High Current Rectification Research Articles

Lateral WSe2 p–n Junction Device Electrically Controlled by a Single‐Gate Electrode

AbstractSemiconductor p–n junctions are building blocks for optoelectronic devices. Recently, p–n junction devices based on 2D transition metal dichalcogenides (TMDCs) have been demonstrated in optoelectronic applications due to their thin thickness, flexibility, high carrier mobility, and high light‐absorption properties. To fabricate 2D semiconductor p–n junction devices, various methods are demonstrated, such as heterojunction structures, chemical doping, and electrostatic doping. In this work, lateral both p–n and n–p junctions in WSe2 devices, electrically controlled by using only a single‐gate electrode, are first reported. It is demonstrated that the single‐gated WSe2 p–n and n–p diodes form an internal built‐in electrical field, showing strong diode‐like current rectifying behavior and photovoltaic effect under the illumination. The resultant device exhibits a high current rectification ratio up to ≈106 and a power conversion efficiency of 0.1% under AM 1.5 illumination. A logical inverter based on the lateral and in‐plane contacted WSe2 device with a hexagonal boron nitride (h‐BN) gate dielectric is also presented. The electrode architecture engineering based on a single TMDC will be useful for applications that are more complicated such as p–n junction optoelectronic devices and inverters.

High‐Performance Photovoltaic Effect with Electrically Balanced Charge Carriers in Black Phosphorus and WS2 Heterojunction

Abstract2D van der Waals materials are promising for various electronic and optoelectronic devices because of their thickness‐dependent mobility and tunable bandgap. Recently, heterojunction structures based on 2D van der Waals materials have exhibited their potential for photovoltaic applications as ultrathin p–n diodes. In this study, the photovoltaic effect of a multilayer black phosphorus (BP)/WS2 p–n heterojunction device is demonstrated under the 405 nm laser illumination and the AM 1.5 solar spectrum. The diode‐like characteristics and photovoltaic effect rely on balance between charge carriers in the heterojunction device, by showing the highest performance at the balance position. The gate‐tunable heterojunction device shows a high current rectification of 103 and an external quantum efficiency of 4.4% under the 405 nm laser illumination, and a photovoltaic efficiency of 4.6% under the AM 1.5 solar spectrum. This work suggests that the balancing of the charge carriers in the 2D heterojunction p–n diode is highly prioritized to fabricate high‐performance photovoltaic device.

Ultrasensitive zinc magnesium oxide nanorods based micro-sensor platform for UV detection and light trapping

A cost effective unique approach has been employed to realize high quality zinc magnesium oxide (Zn1-xMgxO) based nanostructures which can replace conventional material i.e. Zinc oxide (ZnO) in detector and sensor applications. High resolution structural studies showed the formation of vertically aligned hexagonal and truncated hexagonal nanorods on RF sputtered ZnO and Zn1-xMgxO seed layer. Single crystal Zn1-xMgxO and ZnO nanorods with dominant c-axis growth along (002) crystal axis and d-spacing of 0.262 and 0.266 nm, respectively were obtained from high resolution transmission electron microscopy, selective area diffraction pattern and high resolution X-ray diffraction spectra. The high external quantum efficiency of 12.054% with narrow full width half maxima of 12.63 nm was achieved from photoluminescence spectroscopy (Zn1-xMgxO nanorods) performed at room temperature. Approximately two order of enhancement in current density was observed from photodetector fabricated using Zn1-xMgxO nanorods. A state of art peak responsivity of 62.19 A/W at 350 nm attributed to near band edge emission peak and high current rectification capability with a maximum internal gain of 185 was measured from Zn1-xMgxO detector. Zn1-xMgxO detector exhibited temporal response with reasonable rise and fall time constants along with three order higher detectivity values in comparison to ZnO detector. A DC current based humidity sensor was fabricated and characterized using sample synthesized for longer growth duration. High sensitivity of 1.612 /% RH was achieved and low recovery time was observed as the sample was subjected to annealing treatment.

(Invited) Vertical Transport through Multi-Layer Van Der Waals Structures

Electronic transport in low-dimensional systems such as carbon nanotubes, nanowires and more recently transition metal dichalcogenides (TMDs) and black phosphorus (BP) has attracted considerable attention over the years due to the unique properties that these classes of materials offer. In particular, the fact that these materials can be semiconducting with body thicknesses of sub-nanometers with appreciable carrier mobilities makes them relevant for various electronic applications. Consequently, many research activities have been focusing on the lateral transport through these structures, often in three-terminal device geometries. By utilizing a gate, ideal electrostatics conditions can be enabled in the low-dimensional semiconducting channel, due the aforementioned thin body, and field-effect transistors (FETs) can be built to explore their transport properties as a function of gate and drain voltage. In doing so, it was found that most devices behave as Schottky barrier transistors and that the total number of layers in case of 2D systems plays an important role for the effective bandgap of the FET, an aspect that I will discuss in greater detail in my talk. Multi-layer TMDs and BP are also particular in that the hybridization between layers is weak due to the fact that van der Waals interaction – rather than covalent or ionic bonds – is the “glue” between layers, making the system highly anisotropic for charge transport. In fact, this anisotropy is considered to be the key for the formation of so-called “atomically abrupt” p-n junctions, in particular in heterostructures from 2D materials. In my talk, I will discuss in how far lateral and vertical transport is rather entangled in many device geometries and will offer an alternative interpretation – other than an abrupt p-n junction – for transport data that show high current rectification ratios in vertical 2D heterostructures. Last, I will discuss a model to describe transport across multi-layer 2D structures that allows extracting the vertical transport mass in MoS2 and WSe2.

Charge Transport through Single Molecules Connected to Semiconductor Electrodes

Single molecule electrical measurements have revolutionised our understanding of how charge flows through individual molecules and biomolecules in a wide variety of environments and are important for applications ranging from energy technology, DNA sequencing and interfacial electrochemistry. Such measurements have been typically made with metallic contacts but lately we have managed to extend this to the semiconductor gallium arsenide. This advance is important since future applications of molecular electronics will rely on integrating molecules alongside conventional semiconductor electronic technologies. In a recent publication in Nano Letters [1] we have shown that it is possible to make measurements of single molecules connected at one end to gallium arsenide and at the other end to a gold scanning tunnelling microscope tip. Using this methodology we can record current-voltage response of semiconductor – molecule – metal devices and measure the electrical conductance of single molecules in such junctions. For these measurements of charge transfer at the at the semiconductor-molecule interface the gallium arsenide surfaces are modified with organic self-assembled monolayers. Semiconductor / molecule(s) / metal junctions are formed by bringing a gold STM tip into contact with the molecular monolayer. Such junctions behave as a Schottky diode with the rectification ratio of the fabricated device being dependent on both the nature of the organic backbone and the electronic properties of the semiconductor. As well as recording I-V characteristics with the STM tip contacting the molecular monolayer, by using a time-dependent STM technique we were also able to detect the contribution of a single molecule to charge transport across the molecular junction. As well as showing that it is possible to form single molecule devices contacted to the semiconductor gallium arsenide we have also recently demonstrated that such single molecule devices show a strong photocurrent response.[2] Gallium arsenide is able to efficiently absorb visible light as it is a III-V direct bandgap (1.42 eV at room temperature) semiconductor and illumination of the molecular junction led to large changes in the current-voltage response. The junction configuration is schematically illustrated in the figure from reference [2] which shows a molecule bridging between the GaAs surface and the gold STM tip. The hemispheres in the GaAs substrate in this illustration depict the space charge layer (SCL) from which the charge carriers are collected. The resulting photo-generated charge carriers are transported to the metal electrode (STM tip) through the bridging organic dithiol giving the photo-current. Importantly, the photo-current response in these hybrid junctions can be controlled through the choice of molecular bridge as well as the doping density of the semiconductor amd the light intensity and wavelength.[2] In the dark high current rectification (> 103) is achieved with low-doping GaAs and saturated alkyl bridges and appreciable photocurrent is achieved upon illumination of these junctions. The response of the device is greatly influenced by molecular orbital alignment and this can be controlled by switching from saturated molecular bridges to conjugated ones with lower HOMO-LUMO gaps. Good photodiode functionality can be achieved with a space charge layer of only a few nanometres. Since the response of the molecular junctions is controlled by the combined properties of the semiconductor and the molecular wire this significantly extend of the single-molecule electronics “tool-box” as well as providing new methodologies for studying charge transfer at semiconductor-molecule interfaces. The presentation concludes with discussion of the perspective of single molecule photo-electrochemistry.

Why one can expect large rectification in molecular junctions based on alkane monothiols and why rectification is so modest.

Many attempts to obtain high current rectification ratios (RRs) in molecular electronics are triggered by a potentiometer rule argument, which predicts that a strongly asymmetric location of the dominant molecular orbital yields large RR-values. Invoking this argument, molecular junctions based on alkane monothiols (CnT) can be expected to exhibit high RRs; the HOMO of these molecules is localized on the thiol terminal group bonded to one electrode. The extensive current-voltage (I-V) results for CP-AFM (conducting probe atomic force microscope) CnT junctions of various molecular lengths (n = 7, 8, 9, 10, and 12) and different metallic contacts (Ag, Au, and Pt) are consistent with conduction dominated by the HOMO, but the measured RR ∼ 1.5 is much smaller than that predicted by the potentiometer rule framework. Further, the linear shift in the HOMO position with applied bias, γ, which gives rise to rectification, is also smaller than expected, and critically, γ has the opposite sign from potentiometer rule predictions. Companion ab initio OVGF (outer valence Green's function) quantum chemical calculations provide important insight. Namely, a linear Stark shift γm is calculated for the HOMO of CnT molecules for electric field strengths (106-107 V cm-1) typical of molecular junctions, and the sign of γm matches the sign of the experimental γ for junctions derived from transport measurements, suggesting that the Stark effect plays an important role. However, the magnitude of the measured γ is only 10-15% of the computed value γm. We propose that this implies that the contacts are far from optimal; they substantially screen the effect of the applied bias, possibly via molecule-electrode interface states. We predict that, with optimized contacts, the rectification ratios in CnT-based junctions can reach reasonably high values (RR ≈ 500). We believe that Stark shifts and limited current rectification due to non-ideal contacts discussed here for the specific case of alkane monothiol junctions are issues of general interest for molecular electronics that deserve further consideration.

Size-Controllable Gold Nanopores with High SERS Activity.

Nanopore structures have been successfully employed in next-generation DNA sequencing. For more complicated protein which normally contains 20 different amino acids, identifying the fluctuation of ionic current caused by different amino acids appears inadequate for protein sequencing. Therefore, it is highly desirable to develop size-controllable nanopores with optical activity that can provide additional structural information. Herein, we discovered the novel nanopore properties of the self-assembled ultramicroelectrodes originally developed by Bard and co-workers. Using a slightly modified method, the self-assembly of 7 ± 1 nm gold nanoparticles (AuNPs) can be precisely controlled to form a gold nanoporous sphere (GPS) on the tip of a glass capillary. Different dithiol linker molecules (1,3-propanedithiol, C3; 1,6-hexanedithiol, C6; and 1,9-nonanedithiol, C9) reproducibly led to rather similar nanopore sizes (5.07 ± 0.02, 5.13 ± 0.02, and 5.25 ± 0.01 nm), respectively. The GPS nanostructures were found to exhibit high ionic current rectification as well as surface-enhanced Raman scattering (SERS) activity due to the presence of nanopores and numerous "hot spots" among the cross-linked AuNPs on the surface of GPS. The rectification effect of the small nanopores was observed even under high concentration of electrolyte (290 mM), along with SERS enhancement factors well above 1 × 105. The GPS nanostructures were successfully applied for SERS-based detection of glutathione from a single HeLa cell.

Transparent p-CuI/n-BaSnO3−δ heterojunctions with a high rectification ratio

Transparent p-CuI/n-BaSnO3−δ heterojunction diodes were successfully fabricated by the thermal evaporation of a (1 1 1) oriented γ-phase CuI film on top of an epitaxial BaSnO3–δ(0 0 1) film grown by the pulsed laser deposition. Upon the thickness of the CuI film being increased from 30 to 400 nm, the hole carrier density was systematically reduced from 6.0 × 1019 to 1.0 × 1019 cm−3 and the corresponding rectification ratio of the pn diode was proportionally enhanced from ~10 to ~106. An energy band diagram exhibiting the type-II band alignment is proposed to describe the behavior of the heterojunction diode. A shift of a built-in potential caused by the hole carrier density change in the CuI film is attributed to the thickness-dependent rectification ratio. The best performing p-CuI/n-BaSnO3−δ diode exhibited a high current rectification ratio of 6.75 × 105 at ±2 V and an ideality factor of ~1.5.

Analysis of Schottky Contact Formation in Coplanar Au/ZnO/Al Nanogap Radio Frequency Diodes Processed from Solution at Low Temperature.

Much work has been carried out in recent years in fabricating and studying the Schottky contact formed between various metals and the n-type wide bandgap semiconductor zinc oxide (ZnO). In spite of significant progress, reliable formation of such technologically interesting contacts remains a challenge. Here, we report on solution-processed ZnO Schottky diodes based on a coplanar Al/ZnO/Au nanogap architecture and study the nature of the rectifying contact formed at the ZnO/Au interface. Resultant diodes exhibit excellent operating characteristics, including low-operating voltages (±2.5 V) and exceptionally high current rectification ratios of >10(6) that can be independently tuned via scaling of the nanogap's width. The barrier height for electron injection responsible for the rectifying behavior is studied using current-voltage-temperature and capacitance-voltage measurements (C-V) yielding values in the range of 0.54-0.89 eV. C-V measurements also show that electron traps present at the Au/ZnO interface appear to become less significant at higher frequencies, hence making the diodes particularly attractive for high-frequency applications. Finally, an alternative method for calculating the Richardson constant is presented yielding a value of 38.9 A cm(-2) K(-2), which is close to the theoretically predicted value of 32 A cm(-2) K(-2). The implications of the obtained results for the use of these coplanar Schottky diodes in radio frequency applications is discussed.

Radio Frequency Coplanar ZnO Schottky Nanodiodes Processed from Solution on Plastic Substrates.

Coplanar radio frequency Schottky diodes based on solution-processed C60 and ZnO semiconductors are fabricated via adhesion-lithography. The development of a unique asymmetric nanogap electrode architecture results in devices with a high current rectification ratio (10(3) -10(6) ), low operating voltage (<3 V), and cut-off frequencies of >400 MHz. Device fabrication is scalable and can be performed at low temperatures even on plastic substrates with very high yield.

Semi‐transparent NiO/ZnO UV photovoltaic cells

Semi‐transparent pn‐heterojunctions were fabricated from pulsed laser deposited (PLD) n‐type ZnO and DC magnetron sputtered p‐type NiO, working as UV‐active solar cells. The complete cell stack has an average transmission of 46% in the visible spectral range and an optical absorption edge at 380 nm. The diodes exhibit high current rectification of up to eight orders of magnitude at 2 V. Upon illumination with a solar simulator, the devices show photovoltaic activity with open‐circuit voltages of up to 520 mV, short‐circuit current densities of 0.5 mAcm, and a maximum external quantum efficiency of 55%. However, we observed rather low fill factors of the current–voltage characteristics of around 40%, resulting in total power conversion efficiencies of around 0.1% and efficiencies in the UV range of 3.1%. To identify possible loss mechanisms, the voltage‐dependent efficiency of carrier collection was calculated. The data were fitted using a model that considers recombination losses at the NiO/ZnO interface as well as within the electric field region, yielding a high hole interface recombination velocity of 1cm s. We conclude that the carrier collection efficiency is strongly deteriorated by recombination losses at the NiO/ZnO interface, causing a strong bending of the jV characteristics under illumination and thereby low fill factors.

Fabrication of single cylindrical Au-coated nanopores with non-homogeneous fixed charge distribution exhibiting high current rectifications.

We designed and characterized a cylindrical nanopore that exhibits high electrochemical current rectification ratios at low and intermediate electrolyte concentrations. For this purpose, the track-etched single cylindrical nanopore in polymer membrane was coated with a gold (Au) layer via electroless plating technique. Then, a non-homogeneous fixed charge distribution inside the Au-coated nanopore was obtained by incorporating thiol-terminated uncharged poly(N-isopropylacrylamide) chains in series to poly(4-vinylpyridine) chains, which were positively charged at acidic pH values. The functionalization reaction was checked by measuring the current-voltage curves prior to and after the chemisorption of polymer chains. The experimental nanopore characterization included the effects of temperature, adsorption of chloride ions, electrolyte concentration, and pH of the external solutions. The results obtained are further explained in terms of a theoretical continuous model. The combination of well-established chemical procedures (thiol and self-assembled monolayer formation chemistry, electroless plating, ion track etching) and physical models (two-region pore and Nernst-Planck equations) permits the obtainment of a new nanopore with high current rectification ratios. The single pore could be scaled up to multipore membranes of potential interest for pH sensing and chemical actuators.

Current rectification by nanoparticle blocking in single cylindrical nanopores.

Blocking of a charged pore by an oppositely charged nanoparticle can support rectifying properties in a cylindrical nanopore, as opposed to the usual case of a fixed asymmetry in the pore geometry and charge distribution. We present here experimental data and model calculations to confirm this fundamental effect. The nanostructure imaging and the effects of nanoparticle concentration, pore radius, and salt concentration on the electrical conductance-voltage (G-V) curves are discussed. Logic responses based on chemical and electrical inputs/outputs could also be implemented with a single pore acting as an effective nanofluidic diode. To better show the generality of the results, different charge states and relative sizes of the nanopore and the nanoparticle are considered, emphasizing those physical concepts that are also found in the ionic drug blocking of protein ion channels.

An Electrical Rectifier Based on Au Nanoparticle Array Fabricated Using Direct‐Write Electron Beam Lithography

AbstractClose‐packed arrays of Au nanoparticles are produced in patterned regions by electron beam (e‐beam) lithography using a highly sensitive direct–write resist, N+AuCl4−(C8H17)4Br. While the e–beam causes dewetting of the resist to nucleate Au nanoparticles, the following step of thermolysis aids particle growth and removal of the organic part. Thus formed arrays contain Au nanoparticles. Such arrays are patterned into ≈10 μm wide stripes between Au contact pads on SiO2/Si substrates to realize electrical rectification. Under forward bias, the device exhibits a threshold voltage of +4.3 V and a high current rectification ratio of 3 × 105, which are stable over many repetitive measurements. The threshold voltage of the rectifier can be reduced by applying an electric stress or by varying the electron dosage used for array formation. The nanoparticle rectifier element could be transferred onto flexible substrates such as PDMS, where the nanoparticle coupling is influenced by swelling of the substrate. Obviously, the nanoparticle size, shape, and the spacing in array are all important for the rectifier device performance. Based on the electrical measurements the mechanism of rectification is found to be due to switching of electrical conduction with applied bias, from short–distance tunneling to F–N type tunneling followed by transient filament formation.

Ion diode logics for pH control

Electronic control over the generation, transport, and delivery of ions is useful in order to regulate reactions, functions, and processes in various chemical and biological systems. Different kinds of ion diodes and transistors that exhibit non-linear current versus voltage characteristics have been explored to generate chemical gradients and signals. Bipolar membranes (BMs) exhibit both ion current rectification and water splitting and are thus suitable as ion diodes for the regulation of pH. To date, fast switching ion diodes have been difficult to realize due to accumulation of ions inside the device structure at forward bias--charges that take a long time to deplete at reverse bias. Water splitting occurs at elevated reverse voltage bias and is a feature that renders high ion current rectification impossible. This makes integration of ion diodes in circuits difficult. Here, we report three different designs of micro-fabricated ion bipolar membrane diodes (IBMDs). The first two designs consist of single BM configurations, and are capable of either splitting water or providing high current rectification. In the third design, water-splitting BMs and a highly-rectifying BM are connected in series, thus suppressing accumulation of ions. The resulting IBMD shows less hysteresis, faster off-switching, and also a high ion current rectification ratio as compared to the single BM devices. Further, the IBMD was integrated in a diode-based AND gate, which is capable of controlling delivery of hydroxide ions into a receiving reservoir.

Mixed mode, ionic-electronic diode using atomic layer deposition of V2O5 and ZnO films

We demonstrate high current rectification in a new system comprising 30 nm of hydrated vanadium pentoxide and 100 nm of zinc oxide (V2O5·nH2O–ZnO) thin film structures. The devices are prepared using a low temperature (<150 °C), all atomic layer deposition process. A key element in the rectifying properties comes from anomalous p-type conductivity in V2O5 – an otherwise well known n-type semiconductor. Experimental evidence points to protonic (H+) conductivity due to intercalated water in V2O5 as the source for p-type behaviour, while the ZnO is known to be electronically n-type. Thus, the diode behaves as a novel, mixed mode ionic-electronic rectifier. Further, we show that the diode characteristics are strongly dependent on the electrode material in contact with V2O5·nH2O. A high Ion/Ioff ratio (598) at ± 2 V is obtained for oxygen-free Pt electrodes, whereas a low Ion/Ioff ratio (19) is obtained for oxygen-rich ITO electrodes, suggesting the deleterious effects of oxygen atom reactivity to device characteristics.

Electrochemically superfilling of n-type ZnO nanorod arrays with p-type CuSCN semiconductor

The primary problem for constructing three-dimensional (3D) heterojunctions lies in poor pore filling and interface contact quality. An electrochemical superfilling technique is developed to construct well-organized heterojunctions based on a bottom-up filling mechanism. Morphology observation shows that ZnO nanorod arrays are completely filled with CuSCN and intimate interface contact is formed between ZnO and CuSCN. Electrical test confirms that as-fabricated 3D heterojunction has high diode current density and high rectification ratio of 154. This superfilling technique has promising applications in other 3D heterojunctions.

Tunneling Spectroscopy of Magnetic Double Barrier Junctions

Scanning tunneling microscopy (STM) is used to study transport in magnetic double tunnel junctions (DTJs) formed using a fixed transparency barrier of a patterned tunnel junction (TJ), and a variable tunnel barrier between the top electrode of the patterned junction and the STM tip. A sufficiently thin top electrode has been predicted to result in a rectification of charge current through a DTJ when the two barriers have different transparency. Our measurements indeed show a high current rectification ratio for 3-nm-thick, continuous film top electrodes, which is observed for junctions with asymmetric tunnel barriers

Solution-deposited ZnO-organic diodes with high current density and high frequency rectification under ambient conditions

Abstractn-ZnO/p-Pentacene and n-ZnO/poly(bis(dodecyl)quaterthiophene) (p-PQT-12) vertical p-n junction diodes were prepared on ITO-coated glass. A continuous film of ZnO nanoparticles was grown on the ITO glass by dip-coating and subsequent heat treatment of a zinc acetate film. Pentacene was then thermally evaporated to form the ZnO/Pentacene diode, whereas PQT-12 was spin coated for the ZnO/PQT-12 diode. Based on the band energies of ZnO, pentacene and PQT-12, for efficient carrier injection, gold was chosen as the top electrode to complement the ITO for both diodes. The microstructures of ZnO and pentacene films are studied by AFM and show a layer of pentacene grains with about twice the extent of the underlying layer of ZnO grains, implying substantial interfacial contact between them The current density-voltage (J-V) measurement shows that the maximum current density for ZnO/Pentacene and ZnO/PQT12 are 160 A/cm2 and 350 A/cm2 respectively. The rectification was characterized by observation of full input-half output waves. Data indicate that these devices can operate up to frequencies of 20 MHz and 9 MHz for ZnO/Pentacene and ZnO/PQT12, respectively, under ambient environment conditions. This rectification frequency is higher than other reported organic and polymer Schottky diodes under these conditions. Turn on voltages of these diodes are also much lower than for the reported organic and polymer diodes.

Diode effect in asymmetric double-tunnel barriers with single-metal nanoclusters

Asymmetric double-tunnel barriers with the center electrode being a metal cluster in the quantum regime are studied. The zero dimensionality of the clusters used and the associated quantized energy spectra are manifest in well-defined steps in the current-voltage characteristic. Record high current rectification ratios of ∼104 for tunneling through such clusters are demonstrated at room temperature. We are able to account for all of the experimentally observed features by modeling our double-barrier structures using a combination of discrete states and charging effects for tunneling through quantum dots.