Influence of structural characteristics on the thermal conductivity of polycrystalline diamond films

By analyzing the signal formed by the photoacoustic effect as a function of the light modulation frequency, it is shown that this effect may be used to determine the thermal conductivity of diamond materials. The method is checked experimentally for two types of polycrystalline diamond films grown by chemical vapor deposition with the gaseous medium activated by a dc discharge and a microwave discharge. The data obtained on the thermal conductivity of the films are discussed with reference to the results of an investigation of the optical absorption, Raman light scattering, and cathodoluminescence of similar films. It is shown that the thermal conductivity of polycrystalline diamond films depends on the structural characteristics, which are determined by the deposition conditions. Fiz. Tverd. Tela (St. Petersburg) 40, 1221–1225 (July 1998)     

Determination of the thermal conductivity of polycrystalline diamond films by means of the photoacoustic effect

A new method of determining the heat-conducting properties of diamond films is proposed, based on the photoacoustic effect. This method is used to study diamond polycrystalline films grown on silicon by chemical vapor deposition in a microwave discharge plasma. The thermal conductivity obtained was approximately half that for single-crystal diamond. Zh. Tekh. Fiz. 69, 97–101 (April 1999)     

On the mechanism of electronic transport in polycrystalline CdO thin films

Cadmium Oxide (CdO) thin films (d = 0.16−0.62 μm) were deposited onto glass substrates by thermal evaporation under vacuum (quasi closed volume technique) of high purity (99.99%) CdO polycrystalline powders. The substrate temperature was 300 and 473 K, respectively. After a post-deposition heat treatment, the temperature dependence of the electrical conductivity becomes reversible. The electronic transport mechanism in studied samples is explained in terms of Seto’s model for polycrystalline semiconducting films. The values of optical bandgap have been determined from absorption spectra.     

Determination of thermal conductivity of CVD diamond films via photoacoustic measurements

A novel method of determination of polycrystalline diamond films’ thermoconductive properties using the photoacoustic effect is proposed. By this method, we studied the diamond films grown on silicon substrates using chemical vapor deposition technique. A value of thermal conductivity obtained for the films was less than half that for diamond single crystal. The decrease of thermal conductivity, as well as characteristic features of optical properties of the films, is explained by the presence of a large amount of intercrystallite boundaries and other structure defects and admixures detected using Raman and photoluminescent spectroscopies. Received: 22 October 1998 / Accepted: 27 January 1999 / Published online: 28 April 1999     

Investigation of CuInSe2 films by the method of photoacoustic spectroscopy

We investigated the photoacoustic spectra of polycrystalline thin CuInSe₂ films obtained by the method of pulsed laser evaporation onto glass substrates at 100–450°C. The spectra were taken near the fundamental optical absorption edge using a high-resolution spectrometer and microphone-type sensor. We show the relation between the photoacoustic spectrum and the structural properties of films. The interference effect observed is discussed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 583–586, July–August, 1999.     

Synchrotron radiation interference in front of the silicon absorption edge for silicon-on-insulator structures

The synchrotron radiation (SR) interference phenomenon has been for the first time observed in a strained silicon nanolayer deposited on a dielectric SiO₂ layer (∼150 nm) on Si (100) single crystalline substrates (silicon-on-insulator (SOI) structures). Strong oscillations of spectra intensity depending on photon energy have been detected in the energy range preceding the elementary silicon Si L _2,3 absorption edge (≤100 eV) at grazing angles of SR smaller than 21° in the X-ray photoeffect quantum yield structure. The phase of the spectra oscillation structure is reversed for small variations of grazing angle in the 4°–21° range. The silicon nanolayer thickness (∼180 nm) has been estimated in the three-layer, Si nanolayer-SiO₂-Si substrate structure with the use of neighbor maxima positions of ultrasoft X-ray radiation interference in XANES (X-ray absorption near edge structure) spectra. A decrease in the crystal lattice parameter of a strained silicon layer along the normal to substrate has been determined by X-ray diffraction. An increase in the Si-Si interatomic distances in the strained silicon nanolayer lattice of SOI structure has been found using ultrasoft X-ray emission spectroscopy data.     

Enhanced field emission characteristics of boron doped diamond films grown by microwave plasma assisted chemical vapor deposition

Boron doped diamond films were synthesized on silicon substrates by microwave plasma chemical vapor deposition (MPCVD) technique. The effect of B₂O₃ concentration varied from 1000 to 5000 ppm on the field emission characteristics was examined. The surface morphology and quality of films were characterized by scanning electron microscope (SEM) and Raman spectroscopy. The surface morphology obtained by SEM showed variation from facetted microcrystal covered with nanometric grains to cauliflower of nanocrystalline diamond (NCD) particles with increasing B₂O₃ concentration. The Raman spectra confirm the formation of NCD films. The field emission properties of NCD films were observed to improve upon increasing boron concentration. The values of the onset field and threshold field are observed to be as low as 0.36 and 0.08 V/μm, respectively. The field emission current stability investigated at the preset value of ∼1 μA is observed to be good, in each case. The enhanced field emission properties are attributed to the better electrical conductivity coupled with the nanometric features of the diamond films.     

On the possibility of developing thermoelectric sensors based on multielement higher manganese silicide film structures

The possibility of designing thermoelectric sensors based on multielement structures of higher manganese silicide (HMS) polycrystalline films is considered. Test structures with various configurations are developed for studying electrical and thermoelectric parameters of polycrystalline HMS films. The geometrical sizes of the elements of test structures are chosen to match the grain size in polycrystalline HMS films. The test structures are prepared using the planar silicon technology. In these structures, the current-voltage characteristics, Hall constant, charge carrier concentration, and mobility are measured. The thermopower (α) and electrical conductivity (σ) are studied in a temperature range of T = 77–600 K, where α > 250 μV/K and electrical conductivity σ ∼ 20 (Ω cm)⁻¹. It is shown that the sensitivity and thermopowers increase upon a decrease in the cross-sectional area of the elements.     

Specific features and mechanisms of photoluminescence of nanostructured silicon carbide films grown on silicon in vacuum

The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates under conditions of decreased growth temperatures (T _gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously, a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium laser (λ_excit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV) region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal structure of the grown silicon carbide layers has been analyzed.     

Optical absorption in PECVD deposited thin hydrogenated silicon in light of ordering effects

We report results obtained from measurements of optical transmittance spectra carried out on a series of silicon thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane diluted with hydrogen. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (μc-Si:H). Spectral refractive indices and absorption coefficients were determined from transmittance spectra. The spectral absorption coefficients were used to determine the Tauc optical band gap energy, the B factor of the Tauc plots, E ₀₄ (energy at which the absorption coefficient is equal to 10⁴ cm⁻¹), and the Urbach energy as a function of the hydrogen dilution. The results were correlated with microstructure, namely volume fractions of the amorphous and crystalline phase with voids, and with the grain size.      

Persistent spin splitting of a two-dimensional electron gas in tilted magnetic fields

The effect of atomic disorder on the electron transport and the magnetoresistance (MR) of Co₂CrAl Heusler alloy (HA) films has been investigated. We show that Co₂CrAl films with L2₁ order exhibit a negative value for the temperature coefficient of resistivity (TCR) in a temperature range of 10 < T < 290 K, and the temperature dependence of electric conductivity varies as T ^3/2 similarly to that of the zero-gap semiconductors. The atomic or the site disorder on the way of L2₁ → B2 → A2 → amorphous state in Co₂CrAl HA films causes the deviation from this dependence: reduction in the absolute value of TCR as well as decrease in the resistivity down to ϱ(T = 293 K) ∼ 200 μΩ cm in comparison to ϱ(T = 293 K) ∼ 230 μΩ cm typical for the Co₂CrAl films with L2₁ order. The magnetic-field dependence of MR of the Co₂CrAl films with L2₁ order is determined by two competing contributions: a positive Lorentz scattering and a negative s-d scattering. The atomic disorder in Co₂CrAl films drastically changes MR behavior due to its strong influence on the magnetic properties.     

Aerosol deposition of detonation nanodiamonds used as nucleation centers for the growth of nanocrystalline diamond films and isolated particles

The aerosol deposition of detonation nanodiamonds (DNDs) on a silicon substrate is comprehensively studied, and the possibility of subsequent growth of nanocrystalline diamond films and isolated particles on substrates coated with DNDs is demonstrated. It is shown that a change in the deposition time and the weight concentration of DNDs in a suspension in the range 0.001–1% results in a change in the shape of DND agglomerates and their number per unit substrate surface area N _s from 10⁸ to 10¹¹ cm⁻². Submicron isolated diamond particles are grown on a substrate coated with DND agglomerates at N _s ≈ 108 cm⁻² using microwave plasma-enhanced chemical vapor deposition. At N _s ≈ 10¹⁰ cm⁻², thin (∼100 nm) nanodiamond films with a root-mean-square surface roughness less than 15 nm are grown.     

Effect of melt compounding temperature on dielectric relaxation and ionic conduction in PEO–NaClO<Subscript>4</Subscript>–MMT nanocomposite electrolytes

Polymer nanocomposite electrolytes (PNCEs) of poly(ethylene oxide) and sodium perchlorate monohydrate complexes with montmorillonite (MMT) clay up to 20 wt.% MMT concentration of poly(ethylene oxide) (PEO) are synthesized by melt compounding technique at melting temperature of PEO (∼70 °C) and NaClO₄ monohydrate (∼140 °C). Complex dielectric function, electric modulus, alternating current (ac) electrical conductivity, and impedance properties of these PNCEs films are investigated in the frequency range 20 Hz to 1 MHz at ambient temperature. The direct current conductivity of these materials was determined by fitting the frequency-dependent ac conductivity spectra to the Jonscher power law. The PNCEs films synthesized at melting temperature of NaClO₄ monohydrate have conductivity values lower than that of synthesized at PEO melting temperature. The complex impedance plane plots of these PNCEs films have a semicircular arc in upper frequency region corresponding to the bulk material properties and are followed by a spike in the lower frequency range owing to the electrode polarization phenomena. Relaxation times of electrode polarization and ionic conduction relaxation processes are determined from the frequency values corresponding to peaks in loss tangent and electric modulus loss spectra, respectively. A correlation is observed between the ionic conductivity and dielectric relaxation processes in the investigated PNCEs materials of varying MMT clay concentration. The scaled ac conductivity spectra of these PNCEs materials also obey the ac universality law.     

Structural, optical and electrical properties of tin oxide thin film deposited by APCVD method

Tin oxide (SnO₂) thin films have been grown on glass substrates using atmospheric pressure chemical vapour deposition (APCVD) method. During the deposition, the substrate temperature was kept at 400°C–500°C. The structural properties, surface morphology and chemical composition of the deposited film were studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and Rutherford back scattering (RBS) spectrum. XRD pattern showed that the preferred orientation was (110) having tetragonal structure. The optical properties of the films were studied by measuring the transmittance, absorbance and reflectance spectra between λ = 254 nm to 1400 nm and the optical constants were calculated. Typical SnO₂ film transmits ∼ 94% of visible light. The electrical properties of the films were studied using four-probe method and Hall-voltage measurement experiment. The films showed room temperature conductivity in the range 1.08 × 10² to 1.69 × 10² Ω⁻¹cm⁻¹.     

Effect of normal processes on thermal conductivity of germanium,silicon and diamond

The effect of normal scattering processes is considered to redistribute the phonon momentum in (a) the same phonon branch — KK-S model and (b) between different phonon branches — KK-H model. Simplified thermal conductivity relations are used to estimate the thermal conductivity of germanium, silicon and diamond with natural isotopes and highly enriched isotopes. It is observed that the consideration of the normal scattering processes involving different phonon branches gives better results for the temperature dependence of the thermal conductivity of germanium, silicon and diamond with natural and highly enriched isotopes. Also, the estimation of the lattice thermal conductivity of germanium and silicon for these models with the consideration of quadratic form of frequency dependences of phonon wave vector leads to the conclusion that the splitting of longitudinal and transverse phonon modes, as suggested by Holland, is not an essential requirement to explain the entire temperature dependence of lattice thermal conductivity whereas KK-H model gives a better estimation of the thermal conductivity without the splitting of the acoustic phonon modes due to the dispersive nature of the phonon dispersion curves.      

Surface energy and pressure of diamond and silicon nanocrystals

Based on the pair potential of interatomic interaction, we study the dependence of various properties of diamond and silicon nanocrystals with a free surface on size, surface shape, and temperature. A model nanocrystal has the form of a parallelepiped faceted by {100} planes with a square base. The number of atoms N in the nanocrystals is varied from 5 to infinity. The Debye temperature, Gruneisen parameter, specific surface energy, isochoric derivative of specific surface energy with respect to temperature, and surface pressure are calculated as a function of the size and shape of diamond and silicon nanocrystals at temperatures ranging from 20 K to the melting point. The surface pressure P _sf(N) ∼ N ^−1/3 is much lower than the pressure calculated by the Laplace formula for similar nanocrystals for given values of density, temperature, and number of atoms. As the temperature increases from 20 K to the melting point, the isotherm P _sf(N) lowers and changes the shape of the dependence on N; at high temperatures, it goes to the region of extension of small nanocrystals of diamond and silicon.     

Contribution of carriers to optical conductivity spectra of lanthanum manganites

To study the band structure and carriers in lanthanum manganites, measurements have been made of the reflectance spectra of single crystals and polycrystals in the 0.04–1.6-eV range and of the optical conductivity σ _opt calculated by the Kramers-Kronig method as functions of the concentration and species of divalent ions in the paramagnetic (PM) and ferromagnetic (FM) regions. The optical gap for single-crystal La_0.9Sr_0.1MnO₃ is ∼0.17 eV, and the polaronband energy is 0.12 eV. In the PM region, σ _opt spectra do not indicate a band-carrier contribution, and conduction is dominated by polaron hopping and activation to the mobility edge. In the FM region, the variation in the σ _opt and absorption spectra of La_0.7Sr_0.3MnO₃ epitaxial films indicate the appearance of band carriers and a red shift of the absorption edge. The two band-carrier contributions, with weak and strong dependences on photon energy, are related to conduction in the antiferromagnetic matrix and the ferromagnetic regions. Fiz. Tverd. Tela (St. Petersburg) 41, 475–482 (March 1999)     

Permittivity and conductivity of triglycine sulfate films on Al/SiO2 and α-Al2O3 substrates

Triglycine sulfate (TGS) films have been prepared by evaporation from a saturated aqueous solution on substrates of fused quartz coated by a layer of thermally deposited aluminum (Al/SiO₂) and white sapphire (α-Al₂O₃) on whose surface interdigital electrodes have been deposited by photolithography. The TGS films have a polycrystalline structure made up of blocks measuring 0.1–0.3 mm (Al/SiO₂) and 0.1 × 1.0 mm (α-Al₂O₃). The polar axis in the blocks is mostly confined to the substrate plane. The temperature dependences of the capacitance and dielectric losses normal to and in the film plane have maxima at the temperature coinciding with that of the ferroelectric phase transition in a bulk crystal, T _c . The low-frequency conductivity G in TGS/Al/SiO₂ structures displays a frequency dispersion described by the relation G ∼ ω ^s (s ≈ 0.82). The conduction can be tentatively ascribed to the hopping mechanism involving localized carriers with a ground state energy of 0.8–0.9 eV. At temperatures above and below T _c , the low-frequency conductivity in TGS/α-Al₂O₃ films operates through a thermally-activated mechanism with an activation energy of 0.9–1.0 eV. At the phase transition, an additional contribution to conductivity appears in TGS/α-Al₂O₃ films with a dispersion G ∼ ω^0.5, which can be associated with domain-wall relaxation.     

Study of polycrystalline boron-doped diamond films by Raman spectroscopy and optical absorption spectroscopy

The effect of the degree of doping polycrystalline diamond films by boron on their Raman and absorption spectra has been studied in the visible region (from 200 to 1000 nm). As the boron concentration increases in a polycrystalline diamond film, its Raman spectrum exhibits a number of new specific features caused by the effect of boron atoms on the diamond lattice. The dependences that relate these features to the boron concentration in the films are given. Moreover, the absorption spectra of the films have revealed a peak whose maximum corresponds to photons with an energy near 2 eV.     

Miniaturized resonant photoacoustic cell of inclined geometry for trace-gas detection

A photoacoustic cell intended for laser detection of trace gases is represented. The cell is adapted so as to enhance the gas-detection performance and, simultaneously, to reduce the cell size. The cell design provides an efficient cancellation of the window background (a parasite response due to absorption of laser beam in the cell windows) and acoustic isolation from the environment for an acoustic resonance of the cell. The useful photoacoustic response from a detected gas, window background and noise are analyzed in demonstration experiments as functions of the modulation frequency for a prototype photoacoustic cell with the internal volume ∼0.6 cm³. The minimal detectable absorption for the prototype is estimated to be ∼1.2×10⁻⁸ cm⁻¹ W Hz^−1/2.