Surface States and Doping in Aluminum Prototypes for NASA Detector Development

In NASA astrophysics missions, particularly at long wavelengths in the far infrared to microwave, extremely sensitive detectors are required to measure the faint signals from astronomical sources. In order to avoid thermal fluctuation noise, the detectors typically need to operate at sub-Kelvin temperatures, in which case superconducting detectors can be used to achieve high sensitivity even for low energy photons. We will focus this proposal on two types of superconducting photon detectors: microwave kinetic inductance detectors (MKID) and transition-edge sensors (TES). Both detector technologies are used in current NASA missions, and NASA is working to improve both technologies for instruments in future missions, such as the Origins Space Telescope. Aluminum is an important low temperature detector material because of its well understood superconducting properties. However, there are a few scientific challenges that are problematic to repeatable control of these systems, which can be answered by the team at West Virginia University. Holcomb, Johnson, Romero and Bristow form the ideal team for this work based on their expertise and already established research experience together and with NASA’s Detector Development Lab. One of these challenges is the formation of various phases of oxides and fluorides on the superconducting metal surfaces and nearby dielectric surfaces during detector fabrication. Two Level System (TLS) defects in those disordered dielectrics add noise and reduce the responsivity of MKIDS. The oxides and fluorides formed can be studied and characterized in new ways through x-ray photoemission, x-ray absorption and ultrafast optics. Initial work demonstrates high sensitivity to these phases. Another challenge is that you would like to be able to tune (i.e. engineer) the superconducting transition temperature (Tc) of the material used in these photon detectors so that the sensitivity can approach the photon noise background limit for the range of optical load powers and optical frequencies encountered in a particular space mission application. Lowering the Tc of a TES bolometer helps optimize it for lower optical powers and lower photon energies. Lowering the Tc of an MKID lowers the minimum photon frequency to which it responds. Adding a dilute solution of manganese atoms to pure aluminum is known to decrease its Tc, without unfavorably broadening the shape of the resistivity versus temperature curve, and this effect has been used in both MKID and TES detectors. However, testing has demonstrated that sometimes the resulting transition temperature can change upon annealing. It is hypothesized that the Mn upon annealing may be clustering, which could be determined spectroscopically through x-ray absorption and ultrafast optics. To address these challenges, the proposed program will identify the alternative phases on these Al devices, determine the effect of annealing and film thickness on Mn ordering in doped systems, reveal the underlying physics by means of ultrafast optical characterization, and utilize theoretical methods (such as density of states, band structure and vibrational spectroscopy) to identify the lowest energy states and the nature of the Mn clustering. All objective findings will be correlated to device performance. By understanding the alternative phases and Mn ordering in these systems, the detectors can be better engineered, allowing reliable, high resolution astrophysics measurements. Relevant NASA MD: Science Mission Directorate Relevant NASA Center: NASA Goddard Space Flight Center

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