Ph.D Thesis


Ph.D StudentZeevi Gilad
SubjectElectrostatic Interactions and Charge Transfer between
Carbon Nanotube and its Environment
DepartmentDepartment of Electrical and Computers Engineering
Supervisor DR. Yuval Yaish
Full Thesis textFull thesis text - English Version


Abstract

Since the discovery of carbon nanotubes (CNTs) by Iijima et al. in 1991, an extensive research was conducted by the scientific and technological communities due to their extraordinary electrical, optical, thermal, and mechanical properties. Regardless of the structure or application the CNT will be incorporated in, the nearby environment will play a significant role on the CNT’s function and operation.
In the first part of this research, we use a single, small bandgap CNT incorporated in a field effect transistor (FET) structure with a thin HfO2 as gate dielectric (6nm). A substantial hysteresis is measured in our devices and its principal contributor originates from charge injection from the CNT into the gate dielectric. We used short gate-voltage (VG) pulse-train to study the dynamics of this charge transfer for both positive and negative VG, and further extend our analysis to estimate the trap density inside the HfO2. We have also investigated the recovery of the charge trapped inside the HfO2 dielectric and found substantial retention which may indicate that traps and defects in the dielectric layer should be considered in CNT based memory cells. In the second part of the research, we use the ability of trapping and de-trapping of charge inside the gate dielectric for achieving electrostatic doping of the CNT. Together with our double gated CNTFET (local and global gate) we manage to produce intramolecular PN junction along the CNT. The PN junction is a fundamental building block in electronics, and the application of PN junction in a 1D semiconductor is a step towards optimal device miniaturization. As far as we aware this is the first time this type of electrostatic doping (charged traps - very close to the CNT) was utilized for preparation of intramolecular PN junction along a CNT. Our results confirm a large band to band tunneling (BTBT) through the PN junction. The BTBT is so large that the total device conductance is dominated by the sum of conductance of the individual CNT segments. The last part of this research includes a study of the light induced charge transfer (LICT) between CNT and semiconducting nanocrystals (SC-NCs). Here we present a hybrid device consist of CdSe/CdS - core/shell SC-NCs and CNTs in a FET structure. The SC-NCs demonstrate large flexibility in their bandgap (size and material dependent) with excellent optical response, while
the CNT contribute High electrical mobility and exceptional electrostatic sensitivity. We investigate the effect of gate voltage on the LICT. We use electrical measurements that probe the effect of light on the drain-source current (IDS). As far as we know this is the first attempt to describe the LICT dependence on electrical stress. By fitting the data to physical-based charge transfer dynamics equation we extract characteristics parameters of the process. We further compare the LICT of four different CdSe/CdS NCs (different dimensions and sizes). Our results show large optical effect on the IDS of the hybrid device. This effect is reversible and
can be tuned
by application of VG.