Carbon nanotubes are a unique material that can be either metallic or semiconducting, usually with a small bandgap inversely proportional to tube diameter and with interesting optical properties. However, their general randomness in length, diameter, and chirality, and the challenges in aggregating sufficiently large quantities of precisely uniform nanotubes, render its applications in optical detection so far unattainable beyond simple absorptive coating. The highly-ordered carbon nanotube array, as grown by the non-lithographic methods described here, surmounts many of these obstacles while presenting a geometry that is useful for focal plane array applications. A nanoporous alumina template assists the nanotube growth, which proceeds by carbon vapor deposition in a technique that is compatible with integration on silicon. We report on the experimental treatment of one possible platform for applying carbon nanotubes in infrared detection: a heterojunction photodiode with silicon. The nanotube-silicon heterojunction has rectifying characteristics that are consistent with silicon doping type, nanotube work function, and silicon-nanotube bandgaps. We investigate this hybrid nanostructure with spectral photocurrent measurements in the near and mid-infrared regime in both cooled and uncooled modes of detection. Transient photocurrent analysis suggests that both pyroelectric and direct optoelectronic effects are sources of photoresponse. First-principle theoretical treatments of nanotube-silicon heterojunction detection imply that performance parameters such as D* could be greatly optimized in future generations of samples. We explore the suitability of this detector prototype for spaceborne applications where many known properties of carbon, such as chemical and mechanical durability as well as strong covalent bonding and therefore radiation hardness, merit its consideration.
We will present our advance in the utilization of a non-lithographic approach for formation of periodic nanosized arrays and formation of hybrid structures suitable for light detection. We explore a self-organization process for formation of periodical nanopores in anodized aluminum oxide, the transfer of this pattern, and the subsequent growth within the pores. This approach was successfully demonstrated for a system having carbon nanotubes as kernel. The carbon nanotubes by themselves are very attractive for detector applications. It is theoretically predicted and experimentally proven that their band gap is adjustable in broad spectral range, their charge carrier mobility is high, and their thermal and mechanical properties are unmatched by other materials. The nanotemplate we use for growth of the nanotubes allows their controlled placement in a regular array, without restriction of the curvature of the surface to be covered. There are no principal limitations for scaling of the process. The third element of our approach is the integration with silicon which provides the compatibility with the well elaborated silicon technology. We will demonstrate the suitability of these structures for light detection.
Carbon nanotubes are found to have versatile properties ranging from exceptional mechanical strength to semiconductor behavior with varying band gap, ranging from 0 eV to ~1 eV. In this work, we have explored the applicability of the carbon nanotubes for optical and IR sensing. Our platform is a vertical integration of highly uniform carbon nanotube arrays with silicon, forming a heterojunction structure. The heterojunction structure exhibits very good current rectification, voltage dependent capacitance and photocurrent response, all suggestive of an electronic diode function. However, we found, that the photogeneration in the first generation test devices is so far dominated by the silicon part of the heterojunction. Of the possible reasons for the elusive mid-IR photocurrent expected in the carbon nanotube is the presence of a thin barrier layer at the heterojunction. Further optimization of the devices is possible by modifying the technology to avoid the barrier layer formation and by improving the quality of the aluminum oxide matrix.