Bulk Junction Research Articles

Nambu-Goto equation from three-dimensional gravity

We demonstrate that the solutions of three-dimensional gravity obtained by gluing two copies of a spacetime across a junction constituted of a tensile string are in one-to-one correspondence with the solutions of the Nambu-Goto equation in the same spacetime up to a finite number of rigid deformations related to worldsheet and spacetime isometries. The non-linear Nambu-Goto equation satisfied by the average of the embedding coordinates of the junction emerges directly from the junction conditions along with the rigid deformations and corrections due to the tension. Therefore, the equivalence principle generalizes non-trivially to the string. Our results are valid both in three-dimensional flat and AdS spacetimes. In the context of AdS3/CFT2 correspondence, our setup could be used to describe a class of interfaces in the conformal field theory featuring relative time reparametrization at the interface which encodes the solution of the Nambu-Goto equation corresponding to the bulk junction.

Restoring the surface layer of Bi2Sr2CaCu2O8+δ single crystals by current injection

In 2010, we reported a new method to electronically tailor the hole concentration of the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212). We discovered that current injection along the c-axis of Bi-2212 provides a convenient tool to reversibly increase the carrier concentration. In this study, we have investigated the current injection effect on the surface junction of Bi-2212 by a series of current injection experiments carried out in samples of slightly overdoped and of extremely underdoped Bi-2212 crystals. The c-axis transport characteristics of bulk and surface intrinsic Josephson junctions are examined in a wide range of doping by current injection. Before injection treatment, typically, our samples demonstrate non-superconducting characteristics across their surface junctions, because standard preparation procedures cause the topmost CuO2 layers to be heavily degraded by oxygen loss. The electronic doping enables to substantially restore the hole concentration of the topmost CuO2 layers. The contact resistance is significantly reduced, and the intrinsic junction at the surface gets back its superconducting tunneling characteristic. We examine systematically the doping results of surface and bulk junction stacks and discuss the enhancement of their superconducting properties.

Colloidal InSb Quantum Dots/Organic Integrated Bulk Heterojunction for Fast and Sensitive Near‐Infrared Photodetectors

It remains urgent to integrate conductive InSb colloidal quantum dots (CQDs) for sensitive and fast near‐infrared (NIR) photodetection applications. Herein, nanocrystallized InSb CQDs (<12 nm in diameter) have been successfully obtained via hot‐injection procedure, and demonstrated a very narrow absorption peak centered at 1406 nm with a full width at half maximum (FWHM) of 10.3 nm and a broader absorption peak at 1702 nm, indicating strong quantum‐confined effect. After integrating these InSb CQDs with [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) and polymeric triphenyldiamine (poly(N,N′‐diphenylben‐zidine diphenylether), poly‐TPD) bulk junction, the obtained Si/SiO2/InSb CQDs:PCBM:poly‐TPD/Ag photodetector has reached long‐wavelength response up to 1400 nm, fast response time (<80 ms), and superior on/off ratio. In specific, charge carriers can be effectively transported due to favorable energy alignment and interpenetrating network formed in the inorganic/organic blend films. The work provides a new strategy to synthesize high‐quality InSb CQD and reveal its starting point toward low‐cost, practical, and sensitive next‐generation NIR detection.

Terrestrial neutron-induced single events in GaN

We study the physical mechanisms of single event production in GaN wide-bandgap semiconductor subjected to atmospheric high-energy (>1 MeV) neutron irradiation. The interactions of incident neutrons with the target material are investigated with Geant4 and the transport of the deposited charge simulated using our random-walk drift-diffusion (RWDD) modeling approach in a generic reversely biased bulk junction.

Bulk Junctionless Transistor (JLT) with non-uniform doping: A high performance and scalable device

In this paper we have presented the non-uniformly doped bulk Junction less transistor (JLT) and investigated bulk-JLT and SOI-JLT with non-uniform doping in terms of its electrical performance parameters and short channel effects (SCEs) parameters comparatively. Effective thickness of channel depends on non-uniform doping distribution parameters and this affects the performance of bulk-JLT notably, however it is not so in case of SOI-JLT. The effect of non-uniform doping on electrical characteristics of JLTs (bulk and SOI) in terms of Subthreshold Slope (SS), ON-current, OFF-Current and ON/OFF current ratio has been investigated, and the non-uniformly doped bulk-JLT exhibits high ON/OFF ratio (109 for 20 nm Gate Length). Moreover, the non-uniformly doped bulk-JLT also shows improved short-channel effects (SCEs) parameters (such as Drain Induced Barrier Lowering, Threshold Voltage variations etc.) compared to SOI-JLT. Lastly, the effect of standard deviation, dielectric constant, substrate doping, and well biasing on the device performance are examined to further improve the performance of bulk-JLT independently.

Enhanced Organic Photovoltaic Performance through Modulating Vertical Composition Distribution and Promoting Crystallinity of the Photoactive Layer by Diphenyl Sulfide Additives

To understand the vertical phase separation in the bulk junction active layer of organic photovoltaic devices is essential for controlling the charge transfer behavior and achieving effective charge collection. Here, diphenyl sulfide (DPS) was introduced as a novel additive into the PTB7-Th:PC71BM-based inverted polymer solar cells (PSCs), and the effect of additives on active blend films and photovoltaic characteristics was carefully studied. The results show that DPS could not only modulate the vertical composition distribution but also promote the ordered molecular packing of the photoactive layer, thus effectively improving exciton dissociation, charge transport, and collection, and thus exhibit an excellent power conversion efficiency of 9.7% with an improved fill factor (>70%) after using 3% DPS additive. The results show that the DPS solvent additive can effectively adjust the vertical phase distribution and crystallinity of blend films and improve the photovoltaic performance of the inverted organic photovoltaic devices.

Field-Effect Charge Transport in Doped Polymer Semiconductor-Insulator Alternating Bulk Junctions with Ultrathin Transport Layers.

Conjugated-polymer field-effect transistors are attractive for flexible electronics. However, relatively high chemical doping (oxidation) concentration of p-type polymer semiconductors is usually not compatible with good transistor performance, due to poor switching-off capability and short-channel performance. Here, we propose a combined simulation and experimental investigation on charge transport in a semiconductor-insulator alternating bulk junction composed of repeating semiconductor and insulator regions, which shows better transistor performance at higher doping levels, as compared with traditional planar transistors. Moreover, the doped semiconductor transport layers in the junction should be less than 2 nm thick to ensure sufficient pinch-off capability. Using some semiconductors including poly(3-hexylthiophene), we utilize a fast solvent evaporation approach to obtain semiconductor-insulator alternating bulk junctions with ultrathin (thickness < 2 nm) semiconductor crystallites and with vertical gradients of both morphology and electronic properties. Doping with a concentration of up to 1019 cm-3 simultaneously induces the improvement of field-effect mobility, on/off ratio, and subthreshold swing, which leads to long-term (>1 year) stability, without lowering the short-channel performance. Moreover, these heterojunctions are optically transparent, nearly colorless, and flexible, thus could be exploited for wide electronic and photonic applications.

Large surface charge accumulation in 2D MoS2/Sb2Te3 junction and its effect on junction properties: KPFM based study

In the present study, 2D-3D MoS2/Sb2Te3 (n-p) and MoS2/Bi2Te3 (n-n) heterojunctions with varying MoS2 thicknesses have been investigated using the Kelvin Probe Force Microscopy technique. Nanoscale maps of interface measurements based on the difference of surface potential (SP) maps in surface charge and back natural modes have been carried out. The 2D-3D heterojunctions with lower MoS2 thickness show a large difference in SP values in the two modes, which is observed to increase with a decrease in the MoS2 thickness. In comparison, samples with larger (bulk-like) MoS2 thickness show negligible SP differences, indicating complete Fermi level alignment, as expected in a normal bulk junction. The difference in the SP value in two modes represents large surface charge accumulation in the 2D layer due to a relatively high value of the depletion width required for achieving equilibrium in comparison to the atomic scale thickness of 2D MoS2. In limited earlier reports, the current-voltage behavior of metal—2D MoS2 junctions is explained on the basis of the Fermi level pinning effect, which is a very generic explanation given for bulk p-n heterojunctions and may not be applicable in 2D materials. The present study shows that surface charge accumulation has a large influence on the I-V characteristic of 2D junctions, and this may be a key factor influencing the physics of the 2D interface and their potential applications.

Magnetoconductance Study on Nongeminate Recombination in Solar Cell Using Poly(3-hexylthiophene) and [6,6]-Phenyl-C61-butyric Acid Methyl Ester.

The magnetoconductance (MC) effect was investigated for two types of organic solar cells with single junction (SJ) and bulk junction (BJ) of poly(3-hexylthiophene) (P3HT) as donor (D) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor (A). Three components with different half-field-at-half-maximums (B1/2) of 4 ± 1, 20 ± 15 and >400 mT, hereafter referred to MCS,M,B in a sequence, were observed in the magnetic field dependence of the MC effects measured under dark and light conditions. The magnitude of the MCS,M,B components is sensitive to not only the junction structure of the cell but also the presence or absence of incident light. The bias voltage (V) dependence of the MC effect in the dark for the SJ-cell is maximized around the turn-on voltage (VON) of the dark current, where a flat band condition of the active layer is achieved. The B1/2 for the MCM component of the SJ-cell increases with V beyond VON. In light, the BJ-cell exhibits the MC effect, whereas no effect is detected for the SJ-cell. The MCS,M components for the BJ-cell in light increase with the incident light power. The transient MCS,M components for the BJ-cell measured using a nanosecond pulse laser increases with the delay time after the flash. By integrating these phenomena and the phase of the MC effect, it is concluded that all of the MC components arise from the magnetic field effect on the spin conversion of nongeminate electron (e)–hole (h) pairs with spin-dependent charge recombinations at the D/A-interface. The B1/2 values for MCS,M,B are, respectively, understood by the spin conversion due to the hyperfine interaction, the spin relaxation, and the g-factor difference for e (PCBM–) and h (P3HT+). Kinetic simulations of the MCS,M components for the BJ-cell observed at the short-circuit condition in light yield an efficiency of ca. 40% for the nongeminate recombination, which is accompanied by the generation of triplet excitons as well as relaxation to a ground singlet state. The loss mechanism of moderate triplet recombination suggests an important possibility to improve the power conversion efficiency by harvesting of the triplet excitons.

Highly transparent and thermal-stable silver nanowire conductive film covered with ZnMgO by atomic-layer-deposition

Solution-processed silver nanowires (AgNWs) have been considered as a promising material for next generation flexible transparent conductive electrodes. However AgNWs films have several intrinsic drawbacks, such as thermal stability and storage stability. Herein, we demonstrate a laminated ZnO/MgO (ZnMgO, ZMO) as a protective layer on the AgNWs films using atomic layer deposition (ALD). The fabricated films exhibited a low sheet resistance of 16 Ω/sq with high transmittance of 91% at 550 nm, an excellent thermal stability and bending property. The ZMO film grows perpendicularly on the surface of the AgNWs, making a perfect coverage of bulk silver nanowires and junction, which can effectively prompt the electrical transport behavior and enhance stability of the silver nanowires network.

Effective medium theory based analytical models for the potential and field distributions in arrays of nanoscale junctions

Recently, we developed an Effective Medium Theory (EMT) for the Space-Charge Region electrostatics of Schottky and p-n junctions in arrays of nanofilms (NFs), nanowires, and nanotubes in a dielectric ambient and gave formulas for their junction depletion width and screening length characterizing the space-charge tail. In the present work, we develop this EMT further and derive simple formulas for the potential and field distributions in the semiconductor and dielectric media of the array. The formulas derived are validated with numerical simulations. It is shown that the potential and field distributions perpendicular to the junction plane in the array correspond to those in a bulk junction with an effective semiconductor medium, whose permittivity and doping are their weighted averages over the cross-sectional areas of the semiconductor and dielectric; the shapes of the cross-sections are immaterial. We also analyze a single NF junction, treating it as a limiting case of an array, and obtain the following key results. For negligible film thickness, the depletion width depends linearly on applied voltage and inverse of doping; the peak electric field depends linearly on doping and inverse of ambient permittivity and varies very gradually with applied voltage. These features of a thin film junction are remarkably different from the bulk junction, wherein the depletion width and peak field have a square-root dependence on applied voltage.

Probing film-depth-related light harvesting in polymer solar cells via plasma etching

Light harvesting is the first step of photovoltaic process in polymer solar cells. However, such donor: acceptor bulk junction layers are usually featured with vertical phase segregation as well as film-depth-dependent molecular aggregation, chain orientation and crystallinity, leading to a significant variation of photon absorption and exciton generation at different film-depths. We propose an experimentally and numerically accessible method to investigate the depth-dependent light harvesting behaviors in the active layer in polymer solar cells. A low-pressure oxygen plasma is utilized to etch the active layer gradually which is monitored by a light absorption spectrometer. Including the obtained sublayer absorption spectra into transfer matrix optical model yields depth-dependent optical properties and exciton generation profiles, which contribute to quantum efficiency and short-circuit current. This approach is helpful to optimize vertical material variation and provide insights into photovoltaic process.

Effective medium theory of the space-charge region electrostatics of arrays of nanoscale junctions

We develop an Effective Medium Theory for the electrostatics of the Space-Charge Region (SCR) of Schottky and p-n junctions in arrays of nanofilms (NFs), nanowires (NWs), and nanotubes (NTs) in a dielectric ambient. The theory captures the effects of electric fields in both the semiconductor, i.e., NF/NW/NT, and the dielectric media of the array. It shows that the depletion width and the screening length characterizing the SCR tail in the array correspond to those in a bulk junction with an effective semiconductor medium, whose permittivity and doping are their weighted averages over the cross-sectional areas of the semiconductor and dielectric; the shapes of the cross-sections are immaterial. Further, the reverse bias 1/C2-V behavior of junctions in NF/NW/NT arrays is linear, as in bulk junctions, and is useful to extract from measurements the built-in potential, effective doping including the semiconductor-dielectric interface charge, and NF/NW/NT length. The theory is validated with numerical simulations, is useful for the experimentalist, and yields simple formulas for nano-device design which predict the following. In the limiting case of a single sheet-like NF, the junction depletion width variation with potential drop is linear rather than square-root (as in a bulk junction). In arrays of symmetric silicon p-n junctions in oxide dielectric where NF/NW thickness and separation are 5% and 100% of the bulk depletion width, respectively, the junction depletion width and the screening length are scaled up from their bulk values by the same factor of ∼2 for NF and ∼10 for NW array.

Controlling radiative loss in quantum well solar cells

The inclusion of quantum well layers in a solar cell provides a means for extending the absorption and therefore increasing the photocurrent of the cell. In 2009, a single-junction GaAsP/InGaAs quantum well solar cell attained a peak efficiency of 28.3% under solar concentration. Since then InGaP/MQW/Ge quantum well devices have attained efficiencies in excess of 40% under concentration and over 30% under AM0. The principle motivation for incorporating a quantum well stack into a multi-junction solar cell is to increase the photocurrent delivered by the middle junction over the conventional In0.01GaAs bulk junction. This enables additional current to flow through the top and middle cells, resulting in a sharp rise in efficiency. However, quantum wells also provide some freedom to manipulate the radiative recombination in the quantum well solar cell. We show that under radiatively dominated, anisotropic emission, strong radiative coupling between sub-cells takes place, resulting in a multi-junction solar cell that is tolerant to daily and seasonal changes to the solar spectrum.

Microstructurally Explicit Simulation of Intergranular Mass Transport in Oxide Nuclear Fuels

Transport of fission products (FPs) inside fuel pellets is an important mechanism that affects microstructure evolution as well as fuel performance. To study this phenomenon for low fuel burnups, when solid-state diffusion is likely to be the controlling mechanism that sets the stage for subsequent phenomena, e.g., fission gas bubble formation and linkage, we created a three-dimensional (3-D) finite element model based on the real microstructure of a depleted UO2 sample. The model couples grain bulk, grain boundary (GB), and triple junction (TJ) diffusion by using 3-D elements for grain bulks, two-dimensional elements for GBs, and one-dimensional elements for TJs. Grain boundary percolation theory is applied in one case study, and the result shows that the presence of high-diffusivity TJs reduces the effect of GB percolation. The model is also used with mass generation from grain bulks, and it is found that localized regions with a high concentration of FPs can form in the presence of a dominant GB percolation path. The work introduces an approach to model diffusion through GBs and TJs at a fair computational cost that can be applied to study the effects of microstructure on FP transport.

Theoretical investigation of direct and phonon-assisted tunneling currents in InAlGaAs/InGaAs bulk and quantum-well interband tunnel junctions for multijunction solar cells

Direct and phonon-assisted tunneling currents in InAlGaAs-InGaAs bulk and double-quantum-well interband tunnel heterojunctions are simulated rigorously using the nonequilibrium Green's function formalism for coherent and dissipative quantum transport in combination with a simple two-band tight-binding model for the electronic structure. A realistic band profile and the associated built-in electrostatic field are obtained via self-consistent coupling of the transport formalism to Poisson's equation. The model reproduces experimentally observed features in the current-voltage characteristics of the devices, such as the pronounced current enhancement in the quantum-well junction as compared to the bulk junction and the structure appearing in the negative-differential resistance regime due to quantization of emitter states. Local maps of density of states and the current spectrum reveal the impact of quasibound states, electric fields, and electron-phonon scattering on the interband tunneling current. In this way, resonances appearing in the current through the double-quantum-well structure in the negative-differential resistance regime can be related to the alignment of subbands in the coupled quantum wells.

Can Luminescence Imaging Replace Lock-in Thermography on Solar Cells?

The purpose of this paper is a detailed comparison of selected luminescence and lock-in thermography (LIT) results on one exemplary sample and the drawing of corresponding conclusions. Our focus is on solar cells, but some investigations on wafers will be discussed as well. The comparison will help to decide which characterization tools are needed to solve technological problems. It will be demonstrated that luminescence imaging may widely replace LIT with respect to the analysis of recombination-active bulk defects, cracks, series resistance, and junction breakdown sites. However, some important investigations can be done only by LIT. LIT allows for a quantitative analysis of different kinds of leakage currents both under forward and under reverse bias, enabling a reliable analysis of local I-V characteristics. It is shown that LIT and luminescence imaging are complementary to each other and should be used in combination.

Design optimization of GaAs betavoltaic batteries

GaAs junctions are designed and fabricated for betavoltaic batteries. The design is optimized according to the characteristics of GaAs interface states and the diffusion length in the depletion region of GaAs carriers. Under an illumination of 10 mCi cm−2 63Ni, the open circuit voltage of the optimized batteries is about ∼0.3 V. It is found that the GaAs interface states induce depletion layers on P-type GaAs surfaces. The depletion layer along the P+PN+ junction edge isolates the perimeter surface from the bulk junction, which tends to significantly reduce the battery dark current and leads to a high open circuit voltage. The short circuit current density of the optimized junction is about 28 nA cm−2, which indicates a carrier diffusion length of less than 1 µm. The overall results show that multi-layer P+PN+ junctions are the preferred structures for GaAs betavoltaic battery design.

Low substrate-current and high breakdown voltage JI-LIGBT

A novel bulk junction isolated lateral insulated gate bipolar transistor (JI-LIGBT) with a heavily doped buried-layer in substrate is presented. Due to electric field modulation and current-gain reduction of the vertical parasitic transistor from the buried-layer, in comparison with the conventional LIGBT without a buried-layer, the novel structure offers not only high breakdown voltage but also short turn-off time and significantly low substrate leakage current.

Experimental approach to the laser machining of PMMA substrates for the fabrication of microfluidic devices

This paper deals with CO 2 laser machining of a suitable amorphous polymer (PMMA) as a flexible technique for the rapid fabrication of miniaturized structures such as microfluidic devices. A model to estimate the main dimensions (depth and width) of the grooves produced by the laser on PMMA is presented, taking into account the influence of the main process parameters (incident power, scanning speed and spot diameter). This theoretical model allows to control the engraving process showing that laser could represent a valid alternative for the production of microchannels. PMMA single-use devices are found to be easier to manufacture with respect to the conventional glass or silicon products. In a second step, IR laser vaporization is adopted for the removal of a single layer of PMMA. This is achieved using multiple overlapping sequences of straight grooves with different scanning directions. The proposed technique showed that the removal depth varied proportionally with the number of layers machined, while surface roughness is influenced by the grooves spacing and the orientation of the scanning direction between successive layers. A method for thermally bonding the PMMA sheets, constituting the 3D structure of the chip, is also presented. The combination of high temperatures and low bonding pressures makes it possible to generate a bulk junction enabling good performances in terms of sealing characteristics.