Resistive switching characteristics of multilayered (HfO2/Al2O3)n n = 19 thin film

A transparent resistive random access memory used as Indium Tin Oxide (ITO) electrode, ITO/HfO₂/Al₂O₃/…/HfO₂/Al₂O₃/ITO capacitor structure is fabricated on glass substrate by atomic layer deposition. The unipolar resistive switching characteristics can be performed by applying the positive- or negative-bias through top electrode, however, the differences of switching and stability in the two different operations can be observed. The diversities of electrical property are attributed to different oxide/ITO interface materials, which influence the current flow of the injected electrons.     

Reproducible resistance switching of a relatively thin FeOx layer produced by oxidizing the surface of a FePt electrode in a metal-oxide-metal structure

In this study, reproducible resistance switching effects were demonstrated on a relatively thin FeO_x layer in the TiN/SiO₂/FeO_x/FePt structure produced by oxidizing the surface of a FePt electrode during a plasma-enhanced tetraethyl orthosilicate oxide deposition process. Characteristics of the non-stoichiometric FeO_x transition layer were examined by Auger electron spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy analyses. In addition, characteristics of the forming process as well as the current transport characteristics after forming process in the TiN/SiO₂/FeO_x/FePt structure were discussed. Moreover, the role of the silicon oxide layer was also experimentally demonstrated to act as an additional supplier of oxygen ions for the switching requirement by biasing high voltage bias conditions.     

Effects of (NH4)2Sx treatment on the electrical and optical properties of indium tin oxide/conducting polymer electrodes

In this study, the effects of (NH₄)₂S_x treatment on the electrical and optical properties of the indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) electrodes were researched. The authors found that (NH₄)₂S_x treatment could result in suppressing the hysteresis-type current-voltage characteristics related to the interfacial capacitance variation and a reduction in the equivalent refractive index of the ITO/PEDOT:PSS electrodes, owing to the improvement in the interfacial stability of the ITO/PEDOT:PSS electrodes and a reduction in the interface trap-states related charge store at the ITO/PEDOT:PSS interface. This implies that the ITO/PEDOT:PSS electrodes fabricated using the (NH₄)₂S_x-treated ITO may produce a higher extraction efficiency for ITO/PEDOT:PSS-based optoelectronic devices.     

High performance metal-insulator-metal capacitors with atomic vapor deposited HfO2 dielectrics

Thin HfO₂ films were grown as high-k dielectrics for Metal-Insulator-Metal applications by Atomic Vapor Deposition on 8 inch TiN/Si substrates using pure tetrakis(ethylmethylamido)hafnium precursor. Influence of deposition temperature (320-400 °C) and process pressure (2-10 mbar) on the structural and electrical properties of HfO₂ was investigated. X-ray diffraction analysis showed that HfO₂ layers, grown at 320 °C were amorphous, while at 400 °C the films crystallized in cubic phase. Electrical properties, such as capacitance density, capacitance-voltage linearity, dielectric constant, leakage current density and breakdown voltage are also affected by the deposition temperature. Finally, TiN/HfO₂/TiN stacks, integrated in the Back-End-of-Line process, possess 3 times higher capacitance density compared to standard TiN/Si₃N₄/TiN capacitors. Good step coverage (> 90%) is achieved on structured wafers with aspect ratio of 2 when HfO₂ layers are deposited at 320 °C and 4 mbar.     

Characterization of the Polarity Dependent Memory and Switching effect in sandwiches of metal/a-Si/ITO

The Polarity Dependent Memory and Switching effect (PDMS) has been observed in sandwiches of metal/a-Si/ITO layers and metal/a-Si/In. The film composition has been analysed by Auger Electron Spectroscopy and peak heights of the metals, Si and SiO_x have been measured as a function of film depth. No penetration of electrode material into the a-Si film was observed and the interfaces were abrupt even after many cycles of switching. A mechanism based on dendrite formation does not therefore operate in this type of PDMS. Traces of SiO_x have been found at the ITO/a-Si interface which are attributed to the presence of ITO or the deposition of In after exposure of a-Si to air. The SiO_x layer appears only in the OFF state and not in the ON state of the device. Hence a model based on voltage formation and destruction of an approximately 500 Å thick SiO_x layer at the a-Si/ITO interface is given.     

HfO2 gate insulator formed by atomic layer deposition for thin-film-transistors

We have investigated the effects of annealing temperature on the physical and electrical properties of the HfO₂ film deposited by an atomic layer deposition (ALD) method for high-k gate oxides in thin-film-transistors (TFTs). The ALD deposition of HfO₂ directly on the Si substrate at 300 °C results in the formation of thin HfSi_xO_y interfacial layer between Si and HfO₂. The subsequent low temperature N₂-annealing of HfO₂ films (i.e., 300 °C) using a rapid thermal processor (RTP) improves the overall electrical characteristics of HfSi_xO_y-HfO₂ films. Based on the current work, we suggest that HfO₂ film deposited by the ALD method is suitable for high-k gate oxides in TFTs, which have to be fabricated at low temperature.     

Interfacial microstructure of NiSix/HfO2/SiOx/Si gate stacks

Integration of NiSi_x based fully silicided metal gates with HfO₂ high-k gate dielectrics offers promise for further scaling of complementary metal-oxide- semiconductor devices. A combination of high resolution transmission electron microscopy and small probe electron energy loss spectroscopy (EELS) and energy dispersive X-ray analysis has been applied to study interfacial reactions in the undoped gate stack. NiSi was found to be polycrystalline with the grain size decreasing from top to bottom of NiSi_x film. Ni content varies near the NiSi/HfO_x interface whereby both Ni-rich and monosilicide phases were observed. Spatially non-uniform distribution of oxygen along NiSi_x/HfO₂ interface was observed by dark field Scanning Transmission Electron Microscopy and EELS. Interfacial roughness of NiSi_x/HfO_x was found higher than that of poly-Si/HfO₂, likely due to compositional non-uniformity of NiSi_x. No intermixing between Hf, Ni and Si beyond interfacial roughness was observed.     

Resistive switching in Au/SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/TiN/Ti memristive structures with varied geometric parameters and stoichiometry of dielectric film

We have studied Au/SiO _x /TiN/Ti memristive structures obtained by magnetron sputtering, which exhibit a reproducible resistive switching effect. The influence of the thickness and stoichiometry of SiO _x layer and the area of Au electrode on the parameters of switching has been analyzed. The obtained results show evidence in favor of the filament model of resistive switching in SiO _x films.     

Resistive switching in metal-insulator-metal structures based on germanium oxide and stabilized zirconia

Bipolar resistive switching in metal-insulator-metal structures based on a double-layer insulator composed of a layer of yttria-stabilized zirconia (YSZ) containing 12 mol % Y₂O₃ and a layer of GeO _x is studied. It is shown that the incorporation of an additional GeO _x layer into the structure leads to a significant decrease in the variation of resistive switching parameters at both negative and positive voltages. Au/Zr/GeO _x /YSZ/TiN structures exhibit a high stability of the resistance ratio in high-resistance and low-resistance states during cyclic switching. The studied structures can be used for designing next-generation nonvolatile memory elements.     

Tribological and mechanical behaviors of TiN/CNx multilayer films deposited by magnetron sputtering

TiN/CN_x multilayer films with bilayer periods of 4.5-40.3 nm were deposited by direct-current magnetron sputtering. Layer morphology and structure of the multilayered films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. The TiN/CN_x multilayers exhibited coherent epitaxial growth due to the mutual growth-promoting effect at small bilayer period and some crystalline regions going through the interface of TiN/CN_x. Nanoindentation tests showed that the hardness of the multilayers varied from 12.5 to 31 GPa, with the highest hardness being obtained with a bilayer period of 4.5 nm. The tribological properties of the films were investigated using a ball-on-disk tribometer in humid air, and the TiN/CN_x multilayer with a bilayer period of 4.5 nm also exhibited the lowest friction coefficient and the highest wear resistance.     

Hardness enhancement and oxidation resistance of nanocrystalline TiN/MoxC multilayer films

In this paper the influence of the layer's microstructure on the hardness enhancement in multilayer nanocrystalline films and the oxidation resistance are studied. The TiN/Mo_xC multilayer films at different modulation period, and Mo_xC and TiN monolayer films were deposited on the (0 0 1) silicon wafers and molybdenum sheets by rf and dc magnetron sputtering. The monolayer TiN films with a thickness of about 2 μm are of pure face-center cubic TiN phase, while the monolayer Mo_xC films consist of two phases, one of which is body-center cubic Mo and the other is hexagonal Mo₂C as determined by XRD. The coarse columnar grains of about 200 nm in the monolayer TiN films become much smaller or disappear in the multilayer films. The hardness enhancement of the multilayer films takes place at the modulation period of 320 nm, which can reach to 26 GPa and is much higher than the values of Mo_xC and TiN monolayer films. This enhancement in hardness can be explained as the decrease in the size and/or disappearance of columnar grains in the TiN layer. The Young's modulus in the temperature range from 100 to 400 °C increases with decreasing modulation period. It is found that about 100 nm thick TiN films can increase largely the oxidation resistance of Mo_xC films.     

Forming wrinkled stiff films on polymeric substrates at room temperature for stretchable interconnects applications

Periodically wrinkled stiff thin films on elastomeric substrates have been found extensive applications, such as in stretchable electronics. This paper presents a cost-effective and simple method to form wrinkled stiff SiO_x thin films on polydimethylsiloxane (PDMS) substrates at room temperature by ultraviolet/ozone (UV/O) radiation on pre-strained PDMS. Systemic studies have been conducted to understand the dependence of the wavy profile on the PDMS pre-strain, UV/O exposure time, and PDMS modulus. The mechanics analysis has been verified to be quantitatively or qualitatively accurate by experimental comparisons. The wrinkled SiO_x/PDMS system is stretchable and provides a wavy mold for stretchable electrodes. The constant electrical resistance during mechanical stretching shows the stretchability of this system.     

Reproducible resistance switching of defect-engineered NiOx with metallic Nb impurity

The effect of Nb doping on the resistance switching characteristics of NiO_x films was investigated. Pt/Nb-doped NiO_x/Pt metal-insulator-metal stacks were fabricated using NiO_x films with various Nb contents sputtered by reactive dc magnetron sputtering. The resistance switching behaviors of the metal-insulator-metal stacks were then examined in conjunction with a study on the physical properties such as the chemical bonding of NiO_x films.Nb doping of NiO_x at a T_dep of 400 °C and an O₂ partial pressure of 5% resulted in an improved endurance of SET/RESET processes with a narrower distribution of V_SET, and a larger memory window compared to un-doped NiO_x films. NiO_x with 5.47% Nb deposited at an O₂ partial pressure of 15% showed bistable resistance switching behavior while undoped NiO_x material, deposited under the same condition did not. A study of the chemical bonding states by X-ray photoelectron spectroscopy showed that the Nb-doping of NiO_x films produced an increase in the density of Ni⁰ and a reduction in the density of Ni³⁺, compared to corresponding values for undoped NiO_x films deposited under the same condition. The resistive switching behavior of NiO_x was enhanced by defect engineering with metal impurity with different oxidation valence.     

Microstructural size effects on the hardness of nanocrystalline TiN/amorphous-SiNx coatings prepared by magnetron sputtering

It has been postulated that equiaxed nanocrystalline (<10 nm) TiN grains embedded in a thin amorphous silicon nitride (a-SiN_x) phase are a prerequisite to obtain ultrahard TiN/a-SiN_x coatings. The present study correlates hardness and microstructure of TiN/a-SiN_x coatings with Si contents between 0 and 17 at.%. The coatings have been deposited by magnetron sputtering in industrial-scale physical vapour deposition systems. Transmission electron microscopy studies revealed that increasing the silicon content causes the TiN grain size to decrease. This is accompanied by a change in grain morphology: At Si contents lower than 1 at.% TiN grains become columnar, while at Si contents higher than 6 at.% equiaxed grains with diameters of 6 nm form. For silicon contents between 1 and 6 at.%, a transition region with nanocrystalline columnar grains exists. This nanocrystalline columnar microstructure causes maximum hardness values of more than 45 GPa for TiN/a-SiN_x coatings as determined by nanoindentation. The elongated and equiaxed nanocrystalline TiN grains exhibit almost theoretical strength as dislocation-based deformation mechanisms are constrained.     

Electrical properties of AlNx/Al/Mo composite film prepared for use in thin-film transistors

In the present study, AlN_x/Al/Mo composite films with various thicknesses of AlN_x and Al layers were prepared to replace commercial AlNd/Mo composite film as the gate metal of the two metal layers (namely the gate metal and the source-drain metal) in thin-film transistor (TFT) specimens. The prerequisite for the TFT device is that no hillock is formed. The electrical properties of the AlN_x/Al/Mo TFT device rival those of the AlNd/Mo TFT device. One of eight kinds of AlN_x/Al/Mo composite films (0.05 µm/0.2 µm/0.07 µm) without hillocks was compared with the AlNd/Mo (0.25 µm/0.07 µm) composite film. The line width after development and strip inspections, the I_g (gate leakage current)-V_g (gate voltage) curve, the coating film resistance to electricity, the contact resistance between the indium tin oxide (ITO) film and the metal film, the I_d-V_g curve, and the critical dimension loss (CD loss) were compared. The experimental results indicate that the metal line widths for these two composite films are similar. The coating film resistance, the contact resistance between the ITO film and the metal film, and the I_d-V_g curve for the AlN_x/Al/Mo TFT device were similar to those for the AlNd/Mo TFT device. The CD loss shown in the AlN_x/Al/Mo TFT device was lower than that for the AlNd/Mo TFT device.     

Characterization of SiNx/a-Si:H crystalline silicon surface passivation under UV light exposure

One of the most promising solution for crystalline silicon surface passivation in solar cell fabrication consists in a low temperature (< 400 °C) Plasma Enhanced Chemical Vapor Deposition of a double layer composed by intrinsic hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (SiN_x). Due to the high amount of hydrogen in the gas mixture during the double layer deposition, the passivation process results particularly useful in case of multi-crystalline silicon substrates in which hydrogenation of grain boundaries is very needed. In turn the presence of hydrogen inside both amorphous layers can induce metastability effects. To evaluate these effects we have investigated the stability of the silicon surface passivation obtained by the double layer under ultraviolet light exposure. In particular we have verified that this double layer is effective to passivate both p- and n-type crystalline silicon surface by measuring minority carrier high lifetime, via photoconductance-decay. To get better inside the passivation mechanisms, strongly connected to the charge laying inside the SiN_x layer, we have collected the Infrared spectra of the SiN_x/a-Si:H/c-Si structures and we have monitored the capacitance-voltage profiles of Al/SiN_x/a-Si:H/c-Si Metal Insulator Semiconductor structures at different stages of UltraViolet (UV) light exposure. Finally we have verified the stability of the double passivation layer applied to the front side of solar cell devices by measuring their photovoltaic parameters during the UV light exposure.     

Influence of Hf→Zr substitution on optical and refractometric parameters of Hf1−xZrxO2 thin films

Ellipsometric and spectroscopic investigations of Hf_1−xZr_xO₂ thin films were performed. Dispersion dependences of refractive indices and extinction coefficients in the wavelength interval 0.2-0.7 μm were obtained by optical-refractometric synthesis of absorption spectra. Optical-refractometric relation is applied to describe the dispersion of the refractive indices. Compositional behaviour of optical pseudogap and refractive indices of HfO₂-ZrO₂ thin films is studied.     

Annealing effect on the formation of high-k dielectric in the W/ultrathin HfO2/Si-substrate system

Using a method of high-frequency magnetron sputtering, the structures W(150 nm)/HfO₂(5 nm)/Si(100) were prepared and were further annealed either at 500°C in vacuum for 30 min or in an atmosphere of argon at 950°C for 12 s. Study of the capacitance-voltage characteristics of these structures showed that high-temperature annealing leads to a decrease in the maximum specific capacitance in accumulation (from 4.8 × 10^?6 to 3.2 × 10^?6 F/cm²) and dielectric constant (from 27 to 23). Using time-of-flight secondary-ion mass spectrometry, it is established that growth of a WO _x phase at the W/HfO₂ interface and a HfSi _x O _y silicate phase at the HfO₂/Si (100) interface occurs during annealing. In addition, the total thickness of the intermediate oxide layer exceeds the thickness of the initial HfO₂ film by 30%.     

Resistive switching of HfO2-based Metal-Insulator-Metal diodes: Impact of the top electrode material

This paper deals with the impact of the top metal electrode on the resistive switching properties of HfO₂-based Metal-Insulator-Metal diodes. By screening five different metals as top electrode, Al-Cu-Hf-Pt-Ti, we have demonstrated the feasibility of the resistive switching effect on HfO₂. Metals with a low enthalpy of formation of oxides ΔH_f⁰ (Pt and Cu) lead to uni-polar switching whereas easily oxidizable metals with a higher ΔH_f⁰ (Al, Hf and Ti) lead to bipolar switching. Cu-, Hf- and Pt-based devices show a degradation of the top electrode after the forming step by the formation of bubbles whereas such phenomenon was not observed in Al- and Ti-based devices. 200 switching cycles were performed on each device in order to extract the main parameters of the resistive switching effect: I_ON and I_OFF currents in the mA range, R_OFF/R_ON resistance ratio up to 5, V_set and V_reset, voltage levels around 1 V, and powers dissipated during read and write operations in the μW and mW range, respectively. For all systems, the reset process dissipates higher power than the set process. From these results, the Ti top adlayer shows the best performance in terms of stability and resistive switching characteristics.     

Surface chemistry and film growth during TiN atomic layer deposition using TDMAT and NH3

Surface chemistry and film growth were examined during titanium nitride (TiN) atomic layer deposition (ALD) using sequential exposures of tetrakis-dimethylamino titanium (TDMAT) and NH₃. This ALD system is shown to be far from ideal and illustrates many potential problems that may affect ALD processing. These studies were performed using in situ Fourier transform infrared (FTIR) techniques and quartz crystal microbalance (QCM) measurements. Ex situ measurements also analyzed the properties of the TiN ALD films. The FTIR studies revealed that TDMAT reacts with NH_x* species on the TiN surface following NH₃ exposures to deposit new Ti(N(CH₃)₂)_x* species. Subsequent NH₃ exposure consumes the dimethylamino species and regenerates the NH_x* species. These observations are consistent with transamination exchange reactions during the TDMAT and NH₃ exposures. QCM studies determined that the TDMAT and NH₃ reactions are nearly self-limiting. However, slow continual growth occurs with long TDMAT exposures. In addition, the TiN ALD growth rate increases progressively with growth temperature. The resistivities of the TiN ALD films were ?10⁴ μΩ cm and the densities were ?3 g/cm³ corresponding to a porosity of ∼40%. The high porosity allows facile oxidation of the TiN films and lowers the film resistivities. These high film porosities will seriously impair the use of these TiN ALD films as diffusion barriers.