Modulating the Function of GeAs/ReS2 van der Waals Heterojunction with its Potential Application for Short-Wave Infrared and Polarization-Sensitive Photodetection

Van der Waals heterojunction (vdWs) of 2D materials with integrated or extended superior characteristics, opening up new opportunities in functional electronic and optoelectric device applications. Exploring methods to achieve multifunctional vdWs heterojunction devices is one of the most promising prospects in this area. Herein, a diverse function of forward rectifying diode, Zener tunneling diode, and backward rectifying diodes are realized in GeAs/ReS₂ heterojunction by modulating the doping level of GeAs. The tunneling diode presents an interesting trend forward negative differential resistance (NDR) behavior which may facilitate the application of multi-value logic. More importantly, the GeAs/ReS₂ forward rectifying diode exhibits highly sensitive photodetection in the wide-spectrum range up to 1550 nm corresponding to a short-wave infrared (SWIR) region. In addition, as two strong anisotropic 2D materials of GeAs and ReS₂, the heterojunction exhibits strong polarization-sensitive photodetection behavior with a dichroic photocurrent ratio of 1.7. This work provides an effective strategy to achieve multifunctional 2D vdW heterojunction devices and develops more possibilities to broaden their functionalities and applications.     

Gate‐Controlled BP–WSe2 Heterojunction Diode for Logic Rectifiers and Logic Optoelectronics

p–n junctions play an important role in modern semiconductor electronics and optoelectronics, and field‐effect transistors are often used for logic circuits. Here, gate‐controlled logic rectifiers and logic optoelectronic devices based on stacked black phosphorus (BP) and tungsten diselenide (WSe₂) heterojunctions are reported. The gate‐tunable ambipolar charge carriers in BP and WSe₂ enable a flexible, dynamic, and wide modulation on the heterojunctions as isotype (p–p and n–n) and anisotype (p–n) diodes, which exhibit disparate rectifying and photovoltaic properties. Based on such characteristics, it is demonstrated that BP–WSe₂ heterojunction diodes can be developed for high‐performance logic rectifiers and logic optoelectronic devices. Logic optoelectronic devices can convert a light signal to an electric one by applied gate voltages. This work should be helpful to expand the applications of 2D crystals.     

Tunable Polarity Behavior and Self‐Driven Photoswitching in p‐WSe2/n‐WS2 Heterojunctions

Van der Waals (vdW) p–n heterojunctions consisting of various 2D layer compounds are fascinating new artificial materials that can possess novel physics and functionalities enabling the next‐generation of electronics and optoelectronics devices. Here, it is reported that the WSe₂/WS₂ p–n heterojunctions perform novel electrical transport properties such as distinct rectifying, ambipolar, and hysteresis characteristics. Intriguingly, the novel tunable polarity transition along a route of n‐“anti‐bipolar”–p‐ambipolar is observed in the WSe₂/WS₂ heterojunctions owing to the successive work of conducting channels of junctions, p‐WSe₂ and n‐WS₂ on the electrical transport of the whole systems. The type‐II band alignment obtained from first principle calculations and built‐in potential in this vdW heterojunction can also facilitate the efficient electron–hole separation, thus enabling the significant photovoltaic effect and a much enhanced self‐driven photoswitching response in this system.     

MoTe2 p–n Homojunctions Defined by Ferroelectric Polarization

Doped p–n junctions are fundamental electrical components in modern electronics and optoelectronics. Due to the development of device miniaturization, the emergence of two-dimensional (2D) materials may initiate the next technological leap toward the post-Moore era owing to their unique structures and physical properties. The purpose of fabricating 2D p–n junctions has fueled many carrier-type modulation methods, such as electrostatic doping, surface modification, and element intercalation. Here, by using the nonvolatile ferroelectric field polarized in the opposite direction, efficient carrier modulation in ambipolar molybdenum telluride (MoTe₂) to form a p–n homojunction at the domain wall is demonstrated. The nonvolatile MoTe₂ p–n junction can be converted to n–p, n–n, and p–p configurations by external gate voltage pulses. Both rectifier diodes exhibited excellent rectifying characteristics with a current on/off ratio of 5 × 10⁵. As a photodetector/photovoltaic, the device presents responsivity of 5 A W⁻¹, external quantum efficiency of 40%, specific detectivity of 3 × 10¹² Jones, fast response time of 30 µs, and power conversion efficiency of 2.5% without any bias or gate voltages. The MoTe₂ p–n junction presents an obvious short-wavelength infrared photoresponse at room temperature, complementing the current infrared photodetectors with the inadequacies of complementary metal-oxide-semiconductor incompatibility and cryogenic operation temperature.     

纳米硅/单晶硅异质结二极管的电学特性

利用高真空PECVD系统在p型单晶硅上沉积掺磷n型纳米硅薄层（nc-Si:H),形成纳米硅／单晶硅Np异质结二极管,通过C－V和J－V测试研究了二极管的电学性质。C－V特性指出该异质结为突变型。J－V特性表明二极管具有很好的温度稳定性和整流特性。正偏压时二极管存在两种输运机制：小偏压时（＜0．8V）二极管电流由耗尽层纳米硅薄层一侧的载流子复合过程决定,纳米硅薄层由于能带弯曲而减小了禁带宽度,这是该二极管温度稳定性好的根本原因;大偏压（＞1．0V）时电输运符合电荷限制电流（SCLC）模型。负偏压时电流主要来自空间电荷区中的产生电流。     

Anisotropic Spectroscopy and Electrical Properties of 2D ReS2(1–x)Se2x Alloys with Distorted 1T Structure

2D black phosphorus (BP) and rhenium dichalcogenides (ReX₂, X = S, Se) possess intrinsic in‐plane anisotropic physical properties arising from their low crystal lattice symmetry, which has inspired their novel applications in electronics, photonics, and optoelectronics. Different from BP with poor environmental stability, ReX₂ has low‐symmetry distorted 1T structures with excellent stability. In ReX₂, the electronic structure is weakly dependent on layer numbers, which restricts their property tunability and device applications. Here, the properties are tuned, such as optical bandgap, Raman anisotropy, and electrical transport, by alloying 2D ReS₂ and ReSe₂. Photoluminescence emission energy of ReS_{2(1? x )}Se_{2 x} monolayers (x from 0 to 1 with a step of 0.1) can be continuously tuned ranging from 1.62 to 1.31 eV. Polarization behavior of Raman modes, such as ReS₂‐like peak at 212 cm^?1, shifts as the composition changes. Anisotropic electrical property is maintained in ReS_{2(1? x )}Se_{2 x} with high electron mobility along b‐axis for all compositions of ReS_{2(1? x )}Se_{2 x} .     

Monolayer Graphene Film on ZnO Nanorod Array for High‐Performance Schottky Junction Ultraviolet Photodetectors

A new Schottky junction ultraviolet photodetector (UVPD) is fabricated by coating a free‐standing ZnO nanorod (ZnONR) array with a layer of transparent monolayer graphene (MLG) film. The single‐crystalline [0001]‐oriented ZnONR array has a length of about 8–11 μm, and a diameter of 100～600 nm. Finite element method (FEM) simulation results show that this novel nanostructure array/MLG heterojunction can trap UV photons effectively within the ZnONRs. By studying the I–V characteristics in the temperature range of 80–300 K, the barrier heights of the MLG film/ZnONR array Schottky barrier are estimated at different temperatures. Interestingly, the heterojunction diode with typical rectifying characteristics exhibits a high sensitivity to UV light illumination and a quick response of millisecond rise time/fall times with excellent reproducibility, whereas it is weakly sensitive to visible light irradiation. It is also observed that this UV photodetector (PD) is capable of monitoring a fast switching light with a frequency as high as 2250 Hz. The generality of the above results suggest that this MLG film/ZnONR array Schottky junction UVPD will have potential application in future optoelectronic devices.     

A study of the effect of electron and proton irradiation on 4H-SiC device structures

The changes of the current–voltage characteristics and the uncompensated donor-impurity concentration (N _d –N _a ) in the base electrode of Schottky diodes and JBS diodes based on 4H-SiC have been studied upon their irradiation with 0.9-MeV electrons and 15-MeV protons. The carrier-removal rate was 0.07–0.15 cm^–1 under electron irradiation and 50–70 cm^–1 under proton irradiation. It was shown that the current–voltage characteristics of the devices under study remain rectifying at electron irradiation doses of up to ~1017 cm^–2. It was demonstrated that the radiation hardness of the SiC-based devices under study substantially exceeds that of silicon p–i–n diodes with similar breakdown voltages.

     

High current density Esaki tunnel diodes based on GaSb-InAsSb heterostructure nanowires

We present electrical characterization of broken gap GaSb-InAsSb nanowire heterojunctions. Esaki diode characteristics with maximum reverse current of 1750 kA/cm(2) at 0.50 V, maximum peak current of 67 kA/cm(2) at 0.11 V, and peak-to-valley ratio (PVR) of 2.1 are obtained at room temperature. The reverse current density is comparable to that of state-of-the-art tunnel diodes based on heavily doped p-n junctions. However, the GaSb-InAsSb diodes investigated in this work do not rely on heavy doping, which permits studies of transport mechanisms in simple transistor structures processed with high-κ gate dielectrics and top-gates. Such processing results in devices with improved PVR (3.5) and stability of the electrical properties.     

Fabrication and characterization of transparent MEH-PPV/n-GaN (0 0 0 1) heterojunction devices

n-GaN/MEH-PPV thin film heterojunction diode was fabricated by depositing MEH-PPV thin film using spin-coating process on n-GaN (0 0 0 1). The junction properties were evaluated by measuring I-V characteristics. I-V characteristics exhibited well defined rectifying behavior with a barrier height of 0.89 eV and ideality factor of 1.7. The optical band gap of the MEH-PPV film using optical absorption method was found to be 2.2 eV and the fundamental absorption edge in the film is formed by the direct allowed transitions. At higher electric fields, the conductivity mechanism of the film shows a trap charge limited current mechanism. The obtained results indicate that the electronic parameters of the heterojunction diode are affected by properties of MEH-PPV organic film.     

Inorganic Nano Light‐Emitting Transistor: p‐Type Porous Silicon Nanowire/n‐Type ZnO Nanofilm

An inorganic nano light‐emitting transistor (INLET) consisting of p‐type porous Si nanowires (PoSiNWs) and an n‐type ZnO nanofilm was integrated on a heavily doped p‐type Si substrate with a thermally grown SiO₂ layer. To verify that modulation of the Fermi level of the PoSiNWs is key for switchable light emitting, I–V and electroluminescent characteristics of the INLET are investigated as a function of gate bias (V _g). As the V _g is changed from 0 V to ?20 V, the current level and light‐emission intensity in the orange–red range increase by three and two times, respectively, with a forward bias of 20 V in the p–n junction, compared to those at a V _g of 0 V. On the other hand, as the V _g approaches 10 V, the current level decreases and the emission intensity is reduced and then finally switched off. This result arises from the modulation of the Fermi level of the PoSiNWs and the built‐in potential at the p–n junction by the applied gate electric field.     

Black‐Phosphorus‐Based Orientation‐Induced Diodes

Despite many decades of research of diodes, which are fundamental components of electronic and photoelectronic devices with p–n or Schottky junctions using bulk or 2D materials, stereotyped architectures and complex technological processing (doping and multiple material operations) have limited future development. Here, a novel rectification device, an orientation‐induced diode, assembled using only few‐layered black phosphorus (BP) is investigated. The key to its realization is to utilize the remarkable anisotropy of BP in low dimensions and change the charge‐transport conditions abruptly along the different crystal orientations. Rectification ratios of 6.8, 22, and 115 can be achieved in cruciform BP, cross‐stacked BP junctions, and BP junctions stacked with vertical orientations, respectively. The underlying physical processes and mechanisms can be explained using “orientation barrier” band theory. The theoretical results are experimentally confirmed using localized scanning photocurrent imaging. These orientation‐induced optoelectronic devices open possibilities for 2D anisotropic materials with a new degree of freedom to improve modulation in diodes.     

Schottky and heterojunction diodes based on poly(3-octylthiophene) and poly(3-methylthiophene) films of high tensile strength

Schottky and heterojunction diodes were fabricated using high tensile strength polymers. The heterojunction diode was fabricated by sequential electrochemical polymerisation of 3-methyl thiophene and 3-octyl thiophene on an indium-tin oxide (ITO) coated glass substrate. The high tensile strength enabled the bilayer (used in heterojunction diodes) or the poly 3-octyl thiophene films (used in the Schottky diodes) to be peeled of from the substrate and sandwich it between any two desired metals. It was found that the Schottky diodes of ITO (or Si)/POT/Al (or Zn) exhibit moderate rectifying behaviour and ITO (or Si)/POT/Cu devices exhibit ohmic contact. The POT/PMT heterojunction diode showed excellent rectification effect when sandwiched between any two metals irrespective of their work function. This shows that the results observed were solely due to the polymer/polymer interface. The Cu/POT/PMT/Cu heterojunction system was used in this study. The carrier-flow of the two semiconductors in the Cu/POT/PMT/Cu heterojunction diode was discussed in details. The rectification ratio, the barrier height, and the ideality factor for the heterojunction diode were found to be 64 (±1.2 V), 0.81 eV, and 5.7 under ambient conditions, respectively. Some of the important energy band parameters were also determined.     

SnSe/MoS2 van der Waals Heterostructure Junction Field‐Effect Transistors with Nearly Ideal Subthreshold Slope

The minimization of the subthreshold swing (SS) in transistors is essential for low‐voltage operation and lower power consumption, both critical for mobile devices and internet of things (IoT) devices. The conventional metal‐oxide‐semiconductor field‐effect transistor requires sophisticated dielectric engineering to achieve nearly ideal SS (60 mV dec^?1 at room temperature). However, another type of transistor, the junction field‐effect transistor (JFET) is free of dielectric layer and can reach the theoretical SS limit without complicated dielectric engineering. The construction of a 2D SnSe/MoS₂ van der Waals (vdW) heterostructure‐based JFET with nearly ideal SS is reported. It is shown that the SnSe/MoS₂ vdW heterostructure exhibits excellent p–n diode rectifying characteristics with low saturate current. Using the SnSe as the gate and MoS₂ as the channel, the SnSe/MoS₂ vdW heterostructure exhibit well‐behavioured n‐channel JFET characteristics with a small pinch‐off voltage V_P of ?0.25 V, nearly ideal subthreshold swing SS of 60.3 mV dec^?1 and high ON/OFF ratio over 10⁶, demonstrating excellent electronic performance especially in the subthreshold regime.     

Synthesis of 2D Layered BiI3 Nanoplates,BiI3/WSe2van der Waals Heterostructures and Their Electronic,Optoelectronic Properties

Two–dimensional layered materials (2DLMs) have attracted considerable recent interest as a new material platform for fundamental materials science and potential new technologies. Here we report the growth of layered metal halide materials and their optoelectronic properties. BiI₃ nanoplates can be readily grown on SiO₂/Si substrates with a hexagonal geometry, with a thickness in the range of 10–120 nm and a lateral dimension of 3–10 µm. Transmission electron microscopy and electron diffraction studies demonstrate that the individual nanoplates are high quality single crystals. Micro‐Raman studies show characteristic A _g band at ≈115 cm^?1 with slight red‐shift with decreasing thickness, and micro‐photoluminescence studies show uniform emission around 690 nm with blue‐shift with decreasing thickness. Electrical transport studies of individual nanoplates show n‐type semiconductor characteristics with clear photoresponse. Further, the BiI₃ can be readily grown on other 2DLMs (e.g., WSe₂) to form van der Waals heterostructures. Electrical transport measurements of BiI₃/WSe₂ vertical heterojunctions demonstrate p–n diode characteristics with gate‐tunable rectification behavior and distinct photovoltaic effect. The synthesis of the BiI₃ nanoplates can expand the library of 2DLMs and enable a wider range of van der Waals heterostructures.     

Lateral Monolayer MoSe2–WSe2 p–n Heterojunctions with Giant Built‐In Potentials

2D transition metal dichalcogenides (TMDs) have exhibited strong application potentials in new emerging electronics because of their atomic thin structure and excellent flexibility, which is out of field of tradition silicon technology. Similar to 3D p–n junctions, 2D p–n heterojunctions by laterally connecting TMDs with different majority charge carriers (electrons and holes), provide ideal platform for current rectifiers, light‐emitting diodes, diode lasers and photovoltaic devices. Here, growth and electrical studies of atomic thin high‐quality p–n heterojunctions between molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) by one‐step chemical vapor deposition method are reported. These p–n heterojunctions exhibit high built‐in potential (≈0.7 eV), resulting in large current rectification ratio without any gate control for diodes, and fast response time (≈6 ms) for self‐powered photodetectors. The simple one‐step growth and electrical studies of monolayer lateral heterojunctions open up the possibility to use TMD heterojunctions for functional devices.     

Various and Tunable Transport Properties of WSe2 Transistor Formed by Metal Contacts

The abundant electronic and optical properties of 2D materials that are just one‐atom thick pave the way for many novel electronic applications. One important application is to explore the band‐to‐band tunneling in the heterojunction built by different 2D materials. Here, a gate‐controlled WSe₂ transistor is constructed by using different work function metals to form the drain (Pt) and source (Cr) electrodes. The device can be gate‐modulated to exhibit three modes of operation, i.e., the tunneling mode with remarkable negative differential resistance, the transition mode with a second electron tunneling phenomenon for backward bias, and finally the conventional diode mode with rectifying characteristics. In contrast to the heterojunctions built by different 2D materials, these devices show significantly enhanced tunneling current by two orders of magnitude, which may largely benefit from the clean interfaces. These results pave the way toward design of novel electronic devices using the modulation of metal work functions.     

Fabrication and characterization of all-oxide heterojunction p-CuAlO2 + x/n-Zn1 − xAlxO transparent diode for potential application in “invisible electronics”

Transparent p-n heterojunction diodes have been fabricated by p-type copper aluminum oxide (p-CuAlO_2 + x) and n-type aluminum doped zinc oxide (n-Zn_1 − xAl_xO) thin films on glass substrates. The n-layers are deposited by sol-gel-dip-coating process from zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) and aluminum nitrate (Al(NO₃)₃·9H₂O). Al concentration in the nominal solution is taken as 1.62 at %. P-layers are deposited onto the ZnO:Al-coated glass substrates by direct current sputtering process from a prefabricated CuAlO₂ sintered target. The sputtering is performed in oxygen-diluted argon atmosphere with an elevated substrate temperature. Post-deposition oxygen annealing induces excess oxygen within the p-CuAlO_2 + x films, which in turn enhances p-type conductivity of the layers. The device characterization shows rectifying current-voltage characteristics, confirming the proper formation of the p-n junction. The turn-on voltage is obtained around 0.8 V, with a forward-to-reverse current ratio around 30 at ± 4 V. The diode structure has a total thickness of 1.1 μm and the optical transmission spectra of the diode show almost 60% transmittance in the visible region, indicating its potential application in ‘invisible electronics’. Also the cost-effective procedures enable the large-scale production of these transparent diodes for diverse device applications.     

Current transport mechanism of heterojunction diodes based on the sol–gel p-type ZnO and n-type Si with H2O2 treatment

The present work reports the fabrication and detailed electrical properties of heterojunction diodes based on the sol–gel p-type ZnO and n-type Si. The p-type ZnO/n-type Si diode without H₂O₂ treatment showed a poor rectifying behavior with an ideality factor (n) of 6.4 and high leakage. n > 2 implies that the interfacial defects influence the electronic conduction through the device. However, the p-type ZnO/n-type Si diode with H₂O₂ treatment showed a good rectifying behavior with n of 1.6 and low leakage. Such an improvement indicates that a good passivation is formed at the interface as a result of the reduction of the defect density. These experimental demonstrations suggest that it may be possible to minimize the adverse effects of the interface states to obtain functional devices using H₂O₂ treatment.     

Backward-Like Rectifying Behavior of La1.85Sr0.15CuO4/Nb:SrTiO3 Heterojunctions

La_1.85Sr_0.15CuO_4+?? (LSCO)/Nb:SrTiO₃ (NSTO) heterojunction, in which the LSCO film was excessively annealed in pure oxygen, was created by a combined technique including pulsed electron deposition, photolithography, and Ar-ion milling. Subsequent characterizations via measuring the current?Cvoltage (I?CV) relations revealed great sensitivity of the rectifying properties to the annealing, where a backward-like diode behavior was observed, which probably can be interpreted in connection with the band diagram of the heterojunction, and more insulating interface resulted from over-annealing effect on both LSCO and NSTO was presumably the primary cause. This study will provide an opportunity to understand more about the complicated physics underlying the rich properties in such heterojunctions.