Carbon Nanomaterials for Designing Next-Generation Green Electronics

Low-dimensional allotropes of carbon (including two-dimensional graphene and one-dimensional carbon nanotubes and graphene nanoribbons), collectively known as carbon nanomaterials, have extraordinary physical properties that can be exploited for their exciting prospects in a variety of electronics applications.

In particular, these nanomaterials can be used to design low-power, low-loss and ultra energy-efficient active and passive nanoelectronic devices, which can consequently lead to unprecedented levels of integration density and energy-efficiency in coming generations of integrated circuits and electronic products.

The structures of all carbon nanomaterials can be derived from that of graphene, which is a single-atom-thick planar sheet of sp²-bonded carbon atoms packed in a honeycomb crystal lattice (Fig. 1a).

A graphene nanoribbon (GNR) can be obtained by patterning graphene into a ribbon, while a carbon nanotube (CNT) can be thought of as a rolled up ribbon to form a seamless tube.

The band structure of graphene has unique characteristics: the energy dispersion (or simply E-k) relation is linear near the Fermi energy level, leading to zero effective mass for electrons and holes, and thereby to extremely high carrier mobility (> 10000 cm­²/V-s).

Some key properties of these carbon nanomaterials are outlined in Table I along with those of some common semiconductors and a metal (Cu), indicating their great potential in next-generation electronics applications¹. In the following paragraphs, we briefly outline and highlight the prospects of carbon nanomaterials for designing next generation “green” electronics.

Figure 1. (a) The atomic structure of CNT and GNR derived from a graphene sheet. (b) A 2-inch wafer size graphene grown on nickel in the Nanoelectronics Research Laboratory (NRL) at UCSB. (c) Selective carbon nanotube growth used to form the pattern of NRL logo.

Table I. Properties of carbon nanomaterials (single-walled CNT (SWCNT), multi-walled CNT (MWCNT), and graphene nanoribbon (GNR)) in comparison to those of some semiconductors (Si, GaAs, and GaN) and metal (Cu) used for various electronics applications.

Low-Power High-Speed Interconnects

Metal interconnections used as communication links between billions of transistors in modern integrated circuits (ICs) have come to occupy the center stage in determining the performance and power dissipation of electronic chips such as microprocessors^2,3.

A typical high-performance IC employs several layers of metal interconnects separated by insulating materials, with “short-wires” employed for local communication and “long-wires” used for global communication within a chip.

Interconnects are also used for distributing the clock signals throughout the chip and are known to be responsible for over 50% of the power dissipation in most ICs. It has been shown that the delay of global interconnects can be reduced by up to 50% if CNT bundles or multi-layer Graphene interconnects are employed^4-6. Meanwhile, if the delay of CNT/Graphene interconnects are kept at the same optimal delay of metal (Cu) interconnects, CNT/Graphene interconnects would reduce the global interconnect power consumption by 50% compared to that of Cu interconnects⁷.

Low-Loss Passive Devices

In ultra high-frequency (millimeter wave and radio-frequency) applications, increased losses due to skin and proximity effects in IC interconnects and passive devices can also contribute significantly to reducing the energy-efficiency of electronic circuits and products.

CNT/Graphene interconnects have been shown to exhibit unique high-frequency behavior (reduced skin effect) owing to their large momentum relaxation time, indicating promising high-frequency applications^8,9. For example, it has been shown that the maximum Q-factor (a metric quantifying inductor efficiency) of a ¾-turn inductor can be increased by as much as 230 % (3.3 times) and 32% (1.3 times) by replacing Cu with CNTs and graphene, respectively ^8,10.

CNTs are also known to have excellent thermal properties (Table 1) and in light of the benefits they offer for ultra high-frequency operation, they are also promising as Through-Silicon Vias (TSV) ¹¹ --a key enabling technology for 3-dimensional (3D) -ICs.

3-D ICs allow stacking and bonding of multiple active layers (substrates) and are being pursued in many semiconductor companies around the world due to their low-power prospects (resulting from their reduced interconnect lengths) and feasibility of heterogeneous integration of disparate technologies (Si, III-V, Graphene) and circuits (digital, analog, RF, optics, etc) ¹².

High-Density Energy-Storage Devices

For high-density metal-insulator-metal (MIM) capacitor designs, it has been shown that the capacitance density of CNT-based capacitors can reach as high as 38.39 fF/μm², much larger than the International Technology Roadmap for Semiconductors (ITRS) requirement of 12fF/μm² for the year 2022¹, indicating their excellent potential to replace current MIM capacitors as well as other on-chip charge-storage based devices.

Graphene has also shown promise as electrode materials in supercapacitor applications due to its large surface-area to volume ratio. It has been recently reported that graphene based supercapacitors exhibit a specific energy density of 85.6 W·h/kg at room temperature, much higher than that of conventional lead-acid batteries (typically 30 to 40 W·h/kg)¹³.

Ultra Energy-Efficient Active Devices

More recently, the semiconductor industry has witnessed renewed interest in ultra energy-efficient transistors. This is driven by the need to find a replacement switch for the nanoscale MOSFET, which forms the work horse of the IC industry but suffers from increasingly high off-state leakage, thereby making it very energy inefficient.

A key research goal in the area of ultra energy-efficient transistor design is the design and demonstration of small subthreshold swing (SS) switches (including tunnel-field effect transistors (T-FET) and Nanoelectromechanical (NEM)-FET) that makes ON to OFF switching more abrupt and reduces leakage current, as an eventual replacement for the MOSFET^14,15.

However, to make this a reality, demonstration of compact, scalable, and reliable T-FETs/NEM-FETs with high (MOSFET like) ON currents, and low OFF currents at low power-supply voltages, that are suitable for building large-scale logic circuits and systems, are highly desirable.

CNT and GNR are excellent materials for designing such energy-efficient active devices for next-generation green electronics.

For example, the high mobility and low bandgap of graphene has been exploited to design a GNR based heterojunction T-FET exhibiting I_ON as high as 1 mA/μm, I_ON/I_OFF ratio as high as 10⁹, and SS as small as 10 mV/dec at V_DD=0.5 V, and L_ch=20 nm¹⁶. Moreover, understanding the physics of band-to-band-tunneling (BTBT), which is key to designing T-FETs with any material, has been shown to be easily enabled by GNR based T-FETs¹⁷. On the other hand, CNT is an excellent material for designing NEM-FETs due to their low mass density and high Young’s modulus¹⁸.

High-Efficiency Photovoltaics

Efficient harvesting of solar energy through novel photovoltaic devices is critical for global scale reduction of green house gases. Hence, increasing the efficiency of photovoltaic devices has become a key research goal in the area of solar cell design.

A two orders of magnitude enhancement of photocurrent resulting from adding Single-Walled CNTs (SWCNTs) to a poly-3-octylthiophene (P3OT) matrix has been reported in an organic solar cell¹⁹.

It has also been reported that using SWCNTs as conducting scaffolds in a TiO₂ based dye-sensitized solar cell can double the photoconversion efficiency²⁰.

More interestingly, extremely efficient multiple electron-hole pair generation has been observed in CNT due to optical excitation into the second subband suggesting the possibility of exceeding the thermodynamic (Shockley-Queisser) limit²¹. There is also lots of interest in employing CNT/graphene as transparent electrodes for solar cell and LED applications^22-24.

References

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