Sharma, Pranav (2015-12). Fabrication and Characterization of Al / AlOx / Al Josephson Junctions. Master's Thesis.
This thesis works deals with the fabrication and characterization of Josephson Junctions. An upgrade over the existing fabricating process with the aim of integrating Josephson Junctions with hetero-structure based HEMT devices was the main aim of this research work. In order to achieve this, new fabrication technique with aim of fabricating small area overlap junction is presented. Josephson Junctions are a Metal-Insulator-Metal capacitor, with both of its electrodes as superconductor metals having femto Farad capacitance and low resistance. The current transfer mechanism across very thin junctions was discussed and the related tunneling current equations were deduced after extracting the equations from the Wentzel-Kramers-Brillouin (WKB) approximation. These junctions are prone to various kinds of tunneling current phenomenon. A relationship between the room temperature tunneling current for a Metal-Insulator-Metal (MIM) capacitor and that to the resistance of the Josephson Junctions above its critical temperature was derived. A new fabrication method consists of a side-wall overlap area was presented in order to fabricate these Josephson Junctions. The dielectric / insulator layer was also deposited using a relatively new approach of Atomic Layer Deposition technique. Using this new fabrication approach, Josephson Junctions with the small area overlap, lower capacitance and resistance values were obtained. Capacitances for these devices were measured using an E4980 Agilent Precision LSR meter system with a 2 point probe setup. The same setup was used in order to obtain Current (I) vs. Voltage (V) sweep values for the Josephson Junctions, in order to identify the tunneling mechanism occurring in the junctions. Barrier height and effective mass of the dielectric were calculated from these tunneling plots. It was observed that these junctions had an area overlap of ~ 0.75 ?m^2 with capacitance ~ 100 fF and resistance ~ 150 Ohm ?m^2. These values where in the same range to the value from literature which utilized the dolan bridge fabrication technique. Hence, this concluded that the step-edge technique is better approach towards fabrication of small overlap Josephson Junctions with the aim of integrating them with HEMT based devices.