Characteristics of Nb junctions with additional Al/AlOx

We systematically investigated the characteristics of Nb/Al/AlO_x/Al/AlO_x/Nb junctions having a structure in which Al/AlO_x films are placed in a Nb/Al/AlO_x/Nb trilayer junction. The junction characteristics reflect superconductivity in the second Al film due to the proximity effect. As a result, the critical current density J_c and the hysteresis on its current–voltage (I–V) curve strongly depend on the transparencies of the AlO_x barriers. The hystereses of the junctions are smaller than those of Nb/Al/AlO_x/Nb junctions at the same J_c. Intrinsically overdamped junctions are also realized. The intrinsically overdamped junctions can considerably reduce an area occupied by a single gate. Furthermore, the spread of critical current I_c for the junctions, which have hystereses, connected in series, is ±1.2%. This shows that the junctions with additional Al/AlO_x have uniform enough characteristics to be used in integrated circuits.     

Thickness determination of ultrathin oxide films and its application in magnetic tunnel junctions

In this study we propose a method for utilizing x-ray photoelectron spectroscopy (XPS), a surface sensitive technique, coupled with a wedge-shaped sample to determine the thickness of an ultrathin aluminum oxide tunnel barrier layer (∼2 nm) in a magnetic tunnel junction (MTJ). The uncertainty of the measured thickness is analyzed and the factors affecting the accuracy of this measurement are discussed as well as the advantages over the use of high-resolution transmission electron microscopy. Using this approach, we were able to quickly optimize the thickness of an aluminum oxide layer in a fabricated MTJ, yielding a high magnetoresistance ratio. In addition to XPS, one can also use Auger electron spectroscopy to determine the thickness of the oxidized tunnel barrier layer. This method can also be applied to other tunnel barrier materials such as the nitrides.     

Tunnel magnetoresistance devices processed by oxidation in air andUV assisted oxidation in oxygen

Tunnel magnetoresistance (TMR) devices were processed by sputter deposition of Co, Al and NiFe on oxidized Si wafers. After the Al deposition, an ex-situ oxidation in air at room temperature or an in-situ oxidation enhanced by ultraviolet (UV) irradiation in high purity oxygen at 100 mbar follows. The electrical and magnetic properties of the junctions are measured and discussed concerning specific junction resistance, magnetoresistance ratio, long time stability of the junctions, and failure rate of the processes. Some microscopic experiments provided consistent information of the tunnel barrier. MR ratios between 15% and 20% were measured for the different oxidation processes     

Single‐Electron Pass‐Transistor Logic with Multiple Tunnel Junctions and Its Hybrid Circuit with MOSFETs

To improve the operation error caused by the thermal fluctuation of electrons, we propose a novel single‐electron pass‐transistor logic circuit employing a multiple‐tunnel junction (MTJ) scheme and modulate a parameters of an MTJ single‐electron tunneling device (SETD) such as the number of tunnel junctions, tunnel resistance, and voltage gain. The operation of a 3‐MTJ inverter circuit is simulated at 15 K with parameters C_g=C_T=C_clk=1 aF, R_T=5 MΩ, V_clk=40 mV, and V_in=20 mV. Using the SETD/MOSFET hybrid circuit, the charge state output of the proposed MTJ‐SETD logic is successfully translated to the voltage state logic.     

230 and 492 GHz low noise sis waveguide receivers employing tuned Nb/AlOx/Nb tunnel junctions

We report results on two full height waveguide receivers that cover the 200–290 GHz and 380–510 GHz atmospheric windows. The receivers are part of the facility instrumentation at the Caltech Submillimeter Observatory on Mauna Kea in Hawaii. We have measured receiver noise temperatures in the range of 20K–35K DSB in the 200–290 GHz band, and 65–90K DSB in the 390–510 GHz atmospheric band. In both instances low mixer noise temperatures and very high quantum efficiency have been achieved. Conversion gain (3 dB) is possible with the 230 GHz receiver, however lowest noise and most stable operation is achieved with unity conversion gain. A 40% operating bandwidth is achieved by using a RF compensated junction mounted in a two-tuner full height waveguide mixer block. The tuned Nb/AlO _x /Nb tunnel junctions incorporate an “end-loaded” tuning stub with two quarter-wave transformer sections to tune out the large junction capacitance. Both 230 and 492 GHz SIS junctions are 0.49µm² in size and have current densities of 8 and 10 kA/cm² respectively. Fourier Transform Spectrometer (FTS) measurements of the 230 and 492 GHz tuned junctions show good agreement with the measured heterodyne waveguide response.     

Anti‐Ferromagnet Controlled Tunneling Magnetoresistance

The requirement for high‐density memory integration advances the development of newly structured spintronic devices, which have reduced stray fields and are insensitive to magnetic field perturbations. This could be visualized in magnetic tunnel junctions incorporating anti‐ferromagnetic instead of ferromagnetic electrodes. Here, room‐temperature anti‐ferromangnet (AFM)‐controlled tunneling anisotropic magnetoresistance in a novel perpendicular junction is reported, where the IrMn AFM stays immediately at both sides of AlO_x tunnel barrier as the functional layers. Bi‐stable resistance states governed by the relative arrangement of uncompensated anti‐ferromagnetic IrMn moments are obtained here, rather than the traditional spin‐valve signal observed in ferromagnet‐based tunnel junctions. The experimental observation of room‐temperature tunneling magnetoresistance controlled directly by AFM is practically significant and may pave the way for new‐generation memories based on AFM spintronics.     

Coherent response of two nearly identical stacked Josephson junctions to mm wave irradiation

Double-barrier Nb/Al-AlO_x-Nb/Al-AlO_x-Nb tunnel devices with nearly identical Josephson critical current densities of the single junctions have been investigated under influence of the electromagnetic irradiation in the frequency range from 78 to 119 GHz. Odd, even, and fractional Shapiro steps have been observed in the weak-signal regime. The switching of the device to the double gap-sum voltage occurs from even steps, whereas the switching to the single gap-sum voltage has usually been observed from odd steps. The even steps have a much larger amplitude than the odd steps. This behavior is explained as a manifestation of the coherent phase-locking of the two junctions to the applied mm wave signal. Fractional steps may be evidence of a nonlinear interaction between the junctions.     

Ultra-thin alumina layer encapsulation of bulk heterojunction organic photovoltaics for enhanced device lifetime

Successful organic photovoltaic (OPV) device fabrication is contingent on selecting an effective encapsulation barrier layer to preserve device functionality by inhibiting atmosphere-induced degradation. In this work, ultra-thin AlO_x layers are deposited by atomic layer deposition (ALD) to encapsulate pre-fabricated OPV devices. A summary of ALD recipe effects (temperature, cycling time, and number of cycles) on AlO_x film growth and device longevity is presented. First, AlO_x film growth on the hydrophobic OPV surface is shown to occur by a 3D island growth mechanism with distinct nucleation and cluster growth regions before coalescence of a complete encapsulation layer with a thickness ⩾7 nm by 500 cycles. Encapsulated device performance testing further demonstrates that reducing ALD processing temperature to 100 °C minimizes OPV phase segregation and surface oxidation loss mechanisms as evidenced by improved short circuit current and fill factor retention when compared with the conventional 140–150 °C range. Ultra-thin AlO_x encapsulation by ALD provides significant device lifetime enhancement (∼30% device efficiency after 2000 h of air exposure), which is well beyond other ALD-based encapsulation works reported in the literature. Furthermore, the interfacial bonding strength at the OPV–AlO_x interface is shown to play a crucial role in determining film failure mode and therefore, directly impacts ultimate device lifetime.     

The impact of self-assembled monolayer thickness in hybrid gate dielectrics for organic thin-film transistors

We have investigated the electrical characteristics of hybrid dielectrics with a thickness of 6 nm or less that are composed of a plasma-grown aluminum oxide (AlO_x) layer and a self-assembled monolayer (SAM) of an aliphatic phosphonic acid. The impact of the quality of the AlO_x layer on the insulating properties of the double-layer dielectrics was assessed by comparing two different oxidation procedures, and the influence of the thickness of the organic SAM was evaluated by employing molecules with five different chain lengths. In order to decouple the relative contributions of the oxide and the SAM to the performance of the double-layer dielectrics we have also performed cyclic voltammetry measurements on indium tin oxide (ITO)/SAM devices without AlO_x layer. Finally, we have evaluated how the quality of the AlO_x layer and the thickness of the SAM affect the performance of low-voltage organic thin-film transistors (TFTs) that employ the thin AlO_x/SAM dielectrics as the gate dielectric. The results confirm the important role of the SAM in determining the breakdown voltage, in limiting the current density, and in compensating the somewhat lower quality of AlO_x layers produced under mild plasma conditions.     

A low-noise SIS receiver measured from 480 GHz to 650 GHz using Nb junctions with integrated RF tuning circuits

A heterodyne receiver using an SIS waveguide mixer with two mechanical tuners has been characterized from 480 GHz to 650 GHz. The mixer uses either a single 0.5 × 0.5 µm² Nb/AlO_x/Nb SIS tunnel junction or a series array of two 1 µm² Nb tunnel junctions. These junctions have a high current density, in the range 8 – 13 kA/cm². Superconductive RF circuits are employed to tune the junction capacitance. DSB receiver noise temperatures as low as 200 ± 17 K at 540 GHz, 271 K ± 22 K at 572 GHz and 362 ± 33 K at 626 GHz have been obtained with the single SIS junctions. The series arrays gave DSB receiver noise temperatures as low as 328 ± 26 K at 490 GHz and 336 ± 25 K at 545 GHz. A comparison of the performances of series arrays and single junctions is presented. In addition, negative differential resistance has been observed in the DC I–V curve near 490, 545 and 570 GHz. Correlations between the frequencies for minimum noise temperature, negative differential resistance, and tuning circuit resonances are found. A detailed model to calculate the properties of the tuning circuits is discussed, and the junction capacitance as well as the London penetration depth of niobium are determined by fitting the model to the measured circuit resonances.     

Ozone oxidation methods for aluminum oxide formation: Application to low-voltage organic transistors

Four atmospheric pressure ozone oxidation methods were used to produce ultra-thin layers of aluminum oxide for organic thin-film transistors. They are UV/ozone oxidation in ambient (UV-AA) and dry (UV-DA) air, UV/ozone oxidation combined with high-voltage discharge-generated ozone in dry air (UV+O₃-DA), and discharge-generated ozone in dry air (O₃-DA). The lack of the high-energy UV photons during the O₃-DA oxidation led to low relative permittivity and high leakage current density of the AlO_x layer that rendered this method unsuitable for transistor dielectrics. Although this oxidation method led to the incorporation of oxygen into the film, the FTIR confirmed an increased concentration of the subsurface oxygen while the XPS showed the highest portion of the unoxidized Al among all four methods. The remaining three oxidation methods produced AlO_x films with thicknesses in excess of 7 nm (2-h oxidation time), relative permittivity between 6.61 and 7.25, and leakage current density of (1–7) × 10⁻⁷ A/cm² at 2 MV/cm, and were successfully implemented into organic thin-film transistors based on pentacene and DNTT. The presence of –OH groups in all oxides is below the detection limit, while some carbon impurities appear to be incorporated.     

Josephson array oscillator with microstrip resonators for submillimeter wave region

Shunted tunnel junctions with a small parasitic inductance have been developed for improving the operating frequency of Josephson array oscillators. The inductance was minimized by reducing the inductive length to 1 μm and was estimated to be about 40 fH. The analysis of resonant properties for the shunted junctions gave a high resonant frequency up to 1.4 THz. Josephson array oscillators were designed and fabricated to operate at near Nb gap frequency (700 GHz) using 11 shunted Nb/AlOx/Nb tunnel junctions with Nb microstrip resonators. Shapiro steps induced by Josephson oscillation were clearly observed above the Nb gap frequency (up to 830 GHz). By fitting the step height to the simulation result using the RLCSJ model, the output power of the Josephson oscillator to the load resistor was estimated to be about 0.1 μW at 680 GHz.     

Epitaxial Growth and Surface Roughness Control of Ferromagnetic Thin Films on Si by Sputter Deposition

Epitaxial growth of ferromagnetic (FM) thin-film electrodes on Si substrates is attracting increasing attention for magnetic tunnel junction (MTJ) studies as it enables both a high tunneling magnetoresistance (TMR) for practical applications and a well-defined junction structure for fundamental research. In this paper, we report epitaxial growth of both conventional FM and emerging Heusler FM electrodes by magnetron sputter deposition on Si substrates via appropriate buffer layers. Our primary focus is not only on the ability to grow epitaxial structures of the FM electrodes but also to achieve the desired smooth surfaces of these electrodes. The crystalline structures of the␣FM electrodes are known to play a critical role on the TMR of an MTJ. On the other hand, surface roughness of the FM electrode significantly degrades the performance of these junctions by interface scattering of spin-polarized electrons. We successfully grew various epitaxial face-centered cubic, hexagonal close-packed and body-centered cubic structures for Ni, Co, Fe, and their alloys in addition to the ordered intermetallic Heusler alloys, Co₂MnSi and Co₂MnAl, with appropriate buffer layers such as Ag between the FM layers and the Si substrates, followed by post-annealing. This annealing process greatly improves the quality of the epitaxy and the surface roughness of the FM layers deposited at room temperature. The crystallographic orientations of the ferromagnetic layers as well as the surface roughness were also found to vary with the orientation of the Si substrate.     

Ti/TiO x single-electron devices produced with a step-edge-cut-off (SECO) method

Single-electron current oscillations were for the first time observed in metal nanostructures produced by a step-edge-cut-off (SECO) method. The capacitance of an SECO-made Ti/TiO_x tunnel junction was found to be as small as 10 aF at a linewidth of about 150 nm, which is nearly 50 times less than the capacitance obtained by shadow masking at the same linewidth. It is suggested that the produced lateral tunnel junctions are mesoscopic conductors with hopping conduction.     

Effect of an applied magnetic field on the performance of a sis receiver near 300 GHz

We have successfully constructed and tested a superconductor-insulator-superconductor (SIS) receiver for operation at 265–280 GHz using 1 μm² area Nb?AlO _x ?Nb tunnel junctions fabricated at Stony Brook. The best performance to date is a double sideband (DSB) receiver noise temperature of 129 K at 278 GHz. We find that suppression of the Josephson pair currents with a magnetic field is essential for good performance and a stable DC bias point. Fields as high as 280 gauss have been used with no degradation of mixing performance. We illustrate the improvement in the intermediate frequency (IF) output stability with progressively increasing magnetic fields.     

Spin injection and transport in organic spin-valves based on fullerene C60

We report the fabrication and magnetoresistance (MR) of the La_0.67Sr_0.33MnO₃/C₆₀/Co spin valves. The introduction of 1.5 nm AlO_x barrier between the C₆₀ layer and cobalt top electrode prevents effectively interfacial diffusion and Co penetration, and thus an appreciable positive MR (as large as 3.65%) at room temperature was exhibited for the devices in the thickness range (5–40 nm) of C₆₀ studied. Possible mechanisms on the MR polarity are proposed. Furthermore, based on the temperature- and thickness- dependent MR and I–V characteristics, we have obtained clear evidences that the MR of C₆₀-based spin-valves originates from the tunneling of spin-polarized electrons at the low thickness of C₆₀, however at the larger thickness (>20 nm) the electrons are injected into and subsequently hopping transport within the C₆₀ spacer.     

A lownoise 665 GHz SIS quasi-particle waveguide receiver

We report recent results on a 565–690 GHz SIS heterodyne receiver employing a 0.36µm² Nb/AlO _x /Nb SIS tunnel junction with high quality circular non-contacting backshort and E-plane tuners in a full height waveguide mount. No resonant tuning structures have been incorporated in the junction design at this time, even though such structures are expected to help the performance of the receiver. The receiver operates to at least the gap frequency of Niobium, ≈ 680 GHz. Typical receiver noise temperatures from 565–690 GHz range from 160K to 230K with a best value of 185K DSB at 648 GHz. With the mixer cooled from 4.3K to 2K the measured receiver noise temperatures decreased by approximately 15%, giving roughly 180K DSB from 660 to 680 GHz. The receiver has a full 1 GHz IF passband and has been successfully installed at the Caltech Submillimeter Observatory in Hawaii.     

Double relaxation oscillation SQUID with reference junction for biomagnetic multichannel applications

We have fabricated a four-channel SQUID gradiometer system based on double relaxation oscillation SQUIDs (DROSs) with a reference junction. For a biomagnetic multichannel system, we simplified the DROS design by using a single reference junction instead of the reference SQUID. The SQUIDs were fabricated from hysteretic Nb/AlO_x/Nb junctions using a simple four-level process. The DROSs provided very large flux-to-voltage transfers of typically 3 mV/Φ₀, enabling direct readout by a room-temperature dc preamplifier, and consequently simple flux-locked loop electronics were used. To realize a compact and reliable gradiometer, a first-order planar pickup coil was integrated on the same wafer with the SQUID. The flux noise of the gradiometer is about 5 μΦ₀/√Hz at 100 Hz, corresponding to a field noise of 8 fT/√Hz, measured inside a magnetically shielded room. A compact four-channel planar gradiometer system was implemented and operated to measure auditory evoked fields.     

四结叠层太阳电池中AlGaAs/GaAs隧穿结的特性和表现

III-V族化合物叠层太阳电池是具有超高转换效率的第三代新型太阳电池。四结叠层电池GaInP/GaAs/InGaAs/Ge,各子电池的带隙分别为1.8, 1.4, 1.0, 0.7(ev)。为了使各子电池之间电流匹配,在各子电池之间以隧穿结互相连接。本文主要探索研究了四结叠层电池GaInP/GaAs/InGaAs/Ge隧穿结的特性,三个隧穿结的材料选取,探讨了隧穿结对整体叠层电池的特性的补偿作用,对各子电池电流密度的影响,以及在此基础上对整体电池效率的增加。选用AlGaAs/GaAs作为隧穿结运用PC1D进行电池的整体模拟仿真,得到各子电池电流密度分别为16.02mA/cm²,17.12 mA/cm²,17.75 mA/cm²,17.45 mA/cm²,电池在AM0下的开路电压Voc为3.246V,转换效率为33.9%。     

Fabrication of High-Quality Co2FeSi/SiOxNy/Si(100) Tunnel Contacts Using Radical-Oxynitridation-Formed SiOxNy Barrier for Si-Based Spin Transistors

A high-quality Co₂FeSi (CFS)/SiO _x N _y /Si tunnel junction was fabricated, in which the SiO _x N _y barrier layer was formed by radical oxynitridation of an Si(100) substrate and the CFS electrode was formed by silicidation of an Fe/Co/amorphous-Si multilayer deposited on the barrier layer. The ultrathin SiO _x N _y barrier layer completely blocked diffusion of Co and Fe atoms into the Si substrate during rapid thermal annealing (RTA) for the silicidation. X-ray diffraction investigations clarified that the CFS film on the ultrathin SiO _x N _y barrier layer exhibited a highly (110)-oriented texture structure and that the film had the L2₁ structure with a high degree of L2₁ order. High resolution cross-sectional transmission electron microscopy investigations revealed that the CFS/SiO _x N _y interface was atomically flat and that the crystal lattice of the CFS film was directly grown on the SiO _x N _y surface without degradation of the crystallinity at the interface.