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Energy-Level Control at Hybrid Inorganic/Organic Semiconductor Interfaces pp 91–180Cite as

Results and Discussion

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Part of the book series: Springer Theses ((Springer Theses))

Abstract

The main topical complex of the thesis is the energy-level alignment at HIOS interfaces comprising ZnO. In particular it is investigated how one can modify HIOS energy-level alignment by introducing thin donor/acceptor interlayers. For details on this working principle consult to the introduction or Sect. 5.2.

God made solids, but surfaces were the work of the Devil.

—Wolfgang Pauli

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Notes

  1. 1.

    Standardly samples were sputtered with 0.5 keV Ar ions for 40 min, annealed to 400 \(^{\circ }\mathrm{C}\) for 1 h (including ramping up and down) and 10L O\(_\mathrm{{2}}\) were dosed at 400 \(^{\circ }\mathrm{C}\) (for details of the preparation see Sect. 4.1.1.1).

  2. 2.

    Meaning that \(E_\mathrm {ph}\simeq \) 615 eV and measurements using Al K\(\upalpha \) or Mg K\(\upalpha \) radiation are excluded.

  3. 3.

    The particularly large scatter in the datapoints of the width of the surface oxygen peak (Fig. 5.3d), is in part due to instability of the fit.

  4. 4.

    In a simple ansatz, which is similar to the concept of the Madelung constant, one assumes O1s emission to be emitted point-like at the lattice sites. IMFP of adlayer and bulk are presumed to be the same and the adlayer thickness set to 1.95 Å (one Zn–O bondlength). Furthermore, one sums up the intensity of oxygens from different depths corresponding with regularly spaced lattice planes but weighted by the attenuation according to \(\lambda =5.3\) Å. For a 1 \(\times \) 1 adlayer on ZnO(10\(\bar{1}\)0) such methodology (lattice plane spacings 0.9 and 1.9 Å)

    $$\begin{aligned} \left[ 1/\sum \limits _\mathrm {i=1}^\infty \left( e^{-(i\cdot 2.8{\AA }+1.95{\AA })/5.3{\AA }} + e^{-(0.9{\AA }+i\cdot 2.8{\AA }+1.95{\AA })/5.3{\AA }}\right) \right] \end{aligned}$$

    yields an expected OH/bulk oxygen peak area ratio of 0.4. Comparison with Fig. 5.3a thus advises an actual coverage on ZnO(10\(\bar{1}\)0) of one adsorbant per surface unit cell.

  5. 5.

    Equation (5.2) explicitly depends as \(1/N_\mathrm {D}\). But for every change of \(N_\mathrm {D}\), implicitly also \(\delta q\) must be changed to match \(\varDelta \phi ^{BB}+\varDelta \phi ^{ID}=\varDelta \phi \) (see Appendix A).

  6. 6.

    Note that actually two different n doping species, one of which must not be shallow, could also resolve the issue.

  7. 7.

    Note that the 1/\(N_\mathrm {D}\) dependence is only an approximation, since in principle \(N_\mathrm {D}\) also influences the amount of charge transfer \(\delta q\) itself and by this also the \(\varDelta \phi ^{ID}\) part. The correct numerical solution \(\delta q(N_\mathrm {D})\) for \(\varDelta \phi ={2.8}\) eV is shown in Fig. 5.12.

  8. 8.

    In Schottky-depletion-layer approximation all donors are ionized.

  9. 9.

    Note that Fig. 5.12 is calculated for the 2.8 eV total \(\varDelta \phi \) on ZnO(0001). On ZnO(000\(\bar{1}\)), where \(\varDelta \phi ={1.4}\) eV, the extend of the space charge region and the amount of charge transfer are \({\sim }\sqrt{2}\) smaller, since \(\varDelta \phi ^{BB}\propto \delta q^2\). For \(\varDelta \phi ^{ID}\) this means that it is \({\sim }\sqrt{2}\) enhanced. This does not change the general consideration that if using highly doped ZnO crystals the band bending contribution should be significantly reduced.

  10. 10.

    E.g., from Aldrich.

  11. 11.

    In the C K-edge region TEY currents are particularly low due to carbon contaminations on the optical parts of the beamline.

  12. 12.

    All NEXAFS spectra of this thesis have been referenced in energy to a single carbon and nitrogen mirror current signal. To compare absolute NEXAFS peak positions of this work with externally reported data one should mind a constant difference in photon energy due to different references.

  13. 13.

    Since the parameter space of the fit is too large for only three angles recorded.

  14. 14.

    The discrepancy of the band bending as evidenced by Figs. 5.20 and 5.21a is due to light induced energy level differences between UPS and XPS spot.

  15. 15.

    It should be mentioned that one can also fit the O1s spectra (lesser spacing) in the adsorbed state with a shape representative of the bare O1s spectra and an additional peak. In order to decide whether this is indeed physically meaningful much better statistics are necessary and preferably even narrower lineshapes. Until then, the fitting problem is underdetermined and the occurrence of an adsorption induced third O1s core level peak must be considered speculation.

  16. 16.

    Here not the depletion approximation was used, but rather correct Fermi-Dirac occupation statistics applied to a continuum band bending model. This is because at \(\varDelta \phi ^{BB}_\infty ={0.3}\) eV the region, where the depletion approximation is good, is only \(\thicksim \)20 nm large, but the space charge, i.e., the non neutral region, is distributed \(\thicksim \)100 nm inside the crystal. The Mathematica code implemented for this more elaborate calculation is listed in Appendix E.

  17. 17.

    Assuming the intra ZnO charge density to be uniformly distributed 1 nm deep inside the crystal and all 0.6 transferred electrons to reside on nitrogens only.

  18. 18.

    There exist some uncertainty of the core level positions due to UV irradiation related energy-level shifts within ZnO (see Fig. F.4) and screening effects due to the extra charge density at the surface. However, this does not change the general fact that \(\varDelta \phi \gg \varDelta \phi ^{BB}\).

  19. 19.

    Note that for HATCN the band bending as determined by XPS (O1s and Zn3p shift) and UPS (shift of the Zn3d derived band) differ by up to 0.4 eV, which is probably related to the UV-induced energy level shifts within ZnO [49].

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Schlesinger, R. (2017). Results and Discussion. In: Energy-Level Control at Hybrid Inorganic/Organic Semiconductor Interfaces. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-319-46624-8_5

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