Cosmin Roman

Hysteresis-free operation of suspended carbon nanotube transistors

Single-walled carbon nanotubes offer high sensitivity and very low power consumption when used as field-effect transistors in nanosensors. Suspending nanotubes between pairs of contacts, rather than attaching them to a surface, has many advantages in chemical, optical or displacement sensing applications, as well as for resonant electromechanical systems. Suspended nanotubes can be integrated into devices after nanotube growth, but contamination caused by the accompanying additional process steps can change device properties. Ultraclean suspended nanotubes can also be grown between existing device contacts, but high growth temperatures limit the choice of metals that can be used as contacts. Moreover, when operated in ambient conditions, devices fabricated by either the post- or pre-growth approach typically exhibit gate hysteresis, which makes device behaviour less reproducible. Here, we report the operation of nanotube transistors in a humid atmosphere without hysteresis. Suspended, individual and ultraclean nanotubes are grown directly between unmetallized device contacts, onto which palladium is then evaporated through self-aligned on-chip shadow masks. This yields pairs of needle-shaped source/drain contacts that have been theoretically shown to allow high nanotube-gate coupling and low gate voltages. This process paves the way for creating ultrasensitive nanosensors based on pristine suspended nanotubes.

Long term investigations of carbon nanotube transistors encapsulated by atomic-layer-deposited Al2O3 for sensor applications

Single-walled carbon nanotube field-effect transistors (CNFETs) are promising functional structures in future micro- or nanoelectronic systems and sensor applications. Research on the fundamental device concepts includes the investigation of the conditions for stable long term CNFET operation. CNFET operation in ambient air leads to on-state current degradation and fluctuating signals due to the well-known sensitivity of the electronic properties of the CNT to many environmental condition changes. It is the goal of device and sensor research to understand various kinds of sensor-environment interactions and to overcome the environmental sensitivity. Here, we show that the encapsulation of CNFETs by a thermal atomic-layer-deposited (ALD) aluminium oxide (Al(2)O(3)) layer of approximately 100 nm leads to stable device operation for 260 days and reduces their sensitivity to the environment. The characteristics of CNFETs prior to and after Al(2)O(3) encapsulation are comparatively investigated. It is found that encapsulation improves the stability of the CNFET characteristics with respect to the gate threshold voltage, hysteresis width and the on-state current, while 1/f noise is lowered by up to a factor of 7. Finally, CNFETs embedded in a dielectric membrane are employed as pressure sensors to demonstrate sensor operation of CNFETs encapsulated by ALD as piezoresistive transducers.

Pulsed gate sweep strategies for hysteresis reduction in carbon nanotube transistors for low concentration NO2 gas detection

Carbon-nanotube-based field effect transistors (CNFETs) have been employed as highly sensitive chemical sensors. Often used as the sensor output signal, the gate threshold voltage (V(th)) is subject to concentration-dependent shifts upon exposure to target analytes. However, an unambiguous determination of the intrinsic V(th) is usually hampered by substantial hysteresis in CNFET gate characteristics. In this study we show that short gate voltage (V(gd)) pulses can be used for hysteresis reduction in CNFETs as chemical sensors, in particular for NO(2) detection. In the pulsed operation regime, even small shifts of V(th) upon sub-ppm NO(2) exposure remain resolvable. Furthermore, the hysteretic behaviour is systematically investigated by varying the pulse waveforms and timing parameters. Finally, we use an adapted hysteresis model for pulsed V(gd) and employ it to discuss the measurement data.

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.

Sub-ppm NO2 detection by Al2O3 contact passivated carbon nanotube field effect transistors

We investigate carbon nanotube field effect transistors (CNFETs) with aluminum oxide (Al2O3) passivated contacts for NO2 detection. For the CNFETs, consisting of one individual pristine single walled carbon nanotube (SWNT), the measurements indicate a strong influence of adsorbed NO2 gas molecules on the exposed CNFET channel and NO2 concentrations as low as 100 ppb were detected. Applied to the contact-SWNT interfaces, Al2O3 is a suitable material to protect the metal contacts from NO2 molecules and other undesired environmental influences. We discuss the effect of the different processing steps on the CNFET characteristics and show device recovery after short heat treatment.

Signal-to-Noise Ratio in Carbon Nanotube Electromechanical Piezoresistive Sensors

The signal-to-noise ratio (SNR) of piezoresistive transducers based on carbon nanotube field effect transistors (CNFETs) is an essential yet unexplored performance metric. Here, we show that the SNR of CNFET piezoresistors made of small band gap semiconducting SWNTs (SGS-SWNTs) depends strongly on the gate bias voltage. The SNR is found by combining low frequency 1/f noise with the piezoresistive signal. We find that SGS-CNFET piezoresistors are best operated at device off-state, where strain resolution is maximal.

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.

Ultra Small Single Walled Carbon Nanotube Pressure Sensors

This paper reports on the fabrication and characterization of ultra small pressure sensors with individual single-walled carbon nanotube (SWNT) field effect transistors (CNFETs) as strain gauges. The smallest piezoresistive pressure sensor with a membrane diameter of d~40¿m is fabricated and characterized. This miniaturization is made possible due to the nanoscaled size, electronic properties, and high piezoresistive gauge factors (GF) of SWNTs and is currently limited by the membrane fabrication capabilities using a 200¿m thick Si wafer. In summary the sensor performance is: Gauge factor: ~450 to 700 (strain dependent), sensitivity: -54pA/mbar (Vds=200mV), resolution: 15mbar, power consumption: ~100nW.

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.

Gauge Factor Tuning, Long-Term Stability, and Miniaturization of Nanoelectromechanical Carbon-Nanotube Sensors

Integrated piezoresitive strain gauges are established transducers for measuring displacements in microelectromechanical systems (MEMS). Due to large gauge factors (GFs) and low power operation and nanometer dimensions, carbon nanotubes (CNTs) are ideal candidates for further downscaling strain-gauge-based MEMS devices. Here, we present zero-level packaged strain gauges based on individual single-walled CNTs in a field-effect transistor configuration, which can be utilized as long-term stable and tunable transducers for measuring membrane deflections in ultraminiaturized pressure sensors. The gate electrode allows adjusting GFs of nanotube strain gauges by almost a factor of 10. Studies on nanotube segments of different lengths show highly reproducible GFs along the same CNT. The zero-level packaged pressure sensors show stable GFs over a period of at least 14 months. This paper is an important step toward reliable nanoscaled strain gauges with many potential applications, such as ultraminiaturized pressure-sensitive membranes or cantilever-based transducers.