Florian Gstrein

California Institute of Technology, Pasadena, CA, United States

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Publications (7)23.31 Total impact

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    ABSTRACT: The role of band bending in affecting surface recombination velocity measurements has been evaluated by combining barrier height data with charge-carrier lifetime measurements for Si(111) surfaces in contact with a variety of acidic aqueous electrolytes. Charge-carrier lifetimes and thus surface recombination velocities have been measured by contactless radio frequency photoconductivity decay techniques for long bulk lifetime n-Si(111) samples in contact with 11 M (40% by weight) NH4F(aq), buffered (pH = 5) HF(aq), 27 M (48% by weight) HF(aq), or concentrated 18 M H2SO4. Regardless of the sample history or surface condition, long charge-carrier lifetimes were observed for n-Si(111) surfaces in contact with 11 M NH4F(aq) or buffered HF(aq). On the basis of previous barrier height measurements, this behavior is consistent with the formation of an electrolyte-induced surface accumulation layer that reduces the rate of steady-state surface recombination even in the presence of a significant density of surface trap sites. A straightforward evaluation of the surface trap state density from the measured surface recombination velocities, S, is thus precluded for such Si/liquid contacts. In contrast, a wide range of S values, depending on the history of the sample and the state of the surface, were observed for n-Si(111) surfaces in contact with 27 M HF(aq). These results in conjunction with previously measured barrier height data indicate that the charge-carrier lifetimes measured for n-Si(111) in contact with 27 M HF(aq) can be directly correlated with the surface condition and the effective surface-state trap density. These conclusions were confirmed by measurements of the apparent S values of n-Si(111) surfaces in contact with various solutions in the presence of the known deep trap, Cu. For Si(111)/HF(aq) contacts, very high (≥920 ± 270 cm s-1) surface recombination velocities were observed when 0.16 mM (10 ppm) Cu2+ was in the solution and/or adsorbed onto the Si(111) surface as Cu0 deposits, whereas low (100 ± 75 or 225 ± 20 cm s-1) apparent surface recombination velocities were measured for Cu-contaminated Si(111) samples in contact with 0.16 mM (10 ppm) Cu2+-containing 11 M NH4F(aq) or BHF(aq) solutions, respectively.
    Journal of Physical Chemistry C - J PHYS CHEM C. 03/2008; 112(15).
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    ABSTRACT: Open-circuit impedance spectra, channel impedance spectroscopy on solution-gated field-effect devices, and differential capacitance vs potential (Mott−Schottky) measurements were used to determine the energetics of n-Si(111), n-Si(100), and p-Si(111) electrodes in contact with aqueous 11 M (40% by weight) NH4F, buffered HF (BHF), 27 M (48%) HF(aq), and concentrated (18 M) H2SO4. A Mott−Schottky analysis of As2Csc-2-vs-E (where As is the interfacial area, and Csc is the differential capacitance as a function of the electrode potential, E) data yielded reliable barrier heights for some silicon/liquid contacts in this work. Performing a Mott−Schottky analysis, however, requires measurement of the differential capacitance under reverse bias, where oxidation or etching can occur for n-Si and where electroplating of metal contaminants can occur for p-Si. Hence, open-circuit methods would offer desirable, complementary approaches to probing the energetics of such contacts. Accordingly, open-circuit, near-surface channel conductance measurements have been performed using solution-gated n+-p-Si(111)-n+ and p+-n-Si(100)-p+ devices. Additionally, open-circuit impedance spectra were obtained for silicon electrodes in contact with these solutions. The combination of the three techniques indicated that the surfaces of n-Si(111) and n-Si(100) were under accumulation when in contact with either 11 M NH4F(aq) or BHF(aq). The barrier heights for n-Si(111) and n-Si(100) in 11 M NH4F(aq) were −0.065 ± 0.084 V and −0.20 ± 0.21 V, respectively, and were −0.03 ± 0.19 V and −0.07 ± 0.24 V, respectively, for these surfaces in contact with buffered HF(aq). Consistently, p-Si(111) surfaces were determined to be in inversion in contact with these electrolytes, exhibiting barrier heights of 0.984 ± 0.078 V in contact with 11 M NH4F(aq) and 0.97 ± 0.22 V in contact with buffered HF(aq). In contact with 27 M HF(aq), n-Si(111) and n-Si(100) were in depletion, with barrier heights of 0.577 ± 0.038 V and 0.400 ± 0.057 V, respectively, and p-Si(111) was under inversion with a barrier height of 0.856 ± 0.076 V. Measurements performed in 18 M H2SO4 revealed barrier heights of 0.75 ± 0.11 V, 0.696 ± 0.043 V, and 0.889 ± 0.018 V for n-Si(111), n-Si(100), and p-Si(111), respectively, demonstrating that in 18 M H2SO4, the band edge positions of Si were different for different doping types. The barrier height data demonstrate that the observed low recombination rates of silicon in contact with 11 M NH4F, BHF, or 18 M H2SO4 cannot necessarily be attributed to a reduction in the number of surface trap states. In part, low surface recombination rates are expected for such systems because the very large or very small barrier height for silicon in contact with these liquids provides a potential barrier that prevents one type of photogenerated carrier (either electrons or holes) from reaching the surface, thereby producing a low steady-state surface recombination rate.
    Journal of Physical Chemistry C - J PHYS CHEM C. 10/2007; 111(44).
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    ABSTRACT: Near-surface channel impedance measurements, open-circuit impedance spectra, and differential capacitance vs potential measurements have been used to determine the barrier height of liquid contacts formed with n-type and p-type Si electrodes. Barrier heights were measured as the redox potential, E(A/A-), of a metallocene-based, one-electron, outer-sphere, acceptor/donor (A/A-) pair was varied in CH3CN solvent. The barrier heights of p-Si(111) electrodes in contact with CH3CN−Me10Fc+/0 (where Me10Fc is decamethylferrocene) or CH3CN-CoCp2+/0 (where CoCp2 is cobaltocene) were 0.69 ± 0.1 and 1.1 ± 0.1 V respectively. In contrast, barrier heights for n-Si(111)/CH3CN−Me10Fc+/0 and n-Si(111)/CH3CN-CoCp2+/0 contacts were 0.66 ± 0.1 and 0.09 ± 0.01 V, respectively. These measurements indicate that the barrier heights closely track changes in the electrochemical potential of the contact, instead of being relatively invariant to changes in the Fermi level of the contacting phase, as is observed for Si/metal Schottky barriers. These measurements also demonstrate that the low effective surface recombination velocity, S, for silicon in contact with CoCp2+/0 is primarily the result of an accumulation layer rather than solely being due to a low density of surface electrical defects.
    Journal of Physical Chemistry C - J PHYS CHEM C. 05/2007; 111(22).
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    ABSTRACT: Changes in pH have been used to shift the band-edge positions of n-type ZnO electrodes relative to solution-based electron acceptors having pH-independent redox potentials. Differential capacitance vs. potential and current density vs. potential measurements using [Co(bpy)3]3+/2+ and [Ru(bpy)2(MeIm)2]3+/2+ (where bpy = 2,2′-bipyridyl and MeIm = 1-methyl-imidazole) allowed investigation of the pH-induced driving-force dependence of the interfacial electron-transfer rate in the normal and inverted regions of electron transfer, respectively. All rate processes were observed to be kinetically first-order in the concentration of electrons at the ZnO surface and first-order in the concentration of dissolved redox acceptors. Measurements using [Co(bpy)3]3+/2+, which has a low driving force and a high reorganization energy in contact with ZnO electrodes, and measurements of [Ru(bpy)2(MeIm)2]3+/2+, which has a high driving force and a low reorganization energy in contact with ZnO electrodes, allowed for the evaluation of both the normal and inverted regions of interfacial electron-transfer processes, respectively. The rate constant at optimum exoergicity was observed to be approximately 5 × 10−17 cm4 s−1. The rate constant vs. driving-force dependence at n-type ZnO electrodes exhibited both normal and inverted regions, and the data were well-fitted by parabolas generated using classical electron-transfer theory.
    Chemical Physics 01/2006; · 1.96 Impact Factor
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    ABSTRACT: The interfacial energetic and kinetics behavior of n-ZnO/H2O contacts have been determined for a series of compounds, cobalt trisbipyridine (Co(bpy)3(3+/2+)), ruthenium pentaamine pyridine (Ru(NH3)5 py(3+/2+)), cobalt bis-1,4,7-trithiacyclononane (Co(TTCN)2(3+/2+)), and osmium bis-dimethyl bipyridine bis-imidazole (Os(Me2bpy)2(Im)2(3+/2+)), which have similar formal reduction potentials yet which have reorganization energies that span approximately 1 eV. Differential capacitance vs potential and current density vs potential measurements were used to measure the interfacial electron-transfer rate constants for this series of one-electron outer-sphere redox couples. Each interface displayed a first-order dependence on the concentration of redox acceptor species and a first-order dependence on the concentration of electrons in the conduction band at the semiconductor surface, in accord with expectations for the ideal model of a semiconductor/liquid contact. Rate constants varied from 1 x 10(-19) to 6 x 10(-17) cm4 s(-1). The interfacial electron-transfer rate constant decreased as the reorganization energy, lambda, of the acceptor species increased, and a plot of the logarithm of the electron-transfer rate constant vs (lambda + deltaG(o)')(2)/4lambda k(B)T (where deltaG(o)' is the driving force for interfacial charge transfer) was linear with a slope of approximately -1. The rate constant at optimal exoergicity was found to be approximately 5 x 10(-17) cm4 s(-1) for this system. These results show that interfacial electron-transfer rate constants at semiconductor electrodes are in good agreement with the predictions of a Marcus-type model of interfacial electron-transfer reactions.
    Journal of the American Chemical Society 11/2005; 127(40):13949-54. · 10.68 Impact Factor
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    ABSTRACT: The dependence of electron-transfer rate constants on the driving force for interfacial charge transfer has been investigated using n-type ZnO electrodes in aqueous solutions. Differential capacitance versus potential and current density versus potential measurements were used to determine the energetics and kinetics, respectively, of the interfacial electron-transfer processes. A series of nonadsorbing, one-electron, outer-sphere redox couples with formal reduction potentials that spanned approximately 900 mV allowed evaluation of both the normal and Marcus inverted regions of interfacial electron-transfer processes. All rate processes were observed to be kinetically first-order in the concentration of surface electrons and first-order in the concentration of dissolved redox acceptors. The band-edge positions of the ZnO were essentially independent of the Nernstian potential of the solution over the range 0.106-1.001 V vs SCE. The rate constant at optimal exoergicity was observed to be approximately 10(-)(16) cm(4) s(-)(1). The rate constant versus driving force dependence at n-type ZnO electrodes exhibited both normal and inverted regions, and the data were well-fit by a parabola generated using classical Marcus theory with a reorganization energy of 0.67 eV. NMR line broadening measurements of the self-exchange rate constants indicated that the redox couples had reorganization energies of 0.64-0.69 eV. The agreement between the reorganization energy of the ions in solution and the reorganization energy for the interfacial electron-transfer processes indicated that the reorganization energy was dominated by the redox species in the electrolyte, as expected from an application of Marcus theory to semiconductor electrodes.
    Journal of the American Chemical Society 07/2005; 127(21):7815-24. · 10.68 Impact Factor
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    ABSTRACT: Photoconductivity decay data have been obtained for NH4F(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of methanol, tetrahydrofuran (THF), or acetonitrile containing either ferrocene+/0 (Fc+/0), [bis(pentamethylcyclopentadienyl)iron]+/0 (Me10Fc+/0), iodine (I2), or cobaltocene+/0 (CoCp2+/0). Carrier decay measurements were made under both low-level and high-level injection conditions using a contactless rf photoconductivity decay apparatus. When in contact with electrolyte solutions having either very positive (Fc+/0, I2/I-) or relatively negative (CoCp2+/0) Nernstian redox potentials with respect to the conduction-band edge of Si, Si surfaces exhibited low effective surface recombination velocities. In contrast, surfaces that were exposed only to N2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me10Fc+/0) showed differing rf photoconductivity decay behavior depending on their different surface chemistry. Specifically, surfaces that possessed Si−OCH3 bonds, produced by reaction of H-terminated Si with CH3OH−Fc+/0, showed lower surface recombination velocities in contact with N2(g) or in contact with CH3OH−Me10Fc+/0 solutions than did NH4F(aq)-etched, air-exposed H-terminated Si(111) surfaces in contact with the same ambients. Furthermore, the CH3OH−Fc+/0-treated surfaces showed lower surface recombination velocities than surfaces containing Si−I bonds, which were formed by the reaction of H-terminated Si surfaces with CH3OH−I2 or THF−I2 solutions. These results can all be consistently explained through reference to the electrochemistry of Si/liquid contacts. In conjunction with prior measurements of the near-surface channel conductance for p+−n−p+ Si structures in contact with CH3OH−Fc+/0 solutions, the data reveal that formation of an inversion layer (i.e., an accumulation of holes at the surface) on n-type Si, and not a reduced density of surface electrical trap sites, is primarily responsible for the long charge carrier lifetimes observed for Si surfaces in contact with CH3OH or THF electrolytes containing I2 or Fc+/0. Similarly, formation of an accumulation layer (i.e., an accumulation of electrons at the surface) consistently explains the low effective surface recombination velocity observed for the Si/CH3OH−CoCp2 and Si/CH3CN−CoCp2 contacts. Detailed digital simulations of the photoconductivity decay dynamics for semiconductors that are in conditions of inversion or depletion while in contact with redox-active electrolytes support these conclusions.
    Journal of Physical Chemistry B - J PHYS CHEM B. 02/2002; 106(11).