Youn-Seon Kang

Thermoelectric heating of Ge2Sb2Te5 in phase change memory devices

We report on the demonstration of the active thermoelectric application to nanometer-scaled semiconductor devices. The thermoelectric heating already exists during programming in conventional phase change memory (PRAM) cells, which is only a minor supplement to Joule heating. Here, by rigorously designing devices, we have demonstrated an unprecedentedly high efficiency of PRAM, where the majority of the heat is supplied by the thermoelectric effect.

Nonvolatile switching characteristics of laser-ablated Ge2Sb2Te5 nanoparticles for phase-change memory applications

Electrical characteristics of Ge2Sb2Te5 (GST) nanoparticles have been examined for a phase-change memory applications. The GST nanoparticles were generated by in situ pulsed laser ablation and their crystal structure formation was confirmed [H. R. Yoon et al., J. Non-Cryst. Solids 351, 3430 (2005)]. A stacked structure of the GST nanoparticles with 10nm of average diameter shows reversible nonvolatile switching characteristics between a high resistance state and a low resistance state as in the phase-change memory consisting of bulk GST thin film. Experimental results indicate that it is highly probable to test scaling issues of the phase-change memory with well-defined GST nanoparticles.

Rapid crystallization of GeTe–Bi2Te3 mixed layer

We report rapid crystallization of GeTe–Bi2Te3 mixed layers. The as-deposited (GeTe)1−x(Bi2Te3)x (GBT) layers with x>0.5 are fcc crystalline, while the layers with x<0.5 are amorphous, for cosputter deposition at room temperature. We found that Bi2Te3 significantly enhances the crystallization of the GBT layers. Furthermore, both temperature and minimum time required for crystallization (Tc and tc,min) of GBT layers are smaller than those of (GeTe)1−x(Sb2Te3)x (GST) layers. For example, crystallization of GBT layer with x=0.12 occurs at 155.0°C within 30.9ns, which is around 1∕3 of 95.7ns for Ge2Sb2Te5 with Tc=168.5°C.

Effect of indium on phase-change characteristics and local chemical states of In–Ge–Sb–Te alloys

We introduce single-phase In–Ge–Sb–Te (IGST) quaternary thin film (fcc structure when crystallized) deposited by cosputtering from Ge2Sb2Te5(GST) and In3Sb1Te2 targets. This film, compared with the GST ternary system, provides a significant increase of amorphous-to-crystalline transformation temperature. High-resolution x-ray photoelectron spectroscopy (HRXPS) revealed that, with increasing In amounts, the Sb 4d and Ge 3d core peaks shift toward lower binding energies (BEs), with negligible changes in spectral linewidths, whereas the In 4d and Te 4d core peaks show insignificant changes in BEs. HRXPS interpretation suggests that the Na site in IGST can be occupied by Te, Sb, In, and vacancy, whereas in GST it is occupied only by Te.

Reliability perspectives for high density PRAM manufacturing

This paper discussed the key reliability issues for manufacturing high density phase change memory (PRAM). There are its own unique phenomena, such as resistance fluctuation, structural relaxation and crystallization, which are closely correlated with the device reliability characteristics, including data retention, cycling endurance, and write disturbance. Optimizing material composition and controlling doping concentration and minimizing variability of physical dimensions can improve the reliability issues. Above all, isotropic dimension scaling along with writing current scaling is essential for continuing scaling down below 20nm node.

Valence band structures of the phase change material Ge2Sb2Te5

We report the experimental evidence of significant change of the valence band structure during crystallization of Ge2Sb2Te5 (GST). Amorphous GST, prepared by sputter deposition at room temperature (RT), transforms successively into face-centered-cubic (fcc) and a hexagonal-close-packed (hcp) structures at around 150 and 300°C, respectively, during a stepwise temperature increase from RT to 350°C. During temperature increase, ultraviolet photoemission spectra were in vacuo obtained using synchrotron radiation. The measurement of the amorphous and fcc GST shows that the difference between the maximum valence band edge and the Fermi level reduces by 0.35eV during crystallization. For the fcc to hcp phase transformation, no band gap reduction was observed.

Current status and future prospect of Phase Change Memory

This paper reviews recent progress and future outlook of PRAM as a promising candidate for emerging non-volatile memory. Electrical characteristics and reliability issues of PRAM with scale-down of the device dimension are discussed. Despite remarkable progress of PRAM properties in recent last decades, there are still several fundamental issues to resolve for broadening its application area. Several suggestions to overcome these property issues are introduced with recent experimental results.

Electronic structure of (Ge2Sb2Te5)1−x(In3SbTe2)x investigated by x-ray photoelectron spectroscopy

We have investigated the core levels and the valence band of (Ge2Sb2Te5)1−x(In3Sb1Te2)x quaternary phase system (IGST) by means of x-ray photoelectron spectroscopy. A systematic shift of Sb 3d and Ge 2p core-level peaks toward lower binding energies side was observed with increasing indium amount, whereas the In 3d and Te 3d core peaks showed less change. The Sb 3d and Ge 2p core-level shift is attributed to an increase in the electronic charge of p-electrons dependent of indium amount. The valence band spectra show a distinct change in the sp configuration with indium concentration change. The change in the local bonding as the indium amount increase has a profound impact on both local atomic arrangement and amorphous-to-crystalline transformation temperature. The difference in the photoemission spectra have been discussed according to a simple structural model suggesting that the Na site in IGST can be occupied by Te, Sb, In, and vacancy, whereas in GST it is occupied only by Te.