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.     

Formation and stability criteria for amorphous Ge alloy films

Formation and stability of room-temperature deposited amorphous (a-) alloy films of Ge with Ag, Au, Cu, Ni, Fe, Ga, In, Sn, Sb, Ni, Bi and Nb have been studied, using differential thermal analysis and electron microscopy, as a function of various parameters such as: solubility of alloying element (in c-Ge), its melting point, atomic size and electonegativity. It has been shown that the composition range R, up to which amorphous alloy films are obtained, is affected by the solubility of the alloying element and its melting point. The effect of solubility S is dominant when S > 1 at%. However, for $\text{S ? 1}\text{at}\text{\%}$, the range is entirely governed by the melting point of the alloying element. Solubility and melting point have also been found to play a decisive role in stabilizing the amorphous phase of Ge. However, atomic size and electronegativity differences do not seem to play any important role in either the formation or stability of the amorphous phase.     

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.     

The network structure of amorphous silicon-carbon alloy

The network structure of amorphous silicon-carbon alloy (a-Si_1−xC_x) has been studied over a wide range of x. The a-Si_1−xC_x thin films were prepared by sputtering silicon and carbon target with argon in radio-frequency magnetron sputtering equipment. The films were characterized by X-ray photoelectron spectroscopy, optical absorption, infrared absorption, and mechanical measurements. The results showed that the network structure could be classified neither as the random covalent network nor as the chemically ordered covalent network. The structure as a whole was close to the random covalent network, but the Si-Si combination at x>0.5 showed a feature of the chemically ordered covalent network. The film at 0.6<x<0.8 was hard and showed a high energy gap, due to the sp³ configuration in Si-C combinations.     

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.     

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 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.     

Seebeck coefficient in amorphous chalcogenide films

The Seebeck coefficient determined for a number of amorphous chalcogenide films is linear in 1/T. Slopes were typically 0.1 to 0.2 eV less than the conductivity activation energies. A parameter A in the expression for thermopower appears to be a measure of the disorder, being large for highly disordered materials and small for annealed stoichiometric materials.     

Conduction mechanism in amorphous Si films

dc conductivity as a function of temperature has been measured for as-evaporated and annealed films of amorphous Si, grown by the vacuum evaporation technique. The experimental data suggest that conduction in the higher temperature range (～175–300 K) is by the thermally activated holes in the localized states near the valence band edge while conduction in the lower temperature range (～77–175 K) is found to be thermally assisted tunnelling in the localized states near the Fermi level. The activation energy for both the processes is found to increase with an increase in the annealing temperature. The average hopping distance, calculated for conduction near the Fermi level, is also found to increase with an increase in the annealing temperature.     

Crystallization effects in amorphous Cd3As2 films

The amorphous to crystalline transformation due to electron beam or heat treatments was studied. From the results it was concluded that the temperature of the transformation is higher than 300°C - higher than the epitaxy temperature for the deposition process. The a→c transition proceeds in several stages. By using the “cross-cut” method the activation energy for crystallization was estimated to be 2 eV.     

Structure of multicomponent amorphous semiconductor films

Composition of multicomponent amorphous semiconductor films varies with the preparation technique and the chemical nature of component atoms. Inhomogeneous structures due to phase separation of compositional fluctuation have been observed in these films. It seems that there are some regularities governing the film growth and the structure of liquid phase condensation.     

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.     

Holographic recording in amorphous chalcogenide thin films

Holographic recording by He–Ne laser (line 632.8 nm) light in amorphous As_0.55Se_0.45 thin films for different film thickness and grating period was studied. A strong dependence of the diffraction efficiency of the gratings on the readout light wavelength (650 nm, 805 nm and 1150 nm) was observed. A decrease in diffraction efficiency for longer wavelengths is explained by a decrease in the photoinduced changes of refractive index. It is shown that high efficiency gratings can be recorded in As_0.55Se_0.45 films with a thickness of ∼1 μm.     

Optical absorption in amorphous InN thin films

Amorphous indium nitride (a-InN) thin films were deposited onto different substrates at temperatures <325 K using RF magnetron sputtering at a rate 0.3–0.4 Å/s. X-ray diffraction patterns reveal that the films grown on the substrates are amorphous. The optical absorption edge, ‘bandgap’ energy, E_g, of a-InN has been determined by spectroscopic ellipsometry over the energy range 0.88–4.1 eV. The absorption coefficient was obtained by the analysis of the measured ellipsometric spectra with the Tauc–Lorentz model. The E_g was determined using the modified Tauc and Cody extrapolations. The corresponding Tauc and Cody optical bandgaps were found to be 1.75 and 1.72 eV, respectively. These values are in excellent agreement with the values of the bandgap energy obtained as fitting parameters in the Tauc–Lorentz model: 1.72 ± 0.006 eV as well as by using spectrophotometry (1.74 eV) and photoluminescence (1.6 eV). The spectral dependence of the polarized absorptivities was also investigated. We found that there was a higher absorptivity for wavelengths <725 nm. This wavelength, ∼725 nm, therefore indicates that the absorption edge for a-InN is about 1.70 eV. Thus, the average value of the measured optical absorption of a-InN film is approximately 1.68 ± 0.071 eV.     

Coordination of aluminum in amorphous sodium aluminosilicate films

Amorphous sodium aluminosilicate thin films containing large amounts of Al₂O₃ were deposited on fused silica substrates by rf-sputtering, and their aluminum K-band X-ray emission spectra were measured by using an electron probe X-ray microanalyser in order to determine the coordination number of aluminum ions in the amorphous thin films.

The chemical shifts for the amorphous films with Al₂O₃/Na₂O<1 were almost identical with those of tetrahedrally coordinated aluminum ions in microline. On the other hand, for the amorphous films with Al₂O₃/Na₂O>1, the chemical shifts increased with increasing Al₂O₃/NA₂OF ratio, approaching that of amorphous alumina. From the comparison with the chemical shifts of -Al₂O₃ and mullite, the coordination state of aluminum ions in amorphous alumina was found to be about 5, and its structure was found similar to be that in crystalline Al₂O₃ with spinel-type structure. These results indicate that in amorphous sodium aluminosilicate thin films aluminum ions exist in the tetrahedrally coordinated state when the Al₂O₃/Na₂O ratio is nearly equal to or less than unity. However, when the Al₂O₃/Na₂O ratio exceeds unity, some of the aluminum ions begin to assume the octahedrally coordinated state and increase in number with increasing Al₂O₃/Na₂O ratio.     

Growth instabilities in the deposition of amorphous films

It is proposed that during the atom-by-atom deposition of a film, the attractive forces between the oncoming atoms and those deposited become important in determining the growth of the film when surface diffusion and many atom rearrangement processes are suppressed (precisely the conditions that lead to the formation of amorphous films). The trajectories of the oncoming atoms are distorted in such a way that the formation and growth of surface irregularities are favored. Since the deflection depends inversely upon the incident kinetic energy of the atoms, sputtered films should be smoother than evaporated films for equal substrate temperatures. It is also argued that this mechanism can lead to the formation of voids.