Laser-induced amorphization and crystallization on Se80Te20−xPbx thin films

Thin films of glassy alloys of a-Se₈₀Te_20−xPb_x (x=2, 6 and 10) was crystallized in a specially designed sample holder under a vacuum of 10⁻² Pa. The amorphous and crystallized films were induced by pulse laser (wavelength: 337.1 nm, frequency: 10 Hz, pulse duration: 4 ns and pulse energy: 0.963 mJ). After laser irradiation on amorphous and crystalline films: optical band gaps were measured. Crystallization and amorphization of chalcogenide films is accompanied by the change in the optical band gap. The change in optical energy gap could be determined by identification of the transformed phase. This change in the optical band gap may be due to the increase in the grain size and the reduction in the disorder of the system.     

Crystallization and electrical characteristics of Ge1Cu2Te3 films for phase change random access memory

Phase change random access memory (PCRAM) requires an advanced phase change material to lower its power consumption and to enhance its data retention and endurance abilities. The present work investigated the crystallization behaviors and electrical properties of Ge₁Cu₂Te₃ compound films with a low melting point of about 500 °C for PCRAM application. Sputter-deposited Ge₁Cu₂Te₃ amorphous films showed a high crystallization temperature of about 250 °C. The Ge₁Cu₂Te₃ amorphous film showed an electrical resistance decrease of over 10²-fold and exhibited a small increase in thickness of 2.0% upon crystallization. The Ge₁Cu₂Te₃ memory devices showed reversible switching behaviors and exhibited a 10% lower power consumption for the reset operation than the conventional Ge₂Sb₂Te₅ memory devices. Therefore, the Ge₁Cu₂Te₃ compound is a promising phase change material for PCRAM application.     

Crystallization behavior and thermoelectric characteristics of the electrodeposited Sb2Te3 thin films

Crystallization behavior of the electrodeposited Sb₂Te₃ film was characterized and the effect of the amorphous-crystalline transition on the Seebeck coefficient was evaluated. The as-electrodeposited Sb₂Te₃ film was amorphous and exhibited the Seebeck coefficient of 268-322 μV/K, which was much larger than the value of the crystalline Sb₂Te₃ film. When annealed at temperatures above 100 °C, the Seebeck coefficient of the Sb₂Te₃ film dropped significantly to 78-107 μV/K due to the amorphous-crystalline transition at 94 °C. The thermal stability of the electrodeposited Sb₂Te₃ film was improved by the addition of Cu, and the crystallization temperature of the CuSbTe film increased up to 149.5 °C.     

Resistivity studies on Ga2Te3 and In2Te3 under high pressures

The resistivities of III–VI semiconductors Ga₂Te₃ and In₂Te₃ were found to decrease abruptly under hydrostatic pressures between 1 and 7 GPa. The transition was reversible for Ga₂Te₃ but irreversible in the case of In₂Te₃ due to decomposition. These results are compared with the behaviour of II–VI and III–V semiconductors and transition pressures correlated with lattice constants.     

Phase transition characteristics of Bi/Sn doped Ge2Sb2Te5thin film for PRAM application

The Bi and Sn were doped to Ge₂Sb₂Te₅ (GST) to investigate and modify the phase transition characteristics. The Bi/Sn doped GST thin film was prepared by RF magnetron co-sputtering and its crystal structure, sheet resistance, and phase transition kinetics were analyzed. By the doping of Bi/Sn, the crystallization temperature or stable phase was changed slightly compared with GST. For the PRAM application, the optimum doping concentration was Bi 5.9 and Sn 17.7 at.%, and its minimum time for crystallization was shorten more than 30% compared with GST. The sheet resistance difference between amorphous and crystalline state was higher than 10⁴ Ω/□.     

Characterization of phase transition in silver photo-diffused Ge2Sb2Te5 thin films

Surface activity of thermally evaporated amorphous chalcogenide films of Ge₂Sb₂Te₅ has been investigated. Silver (Ag) is readily deposited on such films from appropriate aqueous ionic solution and Ag diffuses into the films upon irradiation with energetic photons. The composition of Ge₂Sb₂Te₅ thin films and the amount of Ag photo-diffused has been gathered from electron probe micro-analyzer having a wavelength dispersive spectrometer. The composition of the films was found to be very close to the bulk used to deposit films and the amount of Ag photo-diffused was ∼ 0.38 at. %. X-ray diffraction and temperature dependent sheet resistance studies have been used for the structural analysis of the bulk alloy, as-deposited, Ag photo-diffused and annealed films at different temperatures. The films remain amorphous after Ag photo-diffusion into the amorphous Ge₂Sb₂Te₅ films. The reflectivity, reflectivity contrast and extinction coefficient of the crystalline and amorphous photo-diffused thin films are presented. The optical band gaps of the amorphous and crystalline photo-diffused (Ge₂Sb₂Te₅)_100−xAg_x=0.38 phase change thin films have also been calculated from absorption data using UV-VIS spectroscopy.     

Crystallization behavior and resistance change in eutectic Si15Te85 amorphous films

Crystallization process and the corresponding electrical resistance change were investigated in eutectic Si₁₅Te₈₅ amorphous thin films. The Si₁₅Te₈₅ amorphous film showed two-stage crystallization process upon heating. In the first stage, the Si₁₅Te₈₅ amorphous crystallized into Te crystals at 175 °C. In the second stage, the residual amorphous phase crystallized into Si₂Te₃ crystals at above 300 °C accompanying the resistance drop. Before the second crystallization, the electrical resistance once increased in the temperature range of about 250-295 °C. This phenomenon can be explained by considering the formation of amorphous phase with a high electrical resistivity.     

Laser cladding of Zr65Al7.5Ni10Cu17.5 amorphous alloy on magnesium

Laser cladding of amorphous alloy Zr₆₅Al_7.5Ni₁₀Cu_17.5 on magnesium substrate was conducted using the blown powder method. The thickness of the coating was about 1.5 mm. The resulting microstructure, wear resistance and corrosion resistance of the coating were studied. The results of the XRD and TEM analyses showed that up to a depth of 1.1 mm, the coating had an amorphous structure, and no apparent crystalline structures were found. The coated specimen exhibited wear and corrosion resistance superior to that of the uncoated specimen: the wear loss was significantly reduced, some thirteen-fold; and the corrosion current was lowered by three orders of magnitude.     

Electronic and atomic structure of Ge2Sb2Te5 phase change memory material

Electronic structure calculations are presented for various model structures of the crystalline and amorphous phases of Ge₂Sb₂Te₅. The structures are all found to possess a band gap of order 0.5 eV, indicating closed shell behaviour. It is pointed out that structural vacancies in A7-like Ge₂Sb₂Te₅ are not electronically active. In addition, A7-like structures do not support valence alternation pair defects, which are one model of the conduction processes in the amorphous phase in non-volatile memories.     

Structure and microstructure of In4Te3 nanopowders prepared by solid state reaction

In this paper, we investigated the structure and microstructure of In₄Te₃ nanopowders obtained by mechanically alloying an In₇₅Te₂₅ powder mixture. Structural, chemical, thermal and vibrational studies of the In₇₅Te₂₅ powder mixture were carried out using X-ray diffraction, energy dispersive spectroscopy, transmission electron microscopy, differential scanning calorimetry and Raman spectroscopy. The orthorhombic In₄Te₃ phase (In₃Se₄-type) was nucleated in 2 h of synthesis, although non-reacted tetragonal indium (In) was still present at that time. Small amounts of cubic In₂O₃ phase were observed after 31 h of synthesis. Rietveld analyses allowed the measurement of mean crystallites sizes and phase fraction variations when milling times were increased. These analyses showed that, after 31 h of synthesis, about 65 wt% of In₄Te₃ phase contained mean crystallite sizes smaller than 27 nm and microstrains greater than 1.5%. The crystallite and interfacial components sizes were determined by high resolution transmission electron microscopy. Differential scanning calorimetry measurements showed the influence of nanometric crystallite sizes on the melting of the In₄Te₃ and non-reacted In phases. Raman measurements showed that the trigonal Te and α-TeO₂ modes, observed for the precursor Te powder, are absent for the sample milled for 31 h. The structural stability of the nanocrystalline phases of the In₇₅Te₂₅ sample milled for 31 h was attested by X-ray diffraction measurements performed twelve months after its production.     

Structural study of amorphous In2O3 film by grazing incidence X-ray scattering (GIXS) with synchrotron radiation

Grazing incidence X-ray scattering (GIXS) using synchrotron radiation is a very useful method for structural analysis of amorphous films. We investigated the structure of amorphous In₂O₃ film utilizing GIXS at BL19B2 in SPring-8. Radial distribution function (RDF) was obtained from the measurement data. Structural models were constructed by molecular dynamics (MD) and reverse Monte-Carlo (RMC) simulations, and the calculated RDFs from the simulations were compared with that observed. It was found that the average oxygen coordination number around In ions was almost 6 and the average length 2.12 Å, which was smaller by about 3% than that of 2.18 Å in crystalline In₂O₃. It was concluded that the atomic arrangement of the amorphous In₂O₃ was characterized by the increase in the number and the boarder angle of distribution of corner-sharing In-O-In bond compared with crystalline In₂O₃.     

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.     

Medium-term thermal stability of amorphous Ge2Sb2Te5 flash-evaporated thin films with regards to change in structure and optical properties

The stability of flash-evaporated amorphous Ge₂Sb₂Te₅ thin films has been studied under medium-term temperature treatment (30 - 80 °C, with a step of 10 °C) in ten subsequent heating and cooling cycles. The significant changes in structure and optical properties are reported. The temperature cycling of the films resulted in formation of an isolated 5 - 7 nm nano-crystalline phase in the amorphous phase. The corresponding increase in refractive index and change in optical bandgap energy and sheet resistance are also presented. The formation of Ge₂Sb₂Te₅ nano-crystals (~ 5 - 7 nm) even under temperature below 80 °C could contribute to the explanation of mechanism of resistivity fluctuation (drift) of the “amorphous phase” films. We also show that the optical and electrical properties of flash evaporated Ge₂Sb₂Te₅ thin films are very similar to those reported for sputtered films.     

An X-ray diffraction and electron spectroscopic study of metallic glass In20Te80

Results of X-ray diffraction and X-ray photoelectron spectroscopy investigations on the crystallization behaviour of an amorphous In₂₀Te₈₀ system and the effects of crystallization on the electronic core levels of In and Te atoms are presented. During controlled heat treatments three crystalline phases, Te, -In₂Te₃, and In₂Te₅-I, were observed in this system. In addition, a few splat-cooled samples were found to exhibit a new metastable crystalline phase. Photoelectron measurements revealed that the Te 3d and 4d core levels of amorphous In₂₀Te₈₀ were shifted downwards in energy from their characteristic values of pure Te metal. The In 3d and 4d levels experienced large energy shifts due to alloying, but remained unaffected by heating at temperatures below 520 K.     

Effects of annealing temperatures on optical and electrical properties of vacuum evaporated Ga15Se77In8 chalcogenide thin films

The optical constants (absorption coefficient, optical band gap, refractive index, extinction coefficient, real and imaginary parts of dielectric constants) of amorphous and thermally annealed thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses with thickness 4000 Å have been investigated from absorption and reflection spectra as a function of photon energy in the wave length region 400-800 nm. Thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses were thermally annealed for 2 h at three different annealing temperatures 333 K, 348 K and 363 K, which are in between the glass transition and crystallization temperature of Ga₁₅Se₇₇In₈ glasses. Analysis of the optical absorption data shows that the rule of non-direct transitions predominates. It was found that the optical band gap decreases with increasing annealing temperature. It has been observed that the value of absorption coefficient and extinction coefficient increases while the values of refractive index decrease with increasing annealing temperature. The decrease in optical band gap is explained on the basis of the change in nature of films, from amorphous to crystalline state. The dc conductivity of amorphous and thermally annealed thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses is also reported for the temperature range 298-393 K. It has been observed that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges. The dc conductivity was observed to increase with the corresponding decrease in activation energy on increasing annealing temperature in the present system. These results were analyzed in terms of the Davis-Mott model.     

Diode-end-pumped passively Q-switched 1.34 μm Nd:Gd0.5Y0.5VO4 laser with Co:LMA saturable absorber

In this paper, the output performances at 1.34 μm in continuous wave operation and passive Q-switching regime of a diode-end-pumped Nd:Gd_0.5Y_0.5VO₄ laser have been investigated. The passive Q-switching regime was achieved with Co²⁺:LaMgAl₁₁O₁₉ (Co²⁺:LMA) saturable absorbers crystals. A maximum average output power of 230 mW was recorded with a Co²⁺:LMA with initial transmission of 81%. The minimum pulse duration was 116 ns, which corresponded to a repetition rate of 360 kHz, the single pulse energy of 2.1 μJ and the pulse peak power of 5.5 W.     

Effects of oxygen or nitrogen doping on the microstructures, bonding, electrical, thermal properties and phase change kinetics of GeSb9 films

This study investigated the phase change kinetics, thermal, structural, electrical properties, and chemical bonding state of 50-nm-thick oxygen or nitrogen doped GeSb₉ films prepared by sputtering. Up to 6.8 at.% O or 8.7 at.% N doping, as measured by secondary ion mass spectrometry, the crystallization temperatures, lattice constants, and the resistivity of both crystalline and amorphous films increased with oxygen or nitrogen concentration , while the grain sizes decreased. Nitrogen atoms bonded to Ge and formed GeN₄ nitrides, and both the GeO₂ and Sb₂O₃ co-existed when oxygen concentration reached 8.5 at.%. Crystallization could be triggered by a laser pulse of duration 12 ns for all films. Both GeSb₉-O and GeSb₉-N films had fast crystallization speed, good thermal stability, and improved resistivity and are therefore potential candidates as active layers for phase-change memory.     

Electron beam evaporation deposition and properties of Abrupt GST/Si heterojunction structure

The ternary alloy, Ge₂Te₂Sb₅ is one of the most important compounds of the GeTe-Sb₂Te₃ pseudobinary systems. Ge₂Te₂Sb₅ thin films of thickness of 100 nm-300 nm were deposited by electron beam evaporation. After annealing at different temperatures, we did X-ray diffraction measurement to characterize the structure transformation of the material. In-situ resistance measurement depending on the temperature shows that there is about three orders of magnitude change between the high resistance state (amorphous state) and the low resistance state (face-centered cubic state). To construct a heterojunction diode, we deposited Ge₂Te₂Sb₅ thin films on n-type silicon wafers. Rectification effects were observed in voltage-current measurements of the abrupt heterojunctions. Traditional voltage-current relationship of p-n junctions and metal-semiconductor junctions are used to explain the characteristics of Ge₂Te₂Sb₅/n-Si heterojunctions.     

Electronic structural analysis of transparent In2O3-ZnO films by hard X-ray photoelectron spectroscopy

The electronic structural analysis of the conductive transparent films was carried out using bulk sensitive hard X-ray photoelectron spectroscopy (HAXPES). The In₂O₃-ZnO film has amorphous structure before and after annealed, and the conduction band spectrum around Fermi level showed the similar spectra with that of as-deposited amorphous In₂O₃ film. In these amorphous films, the conduction band minimum locates at the deeper level than the crystalline In₂O₃ film. The electronic state which comes from randomness of amorphous structure possibly exists around this level or below. These electrons are expected to act as scattering center. We concluded that the electron mobility depends on the density of this electronic state.     

Nucleation and (Zn1 ? xCdxTe)1 ? y(In2Te3)y/ZnSe heteroboundary formation in elasticity field under strongly nonequilibrium conditions

Results of electron-microscopic, electron-diffraction, and X-ray-phase studies of the heteroboundary between the layers of zinc selenide and solid solution (Zn_{1 −x} Cd _x Te)_{1 − y} (In₂Te₃) _y obtained in synthesis from the vapor phase in an elasticity field under strongly nonequilibrium conditions (upon condensation on the cooled substrate) are presented. Mechanisms of nucleation (via spinodal decay) and ordering (due to propagation of elastic waves) are determined. It is demonstrated that high wave propagation velocity and the absence of diffusion provides fast orientation of nuclei of the new phase and high perfection of the crystalline layer of the solid solution. Original Russian Text ? A.P. Belyaev, V.P. Rubets, V.V. Antipov, Kh.A. Toshkhodzhaev, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 4, pp. 404–407.