Density Of Band Tail States Research Articles

Enhanced hole injection and transport property of thermally deposited copper thiocyanate (CuSCN) for organic light-emitting diodes

Due to its high conductivity, broad bandgap, and low manufacturing cost, copper thiocyanate (CuSCN) thin film has been considered a good hole injection layer (HIL)/transport layer in many opto/electronic devices. However, the CuSCN thin films made by the conventional wet process have limitations in terms of interfacial deterioration due to solvent incorporation and charge trap formation due to impurities. We produce a CuSCN film for HIL in organic light-emitting diode (OLED) devices using vacuum deposition and compare it to a solution-processed film. In comparison to the control device, the CuSCN HIL insertion lowers the driving voltage of fabricated OLEDs by 1.0–1.5 V. The low-voltage operation corresponds to better hole transport in hole-only devices. The vacuum-deposited CuSCN, in particular, has an impurity-free composition and a high density of band-tail states, according to chemical and electronic structural studies. Because of their outstanding quality, vacuum-deposited CuSCN films show clear advantages over solution-based films in device performance and manufacture.

Observation of negative differential resistance in SiO2/Si heterostructures

The negative differential resistance (NDR) effect been widely reported in the physical systems of traditional semiconductors, and corresponding theoretical mechanisms are known and applied. However, exploring the origin of NDR phenomena emerging in new structures or conditions remains a worthwhile challenge. Here, we report an experimental observation of an NDR effect based on SiO 2 /Si heterojunctions by injecting photo-generated carriers via vacuum ultraviolet (VUV) irradiation. We propose a complex competition mechanism between trap centers’ and recombination centers’ dominating carrier transport behavior. This is caused by the difference in spatial distribution of density of band-tail states and defect states in SiO 2 /Si. This work may inspire further consideration of SiO 2 /Si heterojunctions for electronic applications. • NDR effect is observed in SiO 2 /Si heterojunction via vacuum VUV irradiation • NDR effect originates from difference in spatial distribution of density of states • SiO 2 /Si heterojunction with NDR effect shows potential for space applications Using vacuum ultraviolet irradiation, Jia et al. observe a negative differential resistance effect in the structure of SiO2/Si heterojunction. This work may bring new opportunities for traditional SiO 2 /Si heterojunctions in electronic applications, such as switches and amplifying elements in space detection systems.

Large mobility modulation in ultrathin amorphous titanium oxide transistors

Recently, ultrathin metal-oxide thin film transistors (TFTs) have shown very high on-off ratio and ultra-sharp subthreshold swing, making them promising candidates for applications beyond conventional large-area electronics. While the on-off operation in typical TFTs results primarily from the modulation of charge carrier density by gate voltage, the high on-off ratio in ultrathin oxide TFTs can be associated with a large carrier mobility modulation, whose origin remains unknown. We investigate 3.5 nm-thick TiOx-based ultrathin TFTs exhibiting on-off ratio of ~106, predominantly driven by ~6-decade gate-induced mobility modulation. The power law behavior of the mobility features two regimes, with a very high exponent at low gate voltages, unprecedented for oxide TFTs. We find that this phenomenon is well explained by the presence of high-density tail states near the conduction band edge, which supports carrier transport via variable range hopping. The observed two-exponent regimes reflect the bi-exponential distribution of the density of band-tail states. This improved understanding would be significant in fabricating high-performance ultrathin oxide devices.

Defect Assisted Carrier Multiplication in Amorphous Silicon

Recent studies show disordered materials such as amorphous silicon (a-Si), in spite of their low mobility, can efficiently amplify photocurrent via carrier multiplication process. Detectors with a thin (~40nm) a-Si multiplication layer has demonstrated single photon sensitivity at high speed under room temperature. It is believed that the abundance of bandtail states that cause low mobility of the amorphous materials actually contribute to the detector’s superior performance because carrier impact ionization involving these states, which can be modeled by donor-acceptor pairs (DAPs), relax the k-selection rule of the many-body carrier multiplication process. Our paper presents a theoretical framework to calculate the carrier multiplication process in a-Si or other disordered materials involving DAPs. Our analysis also establishes the relations between detector characteristics and key parameters such as the density of band tail states, layer thickness, and applied electric field. DAP assisted carrier multiplication rate is computed first. Carrier multiplication coefficients for electrons and holes under given applied field are then calculated using a trial distribution function that satisfies both the continuity equation and the energy balance equation. Our analysis shows that thinner a-Si gives rise to higher multiplication efficiency than thicker a-Si because of its higher carrier kinetic energy when the layer thickness is shorter than the length of energy relaxation by phonon scattering. Using the calculated carrier multiplication coefficients, voltage dependent gain of the device is computed and the results agree well with the measured results of a-Si cycling excitation process (CEP) detectors.

Modeling the effects of defect parameters on the performance of a p-BaSi2/n-Si heterojunction solar cell

Barium disilicide (BaSi2) in thin-film solar cell applications has drawn considerable interest owing to its promising optical and electrical properties. We have achieved an efficiency of 9.9% in p-BaSi2/n-Si heterojunction solar cells, which is the highest performance reported among semiconducting silicide devices; however, this value remains much lower than the theoretical limit as defined by the material's band gap. In this paper, we performed numerical simulations based on Silvaco ATLAS to investigate the effects of defect parameters on the performance of heterojunction solar cells. The defects were modeled by introducing two tail bands around the edge of the conduction and valence bands, and an acceptor-like Gaussian distributed localized energy level within the band gap of BaSi2. The influence of the band tail density of states and the parameters of the localized energy levels on the short-circuit current density, open-circuit voltage, fill factor, and efficiency were evaluated. These results enabled us to reproduce the measured J-V characteristics by simulation.We propose a defect model of p-BaSi2 films to reproduce experimentally obtained current-voltage characteristics of p-BaSi2/n-Si heterojunction solar cell under AM1.5 illumination. Two band tails and one acceptor-like localized energy level reproduce the experimental result.

Bismuth-induced band-tail states in GaAsBi probed by photoluminescence

Band-tail states in semiconductors reflect the effects of material growth and/or treatment, affect the performance of optoelectronic applications, and are hence a well-concerned issue. Dilute-Bi GaAs is considered very competitive though the role of Bi is yet to be well clarified. We in this letter investigate the effect of Bi incorporation on the band-tail states in GaAs1−xBix by excitation power- and magnetic field-dependent photoluminescence (PL) measurements at low temperatures. Three PL features are identified from a broad PL peak, which blue-shift monotonically with the increase in excitation power. None of the PL features correlate with single Bi-content free-exciton recombination, and band-tail filling rather than the donor-acceptor pair process is responsible for the power-induced blueshift. The density of band-tail states gets enhanced with the increase in the Bi incorporation level and affects the determination of Bi-induced bandgap reduction. The results indicate that joint analysis of excitation- and magneto-PL may serve as a good probe for band-tail states in semiconductors.

Charge transfer-induced enhancement of a Raman signal in a hybrid Ag-GaN nanostructure.

A hybrid system consisting of Ag nanoparticles dispersed onto a GaN nanowall network (GaN NWN) exhibited characteristic optical properties and electronic band structure. Surface-sensitive XPS studies of this high-surface-area system revealed the presence of a high surface charge carrier concentration due to dangling bonds, which resulted in a high metal-like surface conductivity. The low coverage of absorbed Ag led to the nanocluster formation, facilitating charge transfer from GaN to Ag, and thereby further increasing the surface charge carriers. Photoluminescence studies revealed the presence of a high density of band tail states at the conduction band, which is significantly (14-fold) larger than in the GaN epilayer. Raman studies show an increase (2.46-fold) in the interfacial strain at the Ag/GaN interface after the deposition of the Ag nanoparticles. We show that these surface modifications increase the density of hot spots, resulting in an intense Raman signal with an enhancement factor of 107. The role of the charge transfer between Ag nanoparticles and GaN NWN in the enhancement of Raman signal has been demonstrated.

Optical and Structural Analysis of Zinc Oxysulfide Digital Alloys Grown by Atomic Layer Deposition

We investigated the optical and structural properties of ZnOS digital alloy grown on Si by atomic layer deposition. The refractive index and extinction coefficient were evaluated using spectroscopic ellipsometry. ZnOS alloy film shows a strong absorption tail extended to the below band gap regime, which is attributed to a high density of band tail states formed within the band gap owing to the amorphous crystallinity. The ZnOS alloy exhibits a large Urbach energy of 210–540 meV. After high-temperature annealing, the band tail states are reduced with a decrease in lattice disorder, which is confirmed by enhanced X-ray diffraction peaks.

Localization and Stabilization of Photogenerated Electrons at TiO2 Nanoparticle Surface by Oxygen at Ambient Temperature.

Understanding the mechanism by which oxygen adsorption influences the separation behavior of charge carriers is important in photocatalytic removal of air pollutants. In this study, we performed steady-state surface photovoltage and surface photocurrent spectroscopy combined with an atmosphere control system to determine the effect of oxygen on the charge separation behavior at the surface of anatase TiO2 nanoparticles at ambient temperature. Results showed that photogenerated electrons were movable in N2 atmosphere but were localized in O2 atmosphere. O2 obviously enhanced the stabilization of the localized photogenerated electrons when the surface defects of TiO2 were fully occupied by adsorbed O2. Moreover, O2 adsorption increased the energy demand for exciting electrons from the valence band to localized surface defect states and reduced the density of band tail states. These findings suggest us that the effect of gaseous species on the mobility and stability of charge carriers should be considered to understand the photocatalytic degradation of air pollutants.

Real- and reciprocal-space attributes of band tail states

Band tail states are localized electronic states existing near conduction and valence band edges. Band tail states are invariably found to exhibit an exponential distribution defined by a characteristic (Urbach) energy. To a large extent, the band tail state density of states determines the electronic performance of an amorphous semiconductor (or insulator) in terms of its mobility. Real-space assessment of a suitable density of states model for extended (delocalized) conduction or valence band states and nearby localized band tail states leads to an expression for the peak density of band tail states at the mobility edge and for the total band tail state density. Assuming a continuous density of states and its derivative with respect to energy across the mobility edge, these densities are found to depend on only two parameters – the Urbach energy and an effective mass characterizing the extended state density above the mobility edge. Reciprocal-space assessment is then employed to deduce a probability density function associated with band tail states. The full width at half maximum of the resulting Gaussian probability density function is found to be equal to the average real-space distance of separation between band tail states, as estimated from the total band tail state density. This real- and reciprocal-space insight may be useful for developing new high-performance amorphous semiconductors and for modeling their electronic properties.

Localized tail state distribution and hopping transport in ultrathin zinc-tin-oxide thin film transistor

Carrier transport properties of solution processed ultra thin (4 nm) zinc-tin oxide (ZTO) thin film transistor are investigated based on its transfer characteristics measured at the temperature ranging from 310 K to 77 K. As temperature decreases, the transfer curves show a parellel shift toward more postive voltages. The conduction mechanism of ultra-thin ZTO film and its connection to the density of band tail states have been substantiated by two approaches, including fitting logarithm drain current (log ID) to T−1/3 at 310 K to 77 K according to the two-dimensional Mott variable range hopping theory and the extraction of density of localized tail states through the energy distribution of trapped carrier density. The linear dependency of log ID vs. T−1/3 indicates that the dominant carrier transport mechanism in ZTO is the variable range hopping. The extracted value of density of tail states at the conduction band minimum is 4.75 × 1020 cm−3 eV−1 through the energy distribution of trapped carrier density. The high density of localized tail states in the ultra thin ZTO film is the key factor leading to the room-temperature hopping transport of carriers among localized tail states.

Capacitance Spectroscopy of Light Induced Trap States in Organic Solar Cells

The light-induced localized trap state density of states in organic bulk heterojunction solar cells is reported, based on capacitance–frequency and photocurrent spectroscopy measurements. Several different cell materials show qualitatively the same behavior. A deep trap is induced by prolonged white light illumination with state density of 1016–1017 cm–3 and is observed by both experimental techniques. The trap depth is 0.5–0.7 eV. The light soaking has no measurable effect on the localized band tail density of states. The observation of the effect in many materials is consistent with the model that traps are created by light-induced dissociation of C–H bonds. The data suggests that disorder enhances the trap creation. The data also allow an estimate of the trap capture cross section, which is discussed in terms of local transport anisotropy.

Thin‐film transistor mobility limits considerations

AbstractThe mobility limiting scattering mechanisms for amorphous semiconductors and polar polycrystalline semiconductors are studied in the context of developing new high‐performance thin‐film transistor (TFT) channel layer materials for large‐area electronics. A physics‐based model for carrier transport in an amorphous semiconductor is developed to estimate the mobility limits of amorphous semiconductor TFTs. The model involves band tail state trapping of a diffusive mobility. Simulation reveals a strong dependence on the band tail density of states. This consideration makes it difficult to realize a high‐performance p‐type oxide TFT. A polar crystalline semiconductor may offer a higher mobility but is fundamentally limited by polar optical phonon scattering. Any crystalline TFT channel layer for practical large‐area applications will not be a single crystal but polycrystalline, and therefore, grain size and grain boundary‐dependent scattering will further degrade the transport properties.

New insights on traps states in organic semiconductor applying illumination-free transient current method

A transient current measurement technique which does not need light stimulus is developed. This illumination-free transient current (IFTC) technique offers insights on trapping states characteristics such as the capture time of the trapping states, their density of states (DOS) and the attempt-to-escape frequency of trapped carriers. For the analysis of IFTC measurements carried out on metal–insulator–semiconductor (MIS) capacitors, numerical simulation of the transient current were performed as a way around usual assumptions of short transit times which can considerably distort the values from transient current measurements. A combination of experimental data and simulations revealed the need to take into account retrapping events in order to extract the correct band tail density of states and corresponding characteristic parameters. An exponential density of states with a total density of 4·1023m−3 and a width of 50meV was found to be representative of the band tail of spin-coated poly(3-hexlythiophene) (P3HT). A broader exponential distribution with a width of 83meV is extracted for the interface states towards SiO2. Using a multiple trap and release model, the capturing time and the attempt-to-escape frequency of the band tail states were found to be 10-10s and 108s−1, respectively. With this technique, we are able to also pinpoint the energy position of the Fermi level relative to the transport energy level.

Charge transport in solution-processed zinc tin oxide thin film transistors

Abstract

Photoluminescence analyses of hydrogenated amorphous silicon nitride thin films

Silicon-rich hydrogenated amorphous silicon nitride (a-SiN x :H) films were grown by plasma enhanced chemical vapor deposition (PECVD) with different r=NH 3/SiH 4 gas flow ratios. The optical absorption characteristics were analyzed by Fourier transform infrared (FTIR) and UV-visible transmittance spectroscopies. The recombination properties were investigated via photoluminescence (PL) measurements. As r was increased from 2 to 9, the PL emission color could be adjusted from red to blue with the emission intensity high enough to be perceived by naked eye at room temperature. The behaviors of the PL peak energy and the PL band broadness with respect to the optical constants were discussed in the frame of electron–phonon coupling and band tail recombination models. A semiquantitative analysis supported the band tail recombination model, where the recombination was found to be favored when the carriers thermalize to an energy level at which the band tail density of states (DOS) reduces to some fraction of the relevant band edge DOS. For the PL efficiency comparison of the samples with different nitrogen contents, the PL intensity was corrected for the absorbed intensity fraction of the incident PL excitation source. The resulted correlation between the PL efficiency and the subgap absorption tail width further supported the band tail recombination model.

Photoluminescence energy and open circuit voltage in microcrystalline silicon solar cells

The interrelation of photoluminescence (PL) properties and open circuit voltage ( V oc) of thin film p-i-n microcrystalline silicon ( μc-Si:H) solar cells with efficiencies above 9% was studied. Hot wire chemical vapour deposition (HW-CVD) technique was used for the preparation of intrinsic layer at various silane concentrations (SC). The effect of the tail states on the splitting of the quasi-Fermi energies and carrier distributions was studied by monitoring V oc and PL energy as a function of temperature and optical generation rate ( g o). An increase of the PL energy and V oc is observed for (i) increasing SC, (ii) increasing g o and (iii) decreasing temperature. It is also found that (i) V oc and PL energy are limited by the band tail states and (ii) short circuit current density drops at low temperatures. The latter effect is attributed to a reduced carrier extraction due to trapping of the carriers in band tail states and subsequent recombination. It is proposed that increasing SC leads to a reduction of the density of band tail states due to structural relaxation of the μc-Si:H network by the presence of hydrogen or hydrogenated amorphous silicon.

A comparison of microcrystalline silicon prepared by plasma-enhanced chemical vapor deposition and hot-wire chemical vapor deposition: electronic and device properties

The optoelectronic properties of undoped μc-Si : H have been investigated, with emphasis on the states close to the edges of the band gap. The usefulness of the constant photocurrent method (CPM) for the determination of the absorption coefficient, α(E), is critically described. Combined with carefully evaluated photothermal deflection spectroscopy data, CPM spectra yield valuable information on the transport and dynamics of photo-generated carriers. By comparing photoluminescence and Raman spectra on high-quality samples prepared by plasma-enhanced and hot-wire chemical vapor deposition, with different silane concentrations in the gas stream, a correlation between the microstructure and photoluminescence energy is obtained. It is proposed that the density of band tail states is reduced with increasing silane concentration, leading to an increase in the photoluminescence energy and in the open-circuit voltages of solar cells.

Electronic Properties of Microcrystalline Silicon investigated by Photoluminescence Spectroscopy on Films and Devices

AbstractPhotoluminescence spectroscopy has been applied to investigate localized states in microcrystal-line silicon (μc-Si:H) films and to address the problem of the changes of the electronic properties of this material upon changes of the hydrogen dilution during film growth. By a comparison of photoluminescence and Raman spectra on device grade sample series prepared at different silane concentration in hydrogen (SC) by PE-CVD and HW-CVD a correlation between the micro-structure and the photoluminescence energy is found. It is proposed that the density of band tail states is reduced with increasing SC leading to the increase of the PL energy as well as to the increase of Voc of solar cells. The reason for the tails and their reduction is not clear but strain might play a crucial role and the amorphous hydrogenated phase might be effective for strain reduction.

Metalorganic chemical vapor deposition of quaternary AlInGaN multiple quantum well structures for deep ultraviolet emitters

AbstractWe report on the growth of quaternary AlInGaN layers and MQWs by two different metalorganic chemical vapor deposition (MOCVD) techniques such as pulsed atomic layer epitaxy (PALE) and pulsed MOCVD (PMOCVD). For both growth processes, emission wavelength of quaternary MQWs can be tuned from 350 nm to 300 nm by simply changing the unit growth cell configurations. The PALE grown AlInGaN MQWs have a very smooth surface, few band tail states and exhibit a band-to-band emission. The PMOCVD grown AlInGaN MQWs exhibit a high density of band tail states, which strongly enhance spontaneous emission. Based on the characterization by photoluminescence, X-ray diffraction and AFM, both MOCVD techniques grown quaternary samples are shown to be promising for fabricating the active region of deep UV LEDs.