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A new non-chalcogenide inorganic Blu-ray recordable medium has been developed using a Pd-based alloy. Optical properties and thermal properties of the Pd-based alloy were studied. Dynamic recording performance and archival lifetime were evaluated.

Investigation focuses on near-field characteristics and surface plasmon (SP) induced with uniform- and none-uniform-size Ag nanoparticles in mask layers of super-RENS structures. Signal enhancement mechanism of the super-RENS disk is discussed.

The initialization-free Blu-ray Disc was proposed as a candidate for multi-speed recording. Experiment results of the initialization-free Blu-ray Disc showed that the initialization-free disc had a multi-speed recording capability from 1x to 2x speed.

Phase transformations of 100nm Ge1Sb4Te7 films induced by single 130fs pulse at 800nm have been investigated with time-resolved microscope. With an average fluence of 30mJ/cm2, a reflective intensity increase was observed within 1ps in 100nm as-deposited Ge1Sb4Te7 films after excitation by intense femtosecond pulse, which was consistent to an electronically induced non-thermal phase transformation. XRD measurement confirmed that single femtosecond pulse could induce crystalline marks in 100nm as-deposited Ge1Sb4Te7 films. Our results indicated that single femtosecond pulse could trigger both crystalline and amorphous phase in 100nm Ge1Sb4Te7 films. The fluence for crystallization was higher than that for amorphization.

Ultrafast phase transitions triggered by single femtosecond laser pulse in Ge 1 Sb 2 Te 4 films were investigated. By proper control of the film thickness, ultrafast crystalline and amorphous phase transformations have been achieved in Ge 1 Sb 2 Te 4 films. These utlrafast phase transitions were confirmed by reflectivity change and x-ray diffraction measurement.

High power blue light generated by quasi phase matched (QPM) second harmonic generation (SHG) techniques is very attractive for high density optical storage. KTiOPO/sub 4/ (abbreviated as KTP) is one of the most important materials used in QPM SHG. Since 1987 many kinds of waveguides and devices have been demonstrated in KTP substrates. However there still remains some technique problems, for example, the control of the duty cycle of the segmented waveguides as well as cracking of the waveguides. On the other hand the photolithography process used for fabrication of KTP segmented waveguides has not been reported. In this paper we report on the experimental results of segmented optical waveguides fabricated in KTP substrates.

In this paper, a three-dimensional finite-element modeling is performed for the analyses of Chalcogenide Random Access Memory (C-RAM), a non-rotation nonvolatile phase change memory cell. The thermal effect generated by an incident electric pulse was mainly discussed. Thermal performances of the cell as a result of electrical and geometrical variations were quantified. Current density distribution, temperature profiles, temperature history, heating rate, cooling rate, and heat flow characteristics were obtained and analyzed. The study is useful for the failure analysis of the C-RAM.

The initialization-free phase-change optical disk was proposed as a candidate for multi-speed recording. Experiment results of the initialization-free DVD-RAM disk showed that the initialization-free disk was compatible with a broad range of recording speed.

The electrical induced structural transformation of Ge2Sb2Te5 thin film in phase change memory device was investigated using micro-Raman spectroscopy and transmission electronic microscopy (TEM). Selected area electron diffraction (SAD) pattern showed that the electrical-induced Ge2Sb2Te5 film was crystallized into a face-centered cubic structure. Micro-Raman spectra show that the Ge2Sb2Te5 active layer at the high resistance state exhibited two minor peaks superposed on the broad peak after several switch cycles, which is identical to those of the Ge2Sb2Te5 active layer at the low resistance state. This is most likely due to the accumulation of segregated crystallites. TEM results suggest that the existence of nano-sized nuclei clusters resulted in the reduced resistance for the Ge2Sb2Te5 active layer at the high resistance state after first several switches. The dependence of resistance on the cycle number indicates that the deterioration of the Ge2Sb2Te5 active layer is resulted from the incomplete amorphization process, which is consistent with the micro-Raman results.

A method for high-density optical disk structure design using computational electromagnetic FDTD analysis is proposed. The land-groove structures of phase-change optical disk with blue laser and high-NA system are investigated.

Nitrogen-doped titanium-tungsten (N-TiW) was proposed as a tunable heater in Phase Change Random Access Memory (PCRAM). By tuning N-TiW's material properties through doping, the heater can be tailored to optimize the access speed and programming current of PCRAM. Experiments reveal that N-TiW's resistivity increases and thermal conductivity decreases with increasing nitrogen-doping ratio, and N-TiW devices displayed (∼33% to ∼55%) reduced programming currents. However, there is a tradeoff between the current and speed for heater-based PCRAM. Analysis of devices with different N-TiW heaters shows that N-TiW doping levels could be optimized to enable low RESET currents and fast access speeds.

A compositionally matched superlattice-like (SLL) structure comprised of Ge2Sb2Te5 (GST) and nitrogen-doped GST (N-GST) was developed to achieve both low current and high endurance Phase Change Random Access Memory (PCRAM). N-GST/GST SLL PCRAM devices demonstrated ∼37% current reduction compared to single layered GST PCRAM and significantly higher write/erase endurances of ∼108 compared to ∼106 for GeTe/Sb2Te3 SLL devices. The improvements in endurance are attributed to the compositionally matched N-GST/GST material combination that lowers the diffusion gradient between the layers and the lower crystallization-induced stress in the SLL as revealed by micro-cantilever stress measurements.

We show a method for studying diffusion barriers for phase change memory devices. The method involves placing a barrier layer between a phase change material like Ge2Sb2Te5 and Fe and taking note of the magnetization changes of the Fe layer before and after anneal. This results in a fast, sensitive but qualitative measure of various diffusion barriers. Based on this analysis, Ru was found to be good diffusion barrier comparable to the conventional TiW, while Pt is a poor barrier. These results were then reconfirmed by X-ray photoelectron spectroscopy depth studies.

Crystal growth of KTiOPO4(KTP) family with flux method has been summarized. The results of defect observation with synchrotron radiation topography and other methods have been reported. It is shown that KTP crystal is of high-quality and there exist different kinds of defect such as growth striations, multiple domains in doped KTP and KTA crystals. The observation and discussion about the bicrystal of KTP family crystal have been mentioned in the paper. SHG and SFG measurements indicated that KTP family crystals investigated have excellent nonlinear optical properties.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

TiWOx interfacial layer was proposed and implemented to act as both heater and inter-diffusion barrier for phase change memory through a complementary metal-oxide semiconductor compatible oxidization process. Significant reduction of RESET current was obtained due to more efficient Joule heating and better thermal confinement. About one order of magnitude endurance increase was achieved for the device with TiWOx due to suppression of inter-diffusion between Ge2Sb2Te5 and TiW. The change of the minimum RESET voltage against cycling was reduced by TiWOx layer with shorter RESET pulse, which would benefit device cyclability.

The circular transfer length method was employed to extract the specific contact resistivity, ρc of GeTe (amorphous and crystalline state) with metals (Ni, W, TiW) to quantify the series contact resistance. The ρc of amorphous-GeTe with metals was also determined for different annealing conditions. Higher metal work functions produce lower ρc for both GeTe states and the ρc was reduced further for annealing temperatures greater than the GeTe crystallization temperature. This is suggested to be a consequence of the higher temperature required to diffuse sufficient interstitial metallic atoms to transform the GeTe covalent bonding at the interface to metallic bonding.

In this paper, we proposed a new simple structure of dual-layer rewritable phase-change optical discs. The disc consists of only three conventional materials for rewritable disc, a phase-change layer, dielectric layers and a reflective layer. Recording, erasing and overwriting in both recording mediums of the new disc structure are demonstrated and characterized. Both recording mediums can be rewritten using the conventional recording power defined by digital versatile disc (DVD) standard.

Thermal analyses are performed on landed multi-track recording on phase change optical disk using a parallel recording head. A new arrangement of parallel recording head is proposed. Temperature uniformity for adjacent tracks was studied. The minimum angle between the slanting line array and the cross tracking line array to achieve temperature uniformity of adjacent track was investigated. This angle was found to be a function of the power and so on. The simulation results can be used to help the design of parallel recording array head as well as the structure of media.

GeTe materials were characterized using x-ray photoelectron spectroscopy in both the amorphous and crystalline states. Valence and conduction band alignments relative to a SiO2 reference were measured to allow the GeTe band diagram, work function, and electron affinity to be inferred. Hole barrier heights was also studied for several metal/GeTe systems (metal=Al,Ni,W) to extract the charge neutrality level of these interfaces for GeTe in both the crystalline and amorphous states. Near perfect Fermi-level pinning was observed for crystalline GeTe in contact with all of the metals with much less pinning observed for amorphous GeTe.

Phase-change random access memory cells with superlattice-like (SLL) GeTe/Sb(2)Te(3) were demonstrated to have excellent scaling performance in terms of switching speed and operating voltage. In this study, the correlations between the cell size, switching speed and operating voltage of the SLL cells were identified and investigated. We found that small SLL cells can achieve faster switching speed and lower operating voltage compared to the large SLL cells. Fast amorphization and crystallization of 300 ps and 1 ns were achieved in the 40 nm SLL cells, respectively, both significantly faster than those observed in the Ge(2)Sb(2)Te(5) (GST) cells of the same cell size. 40 nm SLL cells were found to switch with low amorphization voltage of 0.9 V when pulse-widths of 5 ns were employed, which is much lower than the 1.6 V required by the GST cells of the same cell size. These effects can be attributed to the fast heterogeneous crystallization, low thermal conductivity and high resistivity of the SLL structures. Nanoscale PCRAM with SLL structure promises applications in high speed and low power memory devices.

A phase change memory device integrated with a nickel monosilicide (NiSi) bottom electrode and a dielectric (Ta2O5) interlayer was investigated. The presence of a low thermal conductivity thin film between the bottom electrode and phase change layer promotes heating efficiency in the device. Reset voltages down to 2.2 and 1.86 V could be achieved for memory device without and with the Ta2O5 interlayer, respectively. In addition, low reset current of 0.66 mA and SET current of 0.2 mA were obtained for devices with Ta2O5 interlayer having a contact dimension of ∼1 μm. Endurance of the devices was also studied.

The energy band alignment between stoichiometric phase change alloys residing along the pseudobinary line of GeTe–Sb2Te3[(GeTe)x(Sb2Te3)1−x] and SiO2 was obtained employing high-resolution x-ray photoelectron spectroscopy. The valence band offsets were determined using both the core-level spectra and valence band spectra in the analysis. The results obtained show that the band offsets vary with the composition of the (GeTe)x(Sb2Te3)1−x alloy, exhibiting a parabolic dependence on the amount of GeTe in the alloy. Increasing the proportion of GeTe in the (GeTe)x(Sb2Te3)1−x alloy was generally found to increase (decrease) the valence band (conduction band) offsets, while the binary alloys (GeTe, Sb2Te3) have similar band offset values. This information could be useful for phase change memory device design and optimization.

Phase change random access memory (PCRAM) cells utilizing nickel monosilicide (NiSi) or platinum monosilicide (PtSi) as the bottom electrode as well as a heater material was demonstrated. Electrical and simulation results demonstrate the feasibility of employing silicides as a bottom electrode/heater in a PCRAM. The memory cells fabricated attained promising results such as low programming currents and sufficient resistance ratio between the crystalline (SET) and amorphous (RESET) states. A low RESET current of 0.8 mA and a SET current of 0.2 mA were obtained for contact dimensions of ∼1 μm, while a resistance ratio of 2 orders of magnitude could be achieved employing PtSi as the bottom electrode. This work therefore enables the integration of PCRAM directly on the silicided drain regions of field effect transistors, facilitating compact integration in complementary metal-oxide-semiconductor (CMOS) technology with reduced process complexity and cost.

Superlatticelike (SLL) dielectric comprising of Ge2Sb2Te5 and SiO2 was employed to reduce the power and increase the speed of phase-change random access memories (PCRAMs). In this study, we found that PCRAM cells with SLL dielectric require lower currents and shorter pulse-widths to switch compared to the cells with SiO2 dielectric. As the thickness of the SLL period reduces, the power and speed of the cells improved further due to the better thermal confinement of the SLL dielectric. Fast phase-change in 5 ns was observed in large cells of 1 μm, showing the effectiveness of SLL dielectric for advanced memory applications.

We study the dependence of the hole barrier height at the metal/α-Ge2Sb2Te5 interface as a function of nitrogen doping in Ge2Sb2Te5 as well as the vacuum work function of the metal. Materials parameters such as the band gap, dielectric constant, and electron affinity values of these nitrogen-doped films were also determined. All Ge2Sb2Te5 films studied in this work are amorphous. Following further physical analysis, the effective work functions of metals on nitrogen-doped Ge2Sb2Te5 films were obtained and found to differ from that of their values in vacuum. This led to the extraction of the slope parameter Sx and charge neutrality level ΦCNL which characterizes Ge2Sb2Te5. Appreciable metal Fermi-level pinning to the charge neutrality level of Ge2Sb2Te5, which is located near the valence band edge, was observed. We then demonstrate application of the extracted parameters to obtain the band alignment of α-Ge2Sb2Te5 with various other materials such as SiO2, giving good agreement with experimental results.

Large-area asymmetric split ring resonator (ASRR) arrays are fabricated by femtosecond laser microlens array lithography. The double square ASRR consists of two gaps at the outer edge. The resonance properties of this ASRR under structural symmetry breaking conditions are investigated in the terahertz region by changing gap size at one side. The resonance of the ASRR shows blueshift from 0.85 to 0.98 THz when the one gap size increases from 2 to 14 μm. Numerical simulations reveal that the electric field enhancement at the fixed gap increases from 152 to 206 times as the degree of asymmetry increases.

Parasitic capacitance has increasing implications on the programming performance of phase change random access memory (PCRAM) devices due to increased scaling and high frequency operation. PCRAM devices with larger parasitic capacitance were found to require higher applied voltage to amorphize due to a larger leakage current. The quenching time is also increased due to a longer voltage fall time during amorphization, resulting in a partially crystallized amorphous state. This partial amorphous state requires a lower applied voltage for crystallization, which means improved crystallization performance at the expense of amorphization. Multilevel devices could be implemented by varying the parasitic capacitance to achieve different amorphous resistance.

The metal/α-Ge2Sb2Te5 interface was examined using x-ray photoelectron spectroscopy. Doping Ge2Sb2Te5 with nitrogen leads to an increase in hole barrier height at the interface between metals and nitrogen-doped Ge2Sb2Te5. Hole barrier height at metal/α-Ge2Sb2Te5 interface is reduced slightly by increasing the work function of the metal. We observed significant pinning of metal Fermi level toward the valence band energy of undoped or nitrogen-doped Ge2Sb2Te5. This leads to low hole barrier height and good Ohmic contact formed between metals and α-Ge2Sb2Te5.

The transient current waveform during a crystallization process in a phase change memory device was measured and analyzed. It revealed two important time parameters, which were termed as delay time and current recovery time. A link between this transient phase change effect and its crystallization kinetics was established. The delay time was found to be the minimum pulse duration before an onset of resistance change. The current recovery time was the time the device takes to complete its transition from high resistance to low resistance. Real-time crystallization characterization was applied to demonstrate the differences between nucleation and growth dominated materials used in the devices.

A series of superlatticelike (SLL) structure incorporated with two phase-change materials GeTe and Sb7Te3 was applied in lateral phase change memory. Power consumption and lifetime were used as two criteria to optimize the SLL structure. It was found that with the thickness ratio of GeTe to Sb7Te3 at 1.6, the RESET current could be as low as 1.5 mA and the endurance could reach as high as 5.3×106 cycles. By varying the thickness ratio of GeTe to Sb7Te3, the crystallization temperature of SLL structures and the performance of lateral phase change memory with these SLL structures can be controlled.

We have explored a different method to increase the phase switching speed for the nanocell of the non-volatile phase change random memories. The correlation between the nanocell size and the switching speed has been investigated theoretically, and the ultrafast phase switching has been demonstrated experimentally. The ultrafast switching mechanisms are discussed which are due to the contribution of free carriers and defects at the material interface when the cell dimension is sufficiently small. The nanoscaling effects of the phase change materials not only reduce the programming power, but also provide a new approach to enhance switching speed, which is highly essential for the realization of universal memory devices.

The electronic property for a series of nitrogen-doped Ge2Sb2Te5 phase change material was characterized using high-resolution x-ray photoelectron spectroscopy. The Te 3d5/2 and Si 2p core-level spectra as well as valence band spectra were used in the analysis. As the nitrogen content increases, the valence band offset also decreases, while that of the conduction band increases. Our results show that the valence band and conduction band offsets of nitrogen-doped Ge2Sb2Te5 on silicon oxide exhibit a linear dependence on nitrogen content in the film, for nitrogen content of up to 8.4 at. %.

We have studied crystallization-induced stress in phase change films (Ge2Sb2Te5) as a function of thickness and with and without a capping layer, by measuring the deflection of microcantilevers. The stress is found to increase with decreasing film thickness. A thin dielectric capping layer leads to a further increase in stress compared to uncapped films. This observation can be explained by the suppression of stress relaxation in the phase change film in the presence of a capping layer. High stress will affect device performance as the size of phase change memory cells decreases.

The influence of Sn doping on the crystallization and thermal stability of Ge2Sb2Te5 phase change material was studied. Thermal analysis shows that the phase change of Sn7.0Ge20.6Sb20.7Te51.7 occurs slightly higher than that of Ge2Sb2Te5 at 154 °C and has a lower melting point at 536 °C. The activation energy for crystallization for this material is also higher. It has a face-centred-cubic crystal structure. A fast crystallization speed of 60 ns is realized upon irradiation by a blue laser beam of 405 nm. The use of the Sn7.0Ge20.6Sb20.7Te51.7 phase change material as a mask layer in aperture-type super-resolution near-field phase change disk is realized to increase the carrier-to-noise ratio and thermal stability.

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The reversible and fast phase transitions induced by picosecond electrical pulses are observed in the nanostructured GeSbTe materials, which provide opportunities in the application of high speed nonvolatile random access memory devices. The mechanisms for fast phase transition are discussed based on the investigation of the correlation between phase transition speed and material size. With the shrinkage of material dimensions, the size effects play increasingly important roles in enabling the ultrafast phase transition under electrical activation. The understanding of how the size effects contribute to the phase transition speed is of great importance for ultrafast phenomena and applications.

Band alignment of amorphous Ge2Sb2Te5 and various substrates was obtained using high-resolution x-ray photoelectron spectroscopy. The valence band offset of Ge2Sb2Te5 on various complementary-metal-oxide-semiconductor (CMOS) materials, i.e., Si, SiO2, HfO2, Si3N4 and NiSi, were investigated with the aid of the core level, valence band, and energy loss spectra. Energy band lineups of Ge2Sb2Te5 on these materials were thus determined which can be used as for phase change memory device engineering and integration with CMOS technology.

Different definitions of spot size and depth of focus in optical data storage systems are analyzed and compared numerically. It ap- pears that the differences between the definitions become more signifi- cant as the numerical aperture of the optical system increases. The re- lationship between spherical aberration and axial intensity is studied, and a general definition of the depth of focus based on this analysis is proposed. © 2007 Society of Photo-Optical Instrumentation Engineers.

Sn-doped Ge–Sb–Te material was prepared by laser synthesis. It has a rocksalt crystal structure for Sn doping content less than 30at.%. A phase change temperature tester was developed to in situ measure crystallization temperature and melting point of Sn-doped Ge–Sb–Te. The crystalliza-tion temperature of Sn-doped Ge–Sb–Te is close to that of Ge2Sb2Te5 while its melting point is much lower than that of Ge2Sb2Te5. The melting points of Sn9.8Ge20.3Sb28.4Te41.5 and Sn18.8Ge19.5Sb25.3Te36.4 are 475 and 450°C, respectively. The crystallization speed was tested by an ultraviolet light at pulse duration of 30ns. It exhibits a high crystallization speed.

As potential high-k gate dielectric material for metal-oxide-semiconductor field effect transistor, the TiO2 incorporated Al2O3 [(TiO2)x(Al2O3)1−x (TAO)] films have been prepared by pulsed laser deposition. The thermal stability and electrical properties of the TAO films annealed at different temperatures have been systematically investigated by x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and capacitance-voltage measurements. These results show that the TAO films keep amorphous state after annealed at 900°C, and the interfacial reaction between the TAO film and silicon substrate is closely related to the deposition and postannealing temperatures. The dielectric constant of the film is about 30. For a 5nm TAO film after annealed at 400°C, the thickness between interfacial layer and the silicon substrate is only two atomic layers (about 0.6nm) and a small equivalent oxide thickness (1.2nm) is achieved. A possible explanation for interfacial interaction has been propos...

A light spot that is smaller than a half wavelength will subsequently diverge in all directions. In this letter, the authors model a subwavelength (0.42λ) super-resolution light beam which propagates over a long distance without any divergence. This can be achieved by placing a multibelt pure-phase-type binary optical element on the lens pupil. The authors also report a useful approach for designing the optical element, based on vector diffraction theory, which can be used in paraxial and nonparaxial focusing and imaging systems.

A superlattice-like structure (SLL) incorporating two nonpromising phase change materials was applied to phase change random access memory (PCRAM) cell. A properly designed SLL structure could balance both the phase change speed and stability of a PCRAM. Moreover, SLL PCRAM cells exhibited lower programming current and fast working time of 5ns. The main reason for the excellent performances is due to the much lower thermal conductivity of the SLL material compared to that of bulk materials. The thermal conductivity of eight SLL layers cycle was found to be smaller than 30% of that of single layer material.

The phase change memory cells were developed by using a combination system of a femtosecond laser with near-field scanning optical microscopy. The memory cells with feature size varying from 800nm down to 90nm were achieved. The cell functional performances were tested, and the scalability of the programming current as a function of the memory cell features was investigated. The optical near-field distance which is one of the critical factors to achieve high resolution nanostructures was studied experimentally with the consideration of the whole fabrication process for functional devices. The Bethe-Bouwkamp model was employed to study the effects of the optical near-field distance to the nanostructure geometry. The programming current of 0.8mA was observed for the memory nanocell at a feature size of 90nm.