Surface Photovoltaic Effect Research Articles

Roles of excess minority carrier recombination and chemisorbed O2 species at SiO2/Si interfaces in Si dry oxidation: Comparison between p-Si(001) and n-Si(001) surfaces.

This study provides experimental evidence for the following: (1) Excess minority carrier recombination at SiO2/Si interfaces is associated with O2 dissociative adsorption; (2) the x-ray induced enhancement of SiO2 growth is not caused by the band flattening resulting from the surface photovoltaic effect but by the electron-hole pair creation resulting from core level photoexcitation for the spillover of bulk Si electronic states toward the SiO2 layer; and (3) a metastable chemisorbed O2 species plays a decisive role in combining two types of the single- and double-step oxidation reaction loops. Based on experimental results, the unified Si oxidation reaction model mediated by point defect generation [S. Ogawa et al., Jpn. J. Appl. Phys., Part 1 59, SM0801 (2020)] is extended from the viewpoints of (a) the excess minority carrier recombination at the oxidation-induced vacancy site and (b) the trapping-mediated adsorption through the chemisorbed O2 species at the SiO2/Si interface.

X-ray photoelectron spectroscopy study on effects of ultra-high-pressure annealing on surface of Mg-ion-implanted GaN

The effects of ultra-high-pressure annealing (UHPA) on the surface of Mg-ion-implanted GaN were investigated by X-ray photoelectron spectroscopy (XPS). After Mg ion implantation or Mg–N co-implantation, GaN was annealed at 1400 °C for 5 min under a nitrogen pressure of 1 GPa. No deterioration of the surface stoichiometry occurred after UHPA despite the extremely high annealing temperature. The angle-resolved XPS with calibration showed that the surface Fermi level was pinned at 0.5 eV from the conduction band edge after dehydrogenation subsequent to UHPA. However, the absence of pinning at the charge neutrality level showed that surface disorder was absent after UHPA. The surface photovoltaic effect as evidence of the achievement of p-type conduction even in the near-surface region was more remarkable for Mg–N-ion-implanted samples after dehydrogenation subsequent to UHPA. There is the possibility that the density of N-vacancy-related defects was reduced more by Mg–N co-implantation.

Surface photovoltaic effect and electronic structure of β -InSe

Using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structure of \ensuremath{\beta}-InSe, a van der Waals semiconductor with a direct band gap. Our measurements show a good agreement with ab initio calculations, which helps reveal the important impact of spin-orbit coupling on the electronic structure of \ensuremath{\beta}-InSe. Using surface potassium doping, we tune the chemical potential of the system and observe the unoccupied conduction band. The direct band gap is determined to be about 1.3 eV. Interestingly, we observe a global band shift when the sample is illuminated by a continuous-wave laser at 632.8 nm, which can be understood by an efficient surface photovoltaic effect. The surface photovoltaic can be tuned by in situ surface potassium doping. Our results not only provide important insights into the semiconducting properties of InSe, but also suggest a feasible method to study and engineer the surface photovoltaic effect in InSe-based devices.

Non-equipotential effect observed on the surface of Ti/Si under uniform illumination

In this letter, a non-equipotential surface photovoltaic effect is reported in nano metal–semiconductor structures. When the surface of the Ti/Si is uniformly illuminated by a beam of light, a controllable surface photovoltaic effect is observed on the metal side. The center of the surface presents a remarkably higher metallic potential than the surrounding region. The surface photovoltage is detected to be as high as 53 mV. Besides, it depends sensitively on the thickness and size of the metal films, demonstrating it is a unique feature of nano metal films. We ascribe this phenomenon to the boundary effect of photon-generated carriers in the ultrathin metal thickness. The theoretical calculations based on equivalent electron diffusion model are in great agreement with the experimental results. The results may promise some novel applications based on the nanoscale metal–semiconductor systems.

Spin-ARPES EUV Beamline for Ultrafast Materials Research and Development

A new femtosecond, Extreme Ultraviolet (EUV), Time Resolved Spin-Angle Resolved Photo-Emission Spectroscopy (TR-Spin-ARPES) beamline was developed for ultrafast materials research and development. This 50-fs laser-driven, table-top beamline is an integral part of the “Ultrafast Spintronic Materials Facility”, dedicated to engineering ultrafast materials. This facility provides a fast and in-situ analysis and development of new materials. The EUV source based on high harmonic generation process emits 2.3 × 1011 photons/second (2.3 × 108 photons/pulse) at H23 (35.7 eV) and its photon energy ranges from 10 eV to 75 eV, which enables surface sensitive studies of the electronic structure dynamics. The EUV monochromator provides the narrow bandwidth of the EUV beamline while preserving its pulse duration in an energy range of 10–100 eV. Ultrafast surface photovoltaic effect with ~650 fs rise-time was observed in p-GaAs (100) from time-resolved ARPES spectra. The data acquisition time could be reduced by over two orders of magnitude by scaling the laser driver from 1 KHz, 4W to MHz, KW average power.

Spin-ARPES EUV beamline for ultrafast materials research and development

A new femtosecond, Extreme Ultraviolet (EUV), Time Resolved Spin-Angle Resolved Photo-Emission Spectroscopy (TR-Spin-ARPES) beamline was developed for ultrafast materials research and development. This 50-fs laser-driven, table-top beamline is an integral part of the "Ultrafast Spintronic Materials Facility", dedicated to engineering ultrafast materials. This facility provides a fast and in-situ analysis and development of new materials. The EUV source based on high harmonic generation process emits 2.3 × 1011 photons/second (2.3 × 108 photons/pulse) at H23 (35.7 eV) and its photon energy ranges from 10 eV to 75 eV, which enables surface sensitive studies of the electronic structure dynamics. The EUV monochromator provides the narrow bandwidth of the EUV beamline while preserving its pulse duration in an energy range of 10-100 eV. Ultrafast surface photovoltaic effect with ~650 fs rise-time was observed in p-GaAs (100) from time-resolved ARPES spectra. The data acquisition time could be reduced by over two orders of magnitude by scaling the laser driver from 1 KHz, 4W to MHz, KW average power.

Enhanced photovoltage on the surface of topological insulator via optical aging

The efficient generation of spin-polarized current is one of the keys to realizing spintronic devices with a low power consumption. Topological insulators are strong candidates for this purpose. A surface photovoltaic effect can be utilized on the surface of a topological insulator, where a surface spin-polarized current can flow upon illumination. Here, we used time- and angle-resolved photoelectron spectroscopy on the surface of Bi2Te3 to demonstrate that the magnitude of the surface photovoltage is almost doubled in optically aged samples, i.e., samples whose surface has been exposed to intense infrared light illumination. Our findings pave the way for optical control of the spin-polarized current by utilizing topological insulators.

Polarization Photoelectric Conversion in Layered GeS

AbstractPolarization photovoltaic effect is a unique character for an energy material with specific in‐plane anisotropy. Especially, if the energy material has a direct bandgap close to 1.6 eV, it will efficiently absorb full sunlight spectrum with specific axial polarization. In this study, polarized microtransmittance measurements of GeS multilayer with polarization angles ranging from θ = 0° (E || a) [through 90° (E || b)] to θ = 180° (E || a) have been studied near band edge. The polarized absorption edge follows a sinusoidal variation of E(θ) = 1.6 + 0.05⋅|sin(θ)| eV with respect to the angle change of the polarized absorption spectra. This anisotropic optical response is well reproduced by first‐principles calculations based on a combined Green's function technique, the GW–Bethe–Salpeter equation (BSE) approach. To characterize highly anisotropic band structure of layered GeS, polarized thermoreflectance measurement and first‐principles quasiparticle band‐structure calculations are also carried out. The interband transitions belonging to E ||a and E ||b polarizations are respectively identified. The polarized surface photovoltaic effects of a GeS Schottky solar cell are also tested. The special in‐plane optical anisotropy (along a and perpendicular to the a axis) renders GeS owning highly bi‐axial responsivity with respect to the c‐plane photoelectric conversion.

Enhanced inhibition of steel corrosion by L‐cysteine under visible‐light illumination

The heteroatoms such as nitrogen, oxygen, and sulfur of organic inhibitor play a crucial role on its corrosion inhibition behavior. In order to reveal their adsorption behavior and inhibition performance under illumination condition on steel surface, this study selects L‐cysteine as study object. On the basis of electrochemical measurement, surface analysis, and theoretical calculation, a promising increase in corrosion inhibition efficiency is observed when visible‐light is impinged on steel surface in sodium chloride solution with L‐cysteine. The enhanced performance is attributed to the sample surface photovoltaic effect, thereby facilitating both the thiol and the amino groups to tilt toward the surface.

Direct measurement of exciton dissociation energy in polymers

Exciton dissociation energy was obtained based on the comparison of thickness of the space charge region estimated from the measurement of capacitance of prepared Schottky diode and from the measurement of photovoltage spectra. While the capacitance measurements provide information about the total width of the space charge region (SCR) the surface photovoltaic effect brings information only about the part of the SCR where electric field is sufficiently high to cause dissociation. For determination of the dissociation energy it is sufficient to find the electric potential in the SCR where the process starts.

On the surface photovoltaic effect in a multivalley semiconductor in an external magnetic field

A theory of the photovoltaic effect in a semi-infinite multvalley semiconductor upon the absorption of polarized light at free carriers caused by the specular reflection and diffuse scattering of electrons at a film surface is developed by us. The kinetic Boltzmann equation in the approximation of time relaxation and boundary conditions, which determine the correlation between the distribution function of electrons reflected from a semi-infinite crystal surface and the distribution function of electrons incident onto a surface for the case of both specular reflection and diffuse scattering is used. The aforementioned takes into account the fact that the distribution function of electrons diffusely scattered from a surface depends only on their energy and is determined from the condition of total electron flux vanishing at the surface. Expressions for analyzing the spectral dependence of the current which is linearly dependent on the magnetic-field strength are obtained.

Bending Photoluminescence and Surface Photovoltaic Effect on Multilayer InSe 2D Microplate Crystals

Recently bendable and flexible optoelectronics have attracted more attentions on display technology and concaved or curvy photovoltaic devices owing to the flexibility and softy in layered materials. The bending photoluminescence (BPL) and surface photovoltaic effect of a thin multilayer InSe 2D crystal are demonstrated here by using PL and surface photovoltage (SPV) experiments. The BPL result of InSe (t ≈ 30 nm) shows an enhancement in light intensity with respect to that of the other flat PL measurement owing to the spreading of emission solid angle for the irradiant Se‐In‐In‐Se basic units under concaved upward condition. The band edge structure of multilayer InSe is analyzed. The SPV measurement of an In mesh‐coated InSe microplate demonstrates that the Schottky‐type solar cell converts sunlight starting from ≈1 eV with the auxiliary of surface states on InSe. An initially formed In‐InSe Schottky solar cell (surface area of ≈2 mm2 and thickness of ≈85 μm) is also tested. The testing result shows a photovoltage of V ≈ 24 mV can be generated from the Schottky solar cell under the illumination of Hg lamp with power density of P ≈ 0.83 mW cm−2. All the experimental results demonstrate flexible light‐emission capability and “1 eV” photoelectric conversion capacity existed in the 2D multilayer ε‐InSe.

The role of the space charge region in surface photovoltaic effect

The surface photovoltage method is an important tool for semiconductor characterization. Evaluation of photocarrier diffusion length represents one of the most essential applications of this method. The technique is usually limited to samples with thick neutral bulk and thin space charge region (SCR) at the illuminated surface. The purpose of this paper is to remove these restrictions by introducing a more general model. Two different transport processes for the photogenerated carriers are assumed: drift in the SCR and diffusion in the neutral bulk. It was found that the SCR and the bulk contribute to the overall signal independently even in the case of recombination in the depletion region. The presented surface photovoltage technique was successfully applied to samples of different thickness and with differently thick space charge region at the surface. The diffusion length of the minority carrier in inorganic semiconductors as well as the diffusion length of excitons in organic materials were extracted. The thickness of the space charge region is another important parameter obtained by using the model.

Influence of pH Value in Electrolyte on Photovoltaic Performance of CIGS Thin-films Prepared by Electrochemical Method

Cu(In1-x,Ga-x)Se-2 (CIGS) films were prepared by one-step electrodeposition method on FTO glass substrates, and the influence of electrolytic pH on the chemical composition, structure and photovoltaic performance of CIGS thin-films were studied in detail. The results showed that the stoichiometry of In and Ga in the film could be effectively regulated by changing the pH value of the electrolyte. The X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) results showed that the CIGS thin-films had good crystallinity and uniform particle size distribution when the pH value was 2.0. The effect of different stoichiometries of the CIGS thin-films on the kinetics of photo-charges was also studied. The CIGS thin film with a stoichiometric ratio of 0.3 for Ga/(In + Ga) presented the strongest surface photovoltaic effect.

High open‐circuit voltage of graphene‐based photovoltaic cells modulated by layer‐by‐layer transfer

Graphene has shown great application opportunities in future nanoelectronic devices because of its outstanding electronic properties. Moreover, its impressive optical properties have been attracting the interest of researchers, and, recently, the photovoltaic effects of a heterojunction structure embedded with few layer graphene (FLG) have been demonstrated. Here, we report the photovoltaic response of graphene–semiconductor junctions and the controlled open‐circuit voltage (Voc) with varying numbers of graphene layers. After unavoidably adsorbed contaminants were removed from the FLGs by means of in situ annealing, prepared by layer‐by‐layer transfer of the chemically grown graphene layer, the work functions of FLGs showed a sequential increase as the graphene layers increase, despite random interlayer‐stacking, resulting in the modulation of photovoltaic behaviors of FLGs/Si interfaces. The surface photovoltaic effects observed here show an electronic realignment in the depth direction in the FLG heterojunction systems, indicating future potential toward solar devices utilizing the excellent transparency and flexibility of FLG. Copyright © 2011 John Wiley & Sons, Ltd.

A novel approach to the surface photovoltage method

The surface photovoltage technique allows contactless measurement of some electrical parameters of semiconductors. Unfortunately, a contemporaneous approach to steady-state surface photovoltaic (SPV) effect cannot explain the photovoltage spectra, and its application to the determination of the diffusion length is limited to thick samples with thin space charge region (SCR). In this paper a complete theory of steady-state SPV effect is presented that agrees well with the experiment. Consequently, important parameters can be evaluated from the measurements independently of thickness and resistivity of samples. The use of the theory for determining the diffusion length and thickness of the SCR is shown.

Synthesis, Photophysical Characterization, and Surface Photovoltage Spectra of Windmill‐Shaped Phthalocyanine–Porphyrin Heterodimers and Heteropentamers

AbstractThe synthesis and characterization of three nonperipherally substituted, covalently‐linked phthalocyanine–porphyrin hybrid molecules is described. The optical and photophysical properties of these compounds are studied in detail. The intramolecular energy transfer process is investigated and quantified in terms of the quantum yield of the energy transfer and is found to be highly effective in these molecules. The photophysical properties of the phthalocyanine core of these three compounds are significantly modified by the presence of the porphyrin moieties. These molecules display interesting optical‐power‐limiting behavior and a surface photovoltaic effect. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Surface photovoltaic effect in CdTe as influenced by recombination in the space‐charge region

AbstractDiffusion length of minority carriers in CdTe wafers was measured by the surface photovoltage (SPV) method. A model including recombination in the space‐charge region (SCR) is presented to explain the SPV spectra. Assuming a thick neutral bulk and an electric field only in the SCR, five parameters are needed for characterization of the system. On analyzing the problem we have found that the most important of them – the diffusion length of minority carriers L – is not influenced by recombination in the SCR. The results are verified on samples with different resistivities, and extracted diffusion length is compared with literature data. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoacoustic and Surface Photovoltaic Characters of Nanocrystalline TiO<SUB>2</SUB>

Nanocrystalline TiO_2 was synthesized by sol-gel method. The surface state properties and transport characters of photogenic free charge carriers were probed by the combination of photoacoustic and surface photovoltaic techniques. The results showed that the surface photovoltage (SPV) response appearing at 380 nm of the samples was closely related to the surface electron structure of anatase-type TiO_2 nanoparticles, which belonged to a band-band transition; and that the shoulder peak appearing at 470 nm was related to the surface states of TiO_2 nanoparticles, which was a sub-band-gap charge transfer (CT) transition, and had an obvious donor character. By the energy complementarity between photoacoustic and surface photovoltaic effects of the samples, it was experimentally proved that the nonradiative transitions were responsible for the increasing intensity of photoacoustic signals with decreasing average particle size of the sample; but the number of the nonradiative transitions could be dramatically decreased by a little increase of anatase content in the sample, resulting in increasing quantum-optical efficiency.

Probing Surface Electronic Potentials and Photovoltaic Effects of Self-Assembled Multilayers of Metal Phthalocyanine and Oligomeric Viologen on Conductive Substrates

To probe the effects that self-assembled multilayers may have on electronic properties of a conductive surface, we carried out layer-by-layer depositions of organic/polymeric thin films on indium tin oxide (ITO) substrates. The organic/polymeric thin films were prepared by alternatively dipping ITO substrates in an oligomeric viologen or oligo(hexylene 4,4‘-bipyridinum dibromide) solution and a macrocyle NiPc or nickel phthalocyanine solution. A linear relationship, between the optical absorption at 634 nm from NiPc and 274 nm from oligomeric viologen and the number of depositions, was observed in the UV−vis spectra. The multilayer structures were characterized with X-ray reflectometry, which yielded an average mean thickness of 1.5−2.0 nm per layer-pair. Taking advantage of the conductive surfaces, we performed surface infrared (IR) spectroscopy on oligomeric viologen and NiPc multilayers at an external grazing-angle reflection configuration. IR spectra with polarization perpendicular to the conductive I...