A.C. Rastogi

Electrical behaviour of electron-beam-evaporated yttrium oxide thin films on silicon☆

Abstract The electrical properties of electron-beam-deposited Y2O3 film interfaced with Si(100) have been investigated. The thickness-dependent effective dielectric constant of the Y2O3 film increases to 18–27 and bulk trapped charge density decreases to (10−4) × 1011cm-2 after oxygen annealing at approximately 800 K temperatures. These changes are attributed to annealing out of defects and improvement in the stoichiometry of the Y2O3 film by oxygen intake. In as-deposited Y2O3 (400 A) films, the leakage current density is about 7 × 10-8Acm-2 at 0.1 MV cm-1. Moderate initial annealing causes it to increase as a result of localized field regions caused by structural changes at macroscopic defects in the polycrystalline Y2O3 film. A decrease later to 4 × 10-9Acm-2 on high temperature annealing is caused by reduction in defects and growth of amorphous SiO2 film at the interface. The current transport across the Y2O3 film is by a Poole-Frankel emission process. High dielectric constant, low trap density and well-behaved capacitance-voltage characteristics exhibiting metal-oxide-semiconductor (MOS) action suggest possible applications of Y2O3 films as high density MOS capacitors and gate dielectric in very-large-scale integration technology.

Optical absorption behaviour of evaporated CuxS thin films

The optical absorption behaviour of thin films of copper sulphide (CuxS) formed by a vacuum deposition process at substrate temperatures of 27, 125 and 200°C was studied with a view to establishing a correlation with their structure. From the reflection and transmission data, the optical constants and the absorption coefficient were computed. Both the direct edge at 1.8 eV and the indirect edge at 0.855 eV, 1.075 eV and 1.105 eV respectively for these films were observed. These transitions explain the observed dependence of the absorption coefficient on the photon energy. The unusually high values of the absorption coefficient for the film deposited at room temperature were due to scattering-absorption and are explained on the basis of Maxwell Garnett theory. For CuxS compositions with 1.96 < x < 2.0, no changes in the direct edge were observed. The indirect edge shifts upwards by 0.22 eV when disordering of the copper sublattice occurs in the Cu2S phase. The indirect transitions are associated with the changes in lattice structure and stoichiometry.

Novel two-stage selenization process for the preparation of ZnSe films

The preparation of homogeneous single-phase ZnSe films using a two-stage selenization process is demonstrated. Vacuum-evaporated zinc precursor films (stage 1) are selenized in a specially designed reactor, using elemental selenium vapour as the reactant (stage 2). The ZnSe films are polycrystalline with a cubic structure, as confirmed by X-ray diffraction. The films are optically transparent and have an optical band gap of 2.65 ± 0.05 eV, confirming the formation of the wide band gap semiconductor. The ZnSe films have very small crystallites (size <0.3 μm) and their morphology is determined by that of the deposited zinc film. X-ray photoelectron spectroscopy analysis of the films shows a chemical shift of ≈0.9−1.0 eV for selenium (3d), which suggests a transfer of electronic charge to selenium. This and the observed Auger parameter of 2011.1 eV for Zn (2p32) confirm the formation of ZnSe. The absence of multiple phases is ascertained by the sharpness of the X-ray photoelectron spectroscopy peaks.

Structural and magnetic properties of chemically vapour deposited iron oxide thin films

Abstract Iron oxide thin films have been deposited by a chemical vapour deposition process from a metal-organic iron acetyl acetate source using ArO2 gas mixture transport. As-deposited films are essentially α-Fe2O3. They are transformed by a reduction-oxidati on process to Fe3O4 and γ-Fe2O3 phases as revealed by electron diffraction studies. Microstructural changes and the magnetic properties of the iron oxide thin films as they transform into different states have been studied. These provide new information on the mechanism and transformation kinetics.

Phase evolution in low‐pressure Se vapor selenization of evaporated Cu/In bilayer precursors

Evolution of elemental binaries and single‐phase copper indium diselenide, CuInSe2 (CIS) during Se vapor selenization of evaporated Cu/In bilayer metal precursors at pressures of 0.3–10 mbar and temperatures in the range of 260–400 °C has been investigated. At low pressures, the relative kinetics of selenization of Cu and In are changed resulting in the formation of single‐phase CIS even at very low temperatures (260 °C). Optical, Auger, and x‐ray photoelectron spectroscopy investigations are employed to characterize the chalcopyrite absorber layer. At higher pressures (≊7–10 mbar), simultaneous formation of the equilibrium binaries, CuSe and In2Se3 at low temperatures leads to the formation of CIS through a diffusion limited reaction of the binaries at higher temperatures. The availability of Se reacting species varies significantly in the pressure regime. At low reactor pressures and Se availability, the reaction CuSe+In(l)+Se→CIS, proceeds to completion even at low temperatures. The detailed study of t...

Structure and composition of interfacial silicon oxide layer in chemical vapor deposited Y2O3‐SiO2 bilayer dielectrics for metal‐insulator‐semiconductor devices

A silicon oxide layer is formed at the interface of low pressure chemical vapor deposited Y2O3 film on Si after an annealing in O2 at 580 °C for 45 min. It shows a graded composition varying from SiO2 to SiOx (x∼1) which depends on the thickness of Y2O3 film as revealed by infrared and Auger electron spectroscopic studies. With 52‐nm‐thick Y2O3 film, a more ordered coesite‐like (c‐) SiO2 forms at the interface whose structure changes gradually to an amorphous (a‐) SiO2, SiO2−x near the Si interface. With thicker Y2O3 film (∼110 nm) the formation of a‐SiO2 is characterized by SiO4 linkage having short range order with larger size rings. The composition of silicon oxide changes gradually to SiOx with 1<x<2 near the Si interface. In thinner Y2O3 film (∼30 nm), in addition to c‐SiO2 and a‐SiO2, quasi‐SiO2 with unlinked SiO4 tetrahedra are also observed. The interfacial growth of silicon oxide is due to the oxidation of Si by quasi atomic oxygen which migrates from its entrapped position in the Y2O3 film towar...

Kinetics, growth, structure, and atmospheric chemical vapor deposition of ferrimagnetic iron oxide thin films from metallorganic acetyl acetonate precursor

Growth of magnetic iron oxide thin films by a chemical vapor deposition process from metallorganic iron acetyl acetonate precursor under atmospheric Ar-O{sub 2} transport is described. Kinetics of film growth depends on deposition parameters, particularly substrate temperature, gas flow rate, and location of substrate away from the source which is a direct effect of mass transfer and pyrolyzing efficiency of precursor vapor. As-deposited films essentially have {alpha}-Fe{sub 2}O{sub 3} phase which is transformed by a sequential reduction-oxidation process into Fe{sub 3}O{sub 4} and {gamma}-Fe{sub 2}O{sub 3} phases as revealed by electron diffraction studies. The modification in the crystalline phase of the as-deposited film is observed by varying the substrate orientation and Ar to O{sub 2} gas-phase ratios. The {gamma}-Fe{sub 2}O{sub 3} phase of iron oxide films in the as deposited form, is realized by using pure Ar as transport gas. Microstructural changes as well as magnetic properties of iron oxide thin films on transformation into different polymorphic crystallographic phases are described.

Growth patterns of chemiplated Cu2−xS layers in CdS/Cu2S thin film solar cells

Abstract The kinetics of growth of chemiplated Cu 2− x S films on thermally deposited and chemically etched CdS layers were investigated. Scanning electron microscopy examination of parent films revealed the usual columnar growth and pyramidal tops as in typical high efficiency cells, with an enhanced area factor of 2.5. During chemiplating, Cu 2− x S growth occurs conformal to the grain surface but deep penetrations result along the grain edges. In the present studies, quantitative estimates of such growth behaviour are obtained by varying the ion exchange reaction parameters, particularly the dip period and the pH of the CuCl bath. Results suggest that for a pH value of 4.6 the growth is parabolic in nature whereas in solutions of lower pH (about 3.4) a fast linear growth mechanism dominates. The textured morphology of the grain yields an expression for net Cu 2− x S growth from which grain surface thickness and grain boundary penetration depths are evaluated. Resultant changes in junction area are shown to affect the normalized open-circuit voltage of the CdSCu 2 S junction. The spectral responses of encapsulated cells are given after growth patterns of the Cu 2− x S layers. These studies reveal that the pH of the solution plays an active role in controlling the Cu 2− x S growth processes and also in obtaining superior cell characteristics.

Electrical conduction and phase transitions in vacuum-deposited Cu2−xS films

The structure, phase transitions and electrical conductivity of Cu2−xS films deposited by vacuum evaporation at different substrate temperatures were studied. It was found that the deposition parameters significantly affect the composition and the structure of evaporated Cu2−xS films. The stoichiometry changes from copper rich to copper deficient as the substrate temperature is increased. This is explained in terms of the mechanism of growth of the Cu2−xS films. The occurence of phase transitions in these films was studied through measurement of the electrical conductivity as a function of temperature. Films deposited at 300 K exist in the γ phase and undergo a γ → β phase transition at 325 K and a β → α phase transition at 350 K. Films deposited at 400 K exhibit only one phase transition, the β → α transition at 345 K. A very sluggish transition from the orthorhombic to the tetragonal phase of Cu1.96S was observed at 340 K in films deposited at 475 K. These results were corroborated by electron microscope and diffraction studies on annealed Cu2−xS films.

Magnetic studies of Ce4+ compensated Co‐doped yttrium iron garnet thin film grown by chemical vapor deposition

A low pressure metalo‐organic chemical vapor deposition (LPMOCVD) technique to form Co2+ and Ce4+ doped yttrium iron garnet (Y3−xCexFe5−xCoxO12: Co,Ce:YIG) films is described. A large concentration of Co2+ doping with x=0.3–0.7 results in uniaxial anisotropy perpendicular to the plane of the film with high coercivity values and sufficiently high saturation magnetization values. This has been possible by an alternate doping scheme where commonly used compensator, Ge4+ at Fe3+ sites has been replaced by a new compensator Ce4+ at Y3+ sites in the Co‐doped YIG films. The structural and compositional aspect of stabilized garnet phase in Co2+,Ce4+:YIG thin films and optical, thermomagnetic and magnetic hysteresis properties are presented.