Kiran Chikkadi

Ultra-low power operation of self-heated, suspended carbon nanotube gas sensors

We present a suspended carbon nanotube gas sensor that senses NO2 at ambient temperature and recovers from gas exposure at an extremely low power of 2.9 μW by exploiting the self-heating effect for accelerated gas desorption. The recovery time of 10 min is two orders of magnitude faster than non-heated recovery at ambient temperature. This overcomes an important bottleneck for the practical application of carbon nanotube gas sensors. Furthermore, the method is easy to implement in sensor systems and requires no additional components, paving the way for ultra-low power, compact, and highly sensitive gas sensors.

Reduction of gate hysteresis above ambient temperature via ambipolar pulsed gate sweeps in carbon nanotube field effect transistors for sensor applications

We investigate the hysteresis behavior in carbon nanotube (CNT) field effect transistors (CNFETs) upon pulsed gate voltages (Vg) above ambient temperature within 300–390 K. Assuming charge trapping near the CNT channel to be the major mechanism behind gate hysteresis, we perform charge trapping experiments based on Vg pulses and find that CNFET charge trapping is increasing with temperature. We assess the impact of thermally enhanced charge trapping on the hysteresis reduction performance of two different pulsed Vg sweeps. One of the two sweeps, consisting of alternating polarity pulses, is shown to essentially eliminate gate hysteresis in the studied temperature range.

Suspended CNT-FET piezoresistive strain gauges: Chirality assignment and quantitative analysis

Carbon nanotubes can withstand large elastic deformation and show pronounced piezoresistive effects which make them promising candidates for low-power strain sensors in the large strain regime. Integration of individual suspended nanotubes into complex microelectromechanical devices, including gate structures, is still a challenge. Here, ultraclean carbon nanotubes spanning across actuated MEMS electrodes are operated as suspended Carbon Nanotube Field-Effect Transistors (CNT-FETs) under repeated variable strain up to 4.5%, thus acting as small-sized piezoresistive strain gauges whose gauge factor is electrostatically tuned by the gate. We present the first quantified electromechanical analysis of suspended CNT-FETs to uniaxial strain applied by on-chip micro actuators.

Advances in NO2 sensing with individual single-walled carbon nanotube transistors

Summary The charge carrier transport in carbon nanotubes is highly sensitive to certain molecules attached to their surface. This property has generated interest for their application in sensing gases, chemicals and biomolecules. With over a decade of research, a clearer picture of the interactions between the carbon nanotube and its surroundings has been achieved. In this review, we intend to summarize the current knowledge on this topic, focusing not only on the effect of adsorbates but also the effect of dielectric charge traps on the electrical transport in single-walled carbon nanotube transistors that are to be used in sensing applications. Recently, contact-passivated, open-channel individual single-walled carbon nanotube field-effect transistors have been shown to be operational at room temperature with ultra-low power consumption. Sensor recovery within minutes through UV illumination or self-heating has been shown. Improvements in fabrication processes aimed at reducing the impact of charge traps have reduced the hysteresis, drift and low-frequency noise in carbon nanotube transistors. While open challenges such as large-scale fabrication, selectivity tuning and noise reduction still remain, these results demonstrate considerable progress in transforming the promise of carbon nanotube properties into functional ultra-low power, highly sensitive gas sensors.

Thin film transistors with a ZnO channel and gate dielectric layers of HfO2 by atomic layer deposition

Thin film transistors (TFTs) have been fabricated with a zinc oxide (ZnO) channel layer and a hafnium dioxide (HfO2) gate dielectric layer. The oxide layers were deposited using an atomic layer deposition (ALD) system. The use of ALD for ZnO deposition allows subnanometer thickness control of the deposited layer, and thereby provides a means to vary TFT threshold voltage by controlling the carrier density in the ZnO channel: the carrier density is dependent on the layer thickness because band structure changes result in charge depletion in thinner layers. Enhancement-mode devices have been fabricated and have an on-off current ratio above 106. The enhancement-mode devices of the inverted (gate down) TFT structure were realized by decreasing the ZnO channel layer thickness to 15 nm and below, thereby reducing the carrier density of the as-deposited n-type ZnO layer. An important aspect of the fabrication of the inverted TFTs was the use of either an aluminum sacrificial layer or a thin HfO2 cap layer to el...

Low-Power Gas Sensing Using Single Walled Carbon Nano Tubes in Wearable Devices

Air quality monitoring is gaining importance in public health due to the increasing level of the pollution in cities. In addition, people are more concerned about their personal exposure and they are interested to know the concentration levels of the pollutants, which surround them. In recent years, a wearable technology can be useful for continuous air quality monitoring when people are moving in urban and industrial environments. As wearable systems are usually battery-powered and gas sensors are power-hungry, energy-efficient design and power management are required. In this paper, we present a two-stage gas sensing concept where novel multiple-single-walled carbon nanotubes (SWCNT) are proposed as detectors for an energy-hungry metal-oxide (MOX)-semiconductor gas sensor. We simulate the system performance combining the low power consumption of SWCNT gas sensors and the more mature MOX sensor to achieve an energy-efficient wearable device able to monitor the air quality continuously while achieving long lifetime. We perform the simulations using measured power consumptions for two event-driven scenarios to evaluate the power consumption reduction and lifetime extension in a wearable mobile context. Our results show that the proposed approach prolongs node lifetimes by 30 times compared with adaptive duty-cycling with only MOX gas sensors. We also propose that the nanotube recovery time issue can be overcome by using four single nanotubes on the same chip, which results in an extension of lifetime.

The response of single-walled carbon nanotubes to NO2 and the search for a long-living adsorbed species

Contact-passivated sensor devices allow one to measure the response of individual ultraclean single-walled carbon nanotubes to 1 ppm NO2, and show that the activation energies for desorption from nanotubes of diameters in the 1.5–3.5 nm range are of the order of 1 eV. DFT calculations based on several exchange-correlation functionals are presented and critically examined. The nature of the molecular binding is thus clarified for NO2, N2O4, and NO3, and also the dependence on the size of the nanotube. The binding strength of physisorbed NO3 is consistent with the experimental data on desorption.

Process control monitors for individual single-walled carbon nanotube transistor fabrication processes

The manufacturing yield of carbon nanotube transistors is very sensitive to changes in fabrication process parameters, while controlling length, density and orientation of nanotubes simultaneously is still proving elusive in batch fabrication processes. Here, we show an electrode design with a yield of up to 45% working transistors despite our batch fabrication process being based on randomly grown nanotubes. Transistor parameter distributions of 765 devices are shown, demonstrating the potential of our design for process monitoring and control.

The impact of Cr adhesion layer on CNFET electrical characteristics.

The effect of a Cr adhesion layer on the transfer characteristics of Cr/Au-contacted carbon nanotube field-effect transistors (CNFETs) based on individual single-walled carbon nanotubes (SWNTs) is presented in this paper. We show that a very thin Cr layer (≈0.4 nm) already has an impact on the carrier transport in Schottky-barrier-modulated CNFETs. The ratio of the p- and n-branch current is reduced by eight times when the Cr adhesion layer thickness is increased from 0 to 8 nm. We suggest a change in Schottky barrier height at the contact as the determining mechanism for this result. Additionally, superior lifetime of devices is observed even for non-passivated CNFETs with preserved clean SWNT/Cr/Au-contacts using Cr layer thinner than 2 nm. Our experiments show that the role of the adhesion layer in metal/nanotube contacts should be explicitly considered when designing CNTFET-based circuits, developing CNFET fabrication processes, and analyzing the corresponding properties of the electrical contacts.

Towards Internet of Things for event-driven low-power gas sensing using carbon nanotubes

One of most important applications of sensing devices under the Internet of Things paradigm is air quality monitoring, which is particularly useful in urban and industrial environments where air pollution is an increasing public health problem. As these sensing systems are usually battery-powered and gas sensors are power-hungry, energy-efficient design and power management are required to extend the devices lifetime. In this paper, we present a two-stage concept where a novel low-power carbon nanotube is used as a gas detector for an energy-consuming metal-oxide (MOX) semiconductor gas sensor. We propose a design of a heterogeneous sensor node where we exploit the low-power nanotube gas sensor and the more accurate MOX sensor. This work performs energy consumption simulations for three event-driven scenarios to evaluate the power consumption reduction, as well as the limitations of carbon nanotubes. Our results show the benefits of the proposed approach over the scenarios with adaptive duty-cycling with only MOX gas sensors, proved with 20%-35% node lifetime prolongation. The delay introduced due to the nanotube recovery time can be overcome by radio duty-cycled activity for detecting alarm messages from the neighbour nodes.