Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Feasibility of Er-doped, Ga5Ge20Sb10S65 chalcogenide microstructured optical fiber amplifiers

The feasibility of a microstructured optical fiber (MOF) amplifier, made of a novel Er³⁺-doped chalcogenide glass, has been demonstrated via accurate simulations performed by employing an oppositely implemented computer code. The optical and geometrical parameters measured on the first MOF sample together with other physical constants from literature have been taken into account in the simulations. The calculated optical gain of the optimized MOF amplifier, 2.79 m long, is close to 23 dB at the signal wavelength of 1.538 μm, by using a pump power of 200 mW and a signal power of 0.1 μW.     

Ge20Sb15Se65薄膜的热致光学特性变化研究

采用磁控溅射法制备了Ge₂₀Sb₁₅Se₆₅薄膜, 研究热处理温度(150—400 ℃)对薄膜光学特性的影响. 通过分光光度计、X射线衍射仪、显微拉曼光谱仪对热处理前后薄膜样品 的光学特性和微观结构进行了表征, 并根据Swanepoel方法以及Tauc公式分别计算了薄膜折射率色散曲线和光学带隙等参数. 结果表明当退火温度(T_a)小于薄膜的玻璃转化温度(T_g)时,薄膜的光学带隙(E_g^opt)随着退火温度的增加由1.845 eV上升至1.932 eV, 而折射率由2.61降至2.54; 当退火温度大于薄膜的玻璃转化温度时,薄膜的光学带隙随退火温度的增加由1.932 eV降至1.822 eV, 折射率则由2.54增至2.71. 最后利用Mott和Davis提出的非晶材料由非晶到晶态的结构转变模型对结果进行了解释, 并通过薄膜XRD和Raman光谱进一步验证了结构变化是薄膜热致变化的重要原因. 关键词： 20Sb₁₅Se₆₅薄膜')" href="#">Ge₂₀Sb₁₅Se₆₅薄膜 热处理 光学带隙 折射率     

Studying the crystallization behavior of the Se85S10Sb5 chalcogenide semiconducting glass by DSC and X-ray diffraction

Crystallization kinetics of the Se₈₅S₁₀Sb₅ chalcogenide glassy alloy is studied by differential scanning calorimeter (DSC) non-isothermally. The glassy state of the as-prepared sample and the crystalline phases of the heat treated sample are characterized using X-ray diffraction. The glass transition activation energy E_g is found to be 65.2±0.8 kJ/mol and the crystallization activation energies for the first and the second crystallization peaks (E_c1 and E_c2) are found to be 70±0.8 and 85.2±0.8 kJ/mol, respectively. The determined kinetic parameters have made it possible to postulate the type of crystal growth exhibited in the crystallization process. The phases at which the alloy crystallizes after the thermal process have been identified by X-ray diffraction. The diffractogram of the transformed material indicates the presence of nanocrystallites of Sb₂Se₃, Se-S and Se, with a remaining additional amorphous matrix.     

Ultrafast non-linear optical properties of Ge20As25Se55 chalcogenide films

The non-resonant third-order non-linear optical properties of amorphous Ge₂₀As₂₅Se₅₅ films were studied experimentally by the method of the femtosecond optical heterodyne detection of optical Kerr effect. The real and imaginary parts of complex third-order optical non-linearity could be effectively separated and their values and signs could be also determined, which were 6.6 × 10⁻¹² and −2.4 × 10⁻¹² esu, respectively. Amorphous Ge₂₀As₂₅Se₅₅ films showed a very fast response in the range of 200 fs under ultrafast excitation. The ultrafast response and large third-order non-linearity are attributed to the ultrafast distortion of the electron orbitals surrounding the average positions of the nucleus of Ge, As and Se atoms. The high third-order susceptibility and a fast response time of amorphous Ge₂₀As₂₅Se₅₅ films makes it a promising material for application in advanced techniques especially in optical switching.     

Crystallization of amorphous Ge2Sb2Te5 films induced by a single femtosecond laser pulse

Crystallization is achieved in amorphous Ge₂Sb₂Te₅ films upon irradiation with a single femtosecond laser pulse. Transmission electron microscopy images evidence the morphology of the crystallized spot which depends on the fluence of the femtosecond laser pulse. Fine crystalline grains are induced at low fluence, and the coarse crystalline grains are obtained at high fluence. At the damage fluence, ablation of the films occurs.     

Thermally quenched and generated paramagnetism in Ge42S58 glass

Two kinds of paramagnetic defects whose density increases and decreases with temperature have been observed in Ge₄₂S₅₈ glass using ESR technique. The increasing and decreasing defects are interpreted as resulting from Ge dangling bonds with negative and positive effective electron correlation energies, respectively. Unlike usual amorphous semiconductors, Ge₄₂S₅₈ glass contains defects with the effective electron correlation energy whose value ranges from positive to negative depending on environment around the defect. The densities of the defects with the positive and negative values are about 4 × 10¹⁵ and 3 × 10¹⁷ cm^?3 respectively.     

Morphological characteristics of amorphous Ge2Sb2Te5 films after a single femtosecond laser pulse irradiation

The morphology of materials resulting from laser irradiation of the single-layer and the multilayer amorphous Ge₂Sb₂Te₅ films using 120 fs pulses at 800 nm was observed using scanning electron microscopy and atomic force microscopy. For the single-layer film, the center of the irradiated spot is depression and the border is protrusion, however, for the multilayer film, the center morphology changes from a depression to a protrusion as the increase of the energy. The crystallization threshold fluence of the single-layer and the multilayer film is 22 and 23 mJ/cm², respectively.     

Femtosecond optical Kerr effect study of Ge10As40S30Se20 film

The results of the femtosecond optical heterodyne detection of optical Kerr effect at 805 nm with the 80 fs ultrafast pulses in amorphous Ge₁₀As₄₀S₃₀Se₂₀ film is reported in this paper. The film shows an optical non-linear response of 200 fs under ultrafast 80 fs-pulse excitation, and the values of real and imaginary parts of non-linear susceptibility χ⁽³⁾ were 9.0×10⁻¹² and −4.0×10⁻¹² esu, respectively. The large third-order non-linearity and ultrafast response are attributed to the ultrafast distortion of the electron orbits surrounding the average positions of the nucleus of Ge, As, S and Se atoms. This Ge₁₀As₄₀S₃₀Se₂₀ chalcogenide glass would be expected as a promising material for optical switching technique.     

Study of the gain saturation effect on the propagation of dark soliton in Er-doped, Ga5Ge20Sb10S65 chalcogenide fiber amplifier

We study the effect of gain saturation on the propagation of fundamental dark soliton in a nonlinear, dispersive and amplifying medium. The Er^+ 3-doped, Ga₅Ge₂₀Sb₁₀S₆₅ chalcogenide glass is used for dark and erbium doped silicon glass for bright solitons. The numerical simulations show that dark soliton doesn't split to subpulses unlike bright soliton and also the dark soliton is more stable in the presence of gain saturation and gain dispersion effects. So the chalcogenide glasses are suitable for designing all optical devices.     

Crystallization of Ge2Sb2Te5 phase—change optical disk media

In this paper, the crystallization behaviour of amorphous Ge₂Sb₂Te₅ thin films is investigated using differential scanning calorimetry), x-ray diffraction and optical transmissivity measurements. It is indicated that only the amorphous phase to face-centred-cubic phase transformation occurs during laser annealing of the normal phase-change structure, which is a benefit for raising the phase-change optical disk's carrier-to-noise ratio (CNR). For amorphous Ge₂Sb₂Te₅ thin films, the crystallization temperature is about 200℃ and the melting temperature is 546.87℃. The activation energy for the crystallization, E_a, is 2.25eV. The crystallization dynamics for Ge₂Sb₂Te₅ thin films obeys the law of nucleation and growth reaction. The sputtered Ge₂Sb₂Te₅ films were initialized by an initializer unit. The initialization conditions have a great effect on the reflectivity contrast of the Ge₂Sb₂Te₅ phase-change optical disk.     

Short-to-medium-range order in Mg65Cu25Y10 metallic glass

The atomic configurations of liquid and glassy Mg₆₅Cu₂₅Y₁₀ alloy have been simulated in the temperature range of 300 K to 2000 K via ab initio molecular dynamics. The variations of pair correlation function (PCF), structure factor (SF), coordination number (CN) and bond pairs with the temperature for this alloy are characterized. It has been shown that the atoms are near densely packed and icosahedral type of short-range order (SRO) is predominant in the glass state. Icosahedral medium range order (MRO) can be formed by vertex or intercross connection of icosahedral SROs. In this work, an icosahedral MRO which is composed of 55 atoms has been found. It has been also clarified that Mg and Cu occupy the centre or vertex, and Y atoms only occupy the vertex of the icosahedron in this glassy alloy. It is believed that these findings have implication for understanding the glass forming mechanism of magnesium based metallic glasses.     

Formation of Ge35In8S57 amorphous films for optical applications

Thin films of In-doped Ge-S in the form of Ge₃₅In₈S₅₇ with different film thickness were deposited using an evaporation method. The X-ray diffraction studies demonstrate that the as-prepared films are amorphous in nature for these films. Some optical constants were calculated at a thickness of 150, 300, 450 and 900?nm and annealing temperature of 373, 413, 437 and 513?K. Our optical observations show that the mechanism of the optical transition obeys the indirect transition. It was found that the energy gap, E_g, decreases from 2.44 to 2.20?eV with expanding the thickness of the film from 150 to 900?nm. On the other hand, it was found that E_g increases with annealing temperature from 373 to 513?K. The increment in the band gap can be attributed to the gradual annealing out of the unsaturated bonds delivering a decreasing the density of localized states in the band structure. Using the single oscillator model, the dispersion of the refractive index is described. The dispersion constants of these films were calculated with different both thickness and annealing temperatures. Additionally, both of nonlinear susceptibility, χ⁽³⁾ and nonlinear refractive index, n₂ were calculated.     

Ge20Sb15Se65硫系玻璃光子晶体光纤的中红外色散特性

硫系玻璃与石英玻璃相比具有高折射率(2.0~3.5)、低声子能量 (<350 cm^-1)、优良的中远红外透过性能(可至25 μm)等特性.本文制备了一种在中红外具有优良透过特性的无As环保型Ge₂₀Sb₁₅Se₆₅硫系玻璃材料,以此为基质材料设计了一种三层空气孔结构光子晶体光纤,利用多极法对光纤的中红外色散特性进行了数值模拟,系统研究了结构参量孔径d、孔间距Λ 以及d/Λ 对其色散特性的影响.分析表明:通过改变包层空气孔直径d或空气孔间距Λ,可灵活的调节光子晶体光纤的零色散波长向短波或长波方向移动.通过优化结构参量发现,当Λ=3 μm,d/Λ=0.35 附近变化时,可获得3~5 μm色散平坦,且色散值小于5 ps·nm^-1·km^-1的光子晶体光纤.     

Study on the structure dependent ultrafast third-order optical nonlinearity of GeS2–In2S3 chalcogenide glasses

The results of the femtosecond time-resolved optical Kerr at 820 nm in GeS₂–In₂S₃ chalcogenide glasses indicate that the response time in GeS₂–In₂S₃ glasses is subpicosecond, which is predominantly due to the distortion of the electron cloud. The value of χ⁽³⁾ in 0.95GeS₂–0.05In₂S₃ glass is also as large as 2.7 × 10⁻¹³ esu, and it reduces with the addition of In₂S₃, which may be ascribed to the microstructure evolution of GeS₂–In₂S₃ glasses. It is deduced that the intrinsic [Ge(In)S₄] tetrahedral structure units that possess the high hyperpolarizability may do great contribution to the enhancement of third-order optical nonlinearity while [S₃Ge–GeS₃] ethane-like molecular units make no considerable contribution to that in femtosecond time scale. These GeS₂–In₂S₃ and GeS₂–In₂S₃-based chalcogenide glasses would be expected to be the promising materials for all-optical switching devices.     

Second-order optical nonlinearity in Sb2S3 microcrystal doped glasses by electron beam irradiation

With Maker fringe measurements, the prominent second harmonic generation was observed in the sol-gel-derived Sb₂S₃ doped silica glasses irradiated by electron beam, which was related to the space-charge electrostatic field that makes the glasses poled and broken the centrosymmetry of the glasses. By sides, the interface of nanoparticles contributed to the second harmonic intensity as well. The second harmonic intensity increased with the increasing of the irradiating current, accelerating voltage and the concentration of the dopant due to the enhanced built-in electrostatic field. The poled region was located in the surface of the sample about several microns by the TSDC measurements. The second harmonic intensity is almost 10 times larger than that of the base glass due to the existence of microcrystal.     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Nanoscale observations of the operational failure for phase-change-type nonvolatile memory devices using Ge2Sb2Te5 chalcogenide thin films

In this study, a phase-change memory device was fabricated and the origin of device failure mode was examined using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). Ge₂Sb₂Te₅ (GST) was used as the active phase-change material in the memory device and the active pore size was designed to be 0.5 m. After the programming signals of more than 2×10⁶ cycles were repeatedly applied to the device, the high-resistance memory state (reset) could not be rewritten and the cell resistance was fixed at the low-resistance state (set). Based on TEM and EDS studies, Sb excess and Ge deficiency in the device operating region had a strong effect on device reliability, especially under endurance-demanding conditions. An abnormal segregation and oxidation of Ge also was observed in the region between the device operating and inactive peripheral regions. To guarantee an data endurability of more than 1×10¹⁰ cycles of PRAM, it is very important to develop phase-change materials with more stable compositions and to reduce the current required for programming.     

Second-order optical nonlinearity in bulk PbO/B2O3 glass

By investigating the second-harmonic generation (SHG) of the bulk PbO/B₂O₃ glass samples with different compositions after thermal poling, it was found that there was an optimal poling temperature for each sample with different compositions and there was also a relation between optimal poling temperature and glass transition temperature. At their own optimal poling temperatures, the samples had different frequency doubling efficiencies with the same applied voltage. We also found that the frequency doubling efficiency of PbO/B₂O₃ glass increased with the increase of poling voltage. An induced dipole model was proposed to explain the super-quadratic relation between the SHG intensity and the poling voltage.     

ZnSb掺杂的Ge2Sb2Te5薄膜的相变性能研究

本文采用双靶(ZnSb靶和Ge₂Sb₂Te₅靶)共溅射制备了系列ZnSb掺杂的Ge₂Sb₂Te₅(GST)薄膜. 利用X射线衍射、透射电子显微镜、原位等温/变温电阻测量、X射线光电子能谱等测试研究了薄膜样品的非晶形态、电学及原子成键特性. 利用等温原位电阻测试表明ZnSb掺杂的Ge₂Sb₂Te₅薄膜具有更高的结晶温度. 采用Arrhenius 公式计算发现ZnSb掺杂的Ge₂Sb₂Te₅薄膜的十年数据保持温度均高于传统的Ge₂Sb₂Te₅薄膜的88.9℃. 薄膜在200, 250, 300和350℃ 下退火后的X射线衍射图谱表明ZnSb的掺杂抑制了Ge₂Sb₂Te₅薄膜从fcc态到hex态的转变. 通过对薄膜的光电子能谱和透射电镜分析可知Zn, Sb, Te原子之间键进行重组, 形成Zn–Sb 和Zn–Te 键, 且构成非晶物质存在于晶体周围. 采用相变静态检测仪测试样品的相变行为发现ZnSb掺杂的Ge₂Sb₂Te₅薄膜具有更快的结晶速度. 特别是(ZnSb)_24.3(Ge₂Sb₂Te₅)_75.7薄膜, 其结晶温度达到250℃, 十年数据保持温度达到130.1℃, 并且在70 mW激光脉冲功率下晶化时间仅～64 ns, 远快于传统Ge₂Sb₂Te₅薄膜的晶化时间～280 ns. 以上结果表明(ZnSb)_24.3(Ge₂Sb₂Te₅)_75.7薄膜是一种热稳定性好且结晶速度快的相变存储材料.