Surface state properties of clean cleaved silicon as derived from the temperature dependence of the surface photovoltage

The temperature dependence of the surface photovoltage of clean silicon surfaces cleaved in UHV was investigated with a special emphasize on that temperature where surface photovoltage changes its sign. The surface photovoltage was measured by the electron beam method. Chopped light with an energy larger than the bandgap of silicon was used. The ratio of the transition probabilities $\text{r}{}_{\mathrm{V}}\text{r}{}_{\mathrm{C}}$ for transitions from and into the surface states has been derived from the surface photovoltage and the value for the band bending at the clean silicon surface. It is concluded from model calculations that at temperatures below the sign change of the surface photovoltage the signal height of the surface photovoltage is essentially determined by the surface recombination whereas the contribution due to band bending is negligible.     

Electron propagation from a photo-excited surface: implications for time-resolved photoemission

We perform time- and angle-resolved photoelectron spectroscopy on p-type GaAs(110). We observe an optically excited population in the conduction band, from which the time scales of intraband relaxation and surface photovoltage decay are both extracted. Moreover, the photovoltage shift of the valence band intriguingly persists for hundreds of picoseconds at negative delays. By comparing to a recent theoretical study, we reveal that the negative-delay dynamics reflects the interaction of the photoelectrons with a photovoltage-induced electric field outside the sample surface. We develop a conceptual framework to disentangle the intrinsic electron dynamics from this long-range field effect, which sets the foundation for understanding time-resolved photoemission experiments on a broad range of materials in which poor electronic screening leads to surface photovoltage. Finally, we demonstrate how the long-lasting negative-delay dynamics in GaAs can be utilized to conveniently establish the temporal overlap of pump and probe pulses in a time-resolved photoemission setup.     

The influence of atomic steps on the photoelectric properties of clean cleaved silicon as derived from the surface photovoltage

The correlation of structural and electrical properties of clean silicon surfaces cleaved in UHV was investigated quantitatively by the surface photovoltage, using light with an energy larger than the band gap of silicon. The surface photovoltage, which is a function of band bending and recombination probability, depends strongly on the appearance of atomic steps. The additional surface states vary with density and crystallographic orientation of the steps as well as with adsorption of oxygen. The experimental facts can be explained by accepting a shift of the Fermi level at the surface towards the valence band due to edge atoms. By measuring the change of sign of the surface photovoltage of crystals with various dopings an exponential temperature dependence of the ratio of the recombination probabilities r_v/r_c for transitions from and into the Surface states has been derived.     

Surface electronic properties of sulfur-treated GaAs determined by surface photovoltage measurement and its computer simulation

The Kelvin method together with the simulations of surface photovoltage has been used to determine the surface electronic properties, i.e. the surface band bending (qV_S), surface state density (N_SS0) and surface fixed charge (Q_Fx) of S₂Cl₂-treated GaAs (100) surfaces. The measured values of surface photovoltage (SPV) do not show saturation at high photon flux densities in contradiction to the simple theory of SPV. This behavior of SPV agrees very well with the rigorous computer simulations and can be explained in terms of the Dember effect. Moreover, the SPV values become insensitive to surface states at moderate photon flux densities. On this basis, the surface band bending of untreated (0.79 eV) and S₂Cl₂-treated (0.60 eV) GaAs surfaces was determined. The band diagrams summarizing the obtained results proved the influence on the potential variations not only from the ionized surface states and surface fixed charge but also from the surface dipole layer on the S₂Cl₂-treated GaAs surface. The dipole arises most probably due to the S-Ga bonding on the surface. The presented results offer an alternative explanation for increased PL commonly observed after the sulfidation in the absence of substantial reduction in the band bending.     

Photovoltaic properties of a heterojunction based on copper phthalocyanine films on the surface of polycrystalline cadmium sulfide

The photovoltaic effect has been detected and studied in thin-film structures based on thermally deposited 200-nm-thick copper phthalocyanine (CuPc) films on the surface of polycrystalline CdS. The structures under study demonstrate the linear current-voltage characteristics at external electric fields to 3.5 × 10⁴ V/cm. Two components of the photovoltage of different signs have been revealed when the sample is illuminated in the wavelength range from 350 to 700 nm. The first component has the positive sign on the CuPc film side and is observed when using the radiation with a wavelength lesser than 500 nm, i.e., in conditions of predominant absorption of the radiation in the CdS layer. The second component has the negative sign on the CuPc film side and is observed when using the radiation with a wavelength in the range from 500 to 570 nm, corresponding to the spectral region of the absorption edge of the CuPc films. The dependences of the photovoltage on the radiation intensity studied in the range from 5 × 10¹² to 10¹⁴ photons cm^?2 s^?1 are different in the cases of the two detected components. Mechanisms of generation of the photovoltage components associated with a change in the band bending during photogeneration of charge carriers in the region of space charge in CdS and a change in conditions of the charge transfer in the interfacial CuPc/CdS region during the radiation absorption in the CuPc film have been proposed.     

Observation of extrinsic surface states on (112̄0) CdS

Surface states have been detected by surface photovoltage spectroscopy on (112̄0) CdS surfaces subjected to various treatments in UHV and studied by Auger electron spectroscopy and LEED. All surface electronic features can be related to chemical contamination or lattice nonstoichiometry. Energy level spectra of air-exposed CdS exhibit a set of discrete states due to adsorption of C, O, and Cl. Ion bombardment generates a pair of states 2.35 eV and ~0.8 eV above the valence band edge due to S interstitials and vacancies, respectively. Oxygen adsorption produces a broad continuum of states. Changes in surface atomic order show no direct effect on these electronic features. No intrinsic surface states, filled or empty, are observed by surface photovoltage spectroscopy on clean, stoichiometric (112̄0) faces of CdS.     

Determination of surface state parameters from surface photovoltage transients

Photo-stimulated electron transitions from the surface states into the conduction band which are involved in surface photovoltage spectroscopy are analyzed in terms of phenomenological surface state parameters. The surface state parameters are determined frolm photovoltage transients on the basis of relationships derived for a general case where the mechanism of electron transients is not specified, for the case where the surface states are in equilibrium with the bulk and the case where the surface states are not in equilibrium with tpe bulk. The procedure is illustrated utilizing experimental data obtained on CdS surfaces.     

Determination of surface properties by means of large signal photovoltage pulses and the influence of trapping

Large signal photovoltage pulses, measured on real surfaces of high-resistivity p-type silicon as a function of excitation intensity and induced charge, show characteristic features, especially smooth minima in the negative pulses. It is shown how the results can be used for a determination of surface potentials (band bendings) and surface- or interface-state distributions. Comparisons between theoretically and experimentally determined photovoltages show that the exchange of charge carriers between surface states and space charge layer during electron-hole nonequilibrium was not negligible and has to be taken into account for an accurate determination of surface potentials. The influence of such trapping processes is analysed graphically and analytically for continuously distributed surface states and the modifications due to trapping are determined. It is shown that the bands become asymptotically flat for the limit of large excitation even when strong trapping in a system of continuously distributed surface states with arbitrarily large concentrations prevails.     

Surface relaxation influenced by surface states

A detailed theoretical investigation of the relaxation of the simple metal surface Al(100) is presented. We show the influence of electronic surface states in this context. The sign and magnitude of the relaxation of the topmost atomic layers is mainly determined by the rearrangements of the surface state charge. The degree of surface relaxation convergence, with respect to the number of slab layers, is determined by the location of the surface state band relative to bulk bands.     

Increasing the quantum efficiency of nickel phthalocyanine films by oxygen adsorption

An examination of the surface photovoltage indicates that when oxygen adsorbs on a nickel phthalocyanine polycrystalline film, one form adsorbs irreversibly with a sticking probability of 9 × 10^?3 and a second form adsorbs reversibly with a sticking probability >0.1. The reversibly adsorbed oxygen can be removed by evacuating the ambient oxygen, while the irreversible form can only be removed by heating the sample to 433 K. The irreversibly adsorbed oxygen causes an order of magnitude increase in the photovoltage, even though a comparison of the photovoltaic relaxation times indicates that this oxygen has actually slightly lowered the energy band bending at the surface depletion layer. This increase in the photovoltage is therefore attributed to an increased quantum efficiency of minority carrier injection in a process which is analogous to that observed for oxygen in the bulk.     

Studies of photo-induced charge transfer properties of ZnWO4 photocatalyst

Different sizes of ZnWO₄ photocatalysts were synthesized by a hydrothermal method. The as-prepared sample shows highly efficient photocatalytic activity for the degradation of RhB under UV irradiation, which significantly vary with the increase of the hydrothermal temperatures. Surface photovoltage spectrum (SPS), field-induced surface photovoltage spectrum (FI-SPS) and surface photovoltage transient (TPV) techniques are used to investigate the detailed photoinduced charge transfer behavior. Results indicate that the ZnWO₄ synthesized at 413 K possess the largest BET surface area and the abundant donor surface states which are assumed to inhibit the recombination of the photogenerated electron-hole pairs, and thus a significant enhancement in the reaction rate is observed.     

多孔硅的电荷存贮特性与光电压滞后衰减

自从Canham[1]在1990年报道了多孔硅的光致发光以来,人们便开始了多孔硅的发光特性及其发光机理的研究[2,3].我们在获得多孔硅材料的基础上,曾首次报道了多孔硅光电压的滞后衰减现象[4].在本文中主要对如上实验结果进行了分析和探讨.     

Reflectometric study of surface states and oxygen adsorption on clean Si(100) and (110) surfaces

External differential reflection measurements were carried out on clean Si(100) and (110) surfaces in the photon energy range of 1.0 to 3.0 eV at 300 and 80 K. The results for Si(100) at 300 K showed two peaks in the joint density of states curve, which sharpened at 80 K. One peak at 3.0 ± 0.2 eV can be attributed to optical transitions from a filled surface states band near the top of the valence band to empty bulk conduction band levels. The other peak at 1.60 ± 0.05 eV may be attributed to transitions to an empty surface states band in the energy gap. This result favours the asymmetric dimer model for the Si(100) surface. For the (110) surface at 300 K only one peak was found at 3.0 ± 0.2 eV. At 80 K the peak height diminished by a factor of two. Oxygen adsorption in the submonolayer region on the clean Si(100) surface appeared to proceed in a similar way as on the Si(111) 7 × 7 surface. For the Si(110) surface the kinetics of the adsorption process at 80 K deviated clearly. The binding state of oxygen on this surface at 80 K appeared to be different from that on the same surface at 300 K.     

Au钝化Si(100)表面的电子特性

用TB-LMTO方法研究单层的Au原子在理想的Si(100)表面的化学吸附.计算了Au原子在不同位置的吸附能,吸附体系与清洁Si(100)表面的层投影态密度, 以及电子转移情况.结果表明, Au原子在吸附面上方的A位(顶位)吸附最稳定, Au钝化Si(100)表面可以取得明显的钝化效果, 这一结论与实验事实相符合.     

Ab-initio calculation of the electronic structure and energetics of the unreconstructed Au(001) surface

The electronic structure, surface and relaxation energies, and the electric field gradient for the unreconstructed Au(001) surface were calculated by means of the ab-initio all-electron full-potential linearized augmented plane wave slab method. The valence states were calculated within the standard semi-relativistic approach whereas the core states are treated in a fully relativistic way. The Au(001) surface was modelled by free slabs of 5, 7, and 9 layers. From the 9-layer calculation a work function of 5.39 eV was obtained. For the surface energy a value of 1.30 J/m² for the unrelaxed geometry was derived from the total energies of the 7- and the 9-layer slabs. From total energy minimization of the 7-layer slab, a negative, inward relaxation of −2.6% and a relaxation energy of 14.3 × 10⁻³ J/m² were derived. To discuss a mechanism of reconstruction, particular surface states were analyzed in detail in terms of the band structure, layer-dependent density of states and the charge density distribution. Differences of surface and central-layer charge densities show a gain of charge in z-direction localised below and also, to a smaller extent, above the surface atoms. We find a very small gain of delocalised charge in the surface plane between the nearest neighbour positions at the expense of more localised s-d hybridised states. The electric field gradient component Φ_zz was obtained in a two energy window calculation for which the Au5p states were also treated as band states. The resulting Φ_zz values are −16.50 × 10¹⁷ V/cm² surface layer, and −3.3 × 10¹⁷ V/cm² for the subsurface layer.     

不同密度氢吸附金刚石(100)表面的微观结构

利用基于广义梯度近似的密度泛函理论,计算了金刚石(100)表面不同氢吸附密度的平衡态几何结构和态密度.结果表明对于2×1构型,在平行和垂直表面两个方向上发生弛豫,而1×1构型仅在垂直表面方向上发生弛豫.另外,清洁2×1,2×1 ∶0.5H和1×1 ∶1.5H表面,带隙中存在空表面态;而对于1×1 ∶2H和2×1 ∶H两种表面结构,空表面态上移进入导带,带隙中不存在表面态.结合电荷密度分布,探讨了金刚石(100)不同构型和氢吸附密度表面的表面态诱发机理. 关键词： 氢吸附 金刚石 弛豫 表面态     

Determination of surface state parameters from surface photovoltage transients: CdS

A method was developed for determining surface state parameters such as density, fractional occupancy and capture cross section for electrons and for photons from surface photovoltage transients. These transients were found to be associated with the photostimulated transition of electrons from surface states to the conduction band. Suitable analytical expressions were derived from basic semiconductor surface equations. The application of the method to CdS surfaces is presented.     

Hydrogen action in the surface space charge region of highly doped silicon

The action of atomic hydrogen on clean cleaved (1 1 1) surfaces of highly doped silicon samples, both phosphorus ([n] = 2 × 10¹⁹ cm^-3) and boron ([p] = 4 × 10¹⁹ cm^-3) doped has been compared to the case of lightly doped samples ([n] = 1 × 10¹⁴ cm^-3). Once cleaved under ultra high vacuum, the samples were exposed to increasing doses of atomic hydrogen up to saturation. Before and after each hydrogen exposure, the Si(1 1 1) 2 × 1 surface was studied by low energy electron diffraction (LEED) and photoemission yield spectroscopy (PYS). The compared PYS measurements show that H atoms adsorbed on the Si(1 1 1) surface at room temperature do totally compensate the shallow-acceptor impurities (boron) and only partially the shallow-donor impurities (phosphorus) in the space charge region. They also remove the surface dangling bond states. These effects are reversible upon heating under vacuum. Both surface stresses and space charge electric field play a role in this compensation effect.     

Surface photovoltage studies of n-type and p-type InP

Surface photovoltage spectroscopy (SPV) was used to study the initial stages of oxidation of single crystal InP(110) in an attempt to understand the nature and origin of the surface states that develop. Distinct surface states were seen to develop on n-type as the surface was exposed to oxygen. A surface state, associated with cleavage damage, was also observed on p-type. A detailed fit to the experimental data was made by using a model of the dependence of surface charge on photon energy. This was used to unfold the position and intensity of the states. States trailing into the band gap from the bulk bands were seen on both n- and p-types. The analysis also indicated that pairs of isolated states, a donor and an acceptor state, were produced. On p-type, these were present on the clean, cleaved surface while they developed with oxygen exposure on n-type. These states are consistent with the point defect states proposed by the unified defect model. The time response of the SPV signal was also recorded for these surfaces. They were analysed by careful fitting to a model describing the charging and discharging characteristics. This revealed that the midgap state on n-type had a fast and a slow component.     

Surface photovoltage spectroscopy of electronic surface states on cleaved germanium(111) surfaces

Surface photovoltage (SPV) measurements on UHV cleaved Ge(111) surfaces at 100 K are reported for photon energies 0.4 < ?ω < 1 eV. The SPV spectra are sensitive to surface treatment. Upon annealing to temperatures above 200°C, which is accompanied by a reconstruction change from the (2 × 1) to an (8) superstructure, the SPV spectrum shows 2 shoulders below band gap energy with threshold energies near 0.4 and 0.45 eV. These structures are interpreted in terms of electronic transitions from the valence band into empty surface state levels which are related to the (8) superstructure. Adsorbed oxygen and water vapor both cause new similar transitions from the valence band into empty surface states at 0.08 eV below the bottom of the conduction band.