Moritz Mattmann

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.

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.

Fully integrated detection of single magnetic beads in complementary metal-oxide-semiconductor

Microcoils are integrated with n-well Hall sensors in a complementary metal-oxide-semiconductor (CMOS) integrated circuit for the detection of individual superparamagnetic beads. The 4.2-μm-wide microcoils, generating magnetic fields of 800μT with 10mA of current, are used to polarize beads of 2.8μm in diameter. The resulting 10.8μT magnetization field induced in the bead is measured by a 4.7μm Hall sensor, stacked below the microcoil. The detection system has a sensitivity and resolution of 34V∕AT and 300nT∕√Hz, respectively. Integration of the magnetic bead detection onto a CMOS platform drastically reduces system cost, complexity, and power consumption, and marks an important milestone on the road to implementing low-cost, easy-to-use, point-of-care diagnostic assay.

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.