Temperature dependent optical constants of amorphous Ge2Sb2Te5 thin films

This study focuses on the temperature dependent optical band gap changes in the amorphous Ge2Sb2Te5 (GST) films. The behavior of the amorphous GST thin films at low temperatures has been studied. The band gap increment of around 0.2 eV is observed at low temperature (4.2 K) compared to room temperature (300 K). The band gap changes associated with the temperature are completely reversible. The other optical parameters like Urbach energy and Tauc parameter (B-1/2) are studied for different temperatures and discussed. The observed changes in optical band gap (E-g) are fitting to Fan's one phonon approximation. Phonon energy ((h) over bar omega) corresponding to a frequency of 3.59 THz is derived from Fan's approximation, which is close to the reported value of 3.66 THz. (C) 2010 Elsevier B.V. All rights reserved.

Signatures of Ge2Sb2Te5 film at structural transitions

Ge2Sb2Te5 (GST) films, one of the most suitable Chalcogenide alloys for Phase change Random Access Memory applications are studied for changes in sheet resistance, optical transmission, morphology and surface science by annealing at various transition temperatures. The crystallization leads to an increase of grain size and roughness in the films and the resistance changes to three orders of magnitude. Optical studies on GST films show distinct changes during phase transitions and the optical parameters are calculated. An increase of Tauc parameters B-1/2 indicates a reduction in disorder during phase transition. It is confirmed from XPS studies that Ge-Te, Sb-Te bonds are present in both amorphous and crystalline phases whereas Sb-Ge, Te-Te, Sb-Sb bonds are absent. (C) 2012 Elsevier B.V. All rights reserved.

Direct hexagonal transition of amorphous (Ge2Sb2Te5)(0.9)Se-0.1 thin films

Ge2Sb2Te5 (GST) is well known for its phase change properties and applications in memory and data storage. Efforts are being made to improve its thermal stability and transition between amorphous and crystalline phases. Various elements are doped to GST to improve these properties. In this work, Se has been doped to GST to study its effect on phase change properties. Amorphous GST film crystallized in to rock salt (NaCl) type structure at 150 degrees C and then transformed to hexagonal structure at 250 degrees C. Interestingly, Se doped GST ((GST)(0.9)Se-0.1) film crystallized directly into hexagonal phase and the intermediate phase of NaCl is not observed. The crystallization temperature (T-c) of (GST)(0.9)Se-0.1 is around 200 degrees C, which is 50 degrees C higher than the T-c of GST. For (GST)(0.9)Se-0.1, the threshold switching occurs at about 4.5V which is higher than GST (3 V). Band gap (E-opt) values of as deposited films are calculated from Tauc plot which are 0.63 eV for GST and 0.66 eV for (GST)(0.9)Se-0.1. The E-opt decreases for the films annealed at higher temperatures. The increased T-c, E-opt, the contrast in resistance and the direct transition to hexagonal phase may improve the data readability and thermal stability in the Se doped GST film. (C) 2014 AIP Publishing LLC.

Structural transition and enhanced phase transition properties of Se doped Ge2Sb2Te5 alloys

Amorphous Ge2Sb2Te5 (GST) alloy, upon heating crystallize to a metastable NaCl structure around 150 degrees C and then to a stable hexagonal structure at high temperatures (>= 250 degrees C). It has been generally understood that the phase change takes place between amorphous and the metastable NaCl structure and not between the amorphous and the stable hexagonal phase. In the present work, it is observed that the thermally evaporated (GST)(1-x)Se-x thin films (0 <= x <= 0.50) crystallize directly to the stable hexagonal structure for x >= 0.10, when annealed at temperatures >= 150 degrees C. The intermediate NaCl structure has been observed only for x, 0.10. Chemically ordered network of GST is largely modified for x >= 0.10. Resistance, thermal stability and threshold voltage of the films are found to increase with the increase of Se. The contrast in electrical resistivity between the amorphous and crystalline phases is about 6 orders of magnitude. The increase in Se shifts the absorption edge to lower wavelength and the band gap widens from 0.63 to 1.05 eV. Higher resistance ratio, higher crystallization temperature, direct transition to the stable phase indicate that (GST)(1-x)Se-x films are better candidates for phase change memory applications.

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 ferntosecond 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. (C) 2004 Elsevier Ltd. All rights reserved.

Femtosecond laser-induced crystallization in amorphous Ge2Sb2Te5 films

Crystallization in amorphous Ge2Sb2Te5 films by irradiation with femtosecond laser was investigated. The reflectivity and X-ray diffraction measurements confirmed that the crystalline state has been achieved in amorphous Ge2Sb2Te5 films under the irradiation of fermosecond laser with an average power of 65 mW at a frequency of 1000 Hz and a pulsed width of 120 fs. The surface morphology before and after femtosecond laser irradiation was studied by scanning electron microscope; results showed that the surface of films with irradiation of femtosecond laser was composed of some the crystallized micro-region. (C) 2004 Elsevier B.V. All rights reserved.

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/cm(2), respectively. (c) 2005 Elsevier B.V. All rights reserved.

Observation of ultrafast carrier dynamics in amorphous Ge2Sb2Te5 films induced by femtosecond laser pulses

The femtosecond pump-probe technique was used to study the carrier dynamics of amorphous Ge2Sb2Te5 films. With carrier density at around 10(20)-10(21) cm(-3), carriers were excited within 1 ps and recovered to the initial state for less than 3 ns. On the picosecond time scale, the carrier relaxation consists of two components: a fast process within 5 ps and a slow process after 5 ps. The relaxation time of the fast component is a function of carrier density, which increases from 1.9 to 4.3 ps for the carrier density changing from 9.7x10(20) cm(-3) to 3.1x10(21) cm(-3). A possible interpretation of the relaxation processes is elucidated. In the first 5 ps the relaxation process is dominated by an intraband carrier relaxation and the carrier trapping. It is followed by a recombination process of trapped carriers at later delay time. (c) 2007 American Institute of Physics.

激光辐照引起Ge2Sb2Te5非晶态薄膜的电/光性质变化

研究了激光辐照引起Ge2Sb2Te5非晶态薄膜的电/光性质变化,当激光功率为580mW时薄膜的方块电阻有四个数量级（10^7～10^3Ω/□）的突变;对电阻发生突变前、中、后的三个样品进行了XRD测试,结果表明,随着激光功率的增大,薄膜由非晶态向晶态转变,用椭偏仪测试了结构转变前、中、后三个样品的光学常数,在可见光范围内薄膜的光学常数在波长相同情况下有：n（非晶态）〉n（中间态）〉n（晶态）,k（晶态）〉k（中间态）〉k（非晶态）,α（晶态）〉α（中间态）〉α（非晶态）,结合电阻变化曲线和XRD图谱讨论了激光辐照Ge2Sb2Te5非晶态薄膜的电/光性质变化同激光功率和结构转变之间的关系.

Simulative Investigation on the Electronic, Vibrational and Optical Properties of the Ge2Sb2Te5 Chalcogenide

Chalcogenides are chemical compounds with at least one of the following three chemical elements: Sulfur (S), Selenium (Sn), and Tellurium (Te). As opposed to other materials, chalcogenide atomic arrangement can quickly and reversibly inter-change between crystalline, amorphous and liquid phases. Therefore they are also called phase change materials. As a results, chalcogenide thermal, optical, structural, electronic, electrical properties change pronouncedly and significantly with the phase they are in, leading to a host of different applications in different areas. The noticeable optical reflectivity difference between crystalline and amorphous phases has allowed optical storage devices to be made. Their very high thermal conductivity and heat fusion provided remarkable benefits in the frame of thermal energy storage for heating and cooling in residential and commercial buildings. The outstanding resistivity difference between crystalline and amorphous phases led to a significant improvement of solid state storage devices from the power consumption to the re-writability to say nothing of the shrinkability. This work focuses on a better understanding from a simulative stand point of the electronic, vibrational and optical properties for the crystalline phases (hexagonal and faced-centered cubic). The electronic properties are calculated implementing the density functional theory combined with pseudo-potentials, plane waves and the local density approximation. The phonon properties are computed using the density functional perturbation theory. The phonon dispersion and spectrum are calculated using the density functional perturbation theory. As it relates to the optical constants, the real part dielectric function is calculated through the Drude-Lorentz expression. The imaginary part results from the real part through the Kramers-Kronig transformation. The refractive index, the extinctive and absorption coefficients are analytically calculated from the dielectric function. The transmission and reflection coefficients are calculated using the Fresnel equations. All calculated optical constants compare well the experimental ones.