Surface Photovoltage Signal Research Articles

In-Line Charaterization of HPSI SiC Wafers Using High Resolution Surface Photovoltage Spectroscopy (HR-SPS)

High purity semi-insulating (HPSI) 4H-SiC wafers from different vendors have been studied by surface photovoltage spectroscopy (SPV). It is demonstrated that the surface photovoltage signal height can be used to discriminate between non-compensated and compensated material, and that the SPV signal is also proportional to the bulk resistivity, at least for non-compensated 4H-SiC material.

Anchoring Pt single atoms on specific nitrogen vacancies of carbon nitride to accelerate photogenerated carrier transfer

The anchoring sites of metal single atoms are closely related to photogenerated carrier dynamics and surface reactions. Achieving smooth photogenerated charge transfer through precise design of single-atom anchoring sites is an effective strategy to enhance the activity of photocatalytic hydrogen evolution. In this study, Pt single atoms were loaded onto ultra-thin carbon nitride with two-coordination nitrogen vacancies (VN2c-UCN-Pt) and ultra-thin carbon nitride with three-coordination nitrogen vacancies (VN3c-UCN-Pt). This paper investigated the photocatalytic hydrogen evolution performance and photogenerated carrier behavior of Pt single atoms at different anchoring sites. Surface photovoltage measurements indicated that VN2c-UCN-Pt exhibits a superior carrier separation efficiency compared to VN3c-UCN-Pt. More importantly, the surface photovoltage signal under the presence of H2O molecules revealed a significant decrease. Theoretical calculations suggest that VN2c-UCN-Pt exhibits superior capabilities in adsorbing and activating H2O molecules. Consequently, the photocatalytic hydrogen evolution efficiency of VN2c-UCN-Pt reaches 1774 µmol g−1h−1, which is 1.8 times that of VN3c-UCN-Pt with the same Pt loading. This work emphasized the structure–activity relationship between single-atom anchoring sites and photocatalytic activity, providing a new perspective for designing precisely dispersed single-atom sites to achieve efficient photocatalytic hydrogen evolution.

Electronic states near surfaces and interfaces of β-Ga2O3 and κ-Ga2O3 epilayers investigated by surface photovoltage spectroscopy, photoconductivity and optical absorption

Transient surface photovoltage (SPV) spectroscopy, optical absorption, and photoconductivity (PC) were applied to study electronic transitions in (-201) β-Ga2O3 and (001) κ-Ga2O3 epitaxial layers on c-plane sapphire substrates. SPV signals were distinguished for charge separation near the surface and near the layer/substrate interface. The bandgaps of β-Ga2O3 and κ-Ga2O3 epitaxial layers were found to be, respectively, about 4.75 and 4.79 eV from optical absorption measurements, about 4.8 and 4.9 eV from PC, and 4.65 and 4.9 (near the surface) from SPV measurements. Near the κ-Ga2O3 layer/substrate interface SPV instead gives a value of 4.75 eV, possibly related to the presence of an interlayer. Defect-related transitions with onset energies at about 4.5, 4.1, and 3.5 eV were observed along the whole β-Ga2O3 layer cross-section. In contrast, defects related to different transitions were differently distributed in κ-Ga2O3 epitaxial layers: transitions setting on at about 4.3 eV originate from defects uniformly distributed across the layer, while transitions starting at 2.4 eV and 1.7 eV were respectively connected with defects located near the surface or near the layer/substrate interface. The PC measurements at photo-excitation energy above the bandgap indicated that defect densities and recombination losses were much larger in κ-Ga2O3 than in β-Ga2O3.

Surface photovoltage microscopy for mapping charge separation on photocatalyst particles.

Solar-driven photocatalytic reactions offer a promising route to clean and sustainable energy, and the spatial separation of photogenerated charges on the photocatalyst surface is the key to determining photocatalytic efficiency. However, probing the charge-separation properties of photocatalysts is a formidable challenge because of the spatially heterogeneous microstructures, complicated charge-separation mechanisms and lack of sensitivity for detecting the low density of separated photogenerated charges. Recently, we developed surface photovoltage microscopy (SPVM) with high spatial and energy resolution that enables the direct mapping of surface-charge distributions and quantitative assessment of the charge-separation properties of photocatalysts at the nanoscale, potentially providing unprecedented insights into photocatalytic charge-separation processes. Here, this protocol presents detailed procedures that enable researchers to construct the SPVM instruments by integrating Kelvin probe force microscopy with an illumination system and the modulated surface photovoltage (SPV) approach. It then describes in detail how to perform SPVM measurements on actual photocatalyst particles, including sample preparation, tuning of the microscope, adjustment of the illuminated light path, acquisition of SPVM images and measurements of spatially resolved modulated SPV signals. Moreover, the protocol also includes sophisticated data analysis that can guide non-experts in understanding the microscopic charge-separation mechanisms. The measurements are ordinarily performed on photocatalysts with a conducting substrate in gases or vacuum and can be completed in 15 h.

Surface Photovoltage Method for Photovoltaic Quality Control of GaAs-Based Solar Cells

In this paper, we demonstrate the potential of the contactless surface photovoltage (SPV) method for fast and reliable control of GaAs-based solar cells directly on epitaxial heterostructures before metallization and photolithography processes. The magnitude of the SPV corresponds to the generated photovoltage in the photoactive region, which is related to the open circuit voltage of the cell. The focus of this investigation is the potential of dilute nitride compounds grown by low-temperature liquid-phase epitaxy (LPE) for application as intermediate cells in multijunction solar cells. First, SPV spectroscopy is used to determine the photosensitivity spectral range and bandgap of the grown dilute nitride compound layers. Further, the photovoltaic quality of the grown solar cell heterostructures is evaluated by comparing the magnitude of their SPV signals with that of a reference GaAs solar cell. A drastic reduction in the measured SPV is observed for nitrogen-containing solar cell structures, which correlates with the lowering of solar cell open-circuit voltage values measured under standard test conditions. Finally, solar cell structures based on nitrogen-free GaAsSb compounds with the same long-wavelength photosensitivity limit as GaAsSbN are grown by LPE. They show one order of magnitude higher SPV signal and, therefore, have a great potential for solar cell application.

Dual-heterodyne Kelvin probe force microscopy.

We present a new open-loop implementation of Kelvin probe force microscopy (KPFM) that provides access to the Fourier spectrum of the time-periodic surface electrostatic potential generated under optical (or electrical) pumping with an atomic force microscope. The modulus and phase coefficients are probed by exploiting a double heterodyne frequency mixing effect between the mechanical oscillation of the cantilever, modulated components of the time-periodic electrostatic potential at harmonic frequencies of the pump, and an ac bias modulation signal. Each harmonic can be selectively transferred to the second cantilever eigenmode. We show how phase coherent sideband generation and signal demodulation at the second eigenmode can be achieved by using two numerical lock-in amplifiers configured in cascade. Dual-heterodyne KPFM (DHe-KPFM) can be used to map any harmonic (amplitude/phase) of the time-periodic surface potential at a standard scanning speed. The Fourier spectrum (series of harmonics) can also be recorded in spectroscopic mode (DHe-KPFM spectroscopy), and 2D dynamic images can be acquired in data cube mode. The capabilities of DHe-KPFM in terms of time-resolved measurements, surface photovoltage (SPV) imaging, and detection of weak SPV signals are demonstrated through a series of experiments on difference surfaces: a reference substrate, a bulk organic photovoltaic heterojunction thin film, and an optoelectronic interface obtained by depositing caesium lead bromide perovskite nanosheets on a graphite surface. The conclusion provides perspectives for future improvements and applications.

Molecular Heptazine-Triazine Junction over Carbon Nitride Frameworks for Artificial Photosynthesis of Hydrogen Peroxide.

Revealing the photocatalytic mechanism between various junctions and catalytic activities has become a hotspot in photocatalytic systems. Herein, an internal molecular heptazine/triazine (H/T) junction in crystalline carbon nitride (HTCN) is constructed and devoted to selective two-electron oxygen reduction reaction (2e- ORR) for efficient hydrogen peroxide (H2 O2 ) production. In-situ X-ray diffraction spectra under various temperatures authenticate the successful formation of molecular H/T junction in HTCN during the calcining process rather than physically mixing. The increased surface photovoltage and transient photovoltage signals, and the decreased exciton binding energy undoubtably elucidate that an obvious increasement of carrier density and diffusion capability of photogenerated electrons are realized over HTCN. Additionally, the analyses of in situ photoirradiated Kelvin probe force microscopy and femto-second transient absorption spectra reveal the successful construction of the strong internal built-in-electric field and the existence of the majority of long-lived shallow trapped electrons associated with molecular H/T junction over HTCN, respectively. Benefiting from these, the photocatalytic results exhibit an incredible improvement (96.5-fold) for H2 O2 production. This novel work provides a comprehensive understanding of the long-lived reactive charges in molecular H/T junctions for strengthening the driving-force for photocatalytic H2 O2 production, which opens potential applications for enhancing PCN-based photocatalytic redox reactions.

Anomalous p-type characteristic and recrystallization upon aging of hot-cast CH3NH3PbI3 perovskite thin films grown under atmospheric air

Hot-cast CH3NH3PbI3 perovskite thin films are fabricated in ambient air using heated substrates. All the perovskite films prepared are stored in a low-humidity environment. Interestingly, it has been observed that the perovskite films fabricated with high-temperature substrates degrade at a slower rate compared to those prepared at lower temperatures. Even after one week, the morphology of the films remains unchanged when produced with high-temperature substrates under 35–40 % relative humidity (RH). The hot-cast films exhibit p-type characteristics, as evidenced by their surface photovoltage signals. During the aging process, the iodine ions seem to vaporize as evidenced by the reduction of the I/Pb ratios in the energy-dispersive X-ray spectroscopy (EDX) spectra. The hot-cast perovskite films show relatively low surface trap states, ranging from 1015–1016 cm−3, along with low energy band tails of approximately 20 meV.

Interfacial Mediation by Sn And S Vacancies of p‐SnS/n‐ZnIn2S4 for Enhancing Photocatalytic Hydrogen Evolution with New Scheme of Type‐I Heterojunction

AbstractThe construction of interfacial electric field (IEF) in semiconductor heterojunction is of great significance in boosting photocatalytic hydrogen evolution through efficient separation of photogenerated charge‐carriers. However, the exploitation of IEF in type‐I heterojunction has not been proposed for designing photocatalysts. Herein, based on the density functional theory prediction, p‐SnS with different work functions modulated by Sn‐vacancy are compounded with n‐ZnIn2S4 containing S‐vacancy to form type‐I heterojunction. The optimized SnS/ZnIn2S4 photocatalyst without co‐catalysts exhibits an impressive hydrogen evolution rate of 22.75 mmol g−1 h−1, 6.23 times of ZnIn2S4. Systematic investigations reveal that the interfacial Sn‐S bond acts as a transport channel that accelerates the interface charge‐carriers transfer under the promotion of IEF originating from the significant Fermi level difference. A large difference in the surface photovoltage signal of SnS/ZnIn2S4 and ZnIn2S4 is achieved from effective photogenerated charge‐carriers separation by IEF. The new p‐n type‐I scheme of SnS/ZnIn2S4 induced by the interfacial mediation can separate the photogenerated charge‐carriers, and retain the highly reductive electrons of ZnIn2S4 for hydrogen evolution, overcoming the disadvantage of reduction potential decline in the typical type‐I scheme. This study will afford a new theoretical basis for the achievement of high‐efficiency photocatalytic hydrogen evolution through interface modulation.

Ferroelectric Polarization in BaTiO3 Nanocrystals Controls Photoelectrochemical Water Oxidation and Photocatalytic Hydrogen Evolution.

Ferroelectric (FE) semiconductors such as BaTiO3 support a remnant polarization after the application of an electric field that can promote the separation of photogenerated charge carriers. Here, we demonstrate FE-enhanced photocatalytic hydrogen evolution and photoelectrochemical water oxidation with barium titanate nanocrystals for the first time. Nanocrystals of the ferroelectric tetragonal structure type were obtained by a hydrothermal synthesis from TiO2 and barium hydroxide in 63% yield. BaTiO3 nanocrystal films on tantalum substrates exhibit water oxidation photocurrents of 0.141 mA cm-2 at 1.23 V RHE under UV light (60 mW cm-2) illumination. Electric polarization at 52.8 kV cm-1 normal to the film plane increases the photocurrent by a factor of 2 or decreases it by a factor of 3.5, depending on the field polarity. It also shifts the onset potential by -0.15 or +0.09 V and it modifies the surface photovoltage signal. Lastly, exposure to an electric field increases the H2 evolution rate of Pt/BaTiO3 by a factor of ∼1.5, and it raises the selectivity of photodeposition of silver onto the (001) facets of the nanocrystal. All FE enhancements can be removed by heating samples above the Curie temperature of BaTiO3. These findings can be explained by FE dipole-induced changes to the potential drop across the space charge layer of the material. The ability to use the ferroelectric effect to enhance hydrogen evolution and water oxidation is of potential interest for the development of improved solar energy for fuel conversion systems.

Insights on CaTiS3 films grown by pulsed laser deposition

We attempt to grow perovskite CaTiS3 by pulsed laser deposition (PLD) on various substrates using a CaS:TiS2 target. Using Al2O3 (0001) substrates at 600 °C deposition temperature in vacuum, Ca1.0Ti1.05S2.48Oy stoichiometry is measured by Energy-dispersive X-ray spectroscopy (EDS) and Ca1.0Ti1.0S2.0O0.7 by Rutherford Backscattering Spectrometry (RBS). This indicates that when the films are grown in vacuum at moderate temperature, a high amount of S can be transferred from the target to the film. While no phase segregation of CaS and TiS2 could be observed, no perovskite CaTiS3 phase could be obtained, but rather a phase governed by van der Waals interactions. The films show a highly doped n-type semiconductor behavior with absorption coefficient in the 105 cm−1 range at 350–2500 nm but no surface photovoltage signal. This work should stimulate further experimental efforts in PLD growth of chalcogenides and chalcogenide perovskites.

Photovoltaic Performance of Si and SiGe Surfaces Sonochemically Activated in Dichloromethane

Aims: To activate Si and SiGe surfaces by employing the sonochemical treatment at different operating frequencies in dichloromethane to improve the surface photovoltage signal. Background: To produce integrated electronic devices, one needs to achieve low surface and interface trap densities. In this respect, placing a passivating thin layer on Si and Ge surfaces, which saturates the electronic levels of traps and therefore affects the carrier recombination velocities at the surface, is of great interest. Objective: To demonstrate that the effectiveness of the treatment of Si and SiGe surfaces depends on the ultrasonic frequency used. Methods: Photovoltaic transients, electron microscopy, EDX spectroscopy. Result: The surface photovoltage (SPV) decay curves can be divided into rapid (τ_1) and slow (τ_2) components. The sonication effect on the SPV is different for the treatment done at about 25 and 400 kHz. The SPV signal in Si gradually increases with increasing lower-frequency sonication time, whereas the SPV enhancement on SiGe is somewhat smaller. Increasing the sonication time increases the amplitude of the τ_2 component in Si. In SiGe, the lower-frequency sonication quenches the τ_2 component yielding a nearly single-exponential decay form. This trend is even more pronounced at the higher-frequency sonication. Conclusion: The sonochemical treatments greatly intensify the formation of CxHy–Si and CxHy–Ge bonds on Si and Si1-xGex surfaces, resulting in increased SPV signals and prolonged SPV decay times. These results demonstrate that sonochemical treatment is a more effective technique to obtain stable highly passivated Si and Si1-xGex surfaces in comparison with wet chemical treatments in hydrocarbon solutions.

Hydrogen effects at sputtered Tb-doped AlNxOy:H / c-Si(p) interfaces: A transient surface photovoltage spectroscopy study

In the present work, we studied the interface of terbium doped aluminum oxynitride (Tb-doped AlNxOy:H) deposited under different hydrogen flows with p-type doped crystalline silicon by applying transient surface photovoltage spectroscopy. We observed strong accumulation with concomitant passivation of boron acceptors in the crystalline silicon and defect generation near the interface. With increasing hydrogen flows, the net negative charge in the Tb-doped AlNxOy:H layer decreased, surface photovoltage signals related to defects increased, surface photovoltage transients decayed faster, and the slowest relaxation of charge carriers separated in space changed from trap limited to hopping transport via an exponential distribution of trap states in energy.

Transient surface photovoltage spectroscopy of diamond

Contactless and highly sensitive probing of electronic transitions in diamond over a wide spectral range from near infrared to deep ultraviolet is still challenging. Surface photovoltage (SPV) signals depend on electronic transitions and transport phenomena leading to charge separation in space and allow for a contactless study of electronic transitions. Here, transient SPV spectroscopy in an arrangement with a charge amplifier and a laser tunable over a wide range was applied to study an undoped diamond single crystal between 0.8 and 5.9 eV at room temperature in ambient air. SPV transients were measured without and with weak visible bias light, which allowed for suppression of possible parasitic contributions in SPV signals not related to diamond and distinction of processes of charge separation that were independent of band bending. Transitions at 1.0 and 3.1 eV led to preferential separation of photogenerated holes toward the surface. In contrast, a transition at 1.8 eV caused preferential separation of photogenerated electrons toward the surface. Transitions near the indirect bandgap of diamond were observed at 5.27, 5.32, 5.48, and 5.53 eV and could be assigned to absorption assisted (i) by an indirect exciton and absorption of longitudinal optical and acoustic phonons, (ii) by absorption of transverse acoustic phonons, (iii) by emission of transverse acoustic phonons, and (iv) by emission of longitudinal optical and acoustic phonons, respectively. Charge separation under excitation at 5.27 eV was caused by directed charge transfer at/near the diamond surface after exciton diffusion followed by exciton dissociation.

Enhanced Light Absorption and Efficient Carrier Collection in MoS2 Monolayers on Au Nanopillars.

We fabricated hybrid nanostructures consisting of MoS2 monolayers and Au nanopillar (Au-NP) arrays. The surface morphology and Raman spectra showed that the MoS2 flakes transferred onto the Au-NPs were very flat and nonstrained. The Raman and photoluminescence intensities of MoS2/Au-NP were 3- and 20-fold larger than those of MoS2 flakes on a flat Au thin film, respectively. The finite-difference time-domain calculations showed that the Au-NPs significantly concentrated the incident light near their surfaces, leading to broadband absorption enhancement in the MoS2 flakes. Compared with a flat Au thin film, the Au-NPs enabled a 6-fold increase in the absorption in the MoS2 monolayer at a wavelength of 615 nm. The contact potential difference mapping showed that the electric potential at the MoS2/Au contact region was higher than that of the suspended MoS2 region by 85 mV. Such potential modulation enabled the Au-NPs to efficiently collect photogenerated electrons from the MoS2 flakes, as revealed by the uniform positive surface photovoltage signals throughout the MoS2 surface.

Surface photovoltage characterisation of metal halide perovskite on crystalline silicon using Kelvin probe force microscopy and metal-insulator-semiconductor configuration

In this study we analysed halide perovskite films deposited directly on crystalline silicon by means of two set-ups using different operating modes of the surface photovoltage (SPV) methods, i.e., the Kelvin probe force microscopy (KPFM) and the metal-insulator-semiconductor (MIS) technique. The KPFM allowed to visualize surface potential distribution on a microscale while MIS technique allowed to study SPV spectral dependence. We studied wavelength dependent SPV of these samples, which allowed us to effectively vary the probe depth in the sample and discern the contribution from each interface to the overall effect measured under white light illumination. Depending on where the photocarriers are generated, different SPV signals are observed: at the perovskite/Si interface, the signal depends on Si doping type, while at the surface the SPV is always negative indicating downward surface band bending. This is confirmed by analysing SPV phase measured in the AC MIS mode. In addition, distinction between slow and fast processes contributing to measured SPV was possible. It has been observed, that with decreasing the illumination wavelength, the processes causing SPV become slower, which can indicate that high energy photons not only generate electronic photocarriers but can also induce chemical changes with creation of defects or ionic species that also modify the measured SPV.

Surface photovoltage dynamics at passivated silicon surfaces: influence of substrate doping and surface termination.

We have monitored the temporal evolution of the band bending at controlled silicon surfaces after a fs laser pump excitation. Time-resolved surface photo-voltage (SPV) experiments were performed using time resolved photoemission spectroscopy with time resolution of about 30 ns. To disentangle the influence of doping and surface termination on SPV dynamics, we compare the results obtained on two surface terminations: the water saturated (H,OH)-Si(001) surface and the thermally oxidized Si(001) one. The SPV dynamics were explored as a function of laser fluence and as a function of time for the two surface terminations at given doping levels. The return to equilibrium involves a characteristic time in the 0.1 μs to 10 μs range, depending on the surface termination and bulk doping. Exploring several laser fluences, different SPV regimes were found for the two surface terminations at given doping levels. For low laser fluence the SPV dynamic follows the commonly accepted thermionic model. At higher fluence, the SPV signal reaches a saturation value, and if the fluence is further increased, the decay time of the SPV increases and can no longer be explained by a thermionic model alone.

Surface Photovoltage Spectroscopy over Wide Time Domains for Semiconductors with Ultrawide Bandgap: Example of Gallium Oxide

A nonconventional approach is proposed for the measurement of surface photovoltage (SPV) signals over very wide ranges in photon energy and time. Regimes for AC, DC, and combined AC–DC measurements are defined and applied for the characterization of a β‐Ga2O3 crystal by transient and modulated SPV spectroscopy, spectroscopy in the mode of a Kelvin probe and single‐pulse SPV transients from 10 ns to 1000 s with the same electrode. Numerous electronic transitions are distinguished in β‐Ga2O3 depending on the measurement regime, the time response and the history of measurement. An accumulation of negative charge at the surface of β‐Ga2O3 is observed at long times independent whether the SPV signals are positive or negative before. The nonconventional approach of measuring SPV signals opens new opportunities for investigating electronic properties of semiconductors with ultrawide bandgaps and any other photoactive materials.

Frequency-Dependent Sonochemical Processing of Silicon Surfaces in Tetrahydrofuran Studied by Surface Photovoltage Transients.

The field of chemical and physical transformations induced by ultrasonic waves has shown steady progress during the past decades. There is a solid core of established results and some topics that are not thoroughly developed. The effect of varying ultrasonic frequency is among the most beneficial issues that require advances. In this work, the effect of sonication of Si wafers in tetrahydrofuran on the photovoltage performance was studied, with the specific goal of studying the influence of the varying frequency. The applied ultrasonic transducer design approach enables the construction of the transducer operating at about 400 kHz with a sufficient sonochemical efficiency. The measurements of the surface photovoltage (SPV) transients were performed on p-type Cz-Si(111) wafers. Sonication was done in tetrahydrofuran, methanol, and in their 3:1 mixture. When using tetrahydrofuran, the enhanced SPV signal (up to ≈80%) was observed due to increasing sonication frequency to 400 kHz. In turn, the signal was decreased down to ≈75% of the initial value when the frequency is lowered to 28 kHz. The addition of methanol suppressed this significant difference. It was implied that different decay processes with hydrogen decomposed from tetrahydrofuran could be attempted to explain the mechanism behind the observed frequency-dependent behavior.

A synergistic boost of photo-activity of ZnO for photocatalytic degradation of methylene blue by Ag decoration and Fe doping

Fe-doped Ag-ZnO (Zn0.97FexAg0.03-xO) nanoparticles (NPs) were synthesized by a two-step polymer network gel method. By modulating the Fe, Ag concentration to acquire the strongest spectral absorption and the most efficient charge separation efficiency, the photocatalytic performance of ZnO for methylene blue (MB) degradation can be improved synergistically and remarkably. The spectral absorption and the photo-generated charge carrier separation properties of ZnO are enhanced by introducing Fe and Ag. The Fe2+ to Fe3+ valence state transformation is observed in the case of increasing Fe doping concentration. Especially, sample Zn0.97Fe0.005Ag0.025O (FZA-0.5) exhibits the strongest spectral absorption, the weakest photoluminescence (PL) emission, and displays the strongest surface photo-voltage (SPV) signal. Compared with pure ZnO, these favorable features of FZA-0.5 lead to the degradation efficiency of MB under UV and visible light being ~2.7 and ~5.0 times higher, respectively. Besides, the introduced Fe2+ on ZnO surface seems more beneficial for the photocatalytic enhancement. We show a strategy for synthesizing high performance and low-cost ZnO-based photocatalysts by trace transition metal doping to reduce the decoration of noble metal.