Effect of structural transformation on the electrical properties for Ge1Sb2Te4 thin film

Dependence of electrical properties of phase change Ge₁Sb₂Te₄ thin film on structural transformation was investigated. The electrical resistivity of the film decreases with increasing annealing temperature with a steep drop at ∼ 230 °C (the second crystallization temperature), at which the structure of Ge₁Sb₂Te₄ changes from face-centered cubic to trigonal state. The steep drop of resistivity at the second crystallization temperature is mainly due to the increase of hole density within the p-type film, according to Hall measurement. The crystallization process has been followed by in situ resistance measurement at various annealing temperatures. Transmission electron microscope and atomic force microscope were also employed to study the film.     

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.     

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.     

Electrical and structural properties of TaSiN electrode for phase change random access memory

Characteristics of tantalum silicon nitride (TaSiN) thin films have been investigated as an electrode material for Ge₂Sb₂Te₅ chalcogenide phase change material. The films were deposited by co-sputtering system in which the ratio of tantalum nitride to silicon was controlled by the plasma power on each target. The TaSiN films showed tunable resistivity from 260 to 560 μΩ cm with increasing Si content. From the evaluation of PRAM cell structures consisting of the TaSiN and the Ge₂Sb₂Te₅, we found that the SET voltages are nicely correlated with the resistivity of the TaSiN. Moreover, the sensing margin (resistance ratio: R_SET/R_RESET) turned out to be good for practical application.     

Influence of the additive Ag for crystallization of amorphous Ge-Sb-Te thin films

We have investigated the optical and amorphous-to-crystalline transition properties in four-types of chalcogenide thin films; Ge₂Sb₂Te₅, Ge₈Sb₂Te₁₁, Ag-Ge₂Sb₂Te₅ and Ag-Ge₈Sb₂Te₁₁. Crystallization was caused by nano-pulse illumination (λ = 658 nm) with power (P) of 1-17 mW and pulse duration (t) of 10-460 ns, and the morphologies of crystallized spots were observed by SEM and microscope. It was found that the crystallized spot nearby linearly increases in size with increasing the illuminating energy (E = P ? t) and eventually ablated out by over illumination. Changes in the optical transmittance of as-deposited and annealed films were measured using a UV-vis-IR spectrophotometer. In addition, a speed of amorphous-to-crystalline transition was evaluated by detecting the reflection response signals for the nano-pulse scanning. Conclusively, the Ge₈Sb₂Te₁₁ film has a faster crystallization speed than the Ge₂Sb₂Te₅ film despite its higher crystallization temperature. The crystallization speed was largely improved by adding Ag in Ge₂Sb₂Te₅ film but not in Ge₈Sb₂Te₁₁ film. To explain these results, we considered a heat confinement by electron hopping.     

Characterization of Agx(Ge2Sb2Te5)1 − xthin film by RF magnetron sputtering

We reported the Ag adding effects on the crystallization behavior in Ge₂Sb₂Te₅ film. Ag_x(Ge₂Sb₂Te₅)_1 − x films (where x = 0–0.2) were deposited on SiO₂ wafer and glass substrate by RF magnetron co-sputtering and annealed by RTA (rapid thermal annealing) at various temperature to crystallize. The effects of Ag adding on the structural, thermal and electrical properties were measured by X-ray diffraction, X-ray reflectivity, AFM, SEM, DSC and 4-point probe analysis. It was found that the crystallization temperature increased by Ag adding in Ge₂Sb₂Te₅ films. However, the surface of Ag_x(Ge₂Sb₂Te₅)_1 − x films got rough when annealing temperature and Ag contents increased. According to the Kissinger method, the activation energy for crystallization increased as the Ag content increased. It is thought that Ag atoms in Ge₂Sb₂Te₅ act as an amorphous stabilizer and they make it hard to switch from amorphous to crystalline phase. From this study, we would show the Ag_0.06(Ge₂Sb₂Te₅)_0.94 film is suitable for phase change memory material because of its higher crystallization temperature and structural stability.     

Nano composite Si2Sb2Te film for phase change memory

Growth-dominant Sb₂Te material with large crystal grain is converted to the nano composite material after Si doping. The increase of Si content in Si_xSb₂Te material helps to further diminish the grain size, form more uniform grain distribution, and enhance the thermal stability of the amorphous phase. Si₂Sb₂Te crystallizes into a nano composite structure [amorphous Si + crystalline Sb₂Te (< 20 nm grain size)] without any Te or Sb phase segregation, which ensures better operation stability for the application in T-shaped phase change memory device. Comparing to Ge₂Sb₂Te₅ film, Si₂Sb₂Te film shows better data retention ability (10 years at 397 K). Meanwhile, electrical measurements prove that phase change memory cell based on Si₂Sb₂Te film also has low power consumption than that of the Ge₂Sb₂Te₅ film based cell.     

Phase-change characteristics of nitrogen-doped Ge2Sb2Te5 films during annealing process

The microstructures and electrical properties of nitrogen-doped Ge₂Sb₂Te₅ thermally annealed in an N₂ atmosphere were investigated. The 5.4% nitrogen-doped Ge₂Sb₂Te₅ thin films showed discontinuous changes in resistance with annealing temperature, and corresponding changes in crystal structure. The phase transitions went through three states, amorphous??cubic??hexagonal, after annealing at 200 and 375???C. No chemical compositional change occurred after 400???C annealing. But the 20.1% nitrogen-doped Ge₂Sb₂Te₅ thin films showed continuous changes in microstructure and resistance. According to XRD and TEM analyses, the hexagonal-type Ge?CSb?CTe phase should be directly crystallized from the amorphous phase. Also, the SIMS and XPS spectra indicate that the oxygen in-diffusion and Sb and Te out-diffusion should have occurred simultaneously.     

High speed and low power consumption of superlattice-like Ge/Sb<Subscript>70</Subscript>Se<Subscript>30</Subscript> thin films for phase change memory application

In comparison to Ge₂Sb₂Te₅ (GST) and pure Sb₇₀Se₃₀ (SbSe) thin films, superlattice-like (SLL) Ge/Sb₇₀Se₃₀ (Ge/SbSe) has a higher crystallization temperature, larger crystallization activation energy, better data retention and lower power consumption. SLL Ge/SbSe thin films with different thickness of Ge and SbSe layers were prepared by magnetron sputtering system. The amorphous-to-crystalline transitions of SLL Ge/SbSe thin films were investigated through in situ film resistance measurement. The crystallization activation energy of SLL Ge/SbSe thin films was calculated from a Kissinger plot. The data retention time was estimated through isothermal time-dependent resistance measurement by Arrhenius equation. The phase structure of the thin films annealed at different temperatures was investigated by using X-ray diffraction. Phase change memory cells based on the SLL [Ge(8 nm)/SbSe(5 nm)]₄ thin films were fabricated to test and evaluate the switching speed and operation consumption.     

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).     

Fabrication of nano-scale phase change materials using nanoimprint lithography and reactive ion etching process

Phase change random access memory (PRAM) is one of the most promising non-volatile memories due to its ability to store digital data in the form of crystalline and amorphous phases of phase change materials. As a phase change material, Ge₂Sb₂Te₅ (GST225) is usually used, due to its reversible phase transition capability with speeds of less than 100 ns between the crystalline and amorphous phases. In order to fabricate highly integrated PRAM devices, sub micron- to nanometer-sized patterning of GST225 layer must be accomplished. In this study, 70 nm-sized polymer patterns were fabricated using partial filling nanoimprint lithography (NIL) on a GST225 layer, which was deposited by RF sputtering. Then GST225 was etched using Ar/Cl₂ plasma with an ICP etcher. Etch conditions, including Cl₂ concentration, were optimized to obtain the vertical etch profile of the GST patterns.     

Phase transition behaviors and thermal conductivity measurements of nitrogen-doped Sb2Te3 thin films

Crystallization temperature of nitrogen-doped Sb₂Te₃ (ST) thin films increased with increasing nitrogen doping concentration, which indicates that the long-term stability of the metastable amorphous state can be improved by nitrogen doping. The root-mean-square (rms) roughness values of the films showed a significant decrease with nitrogen doping. Thermal conductivity of nitrogen-doped ST thin films was measured using a transient thermoreflectance (TTR) technique. It was found that the thermal conductivity decreased with increasing nitrogen doping concentration and increased with increasing annealing temperature. Nitrogen-doped ST thin films are suitable phase-change materials for low programming power consumption applications of phase-change random access memory (PCRAM).     

Crystallization behavior caused by N doping in Ge1Sb4Te7 for PCRAM application

In this paper, we reported the effect of N doping on the crystallization behavior of Ge₁Sb₄Te₇ thin films. It was clearly shown that the phase transition of Ge₁Sb₄Te₇ occurred from amorphous to hexagonal state and meta-stable FCC state is shown between these phase transition processes. N doping effectively suppressed crystallization process and the crystal grain size was decreased from 21 nm to 14 nm with increasing N doping contents. N-doped film was found to have nucleation dominant crystallization process and the time demanded to start phase transition is shorter compared to un-doped film. In the case of the film deposited at 9 × 10⁻³ Torr without N doping, 70 ns is required for crystallization to occur at 36 mW of laser power, however, 30 ns is required in the case of N-doped film. These results are demonstrated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and static test.     

Crystallization process in Ge2Sb2Te5 amorphous films

The aim of this work is to investigate the isokinetic and isothermal amorphous-to-crystalline phase transformation process in Ge₂Sb₂Te₅ ternary alloys. The experiments were carried out using electrical impedance, X-ray diffraction and reflection measurements. The results have shown that, upon annealing, the crystallization process in amorphous Ge₂Sb₂Te₅ films starts with nuclei which were identified as the Ge₁Sb₄Te₇ crystalline phase. As temperature increases (or time of isothermal annealing) these nuclei are transformed into the fcc-Ge₂Sb₂Te₅ phase. In order to establish the mechanism of crystallization for this system, a stochastic lattice model was implemented to analyze nucleation and growth of the two phases involved (i.e., the metastable Ge₁Sb₄Te₇ nuclei followed by the stable fcc-Ge₂Sb₂Te₅). The results of the simulations demonstrate close agreement with experimental results. Furthermore, the crystallization process in amorphous films with the Ge₁Sb₄Te₇ composition shows the existence of only one phase during the whole process and can be described by the classical Johnson-Mehl-Avrami-Kolmogorov model.     

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.     

Spectroscopic investigation on phase transitions for Ge2Sb2Te5 in a wide photon energy and high temperature region

We investigated the electronic properties of phase-change material Ge₂Sb₂Te₅ (GST) films using spectroscopic ellipsometry in a wide photon energy and high temperature region. Apart from the charge carrier response, the totality of optical conductivity spectra for three phases of GST films, i.e., amorphous (AM), face-centered-cubic (FCC), and hexagonal (HEX), is quite similar, composed of two interband transitions in visible and UV regions. From optical analysis in a wide photon energy region up to 8.7 eV, we found that the intensity as well as the position of the interband transition in the visible region changes significantly as the phase of GST films turns from the amorphous to the crystalline phase, which is consistent with previous theoretical studies. In high temperature measurements above room temperature for the three phases of GST films, we found that the change of optical response for the AM phase of GST film occurs abruptly through two successive phase transitions near 150 °C and 270 °C, while the optical spectra of the FCC phase shows a change only near 270 °C. In contrast to the two above-mentioned cases, a slight change in optical spectra is observed for the HEX phase with the increasing temperature. From the measured optical spectra, we derived the temperature dependence of optical bandgap for the three phases, which are closely correlated to the change of the transport property for the GST films.     

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/□.     

Influence of effective surface area on gas sensing properties of WO3 sputtered thin films

WO₃ thin films having different effective surface areas were deposited under various discharge gas pressures at room temperature by using reactive magnetron sputtering. The microstructure of WO₃ thin films was investigated by X-ray diffraction, scanning electron microscopy, and by the measurement of physical adsorption isotherms. The effective surface area and pore volume of WO₃ thin films increase with increasing discharge gas pressure from 0.4 to 12 Pa. Gas sensors based on WO₃ thin films show reversible response to NO₂ gas and H₂ gas at an operating temperature of 50-300 °C. The peak sensitivity is found at 200 °C for NO₂ gas and the peak sensitivity appears at 300 °C for H₂ gas. For both kinds of detected gases, the sensor sensitivity increases linearly with an increase of effective surface area of WO₃ thin films. The results demonstrate the importance of achieving high effective surface area on improving the gas sensing performance.     

Integral isoconversional method for evaluating crystallization parameters of thin films of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> phase change memory materials

We propose a method for evaluating kinetic parameters for the crystallization of thin films of phase change materials. Its basic principle is to jointly use model-free and model isoconversional methods in analyzing differential scanning calorimetry results. Using this method, we have identified the reaction model and evaluated the activation energy for crystallization and pre-exponential factor as a function of the degree of conversion for Ge₂Sb₂Te₅-based thin films.     

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.