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.     

Photoluminescence analyses of hydrogenated amorphous silicon nitride thin films

Silicon-rich hydrogenated amorphous silicon nitride (a-SiN_x:H) films were grown by plasma enhanced chemical vapor deposition (PECVD) with different r=NH₃/SiH₄ gas flow ratios. The optical absorption characteristics were analyzed by Fourier transform infrared (FTIR) and UV-visible transmittance spectroscopies. The recombination properties were investigated via photoluminescence (PL) measurements. As r was increased from 2 to 9, the PL emission color could be adjusted from red to blue with the emission intensity high enough to be perceived by naked eye at room temperature. The behaviors of the PL peak energy and the PL band broadness with respect to the optical constants were discussed in the frame of electron-phonon coupling and band tail recombination models. A semiquantitative analysis supported the band tail recombination model, where the recombination was found to be favored when the carriers thermalize to an energy level at which the band tail density of states (DOS) reduces to some fraction of the relevant band edge DOS. For the PL efficiency comparison of the samples with different nitrogen contents, the PL intensity was corrected for the absorbed intensity fraction of the incident PL excitation source. The resulted correlation between the PL efficiency and the subgap absorption tail width further supported the band tail recombination model.     

Wet-chemical passivation of atomically flat and structured silicon substrates for solar cell application

Special sequences of wet-chemical oxidation and etching steps were optimised with respect to the etching behaviour of differently oriented silicon to prepare very smooth silicon interfaces with excellent electronic properties on mono- and poly-crystalline substrates. Surface photovoltage (SPV) and photoluminescence (PL) measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were utilised to develop wet-chemical smoothing procedures for atomically flat and structured surfaces, respectively. Hydrogen-termination as well as passivation by wet-chemical oxides were used to inhibit surface contamination and native oxidation during the technological processing. Compared to conventional pre-treatments, significantly lower micro-roughness and densities of surface states were achieved on mono-crystalline Si(100), on evenly distributed atomic steps, such as on vicinal Si(111), on silicon wafers with randomly distributed upside pyramids, and on poly-crystalline EFG (Edge-defined Film-fed-Growth) silicon substrates.The recombination loss at a-Si:H/c-Si interfaces prepared on c-Si substrates with randomly distributed upside pyramids was markedly reduced by an optimised wet-chemical smoothing procedure, as determined by PL measurements. For amorphous-crystalline hetero-junction solar cells (ZnO/a-Si:H(n)/c-Si(p)/Al) with textured c-Si substrates the smoothening procedure results in a significant increase of short circuit current I_sc, fill factor and efficiency η. The scatter in the cell parameters for measurements on different cells is much narrower, as compared to conventional pre-treatments, indicating more well-defined and reproducible surface conditions prior to a-Si:H emitter deposition and/or a higher stability of the c-Si surface against variations in the a-Si:H deposition conditions.     

Surface photovoltage and internal photoemission at the anodized InSb surface

Surface photovoltage measurements have been made as a function of wavelength and temperature on a number of variously doped samples of n- and p-type InSb in the carrier concentration range of 8.9 × 10¹³ to 1.0 × 10¹⁸ cm^?3. The measurements were made using an MIS sandwich employing for the dielectric an anodically formed layer of In₂ O₃. Differential capacitance measurements have shown that, when cooled in the dark, the surface of the n-type material is near flat band whereas that of the p-type is depleted. Illumination with photons of energy in excess of ～ 1.5 eV leads to a shift of the surface potential to larger negative values presumably as a result of optical activation of electrons from fast interfacial surface states to slow states near the InSb surface. Internal photoemission measurements lend support to this model and suggest that, in the absence of any applied bias an internal field within the oxide causes the electrons excited from the semiconductor to move towards the metal. A theory for the surface photovoltage in the presence of a continuum of surface states is developed. It is concluded from theory, and supported by experiment, that surface trapping as well as recombination can exert a considerable influence on the photovoltaic response.     

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.     

Electrical properties of surface barrier structures with a dielectric having a formed channel

We have studied the effect of channel formation in an Ni/n-Si/V₂O₅−B₂O₃−CaO surface barrier structure on the capacitance-voltage characteristics, the dependence of the small-signal photovoltage on the state of the surface and the modulation frequency of the test signal, and the planar distribution of the photovoltage and the noise properties for the high-resistance and low-resistance states of the formed channel. It has been established that forming of the dielectric causes the appearance of regions of thermal-field recombination of the charge carriers, burst noise, strengthened heterogeneity of the photoelectric properties, and the appearance of two peaks in the density of surface states from silicon divacancies at the interphase boundary. It is proposed that an important role in the creation of the properties of the electrically formed structures is played by shock waves which propagate from the forming channel, the additional lattice defects formed during dissipation, and the phase state of the crystallites in the channel. Tomsk State University. Systems for Control and Radio Electronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 85–91, May, 1997.     

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.     

The spectrum and decay of the recombination radiation from strongly excited silicon

The spectral distribution of the recombination radiation from silicon during and after excitation by a Q-switched ruby laser has been measured and analyzed. The interpretation assumes a third-order (Auger) recombination process and a simple parabolic band structure. It takes into account the heating of the sample at the surface and the reduction of the band gap due to the high carrier density. Measurements of the spectral distribution as a function of time gives a value of the Auger transition rate factor γ₃ = 2·10⁻³¹ cm⁶sec⁻¹.     

Wet-chemical treatment and electronic interface properties of silicon solar cell substrates

On textured n-type silicon substrates for solar cell manufacturing, the relation between light trapping behavior, structural imperfections, energetic distribution of interface state densities and interface recombination losses were investigated by applying surface sensitive techniques. The field-modulated surface photovoltage (SPV), in-situ photoluminescence (PL) measurements, total hemispherical UV-NIR-reflectance measurements and electron microscopy (SEM) were employed to yield detailed information on the influence of wet-chemical treatments on preparation induced micro-roughness and electronic properties of polished and textured silicon substrates. It was shown that isotropic as well as anisotropic etching of light trapping structures result in high surface micro-roughness and density of interface states. Removing damaged surface layers in the nm range by wet-chemical treatments, the density of these states and the related interface recombination loss can be reduced. In-situ PL measurements were applied to optimise HF-treatment times aimed at undamaged, oxide-free and hydrogen-terminated substrate surfaces as starting material for subsequent solar cell preparations.      

Excitonic photoluminescence characteristics of amorphous silicon nanoparticles embedded in silicon nitride film

We have investigated the photoluminescence (PL) properties of amorphous silicon nanoparticles (a-Si NPs) embedded in silicon nitride film (Si-in-SiN_x) grown by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique. The PL spectrum of the film exhibits a broad band constituted of two Gaussian components. From photoluminescence excitation (PLE) measurements, it is elucidated that the two PL bands are associated with the a-Si NPs and the silicon nitride matrix surrounding a-Si NPs, respectively. The existence of Stokes shift between PL and absorption edge indicates that radiative recombination of carriers occurs in the states at the surface of the Si NPs, whereas their generation takes place in the a-Si NPs cores and the silicon nitride matrix, respectively. The visible PL of the film originates from the radiative recombination of excitons trapped in the surface states. At decreasing excitation energy (E_ex), the PL peak energy was found to be redshifted, accompanied by a narrowing of the bandwidth. These results are explained by surface exciton recombination model taking into account there existing a size distribution of a-Si NPs in the silicon nitride matrix.     

Non-equilibrium surface state properties at clean cleaved silicon surface as measured by surface photovoltage

Surface photovoltage transients were measured at clean cleaved silicon (111) faces in ultrahigh vacuum. The temperature and doping of the samples, the intensity of the stimulating light pulses (energy less than band gap), and the surface coverage (clean and adsorbed water vapor) were varied systematically. The results yield information on the charge transfer at the surface and on surface recombination. The calculation of the surface photovoltage (using only the generation rates into and out of the surface states and data of thermal equilibrium) shows, that only one bulk band (conduction band for n-doped samples and valence band for p-doped samples) controls completely signal height and its relaxation via charge transfer to the surface states. The determined surface state parameters are: relaxation time constants, capture cross-sections for photons and transition probabilities. On the basis of the model all decay curves can be reproduced quantitatively.     

Progressive changes in atomic structure and gap states on Si(0 0 1) by Bi adsorption

From ab initio studies employing the pseudopotential method and the density functional scheme, we report on progressive changes in geometry, electronic states, and atomic orbitals on Si(0 0 1) by adsorption of different amounts of Bi coverage. For the 1/4 ML coverage, uncovered Si dimers retain the characteristic asymmetric (tilted) geometry of the clean Si(0 0 1) surface and the Si dimers underneath the Bi dimer have become symmetric (untilted) and elongated. For this geometry, occupied as well as unoccupied surface states are found to lie in the silicon band gap, both sets originating mainly from the uncovered and tilted silicon dimers. For the 1/2 ML coverage, there are still both occupied and unoccupied surface states in the band gap. The highest occupied state originates from an elaborate mixture of the p_z orbital at the Si and Bi dimer atoms, and the lowest unoccupied state has a ppσ* antibonding character derived from the Bi dimer atoms. For 1 ML coverage, there are no surface states in the fundamental bulk band gap. The highest occupied and the lowest unoccupied states, lying close to band edges, show a linear combination of the p_z orbitals and ppσ* antibonding orbital characters, respectively, derived from the Bi dimer atoms.     

Relation between interface states and temperature behavior of the barrier height of silver contacts on clean cleaved n-type silicon

The properties of silver-silicon interfaces formed by cleaving n-type silicon in ultra high vacuum (UHV) in a stream of evaporating silver atoms were studied. The barrier heights of these contacts were measured at different temperatures by using C-V techniques. All measurements were performed in UHV. The dependence of the barrier height upon temperature did not follow the temperature dependence of the Si band gap as it is usually found. The measured temperature behavior depended on the roughness of the Si surface. The temperature behavior can be explained by assuming a specific band structure of the interface states. For Ag contacts on atomically smooth n-type Si, the interface states were found to be arranged in two bands, one band 4 × 10^?3 eV wide with acceptor type states 0.18 eV below the intrinsic level E_i and a density of 10¹⁷ states/cm² eV, and the other 1 eV wide with donor type states with its upper edge 0.28 eV below E_i, and a density of 4 × 10¹⁴ states/cm²eV.     

Surface photovoltage spectroscopy and surface piezoelectric effect in GaAs

“Real” (111) surfaces of n-type GaAs were investigated employing surface photovoltage spectroscopy and the surface piezoelectric effect. Surface states at the energy position E_c ? E_t ? 0.72 eV were found on both the Ga and the As surfaces. Both types of surfaces exhibited a barrier of about 0.55 V. No variations in the surface barrier or the energy position of the surface states were observed in various ambients at atmospheric pressure (dry air, wet air, ammonia and ozone). However, the capture cross-section of the surface states for electrons, as determined from the surface piezoelectric effect transients (of the order of 10^?13 cm²), was found to be sensitive to the ambient. It decreased in wet air and increased in ozone. This effect was more pronounced on the As than on the Ga surfaces. Additional surface states were found to be present in the energy region of 0.9 to 1.0 eV, below the bottom of the conduction band. However, their exact energy positions could not be determined due to interference caused by the carrier trapping of the surface states at E_c ? E_t ? 0.72 eV.     

Band engineering of ZnS by codoping for visible-light photocatalysis

Codoping is demonstrated as an efficient approach to narrow the band gap of ZnS and enhance its photocatalytic activity. Herein, we perform the density-function theory calculations of ZnS by codoping of X (N, F) with transition metals (TM = V, Cu). The band gap is reduced in four different types of codoped ZnS. In particular, Cu_ZnF_S codoping, a charge-compensated donor–acceptor pair, leads to an about 32 % reduction of the energy gap, thus extending the absorption edge to visible-light region. The band gap reduction is due to the upshift of the top valence band comprised with the delocalized hybridizing levels of Cu 3d and S 3p states, and the downshift of the bottom conduction band consisting of F 2s states. Moreover, the larger value of m _e*/m _h* in Cu_ZnF_S–ZnS would result in a lower recombination rate of the electron–hole pairs. Both band gap reduction and low recombination rate are critical elements for efficient light-to-current conversion in codoped ZnS. These findings raise the prospect of using codoped ZnS with specifically engineered electronic properties in a variety of photocatalytic applications.     

Stimulated recombination from the defect level in doped lead telluride

The relaxation processes of the photoexcited carriers from the defect level in the band gap to the valence band states were investigated in Na and Tl doped p-type PbTe single crystals at T = 77 K. The observed photosignal oscillations were proved to be induced by stimulated recombination of photoexcited carriers from the defect level E_d ≈ 50 meV above the top of the valence band. Non-equilibrium carrier inversion population was produced by impulses of a TEA CO₂-laser. The observed stimulated recombination may presumably be used for designing IR semiconductor lasers operating in the wavelength range of λ ∼ 25 μm at T = 77 K.     

Photocurrent and photovoltage spectroscopy of amorphous silicon nanoclusters

Photoelectric properties (photocurrent efficiency and photovoltage) of the silicon suboxide films containing amorphous silicon nanoclusters were investigated in the spectral range of 300–1100 nm. A strongly pronounced increase of the photocurrent efficiency of the sandwich-like structures with such films on c-Si substrates was observed in the short-wavelength region. The possible mechanisms of the increase were discussed, and carrier multiplication due to impact ionization of the defect states was considered to be the most probable. Impact ionization of the defect states involves two main steps: (i) trapping of the photogenerated electron in a defect state, and (ii) impact ionization of this state by another photoexcited electron that has got sufficient energy due to absorption of a high-energy photon.     

Spectroscopy of electroreflection, the electron band structure, and the mechanism of visible photoluminescence of anisotropically etched silicon

We present the results of studies of electroreflection in the 1.1–4.4 eV spectral range, of electron Auger spectroscopy, and of electron diffraction involving the photoluminescent Si-SiO₂ system prepared via anisotropic chemical etching of the Si(100) surface. These results are explained on the basis of a four-layer model of the band structure and energy transition diagram for a system with a quantum well at the silicon surface, surface electron states at the boundary, and a gradient of the band potential in the transition layer. We find that light-emitting silicon remains an indirect-gap semiconductor and that the visible photoluminescence is due to direct recombinations of the light-excited electrons and holes in the quantum well at the center of the Brillouin zone with the participation of the band of deep localized states, which is due to the presence of oxygen at the silicon surface. Zh. éksp. Teor. Fiz. 116, 1750–1761 (November 1999)     

Contact potential barriers and characterization of Ag-doped composite TiO2 nanotubes

Ag-doping TiO₂ composite nanotubes (Ag-TNTs) were synthesized by alkaline fusion followed by hydrothermal treatment. The microstructure and morphology of the materials were characterized by XRD, TEM, XPS, SPS (surface photovoltage spectroscopy), FISPS (electric field-induced surface photovoltage spectroscopy) and Raman spectroscopy. First-principles calculations based on density-functional theory (DFT) showed the formation of several impurity levels near the top of the valence band in the band gap (E_g) of rutile TiO₂ due to Ag doping. A “double junction” is proposed, involving a Schottky junction and p–n junction (denoted as “Ag-p–n junction”) occurring between the Ag particles and the nanotube surface, as well as forming inside TiO₂ nanotubes, respectively. The strongly built-in electric field of the junctions promotes the separation of photo-holes and photoelectrons, enhancing the photocatalytic efficiency. XRD results indicated that the composite Ag-TNTs exist as a mixture of anatase and rutile phases. XPS results showed that Ti⁴⁺ is the primary state of Ti. Raman spectral analysis of Ag-TNTs revealed the presence of a new peak at 271 cm⁻¹. The red-shift of the absorption light wavelength of Ag-TNTs was 0.16 eV (20 nm) due to a considerable narrowing of E_g by the existing impurity levels.     

Influence of surface recombination and interface states on the performance of β-FeSi2/c-Si heterojunction solar cells

To explore the origin of low conversion efficiency for novel β-FeSi₂/c-Si heterojunction solar cells, the effect of surface recombination and interface states on the cell performance has been investigated by numerical simulation. The present results show that surface recombination of β-FeSi₂ film plays an important role in limiting the cell property since the photovoltaic behavior of β-FeSi₂ is quite sensitive to surface recombination due to its especial characteristic of very high optical absorption coefficient. Surface quality of β-FeSi₂ film should be much improved for better cell performance. In addition, it is shown that interface states between β-FeSi₂ film and crystalline silicon are critical to device characterization. Interface states should be minimized to obtain higher conversion efficiency. If surface recombination and interface states can be best suppressed, potential conversion efficiency for the cell may be up to 28.12% at 300 K under illumination of AM 1.5, 100 mW/cm².