Localized Band Tail States Research Articles

Optoelectronic Parameters of 2-oxo-2-(1-oxo-1H-isochromen-3-yl)ethyl methacrylate Compound Thin Film

The UV/VIS spectrophotometric scans were obtained to determine the opto-electronic properties of 2-oxo-2-(1-oxo-1H-isochromen-3-yl)ethyl methacrylate (other name: 2-(isocoumarin-3-yl)-2-oxoethyl methacrylate) compound thin film. The refractive index of compound at 700 nm was found to be 1.61. The values of the real part of the dielectric constant were higher than those of the imaginary part. The results indicated that the electronic transition responsible for the absorption was the indirect allowed one. The optical band gap constant and the Urbach energy corresponding to the width of the band tails of localized states were calculated to be 3.19 eV and 1.05 eV, respectively. From the results obtained, it is predicted that 2-oxo-2-(1-oxo-1H-isochromen-3-yl)ethyl methacrylate (OICEMA) compound can take place in the semiconductor class and play a role in the design of some electro-optic materials.

Dominance of c-axis orientation on the carrier transport properties of Sn doped ZnO thin films

Sol-gel derived pure and Tin (Sn) doped ZnO thin films were deposited on the glass substrate by cost-effective spin-coating method. The influence of Sn incorporation on the structural, morphological, optical, and electrical properties of ZnO films was investigated by varying Sn concentration. X-ray diffraction reveals the hexagonal crystal structure with a preferential orientation of the crystallites along c-axis in ZnO film after doping with Sn. The transparency of ZnO thin film in the visible region is increased significantly from 72% to 93% after Sn incorporation into the ZnO matrix. The grain size of the film decreased from 27 nm to 20 nm after doping with Sn. The widening of optical band gap from 3.23 to 3.29 eV i.e., a blue shift of absorption edge with Sn doping is attributed to a combination of the Burstein-Moss effect and electron-impurity scattering. Urbach energy values have shown that Sn dopant decreases the width of the band tail of localized states. The electrical properties of the films revealed the decrement of the conductivity in the doped film. The decrement in the conductivity upon Sn doping is explained considering the grain-boundary conduction model and piezoelectric scattering mechanism. The Figure of Merit was calculated to assess the efficiency of the prepared film and was found that the film Sn: ZnO (3 at.%) shows the highest value. The obtained results confirm that Sn dopant has considerable effects on the properties of ZnO thin films and can be used as promising transparent conducting oxide films for optoelectronic applications.

Improving the Electrical Parameters of Se80Te20 Films by the Sn Substitution for Te and Thermal-Induced Effect

Films of Se80Te20−xSnx (where x = 6 at.%, 12 at.%, and 15 at.%) were prepared by thermal evaporation in a certain thickness of ~ 450 nm. Both the structural and electrical properties of as-prepared and annealed Se80Te20−xSnx films were studied. The annealing process was performed at 373 K, 393 K, and 413 K to cover the amorphous-crystalline region. Structural analyses show an improvement of film crystallinity with increasing both Sn content and annealing temperature. On the other hand, the electrical conductivity also shows significant enhancement with increasing Sn content and annealing temperature. The electrical results reveal that, at low temperature, conduction takes place through variable range hopping in localized states close to the Fermi level. Consequently, it has been shown in high temperature was explained by thermal charge carrier activation tunneling in the band tails of localized states. More structural and electrical parameters were evaluated for the Se80Te20−xSnx films to increase the understanding of the electronic properties of the Se-Te-Sn system.

Effect of annealing temperature on the structural and optical properties of Ge15Se75Zn10 thin films

Ge15Se75Zn10 thin films were deposited onto cleaned glass substrates by the vacuum thermal evaporation technique. The as-deposited and annealed thin films were characterized by scanning electron microscopy, X-ray diffraction and optical spectroscopy. The average particle sizes of the crystallized GeSe and ZnSe phases increase with increasing annealing temperature. Some optical parameters of the as-deposited and annealed Ge15Se75Zn10 thin films were studied using both transmittance T(?) and reflectance R(?) measurements. The analysis of T(?) and R(?) data revealed the existence of direct optical band gap (EG) and the corresponding values were calculated to be 2.913, 2.802 and 2.692 eV for the as-deposited and annealed thin films at 373 and 423 K, respectively. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple-Didomenico (WD) model. The width of band tails of localized states (Ee), the single oscillator energy (E0), the dispersion energy (Ed), the high frequency dielectric constant (?L) and the steepness parameter (S) were estimated.

Study of ac conductivity mechanism and impedance spectroscopy in CNT-added Cu5Se75Te10In10 chalcogenide system

This study is devoted to the investigation of electrical properties of multi-walled carbon nanotube (MWCNT)-contaminated Cu-Se-Te-In chalcogenide glassy composite in the temperature range 303–373 K and frequency interval from 1 Hz to 1 MHz. The MWCNT/chalcogenide glass was characterized by means of X-ray diffractometer, field emission scanning electron microscope, impedance spectroscopy and electrical measurements. Electrical conductivity was increased by 10–100 times of magnitude by adding 1 and 2 wt% of MWCNT to it, changing the behaviour from insulator to the semiconductor. This rapid change in the electrical conductivity for carbon nanotube-added glasses is due to the highly conducting behaviour of carbon nanotubes. The data observed from dc conductivity measurement in the temperature range 303–373 K suggest that thermally activated hopping is the dominant conduction mechanism between localized states in band tails, which is explained by Mott’s model. The temperature-dependent relaxation phenomenon has also been examined by a detailed analysis of impedance spectra.

Potential Fluctuations and Localization Effects in CZTS Single Crystals, as Revealed by Optical Spectroscopy

Reports on Cu $$_2$$ ZnSn(S $$_x$$ Se $$_{1-x}$$ ) $$_4$$ (CZTSSe) solar cell devices all show an open-circuit voltage lower than expected, especially when compared to CuIn $$_x$$ Ga $$_{1-x}$$ (S,Se) $$_2$$ devices, which reduces their power efficiency and delays their development. A high concentration of intrinsic defects in CZTSSe, and their stabilization through neutral complex formation, which induces some local fluctuations, are at the origin of local energy shifts in the conduction and valence band edges. The implied band tail in Cu $$_2$$ ZnSnS $$_4$$ is studied in this work by combining three types of optical spectroscopy data: emission spectra compared to photoluminescence excitation spectroscopy, emission spectra as a function of excitation power, and time-resolved photoluminescence spectra. All these data converge to show that both the bandgap and the band tail of localized states just below are dependent on the degree of order/disorder in the Cu/Zn cation sublattice of the quaternary structure: in the more ordered structures, the bandgap increases by about 50 meV, and the energy range of the band tail is decreased from about 110 to 70 meV.

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.

Physical properties of neodymium tin oxide pyrochlore ceramics

AbstractIn this work, physical properties of neodymium tin oxide pyrochlore ceramics prepared by solid state reaction technique are investigated by means of X-ray diffraction, scanning electron microscopy, ultraviolet-visible light (UV-Vis) spectrophotometry and temperature dependent electrical resistivity measurements. The pyrochlore is observed to have a cubic FCC crystal lattice with lattice parameter of 10.578 Å. The planes of the cubic cell are best oriented in the [2 2 2] direction. From the X-ray, the UV-Vis spectrophotometry and the electrical resistivity data analysis, the grain size, strain, dislocation density, optical and thermal energy band gaps, localized energy band tail states and resistivity activation energies are determined and discussed. The pyrochlore is observed to have an optical energy band gap of ~3.40 eV. This value corresponds to 365 nm UV light spectra which nominates the neodymium tin oxide pyrochlore ceramics for the use as UV sensors.

Influence of polyvinylpyrrolidone on optical, electrical, and dielectric properties of poly(2-ethyl-2-oxazoline)-polyvinylpyrrolidone blends

Poly(2-ethyl-2-oxazoline) [PEOX] is blended with polyvinylpyrrolidone [PVP] having a relatively high dielectric constant to improve the optical and electrical properties of the material. PEOX-PVP polymer blends with 0, 20, 40, 60, and 80wt% PVP are characterized by their structural, optical, electrical and dielectric properties. SEM images and XRD spectra show that PEOX and PVP have a good miscibility and compatibility. XRD also confirms the amorphous structure of the samples. FTIR spectra indicate the presence of hydrogen bonding between PEOX and PVP. The optical energy band gap, Egopt, and the width of the band tail of localized states in the forbidden band gap, ΔE, as determined by UV–Vis spectroscopy, are changing with PVP content. Electrical and dielectric properties are measured at frequencies from 10Hz to 8MHz using LCR meter. The dielectric constant, the dielectric loss, and the loss tangent (tanδ) decrease, whereas the AC conductivity increases with increasing frequency. PEOX:PVP (80:20wt%) is an optimum blend with superior properties as compared with pure PEOX. This flexible and high-dielectric-constant polymer blend may have potential application in energy storage.

Hopping conduction and random telegraph signal in an exfoliated multilayer MoS2 field-effect transistor

We investigate the hopping conduction and random telegraph signal caused by various species of interface charge scatterers in a MoS2 multilayer field-effect transistor. The temperature dependence of the channel resistivity shows that at low temperatures and low carrier densities the carrier transport is via Mott variable range hopping with a hopping length changing from 41 to 80 nm. The hopping conduction was due to electron tunneling through localized band tail states formed by the scatterers located in the vicinity of the MoS2 layer. In the temperature range of 40–70 K, we observed random telegraph signal (RTS) that is caused by the capture and emission of a carrier by the interface traps that are located away from the layer. These traps form strong potential that interact with the layer and change the potential profile of the electron system. The characteristics of RTS depend strongly on gate bias and temperature, as well as the application of a magnetic field.

Localized Mg-vacancy states in the thermoelectric material Mg2−δSi0.4Sn0.6

Mg2SixSn1−x has been widely studied as a thermoelectric material owing to its high figure-of-merit, low cost, and non-toxicity. However, its electronic structure, particularly when the material contains Mg vacancies, has not been adequately described. The n-type nature of Mg2−δSi0.4Sn0.6 has been a puzzle. Mg deficiency can be present in Mg2SixSn1−x due to Mg evaporation and oxidation. Therefore, an investigation of the role of Mg vacancies is of great interest. In this work, we have prepared a series of samples with various Mg deficiency and Sb doping levels and measured their transport properties. The Seebeck coefficient of these samples all reveals n-type conduction. We propose that Mg vacancies in Mg2−δSixSn1−x create localized hole states inside the band gap instead of simply moving the Fermi-level into the valence band as would be predicted by a rigid band model. Our hypothesis is further confirmed by density-functional theory calculations, which show that the hole states are trapped at Mg vacancies above the valence band. Moreover, this localized hole-states model is used to interpret electrical transport properties. Both the Seebeck coefficient and resistivity of Mg2−δSi0.4Sn0.6 indicate an electron-hopping transport mechanism. In addition, the data suggest that localized band-tail states may exist in the conduction-band edge of Mg2SixSn1−x.

Electronic Structure and Properties of Organic Bulk-Heterojunction Interfaces.

The electronic structure and physical mechanisms of carrier generation and transport in the organic bulk heterojunction are reviewed. The electronic structure describes the bands and band-tail states, the band alignment at the bulk-heterojunction interface, and the overall density-of-states model. The different electronic character of excitons and mobile charge is discussed, the former being highly molecular and the latter more delocalized. Dissociation of the exciton via the charge-transfer (CT) states is attributed to weak binding of the CT state arising from charge delocalization. Carrier transport and charge collection is strongly influenced by the presence of localized band-tail states. Recombination is attributed primarily to transitions from mobile carriers to band-tail or deep trap states.

Band tail recombination in polymer:fullerene organic solar cells

Recombination through band tail localized states is studied analytically and by measurement of the forward-bias dark current as a function of temperature in three different organic bulk-heterojunction solar cells. The Shockley-Read-Hall mechanism is analyzed for the specific case of recombination between mobile carriers and an exponential distribution of localized band tail states. The analysis gives a simple relation between the dark current ideality factor and the band tail slope. Assumptions of the model are verified by numerical drift-diffusion modeling. Diode current-voltage measurements give good agreement with the analytical model, confirming that the band tail recombination mechanism applies to at least some organic solar cells. Deep traps provide a secondary recombination channel in some devices.

Blending of Poly(ethyl methacrylate) with Poly(2-hydroxy-3-phenoxypropyl methacrylate): Thermal and Optical Properties

The miscibility behavior of poly(ethyl methacrylate) [poly(EMA)] with poly(2-hydroxy-3-phenoxypropyl methacrylate) [poly(PPMA)] was studied by DSC technique. Thermogravimetric analysis was used to investigate the thermal degradation mechanism and activation energies for initial decomposition process under non-isothermal conditions by integral approximation methods. The reaction mechanism of decomposition process using the kinetic equations of Coats–Redfern method was described by kinetic equations of phase-boundary-controlled reaction, R 3 mechanism. The activation energies for R 3 mechanism are very close to the values obtained in both Kissinger (115.6 kJ/mol) and Flyn–Wall–Ozawa (121.1 kJ/mol) methods. The optical and dispersion parameters such as refractive index, extinction coefficient, steepness parameter, single-oscillator energy and dispersion energy of blend polymer were determined in the visible range as a function of wavelength. The energy of Urbach corresponding to the width of the band tails of localized states and the optical band gap constant were calculated to be 1.08 and 3.33 eV, respectively.

Optical characterization of organic material bromoindium phthalocyanine thin films grown by electron beam evaporation

This paper presents the optical properties of organic material bromoindium phthalocyanine (BrInPc) thin films grown by electron beam evaporation technique. The paper describes the optical characteristics of BrInPc thin films, which have been determined using spectrophotometric measurements of the absorbance, transmittance and reflectance at normal incident of light in the spectral range 300–1,100 nm. The optical band gap energy and type of the electronic transition have been determined by analysis of spectral behavior of absorption coefficient, which reveals the probability of both direct and indirect transitions. Other optical constants, such as refractive index, extinction coefficient, complex dielectric constant and optical conductivity of thin films have been evaluated. Moreover, the width of band tails of localized states (Urbach energy), steepness parameter and width of the defect states have been determined by studying the absorption coefficient spectra just below the fundamental absorption edge.

Spectroscopic ellipsometry study of N+ ion-implanted ethylene–norbornene films

The optical properties of 150keV N+ implanted ethylene–norbornene (TOPAS 6017S-04) copolymer were investigated using phase modulated spectroscopic ellipsometry (PMSE) and ultraviolet–visible (UV–Vis) spectroscopy in the ranges of 0.6–6.5eV and of 1.5–6.2eV, respectively. The single-effective-oscillator model was used to fit the calculated data to the experimental ellipsometric spectra. The results show that the oscillator and dispersion energies decrease with increasing ion fluence up to 1015cm−2, and then these parameters increase with further fluence increasing. Analysis of the UV–Vis absorption spectra revealed the presence of indirect electronic transitions with the band gap energy in the range of 1.3 to 2.8eV. It was found that both the band gap energy and the energy width of the distribution of localized band tail states decrease, while the values of Tauc coefficient increase with increasing the ion fluence. From the ellipsometric data we found that the real part of the dielectric function increased about 7% after irradiation with 1015cm−2, and decreased about 10% in samples modified with 1016cm−2.

Optical studies of TiO2–lead phthalocyanine nanocomposite thin films prepared by electron beam evaporation

The nanocomposite thin films of titanium dioxide (TiO2)–lead phthalocyanine (PbPc) have been prepared on glass substrates by the electron beam evaporation technique. The optical properties of TiO2/PbPc nanocomposite thin films have been investigated using a spectrophotometric measurement of the absorbance and transmittance at normal incident of light in the wavelength region 300–800nm. Surface morphology of thin films has been characterized using field emission scanning electron microscopy (FESEM). The UV–vis analysis has been performed to determine the type of electronic transition and the optical energy band gap. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region reveals that the absorption mechanism is due to direct transition. Moreover, by studying the absorption coefficient spectra just below the fundamental absorption edge, the width of band tails of localized states (Urbach energy), steepness parameter and width of the defect states have been evaluated. The obtained results of this novel nanocomposite (TiO2/PbPc) support the desirable features for the optoelectronic devices.

Optical characterization of electron beam evaporated chloroindium phthalocyanine thin films

In present study, the optical properties of chloroindium phthalocyanine (ClInPc) thin films prepared by electron beam evaporation have been investigated. The optical characteristics of the prepared thin films have been determined using spectrophotometric measurements of the absorbance, transmittance and reflectance at normal incidence in the spectral range 300–1,100 nm. Surface morphology of thin films is studied using field emission scanning electron microscopy. The absorption spectra recorded in UV-Visible region for the deposited films show two well defined intense absorption bands of phthalocyanine molecule; namely the Q-band and the Soret (B-band). The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region have been performed to determine the optical band gap energy and type of the electronic transition, which reveals the probability of direct and indirect transitions. Moreover, by studying the absorption coefficient spectra just below the fundamental absorption edge, the width of band tails of localized states (Urbach energy), steepness parameter and width of the defect states have been evaluated. The obtained results of this novel grown ClInPc thin films support the desirable feature for the optoelectronic devices.

Morphological, Optical and DC Conduction Properties of a-GaSe Semiconductor Nanoparticle Thin Films

Amorphous gallium selenide (a-GaSe) semiconductor nanoparticle thin films were deposited onto well cleaned glass substrates by inert gas condensation (IGC) technique under a vacuum of 400 × 10−6 Pa (3 × 10−6 Torr). The films were characterized by different structural and optical techniques, including X-ray diffraction, field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), UV–visible absorption spectroscopy and I–V measurements. The particle size and size distribution were determined by TEM images which show the presence of spherical particles in the range of 5–50 nm in size. SEM images indicate that the a-GaSe film grown on glass substrate is almost smooth and dense. The optical properties of a-GaSe nanoparticle thin films were determined by optical absorption spectra. The optical bandgap of the film was estimated to be 2.19 eV and the transitions are allowed direct type. The electrical conductivity of the deposited films has been studied as a function of temperature. In the higher temperature range the dominance of thermally activated band conduction was observed; whereas in the low temperature range the hopping conduction in the band tails of localized states was found to be dominated.

Effect of localized tail states in InGaN on the efficiency droop in GaN light-emitting diodes with increasing current density

The mechanism of the internal quantum efficiency droop in InGaN/GaN structures with multiple quantum wells at current densities of up to 40 A cm−2 in high-power light-emitting diodes is analyzed. It is shown that there exists a correlation between the efficiency droop and the broadening of the high-energy edge of the emission spectrum with increasing current density. It is also demonstrated that the efficiency is a spectrum-dependent quantity and the emission of higher energy photons starts to decrease at higher current densities. The effect of tunneling and thermally activated mechanisms of thermalization of carriers captured into shallow band-tail states in the energy gap of InGaN on the efficiency and the emission spectrum’s shape is considered. Analysis of the results obtained suggests that the efficiency droop occurs at high current densities because of the relative rise in the contribution from nonradiative recombination via defect states as a result of the increasing occupancy of deep band-tail states in InGaN. It is shown that power efficiency close to the theoretical limit can be obtained in the case of low-voltage tunnel injection into localized band-tail states in the InGaN active region.