(Nanowerk Spotlight) The miniaturization of electronic components has been a driving force behind computing advancements for over half a century. However, as silicon-based transistors approach their physical limits, researchers are exploring alternative materials to continue progress in semiconductor technology. Carbon nanotubes (CNTs) have long been considered promising candidates for next-generation electronics due to their exceptional electrical properties and nanoscale dimensions. Yet, the challenge of precisely controlling the electronic characteristics of CNTs has hindered their widespread adoption in practical applications.
Semiconducting CNTs possess several advantages over traditional silicon, including higher carrier mobility and better electrostatic control at nanoscale dimensions. These properties make them potentially ideal for creating ultra-small, high-performance transistors. However, the perfect sp2 carbon-carbon bonds that give CNTs their remarkable strength and conductivity also make them resistant to conventional doping techniques used in semiconductor manufacturing.
This resistance to doping has been a significant obstacle in the development of CNT-based electronics. Doping is crucial for creating both n-type and p-type semiconductors, which are essential for building complementary metal-oxide-semiconductor (CMOS) circuits – the foundation of modern digital electronics. While p-type CNT transistors have been relatively easy to achieve, stable and high-performance n-type CNT transistors have remained elusive.
Previous attempts to modify the electronic properties of CNTs have included chemical functionalization, electrostatic doping, and the use of different metal contacts. However, these methods often resulted in unstable or inconsistent performance, limiting their practical utility. The inability to reliably create both n-type and p-type CNT transistors has been a major roadblock in developing CNT-based CMOS circuits that could potentially outperform silicon-based technology.
Recent advancements in materials science and nanotechnology have opened up new possibilities for manipulating the properties of CNTs. The emergence of perovskite materials, known for their unique optoelectronic properties, has caught the attention of researchers in various fields. Concurrently, progress in precise nanoscale fabrication and characterization techniques has enabled scientists to explore novel ways of combining different nanomaterials to create hybrid structures with tailored properties.
Against this backdrop, a team of researchers in China has developed an innovative approach to modifying the electronic characteristics of CNTs by filling them with one-dimensional halide perovskites. This method, described in a recent paper in Advanced Materials (“Inner Doping of Carbon Nanotubes with Perovskites for Ultralow Power Transistors”), offers a potential solution to the long-standing challenge of creating stable and controllable n-type CNT transistors, as well as enabling the fabrication of advanced electronic devices with unprecedented performance.
Confined atomic structure and the calculated band structure of the coaxial CsPbBr3/CNT. a) Orthogonal CsPbBr3 halide perovskite encapsulated in 1.8–2.0 nm CNT. b,c) Cubic CsPbBr3 halide perovskite encapsulated in 1.2–1.4 nm CNT. d) Encapsulated the smallest CsPbBr3 halide perovskite structure derived from cubic CsPbBr3 in 0.8–1.0 nm CNT. There are experimental HAADF-STEM images, simulation results, the corresponding side view from left to right in (a–d), respectively. e) Schematic of the coaxial CsPbBr3/CNT. f) Longitudinal profiles as indicated in (b). (Image: Adapted from DOI:10.1002/adma.202403743 with permission by Wiley-VCH Verlag)
The research team’s approach involves using perovskite materials, specifically CsPbBr3 and CsSnI3, to fill the hollow interior of CNTs. By carefully controlling the filling process, the researchers were able to create different configurations of perovskite-filled CNTs, including partial-filling and full-filling. The perovskite material inside the CNT forms a coaxial heterojunction with the carbon nanotube, allowing for precise tuning of the electrical properties.
One of the key findings of the study is the ability to create stable n-type CNT field-effect transistors (FETs) using this filling method. N-type semiconductors, which conduct electricity using negatively charged electrons as the primary charge carriers, are essential for creating complementary circuits in modern electronics. Previous attempts to create n-type CNT transistors often resulted in devices with poor stability or performance. The perovskite-filled CNTs, however, demonstrated stable n-type behavior with good electrical characteristics, including high on-state current and low subthreshold swing.
Perhaps the most significant achievement of this research is the demonstration of a quasi-broken-gap (BG) tunnel field-effect transistor (TFET) based on a single partial-filling CsPbBr3/CNT heterojunction. TFETs are a class of transistors that operate on the principle of quantum tunneling rather than thermionic emission, allowing them to potentially overcome the fundamental limits of conventional transistors in terms of power consumption and switching speed.
The quasi-BG TFET created by the research team exhibited remarkable performance metrics. It achieved a subthreshold swing of approximately 35 millivolts per decade, which is significantly below the theoretical limit of 60 millivolts per decade for conventional transistors at room temperature. This low subthreshold swing indicates that the device can switch between its on and off states with very little change in gate voltage, potentially enabling ultra-low power operation.
Moreover, the quasi-BG TFET demonstrated a high on-state current of up to 4.9 microamperes per tube and an on/off current ratio of up to 105. These characteristics suggest that the device can provide both low power consumption and high performance, a combination that has been difficult to achieve in previous TFET designs.
The researchers conducted extensive characterization of the perovskite-filled CNTs using advanced microscopy and spectroscopy techniques. High-resolution scanning transmission electron microscopy (STEM) revealed the atomic-scale structure of the perovskite material inside the CNTs, showing how the confined space affects the crystal structure of the perovskite. Density functional theory (DFT) calculations provided insights into the electronic interactions between the perovskite and the CNT, explaining the observed n-type doping effect.
The team also investigated the temperature dependence of the device performance, confirming that the primary mechanism of carrier transport in the quasi-BG TFET is indeed band-to-band tunneling rather than thermionic emission. This finding supports the potential of these devices to operate with high efficiency at low voltages, which is crucial for reducing power consumption in future electronic systems.
Schematic of the top-gate coaxial CsPbBr3/CNT FET. (Image: Adapted from DOI:10.1002/adma.202403743 with permission by Wiley-VCH Verlag)
The implications of this research extend beyond the creation of individual transistors. The ability to precisely control the electrical properties of CNTs through internal doping with perovskites opens up new possibilities for designing complex integrated circuits. The researchers suggest that their approach could enable the development of high-performance and ultra-low power consumption CNT-based CMOS circuits, potentially surpassing the capabilities of current silicon-based technologies.
While the results are promising, there are still challenges to overcome before this technology can be implemented in practical applications. Scaling up the production of perovskite-filled CNTs, ensuring uniformity and reproducibility across large numbers of devices, and integrating these novel transistors into existing semiconductor manufacturing processes are all areas that will require further research and development.
This work represents a significant step forward in the field of nanoelectronics, offering a new approach to overcoming the limitations of traditional semiconductor devices. By combining the unique properties of carbon nanotubes with the versatility of perovskite materials, the researchers have created a platform for developing next-generation electronic devices that could potentially revolutionize computing, communications, and energy-efficient technologies. As research in this area continues, we may see the emergence of new classes of electronic devices that push the boundaries of performance and efficiency, driving innovation across a wide range of industries and applications.
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