Electron microscope study of aluminium films evaporated on air-cleaved sodium chloride

The article deals with the structure of thin aluminium films evaporated on air-cleaved NaCl substrates. The effect of the substrate temperature and the film thickness on the structure, the orientation and the grain size of the films was studied at a deposition rate of 10 Å/see (ev. 50 Å/see) in a vacuum of about 7. 10^–5 torr. The optimum temperature for epitaxial growth was found to be about 420°C. An almost regular arrangement of grains was found in the thickness region of 90–150 Å.     

Temperature accelerated dielectric breakdown of PECVD low-k carbon doped silicon dioxide dielectric thin films

The dielectric breakdown strength of carbon doped silicon dioxide thin films with thickness d from 32 nm to 153 nm is determined at 25 °C, 50 °C, 100 °C, 150 °C and 200 °C, using I–V measurements with metal-insulator-semiconductor (MIS) structures. It is found that the dielectric breakdown strength, E_B, decreases with increasing temperature for a given film thickness. In addition, a film thickness dependence of breakdown is also observed, which is argued to show an inverse relation to thickness d in the form of E_B∝(d-d_c)^-n. The exponential parameter n and critical thickness limit d_c also exhibit temperature dependent behavior, suggesting a temperature accelerated electron trapping process. The activation energy for the temperature acceleration was shown to be thickness dependent, indicating a thickness dependent conduction mechanism. It is thereafter demonstrated that for relatively thick films (thickness >50 nm), the conduction mechanism is Schottky emission. For relatively thin films (thickness <50 nm), the Schottky conduction mechanism was obeyed at low field region while FN tunnelling was observed as a prevail one in the high field region. PACS 73.40.Qv     

Exchange-coupling across SiO

Strongly temperature-dependent ferromagnetic exchange coupling is observed between Fe films separated by an amorphous SiO barrier. Heating reversibly enhances the coupling strength which monotonically decreases with increasing SiO thickness. AtT=40K the coupling disappears for d_SiO20Å whereas at room temperature it persists up to a barrier thickness of 60 Å. The coupling strengthJ is determined by externally compensating the exchange field of samples grown on an antiferromagnetically biased Fe/Cr/Fe structure. It amounts toJ2·10^–6 J/m² for a sample with d_SiO=25 Å atT=300K. As a tentative explanation we propose that impurity or defect states within the large mobility gap of SiO carry the magnetic interaction across the insulating barrier.     

Electronic structure of square planar CuO 4 6− clusters

The results of spin-polarized MSX calculations show that the ground state of the CuO ₄ ^6– cluster is essentially non-magnetic in spite of odd number of electrons in the system for short Cu–O distances (1.90 Å) as found in the highT _c superconductors. This arises due to the fact that the unpaired electron resides in a molecular orbital with primarily oxygen 3s character. The stability of this molecular orbital is found to be sensitive to the cluster geometry and thus, increase in Cu–O distance (as well as other changes affecting oxygen-oxygen distance) tend to favour a magnetic state. From these calculations we have also estimated the Coulomb correlation strength within the Cu 3d to be about 5.3 eV.     

Rapid growth of ultra thin SiO2 films by a large-area electron beam

Rapid growth of ultra thin oxide films (40–180Å) of silicon using a low-energy large-area electron beam has been performed with a pressure ratio of 31 (O₂/He) and a total pressure of 0.5–0.7 Torr. A higher oxidation rate of about 625Ų/s is found for shorter irradiation time of the e-beam in the e-beam dose range 0.75–3 Coulomb/cm² and at lower substrate temperature 540–740°C. AES and XPS demonstrated a rapid electron-stimulated oxidation process of the Si surface. For the grown ultra thin oxide films, C-V characteristics, dielectric strength, uniformity of the film over the entire Si wafer and its thickness as a function of the processing time of the e-beam are also presented.     

Nickel-iron-gold magnetic films

Ni-Fe-Au films can be made by vacuum evaporation from a single melt since Au has an evaporation rate comparable to that of Ni and Fe. In films 400 Å thick the addition of Au does not changeH _c , butH _k increases 0·1 Oe per 1% Au until 10% Au is reached and then remains constant for Au percentages up to 26%. The change ofH _k with substrate deposition temperature is about the same as for Ni-Fe films (–0·01 Oe/°C). Films with 8% Au have the same skew as Ni-Fe films when made at a substrate temperature of 300 °C, but for Ni-Fe-Au films the change of skew with substrate temperature is about one fourth that of Ni-Fe films. The small area dispersion is reduced by a factor of 2 to 3 for Au additions of 4 to 26 percent.     

Influence of heat treatment on the structure and magnetic properties of the Co/Zr and Co/Hf multilayer amorphous films

The influence of annealing on the structure and magnetic properties of amorphous Co/Zr and Co/Hf multilayer films was studied with particular attention to the dependence of the magnetic properties, thermal stability and crystallization process on layer composition and thickness. The temperature at which crystallization commences increases from 400 to 460 °C as the layer thickness d_Zr or d_Hf increases from 6 to 18 Å, and decreases from 450 to 400 °C as d_Co increases from 12 to 18 Å. Multilayers containing 19–60 at% Zr were studied. The specific magnetization was found to increase even below the temperature at which crystallization commences. Our data are compared with non-multilayer Co–Zr amorphous films and rapidly quenched metallic glasses.     

Effect of poly silicon thickness on the formation of Ni-FUSI gate by using atomic layer deposited nickel film

The effect of poly-Si thickness on silicidation of Ni film was investigated by using X-ray diffraction, auger electron spectroscopy, cross-sectional scanning transmission electron microscopy, resistivity, I–V, and C–V measurements. The poly-Si films with various thickness of 30–200 nm were deposited by LPCVD on thermally grown 50 nm thick SiO₂, followed by deposition of Ni film right after removing the native oxide. The Ni film was prepared by using atomic layer deposition with a N²-hydroxyhexafluoroisopropyl-N¹ (Bis-Ni) precursor. Rapid thermal process was then applied for a formation of fully silicide (FUSI) gate at temperature of 500 °C in N₂ ambient during 30 s. The resultant phase of Ni-silicide was strongly dependent on the thickness of poly-Si layer, continuously changing its phase from Ni-rich (Ni₃Si₂) to Si-rich (NiSi₂) with increasing the thickness of the poly-Si layer, which is believed to be responsible for the observed flat band voltage shift, ΔV_FB, in C–V curves.     

Preparation and properties of the single 110 K phase Bi(Pb)-Sr-Ca-Cu-O superconductor

The single 110 K phase Bi(Pb)–Sr–Ca–Cu–O superconductor was made by sintering in air. Its zero-resistance temperature is 106 K, and it is an orthorhombic cell witha=5.403 Å,b=5.412 Å andc=37.062 Å. It formed around the low-T _c phase. The forming period of the 110 K phase can be shortened by multiple sintering and grinding. If PbO is mixed into the samples after the 85 K phase has formed, the forming rate of the 110 K phase can be increased significantly. EDAX analysis of grain composition confirms that the Pb atoms enter the unit cell, and probably Pb cations replace some Ca cations.     

Structure and electrical properties of thin films of copper selenide

Electron-diffraction and electron-microscope methods were used to investigate the structure of Cu₂Se films of close to stoichiometric composition. It is shown that in polycrystalline and single-crystal films of thickness >400Å at room temperature, the tetragonal modification is stable, which at temperatures above 400°K is transformed into the cubical modification. In thinner films d<400 Å the cubical modification of copper selenide is stable at room temperature. A sharp peak is observed at 400°K on the temperature dependence of the resistance; this is connected with the phase transition. At room temperature, copper selenide is a degenerate p-type semiconductor with carrier concentration 5 · 10²²–8 · 10²⁰ cm^–3, depending on the thickness of the film.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 90–94, August, 1973.     

Long-wavelength behavior of the optimal HNC/O solution for the ground-state of homogeneous liquid4He atT=0 K

The long-wavelength behavior of the two-body correlation factor,u(k), of the ground-state wave function for an homogeneous liquid⁴He at zero temperature is studied. The solutions are obtained from a paired-phonon-analysis within the HNC/O approximation. The cut-off for the phonons is investigated analyzingu(k) at small momenta. It is shown that the numerical results: (a) rule out a pure gaussian cut-off and (b) are in good agreement with the exponential cut-off suggested by Chester and Reatto. The first-sound velocityc ₁ and the cut-off momentumk _c are determined at several densities in the range 0.016–0.025 Å^–3. In addition, a parametrization of these quantities as a function of the density is provided.     

Correlations between frequency of infra-red active vibrational modes and copper-oxygen distance in copper oxides. Application to superconductors

The infra-red spectra of a large number of ternary Cu(II) oxides with at least a quasi square-planar coordination of oxygen around the copper ions have been studied. The frequency of the bands with the highest frequency,v _max, is found to correlate extremely well with the shortest Cu–O distance.v _max increases at an impressive rate of 20 cm^–1 per 0.01 Å when the Cu–O distance becomes less than 1.97 Å, which is the Cu²⁺–O^2– distance in square-planar CuO₄ complexes as obtained from empirical ionic radii considerations. The marked sensitivity may be used as a titration procedure not only to assign bands but also to obtain diagnostic information about local coordination in compounds derived, for example, from the YBa₂Cu₃O_7–d structure such as LaCaBaCu₃O_7–d . The only example where this correlation fails is in the two-layer non-superconducting oxides derived from La₂(Ca, Sr)Cu₂O₆. The significance of this result is discussed. The marked dependence of frequency on the bond-distance is qualitatively examined in terms of an increased electron-phonon coupling to account for the observed tendency of the superconducting transition temperature to go through a maximum as the average basal plane Cu–O distance is decreased.     

GaN/AlN quantum dot photodetectors at 1.3–1.5 μm

We have demonstrated GaN/AlN quantum dots (QD) photodetectors, relying on intraband absorption and in-plane carrier transport in the wetting layer. The devices operate at room temperature in the wavelength range 1.3–1.5 μm. Samples with 20 periods of Si-doped GaN QD layers, separated by 3 nm-thick AlN barriers, have been grown by plasma-assisted molecular-beam epitaxy on an AlN buffer on a c-sapphire substrate. Self-organized dots are formed by the deposition of 5 monolayers of GaN under nitrogen-rich conditions. The dot height is 1.2±0.6 to 1.3±0.6 nm and the dot density is in the range 10¹¹–10¹² cm⁻². Two ohmic contacts were deposited on the sample surface and annealed in order to contact the buried QD layers. The dots exhibit TM polarized absorption linked to the s–p_z transition. The photocurrent at 300 K is slightly blue-shifted with respect to the s–p_z intraband absorption. The responsivity increases exponentially with temperature and reaches a record value of 10 mA/W at 300 K for detectors with interdigitated contacts.     

Magnetic behavior of amorphous Sm-Co alloy films

Amorphous films of Sm_100–x Co _x with 70x90 were made by vapor deposition on flat glass substrates kept at 300 K. The films crystallize above 700 K. The film plane is the easy plane with the planar anisotropy of the order of 10⁷ erg cm^–3, indicating a local anisotropy of the order 10⁸ erg cm^–3. With regard to local anisotropy and magnetization, the films appear to correspond to the random anisotropy concept of amorphous magnets. Films evaporated in a magnetic field parallel to the film plane have an induced uniaxial anisotropy of the order of 10⁶ erg cm^–3. Hysteresis loops in fields parallel to the easy axis are perfectly rectangular with coercive fieldsH _c in the range 30 Oe<H _c<3000 Oe, depending on composition, temperature and heat treatment. The magnetization reverses by a thermally activated domain nucleation and growth process having a narrow distribution of time constants. Aging at temperatures below the crystallization temperature reduces the anisotropies andH _c.     

Superconductivity and normal electrical conductivity of amorphous lead films nucleated with molybdenum

Lead films vapor quenched onto nucleating monolayers of Mo or W exhibit strong lattice disorder and can be considered to be amorphous. The amorphous-to-crystalline transformation temperatureT _tr is indicated by a sharp drop of the electrical resistivity in the course of annealing.T _tr is found to be proportional tod ^–2 for Pb thicknessd smaller than 30 nm. The superconducting transition temperatureT _c is by 0.6 to 1 K smaller in the amorphous state than after crystallization. In both states,T _c is proportional tod ^–1. Prenucleation with about half a monolayer of Mo leads to quite the sameT _c depression as observed earlier by Strongin et al. on Pb films vapor quenched onto predeposited films of SiO, Ge or Al₂O₃. For comparison, experiments have been carried out with 2.5 nm Ge predeposits. As with Mo prenucleation, a well defined transformation temperatureT _tr of about the same value has been observed.T _c of bulk amorphous Pb can be extrapolated to be about 6.6 K.     

Soft X-ray amplification by Li-like Al10+ and Si11+ ions in recombining plasmas

Experimental studies of soft X-ray lasers were carried out on the six-beam laser facility and the LF 12 laser facility of SIOM. Using a home-made one-dimensional spatially resolved grazing incidence grating spectrograph, XUV amplification has been observed in Li-like aluminum and silicon ions, by irradiation of slab targets with a line-focused laser. Based on time-integrated measurement, gain coefficients are 3.1 cm^–1 for the 105.7 Å 5f–3d transition in Li-like Al ions, and 1.5 cm^–1 and 1.4 cm^–1 for the 88.9 Å 5f–3d and the 87.3 Å 5d–3p transitions in Li-like Si ions, respectively. The maximum gain × length products (GL) are about 2.5.     

Reactive pulsed laser deposition and laser induced crystallization of SnO2 transparent conducting thin films

Transparent conducting SnO₂ thin films with a thickness between 1000–2000 Å were deposited on glass, quartz and silicon substrates using standard pulsed laser deposition techniques with two different targets (Sri and SnO₂) and with three different laser wavelengths (1.06, 0.532 and 0.266 ) from a Q-switched Nd: YAG laser. Tin dioxide films with optical transmission over most of the visible spectrum exceeding 80% were obtained using a Sn target and a background oxygen pressure of 20 Pa. The electrical resistivity () depended strongly on the substrate temperature during deposition, with the lowest values of of about 10^–2 -cm obtained when the substrate was maintained at 400°C during deposition. Using SnO₂ targets, predominantly amorphous phase SnO₂ films were deposited on Si substrates and then transformed into polycrystalline Sn₃O₄ by laser induced crystallization ( = 1.06 m). Whereas these later films were essentially non-conducting as deposited ( > 400 -cm), the electrical resistivity was permanently reduced after laser induced crystallization by a factor greater than 1000 to a value of approximately 4 × 10^–1 -cm.     

Kinetic Monte Carlo simulation of thin film growth

A three-dimensional kinetic Monte Carlo technique has been developed for simulating growth of thin Cu films. The model involves incident atom attachment, diffusion of the atoms on the growing surface, and detachment of the atoms from the growing surface. The related effect by surface atom diffusion was taken into account. A great improvement was made on calculation of the activation energy for atom diffusion based on a reasonable assumtion of interaction potential between atoms. The surface roughness and the relative density of the films were simulated as the functions of growth substrate temperature and film thickness. The results showed that there exists an optimum growth temperatureT _opt at a given deposition rate. When the substrate temperature approaches toT _opt, the growing surface becomes smoothing and the relative density of the films increases. The surface roughness minimizes and the relative density saturates atT _opt. The surface roughness increases with an increment of substrate, temperature when the temperature is higher thanT _opt.T _opt is a function of the deposition rate and the influence of the deposition rate on the surface roughness depends on the substrate temperatures. The simulation results also showed that the relative density decreases with the increasing of the deposition rate and the average thickness of the film.     

Anomalous plasma phenomena of hollow cathode arc discharge

Magnetically confined argon plasma produced by hollow cathode arc discharge has been studied in different experimental conditions, with discharge current from 10–50 A, vessel argon pressure between 10^–3 and 10^–4 torr (1 torr=133·32 Pa) and axial magnetic field up to 0·12 T. The plasma density measured by a cylindrical Langmuir probe is found to be 10¹⁹ to 4 × 10¹⁹ m^–3 and the electron temperatureT _e varies between 2·5 and 4·8 eV. When an external axial magnetic field is applied the plasma temperature decreases with the increase in the magnetic field intensity until it reaches a minimum value at 0·075T and then increases with the same rate. This has been interpreted as high frequency waves excitation due to electron beam-plasma interaction, which explains the electron density jumps with the magnetic field intensity. Enhanced plasma transport across the magnetic field is studied and classified as anomalous diffusion.     

Laser-induced changes in the electron density distribution of crystals: Ca3Cr2Si3O12

Difference of electron density distribution between a crystal being He–Ne laser-irradiated and non-irradiated has been investigated by in situ X-ray diffraction methods. Difference Fourier analysis between these data revealed electron density decrease of 2.1 eÅ^–3 at the trivalent site occupied 64% by Cr³⁺, the absorbant; while the calcium, silicon, and oxygen sites remained practically unchanged. Crystal data: [Natural uvarovite from Outokumpu, Finland; Ca₃(Cr_1.284Al_0.692Fe_0.024)Si₃O₁₂ (EPMA analysis); Cubic garnet structure; ;Z=8;a ₀=11.936(1) Å;V=1700.5 ų D _x =3.775 Mgm^–3; (MoK)=0.71069 Å; =4025 m^–1;F(000)=1900;T=293 K].