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We investigated the effect of Se doping on the physical and electrical properties of Ge 10 Sb 90 films prepared by sputtering. We examined the crystal structure, chemical bonding, electrical and thermal properties, and volume shrinkage during the crystallization of Ge 10 Sb 90 films doped concentrations of Se between 0 and 20 at.%. X-ray diffraction patterns showed that incorporating Se leads to phase separation at high Se concentrations. As the Se concentration increased, both the crystallization temperature (T c ) and the sheet resistance increased in both the amorphous and crystalline states. In contrast, the melting temperature (T m ) and grain size both decreased with the increasing Se. The volume shrinkage of the Se-doped Ge 10 Sb 90 films was analyzed by X-ray reflectivity measurements. Our results demonstrate that Se-doped Ge 10 Sb 90 films show a favorably low operation current and good thermal and mechanical stability, which strongly indicate their feasibility for phase-change memory applications. [ABSTRACT FROM AUTHOR]

Phase change memory (PCM) that is operated on resistance changes caused by joule heating has been suggested as the next-generation memory for scaling since its programming current scales linearly. We propose a U-shaped cell design to further reduce the reset current in PCM devices, which enables easier and more efficient scaling than conventional PCMs. Simulation studies of heat transfer demonstrated that our U-shaped design with a dashed heater has a higher thermal efficiency of 4.97 K/μA compared to 3.36 K/μA in a lance cell with a ring heater for the same storage node. The reset current can be better scaled proportionate to k 2.0 in which the exponent is higher than the lance cell of k 1.5 in non-isotropic scaling. This better scalability is attributed to the small programming volume of the U-shaped cell, which was verified by transmission electron microscopy analysis. Furthermore, the cyclic endurance of the U-shaped cell was enhanced by 1 order of magnitude compared to a lance cell and the thinner CGeSbTe films reduced the reset current further. Our results show that a U-shaped cell is a highly promising design to scale reset current in next-generation PCM devices. [ABSTRACT FROM AUTHOR]

As the cell size of phase change memory devices decreases to less than 100 nm, the dry etch used for cell patterning becomes extremely critical because of its impact on the properties of memory cells. HBr gas has been known as the etchant that can minimize surface etching damage to GeSbTe-based phase change materials. However, the findings reported herein show that the HBr etch of CGeSbTe (CGST) films causes voids after annealing at temperatures below 400 °C due to the vaporization of volatile bromides (GeBr 4 , SbBr 2 , and TeBr 2 ) that form when bromine diffuses during the etch. In this investigation, we report on the use of NH 3 etchants to suppress the formation of volatile compounds and thereby eliminate void formation in CGST materials. The properties of NH 3 etchants were compared to those of HBr etchants as a function of both etch rate and profiles. The effects of void suppression as observed in transmission electron microscopy images of as-etched CGST film after annealing indicate that phase change memory devices etched using an NH 3 exhibited enhanced set cycles that were over 10 8 superior to results of HBr etchants by 2 orders of magnitude. [ABSTRACT FROM AUTHOR]

Phase Change Memory (PCM) has the potential for use as the flash memories for the next generation due to its scalability, long endurance, high speed, and the possibility of random access. To successfully integrate phase change materials of a GeSbTe (GST) alloy in the device fabrications, the presence of a Ti layer is required to obtain proper contact resistance characteristics and also to improve the adhesion property between the GST alloys and the top TiN electrode. The findings reported herein indicate that Ti can readily diffuse into the GST layer and reduce the set resistance when the subsequent device fabrication involves the heating process. As a result, the reset current significantly increases and the endurance cycles of the PCM device are significantly degraded. We proposed the use of a Highly Nitrogen doped GST (HNGST) layer as a barrier to Ti diffusion, in which the presence of the barrier was able to restore the reset current of the PCM devices down to the level of as-etched PCM cell by the use of a 9 nm thick HNGST barrier. In addition, endurance cycles were significantly enhanced from 10 6 to over 10 8 by blocking the diffusion of Ti into the program volume of the cell with the HNGST barrier layer. [ABSTRACT FROM AUTHOR]

• The Sb 54 Zn 46 thin film is fabricated by antimony and zinc targets co-sputtering. • The thermal stability of Sb 54 Zn 46 can satisfy the electro-mobile application. • The strong Sb-Zn bond stretching mode accounts for the high thermal stability. • The phase change speed of Sb 54 Zn 46 (10 ns) is much faster than that of Ge 2 Sb 2 Te 5. • Growth-dominant crystallization mechanism explains the fast phase change speed. In this work, we fabricate the Sb 54 Zn 46 thin film by pure antimony and zinc targets co-sputtering and analyze its phase change properties. The ten-years data retention of Sb 54 Zn 46 thin film, being a good reflection of thermal stability, is improved to 119 °C from 24 °C of pristine antimony thin film, which is much higher than that of the traditional phase change material Ge 2 Sb 2 Te 5 (85 °C). The increased thermal stability of Sb 54 Zn 46 thin film mainly derives from the formation of the strong Sb-Zn bond stretching mode during crystallization. In addition, the phase change speed of Sb 54 Zn 46 thin film measured in device test increases to 10 ns, which is also much faster than that of the traditional phase change material Ge 2 Sb 2 Te 5 (100 ns). These results illustrate that the fast phase change speed of Sb 54 Zn 46 thin film is mainly ascribed to the Sb-rich atomic environment and the growth-dominant crystallization mechanism. Our work substantiates the co-sputtering Sb 54 Zn 46 thin film can ensure fast phase change speed while effectively increasing its thermal stability. [ABSTRACT FROM AUTHOR]

The crystallization behavior of ultrathin Ti 0.43 Sb 2 Te 3 (TST) films dependent on thickness was studied. As the film thickness decreases, the crystallization temperature ( T c ) firstly increases in the thickness range of 10–50 nm, and then decreases when the thickness is less than 10 nm. Two factors can explain the variation trend of T c with decreasing thickness of TST films. The increment of T c is mainly due to the crystallization suppression originating from the increment of surface-to-volume in 10–50 nm TST films, and the decrement is dominantly attributed to the crystallization promoter of substrate/film interface in sub-10 nm TST films. The current–voltage and resistance–voltage measurements of 50 nm-TST-based and 10 nm-TST-based devices exhibit that 10 nm-TST-based device can realize low voltage and low power operation, but owns relatively lower speed for its longer incubation time. [ABSTRACT FROM AUTHOR]

In this paper, phase change behaviors of GaSb doped Sb–Se films were investigated systematically for phase change memory. It is shown that the precipitation of Sb 2 Se 3 phase from Ga–Sb–Se film was suppressed to some extent with the increasing Ga content. The crystallization temperature enhances from 245 to 320 °C, and activation energy of crystallization increases from 3.49 to 5.52 eV as Ga content increases from 8 to 34 at.%. Correspondingly, the temperature for 10-year data retention of as-deposited amorphous state increases from 154 to 230 °C. The minimum reset voltage and set resistance of Ga 13 Sb 32 Se 55 are 0.7 V lower and about one order of magnitude higher than those of Ge 2 Sb 2 Te 5 , respectively. The low power of Ga 13 Sb 32 Se 55 results from both the low thermal conductivity (0.33 W/m K) and low electrical conductivity (243.9 Ω − 1 m − 1 ) of crystalline state as well as low melting point (497 °C). [ABSTRACT FROM AUTHOR]

The bulk samples of Sb x Se 100 − x system (x = 90, 85, 80) and their amorphous thin films were prepared by flash evaporation method. The films possess good thermal stability and high values of the activation energy of crystallization. The activation energy of crystallization (E a ) of the thin films Sb 85 Se 15 and Sb 80 Se 20 was found to be 2.97 and 2.55 eV, respectively. The E a of Sb 90 Se 10 was found to be 1.79 eV for first step and 2.95 eV for second step. The homogeneous thin amorphous films were crystallized by nanosecond laser pulses, by annealing at 235 and 360 °C and during the measurement of temperature dependence of electrical resistivity. The crystalline phase possesses much lower electrical resistivity than the amorphous one (ΔR s ≈ 10 3 ); the temperature of crystallization T c is from the region of 150–161 °C. The structure of prepared films as determined by X-ray diffraction analysis revealed that the films crystallized by laser pulses and by annealing at 235 °C contain only one phase of Sb-like structure. The films of composition Sb 80 Se 20 and Sb 85 Se 15 annealed at 360 °C contain two phases of Sb and Sb 2 Se 3 . The film of composition Sb 90 Se 10 annealed at 360 °C contains one phase of Sb-like structure. The optical reflectivity change due to crystallization is also large (ΔR opt. ≈ 20–30%). The refractive index n of crystalline samples is higher than that of amorphous films (Δ n ≈ 1.5–3.1 for transparent part of spectrum); it is higher for films with higher Sb content. The real and imaginary parts of permittivity, evaluated from ellipsometric measurement, are also much higher for crystalline state (up to 55) than for the amorphous one (< 33). The amorphous films are semiconductive, and their E g opt. (0.28–0.35 eV) is decreasing with an increasing content of Sb. In spectral region of λ = 360–830 nm, the real part of permittivity of crystalline phase is negative and the imaginary part of permittivity is not high, especially for the Sb 90 Se 10 samples. Such films are perspective not only as the phase change materials, but also as the plasmonic materials. [ABSTRACT FROM AUTHOR]

Abstract: Selenium doped thin films of GeTe alloys were investigated for their structural modifications by X-ray Diffraction, Fourier Transform Infrared Spectroscopy, X-ray photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The band gap increase from 0.69 to 1.10eV with increasing Se addition signifies the possibility of band gap tuning in the material. Disorder decreases, band widens and conductivity saturates about 0.20at.% of Se addition. Structural changes are explained by the bond theory of solids. The as-deposited films are amorphous and 0.50at.% Se alloy forms a homogeneous amorphous phase with a mixture of Ge Se and Te Se bonds. The XPS core level spectra and Raman spectra investigation clearly indicate the formation of Ge Se, GeTe2 and Te Se bonds with Se addition. Crystallization temperature is found to be increasing with Se and the 0.10at.% Se alloy is found to have a higher resistance contrast compared to other Se concentration alloys. Up to 0.10at.% of Se addition can enhance GeTe phase change memory properties. [Copyright &y& Elsevier]

We studied Ga19Sb81 film deposited on SiOx/Si(100) by RF co-sputtering using GaSb and Sb targets aiming for applications in multi-level phase-change memory. Crystallization temperature (Tx ) is 228°C–235°C determined by electrical resistance versus temperature measurements at various heating rates. The dramatic resistance drop is attributed to increased carrier concentration based on Hall measurements. The films show p-type conduction behavior at all temperature regimes. The temperature for ten-year failure-time (T 10y) and the activation energy of crystallization (E a) is 156°C and 4.2eV, respectively, as deduced using Arrhenius plots from as-deposited films. The two-step drop in resistance is attributed to the two-step phase formation as depicted by X-ray diffractometry. Density changes before and after crystallization is 5.3% (Ge2Sb2Te5: 9.5%) as analyzed by the X-ray reflectivity method. Memory switching is carried out by using test cells with a via hole size of 600nm. Set and reset processes can be achieved by using pulse widths of 20–100ns. The cycling test showed performance at least 104 set–reset times. Furthermore, two stable set states are routinely achieved by controlling set voltages. The two states can be operated at least 102 cycles steadily with resistance ratios ~5 and ~22 times, respectively. The two set states arise from the formation of two phases (Sb then GaSb) during set operations. [ABSTRACT FROM AUTHOR]

The amorphous to face-centered cubic (fcc) and fcc to hexagonal close-packed (hcp) crystallization temperatures of GeSbTe thin films on underlying silicon nitride and silicon dioxide films were studied through slow (1K/min) resistance versus temperature measurements. The amorphous to fcc phase transition is observed at ~170°C for both cases but the fcc to hcp phase transition temperature for GeSbTe films on silicon nitride is observed to be ~80°C lower than for GeSbTe films on silicon dioxide, possibly due to the hexagonal symmetry of silicon nitride. [ABSTRACT FROM AUTHOR]

Abstract: A systematic study of the deposition parameters for the metal–organic chemical vapour deposition growth of In–Sb–Te (IST), of interest for phase change memory applications, was performed. Samples were grown on Si/SiO2 and patterned substrates in the (220÷350)°C temperature range and working pressure from 35 to 100×102 Pa, which resulted in the formation of thin films (down to 30nm) or IST crystals. The chemical composition of the IST films was mainly dependent on the deposition temperature. We have demonstrated the possibility to obtain a conformal and smooth morphology with improved surface roughness for films grown at 260°C when the substrate surface is treated with the TrisDimethylaMinoAntimony ([N(CH3)2]3Sb) precursor. The IST-based chalcogenide films exhibited different crystalline and partially amorphous phases, which may be favourable for multilevel data storage. The IST growth mechanism was analysed in terms of the structural, compositional and electrical properties. [Copyright &y& Elsevier]

Abstract: A phase change alloy has been synthesized and characterized. The reversible phase transitions between amorphous and crystalline states of SnSe4 films have been studied using variable electrical pulses and X-ray diffraction. Temperature dependent sheet resistance measurements have shown two distinct resistivity states of more than two orders of magnitude. This high electrical contrast makes the alloy suitable for nonvolatile phase change memory applications. X-ray diffraction has attributed the large electrical contrast to an amorphous–crystalline phase transition. The nonvolatile memory cells have been fabricated using a simple sandwich structure (metal/chalcogenide thin film/metal). A threshold voltage of 3.71V has been determined for this phase change random access memory cell. Memory switching was initiated using the voltage pulses of 3.71V, 90ns, 1.3V and 26μs, for the crystallization and amorphization process, respectively. [Copyright &y& Elsevier]

Abstract: To meet increasing demands for chemical vapor deposition methods for high performance phase-change memory, cyclic plasma enhanced chemical vapor deposition of Sb and GexSby phase-change films and characterization of their properties were performed. Two cycle sequences were designed to investigate the role of hydrogen gas as a reduction gas during Sb film deposition. Hydrogen gas was not introduced into the reaction chamber during the purge step in cycle sequence A and was introduced during the purge step for cycle sequence B. The role of hydrogen gas was investigated by comparing the results obtained from these two cycle sequences and was concluded to exert an effect by a combination of precursor decomposition, surface maintenance as a hydrogen termination agent, and surface etching. These roles of hydrogen gas are discussed through consideration of changes in deposition rates, the oxygen concentration on the surface of the Sb film, and observations of film surface morphology. Based on these results, GexSby phase-change films were deposited with an adequate flow rate of hydrogen gas. The Ge and Sb composition of the film was controlled with the designed cycle sequences. A strong oxygen affinity for Ge was observed during the X-ray photoelectron spectroscopy analysis of Sb 3d, Sb 4d, and Ge 3d orbitals. Based on the XPS results, the ratios of Ge to Sb were calculated to be Ge0.32Sb0.68, Ge0.38Sb0.62, Ge0.44Sb0.56, Ge0.51Sb0.49 and Ge0.67Sb0.33 for the G1S7, G1S3, G1S2, G1S1, and G2S1 cycles, respectively. Crystal structures of Sb, Ge, and the GeSb metastable phase were observed with various GexSby film compositions. Sb crystallinity decreased with respect to Ge crystallinity by increasing the Ge fraction. A current–voltage curve was introduced, and an electro-switching phenomenon was clearly generated at a typical voltage, Vth. Vth values increased in conjunction with an increased proportion of Ge. The Sb crystallinity decrease and Vth increase were explained via the bonding characteristics of each element. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: A metal-chalcogenide layer, SnSe, is inserted between the memory layer GeTe and the top electrode to form a phase change memory cell. The GeTe layer exhibits ovonic threshold switching at a threshold field of ~110V/μm. For subsequent implementation into applications and reliability, material inter-diffusion and sublimation are examined in bilayer phase change films of GeTe/SnSe. Transmission electron microscopy and parallel electron energy loss spectroscopy analyses reveal Sn migration to the GeTe layer, which is responsible for lowering the rhombohedral to cubic structural transformation temperature in GeTe. Incongruent sublimation of SnSe and GeTe is observed at temperatures higher than 500°C. Severe volatilization of Se results in the separation of a metallic Sn phase. The use of Al2O3 as a capping layer has been found to mitigate these effects. [Copyright &y& Elsevier]

Abstract: Phase-transition temperature of GeSbTe (GST) chalcogenide film was drastically increased from 159 to 236°C by cerium (Ce) doping (up to 8.6at.%) without altering the resistivity property of GST. Grain refinement via the solid-solution mechanism and the amplification of p-type semiconducting behavior in Ce-doped GST were observed. They were correlated with the enhancement of thermal stability and data retention property of GST as revealed by exothermal and isothermal analyses. Phase-change memory (PCM) device characterized at various temperatures revealed an effective thermal stability improvement on the threshold voltage of PCM device by Ce doping. [Copyright &y& Elsevier]

Abstract: Growth-dominant Sb2Te material with large crystal grain is converted to the nano composite material after Si doping. The increase of Si content in SixSb2Te 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 (<20nm 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 (10years at 397K). 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. [Copyright &y& Elsevier]

Abstract: This study investigated the phase change kinetics, thermal, structural, electrical properties, and chemical bonding state of 50-nm-thick oxygen or nitrogen doped GeSb9 films prepared by sputtering. Up to 6.8 at.% O or 8.7 at.% N doping, as measured by secondary ion mass spectrometry, the crystallization temperatures, lattice constants, and the resistivity of both crystalline and amorphous films increased with oxygen or nitrogen concentration , while the grain sizes decreased. Nitrogen atoms bonded to Ge and formed GeN4 nitrides, and both the GeO2 and Sb2O3 co-existed when oxygen concentration reached 8.5 at.%. Crystallization could be triggered by a laser pulse of duration 12 ns for all films. Both GeSb9–O and GeSb9–N films had fast crystallization speed, good thermal stability, and improved resistivity and are therefore potential candidates as active layers for phase-change memory. [Copyright &y& Elsevier]

Abstract: Ga–Sb–Te ternary thin films were prepared by co-sputtering using targets GaSb and Sb8Te2. Crystallization processes of these films were studied by measuring the temperature-dependent electrical resistance during non-isothermal heating. The activation energy (E a), rate factor (K o), and kinetics exponent (n) were deduced from Kissinger and Ozawa''s plots, respectively. The crystallization temperatures (T x =108∼319°C) increase with increasing GaSb contents. The activation energy (1.82∼7.52 eV) increases with increasing Ga content until 31.6 at.% then it decreases. The crystallization mechanisms of compositions A∼D (Ga17∼32Sb71∼62Te12∼6), as evidenced from n values, are nuclei formation and subsequent crystal growth; the nucleation rate decreases with grain growth. While composition E (Ga38.2Sb57.7Te4.1) reveals an n value lower than 1.5, it implies the one-dimensional crystal growth from the nuclei. The crystallization time of each composition was evaluated using JMA equation using all derived kinetics parameters. Composition C (Ga26.4Sb65.2Te8.4), showing the shortest crystallization time, is suggested for phase change RAM applications. [Copyright &y& Elsevier]

Abstract: The structural and electrical resistance properties of excess-Sb Ge2Sb2+ x Te5 (Sb-GST) films were investigated. As the Sb-doping concentration was increased, the face center cubic structure of Sb-GST films was no longer observed, at Sb concentrations exceeding 27%, the amorphous phase directly changed to hexagonal closed-packed structures. The crystallization temperature of the 27% Sb-GST film was 15 °C higher than that of the Ge2Sb2Te5 (GST) film. The activation energies of the Sb-GST films also were greater than those of the GST films. These results indicate an increase in the stability of the Sb-doped films in the amorphous state compared with GST films. [Copyright &y& Elsevier]