Annealing effects on the structural and optical characteristics of electron beam deposited Ge-Se-Bi thin films

Measurements of absorbance and transmittance in Ge₂₀Se_80-xBi_x (0 x 10 at %) chalcogenide thin films in the visible range at room temperature were carried out. The dependence of the optical absorption on the photon energy is described by the relation hv=B(hv-E₀)². It was found that the optical energy gap E₀ decreases gradually from 1.93 to 1.205 eV with increasing Bi content up to 10 at %. The rate of the change decreases with increasing Bi content. The composition dependence of the optical energy gap is discussed on the basis of the concentration of covalent bonds formed in the chalcogenide film. The decrease of optical energy gap with increasing Bi content is related to the increase of Bi–Se bonds and the decrease of Se-Se bonds. The effect of thermal annealing for different periods of time on the behavior of optical absorption of the as-deposited films was investigated. The optical gap increases with increasing annealing time. The rate of change decreases with increasing annealing time, then E₀ reaches a steady state. The increase in the values of the optical gap of the amorphous films with heat treatment is interpreted in terms of the density of states model. The structure of the as-prepared and thermally annealed films were investigated using transmission electron microscopy, energy dispersive analysis and X-ray diffraction. It was confirmed that the as-prepared films were in the amorphous state. Phase separation was observed after thermal annealing. The separated crystalline phases were identified.     

Optical properties of tin diselenide films

SnSe₂ films were deposited on substrates at 300 K by a conventional thermal evaporation technique. The as-deposited films were amorphous and transformed to the crystalline phase on post-deposition annealing above 573 K in an inert atmosphere. The optical properties of the films were investigated, using spectrophotometric measurements of the transmittance and reflectance at normal incidence in the wavelength range 400–2000 nm. The refractive index data fit a single oscillator model with a dispersion parameter 5.149×10^–14 and 5.773×10^–14 eVm² for the amorphous and crystalline films, respectively. The high-frequency dielectric constant of the amorphous films decreased from 9.871 to 7.475 for the crystalline films. The analysis of the spectral behaviour of the absorption coefficient in the intrinsic absorption region revealed an indirect forbidden and a direct allowed transition with energy gaps 0.99 and 2.05 eV for the amorphous films and 0.96 and 2.02 eV for the crystalline films, respectively.     

Electrical, optical, and thermal properties of Sn-doped phase change material Ge2Sb2Te5

In this article, effect of Sn on the electrical, optical, and thermal properties of Ge₂Sb₂Te₅ is studied. Ge₂Sb₂Te₅, Ge_1.55Sb₂Te₅Sn_0.45, and Ge_1.1Sb₂Te₅Sn_0.9 alloys are prepared by melt quenching technique and their thin films are prepared by thermal evaporation on glass substrates. These materials are then characterized by differential scanning calorimetry, X-ray diffraction, optical method, and impedance measurements. Doping with Sn maintains the NaCl-type crystalline structure of Ge₂Sb₂Te₅. Activation energy (E _a) for crystallization is calculated by Kissinger’s method. E _a decreases slightly from 2.56 eV for Ge₂Sb₂Te₅ to 2.24 eV for Ge_1.1Sb₂Te₅Sn_0.9. The distinct change in extinction coefficient (k) of Ge₂Sb₂Te₅ and Sn-doped amorphous films is found in the visible region. A large increase in optical contrast (C) is observed in the Sn-doped phase change materials. The phase change transition is studied using impedance measurements as a function of temperature. Impedance measurements show the appearance of nucleation centers in samples heated at temperatures below crystallization temperature (T _c) and above glass transition temperature (T _g).     

Thermal induced transformations in glassy chalcogenides SixTe60−xAs30Ge10

Temperature-induced transformations are considered to be interesting characteristic properties of amorphous materials including the Si _x Te_60–x As₃₀Ge₁₀ system, withx=5, 10, 12 and 20. Density (), X-ray diffraction and differential thermal analysis (DTA) were used to characterize the compositions. DTA traces of each glass composition at different heating rates from 5 to 30° C min^–1 were obtained and interpreted. Fast and slow cooling cycles were used to determine the rate of structure formation. Cycling studies of materials show no memory effect but only ovonic switching action. The compositional dependence of the crystallization activation energy (E) and the coefficient of glass-forming tendency (K _gl) have been calculated. The thermal transition temperatures and associated changes in specific heat have been examined as a function of the Te/Si ratio by differential scanning calorimetry. It was found that andE increase linearly with increasing tellurium content, while the heat capacity (c _p) andK _gl, decrease with increasing tellurium content.E=1.54 eV andc _p=0.246 J g^–1 K^–1 forx=20 whileE=2.74eV andc _p=0.22 J g^–1 K^–1 forx=5.     

Effect of Sn Doping on the Crystallization Kinetics of Amorphous TlInS<Subscript>2</Subscript> Films

The crystallization of amorphous Sn-doped TlInS₂ films into three polymorphs has been studied by kinematic electron diffraction. The results demonstrate that the crystallization of 30-nm-thick amorphous films produced by thermal evaporation in high vacuum can be described by the Avrami–Kolmogorov equation: V_τ = V₀[1–exp(–kτ^m)]. Kinematic electron diffraction patterns of the TlIn_1–хSn_xS₂ films have been used to assess the effect of doping with Sn on the growth dimensionality and the activation energy for the crystallization of the amorphous films and the unit-cell parameters of the resultant crystalline materials. Doping extends the temperature range and effective activation energy for the crystallization of the amorphous films.     

Electrical conductivity,thermoelectric power and optical absorption studies of amorphous and crystalline gallium antimonide thin films

Amorphous films (240–1740 Å thick) of gallium antimonide were formed by vacuum evaporation onto goldseal glass slides and onto sodium chloride crystals. They were annealed in successive cycles to obtain the ideal amorphous state. The heat treatment was continued until the crystalline phase was obtained. The electrical conductivity, the thermoelectric power with respect to bulk silver and the optical absorption were measured in all these films before and after crystallization. The electron diffraction patterns of the as-deposited, the annealed and the crystalline films were obtained using a transmission electron microscope. The activation energy for conduction ΔE, the amorphous-to-crystalline transition temperature, the density of states at the Fermi level N(_EF) and the optical energy gap E_o were evaluated.     

Characterization of (Pb,La) TiO3 thin films prepared by the sol-gel method

Pb_(1–1.5x)La_xTiO₃ thin films were synthesized by the sol-gel spin-coating technique. The films became crystallized when the spin-coated films were annealed at 600 °C and at higher temperature, and became amorphous when annealed at 550 °C. The breakdown voltage, V _B, was recorded at around 30 V for 600–650 °C annealed films and varied only slightly with the composition. The V _B value of the amorphous films was observed to be higher than that of the crystalline films. The ferroelectric properties of both the amorphous and crystalline films were found to be similar. The dielectric constant, charge storage density and optical index of refraction of the films were _r =5–20, Q _c=0.12–0.54 C cm^–2 and n=1.6–2.3, respectively. They all increased moderately with La³⁺ content in the films. One possible reason why the ferroelectric properties are not modified as the amorphous films crystallize, may be that the octahedra are equilateral, whether the films are amorphous or crystalline. Additionally, a possible cause which lowers the breakdown voltage in crystalline film, is the formation of lead vacancies due to lead loss. The electrical properties of films coated on bare silicon become significantly lowered due to interdiffusion between films and substrate. The diffusion of Si⁴⁺ ions into-the films can be prevented by coating SrTiO₃ on the silicon substrate as a buffer layer. The charge storage capacity consequently becomes substantially enhanced.     

Optical constants and electrical conductivity of Ge20Se60Sb20 thin films

Optical absorption and electrical resistivity of amorphous Ge₂₀Se₆₀Sb₂₀ films are investigated as a function of the thermal annealing. The dependence of the optical absorption coefficient on the photon energy is ascribed by the relation (h) = B(h–E_o)². Increasing the annealing temperature from 423 K to 553 K, decreases the optical gap of the film from 1.25 eV to 0.78 eV. The effect of annealing temperature on high frequency dielectric constant () and carrier concentration (N) was also studied. As a result of annealing the film at 533 K, the electrical resistivity and activation energy for conduction decreased from 5.7 × 10⁷ to 2.9 × 10² ·cm and from 0.94 to 0.34 eV, respectively. The crystalline structures resulting from heat treatment at different elevated temperatures have been studied by X-ray Diffraction (XRD). The optical and electrical changes were attributed to the amorphous-crystalline transformations in the chalcogenide films.     

Optical and electrical properties of carbon nitride films deposited by cathode electrodeposition

Carbon nitride (CN _x ) films were prepared on silicon (100) wafers and ITO conductive glasses by cathode electrodeposition, using dicyandiamide (C₂H₄N₄) in acetone as precursors. The composition ratios (N/C) were approximated to or larger than 1 from XPS. The optical properties and electrical resistivities of the films were investigated. Intense PL with two bands in the range 2.5–3.5 eV was observed on the CN _x films. The band gaps (E _opt) deduced from measurements of the optical absorption coefficients in the UV-VIS spectra were found to be in the range of 1.1–1.6 eV. From the PL and UV-VIS spectra, the nitrogen content has a large effect on the PL band gap and E _opt. The electrical resistivities of the films on Si wafers are in the 10⁹–10¹⁰ · cm range.     

Structural and optical properties of CdGa2Se4 thin films

CdGa₂Se₄ thin films were prepared by vapour deposition onto either room temperature or preheated quartz and glass substrates (T _S) or they were deposited at room temperature and then annealed at about (T _A) 623 K. The films thus prepared were crystalline with a thiogallate tetragonal structure. The optical constants (the refractive index n and the absorption index, k) were determined for CdGa₂Se₄ films deposited onto quartz substrates held at either room temperature or at T _S = 573 K. These constants were also determined for preannealed films (T _A = 623 K). Plots of (hv)² = f(hv) and (hv)^1/2 = g(hv) were linear, indicating the existence of both direct and indirect optical transitions. It was found that the values of E _g ^d and E _g ^ind for as-deposited CdGa₂Se₄ films were 2.46 and 1.91 eV, respectively. The corresponding values for the annealed films and the films deposited at T _S = 573 K were 2.56 and 2.06 eV, respectively.     

Phase change and optical band gap behavior of Ge-Te-Ga thin films prepared by thermal evaporation

The amorphous Ge_11.4Te_86.4Ga_2.2 chalcogenide thin films were prepared by thermal evaporation onto chemically cleaned glass substrates. Properties measurements include X-ray diffraction (XRD), Scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), Four-point probe and VIS-NIR transmission spectra. The allowed indirect transition optical band gap and activation energy of samples were calculated according to the classical Tauc equation and Kissinger's equation, respectively. The results show that there is an amorphous-to-crystalline phase transition of Ge_11.4Te_86.4Ga_2.2 thin film. The investigated film has high crystallization temperature (∼200 °C) and activation energy (2.48 eV), indicating the film has good amorphous stability. The sheet resistance of the crystalline state is ∼10 Ω/and the amorphous/crystalline resistance ratio is about 10⁵. Besides, a wide optical band gap (0.653 eV) of Ge_11.4Te_86.4Ga_2.2 is obtained, indicating that the material possesses a low threshold current from amorphous-to-crystalline state for phase-change memory application.     

Nanostructure, electrical and optical properties of p-type hydrogenated nanocrystalline silicon films

In this paper, p-type hydrogenated nanocrystalline (nc-Si:H) films were prepared on corning 7059 glass by plasma-enhanced chemical vapor deposition (PECVD) system. The films were deposited with radio frequency (RF) (13.56 MHz) power and direct current (DC) biases stimulation conditions. Borane (B₂H₆) was a doping agent, and the flow ratio η of B₂H₆ component to silane (SiH₄) was varied in the experimental. Films’ surface morphology was investigated with atomic force microscopy (AFM); Raman spectroscopy, X-ray diffraction (XRD) was performed to study the crystalline volume fraction X_c and crystalline size d in films. The electrical and optical properties were gained by Keithly 617 programmable electrometer and ultraviolet-visible (UV-vis) transmission spectra, respectively. It was found that: there are on the film surface many faulty grains, which formed spike-like clusters; increasing the flow ratio η, crystalline volume fraction X_c decreased from 40.4% to 32.0% and crystalline size d decreased from 4.7 to 2.7 nm; the optical band gap E_g^opt increased from 2.16 to 2.4 eV. The electrical properties of p-type nc-Si:H films are affected by annealing treatment and the reaction pressure.     

Synthesis and Optical Studies of Superconducting MgB<Subscript>2</Subscript> Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealedex situ and rendered superconducting withT _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼ 10 min at 550°C. The optical absorption coefficient,α and photon energy,E followed a Tauc-type behavior, (αE)^1/2=β _T(E −E _g). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameterβ _Tbehaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Deposition of luminescent a-SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:H Films with SiH<Subscript>4</Subscript>–N<Subscript>2</Subscript> gas mixture by VHF–PECVD using novel impedance matching method

Hydrogenated amorphous silicon nitride (a-SiN _x :H) films are produced from a SiH₄–N₂ gas mixture by plasma enhanced chemical vapor deposition (PECVD) system with a newly developed impedance matching method at frequencies 13.6–150 MHz. An increase in the rf power from 35 to 350 mW/cm² at the highest frequency of 150 MHz increases the optical bandgap (E _opt) from 2.0 to 4.5 eV. Optical emission spectroscopy (OES) of the SiH₄–N₂ plasma shows that the emission intensity of SiH* (414 nm) is almost proportional to deposition rate. Films of a-SiN _x :H deposited at 150 MHz and 210 mW/cm² has an optical bandgap of E _opt ≈ 4.1 eV and emits visible photoluminescence (PL) at room temperature (RT).     

Optical properties of silver containing As–S–Se thin films

The As₃₃S_{67- y} Se _y , where y = 0, 16.75, 33.5, 50.25 and 67, amorphous thin films were prepared by vacuum thermal evaporation technique. Range of the silver content dissoluted in films was x = 0 – 25 at. %. The refractive index increase with increasing silver and selenium content. The difference of the refractive index (Δn) between undoped and silver doped films was ～ 0.4 and between As₃₃S₆₇ and As₃₃Se₆₇ was films ～ 0.42. The values of non-linear refractive index grow with increasing silver and selenium content. The difference of optical bandgap, ΔE _g ^opt , between undoped As₃₃S₆₇ and fully dopped films with Ag and Se was ～ 1 eV.     

Phase diagram study of the formation and crystallization of Ge-metal amorphous alloy films

The Ge-Au and Ge-Ag alloy films were deposited in vacuum at room temperature and then systematically observed in the TEM. The maximum metallic concentrations in the alloy films,C _max, which form the stable amorphous alloy phases of germanium with gold and silver, were obtained. The annealed crystallization temperatureT _c, which falls with increasing metallic content in these films was also found. The structures of these films and their annealed specimens were also studied. There are various factors which influence the formation of amorphous alloy films deposited in vacuum for Ge-metal systems. A new formula forC _max has been derived. The annealed crystallization character has been explained by means of the variation of the free energy and the activation energy of crystallization. The activation energy of crystallization,E _a, can be obtained from the data values ofT _c. For Ge-Au films,E _a (Au)=E _a ^o /(–18.66C _Au ² +16.83C _Au+1)±3.3 (kcal mol^–1); for Ge-Ag films,E _a (Ag)=E _a ^o /(–2.754C _Ag ² +3.815C _Ag+ 1)±2.6 (kcal mol^–1). In order to explain all these results, two kinds of phase diagram for the alloy films have been introduced. One is the three-dimensional relationship diagrams of phase formation in semiconductor-metallic alloy films; it was introduced to explain the influencing factors. The other is the three-dimensional phase diagrams of annealed semiconductor-metallic films systems. From this diagram all the phase transitions can be found.     

Effect of annealing on the superconducting properties of two amorphous alloys: Nb70Zr15Si15 and Zr85Si15

The changes in the superconducting and electronic properties of amorphous Nb₇₀Zr₁₅Si₁₅ and Zr₈₅Si₁₅ alloys with annealing were examined with an aim to evaluate the effect of structural relaxation on the superconductivity of metal-metalloid type amorphous alloys.T _c rises once from 3.99 to 4.42 K on annealing at temperatures below about 473 K for the Nb-Zr-Si alloy and from 2.71 to 2.75 K at temperatures below about 373 K for the Zr-Si alloy, and with further rising annealing temperature,t _d, lowers monotonically to a final relaxed value (3.15 K for Nb₇₀Zr₁₅Si₁₅ and 2.49 K for Zr₈₅Si₁₅), which is independent of the previous thermal cycling. These results indicate that the thermal relaxation of an amorphous phase occurs through at least two stages. The lowering ofT _c occurs exponentially witht _d, and an activation energy for the relaxation process and the frequency of jump over the barrier were estimated to be about 2.03 eV and 2.4×10¹⁴ sec^–1 for Nb₇₀Zr₁₅Si₁₅ and about 1.28 eV and 1.2×10¹¹ sec^–1 for Zr₈₅Si₁₅, respectively. The high frequencies indicate that the relaxations occur more or less independently of each other in a non-co-operative manner. The dressed density of electronic states at the Fermi level,N(E _f) (1+), which was calculated from the measured values of _n and (dH _c2dT)T_c, exhibited a similar annealing temperature dependence to that ofT _c. From this the change inT _c on thermal relaxation was interpreted as due to the changes in and/orN(E _f). From the depressions ofJ _c(H) and fluxoid pinning force on annealing in a temperature range of 473 to 873 K, it was concluded that the structural relaxation from a less homogeneous quenched-in state to a homogeneous stable state occurred on the scale of coherence length (7.5 nm) during the annealing.     

Crystallization kinetics in (AgSbTe)x(In1− ySby)1− x films used in optical data storage

In phase change recording, higher linear densities can be achieved with materials in which crystallization is dominated by growth. AgInSbTe alloy based thin films appear to be the latest promising materials for optical data storage that has drawn worldwide attention. In these films, marks can be written with sharper edges and lower jitter. Films of (AgSbTe)_x (In_1−ySb_y)_1−x material with different compositions (x = 0.2, 0.3, 0.4 keeping y = 0.7) were deposited using thermal evaporation technique under a high vacuum of 10⁻⁶ torr. The results of Differential Scanning Calorimetry (DSC) under non-isothermal conditions with different heating rates (5, 10, 15, 20° K/min) are reported and discussed here. The glass transition temperature T_g and the onset crystallization temperature T_c were found to be dependent on the composition as well as on the heating rate. The activation energy for glass transition E_g and the activation energy for crystallization E_c are calculated using Kissinger’s equation and their compositional dependence is discussed. The glass forming ability lies in the range 0.4–0.6.The present investigations indicate that the above-mentioned quaternary material with a typical composition (x = 0.2) is good for phase change optical memory.     

Fabrication and Transport Properties of MgB2 Thin Films and Tunnel Junctions

We report on fabrication and characterization of MgB₂ thin films and tunnel junction structures. The MgB₂ films were prepared on Al₂O₃, Si, glass, and plastic foil substrates by either vacuum codeposition of boron and magnesium, or high-temperature magnesium annealing of boron films. The crystalline structure of our films depended directly on the method of preparation. The films prepared by codeposition and postannealed in Ar atmosphere were amorphous with nanocrystal inclusions and were characterized by very smooth surfaces. On the other hand, the boron-precursor films annealed in magnesium vapor were rough, polycrystalline with approximately 1-m-diameter single-crystal blocks. Because of their surface quality, the amorphous films were used for preparation of point contact junctions and for optical characterization. The point-contact spectra of tested junctions exhibited a two-gap structure. The MgB₂ polycrystalline films was used for bulk transport studies. The best films were characterized by the critical temperature T _c of up to 39 K and the current density j _c at 4.2 K of about 10⁷ A/cm².     

Material science aspects of phase change optical recording

Chalcogenide thin films are used as the recording medium for phase change-type optical memory discs. The films are switched between amorphous and crystalline states using the heat of a focussed laser beam. Large reflectivity differences between amorphous and crystalline states are then used to store and retrieve the information. An active chalcogenide layer for this purpose should have a high optical absorption coefficient (α), and good structural and thermal stability. It should be possible to switch the chalcogenide layer between amorphous and crystalline states repeatedly within a short duration, the optical contrast should be high, and the material must have large cycling capability. Keeping the above requirements in mind, we have carried out systematic investigation of structural, optical and crystallization behaviour of thin films of various compositions of GaGeTe, Sb₂Te₃ and BiSe. These studies have shown that these materials can be good candidates for use as recording media in erasable phase-change optical recording.