Infrared detection and electron transport characteristics of carbon nanotubes/silicon heterodimensional heterostructure

In this thesis, we explore the underlying science and device potential of carbon nanotube arrays in infrared photodetection. In Chapter 2, we will present the physical formation of electronically functional heterojunction between carbon nanotubes (CNTs) and Silicon by controlled growth of vertical and highly ordered array of CNTs directly on a silicon substrate. The nanotubes grown this way proved to have uniform diameter, length, spacing and alignment. The spectral dependence of the photocurrent was measured successfully for the first time. The results allowed for an estimate of the band gap of the nanotubes, which is found to be in good agreement with that determined from an independent temperature dependent conductivity measurement. Mechanism of the infrared photocurrent response is elucidated by detailed current-voltage (I-V) measurement and analysis. The limiting factor for forward bias conduction is found to be the space charge limited current. The reverse current reveals quantum transport dominated phenomena, including a tunneling current that is made larger than the forward current under similar small biases due to the intensified field at the nanotube tip and to the consequent barrier lowering. Those effects are amplified by the unique hetero-dimensionality (3D-to-1D) of the CNT/Si junction. We also observed that the array of point contacts between CNTs and Si transform into an effective single planar contact with changing temperatures, another intriguing aspect of the system of the regular array of heterojunctions between a one-dimensional (1-D) narrow bandgap system and a three-dimensional (3-D) wider bandgap material.