Effects of Interfacial Energetics on the Effective Surface Recombination Velocity of Si/Liquid Contacts

Photoconductivity decay data have been obtained for NH_4F_((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) (Me_(10)Fc^(+/0)), iodine (I_2), or cobaltocene^(+/0) (CoCp_2^(+/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), I_2/I^-) or relatively negative (CoCp_2^(+/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 N_2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me_(10)Fc^(+/0)) showed differing rf photoconductivity decay behavior depending on their different surface chemistry. Specifically, surfaces that possessed Si−OCH_3 bonds, produced by reaction of H-terminated Si with CH_3OH−Fc^(+/0), showed lower surface recombination velocities in contact with N_(2(g)) or in contact with CH_3OH−Me_(10)Fc^(+/0) solutions than did NH_4F_((aq))-etched, air-exposed H-terminated Si(111) surfaces in contact with the same ambients. Furthermore, the CH_3OH−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 CH_3OH−I_2 or THF−I_2 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 CH_3OH−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 CH_3OH or THF electrolytes containing I_2 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/CH_3OH−CoCp_2 and Si/CH_3CN−CoCp_2 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

Measurement of the Free-Energy Dependence of Interfacial Charge-Transfer Rate Constants using ZnO/H_2O Semiconductor/Liquid Contacts

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) cm4 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

Electron-Transfer Processes at Semiconductor/Liquid Interfaces and Metal/Nanogap Junctions

It is shown that n-ZnO/H₂O-A/A⁻ junctions (A/A⁻ = [Co(bpy)₃]³⁺/²⁺ or [OsL₂L']³⁺/²⁺) display energetic and kinetic behavior of unprecedented ideality. The rate constant of the junction with the highest driving force increased when the driving force was lowered, which indicates that the junction operated in the inverted regime. The driving force was varied by shifting the conduction-band edge of the semiconductor with pH. The contact with the lowest driving force was found to operate in the normal regime of charge transfer. These results provide the first experimental indication that semiconductor/liquid contacts can operate in the inverted regime. Junctions having a similar driving force but different reorganization energies show the expected dependence of the rate constant on the reorganization energy. Low surface-recombination velocities (SRVs) were observed for systems with an accumulation of holes or electrons at the Si surface. Formation of the charge-carrier accumulation layer was confirmed by a solution-gated transistor method. Digital simulations revealed that SRVs &#60; 10 cm s⁻¹ can be produced by surfaces with trap densities as large as 10¹² cm⁻² provided that the surface is in accumulation or inversion. The degree of band bending and SRVs of Si(111) in contact with a variety of aqueous fluoride solutions were determined for the first time at open circuit. An accumulation of electrons at the surface is responsible for the low effective SRVs in NH₄F and buffered HF solutions. The protonation of basic defect sites is important for the low SRV of Si(111)/H₂SO4(aq) and Si(111)/HF(aq) contacts. The J-E characteristics of electron-tunnel junctions formed by the electromigration of metal nanowires without a molecule bridging the gap were explored in detail. The low-temperature J-E curves of some junctions showed regions of zero conductivity near zero bias, while such features were absent in the data collected for other junctions. A common pattern was discerned in that the low-bias resistances of all junctions decreased by at least an order of magnitude with increasing temperature according to Abeles' model for electron tunneling in granular metal junctions. These findings were consistent with the Coulomb blockade effect and can be attributed to metal islands in the gap.</p

The Role of Band Bending in Affecting the Surface Recombination Velocities for Si(111) in Contact with Aqueous Acidic Electrolytes

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) NH_4F(aq), buffered (pH = 5) HF(aq), 27 M (48% by weight) HF(aq), or concentrated 18 M H_2SO_4. Regardless of the sample history or surface condition, long charge-carrier lifetimes were observed for n-Si(111) surfaces in contact with 11 M NH_4F(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) Cu^(2+) was in the solution and/or adsorbed onto the Si(111) surface as Cu^0 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) Cu^(2+)-containing 11 M NH_4F(aq) or BHF(aq) solutions, respectively

Interfacial Energetics of Silicon in Contact with 11 M NH_4F(aq), Buffered HF(aq), 27 M HF(aq), and 18 M H_2SO_4

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) NH_4F, buffered HF (BHF), 27 M (48%) HF(aq), and concentrated (18 M) H_2SO_4. A Mott−Schottky analysis of A_s^2C_(sc)^(-2)-vs-E (where As is the interfacial area, and C_(sc) 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 NH_4F(aq) or BHF(aq). The barrier heights for n-Si(111) and n-Si(100) in 11 M NH_4F(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 NH_4F(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 H_2SO_4 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 H_2SO_4, 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 NH_4F, BHF, or 18 M H_2SO_4 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

Measurement of the Dependence of Interfacial Charge-Transfer Rate Constants on the Reorganization Energy of Redox Species at n-ZnO/H_2O Interfaces

The interfacial energetic and kinetics behavior of n-ZnO/H_2O contacts have been determined for a series of compounds, cobalt trisbipyridine (Co(bpy)_3^(3+/2+)), ruthenium pentaamine pyridine (Ru(NH_3)_5py^(3+/2+)), cobalt bis-1,4,7-trithiacyclononane (Co(TTCN)_2^(3+/2+)), and osmium bis-dimethyl bipyridine bis-imidazole (Os(Me_2bpy)_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 × 10^(-19) to 6 × 10^(-17) cm^4 s^(-1). The interfacial electron-transfer rate constant decreased as the reorganization energy, λ, of the acceptor species increased, and a plot of the logarithm of the electron-transfer rate constant vs (λ + ΔG°‘)^2/4λk_BT (where ΔG°‘ is the driving force for interfacial charge transfer) was linear with a slope of ∼ −1. The rate constant at optimal exoergicity was found to be ∼5 × 10^(-17) cm^4 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