AVS2013 Session EM+PS-WeM: Oxides and Dielectrics for Novel Devices and Ultra-dense Memory II

Flash memories and DRAM are ubiquitous today. However, a potential leap beyond the limits of Flash (with respect to write speed, write energies) and DRAM (with respect to scalability, retention times) emerges from nanoionic redox-based switching effects encountered in metal oxides (ReRAM). A range of systems exist in which ionic transport and redox reactions on the nanoscale provide the essential mechanisms for memristive switching. One class relies on mobile cations which are easily created by electrochemical oxidation of the corresponding electrode metal, transported in the insulating layer, and reduced at the inert counterelectrode (so-called electrochemical metallization memories, ECM). Another important class operates through the migration of anions, typically oxygen ions, towards the anode, and the reduction of the cation valences in the cation sublattice locally providing metallic or semiconducting phases (so-called valence change memories, VCM). The electrochemical nature of these memristive effects triggers a bipolar memory operation. In yet another class, the thermochemical effects dominate over the electrochemical effects in metal oxides (so-called thermochemical memories, TCM) which leads to a unipolar switching as known from the phase-change memories. In all systems, the defect structure turned out to be crucial for the switching process.

The presentation will cover fundamental principles in terms of microscopic processes, switching kinetics and retention times, and device reliability of the VCM-type ReRAM variant. It will describe what can be learnt about the nature of the redox-based switching process on open surfaces (e.g. system with top electrodes removed) and embedded electrode interface.

Despite exciting results obtained in recent years, several challenges have to be met before these physical effects can be turned into a reliable industrial technology.

Materials exhibiting reversible resistance changes are essential elements of resistance random access memory (R-RAM), which has a simpler structure, lower energy consumption, higher operating speed and higher endurance than conventional RAM. The monoclinic transparent conducting oxide β-Ga₂O₃ is a promising candidate for R-RAM due to its open-channel structure that enables large scale defect migration. The resistive switching mechanism in pulsed-laser-deposited Ga₂O₃ was investigated by combining electrical measurements with x-ray diffraction (XRD) and sputter-profiling x-ray photoemission spectroscopy (SP-XPS), revealing a strong correlation of oxide-metal interface conductance with electrically and thermally driven defect migration and agglomeration near the interface.

Electrically-activated reversible resistance switching is observed in thin-film Ni/Ga₂O₃/Ir, while irreversible changes can be observed upon annealing either single-crystal or thin film gallium oxide. Room-temperature Ni deposition on single crystal β-Ga₂O₃ results in a rectifying contact with barrier height ~ 0.8 eV and SP-XPS reveals no interface reaction; annealing this structure to 500°C irreversibly creates an ohmic contact, as well as oxidized Ni and reduced Ga at the interface.

A reversible resistance change can be triggered by an electrical pulse in polycrystalline gallium oxide films grown by pulsed laser deposition. Ni/Ga₂O₃/Ir heterostructures were fabricated and then investigated at different points in their electrical cycling history. Application of less than about 10 V across a 100 nm film (~10⁶ V/cm) maintains the initial Schottky behavior; a 1 sec, 30 V electric pulse switches the metal/oxide contact from Schottky to Ohmic and increases the device conductance by two orders of magnitude. X-ray diffraction shows the film recrystallizes into α and β phases of Ga₂O₃; further electric field treatment increases the β-phase fraction. Sputter-profiling XPS shows an increase in the near-surface Ga:O ratio and introduction of reduced Ga within 2 nm of the metal-oxide interface. The film remains Ohmic under low voltage cycling, but a high-voltage pulse with the opposite polarity both reverses the interface chemical changes and reverts the electrical characteristic to a Schottky contact. Further cycling between Ohmic and Schottky behavior continues with additional voltage pulses. The results are consistent with Ga interstitial migration and/or an interface redox reaction.

This project is supported by the National Science Foundation under DMR 1104628 and the Micron Foundation.

High pressure annealing (HPA) is known as an effective way to control the fundamentals of oxide system through the modification of stoichiometry, thermal decomposition, and compression.[1] Here, we report the effect of N₂ HPA on NiO_x based resistive random access memory (RRAM) device in terms of applied pressures in NiO_x system. In this research, the annealing temperature was fixed at 350 ^oC while the applied pressures were varied to 1, 20, and 50 atm. Pt was commonly used as bottom and top layer of metal-insulator-metal structure. As the N₂ pressure increased, the on- and off- resistance ratio was decreased from ~10⁵ to ~10⁴. However, the operation voltages (reset and set voltage) were reduced followed by increment of N₂ pressure. Specifically, the 50 atm HPA sample shows the lowest reset voltage of 0.95 V and set voltage of 2.12 V. This result implies that enhanced grain size was induced by the N₂ pressure as the grain boundaries are preference sites for conduction filament formation.[2] Furthermore, notable increment of non-lattice oxide component was confirmed which may cause the reduced driving voltages by x-ray photoelectron spectroscopy. These findings can enhance the understanding of low-power driving RRAM for next generation memory device.

References:

[1] Y. S. Rim, W. H. Jeong, D. L. Kim, H. S. Lim, K. M. Kim and H. J. Kim, J. Mater. Chem. 22, 12491 (2012).

[2] S. Kim, D. Lee, J. Park, S. Jung, W. Lee, J. Shin, G. Choi, and H. Hwang, Nanotech. 23, 325702 (2012).

Semiconductor conduction band edge states (CBES), and symmetries and singlet transport and/or trapping states of vacated O-atom defects are quantitatively different for nc-Si(Ge)O₂ and nano-grain (ng-) TM oxides. CBES in Si(Ge)O₂ and Si/Ge an zinc-blende III-V semiconductors are "s-like" with high electron mobilities. Combined with intrinsic band edge triplets in SiO₂, energies of vacated O-atom singlet defect states are deeper into the band-gap. Injection into, and transport through singlet defects localized in 1 nm nc-SiO₂ clusters with "crystalline-like" medium range order (MRO) extends to self-organized symmetric dihedral angles. Hopping transport through vacated O-atom sites is determined by percolation. It is l\eiminated through nc-Si(Ge)O₂ interfacial transition regions by the 0.6 eV spectral extent of the intrinsic band edge triplets explaining the absence of bulk negative space-charge in devices, and after accelerated bias-controlled stress testing. CBES and vacated O-defects in X-ray absorption spectra (XAS).

In thicker films O-vacancy singlet states are below CBES with bias-controlled injection and transport. Combined with metal gates, and n- and p-type doping, for Si, Ge and III-V semiconductor substrates, memory and switching functionality is established. 300° CVD devices are obtained in RPD ~2 nm thick TiO₂ or HfO₂ onto nitrided Ge substrates with ~0.5 nm of Ge-N detected by in-line AES. 400-500°C post-deposition annealling removes Ge-N bonds at nitrided interfaces, and provides a template for deposition of nc- and ng-dielectrics. Nc-HfSiON on Si-passivated and nitrided Ge (100) and ng-TiO2 yield Dit levels of 2-5x10¹⁰ cm^-2. C-V characteristics are symmetric with respect to flat-band voltages, V_FB yielding fixed charge levels ~2-3x10¹¹ cm^-3. J-V characteristics yield currents at 1 V > V_FB, of 3-5x10^-6 A-cm^-2. TiO2 capacitors on Ge have EOT values of ~ 4.5 nm, as well as low values of Dit .

[1] G. Lucovsky, et al: Many-Electron Multiplet Theory Applied to O-Vacancies in (i) Nanocrystalline HfO2 and (ii) Non-crystalline SiO2 and Si Oxynitride Alloys, Progress in Theoretical Chemistry and Physics 23 (2010) 193-211.

[2] G. Lucovsky, D. Zeller, Remote Plasma Enhanced Chemical Deposition of Non-Crystalline GeO2 on Ge and Si Substrates, Journal of Nanoscience and Technology 11 (2011) 7974-7981

[3] Y. Tanabe, S. Sugano, On the absorption spectra of complex ions, Journal of the Physical Society of Japan (1956) 11 (8) (1956) 864–877; 9(5) (1954) 9(5) 766-779.

Thin film metal-insulator-metal (MIM) tunnel devices have seen renewed interest for high speed applications such as infrared (IR) detectors, optical rectennas for energy harvesting, and hot electron transistors. For many of these applications, desired properties include high asymmetry (η) and non-linearity (f_NL) of current vs. voltage (I-V) behavior and small turn-on voltage (V_ON). The standard approach to achieving these characteristics in tunnel devices is M₁IM₂ diodes - the use of electrodes with different workfunctions to induce a built-in field across the insulator. V_ON is influenced by the choice of the dielectric layer. In theory, small band-gap dielectrics with large electron affinity (χ) are desired to achieve small V_ON in tunnel diodes as they make small energy barriers with electrodes. Regarding this, Nb₂O₅ and Ta₂O₅ are widely considered to be promising candidates as tunnel dielectrics. In this work, we investigate Nb₂O₅ and Ta₂O₅ MIM diodes. Atomic layer deposition (ALD) was used to deposit 5 nm and 10 nm thick Nb₂O₅ and Ta₂O₅ tunnel barriers with sputtered amorphous bottom electrodes (ZrCuAlNi) and evaporated Al top electrodes. The I-V responses were found to be asymmetric for diodes made with 5 nm and 10 nm of either of these dielectrics. Although a lower V_ON was observed, the maximum asymmetry was 3 orders of magnitude smaller than what we previously reported for similar diodes with ALD Al₂O₃ as the dielectric layer. High speed operation of MIM devices is typically based on Fowler-Nordheim tunneling (FNT). Conduction mechanisms were investigated as a function of temperature and electric field. By fitting I-V curves to FNT, Schottky emission (SE), and Frenkel-Poole (FP) emission plots, the dominant conduction mechanism in these diodes was found to be SE in the small bias regime (0.1 V to 0.3 V) and FP emission in the large bias regime (≥ 0.75 V). These assignments were confirmed by the close match of the optical dielectric constants extracted from the SE and FP plots with spectroscopic ellipsometry. Finally, Arrhenius plots show temperature dependence of current at both small and large bias regimes, indicating that tunneling is not the dominant conduction mechanism. In conclusion, we find that the small low-voltage asymmetric I-V behavior in MIM diodes made with Nb₂O₅ and Ta₂O₅ dielectrics is due to SE, rather than FN tunneling. A comparison will be made to bi-layer dielectric MIIM diodes, which we recently reported to show improved low voltage η and f_NL. This work indicates that the choice of dielectric is critical as high χ dielectrics may not exhibit conduction mechanisms appropriate for high speed applications of MIM tunnel diodes.

Currently, light emitting devices (LEDs) are made of inorganic or organic semiconductor materials prepared in crystalline thin films or nanocrystalline dots. They usually emit narrow band light due to the limit of the band gap energy of the semiconductor material. Commercial p-n junction or quantum well LEDs are fabricated with the MOCVD process involving toxic chemicals on the expensive single crystal substrate. Compared with the incandescent or the fluorescent light bulb, these LEDs have many advantages, such as the compact size, long lifetime, and low power consumption, but at the expense of the higher price, e.g., by more than one order of magnitude. On the other hand, the good-quality white light cannot be emitted from a single LED chip. It requires a set of three different LEDs or one LED in combination with a phosphor layer.

Recently, it has been reported that the broad band white light could be emitted from a new type of LED that has a MOS structure with a very thin amorphous metal oxide high-k dielectric layer (1,2,3). The complete device can be easily fabricated on the Si wafer using the IC compatible process at a low thermal budget. In this talk, authors will discuss 1) the light emission mechanism, which is from the thermal excitation of the conductive paths formed during the dielectric breakdown, 2) the optical characteristics, which covers the whole visible and part of the IR wavelength range of 400 nm to 1,000nm with a high color rendering index of 95, and 3) the lifetime, which is longer than 1,500 hours under the continuous operation at room temperature in the air. The unique physical structure and material properties of the sub 10 nm thick metal oxide layer are responsible for the light emitting phenomenon. Experimental results on this kind of device will be shown and discussed. In summary, this type of LED is applicable to a large range of industry, consumer, medical, etc. products.

(1) Y. Kuo and C.-C. Lin, Appl. Phys. Lett.,102, 031117 (2113).

(2) Y. Kuo and C.-C. Lin, ECSSolid State Lett., 2(8) Q59-Q61 (2013).

(3) Y. Kuo and C.-C. Lin, Solid State Electronics, accepted, August 2013.