Thickness dependence of crystallization and melting kinetics of eutectic Sb70Te30 phase change recording film

As the thickness of eutectic Sb₇₀Te₃₀ phase change recording film was reduced, the crystallization temperature and activation energy for crystallization would increase, while the melting temperature and activation energy for melting would decrease. Accordingly, the archival stability and recording sensitivity can be improved. However, the recording speed for direct over write will be slowed down. Based upon the Johnson-Mehl-Avrami equation, it was found that the increase of film thickness would increase the nucleation rate of eutectic Sb₇₀Te₃₀ recording film and make the crystallization process become more nucleation-dominated.     

Apparent activation energy of structural relaxation for Se70Te30 glass

Differential scanning calorimetry and dc conductivity measurements were used to study structural relaxation of Se₇₀Te₃₀ glass. A single set of Tool-Narayanaswamy-Moynihan (TNM) parameters was obtained from the curve-fitting procedure. The value of apparent activation energy Δh∗ was further confirmed by two non-fitting techniques. Results of the Δh∗ evaluation from the T_g dependence on cooling rate are discussed in terms of how the T_f determination might be influenced by the material’s structure type and by the interference of the crystallization process.     

Crystallization process on amorphous GeTeSb samples near to eutectic point Ge15Te85

One of most important properties of some tellurium-based chalcogenide glasses is the optical and electrical switching between two states: the glass and the crystalline state. The understanding in these systems of the glass to crystal transition and its transformation kinetics is essential for their application in non-volatile memories. GeTeSb and GeTe amorphous samples of compositions close to the eutectic point Ge₁₅Te₈₅ were obtained by rapid solidification from the liquid state employing melt spinning technique. The glass forming ability of this system, for this cooling technique, is restricted to a small composition range nearby the binary eutectic. The crystallization kinetics of the samples was studied by means of differential scanning calorimetry (DSC) under both isothermal and continuous heating regimes. The quenched samples and the crystallization products have been characterized by X-ray diffraction with Cu(Kα) radiation. The crystallization temperature, activation energy, crystallization enthalpy and the dependence of these properties on concentration are reported. The crystallization study of Ge₁₅Te₈₅ glasses shows: a primary crystallization of Te superimposed with a secondary crystallization of GeTe. The addition of Sb (5 at.%) to the eutectic point Ge₁₅Te₈₅ modifies this behavior: the crystallization of Ge₁₃Sb₅Te₈₂ glasses consists on the crystallization of Te and Ge₂Sb₂Te_5. The crystallization of the ternary glasses was modeled.     

Structure, electrical, optical and thermal properties of Ge4Sb4Tex (x = 8, 9 and 10) thin films

The crystalline samples of Ge₄Sb₄Te₁₀, Ge₄Sb₄Te₉, and Ge₄Sb₄Te₈ were prepared and their amorphous semiconducting thin films obtained by flash evaporation. Their sheet resistance decreased slowly with temperature up to 147–160 °C with activation energy of electrical conductivity ΔE = 0.40–0.44 eV. Above these temperatures, the sheet resistance drops abruptly by several orders due to crystallization. The drop of resistivity proceeds in two steps. Two steps of phase change were also found on curves of DSC and on the temperature dependence of index of refraction. It pays for slow heating rates, crystallization induced by short (≈30 ns) laser pulses proceeds probably in one step only for all studied samples (as it follows from X-ray diffraction), not only for Ge₂Sb₂Te₅ in which a single phase formation was confirmed. The crystallization temperatures are increasing slightly with decreasing Te content in the series Ge₄Sb₄Te₁₀–Ge₄Sb₄Te₉–Ge₄Sb₄Te₈ from 147 to 160 °C. The X-ray diffractograms revealed that in laser crystallized samples can be found only cubic modification of Ge₂Sb₂Te₅ type (a = 0.6 nm), while the samples annealed (230 °C, 2 h) or annealed after the crystallization with laser pulse, contain also small amounts of hexagonal phase.     

Phase transitions and electrical properties of As2Te3

This study identifies and characterizes the phase transitions induced thermally in bulk amorphous and crystalline As₂Te₃. In addition, it examines the nature and the electrical properties of the various As₂Te₃ phases, and attempts to correlate the memory electrical switching exhibited by these materials with their phase transitions.     

Structural and electrical characterization of ALCVD ZrO2 thin films on silicon

We report on the structural and electrical properties of ZrO₂ thin layers grown on Si by atomic layer chemical vapour deposition. Atomic force microscopy, X-ray diffraction, X-ray reflectivity and time-of-flight secondary ion mass spectrometry have been used to characterize as-grown and annealed samples. High frequency capacitance-voltage measurements have been performed to determine the capacitance of the gate dielectric stack. The ZrO₂ film is found to be polycrystalline. Electrical and structural data suggest a coherent picture of film modification upon annealing.     

Effects of Ge addition on the optical and electrical properties of eutectic Sb70Te30 films

The aim of this work is to study the influence of Ge addition on the optical and electrical properties in eutectic SbTe thin films (with the compositions: Sb₇₀Te₃₀, Ge₂Sb₇₀Te₂₈, Ge₅Sb₇₀Te₂₅ and Ge₁₀Sb₆₅Te₂₅) using visible optical reflectance, ellipsometry measurements, near infrared transmittance spectra, and four probe electrical resistivity measurements. From near infrared transmittance measurements the optical band gap was determined using Tauc’s expression for amorphous materials, a value of about 0.47 eV was obtained without any clear dependence on the Ge content. All amorphous films have approximately the same reflectance value, however the contrast ratio between the crystalline and amorphous phases decrease with increase of Ge. Using in situ four probe measurements versus temperature the dependence of the activation energy of conductance and the onset of the crystallization temperature have been determined for different materials. Four probe measurements have shown that the resistivity of amorphous films increases with increase of Ge. The results obtained have shown that optical and electrical properties of SbTe films with near eutectic composition change with the Ge content and depending on the application, in either optical or electrical memory devices, the most suitable composition needs to be determined.     

Impurity effects of some metals on electrical properties of amorphous As2Se1Te2 films

Amorphous As₂Se₁Te₂ (a-As₂Se₁Te₂) films, into which Cu or Cd ions were doped by thermal diffusion below the glass transition temperature exhibited the increase of conductivity by several orders of magnitude. In addition, these samples exhibited enhanced photoconductivity and the activation type conduction. From the measurements of the thermoelectric power, it was found that Cu doped samples were p-type and Cd doped samples were n-type. The impurities was also studied by SIMS.     

Structural peculiarities, and electrical and optical properties of 70TeO2·30PbCl2 glasses doped with Pr, prepared in Pt or Au crucibles

The influence of crucibles (Au or Pt) on the structure, electrical, dielectric and optical properties of 70TeO₂·30PbCl₂ glasses doped with Pr³⁺ added as a metal, chloride, or oxide, in concentrations of 500–1500 wt-ppm, is reported. The dc conductivity of ‘pure’ glasses prepared in Au crucibles is two orders of magnitude higher than that of those prepared in Pt crucibles. Upon doping, the dc conductivity of glasses prepared in Pt and Au crucibles increases or decreases, respectively. The static relative permittivity is equal to 33 ± 2. In the range of 640–700 nm, six photoluminescence (PL) peaks were observed, at 641.5, 647.1, 652.4, 660.8, 662.9, and 664.5 nm. In the range of 200–1200 cm⁻¹, seven Raman scattering (RS) peaks were observed at 184, 217, 321, 468, 654, 735 cm⁻¹, and a small peak at 650 cm⁻¹. Both spectra were deconvoluted using symmetrical Gaussian functions. Relative intensities of PL and RS bands depend on the concentration and chemical form of Pr³⁺ and on the material of the crucible. However, positions of these bands are independent of these conditions.     

Optimal crystal growth conditions of thin films of Bi2Te3 semiconductors

Crystal growth conditions of Bi₂Te₃ narrow bandgap semiconductors have been studied using molecular beam epitaxy method. It was applied to the growth of Bi₂Te₃ on Bridgman single-crystal substrate Sb₂Te₃. Substrate ingots were taken from the natural cleavage along the (0001) plane. The deposited conditions have been studied as a function of substrate temperature (T_s) and flux ratio (F_R=F(Te)/F(Bi)). The quality of deposited layers was controlled by X-ray diffraction, scanning electron microscope (SEM), secondary ion mass spectroscopy (SIMS) depth profiling and energy-dispersive X-ray (EDX) microanalyser. The sticking coefficients K_s(Te) and K_s(Bi) of the elements that compose Bi₂Te₃ were determined. It was found that the stoichiometry of deposited layers depended on substrate temperature and flux ratio. It was observed that all deposited layers were single-crystal in the orientation of their substrates with a small shift due to the stress in layer.     

Capacitance-voltage characteristics of In0.3Ge2Sb2Te5 thin films

The influence of indium doping on the capacitance variation with temperature and applied bias voltage of Ge₂Sb₂Te₅ is investigated. The capacitance-voltage (C-V) measurements of In_0.3Ge₂Sb₂Te₅ and Ge₂Sb₂Te₅ thin films were performed for a sweep of voltages from −20 to +20 V at different temperatures. The results show different capacitance behavior of In_0.3Ge₂Sb₂Te₅ and Ge₂Sb₂Te₅ films. As the temperature increases the capacitance of the indium-doped sample decreases and becomes negative. The negative capacitance effect might be attributed to a significant increase of the film’s conductivity due to temperature and applied bias voltage. The nonlinearity in the capacitance and conductivity could be related to the nucleation mechanism as the temperature becomes close to the amorphous-crystalline transition temperature.     

Structural and optical properties of thin films prepared from surface modified ZrO2

This paper describes the preparation and characterization of ZrO₂ thin films deposited on silicon wafer by spin coating method. Nanocrystalline ZrO₂ was synthesized by hydrothermal method using zirconium (IV)-n-propoxide as a precursor material. Surface of the ZrO₂ particles was then modified with 2-acetoacetoxyethyl methacrylate used as a copolymer for coatings. The optical properties, nanostructure and surface morphology of the thin films prepared from surface modified ZrO₂ nanoparticles were examined by optical spectroscopy, X-ray diffraction and scanning electron microscopy, respectively. It was found that the films deposited on silicon wafer have crystalline structure of monoclinic (111) at temperature of 150 °C. It was observed that films depict very dense material that does not present any granular or columnar structure. It was found that optical transparency of thin ZrO₂ films distributed in the range of 30-40 percent in the spectral range 400-800 nm. Refractive index of ZrO₂ films were determined as functions of ZrO₂ content and it was found that the refractive index increases from 1.547 to 1.643 with increased ZrO₂ content.     

Structural analysis and devitrification of glasses based on the CaO-MgO-SiO2 system with B2O3, Na2O, CaF2 and P2O5 additives

Glasses, whose basic composition was based on the CaO-MgO-SiO₂ system and doped with B₂O₃, P₂O₅, Na₂O, and CaF₂, were prepared by melting at 1400 °C for 1 h. Raman and infrared (IR) spectroscopy revealed that the main structural units in the glass network were predominantly Q¹ and Q² silicate species. The presence of phosphate and borate units in the structure of the glasses was also evident in these spectra. X-ray analysis showed that the investigated glasses devitrified at 750 °C and higher temperatures. The crystalline phases of diopside and wollastonite dominated, but weak peaks, assigned to akermanite and fluorapatite, were also registered in the diffractograms. The presence of B₂O₃, Na₂O, and CaF₂ had a negligible influence on the assemblage of the crystallized phases, but it caused a reduction of crystallization temperature, comparing to similar glasses of the CaO-MgO-SiO₂ system.     

Structural and transport properties of quaternary glass system: LiF-Li2O-SrO-Bi2O3

Bismuth based glasses containing LiF, Li₂O and SrO were investigated by different physical, spectroscopic and transport techniques. The results show that density of the glass system increases whereas glass transition temperature decreases with increase in LiF content. The decrease in glass transition temperature is attributed to the entry of the fluoride ions into the glass network mainly substitutionally in place of oxygen ion. The increase in dc electrical conductivity in the present glasses with increase in the fluorine ions is due to the mixed contribution of the positively charged lithium cations throughout the glass network and the negatively charged fluorine, which may act as impurity and/or as terminal non-bridging halide ion. Infrared and Raman spectroscopic results indicate that the glass network consists of BiO₆ octahedral and BiO₃ pyramidal units.     

Electrical and optical properties of Sn10Sb20−xBixSe70 (0 x 8) glassy films

Chalcogenide glasses are interesting materials due to their infrared transmitting properties and photo-induced effects exhibited by them. Thin films of the glasses Sn₁₀Sb_20−xBi_xSe₇₀ (0 x 8) prepared by melt quenching technique were evaporated in a vacuum better than 10⁻⁵ mbar. Optical transmission spectra of all the deposited films were obtained in a range 400–2500 nm. The optical band gap and the absorption coefficient were calculated from the transmission data and refractive index was calculated using the swanepoel method. The optical band gap initially increases with increase in Bi content (for x = 2) and then decreases sharply for higher Bi concentrations. The refractive index as well as absorption coefficient decrease with increase in wavelength. The dark activation energy initially increases with increase in Bi content and then decreases with further addition.     

Structural transformation on Se0.8Te0.2 chalcogenide glass

Using X-ray diffraction and differential scanning calorimetry (DSC), the structure and the crystallization mechanism of Se_0.8Te_0.2 chalcogenide glass has been studied. The structure of the crystalline phase has been refined using the Rietveld technique. The crystal structure is hexagonal with lattice parameter a = 0.443 nm and c = 0.511 nm. The average crystallite size obtained using Scherrer equation is equal 16.2 nm, so it lies in the nano-range. From the radial distribution function, the short range order (SRO) of the amorphous phase has been discussed. The structure unit of the SRO is regular tetrahedron with (r₂/r₁) = 1.61. The Se_0.8Te_0.2 glassy sample obeys the chemical order network model, CONM. Some amorphous structural parameters have been deduced. The crystallization mechanism of the amorphous phase is one-dimensional growth. The calculated value of the glass transition activation energy (E_g) and the crystallization activation energy (E_c) are 159.8 ± 0.3 and 104.3 ± 0.51 kJ/mol, respectively.     

Structural study of amorphous Al0.20As0.50Te0.30, Al0.10As0.40Te0.50 and Al0.10As0.20Te0.70 by x-ray diffraction (I)

An analysis of the atomic radial distribution function of Al_0.20As_0.30Te_0.30, Al_0.10As_0.40Te_0.50 and Al_0.10As_0.20Te_0.70 amorphous alloys obtained from quenching of the molten mixture of the elements was performed. A structure in which all the Al atoms are tetrahedrally bonded to the other types of atoms in the material, would satisfy the requirements of the experimental curve. Tetrahedral groups might be linked to each other by As and Te atoms, or directly through a Te or As atom belonging to more than one tetrahedra.     

Structural study of amorphous Al0.20As0.50Te0.30, Al0.10As0.40Te0.50 and Al0.10As0.20Te0.70 by X-ray diffraction (II)

A spherical-shaped model of Al_0.20As_0.50Te_0.30, Al_0.10As_0.40Te_0.50 and Al_0.10As_0.20Te_0.70 amorphous alloys has been performed by the random Monte Carlo method. These models describe quite well the experimental radial distribution functions and abide by the expected coordination numbers apart from the threefold coordinated Te, of which an excess has appeared. The structures are formed, basically, of distorted tetrahedra around the Al atoms whose corners are occupied by As or Te atoms. Also, a separated phase model for Al_0.10As_0.20Te_0.70 alloy has been built taking into account the results of thermodynamical study on this amorphous alloy system. The fitting of this model was better than that of the model generated under the hypothesis of a continuous phase.     

Structural and electrical properties of CuGaSe2 thin films on GaAs substrates

CuGaSe₂ thin films with thicknesses in the range of 0.1 μm were deposited onto semiinsulating (111) A-oriented GaAs substrates by flash evaporation under controlled growth conditions. Epitaxial growth begins at a substrate temperature of T_sub = 835 K. Besides the chalcopyrite phase a hexagonal modification was observed in the range T_sub = 835 to 855 K. At T_sub ≧ 860 K the films have sphalerite structure. All epitaxial films were p-type conducting. Two acceptor levels with ionization energies of about 150 meV and 550 meV, respectively, were obtained from an analysis of electrical and photoluminescence measurements.     

Structural and electrical properties of CdS thin films evaporated onto GaAs substrates

CdS thin films on GaAs substrates are prepared by single source thermal evaporation under high vacuum conditions. Films on (111)A-oriented substrates have the wurtzite structure, whereas in the case of (1 1 1 )B-oriented substrates the sphalerite structure predominates. Depositing films on pre-annealed substrates a tetragonal phase with a composition near CdGa₂S₄ was found. The electron concentration in the CdS films is influenced by the substrate temperature, the growth rate of the films, and the substrate orientation.