Probing the Orbital Levels of Engineered Fullerenic Molecules from a Nonvolatile Memory Cell
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Probing the Orbital Levels of Engineered Fullerenic Molecules from a Nonvolatile Memory Cell
Sarah Q. Xu1, Jonathan Shaw1 and Edwin C. Kan1 1
School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, USA.
ABSTRACT The Coulomb blockade behavior was observed for both C60-PCBM and C70-PCBM at room temperature utilizing a nonvolatile memory cell fabricated through a liquid-transfer process. Room-temperature and low-temperature (10K) electrical characterizations verified the blockade effect was originated from both molecular energy levels and single electron charging energy. Molecular orbital energy was extracted and shown good agreement with the literature [1].The successful integration and operation of this hybrid structure signified a strong potential for molecule-based electronic device design.
INTRODUCTION Engineered fullerenic molecules (EFM) are chemical derivatives of neat fullerene molecules with multiple functionalities. The mono-dispersed nanoscale size of EFM brings forth improved scalability and reduced device variations. The redox capability [2] and electrical conductivity [3] of EFM are notably different from pristine fullerenes and offer more flexibility in tailoring the fabrication process and device characteristics. Most importantly, chemical functionalization in EFM alters the electronic structure of the molecule, creating programmable HOMO-LUMO levels [4] which are crucial for designing resonant tunneling barrier for Flash memory to overcome the scaling bottleneck [5]. This chemical derivation also grants EFM large solubility at room temperature that enables wafer-level fluid-transfer process, which may ease both the process control and manufacture cost in the case of commercialization. EXPERIMENT A Metal-Oxide-Semiconductor (MOS) capacitor structure in the gate stack of conventional flash memory cell [6] was fabricated and electrically characterized. The experimental splits are listed in Figure 1. The device was fabricated on top of a 4” p-type silicon substrate, with a 3nm thermally grown tunnel oxide. Floating gates for charge storage including design splits of two different EFM species were formed on top of the tunnel oxide.
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Figure 1. (a) Design splits of the EFM gate stack: S1 is the control sample, S2 contains C60PCBM floating gate, and S3 contains C70-PCBM floating gate. (b) Chemical structure of the C60PCBM molecule. (c) Chemical structure of the C70-PCBM molecule. All EFMs used in the experiment were chemically synthesized at Nano-C Inc and received as powder form with over 99% purity. Sample S1 is the control device without any embedded EFM. Samples S2 and S3 integrate fluid-transferred C60-PCBM and C70-PCBM through the room-temperature spin-coating method using toluene as the solvent. The pristine C60 and C70 are not included in the control samples due to the low solubility. The initial spin-coating recipe was developed based on the model of Newtonian liquid on a rotating disk [7], and later improved through experimental trials. The targeted c
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