Tail States Research Articles

Temperature dependent aluminum induced crystallization of amorphous germanium thin films

In this work, we present the sequential growth of metal–semiconductor bilayer structure on HF-treated textured glass substrates by thermal evaporation method. The process of aluminum-induced crystallization of amorphous germanium thin films was critically observed under heat treatment. Temperature assisted structural modifications triggered the nucleation phenomenon for controlled grain growth which transformed the nature of Ge-thin film from amorphous-to-polycrystalline. Interestingly, the electrical resistivity also decreased by subsequent annealing which promoted the conduction mechanism. Optical measurements indicated a dwindling in band tail states and related structural defects, leading to better crystallinity.

Optical properties of chromium-selenide films designed for terahertz applications

Herein the effects of indium substrates on the properties of chromium selenide thin films are reported. Chromium selenide thin films and indium substrates are prepared by the thermal evaporation technique under a vacuum pressure of 10−5 mbar. It is observed that indium sublayers alters the atomic stoichiometry of chromium selenide. It induces the formation of Cr2Se3 phase instead of CrSe2 phase which grow onto glass substrates. Both of the glass/CrSe2 and In/Cr2Se3 films displayed direct and indirect energy band gaps. The respective gaps are 2.60 eV and 3.19 eV and 2.25 eV and 2.83 eV. The Urbach tails states exhibited a width of 2.24 eV in glass/CrSe2 and showed value of 0.85 eV in In/Cr2Se3. In addition a light absorbability enhancement of more than 45% is reached at the In/Cr2Se3 interfaces in the visible range of light. Moreover, as terahertz resonators, In/Cr2Se3 films showed larger optical conductivity. It also displayed a spectral terahertz cutoff frequency in the range of 3.0–35 THz. Furthermore, when utilized as terahertz resonators, In/Cr2Se3 films demonstrate heightened optical conductivity and a spectral terahertz cutoff frequency ranging from 3.0 to 35 THz. Analyzing these terahertz oscillators using Drude-Lorentz models reveals that the density of free charge carriers increases with higher oscillator energy, while the drift mobility of these carriers within the terahertz resonators varies between 1.10 and 3.09 cm²/Vs. The array of features showcased by In/Cr2Se3 thin films, including their performance as terahertz resonators, positions them as strong contenders for applications in optical and terahertz technologies.

Theoretical analysis of the charge carrier transport in thin layers of disordered materials with exponential band tail states

Analytic solutions of the convection–diffusion equation (CDE) have been obtained in the case of the exponential band tail states for a reflecting front electrode. Analytic expressions for the transient current density in time-of-flight (TOF) experimental conditions have been obtained. An assessment of the distortion by diffusion has been made for a data analysis of the common TOF method. A new method for calculating a charge carrier mobility and other transport parameters in thin layers of disordered materials has been proposed using the weak-field TOF experiments. The obtained results can provide the use of TOF method for optimizing the characteristics and designing thin-film electronic devices.

The tailless gecko gets the worm: prey type alters the effects of caudal autotomy on prey capture and subjugation kinematics.

Prey capture and subjugation are complex behaviors affected by many factors including physiological and behavioral traits of both the predator and the prey. The western banded gecko (Coleonyx variegatus) is a small generalist predator that consumes both evasive prey items, such as spiders, wasps, and orthopterans, and non-evasive prey items, including larvae, pupae, and isopterans. When consuming certain prey (e.g., scorpions), banded geckos will capture and then rapidly oscillate, or shake, their head and anterior part of their body. Banded geckos also have large, active tails that can account for over 20% of their body weight and can be voluntarily severed through the process of caudal autotomy. However, how autotomy influences prey capture behavior in geckos is poorly understood. Using high-speed 3D videography, we studied the effects of both prey type (mealworms and crickets) and tail autotomy on prey capture and subjugation performance in banded geckos. Performance metrics included maximum velocity and distance of prey capture, as well as velocity and frequency of post-capture shaking. Maximum velocity and distance of prey capture were lower for mealworms than crickets regardless of tail state. However, after autotomy, maximum velocity increased for strikes on mealworms but significantly decreased for crickets. After capture, geckos always shook mealworms, but never crickets. The frequency of shaking mealworms decreased after autotomy and additional qualitative differences were observed. Our results highlight the complex and interactive effects of prey type and caudal autotomy on prey capture biomechanics.

Superior Interfacial Contact Yields Efficient Electron Transfer Rate and Enhanced Solar Photocatalytic Hydrogen Generation in M/C3N4 Schottky Junctions.

Various literature studies (Table 6) have reported that dispersion of metal nanoparticles (NPs) on graphitic carbon nitride g-C3N4 (M/CN) has considerably improved the photocatalytic hydrogen yield. It is understood that metal NPs create active sites on the surface of CN and act as a cocatalyst. However, the precise changes induced by different metal NPs on the surface of CN still elude us. Here, we report a thorough understanding and comparison of the morphology, metal-support interactions, interfacial charge transfer kinetics, and band characteristics in different M/CN (M = Pt, Pd, Au, Ag, Cu) correlated with photocatalytic activity. Among all metals, Pt/CN was found to be the best performer both under sunlight and UV-visible irradiation. Under sunlight, maximum H2@ 2.7 mmol/h/g was observed over Pt/CN followed by Pd/CN > Au/CN > Ag/CN > Cu/CN ≈ CN. The present study revealed that among all metals, Pt formed superior interfacial contact with g-C3N4 as compared to other metals. The maximum Schottky barrier height (Φb,Pt) of 0.66 V was observed at Pt/CN followed by Φb,Au/CN (0.46 V) and Φb,Pd/CN (0.05 V). The presence of electron-deficient Pt in Pt-XPS, decrease in the intensity of d-DOS of Pt near the Fermi level in VB-XPS, increase in CB tail states, and cathodic shift in Vfb in MS plots sufficiently confirmed strong metal-support interactions in Pt/CN. Due to the SPR effect, Au and Ag NPs suffered from agglomeration and poor dispersion during photodeposition. Finely dispersed Pt NPs (2-4 nm, 53% dispersion) successfully competed with shallow/deep trap states and drove the photogenerated electrons to active metallic sites in a drastically reduced time period as investigated by femtosecond transient absorption spectroscopy. Typically, an interfacial electron transfer rate, KIET,avg, of 2.5 × 1010 s-1 was observed for Pt/CN, while 0.087 × 1010 s-1 was observed in Au/CN. Band alignment/potentials at M/CN Schottky junctions were derived and most favorable in Pt/CN with CB tail states much above the water reduction potential; however, in the case of Pd, these extend much below the H+/H2 potential and hence behave like deep trap states. Thus, in Pd/CN (τ0 = 4200 ps, 49%) and Ag/CN (3870 ps, 53%), electron deep trapping dominates over charge transfer to active sites. The present study will help in designing futuristic new cocatalyst-photocatalyst systems.

Structural and optical properties of InP1-xSbx/n-InAs epilayers grown by gas source molecular beam epitaxy

High-resolution x-ray diffraction (HR-XRD), photoluminescence (PL), synchrotron radiation extended x-ray absorption fine-structure (SR-EXAFS) measurements are methodically analyzed for assessing the optical and structural properties of InP1−xSbx/n-InAs epifilms grown by gas-source molecular-beam epitaxy method. For InP0.63Sb0.37/n-InAs sample, the PL study has revealed two A1, A2 energy bands. The A1 band prevalent at low temperature is attributed to the recombination of carriers trapped in the tail states. The A2 band with nearly constant transition energy ∼ 0.46 eV is virtually temperature independent. In InP1−xSbx alloys, the energy peak of A1 band redshifted with temperature and showed strong compositional disorder. The A2 band, dominant at higher temperature with Gaussian-like line shape is ascribed to the deep-level transition as its behavior coincided with the signature of a configuration coordinate model. The deep level responsible for A2 band is possibly linked to the “vacancy-impurity” like complexes having ground and excited states in the energy band gap. The composition dependent analysis of SR-EXAFS data for InP1−xSbx/InAs samples has confirmed the maintenance of nearest neighbor In-P, In-Sb shell distances within the range of bulk binary materials’ bond lengths. We feel that our results of HR-XRD, PL and SR-EXAFS techniques have provided valuable information on the structural and optical characteristics of the InP1−xSbx/n-InAs (001) epilayers and can be extended to many other technologically important materials.

Direct-current-sputtered nickel oxide (NiO) films with improved p-type conduction characteristic: impact of substrate temperature on structure, optical and electrical properties

Nickel oxide (NiO) films are direct current sputtered at different substrate temperatures (Tsub) using high sputtering power density and sputtering pressure. The improvement in crystallization of the films with Tsub results in the decrease of concentration of the coexisting Ni vacancies and interstitial O atoms, thereby increasing the hole mobility and resistivity. All the films have an approximate near-violet absorption edge of ca. 3.6 eV. The only difference is the rate of increase of transmittance that is attributed to the tail states produced by the tensile stress. The polycrystalline NiO film with 200 °C Tsub can be used as the hole-transporting layer material of inverse-architecture perovskite solar cells due to the maximum free hole concentration, minimum resistivity and intermediate hole mobility.

Wavelength-Dependent Excitonic Properties of Covalent Organic Frameworks Explored by Theory and Experiments

Many aspects of the correlation between the physical structure, light harvesting, and excitonic properties of covalent organic frameworks (COFs) remain unclear despite being key properties determining their photocatalytic function. One area of COF research that could bring clarity is using both electronic structure theory and time-resolved spectroscopic analysis over a series of systematically varied COFs. Here, we show structure–property relationships between four imine COFs built from a combination of ditopic and tritopic monomers using transient absorption spectroscopy together with time-dependent density functional theory. We find that monomer selection only moderately affects the charge transfer (CT) behavior of the COFs. Instead, we infer that imine chemistry profoundly impacts CT by acting as a CT mediator. Moreover, we discover two distinct valence bands arising from varying degrees of locally excited/CT mixing, which is responsible for energy-dependent exciton dynamics. Finally, we use theory to hypothesize that interlayer interactions can modify excitonic properties that we correlate with tail states commonly observed but rarely investigated in COFs. These results reveal that imine chemistry should be recognized as a very important factor to consider in the development of COF photocatalysts and the correlation of their structural environment with light-harvesting and CT properties that should ultimately determine their photocatalytic function.

Power-Law Density of States in Organic Solar Cells Revealed by the Open-Circuit Voltage Dependence of the Ideality Factor.

The density of states (DOS) is fundamentally important for understanding physical processes in organic disordered semiconductors, yet hard to determine experimentally. We evaluated the DOS by considering recombination via tail states and using the temperature and open-circuit voltage (V_{oc}) dependence of the ideality factor. By performing Suns-V_{oc} measurements, we find that the energetic disorder increases deeper into the band gap, which is not expected for a Gaussian or exponential DOS. The linear dependence of the disorder on energy reveals the power-law DOS in organic solar cells.

Study of Electrical Characteristics for Dual-Gate TFTs With Asymmetric Defect Distributions and Gate Work Functions

Combined effects of asymmetric defect distributions and asymmetric gate work functions (WFs) on the performances of self-aligned dual-gate poly-Si TFTs are investigated. Normally, small grains with plentiful grain boundaries (GBs) or other structural defects appear at different positions of the poly-Si film, which is dependent on the growth process of the film. Subgap states of acceptor-like tail, acceptor-like deep-level, donor-like tail, and donor-like deep-level states are used to emulate the defects. Two sets of density of states (DOS) are employed. We find that defects at different positions of the source-side and drain-side channels exhibit different influences on TFT performance and the influences are dependent on the WFs of the gates. TFTs with a higher gate WF can have a higher tolerance to the depth of the defect region. Besides the electrical characteristics, the combined effects of defects and gate WFs on current density distributions and electric field distributions in the channel regions are explored. The performance variations caused by the asymmetric defects along with asymmetric gate WFs can be explained.

Photocurrent enhancement by below bandgap excitation in GaPN

This study presents two-wavelength excited photocurrent (TWEPC) measurements in GaP1−x N x grown by metalorganic vapor phase epitaxy. TWEPC measurements reveal that photocurrent generation is significantly enhanced when above gap excitation and below gap excitation (BGE) sources are applied simultaneously. With increasing BGE photon energy, a large increase in photocurrent is observed. The external quantum efficiency measurements show that the effect of BGE light is higher with a higher density of tail states present. The extended numerical study by rate equations reproduced the results in a good manner. Furthermore, the simulation results showed that the addition of the BGE light affects the electron occupancy as well as the electron lifetime, which is found to be 0.1 ns in this study.

On the Origin of Tail States and Open Circuit Voltage Losses in Cu(In,Ga)Se2

The detrimental effect of tail states on the radiative and non‐radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage (VOC) in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of compositional variations and alkali incorporation into single‐crystal films. The results demonstrate that sodium and potassium decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of sodium or potassium incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high‐efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2 based on the combined effect of doping on tail states and VOC.

Bismuth sulfide nanorods by thermal decomposition of a complex: Effect of reaction temperature on microstructural and optical properties

Bismuth sulfide nanorods have been prepared by thermolysis of a bismuth dithiocarbamate complex, as a single-source precursor, in a mixture of solvents (oleylamine, 1-dodecanethiol, and 1-octadecene) at various temperatures. Bismuth sulfide nanorods denoted as BS(120), BS(150), BS(180), BS(200), and BS(220) have been obtained at the specified temperatures (°C). They have been characterized by X-ray diffraction analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV/Vis spectrophotometry, and photoluminescence (PL) spectroscopy. The effect of temperature on the microstructural and optical properties of the obtained Bi2S3 has been studied. The results show that interactions of the N–H group of oleylamine and the S–H group of dodecanethiol with the nanoparticles are temperature-dependent. The crystallite sizes of the nanoparticles were calculated using the Scherrer equation, Williamson–Hall (W–H) analysis, and Rietveld analysis, which yielded values in the ranges 20.1–45.9, 20.9–26.6, and 23.0–29.6 nm, respectively. Analysis of the microstrain in crystals of the nanoparticles showed that while the strain therein was due to contraction of the nanoparticles obtained at 180 °C, the strains at other temperatures were due to expansion, thereby confirming the effect of temperature on the nature of the defects in Bi2S3 crystals. The lattice strains were estimated by the W–H and Rietveld analyses to be in the ranges 3.19 × 10−5–1.04 × 10−4 and 0.008–0.067, respectively. The crystallite sizes and lattice strains estimated by both W–H and Rietveld analyses were mutually consistent, showing similar trends. SEM and TEM analyses showed that the nanorods were aggregated into longer rods at 120 and 150 °C, whereas shorter rods were obtained at 180 and 220 °C, suggesting a change in preferential growth direction. The widths and lengths of the nanorods were estimated as 20.97–41.25 nm and 79.4–111.0 nm, respectively, giving aspect ratios in the range 1.92–5.52. The band-gap energies of the nanorods varied in the range 1.08–0.78 eV, which can be related to differences in tail state width as observed in the absorption spectra. Calculation of texture coefficients showed preferential growth in the (111), (020), and (321) directions.

Green Synthesis of TiO2 Nanoparticles in a Deep Eutectic Solvent for High‐Performance Photocatalysis: The Role of the Cosolvent

AbstractHerein, we report a unique surfactant‐free synthetic pathway for the synthesis of anatase TiO2 catalysts in the size range of 13 to 20 nm by solvothermal method using choline chloride‐urea deep eutectic solvent (DES) as green media and 2‐propanol as cosolvent. The composition of reline was tuned by adding 0 (pure reline), 5, 10, 20 and 50 wt% of 2‐propanol. The amount of cosolvent was found to have a significant influence on TiO2 particle size, N2 isotherms and the magnitude of Urbach energy formed within the band gap of mesoporous TiO2 nanostructures. The sample obtained with 10 wt% of propanol showed the best photocatalytic activity in the degradation of Rhodamine B (RhB), which is attributed to the combined effect of Urbach tail states, specific surface area, particle size and band gap energy of TiO2 NPs. Radical trapping experiments with scavengers revealed that OH⋅ radicals are primarily responsible for RhB decomposition.

Characterization of N Rich-Silicon Nitride Thin Films Deposited by PECVD

Silicon nitride thin films are very important for their possible use in semiconductor industry and electronic applications. Changing the deposition parameters, silicon nitrides which have many varying optical properties can be produced. In this work, silicon nitride (SiNx) thin films were deposited on silicon substrates using Plasma enhanced chemical vapor deposition (PECVD) technique. The silane (SiH4) and ammonia (NH3) were used as reactant gases. Using these reactant gases, nitrogen (N) rich SiNx films were obtained. In order to get information about absorption and bond types in the films, films were analyzed by the help of Fourier transform infrared spectroscopy (FTIR) was performed. The refractive index, extinction coefficient and band gap energy of the films were changed from 1.86, 0 and 5.38 eV to 2.05, 0.0048 and 4.26 eV, respectively. Using the refractive index, composition of the films were estimated that is [N]/[Si] ratio of the films varied from 1.38 to 1.62. For possible applications of the SiNx films, learning the origin of the light-emission of the films is very important so the photoluminescence (PL) measurements were also used to see the luminescent of the SiNx films which is related to the electronic transitions between the K-center level and the conduction band tail states.

Understanding the Excess 1/f Noise in MOSFETs at Cryogenic Temperatures

Characterization, modeling, and development of cryo-temperature CMOS technologies (cryo-CMOS) have significantly progressed to help overcome the interconnection bottleneck between qubits and the readout interface in quantum computers. Nevertheless, available compact models still fail to predict the deviation of 1/f noise from the expected linear scaling with temperature ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}$ </tex-math></inline-formula> ), referred to as “excess 1/f noise,” observed at cryogenic temperatures. In addition, 1/f noise represents one of the main limiting factors for the decoherence time of qubits. In this article, we extensively characterize low-frequency noise on commercial 28-nm CMOS and on research-grade Ge-channel MOSFETs at temperatures ranging from 370 K down to 4 K. Our investigations exclude electron heating and bulk dielectric defects as possible causes of the excess 1/f noise at low temperatures. We show further evidence for a strong correlation between the excess 1/f noise and the saturation of the subthreshold swing (SS) observed at low temperatures. The most plausible cause of the excess noise is found in band tail states in the channel acting as additional capture/emission centers at cryogenic temperatures.

Subgap Absorption in Organic Semiconductors.

Organic semiconductors have found a broad range of application in areas such as light emission, photovoltaics, and optoelectronics. The active components in such devices are based on molecular and polymeric organic semiconductors, where the density of states is generally determined by the disordered nature of the molecular solid rather than energy bands. Inevitably, there exist states within the energy gap which may include tail states, deep traps caused by unavoidable impurities and defects, as well as intermolecular states due to (radiative) charge transfer states. In this Perspective, we first summarize methods to determine the absorption features due to the subgap states. We then explain how subgap states can be parametrized based upon the subgap spectral line shapes. We finally describe the role of subgap states in the performance metrics of organic semiconductor devices from a thermodynamic viewpoint.

Electron conduction mechanisms in magnetic tunnel junctions fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier

CoFeB/ Si-Zn-Sn-O /CoFeB magnetic tunnel junctions (MTJs) have been fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier. In the low bias voltage range (up to ∼0.2 V), direct tunneling is found to be the dominant transport mechanism in MTJs. Tunneling conduction is further verified by simulation of tunnel current density and differential conductance using Simmon’s and Brinkmann model, respectively. Simulated results provided valuable insights into the barrier properties, including interfacial barrier height, thickness, and barrier asymmetry. Above the direct tunneling regime, electron transport in MTJs is governed by Pool Frenkel emission, which possibly arises due to the presence of high-density localized tail states below the conduction band of amorphous Si-Zn-Sn-O. Tunnelling magnetoresistance value of MTJs is found to be very low, which is attributed to the presence of various inelastic conduction channels. The results of this study might be useful to explore the potential of amorphous Si-Zn-Sn-O for fabricating low-resistive MTJ based spintronic devices.

Comparing Methods of Characterizing Energetic Disorder in Organic Solar Cells

AbstractThe energetic disorder has been known for decades to limit the performance of structurally disordered semiconductors such as amorphous silicon and organic semiconductors. However, in the past years, high‐performance organic solar cells have emerged showing a continuously reduced amount of energetic disorder. While searching for future high‐efficiency material systems, it is therefore important to correctly characterize this energetic disorder. While there are several techniques in the literature, the most common approaches to probe the density of defect states are using optical excitation as in external quantum efficiency measurements, or sequential filling of the tail states by applying an external voltage as in admittance spectroscopy. A metanalysis of available literature, as well as the experiments using four characterization techniques on two material systems, reveal that electrical, voltage‐dependent measurements frequently yield higher values of energetic disorder than optical measurements. With drift‐diffusion simulations, it is demonstrated that the approaches probe different energy ranges of the subband‐gap density of states. The limitations of the techniques are further explored and it is found that extraction of information from a capacitance‐voltage curve can be inhibited by internal series resistance. Thereby, the discrepancies between measurement techniques with sensitivity to different energy ranges and electronic parameters are explained.

Cryogenic Characteristics of InGaAs MOSFET

We present an investigation of the temperature dependence of the current characteristic of a long-channel InGaAs quantum well MOSFET. A model is developed, which includes the effects of band tail states, electron concentration-dependent mobility, and interface trap density to accurately explain the measured data over all modes of operation. The increased effect of remote impurity scattering is associated with mobility degradation in the subthreshold region. The device has been characterized down to 13 K, with a minimum inverse subthreshold slope of 8 mV/dec and a maximum ON-state mobility of 6700 cm2/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}\cdot \text{s}$ </tex-math></inline-formula> and with values of 75 mV/dec and 3000 cm2/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}\cdot \text{s}$ </tex-math></inline-formula> at room temperature.