QUANTUM EFFICIENCY OF ASTRONOMICAL CHARGE COUPLED DEVICES FOR DEEPER EXPLORATION OF OUR COSMOS

Abstract

When one looks up at the sky and counts the number of stars and observes constellations, they are not only appreciating the beauty of the universe but doing an astronomer’s job as well. Astronomy is the oldest science that humans have dealt with, and it is rather the simplest science that can be done with the naked eye. Our quest to understand the nature of our existence started with the simple observation of celestial objects. With time humans were able to solve a lot of such mysteries and as we move on to the next job, it becomes more complex. In today’s astronomical scenario, this quest is driven by the advancement in the technology used for observing our universe. Charge-coupled devices (CCDs) are the backbone of all space study missions today, such as Gaia, Hubble Space Telescope, James Webb Telescope, etc. They are the basic piece of machinery used by any space mission to capture the light in space and help us create images that can be studied on Earth. Their functionality is what makes them an imperative instrument whose efficiency needs to be as high as possible. Hence this project is focused on finding ways to enhance ⅵ the quantum efficiency of Gaia Astrometric Field (AF) CCDs by altering some of their structural parameters. CCDs lose a lot of incident photons due to optical losses. To reduce the loss anti-reflection (AR) coatings are applied on the substrate. Currently, hafnium dioxide (HfO2) is being used as an AR coating for the Gaia AF CCDs. Through our studies, we contemplated alternative AR coatings that could increase the QE of the CCDs. Aluminium oxide (Al2O3), zinc sulphide (ZnS), zirconium dioxide (ZrO2), and tantalum pentoxide (Ta2O5) are some promising materials that have been selected and tested by us for the Gaia AF CCDs. Further, gallium nitride (GaN) has been analysed as an alternative to silicon for its use in broadband astronomical CCDs. The optical and electronic performance of GaN and silicon CCD pixel models have been compared for this purpose. Simulations have been conducted using the SILVACO TCAD software to test these coatings on the Gaia pixel structure. The Monte Carlo method has been implemented by SILVACO for these simulations. ATLAS module of the SILVACO software is employed for the model. Upon contemplation, two AR coatings (Ta2O5 and ZrO2) turn out to produce better results than HfO2. They give better QE towards the lower end of the Gaia AF CCDs’ operational spectrum (from 0.330 µm to 0.575 µm) and prove to be better AR coatings for broadband astronomical CCDs. GaN also proves to be substantially better than silicon for use in such devices. These studies will open new avenues for understanding the evolution of the Milky Way and our universe.