A. K. Barua

Properties of tin doped indium oxide thin films prepared by magnetron sputtering

Indium tin oxide (ITO) films have been prepared by the magnetron sputtering technique from a target of a mixture of In2O3 and SnO2 in the proportion 9:1 by weight. By optimizing the deposition conditions it has been possible to produce highly transparent (transmission ∼90%) and conducting (resistivity ∼10−5 Ω cm) ITO films. A resistivity ∼10−4 Ω cm has been obtained for films of thickness ∼1000 A at a comparatively low substrate temperature of 50 °C and without using oxygen in the sputtering chamber. To characterize the films, the following properties have been studied, viz., electrical conductivity, thermoelectric power, Hall effect, optical transmission, and band gap. The effect of annealing in air and vacuum on the properties of the films have also been studied.

Degradation of tin‐doped indium‐oxide film in hydrogen and argon plasma

The degradation of tin‐doped indium‐oxide (ITO) films in glow‐discharge plasmas of hydrogen and argon have been investigated. Parameters which have been varied for the study include the temperature of ITO under ion bombardment, the rf power density, the time of exposure to plasma, and the gas flow rate. After bombardment, scanning electron micrograph observation, measurement of sheet resistance, transmittance and reflectance, and Auger analysis have been carried out to decide the extent of degradation. Magnetron‐sputtered ITO films have been found to be more resistant to ion bombardment damage compared to electron‐beam evaporated films. The degradation of ITO under the plasma of the reducing species such as hydrogen has been found to take place at lower temperature and power density compared to argon plasma.

Low band gap amorphous silicon deposited under He dilution in the γ regime of an rf glow discharge: properties and stability

Abstract A new type of hydrogenated amorphous silicon film having variable bandgap (1.7–1.5 eV) has been developed in an rf powered plasma enhanced chemical vapor deposition system using a mixture of silane and helium at a subtrate temperature of 210°C. The deposition conditions were chosen so that the rf glow discharge occurs in the γ regime, usually avoided because of powder formation. The influence of the chamber pressure, on the optical gap, the hydrogen content and the electronic properties is presented. Increasing the pressure up to 1.8 Torr is found to decrease the optical gap down to 1.5 eV. The densities of states of these films were measured by electron spin resonance, constant and modulated photocurrent techniques. The density of states above the Fermi level is found to be two orders of magnitude less than that of standard amorphous silicon. Moreover, unusually fast kinetics of degradation are observed. This new material could be a good alternative to amorphous silicon germanium alloys.

Characterization of tin doped indium oxide films prepared by electron beam evaporation

Abstract Tin doped indium oxide (ITO) films have been prepared by electron beam evaporation of hot pressed powder of 90% In 2 O 3 10% SnO 2 by weight. The parameters varied for optimization of film properties have been the substrate temperature and the partial pressure of the oxygen added. Properties which have been studied for characterization are the resistivity, Hall effect, transmittance and optical band gap. The structural studies have been made by X-ray diffraction and transmission electron microscopy. D.c. resistivity in the range 10 −3 −10 −4 Ω cm and visible transmittance in excess of 90% have been obtained for the films, with proper parametric adjustments. A 〈111〉 texture has been generally exhibited by the ITO films, using X-ray diffraction. This has been corroborated by electron diffraction studies.

Characterization of undoped μc-SiO:H films prepared from (SiH4+CO2+H2)-plasma in RF glow discharge

Undoped hydrogenated microcrystalline silicon oxygen alloy films (μc-SiO:H) have been prepared from (SiH4+CO2+H2)-plasma in RF glow discharge at a high H2 dilution, moderately high RF power and substrate temperature. A detailed characterization of the films has been done by electrical, optical as well as structural studies, e.g., IR absorption spectroscopy, Raman scattering and transmission electron microscopy. The presence of a very small amount of oxygen induces the crystallization process, which fails to sustain at a higher oxygen dilution. At higher deposition temperature and in improved μc-network H content reduces, however, O incorporation is favoured. Sharp crystallographic rings in the electron diffraction pattern identify several definite planes of c-Si and no such crystal planes from c-SiOX is detected.

Infrared vibrational spectra of hydrogenated amorphous silicon carbide thin films prepared by glow discharge

Abstract The IR vibrational spectra of a-SiC:H films prepared by rf glow discharge decompositon of a mixture of silane and methane gases have been studied. The films have been prepared under different conditions which include variation of methane concentration in the gas mixture, rf power and substrate temperature. The structures of the a-SiC:H films depend very sensitively on the deposition parameters. The spectra of p-type a-SiC:H films prepared with boron doping have also been studied. The effect of annealing on the IR spectra of the films has been investigated. Attempts have been made to analyse all these data in order to obtain information about different bonded configurations in the films.

Heterogeneity in microcrystalline-transition state: Origin of Si-nucleation and microcrystallization at higher rf power from Ar-diluted SiH4 plasma

Using very high Ar-dilution to the SiH4 plasma, good quality amorphous Si:H films could be obtained at very low rf power. The a-Si:H film, prepared at a very low deposition rate of ∼10 A/min, exhibited a σPh∼1×10−4 S cm−1, σPh/σD∼105, a notably wide optical gap of 2.10 eV and a very good stability against thermal annealing effects with reasonable light induced degradation. At higher rf power undoped μc-Si:H films were prepared with a high σD∼1×10−4 S cm−1, at a deposition rate of 30 A/min from <1 sccm of SiH4. Micrograins were identified with several well-defined crystallographic orientations. However, porosity in the grain boundary zone contributed a significant amount of adsorbed effects on the electrical properties. At very high powers, the growth of a columnar network structure was demonstrated. Long-range structural relaxation permitted by the non-rigid and heterogeneous network structure associated with the physical vapor deposition-like growth at the microcrystalline-transition state, has been iden...

Wide optical-gap a-SiO:H films prepared by rf glow discharge

Abstract Hydrogenated amorphous silicon oxygen alloy (a-SiO:H) films have been prepared at a temperature of 200°C by a rf plasma deposition process. A controlled widening in the optical gap (Eg), has been obtained by diluting SiH4 with CO2 in the rf plasma. The refractive index (n) decreases, dark conductivity (σd) as well as photoconductivity (σph) decreases, the Si-dangling bond density (Ns) increases, and the widening of the optical gap is caused by O-incorporation in the a-Si:H network. However, up to an optimum level of O-incorporation σph decreases by a factor of 5 only, photosensitivity (σph/σd) increases and Ns remains virtually unchanged. Further O-incorporation results in a degradation of optoelectronic properties. A correlation between optical and electrical properties suggests that it occurs when the Fermi level passes through the mid gap (ΔE=Eg/2) and σo, the prefactor for σd, attains a magnitude similar to that for a-Si:H. Ns has been observed to be less than that reported earlier for a similar optical gap.

Properties of vacuum-evaporated CdS thin films

Thin films of highly purified CdS have been prepared by the vacuum evaporation method. The source has been maintained at 850°C and a semi-closed system has been used for evaporation. The following properties of the films have been studied, viz: (1) dark and photoconductivity as functions of temperature and thickness of the films (2) Hall mobility and carrier concentration at room temperature (3) optical absorption and spectral response and (4) structural studies by X-ray diffraction. The resistivity of the films varies from 0.15 to 3.7 Ωcm as the thickness of the films decreases from 13600 to 1200 A. The transmittance of the films has been found to be 80-90%. The results for Hall mobility and carrier concentration show that the comparatively low resistivity of CdS films obtained by us is primarily due to high carrier concentration. The photoconductive gain increases with a decrease in film thickness. The effect of heat treatment in air on dark and photoconductivity has been studied.

Role of hydrogen dilution and diborane doping on the growth mechanism of p‐type microcrystalline silicon films prepared by photochemical vapor deposition

Boron‐doped microcrystalline hydrogenated silicon (μc‐Si:H) films were grown from a gas mixture of silane, diborane, and hydrogen employing mercury sensitized photochemical vapor deposition. At a low diborane doping ratio (1.1×10−3) hydrogen dilution of source gases resulted in films exhibiting a maximum conductivity of 7.4 S cm−1. For a higher doping ratio (10−2) the value of conductivity remained almost unchanged (10−5–10−6 S cm−1) with hydrogen dilution indicating nonexistence of microcrystallinity under such conditions. Transmission electron microscopy of B‐doped μc‐Si:H films revealed formation of crystallites possessing different crystallographic orientations, e.g., (111), (220), and (311) along with other planes. X‐ray spectra confirmed a large number of crystallites with (220) orientations.