Photoconductivity Spectra Research Articles

Conversion of n-type to p-type conductivity in ZnO by incorporation of Ag and Ag-Li

Elemental doping is an efficient strategy to modulate different properties of semiconductors. Conversion from n-type to p-type and band gap modulation of ZnO are investigated by theoretical and experimental pathways studies. The structural, electronic and optical properties of undoped, Ag and Ag-Li doped ZnO are studied in the framework of density functional theory (DFT). The value of direct band gap is found to be ~ 0.730, 0.440, 0.274, and 0.870 eV for undoped, Ag = 6.25% and 12.5% doped, and Ag= 6.25, Li= 6.25% co-doped ZnO, respectively. Acceptor levels are created at the top of the valence band and above the Fermi level in Ag and Ag-Li doped ZnO which reveals that Ag and Ag-Li are promising dopants for generating p-type ZnO. Importantly, a remarkable change in photoconductivity and optical properties are observed. The surface morphology of the spray deposited Zn100-xAgxO (x = 0.0–20%) and Zn100-x-yAgxLiyO (x = 5, y = 0.0–10%) thin films are changed with Ag and Ag-Li contents. The XRD patterns confirmed the hexagonal structure of all the deposited films. The band gap decreases from 3.27 to 3.08 eV (for Ag doped ZnO) and increases from 3.16 to 3.38 eV for Ag-Li doping ZnO, respectively. The dielectric constants and photoconductivity spectra support the formation of p-type conductivity of Zn100-xAgxO and Zn100-x-yAgxLiyO, which are in good agreement with the available theoretical and experimental reports. Thus, the studies performed in this work help us to understand the Ag and Ag-Li doping mechanism in ZnO; opening up possible directions toward the fabrication of p-type ZnO for advanced electronic and optoelectronic applications.

BAND GAP DETERMINATION FROM PHOTOCONDUCTIVITY EXCITATION SPECTRA

The possibility to determine the direct and indirect band gap from photoconductivity spectrum is shown. Using various structures as an example, it is shown, that the band gap determined from the photoconductivity spectrum differs from the one determined from the edge optical spectrum. By this method in composite structures it is possible to indicate impurity band gaps.

VERTICAL PURE ELECTRONIC OPTICAL TRANSITION AND SEMICONDUCTOR DIRECT BAND GAP

It is shown that the method of determining the frequency of pure electronic transition from molecular diffuse vibrational spectra is applicable for the estimation of a semiconductor direct band gap. This method is used to determine the direct band gap between the valence band and the photoconductivity band from the
 photoconductivity spectrum.

Features of photoconductivity and luminescence of CdS thin films and Cd-=SUB=-1-x-=/SUB=-Zn-=SUB=-x-=/SUB=-S solid solutions under laser excitation

The features of photoconductivity and luminescence of CdS thin films and Cd1-xZnxS solid solutions obtained by chemical pulverization with subsequent pyrolysis under laser excitation have been studied experimentally. Pulsed liquid (473-547 nm) and nitrogen (337 nm) lasers were used as a radiation source. An experimental study was made of the photoconductivity and photoluminescence spectra of Cd1-xZnxS thin films depending on the composition x at different excitation intensities and the relaxation curves of nonequilibrium photoconductivity. It is shown that the observed features in the photoconductivity and photoluminescence spectra of Cd1-xZnxS thin films are due to direct band-to-band transitions. Amplification of light is observed at high intensities of optical excitation in thin CdS films. Keywords: Thin films CdS, Cd1-xZnxS, laser, photoconductivity spectra, photoluminescence

Особенности фотопроводимости и люминесценции тонких пленок CdS и твердых растворов Cd-=SUB=-1-x-=/SUB=-Zn-=SUB=-x-=/SUB=-S при лазерном возбуждении

Abstract. The features of photoconductivity and luminescence of CdS thin films and Cd1-хZnxS solid solutions obtained by chemical pulverization with subsequent pyrolysis under laser excitation have been studied experimentally. Pulsed liquid (473 - 547) nm and nitrogen (337 nm) lasers were used as a radiation source. An experimental study was made of the photoconductivity and photoluminescence spectra of Cd1-хZnxS thin films depending on the composition х at different excitation intensities and the relaxation curves of nonequilibrium photoconductivity. It is shown that the observed features in the photoconductivity and photoluminescence spectra of Cd1-хZnxS thin films are due to direct band-to-band transitions. Amplification of light is observed at high intensities of optical excitation in thin CdS films.

Фотопроводимость монокристаллов слоистого полупроводника p-GaSe, легированного редкоземельными элементами, и многодиапазонный фотоприемник света на их основе

The main characteristics of intrinsic photoconductivity and the spectra of negative photoconductivity induced by impurity photoconductivity, infrared quenching of intrinsic photoconductivity in doped REE (gadolinium and dysprosium) at N = 0−10−1 at%, p-GaSe single crystals are experimentally investigated. It is shown that the instability and irreproducibility of the photoelectric characteristics of this semiconductor are due to fluctuations in the electronic potential associated with the presence of random macroscopic defects. The dependence of photoelectric parameters and characteristics on N, as well as ensuring a high degree of their stability and reproducibility at N ≈ 10−1 at%, are associated with the corresponding changes in the fluctuations of the electronic potential and the fraction of covalent bonds between the layers, depending on N. Shown is the possibility of creating a multi-range photodetector of light with stable, reproducible parameters and characteristics based on p-GaSe<REE> at N = 10−2−10−1 at%.

Photoluminescence spectroscopy of HgCdTe quantum well heterostructures in the 15-30 μm wavelength range

In recent years, narrow gap heterostructures with Hg1-xCdxTe/CdyHg1-yTe quantum wells have been actively studied both in connection with the topic of topological insulators and from the point of view of their application in optoelectronics in the terahertz domain. In this work, we study the photoconductivity and photoluminescence spectra of structure with a band gap of 40 meV. In addition to interband transitions, the features of the spectra associated with the resonance states of acceptors have been identified. The possibilities of using the structures under study to develop interband emitters at a wavelength of ~30 μm, which is inaccessible to existing quantum-cascade lasers, are discussed. Keywords: narrow-gap semiconductors, HgCdTe, resonance states, acceptors, photoluminescence.

Спектроскопия фотолюминесценции структур с квантовыми ямами на основе HgCdTe в диапазоне длин волн 15-30 мкм

In recent years, narrow gap heterostructures with Hg1-xCdxTe/CdyHg1-yTe quantum wells have been actively studied both in connection with the topic of topological insulators and from the point of view of their application in optoelectronics in the terahertz domain. In this work, we study the photoconductivity and photoluminescence spectra of structure with a band gap of 40 meV. In addition to interband transitions, the features of the spectra associated with the resonance states of acceptors have been identified. The possibilities of using the structures under study to develop interband emitters at a wavelength of ~ 30 μm, which is inaccessible to existing quantum-cascade lasers, are discussed.

Influence of Mn, Fe, Co, and Cu Doping on the Photoelectric Properties of 1T HfS2 Crystals.

Doping plays a vital role in the application of transition-metal dichalcogenides (TMDCs) because it can increase the functionality of TMDCs by tuning their native characteristics. In this study, the influence of Mn, Fe, Co, and Cu doping on the photoelectric properties of HfS2 was investigated. Pristine, Mn-, Fe-, Co-, and Cu-doped HfS2 crystals were grown using the chemical vapor transport method. Scanning electron microscopy images showed that the crystals were layered and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy measurements confirmed that the crystals were in the 1T-phase with a CdI2-like structure. The bandgap of pristine HfS2 obtained from the absorption and photoconductivity spectra was approximately 1.99 eV. As the dopant changed from Mn, Fe, and Co, to Cu, the bandgap gradually increased. The activation energies of the samples were determined using temperature-dependent current-voltage curves. After doping, the activation energy decreased, and the Co-doped HfS2 exhibited the smallest activation energy. Time-resolved photoresponse measurements showed that doping improved the response of HfS2 to light; the Co-doped HfS2 exhibited the best response. The photoresponsivity of HfS2 as a function of the laser power and bias voltage was measured. After doping, the photoresponsivity increased markedly; the Co-doped HfS2 exhibited the highest photoresponsivity. All the experimental results indicated that doping with Mn, Fe, Co, and Cu significantly improved the photoresponsive performance of HfS2, of which Co-doped HfS2 had the best performance.

PHOTOELECTRIC PROPERTIES OF MnIn2S4 SINGLE CRYSTALS

The spectral distribution of the photoconductivity and the temperature dependence of the photocurrent of MnIn2S4 single crystals are investigated. The intrinsic, impurity photoconductivity and a maximum at an energy of 2.69 eV, which is associated with the intracenter transition of Mn2+ ions (6A1→4A1), are revealed
 in the photoconductivity spectrum. The region of the wavelengths of 600–1000 nm appears with an excess of manganese in the crystals and is caused by a donor defect. At temperatures of 80—145 K, the increase in the photocurrent is associated with the thermal depletion of the adhesion levels. The activation energy of the adhesion levels is determined.

Near-far IR photoconductivity damping in hyperdoped Si at low temperatures

Silicon p-n junction photoelement fabricated on a p-doped wafer by sulfur-based n-doping of its sub-micron thick surface layer, exhibits at liquid-helium temperatures impurity-based near-far IR (2–21 μ m) photoconductivity spectra in the form of well-resolved separate bands of neutral and ionized atomic-like and cluster-like sulfur centers. Temperature variation in the range of 5–105 K demonstrates first at lower temperatures < 35 K strong damping of IR photoconductivity related to cluster-like sulfur centers with ultralow activation energy ≈ 4 meV, corresponding to excitation of the lowest energy of Raman-active phonon in orthorhombic crystalline sulfur lattice. Further increase in temperature results in the next damping step above 85 K for all spectral bands above 1800 cm-1 with higher activation energy ≈ 20 meV, representing the lowest energy of Raman-active vibration of octagon molecules in the crystalline sulfur. Broad near-far IR photosensitivity of the hyperdoped material, provided by the concentration-driven sulfur aggregation and quantum-level temperature control of its photoconductivity, paves the way for silicon photonics in far-IR and, possibly in the future, even THz spectral regions.

Enhanced Photoresponsivity of 2H-MoTe2 by Inserting 1T-MoTe2 Interlayer Contact for Photodetector Applications

The 2H molybdenum telluride (MoTe2) photodetector structures were made with inserting 1T-MoTe2 interlayer contacts. The optical response properties such as photoconductivity (PC) spectroscopy, illumination intensity dependent photoresponsivity, frequency dependent photocurrent, and time-resolved photoresponse were carried out in this study. In PC spectra, a much higher photoresponsivity of 2H-MoTe2 were observed by inserting 1T-MoTe2 interlayer contact. The frequency dependent photocurrent and time-resolved photoresponse investigations explore the carrier kinetic decay process of MoTe2 with different electrode contact. The Schottky barrier heights (SBH) extracted by thermionic emission theory were also investigated by inserting 1T-MoTe2 interlayer contacts. The results show the potential applicability for photodetection devices based MoTe2 layered transition metal dichalcogenides semiconductors.

Monolithic Tandem Multicolor Image Sensor Based on Electrochromic Color-Radix Demultiplexing.

Optical data acquisition has been set as one of the milestones to testify the developments aimed at harnessing the full potential of the spatial and temporal data processing capabilities of the advanced semiconductor technology. A highly promising approach to drive the level of acquisition beyond the current technological node is the vertical integration of multiple photodetectors. However, vertical integration so far requires the same level of circuit complexity as lateral integration from the incapability of monolithic integration. Here, an electrochromic device architecture is introduced that enables realization of a monolithic tandem multicolor photodetector. The device, composed of vertically stacked p-type and n-type graphene barristors, is demonstrated to be capable of regulating the balanced charge transport under any desired illumination wavelengths. It exhibits variable anti-ambipolar charge transport behavior, which yields sensitive voltage-controlled photoconductive gain spectra. These electrical behaviors are utilized to fabricate an optoelectronic logic sensor that can demultiplex the desired color coordinate or wavelength in the constituent array with high color accuracy.

Temperature-dependent sandwich and in-plane optical characterization of ternary chalcogenide TlSbS2

We have studied the temperature-dependent optical absorption and emission mechanisms of ternary chalcogenide TlSbS2 by carrying out sandwich and planar photoconductivity, and photoluminescence measurements in the temperature range 30 K to 300 K. Two main transitions were observed in the room temperature photoconductivity spectrum at energies of 1.74 and 1.84 eV, which are attributed to a native acceptor–donor transition and a band-to-band transition, respectively. A spectral anisotropy near the bandgap energy of TlSbS2 is observed between the sandwich and in-plane photoconductivity spectrum. In the photoluminescence spectrum, we observed deep-level broadband emission which is correlated with the native defect-related transition-based free-to-bound recombination between 30 K and 70 K. The Debye temperature of TlSbS2 was found to be 280 K, and the thermal expansion coefficient was determined as 1.62×10-3eV/K from the analyses of its temperature-dependent photoconductivity spectra.

Terahertz Photoconductivity Spectra of Electrodeposited Thin Bi Films.

Electron dynamics in the polycrystalline bismuth films were investigated by measuring emitted terahertz (THz) radiation pulses after their photoexcitation by tunable wavelength femtosecond duration optical pulses. Bi films were grown on metallic Au, Pt, and Ag substrates by the electrodeposition method with the Triton X-100 electrolyte additive, which allowed us to obtain more uniform films with consistent grain sizes on any substrate. It was shown that THz pulses are generated due to the spatial separation of photoexcited electrons and holes diffusing from the illuminated surface at different rates. The THz photoconductivity spectra analysis has led to a conclusion that the thermalization of more mobile carriers (electrons) is dominated by the carrier–carrier scattering rather than by their interaction with the lattice.

Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin-film metal oxide photoanodes for solar water splitting.

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, ξ(λ), represents the wavelength-dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract ξ(λ) through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo-oxidation, and observed that it is self-consistent for different illumination conditions and applied potentials. We found agreement between the extracted ξ(λ) spectrum and the photoconductivity spectrum measured by time-resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite's absorption spectrum. Low-energy non-contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO2 and BiVO4 photoanodes, demonstrating the broader utility of the method for determining ξ(λ).

Measurement of direct and indirect bandgaps in synthetic ultrathin MoS2 and WS2 films from photoconductivity spectra

Exploring the thickness-dependent electronic properties of ultrathin transition metal dichalcogenides is crucial for novel optoelectronic devices. Particularly important is experimental information regarding the bandgap width. This information is scarce and often inconsistent among the several measurement techniques that were employed for this task, such as optical absorption, scanning tunneling spectroscopy, and photoconductivity. Here, we present photoconductivity measurements in large-area synthetic MoS2 and WS2 films (one to five monolayers and the bulk crystal) grown on insulating layers (SiO2, Al2O3, or HfO2). The excitonic peaks of MoS2 and WS2 were detected in both the photocapacitor and traditional in-plane geometries. Their contribution to the photoconductivity is explained by the electric field-assisted dissociation mechanism. We have separated the excitonic and free carrier components in the photocurrent spectra and extracted the direct and indirect bandgaps using the Tauc plot, revealing their dependencies on the number of monolayers.

“RESEARCH OF PHYSICAL PROPERTIES OF NATURAL SEMICONDUCTOR FIBERS”

It was revealed that the photoconductivity spectra of various varieties of cotton fibers doped with iodine are different from each other. It was found that the electrical conductivity of cotton fibers undoped and doped with iodine and KMnO4 increases exponentially with increasing temperature. Infrared quenching and long-term relaxation of the photoconductivity of cotton fibers doped with iodine were detected. For the first time, it was found that with small doses of UV irradiation of silkworm gren, the quality of silk fibers improves, and with large doses it worsens

Ultrafast dynamics of photoconductivity in lead sulfide nanocrystals in terahertz region

The dynamics of carrier relaxation and charge transport of photoinduced carriers in Lead Sulfide nanocrystals with quantum confinement, synthesized by chemical bath deposition method, have been simultaneously determined using time-resolved terahertz spectroscopy with sub-picosecond time resolution. We use both Drude-Smith and Drude Lorenz model to successfully reproduce the photoconductivity spectra, which manifest that localized free-carriers not bounded excitons are responsible to the THz transport in PbS nanocrystals. The pump fluence dependence of carrier dynamics reveals the process of Auger recombination through the interaction between free carriers and excitons.

Time dependence of negative and positive photoconductivity for Si δ-doped AlGaAs/InGaAs/AlGaAs quantum well under various temperatures and various incident photon energies and intensities

Si δ-doped AlGaAs/InGaAs/AlGaAs quantum well (QW) structure is commonly adopted as one of the core elements in modern electric and optoelectronic devices. Here, the time dependent photoconductivity spectra along the active InGaAs QW channel in a dual and symmetric Si δ-doped AlGaAs/InGaAs/AlGaAs QW structure are systematically studied under various temperatures (T = 80–300 K) and various incident photon energies (E in = 1.10–1.88 eV) and intensities. In addition to positive photoconductivity, negative photoconductivity (NPC) was observed and attributed to two origins. For T = 180–240 K with E in = 1.51–1.61 eV, the trapping of the photo-excited electrons by the interface states located inside the conduction band of InGaAs QW layer is one of the origins for NPC curves. For T = 80–120 K with E in = 1.10–1.63 eV, the photoexcitation of the excess ‘supersaturated’ electrons within the active InGaAs QW caused by the short cooling process is another origin.