High Rectification Ratio Research Articles

High-performing MoS2-embedded Si photodetector

Transition metal dichalcogenides (TMDs) are a new class of materials that replace advanced functional materials like graphene, CNT etc., in photovoltaics and sensors. In the present work, the 2-dimensional tri-layer MoS2 is applied for high-performing photodetector. Here, the conventional n-doped p-type Si (n-Si/p-Si) was coated with tri-layer MoS2 film using CVD method. HRTEM reveals the presence of tri-layer with highly ordered lattice planes. Characteristic peaks of Mo and S are obtained in XPS profile. Due to spin-orbit coupling, the 3d band of Mo and 2p band of S are split into two states. Raman spectrum of MoS2 film shows two peaks, corresponding to its in-plane and out-of-plane vibrational modes. The wave number difference between these modes is measured as 22.97cm−1, which ensures that there are three layers in MoS2 film. The splitting of valence band generates multiple excitons which are marked as A and B in the absorption profile. The excitonic transition corresponds to the direct band gap of MoS2 (ie., 1.9eV). The prepared MoS2/n-Si/p-Si photodetector includes two rectifying junctions with considerable built-in potentials. A high rectification ratio was measured as 51.37 to ensure the quality junction formation. The photoresponse ratio of the MoS2/n-Si/p-Si photodetector was obtained as 58.74 to confirm the quality junction formation between MoS2 and Si with high detectivity of 5.42 × 1014 Jones. Moreover, the extremely fast rise and fall times of 33 µs and 30 µs were achieved without any external bias application. The functional use of MoS2 window design would provide the high potential for the enhanced photoelectric devices, such as photodetectors and solar cells.

Tune turn-on voltage of LaFeO3−x/Nb:SrTiO3 p-n junctions by oxygen vacancy

Oxide semiconductors with high expectations are regarded as the best candidate in the next generation thin film transistors. Flexible structure endows perovskite oxide semiconductors with the potential to satisfy different application requirements. In this letter, LaFeO3 epi-films (epitaxial films) were deposited on Nd-SrTiO3 to construct p-n junctions. The junctions show distinct rectification characteristic with a high rectification ratio of ~5 × 103 at ±2 V. Through annealing in deficient oxygen ambience, changes in LaFeO3 structure by introducing oxygen vacancies lead to the different turn-on voltages. Confirmed by XPS data, oxygen vacancies result in the slight reduction of Fe3+ to Fe2+ and the increase in holes concentration. Simultaneously, the location of Fermi Level shifts toward the top of valance band and barrier height for carriers increases gradually. Correspondingly, the turn-on voltage increases as lowering anneal oxygen pressure.

Spin-filter silicene devices induced by asymmetric electrodes

Using nonequilibrium Green's function in combination with the density functional theory, we investigate the spin-dependent transport properties of three silicene-based heterojunctions. The calculated results show that the heterojunctions are promising multifunctional devices in spintronics due to their nearly perfect bipolar spin-filter effect and high rectification ratio. Also shown are the obvious negative differential resistance behaviors for both spin channels. By analyzing the spin-resolved transmission spectra and the band structure of ZSiNR electrodes, as well as the spatial resolved local density of states, the mechanisms for these intriguing properties are demonstrated in detail.

Programmable Electrochemical Rectifier Based on a Thin-Layer Cell.

A programmable electrochemical rectifier based on thin-layer electrochemistry is described here. Both the rectification ratio and the response time of the device are programmable by controlling the gap distance of the thin-layer electrochemical cell, which is easily controlled using commercially available beads. One of the electrodes was modified using a ferrocene-terminated self-assembled monolayer to offer unidirectional charge transfers via soluble redox species. The thin-layer configuration provided enhanced mass transport, which was determined by the gap thickness. The device with the smallest gap thickness (∼4 μm) showed an unprecedented, high rectification ratio (up to 160) with a fast response time in a two-terminal configuration using conventional electronics.

Rectified Schottky diodes based on PEDOT:PSS/InGaZnO junctions

We report the successful use of the high-work-function, high-conductivity transparent conducting polymer PEDOT:PSS as the Schottky contact to form the Schottky junction (and thus Schottky diode) with the n-type semiconductor a-IGZO. The Schottky didoes exhibited a low apparent turn-on voltage, a high rectification ratio of >105 at ±1 V, and a decent ideality factor of ∼1.5–1.6. Detailed junction properties were systematically analyzed from J-V and C-V characteristics of the diodes. We also demonstrated the applications of PEDOT:PSS/a-IGZO Schottky junctions to various types of Schottky diodes, including the flexible, the transparent, and the flexible transparent PEDOT:PSS/a-IGZO Schottky diodes, by using different substrates and different counter electrodes.

Electrical properties of RF-sputtered Zn-doped GaN films and p-Zn-GaN/n-Si hetero junction diode with low leakage current of 10−9 A and a high rectification ratio above 105

Zn-doped GaN films have been deposited on Si (100), SiO2/Si (100), and glass substrates by RF reactive sputtering at 100–400°C with single cermet targets. The Zn-GaN films deposited with a 10% Zn cermet target showed the good electrical properties, wurtzite structure, and the p-type behavior without a thermal annealing process. At room temperature (RT), the 400°C-deposited film with mobility (μ) of 17.7cm2/V·s had the highest hole concentration (np) of 6.1×1017cm−3 and electrical conductivity of 1.72S·cm−1. The values of band gap (Eg) for Zn-GaN films were found in the range of 2.83–3.01eV. Furthermore, to prove the p-type behavior of Zn-doped GaN, the p-GaN/n-Si hetero junction diode was also made by RF reactive sputtering. At room temperature, the device showed the leakage current of 1.65×10−9A at −5V, the turn-on voltage of ∼2.3V, and the breakdown voltage above −20V. The high rectification (on/off) ratio of the diode was calculated to be ∼1.08×105 at bias of ±5V and ∼2.3×105 at the bias of ±20V. The electrical characteristics of the diode were also tested at the temperature range of 25–150°C. By using equations based on the standard thermionic-emission (TE) mode and the Cheungs’ method, the barrier height, ideality factor, and series resistance of the diode were also determined.

Efficient UV photodetector based on heterojunction of n-ZnO nanorods/p-diamond film

ZnO nanorods (NRs) have been successfully fabricated on p-type boron-doped diamond (BDD) substrate by hydrothermal method. The current–voltage characteristics of the ZnO NRs/BDD heterojunction show a typical rectifying behavior with extremely low reverse leakage current and high rectification ratio of around 370 at bias of 3 V. Exposed to ultraviolet (UV) illumination with a wavelength of 365 nm, the time dependent photoresponse manifests high sensitivity and consistent transients with a rise time of 4.9 s and decay time of 6.4 s. Circulating on/off measurement under UV detection demonstrates the excellent stability and repeatability for as-fabricated ZnO NRs/BDD heterojunction. Furthermore, the mechanism for UV detection on the ZnO NRs/BDD heterojunction has also been discussed.

Diamond Schottky-pn diode using lightly nitrogen-doped layer

We successfully fabricated diamond Schottky-pn diodes (SPNDs) using a lightly nitrogen-doped (N-doped) layer. The diamond SPNDs showed a high forward current density of over 20,000A/cm2 at 7V, a high rectification ratio of over 1013, and a high breakdown electric field of 3.3MV/cm at room temperature. These properties mean that the N-doped layer acted as an n-type semiconductor. The N-doped n-type layer is effective in the SPNDs as same as a phosphorus-doped n-type layer.

Design and fabrication of dioxyphenylcoumarin substituted cyclotriphosphazene compounds photodiodes

The present study introduces cyclotriphosphazene compounds substituted by dioxyphenylcoumarin as a photodiode application. Firstly, 7,8-dihydroxy-3-(3-methylphenyl)coumarin (1b) has been obtained by conventional as well as microwave assisted methods. Novel optoelectronic device characteristics for both mono and disubstituted dioxyphenylcoumarin bearing cyclotriphosphazene compounds (HCP-2 and HCP-4) have been synthesized from the reactions of cyclotriphosphazene containing dioxybiphenyl (HCP-1 and HCP-3) with compound 1b, respectively. The structures of compounds HCP 1–4 were identified by using elemental analysis, 1H, 13C-APT, 31P NMR and 2D HETCOR NMR and FT-IR spectroscopy methods. The Al/HCP-2/p-Si/Al and Al/HCP-4/-p-Si/Al photodiodes properties have been investigated from current-voltage (I−V) and capacitance-voltage (C−V) measurements. The electrical parameters of the prepared diodes such as ideality factor n and series resistance Rs were investigated in dark and at room temperature from (I−V) curve and Nord's method. As can be seen, the Al/HCP-2/p-Si/Al diode of high rectification ratio RR and with ideality factor greater than unity. The influence of light illuminations on the diode shows that the device can be used as photodiode with good efficiency. The barrier height ϕb and series resistance Rs have been calculated from the capacitance-voltage (C−V)and conductance-voltage (G−V) measurements under various applied frequencies from 10kHz to 1MHz. the high difference in the results of barrier height obtained from I-V and C-V calculations confirm the influence of series resistance and localized states on transport of charge carriers and the photodiode performance.

Complexes of graphene nanoribbons with porphyrins and metal-encapsulated C28 as molecular rectifiers: a theoretical study

The rectification properties of porphyrin–graphene nanoflake complexes and endohedral complexes of C28 fullerene with metal atoms have been studied using the fully ab initio method. D3 dispersion-corrected PBE/-def2-SVP model was used for the optimisations and the electronic energy evaluation. In porphyrin–graphene nanoflake complexes dispersion dominates, while in the endohedral complexes of C28 dispersion does not play an important role. All studied systems do rectify. In the case of fullerenes, the rectification is possible due to the reduction in the molecular symmetry of the fullerene caused by the interaction with electrodes and the endohedral complex formation. The origin of the rectification is the asymmetrical deformation of the electron density under direct and inverse voltages which creates different currents in opposite directions. It seems that peculiar geometry of Au-TPP-Cd/NF diode is responsible for its high rectification ratio. The Cd ion is notably out of the porphyrin plane making close contact with the neighbouring electrode, increasing the asymmetry of the diode compared to other TPP/NF complexes.

Analysis of electrical characteristics and conduction mechanisms in the Al/(%7 Zn-doped PVA)/p-Si (MPS) structure at room temperature

The electrical properties and current-conduction/transport mechanism of Al/(%7 Zn-doped PVA)/p-Si (MPS) structure was investigated by current–voltage (I-V), capacitance–voltage (C-V) and conductance-voltage (G/ω-V) measurements at room temperature. The energy dependent profile of surface states (Nss) was obtained by taking into account voltage dependent effective barrier height (Φe), ideality factor (n) and series resistance (Rs) of the structure. Voltage dependent profile of resistivity (Ri) and some main electrical parameters such as reverse saturation current (Io), ideality factor (n) and zero-bias barrier height (ΦBo) were also evaluated from the forward bias I-V data. Experimental results reveal that the fabricated MPS structure has a higher rectification ratio with low reverse leakage current. The double logarithmic I-V plot was drawn and it shows a power-law behavior of the current (I ∝ V m). The forward and reverse bias C-V and G/ω-V measurements were carried out at enough high frequency (1 MHz) and then to eliminate of the Rs the measured C-V and G/ω-V plots were corrected. The C-V and G/ω-V plots exhibit inductive behavior at accumulation region due to the effect of Nss and Rs. The other some electrical parameters such as concentration of acceptor atoms (NA) and barrier height (ΦB) were also obtained from the slope and intercept of reverse bias C− 2 vs V plot. Further, both the forward and reverse bias conduction mechanisms of the MPS structure are also discussed compare with the literature.

Ab initio electronic transport study of two-dimensional silicon carbide-based p–n junctions**Project supported by the National Science Foundation of China (Nos. 61474099, 61674127) and the ZJ-NSF (No. Z17F04003).

Two-dimensional silicon carbide (2d-SiC) is a viable material for next generation electronics due to its moderate, direct bandgap with huge potential. In particular, its potential for p–n junctions is yet to be explored. In this paper, three types of 2d-SiC-based p–n junctions with different doping configuration are modeled. The doping configurations refer to partially replacing carbon with boron or nitrogen atoms along the zigzag or armchair direction, respectively. By employing density functional theory, we calculate the transport properties of the SiC based p–n junctions and obtain negative differential resistance and high rectification ratio. We also find that the junction along the zigzag direction with lower doping density exhibits optimized rectification performance. Our study suggests that 2d-SiC is a promising candidate as a material platform for future nano-devices.

Solution-processed, hybrid 2D/3D MoS2/Si heterostructures with superior junction characteristics

We report a theoretical and experimental investigation of the hybrid heterostructure interfaces between atomically thin MoS2 nanocrystals (NCs) on Si platform for their potential applications towards next-generation electrical and optical devices. Mie theory-based numerical analysis and COMSOL simulations based on the finite element method have been utilized to study the optical absorption characteristics and light–matter interactions in variable-sized MoS2 NCs. The size-dependent absorption characteristics and the enhancement of electric field of the heterojunction in the UV-visible spectral range agree well with the experimental results. A lithography-free, wafer-scale, 2D material on a 3D substrate hybrid vertical heterostructure has been fabricated using colloidal n-MoS2 NCs on p-Si. The fabricated p-n heterojunction exhibited excellent junction characteristics with a high rectification ratio suitable for voltage clipper and rectifier applications. The current–voltage characteristics of the devices under illumination have been performed in the temperature range of 10–300 K. The device exhibits a high photo-to-dark current ratio of ∼3 × 103 and a responsivity comparable to a commercial Si photodetector. The excellent heterojunction characteristics demonstrate the great potential of MoS2 NC-based hybrid electronic and optoelectronic devices in the near future.

Self-assembly of regioregular poly (3,3‴-didodecylquarterthiophene) in chloroform and study of its junction properties

This article deals with the study of self-assembly of regioregular poly (3,3‴-didodecylquarterthiophene), rr-PQT-12 into fiber form in chloroform by ageing process. Time dependent fiber growth mechanism is monitored by UV–vis absorption and confirmed by atomic force microscopy technique. It is observed that isolated rr-PQT-12 undergoes self-assembled fibril growth along π-π interaction direction and 45min is sufficient for such assemblies in case of 0.125% w/v of rr-PQT-12 polymer in chloroform. Further the self-assembled fibril polymer is used in fabrication of Schottky diode. It exhibits ten times enhancement in forward current density (with one-fold higher mobility) and high rectification ratio compared to the isolated rr-PQT-12 due to the segmental electronic traps filling within stacking region. Our study provides a facile method of ordering of PQT-12 isolated chains in chloroform solvent and an effective way for improvement of performance of organic polymers based devices through such self-assembly.

Computational simulations for the development of novel solid-state smart NiTi-Al thermal diodes

In this study, NiTi shape memory alloys coupled in series with Al are considered as building blocks for thermal diodes. It is shown that the strong nonlinearity in the temperature-dependent thermal properties of NiTi in conjunction with the very different thermal properties of Al can result into a thermal diode of high thermal rectification ratio. As a first level of study, Ni50Ti50 is considered and the effects of various NiTi-Al geometrical configurations, initial temperature, and temperature difference at two ends on the thermal rectification ratio are studied numerically. Within the adopted temperature range (300–400 K, where phase transformation in NiTi occurs), it is shown that NiTi-Al thermal diodes are feasible with rectification ratio up to 4.8, which is quite higher than the ratios in currently known solid-state thermal diodes. This fundamental computational study could provide an important basis and motivation for the development of the next generation of high-temperature solid-state thermal diodes based on smart material such as NiTi shape memory alloys or others.

Effects of substrate and anode metal annealing on InGaZnO Schottky diodes

By studying different annealing effects of substrate and anode metal, high-performance Schottky diodes based on InGaZnO (IGZO) film have been realized. It is observed that a suitable thermal annealing of the SiO2/Si substrate significantly improves the diode performance. On the contrary, annealing of the Pd anode increases surface roughness, leading to degradation in the diode performance. As such, by only annealing the substrate but not the anode, we are able to achieve an extremely high rectification ratio of 7.2 × 107, a large barrier height of 0.88 eV, and a near unity ideality factor of 1.09. The diodes exhibit the highest performance amongst IGZO-based Schottky diodes reported to date where IGZO layer is not annealed. The capacitance vs. voltage measurements indicate that the surface roughness is correlated with the trap state density at the Schottky interface.

Fast gate-tunable photodetection in the graphene sandwiched WSe2/GaSe heterojunctions.

We investigated electrical and photoelectrical properties of graphene sandwiched WSe2/GaSe van der Waals heterojunctions. The device showed a high rectification ratio up to 300 at Vds = 1.5/-1.5 V, which is attributed to the built-in electric field in the device. Due to the bipolar property of WSe2, gate-tunable rectification inversion was observed. Meanwhile, the graphene sandwiched heterojunction showed excellent performances on photodetection, where the photoresponsivity of (6.2 ± 0.2) A W-1 can be reached under Vds = -1.5 V and P = 0.2 μW. The device also showed great external quantum efficiency of (1490 ± 50)% and fast response time of ∼30 μs. Our study identified the graphene sandwiched heterojunctions based on 2D materials have great potential for gate-tunable electronic and optoelectronic applications.

High performence visble-near infrared photovoltaic detector based on narrow bandgap polymer

Polymer-based visible-near infrared photodetectors have attracted considerable attention in the recent years due to their unique advantages of low cost of fabrication, compatibility with lightweight/flexible electronics, and wide material sources. Current researches mainly focus on high performence visble-near infrared photovoltaic detector based on narrow bandgap polymer. Device structure of the photodetector is ITO/PEDOT:PSS/photosensitive layer/Ca/Al. The weak light (0.4 mW/cm2, 800 nm) and reverse bias (-2 V) induce insignificant differences in photocurrent among the devices. Current values of 1.69×10-4 A/cm2, 7.96×10-5 A/cm2 and 6.98×10-5 A/cm2 are obtained with photosensitive layer thickness values of 100, 200 and 300 nm, respectively. However, the dark current density-voltage characteristics of the detectors with various thickness values of the photosensitive layer show that reverse bias (-2 V) induces significant differences in current among the devices. Current values of 1.35×10-6 A/cm2, 1.13×10-7 A/cm2 and 2.98×10-8 A/cm2 are obtained with photosensitive layer thickness values of 100 nm, 200 nm and 300 nm, respectively. Meanwhile, all detectors possess high rectification ratios over 105(±2 V), indicating good diode rectification characteristics. Photosensitivity measurements show that detection spectral regions of the detectors are extended from 380 nm to 960 nm. The values of detectivity (D*) of detectors with various thickness values of photosensitive layers are investigated, and the obtained values of D* of tested detectors are found to be very stable in a range from 400 nm to 860 nm, and the average D* value for the 300 nm thick device in this spectral range is as high as 6.89×1012 Jones. The latter compares well with values obtained with silicon detectors. In a range from 800 nm to 900 nm, the estimated detectivities of the 300 nm and 200 nm thick detectors are slightly higher than those obtained with InGaAs devices. Through analyzing energy band diagrams of the polymer photodetectors under reverse voltage bias it could be argued that the relatively weak electric field in the thicker device is the origin of the lower noise current density. The capacitance characteristics of polymer based detectors at high frequency (100 kHz) are examined through capacitance-voltage curves, and the resulting data show that capacitances of all devices at reverse and even small positive voltage are constant. This indicates that the device photosensitive layers are fully depleted and fast signal detections are theoretically possible. The time responses of detectors under near-infrared stimulation are also examined. The output signal appears to rise and fall periodically according to the input signal, suggesting a good repeatability. The rise and fall times for the devices are recorded to be ~5 μs and ~50 μs, indicating that the polymer photodetectors have quick response capabilities.

Fabrication of zinc oxide/ polyaniline (ZnO/PANI) heterojunction and its characterisation at room temperature

We report the formation of a diode like ZnO/PANI heterojunction structure by RF sputtering technique with a high rectification ratio in the laboratory. The diode parameters obtained from the I-V data using the established methods of Cheung and Norde's function, based on the thermionic emission model of diode, agrees with each other. The current mechanism in the diode was ohmic at lower voltages (at voltages less than 2.8V) where as space charge limited current (SCLC) dominates at higher voltage (above 3.5V). The measured I-V characteristics remain the same under bending conditions also. This indicates that this heterojunction could be exploited positively in flexible electronics.

Intramolecular p-i-n junction photovoltaic device based on selectively doped carbon nanotubes

Photovoltaic devices show promising applications in detection and energy fields as well as in next-generation optoelectronic circuits. The use of the ideal photosensitive material and device design are critical for achieving a high-performance photovoltaic device. Here, an intramolecular p-i-n junction photovoltaic device based on selectively doped carbon nanotubes is investigated. In this kind of device, the opposite ends of an individual single-walled carbon nanotube (SWCNT) channel are doped selectively by triethyloxonium hexachloroantimonate (OA) and polyethylene imine (PEI) to obtain stable p- and n- type SWCNT segments respectively, while the middle segment of the SWCNT is kept intrinsic, causing the formation of an intra-tube p-i-n junction for the efficient separation of photogenerated electron-hole pairs. The optical-absorption and electrical testing demonstrate that the OA and PEI can dope the SWCNTs into the stable p- and n- types, respectively. In the dark, the prepared p-i-n junction device behaves as a diode with a high rectification ratio >103 that can be tuned by the gate voltage. Under a 1550-nm monochromatic illumination, the device exhibits a good photovoltaic effect with a large open-circuit voltage of 0.41V and an external power conversion efficiency of ~4.2%. The quantum efficiency of the device is estimated to be as high as ~73%.