1s 1050s 128s 64s 32s Sn 3d 5/2 32s Pt 4f VB 16s Intensity 8s 4s 2s 1s 0s Binding energy [ev] Reduction of SnO 2 during deposition Andreas Klein TCO Surfaces and Interfaces 37 Surf. Sci. 602 (2008), 3246.

38 SnO 2 /Pt interface chemistry 150 C 150 C T=150 C Sn 3d 150 C: Sn 0 <-> Sn 4+ with intermediate count rate [arb. units] Sn 4+ Sn binding energy [ev] oxygen pressure binding energy [ev] Sn 2+ state 100 C: Sn 0 <-> Sn 2+ Oxidation/reduction not observable for - large Pt islands - bare SnO 2 surface Reversible oxidation/reduction of Sn Andreas Klein TCO Surfaces and Interfaces 38

39 Chemistry at buried interface Reducing environment Oxidizing environment SnO 2 Pt SnO 2 Pt H O O H O O Sn 0 Sn 4+ SnO 2 Pt SnO 2 Pt E CB E F E CB E F E VB E VB Oxygen is reversibly transported to/from the interface Andreas Klein TCO Surfaces and Interfaces 39

40 Summary Work functions depend on doping, surface orientation (ZnO, In 2 O 3, SnO 2?) and surface termination (In 2 O 3, SnO 2 ) Oxygen exchange generally possible for In 2 O 3 but can be suppressed at stoichiometric SnO 2 surfaces Reactivity of interfaces determined by catalytic activity of surface for dissociation of oxygen Energy band alignment characterized by similar valence band energies but may be affected by Fermi level pinning Schottky barrier heights can depend on oxygen pressure Andreas Klein TCO Surfaces and Interfaces 40

41 Thanks Frank Säuberlich, Yvonne Gassenbauer, Christoph Körber, André Wachau, Mareike Hohmann, Karsten Rachut (Surface Science) Paul Erhart, Péter Ágoston, Karsten Albe (Materials Modelling) S.P Harvey, T.O. Mason (Northwestern University) BMBF (ZnO), DFG (SFB 595), DFG-NSF (Materials World Network) Andreas Klein TCO Surfaces and Interfaces 41