Photoconductivity Data Research Articles

All-optical broadband terahertz modulator based on CdS NWs/Si heterojunction and interface photoconductivity analysis

A high-performance, low-cost terahertz modulator is fabricated by spin-coating CdS nanowires onto silicon. This all-optical modulator achieves broadband modulation (0.4–1.6 THz) with a significant depth of 85% at a low power density (2 W cm−2), exceeding bare silicon by fourfold. THz-TDS confirms its terahertz amplitude modulation capability. Tests of the simple photonic switch further illustrate the modulator’s potential for carrying information on a terahertz transmission wave. The modulation mechanism is explained through energy band theory and photoconductivity data. This work also presents a novel method for probing heterostructure formation using THz-TDS with laser irradiation.

The effects of sub-bandgap transitions and the defect density of states on the photocurrent response of a single ZnO-coated silica nanospring

The electrical and optoelectronic properties of nanometer-sized ZnO structures are highly influenced by its native point defects. Understanding and controlling these defects are essential for the development of high-performance ZnO-based devices. Here, an electrical device consisting of a polycrystalline ZnO-coated silica nanospring was fabricated and used to characterize the electrical and photoconductive properties of the ZnO layer using near-UV (405 nm) and sub-bandgap (532 and 633 nm) excitation sources. We observe a photocurrent response with all three wavelengths and notably with 532 nm green illumination, which is the energy associated with deep oxygen vacancies. The polycrystalline ZnO-coated silica nanospring exhibits a high responsivity of 1740 A W−1 with the 405 nm excitation source. Physical models are presented to describe the photocurrent rise and decay behavior of each excitation source where we suggest that the rise and decay characteristics are highly dependent on the energy of the excitation source and the trapping of electrons and holes in intermediate defect levels in the bandgap. The energy levels of the trap depths were determined from the photoconductive decay data and are matched to the reported energy levels of singly and doubly ionized oxygen vacancies. A phenomenological model to describe the dependence of the saturation photocurrent on excitation intensity is presented in order to understand the characteristics of the observed breaks in the slopes of the saturation photocurrent versus excitation intensity profile.

Terahertz photoconductivity and photocarrier dynamics in graphene–mesoporous silicon nanocomposites

We investigate charge transport and photocarrier dynamics in graphene--mesoporous silicon nanocomposites using optical-pump terahertz-probe measurements. The nanocomposite material consists of a free-standing mesoporous silicon membrane whose specific surface is coated with a few-layer graphene shell. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial decay time of 5 ps and a longer decay time of about 25 ps. These decay times are significantly reduced compared to the values of ${\ensuremath{\tau}}_{1}\ensuremath{\sim}74$ ps and ${\ensuremath{\tau}}_{2}\ensuremath{\sim}730$ ps obtained for the uncoated mesoporous silicon membrane and this is attributed to the introduction of additional surface defects formed during the graphene deposition process. Based on the influence of the laser fluence on the time-resolved differential transmission curves, a capture/recombination model is proposed to describe the photocarrier dynamics in these nanocomposite materials. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz waveforms taken at different optical-pump THz-probe delays. These data curves are well reproduced using a modified Drude-Smith model taking into account diffusive-restoring currents. The $c$ parameter of this model, which describes the degree of carrier localization, is about $\ensuremath{-}0.73$ for the uncoated porous Si membrane and is approaching $\ensuremath{-}1$ for graphene--mesoporous Si nanocomposites formed at temperatures above $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. For all the nanocomposites, the characteristics of the photoconductive material, in terms of photocarrier capture/recombination time and effective mobility, are of interest for the fabrication of pulsed terahertz devices.

The determination of carrier lifetimes and associated mobility magnitudes using photoconductivity recovery dynamics in thin-film amorphous semiconductors

The extraction of trap-limited mobility information using photoconductivity relaxation dynamics in thin-film amorphous semiconductors is evaluated. Simulated photoconductivity dynamics suggest that free carrier decay is characterised by a spectrum of recombination lifetimes, where the spectra are sensitive to the underlying distribution of shallow trap densities and the experimental photoconductivity generation conditions employed. Weighted averaging of the spectral lifetimes is found to return trap-limited lifetime estimates which agree to within 5% of the lifetime values expected from free-to-trapped carrier ratios established under steady-state photoconductivity conditions. The averaged spectral lifetimes may consequently be used with mobility-lifetime magnitudes to determine the associated trap-limited mobility. The analysis procedure is evaluated using photoconductivity data obtained from a set of a-Si:H films deposited over a range of substrate temperatures.

A new approach for determination of free carriers lifetime and density of localised states in disordered semiconductors

ABSTRACTA new method for measuring the free carriers lifetime () and the density of localised states (DOS) in amorphous semiconductors is described. The method is based on the analysis of transient photoconductivity (TPC) data using Laplace transform technique. This technique involves Laplace transform of TPC data, it is a simple analysis of the solution of the basic linearised multiple trapping equations for free and trapped electrons. It is demonstrated by application to simulated and experimental TPC data measured on a typical disordered semiconductor, the hydrogenated amorphous silicon (a-Si:H). An introduced in the computing of the TPC using an arbitrarily proposed DOS model is recovered with high accuracy. For the experimental case, the determination of the exact DOS and the estimated from the experimental TPC data allow to reconstruct accurately this last. The performance and limitations of the technique are studied by means of computer simulations. Limitations are essentially the low temperatures of measurement of TPC when the recombination phenomenon is not observed at short times.

Influence of Defects on Photoconductivity and Photocatalytic Activity of Nitrogen-Doped Titania

Samples of nitrogen-doped titanium dioxide (anatase, $0.2 \leq N \leq 1.0~wt\%$) prepared by the sol-gel method were investigated using X-band EPR spectroscopy, photoconductivity and photocatalysis measurements. $N\bullet$ and $NO\bullet$ paramagnetic defects in N-TiO2 have been observed; their concentrations and spin-Hamiltonian parameters were calculated. An increase both in the rate of the generation of free charge carriers and in the rate of photocatalysis was found in N-TiO2 in contrast with non-doped titania under visible light. The correlation of the density of the observed radicals with the photoconductivity and photocatalysis data is discussed.

Study of the impurity photoconductivity in p-InSb using epitaxial p + contacts

The optical absorption coefficient α in p +-InSb layers (with hole concentrations of p ≈ 1 × 1017–1.2 × 1019 cm–3), grown by liquid-phase epitaxy on p-InSb substrates, is measured in the spectral range of 5-12 µm at 90 K, and the impurity photoconductivity is measured (at 60 and 90 K) in p +–p structures. It is found that a in the p + layers reaches a value of 7000 cm–1 (at p ≈ 2 × 1019 cm–1). It is shown that the measured substrate value of (α ≈1–3 cm–1) is overestimated in comparison with estimates (α ≈ 0.1 cm–1) based on comparing the photoconductivity data. This discrepancy is explained by the fact that the optical transitions of holes responsible for photoconductivity are obscured by the excitation of electrons to the conduction band. The photoionization cross section for these transitions does not exceed 1 × 10–15 cm2.

Optical and photoelectrical properties of nanocrystalline indium oxide with small grains

Optical properties, spectral dependence of photoconductivity and photoconductivity decay in nanocrystalline indium oxide In2O3 are studied. A number of nanostructured In2O3 samples with various nanocrystals size are prepared by sol–gel method and characterized using various techniques. The mean nanocrystals size varies from 7 to 8nm to 39–41nm depending on the preparation conditions. Structural characterization of the In2O3 samples is performed by means of transmission electron microscopy and X-ray powder diffraction. The combined analysis of ultraviolet–visible absorption spectroscopy and diffuse reflectance spectroscopy shows that nanostructuring leads to the change in optical band gap: optical band gap of the In2O3 samples (with an average nanocrystal size from 7 to 41nm) is equal to 2.8eV. We find out the correlation between spectral dependence of photoconductivity and optical properties of nanocrystalline In2O3: sharp increase in photoconductivity was observed to begin at 2.8eV that is equal to the optical bandgap in the In2O3 samples, and reached its maximum at 3.2–3.3eV. The combined analysis of the slow photoconductivity decay in air, vacuum and argon, that was accurately fitted by a stretched-exponential function, and electron paramagnetic resonance (EPR) measurements shows that the kinetics of photoconductivity decay is strongly depended on the presence of oxygen molecules in the ambient of In2O3 nanocrystals. There is the quantitative correlation between EPR and photoconductivity data. Based on the obtained data we propose the model clearing up the phenomenon of permanent photoconductivity decay in nanocrystalline In2O3.

Fabrication of semiconducting pyrite thin films from hydrothermally synthesized pyrite (FeS2) powder

Thin films of pyrite (FeS2) were fabricated using synthesized nano-particles and their structural, electrical and optical properties were characterized. Initially, pyrite-nano-particles were synthesized by an aqueous hydrothermal reaction. The X-ray diffraction confirmed that synthesized pyrite had a phase pure cubic pyrite structure with the average grain size around 30.0 nm. The pyrite films were fabricated by vacuum thermal evaporation of pyrite nanoparticles followed by a thermal sulfurization process. The as-deposited films have an amorphous structure and the sulfurized films have a cubic pyrite structure. The optical properties exhibited the bandgap closer to previously reported bandgap of pyrite. The photoconductivity of pyrite films was measured under AM1.5G sunlight and infrared (850 nm) light illumination. The photoconductivity data showed that the sulfurized pyrite films were more photoconductive than the as-deposited films.

TlHgInS3: An Indirect-Band-Gap Semiconductor with X-ray Photoconductivity Response

The quaternary compound TlHgInS3 crystallizes in a new structure type of space group, C2/c, with cell parameters a = 13.916(3) A, b = 3.9132(8) A, c = 21.403(4) A, β = 104.16(3)°, V = 1130.1(8) A3, and ρ = 7.241 g/cm3. The structure is a unique three-dimensional framework with parallel tunnels, which is formed by ∞1[InS33–] infinite chains bridged by linearly coordinated Hg2+ ions. TlHgInS3 is a semiconductor with a band gap of 1.74 eV and a resistivity of ∼4.32 GΩ cm. TlHgInS3 single crystals exhibit photocurrent response when exposed to Ag X-rays. The mobility-lifetime product (μτ) of the electrons and holes estimated from the photocurrent measurements are (μτ)e ≈ 3.6 × 10–4 cm2/V and (μτ)h ≈ 2.0 × 10–4 cm2/V. Electronic structure calculations at the density functional theory level indicate an indirect band gap and a relatively small effective mass for both electrons and holes. Based on the photoconductivity data, TlHgInS3 is a potential material for radiation detection applications.

Conductivity and magnetoresistance of La0.7Ce0.3MnO3−δ thin films under photoexcitation

La0.7Ce0.3MnO3 thin films of different thicknesses, degrees of CeO2-phase segregation and oxygen deficiency, grown on SrTiO3 single crystal substrates, were comparatively investigated with respect to both their spectral and temperature-dependent photoconductivity (PC) and their magnetoresistance (MR) behaviour under photoexcitation. While as-grown films were insensitive to optical excitation, oxygen reduction appeared to be an effective way to decrease the film resistance, but the film thickness was found to play a minor role. However, from the evaluation of the spectral behaviour of the PC and the comparison of the MR of the LCeMO/substrate-samples with a bare substrate under illumination we find that the photoconductivity data reflects not only contributions from (i) photogenerated charge carriers in the film and (ii) carriers injected from the photoconductive substrate (as concluded from earlier works), but also (iii) a decisive parallel photoconduction in the SrTiO3 substrate. Furthermore—also by analyzing the MR characteristics—the unexpected occurence of a strong electroresistive effect in the sample with the highest degree of CeO2 segregation and oxygen deficiency could be attributed to the electroresistance of the SrTiO3 substrate as well. The results suggest a critical reconsideration and possibly a reinterpretation of several previous photoconductivity and electroresistance investigations of manganite thin films on SrTiO3.

Impact ofn-type doping on the carrier dynamics of silicon nanowires studied using optical-pump terahertz-probe spectroscopy

The n-doped and undoped silicon nanowires grown by chemical vapor deposition on quartz substrate were characterized using optical-pump terahertz-probe transmission experiments. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial photocarrier lifetime of ∼2 ps and a longer decay time of 10 ps to a few tens of picoseconds. Based on the influence of the laser fluence, a carrier capture and recombination scenario is proposed to explain these temporal decay curves. For both samples, the capture of photocarriers by the traps present on the surface of the nanowires plays an important role in the observed photoconductivity dynamics. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz (THz) traces taken at different optical-pump THz-probe delays. These data curves are reproduced using a plasmon resonance model. The fitting procedure allows us to determine carrier scattering times of about 28±6 and 14±4 fs for the undoped and doped samples, respectively. Our results show that defects and ionized impurities introduced by n doping the silicon nanowires reduce the photocarrier mobility and lifetime.Received 2 January 2014DOI:https://doi.org/10.1103/PhysRevB.89.115316©2014 American Physical Society

Anion vacancies in II–VI chalcogenides: Review and critical analysis

We performed critical analysis and comparison of all EPR, photo-EPR, photosensitive optical absorption, photoluminescence, and photoconductivity data taken on various Zn- and Cd-related II–VI chalcogenides compounds, such as ZnO, ZnS, ZnSe, and ZnTe, and CdS, CdSe, and CdTe. We developed a scheme for the electronic transitions and recombination associated with anion vacancies that is common for all these materials. This scheme explains all known facts obtained to date on quenching and excitation of the EPR signal, optical absorption, photoluminescence and photoconductivity. Based on these data we determined that the location of the energy level of the singly charged anion vacancy, VA+, is nearly equal for Zn-related II–VI materials (EC−1.0eV) and EC+0.8eV for Cd-related materials. For Cd-related chalcogenides most of the data were derived only from photoluminescence- and photoconductivity-spectra, so based on the available data, the position of the energy level of a singly charged anion vacancy in these materials was determined not so convincingly. Nonetheless, these materials have attracted much interest for decades because of their industrial applications as luminescent devices, laser filters and other optical elements, infrared, visible- and (X) γ-ray-detectors, solar cells, and the like.

Transient photoresponse in amorphous In-Ga-Zn-O thin films under stretched exponential analysis

We investigated transient photoresponse and Hall effect in amorphous In-Ga-Zn-O thin films and observed a stretched exponential response which allows characterization of the activation energy spectrum with only three fit parameters. Measurements of as-grown films and 350 K annealed films were conducted at room temperature by recording conductivity, carrier density, and mobility over day-long time scales, both under illumination and in the dark. Hall measurements verify approximately constant mobility, even as the photoinduced carrier density changes by orders of magnitude. The transient photoconductivity data fit well to a stretched exponential during both illumination and dark relaxation, but with slower response in the dark. The inverse Laplace transforms of these stretched exponentials yield the density of activation energies responsible for transient photoconductivity. An empirical equation is introduced, which determines the linewidth of the activation energy band from the stretched exponential parameter β. Dry annealing at 350 K is observed to slow the transient photoresponse.

Determination of the transport parameters of nanocrystalline CdSe:Cu thin films

Thin films of copper (Cu)-doped CdSe are prepared by the physical vapor deposition technique. X-ray diffraction analysis shows that the films crystallize in hexagonal structure with (100) as the preferred orientation. Scanning electron microscope studies reveal that the grains are spherical in shape and uniformly distributed all over the surface of the substrates. The transport properties of Cu-doped CdSe thin films have been investigated by the time-of-flight measurement, steady-state photocarrier grating and steady-state photoconductivity techniques. The temperature dependence of dark conductivity and photoconductivity data reflects the existence of two independent donor energy levels. The bimolecular recombination nature of the thin films has been found from the photocurrent-illumination dependence at different applied voltages. The minority carrier diffusion length (Ldiff) is obtained both from the best fit of the experimental photocurrent ratio β versus grating period (Λ) and from the ‘Balberg plot’. A straight line has been obtained from the best fit, which confirms that the measured length is the ambipolar diffusion length.

Charging phenomena in dielectric/semiconductor heterostructures during x-ray photoelectron spectroscopy measurements

The determination of the valence band offset (VBO) by x-ray photoelectron spectroscopy (XPS) is commonly performed using the so-called Kraut’s method that was developed for VBO determination in semiconductor/semiconductor heterojunctions. Although the physical model, which is the basis of the method, can be safely extended to dielectric/semiconductor (D/S) heterojunctions, in these systems a careful evaluation of the experimental results is necessary due to the differential charging phenomena originating at D/S interface during x-ray bombardment. As a consequence, precise determination of the VBO requires an accurate calibration of the energy scale in order to remove artifacts induced by the progressive charging of the oxide during the XPS measurement. In this work a detailed analysis of the band alignment between e-beam evaporated amorphous HfO2 films and Si substrates is reported. The HfO2/Si heterojunction was selected as a prototype for this study since HfO2 based dielectrics have already been implemented as gate dielectrics in real devices and have been the subject of a wide number of publications providing controversial results in terms of VBO values. A clear dependence of the binding energy of the Hf 4f and O 1s core lines on the thickness of the HfO2 film is identified. The time evolution of these signals indicates that different steady states are reached after prolonged x-ray bombardment depending on the thickness of the HfO2 films. On the basis of the original work of Iwata et al. [J. App. Phys. 79, 6653 (1996)], a rigorous method to remove these artifacts and empirically determine the real band offsets in D/S heterojunctions is proposed and validated by comparison with internal photoemission and photoconductivity data obtained on the same set of samples.

Radiative recombination in CdGa2S4 single crystals

We have studied the effects of light polarization and temperature on the photoluminescence spectrum of CdGa2S4 single crystals at temperatures from 77 to 300 K. Based on analysis of the present experimental data in conjunction with earlier band structure, optical absorption, and photoconductivity data, we have identified the mechanism of radiative recombination in CdGa2S4.

Probing electronic defect states in manganite/SrTiO3 heterostructures by surface photovoltage spectroscopy

The energetic distribution of electronic defect states in oxide heterostructures made of ultrathin lanthanum manganite (La0.7Ca0.3MnO3, La0.7Ce0.3MnO3) films on SrTiO3 substrates is investigated by surface photovoltage (SPV) spectroscopy. Within a comparative evaluation of the SPV spectra of both the film/substrate structures and pure substrates at different temperatures we were able to elaborate a map of defect states across the SrTiO3-bandgap and we find that the defect state distribution is mainly affected by the substrate, i.e., no specific film-induced defect states were detected. Possible origins of the defect states are discussed within the framework of a semiconductor band scheme, taking into account complementary photoconductivity and SPV transient data.

Photoconductivity and Relaxation Dynamics in Sonochemically Synthesized Assemblies of AgBiS2 Quantum Dots

The transport properties of nonequilibrium (photoexcited) charge carriers in sonochemically synthesized three-dimensional (3D) assemblies of AgBiS2 quantum dots (QDs) deposited as thin films were studied. To characterize the photoconduction of quantum-confined nanocrystals close packed in thin film form, both stationary and time-resolved experiments were performed. Besides by interband electronic transitions in the bulklike part of the nanocrystals, the photoresponse of nanocrystalline films was found to be also affected to a greater extent by the crystal boundary barrier height modulation upon illumination. The surface and bulk recombination velocities were found to be comparable. Good agreement was obtained between the band gap energy determined by analysis of the photoconductivity data measured by the constant field and the constant photocurrent method (∼1.18 eV). This value is in agreement with the optical spectroscopy data. It is higher than the optical band gap of a bulk specimen of this semiconduct...

Influence of series resistance on the photocurrent analysis of organic solar cells

The series resistance of a bulk heterojunction solar cell diode affects the measurement of the voltage dependence of the diode photocurrent, P( V). An empirical model for the effect is described and shows that P( V) can be significantly modified, particularly at high bias voltages. Experimental measurements on PCDTBT:PCBM solar cells with added series resistance demonstrate the effect and confirm the model. The model provides a method to correct the photoconductivity data using measured quantities.