Carbon-based electronics
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Phaedon Avouris*, Zhihong Chen and Vasili Perebeinos
IBM T. J. Watson Research Center, Yorktown Heights, New York, 10598, USA
*e-mail: avouris@us.ibm.com
In the last few decades we have witnessed dramatic advances in electronics that have found uses in computing, communications, automation and other applications that affect just about every aspect of our lives. To a large extent these advances have been the result of the continuous miniaturization or ‘scaling’ of electronic devices, particularly of silicon-based transistors, that has led to denser, faster and more power-efficient circuitry. Obviously, however, this device scaling and performance enhancement cannot continue forever; a number of limitations in fundamental scientific as well as technological nature place limits on the ultimate size and performance of silicon devices.
Electronic structure
Graphite is a well-known allotropic form of carbon composed of layers of sp2-bonded carbon atoms in a honeycomb arrangement (Fig. 1a). The different carbon layers in graphite interact weakly, primarily by van der Waals forces. This interaction produces a small valence and conduction band overlap of about 40 meV, which makes graphite overall a semi-metal. The electronic structure of graphene, an individual layer of graphite, was first discussed by P. R. Wallace in 1947 (ref. 2). There are about 3 million layers in a millimetre thickness of graphite and the experimental study of graphene has been thwarted by the difficulty of isolating and studying this single atomic layer.
In this article we examine the electronic structure, electrical transport and optoelectronic properties of CNTs, with a focus on the physical phenomena involved. We discuss briefly the emerging area of study involving single graphene layers and narrow graphene nanoribbons (GNRs). We analyse the switching mechanism and characteristics of single-CNT field-effect transistors (FETs), GNR FETs and efforts towards device integration. We also describe the principles of simple CNT optoelectronic devices such as electroluminescent light emitters and photodetectors.
Recently, however, graphene became the object of intense experimental study when it was realized that single layers, or a few layers, could be produced relatively easily by mechanical exfoliation of graphite3, or by heating SiC (ref. 4). Figure 1b shows the peculiar single-particle band structure of this 2D material. The linear dispersion at low energies makes the electrons and holes in graphene mimic relativistic particles that are described by the Dirac relativistic equation for particles with spin 1/2, and they are usually referred to as Dirac Fermions. Their dispersion, E2D = ħνF√kx2 + ky2, is analogous to that of photons, Ek = ħck, but with the velocity of light c replaced by vF ≈ 106 m s–1, the Fermi velocity. Thus, electrons and holes in graphene have zero effective mass and a velocity that is about 300 times slower than that of light. This linear dispersion relationship also means that quasi-particles in graphene display properties quite different to those observed in conventional three-dimensional materials, which have parabolic dispersion relationships. For example, graphene displays an anomalous quantum Hall effect and half-integer quantization of the Hall conductivity5,6. The quantum Hall effect in graphene can be observed even at room temperature7.
Carbon-based electronics
REVIEW ARTICLE
The semiconductor industry has been able to improve the performance of electronic systems for more than four decades by making ever-smaller devices. However, this approach will soon encounter both scientific and technical limits, which is why the industry is exploring a number of alternative device technologies. Here we review the progress that has been made with carbon nanotubes and, more recently, graphene layers and nanoribbons. Field-effect transistors based on semiconductor nanotubes and graphene nanoribbons have already been demonstrated, and metallic nanotubes could be used as high-performance interconnects. Moreover, owing to the excellent optical properties of nanotubes it could be possible to make both electronic and optoelectronic devices from the same material.
The realization of the approaching limits has inspired a worldwide effort to develop alternative device technologies. Some approaches involve moving away from traditional electron transport-based electronics: for example, the development of spin-based devices. Another approach, on which we focus here, maintains the operating principles of the currently used devices, primarily that of the field-effect transistor, but replaces a key component of the device, the conducting channel, with carbon nanomaterials such as one-dimensional (1D) carbon nanotubes (CNT) or two-dimensional (2D) graphene layers, which have superior electrical properties1. Furthermore, semiconducting carbon nanotubes are direct bandgap materials providing an ideal system to study optics and optoelectronics in one dimension and explore the possibility of basing both electronics and optoelectronic technologies on the same material.
Phaedon Avouris*, Zhihong Chen and Vasili Perebeinos
IBM T. J. Watson Research Center, Yorktown Heights, New York, 10598, USA
*e-mail: avouris@us.ibm.com
In the last few decades we have witnessed dramatic advances in electronics that have found uses in computing, communications, automation and other applications that affect just about every aspect of our lives. To a large extent these advances have been the result of the continuous miniaturization or ‘scaling’ of electronic devices, particularly of silicon-based transistors, that has led to denser, faster and more power-efficient circuitry. Obviously, however, this device scaling and performance enhancement cannot continue forever; a number of limitations in fundamental scientific as well as technological nature place limits on the ultimate size and performance of silicon devices.
Electronic structure
Graphite is a well-known allotropic form of carbon composed of layers of sp2-bonded carbon atoms in a honeycomb arrangement (Fig. 1a). The different carbon layers in graphite interact weakly, primarily by van der Waals forces. This interaction produces a small valence and conduction band overlap of about 40 meV, which makes graphite overall a semi-metal. The electronic structure of graphene, an individual layer of graphite, was first discussed by P. R. Wallace in 1947 (ref. 2). There are about 3 million layers in a millimetre thickness of graphite and the experimental study of graphene has been thwarted by the difficulty of isolating and studying this single atomic layer.
In this article we examine the electronic structure, electrical transport and optoelectronic properties of CNTs, with a focus on the physical phenomena involved. We discuss briefly the emerging area of study involving single graphene layers and narrow graphene nanoribbons (GNRs). We analyse the switching mechanism and characteristics of single-CNT field-effect transistors (FETs), GNR FETs and efforts towards device integration. We also describe the principles of simple CNT optoelectronic devices such as electroluminescent light emitters and photodetectors.
Recently, however, graphene became the object of intense experimental study when it was realized that single layers, or a few layers, could be produced relatively easily by mechanical exfoliation of graphite3, or by heating SiC (ref. 4). Figure 1b shows the peculiar single-particle band structure of this 2D material. The linear dispersion at low energies makes the electrons and holes in graphene mimic relativistic particles that are described by the Dirac relativistic equation for particles with spin 1/2, and they are usually referred to as Dirac Fermions. Their dispersion, E2D = ħνF√kx2 + ky2, is analogous to that of photons, Ek = ħck, but with the velocity of light c replaced by vF ≈ 106 m s–1, the Fermi velocity. Thus, electrons and holes in graphene have zero effective mass and a velocity that is about 300 times slower than that of light. This linear dispersion relationship also means that quasi-particles in graphene display properties quite different to those observed in conventional three-dimensional materials, which have parabolic dispersion relationships. For example, graphene displays an anomalous quantum Hall effect and half-integer quantization of the Hall conductivity5,6. The quantum Hall effect in graphene can be observed even at room temperature7.
Carbon-based electronics
REVIEW ARTICLE
The semiconductor industry has been able to improve the performance of electronic systems for more than four decades by making ever-smaller devices. However, this approach will soon encounter both scientific and technical limits, which is why the industry is exploring a number of alternative device technologies. Here we review the progress that has been made with carbon nanotubes and, more recently, graphene layers and nanoribbons. Field-effect transistors based on semiconductor nanotubes and graphene nanoribbons have already been demonstrated, and metallic nanotubes could be used as high-performance interconnects. Moreover, owing to the excellent optical properties of nanotubes it could be possible to make both electronic and optoelectronic devices from the same material.
The realization of the approaching limits has inspired a worldwide effort to develop alternative device technologies. Some approaches involve moving away from traditional electron transport-based electronics: for example, the development of spin-based devices. Another approach, on which we focus here, maintains the operating principles of the currently used devices, primarily that of the field-effect transistor, but replaces a key component of the device, the conducting channel, with carbon nanomaterials such as one-dimensional (1D) carbon nanotubes (CNT) or two-dimensional (2D) graphene layers, which have superior electrical properties1. Furthermore, semiconducting carbon nanotubes are direct bandgap materials providing an ideal system to study optics and optoelectronics in one dimension and explore the possibility of basing both electronics and optoelectronic technologies on the same material.