The photo-crystallization of amorphous selenium thin films

The photocrystallization of amorphous selenium under the influence of light or electrons used to produce hole-electron pairs has been studied. Illumination increases the growth rate of crystallites and modifies their morphology. Conversely, electron irradiation alters the structure of the amorphous material and induces a decrease of the nucleation rate. An explanation is proposed, which takes into account recent publications on the ‘band structure’ and the nature of bonds in amorphous selenium.     

Dark conductivity in amorphous undoped silicon films

The temperature dependence of the dark conductivity was investigated in amorphous undoped silicon films deposited by glow-discharge in a SiCl₄H₂ mixture. Different transport processes were indentified according to the investigated temperature range. The dependence of the dark conductivity was also examined as a function of some deposition parameters. The experimental results are discussed in terms of the two-phase structure of the film.     

Study of the conductivity of nitrogen doped tetrahedral amorphous carbon films

The conductivity of nitrogen incorporated tetrahedral amorphous carbon (taC:N) films prepared by filtered cathodic vacuum arc (FCVA) system was studied. The film resistivity varied as a function of nitrogen content in a wide nitrogen content range from 1 to 23 at.%. An interesting phenomenon of the photoconductivity for the films was found, that the resistivity of those films with low nitrogen content (<5 at.%) increased due to the light irradiation. This kind of behavior of photoconductivity had not been discussed in previous papers on taC:N films.     

Photodarkening of amorphous selenium

Various parameters associated with photodarkening of amorphous selenium have been measured at 100 K. The spectral response of photodarkening shows a step at the absorption edge indicating that electron-hole pair creation is a basic feature of the process. It is shown that the slope of the Urbach edge decreases during photodarkening, and it is found that the change in the optical joint-density-of-states peaks in the region of photon energy between 2.0 and 2.5 eV. A film of a-Se subjected to ion bombardment at 100 K does not then show photodarkening, and this and the other observations are discussed in terms of a model of local change in atomic arrangement.     

Glass transition temperature dependence on heating rate and on ageing for amorphous selenium films

The study, by differential thermal analysis (DTA) of the glass transition of thermally evaporated amorphous selenium, allows the effect of ageing on structural properties to be observed. An enthalpic diagram is used to explain the shift of the glass transition region, with heating rate, and also with ageing. These shifts are directly dependent on structural relaxation in amorphous selenium. The model used, to express the change of the structural time relaxation with temperature, and with the departure of thermodynamic equilibrium leads to a slightly different expression for the dependence of T_g on heating rate than that previously published (by Moynihan et al.).The activation enthalpies of the studied phenomena compared with other results, seem to show that structural relaxation in a-Se principally involves bond breaking and reformation of chains.     

Acoustic behaviour of amorphous selenium

The attenuation of longitudinal sound waves in amorphous selenium has been measured at frequencies of 15 and 25 MHz and in the range of temperature between 10 and 310 K. It is shown that absorption exhibits a minimum around Tτ150 K, while a pronounced maximum takes place around 30 K. Such a behaviour is briefly discussed in the frame of acoustic properties of amorphous material below the glass temperature. It is shown that the experimental results can be described, almost qualitatively, in terms of a recently proposed theoretical model involving a stochastic resonance that seems to be characteristic of the disordered systems.     

The effects of oxygen,hydrogen and fluorine on the conductivity of high purity evaporated amorphous silicon films

Electron bombardment evaporation was used to deposit amorphous silicon (α-Si) films in an evaporator with a base pressure of 2 × 10^?10 Torr. Rutherford backscattering analysis was used to establish the conditions necessary for deposition of pure films.The DC conductivity was measured as a function of temperature (? 150°C to + 140°C). Pure films, which were deposited between room temperature and 400°C, were found to have a room temperature conductivity (σ_RT) in the region of 10^?3μ^?1cm^?1 and a log $\text{σα}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ dependence. The value of σ_RT could be reduced by annealing reaching a minimum of 2 × 10^?7μ^?1 cm^?1 for an anneal temperature (T_A) of 520°C, but activated conduction was not observed.The implantation of hydrogen or fluorine (or contamination with oxygen) had the effect of reducing σ_RT, with a minimum value of less than 10^?8μ^?1cm^?1 (T_A = 400°C) for fluorine implantation to a dose of ≈ 10¹⁶ cm² (≈ 0.4 at% concentration). These films had high temperature (50°C) activation energies typical of activated conduction in extended states on the edge of the mobility gap. Implantation of fluorine to a dose of 1.5 × 10¹⁷ resulted in a rise of σ_RT (T_A = 400°C) to nearly 10⁴μ^?1 cm^?1 and log $\text{σα}\text{T}{}^{\mathrm{<mtext>?1</mtext><mtext>4</mtext>}}$ behaviour.X-ray analysis revealed that some crystallization occurred in films annealed at 600°C. This is correlated with a rise in σ_RT of the pure films and the disappearance of the effects of the introduced impurities.     

The crystallization of amorphous germanium films

Germanium thin films were prepared in an amorphous form by vacuum deposition onto room temperature fused silica substrates. The amorphous—crystalline transition was studied as a function of time and temperature by measuring the optical transmission near 0.65 μm, where the absorption constant is most sensitive to the phase transformation. At a fixed temperature, the time for half the volume of the sample to become crystallized was found to be consistent with the relation t_c = τ exp(E₀/kT), with an activation energy E₀ = 2.96 eV.     

DC conductivity of amorphous germanium

A critical review of the phonon-assisted hopping theory of the dc conductivity σ is given. It is argued that the current theories either contain uncontrollable approximations, or they yield the result of the same order of magnitude as the approximations used or they may also lead to temperature dependence of σ other than that described by the Mott law. For the simplest usually accepted model of amorphous Ge (a-Ge), the low-temperature asymptote of σ is obtained in the form of a power law. In the experimental part of the work, careful measurements from 300 down to 8 K are reported, both for the pure and the gold-doped a-Ge films. Since the results of such measurements yield a straight line in a wide interval of temperature both in the log σ versus $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ and log σ versus log T scale, the power law appears to be equally well justified as the Mott law, both from the point of view of theory and that of experiment.     

Dielectric study and ac conductivity of iron sodium silicate glasses

A series of glasses with formula (SiO₂)_0.7−x(Na₂O)_0.3(Fe₂O₃)_x with ( 0.0 ≤ x ≤ 0.20) were prepared and studied by means of AC measurements in the frequency range 20 kHz to 13 MHz at room temperature. The study of frequency dependence of both dielectric constant ε' and dielectric loss ε" showed a decrease of both quantities with increasing frequency. The results have been explained on the basis of frequency assistance of electron hopping besides electron polarization. From the Cole-Cole diagram the values of the static dielectric constant ε_s, infinity dielectric constant ε_∞, macroscopic time constant τ, and molecular time constant τ_m are calculated for the studied amorphous samples. The frequency dependence of the ac conductivity obeys a power relation, that is σ_ac (ω) = Α ω^s. The obtained values of the constant s lie in the range of 0.7 ≤ s ≤ 1 in agreement with the theoretical value which confirms the simple quantum mechanical tunneling (QMT) model. The increase in ac conductivity with iron concentration is likely to arise due to structural changes occurring in the glass network. The structure of a glass with similar composition was published and showed clustering of Fe²⁺ and Fe³⁺ ions which favor electron hopping and provide pathways for charge transport.     

Electrical conductivity and crystallization in amorphous germanium

A simple model is presented which interprets conductivity variations due to crystallization in amorphous layers and allows the determination of the growth rate of crystallization. The validity of this model is demonstrated in the case of germanium.     

Preparation and characterization of amorphous cadmium–selenium ribbons

Chalcogenide glasses based on the cadmium–selenium system, with the selenium composition varying from 0 to 7.5 wt% have been prepared using melt-quenching method i.e., single-roller quenching technique. The X-ray diffraction (XRD) and selected area electron diffraction (SAD) patterns of the CdSe ribbons indicate that the ribbons are amorphous. The transmission electron microscopy (TEM) studies carried out on these ribbons reveal that the constituents are inhomogeneously distributed in these ribbons. The temperature dependence of the electrical resistivity, ρ and thermoelectric power (TEP) of these ribbons has been studied in the temperature range 30–350 °C. The sudden jump in the values of electrical resistivity at a specific temperature for each case in these ribbons has been correlated with the phase transition i.e., the onset of crystallization in these materials during heating. The crystallization temperature, T_c has been found to be a function of Se content of these ribbons. The phase change in these ribbons as a result of heating does not seem to affect the variation of TEP with temperature. However, the slope of TEP versus temperature curves depends on Se content in these ribbons. The differential scanning calorimetry (DSC) of these ribbons indicates that the supercooled region in these ribbons extends from 50 to 70 °C. The composition CdSe ribbon with 0.5 wt% Se has the highest value of T_c and glass forming ability, K_g = 0.7.     

Crystallization of amorphous germanium films

Kinetics of isothermal crystallization has been studied in the temperature range from 375 to 525°C. The kinetic curves are obtained and the rate of isothermal transformation of amorphous films into crystalline ones has been determined. Using experimentally determined kinetic curves the stability diagram of the amorphous films has been plotted in the temperature range from 400 to 525°C. The value of effective activation energy has been defined.     

Photoconductivity of amorphous CdS films

The photoconductivity of amorphous CdS films, vacuum evaporated onto cooled substrates, has been investigated. The conditions and possible reasons for the appearance of a low temperature maximum in the photoconductivity temperature dependence of chalcogen rich samples have been found. The presence of a slow recombination centre related to the lone pair level of the one-fold coordinated negative chalcogen has been assumed. The energy necessary for both the electron (2 eV) and hole (0.4 eV) escape from this centre has been determined. Photoinduced changes in photoconductivity have been examined as well.     

Crystallization and decomposition of amorphous silicon-aluminium films

The crystallization and decomposition of vacuum-deposited amorphous silicon-aluminium films have been examined by means of transmission electron microscopy. Depending on the aluminium concentration, the transformation of the metastable amorphous phase into the stable phases of aluminium and silicon proceeds by different reactions such as pre-crystallization of aluminium, polymorphous transformation into supersaturated crystalline solid solutions or eutectic decomposition. The temperature dependence of the eutectic crystallization was measured. The results are discussed in terms of the thermodynamics of amorphous-to-crystalline transformation.