Josephson diode (JD) effect in Josephson tunnel junctions (JTJs) has attracted a great deal of attention due to its importance for developing superconducting-circuitry-based quantum technologies. Even though the preparation of high-quality JTJs by techniques employed in the semiconductor industry has been demonstrated, which was an important milestone because JTJs are the building blocks of superconducting electronics even before the quantum era, the JD effect has not been accomplished in them, nor has the highly desirable electrical control of the effect. We report here the fabrication of JTJs featuring a composite tunnel barrier of Al2O3 and Hf0.8Zr0.2O2 using complementary-metal-oxide-semiconductor compatible atomic layer deposition. These JTJs were found to show the JD effect in nominally zero magnetic fields with nonreciprocity controllable via an electric training current, yielding a surprisingly large diode efficiency. The quasiparticle tunneling, through which the Josephson coupling in a JTJ is established, was found to show theoretically expected gap features but no nonreciprocity. We attribute these observations to the simultaneous presence of positive and negative local Josephson couplings in the JTJs, with the negative Josephson coupling originating from indirect tunneling, which results in spontaneous time-reversal symmetry breaking. The double-minima washboard potential for the ensemble-averaged phase difference in the resistively and capacitively shunted junction model is shown to fully account for the experimentally observed JD effect.

A bottom-up fabrication approach for flexible piezoelectric micromachined ultrasound transducer (PMUT) arrays on stainless-steel substrates was developed. Devices were fabricated using chemical solution deposition of a 700 nm-thick layer of Pb0.99□0.01(Zr0.52Ti0.48)Nb0.02O3, where □ denotes a vacancy on the Pb site, on 50 μm-thick LaNiO3/HfO2/stainless-steel foils. Lithography for definition of the electrode and piezoelectric layers was completed on the front of the wafer. Ni electroplating on the back side of the foil was used to create locally stiff areas to define the deflection area. PMUT devices were successfully fabricated using this method. The permittivity and loss tangent of the fabricated device at 1 kHz were 283 ± 9 and <1.5%, respectively. The remanent polarization was measured to be 38 ± 0.3 μC/cm2.

This letter reports the concept and implementation of a novel AlGaN/GaN sidewall gated HEMT design. This new device structure can achieve a high threshold voltage, based on a p-GaN/AlGaN/GaN epitaxy stack, by forming a MOS gate on the p-GaN sidewall. Incorporation of composition grading in the AlGaN layer eliminates electron injection barrier between the vertical channel on the p-GaN sidewall and the lateral 2DEG channel, resulting in favorable output I-V characteristics. Experimental implementation of this new device design resulted in a high threshold voltage of 3.5 V, favorable on-state I-V characteristics, low value of ${C}_{ extit {gd}}$ , breakdown voltage of 1050 V, and controlled degradation of dynamic on-resistance. Further improvements in on-resistance can be expected by optimizing device geometries; and further improvements in breakdown voltage and dynamic on-resistance can be expected with improved charge balance in the drift region.

Growing interest in nonvolatile memory compatible with back-end-of-line integration has driven advancements in ferroelectric field-effect transistors (FeFETs) with oxide semiconductors. Recently, we demonstrated $15 \mathrm{~nm} \mathrm{Al}_{1-\mathrm{x}} \mathrm{B}_{\mathrm{x}} \mathrm{N}$ FeFETs with ZnO semiconductor channels, but the memory window (MW) was small, $\sim 1 \mathrm{~V}$ at room temperature, due to a negative shift in the turn-on voltage ($\mathrm{V}_{\text {on }}$) with cycling, as shown in Fig. 3 [1]. We attribute this shift to the thinfilm transistor structure. For sufficiently large $+V_{G S}$, polarization in the $\mathrm{Al}_{1-x} \mathrm{~B}_{x} \mathrm{~N}$ layer from source to drain can be switched upward due to electron accumulation in the ZnO channel. However, for $-\mathrm{V}_{\mathrm{GS}}$, the $\mathrm{Al}_{1-\mathrm{x}} \mathrm{B}_{\mathrm{x}} \mathrm{N}$ polarization can only be switched downward in the FET contact regions. Because holes are not accumulated in the ZnO, the electric field in the channel region (between contacts) is not confined. In this region, once switched upward, the polarization will remain in this state. The unswitched polarization results in a negative shift in the gate voltage required for electron accumulation in the channel region (between the contacts). Combined with significant electron density at the source and drain metal contacts due to Fermi level pinning near the ZnO conduction band minimum at the $\mathrm{ZnO}-\mathrm{Ti}$ interface ($\mathrm{E}_{\mathrm{C}}-\mathrm{E}_{\mathrm{F}} \sim 0.1 \mathrm{eV}$), this results in a negative shift in the threshold and turn-on voltages for the FeFET (Fig. 5). By introducing a thin Al2 O3 dielectric layer between the ZnO channel and Ti source/drain contacts, we are able to unpin the Fermi level, resulting in a large increase in memory window (to $\sim 10 \mathrm{~V}$ for FeFETs with 15 nm thick $\mathrm{Al}_{1-\mathrm{x}} \mathrm{B}_{\mathrm{x}} \mathrm{N}$ and 10 nm thick ZnO channel).

The discovery of ferroelectricity in hafnium zirconium oxide (HfxZr1-xO2) and related fluorite materials has spurred interest in ferroelectric devices suitable for integration with silicon integrated circuits (ICs), especially those that can be embedded in the back-end-of-line (BEOL) process. More recently, ferroelectricity has been found in wurtzite aluminum nitride-based materials, such as scandium and boron-doped aluminum nitride (Al1-xScxN and Al1-xBxN). Although these materials currently have undesirably large coercive electric fields, and small breakdown electric field-to-coercive electric field ratio, their low processing temperatures and large remanent polarization offer intriguing possibilities for device applications. Here, we report ferroelectric field-effect transistors (FeFETs) with a 15 nm thick Al0.88B0.12N layer and an 11 nm ZnO semiconductor channel, achieving a memory window >1 V and switching voltages (V-switch) <+/- 10 V.

Several proposed future X-ray missions will require thin (<= 0.5 mm thick) mirrors with precise surface figures to maintain high angular resolution (<= 0.5 arcsec). To study methods of meeting these requirements, adjustable X-ray optics have been fabricated with thin-film piezoelectric actuators to perform figure correction. The fabrication and actuator performance for an adjustable X-ray mirror that forms a conical approximation to a Wolter-I telescope are reported. The individual responses of actuator cells were measured and shown to induce a figure change of 870 nm peak-to-valley on average. These measured responses were compared with predicted responses generated using a finite-element analysis algorithm. On average, the measured and predicted cell responses agreed to within 60 nm root mean square. A set of representative mirror distortions and the measured cell responses were used to simulate figure corrections and calculate the half-power diameter (HPD, single reflection at 1 keV) achieved. These simulations showed an improvement in 4.5 to 9 arcsec mirrors to 0.5 to 1.5 arcsec HPD. The disagreements between the predicted and measured cells' performance in actuation and figure correction were attributed to a high spatial frequency metrology error and differences in mirror bonding considerations between the finite-element analysis model and the as-built mirror mount.

B -doped AlN ferroelectric thin films are promising candidates for non-volatile memory device applications, due to their relatively low deposition temperatures, large remanent polarizations (up to 120 mu C/cm2), and outstanding polarization retention properties. In this research, the polarization switching fatigue characteristics in epitaxial B -doped AlN thin films were investigated. Films were deposited via reactive RF magnetron sputtering onto W -coated c -plane sapphire substrates. Polarization switching fatigue measurements were conducted with triangular waveforms for frequencies between 50 and 1000 Hz from room temperature to 200 degrees C. On repeated cycling, the films undergo wake-up; the wake-up process was characterized by an activation energy of 0.15 +/- 0.05 eV. After a period of normal switching, the film leakage current increased with additional cycling, and finally the films underwent dielectric breakdown. Unpatterned Al0.93B0.07N films with 100 nm thick Pt top electrodes survived -104 bipolar cycles, whereas films with 1000 nm W top electrodes survived -105 cycles before dielectric breakdown. Modeling was used to design a field plate, which improved the performance to -106 fatigue cycles. It was found that dielectric failure during fatigue was not due to surface flashover but was associated with hard breakdown events in the dielectric. Failure for 100 Hz cycling was not associated with device self -heating but was exacerbated by field concentrations; at higher frequencies, the instantaneous temperature rise during the switching becomes prominent. Based on the fatigue process under 100 Hz under various ambient temperatures, the activation energy of the polarization switching fatigue of B -doped AlN thin films was found to be Ea = 0.10 +/- 0.03 eV.

Wurtzite ferroelectrics are an emerging material class that expands the functionality and application space of wide bandgap semiconductors. Promising physical properties of binary wurtzite semiconductors include a large, reorientable spontaneous polarization, direct band gaps that span from the infrared to ultraviolet, large thermal conductivities and acoustic wave velocities, high mobility electron and hole channels, and low optical losses. The ability to reverse the polarization in ternary wurtzite semiconductors at room temperature enables memory and analog type functionality and quasi-phase matching in optical devices and boosts the ecosystem of wurtzite semiconductors, provided the appropriate combination of properties can be achieved for any given application. In this article, advances in the design, synthesis, and characterization of wurtzite ferroelectric materials and devices are discussed. Highlights include: the direct and quantitative observation of polarization reversal of ∼135 μC/cm2 charge in Al1−xBxN via electron microscopy, Al1−xBxN ferroelectric domain patterns poled down to 400 nm in width via scanning probe microscopy, and full polarization retention after over 1000 h of 200 °C baking and a 2× enhancement relative to ZnO in the nonlinear optical response of Zn1−xMgxO. The main tradeoffs, challenges, and opportunities in thin film deposition, heterostructure design and characterization, and device fabrication are overviewed.

The effective large signal longitudinal piezoelectric coefficient (d33,f∗) of piezoelectric thin films on rigid substrates has been widely investigated. Unclamped piezoelectric thin films are predicted to have a higher d33,f∗ coefficient due to reduced constraints on piezoelectric strain, domain reorientation, and domain wall motion, but quantitative measurements of this coefficient have been limited. This study uses microfabrication techniques along with double-beam laser interferometry (DBLI) to accurately determine the longitudinal piezoelectric coefficient of Pb(Zr,Ti)O3 thin films in partially released piezomicroelectromechanical structures. A two-step backside release process was used: first, deep reactive ion etching to create backside vias and second, wet etching of the ZnO sacrificial layer to release the area beneath the Pb(Zr,Ti)O3 thin films. Post wet etching, optical profilometry showed concavely deformed diaphragms resulting from asymmetrical stress profiles through the diaphragm thickness. DBLI was then used to examine diaphragm deflection under an applied unipolar voltage ranging from 0 to 10 V. Devices with 50% and 75% of the area beneath the top electrode released exhibited large signal d33,f∗ values of 410 ± 6 and 420 ± 8 pm/V, respectively, more than three times higher than the d33,f∗ value of the clamped samples: 126 ± 13 pm/V. The reasons contributing to the large d33,f∗ include (i) the change in stress levels due to the release process, (ii) the elimination of mechanical constraints from substrate clamping, and (iii) enhanced domain reorientation. These findings confirm that substrate declamping significantly boosts the piezoelectric coefficient, bringing d33,f∗ closer to the bulk longitudinal piezoelectric coefficient (d33).

Abstract Here, a polymer blend active layer that exhibits both electronic and adhesive properties is introduced. Various conjugated polymers are blended with a catechol‐based polymer that shows high adhesion, such that blends serve as the active layer of multifunctional sticky organic thin‐film transistors (OTFTs). Blend films maintain relatively constant field‐effect charge carrier mobility in OTFTs regardless of composition. Lap shear adhesion strength tests show that all blend films exhibit adhesive properties with adhesion values ranging from 0.05 to 4.30 MPa. With relatively consistent mobility and the presence of adhesive properties at different compositions, blends of conjugated and adhesive polymers can lead to next‐generation organic transistors for stable 3D stacking and waterproof adhesive sensors.

As scaling becomes increasingly difficult, there is growing interest in vertical or three-dimensional stacking of transistors and especially memory. Ferroelectric semiconductor field effect transistors can be key enablers to improve energy efficiency and overall chip and memory performance. In this work, low-temperature processed, back-end-of-the-line compatible transistors were demonstrated by depositing a layered chalcogenide ferroelectric semiconductor, beta-phase In2Se3, at temperature as low as 400 °C. Top gate n-channel In2Se3 thin film transistors were fabricated with field-effect mobility ∼1 cm2 V−1 s−1, and simple polarization switching based memory results are presented.

This paper reports the fatigue and retention behavior for Al1-xBxN thin films, a member of the novel family of wurtzite ferroelectrics, with an emphasis on the role of capacitor architecture. By modifying the capacitor architecture, and thus thermal and electrical boundary conditions, we create insight regarding the relative importance of intrinsic and extrinsic contributors to the degradation tendencies. Our experiments suggest that bipolar cycling of metal (Pt/W)/Al0.93B0.07N/W/Al2O3 film stacks first induced wake-up, then a region of constant switchable polarization. On additional cycling, the film leakage current increased, and then films underwent dielectric breakdown. For unpatterned first generation Al0.93B0.07N films with 100 nm thick Pt top electrodes survive 104 bipolar cycles, whereas films with 1000 nm W top electrodes survive 10^5 cycles before thermal dielectric breakdown. Sentaurus modeling was used to design an SU8 field plate which improved the performance to 10^6 fatigue cycles. It was found that the thermal failures during fatigue were not due to surface flashover events but were associated with hard breakdown events in the dielectric. The films showed excellent retention of the stored polarization state. As expected, data retention was slightly inferior in the opposite state (OS) measurements. However, it is noted that even after 3.6x10^6 sec (1000 hr). at 200C, the OS signal margin still exceeded 200 uC/cm2. The predicted OS retention is 82 after 10 years baking at 200oC.

Objective and Impact Statement. Simultaneous imaging of ultrasound and optical contrasts can help map structural, functional, and molecular biomarkers inside living subjects with high spatial resolution. There is a need to develop a platform to facilitate this multimodal imaging capability to improve diagnostic sensitivity and specificity. Introduction . Currently, combining ultrasound, photoacoustic, and optical imaging modalities is challenging because conventional ultrasound transducer arrays are optically opaque. As a result, complex geometries are used to coalign both optical and ultrasound waves in the same field of view. Methods . One elegant solution is to make the ultrasound transducer transparent to light. Here, we demonstrate a novel transparent ultrasound transducer (TUT) linear array fabricated using a transparent lithium niobate piezoelectric material for real-time multimodal imaging. Results . The TUT-array consists of 64 elements and centered at ~6 MHz frequency. We demonstrate a quad-mode ultrasound, Doppler ultrasound, photoacoustic, and fluorescence imaging in real-time using the TUT-array directly coupled to the tissue mimicking phantoms. Conclusion . The TUT-array successfully showed a multimodal imaging capability and has potential applications in diagnosing cancer, neurological, and vascular diseases, including image-guided endoscopy and wearable imaging.

When utilizing double-beam laser interferometry to assess the piezoelectric coefficient of a film on a substrate, probing both top and bottom sample surfaces is expected to correct the erroneous bending contribution by canceling the additional path length from the sample height change. However, when the bending deformation becomes extensive and uncontrolled, as in the case of membranes or fully released piezoelectric films, the double-beam setup can no longer account for the artifacts, thus resulting in inflated film displacement data and implausibly large piezoelectric coefficient values. This work serves to identify these challenges by demonstrating d33,f measurements of fully released PZT films using a commercial double-beam laser interferometer. For a 1 μm thick randomly oriented PZT film on a 10 μm thick polyimide substrate, a large apparent d33,f of 9500 pm/V was measured. The source of error was presumably a distorted interference pattern due to the erroneous phase shift of the measurement laser beam caused by extensive deformation of the released sample structure. This effect has unfortunately been mistaken as enhanced piezoelectric responses by some reports in the literature. Finite element models demonstrate that bending, laser beam alignment, and the offset between the support structure and the electrode under test have a strong influence on the apparent film d33,f.

Abstract. Next-generation x-ray observatories require lightweight, high throughput optics that maintain a <0.5  arcsec resolution to probe the physics of black holes and gain understanding of the early universe. One potential type of x-ray mirror consists of a 400-μm thick curved Corning EAGLE XG® glass substrate with a Cr/Ir x-ray mirror coating deposited on the front (concave) side and an array of radio frequency sputtered Pb0.995  (  Zr0.52Ti0.48)0.99Nb0.01O3 piezoelectric thin film actuators on the back (convex) side to enable correction of figure errors. A stress-balancing process was developed to correct the figure distortion arising from thin film stresses in the actuator layers. Compressively stressed SiO2 films were deposited on the convex side of the mirror to balance the tensile integrated stress of the actuator array while also matching the film thickness distribution. Finite-element methods were used to assess the impact of film thickness distributions on the convex and concave substrate surfaces. The resulting models show peak-to-valley figure errors of 105 nm, well within the 1-μm peak-to-valley dynamic range of the piezoelectric adjusters. In contrast, when stress compensation was done with an iridium mirror film deposited on the front side, the mismatched thickness distribution results in peak-to-valley figure errors over 3  μm.

Adjustable X-ray optics represent a potential mirror technology for the NASA Lynx X-ray observatory mission concept. Adjustable optics employ an integrated micron-thick piezoelectric film deposited on the convex side of silicon Wolter-type mirror segments. Discrete, independently addressable electrodes on the convex surface form individual actuators; the applied voltages are used to correct the shape of the mirror segments for figure errors resulting from a change in thermal environment, epoxy creep, or failure of an epoxy bond. On-orbit correction requires a metrology system to provide real-time feedback of mirror figure. We are examining the use of deposited semiconductor strain gauges to monitor mirror mechanical strains and surface temperatures. To establish requirements for monitoring we modeled a variety of thermal and mechanical disturbances to a mirror segment such as might occur on-orbit or from launch. Models are described and resulting requirements and performance discussed.

Thin adjustable X-ray mirrors can correct deformations generated from fabrication, gravity release, mounting stresses, drifting stresses in the reflecting layer(s) and thermal variations while maintaining high angular resolution (< 0.5 arcsecond) and large effective area (< 2 m2) required for future X-ray missions. This work presents fabrication process developments for adjustable mirror segments with actuators for the Lynx X-ray observatory mission concept. Piezoelectric actuator arrays were fabricated on the convex side of precision slumped glass or curved silicon mirror segments using a 1.5 μm thick lead zirconate titanate (PZT) film. A two-layer metal routing scheme with a polymeric insulator was used to independently address 288 actuators on the mirror. The two-layer metal allows narrow kerfs between actuators and increased actuator density. Anisotropic conductive film was used to bond thin flexible copper cables to flat edges of the mirror to interface with external control electronics. This prototype mirror has eight cables with a total of 290 connections to access the array. To reduce the cabling complexity for future mirrors, thin film transistors have been fabricated on the curved mirror to function as access switches. To facilitate this, a mask aligner that allows precision alignment on curved mirror segments was developed and arrays of thin film transistors (TFT) on curved substrates have been tested. TFT and actuator integration on future mirrors will reduce external connections to just two cables with a total of 30 connections. Keywords: Lynx, adjustable optics, X-ray optics, thin film piezoelectric, curved substrate aligner

Lift-off is a useful method to pattern metals or other materials in semiconductor device fabrication. Polymer photoresists are commonly used for lift-off; however, these resists cannot be used when the device fabrication requires high temperature with the lift-off materials in place. Here, we have used bilayer stacks of metal oxides deposited by plasma-enhanced atomic layer deposition (PEALD) to provide a lift-off process compatible with high temperatures (>500 C). By using selective wet-etching techniques, we successfully created reentrant or undercut structures and achieved lift-off for micro- and nano-scale devices. We also demonstrated that the metaloxide based inorganic lift-off process allows high-temperature material deposition and/or annealing as a part of the fabrication process. The inorganic lift-off technique overcomes temperature limits of polymer resists and provides a photoresist-residue-free and high-temperature capable process.