Thin-film Junctions Research Articles

Kinetics of electromigration mass transfer in micro- and nanoelectronics interface elements depending on the strength of thin-film junctions

The theoretical model proposed earlier by the authors, which describes the interrelation of strength and electromigration (diffusion) properties of interfaces formed by joined materials, has been perfected and extended. Within the framework of the developed model, a linear relationship between the values of the work of reversible interface separation Wa and the activation energy of electromigration in the interface HEM was established. The coefficients of the obtained relation are estimated and compared with experimental data on the study of electromigration in a copper conductors covered with protective dielectrics. Using also the model developed earlier by the authors, which describes the dependence of the value on the concentrations of non-equilibrium lattice defects in the volumes of joined materials, a number of effects due to the influence of such defects on the processes caused by electromigration have been predicted and investigated. In the paper we obtained that by introducing non-equilibrium lattice defects in the volumes of bonded materials in the form of atomic impurities of interstition or substitution it is possible to effectively influence the characteristics of electromigration instability of the shape of the interlayer interface. For the introduction impurities, quantitative analytical estimates of the impurity concentration necessary for a significant change (both increase and decrease) in the characteristic rise time of the instability of the shape of the initially flat interface have been obtained.

Investigating the effect of lead substitution on the optical, electrical, and photoresponse properties of Quasi-2D double perovskites

Quasi-2-dimensional (2D) halide perovskites have recently attracted attention due to their higher operational stability as alternatives to 3-dimensional (3D) perovskites having exceptional optoelectronic and charge transport properties. To reduce the lead content, here following the double perovskite approach lead is substituted with silver and bismuth simultaneously, and three quasi-2D perovskites, with general formula (C7H10N)2Pb(1-2x)AgxBixBr4, (0 ≤ x ≤ 0.5) were prepared. The optical studies show that the partially lead substituted sample has the lowest optical band gap, aptly supported by the theoretical calculations. The powder X-ray diffraction technique along with field-emission scanning electron microscopy suggests enhancement in crystallinity along with the decrease in grain boundaries with the substitution of lead. The improvement in crystallinity with concomitant reduction in grain boundaries has led to the decrease in point defects as identified from the positron annihilation lifetime spectroscopy and coincidence Doppler broadening analysis. The tuned band gap, improved crystal quality along with lower defects jointly contributed to the enhancement in electrical properties of the perovskites with varying lead percentages. Finally, the photoresponse of all the materials was studied after fabricating metal (Al)-semiconductor (MS) junction thin film photodetector devices.

Thermoelectric response of textured Sb2Te3–BiSb and Sb2Te3–Bi2Te3 thin film junctions

Surface mounted thin film thermoelectric (TE) devices for localized cooling, power generation, and sensing are topics of immense current interest. Here, we establish the superior TE performance of thin film junctions made of topological insulators (TIs) Bi85Sb15 (BiSb) and Sb2Te3 by comparing their performance with those of Bi2Te3 and Sb2Te3. Thin films of these TIs were first evaluated for their carrier concentration, Hall mobility, resistivity, and thermopower. The Seebeck coefficient of BiSb, Sb2Te3, and Bi2Te3 measured against copper at ambient temperature is −100, +160, and −70 (±10) μV/K, while their power factors are 0.5, 0.5, and 0.45 (±0.05) 10−3 W m−1 K−2, respectively. Single TE junctions of BiSb–Sb2Te3 and Bi2Te3–Sb2Te3 yield a response of 272 and 240 (±10) μV/K, respectively. This comparative study shows that BiSb is a superior n-type counter electrode for Sb2Te3 compared to the n-type Bi2Te3. Moreover, Bi2Te3 is prone to tellurium antisite disorder, which affects its TE properties significantly.

Analysis of two-terminal perovskite/silicon tandem solar cells with differing texture structure, perovskite carrier lifetime, and tunneling junction quality

Presented here is the optimization of a planar two-terminal perovskite/silicon tandem solar cell with a texture structure. The developed simulation model is fitted to published experimental results, and the importance of current matching in the two-terminal structure is discussed. With the texture structure optimized and considering current matching, the optimal texture structure improves Jsc from 17.9 to 20.87 mA/cm2 compared to the planar structure, as well as improving the power conversion efficiency from 25.8% to 35.9%. Furthermore, if the quality of the perovskite thin film and tunneling junction efficiency with a smaller voltage penalty can be improved, then the efficiency can be further improved to 38.13%. This indicates that this tandem solar cell still has much room for improvement.

Metasurface absorber based single junction thin film solar cell exceeding 30% efficiency.

In this article, we report, as per our knowledge, for the first time, a thin film single junction solar cell with a metasurface absorber layer directly incorporated. We have used an interconnected dual inverted split ring resonator pattern in the InAsP absorber layer. The structure eliminated patterns of conventional metals, such as silver, aluminum, and gold, from the active layer, a common drawback in conventional solar absorbers, hindering their direct integration into solar cells. Optical simulation results show a peak ideal short circuit current density of 76.23mA/cm2 for the meta-absorber structure under solar illumination. This current is the highest among previously reported absorbers based on Group IV materials and III-V compounds, overcoming the low solar absorption of such metasurfaces. The final proposed solar cell structure combines this meta-absorber layer with traditional efficiency enhancement methods namely anti-reflecting coating, textured back reflector, and transparent top electrode. This novel single junction structure shows a solar absorption efficiency of 97.86% and a power conversion efficiency of 30.87%, the highest for III-V solar cells. Our device proves the ability of metasurface absorber layers to produce high-efficiency solar cells and is expected to pave the way for integrating novel meta-devices into state-of-the-art photovoltaic devices, aiding the global transition towards clean energy sources.

Photosensitivity reversal in solution-processed Copper Zinc Tin Sulphide/Cadmium Sulphide superstrate junction

Inverse photosensitivity is a sought-after property for the development of photonic devices and non-volatile memories with low power conversion. Herein, we report on the development of a superstrate-type thin film junction exhibiting inverse photosensitivity which does not involve the use of a pricy transparent conducting oxide layer, providing a method for engineering low-cost thin film devices. Sequential ionic layer adsorption reaction (SILAR) was used to grow Copper Zinc Tin Sulphide (CZTS) and Cadmium sulphide (CdS) large area thin film p-n diode junction. Uniform CZTS thin films with an area of ∼10 cm2 which were free of pin holes or voids could be deposited using an optimized SILAR process recipe. The grown CZTS films exhibited an electrical resistivity of 4.4 × 10−4 Ωm using silver (Ag) electrodes. CdS thin films were sub-sequentially grown over the CZTS layer to form a p-n junction. A knee voltage of 0.2 V in the device configuration “glass/CZTS/CdS/Ag” was observed in the current voltage characteristics. The as-prepared and the vacuum-annealed p-n junction exhibited negative photosensitivity of magnitude 0.21 and 0.16 respectively. The air-annealed device structure exhibited photosensitivity of 216.

Gauge Theories of Josephson Junction Arrays: Why Disorder Is Irrelevant for the Electric Response of Disordered Superconducting Films

We review the topological gauge theory of Josephson junction arrays and thin film superconductors, stressing the role of the usually forgotten quantum phase slips, and we derive their quantum phase structure. A quantum phase transition from a superconducting to the dual, superinsulating phase with infinite resistance (even at finite temperatures) is either direct or goes through an intermediate bosonic topological insulator phase, which is typically also called Bose metal. We show how, contrary to a widely held opinion, disorder is not relevant for the electric response in these quantum phases because excitations in the spectrum are either symmetry-protected or neutral due to confinement. The quantum phase transitions are driven only by the electric interaction growing ever stronger. First, this prevents Bose condensation, upon which out-of-condensate charges and vortices form a topological quantum state owing to mutual statistics interactions. Then, at even stronger couplings, an electric flux tube dual to Abrikosov vortices induces a linearly confining potential between charges, giving rise to superinsulation.

Beyond the effective length: How to analyze magnetic interference patterns of thin-film planar Josephson junctions with finite lateral dimensions

The magnetic-field-dependent critical current ${I}_{\text{c}}(B)$ of a Josephson junction is determined by the screening currents in its electrodes. In macroscopic junctions, a local vector potential drives the currents; however, in thin-film planar junctions, with electrodes of finite size and various shapes, they are governed by nonlocal electrodynamics. This complicates the extraction of parameters such as the geometry of the effective junction area, the effective junction length, and the critical current density distribution from the ${I}_{\text{c}}(B)$ interference patterns. Here, we provide a method to tackle this problem by simulating the phase differences that drive the shielding currents and use those to find ${I}_{\text{c}}(B)$. To this end, we extend the technique proposed by Clem [Phys. Rev. B 81, 144515 (2010)] to find ${I}_{\text{c}}(B)$ for Josephson junctions separating a superconducting strip of length $L$ and width $W$ with rectangular, ellipsoid, and rhomboid geometries. We find the periodicity of the interference pattern ($\mathrm{\ensuremath{\Delta}}B$) to have geometry-independent limits for $L\ensuremath{\gg}W$ and $L\ensuremath{\ll}W$. By fabricating elliptically shaped superconductor--normal-metal--superconductor junctions with various aspect ratios, we experimentally verify the $L/W$ dependence of $\mathrm{\ensuremath{\Delta}}B$. Finally, we incorporate these results to correctly extract the distribution of critical currents in the junction by the Fourier analysis of ${I}_{\text{c}}(B)$, which makes these results essential for the correct analysis of topological channels in thin-film planar Josephson junctions.

Exploring domain continuity across BaTiO3 grain boundaries: Theory meets experiment

Polycrystalline ferroelectrics constitute the basis of many advanced technologies, including sensors and actuators. Their intricate domain patterns, and switching, drive the macroscopic electrical and mechanical properties of the material, where the domain switching behaviour is largely influenced by the grain-grain interaction of the domain walls. Domain wall continuity across grain boundaries is speculated to affect the domain wall – grain boundary interaction, although the true impact of this phenomenon on the ferroelectric properties, and the conditions under which continuity occurs, are not yet well understood. Whilst there are some theoretical reports, the link to experimental evidence is limited, greatly hindering the applicability and fundamental understanding of current polycrystalline based devices. In this work, we close this gap by studying several grain junctions in free-standing BaTiO3 thin films using microscopy techniques and rationalising the domain configurations with reference to martensite theory. A pleasing agreement of minimal strain and polarisation mismatch for a pair of domain variants were found in cases where domain wall continuity across grain boundaries was observed, confirming that domain continuity is related to the compatibility conditions at the grain boundary. Following this experimental validation, the mismatches for various combinations of Euler angles in bi-grain junctions were theoretically explored, offering valuable insights into specific cases where domain continuity can be expected. These results offer an advancement in the understanding of grain-grain-domain interactions and provides a template for the prediction and control of domain wall continuity in polycrystalline ferroelectrics, appealing to those working in polycrystal design and domain engineering.

Ultrathin 3D radial tandem-junction photocathode with a high onset potential of 1.15 V for solar hydrogen production

Combining a progressive tandem junction design with a unique Si nanowire (SiNW) framework paves the way for the development of high-onset-potential photocathodes and enhancement of solar hydrogen production. Herein, a radial tandem junction (RTJ) thin film water-splitting photocathode has been demonstrated experimentally for the first time. The photocathode is directly fabricated on vapor-liquid-solid-grown SiNWs and consists of two radially stacked p - i - n junctions, featuring hydrogenated amorphous silicon (a-Si:H) as the outer absorber layer, which absorbs short wavelengths, and hydrogenated amorphous silicon germanium (a-SiGe:H) as the inner layer, which absorbs long wavelengths. The randomly distributed SiNW framework enables highly efficient light-trapping, which facilitates the use of very thin absorber layers of a-Si:H (~50 nm) and a-SiGe:H (~40 nm). In a neutral electrolyte (pH = 7), the three-dimensional (3D) RTJ photocathode delivers a high photocurrent onset of 1.15 V vs. the reversible hydrogen electrode (RHE), accompanied by a photocurrent of 2.98 mA/cm 2 at 0 V vs. RHE, and an overall applied-bias photon-to-current efficiency of 1.72%. These results emphasize the promising role of 3D radial tandem technology in developing a new generation of durable, low-cost, high-onset-potential photocathodes capable of large-scale implementation. A radial tandem junction thin-film photocathode with a V onset of 1.15 V vs. RHE has been demonstrated directly on VLS-grown SiNWs for the first time, using a-Si:H (~50 nm) as the outer absorber layer and a-SiGe:H (~40 nm) as the inner layer.

Magnetic Field Resilience of Three-Dimensional Transmons with Thin-Film Al/AlOx/Al Josephson Junctions Approaching 1 T

Magnetic-field-resilient superconducting circuits enable sensing applications and hybrid quantum computing architectures involving spin or topological qubits and electromechanical elements, as well as studying flux noise and quasiparticle loss. We investigate the effect of in-plane magnetic fields up to 1 T on the spectrum and coherence times of thin-film three-dimensional aluminum transmons. Using a copper cavity, unaffected by strong magnetic fields, we can probe solely the effect of magnetic fields on the transmons. We present data on a single-junction and a superconducting-quantum-interference-device (SQUID) transmon that are cooled down in the same cavity. As expected, the transmon frequencies decrease with increasing field, due to suppression of the superconducting gap and a geometric Fraunhofer-like contribution. Nevertheless, the thin-film transmons show strong magnetic field resilience: both transmons display microsecond coherence up to at least 0.65 T, and ${T}_{1}$ remains above $1\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\text{s}$ over the entire measurable range. SQUID spectroscopy is feasible up to 1 T, the limit of our magnet. We conclude that thin-film aluminum Josephson junctions are suitable hardware for superconducting circuits in the high-magnetic-field regime.

Enhanced photocatalytic activity of nZnO/n+Al:ZnO homojunction with an overlayer of Al2O3 nanoballs

This paper reports improvement in the degradation efficiency of ZnO for the organic dye methylene blue by constructing a homojunction of nZnO and n+Al doped ZnO with an overlayer of alumina nanoballs. The thin film junction is fabricated by the simple, cost-effective two stage electrochemical method of anodization followed by electrochemical doping at room temperature. Structural, optical, morphological and electrical analyses are done to elucidate the corresponding properties of each layer as well as of the junction. The compositional depth profile is obtained by the Rutherford backscattering technique. Valence band x-ray photoelectron spectroscopy in conjunction with optical data is used for designing the schematic of the junction formation. The rectification ratio of the thin film junction is determined to be ∼102 from voltage–current data.

Interface Engineering of Electrical Contacts

Highly conductive nanoscale electrical contacts suffer from strong current crowding at the contact edges, which can lead to nonuniform heat deposition; the formation of local hot spots, aggravation of electromigration; and, in the worst-case scenario, lead to thermal runaway and breakdown of the device. These effects severely affect the overall device properties, reliability, and lifetime. Devices based on thin-film junctions, nanotubes or nanowires, and two-dimensional (2D) materials are especially sensitive to current transport at electrical contacts, due to their reduced dimensions and increased geometrical confinement for current flow. Here, we demonstrate a method to mitigate current crowding, by engineering the interface layer properties and geometry. Based on a self-consistent transmission-line model, we show that the distribution of the contact current greatly depends on the properties of the interfacial layer between two contacting members. Current steering and redistribution can be realized by strategically designing the specific contact resistivity, ${\ensuremath{\rho}}_{c}$, along the contact length. For similar contact members, parabolically varying ${\ensuremath{\rho}}_{c}$ along the contact interface significantly reduces the edge-current crowding in ohmic contacts. Similarly, the nonuniform current distribution of 2D semiconductor-3D metal contacts can be decreased, and the current-transfer length can be increased by varying the Schottky barrier height along the interface. It is also found that introducing a nanometer- or subnanometer-scale thin insulating tunneling gap between contact members can greatly reduce current crowding, while maintaining a similar total contact resistance.

Fabrication of Josephson parameter amplifier and its application in squeezing vacuum fluctuations* *Project supported by the State Key Development Program for Basic Research of China (Grant No. 2017YFA0304300), the Key-Area Research and Development Program of Guangdong Province, China (Grant No. 2020B0303030001), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB28000000).

Josephson parameter amplifier (JPA) is a microwave signal amplifier device with near-quantum-limit-noise performance. It has important applications in scientific research fields such as quantum computing and dark matter detection. This work reports the fabrication and characterization of broadband JPA devices and their applications in multi-qubit readout and squeezing of vacuum state. We use a process in which transmission lines and electrodes are made of niobium thin film and aluminum Josephson junctions are made by Dolan bridge technique. We believe this process is more convenient than the process we used previously. The whole production process adopts electron beam lithography technology to ensure high structural resolution. The test result shows that the gain value of the manufactured JPA can exceed 15 dB, and the amplification bandwidth is about 400 MHz. The noise temperature is about 400 mK at the working frequency of 6.2 GHz. The devices have been successfully used in experiments involving superconducting multi-qubit quantum processors. Furthermore, the device is applied to squeeze vacuum fluctuations and a squeezing level of 1.635 dB is achieved.

Highly flexible radial tandem junction thin film solar cells with excellent power-to-weight ratio

High power-to-weight ratio (PTWR) is an important figure-of-merit for high performance flexible/portable solar cells. Marrying advanced tandem junction design with three-dimensional (3D) Si nanowire (SiNW) framework enables a promising route to boost the PTWR. In this work, a radial tandem junction (RTJ) thin film solar cell has been demonstrated, for the first time, over SiNWs, which consist of radially deposited p-i-n multilayers with hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon germanium (a-SiGe:H) absorption layers in the outer and the inner junctions, respectively. The strong light trapping within the 3D SiNW framework allows for the use of a very thin a-SiGe:H (~45 nm) absorption layer to harvest efficiently the long wavelengths. The RTJ cells fabricated via a one-pump-down process in a single PECVD chamber, directly upon 15 µm thick aluminum foils demonstrate an excellent flexibility that can bend to 10 mm radius and achieve a record PTWR~1628 W/kg, and accomplish a high open-circuit voltage, filling factor and conversion efficiency of 1.2 V, 61.5% and 8.1% on glass, respectively, substantially improved compared to those accomplished by radial single junction cells. These results highlight the unique potential of 3D radial tandem technology to enable a new generation of high performance and durable flexible photovoltaics.

Fabrication of Corner-Like Josephson Junctions Based on Pnictide Single crystals

In order to study fundamental properties of pnictide superconductors such as the pairing symmetry, we have fabricated corner-like Josephson devices. Their configuration contains two SNS* (superconductor/barrier/superconductor*) thin-film junctions in different crystal directions. We have used multi-band Co-doped Ba-122 single crystals as the base electrode (S), evaporated lead films as a counter electrode (S*) made of a conventional s-wave superconductor and normal-metal (N) layers as the barrier. The junctions exhibit symmetric I-V characteristics with IcRN values about 0.975 μV. Ic is the critical current, and RN is the normal-state resistance. The observed Fraunhofer pattern-like dependence of Ic on the magnetic field indicates homogeneity of the junctions studied.

Investigations on Optical, Material and Electrical Properties of aSi:H and aSiGe:H in Making Proposed n+aSi:H/i-aSi:H/p+aSiGe:H Graded Bandgap Single-junction Solar Cell

Objective: This work identifies materials that satisfy refractive index, optical band gap, composition profile, conductivity, hall mobility, carrier type and carrier concentration to utilize them in making thin film photovoltaic cells. Methods: We fabricated phosphorous doped amorphous silicon (n+ aSi:H), boron doped amorphous silicon germanium(p+ aSiGe:H) and intrinsic amorphous silicon (i-aSi:H). A detailed and systematic characterization of the fabricated layers was done. The phosphorous doped amorphous silicon (n+ aSi:H) showed an optical band gap of 1.842 eV and an electron mobility of 295.45 cm2V-1s-1. The boron doped amorphous silicon germanium (p+ aSiGe:H) exhibited an optical band gap of 1.74 eV and a hole mobility of 158.353 cm2V-1s-1. The intrinsic amorphous silicon (i-aSi:H) has an optical band gap of 1.801 eV. The films of n+ aSi:H, i-aSi:H and p+ aSiGe:H can be utilized for fabricating graded band gap single junction thin film solar cells, as they are semiconducting materials with varying band gaps in the range of 1.74 eV to 1.84 eV. The tailoring of band gap achieved by the proposed material combination has been presented using its energy band diagram. Results: In this work, we are proposing a single junction graded band gap solar cell with aSi:H and aSi- Ge:H alloys of varying doping to achieve grading of band gap, which improves the efficiency while keeping the cell compact and light. Conclusion: As a first step in the validation, we have simulated a thin film solar cell using SCAPS1D simulation software with the measured parameters for each of the layers and found that it successfully performs as solar cell with an efficiency of 14.5%.

IR hot carrier based photodetection in titanium nitride oxide thin film-Si junctions

Abstract:Hot carrier based methods constitute a valuable approach for efficient and silicon compatible sub-bandgap photodetection. Although, hot electron excitation and transfer have been studied extensively on traditional materials such as Au and Ti, reports on alternative materials such as titanium nitride (TiN) are rare. Here, we perform hot hole photodetection measurements on a p-Si/metal thin film junction using Ti, Au and TiN. This material is of interest as it constitutes a refractory alternative to Au which is an important property for plasmonic applications where high field intensities can occur. In contrast to Au, a TiN/Si junction does not suffer from metal diffusion into the Si, which eases the integration with current Si-fabrication techniques. This work shows that a backside illuminated p-Si/TiN system can be used for efficient hot hole extraction in the IR, allowing for a responsivity of 1 mA/W at an excitation wavelength of 1250 nm and at zero bias. Via a comparison between TiN and other commonly used materials such as Au, the origin of this comparably high photoresponse can be traced back to be directly linked to a thin TiO2-x interfacial layer allowing for a distinct hot-hole transfer mechanism. Moreover, the fabrication of TiN nanodisk arrays is demonstrated which bears great promise to further boost the device efficiency.

Source engineering for bilayer tunnel field-effect transistor with hetero tunnel junction: thickness and impurity concentration

We have investigated the impacts of source layer thickness, source impurity concentration and channel thickness in a bilayer tunneling field-effect transistor with a thin-film hetero tunneling junction on electrical characteristics. Device simulation has revealed that thinning of the source layer significantly degrades on-state current (Ion) due to non-uniform band-to-band tunneling over the tunneling junction, while channel layer thinning is effective for increasing Ion. On the other hand, source/channel impurity concentrations of around 3 × 1018 cm‒3 are found to be optimal for high Ion, high Ion/Ioff and suppression of transfer characteristic shifts with changing Vd.

Solvothermal growth of aligned SnxZn1-xS thin films for tunable and highly response self-powered UV detectors

In this study, well-aligned SnxZn1-xS (0 ≤ x ≤ 0.12 mol) nanostructured thin films were successfully grown on the glass substrate by a solvothermal method for ultraviolet detector applications. XRD results revealed a hexagonal wurtzite structure for all the SnxZn1-xS thin films. Through structural and morphological studies, as the Sn-content increases, the c-axis orientation became stronger. A comprehensive investigation of the morphological evolution also confirmed a considerable influence of Sn2+ incorporation on the growth mechanism and shape of the SnxZn1-xS nanostructures. The optical measurements indicated that as the Sn-content increases, a linear relationship between the band gap and Urbach energy was found, in which the band gap energy experienced a significant decrease from 4.20 to 3.83 eV; whereas the Urbach energy was dramatically broadened. A gradual increase was observed in the intensity of photoluminescence spectra, in which three prominent emission peaks were found, indicating a successful formation of the SnxZn1-xS nanostructures. The photosensing performance of the samples was studied for the first time. The ultraviolet detectors based on the Au/SnxZn1-xS thin film junctions featured an excellent rectification behavior and attractive photovoltaic effect under UVB exposure. The enhancing effects of annealing treatment on the photoelectric behavior of the samples were also noted. The photoresponse characteristics of the self-powered ultraviolet detectors exhibited a high Ion/Ioff ratio > 103 and a fast photoswitching speed with superior stability and reproducibility at zero voltage. Additionally, notable photosensitivity and detectivity of more than ∼20 times are achieved at the self-powered mode.