Lift-off Technique Research Articles

Microfluidic Device for in Situ Observation of Bottom-up Copper Deposition in a TSV-like Structure- Improved Adhesion of Working Electrode Edge

The high performance of modern integrated circuits is now owing to packaging technologies. 3d-packaging using TSVs (Through Silicon Vias) has attracted large attention. Copper electroplating is used for filling conductor into the vias, because extreme bottom-up is available using a combination of additives. Even though extreme bottom-up is available with single addition of leveler without other additives and theoretical model, such as S-NDR model proposed by Moffat and Josell, can predict the filling behavior, still combination of leveler, suppressor and accelerator is generally used in industry and filling mechanism with the combination is not clear yet. To understand the mechanism of copper filling and the function of each additive, we have proposed microfluidic devices. In our previous study, a TSV-like structure was built in a micro channel as shown in figure 1, and real-time observation of copper bottom-up deposition was demonstrated. Though the observation was successful, observation period was limited to short periods of time, because abnormal deposition occurred at mouse of the TSV-like structure. In this study, we assumed that poor adhesion around the edge of the working electrode on a substrate caused the abnormal deposition, because insufficient adsorption of leveler is expected if the working electrode thin film was floating from the substrate. Then, fabrication process of the working electrode was changed from etching to lift-off patterning technique. Several processes were optimized and abnormal deposition along the edge was successfully suppressed as show in figure 2. Using the lift-off technique, a TSV-like structure was built and electrodeposition was examined. Electrodeposition was started at 30 s after feeding a plating solution containing the leveler. Results are shown in figure 3. No abnormal deposition around the opening of TSV-like structure more than 10 min, and we succeeded in the observation of suppression breakdown inside the via. Further study with various combination of additives will be planned. Figure 1

Facile Y-type Micro Ag2Se/MgAgSb flexible thermoelectric device based on lift-off technology

The development of thermoelectric devices provides an ideal solution for self-sustaining Internet of Things (IoT) applications. However, existing thermoelectric devices face issues such as low integration levels and poor stability. To address these challenges, this paper presents a novel method for fabricating flexible thermoelectric devices using Ag2Se/MgAgSb, which combines MEMS lithographic lift-off techniques to achieve precise control over the dimensions of the thermoelectric functional layer while also reducing fabrication costs. The optimal cell dimensions were identified through simulations. The device achieved an open circuit voltage of 0.49 mV at a temperature difference of 50 K. Additionally, the output power density of the Ag₂Se/MgAgSb flexible Y-type thermoelectric device reached 3.39 µW/m² at a temperature difference of 5 K. The device also demonstrated excellent flexibility; even after being bent 800 times around a glass rod with a diameter of 4 mm, its conductivity decreased by only 12%. Furthermore, it maintained stability under high humidity conditions. This work offers a novel strategy for the miniaturization and large-area controllable fabrication of flexible and reliable thermoelectric devices, as well as an enhancement in their reliability.

Reliable fabrication of 3D freestanding nanostructures via all dry stacking of incompatible photoresist

Three-dimensional (3D) free-standing nanostructures based on electron-beam lithography (EBL) have potential applications in many fields with extremely high patterning resolution and design flexibility with direct writing. In numerous EBL processes designed for the creation of 3D structures, the multilayer resist system is pivotal due to its adaptability in design. Nevertheless, the compatibility of solvents between different layers of resists often restricts the variety of feasible multilayer combinations. This paper introduces an innovative approach to address the bottleneck issue by presenting a novel concept of multilayer resist dry stacking, which is facilitated by a near-zero adhesion strategy. The poly(methyl methacrylate) (PMMA) film is stacked onto the hydrogen silsesquioxane (HSQ) resist using a dry peel and release technique, effectively circumventing the issue of HSQ solubilization by PMMA solvents typically encountered during conventional spin-coating procedures. Simultaneously, a dry lift-off technique can be implemented by eschewing the use of organic solvents during the wet process. This pioneering method enables the fabrication of high-resolution 3D free-standing plasmonic nanostructures and intricate 3D free-standing nanostructures. Finally, this study presents a compelling proof of concept, showcasing the integration of 3D free-standing nanostructures, fabricated via the described technique, into the realm of Fabry-Perot cavity resonators, thereby highlighting their potential for practical applications. This approach is a promising candidate for arbitrary 3D free-standing nanostructure fabrication, which has potential applications in nanoplasmonics, nanoelectronics, and nanophotonics.

Fabrication of thick Cr masks for reactive ion substrate etching by electron beam lithography and lift-off techniques

Fabrication of tall Cr nanostructures of different shapes by lithography and lift-off processes is demonstrated. By varying resist thickness, metal layer thickness, and diameter of holes in the resist mask, it is demonstrated that metal structures with shapes ranging from sharp-tipped conical over flat-top cones to nearly cylindrical can be fabricated. A comparison of resist layer dissolution in acetone, covered by Ag and Cr films, reveals that Cr films grow with an open structure of particles that allow rapid solvent diffusion through Cr layers that are several hundred nanometers thick. On the other hand, the 2D growth of Ag on the resist forms a barrier against acetone diffusion. The open structure of Cr enables the lift-off process to fabricate several-μm-high nanostructures using a single resist layer. As an example, high-aspect-ratio Si structures are demonstrated by reactive ion etching using thick Cr layers as a mask, fabricating nanopillars with 3 μm height at room temperature.

Wafer-scale N-polar GaN heterogeneous structure fabricated by surface active bonding and laser lift-off

N-polar Gallium nitride (GaN) technology is one of the most important technical routes for millimeter-wave devices. This paper introduces a new approach based on reverse epitaxial layer transfer technology to fabricate N-polar GaN materials. By combining low-damage surface-activated bonding and laser lift-off techniques, a wafer-scale N-polar GaN heterogeneous structure with low O impurities, high two-dimensional electron gas density, and high carrier mobility was obtained. The thin (∼2 nm) crystal bonding interlayer, coupled with the low thermal boundary resistance of only 6.8 m2K/G·W, provides evidence for the low interfacial damage caused by this approach. These results demonstrate its superiority as an attractive method for fabricating high-performance N-polar GaN millimeter-wave devices.

Patterned flexible Silver Nanowire/2D MXene transparent conducting electrode for organic light-emitting diodes

With the advent of numerous flexible optoelectronic applications, the importance of flexible and stable transparent conducting electrodes (TCEs) has been considerable. The conventional silver nanowire (AgNW) is a promising candidate as TCEs due to its decent opto-electrical properties and solution processibility. However, AgNW's high surface root mean square (RMS) roughness, relatively high sheet resistance, and low work function (∼4.5 eV) limit the usage for display applications. In this work, we design and study the AgNW and MXene (Ti3C2Tx) hybrid TCEs for flexible organic light-emitting diodes (OLEDs) to enhance the optoelectrical properties and the hole injection within the device. Experiment investigation is carried out via an application of AgNW/MXene (AgMX) hybrid TCEs as a bottom electrode for OLEDs devices in which the photolithography process and lift-off technique are employed for such fabrication. The hybrid TCEs indicate high optical transparency (86.8 % at 550 nm) and low sheet resistance (24.1 Ω/cm2) as well as improved surface morphology, enabling the efficient OLEDs operation. The optimized OLEDs exhibit a clear and stable red electroluminescence (EL) peak with an improved maximum external quantum efficiency (EQE) and luminance of 5.25 % and 3421.7 cd/m2, respectively. Furthermore, the hybrid TCEs are readily formed on poly(ethylene terephthalate) (PET) substrate, showing high durability under the bending test up to 100,000 cycles. The results demonstrate that adopting AgMX TCEs is a reasonable strategy for obtaining proper flexible optoelectronic devices.

Biofilm Spatial Monitoring By Multiple Electrochemical Techniques Integrated with Optical Microscopy and a Microfluidic Device

Biofilm represents the inherent structure of bacterial populations, consisting of bacteria encapsulated within extracellular polymeric substances they secrete. The natural formation of a biofilm acts as a physical barrier, offering protection against environmental changes and increasing bacterial antibiotic resistance, often up to 1,000 times. To enhance our understanding of these intricate biosystems, electrochemical analytical micro-systems are crucial to mimic the biofilm’s behavior in nature and to monitor responses to diverse treatments. Current electrochemical micro-systems, however, face a fundamental limitation, employing a single electrode to characterize biofilm behavior [1], despite biofilms being inherently three-dimensional and non-uniformly distributed across the electrode. Here, we introduce a novel electrochemical micro-systems that is based on an array of microelectrodes and microfluidics to provide spatial monitoring of the biofilm. Fabricated through conventional photolithography and lift-off techniques, the micro-system comprises five individual gold microelectrode arrays covered with a polydimethylsiloxane microfluidic channel (Fig. 1A) and linked to a reference Ag/AgCl electrode (Fig. 1B). In a proof-of-concept study focusing on monitoring the biofilm formation and functionality, we cultivated Pseudomonas aeruginosa, a pathogenic bacterium secreting electroactive molecules known as phenazines when reaching high cell density. Subsequently, we monitored biofilm formation over 30 hours by repetitively measuring the electrochemical signal using differential pulse voltammetry (DPV). The analysis of anodic (Fig. 1C) and cathodic (Fig. 1D) currents unveiled an increasing oxidation peak at -0.28V vs Ag/AgCl, varying among the five electrodes, attributed to phenazine formation (Fig. 1E). Furthermore, both anodic and cathodic currents decreased at the negative applied potential (-0.8 V vs Ag/AgCl), correlating with bacterial growth and phenazines production (Fig. 1F). Electrochemical impedance spectroscopy (EIS) at different DC potentials (-400, +50, and +400 mV vs Ag/AgCl; Fig. 1G, +400mV) was performed, and the impedance generated was correlated with 10 different equivalent electrical circuits (optimal circuit with the best fitting score is shown in Fig. 1H) [2]. Hourly monitoring of biofilm growth was conducted using microscopy imaging at multiple locations along the microfluidic channel (Fig. 1I, image after 10hr). The experiment's conclusion involved validating biofilm formation through fluorescent staining of the biofilm's sugars (Figs. 1J – 1L, phase image, fluorescent image, and overlay image). By integrating the developed electrochemical micro-system with microscopy imaging, real-time and direct monitoring of biofilm formation becomes achievable. Furthermore, this integrated system can be employed across electrochemical and microscopic platforms to explore biofilm properties in response to diverse conditions. Figure 1. (A) Microfluidic electrochemical array illustration (B) photograph of the experimental setup (C) 30-hours anodic and cathodic (D) voltammograms recorded in the presence of Pseudomonas aeruginosa (PA14). (E) transient oxidation peak current at -0.28V (F) the anodic and cathodic currents at -0.8V (G) transient impedance spectrographs recorded from PA14 (H) raw data and fitted electric circuit. Microscopic imaging of biofilm growth at (I) 10 hr, end of experiment (J) phase image, (K) biofilm polysaccharides fluorescent staining and (L) overlayed image.[1] Estrada-Leypon, O. et. al, Bioelectrochemistry 2015, 105, 56–64.[2] Ben-Yoav, H. et.al, Electrochim. Acta 2011, 56 (23), 7780–7786. Figure 1

Optimization of e-beam lithography parameters for nanofabrication of sub-50 nm gold nanowires and nanogaps based on a bilayer lift-off process

Electron beam lithography (EBL) stands out as a powerful direct-write tool offering nanometer-scale patterning capability and is especially useful in low-volume R&D prototyping when coupled with pattern transfer approaches like etching or lift-off. Among pattern transfer approaches, lift-off is preferred particularly in research settings, as it is cost-effective and safe and does not require tailored wet/dry etch chemistries, fume hoods, and/or complex dry etch tools; all-in-all offering convenient, ‘undercut-free’ pattern transfer rendering it useful, especially for metallic layers and unique alloys with unknown etchant compatibility or low etch selectivity. Despite the widespread use of the lift-off technique and optical/EBL for micron to even sub-micron scales, existing reports in the literature on nanofabrication of metallic structures with critical dimension in the 10–20 nm regime with lift-off-based EBL patterning are either scattered, incomplete, or vary significantly in terms of experimental conditions, which calls for systematic process optimization. To address this issue, beyond what can be found in a typical photoresist datasheet, this paper reports a comprehensive study to calibrate EBL patterning of sub-50 nm metallic nanostructures including gold nanowires and nanogaps based on a lift-off process using bilayer polymethyl-methacrylate as the resist stack. The governing parameters in EBL, including exposure dose, soft-bake temperature, development time, developer solution, substrate type, and proximity effect are experimentally studied through more than 200 EBL runs, and optimal process conditions are determined by field emission scanning electron microscope imaging of the fabricated nanostructures reaching as small as 11 nm feature size.

Monolithic integrated optoelectronic chip for vector force detection

Sensors with a small footprint and real-time detection capabilities are crucial in robotic surgery and smart wearable equipment. Reducing device footprint while maintaining its high performance is a major challenge and a significant limitation to their development. Here, we proposed a monolithic integrated micro-scale sensor, which can be used for vector force detection. This sensor combines an optical source, four photodetectors, and a hemispherical silicone elastomer component on the same sapphire-based AlGaInP wafer. The chip-scale optical coupling is achieved by employing the laser lift-off techniques and the flip-chip bonding to a processed sapphire substrate. This hemispherical structure device can detect normal and shear forces as low as 1 mN within a measurement range of 0–220 mN for normal force and 0–15 mN for shear force. After packaging, the sensor is capable of detecting forces over a broader range, with measurement capabilities extending up to 10 N for normal forces and 0.2 N for shear forces. It has an accuracy of detecting a minimum normal force of 25 mN and a minimum shear force of 20 mN. Furthermore, this sensor has been validated to have a compact footprint of approximately 1.5 mm2, while maintaining high real-time response. We also demonstrate its promising potential by combining this sensor with fine surface texture perception in the fields of compact medical robot interaction and wearable devices.

Electrical actuation of single-crystal diamond MEMS resonators at high temperatures

Achieving efficient low-voltage actuation of microelectromechanical system (MEMS) resonators in high-temperature environments poses a difficult topic due to the thermal interference and the risk of high-temperature failure. In this work, the single-crystal diamond (SCD) resonators fabricated through the ion implantation-assisted lift-off (IAL) technique exhibit a SCD-on-SCD cantilever structure. We propose an electrical actuation system based on the electrostatic effect specifically designed for SCD MEMS resonators with a low radio-frequency amplitude of ∼ 100 mV. The SCD resonators demonstrate stable and efficient actuation across a wide temperature range, from room temperature to 500 °C. Importantly, the actuation voltage exhibits little impact on the resonance frequency and the Q factor of the resonator. The SCD resonator showcases exceptional thermal stability in resonance frequency, with a low temperature coefficient of frequency (TCF) below −12 ppm/°C up to 500 °C. The developed actuation scheme holds tremendous potential as a robust platform for realizing SCD MEMS devices, particularly in applications requiring high integration at high temperatures.

Investigation of Power IR (850 nm) Light-Emitting Diodes Manufacturing by Lift-Off Technique of AlGaAs–GaAs-Heterostructure to Carrier-Substrate

Investigation of Power IR (850 nm) Light-Emitting Diodes Manufacturing by Lift-Off Technique of AlGaAs–GaAs-Heterostructure to Carrier-Substrate

Impact of epitaxial strain relaxation on ferromagnetism in a freestanding La2 / 3Sr1 / 3MnO3 membrane

Manganite perovskites host emerging physical properties of strongly-correlated electrons with charge, spin, and lattice degrees of freedom. Using the epitaxial lift-off technique, we report enhancement of saturation magnetization and the ferromagnetic transition temperature of freestanding La2/3Sr1/3MnO3 membrane compared with as-grown film on SrTiO3 substrate involving lateral tensile strain. Structural analysis reveals shrinkage of unit-cell volume by tensile strain relaxation in the freestanding membrane, which causes enhancement of the ferromagnetic interaction. The impact of the microscopic lattice deformation on the ferromagnetism of La2/3Sr1/3MnO3 indicates a high potential of this material for flexible electronics applications with intriguing functionalities in strongly-correlated electron systems.

Advanced manufacturing of microscale light-emitting diodes and their use in displays and biomedicine

Thin-film, microscale light-emitting diodes (micro-LEDs)-based III–V compound semiconductors are emerging as a promising technology for next-generation light sources with various applications including lighting, display and biomedicine. With this perspective, we provide an overview and highlight our own progress on the recent developments in advanced manufacturing techniques and potential applications of micro-LEDs. The combination of epitaxial lift-off and transfer printing techniques have produced ultraminiaturized, highly luminous, and energy-efficient thin-film micro-LEDs, with a wide range of applications. To achieve full-color displays, several approaches have been developed, including horizontally and vertically assembled red-green–blue (RGB) pixels, and quantum dot conversion. For neuroscience studies, these micro-LEDs can be integrated in flexible probes as implantable light sources, which facilitate light delivery into the deep brain tissue for optogenetic stimulations and biological sensing. Additionally, micro-LEDs have the potential to function as wireless biosensors to detect changes in the biological environment, through their luminescence or in combination with other devices. Overall, advanced manufacturing techniques produce micro-LEDs that promise significant applications not only in displays but also in various biomedical fields.

Size and kink effects on thermal conductivity in nickel nanowires

The potential applications of nanowires in thermal management and thermoelectric energy conversion have sparked extensive research on thermal transport in various nanowires. Nickel nanowires, with their unique properties and promising applications, have been extensively studied. However, the influence of size, particularly the impact of kink structures, on the thermal transport behavior in nickel nanowires remains unclear. In this paper, we employed electron-beam lithography and liftoff techniques to fabricate suspended nickel nanowires with varying sizes and kinks to experimentally investigate the size and kink effect on the thermal conductivity. The experimental results revealed that the thermal transport behavior of nickel nanowires is significantly influenced by both size and kink effects. Notably, as the nanowire size decreases, the thermal conductivity also decreases. Furthermore, we discovered that the thermal conductivity can be adjusted by altering the number and angle of kinks. Increasing the number of kinks from 18 to 36 resulted in a significant decrease in thermal conductivity. In contrast, as the kink angle decreased from 157° to 90°, the thermal conductivity also decreased. However, intriguingly, when the kink angle was further decreased from 90° to 43°, the thermal conductivity increases. This non-monotonic change in thermal conductivity with the kink angle provides an interesting insight into the intricate behavior of heat carriers in kinked nickel nanowires. Additionally, we found that varying the alloy elements can profoundly alter the thermal conductivity of nanowires with kinks. These results offer valuable insights into the behaviors of heat carriers, including electrons and phonons, during heat transfer in nickel nanowires.

Novel TFC membrane with PMIA thin-film as active layer fabricated by the T-FLO technique for dye desalination

Poly (m-phenylene isophthalamide) (PMIA) generally is employed in a thin-film composite (TFC) membrane as the porous substrate via the non-solvent induced phase separation technique. Relatively little research has been done on PMIA thin-films. Here, dense PMIA thin-film was deposited via the cast-coating coupled solvent evaporation and was modified systematically. The ion permeance of PMIA thin-film has been explored for the first time. Furthermore, the novel TFC membrane with PMIA active layer (TFC-P10) was successfully fabricated for the first time via the thin-film liftoff (T-FLO) technique. The mechanism of applying PMIA thin-film as the active layer was illustrated by SEM, FTIR and XPS as the secondary amine group of PMIA reacted with the epoxide group of the epoxy layer. TFC-P10 performed remarkable dye desalination. Specifically, Congo red (CR) rejection of 99.55 % (50 mg L−1), NaCl rejection of 7.34 % (1000 mg L−1), high-temperature resistance under 90 °C, good antifouling with FRR > 90 %, and 96 h long-term stability. Besides, the dye/salt selectivity coefficient α of TFC-P10 was 207.9. This work endows a guideline for in-situ dye effluent treatment and the novel TFC membrane fabrication.

Direct synthesis of nanopatterned epitaxial graphene on silicon carbide

This article introduces a straightforward approach for the direct synthesis of transfer-free, nanopatterned epitaxial graphene on silicon carbide on silicon substrates. A catalytic alloy tailored to optimal SiC graphitization is pre-patterned with common lithography and lift-off techniques to form planar graphene structures on top of an unpatterned SiC layer. This method is compatible with both electron-beam lithography and UV-lithography, and graphene gratings down to at least ∼100 nm width/space can be realized at the wafer scale. The minimum pitch is limited by the flow of the metal catalyst during the liquid-phase graphitization process. We expect that the current pitch resolution could be further improved by optimizing the metal deposition method and lift-off process.

Optimized photoresponse performances in vertical and horizontal photodetectors based on freestanding GaN membranes

GaN has become an important material in the optoelectronics due to its suitable band gap and excellent physical and chemical properties, however, the grown substrates limit device geometry owing to their poor electrical and thermal conductivity. Herein, horizonal and vertical GaN ultraviolet photodetectors (UV PDs) were fabricated using the freestanding GaN membranes obtained by an epitaxial lift-off technique. The photoresponse behaviors in the fabricated PDs were then optimized by the piezo-phototronic effect. Owing to the synergistic effect of spontaneous polarization and asymmetric electrodes, the vertical UV PD exhibits unique self-powered characteristics with the light on/off of 3.3 × 103 and the responsivity (R) of 114 mA/W at the bias of 0 V. Optimized by the piezo-phototronic effect, the R increases to 168 mA/W at a tension strain of 0.25 %. In comparison, the R of horizontal UV PD decreases, regardless of the compression or tension. Physical mechanisms of the performances optimization in the vertical and horizontal UV PDs are proposed and analyzed. This study opens a pathway to fabricate GaN UV PDs with different device configurations and provides fundamental support for their performance modulation by the piezo-phototronic effect.

Novel TFC membrane with crosslinked PMIA active layer fabricated by the T-FLO technique for dye desalination

Poly (m-phenyleneisophthalamide) (PMIA) is a promising candidate for membrane material resulting from its excellent thermal and chemical stability. Being the linear molecular structure, almost all studies have been carried out on PMIA as a substrate material for thin-film composite (TFC) membranes rather than an active layer. Here, amino-epoxide crosslinkers were introduced into the PMIA dope solution to modify the molecular structure for the first time. Employing the casting coupled solvent evaporation deposited the crosslinked PMIA (CPMIA) thin-film. The effect of crosslinker type and concentration on the morphology and the ion permeance have been investigated systematically. That is, achieving the separate optimization of the active layer. Based on the CPMIA thin-film with the best ion permeation, novel TFC membranes (TFC-CPE) with CPMIA active layer were generated successfully via the thin-film liftoff (T-FLO) technique for the first time. Results demonstrated that TFC-CPE had desirable dye desalination performance. For feeding solution containing 100 ppm CR and 1 g L−1 NaCl, the permeance was 25.17 L m−2 h−1 bar−1. CR rejection was 99.88% and NaCl rejection was 6.30%. The selective coefficient α was up to 793.9. It also demonstrated that the TFC-CPE had excellent thermal resistance, anti-fouling capability and long-term stability. In conclusion, the novel TFC membrane has a promising future for in-situ dye wastewater treatment. This work throws light on the PMIA modification and the preparation of the TFC membrane with a novel active layer.

Influence of buffer layer on nitrogen doped AZO relative humidity sensor with p-n heterojunction structure

A nitrogen doped AlZnO (N-doped AZO) relative humidity (RH) sensing p-n heterojunction structure on n-type silicon substrate using DC sputtering system and lift-off technique has been proposed in this research. N-doped AZO RH sensing film was adopted to enhance photocatalyst effect and realize the heterojunction structure in humidity sensing device. The carrier properties of N-doped AZO RH sensing film were evaluated in the carrier type, carrier mobility, carrier concentration, and carrier conductivity using Hall effect measurement. The N-doped AZO RH sensors were designed with a p-n heterojunction structure and embedded a buffer layer structure. The sensing properties of N-doped AZO RH sensors both with polymer buffer layer and without polymer buffer layer were respectively explored and evaluated in the hysteresis effect, sensitivity, linearity, sensing response and recover response characteristics using environment measurement system and measurement firmware. The research results revealed the carrier type, carrier mobility, carrier concentration, and carrier resistivity of the N-doped AZO RH sensing film deposited on p-n heterojunction structure with buffer layer respectively reached as p-type, 9.6✕101 cm2/Vs, 3.3✕1017 cm−3, and 5.0✕100 Ω-cm. The RH sensing hysteresis error of N-doped AZO RH sensor with buffer layer is approximately 4.57% at 60 %RH which is less than those at other relative humidity values. The sensitivity of N-doped AZO RH sensor with buffer layer reaches as more than 15.7 kΩ/%RH at desorption process which is better than that of sensor without buffer layer either at relative humidity value from 30 %RH to 90 %RH and from 30 %RH to 75 %RH. The adsorption linearity and desorption linearity of N-doped AZO RH sensor with buffer layer are respectively 0.31 and 0.27 between 30 %RH and 75 %RH which are better than those without buffer layer. The adsorption time and desorption time responses of N-doped AZO RH sensor with buffer layer measured during the measurement time of 2000 s with two measuring cycles were estimated about 235 s and 295 s at first cycle respectively less than those of the RH sensor without buffer layer. The design of polymer buffer layer embedded between N-doped AZO RH sensing p-n heterojunction structure and use of n-type silicon substrate revealed the better sensing characteristics and electrical properties both the molecular adsorption/desorption effects in chemical property and the carrier transport phenomena in physical property respectively.

Impact of capacitive effect on the J-V hysteresis in MgZnO/CdTe solar cells

In this study, we demonstrate that the current–voltage (J-V) hysteresis of MgZnO (MZO)/CdTe solar cells mainly originates from the modification of J-V curves by nonsteady-state photocurrent, which is correlative with low-frequency capacitance corresponding to the MZO/CdTe interface. The hysteresis was significantly strengthened by introducing oxygen into the CdTe growth atmosphere, and became more pronounced as the MZO thickness increased. The electrical properties of MZO in CdTe solar cells were analyzed in detail by using thermomechanical lift-off technique. The results showed that oxygen during CdTe growth seriously deteriorated the conductivity of MZO by reducing the concentration of oxygen vacancy. Furthermore, a spike-like conduction band offset (CBO) greater than 0.25 eV was determined at MZO/CdTe interface due to the formation of Mg-rich surface layer. Thus, the low-frequency capacitance originating from electron accumulation at MZO/CdTe interface was increased due to the less probability of electron tunneling through the interface barrier. SCAPS simulations confirmed that the low-frequency capacitance was caused by substantially reduced conductivity of MZO and large CBO at MZO/CdTe interface. The experimental and simulation results indicate that the J-V hysteresis of MZO/CdTe cells can be suppressed by reducing low-frequency capacitance, which is achievable through enhanced electron transport at MZO/CdTe interface.