Hyang Keun Yoo

Oxide double-layer nanocrossbar for ultrahigh-density bipolar resistive memory.

S. H. Chang , S. B. Lee , S. M. ang , Y S. C. Chae , H. K. oo , Y Prof. W Noh . . TReCFI, Department of Physics and Astronomy Seoul National University Seoul 151-747, Korea E-mail: twnoh@snu.ac.kr D. Jeon , . Y S. J. Park , Prof. G. Kim . TSchool of Electrical Engineering Korea University, Seoul 136-701, Korea Prof. B. S. Kang Department of Applied Physics Hanyang University Ansan, Gyeonggi-do 426-791, Korea Dr. M.-J. Lee Semiconductor Device Laboratory Samsung Advanced Institute of Technology Yongin, Gyeonggi-do 446-712, Korea

Interface-modified random circuit breaker network model applicable to both bipolar and unipolar resistance switching

We observed reversible-type changes between bipolar (BRS) and unipolar resistance switching (URS) in one Pt/SrTiOx/Pt capacitor. To explain both BRS and URS in a unified scheme, we introduce the “interface-modified random circuit breaker network model,” in which the bulk medium is represented by a percolating network of circuit breakers. To consider interface effects in BRS, we introduce circuit breakers to investigate resistance states near the interface. This percolation model explains the reversible-type changes in terms of connectivity changes in the circuit breakers and provides insights into many experimental observations of BRS which are under debate by earlier theoretical models.

Conversion from unipolar to bipolar resistance switching by inserting Ta2O5 layer in Pt/TaOx/Pt cells

We observed unipolar resistance switching in Pt/TaOx/Pt cells. We could make the cell have the bipolar resistance switching by inserting a stoichiometric Ta2O5 layer between Pt and TaOx layers. Bipolar resistance switching in Pt/Ta2O5/TaOx/Pt cells occurred reliably without applying an external compliance current. With increase in the Ta2O5 layer thickness, the current value at the low-resistance state became decreased but the forming voltage became increased. We could explain these intriguing phenomena using the interface-modified random circuit breaker network model.

Reduction in high reset currents in unipolar resistance switching Pt/SrTiOx/Pt capacitors using acceptor doping

The high reset current, IR, in unipolar resistance switching is an important issue which should be resolved for practical applications in nonvolatile memories. We showed that, during the forming and set processes, the compliance current, Icomp, can work as a crucial parameter to reduce IR. Doping with Co or Mn can significantly reduce the leakage current in capacitors made using SrTiOx film, opening a larger operation window for Icomp. By decreasing Icomp with acceptor doping, we could reduce IR in SrTiOx films by a factor of approximately 20. Our work suggests that the decrease in Icomp by carrier doping could be a viable alternative for reducing IR in unipolar resistance switching.

Polaron Transport and Thermoelectric Behavior in La‐Doped SrTiO3 Thin Films with Elemental Vacancies

The electrodynamic properties of La-doped SrTiO 3 thin fi lms with controlled elemental vacancies are investigated using optical spectroscopy and thermopower measurement. In particular, a correlation between the polaron formation and thermoelectric properties of the transition metal oxide (TMO) thin fi lms is observed. With decreasing oxygen partial pressure during the fi lm growth ( P (O 2 )), a systematic lattice expansion is observed along with the increased elemental vacancy and carrier density, experimentally determined using optical spectroscopy. Moreover, an absorption in the mid-infrared photon energy range is found, which is attributed to the polaron formation in the doped SrTiO 3 system. Thermopower of the La-doped SrTiO 3 thin fi lms can be largely modulated from ‐120 to ‐260 µV K −1 , refl ecting an enhanced polaronic mass of ≈3 < m polron / m < ≈4. The elemental vacancies generated in the TMO fi lms grown at various P (O 2 ) infl uences the global polaronic transport, which governs the charge transport behavior, including the thermoelectric properties.

Time-dependent current-voltage curves during the forming process in unipolar resistance switching

We investigated the time-dependent current-voltage curves of the forming process in unipolar resistance switching. We applied triggered-voltage triangular-waveform (pulse-waveform) signals with varied sweep rate (amplitude) to Pt/SrTiOx/Pt capacitors. By investigating their temperature dependences, we found that the forming process was driven by two different mechanisms, depending on the sweep rate (amplitude): a purely electrical dielectric breakdown and a thermally assisted dielectric breakdown. For the latter process, we observed precursory changes in the current I(t) before the forming process. By fitting the time-dependent precursory changes with I(t)=Io−A exp(−t/τ), we suggest that the thermally activated migration of oxygen vacancies/ions could help the thermally assisted dielectric breakdown.

Impact of vacancy clusters on characteristic resistance change of nonstoichiometric strontium titanate nano-film

In practical applications to bipolar resistance switching (BRS) memory devices with enhanced performance and high-scalability, oxide materials are commonly fabricated to highly nonstoichiometric and nanometer scale films. In this study, we fabricated ultrathin strontium titanate film, which shows two types of BRS behavior. By using micro-beam X-ray photoemission spectroscopy, the changes of core-level spectra depending on the resistance states are spatially resolved. Experimental and calculated results demonstrated that the fundamental switching mechanism in the two types of BRS is originated from the migration of anion and cation vacancies and the formation of insulating vacancy clusters near vicinity of the interface.

Stabilizing the forming process in unipolar resistance switching using an improved compliance current limiter

The high reset current IR in unipolar resistance switching is a major obstacle to practical applications in memory devices. In particular, the first IR value after the forming process is so high that the capacitors sometimes do not exhibit reliable unipolar resistance switching. We find that the compliance current Icomp is a critical parameter for reducing IR values in polycrystalline Pt/NiOw/Pt, Pt/SrTiOx/Pt, Ti/SrTiOx/Pt, Pt/TiOy/Pt and Pt/FeOz/Pt capacitors, which show unipolar resistance switching. We therefore introduce an improved, simple and easy-to-use Icomp limiter that stabilizes the forming process by drastically decreasing the current overflow so as to precisely control the Icomp and subsequent IR values.

Spatially Resolved Electronic Properties of Single-Layer WS2 on Transition Metal Oxides

There is a substantial interest in the heterostructures of semiconducting transition metal dichalcogenides (TMDCs) among each other or with arbitrary materials, through which the control of the chemical, structural, electronic, spintronic, and optical properties can lead to a change in device paradigms. A critical need is to understand the interface between TMDCs and insulating substrates, for example, high-κ dielectrics, which can strongly impact the electronic properties such as the optical gap. Here, we show that the chemical and electronic properties of the single-layer (SL) TMDC, WS2, can be transferred onto high-κ transition metal oxide substrates TiO2 and SrTiO3. The resulting samples are much more suitable for measuring their electronic and chemical structures with angle-resolved photoemission than their native-grown SiO2 substrates. We probe the WS2 on the micron scale across 100 μm flakes and find that the occupied electronic structure is exactly as predicted for free-standing SL WS2 with a strong spin-orbit splitting of 420 meV and a direct band gap at the valence band maximum. Our results suggest that TMDCs can be combined with arbitrary multifunctional oxides, which may introduce alternative means of controlling the optoelectronic properties of such materials.

Latent instabilities in metallic LaNiO3 films by strain control of Fermi-surface topology

Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by misfit strain to change its fermiology. As tensile strain increases, the hole pocket centered at the A point elongates along the kz-axis and seems to become open, thus changing Fermi-surface (FS) topology from three- to quasi-two-dimensional. Concomitantly, the FS shape becomes flattened to enhance FS nesting. A FS superstructure with Q1 = (1/2,1/2,1/2) appears in all LNO films, while a tensile-strained LNO film has an additional Q2 = (1/4,1/4,1/4) modulation, indicating that some instabilities are present in metallic LNO films. Charge disproportionation and spin-density-wave fluctuations observed in other nickelates might be their most probable origins.