Top Electrode Thickness Research Articles

Understanding the stress effect of TiN top electrode on ferroelectricity in Hf0.5Zr0.5O2 thin films

We conducted a comprehensive investigation on the influence of TiN thickness and stress on the ferroelectric properties of Hf0.5Zr0.5O2 thin films. TiN top electrode layers with varying thicknesses of 2, 5, 10, 30, 50, 75, and 100 nm were deposited and analyzed. It was observed that the in-plane tensile stress in TiN films increased with the thickness of the TiN top electrode. This is expected to elevate the tensile stress in the Hf0.5Zr0.5O2 film, consequently leading to an enhancement in ferroelectric polarization. However, the effect of stress on the ferroelectric behavior of Hf0.5Zr0.5O2 films exhibited distinct stages: improvement, saturation, and degradation. Our study presents novel findings revealing a saturation and degradation phenomenon of in-plane tensile stress on the ferroelectric properties of polycrystalline Hf0.5Zr0.5O2 films, thereby partially resolving the discrepancies between experimental observations and theoretical predictions. The observed phase transformation induced by tensile stress in Hf0.5Zr0.5O2 films played a crucial role in these effects. Furthermore, we found that the impact of the TiN top electrode thickness on other factors influencing ferroelectricity, such as grain size and oxygen vacancies, was negligible. These comprehensive results offer valuable insights into the influence of stress and TiN top electrode thickness on the ferroelectric behavior of Hf0.5Zr0.5O2 films.

Ag2O/β-Ga2O3 Heterojunction-Based Self-Powered Solar Blind Photodetector with High Responsivity and Stability.

Self-powered deep-ultraviolet photodetectors have received considerable attention in recent years because of their efficiency, reliability, and various applications in civilian and military fields. Herein, a Ag/Ag2O layer is continuously deposited on a β-Ga2O3 epitaxial layer by a facing target sputtering system without opening the chamber, which has an advantage in time and cost. A p-n junction photodetector was constructed through the Ag2O/β-Ga2O3 heterojunction and by varying the thickness of the Ag film, which was controlled by the sputtering time. The effect of top electrode thickness on the photoresponse characteristics of photodetectors was studied. Because thin Ag films have low surface roughness, indicating low optical loss and good interfacial conditions, photodetectors using a thin Ag film as the top electrode exhibit high photoresponsivity. However, Ag films that were thinner than the threshold thickness, which is the minimum thickness required to form a continuous, homogeneous surface film, exhibited rather low performance owing to the high reflection and scattering caused by the inhomogeneous surface morphology. The as-fabricated photodetector with a 20 nm Ag film presents a high on/off ratio of 3.43 × 108, responsivity and detectivity of 25.65 mA/W and 6.10 × 1011 Jones, respectively, and comparable rise and decay times of 108 and 80 ms, respectively. Additionally, even after three months of storage in an ambient environment, the photoresponse of the photodetector was maintained, indicating good stability in air. These results suggest that Ag2O/β-Ga2O3 heterojunction-based photodetectors with thin Ag films can be used in various applications requiring deep-ultraviolet detection without an external power supply.

Analysis and Enhancement of Q-Factor in Thin-Film Bulk Acoustic Wave Resonator (FBAR)

In MEMS technology, MEMS Resonators has shown high potential applications in wireless communications such as Bluetooth and WIFI range. The Bulk Acoustic Wave (BAW) Resonator is a device which has more potential to meet the needs of today’s wireless technology. The Acoustic wave technology has attracted more attention and it’s an effective solution for achieving high-performance RF filters applications due to its low cost and small size of the device. The performance analysis of thin film bulk acoustic resonator (FBAR) by varying different piezoelectric material, piezoelectric material’s thickness and electrode materials is demonstrated in this paper. It is understood that the Quality factor is considerable higher for AlN as compared to ZnO and LiNbO3. Among all the materials, Aluminium electrode achieved maximum quality factor. As the thickness of piezo material decreases, it is found that the resonant frequency and Quality factor increases. Along with the Quality Factor, the top and bottom electrode thickness has a significant impact on the resonant frequency.

Evaluating gallium-doped ZnO top electrode thickness for achieving a good switch-ability in ZnO2/ZnO bilayer transparent valence change memory

Evaluating gallium-doped ZnO top electrode thickness for achieving a good switch-ability in ZnO2/ZnO bilayer transparent valence change memory

Perovskite CH3NH3PbI3–XClx Solar Cells. Experimental Study of Initial Degradation Kinetics and Fill Factor Spectral Dependence

Abstract The main drawback of the methylammonium lead halide perovskite solar cells is their degradation in ambient atmosphere. To investigate ambient-air-induced cell degradation, spectral dependencies of open-circuit voltage (V OC ), fill factor (FF) and the power conversion efficiency (PCE) have been acquired (for the first time reported in literature). Our custom-made measurement system allowed us to perform measurements of the abovementioned entities in situ directly in vacuum during and after thermal deposition of the electrode. We also studied how these parameters in vacuum changed after cell exposure to ambient air for 85 min (50 nm top electrode) and for 180 min (100 nm top Ag electrode). For fresh CH3NH3PbI3–xClx cell (never been in open air) with very high shunt resistance of 3·107 Ω·cm2 (with practically no shorts and therefore FF could be determined mainly by charge carrier recombination processes) we found that FF in vacuum increased along with an increase of the incident photon energy from 0.55 at 760 nm up to 0.82 at 400 nm. Hypothesis considering hot polaron participation in charge carrier photogeneration and recombination processes as well as another competing hypothesis were offered as possible explanations for the observed FF increase. The kinetics of short-circuit photocurrent EQE with a change in pressure was also investigated. It was also shown that perovskite solar cell degradation could be noticeably reduced by increasing the top Ag electrode thickness to at least 100 nm, which could possibly facilitate the usual encapsulation process.

Impact of process-induced variability on the performance and scaling of Ge2Sb2Te5 Phase-change memory device

The process-induced variability in nanoscale phase-change memory (PCM) devices is of utmost importance for the development of reliable single-bit/multi-bit data storage devices. In this study, the influence of structural and interfacial parameters on the performance of Ge2Sb2Te5 (GST) PCM device is systematically investigated using Plackett–Burman design of experiment method. Five important structural parameters, (i) heater (TiN) radius (HR), (ii) heater height, (iii) GST radius (WGST), (iv) GST thickness, and (v) top electrode thickness, and along with three interfacial parameters namely, (i) thermal boundary resistance (TBR) between GST and TiN interface (ii) TBR between GST and SiO2 interface, and (iii) electrical interface resistance (EIR) between GST and TiN interface are considered for the study. Furthermore, to understand the impact of scaling, the performance metrics i.e. RESET resistance (RRESET), SET resistance (RSET), RESET power (PRESET) and SET power (PSET) of an isotropically scaled-down device with a HR of 10 nm are extracted and compared against the reference device of 50 nm HR. The TCAD simulation results reveal that HR and WGST are the most dominant structural parameters for the output metrics and the analysis shows that ratio should be maintained between 2.7 and 4.5 to offer reliable RESET operation. Among the interfacial parameters, GST/TiN EIR is the most significant controlling parameter for PRESET/PSET metrics, whereas GST/TiN TBR plays an important role in achieving better RRESET/RSET. Hence, our findings of the most and least sensitive input parameters can be effectively used for the better optimization of RESET/SET pulse parameters to achieve reliable programming of PCM devices in the future technology nodes.

Effect of film thickness on the ferroelectric and dielectric properties of low-temperature (400 °C) Hf0.5Zr0.5O2 films

We report on the effect of the Hf0.5Zr0.5O2 (HZO) film thickness on the ferroelectric and dielectric properties using pulse write/read measurements. HZO films of thicknesses ranging from 5 to 20 nm were annealed at 400 °C for 1 min in a nitrogen ambient to be compatible with the back-end of the line thermal budget. As the HZO film thickness decreases, low-voltage operation (1.0 V or less) can be achieved without the dead layer effect, although switching polarization (Psw) tends to decrease due to the smaller grain size. Meanwhile, for 20-nm-thick HZO films prepared under the identical stress (similar TiN top electrode thickness and thermal budget), the Psw and dielectric constant are reduced because of additional monoclinic phase formation.

Domain nucleation behavior in ferroelectric films with thin and ultrathin top electrodes versus insulating top layers

The effect of varying the thickness of Pt top electrodes made by the electron beam induced deposition technique on the domain nucleation characteristics of Pb(Zr,Ti)O3 films has been investigated by piezoresponse force microscopy. The advantage of this experimental setup is that the nucleation position can be precisely controlled allowing comparison between two capacitor structures with different top electrode thicknesses and negating the role of defect-induced polarization switching scenarios. In the ultrathin limit (<5nm), the coercive biases of polarization switching change substantially relative to thicker electrodes, from −4.6 to −2.6V and from +3.5 to 5.5V respectively, which we link to a work function size effect. The evolution of the piezoresponse and coercive biases as a function of hysteresis loop cycling is presented and contrasted with that obtained on the bare film surface with no top electrode. To provide a counterpoint to the observed effects with top electrodes, we have demonstrated the variation in switching due to a top silicon oxide insulating layer. This work has revealed a method which could be used to control nucleation bias in ferroelectric thin film systems.

Full-Wave Analysis of Ultrahigh Electromechanical Coupling Surface Acoustic Wave Propagating Properties in a Relaxor Based Ferroelectric Single Crystal/Cubic Silicon Carbide Layered Structure

This paper describes a full-wave analysis of ultrahigh electromechanical coupling surface acoustic wave (SAW) of Y-cut X propagating Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (YX-PIMNT) single crystals on a cubic silicon carbide (3C-SiC) substrate. There are several eigenmodes including shear horizontal (SH) and Rayleigh SAWs. Based on the finite-element method (FEM), the phase velocity (vp) and coupling factor (K2) of SAWs varying with the top electrode thickness, thickness, and Euler angle (θ) of the YX-PIMNT substrate have been investigated. K2 of SH SAW can reach an extremely high value of 75.9%. The proper control of structural parameters can suppress unwanted responses caused by other modes without deteriorating the coupling factor. The large K2 value of SH SAW and suppression of unwanted responses have highly promising applications in developing ultrawideband and tunable SAW filters. Finally, the performance of 3C-SiC and 6H-SiC as substrates was investigated, and 3C-SiC was identified as a more attractive substrate candidate than 6H-SiC.

Effects of transparent bottom electrode thickness on characteristics of transparent organic light-emitting devices

We conduct both simulation and experiment studies of impacts of simultaneously varying the thicknesses of transparent bottom electrodes and semi-transparent top thin metal electrodes on optical characteristics (e.g., transmission, reflection) and efficiencies of transparent organic light-emitting devices (OLEDs). For the thickness range of both electrodes studied, the total electroluminescent (EL) efficiencies (including both bottom and top emission), EL spectra, and emission patterns remain similar; yet the ratio of top to bottom emission would be modulated by the semi-transparent top metal electrode thickness. The thickness of the transparent bottom electrode has weak effects on the efficiencies of top/bottom/total emission, but it does have definite effects on optical transmission/reflection spectra (e.g. peak/valley wavelengths). Meanwhile, the thickness of semi-transparent top metal electrodes mainly affect magnitudes of the optical transmission/reflection. Transmissive/reflective hues and appearances of the transparent OLEDs can thus be tuned by the thicknesses of bottom/top electrodes. Overall, we demonstrated efficient transparent green phosphorescent OLEDs exhibiting a high peak transmittance of up to 81% and rather high total external quantum efficiencies of up to 21–21.5% (corresponding to a total current efficiency of 80–82 cd/A and total power efficiency of 95–99 l m/W), among the highest (if not the highest) for planar transparent OLEDs using no other optical out-coupling structures. By varying the thicknesses of transparent bottom electrodes and semi-transparent top metal electrodes, the ratio of top to bottom emission, and the transmissive or reflective hues/appearances of transparent OLEDs in the off state can be tuned, yet without sacrificing total EL efficiencies or changing their EL colors/patterns. Such tunable optical characteristics of transparent OLEDs may find some interesting applications.

Enlarged phase velocities of ultra-wideband surface acoustic wave devices with relaxor based ferroelectric single crystal/diamond layered structure

The surface acoustic wave (SAW) characteristics of Y-cut X propagating Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (YX-PIMNT) single crystals on a diamond substrate have been theoretically calculated. The simulated results show that the phase velocity of the shear horizontal (SH) SAW may be greatly enhanced from the 1350m/s to 3350m/s by reducing the thickness of the PIMNT from 0.5λ to 0.05λ, with a corresponding decrease in the electromechanical coupling factor (K2) from 73.6% to 19.6%. The dispersion curves of phase velocity and K2 as a function of PIMNT thickness are given for the proposed layered structure. Besides the SH SAW, there are also higher order modes that would cause unwanted responses in the pass-band of wideband SAW filters. These were suppressed by properly controlling structural parameters including top electrode thickness, thickness and Euler angle (θ) of PIMNT substrate. The calculated results demonstrate the effectiveness of this approach to enlarge the phase velocity of the SH SAW without dramatically sacrificing its K2, which makes relaxor based ferroelectric single crystals promising for realizing ultra-wideband SAW devices working in ~GHz range.

Effects of High-Pressure Hydrogen Annealing (HPHA) on Reliability Characteristics of RRAM.

Reliability characteristics (retention and endurance) of RRAM are critical for its practical realization and need to be improved. In this work, we confirmed the trade-off between retention and endurance by using various top electrode thickness conditions. The trade-off between retention and endurance characteristics was mainly due to the different amount of oxygen in scavenging layer (Ta) and the amount of oxygen vacancy in switching layer (HfO2). The amount of the oxygen in scavenging layer (Ta) and the amount of the oxygen vacancy in switching layer (HfO2) will be increased with the increase of Ta thickness. Therefore, the thicker Ta cells have worse retention because the large amount of oxygen in scavenging layer (Ta) can diffuse back into switching layer (HfO2) and recombine with oxygen vacancies in the filament. However, it has longer endurance because the large amount of oxygen vacancy in switching layer (HfO2) can be a source of the filament. Hence, there exists a trade-off relation between retention and endurance under the various Ta thickness conditions. To improve both retention and endurance characteristics, we proposed a new method by using high-pressure hydrogen annealing (HPHA). The thin Ta cells have longer retention and worse endurance because it has small amount of both oxygen in scavenging layer (Ta) and oxygen vacancy in switching layer (HfO2). Therefore, to generate more oxygen vacancies in switching layer (HfO2) maintaining small amount of oxygen in scavenging layer (Ta), we treated the samples by HPHA before Ta deposition. Finally, we obtained both improved retention and endurance characteristics in HfO2 based RRAM devices after high-pressure hydrogen annealing treatment.

A numerical study of forming voltage and switching polarity dependence on Ti top electrode thickness in Zr $$\mathrm{O}_2$$ O 2 RRAM

The authors investigate the conduction filament (CF) formation voltage and reset polarity dependence of a Ti/Zr$$\mathrm{O}_{2}$$O2/Pt resistive switching memory device on the Ti top electrode (TE) thickness by means of numerical simulation. A calculation method which accounts for both the initial statistical nature of the resistive switching layer (RSL) and the interaction of oxygen vacancies with oxygen species (O) on the overall conductance is used. The results show a correlation between the TE thickness and the density profile of O species both along the RSL---TE interface and within the TE, and indentify the causes for the previously observed not yet fully comprehended TE thickness dependent functionality of this device. The authors demonstrate that accumulation and reduction of O, during forming, result in a charged interfacial layer which decreases the effective oxide thickness and determines the forming voltage and switching polarity. In addition, local temperature profiles within the CF during forming promote O diffusion into the TE material which reduces the local conductivity and affects the forming voltage magnitude as well. In addition the authors provide forming and reset calculations for TE thicknesses of 5, 20 and 40 nm which are in good agreement with published experimental data.

Impact of inserted Ta ultrathin layer and postdeposition annealing on the forming voltage of Ir/Ti/Ta/HfO2/TiN/Ti/SiO2/Si resistive switching devices

We fabricated resistive switching (RS) devices that possess the Ir/Ti/Ta/HfO2/TiN/Ti/SiO2/Si structure with different Ta and Ti layer thicknesses by a magnetron sputtering method. The lowering of forming voltage was observed when the combination of Ti and Ta layer thicknesses was Ti (45 nm)/Ta (5 nm) and the device was annealed after the deposition of the top electrode. In this sample, the migration of oxygen ions from the HfO2 layer to the Ti layer through the Ta layer upon thermal annealing was confirmed by secondary ion mass spectroscopy measurements. On the basis of the dependence of the forming voltage on the combination of the top electrode thicknesses and the presence or absence of postdeposition annealing, we discuss a method of reducing the oxide layer more effectively without changing the material combination of the metal electrode and the oxide layer.

Influence of Ti top electrode thickness on the resistive switching properties of forming free and self-rectified TiO2 − x thin films

The memory performance of titanium oxide (TiO2−x)-based resistive memories containing an ultra thin reactive Ti top electrode can be greatly enhanced. Very good switching memory characteristics were demonstrated for an Au/Ti/TiO2−x/Au/SiO2/Si structure with the insertion of a Ti nanolayer at the Au/TiO2−x interface. Due to the superb ability of Ti to absorb oxygen atoms from the dielectric matrix, a large amount of oxygen vacancies is created, which are crucial for the stable function of the memory devices. As the Ti thickness increases, a thick interfacial layer is created, which degrades the resistive switching behavior. The induced interface thickness is found also to affect the fluctuation of the ON/OFF processes. The very good switching characteristics which were recorded for the devices containing Ti as top electrode, denote the direct impact that Ti has on the oxygen vacancy density. Oxygen vacancy distribution is also found to be directly associated with the filaments' diameter. Thus, the resistive switching mechanism is proposed to be associated with the formation/rupture of oxygen vacancy-based conducting filaments at the Ti/TiO2−x interface. Self-rectifying characteristics were also recorded for all samples in the low resistance states. Conduction mechanism analysis revealed that trap-assisted-tunneling is the dominant conduction mechanism, which also strongly affects the distribution of the current during SET process.

Highly doped fullerene C60 thin films as transparent stand alone top electrode for organic solar cells

We report on transparent stand alone C60 top electrodes for small molecule based organic photovoltaic cells. All device layers (including bottom and top electrode) are thermally evaporated in vacuum, guaranteeing fast, reliable, high-purity and low-cost processing. Employing an organic compound as electrode avoids refractive index mismatch between contact and photo-active layer which results in an increased number of photons reaching the absorber layers. The conductivity of C60 is improved from 10−8S/cm to remarkable 4.97S/cm by optimized molecular n-type doping. Furthermore, it is shown that high doping concentrations change the absorption behavior of C60, leading to transmittance values over 70% for wavelengths larger than 600nm with a weak dependence on layer thickness. Organic solar cells employing such an optimized n-doped C60 top electrode (thickness 200nm, doping concentration 16wt%) exhibit power conversion efficiencies of 1.53%. Electrical device simulations based on a discretized equivalent circuit model reveal an efficient lateral charge transport in n-doped C60 electrodes over a few 100µm. In combination with a supporting grid electrode (usually employed for large area photovoltaic applications), n-doped C60 might replace ITO and fill the lack of transparent conductive top electrodes required for roll-to-roll processed semi-transparent organic devices or devices on opaque substrates.

Effect of top electrode thickness on the piezoresponse of polycrystalline ferroelectric capacitors

The domain images of ferroelectric capacitors with different top electrode thicknesses in the as-grown and poled states were obtained by piezoresponse force microscopy (PFM). In poled capacitors, a uniform piezoresponse with the same contrast was obtained for top electrodes of different thicknesses. For capacitors in the as-grown state, a lower piezoresponse was observed with thicker top electrodes, due to the different domain orientations, suppression effect of the ferroelectric domains and the damping effect of the electrode. It is shown that the piezoresponse decreases nonlinearly with the increase in electrode thickness. When the top electrode thickness is above 75 nm, the piezoresponse value is almost zero. When the top electrode thickness is increased from less than 10 to 75 nm, the piezoresponse is reduced rapidly to a very small value. Details of the domain wall, grain boundary and defects gradually become indistinguishable. Our investigation suggests that to obtain clear domain information by PFM based on global excitation, the top electrode thickness should be less than 10 nm.

Evidences of oxygen-mediated resistive-switching mechanism in TiN\HfO2\Pt cells

In this letter, we study the influence of the Pt top-electrode thickness and of the chamber atmosphere during cell operation on the resistive switching of TiN\HfO2\Pt cells. The oxygen permeability of the Pt electrode directly in contact with the atmosphere significantly affects the resistive switching and the resistance states of the cell. The results provide strong experimental indications that the electroforming operation leads to oxygen-vacancy formation and that the subsequent reset operation relies on the available oxygen species in the filament neighborhood. Significant implications with respect to endurance and retention assessment of resistive-switching memory devices are discussed.

Investigation of top electrode for PZT thick films based MEMS sensors

In this work processing of screen printed piezoelectric PZT thick films on silicon substrates is investigated for use in future MEMS devices. E-beam evaporated Al and Pt are patterned on PZT as a top electrode using a lift-off process with a line width down to 3 μm. Three test structures are used to investigate the optimal thickness of the top electrode, the degradation of the piezoelectric properties of the PZT film in absence of a diffusion barrier layer and finally how to fabricate electrical interconnects down the edge of the PZT thick film. The roughness of the PZT is found to have a strong influence on the conductance of the top electrode influencing the optimal top electrode thickness. A 100 nm thick top electrode on the PZT thick film with a surface roughness of 273 nm has a 4.5 times higher resistance compared to a similar wire on a planar SiO2 surface which has a surface roughness of less than 10 nm. It is found that the piezoelectric properties of the PZT thick film are degraded up to 1,000 μm away from a region of the PZT thick film that is exposed directly to the silicon substrate without a diffusion barrier layer. Finally, ferroelectric hysteresis loops are used to verify that the piezoelectric properties of the PZT thick film are unchanged after the processing of the top electrode.

Effects of Ti top electrode thickness on the resistive switching behaviors of rf-sputtered ZrO2 memory films

In this study, we propose a simple method to produce the various interface thicknesses within Ti/ZrO2 by changing the thickness of the Ti top electrode. As the Ti thickness increases, the induced interface thickness also increases to degrade the dielectric strength of the ZrO2, further lowering the forming voltage. However, when the interface layer is thick enough, it will trap sufficient charges to build up an opposite electric field to increase the forming voltage. The induced interface thickness is found to obviously affect the bias polarity of the resistive switching behavior and the device reliability. A fluctuant ON process is also demonstrated to be attributed to the competition between the formation and rupture of the conducting filaments.