Negative Photoconductivity Research Articles

Third order nonlinear optical, luminescence and electrical properties of ʟ-phenylalanine ʟ-phenylalaninium bromide bulk crystals

Optical quality ʟ-phenylalanine ʟ-phenylalaninium bromide (LPLPB) bulk crystals were grown by slow evaporation technique. The third order nonlinear refractive index and nonlinear absorption coefficient of the grown crystal were measured by Z-scan studies. The third order nonlinear susceptibility was found to be 1.8792 х 10−4 esu which is fairly higher than the other compounds. The Photoluminescence spectra reveal the emission bands for LPLPB crystals. The Photoconductivity studies were employed to determine the dependence of photocurrent on the applied electric field. Negative photoconductivity was exhibited by the sample. The d. c. conductivity of the grown crystal was measured by the complex impedance analysis wherein the obtained plot in the form of semicircle finds application in Debye relaxation for materials having large dc conductivity.

Negative Photoconductance in van der Waals Heterostructure-Based Floating Gate Phototransistor.

van der Waals (vdW) heterostructures made of two-dimensional materials have been demonstrated to be versatile architectures for optoelectronic applications due to strong light--matter interactions. However, most light-controlled phenomena and applications in the vdW heterostructures rely on positive photoconductance (PPC). Negative photoconductance (NPC) has not yet been reported in vdW heterostructures. Here we report the observation of the NPC in the ReS2/h-BN/MoS2 vdW heterostructure-based floating gate phototransistor. The fabricated devices exhibit excellent performance of nonvolatile memory without light illumination. More interestingly, we observe a gate-tunable transition between the PPC and the NPC under the light illumination. The observed NPC phenomenon can be attributed to charge transfer between the floating gate and the conduction channel. Furthermore, we show that control of NPC through light intensity is promising in realization of light-tunable multibit memory devices. Our results may enable potential applications in multifunctional memories and optoelectronic devices.

Growth, Hirshfeld surfaces, spectral, quantum chemical calculations, photoconductivity and chemical etching analyses of nonlinear optical p-toluidine p-toluenesulfonate single crystal.

A single crystal of p-toluidine p-toluenesulfonate (PTPT) has been grown by slow evaporation solution technique (SEST) at room temperature. Single crystal X-ray analysis confirms that grown crystal belongs to the monoclinic structure with space group P21. Intermolecular interactions and fingerprint plots of PTPT molecules are executed by Hirshfeld surface analysis. It was found that H···H (44.2%) contacts have maximum intermolecular interactions contributions in the total Hirshfeld surface area. The characteristic absorption band occurs at below 290 nm. The functional groups were identified using FTIR and FT-Raman analyses and compared with theoretical values. The title molecule contains fourteen CH bonds and three OH bonds. The calculated HOMO and LUMO energy values are -6.125 eV and -1.157 eV, respectively. The chemical potential (μ) and electronegativity (χ) values are estimated to be -3.4938 eV and 3.4938 eV, respectively. The strongest negative hyperconjugation occurs due to the charge transfer from the occupied orbital (σ) to the unoccupied orbital (π*) which is calculated for the σ(N20-C21) → π*(N20-O18). The green and red lines in the total density of states (TDOS) spectrum indicate the occupied orbital and virtual orbital levels, respectively. Photoconductivity studies have been done for the grown crystal. It is observed that the dark current is greater than photocurrent. It shows negative photoconductivity nature of PTPT crystal. The etching analysis was executed on (001) plane of PTPT crystal. It has rectangular shape etch pits patterns.

Frequency-driven bulk-to-surface transition of conductivity in ZnO nanowires

The frequency of an applied AC voltage dramatically affects the nature of conductivity in zinc oxide nanowires. The nanowires exhibit a continuous transition from positive to negative photoconductivity as a function of the driving frequency with the characteristic transition frequency observed at 48.9 kHz. In addition, ZnO nanowires display a frequency-driven metal-insulator transition at room temperature, which appears almost at the same critical frequency. The responsible mechanism for both transitions is attributed to the switching of conductivity from bulk to surface conduction. A theory is presented describing the experimental observations. The frequency-driven bulk-to-surface transition of conductivity is expected to be a generic character for a broad range of semiconductor nanostructures with the large surface-to-volume ratio.

Ar-Plasma-Modulated Optical Reset in the SiO2/Cu Conductive-Bridge Resistive Memory Stack

Resistive Random Access Memory (RRAM) has been widely studied as a promising next-generation memory with advantageous characteristics such as high-speed and low-power operation[1]. In particular, the SiO2/Cu conductive-bridge random access memory (CBRAM), where electric-field-driven formation/dissolution of Cu metal filaments provides resistance switching, has been drawing much attention because of its compatibility with the backend interconnect process. Recently, we have demonstrated negative photo conductivity (NPC) or light-induced resistance reset (LIR) in the SiO2/Cu stack by disrupting the electrically-formed filamentary breakdown path[2]. It is considered that the NPC is caused by recombination of light-excited oxygen ions and oxygen vacancies created in the breakdown path during electrical forming. Although the exact mechanism is still elusive, the finding suggests a potential to realize an electro-optical control of memory devices. In our pursuit of the mechanism(s) behind the abovementioned NPC phenomenon, we reveal here an impact on NPC by Ar plasma surface treatment, which is reported to increase oxygen vacancies in the oxide electrolyte[3]. The effect of light illumination on the electrical property of the Ar-plasma-treated Cu sample (SiO2 10nm/Cu) and control sample (SiO2 5nm/Si) is investigated via ultra-high vacuum conductive atomic force microscope or CAFM (Fig. 1). During forming, a negative bias voltage is supplied to the probe with a preset current compliance of 1.1 nA. The area of the probe-oxide contact is estimated to be ~26nm2[4]. From the current-voltage (I-V) curves for the Cu or control sample depicted in Fig. 2, it is evident that the switching voltage from the high resistance state (HRS) to the low resistance state (LRS) in the Cu sample is shifted from -6V to -4V by 33% after the Ar plasma treatment, while the breakdown voltage in the control sample is almost unchanged. Therefore, the electrical formation of Cu filaments is considered to be accelerated by Ar plasma treatment. Fig. 3 shows typical response to light in the post-forming Cu sample and percentage of NPC occurrence calculated from 30 tested locations in each of the Cu and control sample. The light intensity used was 1 mW/cm2. Clearly, Ar plasma treatment suppresses the NPC in both samples and especially enlarges the proportion of no-NPC cases in the Cu sample. The FT-IR spectra (Fig. 4) of the pre- and post-plasma-treated SiO2 film show the intensities of HO-H stretching (3300cm-1) and SiO-H stretching (3650cm-1) become higher after the Ar plasma treatment. This rise implies the destruction of the Si-O network near the surface and the increase of various defects like Si dangling bonds and oxygen vacancies where water in the air may be adsorbed. Based on the results, we propose a possible explanation for the mechanism of the lower switching voltage in the Cu sample and the decreased percentage of NPC occurrence with Ar plasma treatment in both samples. For the control sample, Ar plasma treatment may sputter oxygen ions and create more oxygen vacancies near the surface of SiO2 [3], which helps forming a larger conducting filament composed of oxygen vacancies. This can be confirmed by a higher current in the reverse I-V curve after Ar plasma exposure (Fig. 2 (c)(d)). Since oxygen ions are depleted, the NPC response (due to recombination of light-excited oxygen ions and vacancies) is suppressed. In the case of the Cu sample, the oxygen vacancies created in the surface of SiO2 by Ar sputtering may adsorb water molecules. The resultant hydroxyl groups (OH-) reportedly promote Cu ion (Cu+) diffusion into SiO2 [5] and reduce the switching voltage during forming (Fig. 5). When more Cu ions migrate into SiO2, part of the conducting filament made up of oxygen vacancies there is replaced by a Cu filament (Fig. 5 (e)), which is not readily disrupted due to the stronger metal-metal bond (which requires higher energy for cleavage than that of the light used[2]). Therefore, the conducting path composed of more Cu metal filaments remain after light illumination (Fig.5 (f)), leading to the suppression of the NPC response.

Ar-Plasma-Modulated Optical Reset in the SiO2/Cu Conductive-Bridge Resistive Memory Stack

Our study using conductive atomic force microscope shows that the resistive switching voltage in the SiO2/Cu stack is reduced by 33% after Ar plasma treatment of the oxide. Besides, the negative photo-conductivity (NPC) effect, normally observed on many locations following electrical soft-breakdown, is suppressed. The NPC effect arises because the electrically-formed filamentary conductive path, comprising both Cu and oxygen vacancies, may be disrupted by the recombination of the vacancies with nearby light-excited interstitial oxygen ions. Increase of the O-H peak, as seen from FT-IR spectroscopy, indicates that surface defects generated by the Ar plasma may have adsorbed water molecules, which in turn act as counter anions (OH−) accelerating Cu-ion diffusion into the oxide, forming a more complete Cu filament that is non-responsive to light. The finding offers the possibility of both electrical and optical resistance control by a simple surface treatment step.

Photo-Hall-Effect Spectroscopy with Enhanced Illumination in p−Cd1−xMnxTe Showing Negative Differential Photoconductivity

We studied deep levels (DLs) in p-type CdMnTe by photo-Hall effect spectroscopy with enhanced illumination. We showed that the mobility of minority and majority carriers can be deduced directly from the spectra by using proper wavelength and excitation intensity. Four deep levels with ionization energies 0.63 eV, 0.9 eV, 1.0 eV and 1.3 eV were detected and their positions in the bandgap were verified by comparison of photogenerated electron and hole concentrations. Deduced DL model was analyzed by numerical simulations with Shockley-Reed-Hall charge generation-recombination theory and compared with alternative DL models differing in the position of selected DLs relative to Ec and Ev. We showed that the consistent explanation of collected experimental data principally limits the applicability of alternative DLs models. We also demonstrated the importance of the extended operation photon fluxes used in the spectra acquisition for correct determination of DLs character. Negative differential photoconductivity was observed and studied by charge dynamic theoretical simulations.

Surface chemistry and buried interfaces in all-inorganic nanocrystalline solids.

Semiconducting nanomaterials synthesized using wet chemical techniques play an important role in emerging optoelectronic and photonic technologies. Controlling the surface chemistry of the nano building blocks and their interfaces with ligands is one of the outstanding challenges for the rational design of these systems. We present an integrated theoretical and experimental approach to characterize, at the atomistic level, buried interfaces in solids of InAs nanoparticles capped with Sn2S64- ligands. These prototypical nanocomposites are known for their promising transport properties and unusual negative photoconductivity. We found that inorganic ligands dissociate on InAs to form a surface passivation layer. A nanocomposite with unique electronic and transport properties is formed, that exhibits type II heterojunctions favourable for exciton dissociation. We identified how the matrix density, sulfur content and specific defects may be designed to attain desirable electronic and transport properties, and we explain the origin of the measured negative photoconductivity of the nanocrystalline solids.

Optical tuning of the charge carrier type in the topological regime of InAs/GaSb quantum wells

We study the optical tunability of the charge carrier type in InAs/GaSb double quantum wells with its type-II broken band alignment and inverted band structure. Under constant optical excitation, the majority charge carrier type switches from electron to hole. Within the majority charge carrier type transition, the coexisting minority charge carrier contribution indicates electron-hole hybridization with a nontrivial topological insulating phase. The optical tuning is attributed to the negative photoconductivity of antimonide materials in combination with a persistent charge carrier buildup of photogenerated charges at the surface and substrate side of the device, respectively. Our study of the tuning of an InAs/GaSb double quantum well heterostructure reveals that an electro-optical switching is possible and paves the way to an optical control of the phase diagram of InAs/GaSb topological insulators.

Investigation on the key aspects of l-arginine para nitrobenzoate monohydrate single crystal: A non-linear optical material

Abstract In the contemporary days, non-linear optical materials (NLO) are getting higher attention as per the increasing demand in optical communication and data storage technology. Consequently, they explore various physical properties of the NLO material for their industrial applications. In current report, l-arginine p-nitrobenzoate monohydrate (LANB) single crystals were obtained by adopting slow cooling technique. The structural confirmation of grown single crystal was carried out using single crystal X-Ray diffractometer. Presence of strain within single crystal was deduced using Hall–Williamson relation. Various defects associated within single crystal were assessed using high resolution XRD. Additionally, its crystalline quality was again confirmed by time resolved photoluminescence spectroscopy (TRPL). Using transmission spectra, the cut off wavelength and band gap of the host material was determined to be nearly 420 nm and 2.9 eV respectively. The dielectric property has been recorded by varying the frequency ranging from 50 Hz to 100 kHz. The curve suggests that dielectric loss value is less at high frequency ensures the suitability of crystal in photonics and NLO based devices. The laser damage threshold values were also measured for single and multiple shots. Thermal parameters of the titled compound were calculated using PPE. In photoconductivity measurement, negative photoconductivity has been observed in titled compound. Microhardness studies were also performed on single crystal to explore its mechanical properties.

Non-volatile memory and negative photoconductivity in a metal-insulator-semiconductor diode with embedded Co nanoparticles

We describe a new metal-insulator-semiconductor (MIS) device in which cobalt based nano particles (NPs) in a core-shell structure (Co–core and Co3O4-shell) are embedded between a thermally grown SiO2 layer and a HfO2 film deposited by atomic layer deposition. Two additional structures were prepared for comparison. One had no NPs and the other included the Fe NPs, prepared using the same procedure as used for the Co film. All devices exhibited the classic behavior of a voltage variable MIS capacitor with or without a large hysteresis as in non-volatile memory (NVM) systems. However, only the device with the Co core-shell structure exhibits a negative photoconductivity (NPC) effect as well as NVM capabilities in both the capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The dependence of C-V and current voltage I-V characteristics on illumination intensity and wavelength (from ultraviolet to near infrared) as well as on temperature was characterized. Illumination enhances the NPC effect as well as the flat-band voltage shift determined from C-V characteristics and hence the memory width. Illumination in the wavelength range of 735–780 nm caused a current decrease, at a given voltage, by up to a factor of two. The NPC effect stimulates an annihilation of the stored charges and therefore erases the system instantly at a small applied bias. The main cause of the NPC effect under illumination is the photo excitation of supplementary trap channels in the Co3O4 shell, which lowers the free carrier density and hence the conductivity of the MIS structure.

Argon-plasma-controlled optical reset in the SiO2/Cu filamentary resistive memory stack

We show that resistive switching in the SiO2/Cu stack can be modified by a brief exposure of the oxide to an Ar plasma. The set voltage of the SiO2/Cu stack is reduced by 33%, while the breakdown voltage of the SiO2/Si stack (control) is almost unchanged. Besides, the Ar plasma treatment suppresses the negative photoconductivity or optical resistance reset effect, where the electrically formed filamentary conductive path consisting of Cu-ion and oxygen-vacancy clusters is disrupted by the recombination of the oxygen vacancies with nearby light-excited oxygen ions. From the enhanced O-H peak in the Fourier-transform infrared spectrum of the plasma-treated oxide, it is proposed that the Ar plasma has created more oxygen vacancies in the surface region of the oxide. These vacancies in turn adsorb water molecules, which act as counter anions (OH-) promoting the migration of Cu cations into the oxide and forming a more complete Cu filament that is less responsive to light. The finding points to the prospect of a control over the optical resistance reset effect by a simple surface treatment step.

New photoelectrical properties of InN: Interband spectra and fast kinetics of positive and negative photoconductivity of InN

Using a Fourier transform spectrometer and a pulsed laser, we study spectra and kinetics of positive and negative photoconductivity (PC) along with other types of photoexcitation spectra of n-type InN epilayers with Hall concentrations from 3 × 1017 to 5 × 1019 cm−3 at room and low temperatures. The PC, absorption, and photoluminescence spectra of the investigated InN samples correspond to a degenerate homogenous direct band semiconductor. In contrast to previous works, we observed positive and negative PC with relaxation times in the nano- and microsecond ranges. The sign of PC depends on the temperature and the equilibrium carrier concentration.

Negative photoconductivity in metal-organic potassium oxalate oxalic acid dihydrate single crystal

The single crystals of potassium oxalate oxalic acid dihydrate (KH3(C2O4)2.2H2O) were synthesized using the slow solvent evaporation technique at room temperature. The unit cell parameters of the crystal were determined using single crystal X-ray diffraction technique. The crystal belongs to triclinic crystal system, –P1 space group. The structural analysis of synthesized crystal was carried out using powder X-ray diffraction technique. The surface morphology of the crystal was studied using Field Emission Scanning Electron Microscope (FESEM). The elemental composition was determined and mapped using Energy Dispersive X-ray (EDAX) analysis. The crystal was studied using UV-visible transmission spectrum. The photoluminescence properties of the crystal were studied by recording the emission spectrum. The thermal stability of the crystal was analyzed using Differential Scanning Calorimetry (DSC). Furthermore, light induced I-V studies reveal the probability of the synthesized crystal in photo-resistance applications.

Non-equilibrium electron transport induced by terahertz radiation in the topological and trivial phases of Hg1-x Cd x Te.

Terahertz photoconductivity in heterostructures based on n-type Hg1−xCdxTe epitaxial films both in the topological phase (x < 0.16, inverted band structure, zero band gap) and the trivial state (x > 0.16, normal band structure) has been studied. We show that both the positive photoresponse in films with x < 0.16 and the negative photoconductivity in samples with x > 0.16 have no low-energy threshold. The observed non-threshold positive photoconductivity is discussed in terms of a qualitative model that takes into account a 3D potential well and 2D topological Dirac states coexisting in a smooth topological heterojunction.

Tuning positive and negative photoconductivity in Zn1-xCdxO films

Photoconductivity investigation is performed on Zn1-xCdxO films and a transition from positive to negative photoconductivity is observed at room temperature as Cd concentration x is varied. Low and high Cd-content samples are studied by temperature dependent photoconductivity measurements in the temperature range of 77–300 K. Our results exhibited additional transitions manifested by an anomalous persistent effect on sample with x ∼ 0.25 and a transition from negative to positive photoconductivity for sample with x ∼ 0.75 as temperature changes. The observed negative photoconductivity effect is attributed to trap levels while the anomalous behavior of persistent photoconductivity amplitude is caused by random local potential fluctuations, responsible to spatial separation of electrons and holes at high temperatures. This interpretation is further supported by photoluminescence and electrical resistance measurements.

On the Transient Negative Photoconductance in &lt;italic&gt;n&lt;/italic&gt;-type Czochralski Silicon

A transient negative photoconductance phenomenon is observed in n -type Czochralski silicon wafers. After a pulsed light excitation, the measured photoconductance drops to negative values and then increases to the original zero. The paper details how transient negative photoconductance is predicted by the Shockley–Read–Hall rate equations for a system of two defects. For this to occur, one defect must act as a majority carrier trap, and the other must be a relatively faster recombination channel. The specific conditions of these defects for a negative photoconductance to occur are derived analytically. Numerical modeling of this system shows good agreement with the experimental results. Finally, a method to extract the trap parameters from negative photoconductance for a sample with temperature-independent capture cross sections is demonstrated.

Growth and physicochemical properties of organometallic (DL)-trithioureatartrato-O1,O2,O3-cadmium(II) single crystals

Single crystals of organometallic (DL)-trithioureatartrato-O1,O2,O3-cadmium(II) (TUDLC) have been grown from methanol solution by using the slow evaporation of solvent growth technique. The lattice structure and crystalline perfection have been determined by carrying out single crystal X-ray diffraction and high resolution X-ray diffraction measurements. The grown crystal was characterized thermally and mechanically by carrying out thermo-gravimetric and micro hardness measurements. The linear and nonlinear optical characterizations were made by carrying out optical transmittance, surface laser damage threshold, particle size-dependent second harmonic generation (SHG) efficiency and photo conductivity measurements. The grown crystal was electrically characterized by carrying out frequency-dependent dielectric measurements. Chemical etching study was also carried out and the dislocation density was estimated. Results obtained in the present study indicate that the grown TUDLC crystal is optically transparent with lower cut-off wavelength 304 nm, mechanically soft, thermally stable up to 101 °C and NLO active with SHG efficiency 2.13 (in KDP unit). The grown crystal is found to have considerably large size, good crystalline perfection, large specific heat capacity, higher surface laser damage threshold and negative photoconductivity.

Growth and characterization of benzyl 4-hydroxybenzoate single crystal by vertical Bridgman technique for optical applications

The benzyl 4-hydroxybenzoate single crystal has been grown by vertical Bridgman technique. The grown crystal was confirmed by single crystal X-ray diffraction studies. The presence of functional groups in the crystal was confirmed by Fourier transform infrared (FTIR) spectral studies. The thermal behaviour of the grown crystal was analyzed by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetric (DSC) studies. Optical behaviour of the grown benzyl 4-hydroxybenzoate crystal was studied by UV–Vis-NIR spectral analysis. Fluorescence spectrum shows near violet light emission. The second harmonic generation behaviour of benzyl 4-hydroxybenzoate was analyzed. The laser damage threshold value of benzyl 4-hydroxybenzoate was measured as 2.16 GW/cm2. The dielectric measurements of benzyl 4-hydroxybenzoate crystal were carried out with different frequencies 1 kHz to 1 MHz versus different temperatures ranging from 313 to 353 K. Photoconductivity study shows that the grown benzyl 4-hydroxybenzoate crystal belongs to negative photoconductivity property. The mechanical strength of the crystal was calculated by Vickers microhardness study.

Optically reversible electrical soft-breakdown in wide-bandgap oxides—A factorial study

In an earlier work, we found that an electrical soft-breakdown region in wide-bandgap oxides, such as hafnium dioxide, silicon dioxide, etc., could be reversed when illuminated by white light. The effect is evidenced by a decrease in the breakdown leakage current, termed as a negative photoconductivity response. This finding raises the prospect for optical sensing applications based on these traditionally non-photo-responsive but ubiquitous oxide materials. In this study, we examine the statistical distribution for the rate of breakdown reversal as well as the influence of factors such as wavelength, light intensity, oxide stoichiometry (or oxygen content) and temperature on the reversal rate. The rate of breakdown reversal is shown to be best described by the lognormal distribution. Light in the range of ∼400–700 nm is found to have relatively little influence on the reversal rate. On the other hand, light intensity, oxygen content and temperature, each of them has a clear impact; a stronger light intensity, an oxide that is richer in oxygen content and a reduced temperature all speed up the reversal process substantially. These experimental results are consistent with the proposed phenomenological redox model involving photo-assisted recombination of the surrounding oxygen interstitials with vacancy defects in the breakdown path.