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The thermal conduction characteristics of GeTe and Ge2Sb2Te5(GST) nanowires were investigated using an optical method to determine the local temperature by Raman spectroscopy. Since the localization of surface charge in a single-crystalline nanostructure can enhance charge-phonon scattering, the thermal conductivity value (κ) of single crystalline GeTe and GST nanowires was decreased significantly to 1.44 Wm(-1) K(-1) for GeTe and 1.13 Wm(-1) K(-1) for GST, compared to reported values for polycrystalline structures. The SET-to-RESET state in single-crystalline GeTe and GST nanowires are characteristic of a memory device. Unlike previous reports using GeTe and GST nanowires, the SET-to-RESET characteristics showed a bipolar switching shape and no unipolar switching. In addition, after multiple cycles of operation, a significant change in morphology and composition was observed without any structural phase transition, indicating that atoms migrate toward the cathode or anode, depending on their electronegativities. This change caused by a field effect indicates that the structural phase transition does not occur in the case of GeTe and GST nanowires with a significantly lowered thermal conductivity and stable crystalline structure. Finally, the formation of voids and hillocks as the result of the electromigration critically degrades device reliability.

This paper firstly reports key factors which are to be necessarily considered for the successful two-bit (four-level) cell operation in a phase-change random access memory (PRAM). They are 1) the writeand-verify (WAV) writing of four-level resistance states and 2) the moderate-quenched (MQ) writing of intermediate resistance levels, 3) the optimization of temporal resistance increase (so-called resistance drift) and 4) of resistance increase after thermal annealing. With taking into account of them, we realized a two-bit cell operation in diodeswitch phase change memory cells with 90nm technology. All of four resistance levels are highly write endurable and immune to write disturbance above 10 8 cycles, respectively. In addition, they are nondestructively readable above 10 7 read pulses at 100ns and 1uA. IntroductionPhase-change random access memory (PRAM) is most promising to realize a multi-level cell (MLC) operation because it has very wide range of resistance across two orders of magnitude or the higher, with respect to writing current. According to the PRAM road map [1], it is expected that highest memory densities of PRAM become comparable to conventional memories such as NOR Flash and DRAM in coming years when MLC operation is fully accomplished. In this paper, we systematically investigated a four-level (two-bit) cell operation in diode-switch phase change memory cells with 90nm technology and discussed its possibilities and issues as well.

The chemical state of N in N-doped amorphous Ge2Sb2Te5 (a-GST) samples with 0–14.3Nat.% doping concentrations was investigated by high-resolution x-ray photoelectron spectroscopy (HRXPS) and Ge K-edge x-ray absorption spectroscopy (XAS). HRXPS showed negligible change in the Te 4d and Sb 4d core-level spectra. In the Ge 3d core-level spectra, a Ge nitride (GeNx) peak developed at the binding energy of 30.2eV and increased in intensity as the N-doping concentration increased. Generation of GeNx was confirmed by the Ge K-edge absorption spectra. These results indicate that the N atoms bonded with the Ge atoms to form GeNx, rather than bonding with the Te or Sb atoms. It has been suggested that the formation of Ge nitride results in increased resistance and phase-change temperature.

We observed the atomic structures for each reset and set state in a phase-change random access memory fabricated using stoichiometric crystalline Ge 2 Sb 2 Te 5 . The reset state clearly showed a mixture of dome-shaped amorphous and crystal structure surrounding amorphous, but the set state showed abnormally grown large grains due to recrystallization of the amorphous structure. The crystal structure of the recrystallized grain was face-centered cubic. The element analysis indicated that the atomic composition changes to nonstoichiometric phase in the active regions of the reset and the set state, which is Sb-rich and Te-deficient compared to the pristine stoichiometric composition. Analysis showed that thermal interdiffusion of Sb and Te caused nonstoichiometric nature of the material to reach the energetically stable state in the active region.Great efforts to develop next generation memories with fewer technical barriers, longer lifetime, and better performance in comparison with existing memories, such as flash, and dynamic-static random access memories, have been attempted. Recently, there has been a great deal of interest in phase-change random access memory ͑PRAM͒ based on stoichiometric crystalline Ge 2 Sb 2 Te 5 ͓referred to as GST ͑2/2/5͔͒ films. PRAM takes advantage of electrically bistable status of resistance difference between its amorphous ͑reset state with high resistance͒ and crystalline phase ͑set state with low resistance͒. 1,2 Amorphous resistance is at least 10 2 times higher than crystalline resistance. Hence, PRAM is considered to be one of the promising memories owing to its nonvolatility, good scalability, large sensing signal, fast reading and moderately fast writing time, and long data endurance.In the operation, PRAM uses a high current pulse to make the amorphous state, and a low current pulse to get the crystalline state. The current is applied to heat up a contact region between GST and a bottom electrode ͑role of resistive heater͒ using Joule heating. During the high current pulse, the GST loses the polycrystalline ordering as temperature exceeds its melting point. After that, the GST is quenched and stays in a structurally disordered state. The low current pulse and its sufficient duration is used to maintain temperature in the crystallization range for both nucleation and crystallization of the GST. The amorphous and the recrystallized GST right near the contact region is the active region. The typical setreset characteristics of the device in this study are shown in Fig. 1, which is fully integrated in complementary metal-oxidesemiconductor ͑CMOS͒ technology.Because of high current driving operation mechanism, PRAM has suffered from device failure such as delamination. Recently, Ryu et al. reported another important issue regarding compositional alteration of GST͑2/2/5͒ during operation. 3 In their report GST͑2/2/5͒ phase is not stable and has changed to GST͑15/47/38͒. However, they have has not mentioned the detailed phenomena of the GST/TiN contact region, especially element t...

Changes in the electronic structures of Ge2Sb2Te5 (GST) and N-doped Ge2Sb2Te5 film during the phase transition from an amorphous to a crystalline phase were studied using synchrotron radiation high-resolution x-ray photoemission spectroscopy. The changes in tetrahedral and octahedral coordinated Ge 3d peaks are closely related to the changes in the chemical bonding state of GST films. The metallic Sb peak in the Sb 4d spectra of annealed GST films demonstrates that the metallic Sb atoms become segregated during thermal treatment resulting in phase separation. The incorporation of nitrogen into the GST film affects its structure and chemical bonding state, resulting in the suppression of crystallization. The incorporation of nitrogen also increases the optical band gap of the film due to the formation of a nitride.