Photocapacitance Effect Research Articles

Exploring the border traps near the valence band in the SiO2 - SiC system using above-band-gap optical excitation

Studying near-valence-band (EV) defects at the insulator–n-type 4H-SiC interface is challenging due to the low minority carrier concentration. Herein, we present a technique for characterizing the border traps near EV in an n-type 4H-SiC MOS capacitor by generating holes using above-band-gap optical excitation (OE). A rise in capacitance was observed under OE (due to hole capture by border traps), remaining stable long after the optical stimulus was removed, resulting in a persistent photocapacitance effect. We show the dynamics of the capture process and perform a quantification of the captured holes in samples with thermally grown oxide and different postoxidation annealing treatments. Published by the American Physical Society 2024

Impact of Hydrogen Plasma on Electrical Properties and Deep Trap Spectra in Ga2O3 Polymorphs

In this study, the results of hydrogen plasma treatments of β-Ga2O3, α-Ga2O3, κ-Ga2O3 and γ-Ga2O3 polymorphs are analyzed. For all polymorphs, the results strongly suggest an interplay between donor-like hydrogen configurations and acceptor complexes formed by hydrogen with gallium vacancies. A strong anisotropy of hydrogen plasma effects in the most thermodynamically stable β-Ga2O3 are explained by its low-symmetry monoclinic crystal structure. For the metastable, α-, κ- and γ-polymorphs, it is shown that the net result of hydrogenation is often a strong increase in the density of centers supplying electrons in the near-surface regions. These centers are responsible for prominent, persistent photocapacitance and photocurrent effects.

Photo-Chemical Stimulation of Neurons with Organic Semiconductors.

Recent advances in light-responsive materials enabled the development of devices that can wirelessly activate tissue with light. Here it is shown that solution-processed organic heterojunctions can stimulate the activity of primary neurons at low intensities of light via photochemical reactions. The p-type semiconducting polymer PDCBT and the n-type semiconducting small molecule ITIC (a non-fullerene acceptor) are coated on glass supports, forming a p-n junction with high photosensitivity. Patch clamp measurements show that low-intensity white light is converted into a cue that triggers action potentials in primary cortical neurons. The study shows that neat organic semiconducting p-n bilayers can exchange photogenerated charges with oxygen and other chemical compounds in cell culture conditions. Through several controlled experimental conditions, photo-capacitive, photo-thermal, and direct hydrogen peroxide effects on neural function are excluded, with photochemical delivery being the possible mechanism. The profound advantages of low-intensity photo-chemical intervention with neuron electrophysiology pave the way for developing wireless light-based therapy based on emerging organic semiconductors.

Decoupling Acoustoelectric and Thermal Effects of Ultraviolet Responses for Acoustic Wave Sensing Mechanisms

Ultraviolet (UV) detectors based on surface acoustic wave (SAW) technology offer unique advantages of remote or wireless operation capability with a potential of zero power consumption. Frequency shift of SAW devices induced by UV irradiation is mainly caused by acoustoelectric and thermal effects, although mass-loading and photo-capacitive effect might also have minor influences. Currently the individual contribution from either acoustoelectric effect or thermal effect for UV sensing has not been thoroughly studied. In this work, we systematically investigated the contribution of acoustoelectric and thermal effects at different UV light intensities based on a LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> SAW device with its wavelength of 20 μm, and decoupled their individual contributions to the overall ultraviolet responses for acoustic wave sensing. We found that for the LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> SAW device without any surface treatment, the frequency shift was mainly caused by the thermal accumulation induced by UV radiation. Whereas for the LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> SAW devices coated with zinc oxide nanowires (ZnO NWs), the frequency shift is caused by the combined acoustoelectric and thermal effects. We found that the frequency shift caused by acoustic electric effect is dominant at a low UV intensity (< 30 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). Whereas with the UV light intensity larger than 90 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the photogenerated carriers generated by ZnO NWs became saturated, and then the thermal effect became dominant. This study clarifies the nature of individual influences of these two factors on the dominant mechanisms of the SAW based UV sensing at different conditions.

Фоточувствительность полевого транзистора металл-диэлектрик-полупроводник на основе пленки PbSnTe:In с составом вблизи инверсии зон

A prototype of a metal–insulator–semiconductor field-effect transistor based on PbSnTe:In/(111)BaF2 film with an Al2O3 gate dielectric was designed for the first time. With the gate voltage applied in the range – 7.7 V &lt; Ugate &lt; +7.7 V the relative modulation in the drain-source current Ids/Ids attained near five-fold change at T = 4.2 K. When illuminated with rela-tively low (~100 photon/s) fluxes, negative photoconductivity was detected accompanied with a decrease in Ids by ~104 times and a simultaneous decrease in Ids by ~103 times or even more. The estimated detectivity was about 71016 cmHz0.5W-1 at a wavelength  about 25 micron with the accumulation time about 0.5 s. A qualitative model is discussed which assumes the ex-istence of deep traps and a photo-capacitance effect.

Model and data of optically controlled tunable capacitor in silicon single-photon avalanche diode

This paper reports the photocapacitance effect of silicon-based single-photon avalanche diodes (SPADs), and the frequency scattering phenomenon of capacitance. The test results of the small-signal capacitance–voltage method show that light can cause the capacitance of a SPAD device to increase under low-frequency conditions, and the photocapacitance exhibits frequency-dependent characteristics. Since the devices are fabricated based on the standard bipolar-CMOS-DMOS process, this study attributes the above results to the interfacial traps formed by Si–SiO2, and the illumination can effectively reduce the interfacial trap lifetime, leading to changes in the junction capacitance inside the SPAD. Accordingly, an equivalent circuit model considering the photocapacitance effect is also proposed in this paper. Accurate analysis of the capacitance characteristics of SPAD has important scientific significance and application value for studying the energy level distribution of device interface defect states and improving the interface quality.

Bioresorbable thin-film silicon diodes for the optoelectronic excitation and inhibition of neural activities.

Neural activities can be modulated by leveraging light-responsive nanomaterials as interfaces for exerting photothermal, photoelectrochemical or photocapacitive effects on neurons or neural tissues. Here we show that bioresorbable thin-film monocrystalline silicon pn diodes can be used to optoelectronically excite or inhibit neural activities by establishing polarity-dependent positive or negative photovoltages at the semiconductor/solution interface. Under laser illumination, the silicon-diode optoelectronic interfaces allowed for the deterministic depolarization or hyperpolarization of cultured neurons as well as the upregulated or downregulated intracellular calcium dynamics. The optoelectronic interfaces can also be mounted on nerve tissue to activate or silence neural activities in peripheral and central nervous tissues, as we show in mice with exposed sciatic nerves and somatosensory cortices. Bioresorbable silicon-based optoelectronic thin films that selectively excite or inhibit neural tissue may find advantageous biomedical applicability.

Understanding Photocapacitive and Photofaradaic Processes in Organic Semiconductor Photoelectrodes for Optobioelectronics

AbstractPhotoactive organic semiconductor substrates are envisioned as a novel class of bioelectronic devices that transduce light into stimulating biological signals with relevance for retinal implants or guided cellular differentiation. The direct interface between the semiconductor and the electrolyte gives rise to different competing optoelectronic transduction mechanisms. A detailed understanding of such faradaic or capacitive processes and the underlying material science is necessary to develop and optimize future devices. Here, the problem in organic photoelectrodes is addressed based on a planar p‐n junction containing phthalocyanine (H2Pc) and N,N′‐dimethyl perylenetetracarboxylic diimide (PTCDI). The detailed characterization of photoelectrochemical current transients is combined with spectroscopic measurements, impedance spectroscopy, and local photovoltage measurements to establish a model that predicts quantitatively faradaic or capacitive current transients. The decisive elements of the model are the energy levels present at the interface and the voltage building up in the photoelectrode. The result of the efforts is a comprehensive model of photocapacitive and photofaradaic effects that can be applied to developing wireless bioelectronic photostimulation devices.

Untangling Photofaradaic and Photocapacitive Effects in Organic Optoelectronic Stimulation Devices.

Light, as a versatile and non-invasive means to elicit a physiological response, offers solutions to problems in basic research as well as in biomedical technologies. The complexity and limitations of optogenetic methods motivate research and development of optoelectronic alternatives. A recently growing subset of approaches relies on organic semiconductors as the active light absorber. Organic semiconductors stand out due to their high optical absorbance coefficients, mechanical flexibility, ability to operate in a wet environment, and potential biocompatibility. They could enable ultrathin and minimally invasive form factors not accessible with traditional inorganic materials. Organic semiconductors, upon photoexcitation in an aqueous medium, can transduce light into (1) photothermal heating, (2) photochemical/photocatalytic redox reactions, (3) photocapacitive charging of electrolytic double layers, and (4) photofaradaic reactions. In realistic conditions, different effects may coexist, and understanding their role in observed physiological phenomena is an area of critical interest. This article serves to evaluate the emerging picture of photofaradaic vs. photocapacitive effects in the context of our group’s research efforts and that of others over the past few years. We present simple experiments which can be used to benchmark organic optoelectronic stimulation devices.

Experimental Investigation of the Photocapacitance Effect in Organic Heterojunction Devices

Photocapacitance effect refers to a mechanism that the illumination of light gives rise to an alteration in the junction capacitance. We reported the photocapacitance effect in organic heterojunction devices with measurements on various fabricated layer stacks. According to the results, the photocapacitance effect was observed in those organic heterojunction devices constituting a pn junction, in which the illumination leads to increasing the device capacitance at low frequencies. Based on the results, we attributed this effect to charge traps so that the light could effectively reduce the trapping lifetime, leading to changing the junction capacitance at low frequencies.

Photovoltaic, photo-impedance, and photo-capacitance effects of the flexible (111) BiFeO3 film

Ferroelectric polarization allows a depolarization electric field to separate electron-hole pairs excited by lights, and thus, the photovoltaic properties of ABO3-type films on a hard SrTiO3 or Si substrate have been extensively studied recently. However, there are a few reports on the photocapacitance and photoimpedance of these oxide films, especially on flexible substrates. In this work, the strong photovoltaic, giant photocapacitance, and photoimpedance effects were observed in the flexible (111) BiFeO3 films with natural downward polarization. The flexible mica/SrRuO3/BiFeO3/Au cells show a maximum photovoltaic efficiency at 150 °C, a 95.5% decrease in photoimpedance effects, and a 316% increase in photocapacitance effects under 405-nm-wavelength light illumination at 25 °C. Most importantly, these properties do not show obvious degradation when the device is bent to 3 mm radius for 104 times. This work is of vital importance for us to develop new flexible photoelectronic devices.

Compensation and persistent photocapacitance in homoepitaxial Sn-doped β-Ga2O3

The electrical properties of epitaxial β-Ga2O3 doped with Sn (1016–9 × 1018 cm−3) and grown by metalorganic chemical vapor deposition on semi-insulating β-Ga2O3 substrates are reported. Shallow donors attributable to Sn were observed only in a narrow region near the film/substrate interface and with a much lower concentration than the total Sn density. For heavily Sn doped films (Sn concentration, 9 × 1018 cm−3), the electrical properties in the top portion of the layer were determined by deep centers with a level at Ec-0.21 eV not described previously. In more lightly doped layers, the Ec-0.21 eV centers and deeper traps at Ec-0.8 eV were present, with the latter pinning the Fermi level. Low temperature photocapacitance and capacitance voltage measurements of illuminated samples indicated the presence of high densities (1017–1018 cm−3) of deep acceptors with an optical ionization threshold of 2.3 eV. Optical deep level transient spectroscopy (ODLTS) and photoinduced current transient spectroscopy (PICTS) detected electron traps at Ec-0.8 eV and Ec-1.1 eV. For lightly doped layers, the compensation of film conductivity was mostly provided by the Ec-2.3 eV acceptors. For heavily Sn doped films, deep acceptor centers possibly related to Ga vacancies were significant. The photocapacitance and the photocurrent caused by illumination at low temperatures were persistent, with an optical threshold of 1.9 eV and vanished only at temperatures of ∼400 K. The capture barrier for electrons causing the persistent photocapacitance effect was estimated from ODLTS and PICTS to be 0.25–0.35 eV.

Photovoltaic and photo-capacitance effects in ferroelectric BiFeO3 thin film

A polycrystalline BiFeO3 film on Pt/Ti/SiO2/Si was fabricated using the spin coating technique. The film shows diode-like characteristics with and without poling measured under dark conditions. However, it exhibits a switchable photovoltaic effect with light illumination under poled conditions. The measured photovoltaic effect revealed an open circuit voltage of ∼0.47 V and a short circuit current of 3.82 μA/cm2 under the illumination of 165 mW/cm2 irradiance. The studies clarified the dominant role of the depolarization field rather than the interface in the photovoltaic characteristics of the BiFeO3 film. Significantly, the photo-capacitance effect was demonstrated with a substantial enhancement in capacitance (∼45%) in Au/BiFeO3/Pt geometry, which could open up a new window for BiFeO3 applications.

Magneto-capacitance effects induced by air-generated traps in organic semiconductors

The magneto-capacitance effect of photoexcited organic diodes has been investigated for an air-treated polymer diode that is designed to generate plenty of trapped photocarriers. The capacitance in the dark condition is nearly independent of frequency, but the capacitance under photoirradiation is found to be frequency-dependent in the low frequency region. The observed photo-capacitance effect is suggested to result from photocarriers trapped inside the diode. The capacitance under photoexcitation is shown to change depending on the magnetic field, demonstrating magneto-capacitance effects occurring from air-treated organic diodes. The observed magneto-capacitance curve shows a rapid increase in the low magnetic field region. The feature suggests the magneto-capacitance effect to be induced via hyperfine interaction of carriers with surrounding nuclear spins.

Negative photoconductivity in films of alloys of II–VI compounds

Various negative photoelectric effects in films of alloys of II–VI compounds deposited from a solution are studied depending on the deposition mode and heat treatment. A combined electronic-molecular mechanism of the negative photocapacitance effect, which is for the first time found in Cd1 − xZnxS and CdS1 − xSex films, and negative slowly relaxing photoelectric effects is established. The latter are caused by the transition of electrons arranged in a nanoscale surface layer from shallow energy levels of attachment centers to deeper levels with lower polarizability and by the presence of nanoscale clusters playing the role of a “reservoir” for minority carriers in these materials. A model, which makes it possible to interpret the main regularities of negative photoconductivity in Cd1 − xZnxS and CdS1 − xSex films deposited from a solution, is suggested. It is established that the negative residual photoconductivity is explained on the basis of a double barrier profile, while the negative differential photoconductivity is explained by the presence of nanoscalel electric domains.

Photosensitive Capacitance Effect In High-purity Semi-insulating 4H-SiC

AbstractWe demonstrate a novel optically tunable photosensitive capacitor (PSC) made from high-purity semi-insulating 4H-SiC. Photosensitive capacitors can provide continuously variable reactive tuning in RF circuitry or enable capacitive-optical sensing applications. Unlike varactors, PSCs often do not require a DC bias voltage to operate. To demonstrate the effect, we fabricated several 1cm x 1cm square photocapacitor devices from bulk material using metal-evaporated Ti/Au contacts using a simple planar parallel-gap geometry. IV curves were taken of the devices using an HP-4145B semiconductor parameter analyzer to verify Schottky behavior as a function of DC bias. The samples were then illuminated with pulsed below-bandgap 470 nm and 590 nm high intensity LED light sources. The resulting data demonstrated an increase in capacitance, Cs, and a drop in resistance, Rs, with increasing optical intensity incident on the device. The observed shifts in both Cs and Rs were repeatable. At a measurement frequency of 33 kHz. Cs increased from its nominal value of 186.7 pF to 575.6 pF while Rs dropped from 150.0 kΩ to 22.4 kΩ. This demonstrates the existence of the photocapacitance effect in high-purity semi-insulating 4H-SiC and thus warrants further investigation. The underlying phenomenon of the effect is suspected to be light interaction with the dominant deep level traps through the Shockley–Read–Hall (SRH) recombination mechanism.

Capacitive effects in pinpin photodiodes

The application of a-SiC:H/a-Si:H pinpin photodiodes for optoelectronic applications as a WDM demultiplexer device has been demonstrated useful in optical communications that use the WDM technique to encode multiple signals in the visible light range. This is required in short range optical communication applications, where for costs reasons the link is provided by Plastic Optical Fibers. Characterization of these devices has shown the presence of large photocapacitive effects. By superimposing background illumination to the pulsed channel the device behaves as a filter, producing signal attenuation, or as an amplifier, producing signal gain, depending on the channel/background wavelength combination. We present here results, obtained by numerical simulations, about the internal electric configuration of a-SiC:H/a-Si:H pinpin photodiode. These results address the explanation of the device functioning in the frequency domain to a wavelength tunable photo-capacitance due to the accumulation of space charge localized at the bottom diode that, according to the Shockley–Read–Hall model, it is mainly due to defect trapping. Experimental result about measurement of the photodiode capacitance under different conditions of illumination and applied bias will be also presented. The combination of these analyses permits the description of a wavelength controlled photo-capacitance that combined with the series and parallel resistance of the diodes may result in the explicit definition of cut off frequencies for frequency capacitive filters activated by the light background or an oscillatory resonance of photogenerated carriers between the two diodes.

Impact of Photocapacitance on Phase Response of GaN/Sapphire SAW UV Sensor

The response of GaN/sapphire SAW delay line to the focused ultraviolet (UV) illumination was investigated at the SAW frequency 306 MHz and optical wavelength 375 nm. The UV irradiation of an interdigital transducer (IDT) led to 4 to 5 times larger phase change of transmitted signal than the irradiation of the surface acoustic wave (SAW) propagation path between the IDTs at the same UV light spot size of 2.4 mm. The UV-induced phase change has been attributed to the variation in IDT impedance due to photo-capacitive effect. Simulation of the SAW sensor phase response on the basis of this model is in good agreement with experimental results.

Wireless UV sensor based on photocapacitive effect in GaN

The LC oscillator with the interdigitated aluminium-on-GaN/sapphire photo capacitor has been demonstrated. Upon illumination with a 375 nm UV LED, an oscillator frequency downshift owing to the photo capacitive effect attains 5.2 MHz (3.5 of the dark frequency value 149 MHz) with sensitivity up to 170 kHz/( muW/cm 2 ). The oscillator can be employed as a visible-blind UV sensor with a remote wireless pickup of the RF output signal.

Low-frequency photodielectric phenomena in polycrystalline Pb3O4 layers

Photopolarization phenomena in the polycrystalline layers of Pb3O4 with a binder have been studied. The samples exhibited a positive photocapacitive effect, an increase in dielectric losses under illumination, and a low-frequency dispersion of dielectric parameters. Possible mechanisms of relaxation processes in the dark and under illumination and the nature of relaxators are discussed.