14682
J. Phys. Chem. C 2008, 112, 14682–14692
Electron Dynamics at the ZnO (101j0) Surface William A. Tisdale,† Matthias Muntwiler,‡ David J. Norris,† Eray S. Aydil,† and X.-Y. Zhu*,‡ Department of Chemical Engineering & Materials Science, and Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed: March 20, 2008; ReVised Manuscript ReceiVed: June 26, 2008
We use femtosecond time-resolved two-photon photoemission spectroscopy (TR-2PPE) to study the dynamics of electrons excited at the ZnO (101j0) surface. Efficient relaxation of hot electrons within the Γ valley of the bulk conduction band results in sub-30 fs lifetimes for electron energies greater than 0.1 eV above the conduction band minimum (CBM). These relaxation rates, which are among the fastest observed in any semiconductor over the same energy range, are consistent with the emission of longitudinal optical phonons resulting from strong Fro¨hlich coupling. For energy at or below the CBM, the excited electron lifetime increases exponentially with decreasing energy to as long as 1 ps. Dynamics in this region can be described by electronic relaxation within a quasi-continuum of defect-derived surface states whose density decreases exponentially into the band gap. Deliberately increasing defects on the ZnO surface drastically decreases the lifetime of electrons in this energy region. Existence of these states is consistent with observed upward band-bending and Fermi level pinning at the (101j0) surface. Introduction ZnO, a direct-gap wide band gap semiconductor, is a remarkably versatile material with applications ranging from catalysis to optoelectronic devices to other emerging technologies featuring easily synthesized anisotropic nanostructures.1,2 ZnO nanoparticle films3,4 and nanowire arrays5-8 have been used in lieu of TiO2 in dye- and quantum dot-sensitized solar cells as well as in hybrid organic/inorganic bulk heterojunction photovoltaic devices because ZnO has a band gap (3.4 eV) and electron affinity (4.2 eV) similar to those of TiO2, a semiconductor used widely in dye-sensitized solar cells.9 Because of the intrinsic anisotropy of the wurtzite unit cell, ZnO nanowires grow in the c-axis direction. Consequently, the side facets of these nanowires belong to the {101j0} family of planes, also referred to as the prism face of ZnO. Additionally, the (101j0) surface is the most stable of the low-index ZnO surfaces10 and likely constitutes a large fraction of the surface area in sintered ZnO nanoparticle films. As a result, the majority of charge separation and recombination events occur at the (101j0) or equivalent surfaces in optoelectronic and photovoltaic devices. Details on ZnO surface structure and properties are available in an excellent book chapter11 and a recent review article.12 For applications in optoelectronics and solar cells, it is important to understand the dynamics of excited electrons in ZnO and at its surfaces, particularly the predominant (101j0) surface. Developing this understanding requires the application of time-domain techniques capable of elucidating excited-state electron dynamics with femtosecond resolution. Time-resolved two-photon photoemission (TR-2PPE) spectroscopy,13-15 a surface-sensitive technique that offers simultaneous energy and time-domain characterization of electronic excited states at solid surfaces, is well-suited for obtaining this information. TR-2PPE is a pump-probe technique particularly useful for tracking interfacial electron dynamics. In this approach, the first photon * Corresponding author. E-mail:
[email protected]. † Department of Chemical Engineering & Materials Science. ‡ Department of Chemistry.
excites an electron from an occupied state into an unoccupied interfacial state (e.g., across the ZnO band gap in the present study). After a variable time delay, the second photon ionizes the electron for detection, with time, energy, and momentum resolution. TR-2PPE has been successfully applied in the past to probe excited electron dynamics at metal16 and semiconductor17 surfaces. We report in this Article a TR-2PPE study of the ZnO (101j0) surface. Our study reveals that the hot electron cooling rates in ZnO are among the fastest observed in any semiconductor at the same electron energies above the conduction band minimum (CBM).18-21 We show that this is due to long-range Fro¨hlich coupling responsible for longitudinal optical (LO) phonon emission in polar semiconductors. Additionally, we find longer lifetimes for electrons in a continuum of defect-derived surface states (SS) located in the vicinity of the CBM whose densityof-states (DOS) decreases exponentially into the band gap. Existence of these surface states is consistent with the observed upward band-bending and Fermi level pinning at the (101j0) surface. While our experiments provide no direct evidence that these defect states are stable in ambient or an aqueous environment, the nature and location of these states is remarkably similar to that of near-band edge acceptor states, which have been assumed in modeling of solar cells featuring ZnO photoanodes and in the interpretation of electron transport and recombination data in these devices.22,23 Experimental Methods Laser light was generated by a commercial mode-locked Ti: sapphire oscillator (Coherent Mira, 76 MHz, ∼700-900 nm) pumped by a Nd:vanadate solid-state laser at 532 nm (Coherent Verdi). Tunable wavelength Ti:sapphire fundamental emission (typically 840-850 nm, ∼60 fs, 8 nJ for this experiment) was frequency tripled in beta barium borate (BBO) and lithium triborate (LBO) crystals (Inrad) to obtain pulses in the near UV (∼4.4 eV, 0 in the bulk. In the one-dimensional approximation, Poisson’s equation (MKS unit system) is:
F(z, t) ∂ V(z, t) )2 ε ∂z 2
(A1)
where ε is the permittivity of ZnO and F(z,t) is the charge density, given by:
F(z, t) ) e(p(z, t) - n(z, t) + Nd)
∂n ∂ ∂n ∂V µ n - Dn ) G(z, t) + ∂t ∂z n ∂z ∂z
(A3)
∂p ∂ ∂p ∂V -µpp - Dp ) G(z, t) + ∂t ∂z ∂z ∂z
(A4)
(
)
and
(
)
where the diffusivity, Dn,p, and mobility, µn,p, are related through the Einstein relation:
Dn,p kbT ) µn,p e
(A5)
In eqs A3 and A4, G(z,t) is the electron-hole pair generation rate in the bulk (z > 0). Over the short time scales we are interested in ( 0, boundary conditions for solution of Poisson’s equation are:
(A8)
∂V )0 ∂z
(A9)
and
@z ) ∞,
From the 2PPE data, we are able to assign a maximum constant surface potential of Vs ) -0.3 V. Boundary conditions for electrons are
@z ) 0, Jn ) -rn
(A10)
@z ) ∞, n ) Nd
(A11)
@z ) 0, Jp ) -rp
(A12)
@z ) ∞, p) 0
(A13)
and
(A2)
In eq A2, p and n are the concentration of holes and electrons, respectively, Nd is the concentration of ionized donors, and e is the magnitude of the electron charge (defined so that e is positive). Substitution of electron and hole fluxes into their respective continuity expressions leads to the well-known driftdiffusion equations:
@z ) 0, V ) Vs
and for holes,
and In eqs A10-A13, Jn,p is the flux of electrons or holes at the surface and rn,p is the surface electron/hole capture rate. The surface capture rate is often expressed phenomenologically through a surface recombination velocity, that is, rp ) Vp · p, but more detailed expressions may be derived if specific information about the surface states participating in surface recombination is known.66 The most important result of our simulation is that the laser intensities used in this 2PPE study are not strong enough to induce significant changes to the potential profile near the ZnO surface within the time scale of interest for a two-pulse correlation (TPC) measurement (Ej Ei Ej. The second term on the RHS of eq B1 represents the depopulation of states with energies between Ej and Ej + δE by electrons decaying into all states with energy Ei < Ej. In writing eq B1, we have implicitly assumed that saturation effects are unimportant, that is, F(Ej)δE . n(Ej). For a continuum of states, eq B1 is rewritten as an integral-differential equation:
∫EE +σ n(Ei) dEi -
(B9)
where
τj )
2mce(Ec-Ej) ⁄mc kdFoσ2
(B10)
The solution of eq B9 is simply
n(Ej, t) ) no(Ej) · e-t⁄τj
(B11)
where the experimentally observed apparent lifetime, τj, of an electron residing in a state with energy Ej is given by eq B10. Thus, for an exponential distribution of states below the CBM, electron populations at all energies are expected to follow singleexponential decay dynamics where the lifetime increases exponentially with decreasing SS energy. Additionally, the parameter mc, which describes the average depth of the SS distribution, is uniquely determined by a fit of the TPC lifetime, τj, to the form τj ∝ exp[(Ec - Ej)/mc]. References and Notes (1) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98, 041301. (2) Huang, M.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, P.; Yang, P. Science 2001, 292, 1897. (3) Keis, K.; Bauer, C.; Boschloo, G.; Hagfeldt, A.; Westermark, K.; Rensmo, H.; Siegbahn, H. J. Photochem. Photobiol., A 2002, 148, 57–64. (4) Keis, K.; Magnusson, E.; Lindstrom, H.; Lindquist, S. E.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2002, 73, 51–58. (5) Baxter, J. B.; Aydil, E. S. Appl. Phys. Lett. 2005, 86, 053114. (6) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455–459.
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