States In Amorphous Semiconductors Research Articles

Band gap engineering and low temperature transport phenomenon in highly conducting antimony doped tin oxide thin films

A huge band gap tuning and low temperature transport phenomenon in highly transparent antimony doped tin oxide thin film (Sb:SnO2) under the influence of swift heavy ions irradiation (SHII) is reported. Structural analysis shows an enhancement in crystallinity at initial fluence of irradiation followed by amorphization at higher fluences. Films were also well studied for their surface morphology by atomic force microscopy and scanning electron microscopy. Band gap analysis reveals a drastic band gap narrowing around 1.1eV upon SHI irradiation. Transport measurements show that the high conductivity and the carrier concentration decrease upon increase in the fluence of irradiation. The mechanism of charge carrier transport investigated at low temperature is attributed to nearest neighbor hopping (NNH) and variable range hopping (VRH) in different temperature regimes. Origin of the band gap tuning is understood in framework of Burstein–Moss (BM) shift, Quantum Confinement (QC) effect and band-tailing states in amorphous semiconductors.

Thermal Management in Photo-Electrochemical Hydrogen Generation Devices Using Concentrated Solar Irradaiton

Human activity is overloading the atmosphere with global warming gas emissions which create significant and harmful impacts on our health, environment and climate. Utilizing the ample energy available from renewable sources like sunlight and water, and converting it into chemical energy by photoelectrochemical (PEC) processes, is one viable route to reduce these impacts as life cycle global warming emissions associated with renewable energy are minimal. This presents an effective path for renewable fuel processing and energy storage. Integrated PEC devices, which are composed of an integrated traditional photovoltaics (PV) component and an electrolyzer (EC) component, allow to circumvent some of the challenges imposed by solid-liquid interface in traditional PEC devices, and operate at higher efficiencies than externally wired (non-integrated) PV plus EC devices. To make the integrated devices economically competitive, we have employed concentration of irradiation. We proposed a novel integrated design, shown in fig. 1, combining EC, PV and concentrator [1]. We developed, a coupled 2D multi-physics model of the proposed concentrated PEC device. The model includes radiative heat transfer in the concentrator, electromagnetic wave propagation in the semiconductors (a triple/dual junction solar cell), charge generation and transport in the photoabsorbers and the integrated electrolyzer (polymeric electrolyte and solid electrode), electrochemical reaction at the catalytic sites, fluid flow and species transport in the channels delivering the reactant (water) and removing the products (hydrogen and oxygen), and heat transfer in all components. The detailed simulation flow and coupling between different physical nodes have been shown in fig. 2. Detailed treatment was given for various heat source/sink term calculations for effective thermal management. Urbach tails and defect states in amorphous semiconductors have been incorporated using wxAMPS [2].We completely automatized the simulation flow using multiple interactions between MATLAB Inc., COMSOL and wxAMPS. Efficient computational power saving techniques have been rigorously employed making our model one of the most detailed yet computationally economical. The model proves to be a valuable design and optimization tool for integrated PEC devices working with concentrated irradiation and at elevated temperatures. The model allowed to investigate and optimize the thermal management in the integrated PEC, highlighting that the operating point of the integrated device could be readily shifted to the maximum power point (MPP) of the PV. Thermal management therefore allowed for an efficient and cost effective way of tracking the optimum operating point (OOP), e.g. by controlling the flow rate of water in the water channel for a particular irradiation concentration, providing two distinct benefits: i) reducing the losses by utilizing the generated heat, ii) enabling simple tracking of OOP of the device. The model allowed to conclude that the OOP at a particular concentration was the MPP of the PV and that the OPP of the integrated system was decided by the EC’s current-voltage curve’s saturation part. The mass transport limitations in the EC govern the optical concentration for maximized solar-to-fuel efficiency. The top water channel above the PV not only helps in cooling the PV and supplying the heated reactants to the electrolyser, but it also enhances the optical absorption in the PV by reducing the reflections on top cell surface. The absorption in the water for comparable channel widths is found to be minimal within the absorption spectra of the PV. Parametric studies allowed to find the optimum channel widths governing the absorption and the cooling of device for varying irradiance concentrations. We analysed triple/dual junction thin film solar cells as a cost effective alternative to the earlier studied Ga0.51In0.49P-GaAs-Si [1]. Smart thermal management allowed the design of an integrated device with a dual aSi-ucSi cell to operate at efficiencies comparable to those of Ga0.51In0.49P-GaAs-Si device under non-concentrated irradiation. The model proves to be a valuable tool for the design of integrated PEC cells working with concentrated irradiation at elevated temperatures and highlights that the smart thermal management can help in achieving increased low cost production of solar fuel.

Quantum Phenomena in Structural Glasses: The Intrinsic Origin of Electronic and Cryogenic Anomalies

The structural glass transition is often regarded as purely a problem of the classical theory of liquids. The dynamics of electrons enters only implicitly, through the interactions between ionic cores or molecules. Likewise, zero-point effects tied to the atomic masses hardly affect the typical barriers for liquid rearrangements. Yet, glasses do exhibit many quantum phenomena—electronic, optical, and cryogenic peculiarities that seem to have universal characteristics. These anomalies of the glassy state are uncommon or strongly system dependent in crystals and amorphous solids not made by a quasi-equilibrium quench of a melt. These clearly quantum phenomena include midgap electronic states in amorphous semiconductors, the two-level systems, and the Boson peak. Here, we discuss how these quantum phenomena found in glasses are not merely consequences of any kind of disorder but have universal characteristics stemming from the structural dynamics inherent in the glass transition itself. The quantum dynamics ...

Urbach energy and the tails of the density of states in amorphous semiconductors

Formulas are derived for the Urbach energy as a function of the parameters controlling the slope of the exponential tails in the valence and conduction bands of amorphous semiconductors. It is shown that the Urbach energy remains constant, even if one of these parameters varies over a wide range.

What is the origin of tail states in amorphous semiconductors?

The tight-binding molecular dynamics simulations and reverse Monte Carlo structural modeling method were applied in order to investigate the existence of small bond angles (like those in triangles and squares) in amorphous silicon networks. The influence of small bond angles on the electronic density of states was analyzed. The presence of a number of smaller bond angles is necessary for a proper reproduction of the neutron diffraction data of amorphous silicon. Semi-empirical Hartree–Fock calculations show that these arrangements provide higher energy states in electronic density of states which are localized on these local structures. We consider that smaller bond angles are a new type of defects. These defect states are highly energetic and they are the origin of tail in amorphous semiconductors.

Excitonic contribution to photoluminescence in amorphous semiconductors

Abstract Applying the effective-mass approach, the energy eigenvalues of excitonic states in amorphous semiconductors are derived. It is shown that Wannier–Mott-type excitons can indeed be formed in amorphous solids. The results show that the occurrence of the double photoluminescence (PL) lifetime distribution peak, fast and slow, in hydrogenated amorphous silicon (a-Si: H) and hydrogenated amorphous germanium (a-Ge: H) can unambiguously be assigned to radiative recombinations from singlet and triplet excitonic states respectively. The dependence of PL peaks on the temperature and generation rate in a-Si: H and a-Ge: H is also discussed. The approach is general and simple and can be applied to study the charge-carrier transport and PL properties in any amorphous solid.

Excitonic contribution to photoluminescence in amorphous semiconductors

Abstract Applying the effective-mass approach, the energy eigenvalues of excitonic states in amorphous semiconductors are derived. It is shown that Wannier–Mott-type excitons can indeed be formed in amorphous solids. The results show that the occurrence of the double photoluminescence (PL) lifetime distribution peak, fast and slow, in hydrogenated amorphous silicon (a-Si: H) and hydrogenated amorphous germanium (a-Ge: H) can unambiguously be assigned to radiative recombinations from singlet and triplet excitonic states respectively. The dependence of PL peaks on the temperature and generation rate in a-Si: H and a-Ge: H is also discussed. The approach is general and simple and can be applied to study the charge-carrier transport and PL properties in any amorphous solid.

An experimental evaluation of transient and modulated photocurrent density-of-states spectroscopies

Abstract An evaluation of transient photocurrent (TPC) and modulated photocurrent (MPC) spectroscopies as a means of studying the density and capture properties of localized states in amorphous semiconductors is presented. Freauency-domain analysis of TPC data via the discrete Fourier transform (TPC FT) permits a direct comparison with MPC data obtained using conventional lock-in techniaues to be made. Results obtained from undoped hydrogenated amorphous silicon over a wide range of temperatures and optical excitations are used to explore the limits of resolution and applicability, and to highlight the relative merits, of each approach. It is shown that TPC spectroscopy offers significant practical advantages over MPC spectroscopy from the viewpoint of signal-to-noise performance. Discrepancies between TPC FT and MPC data obtained from the same sample under equivalent conditions suggest that the Fourier transform pairing of these methods is not exact, even when the reauirements of low excitation are met. ...

Improved High Resolution Post-Transit Spectroscopy for Determining the Density of States in Amorphous Semiconductors

ABSTRACTWe show that the analysis of post-transit photocurrent i(t) in a multi-trapping context to determine the density of trapping states g(E) is capable of resolving features less than kT in width. A commonly used method uses a Laplace inversion of i(t) data giving the well-known result g(E) ∼ t i(t) but employs a delta function approximation for trap release times, which results in loss of energy resolution. We show that it is possible to retain the exponential distribution function for trap release time and solve the multi-trapping rate equations directly, giving significantly improved resolution. The analysis is performed on computer generated post-transit data for distributed and discrete traps, and compared with the earlier method and other related Fourier transform methods for determining g(E). In addition, the versatility of the new method in handling cases with either distributed traps or with discrete traps means that it can be applied to disordered materials or to crystalline materials with well-defined defect levels.

Influence of the kinetic energy of localization on the distribution of electronic states in amorphous semiconductors

We examine how the kinetic energy of localization associated with the site disorder influences the distribution of electronic states in amorphous semiconductors. Employing an elementary potential well model, in which the disorder characteristic of amorphous semiconductors is represented by an ensemble of potential wells, we evaluate the one-electron density of states function by first determining the binding energy corresponding to each well, and then averaging over the ensemble of wells. We find that the kinetic energy of localization plays an important role in determining both the shape and the magnitude of this one-electron density of states function.

A Brief Study to Clarify Some Aspects Related to Vibrational Density of States for the Far Infrared Range in Amorphous Semiconductors

Phonon density of states of amorphous semiconductors for the far-infrared range is examined analytically. On the basis of this formulation, optical absorption corresponding to structural disorder is evaluated and discussed at the far-infrared range for a-Ge and a-Si.

The influence of the distribution of potential fluctuations on the distribution of states in amorphous semiconductors

Using a simplified but representative model for the physical structure of an amorphous semiconductor, we examine the corresponding distribution of potential fluctuations. We demonstrate that substantial deviations from a Gaussian distribution of potential fluctuations occur for reasonable parameter selections. We then use a semiclassical approach to study how the distribution of states is shaped by this distribution of potential fluctuations. We find that while the distribution of tail states is relatively heavily influenced by the distribution of potential fluctuations, the distribution of band states is not.

Density of states in amorphous semiconductors determined from transient photoconductivity experiment: Computer simulation and experiment

A method for the determination of localized density-of-state distributions in amorphous semiconductors from transient photoconductivity using Laplace transforms is described. The method is applicable to both the pre- and post-monomolecular recombination regimes of transient photoconductivity, and to amorphous semiconductors exhibiting either non-dispersive or dispersive transport. These advantages are demonstrated by the numerical simulation of transient photoconductivity using a fast inverse Laplace transform technique for model localized-state distributions appropriate for amorphous semiconductors. In addition, the numerical technique is proposed for the improvement of the energy resolution of the method. The applicability of the method is experimentally shown in amorphous GeS 2 thin films.

MuSR simulation of hydrogen centres in amorphous semiconductors — the case of germanium selenide glass

MuSR (muon spin rotation/repolarization/resonance/relaxation) is introduced as a useful tool for the study of dilute hydrogen states in amorphous semiconductors. Its application is then exemplified by considering the case of glassy germanium selenide (g-GeSe 2), the first results on which indicate the presence of both paramagnetic as well as diamagnetic centres. Theoretical fits to the data yield a contact hyperfine constant, A, for the paramagnetic centre of about ∼ 3200 MHz at room temperature and ∼ 2000 MHz at low temperatures — values that can be compared with A = 4463 MHz for vacuum-state muonium. The general shape of the experimental repolarization curves suggests that the paramagnetic centre is anisotropic. Differences between the centre in this host and that in g-SiO 2 are discussed.

Potential fluctuations in amorphous silicon

Recent measurements of the electron and hole drift mobilities and the optical absorption in hydrogenated amorphous silicon have strongly supported the presence of long-range potential fluctuations (LRPF) in a-Si:H. Independently, such fluctuations were suggested as the cause of the exponential energy distribution of localized tail states in amorphous semiconductors. These two problems are analyzed theoretically, and it is shown that the experimental results on the optical absorption can, indeed, be well accounted for, even quantitatively, by the classical theory of LRPF. However, this theory does not predict a purely exponential dependence for the density of states. Such a dependence could only appear in the transition range from a Gaussian shape, which for shallow states is caused by typical classical fluctuations, to a weaker dependence caused by quantum mechanical fluctuations for deeper localization energies.

Signature of the Weak Bond-Dangling Bond Conversion Process in a-Si:H as Seen by Total Photoelectron Yield Spectroscopy

Photoelectron Yield Spectroscopy is one of the most direct methods to study the density of gap states of amorphous semiconductors. We have made use of it to investigate the transient changes of the density of gap states of a-Si:H upon illumination with above-band-gap light. An excitation density of 50 mW/cm2 of 532 nm light from a frequency doubled Nd-YAG laser was modulated with frequencies between 0.1 Hz and 10 KHz and the transient response of the photoelectron yield signal was monitored by gated electron counting or a lock-in amplifier. From this we deduce a reversible increase of the occupied density of defect states under illumination with a maximum Δg ∼ 1017 cm3 eV-1 at about 0.3 eV below the Fermi energy and a decrease below EF− 0.75 eV, i. e. in the region of deep tail states. The effect exhibits a wide spectrum of time constants involved. Based on additional transient surface photo voltage measurements as well as numerical simulations we argue that the observed redistribution of states is not due to a light induced change in surface band bending or due to a mere excitation of electrons from occupied to empty states. Instead, we interpret the observed effect as a conversion of weak to dangling bonds, i.e. a precursor of the Staebler-Wronski Effect, lacking however the final step of hydrogen stabilisation necessary for the creation of metastable defects.

Post-transit-time analysis of time-of-flight photocurrents

It is shown how the density of localized gap states in amorphous semiconductors can be deduced from time-of-flight photocurrents on the basis of the trap-limited band transport model, post-transit currents giving information on deep-state densities; and how strong deep-trapping of the photo-induced excess carriers invalidates the analysis. Results from a-Si:H, p-type a-Si:H, a-Si,S:H and a-Si,C:H samples are used to illustrate these points.

A study of the density of gap states in amorphous semiconductors from thermostimulated conductivity spectra

A theoretical analysis of thermostimulated conductivity spectra σTSC(T) has been applied to determine the density of gap states g(E) in a-Si: H and a-Si: H/a-SiNx: H multilayer structures. The results for g(E) are consistent with the results deduced from Fritzsche's analytical approach as well as other methods. A comparison has been made between the two different analytical approaches for σTSC(T). We discuss the relationship between the energy of maximum thermostimulated current emission Em and quasi-Fermi level Eq. We demonstrate that Eq could be a better parameter than Em in the general theoretical treatment of thermostimulated conductivity.

Analysis of high-frequency capacitance of amorphous silicon-crystalline silicon heterojunctions

Abstract High-frequency capacitance measurements as a function of applied bias of n-p amorphous-crystalline heterojunctions have been suggested as a method to evaluate band discontinuities and density of gap states in amorphous semi-conductors. However, the models used for analysing the capacitance are oversimplified and the measured capacitance data do not always follow the form predicted by these models. In this paper, we critically analyse the limitations of the existing models and present models based on a more realistic calculation of space charge in amorphous semiconductors.

An iterative procedure for improved xerographic spectroscopy analysis of gap states in amorphous semiconductors

Xerographic spectroscopy of deep trapping states in amorphous semiconductors with constant voltage charging has been studied by a numerical simulation technique. It has been found that full trap occupation cannot be achieved under charging conditions which are in practice characteristic of most charging devices. The occupation of traps at any stage of the charging-up process has been found to be dependent on parameters governing the trap distribution under investigation. Subsequent analysis of experimental data by conventional methods leads to the mapping of a filled trap distribution which has no direct correlation with the true trap state distribution. An iterative procedure is proposed to restore the ability to probe arbitrary trap distribution under such charging conditions, thereby mapping the true density of deep trap states. The procedure has been applied to experimental results obtained for a-As2Se3, the mapped true density of trap states is consistent with established results, thus indicating the validity of the iterative procedure.