Hayato Sone

Characterization of nitrogen-doped Sb2Te3 films and their application to phase-change memory

In this study, sputtered undoped and nitrogen doped Sb2Te3 (ST and STN) films were systematically investigated by x-ray diffraction (XRD) and resistance measurements. Their application to lateral phase-change memory (PCM) is presented as well. The STN film sputtered at a flow rate ratio (N2∕Ar) of 0.07 proved to have both high stability and low power consumption, implying its high performance in PCM applications. In the STN films (N2∕Ar>0.15), the hexagonal Te phase first appeared at 160 °C, and then the orthorhombic SbN phase appeared at 290 °C. The phase separation made it very difficult for these films to switch reversibly between the crystalline and the amorphous phase.

Picogram Mass Sensor Using Resonance Frequency Shift of Cantilever

A microsensor with a picogram mass sensitivity has been researched using the resonance frequency shift of an atomic force microscope (AFM) cantilever. The mass change of the water molecules adsorbed on the cantilever was measured by analyzing the resonance frequency shift using a laser beam deflection detection system. As experimental results, a sensitivity of about 2 pg/Hz was obtained. The mass change due to increasing humidity showed that the mechanism of water molecular growth changed from an island growth to a layer growth. The critical point of the mechanism change was a humidity of about 60%. The adsorbed mass corresponds to a water layer of about 12 monolayers.

A Novel Lateral Phase-Change Random Access Memory Characterized by Ultra Low RESET Current and Power Consumption

We have fabricated and studied single lateral phase-change random-access-memory (PRAM), which has a confined Ge2Sb2Te5 (GST) channel connected by two wide TiN electrodes of relatively low resistivity. Its switching current for Reset operation could be as low as 4–20 µA, about one or two orders of magnitude lower than that of the conventional bottom contact PRAM cell. Its corresponding switching power for Reset operation is about 2–4 µW. The reason for such ultra low Reset current and power could be that Joule heating occurred mainly in the GST channel, instead of the resistive heater in the conventional PRAM cell.

Simulation of proposed confined-chalcogenide phase-change random access memory for low reset current by finite element modelling

A confined-chalcogenide (CC) cell structure for reducing the reset current of phase-change random access memory (PRAM) is proposed in this investigation. Both single normal-bottom-contact (NBC) (for reference) and proposed CC PRAM cells are simulated by two-dimensional finite element modelling. The simulated amorphous region of the NBC cell after reset operation is generally a semiellipse, which agrees very well with the reported experimental results. The CC cell has a rectangular amorphous region after reset operation. The reset operation current of the CC cell is much lower than that of the NBC cell. The CC cell structure needs a low reset current and a low power consumption and has a simple configuration.

Dependences of Electrical Properties of Thin GeSbTe and AgInSbTe Films on Annealing

We studied the electrical properties of 20- and 50-nm-thick Ge2Sb2Te5 and AgInSbTe films for nonvolatile lateral transistor memory devices. Both kinds of thin films were prepared as film samples and device samples which were then annealed at temperatures from 140 to 415°C. It is known that crystal size can be effectively reduced with film thickness on the basis of X-ray diffraction analysis. The resistances of all film samples annealed at 140–415°C decreased by approximately 5–6 orders of magnitude. In the case of device samples, however, the source-drain resistances of Ge2Sb2Te5 samples were first reduced and then reversely increased and it seemed that the resistances of AgInSbTe samples did not drop. The abnormal resistance increase above the crystallization temperature may be caused by phase change and thermal expansion, as we analyzed in this paper. Finally, the resistance changes of device samples with channel lengths in the range of 0.4–3 µm were discussed from the point of view of miniaturizing the phase change memory device.

Finite Element Analysis of Dependence of Programming Characteristics of Phase-Change Memory on Material Properties of Chalcogenides

The programming characteristics of a phase-change memory (PCM) cell with a chalcogenide layer contacted by a resistive heater are investigated by finite element modelling. As analyzed in this study, the characteristics are markedly affected by the resistivity of the phase-change chalcogenide material. A higher reset current of 1.6 mA is required for the as-fabricated virgin PCM than that of 1.3 mA for the cycled PCM because of the resistivity difference of the chalcogenides in the two cases. More importantly, a chalcogenide layer with a much higher resistivity than the resistive heater is necessarily adopted for a higher energy efficiency to markedly reduce reset current to 0.6 mA or even lower while slightly increasing reset voltage.

Nanosilicon dot arrays with a bit pitch and a track pitch of 25nm formed by electron-beam drawing and reactive ion etching for 1Tbit∕in.2 storage

The formation of very fine Si dots with a bit pitch and a track pitch of less than 25nm using electron-beam (EB) lithography on ZEP520 and calixarene EB resists and CF4 reactive ion etching has been demonstrated. The experimental results indicate that the calixarene resist is very suitable for forming an ultrahigh-packed bit array pattern of Si dots. This result promises to open the way toward 1Tbit∕in.2 storage using patterned media with a dot size of <15nm.

Multilevel Storage in Lateral Top-Heater Phase-Change Memory

A lateral top-heater phase-change memory (LTH-PCM) is proposed and investigated for multilevel storage (MLS). The active layers are composed of a 50-nm-thick TiN layer as a top heater and a 150-nm-thick SbTeN layer. A number of intermediate levels, which are induced by electric currents, are exhibited in experimental results. They are distinct and very stable. The reversible switching of multilevels is successfully demonstrated in the LTH-PCM device. The MLS results from the gradual enlargement of crystalline region between electrodes by Joule heating according to our analysis.

Ultramultiple-level storage in TiN∕SbTeN double-layer cell for high-density nonvolatile memory

We report a phase-change nonvolatile memory (NVM) concept based on a TiN∕SbTeN (N-doped Sb2Te3) double-layer structure, which can be used for ultramultiple-level storage (UMLS). SbTeN shows a gradual resistivity drop and good phase stability with increasing annealing temperature, a characteristic which makes it suitable for UMLS applications. We demonstrate that the number of distinguishable resistance levels can readily reach 16 and even higher. These levels in this study result from the initial threshold switching and the subsequent current-controlled crystallization induced by Joule heating. The latter allows the creation of many distinct levels, thus, enabling the low-cost ultrahigh-density NVM.

Electrical Properties of Phase Change and Channel Current Control in Ultrathin Phase-Change Channel Transistor Memory by Annealing

We studied phase-change channel transistor memory devices with an ultrathin Ge2Sb2Te5 chalcogenide film channel and tried to demonstrate two combined functions (memory: resistance change and selection: channel current control) by annealing in this paper. Drain–source resistance can be markedly decreased by 2 – 3 orders of magnitude after annealing due to the phase change from the amorphous to crystalline phases. A channel current control effect in which the drain current decreases with the gate voltage was clearly observed in 10- and 20-nm-thick Ge2Sb2Te5 devices. The absolute channel current modulation by the gate voltage in the crystalline state is much stronger than that in the amorphous state. Furthermore, the channel current control ability in devices with thin Ge2Sb2Te5 is stronger than that in devices with thick Ge2Sb2Te5. The channel current control effect might result from the potential change of the ultrathin Ge2Sb2Te5 channel by the gate voltage. [DOI: 10.1143/JJAP.45.3238]