Vertical Heterojunction Research Articles

Impact of heterogeneous gate dielectric on DC, RF and circuit-level performance of source-pocket engineered Ge/Si heterojunction vertical TFET

This paper reports the DC, RF and circuit-level performance analysis of short-channel Ge/Si based source-pocket engineered (SPE) vertical heterojunction tunnel field effect transistors (Ge/Si SPE-V-HTFETs) with and without a heterogeneous gate dielectric (HGD) structure for the first time. The DC performance parameters in terms of ION/IOFF and subthreshold swing (SS) are investigated for the proposed V-HTFETs. The average SS for the proposed V-HTFET with an HGD is found to be as low as 20 mV dec−1 compared to V-HTFET without any HGD (26 mV dec−1) at VDS = 0.5 V. The proposed Ge/Si SPE-V-HTFET with an HGD possesses higher cut-off frequency of 502 GHz and maximum frequency of oscillation of 2.33 THz at VDS = 0.5 V over the Ge/Si SPE-V-HTFET without any HGD which possesses cut-off frequency of 273 GHz and maximum frequency of oscillation of 1.47 THz. The proposed Ge/Si SPE-V-HTFET with and without an HGD have then been used for designing a basic current mirror circuit. Device-level study has been carried out using SILVACO ATLASTM TCAD simulator while the circuit-level investigation has been performed using the look up table based Verilog-A models in the CADENCE Virtuoso tool. The performances of the Ge/Si SPE-V-HTFET with HGD based current mirror circuit is observed to be better than the corresponding current mirror circuit designed by Ge/Si SPE-V-HTFET without any HGD.

Black phosphorus-based van der Waals heterostructures for mid-infrared light-emission applications

Mid-infrared (MIR) light-emitting devices play a key role in optical communications, thermal imaging, and material analysis applications. Two-dimensional (2D) materials offer a promising direction for next-generation MIR devices owing to their exotic optical properties, as well as the ultimate thickness limit. More importantly, van der Waals heterostructures—combining the best of various 2D materials at an artificial atomic level—provide many new possibilities for constructing MIR light-emitting devices of large tuneability and high integration. Here, we introduce a simple but novel van der Waals heterostructure for MIR light-emission applications built from thin-film BP and transition metal dichalcogenides (TMDCs), in which BP acts as an MIR light-emission layer. For BP–WSe2 heterostructures, an enhancement of ~200% in the photoluminescence intensities in the MIR region is observed, demonstrating highly efficient energy transfer in this heterostructure with type-I band alignment. For BP–MoS2 heterostructures, a room temperature MIR light-emitting diode (LED) is enabled through the formation of a vertical PN heterojunction at the interface. Our work reveals that the BP–TMDC heterostructure with efficient light emission in the MIR range, either optically or electrically activated, provides a promising platform for infrared light property studies and applications.

The Large-Scale Preparation and Optical Properties of MoS2/WS2 Vertical Hetero-Junction

A variety of hetero-junctions can be constructed to form the basic structural units in the different optoelectronic devices, such as the photo-detectors, solar cells, sensors and light-emitting diodes. In our research, the large-area high-quality MoS2/WS2 vertical hetero-junction are prepared by the two-step atmospheric pressure chemical vapor deposition (APCVD) methods and the dry transfer method, and the corresponding optimal reaction conditions of MoS2/WS2 vertical hetero-junction are obtained. The morphology, composition and optical properties of MoS2/WS2 vertical hetero-junction are systematically characterized by the optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy and the field emission scanning electron microscopy. Compared to the mechanical transfer method, the MoS2/WS2 vertical hetero-junction sample obtained by the APCVD and dry transfer methods have lower impurity content, cleaner interfaces and tighter interlayer coupling. Besides, the strong interlayer coupling and effective interlayer charge transfer of MoS2/WS2 vertical hetero-junction are also further studied. The photoluminescence intensity of MoS2/WS2 vertical hetero-junction is significantly reduced compared to the single MoS2 or WS2 material. In general, this research can help to achieve the large-scale preparation of various Van der Waals hetero-junctions, which can lay the foundation for the new application of optoelectronic devices.

Quantum mechanical analysis of 2D transition metal dichalcogenide material based vertical heterojunction tunnel FET

In this work, a single layer n-doped MoS2 and p-doped WTe2 based vertical heterojunction tunnel FET has been investigated through a well-organized quantum mechanical approach. The key outcome is the design criteria of the device for low subthreshold swing keeping its length as short as possible. Inter-coupled real space model Hamiltonian of the device is formed by introducing the coupling energy of the WTe2 valence band and the MoS2 conduction band in the overlap region. Here, MATLAB based self-consistent analysis is used to numerically solve the device by coupling Schrödinger and Poisson equations taking into account the plane dependence of permittivity in 2D transition metal dichalcogenide materials. For 15 nm channel overlap length and 15 nm gate extension length, a subthreshold slope of as low as 10 mV/decade has been obtained. For 20 nm channel overlap length, an ON current of 18 µA/μm has been obtained as well. The effect of the top gate extension, overlap length, and dielectric layer thickness over the ON and OFF state currents has been explained from the viewpoint of device physics. Thus, the framework presented will help designers to optimize the device for improved performance.

Lead-Free Perovskite/Organic Semiconductor Vertical Heterojunction for Highly Sensitive Photodetectors.

In recent years, photodetectors based on organic-inorganic lead halide perovskites have been studied extensively. However, the inclusion of lead in those materials can cause severe human health and environmental problems, which is undesirable for practical applications. Here, we report high-performance photodetectors with a tin-based perovskite/PEDOT:PSS vertical heterojunction. The device demonstrates a broadband photoresponse from NIR to UV. The maximum responsivity and gain are up to 2.6 × 106 A/W and 4.7 × 106, respectively. Moreover, a much shorter response time and higher detectivity can be achieved by reducing the thickness of PEDOT:PSS. The outstanding performance is due to the excellent optoelectronic properties of the perovskite and the photogating effect originating from the heterojunction. Furthermore, devices fabricated on flexible substrates can demonstrate not only high sensitivity but also excellent bending stability. This work opens up the opportunity of using lead-free perovskite in highly sensitive photodetectors with vertical heterojunctions.

Gate-tunable trion switch for excitonic device applications

Trions are excitonic species with a positive or negative charge, and thus, unlike neutral excitons, the flow of trions can generate a net detectable charge current. Trions under favourable doping conditions can be created in a coherent manner using resonant excitation. In this work, we exploit these properties to demonstrate a gate controlled trion switch in a few-layer graphene/monolayer WS2/monolayer graphene vertical heterojunction. By using a high resolution spectral scan through a temperature controlled variation of the bandgap of the WS2 sandwich layer, we obtain a gate voltage dependent vertical photocurrent strongly relying on the spectral position of the excitation, and the photocurrent maximizes when the excitation energy is resonant with the trion peak position. Further, the resonant photocurrent thus generated can be effectively controlled by a back gate voltage applied through the incomplete screening of the bottom monolayer graphene, and the photocurrent strongly correlates with the gate dependent trion intensity, while the non-resonant photocurrent exhibits only a weak gate dependence -unambiguously proving a trion driven photocurrent generation under resonance. We estimate a sub-100 fs switching time of the device. The findings are useful towards demonstration of ultra-fast excitonic devices in layered materials.

Flexible and Self‐Powered Lateral Photodetector Based on Inorganic Perovskite CsPbI3–CsPbBr3 Heterojunction Nanowire Array

AbstractSelf‐powered perovskite photodetectors mainly adopt the vertical heterojunction structure composed of active layer, electron–hole transfer layers, and electrodes, which results in the loss of incident light and interfacial accumulation of defects. To address these issues, a self‐powered lateral photodetector based on CsPbI3–CsPbBr3 heterojunction nanowire arrays is designed on both a rigid glass and a flexible polyethylene naphthalate substrate using an in situ conversion and mask‐assisted electrode fabrication method. Through adding the polyvinyl pyrrolidone and optimizing the concentration of precursors under the pressure‐assisted moulding process, both the crystallinity and stability of perovskite nanowire array are improved. The nanowire array–based lateral device shows a high responsivity of 125 mA W−1 and a fast rise and decay time of 0.7 and 0.8 ms under a self‐powered operation condition. This work provides a new strategy to fabricate perovskite heterojunction nanoarrays towards self‐powered photodetection.

Excellent performance in vertical graphene-C60-graphene heterojunction phototransistors with a tunable bi-directionality

Array integrated all-carbon phototransistor with high photoconductive gain and fast response speed is very attractive for researchers to realize broadband and tunable photo-imaging. Generally, the photoresponse is limited by the low light absorption of carbon materials while the detector array is limited by processing technology. Here, we patterned vertical graphene-C60-graphene heterojunction using photolithograph and deep reactive ion etching method to achieve a sensitive (3.4×105A/W), fast (23 ms) and broadband (405 nm–1550 nm) response. More interestingly, a bi-directional response photocurrent (positive and negative) has been observed, and the response intensity, speed and direction could adjust by changing gate voltage. The different charges transfer mechanism of positive and negative response of fabricated vertical heterojunction device was explored. What’ more, we fabricated 250×250 vertical graphene/C60 film/graphene phototransistors array, which made it possible to realize a large-scale array detection in wide band with all-carbon material.

ZnS/Carbon Quantum Dot Heterojunction Phototransistors for Solar-Blind Ultraviolet Detection

ZnS/Carbon quantum dots (QDs) vertical heterojunction field-effect phototransistors were fabricated by a solution-based process. The light-absorbing QD layer is deposited by using ZnS QDs with a size of 2.2 nm and a bandgap at 4.5 eV, resulting in a cutoff at wavelengths at ~ 290 nm. The demonstrated phototransistor shows a photoresponsivity of 137 mA/W, EQE of 66.6 %, and response times of 0.2 s, which is attributed to the effective carrier separation at the vertical heterojunction in depletion mode. The results show that the chosen architecture is a promising design for solution-processed UV photodetectors.

Contactless device characterization of transistor structures in silicon using electro optical frequency mapping (EOFM)

Electro-optical frequency mapping (EOFM) is sensitive to carrier densities in electronic devices. Here, parametric measurements of FET and bipolar transistor structures have been performed with EOFM. The Metal-Insulator-Semiconductor (MIS) system of the FET could be characterized for strong inversion and accumulation for estimations of flatband and threshold voltage. Driving the MIS into accumulation also turns on the source/drain pn junctions into forward bias. The carrier profile of the corresponding parasitic bipolar structure is also measured with EOFM showing the different operation modes of a parasitic bipolar junction transistor and the limited performance of the unwanted bipolar parasitic under the FET. For comparison, EOFM results for a vertical high-performance heterojunction bipolar transistors (HBT) in SiGe:C BiCMOS technology are included, showing clearly distinguishable results of a golden and a faulty SiGe:C HBT.

Ambipolar Resistive Switching in an Ultrathin Surface-Supported Metal-Organic Framework Vertical Heterojunction.

Memristors (MRs) are considered promising devices with the enormous potential to replace complementary metal-oxide-semiconductor (CMOS) technology, which approaches the scale limit. Efforts to fabricate MRs-based hybrid materials may result in suitable operating parameters coupled to high mechanical flexibility and low cost. Metal-organic frameworks (MOFs) arise as a favorable candidate to cover such demands. The step-by-step growth of MOFs structures on functionalized surfaces, called surface-supported metal-organic frameworks (SURMOFs), opens the possibility for designing new applications in strategic fields such as electronics, optoelectronics, and energy harvesting. However, considering the MRs architecture, the typical high porosity of these hybrid materials may lead to short-circuited devices easily. In this sense, here, it is reported for the first time the integration of SURMOF films in rolled-up scalable-functional devices. A freestanding metallic nanomembrane provides a robust and self-adjusted top mechanical contact on the SURMOF layer. The electrical characterization reveals an ambipolar resistive switching mediated by the humidity level with low-power consumption. The electronic properties are investigated with density functional theory (DFT) calculations. Furthermore, the device concept is versatile, compatible with the current parallelism demands of integration, and transcends the challenge in contacting SURMOF films for scalable-functional devices.

Ultralow switching voltage slope based on two-dimensional materials for integrated memory and neuromorphic applications

To realize ultrafast and energy-efficient electronic devices, reducing the switching voltage slope for ON and OFF states that scales the supply voltage and device dimensions is critical. Novel device architectures based on two-dimensional (2D) materials have overcome the fundamental thermionic limit of the switching slope (60 mV/dec); however, a versatile switching device required for highly integrated memory and neuromorphic applications has not been achieved with such exceptional switching slope characteristics. Here, we demonstrate a switching voltage slope down to 0.62 mV/dec in a threshold switching device based on a vertical heterojunction of silver/hexagonal boron nitride (h-BN)/graphene. The sub-1 mV/dec switching slope for the first time, maintaining a high ON/OFF ratio (up to 1010), originates from the unique coupling between the migrated silver atoms and the chemically-inert graphene electrode through the 2D insulating h-BN. Moreover, our original switching device enables the evolution from a conventional volatile (threshold switching) to non-volatile memristive state by adequate voltage spikes, which is ideal for selector applications in highly integrated crossbar array architecture and in a novel synaptic device for neuromorphic computing.

Ultrahigh Speed and Broadband Few‐Layer MoTe2/Si 2D–3D Heterojunction‐Based Photodiodes Fabricated by Pulsed Laser Deposition

Abstract2D transition metal dichalcogenides are promising candidates for high‐performance photodetectors. However, the relatively low response speed as well as the complex transfer process hinders their wide applications. Herein, for the first time, the fabrication of a few‐layer MoTe2/Si 2D–3D vertical heterojunction for high‐speed and broadband photodiodes by a pulsed laser deposition technique is reported. Owing to the high junction quality, ultrathin MoTe2 film thickness, and unique vertical n–n heterojunction structure, the photodiode exhibits excellent device performance in terms of a high responsivity of 0.19 A W−1 and a large detectivity of 6.8 × 1013 Jones. The device is also capable of detecting a broadband light with wavelength spanning from 300 to 1800 nm. More importantly, the device possesses an ultrahigh response speed up to 150 ns with a 3‐dB electrical bandwidth approaching 0.12 GHz. This work paves the way toward the fabrication of novel 2D–3D heterojunctions for high‐performance, ultrafast photodetectors.

Solid-state synthesis of stable and color tunable cesium lead halide perovskite nanocrystals and the mechanism of high-performance photodetection in a monolayer MoS2/CsPbBr3 vertical heterojunction

Color tunable cesium lead halide perovskite nanocrystals with high stability and the mechanism of high-performance photodetection in a monolayer MoS2/CsPbBr3 vertical heterojunction.

An in situ assembled WO3–TiO2 vertical heterojunction for enhanced Z-scheme photocatalytic activity

The face-to-face contact of a vertical heterojunction is beneficial to charge interaction in photocatalysis. However, constructing a vertical heterojunction with uncompromised redox ability still remains a challenge. Herein, we report the successful synthesis of a WO3-TiO2 vertical heterojunction via establishing an internal electric field across the interface. Experimental investigation and computational simulations reveal that strong electric coupling occurs at the WO3-TiO2 interface forming an internal electric field. The internal electric field induces a Z-scheme charge-carrier transfer through the heterojunction under light irradiation, which leads to effective charge separation and maintains high reaction potentials of charge-carriers. The improved photocatalytic activity of the WO3-TiO2 heterojunction is proved by enhanced generation of reactive oxygen species and accelerated Escherichia coli (E. coli) disinfection. This study provides new insights into understanding and designing Z-scheme heterogeneous photocatalysts.

Composition-induced type I and direct bandgap transition metal dichalcogenides alloy vertical heterojunctions.

While members of the 2D semiconducting transition metal dichalcogenide (TMD) family MX2 (M = {Mo, W}, X = {S, Se}) are promising for device applications, stacked layer (vertical) heterojunctions exhibit features that make them inappropriate for light-emitting applications. Such vertical heterojunctions exhibit type II, rather than the preferred type I band alignment. Using density functional theory calculations, we identify the pseudo-binary and quaternary alloy composition range for which band alignment is type I. While broad regions of composition space lead to type I band alignment, most light-emitting devices require direct bandgaps. We demonstrate that by taking advantage of alloying and/or twisting between layers, a wide range of type I, direct bandgap stacked layer (vertical) heterojunctions are achievable. These results and the underlying method developed here provide new opportunities for TMD vertical heterojunction device optimization and opens the door to new classes of TMD vertical heterojunction device applications.

Growth of uniform MoS2 layers on free-standing GaN semiconductor for vertical heterojunction device application

The feasibility of van der Waals (VdW) heteroepitaxy of molybdenum disulphide (MoS2) layers on gallium nitride (GaN) semiconductor has attracted significant interest in heterojunction optoelectronic device applications. Here, we report on the growth of uniform MoS2 layers on free-standing GaN semiconductor for vertical heterojunction device application. A uniform MoS2 layer was directly grown on the n-type GaN wafer by sulphurization process of molybdenum oxide thin layer. Raman and scanning electron microscopy (SEM) analyses showed homogenous growth of the few-layers MoS2 forming a continuous film, considering the suitability of GaN semiconductor substrate. The fabricated MoS2/GaN vertical heterojunction showed excellent rectifying diode characteristics with a photovoltaic photoresponsivity under monochromatic light illumination. The X-ray photoelectron spectroscopy (XPS) studies showed the conduction and valence band offset values are around 0.44 and 2.3 eV with type II band alignment in the fabricated heterojunction device. This will facilitate effective movement of photoexcited electrons across the MoS2–GaN junction, while a large valence band offset will prevent movement of holes towards the GaN, resulting in low recombination loss to obtain a photovoltage in the heterojunction device. Our study revealed the formation of large-area homogenous MoS2 layers on GaN wafer for vertical heterojunction device application.

Vertical heterojunction Ge0.92Sn0.08/Ge gate-all-around nanowire pMOSFETs with NiGeSn contact

Vertical heterojunction Ge0.92Sn0.08/Ge gate-all-around (GAA) nanowire pMOSFETs fabricated with a top-down approach are reported. With optimized processes, pFETs with nanowire diameters as small as 32 nm were achieved and a decent ION/IOFF ratio of ~3 × 106 was obtained thanks to the GAA nanowire geometry, the GeSn/Ge heterostructure and NiGeSn metallization. A compensating effect between top source resistance and diameter-related electrostatics was identified for pFETs without NiGeSn source. Devices with NiGeSn source showed significant improved ION, ION/IOFF ratio and subthreshold swing (SS) characteristics compared with those without NiGeSn.

Fabrication of a MoS2/Graphene Nanoribbon Heterojunction Network for Improved Thermoelectric Properties

AbstractA number of 2D materials have been developed that have properties different from bulk materials due to the quantum confinement effect. These 2D materials can also form a vertical van der Waals heterojunction at a large interface with other 2D materials, resulting in unique electronic and thermoelectric properties. However, it is difficult to fabricate a van der Waals heterostructure of 2D materials that can provide a sufficient temperature gradient while also forming carrier paths across the vertical heterojunction. Here, a heterojunction network structure constructed of highly conductive sub‐20‐nm graphene nanoribbons (GNRs) arrays stacked on a semiconducting molybdenum disulfide (MoS2) monolayer is suggested to maximize the heterojunction effect on carrier transport. This heterojunction network allows the carriers inevitably pass back and forth between the GNR and the MoS2 through the vertical heterojunction, effectively utilizing the interfacial properties in thermoelectricity. This structure also can modify the band structure by controlling the linewidth of the nanoribbons, or introduce an interlayer at the heterojunctions to enhance the tunneling effect between the two layers, thus significantly improved thermoelectric properties are achieved such as enhanced electrical conductivity of 700 S m−1 and a high power factor of 222 µW m−1 K−1.

Demonstration of CuI as a P–N heterojunction to β-Ga2O3

Vertical heterojunction p-CuI/n-Ga2O3 diodes were fabricated on commercial β-Ga2O3 substrates using the reaction of epitaxially-sputtered Cu with natural I2 vapor at room temperature followed by either a high temperature I2 gas reaction or an I2 solution treatment to remove iodine vacancies. Results show that using an epitaxially-oriented over a polycrystalline Cu layer produces larger and more strongly bonded CuI crystals. Hall and current–voltage measurements support rectifying behavior consistent with p-CuI/n-Ga2O3 heterojunction characteristics including 8 orders of magnitude Ion/Ioff ratio and an ideality factor of 1.37 measured on the sample with the lowest concentration of iodine vacancies.