J. Iyer, P. Comita, D. Cooke, M. Cheponis, M. McAuliffe, L.H. Cooke
Keywords: horizontally suspended carbon nanotubes, multi-walled, end-on contacts, solar devices
Summary:NovaSolix, a silicon valley based start-up, is developing antenna technology based solar devices used to capture solar energy with twice the efficiency at 20% of the cost and 20% of the weight per watt compared to current photovoltaic technology based products. In the NovaSolix solution, individual multi-wall carbon nanotubes (CNT) are grown into arrays of tiny antennas that are suspended between Aluminum ground/contact lines to maximize gain for the rectified light. The carbon nanotube antennas in the NovaSolix device are in the 80-650nm length range to effectively utilize the entirety of the solar spectrum from ultraviolet to near-infrared frequencies (solar spectrum wavelength range: 300-2500nm) for power generation. End-on contact between carbon nanotubes and aluminum fingers on the contact line results in metal-oxide-carbon diodes with nm2 size and an expected cut-off frequency in THz range. The extensive literature on various methods used to grow horizontally aligned carbon nanotubes typically involve post-growth processing to fabricate devices. However, making end-on contacts to carbon nanotubes through post-growth processing techniques at scale remains a challenge. Here, NovaSolix chip design and carbon nanotube growth process that simultaneously addresses both these challenges - (1) developing arrays of horizontally aligned, independent, individual multi-walled carbon nanotubes and (2) forming scalable contacts to each of these individual carbon nanotubes in the array - to enable in-situ fabrication of solar devices is presented. Optimization of growth parameters for synthesizing multi-walled carbon nanotubes that are suspended over 0.5-1.4µm trenches formed by inter-digitated Aluminum lines, one of the primary steps in building the NovaSolix solar device, is the focus of this presentation. The effect of temperature, NH3/C2H2 gas ratio and Ni catalyst layer in controlling the number of CNTs that bridge the trench is discussed. Use of pulsed acetylene flow along with an in-situ electrical resistance measurement technique to control end-point of carbon nanotube growth is presented. Characterization of the fabricated devices by Scanning electron microscope analysis and electrical resistance measurements is presented.