Thick HfO2 Research Articles

Electronic Passivation of Crystalline Silicon Surfaces Using Spatial‐Atomic‐Layer‐Deposited HfO2 Films and HfO2/SiNx Stacks

Spatial atomic layer deposition (SALD) is applied to the electronic passivation of moderately doped (≈1016 cm−3) p‐type crystalline silicon surfaces by thin layers of hafnium oxide (HfO2). For 10 nm thick HfO2 layers annealed at 400 °C, an effective surface recombination velocity Seff of 4 cm s−1 is achieved, which is below what has been reported before on moderately doped p‐type silicon. The one‐sun implied open‐circuit voltage amounts to iVoc = 727 mV. After firing at 700 °C peak temperature in a conveyor‐belt furnace, as applied in the production of solar cells, still a good level of surface passivation with an Seff of 21 cm s−1 is attained. Reducing the HfO2 thickness to 1 nm, the passivation virtually vanishes after firing (i.e., Seff > 1000 cm s−1). However, by adding a capping layer of plasma‐enhanced‐chemical‐vapor‐deposited hydrogen‐rich silicon nitride (SiNx) onto the 1 nm HfO2, a substantially improved firing stability is attained, as demonstrated by Seff values as low as 30 cm s−1 after firing, which is attributed to the hydrogenation of interface states. The presented study demonstrates that SALD‐deposited HfO2 layers and HfO2/SiNx stacks have the potential to evolve into an attractive surface passivation scheme for future solar cells.

Interfacial passivation by using an amorphous hafnium oxide thin layer toward improved CH3NH3PbI3/Si heterojunction photodetectors

In this paper, we reported the fabrication of improved CH3NH3PbI3/Si heterojunction photodetectors (PDs) achieved by passivating the interfacial defects by a low-temperature atomic layer deposition-grown thin amorphous HfO2 layer. The results suggested that the HfO2 thin layer effectively passivated the surface defects of Si and slightly improved the qualities of CH3NH3PbI3 thin films in terms of increasing the grain sizes. Current–voltage measurements suggested that the HfO2 thin layer suppressed interfacial Shockley–Read–Hall recombination, which decreased the dark current and simultaneously increased the photocurrent. However, a thick HfO2 layer resulted in a decrease in the photocurrent because of the insulting nature of HfO2. A champion performance was obtained by employing a 5 nm HfO2 layer, where the responsivity and detectivity were 0.6 mA/W and 8.0 × 1010 Jones, respectively, which are two times and four times as high as those PDs without the HfO2 layer. The results will provide a simple strategy for improving the performance of perovskite/Si heterojunction PDs in the future.

Influence of XeCl excimer laser annealing on the ferroelectric nondoped HfO2 formation deposited on a Si(100) substrate

In this research, we have investigated the effect of excimer laser annealing (ELA) on the ferroelectric nondoped HfO2 (FeND-HfO2) formation deposited on a Si(100) substrate. The XeCl (λ: 308 nm) ELA was irradiated as post-deposition annealing (PDA) in the N2 ambient to the 10 nm thick HfO2 deposited by RF-magnetron sputtering without substrate heating. The C–V characteristics of Al/HfO2/p-Si(100) metal/ferroelectrics/Si (MFS) diodes were gradually improved with the energy density of ELS from 170 mJ cm−2 to 270 mJ cm−2 irradiated at 200 Hz for 200 shots although the charge-injection type hysteresis of 0.2–0.3 V remained. The post-metallization annealing (PMA) at 400 °C/5 min in N2/4.9%H2 ambient for Al/HfO2/p-Si(100) MFS diodes markedly improved the C–V characteristics, and negligible hysteresis with ideal flat-band voltage (V FB) was realized. The memory window (MW) of 0.42 V was achieved by the program/erase (P/E) operation with the input pulses of +3 V/100 ms and −8 V/100 ms for the MFS diode with an ELA energy density of 270 mJ cm−2 at 200 Hz for 200 shots followed by the PMA.

Harnessing the power of temperature gradient-enhanced pyroelectricity: self-powered temperature/light detection in Ce-doped HfO2 ferroelectric films with downward spontaneous polarization

Ferroelectric materials are ideal for self-powered sensors in Internet of Things (IoT) and high-precision detection systems due to their excellent polarization properties. Compatibility with miniaturization, high-density systems, and complementary metal oxide semiconductor (CMOS) processes is crucial for their widespread adoption. HfO2-based ferroelectric films show potential in self-powered pyroelectric sensors as their thinness enables effective temperature and light detection. However, the disordered ferroelectric domain distribution limits their pyroelectric performance and hampers the development of highly integrated self-powered pyroelectric devices. This report investigates the temperature and light detection capabilities of Ce-doped HfO2 ferroelectric films, which exhibit as-grown spontaneous polarization in the downward direction, making them a promising option for self-powered pyroelectric sensors. The findings provide robust evidence that the introduction of a temperature gradient significantly enhances pyroelectricity. In addition, their applications in the detection of hot/cold wind and breathing have been proved. Notably, the 30 nm thick Ce-doped HfO2 ferroelectric film has a high pyroelectric coefficient of about 894.7 μC·m–2·K–1 and enables high-precision detection of changes in temperature of 0.1 K. This study highlights the potential application of HfO2-based ferroelectric films in self-powered sensors with temperature and light detection capabilities, making them a promising candidate for future IoT-based systems and high-precision detection systems.

VO2-based colorful smart windows with self-cleaning function

Smart windows hold promise for mitigating energy demand for indoor heating or cooling. However, VO2-based thermochromic smart windows face challenges such as high phase transition temperature, limited window color options, and a lack of functional diversity. Herein, we investigated the colorful smart windows utilizing the tungsten-doped VO2 thin films with the phase transition temperature of approximately 23.5 °C. The surface color of these smart windows can be dynamically adjusted from brown to purple, cyan, yellow, and red by tuning the thickness of the wave-impedance matching layer of HfO2 film, resulted from the interference effect of the HfO2 layer and the underlying WxV1-xO2 layer. Moreover, the HfO2-coated WxV1-xO2 thin film with the HfO2 thickness of 132 nm demonstrates superior optical performance with a solar modulation ability of 9.35 %, the low-temperature luminous transmission of 36.81 %, and the high-temperature luminous transmission of 38.03 %. In addition, the incorporation of SiO2 nanoparticles into the HfO2-coated WxV1-xO2 thin films results in the superhydrophobic property with a water contact angle of 162.1° due to the formed rough surface, which is favor to the self-cleaning of the windows surface.

Switching layer optimization in Co-based CBRAM for >105 memory window in sub-100 µA regime

Co/HfO2-based CBRAM stacks are optimized to enlarge the memory window for low-current (50 µA) operation. First, we dope the switching layer with Si to decrease the pristine current, thus enlarging the memory window. Then, we reduce the forming voltage by scaling the Si-doped HfO2 thickness. Finally, we extend the endurance lifetime and reduce the write time by introducing a hygroscopic oxide, LaSiO, in combination with HfSiO, to enhance Co ion hopping through hydroxyl groups. We further outline the important role of the position of the hygroscopic layer with respect to the Co active electrode in enlarging the memory window of the CBRAM device up to > 105.

Enhancement of ferroelectricity in chemical-solution-deposited ZrO2 thin films through fine phase transition control

Recent studies have highlighted the ferroelectric potential of ZrO2 thin films, which offer advantages over HfO2-based ferroelectric materials, such as lower thermal budget and greater natural abundance. In this research, pure ZrO2 ferroelectric thin films were fabricated on Si substrates using chemical solution deposition with all-inorganic precursors. We explored the impacts of film thickness and HfO2 seed layers on phase transition and ferroelectric properties. Notably, an 11 nm thick ZrO2 film exhibited ferroelectric behavior with a remanent polarization of 12.6 μC/cm2. Due to the combined effects of tensile stress and surface energy, the increase in thickness to 30 nm led to a phase transition from the orthorhombic phase to the monoclinic phase and a resultant decrease in remanent polarization. Additionally, the introduction of a HfO2 seed layer facilitated the crystallization of the ferroelectric o-phase, enhancing the remanent polarization to 16.5 μC/cm2 in our ∼8 nm thick ZrO2 thin film with a ∼2 nm thick HfO2 seed layer. These findings provide valuable insights into optimizing ferroelectric properties in ZrO2 thin films through phase transition control.

TiN-Au/HfO2 -Au Multilayer Thin Films with Tunable Hyperbolic Optical Response.

Hyperbolic metamaterials (HMM) possess significant anisotropic physical properties and tunability and thus find many applications in integrated photonic devices. HMMs consisting of metal and dielectric phases in either multilayer or vertically aligned nanocomposites (VAN) form are demonstrated with different hyperbolic properties. Herein, self-assembled HfO2 -Au/TiN-Au multilayer thin films, combining both the multilayer and VAN designs, are demonstrated. Specifically, Au nanopillars embedded in HfO2 and TiN layers forming the alternative layers of HfO2 -Au VAN and TiN-Au VAN. The HfO2 and TiN layer thickness is carefully controlled by varying laser pulses during pulsed laser deposition (PLD). Interestingly, tunable anisotropic physical properties can be achieved by adjusting the bi-layer thickness and the number of the bi-layers. Type II optical hyperbolic dispersion can be obtained from high layer thickness structure (e.g., 20 nm), while it can be transformed into Type I optical hyperbolic dispersion by reducing the thickness to a proper value (e.g., 4 nm). This new nanoscale hybrid metamaterial structure with the three-phase VAN design shows great potential for tailorable optical components in future integrated devices.

Wedge-shaped HfO2 buffer layer-induced field-free spin-orbit torque switching of HfO2/Pt/Co structure

Field-free spin–orbit torque (SOT) switching of perpendicular magnetization is essential for future spintronic devices. This study demonstrates the field-free switching of perpendicular magnetization in an HfO2/Pt/Co/TaO x structure, which is facilitated by a wedge-shaped HfO2 buffer layer. The field-free switching ratio varies with HfO2 thickness, reaching optimal performance at 25 nm. This phenomenon is attributed to the lateral anisotropy gradient of the Co layer, which is induced by the wedge-shaped HfO2 buffer layer. The thickness gradient of HfO2 along the wedge creates a corresponding lateral anisotropy gradient in the Co layer, correlating with the switching ratio. These findings indicate that field-free SOT switching can be achieved through designing buffer layer, offering a novel approach to innovating spin–orbit device.

Superlattice-like Sb70Se30/HfO2 thin films for high thermal stability and low power consumption phase change memory

We have fabricated Sb70Se30/HfO2 superlattice-like structure thin films for phase change memory by magnetron sputtering method, and investigated the effect of the HfO2 layer on the crystalline characteristics and phase change behavior of Sb70Se30/HfO2 thin films. The experimental results show that as the HfO2 thickness increases, the crystallization temperature rises, the data retention capacity increases as well as the band gap widens, which is beneficial for improving the thermal stability and reliability of Sb70Se30/HfO2 thin films. It was also found that the HfO2 composite layer inhibited the grain growth of the Sb70Se30 thin film, reducing the grain size and resulting in a smoother surface. In addition, the volume fluctuation of the Sb70Se30/HfO2 thin films changes by only 5.58% between amorphous and crystalline. The threshold and reset voltages of the cell based on Sb70Se30/HfO2 thin films are 1.52 V and 2.4 V respectively. We found that the HfO2 composite layer plays a significant role in improving thermal stability, refining grain size of Sb70Se30 phase change films and reducing device power consumption.

Theory and Simulation of Metal–Insulator–Semiconductor (MIS) Photoelectrodes

A metal-insulator-semiconductor (MIS) structure is an attractive photoelectrode-catalyst architecture for promoting photoelectrochemical reactions, such as the formation of H2 by proton reduction. The metal catalyzes the generation of H2 using electrons generated by photon absorption and charge separation in the semiconductor. The insulator layer between the metal and the semiconductor protects the latter element from photo-corrosion and, also, significantly impacts the photovoltage at the metal surface. Understanding how the insulator layer determines the photovoltage and what properties lead to high photovoltages is critical to the development of MIS structures for solar-to-chemical energy conversion. Herein, we present a continuum model for charge-carrier transport from the semiconductor to the metal with an emphasis on mechanisms of charge transport across the insulator. The polarization curves and photovoltages predicted by this model for a Pt/HfO2/p-Si MIS structure at different HfO2 thicknesses agree well with experimentally measured data. The simulations reveal how insulator properties (i.e., thickness and band structure) affect band bending near the semiconductor/insulator interface and how tuning them can lead to operation closer to the maximally attainable photovoltage, the flat-band potential. This phenomenon is understood by considering the change in tunneling resistance with insulator properties. The model shows that the best MIS performance is attained with highly symmetric semiconductor/insulator band offsets (e.g., BeO, MgO, SiO2, HfO2, or ZrO2 deposited on Si) and a low to moderate insulator thickness (e.g., between 0.8 and 1.5 nm). Beyond 1.5 nm, the density of filled interfacial trap sites is high and significantly limits the photovoltage and the solar-to-chemical conversion rate. These conclusions are true for photocathodes and photoanodes. This understanding provides critical insight into the phenomena enhancing and limiting photoelectrode performance and how this phenomenon is influenced by insulator properties. The study gives guidance toward the development of next-generation insulators for MIS structures that achieve high performance.

Mechanisms of Silicon Surface Passivation by Negatively Charged Hafnium Oxide Thin Films

We have studied the mechanisms underpinning effective surface passivation of silicon with hafnium oxide (HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) thin films grown <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">via</i> atomic layer deposition (ALD). Plasma-enhanced ALD with O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma and a tetrakis(dimethylamido)hafnium precursor was used to deposit 12 nm thick HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films at 200 °C on high-lifetime 5 Ωcm <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -type Czochralski silicon wafers. The passivation was activated by postdeposition annealing, with 30 min in air at 475 °C found to be the most effective. High-resolution grazing incidence X-ray diffraction measurements revealed the film crystallized between 325 and 375 °C, and this coincided with the onset of good passivation. Once crystallized, the level of passivation continued to increase with higher annealing temperatures, exhibiting a peak at 475 °C and yielding surface recombination velocities of <5 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> at 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> injection. A steady decrease in effective lifetime was then observed for activation temperatures >475 °C. By superacid repassivation, we demonstrated this reduction in lifetime was not because of a decrease in the bulk lifetime, but rather because of changes in the passivating films themselves. Kelvin probe measurements showed the films are negatively charged. Corona charging experiments showed the charge magnitude is of order 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> qcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> and that the reduced passivation above 475 °C was mainly because of a loss of chemical passivation. Our study, therefore, demonstrates the development of highly charged HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films and quantifies their benefits as a standalone passivating film for silicon-based solar cells.

A-InGaZnO Thin-Film Transistors With Novel Atomic Layer-Deposited HfO2 Gate Insulator Using Two Types of Reactant Gases

Plasma-enhanced chemical vapor deposition is often utilized to fabricate amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs) with mobility of approximately 7 cm2/( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}\cdot \text{s}$ </tex-math></inline-formula> ) using a SiOx gate insulator. For use in high-resolution organic light-emitting diode displays, this value must be markedly improved. Therefore, we used various reactants to create a HfO2 bilayer via atomic layer deposition (ALD) and examined the electrical properties of IGZO TFTs, such as their mobility and subthreshold swing (SS). By adjusting the thickness of HfO2 with H2O reactant gas, the amount of hydrogen that diffused into the IGZO channel was controlled. The IGZO TFT with a specific HfO2 bilayer gate insulator exhibited high saturation mobility of 16.75 cm2/( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}\cdot \text{s}$ </tex-math></inline-formula> ) and an improved SS of 159 mV/dec compared to a conventional device with a HfO2 gate insulator formed using only O3 reactant gas. Furthermore, the fabricated HfO2 bilayer devices presented excellent reliability under positive bias stress and were more robust against the short-channel effect. Thus, based on the findings of this study, to improve the electrical properties of a-IGZO TFTs, the ALD process is suggested to be used to deposit a high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}$ </tex-math></inline-formula> gate insulator.

Interfaces in ALD very thin Al2O3/HfO2 stacks studied by ellipsometry

Stacks and laminates of high-k binary oxides as Al2O3/HfO2 deposited by Atomic Layer Deposition (ALD) recently are intensively investigated not only for optical applications and micro/nano- electronics devices but also for replacing the conventional single dielectric layers in charge trapping flash memories. The efforts are focused on low cycle numbers of HfO2 to Al2O3 ALD and the way the sublayers alternate as blocks. Although the growth of the both HfO2 and Al2O3 sublayers in ALD stacks is a result of fixed cycles and intended refractive index dispersions in combination whit fixed optical band gaps, the stack parameters frequently show some flexibility in regard to the thickness in case of low dimensions. Ellipsometry is a known optical method with powerful algorithms for experimental data interpretations. Here the ellipsometric data on very thin Al2O3/HfO2 stacks as active unites in charge trapping memory devices were processed by suitable algorithms in order to determine the individual thickness of HfO2 and Al2O3 sublayers, their variation in stack depth and deviations from the nominal thicknesses. Interface regions were determined in the stack beginning and in an initial growth of every block. Interfaces were identified as Al2O3-HfO2 mixtures with a changeable composition and thickness in the stack depth.

Stable chemical enhancement of passivating nanolayer structures grown by atomic layer deposition on silicon.

Incorporation of carrier-selective passivating contacts is on the critical path for approaching the theoretical power conversion efficiency limit in silicon solar cells. We have used plasma-enhanced atomic layer deposition (ALD) to create ultra-thin films at the single nanometre-scale which can be subsequently chemically enhanced to have properties suitable for high-performance contacts. Negatively charged 1 nm thick HfO2 films exhibit very promising passivation properties - exceeding those of SiO2 and Al2O3 at an equivalent thickness - providing a surface recombination velocity (SRV) of 19 cm s-1 on n-type silicon. Applying an Al2O3 capping layer to form Si/HfO2/Al2O3 stacks gives additional passivation, resulting in an SRV of 3.5 cm s-1. Passivation quality can be further improved via simple immersion in hydrofluoric acid, which results in SRVs < 2 cm s-1 that are stable over time (tested for ∼50 days). Based on corona charging analysis, Kelvin probe measurements and X-ray photoelectron spectroscopy, the chemically induced enhancement is consistent with changes at the dielectric surface and not the Si/dielectric interface, with fluorination of the Al2O3 and underlying HfO2 films occurring after just 5 s HF immersion. Our results show that passivation is enhanced when the oxides are fluorinated. The Al2O3 top layer of the stack can be thinned down by etching, offering a new route for fabrication of ultra-thin highly passivating HfO2-containing nanoscale thin films.

Memristors with Nociceptor Characteristics Using Threshold Switching of Pt/HfO2/TaOx/TaN Devices.

As artificial intelligence technology advances, it is necessary to imitate various biological functions to complete more complex tasks. Among them, studies have been reported on the nociceptor, a critical receptor of sensory neurons that can detect harmful stimuli. Although a complex CMOS circuit is required to electrically realize a nociceptor, a memristor with threshold switching characteristics can implement the nociceptor as a single device. Here, we suggest a memristor with a Pt/HfO2/TaOx/TaN bilayer structure. This device can mimic the characteristics of a nociceptor including the threshold, relaxation, allodynia, and hyperalgesia. Additionally, we contrast different electrical properties according to the thickness of the HfO2 layer. Moreover, Pt/HfO2/TaOx/TaN with a 3 nm thick HfO2 layer has a stable endurance of 1000 cycles and controllable threshold switching characteristics. Finally, this study emphasizes the importance of the material selection and fabrication method in the memristor by comparing Pt/HfO2/TaOx/TaN with Pt/TaOx/TaN, which has insufficient performance to be used as a nociceptor.

Resistance of a corner cube reflector with a dielectric antireflection coating to the space environment

Subject of study. The negative effect of corpuscular radiation of outer space on the optical properties of a corner cube reflector with a multilayer dielectric coating is studied. Aim. The feasibility of applying a bilayer antireflection dielectric coating for radiation with a wavelength of 532 nm on the input face of a corner cube reflector operated in a high-orbit spacecraft is investigated. Method. Electron–proton irradiation simulating the effect of hot magnetospheric plasma on corner cube reflectors with and without antireflection coatings was carried out using a simulation stand. Because the corner cube reflector is made of KU-1 radiation-resistant quartz glass, special attention was devoted to the formation of electrostatic discharges that accompany the irradiation of the samples. Main results. The induced optical density of the coating of the input face of corner reflectors caused by magnetospheric plasma particles increased with the distance from the geometric center of the aperture of the input face to the metal frame. This can be explained by the fact that discharges with the maximum intensity were observed in the vicinity of the frame, resulting in the destruction of the near-surface layers of the irradiated samples. It has been experimentally established that the antireflection effect of a bilayer coating, which comprises HfO2 (thickness of approximately 25 nm) and SiO2 (thickness of approximately 100 nm), leveled with increasing irradiation dose. Practical significance. The results demonstrate that the use of the investigated bilayer antireflection coating is advisable in optical systems of a high-orbit spacecraft with an active lifetime not exceeding 6 years or under conditions of lower dose load.

Effects of sputtering power on the formation of 5 nm thick ferroelectric nondoped HfO2 gate insulator for MFSFET application

In this research, the effects of sputtering power on the ferroelectric property of 5 nm thick ferroelectric nondoped HfO2 were investigated for metal–ferroelectric–semiconductor field-effect-transistor application. The remnant polarization (2P r) was increased to 5.9 μC cm−2, and the density of interface states (D it) at silicon interface was effectively reduced to 1.8 × 1011 cm−2 eV−1 when the sputtering power was 50 W for 5 nm thick nondoped HfO2 formation. The largest Weibull slope (β) of 1.76 was extracted in Weibull distribution plot of the time-dependent dielectric breakdown measurements, and excellent fatigue properties until 1010 cycles were realized. The memory window of 0.56 V was realized by the pulse amplitude and width of −1/6 V and 100 ms, respectively. Furthermore, the memory characteristic was expected to be maintained ever after 10 years of retention time.

Precisely nanostructured HfO2 rear passivation layers for ultra‐thin Cu(In,Ga)Se2

AbstractThe quest for material‐efficient Cu(In,Ga)Se2 (CIGS) solar cells encourages the development of ultra‐thin absorbers. Their use reduces material consumption and energy usage during production by increasing the throughput. Thereby, both the bill of materials as well as the energy and capital costs are reduced. However, because thin absorbers are prone to increase back contact recombination, back surface passivation schemes are necessary to reach a similar or higher conversion efficiency than for absorbers with conventional thickness. Here, we investigate nanostructured hafnium oxide (HfO2) rear passivation layers for ultra‐thin CIGS solar cells. We fabricate regular arrays of point contacts with 200 nm diameter through HfO2 layers with thicknesses between 7 and 40 nm using electron beam lithography and reactive ion etching. The current–voltage curves of solar cells with a 500 nm thick CIGS absorber layer and the nanostructured passivation layer show improved performance concerning Voc and Jsc compared to non‐passivated reference devices. Furthermore, external quantum efficiency and optical reflection confirm an effective passivation behavior, with an average efficiency increase of up to 1.2% for the cells with the 40 nm thick HfO2 layer. In addition, simulation work shows that even 40 nm thick HfO2 passivation layers have only a minimal effect on the optical properties of ultra‐thin CIGS solar cells, and hence, the photocurrent increase verified experimentally stems from electrical improvements caused by the HfO2 layer passivation effect. We also investigate the impact of ultra‐thin (0.3, 0.6, 1.3, and 2.5 nm) non‐patterned HfO2 passivation layers on the same type of solar cells. However, these results showed no improvement in solar cell performance, despite an increase in the current density with layer thickness.

Investigation of deposition technique and thickness effect of HfO2 film in bilayer InWZnO-based conductive bridge random access memory

In recent research on memristor of brain-mimicking integrated circuit (IC) architecture, the electrical performance of electrochemical metallization memory can be significantly improved by applying a bilayer switching structure. In this work, a novel amorphous semiconductor InWZnO (IWZO) is used as the main switching layer, and a bilayer structure with HfO2 is fabricated by different deposition technique. By surface and chemical composition analysis, the effect of deposition process and thickness of HfO2 have been fully explored. The bilayer memory inserted with 3 nm HfO2 formed by atomic layer deposition process exhibits excellent electrical properties, such as high endurance cycle (more than 2 × 103), better retention time (up to 2 × 104 s, 85 °C), relatively uniform resistance state distribution and low compliance current operation. Consequently, the low power electrochemical metallization memory with optimized HfO2 layer has great potential for next generation memory in pixel and three-dimensional brain-mimicking IC technology.