Photocurrent Oscillations Research Articles

Oscillations of photocurrent in the structure of a space-time electro-optical modulator in the presence of monopolar injection from illuminated contact

The effect of injection current on transient photocurrent is studied in the case of arbitrary light absorption in space-time electro-optical modulator in the presence of charge carrier trapping. Photocurrent oscillations caused by the generation of alternating space-charge regions in the form of domains that move from the collector to the emitter, are found. It is shown that the injection current stabilizes an electric field on the emitter.

Bulk photovoltaic effect modulated by ferroelectric polarization back-switching

Short-circuit photocurrent due to bulk photovoltaic effect displays an oscillatory dependence on the polarization state of light. Here, we explore how the ferroelectric polarization direction in h-LuMnO3 crystals affects the oscillating short-circuit photocurrent. It is shown that after prepoling the crystal at saturation, at remanence, the direction and amplitude of photocurrent oscillations are no longer dictated by prepoling voltage but are largely modulated by polarization back-switching, here ruled by the imprint field. Thus, the light polarization dependence of photocurrent is also ruled by the imprint field. The impact of these effects on the determination of the Glass coefficients of the material is discussed.

Photocurrent oscillations in natural dyes-based DSSCs with different mordant and assistants: Their role in oscillations and color stability

Cochineal was used as natural dye together with alum and tin as mordant and five assistants (also called brighteners): citric acid, acetic acid, oxalic acid, sodium metasilicate, and cream of tartar. The mordant improves the adhesion between the dye and the mesoporous, stabilizing and intensifying the color giving it firmness and fastness, with an increment in the lifetime. The cells, prepared using mordant and assistants, were illuminated with an abrupt transition dark-to-light; under these conditions, unstable oscillations appeared on top of the output current. These oscillations were produced because the cell was shifted far from equilibrium by the fast illumination; the dependence of the oscillations with the concentration of mordant and assistants is described; additionally, the color stability was determined based on the additive's concentration and the lifetime on the irradiation time. No other cells parameters were determined in this work.

The role of diffusion of photoexcited electrons from heavily doped layers in the photoconductivity of AlAs/GaAs heterostructures

Based on the study of photoconductivity in GaAs / AlAs p-i-n heterostructures in the visible light range, the dominant role of the diffusion channel of photoexcited electrons from heavily doped layers in the formation of photocurrent oscillations from the bias voltage and the determining contribution of this channel to the total current through the structure is shown. A qualitative model of the transport of excited carriers is considered, which assumes the diffusion channel as the main source of photooscillations. Keywords: heterostructures, photoconductivity.

Роль диффузии фотовозбужденных электронов из сильнолегированных слоев в фотопроводимости гетероструктур AlAs/GaAs

Based on the study of photoconductivity in GaAs / AlAs p-i-n heterostructures in the visible light range, the dominant role of the diffusion channel of photoexcited electrons from heavily doped layers in the formation of photocurrent oscillations from the bias voltage and the determining contribution of this channel to the total current through the structure is shown. A qualitative model of the transport of excited carriers is considered, which assumes the diffusion channel as the main source of photooscillations.

Peculiar spectra and photocurrent oscillation caused by laser interference in WX2 (X = S, Se) bubbles

We report the successful synthesis of WX2 (X = S, Se) crystals by chemical vapor deposition and fabrication of micron-sized bubbles on the WX2 crystals via a facile annealing process in Ar atmosphere. The optical and fluorescence images show evident interference oscillating rings in spherical cap-shaped WX2 bubbles, and such optical interference can cause intensity oscillations in photoluminescence (PL) and Raman mapping images, which are mainly attributed to the local thermal effect. Structure of this bubble profoundly affects the band structure of WX2, such as the appearance of direct bandgap in few-layer WSe2 bubbles. Similar to the monolayer WS2 bubble, the deconvoluted PL and Raman peaks of the WSe2 bubbles exhibit obvious oscillation in normalized integral intensity, position and width as the external pressure decreases. In addition, the photocurrent mapping images of the monolayer WS2 and bilayer WSe2 bubble devices unambiguously reveal the photocurrent oscillation phenomenon due to the optical interference enhancement or decrement on the suspended film. Our results open up a novel avenue in optoelectronics by manipulating 2D material bubbles.

Resonance peak shift in the photocurrent of ultrahigh-mobility two-dimensional electron systems

We report on a theoretical study on the rise of strong peaks at the harmonics of the cyclotron resonance in the irradiated magnetoresistance in ultraclean two-dimensional electron systems. The motivation is the experimental observation of a totally unexpected strong resistance peak showing up at the second harmonic. We extend the radiation-driven electron orbit model (previously developed to study photocurrent oscillations and zero resistance states) to a ultraclean scenario that implies longer scattering time and longer mean free path. Thus, when the mean free path is equivalent, in terms of energy, to twice the cyclotron energy ($2\hbar w_{c}$), the electron behaves as under an effective magnetic field twice the one really applied. Then, at high radiation power and/or low temperature, a resistance spike can be observed {\it at the second harmonic}. For even cleaner samples the energy distance could increase to three or four times the cyclotron energy giving rise to resistance peaks at higher harmonics (third, fourth, etc.), i.e., a resonance peak shift to lower magnetic fields as the quality of the sample increases. Thus, by selecting the sample mobility one automatically would select the radiation resonance response without altering the radiation frequency.

Multivariable Control of Solar Battery Power by Extremum Seeking: Starting from Linear Analysis

In this study, we tried to combine maximum power point trackers (MPPT) and «Extremum Seeking» in a single multi-parameter extremum seekeng system for orienting solar panels and draw attention to the problem of a deeper study of nonlinear adaptive control using appropriate methods for their analysis. MPPT controller becomes one of the extremum seeking loops, and as a result, the maximum power is achieved not only by searching for the optimal voltage value, but also due to the optimal angular position of the solar panel in Euclidean space, because the photocurrent depends on the angle of inclination of the Sun’s rays to the surface. The task of tuning extremum seeking loops becomes more analytically difficult, which is associated with nonlinear and multiply connected properties. This requires starting the solution from a simpler “linear” level. We applied the approach associated with the passage of modulating oscillations with a given frequency and amplitude through an open-loop system. This approach, which is generalized in this work at least for extremum seeking of the solar panels power, should be used for approximate calculations if there are no strict requirements for convergence and energy loss for the search. Research design is as follows: parametric identification of the current-voltage and volt-watt curves; obtaining the transfer function by the semi-automated sparse matrix method; reducing the order of the transfer function of coordinate electric drives by introducing a scaling factor. To the most important theoretical result, we attribute the property of the generalized amplitude of the solar panel power oscillations with multi-parameter control to be a combination of input modulating oscillations superimposed on the signals of the control integrators. Having revealed the relationship of their properties, it becomes possible to eliminate non-linearity from the system and operate only with the analytical relationship of the input modulating oscillations and the generalized oscillation of the controlled parameter. We attribute the prediction of the effect to one of the most interesting physical results, in which, for the same amplitude of modulating oscillations, the amplitudes of the photocurrent oscillations and the power of the solar panel at different angular positions will be generally different.

Magnetic ratchet effects in a two-dimensional electron gas

The effect of the magnetic field on the generation of an electric current in a two-dimensional electronic ratchet is theoretically studied. Mechanisms of the formation of magnetically induced photocurrent are proposed for a structure with a two-dimensional electron gas (quantum well, graphene, or topological insulator) with a lateral asymmetric superlattice consisting of metallic strips on the external surface of the structure. The ratchet with the spatially oscillating magnetic field generated by the ferromagnetic lattice, as well as the nonmagnetic ratchet placed in the uniform magnetic field both classically weak and strong quantizing, is considered. It is established that the ratio of the amplitude of the magnetic oscillations of photocurrent to the ratchet photocurrent in zero field can exceed two orders of magnitude.

Analysis of InGaN/GaN Multiple Quantum Well Heterostructures by Means of Photoconductivity Measurements

A novel method for the characterization of multiple quantum well(MQW) light emitting diodes(LEDs) is investigated. This method involves the measurement of photocurrent generation in an LED when irradiated with photons of energy near the absorption edge of the quantum wells. In this situation the I-V curve of the LED reveals a number of photocurrent oscillation zones corresponding to spatial variations in the heterostructure (Figure 1). The mechanism leading to the voltage-bias dependence of these features is hypothesized to be the expansion of the depletion (space-charge) region. Electrons and holes, from the N and P regions respectively, are drawn to the P-N junction as the reverse bias-voltage is increased. The area occupied by the ionized dopant atoms left behind by these charges is referred to as the ‘depletion region’. As the magnitude of the bias increases this area of depleted atoms becomes larger, engulfing the quantum wells in series. An electric field forms between the charges gathered at the P-N junction and the ionized atoms, giving rise to a potential gradient across the depletion region. The I-V features are then caused by the dynamics of the excited electrons as the potential gradient traverses each quantum well. A preliminary model is constructed by using the SIMS data (Figure 2) for the LED heterostructure supplied by Veeco. The spatial extent of the depleted region is calculated as a function of the applied bias using a doping profile derived from the SIMS data. This allows the estimation of the voltage at which each quantum well will become depleted and thus when a feature will be expected. The resulting curve is compared to the experimentally determined I-V feature spacing. The wavelength and temperature dependencies of this phenomenon are discussed, citing examples from existing literature. The benefits of this method of analysis are examined in terms of acquisition speed, measurement resolution and hardware cost with respect to current techniques such as capacitance-voltage measurement. Further research into this effect would involve the spectral transmission of the MQW devices as a function of voltage bias and the possible applications in spectrometry or optical filtering. A more complete picture of the quantum mechanical interaction between the incident photons, quantum wells, and potential gradient would also be valuable for the interpretation of the I-V curves and possible engineering applications.

Experimental investigation and numerical modelling of photocurrent oscillations in lattice matched Ga1−xIn x N y As1−y/GaAs quantum well p-i-n photodiodes

Photocurrent oscillations, observed at low temperatures in lattice-matched Ga1−x In x N y As1−y /GaAs multiple quantum well (MQW) p-i-n samples, are investigated as a function of applied bias and excitation wavelength and are modelled with the aid of semiconductor simulation software. The oscillations appear only at low temperatures and have the highest amplitude when the optical excitation energy is in resonance with the GaInNAs bandgap. They are explained in terms of electron accumulation and the formation of high-field domains in the GaInNAs QWs as a result of the disparity between the photoexcited electron and hole escape rates from the QWs. The application of the external bias results in the motion of the high-field domain towards the anode where the excess charge dissipates from the well adjacent to anode via tunnelling.

Carrier trapping and escape times in p-i-n GaInNAs MQW structures

We used a semi-classical model to describe carrier capture into and thermionic escape from GaInNAs/GaAs multiple quantum wells (MQWs) situated within the intrinsic region of a GaAs p-i-n junction. The results are used to explain photocurrent oscillations with applied bias observed in these structures, in terms of charge accumulation and resonance tunnelling.

Micro- and nanotopographies for photoelectrochemical energy conversion. I: The photovoltaic mode

The development of an efficient photovoltaic electrochemical solar cell with n-Si (1 0 0) based on the nanoemitter concept is described. The corresponding structure results from the self-organized formation of nanopores in about 10 nm thick silicon oxide during photocurrent oscillations in dilute fluoride containing solutions. The oscillations occur at higher anodic potentials and, due to the stress between Si and its anodic oxide, nanopores form in the oxide. Site selective metal electrodeposition (Pt) into those pores that have contact to the Si substrate and immersion into an acidic iodine-iodide redox electrolyte gives efficiencies in the range of 10%. Model experiments on the electrodeposition of Pt nanoparticles on n-Si (1 1 1) using photoelectron spectroscopy, performed with synchrotron radiation and atomic force microscopy in the contact- and tapping mode show that upon Pt deposition, silicon oxide is formed and step-bunching is observed. Mechanistic models are presented that explain the observed features. Scanning tunnelling experiments are performed with the tip on top of Pt nanoparticles. The resulting local I– V characteristics are interpreted based on a modified MOS (metal-oxide-semiconductor) model.

Ultralow-frequency photocurrent oscillation in ZnO nanowires

We report experimental results of ultralow frequency photocurrent oscillation in individual ZnO nanowires. Consecutive photocurrent and photoluminescence measurements corroborate the process of capture and release of photogenerated holes by surface trap states. The dynamic process results in the oscillation of the thickness of surface depletion region, which is believed to be responsible for the observed photocurrent oscillation.

Phase-Relations between Photocurrent and In Situ Reflectance during Photoelectrochemical Dissolution of Silicon

The phase relation between local maxima of photocurrents and those of in-situ Brewster-angle reflectance (BAR) is investigated upon dissolution of silicon photoelectrodes in diluted NH4F. In the region of photocurrent oscillations, charge flow and optical response can be related by a linear equation which explains the observed positive phase shift of the reflectance. In the transition regime, between porous silicon formation and electropolishing, commencing surface oxidation results in a negative phase shift of the reflectance. It is shown that this observation points to a change in the dissolution mechanism of the surface-near region. A step-function, convoluted with the charge flow, is applied to model the optical behavior. This approach is compared to optical multi-layer analysis in which simultaneously increasing sub-surface porosity and surface roughness are considered.

Photocurrent and Photovoltage Oscillations in the Two-Dimensional Electron System: Enhancement and Suppression of Built-In Electric Fields

We observe microwave-induced photocurrent and photovoltage oscillations around zero as a function of the applied magnetic field in high mobility GaAs 2D electron systems. The photosignals pass zero whenever the microwave frequency is close to a multiple of the cyclotron resonance frequency. They originate from built-in electric fields due to for instance band bending at contacts. The oscillations correspond to a suppression (screening) or an enhancement ("antiscreening") of these fields by the photoexcited electrons.

Model for Current Oscillations at the Si/Electrolyte Contact:Extension to Spatial Resolution

A morphological model is given for the anodic oxidation of n-type silicon in fluoride containing solution in a potentiostatic arrangement. An earlier introduced macroscopic version of the model is extended from temporal to spatio-temporal resolution using a cellular automate. The prediction of the macroscopic model that lattice mismatch between silicon and its oxide leads to stress and stress leads to two types of oxides in the case of photocurrent oscillation is approved by the cellular automate model. The oxide types differ in stability against the etching process. Both oxide types are arranged in clusters which alternate at the electrode surface. Each cluster is in the size of 80-100 nm.

Charge accumulation in ultrathin Cs / n - GaN and Cs / n - InGaN interfaces

We report on the observation of new phenomena that arise under Cs adsorption on n -GaN(0001) and n -InGaN(0001) surfaces. First, an extremely highly quantum efficient photoemission has been found by excitation with visible light in the transparency region of GaN and InGaN. The photoemission is revealed to appear due to the formation of an electron accumulation layer in the vicinity of the surfaces. Second, a large variety of band bending and potential wells are provided by the Cs coverages. The accumulated charge density at the n -InGaN surface is much stronger than that at the n -GaN surface. Third, a new effect is revealed, namely, the appearance of an oscillation structure in the spectral dependences of the threshold photoemission. A model concept is proposed for photocurrent oscillations that takes into account the formation of an accumulation layer and the multiple-beam interference in parallel-sided GaN or InGaN samples.

Brewster-angle analysis of native and photoelectrochemically grown silicon oxide nanotopographies

Brewster-angle reflectometry (BAR) is employed in-situ for the first time for investigation of the silicon oxide/silicon system during photocurrent oscillations in fluorine containing solution. Combining ex-situ Brewster-angle analysis (BAA) and atomic force microscopy (AFM) shows oxide thickness variations between 4.8 and 13.8 nm. The anodic oxide etch rate amounts to 0.1 nm s −1 in contrast to 0.01 nm s −1 for native oxide. The ambient/anodic oxide interface roughness, σ, is 2 ± 0.2 nm while σ for the oxide/silicon interface varies between 1.0 and 1.9 nm. The photocurrent onset, preceding regular current oscillations, is accompanied by roughness changes at the silicon oxide/silicon interface. A quantitative analysis of the overall system by discerning roughness and surface layer contributions is presented.

Ultra-low-frequency photocurrent self-oscillation in strained In xGa 1−xAs quantum well diodes

Ultra-low-frequency photocurrent (PC) self-oscillation has been investigated in an In 0.15Ga 0.85As/Al 0.15Ga 0.85As quantum-well (QW) diode in details as a function of temperature, excitation power and wavelength. The PC oscillation frequency increases with increasing temperature and illumination power at excitation wavelengths below the leading n=1 heavy-hole exciton resonance line under reverse bias conditions. The illumination wavelength dependence shows a clear evidence for beating due to two oscillators when photoexcitation by shorter wavelength below 1050 nm is used. These results suggest that the low-frequency PC self-oscillation with a characteristic frequency of about 0.01–0.1 Hz is caused by oscillating electric fields due to two types of photogenerated charge carriers trapped at deep localized centers within the QW regions.