STUDY OF SUPERCONDUCTING NANOSTRUCTURE TOWARDS ITS APPLICATION AS SINGLE PHOTON DETECTOR

Abstract

This study is about fabrication and characterization of nanostructures towards its application as single photon detector. Since this type of nanostructure can detect infrared radiation up to 2000nm, special care needs to be taken for selection of its dimensions as well as substrate and superconducting material for the growth of thin film. Quantum optical technologies have many applications in the area of quantum teleportation, quantum computation (QC), quantum key distribution (QKD). These applications rely significantly on the performance of a single photon detector. Photo multiplier tube (PMT) is one of the initially invented photon detectors. Si based avalanche photodiode (APD) has better performance metrices in comparison to PMT. However, both these technologies have limitation at telecom wavelengths. Although InGaAs based photodetector shows better detection efficiency at infrared, it has its own shortcomings due to high dark count rate, relatively large time jitter, low count rate and after-pulsing effect. Superconducting nanowire based detectors have shown a lot of potential in recent years as single photon detector at the wavelength of interest for its application in the area of quantum optical technologies due to its better performance metrices. The thesis starts with the study of superconducting properties of materials in case of two dimensional structures as well as one dimensional structure. The superconducting nanowire single photon detector (SNSPD) device fabrication incorporates five main steps, i.e. (1) Device operation principle and modeling. (2) Customized low temperature experimental setup for transport property measurement as well as optical characterization of the sample. (3) Thin film fabrication and optimization using suitable superconducting material and its transport property measurement. (4) Nanostructure fabrication and optimization of its process parameters and its transport property.

vi (5) Optical characterization of the nanowires in meander shape using readout electronics in microwave frequency range. This thesis mainly focuses on first, second, third and fourth points as given above. Discussion on the device operation, modeling involves fundamental physics which help understanding the behavior of the device. The modeling of the device along with other components of the readout electronics has helped simulating the output response of the optical setup which can be verified experimentally in future. The overall fabrication steps of nanowires in meander shape have also been discussed along with design of contact pads in coplanar waveguide shape. The optical mask design for its use in fabrication of contact pads to save machine time of EBL is also discussed. It is obvious that detection and measurement of low level light signal is very difficult. When it comes to detecting single photon and that too in the infrared range, the difficulty level enhances due to lower photon energy. Detecting such weak signal with high detection efficiency requires low operating temperature in the range of mK to few Kelvin depending on the material used for the fabrication of detector. It is found that the detection efficiency of SNSPDs increases and the dark count diminishes significantly with decrease in operating temperature. An experimental setup down to 1.8K is designed using liquid helium flow cryostat along with probes for electrical as well as optical characterizations. Economical consumption of liquid helium, sound temperature stability, and efficient optical coupling, easy and user friendly samples changing option without breaking the vacuum, or warming up the cryostat were some of the important requirements taken into considerations while designing the cryostat. Sample holder with necessary arrangements for precise alignment of laser light with the active area of device to enhance optical coupling efficiency is designed for the optical probe. The INVAR alloy is used for sample holder to ensure that the alignment is not disturbed at low temperature. Single mode fibres due to its high transmission rate, minimum attenuation and least distortion have been used to shine light on the samples. Sample holder with 20-pin LCC socket and matching chip carrier provides convenient and fast sample mounting in electrical insert. Mu-metal is employed to cover the sample

space in both the inserts to attenuate any electromagnetic interference. Further, temperature stability with the passage of time is also monitored. It is found that variation in temperature is less than 10mK at lowest operating temperature. Apart from the above, another important advantage of the system is its very low liquid helium loss rate (~100ml/hr) with all inserts which allows uninterrupted measurements for several days without refilling of liquid helium. Further, the thesis includes fabrication of thin films, patterning of nanowire structure and transport studies both on thin films and nanostructures. The niobium nitride (NbN) films were deposited using reactive magnetron sputtering. Argon to nitrogen ratio played a crucial role in the synthesis of high quality superconducting NbN. Critical temperatures (Tc) of about 15.5K have been measured for films with a thickness of about 10nm. Zero-Field-Cooled (ZFC) magnetization measurements were carried out to optimize the superconducting properties in ultra thin NbN films. The transport behavior down to 2K was studied using conventional resistance vs. temperature and current-voltage characteristics. Fabrication of NbN superconducting nanowires based on focused ion beam milling (FIB) and electron beam lithography (EBL) is presented. The detailed study of fabrication of nanowires with optimized process parameters using EBL is presented. Effect of gallium contamination on superconducting properties is discussed. Superconducting transition temperature as well as the transition width of nanowires do not show any significant impact of processing steps of standard EBL route, there is a significant degradation of superconducting properties for nanowires prepared using FIB. Gallium ion implantation across the superconducting channel may be considered as the root cause for this. The effect of gallium implantation may be the reason of technological limitations in designing fascinating single photon detector architectures, nevertheless provides some interesting manifestation of intrinsic low dimensional superconducting properties.