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.     

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.     

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.     

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.     

Structural and optical properties of heteroepitaxial beta Ga2O3 films grown on MgO (100) substrates

Gallium oxide (Ga₂O₃) films were deposited on MgO (100) substrates by metalorganic vapor phase epitaxy. Structure analyses showed that the films deposited at 550-700 °C were epitaxial β-Ga₂O₃ films with an out of plane relationship of β-Ga₂O₃(100)||MgO(100). The film deposited at 650 °C showed the best crystallinity and the microstructure of the film was investigated by high resolution transmission electron microscopy. A theoretical model of the growth mechanism was proposed and the in-plane epitaxial relationship was given to be β-Ga₂O₃[001]||MgO<011>. A four-domain structure inside the epitaxial film was clarified. The β-Ga₂O₃ film deposited at 650 °C showed an absolute average transmittance of 95.9% in the ultraviolet and visible range, which had an optical band gap of 4.87 eV.     

VO2 thin films deposited on silicon substrates from V2O5 target: Limits in optical switching properties and modeling

Thermochromic VO₂ thin films presenting a phase change at T_c = 68 °C and having variable thickness were deposited on silicon substrates (Si-001) by radio-frequency sputtering. These thin films were obtained from optimized reduction of low cost V₂O₅ targets. Depending on deposition conditions, a non-thermochromic metastable VO₂ phase might also be obtained. The thermochromic thin films were characterized by X-ray diffraction, atomic force microscopy, ellipsometry techniques, Fourier transform infrared spectrometry and optical emissivity analyses. In the wavelength range 0.3 to 25 μm, the optical transmittance of the thermochromic films exhibited a large variation between 25 and 100 °C due to the phase transition at T_c: the contrast in transmittance (difference between the transmittance values to 25 °C and 100 °C) first increased with film thickness, then reached a maximum value. A model taking into account the optical properties of both types of VO₂ film fully justified such a maximum value. The n and k optical indexes were calculated from transmittance and reflectance spectra. A significant contrast in emissivity due to the phase transition was also observed between 25 and 100 °C.     

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.     

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.     

Effects of Si-ion implantation on crystallization behavior of Ge2Sb2Te5 film

Crystallization and thermal stability of Ge₂Sb₂Te₅ (GST), the benchmark working material in phase-change non-volatile memory, were modified via Si-ion implantation. Through 5 × 10¹⁵ Si-ions/cm² ion-implantation, crystallization temperature increases from 165 °C to 177 °C. Furthermore, the activation energy of crystallization increases from 2.9 eV in the pristine film to 3.3 eV and 4.0 eV in films implanted with the doses of 5 × 10¹⁵ and 5 × 10¹⁶ Si-ions/cm², respectively. Temperatures corresponding to a 10-year failure-time increase from 83 °C in the pristine film to 96 °C and 107 °C in films implanted with 5 × 10¹⁵ and 5 × 10¹⁶ Si-ions/cm², respectively. Thermal stability of Si-ion implanted GST thus improves significantly. It was also found that grain growth is inhibited with higher implantation doses. In the case of the 5 × 10¹⁶ ion/cm² dose, the second-phase transition from face-centered cubic to hexagonal closed-packed structure of the GST is completely inhibited. However, crystallization time increases slightly due to Si-ion implantation.     

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.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Low temperature chemical vapor deposition of nanocrystalline V2O5 thin films

Spatially uniform, carbon-free thin films of V₂O₅ were deposited on silicon by chemical vapor deposition using vanadium oxide triisopropoxide and water as gaseous precursors, in the temperature range of 100-300 °C. Films with substantial crystallinity were obtained for deposition temperatures as low as 180 °C. The “neat” chemistry that nominally leaves no fragments of ligand or water in the solid promotes film purity and reduces the deposition temperature needed for crystallization. Such deposition temperatures also open up additional possibilities for using crystalline vanadia on fragile substrates such as polymers for electronics and optical applications.     

High-rate synthesis of microcrystalline silicon films using high-density SiH4/H2 microwave plasma

A high electron density (> 10¹¹ cm^− 3) and low electron temperature (1-2 eV) plasma is produced by using a microwave plasma source utilizing a spoke antenna, and is applied for the high-rate synthesis of high quality microcrystalline silicon (μc-Si) films. A very fast deposition rate of ∼ 65 Å/s is achieved at a substrate temperature of 150 °C with a high Raman crystallinity and a low defect density of (1-2) × 10¹⁶ cm^− 3. Optical emission spectroscopy measurements reveal that emission intensity of SiH and intensity ratio of H_α/SiH are good monitors for film deposition rate and film crystallinity, respectively. A high flux of film deposition precursor and atomic hydrogen under a moderate substrate temperature condition is effective for the fast deposition of highly crystallized μc-Si films without creating additional defects as well as for the improvement of film homogeneity.     

Atomic layer deposition of ZnS via in situ production of H2S

Atomic layer deposition (ALD) of ZnS films utilizing diethylzinc and in situ generated H₂S was performed over a temperature range of 60 °C-400 °C. This method for generating H₂S in situ was developed to eliminate the need to store high pressure H₂S gas. The H₂S precursor was generated by heating thioacetamide to 150 °C in an inert atmosphere, producing acetonitrile and H₂S as confirmed with mass spectroscopy. ALD behavior was confirmed by investigation of growth behavior and saturation curves. The properties of the films were studied with X-ray diffraction, transmission electron microscopy, ellipsometry, atomic force microscopy, scanning electron microscopy, ultraviolet-visible spectroscopy, and X-ray photoelectron spectroscopy. The results show a growth rate that monotonically decreases with temperature, and films that are stoichiometric in Zn and S. The root mean square roughness of the films increases with temperature above 100 °C. A change in crystal phase begins at ∼ 300 °C. The band gap is dependent on the crystal phase and is estimated to be 3.6-4 eV.     

Fabrication and characterization of n-CdSe0.7Te0.3/p-CdSe0.15Te0.85 solar cell

Hot wall deposited CdSe_xTe_1−x where 0 ≤ x ≤ 1 thin films for solar cell applications have been prepared from a compound synthesized by direct reaction of high purity Cd, Se and Te elements. Crystal structure and composition of the films were analyzed by X-ray diffraction, scanning electron microscope and EDAX. X-ray diffraction studies carried out on pseudo-binary system revealed that the films are polycrystalline in nature with CdSe_0.7Te_0.3 film exhibiting hexagonal structure and CdSe_0.15Te_0.85 film exhibiting cubic zinc blende structure. The type of conduction was determined by Hall studies. A novel solar cell with structure n-CdSe_0.7Te_0.3/p-CdSe_0.15Te_0.85 has been fabricated and the efficiency was found to be 3.13%.     

Structural, electrical and optical properties of Bi2Se3 and Bi2Se(3−x)Tex thin films

Thin films of Bi₂Se₃, Bi₂Se_2.9Te_0.1, Bi₂Se_2.7Te_0.3 and Bi₂Se_2.6Te_0.4 are prepared by compound evaporation. Micro structural, optical and electrical measurements are carried out on these films. X-ray diffraction pattern indicates that the as-prepared films are polycrystalline in nature with exact matching of standard pattern. The composition and morphology are determined using energy dispersive X-ray analysis and scanning electron microscopy (SEM). The optical band gap, which is direct allowed, is 0.67 eV for Bi₂Se₃ thin films and the activation energy is 53 meV. Tellurium doped thin films also show strong optical absorption corresponding to a band gap of 0.70-0.78 eV. Absolute value of electrical conductivity in the case of tellurium doped thin film shows a decreasing trend with respect to parent structure.     

Structural and thermal characterization of La5Ca9Cu24O41 thin films grown by pulsed laser deposition on (1 1 0) SrTiO3 substrates

In the present study stoichiometric, b-axis oriented La₅Ca₉Cu₂₄O₄₁ thin films were grown by pulsed laser deposition on (1 1 0) SrTiO₃ substrates in the temperature range 600-750 °C. High resolution transmission electron microscopy was employed to investigate the growth mechanism and the epitaxial relationship between the SrTiO₃ substrates and the La₅Ca₉Cu₂₄O₄₁ films grown at 700 °C. The 3-ω method was used to measure the cross-plane thermal conductivity of La₅Ca₉Cu₂₄O₄₁ films in the temperature range 50-350 K. The observed glass-like behavior is attributed to atomic-scale defects, grain boundaries and an interfacial layer formed between film and substrate.     

Hydrogen sensing properties of Pd-doped SnO2 sputtered films with columnar nanostructures

Pd-doped SnO₂ sputtered films with columnar nanostructures were deposited using reactive magnetron sputtering at the substrate temperature of 300 °C and the discharge gas pressures of 1.5, 12, and 24 Pa. Structural characterization by means of X-ray diffraction and scanning electron microscopy shows that the films composed of columnar nanograins have a tetragonal SnO₂ structure. The films become porous as the discharge gas pressure increases. Gas sensing measurements demonstrate that the films show reversible response to H₂ gas. The sensitivity increases as the discharge gas pressure increases, and the operating temperature at which the sensitivity shows a maximum is lowered. The highest sensitivity defined by (R_a − R_g) / R_g, where R_a and R_g are the resistances before and after exposure to H₂, 84.3 is obtained for the Pd-doped film deposited at 24 Pa and 300 °C upon exposure to 1000 ppm H₂ gas at the operating temperature of 200 °C. The improved gas sensing properties were attributed to the porosity of columnar nanostructures and catalytic activities of Pd doping.     

Leakage current characteristics of Bi3.15Nd0.85Ti3 − xZrxO12 thin films

Thin films of Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) and Bi_3.15Nd_0.85Ti_3 − xZr_xO₁₂ (BNTZ_x, x = 0.1 and 0.2) were fabricated on Pt/TiO₂/SiO₂/Si(100) substrates by a chemical solution deposition (CSD) technique at 700 °C. Structures, surface morphologies, leakage current characteristics and Curie temperature of the films were studied as a function of Zr ion content by X-ray diffraction, atomic force microscopy, ferroelectric test system and thermal analysis, respectively. Experimental results indicate that Zr ion substitution in the BNT film markedly decreases the leakage current of the film, while almost not changing the Curie temperature of the film, which is at about 420-460 °C. The decrease of the leakage current in BNTZ_x films is that the conduction by the electron hopping between Ti⁴⁺ and Ti³⁺ ions is depressed because Zr⁴⁺ ions can block the path between two adjacent Ti ions and enlarge hopping distance.