Showing papers in "Thin Solid Films in 1972"

Recent advances in epitaxy

Abstract: The various growth modes occurring in epitaxy are illustrated by comparing the growth of Au and Ag on a variety of substrates ranging from ionic to homopolar to metallic crystals (LiF, MgO, NaCl, KCl, PbS, PbTe, Si, Cu and W). The growth on these substrates differs markedly in heat of adsorption, activation energy for surface diffusion and chemical reactivity. The techniques suitable for the study of the growth process depend strongly upon the mechanical, thermal and electronic properties of the substrate. By proper combination of various techniques it has become possible to obtain considerable insight into the differences between the various systems and to confirm and refine existing growth models.

Electrical conduction and electron emission of discontinuous thin films

Abstract: The electrical conductivity of discontinuous thin metal films is assumed to be by a substrate-assisted tunnelling. Electrons travel from one island to another via impurity levels of the substrate surface. Electrons emitted into vacuum obey the Fowler-Nordheim equation for field emission. Field factors are calculated and related to the film structure. It is shown that electrons are emitted from small islands in the cathode region where the main voltage drop occurs.

The origins of stress in thin nickel films

Abstract: The intrinsic stress in thin, polycrystalline nickel films vapor-deposited onto polished silicon substrates was measured by viewing the substrate deflection using an optical interference method. Samples were stripped from their substrates, and since they were all approximately 2000 A thick, a 650 kV electron microscope was used to observe the microstructure directly. As a function of thickness, the instantaneous stress in the films was found to be constant except for the development of compressive stresses in the first 500–1000 A of growth at the higher substrate temperatures. The magnitude of the observed tension decreased from 1.5 x 1010 dyn/cm2 at 0°C to 0.4 x 1010 dyn/ cm2 at 200°C. The stress resulting from a constrained grain boundary relaxation was calculated from an average grain boundary potential and the experimental grain sizes. The fall-off of stress with increasing substrate temperature was well matched by the calculated values, and the calculated stress values agreed to within roughly 30% of the measured values. It was concluded that the intrinsic stress was produced by this constrained grain boundary relaxation. The compressive stresses were suspected to be diffusion-induced stresses resulting from grain boundary diffusion of silicon into the nickel film.

Effect of vapour incidence angles on profile and properties of condensed films

Abstract: The profile of evaporated polycrystalline and amorphous films of various materials has been investigated as a function of condensation conditions: angle of vapour incidence, substrate temperature and residual gas pressure. In most cases at oblique vapour incidence films are shown to consist of needle-like formations or columnar crystallites. The crystallites are inclined to the substrate and the angle of inclination фo is defined by the condensation conditions. In oblique columnar structure formation the processes of surface self diffusion are important. Attention is given to the existence of needle-like formations in amorphous films of Ge, Si and GeTe.

Recent developments in the study of mechanical properties of thin films

Abstract: Recent activities in the study of film stress are described. After a brief survey of the stress versus film thickness relations, an analysis of the thermal stress is given, and the diffuculties in thermal stress corrections are pointed out. The models for the origin of the intrinsic stress that have been proposed recently are critically reviewed, and future steps to construct better models are discussed. The complicated stress behaviours observed with Ag films that have been deposited on baked or unbaked mica substrates and kept in a vacuum of 10 −8 torr or 10 −5 torr are described; they present novel problems to the existing models. A Vickers type ultra-microhardness tester for thin films is described. With this instrument, any load between 1 and 100 mg-wt can be applied to a diamond triangular pyramid indenter and its penetration depth determined to ±100 A. The results with LiF, MgF 2 , Cr and Ag films are briefly discussed.

Correlation between film structure and sorption behaviour of vapour deposited ZnS, cryolite and MgF2 films

Abstract: In this paper results are given concerning the sorption of water vapour on freshly deposited films of MgF 2 , cryolite and ZnS. Electron optical observations of the cross section of these films have shown that their microstructure consists of prismatic crystals with grain boundaries nearly parallel to the evaporation direction. Based on the observed microstructure a film model, consisting of close packed cylindrical columns, has been developed. This model enables, together with the results obtained from the water vapour adsorption measurements, the packing density, the crystal radius and the inner surface of these films to be calculated.

Optical filters: Monitoring process allowing the auto-correction of thickness errors

Abstract: It is known that the optical properties of dielectric quarter-wave multilayers are stationary in relation to the thickness of each layer This property is particularly interesting for the purpose of controlling the production of such stacks In fact, if because of inevitable production errors the thickness of the layer that has just been evaporated is not exactly equal to Λ 4 , the consequences of the error can be lowered considerably; it is enough to give to the following layer a thickness not equal to Λ 4 , in such a way that the stack optical properties will be stationary again Obviously, this result can only be obtained if control of the thickness of each layer during evaporation is based on observation of the optical properties of the stack for the wavelength Λ A monitoring process that allows the use of this error correction has been perfected In addition, computer simulation of stack production allowed us to estimate the allowable tolerances For example, we found that even for the difficult case of narrow band interference filters (Fabry-Perot or double half-wave type) the stack optical properties are not seriously affected by errors of 10% in the thickness of each layer The experimental results agree with our predictions

X-ray analysis of sputtered films of beta-tantalum and body-centered cubic tantalum

Abstract: The X-ray diffraction patterns obtained from thin films of β-Ta and b.c.c.-Ta are complicated by variations in type and degree of preferred orientation, and by variations in cell parameters. A discussion of the effects of these variations on the modified Debye-Scherrer X-ray diffraction patterns is given. The diffraction patterns from the commonly observed (200) β-Ta and (110) and (111) b.c.c.-Ta preferred orientations are illustrated, as well as those from mixtures of the two phases. For the case in which the film is a mixture of β-Ta and b.c.c.-Ta phases it is shown that the unit cells of the two phases expand in parallel. The difficulty in determining the orientation and relative amount of a phase present in a film containing both phases on the basis of diffractometer traces alone is emphasized.

The deposition of thin films by the decomposition of tetra-ethoxy silane in a radio frequency glow discharge

Abstract: Thin films of organo-silicon polymer and silicon oxide have been prepared by the decomposition of tetra-ethoxy silane in a radio frequency (1 MHz) glow discharge. The rate of deposition has been studied as a function of time, total pressure, nature of background gas, partial pressure of tetra-ethoxy silane, substrate temperature, subtrate position. With argon as background gas or with no background gas electron impact with the tetra-ethoxy silane molecule in the vapour phase is the dominant factor leading to its decomposition. With oxygen background gas the interaction of the tetra-ethoxy silane vapour and oxygen atoms is the main factor in the reaction mechanism. In an oxygen plasma non-crystalline silicon oxide films are formed, but in an argon plasma or in the absence of any background gas organo-silicon polymer films are formed. Both oxide and polymeric films were transparent, smooth, free from pin holes and strongly adherent to metals and non-metals. The infra-red spectra of the non-crystalline silica films deposited at high oxygen to tetra-ethoxy silane vapour ratios are similar, but not identical, to the spectra of thermally grown silica films. The composition of the silica films becomes oxygen deficient when deposition is carried out under low oxygen to vapour ratios at a temperature of 500°C and above. The nature of the hydroxyl groups present in the films deposited at low substrate temperatures was investigated by infra-red spectroscopy.

Anomalous large grains in alloyed aluminum thin films I. Secondary grain growth in aluminum-copper films

Abstract: Secondary grain growth in vapor-deposited Al-Cu thin films has been investigated as a function of temperature, Cu concentration and distribution. Second phase Al 2 Cu particles were found to have a considerable effect in retarding secondary growth. Nuclei for secondary growth formed, presumably by rearrangement of misfit dislocations, only when Cu was introduced as a layer sandwiched between two equal Al layers.

Ferroelectric films and their device applications

Abstract: Recent reseach on ferroelectric films is discussed, with emphasis on preparative problems, optimization of film structure and of electrical properties and application in devices. Rapid progress has been made in each of these research areas and this fact, together with the emergence of certain new materials such as polyvinylidene fluoride (PVF 2 ), is increasing the variety of possible applications to solid state devices. Typical examples considered here include microwave capacitors, thermistor bolometers, pyroelectrics, piezoelectric transducers, memories and optical display devices. The present state of the art for these applications and materials development needed for further progress are reviewed.

Épitaxie: Phénomène de postnucléation (sur l'exemple des couches minces discontinues d'aluminium et d'or sur (100) KCl

Abstract: Resume Il est montre experimentalement que le processus de formation d'une epitaxie (Al ou Au sur (100) KCl) n'est pas en liaison avec le processus de nucleation heterogene. Les epitaxies (111) et (100) bien connues s'installent par deux mecanismes differents: 1. (a) (111) par translation et rotation Brownienne de petits cristallites ( 2. (b) (100) par chocs efficaces intercristallites, ce qui se traduit par une coalescence a des temperatures plus elevees. Lors de la condensation sur support chaud (≅300°C), l'ensemble des processus, nucleation, migration Brownienne et coalescence par chocs ont lieu simultanement. Il n'est donc pas raisonnable comme cela a ete tente par le passe de formuler une theorie simple de la “nucleation epitaxique”.

Exponential tail of the optical absorption edge of amorphous semiconductors

Abstract: The low energy tail of the absorption edge of amorphous films of Ge, GeTe, As2Se3, Se and Se doped with 1–5 wt. % As, and melt-quenched platelets of Se and doped Se has been found to vary exponentially with photon energy in a manner similar to the Urbach rule observed in various ionic compounds and crystalline semiconductors. Our critical examination of all known data on the Urbach tail suggests that the tail must have similar physical origin in all cases. The tail may be ascribed to the broadening of the absorption edge by random, localized electric fields (Franz-Keldysh effect) arising from the LO phonons of the polar modes of vibration, and/or frozen-in lattice distortions or fluctuations in the nearest neighbour distance for an amorphous material. This mechanism can also explain the observation that amorphous semiconductors lacking long range order exhibit the Urbach tail with slope independent of temperature up to the amorphous→crystalline transformation temperature. The observed temperature variation of the slope of the Urbach tail in liquid semiconductors and amorphous Se (a variation similar to that of crystalline materials) may, however, be understood on the basis that some long range order in the form of molecular or chain structures exists in these materials.

R.F. sputtered β-tantalum and b.c.c. tantalum films

Abstract: Tantalum films have been deposited on both glass and Ta2O5-coated substrates by r.f. sputtering. The structure, resistivity ρ and temperature coefficient of resistance TCR of films sputtered in pure argon or reactively in Ar-O2 or Ar-N2 have been determined. In pure argon on sufficiently preheated substrates a very pure b.c.c. Ta phase (α∗-Ta), with ρ∼25 μΩ cm and TCR∼1500 ppm/°C, is formed. Sputtering on substrates which are only slightly preheated or are heated solely by the sputtering process yields a tetragonal Ta phase (β-Ta) with ρ∼165 μΩ cm and TCR∼-160 ppm/°C. With added oxygen the β−Ta phase is produced over the whole partial pressure range investigated. Low partial pressures of nitrogen lead to β-Ta. At higher nitrogen partial pressures b.c.c. Ta (α-Ta) with ρ∼50 μΩ cm and TCR∼650 ppm/°C is formed and finally the plateau region is observed.

The properties of very thin R.F. sputtered transparent conducting films of SnO2:Sb and In2O3:Sn☆

Abstract: Relatively thick r.f. sputtered films of SnO2 doped with Sb and In2O3 doped with Sn have been reported to exhibit moderate optical transmission and high electrical conductivity. In the present work, the properties of these materials were examined for the range of film thicknesses that yield very high optical transparency and moderately high electrical conductivity. The conductivity of the In2O3:Sn system was found to be unstable for film thicknesses less than 1000 A. The preferred system for high transmission applications is SnO2:Sb, which was found to be useful, stable and reproducible in thicknesses as small as 50 A. Film transmission averaging 98% in the visible has been achieved with a sheet resistivity of 30 kohms/□. The bulk resistivity of films thicker than 300 A is 0.006 ohms cm. The resistivity increases to 0.05 ohms cm at a thickness of approximately 50 A because a high resistivity surface layer becomes an appreciable fraction of the total film volume.

Carrier mobility and field effect in thin indium antimode films

Abstract: Thin polycrystalline InSb films in the thickness range 1000–3000 A, have been prepared by the flash evaporation technique. A linear relation is found to exist between the electron mobility and the mean crystallite size. A positive gate field, applied orthogonally to the film, increases the electron mobility by a factor of 30, at 77°K. It is proposed that a potential barrier exists at the grain boundaries, which dominates the scattering process. This model could also explain the temperature dependence of the electron mobility. Typical potential barriers are found to be ∼0.04 eV.

The electrical resistivity and resistance-temperature characteristics of thin titanium films

Abstract: In this communication are described the results on the electrical resistivity and resistance-temperature characteristics of titanium films, measured in vacuum. They were evaporated onto soda glass microscope slides at room temperature in a sputter-ion pumped glass belljar vacuum system at about 5×10-8 torr. The films varied in their thickness from 50 to 1100 A, and the resistivity was very high for the thinnest films but for the thickest ones it approached approximately double the bulk value. The measured resistivities for the continuous freshly prepared films are too high to be explained on the basis of the Fuchs-Sondheimer theory1, 2 for diffuse scattering, and are attributed to porosity and the gaseous impurities taken down during and after their formation. The temperature coefficient of resistance (TCR) was negative for films less than 50–60 A thick but positive for thicker ones. A bulk mean free path of 285 A in titanium was calculated at room temperature. A brief survey is made of the similar work available on titanium films in the literature, and our results are discussed and compared with it.

Electrical and thermoelectric properties of evaporated copper films

Abstract: The thermoelectric power, resistivity and TCR of vacuum evaporated copper films were measured. The energy dependence of the electron mean free path and the area of the constant energy surface were obtained as 0.7 and -2.1 respectively. The specularity parameter and the electron mean free path became 0.5 and 590 A respectively. The mean free path of the dominant phonon mode was estimated at 500 A. It was inferred that a change of the electronic structure occured in the unannealed films.

A contribution to the dependence of secondary electron emission from the work function and fermi energy

Abstract: In accordance with the calculations of Amelio, we have used the Boltzmann equation to evaluate the energy distribution of secondary electrons of various materials. A comparison between experimental and theoretical results for the variation of the work function and Fermi energy is presented and is found to be quite satisfactory. It can be shown that these parameters strongly influence the energy distribution of secondary electrons in the low energy range (0 eV ≦ E ′≲10 eV).

Energetic electronic processes and negative resistance in amorphous TaTa2O5Au and AlAl2O3Au diodes

Abstract: Voltage-controlled negative resistance (VCNR) can be established in metal-insulator-metal structures by applying a potential to the diode, which is usually in vacuum. Light emission due to electroluminescence (E.L.), and electron emission into vacuum, accompany the formation of conductivity; these energetic electronic phenomena are closely related to the “forming” of VCNR and to the resultant current-voltage characteristics. For Al 2 O 3 diodes with impure oxides, VCNR forms at constant voltage, independent of oxide thickness; for clean oxides, forming depends on field and on the metal counterelectrode. The intensity and energy distribution of light emitted from TaTa 2 O 5 Au diodes have been measured, and are compared to E.L. from AlAl 2 O 3 Au diodes. The voltage threshold for the appearance of E.L. in TaTa 2 O 5 Au diodes is 1.2 V, for AlAl 2 O 3 Au it is 1.4 V. The respective voltages for maximum current, V m , are 1.9 V and 2.8 V. In Al 2 O 3 , the E.L. spectrum covers the visible range with peaks at 1.8 eV, 2.3 eV, and 4.0 eV. For Ta 2 O 5 , the E.L. intensity is constant over most of the visible, but has a maximum between 1.6 eV and 1.8 eV which is also the energy of E.L. of TaTa 2 O 5 Au diodes before the establishment of VCNR. For both Al 2 O 3 and Ta 2 O 5 diodes, electron emission into vacuum is anomalous since emission is detectable at diode voltages of ∼2 V. Electron emission in the two insulators, though qualitatively similar, shows differences that depend on differences in trapping levels and band gaps of the insulators.

Formation of second phase particles in aluminum-copper alloy films

Abstract: The formation of second phase particles in Al+3% Cu alloy films was examined in order to find differences of the precipitation in thin films from that occurring in bulk alloys. Compared to bulk alloys in the thin film material the formation of Guinier-Preston zones is retarded. In well quenched thin films 60 h at 100°C or 6 h at 130°C are necessary to produce G.P. zones. θ′ particles are nucleated after approximately 100 h at 120°C without formation of θ″. The stable θ-Al2Cu precipitate was found at temperatures as low as 150°C on the film surface. The results show that excess vacancies cannot be retained in thin films and that the film surfaces provide preferential nucleation sites for the equilibrium precipitate θ-CuAl2.

Temperature coefficient of resistance of beta-tantalum films and mixtures with b.c.c.-tantalum

Abstract: Beta-tantalum is the preferred thin film material for the anode of thin film tantalum oxide capacitors,but further information is required about its basic electronic characteristics The original report on its properties characterized it as having: a resistivity at room temperature of 200 ± 20 μΩ cm; a temperature coefficient of resistance (α), measured between room and liquid nitrogen temperatures, of ± 100 ppm/°C; and a superconducting transition of 057 -047 K In the present study, beta-tantalum films obtained from various bell jar systems in Bell Telephone Laboratories were studied for their resistivity characteristics down to 42 K Films which had no detectable bcc-tantalum had linear α values of -150±30 ppm/°C over the temperature range of 42 to 380 K Films with increasing amounts of bcc-tantalum tended to have an increasing positive α value in the temperature range of 42–100 K, and with sufficient bcc-tantalum, a maximum in resistivity can occur The pure beta-tantalum films with the negative constant α values had superconducting transitions of 096-067 K Mixed beta- and bcc-tantalum films have been prepared by nitrogen doping, and these films confirm that the presence of bcc-material causes an increasing positive α value at low temperatures The conversion temperature of beta- to bcc-tantalum, determined on stripped film material, has been confirmed to have a high value of 800°–850°C Despite this high value, the shape of the low temperature resistivity curve indicates that mixtures of bcc- and beta-tantalum can be deposited by sputtering at substrate surface temperatures of 400°–465°C The α-values between 42 and 77 K or between 77 and 196 K are particularly sensitive to the amount of bcc-material in the film The α-value method of analysis to detect bcc-material is probably more sensitive than the X-ray method

Electron conduction, electron emission and electroluminescence of MIM sandwich structures with Al2O3 insulating layers

Abstract: The electrical properties of AlAl 2 O 3 Au sandwiches with different Al 2 O 3 structures have been investigated. Sandwich cathodes with Al 2 O 3 layers up to 100 A in thickness, or with thicker thermally oxidized or anodically grown dense Al 2 O 3 layers, give monotonic current-voltage characteristics. The lifetime of such cathodes is limited to a few minutes by uniform Au ion diffusion at higher emission yields. Sandwich cathodes having a multilayered Al 2 O 3 structure with at least one heavily doped porous Al 2 O' 3 layer exhibit current-voltage curves with voltage-controlled negative resistance. At voltages above 5 V the sandwich and the emission currents may be attributed to Fowler-Nordheim tunnelling. The electron-emitting regions of the sandwich structure show faint electroluminescence with peaks of emitted photons at 2.3 and 4.1 eV. From a consideration of different electron conduction mechanisms, related to observed electrical conductivity, electron emission and electroluminescence, a band structure has been derived for amorphous Al 2 O' 3 decorated with Au precipitations. The investigations have led to the construction of a sandwich cathode with extended lifetime. The main features of this cold emitter are a special multilayer structure of the Al 2 O 3 insulator and a discontinuous upper Au electrode.

Quantum size effect and perspectives of its practical application

Abstract: At the present time the intensive development of microelectronics raises many problems in thin film physics. One specific film effect is the quantum size effect, peculiar to films with a thickness comparable with the effective de Broglie wavelength of the carriers: macroscopic characteristics due to the carriers, such as thermodynamic potentials, kinetic coefficients and optical properties, are thus oscillatory functions of the film thickness. The theory of this effect, based on quantization of the component of the carrier momentum normal to the film plane, is discussed and the available experimental evidence is presented. Particular effects exhibiting possible practical applications are discussed in more detail: resonance current tunnelling in dielectrics, resonance intraband optical absorption, injection lasers with discrete levels due to the quantum size effect and magnetic field quantization, and oscillations in the surface conductivity of films of constant thickness or with a wedge shape.

Spontaneous polarization of condensing carbon monoxide and other gases with an electrical dipole moment

Abstract: The electrical polarization which occurs during condensation of gases with a dipole moment is investigated for eight gases at condensatiom temperatures between 3 and 100 K. On condensate layers of 10 -2 cm thickness surface potentials up to 100 V were measured. The sign and the value of the polarization depends on the condensation temperature. By means of resonances with the temperature dependent phonon spectrum this polarization and its temperature dependence can be explained.

Absorption edge of CdS thin films

Abstract: The optical absorption of CdS thin films was measured on a number of sanples deposited under controlled conditions. The broad and nearly exponential absorption edge is attributed to the presence of potential barriers at the microcrystal boundaries. A good agreement is obtained between the experimental results and the predictions of a model based on the Franz-Keldysh effect.

Charge effects in thin film adhesion

Abstract: The adhesion of thin films is usually attributed to van der Waals and chemical forces. In this work the importance of an electrostatic contribution to the adhesion of metallic thin films to insulating substrates has been investigated. The transfer of charge across a film-substrate interface can lead to the formation of an electrical double layer; the resultant electrostatic attraction can contribute to the adhesion of the film. The magnitude of this attractive force can be comparable with that due to van der Waals forces. For two weakly adhering solids where the adhesion is determined only by long-range forces, the electrostatic component may therefore be significant. Vacuum-deposited metallic films are peeled from their substrates (amorphous or crystalline insulators) in vacuo (10 -6 torr) and the work done in this process is used to obtain a measure of the film adhesion. Using a highly sensitive probe technique, the sign and magnitude of the charge remaining on the substrate is determined subsequently at various substrate temperatures. Results obtained over a range of ambient conditions show charge densities of ±10 10 -10 11 electronic charges cm -2 ; comparison with the measured work of peeling suggests an electrostatic component of adhesion of a few percent. However, the possibility of charge decay during peeling of the film cannot be excluded, so that this value must be regarded as a minimum. A model is proposed for the mechanism of charge transfer during formation and separation of the interface.