- Physicists who study semiconductor devices are fascinated by the fundamental limits of device performance. From the sub-threshold swing of transistors to the power conversion efficiency of photocells, performance is limited by the electronic structure of the materials used to build them. To surpass traditional device limits, we must turn to untraditional materials.
Nanomaterials, in which electrons are confined to ≲ 100 nm in at least one dimension, have remarkable optical and electronic properties. The Coulomb interaction energy scale in nanomaterials can exceed the scales of quantum kinetic energy and the thermal energy at room temperature, giving rise to interesting electronic phenomena that are absent in materials with weak interactions. Strong interactions in nanomaterials may open pathways for novel photocurrent generation processes in photovoltaics or electronic-phase-change mediated transistor conductance, perhaps enabling devices that can break traditional performance limits. Electronic phenomena in nanomaterials are complex and the theory for describing them is not yet complete. More experiments are needed to lay the foundation of next-generation, high-performance devices.
Carbon nanotubes (CNTs) are a rare, real-world, one-dimensional electronic system that exhibit strong interactions between charge carriers and offer a unique opportunity to study interaction-driven optoelectronic phenomena. CNTs exhibit a rich variety of behavior even at the level of non-interacting electron theories. A CNT can be metallic or semiconducting depending on its chirality, the specific arrangement of carbon atoms into the nanotube structure. Subtle differences between chiralities also affect the optical absorption spectrum, the quantum interferences in metallic CNTs, and the relationship between mechanical stress and band gap. While non-interacting physics provides a reasonable first-order description of CNT properties, interactions modulate the electronic structure significantly. For example, electron-hole attraction changes optical transition energies due to exciton formation, which dominate the absorption cross section of semiconducting and metallic CNTs at room temperature. Electron-electron repulsion increases band gaps and creates an energy gap in metallic CNTs that can be hundreds of millielectronvolts.
In this thesis, the interplay between structure and interactions is studied experimentally in devices made with ultraclean CNTs. CNTs in these devices are suspended off substrate, leaving them free of electrostatic disorder and limiting dielectric screening from the substrate, maximizing Coulomb interactions between charge carriers. The CNT bridges the gap between two electrodes above dual gate electrodes, which can electrostatically dope the CNT. The voltage applied to the gates can be tuned together to either operate the CNT as a field effect transistor, or to generate a pn junction in the CNT. This device platform allows for multiple styles of optoelectronic measurements with the same CNT.
For the first time, metallic CNTs are chirally identified with spectral photocurrent measurements. Transport measurements are carried out on the same, chirally identified, CNTs. Our collaborators at University of Utah performed low temperature (~ 1 kelvin) quantum transport measurements on our ultraclean CNT. These measurements revealed electronic Fabry-Perot and Sagnac quantum interference that had not been previously measured in CNTs with known chirality. To realize this Sagnac effect, interference between non-identical transmission channels in a single CNT must be observed. A Landauer transport formulation is applied to understand the role the CNT’s atomic structure in creating these non-identical transmission channels.
The nature of the energy gap in “metallic” CNTs (m-CNTs) remains mysterious; it has been attributed to Mott physics, driven by close range electron-electron repulsion, or an excitonic-insulator state, driven by spontaneous exciton formation mediated by long range, electron-hole attraction. To further understand this strongly-interacting system, we performed electronic measurements of suspended m-CNTs with known diameter and chiral angle. Spectrally-resolved photocurrent microscopy was used to determine m-CNT structure. The room-temperature electrical characteristics of 18 individual-contacted m-CNTs were compared to their respective diameter and chiral angle. At the charge neutrality point, we observed a peak in m-CNT resistance that scales exponentially with inverse diameter. Using a thermally-activated transport model, we estimated that the transport gap is 450 meV∙nm/D where D is CNT diameter. We find no correlation between the gap and the CNT chiral angle. Our results add important new constraints to theories attempting to describe the electronic structure of m-CNTs.
CNT photodiodes are a promising system for high-efficiency photocurrent generation due to the strong Coulomb interactions that can drive carrier multiplication. If the Coulomb interactions are too strong, however, exciton formation can hamper photocurrent generation. In this thesis, we explore, experimentally and theoretically, the effect of the environmental dielectric constant (εenv) on the photocurrent generation process in CNTs. We study individual ultra-clean CNTs of known chiral index in both air/vacuum (εenv = 1) and oil (εenv = 2.15). The efficiency of photocurrent generation improves by more than an order of magnitude in oil. Two mechanisms explain this improvement. First, the refractive index of the environment optimizes the interference between incident and reflected light. Second, exciton binding energies are reduced in oil, changing the relaxation pathways of photoexcited carriers. We varied the axial electric field in the pn junction from 4 to 14 V/μm. Our measurements at high field indicate that autoionization of second-subband excitons can coexist with carrier multiplication. Dielectric screening makes this coexistence regime more accessible and allows us to reach photocurrent quantum yields greater than 100%.