Time-dependent properties of organic thin films deposited by glow discharge

Sample capacitors were constructed with dielectrics obtained from completely fluorinated carbons which were polymerized in a plasma glow discharge. The rate of degradation of the capacitor properties depends on the size of the monomer molecule selected, the discharge frequency used during the deposition of the film, the vacuum anneal immediately after deposition, the temperature, the applied electric field, and the air or vacuum environment during life tests. The diversity of the results is explained by plasma acitvation of the carbon-carbon bond in the monomers and the formation of the films from the radicals thus produced.     

Optical properties of hydrogenated amorphous carbon films deposited from glow discharge plasma

Optical transmission and reflection spectra of the layers of hydrogenated amorphous carbon deposited onto fused quartz substrates from a glow discharge plasma are studied on samples of two series, which differ by the substrate temperatures (200 vs. 400°C) during deposition. The dispersion of the imaginary part of the dielectric function and the spectrum of the effective electron density of states involved in optical transitions are reconstructed. It is shown that this spectrum is adequately described by a sum of two Gaussian contours. The maximum of the first contour falls in the range of low frequencies and reflects the contribution of π electrons to the optical absorption of the material, while the second contour with the maximum in the high-frequency region is assigned to the contribution of σ electrons.     

Electrical and switching properties of InSe amorphous thin films

In this work electrical and switching properties of InSe thin films have been studied.

The semiconductor compound InSe was obtained by direct synthesis from stoichiometric amounts of spectroscopically pure indium and selenium. By slow cooling of the synthesized InSe a polycrystalline material is obtained. The amorphous films were obtained by thermal evaporation under vacuum of the polycrystalline material on glass or pyrographite substrates.

From electrical measurements, it was found that for all films the dark electrical resistivity decreases with an increase in film thickness and temperature. The InSe compound exhibits non-linear I–V characteristics and switching phenomena. The threshold voltage decreases with increasing annealing temperature and increases with increasing film thickness.     

Optical absorption in glow discharge amorphous silicon films

Both undoped films and films doped with phosphine were deposited by the r.f. glow discharge decomposition of silane onto 7059 glass and single-crystal silicon wafers. The absorption edge in the near infrared was examined and the refractive index was calculated from transmittance interference fringes. The various silicon- hydrogen infrared absorption bands were examined to see whether hydrogen incorporation varied with the growth conditions. Our films invariably showed the presence of secondary and tertiary bonded hydrogen.     

Characterization of a-Si:H thin films prepared by dc glow discharge of silane

A dc glow discharge apparatus for preparing amorphous silicon films from silane gas is described. The films are characterized by electron microscopy, infrared spectroscopy, electrical conductivity and photoconductivity. The deposition parameters which give good photoconducting films are established. The Staebler-Wronski effect is studied and is found to be smaller in vacuum than in air. A photovoltage is observed in structures with gold as the Schottkybarrier metal. The conversion efficiency of the device is about 1%. The results are compared with those in the literature, and the improvements which might result in a better conversion efficiency are pointed out.     

Some properties of thin aluminium nitride films formed in a glow discharge

The formation of thin AlN films and some of their electrical properties have been investigated. The films were prepared by exposing the surface of evaporated aluminium films to a glow discharge in pure nitrogen gas. It has been confirmed by transmission electron diffraction that the structure of the film is of the wurtzite type, as is that of the bulk material. It was observed that the thickness of the AlN films depends on the voltage of the glow discharge, the distance from the discharge electrode to the films and the treatment time. The electrical properties were measured in the form of a sandwich diode, i.e. Al-AlN-Au. The voltage-controlled negative resistance, which was first discovered in Al-Al₂O₃-metal diodes, has also been observed in this nitride film diode and shows very good reproducibility and stability.     

Investigation of optical and compositional properties of thin SiNx:H films with an enhanced growth rate by high frequency PECVD method

Hydrogenated thin silicon nitride (SiN_x:H) films were deposited by high frequency plasma enhanced chemical vapor deposition techniques at various NH₃ and SiH₄ gas flow ratios [R = NH₃/(SiH₄ + NH₃)], where the flow rate of NH₃ was varied by keeping the constant flow (150 sccm) of SiH₄. The deposition rate of the films was found to be 7.1, 7.3, 9 and 11 Å/s for the variation of R as 0.5, 0.67, 0.75 and 0.83, respectively. The films were optically and compositionally characterized by reflectance, photoluminescence, infrared absorption and X-ray photoelectron spectroscopy. The films were amorphous in nature and the refractive indices of the films were varied between 2.46 and 1.90 by changing the gas flow ratio during the deposition. The PL peak energy was increased and the linear band tails become broad with the increase in R. The incorporation of nitrogen takes place with the increase in R.     

Deposition rate and structural properties of microcrystalline glow discharge Si:H,Cl films

The deposition rate and the structural properties of Si:H, Cl films produced in SiCl₄H₂ glow discharges were studied as functions of the substrate temperature, the gas feed composition and the operating pressure. The experimental results were interpreted on the basis of a discharge equilibrium involving both deposition and etching of the film. A close relationship between the deposition rate and the crystallite size is shown.     

Electrical properties of AlNxOy thin films prepared by reactive magnetron sputtering

Direct current magnetron sputtering was used to produce AlN_xO_y thin films, using an aluminum target, argon and a mixture of N₂ + O₂ (17:3) as reactive gases. The partial pressure of the reactive gas mixture was increased, maintaining the discharge current constant. Within the two identified regimes of the target (metallic and compound), four different tendencies for the deposition rate were found and a morphological evolution from columnar towards cauliflower-type, ending up as dense and featureless-type films. The structure was found to be Al-type (face centered cubic) and the structural characterization carried out by X-ray diffraction and transmission electron microscopy suggested the formation of an aluminum-based polycrystalline phase dispersed in an amorphous aluminum oxide/nitride (or oxynitride) matrix. This type of structure, composition, morphology and grain size, were found to be strongly correlated with the electrical response of the films, which showed a gradual transition between metallic-like responses towards semiconducting and even insulating-type behaviors. A group of films with high aluminum content revealed a sharp decrease of the temperature coefficient of resistance (TCR) as the concentration ratio of non-metallic/aluminum atomic ratio increased. Another group of samples, where the non-metallic content became more important, revealed a smooth transition between positive and negative values of TCR. In order to test whether the oxynitride films have a unique behavior or simply a transition between the typical responses of aluminum and of those of the correspondent nitride and oxide, the electrical properties of the ternary oxynitride system were compared with AlN_x and AlO_y systems, prepared in similar conditions.     

Electrical conduction mechanisms in thin amorphous semiconductor films

An account is given of the different conduction mechanisms possible in thin amorphous semiconductor films. Special attention is paid to the low temperature regime where variable range hopping between states near the Fermi level is supposed to be most important. An attempt is made to assess the significance of fits of the conductivity versus temperature relation to the much-used T^{-$\text{1}\text{4}$} relation proposed by Mott.     

Electrical and optical switching properties of ion implanted VO2 thin films

The metal-insulator transition in vanadium dioxide thin films implanted with O⁺ ions was studied. Ion implantation lowered the metal-insulator transition temperature of the VO₂ films by 12 °C compared to the unimplanted ones, as measured both optically and electrically. The lowering of the transition temperature was accomplished without significantly reducing the mid-wave infrared optical transmission in the insulating state for wavelengths > 4.3 μm. Raman spectroscopy was used to examine changes to the crystalline structure of the implanted films. The Raman spectra indicate that ion implantation effects are not annealed out for temperatures up to 120 °C.     

Electrical and thermal properties of highly quenched amorphous V2O5 thin films

Amorphous thin films of V₂O₅ have been prepared by vapour deposition in high vacuum ( 10^–6 torr). In order to study the role of quenching, various temperatures, ranging from –196 to 260° C, have been selected for the substrate. Differential thermal analysis. X-ray diffraction and conductivity measurements clearly divide the material into two sets, depending on the efficiency of the quenching. Whereas the least-quenched samples resemble those previously obtained by splat-cooling, the better quenched are only barely stable and, as a consequence, exhibit unique features, such as the occurrence of a glassy transition and the highest crystallization temperature ever found for V₂O₅.     

Depth profiling of thin films with pulsed glow discharge atomic emission spectrometry

The application of microsecond pulsed Grimm glow discharge atomic emission spectrometry for depth profiling of thin films is examined. The effects of pulsed conditions including pulse voltage, pulse frequency, pulse width, and Ar pressure on depth profiling performance were characterized for Zn and Cu coatings on steel. Using optimized conditions, linear calibration curves of coating thickness for Zn (6.1-26.9 μm) and Cu (50-500 nm) on steel were achieved. A precision of 2-5% relative standard deviation was determined. An ultrathin coating of Cu (10 nm) on steel was also measured by this technique.     

Electrical properties of Ta2O5 thin films deposited on Cu

The electrical and dielectric properties of reactively sputtered Ta₂O₅ thin films with Cu as the top and bottom electrodes forming a simple metal insulator metal (MIM) structure, Cu/Ta₂O₅/Cu/n-Si, were studied. Ta₂O₅ films subjected to rapid thermal annealing (RTA) at 800°C for 30 s in N₂ ambient crystallized the film, decreased the leakage current density and resulted in reliable time-dependent dielectric breakdown characteristics. The conduction mechanism at low electric fields (<100 kV/cm) is due to Ohmic conduction; however, the Schottky mechanism becomes predominant at high fields (>100 kV/cm). Present studies demonstrate the use of Cu as a potential electrode material to replace the conventional precious metal electrodes for Ta₂O₅ storage capacitors.     

Electrical resistivity of polycrystalline AgInSe2 thin films

Thin films of AgInSe₂ are grown onto glass substrates by flash evaporation technique at temperatures ranging from 30 to 300°C and their electrical resistivity is studied. It is observed that the films deposited at 200°C have minimum resistivity. The variation of resistivity is explained in terms of the compositional variation in the films, which was carried out by EDAX (energy dispersive analysis of X-rays). The activation energies of the compound films grown at different substrate temperatures are determined. The implications are discussed.