Tunable black phosphorus heterojunction transistors for multifunctional optoelectronics

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

The evolutionary success in information technology has been sustained by the rapid growth of sensor technology. Recently, advances in sensor technology have promoted the ambitious requirement to build intelligent systems that can be controlled by external stimuli along with independent operation, adaptivity, and low energy expenditure. Among various sensing techniques, field-effect transistors (FETs) with channels made of two-dimensional (2D) materials attract increasing attention for advantages such as label-free detection, fast response, easy operation, and capability of integration. With atomic thickness, 2D materials restrict the carrier flow within the material surface and expose it directly to the external environment, leading to efficient signal acquisition and conversion. This review summarizes the latest advances of 2D-materials-based FET (2D FET) sensors in a comprehensive manner that contains the material, operating principles, fabrication technologies, proof-of-concept applications, and prototypes. First, a brief description of the background and fundamentals is provided. The subsequent contents summarize physical, chemical, and biological 2D FET sensors and their applications. Then, we highlight the challenges of their commercialization and discuss corresponding solution techniques. The following section presents a systematic survey of recent progress in developing commercial prototypes. Lastly, we summarize the long-standing efforts and prospective future development of 2D FET-based sensing systems toward commercialization.

2D Homojunctions for Electronics and Optoelectronics

In the post-Moore era, 2D materials with rich physical properties have attracted widespread attention from the scientific and industrial communities. Among 2D materials, the 2D homojunctions are of great promise in designing novel electronic and optoelectronic devices due to their unique geometries and properties such as homogeneous components, perfect lattice matching, and efficient charge transfer at the interface. In this article, a pioneering review focusing on the structural design and device application of 2D homojunctions such as p-n homojunctions, heterophase homojunctions, and layer-engineered homojunctions is provided. The preparation strategies to construct 2D homojunctions including vapor-phase deposition, lithium intercalation, laser irradiation, chemical doping, electrostatic doping, and photodoping are summarized in detail. Specifically, a careful review on the applications of the 2D homojunctions in electronics (e.g., field-effect transistors, rectifiers, and inverters) and optoelectronics (e.g., light-emitting diodes, photovoltaics, and photodetectors) is provided. Eventually, the current challenges and future perspectives are commented for promoting the rapid development of 2D homojunctions.

In Situ Cleaning and Fluorination of Black Phosphorus for Enhanced Performance of Transistors with High Stability

Most two-dimensional (2D) semiconductors suffer from intrinsic instability under ambient conditions, especially 2D black phosphorus (BP). Although much effort has been made to study the passivation of 2D materials against corrosion by oxygen and water molecules, facile and effective passivation with long-term stability is still challenging; in particular, selective passivation, which is critical for integration into nanoelectronics, is still lacking. Here, we develop a novel passivation route for BP using a fluorinated self-assembled thin film of PFSA (perfluorosulfonic acid, PFSA), where the surface modifier with high hydrophobicity on BP presents extremely stable characteristics over five months under ambient conditions. Moreover, we report for the first time in situ cleaning and selective fluorination of only BP flakes on a SiO₂/Si substrate by a spin-coating process followed by ultrasonication, which was attributed to the formation of P-F covalent bonds on the BP surface. Selectively fluorinated BP shows not only enhanced stability in air but also electrical properties of the BP field-effect transistor (FET), with the on-current of the BP FET increasing and presenting enhanced carrier mobility (125 cm² V^-1 s^-1) and on/off ratio (10⁴). This significant finding sheds light on fabricating vertical 2D heterostructures to realize high performance and reliability with versatile 2D materials. This work demonstrates an emerging passivation approach for long-term stability together with superior electrical properties, which paves the way for integrating 2D semiconductors into critical channel materials in FETs that are favorable for next-generation digital logic circuits.

Carrier polarity modulation of molybdenum ditelluride (MoTe2) for phototransistor and switching photodiode applications

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are layered semiconductor materials that have recently emerged as promising candidates for advanced nano- and photoelectronic applications. Previously, various doping methods, such as surface functionalization, chemical doping, substitutional doping, surface charge transfer, and electrostatic doping, have been introduced, but they are not stable or efficient. In this study, we have developed carrier polarity modulation of molybdenum ditelluride (MoTe₂) for the development of phototransistors and switching photodiodes. Initially, we treated p-MoTe₂ in a N₂ environment under DUV irradiation and found that the p-type MoTe₂ changed to n-type MoTe₂. However, the treated devices exhibited environmental stability over a long period of 60 days. Kelvin probe force microscopy (KPFM) measurements demonstrated that the values of the work function for p-MoTe₂ and n-MoTe₂ were ∼4.90 and ∼4.49 eV, respectively, which confirmed the carrier tunability. Also, first-principles studies were performed to confirm the n-type carrier polarity variation. Interestingly, the n-type MoTe₂ reversed its polarity to p-type after the irradiation of the devices under DUV in an O₂ environment. Additionally, a lateral homojunction-based p-n diode of MoTe₂ with a rectification ratio of ∼2.5 × 10⁴ was formed with the value of contact potential difference of ∼400 mV and an estimated fast rise time of 29 ms and decay time of 38 ms. Furthermore, a well self-biased photovoltaic behavior upon illumination of light was achieved and various photovoltaic parameters were examined. Also, V_OC switching behavior was established at the p-n diode state by switching on and off the incident light. We believe that this efficient and facile carrier polarity modulation technique may pave the way for the development of phototransistors and switching photodiodes in advanced nanotechnology.

Two-Dimensional Pnictogen for Field-Effect Transistors

Two-dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group-VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next-generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field-effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.

Gate-Tunable and Programmable n-InGaAs/Black Phosphorus Heterojunction Diodes

Semiconductor heterostructures have enabled numerous applications in diodes, photodetectors, junction field-effect transistors, and memory devices. Two-dimensional (2D) materials and III-V compound semiconductors are two representative materials providing excellent heterojunction platforms for the fabrication of heterostructure devices. The marriage between these semiconductors with completely different crystal structures may enable a new heterojunction with unprecedented physical properties. In this study, we demonstrate a multifunctional heterostructure device based on 2D black phosphorus and n-InGaAs nanomembrane semiconductors that exhibit gate-tunable, photoresponsive, and programmable diode characteristics. The device exhibits clear rectification with a large gate-tunable forward current, which displays rectification and switching with a maximum rectification ratio of 4600 and an on/off ratio exceeding 10⁵, respectively. The device also offers nonvolatile memory properties, including large hysteresis and stable retention of storage charges. By combining the memory and gate-tunable rectifying properties, the rectification ratio of the device can be controlled and memorized from 0.06 to 400. Moreover, the device can generate three different electrical signals by combining a photoresponsivity of 0.704 A/W with the gate-tunable property, offering potential applications, for example, multiple logic operator. This work presents a heterostructure design based on 2D and III-V compound semiconductors, showing unique physical properties for the development of multifunctional heterostructure devices.

Efficient and reliable surface charge transfer doping of black phosphorus via atomic layer deposited MgO toward high performance complementary circuits

Black phosphorus (BP), a fast emerging 2D material, has shown great potential in future electronics and optoelectronics owing to its outstanding properties including sizable band gap and ambipolar transport characteristics. However, its hole conduction dominance, featured by a much larger hole mobility and the corresponding on-current than that of the electrons, renders the reliable modulation of its carrier type and density a key challenge, thereby hindering its application to complementary electronics. Here, we demonstrate an efficient and reliable n-type doping for BP transistors via surface functionalization by atomic layer deposited magnesium oxide (MgO) with favorable controllability. By optimizing the MgO thickness, an electron mobility of up to 95.5 cm2 V-1 s-1 is reached with a simultaneous significant suppression of hole conduction. Subsequently, a high-performance complementary logic inverter is demonstrated within a single BP flake, which operates well with a supply voltage as low as <0.5 V, outperforming reported BP inverters in terms of logic level match, power consumption and process feasibility. Our findings suggest that surface charge transfer doping via MgO can be used as a promising technique towards high performance BP-based functional nanoelectronics.