Surface Photovoltage Technique Research Articles

Behaviour of total surface charge in SiO 2–Si system under short-pulsed ultraviolet irradiation cycles characterised by surface photo voltage technique

Effects of time-accumulated ultraviolet (UV) irradiation and surface treatment on thermally oxidized p-type silicon wafers were investigated by using the surface photo voltage (SPV) technique via the direct measurement of the total surface charge, Q SC. The rise and fall times of Q sc curves, as a function of accumulated UV irradiation, depended on the thermal oxide thickness. A simple model was proposed to explain the time-varying characteristics of Q sc based on the UV-induced bond breaking of SiOH and SiH, and photoemission of bulk electrons to wafer surface where O 2 − charges were formed. While these mechanisms resulted in charge variations and hence in Q sc, these could be removed by rinsing the silicon wafers in de-ionized water followed by spin-dry or blow-dry by an ionizer fan. Empirical parameters were used in the model simulations and curve-fitting of Q SC. The simulated results suggested that initial changes in the characteristic behaviour of Q sc were mainly due to the net changes in the positive and negative charges, but subsequently were dominated by the accumulation of O 2 − during the UV irradiation.

Mesoporous SiO2-Modified Nanocrystalline TiO2 with High Anatase Thermal Stability and Large Surface Area as Efficient Photocatalyst

Mesoporous SiO2-modified nanocrystalline TiO2 photocatalysts have been prepared by sol-hydrothermal processes, followed by post-treatment with appropriate amount of surfactant F127-modified silica sol. The resulting photocatalysts were also characterized by X-ray diffraction, Raman spectroscopy, Brunauer−Emmett−Teller spectroscopy, N2 adsorption−desorption, transmission electron microscopy, FT-IR, X-ray photoelectron spectroscopy, UV−vis diffuse reflectance spectroscopy, steady state surface photovoltage (SS-SPV), and transient state surface photovoltage (TS-SPV) techniques. The photocatalytic activities of the samples were evaluated by degrading rhodamine B solution under simulated solar illumination. The results show that the surface modification with mesoporous SiO2 greatly enhances the thermal stability of the nanocrystalline anatase TiO2, even still being with a main anatase phase after calcination at 900 °C, and the more is the amount of SiO2 used, the more obvious is the ehancement in the thermal s...

Characterization of Residual Implant Damage by Generation time and Surface Photovoltage Techniques

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TiO 2–carbon nanotube heterojunction arrays with a controllable thickness of TiO 2 layer and their first application in photocatalysis

TiO 2–carbon nanotube (CNT) heterojunction arrays on Ti substrate were fabricated by a two-step thermal chemical vapor deposition (CVD) method. CNT arrays were first grown on Ti substrate vertically, and then a TiO 2 layer, whose thickness could be controlled by varying the deposition time, was deposited on CNTs. Measured by electrochemical impedance spectroscopy (EIS), the thickness of the TiO 2 layer could affect the photoresponse ability significantly. About 100 nm thickness of the TiO 2 layer proved to be best for efficient charge separation among the tested samples. The optimized TiO 2–CNT heterojunction arrays displayed apparently higher photoresponse capability than that of TiO 2 nanotube arrays which was confirmed by surface photovoltage (SPV) technique based on Kelvin probe and EIS. In the photocatalytic experiments, the kinetic constants of phenol degradation with TiO 2–CNT heterojunctions and TiO 2 nanotubes were 0.75 h −1 ( R 2 = 0.983) and 0.39 h −1 ( R 2 = 0.995), respectively. At the same time, 53.7% of total organic carbon (TOC) was removed with TiO 2–CNT heterojunctions, while the removal of TOC was only 16.7% with TiO 2 nanotubes. These results demonstrate the super capability of the TiO 2–CNT heterojunction arrays in photocatalysis with comparison to TiO 2-only nanomaterial.

Photoelectric method for non-contact characterization of SiGe

A new non-destructive measurement method for characterization of Si 1 − x Ge x based multi-layer structures, including SiGe epitaxial layer and the adjacent strained Si epitaxial layer, has been developed. The method is based on a small signal surface photo-voltage technique and allows measurement of germanium concentration and thickness of top epitaxial layer. A new physical model and a measurement algorithm were developed. The results of the measurement of a set of SiGe samples with various germanium concentration and silicon layer thickness values are presented. The characteristic mixed crystal parameter—the coefficient representing the energy gap dependence on Ge content was calculated.

Surface photovoltage characterization of an oriented α-Fe 2O 3 nanorod array

A large aligned nanorod array of α-Fe 2O 3 grown on FTO substrate was prepared via a simple hydrothermal synthesis method. The photoelectric property of the array was studied by surface photovoltage technique. Based on the photovoltage responses on spectrum, we discussed the transfer characteristics of the photogenerated charge at the surface of the nanorods and the interface between the nanorods and the FTO substrate. The contributions of surface states and built-in electric field were discriminated. Moreover, the quantum size effect on the photovoltage spectra could be observed only when the array was illuminated from the free surface of the nanorods.

Investigation on the electron transfer between anatase and rutile in nano-sized TiO 2 by means of surface photovoltage technique and its effects on the photocatalytic activity

Photoinduced electron transfer between anatase and rutile in nanosized TiO 2, prepared by a sol–gel method, was revealed by means of the surface photovoltage technique, and its effects on the photocatalytic performance in the degradation of a phenol solution were investigated. Also, the role of the surface states during the processes of photo-physics and photochemical reactions was discussed. In the as-prepared TiO 2 sample consisting of anatase and rutile, the photoinduced electrons can easily transfer from anatase surface states to rutile, as well as from anatase conduction band to rutile. These factors are responsible for the strong surface photovoltage response and high photocatalytic activity. Moreover, the surface states related to oxygen vacancies can induce photocatalytic reactions under visible irradiation, especially in the resulting biphase TiO 2, due to the electron transfer from anatase surface states to rutile.

Photovoltaic properties of a ZnO nanorod array affected by ethanol and liquid-crystallineporphyrin

A vertically aligned array of ZnO nanorods, fabricated on conductive ITO substrate inaqueous solution, was characterized by scanning electron microscopy (SEM), x-raydiffraction (XRD), and UV–visible transmission spectroscopy. Surface photovoltage (SPV)techniques based on a lock-in amplifier and a Kelvin probe were both employed to studythe photogenerated charges in the system. The effects of ethanol solvent anda liquid-crystalline porphyrin, [5-(para-dodecyloxy)phenyl-10,15,20-tri-phenyl]porphyrin (DPTPP), on the photovoltage enhancement in the ZnO nanorod arraywere studied via SPV comparison between different irradiation directions on thesystem. We demonstrate that the ethanol adsorption could induce the space chargeregion to expand towards the ZnO/ITO interface. In the absence of ethanol, theZnO nanorod array with the DPTPP adsorption showed enhanced SPV withreduced attenuation rate of photogenerated charge carriers. We found that theseparation of photogenerated charges could be further improved by coating thesurface with DPTPP and ethanol together. Furthermore, the SPV spectra patternsof the composite system with opposite incident-light directions reveal that theDPTPP molecules adsorbed just at the surface of ZnO nanorods adopt a moremonomeric alignment in contrast to the aggregative state in the DPTPP bulk.

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.

Au/N‐type Si Schottky‐barrier contact and oxidation kinetics in Au‐contaminated and thermally oxidized N‐type Si (001) surfaces

AbstractAnomalous Au locations in SiO2/Si system of Au‐contaminated and thermally oxidized n‐type Si(001) have been investigated using XPS and alternating current surface photovoltage (AC SPV) techniques. On the basis of XPS analyses, in Au‐contaminated (2 × 1015 atoms/cm2) and thermally oxidized Si surfaces between 823 and 973 K, the Au existed both on the top of the SiO2 and the SiO2/Si interface as a cluster that did not make bonds with other elements such as O and H. The resulting Au/n‐type Si Schottky barrier causes an occurrence of frequency‐dependent AC SPV, demonstrating that the Si surface was depleted and/or weakly inverted. This result indicates that the Au/n‐type Si Schottky barrier (calculated to be 0.70 eV) survived at the SiO2/Si interface even after the thermal oxidation. As the temperature increased higher than 1023 K, the Au at the SiO2/Si interface diffused into bulk Si, resulting in drastic reduction of AC SPV. On oxidation kinetics between 1023 and 1173 K, Au is thought to act as a catalyst and to promote the SiO2 growth at the Si surface, resulting in enhanced oxidation. Copyright © 2008 John Wiley & Sons, Ltd.

1J2a-2 Investigation of carrier recombination processes in GaAs/AlAs multiple quantum wells using piezoelectric photothermal and surface photovoltage techniques(Measurement techniques)

1J2a-2 Investigation of carrier recombination processes in GaAs/AlAs multiple quantum wells using piezoelectric photothermal and surface photovoltage techniques(Measurement techniques)

Preparation and photoelectric performance of ITO/TiO 2/CdS composite thin films

ITO/TiO 2/CdS composite thin films were successfully prepared by electrodeposition of CdS on ITO/TiO 2 substrate. The prepared ITO/TiO 2/CdS thin films were characterized in detail by X-ray diffraction, surface photovoltage, and scanning electronic microscopy techniques. The results indicated that the deposited CdS layer with about 1:1 stoichiometry of Cd and S distributes not only on the surface with cauliflower-like structure but also the TiO 2 film interior and even in the immediate proximity to the surface of ITO substrate. The UV–vis absorption spectrum results suggested that the absorption peak of the ITO/TiO 2/CdS film shifts from the ultraviolet region to the visible region in comparison to that of the ITO/TiO 2 film. Furthermore, by the photocurrent–potential behavior and incident monochromatic photon to current conversion efficiency measurements, it can be also noted that the deposited ITO/TiO 2/CdS thin films possess higher photoelectric conversion efficiency than the ITO/TiO 2 films. The sensibilization of TiO 2 with CdS can effectively improve steady photocurrent. Therefore, the modification of CdS for ITO/TiO 2 can strongly ameliorate the photoelectric capability of ITO/TiO 2 thin film, and the prepared ITO/TiO 2/CdS thin films were expected to possess more excellent photoelectrochemical performance than the ITO/TiO 2 films.

Investigation of Optical Absorption Spectra of GaAs/AlAs Multiple Quantum Wells Fabricated on a GaAs Substrate Using Surface Photovoltage and Piezoelectric Photothermal Techniques

Two nondestructive techniques of surface photovoltage (SPV) and piezoelectric photothermal (PPT) spectroscopies were adopted to investigate GaAs/AlAs multiple quantum well (MQW) heterostructures fabricated on a GaAs substrate, which is difficult to obtain the absorption spectra using a conventional technique. Excitonic and two-dimensional step-like subband absorptions corresponding to the density of states of MQW were obtained by subtracting the background signal from the substrate signal at room temperature. We also conclude that a difference between SPV and PPT spectra indicates the nonradiative component of the carrier relaxation processes in MQW. We found that the combination of SPV and PPT measurements is a powerful method of investigating the carrier confinement and recombination processes in MQW.

Ion implanters contamination on wafer surface analyzed by ToF-SIMS and SPV analytical techniques

In ULSI processes, metallic contamination controls are very important issues. For the ion implantation process it is known that several sources of contaminations still need to be controlled: metals from sputtering of the apertures or wafer holders, Na+ contaminations from filament impurities and messy maintenance procedure. ToF-SIMS is one of the most promising candidates to perform in-line surface analysis due to its high sensitivity. It is very common to use surface photo-voltage (SPV) techniques to control ion implanter equipments but this kind of analysis is an indirect measure for metallic contamination.The aim of this work is to study the possibility to use ToF-SIMS instead of SPV for in line equipment contamination monitoring. For this reason a comparison between SPV and ToF-SIMS data occurred. Good correlation between the data is shown; moreover ToF-SIMS spectra give detailed information about the other contaminations present on the wafer surface.

Surface photovoltage study of photogenerated charges in ZnO nanorods array grown on ITO

A well-aligned nanorods array of ZnO was chemically grown on conductive ITO substrate at low temperature. The photogenerated charges at surface and interface were examined by surface photovoltage techniques based on both Kelvin probe and lock-in amplifier with dc bias. The photovoltage response bands related to band-to-band transition and bound excitons were discriminated. We demonstrated the spectrum-dependent transfer characteristic of photogenerated charges at the surface of ZnO nanorods array and the interface between ZnO and ITO.

The electronic and optoelectronic properties study of N, N-dimethylperylene-3,4,9,10-dicarboximide/ITO film using surface photovoltage technique

The electronic structure and photo-induced charge carriers transport have been studied in N, N-dimethylperylene-3,4,9,10-dicarboximide (Me-PTCDI) films evaporated on ITO substrate using surface photovoltage (SPV) technique. For the Me-PTCDI film with the thickness of 456 nm, with changing the illumination geometry, the amplitudes as well as the phases of the SPV response bands at the wavelength where Me-PTCDI strongly absorbs significantly changed, while those of the SPV response bands at the wavelength where Me-PTCDI weakly absorbs hardly changed. It was explained based on the different absorption lengths of Me-PTCDI at different wavelengths. The experiment results suggest that the photo-induced charge carriers generated at the surface and the interface region are separated by the action of the built-in electric field and move in the opposite directions. From the dependence of the work function on the film thickness, it is inferred that the interface and surface are all depleted with the interface space charge region extending to be about 200 nm. In addition, it is found that an acceptor state at 0.1 eV above the valence band exists at the surface as well as at the interface.

Surface potential measurements on Ni–(Al)GaN lateral Schottky junction using scanning Kelvin probe microscopy

The surface potential distribution across lateral Ni–(Al)GaN Schottky junctions was measured by scanning Kelvin probe microscopy. The bare surface barrier heights of unintentionally doped Al0.22Ga0.78N and n-GaN in air were estimated to be ∼1.15 and 0.7 eV, respectively. Upon 364.5 nm (band edge for GaN) illumination, the surface barriers of both n-GaN and AlGaN∕GaN were observed to decrease. The minority carrier diffusion length in n-GaN(Si∼3.5×1017cm−3) was extracted from the surface photovoltage profile near the Schottky junction, ∼1.8±0.4μm. The scanning Kelvin probe surface photovoltage technique for measuring minority carrier diffusion length, while similar to the electron beam induced current technique, offers greater accuracy and higher spatial resolution due to separation of the minority carrier excitation source (relatively large area, above band gap light beam) from the nanometer-size probe (scanning force microscope tip).

Bound-to-bound and bound-to-free transitions in surface photovoltage spectra: Determination of the band offsets forInxGa1−xAsandInxGa1−xAs1−yNyquantum wells

We present an extensive study leading to a general understanding about the optical transitions involved in the surface photovoltage (SPV) spectra of type-I quantum well (QW) structures. SPV measurements were carried out on ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}∕\mathrm{GaAs}$ QW samples with varying QW width and on ${\mathrm{In}}_{0.15}{\mathrm{GaAs}}_{0.85}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ QW samples with varying aluminum content. The results are compared with experimental electroreflectance spectra, absorption spectra simulated with multiband $\mathbf{k}∙\mathbf{p}$ computations, and theoretical calculations of the electronic states in the QWs. Thus, the transitions which induce steps in the SPV spectra are unambiguously identified. Most remarkably, the bound electron-to-free hole transition is detected by SPV measurements. This distinguishes the SPV method from many other optical techniques like photoluminescence and photoreflectance spectroscopy that allow only the observation of bound-to-bound and free-to-free transitions. The analogous free electron-to-bound hole transition does not give rise to a feature in the SPV spectra. In addition, the bound-to-bound transitions ${e}_{1}\text{\ensuremath{-}}h{h}_{1}$, ${e}_{1}\text{\ensuremath{-}}l{h}_{1}$, and ${e}_{2}\text{\ensuremath{-}}h{h}_{2}$ are observed, if the respective states are confined. Also, the free-to-free transition $ce\text{\ensuremath{-}}ch$ is measured. We demonstrate how these transitions can typically be identified in the spectra without any further measurements or calculations. With this knowledge, the practical band offsets of the QW structure, i.e., the energy difference between the lowest confined states in the QW and the extended states in the barrier, can be extracted directly from the spectra. An advantage over conventional techniques for the determination of band offsets is that neither any additional knowledge of other QW parameters nor simulations are necessary. As an example for the application of the SPV technique, the electronic states and band offsets of a series of ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}{\mathrm{As}}_{0.98}{\mathrm{N}}_{0.02}∕\mathrm{GaAs}$ QWs with varying QW width are characterized. These experiments directly show that nitrogen affects only the conduction band states.

Generalized models of the spectral response of the voltage for the extraction of recombination parameters in silicon devices

Analytical models of the spectral response of the voltage of silicon devices have been generalized using the concept of the internal quantum efficiency of the semiconductor. This allows the extension of models used in the analysis of the internal quantum efficiency to the spectral response of the voltage. Existing models for the spectral response of the voltage that are largely employed in the surface photovoltage technique are shown to be special cases that approximate the internal quantum efficiency. A more sophisticated model of the internal quantum efficiency, the model of Isenberg, and a model that allows the analysis of the internal quantum efficiency of rear-illuminated devices have been adapted to the spectral response of the voltage. This paves the way to analyze solar cells, Schottky devices, or chemically treated silicon wafers independently of the light intensity and using front or rear illumination. The models have been validated by comparing the analysis of the spectral response of the short-circuit current and the open-circuit voltage with computer simulations for a wide range of solar cells. The model of Isenberg has been found to give in general the best prediction of the recombination parameters for both the spectral photocurrent and photovoltage. The analysis of the spectral response of the voltage can often unveil the surface recombination rate where the spectral photovoltage fails to produce any output. The measurement of rear-junction cells showed that the surface recombination rate can be predicted equally well with either method.

Preparation of room temperature NO 2 gas sensors based on W- and V-modified mesoporous MCM-41 thin films employing surface photovoltage technique

W- and V-modified mesoporous MCM-41 thin films were investigated as an NO 2 gas sensor operable at room temperature employing surface photovoltage (SPV) technique. The self-ordered and structure-controlled of W- and V-modified mesoporous silica MCM-41 were prepared by a molecule surfactant template method using spin coating. The resulting thin films were then applied as a SPV NO 2 gas sensor based on a metal-insulator-semiconductor (MIS) structure. The W- and V-modified mesoporous silica were highly sensitive to a low concentration of NO 2, as low as 350 ppb, at room temperature. The mechanism of NO 2 detection is attributed to both the surface area, which is contributed to the change of dielectric constant, and the amount of the tungsten and vanadium incorporated, which is contributed to the change of semiconductor surface charge. The W-modified mesoporous MCM-41 was found to have better sensing properties than those of the Sn- and V-modified mesoporous MCM-41.