Ultra-thin Niobium Nitride Films for Hot Electron Bolometer and THz Applications
Licentiate thesis, 2016
The part of the electromagnetic spectrum between microwaves and infrared, also known as the terahertz band, is of particular interest for radio astronomy. The radiation intensity of the cold universe peaks at this frequency band, thus defining the demand on sensitive low-noise instruments in this particular frequency range. Phonon-cooled hot electron bolometers based on niobium nitride (NbN) thin films have been demonstrated for the first time in 1990 and evolved since then to the technology of choice for frequencies above 1.2 THz. At those wavelengths their noise temperature is typically between 5 to 10 times the quantum noise hence outperforming any other heterodyne receiver technology. However, the main concern of HEBs is there limited intermediate frequency (IF) of, in practice a few GHz, which is not sufficient for certain astronomical tasks requiring, e.g. 4-12 GHz of bandwidth. The cooling rate of “hot” electrons translates directly into the IF bandwidth of such devices and is intrinsically determined by the quality and thickness of the used NbN film and the substrate, which serves as a heat sink in these phonon-cooled devices .
This thesis deals with the development of NbN films particularly for the use in hot electron bolometers, and is addressing the optimization of superconducting properties of the NbN ultra-thin films. A particular emphasis was put on the influence of the underlying substrate. The suitability of hexagonal AlxGa1-xN as a buffer-layer for the NbN film growth was demonstrated for the first time and enabled the tuning of the superconducting properties by changing the Al content x. Single crystal NbN with 5 nm thickness has been grown onto GaN with Tc of 13.2 K, very narrow superconducting transition width and residual resistivity ratio (R20K/R300K) close to unity. The critical current density Jc was determined to be 3.8MA/cm2, compared to 1.2MA/cm2 of a high quality NbN film deposited onto a silicon substrate.
The GaN buffer-layer is also believed to result in improved acoustic matching compared to commonly used substrates such as silicon or MgO, thus possibly enhancing the IF bandwidth. A first mixer experiment, where the HEB was operated in the bolometric mode at 180 GHz and at elevated bath temperatures, showed clearly that the measured IF roll-off, which is associated with the phonon escape time in this mode of operation was increased almost by 80% while the GaN buffer-layer was used as compared to a bare silicon substrate.