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 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.     

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.     

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.     

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-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.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

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.     

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.     

Catalyst-free growth of Sb2Te3 nanowires

In this study, a catalyst-free growth method was discovered to prepare the high-quality single crystal Sb₂Te₃ nanowires from the Al:Ge:Sb:Te thin films. The diameters of Sb₂Te₃ nanowires were found to be ~ 100 nm and their lengths were as great as tens of micrometers. The Al content and the annealing temperature play an important role in the growth of Sb₂Te₃ nanowires. When the Al content (> 12.4 at.%) was sufficiently contained in Al:Ge:Sb:Te film, Sb₂Te₃ nanowires were extruded spontaneously on the surface of thin film with increase in annealing temperatures. Compared with the vapor-liquid-solid method, our method has advantages of low temperature (~ 300 °C) and no impurities, such as a metal catalyst.     

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.     

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).     

Structural and magnetic properties of Ge0.7Mn0.3 thin films

Ge_0.7Mn_0.3 thin films were fabricated on Al₂O₃ (0001) and glass substrates at growth temperatures ranging from room temperature to 500 °C by a radio frequency magnetron sputtering. We found that the Ge_0.7Mn_0.3 thin films showed a polycrystalline-to-amorphous transition at about 360 °C, and the ferromagnetic transition temperature of each thin film depends on its structure — crystalline or amorphous states. Particularly, the Ge_0.7Mn_0.3 thin films showed room temperature ferromagnetism when they were fabricated at temperatures above the crystallization temperature.     

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.     

Optical and structural properties of CuSbS2 thin films grown by thermal evaporation method

Structural, optical and electrical properties of CuSbS₂ thin films grown by thermal evaporation have been studied relating the effects of substrate heating conditions of these properties. The CuSbS₂ thin films were carried out at substrate temperatures in the temperature range 100-200 °C. The structure and composition were characterized by XRD, SEM and EDX. X-ray diffraction revealed that the films are (111) oriented upon substrate temperature 170 °C and amorphous for the substrate temperatures below 170 °C. No secondary phases are observed for all the films. The optical absorption coefficients and band gaps of the films were estimated by optical transmission and reflection measurements at room temperature. Strong absorption coefficients in the range 10⁵-10⁶ cm^− 1 at 500 nm were found. The direct gaps Eg lie between 0.91-1.89 eV range. It is observed that there is a decrease in optical band gap Eg with increasing the substrate temperature. Resistivity of 0.03-0.96 Ω cm, in dependence on substrate temperature was characterized. The all unheated films exhibit p-type conductivity. The characteristics reported here also offer perspective for CuSbS₂ as an absorber material in solar cells applications.     

Sb2S3:C/CdS p-n junction by laser irradiation

In this paper, we report laser irradiated carbon doping of Sb₂S₃ thin films and formation of a p-n junction photovoltaic structure using these films. A very thin carbon layer was evaporated on to chemical bath deposited Sb₂S₃ thin films of approximately 0.5 μm in thickness. Sb₂S₃ thin films were prepared from a solution containing SbCl₃ and Na₂S₂O₃ at 27 °C for 5 h and the films obtained were highly resistive. These C/Sb₂S₃ thin films were irradiated by an expanded laser beam of diameter approximately 0.5 cm (5 W power, 532 nm Verdi laser), for 2 min at ambient atmosphere. Morphology and composition of these films were analyzed. These films showed p-type conductivity due to carbon diffusion (Sb₂ S₃:C) by the thermal energy generated by the absorption of laser radiation. In addition, these thin films were incorporated in a photovoltaic structure Ag/Sb₂S₃:C/CdS/ITO/Glass. For this, CdS thin film of 50 nm in thickness was deposited on a commercially available ITO coated glass substrate from a chemical bath containing CdCl₂, sodium citrate, NH₄OH and thiourea at 70 °C. On the CdS film, Sb₂S₃/C layers were deposited. This multilayer structure was subjected to the laser irradiation, C/Sb₂S₃ side facing the beam. The p-n junction formed by p-Sb₂S₃:C and n-type CdS showed V_oc = 500 mV and J_sc = 0.5 mA/cm² under illumination by a tungsten halogen lamp. This work opens up a new method to produce solar cell structures by laser assisted material processing.     

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.     

Solar cells with Sb2S3 absorber films

CdS/Sb₂S₃/PbS structures were prepared by sequential chemical deposition of CdS, Sb₂S₃ and PbS thin films on TEC-8 (Pilkington) transparent electrically conductive SnO₂ (TCO) coatings. CdS thin films (100 nm) were deposited with hexagonal structure from Cd-citrate bath and of cubic structure from Cd-ammine/triethanolamine bath. Sb₂S₃ thin films were deposited at 40 °C from a solution mixture of potassium antimony tartrate, triethanolamine, ammonia and thioacetamide(TA) or at 1 to 10 °C from a mixture of antimony trichloride and thiosulfate (TS). These films were made photoconductive by heating at temperatures 250 to 300 °C. When heated in the presence of a chemically deposited Se thin film of 300 nm, a solid solution Sb₂S_1.8Se_1.2 resulted. PbS thin films of 100-200 nm thickness were deposited on the TCO/CdS/Sb₂S₃ or TCO/CdS/Sb₂S_1.8Se_1.2 structure. Graphite paint was applied on the PbS film prior to applying a silver epoxy paint. The cell structures were of area 0.4 cm². The best results reported here is for a cell: TCO/CdS(hex-100 nm)/Sb₂S₃(TS-100 nm)/PbS(200 nm) with open circuit voltage (V_oc) 640 mV, short circuit current density 3.73 mA/cm², fill factor 0.29, and conversion efficiency 0.7% under 1000 Wm^− 2 sunlight. Four series-connected cells of area 1 cm² each gave V_oc of 2 V and short circuit current of 1.15 mA.     

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/□.     

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.