Schottky Junction Devices Research Articles

Two-Dimensional/Three-Dimensional Schottky Junction Photovoltaic Devices Realized by the Direct CVD Growth of vdW 2D PtSe2 Layers on Silicon.

Two-dimensional (2D) platinum diselenide (PtSe2) layers are a new class of near-atom-thick 2D crystals in a van der Waals-assembled structure similar to previously explored many other 2D transition-metal dichalcogenides (2D TMDs). They exhibit distinct advantages over conventional 2D TMDs for electronics and optoelectronics applications such as metallic-to-semiconducting transition, decently high carrier mobility, and low growth temperature. Despite such superiority, much of their electrical properties have remained mostly unexplored, leaving their full technological potential far from being realized. Herein, we report 2D/three-dimensional Schottky junction devices based on vertically aligned metallic 2D PtSe2 layers integrated on Si wafers. We directly grew 2D PtSe2 layers of controlled orientation and carrier transport characteristics via a low-temperature chemical vapor deposition process and investigated 2D PtSe2/Si Schottky junction properties. We unveiled a comprehensive set of material parameters, which decisively confirm the presence of excellent Schottky junctions, i.e., high-current rectification, small ideality factor, and temperature-dependent variation of Schottky barrier heights. Moreover, we observed strong photovoltaic effects in the 2D PtSe2/Si Schottky junction devices and extended them to realize flexible photovoltaic devices. This study is believed to significantly broaden the versatility of 2D PtSe2 layers in practical and futuristic electronic devices.

High sensitivity Si-based photodetection with nanoscale protective layer based on interface states

A large lateral photovoltaic effect has been observed at the interface of SiO2/p-Si/SiO2 structure. Different from the traditional Schottky junction or PN junction device, this photovoltage is mainly dominated by the interface states existing at SiO2/p-Si interface, where the covered nanoscale SiO2 layer brings this device a stable and high photoelectric performance. These interface states can be explained as built-in field caused by the band bending at interface, which regulates the generation and diffusion of photo induced carriers. In this study, we discuss clearly the factors that impact greatly on the photovoltage output and sensitivity, including the oxide thickness, resistivity and tunneling effect. We believe this simple but efficient device will be beneficial for the exploring in photoelectric detection.

High Detectivity from a Lateral Graphene–MoS2 Schottky Photodetector Grown by Chemical Vapor Deposition

Abstract2D material–based photodetectors have demonstrated the great potential in future optoelectric applications and the compatibility with the traditional semiconductor technology. However, low detectivity and difficulty of large‐scale fabrication still limit their application. Here, an ultrasensitive in‐plane lateral graphene–MoS2 heterostructure is successfully constructed using one‐step growth by chemical vapor deposition, which is suitable for large‐scale fabrication. The Schottky junction is formed in the channel with the edge contact of graphene and MoS2. It displays good rectification characteristics with an on/off ratio up to 106. As a photodetector, it exhibits excellent detectivity with the specific detectivity D* up to 1.4 × 1014 Jones and the responsivity of 1.1 × 105 A W−1, which benefit from strong absorption, the efficient separation of the photoexcited carriers, and quick charge transport in the Schottky junction device. Moreover, heterostructure photodetector array is demonstrated here which shows the large‐scale fabrication capacity. All of these results prove the potential of 2D material–based junction devices for optoelectronic devices.

DMSO modified PEDOT:PSS polymer/ZnO nanorods Schottky junction ultraviolet photodetector: Photoresponse, external quantum efficiency, detectivity, and responsivity augmentation using N doped graphene quantum dots

Abstract Schottky junction ultraviolet (UV) photodetector was fabricated by spin coating a hole conducting polymer, poly 3,4-ethylenedioxythiophene: polystyrene sulfonate (PEDOT:PSS) on hydrothermally grown zinc oxide (ZnO) nanorod array. The photodetector performance was improved by increasing the conductivity of PEDOT:PSS polymer using dimethyl sulfoxide (DMSO) additive. The UV detector performance was further enhanced significantly by sensitizing ZnO nanorods with N doped graphene quantum dots (NGQDs). NGQD decorated ZnO NRs/DMSO-PEDOT:PSS Schottky junction device demonstrated superior external quantum efficiency (EQE ∼ 90063%), responsivity ( R λ ∼247 A/W) and detectivity ( D λ ∼2.42 × 1011 Hz1/2/W) at 340 nm wavelength and –1V external bias. The EQE of NGQD modified sample was 56 times higher than pristine PEDOT:PSS/ZnO NR and 4.3 times higher than DMSO modified PEDOT:PSS/ZnO NRs device. NGQD sensitized detector showed superior photocurrent of 80.77 mA/cm2 at 340 nm wavelength, –1V external bias, which was 2 times higher than DMSO modified and 32 times higher than pristine PEDOT:PSS based device. The photocurrent rise and decay time of NGQD sensitized NRs are very fast compared to other photodetectors.

Fabrication of p-type ZnTe NW/In Schottky diodes for high-speed photodetectors

Nitrogen-doped p-type ZnTe nanowires were successfully synthesized by a chemical vapor deposition method. Schottky junctions based on Au/ZnTe NW/In structure were constructed and their device performances were studied. ZnTe/In Schottky junction devices show excellent rectifying characteristics with rectification ratio up to 103 within ±5 V. Photoresponse analysis reveals that such devices were highly sensitive to varying optical signal with excellent stability, reproducibility and fast response speeds of 69/120 μs. These results demonstrate that ZnTe/In Schottky junction devices will promote the applications of ZnTe 1D nanostructures in electronic and optoelectronics.

Near-Unity Absorption in van der Waals Semiconductors for Ultrathin Optoelectronics.

We demonstrate near-unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (<15 nm) van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of the key criteria to enable TMDCs as potential candidates to achieve high optoelectronic efficiency.

Silver nanowires-templated metal oxide for broadband Schottky photodetector

Silver nanowires (AgNWs)-templated transparent metal oxide layer was applied for Si Schottky junction device, which remarked the record fastest photoresponse of 3.4 μs. Self-operating AgNWs-templated Schottky photodetector showed broad wavelength photodetection with high responsivity (42.4 A W−1) and detectivity (2.75 × 1015 Jones). AgNWs-templated indium-tin-oxide (ITO) showed band-to-band excitation due to the internal photoemission, resulting in significant carrier collection performances. Functional metal oxide layer was formed by AgNWs-templated from ITO structure. The grown ITO above AgNWs has a cylindrical shape and acts as a thermal protector of AgNWs for high temperature environment without any deformation. We developed thermal stable AgNWs-templated transparent oxide devices and demonstrated the working mechanism of AgNWs-templated Schottky devices. We may propose the high potential of hybrid transparent layer design for various photoelectric applications, including solar cells.

Semiconductor‐Based Photoelectrochemical Water Splitting at the Limit of Very Wide Depletion Region

In semiconductor‐based photoelectrochemical (PEC) water splitting, carrier separation and delivery largely relies on the depletion region formed at the semiconductor/water interface. As a Schottky junction device, the trade‐off between photon collection and minority carrier delivery remains a persistent obstacle for maximizing the performance of a water splitting photoelectrode. Here, it is demonstrated that the PEC water splitting efficiency for an n‐SrTiO3 (n‐STO) photoanode is improved very significantly despite its weak indirect band gap optical absorption (α &lt; 104 cm−1), by widening the depletion region through engineering its doping density and profile. Graded doped n‐SrTiO3 photoanodes are fabricated with their bulk heavily doped with oxygen vacancies but their surface lightly doped over a tunable depth of a few hundred nanometers, through a simple low temperature reoxidation technique. The graded doping profile widens the depletion region to over 500 nm, thus leading to very efficient charge carrier separation and high quantum efficiency (&gt;70%) for the weak indirect transition. This simultaneous optimization of the light absorption, minority carrier (hole) delivery, and majority carrier (electron) transport by means of a graded doping architecture may be useful for other indirect band gap photocatalysts that suffer from a similar problem of weak optical absorption.

Solution-processed transparent conducting Ag nanowires layer for photoelectric device applications

Electrically conductive and optically transparent Ag nanowires (AgNWs) were spin-coated on a Si substrate for high-performing Schottky junction device. The AgNWs window layer significantly improved transmittance for broad wavelengths, comparing to that of the conventional indium-tin-oxide (ITO) film structure. This optical benefit spontaneously provides significantly improved photodetection for broad wavelengths. The junction properties were investigated using current–voltage and voltage–capacitance characteristics. The AgNWs/Si formed a built-in potential of 0.62eV at the interface and this photodetector provided a considerably high rectification ratio with a low saturation current value. These intrinsic advantages simultaneously induced substantially enhanced signal to noise ratio different from the ITO/Si heterojunction device. This solution-processed architecture of a transparent conductor (TC) is also beneficial for non-vacuum designs. We demonstrated that AgNWs would be the competitive alternative for the conventional TC coating techniques. Electrical analyses and photoresponses were systematically investigated for AgNWs/Si Schottky device.

Efficient organic Schottky junction solar cells with a platinum chloride-treated PEDOT:PSS interfacial layer

We demonstrate highly efficient organic-based Schottky junction solar cells (OSJSCs) obtained by poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) modification with solution-processed platinum chloride (PtCl4) treatment. The effect of PtCl4 on PEDOT:PSS properties and device performances of solar cells was investigated. Kelvin probe and 4-point probe studies demonstrated that PtCl4 decreased the PEDOT:PSS sheet-resistance and increased the PEDOT:PSS work-function, thereby inducing an improved built-in potential and interface resistance. As a result, with the aid of the PtCl4 treatment, the Schottky junction device had a high power conversion efficiency of 3%, which was more than 20% higher than the reference OSJSCs with no PtCl4, indicating that PtCl4 can be a promising PEDOT:PSS modifier for raising the cell-performances of Schottky-junction based organic solar cells.

Fabrication of a Schottky junction diode with direct growth graphene on silicon by a solid phase reaction

We demonstrate fabrication of a Schottky junction diode with direct growth graphene on n-Si by the solid phase reaction approach. Metal-assisted crystallization of a-C thin film was performed to synthesize transfer-free graphene directly on a SiO2 patterned n-Si substrate. Graphene formation at the substrate and catalyst layer interface is achieved in presence of a Co catalytic and CoO carbon diffusion barrier layer. The as-synthesized material shows a linear current–voltage characteristic confirming the metallic behaviour of the graphene structure. The direct grown graphene on n-Si substrate creates a Schottky junction with a potential barrier of 0.44 eV and rectification diode characteristic. Our finding shows that the directly synthesized graphene on Si substrate by a solid phase reaction process can be a promising technique to fabricate an efficient Schottky junction device.

Solution‐Processed Fullerene‐Based Organic Schottky Junction Devices for Large‐Open‐Circuit‐Voltage Organic Solar Cells

A solution-processed fullerene-based organic Schottky junction photovoltaic device is fabricated to produce a large open circuit voltage, 0.85-0.95 V, which is higher than that of most organic bulk-heterojunction devices. A power conversion efficiency of 5% is achieved in fullerene-derivative [6,6]-phenyl-C71-butyric acid methylester-based Schottky junction devices.

Defect-assisted tunneling current-transport mechanism for Schottky diodes of Pt thin film on p-SiNWs tips

The Schottky diodes of a Platinum (Pt) thin film on p-silicon nanowires (p-SiNWs) tips were fabricated. The current-transport mechanism of Pt/p-SiNWs Schottky diodes was defect-assisted tunneling (TU). Each silicon nanowire could be seen as a core/shell structure with a monocrystalline silicon core wrapped by a thin natural amorphous silicon oxide shell, which induced a large number of defects and enhanced the defect-assisted tunneling probability. The experimental I-V data were fitted to the theoretical mode of the thermionic emission (TE), generation-recombination (GR), TU, and leakage (RL) current-transport mechanisms in the temperature range of 300-370K and voltage range of -1 to 1V. The TU fitting data were in excellent agreement with the experimental data. Meanwhile, the tunneling parameter (E0) was independent of the temperature, which closely followed the TU current-transport mechanism. Defect-assisted tunneling mechanism of Pt/p-SiNWs Schottky diodes could be applied to many other Schottky junction devices, especially, for low-dimensional nanostructures.

High‐Sensitivity p–n Junction Photodiodes Based on PbS Nanocrystal Quantum Dots

AbstractChemically synthesized nanocrystal quantum dots (NQDs) are promising materials for applications in solution‐processable optoelectronic devices such as light emitting diodes, photodetectors, and solar cells. Here, we fabricate and study two types of p‐n junction photodiodes in which the photoactive p‐layer is made from PbS NQDs while the transparent n‐layer is fabricated from wide bandgap oxides (ZnO or TiO2). By using a p–n junction architecture we are able to significantly reduce the dark current compared to earlier Schottky junction devices without reducing external quantum efficiency (EQE), which reaches values of up to ∼80%. The use of this device architecture also allows us to significantly reduce noise and obtain high detectivity (&gt;1012 cm Hz1/2 W−1) extending to the near infrared past 1 μm. We observe that the spectral shape of the photoresponse exhibits a significant dependence on applied bias, and specifically, the EQE sharply increases around 500–600 nm at reverse biases greater than 1 V. We attribute this behavior to a “turn‐on” of an additional contribution to the photocurrent due to electrons excited to the conduction band from the occupied mid‐gap states.

Improvement in Resistance Switching and Retention Properties of Pt ∕ TiO2 Schottky Junction Devices

Resistance switching and retention properties of Schottky junction devices are improved by doping anatase thin films with Co ions. Co-doped devices exhibit excellent rectifying current–voltage characteristics and resistance switching compared to undoped devices. Conventional Schottky junction devices such as and devices exhibit very poor resistance retention properties in low resistance state (LRS); the resistance in LRS increases with time and the resistance relaxation accelerates with an increase in temperature. Co-doped devices show excellent resistance retention properties even at . Experimental results indicate that in Schottky junction devices, resistance switching originates from the formation of local tunneling paths, and the oxygen vacancies near the electrode interface play an important role in resistance switching. These results are very important from the point of view of understanding the resistance switching mechanism and improving the resistance switching properties of Schottky junction devices.

A Silicon Nanocrystal Schottky Junction Solar Cell produced from Colloidal Silicon Nanocrystals

Solution-processed semiconductors are seen as a promising route to reducing the cost of the photovoltaic device manufacture. We are reporting a single-layer Schottky photovoltaic device that was fabricated by spin-coating intrinsic silicon nanocrystals (Si NCs) from colloidal suspension. The thin-film formation process was based on Si NCs without any ligand attachment, exchange, or removal reactions. The Schottky junction device showed a photovoltaic response with a power conversion efficiency of 0.02%, a fill factor of 0.26, short circuit-current density of 0.148 mA/cm2, and open-circuit voltage of 0.51 V.

Current–voltage and capacitance–voltage studies of nanocrystalline CdSe/Au Schottky junction interface

CdSe nanocrystalline thin films have been synthesized on indium tin oxide (ITO) substrates by an electrodeposition technique. A Schottky junction device in the configuration, ITO/nano-CdSe/Au has been fabricated to study the device interface properties by current (I)–voltage (V) and capacitance (C)–voltage (V) measurements and compared with the ITO/bulk-CdSe/Au device. The I–V characteristics of the nano-CdSe device shows a series resistance effect and C–V characteristics show the presence of surface/interface traps induced by a thin native oxide layer at the nano-CdSe/Au interface and is responsible to the deviation in the ideal Mott–Schottky behavior. The presence of a thin oxide layer on the CdSe nanocrystal surface has been identified from Rutherford backscattering (RBS) spectrometry. The low frequency capacitance response of the nano-CdSe device characteristics are being compared with the bulk device, which confirms the presence of surface/interface states within the band gap of CdSe nanocrystals. Mott–Schottky plots at different frequencies indicate the formation of a Schottky barrier between nano-CdSe and Au junction.

Resistance switching effect in Nb-doped SrTiO3 (100) bicrystal with (100) ∼45° twist boundary

We have fabricated Nb-doped SrTiO3 bicrystals with (100) ∼45° twist boundary by changing the fabricating conditions and demonstrated that they can exhibit a large resistance switching effect. Nb:SrTiO3 bicrystals cooled at the rate of −300 K/h show an asymmetric large hysteresis in current-voltage characteristics and the magnitude of resistance change attains two orders of magnitude. The resistance state can also be switched by the application of voltage pulses with different polarity and both states can be memorized for over 5 h at room temperature. From experimental results, it can be speculated that the twist boundary could be an origin of large resistance switching and memory effects and it behaves like an electrode interface in SrRuO3/Nb:SrTiO3 Schottky junction devices.

Current Transport Study of Schottky and P-N Junction Solar Cells Using Metal-Induced Growth Poly-Si Thin Films

AbstractPoly-Si thin films deposited at low temperature by using the metal-induced growth (MIG) method have the advantage of less metal impurity contaminations and relative large grains with preferred crystal orientation of (220). In recent research, the Schottky solar diode made of MIG poly-Si shows Jsc of 12 mA/cm2 and Voc of 0.214V. In this paper, current transport mechanisms were studied by current-voltage-temperature (I-V-T) testing from 100 K to 400K. For the samples deposited by a one-step sputtering process, the large value of ideality factor (n) and abnormal increase of barrier height with the temperature implies that the current transport mechanism does not follow the pure thermionic-emission theory, which was proven to be thermionic-field emission due to the highly doped Si film. By using a two-step sputtering process, the ideality factor and Au-Schottky barrier height at room temperature were about 1.5 and 0.7 eV, which was improved from one-step sputtering. Hydrogenation by electron cyclotron resonance (ECR) plasma can further improve the Schottky diode ideality factor and barrier height. Although a low-level Phosphors-doped Si target was used for poly-Si thin film deposition, a thermionic-field emission mechanism was still found by plotting the activation energy (E0) versus the testing temperature range. Capacitance-voltage (C-V) analysis revealed an unexpected carrier density of 1017 cm−3 level, which is 1000 times higher than the doping density level in the Si film. “Oxygen thermal donor” effect was assumed due to high oxygen level (1020 cm−3) detected by SIMS and processing at ∼ 600 °C. Increasing of the total carrier density due to the oxygen donor may cause the transport mechanism change from pure thermionic emission to thermionic-field emission. Reducing oxygen in the Si film by filtering the sputtering gas to 50 ppb oxygen level was proven to be effective. C-V results gave ∼1016 cm−3 level of carrier density after using oxygen filtering. P-N junction solar cells were made by B-ion implantation into n-type Si film and dopant activation at 700 °C. I-V-T study showed similar curves for P-N junction as for Schottky junction devices. This implies that the current transport was dominated by the Si films instead of the junctions for both Schottky and P-N junction devices.

Current responsivity of semiconductor superlattice THz-photon detectors

The current responsivity of a semiconductor superlattice THz-photon detector is calculated using an equivalent circuit model which takes into account the finite matching efficiency between a detector antenna and the superlattice in the presence of parasitic losses. Calculations performed for currently available superlattice diodes show that both the magnitudes and the roll-off frequencies of the responsivity are strongly influenced by an excitation of hybrid plasma-Bloch oscillations which are found to be eigenmodes of the system in the THz-frequency band. The expected room temperature values of the responsivity (2–3 A/W in the 1–3 THz-frequency band) range up to several percents of the quantum efficiency e/ℏω of an ideal superconductor tunnel junction detector. Properly designed semiconductor superlattice detectors may thus demonstrate better room temperature THz-photon responsivity than conventional Schottky junction devices.