Non-crystalline Semiconductors Research Articles

A life dedicated to science: academician Andrei ANDRIEȘ (1933–2012)

The article is dedicated to the memory of the distinguished scientist, the President of the Academy of Sciences of Moldova in 1989–2004, academician Andrei Andrieș. Andrei Andrieș certainly had a lifelong passion – the physics of amorphous chalcogenous materials. On his initiative, a new research field of non-crystalline semiconductor physics was initiated, and a scientific school in the area of non-crystalline semiconductors was formed in Chișinău. Academician Andrei Andries, an outstanding personality in this field, recognized worldwide, is author/ coauthor of numerous scientific publications, participating in international scientific conferences; he was an excellent organizer of science in the Republic of Moldova.

Energy-band-structure calculation by below-band-gap spectrophotometry in thin layers of non-crystalline semiconductors: A case study of unhydrogenated [formula omitted]-Si

We have in depth analyzed the refractive-index behavior and optical absorption of below-band-gap light, in order to calculate the basic parameters of the energy-band structure of thin layers of non-crystalline semiconductors. By carrying out a semi-empirical determination of the influence of the finite (non-zero) width of the valence and conduction electronic bands, we find the dependence of the index of refraction upon the photon energy, n(E), which goes just one order beyond the Wemple–DiDomenico two-level single-oscillator expression, and we simultaneously obtain the spectral dependence of the absorption coefficient, α(E). By model fitting the measured normal-incidence transmittance spectrum, we demonstrate that with a highly-sensitive double-beam spectrophotometer, it can be accurately determined the energy distance, EM,Sol, between the corresponding ‘centers of mass’ of the bonding and anti-bonding electronic bands, and also a reasonable estimate of the so-called effective width, Δeff, of both valence and conduction bands. We have used this devised optical approach with a series of uniform and non-uniform thin layers of unhydrogenated fully a-Si, grown by RF-magnetron-sputtering deposition, onto room-temperature transparent glass substrates. The advantages of our novel approach are mainly due to the additional attention paid to the roles of the weak-absorption Urbach tail and the thickness non-uniformity of the studied a-Si films. We have also used a universal normal-incidence transmission expression reported by the authors in an earlier paper, which can be applied even to strongly-wedge-shaped semiconductor layers. Together with the use of the improved Solomon formula for the normal optical dispersion of the refractive index, the complete approach with all its elements constitutes the main novelty of the present paper, in comparison with other existing works.

Energy-Band-Structure Calculation by Below-Band-Gap Spectrophotometry in Thin Layers of Non-Crystalline Semiconductors: A Case Study of Unhydrogenated A-Si

Energy-Band-Structure Calculation by Below-Band-Gap Spectrophotometry in Thin Layers of Non-Crystalline Semiconductors: A Case Study of Unhydrogenated A-Si

Noise and detectivity limits in organic shortwave infrared photodiodes with low disorder

To achieve high detectivity in infrared detectors, it is critical to reduce the device noise. However, for non-crystalline semiconductors, an essential framework is missing to understand and predict the effects of disorder on the dark current. This report presents experimental and modeling studies on the noise current in exemplar organic bulk heterojunction photodiodes, with 10 donor–acceptor combinations spanning wavelength between 800 and 1600 nm. A significant reduction of the noise and higher detectivity were found in devices using non-fullerene acceptors (NFAs) in comparison to those using fullerene derivatives. The low noise in NFA blends was attributed to a sharp drop off in the distribution of bandtail states, as revealed by variable-temperature density-of-states measurements. Taking disorder into account, we developed a general physical model to explain the dependence of thermal noise on the effective bandgap and bandtail spread. The model provides theoretical targets for the maximum detectivity that can be obtained at different detection wavelengths in inherently disordered infrared photodiodes.

Pulse percolation conduction and multi-valued memory

We develop a theory of pulse conduction in percolation type materials such as noncrystalline semiconductors and nano-metal compounds. For short voltage pulses, the corresponding electric currents are inversely proportional to the pulse length and exhibit significant nonohmicity due to strong local fields in resistive regions of the percolation bonds. These fields can trigger local switching events incrementally changing bond resistances in response to pulse trains. Our prediction opens a venue to a class of multi-valued nonvolatile memories implementable with a variety of materials.

A new approach to the schrodinger equation with position-dependent mass and its implications in quantum dots and semiconductors

In this study, we introduce a new approach to construct the Schrödinger equation with position-dependent mass characterized by an emerging particle's effective mass, in perfect analogy with the problems involving a position-dependent mass particle in semiconductor heterostructures. We have discussed several of its properties and some of its implications in quantum dots and semiconductors. It was observed that the PDM approach affects only semiconductors with large interatomic scaling such as non-crystalline semiconductors and its effect on conventional semiconductors is insignificant.

Temperature Dependence of Urbach Energy in Non-Crystalline Semiconductors

Until now, no analytical relationships have been derived for the temperature dependence of the Urbach energy in non-crystalline semiconductors. Consequently, the problem associated with the theoretical study of the temperature dependence of this energy has not been solved. This paper presents the results of theoretical calculations and attempts to establish the temperature dependence of the Urbach energy in non-crystalline semiconductors. A linear increase in the Urbach energy with increasing temperature is shown.

Nanosized levels of the self-organized structures in the non-crystalline semiconductors As–S(Se) system

Nanosized levels of the self-organized structures in the non-crystalline semiconductors As–S(Se) system

Radical addition polymerization: Enzymatic template-free synthesis of conjugated polymers and their nanostructure fabrication.

Conjugated polymers are attractive for many applications due to their unique properties. Their molecular structure can easily be tuned, making them suitable for an enormous number of specific applications. Conjugated polymers have the potential to achieve electrical properties similar to those of noncrystalline inorganic semiconductors; however, their chemical structure is much more complex and somewhat resembles that of biomacromolecules. The molecular conformation and interactions of conjugated polymers play an important role in their functionality. The use of enzymes has emerged as a highly valuable alternative method to synthesize these polymers and is very useful in the fabrication of their nanostructures. Here, we present established strategies for the synthesis of conjugated polymers in template-free systems that do not interfere with the preparation of their nanostructures. These strategies are based on the use of peroxidases (class III; EC 1.11.1.7, donor: hydrogen peroxide oxidoreductase), which are enzymes that have the ability to catalyze the oxidation of a number of compounds (including aromatics such as aniline, pyrrole, thiophene and some of their derivatives), in the presence of hydrogen peroxide, to obtain conjugated polymers.

Determining a hopping polaron's bandwidth from its Seebeck coefficient: Measuring the disorder energy of a non-crystalline semiconductor

Charge carriers that execute multi-phonon hopping generally interact strongly enough with phonons to form polarons. A polaron's sluggish motion is linked to slowly shifting atomic displacements that severely reduce the intrinsic width of its transport band. Here a means to estimate hopping polarons' bandwidths from Seebeck-coefficient measurements is described. The magnitudes of semiconductors' Seebeck coefficients are usually quite large (>k/|q| = 86 μV/K) near room temperature. However, in accord with the third law of thermodynamics, Seebeck coefficients must vanish at absolute zero. Here, the transition of the Seebeck coefficient of hopping polarons to its low-temperature regime is investigated. The temperature and sharpness of this transition depend on the concentration of carriers and on the width of their transport band. This feature provides a means of estimating the width of a polaron's transport band. Since the intrinsic broadening of polaron bands is very small, less than the characteristic phonon energy, the net widths of polaron transport bands in disordered semiconductors approach the energetic disorder experienced by their hopping carriers, their disorder energy.

High-Current Gain Two-Dimensional MoS2-Base Hot-Electron Transistors

The vertical transport of nonequilibrium charge carriers through semiconductor heterostructures has led to milestones in electronics with the development of the hot-electron transistor. Recently, significant advances have been made with atomically sharp heterostructures implementing various two-dimensional materials. Although graphene-base hot-electron transistors show great promise for electronic switching at high frequencies, they are limited by their low current gain. Here we show that, by choosing MoS2 and HfO2 for the filter barrier interface and using a noncrystalline semiconductor such as ITO for the collector, we can achieve an unprecedentedly high-current gain (α ∼ 0.95) in our hot-electron transistors operating at room temperature. Furthermore, the current gain can be tuned over 2 orders of magnitude with the collector-base voltage albeit this feature currently presents a drawback in the transistor performance metrics such as poor output resistance and poor intrinsic voltage gain. We anticipate our transistors will pave the way toward the realization of novel flexible 2D material-based high-density, low-energy, and high-frequency hot-carrier electronic applications.

Dynamic Charge Carrier Trapping in Quantum Dot Field Effect Transistors.

Noncrystalline semiconductor materials often exhibit hysteresis in charge transport measurements whose mechanism is largely unknown. Here we study the dynamics of charge injection and transport in PbS quantum dot (QD) monolayers in a field effect transistor (FET). Using Kelvin probe force microscopy, we measured the temporal response of the QDs as the channel material in a FET following step function changes of gate bias. The measurements reveal an exponential decay of mobile carrier density with time constants of 3-5 s for holes and ∼10 s for electrons. An Ohmic behavior, with uniform carrier density, was observed along the channel during the injection and transport processes. These slow, uniform carrier trapping processes are reversible, with time constants that depend critically on the gas environment. We propose that the underlying mechanism is some reversible electrochemical process involving dissociation and diffusion of water and/or oxygen related species. These trapping processes are dynamically activated by the injected charges, in contrast with static electronic traps whose presence is independent of the charge state. Understanding and controlling these processes is important for improving the performance of electronic, optoelectronic, and memory devices based on disordered semiconductors.

The Influence of Defects on the Energetic Spectrum of Noncrystalline Semiconductors

The Influence of Defects on the Energetic Spectrum of Noncrystalline Semiconductors

The offset of the quantum interband transitions in non-crystalline semiconductors

The paper proposes an approach to the description of non-crystalline semiconductors, which are promising materials in solar photovoltaic industry. We describe the optical wave interaction with film structure that has an amorphous solid state, its thickness being about 100 nanometers. The edge’s fundamental absorption displacements in semiconductors with the introduction of defects into the periodic lattice have been explained by comparing the crystalline and amorphous material. Averaged experimental data have been compared with the proposed description of the absorption coefficient in single-crystal and amorphous silicon. Optical band gap expansion in silicon, which leads to the absorption spectrum shift from 1.12 to 1.6 eV, has been shown and described.

Thin film uncooled microbolometers based on plasma deposited materials

We make a summary of our research and development efforts made about microbolometers (MBs) based on plasma-enhanced chemical vapor deposition (PECVD) materials, like noncrystalline semiconductors that provide high temperature coefficient of resistivity values, in conjunction with SiOx and SiNx dielectrics used for thermoisolation, which, together with micromachining, are paving new ways for fabrication of MBs, making them promising for 2D imagers in both infrared and tera-Hertz regions. We studied a-Si:H(B), a-Ge:H, a-GeSi:H, and polymorphous p-Ge:H, p-SiGe:H as thermosensing materials (TSMs) for MBs in “bridge” configuration with “planar” and “sandwich” electrodes. This allows placing the read-out circuitry under the bridge, improving use of pixel area. PECVD SiNx films were used as both a support layer and as a coating for improving the response for λ = 8–12 μm. 2D modeling revealed both linear and super linear response to IR intensity. Voltage responsivity RU = (1.2–7) × 105 V/W is observed in both “planar” and “sandwich” MBs. The latter shows current responsivity RI = 0.3–14 A/W higher by about three orders of magnitude than the former. A key issue for any detector is the detectivity. Different TSMs show different noise characteristics. Noise in “sandwich” MBs is several orders of magnitude higher than that in “planar” structures. The best parameters observed with TSM Ge-Si:H are: RU = 7.2 × 105 V/W, RI = 14 A/W voltage and current detectivities [Formula: see text] = 8 × 109 cm Hz1/2W−1 and [Formula: see text] = 4 × 109 cm Hz1/2W−1. Junction structures on top of the “bridge” are also discussed. Finally we describe some reported applications of MBs.

AIM-Spice Integration of a Recursive Model for Threshold Voltage Shift in Thin Film Transistors

Non-crystalline semiconductor based thin film transistors are the building blocks of large area electronic systems. These devices experience a threshold voltage shift with time due to prolonged gate bias stress. In this paper we integrate a recursive model for threshold voltage shift with the open source BSIM4V4 model of AIM-Spice. This creates a tool for circuit simulation for TFTs. We demonstrate the integrity of the model using several test cases including display driver circuits.

WE-G-500-08: Novel Multilayer Detector Design Using Polycrystalline CdTe for Radiation Therapy Imaging Applications

Purpose: In radiation therapy treatments the standard approach of taking a planar portal or CIAO image offers setup verifications crucial to accurate radiation delivery. A substantial detector thickness necessary for MV photon absorption is a limitation due to large signal spreading; the requirements of large areas (up to 40×40 cm) and commercial viability dictate the use of non-crystalline semiconductors. We propose a detector based on thin-film CdTe technologies recently developed for photovoltaic applications. It utilizes a stack of CdTe photovoltaic layers, each having a built-in electric field. This 3D structure adds a depth signal reading to the traditional 2-D pixilation allowing for the photon energy and position resolution. Methods: We modeled the energy deposition under mono-energetic photon sources of 0.5 to 18MV with depth by Monte Carlo simulation package MCNP5. The results yield the charge carrier generation profiles in structures having 3 to 30 layers, and CdTe thickness of 2–10 microns. The profiles were used as input for modeling of current-voltage (I–V) characteristics with device operation modeling software SCAPS-1D. Results: Based on the modeled I–V we established variations of the output signal in each layer with depth (layer number), and developed an algorithm for extracting the position dependent source energy. Our design includes an original read-out approach where, under the global short circuit conditions, the voltages across individual layers are exponentially sensitive to their corresponding generation rates, a ∼1% difference in deposition energies translates into ∼100% difference in the corresponding voltage readings. Conclusion: The micron-range thickness of layers overcomes known limiting factors of the low hole mobility and electron trapping typical of thick crystalline CdTe detectors. A new algorithm allows extracting information on radiation intensity and spectra from the data on the layer photo-voltages leading to enhancement in spatial resolution.Acknowledgment: this research is supported through NRC grant No.: NRC-HQ-12-G-38-0042; This project is part of a faculty development award supported by the NRC grant No.: NRC-HQ-12-G-38-0042

The Meyer–Neldel rule for dc activation processes in mixed isoelectronic chalcogens systems

The origin of the compensation Meyer–Neldel rule (MNR) for an Arrhenius dc conductivity behavior has been elucidated via comparative analytical studies on non-crystalline and crystalline groups of Se-like structure semiconductors (i.e., mixed isoelectronic chalcogens: Se–S, Se–Te and Se–S–Te systems) with a comparison with those of Si:H. Findings and results disclosed a clear appearance of the MNR in all non-crystalline forms and a common deviation for the crystalline structures. The presence (EMN>0) or absence (EMN<0) of the MNR does not depend entirely on the quantity of the activation energy of conduction process, whether it is small or large. This surveillance brings about a fact that defects associated with disorder do behave in a monotonic similar mode in the two non-crystalline semiconductors types. In this consensus, one can realize that the concept of the multiple excitations entropy stimulated by phonon energy is strongly authenticating the correlation between the two MN parameters (true-prefactor σ00 and MN-energy EMN) for both lone-pair (mixed chalcogens) as well as for tetrahedrally bonded (Si:H) semiconductors.

A New Perspective on an Old Problem: The Staebler-Wronski Effect

AbstractPhoto-induced structural changes and defect creation are common phenomena in a large variety of polymeric and non-crystalline semiconductors. The photo-induced degradation of a-Si:H and its alloys, discovered by Staebler and Wronski in 1977, belongs to a special category with quite unique features, which so far has resisted an explanation. Part of the problem is that the near 4-fold coordinated network does not naturally form an amorphous material. It is over-constrained and forms a stress relief void structure. While reviewing the experimental evidence it will be argued that some of our commonly held views regarding the underlying mechanisms of the Staebler-Wronski effect (SWE) may have to be abandoned. First, the internal void surfaces seem to be the principal locations of the photo-structural changes. Second, non-radiative bimolecular recombinations of photo carriers do not seem to be the driving force of defect creation at helium temperatures. Alternative pathways for the photo-induced processes will be suggested.

The barrier–cluster model applied to chalcogenide glasses

The aim of this article is to familiarize the readers with barrier–cluster model of the non-crystalline semiconductors, which is based on the assumption that in non-crystalline semiconductors, there exist micro-regions separated from each other by potential barriers. We suppose that these micro-regions in chalcogenide glasses are created by closed clusters. This model allows explanation not only of a number of important optical and electrical features of chalcogenide glasses, but also the results of X-ray structure measurements and ESR experiments. This concept gives a new look at the density of states within the forbidden band and at the exponential tails of the optical absorption.