Spin Dynamics of Low Sodium Type-II Silicon Clathrates

In this work we examine the relaxation and coherence times of Na in silicon clathrate thin films for potential use in quantum applications. Diamond silicon (d-Si) is under active investigation as a quantum material motivated to a large extent by the depth of knowledge surrounding this material and its dominant position in microelectronics. Type II silicon clathrate represents an interesting alternative Si crystal structure. This cage-like inclusionary compound is made up of a silicon lattice with interstitial “guests” such as sodium situated inside the cages. The Na atoms, which are decoupled from the lattice, act as shallow donor atoms and are potential qubits. In addition, unlike d-Si, type II Si clathrate has a direct or quasi-direct bandgap which may allow easier optical access to Na electronic states which makes it of interest as a potential material for microelectronics, LEDs, and solar cell applications.<br/>Most reported studies of silicon clathrate have utilized powders. Powders, however, are not the preferred form factor for optoelectronic studies or transport measurements and the measurements reported here use low Na content, type II Si clathrate films. To synthesize these materials, a two-step procedure adapted from powder synthesis has been developed. First, sodium is evaporated from a crucible and diffused into a silicon wafer under an inert gas atmosphere to form NaSi. The wafer is then annealed under vacuum to thermally decompose the film into clathrate. The clathrate films are then characterized through a variety of techniques including x-ray diffraction, Raman spectroscopy, scanning electron microscopy, photoluminescence, and, of particular importance to this study, continuous wave and pulsed electron paramagnetic resonance (EPR). EPR gives insight into the electronic properties of the Na donors and their placement and interactions within the silicon cages. The naturally occurring Na isotope, ²³Na, has nuclear spin 3/2 with the EPR spectrum exhibiting four hyperfine lines associated with the interaction of the electronic and nuclear spins. Hyperfine features associated with Na atoms in neighboring cages and clustered Na are also observed as well as a dangling bond signature indicative of a minority phase of disordered Si. Pulsed-EPR spectra clearly exhibit spin echo signals with T1 times in the microsecond regime at temperatures near 10K. By probing the spin echo’s of specific Na hyperfine interactions, we have gained insight into the relaxation and coherence times of electron spins bound to the Na of low Na doped type-II silicon clathrates. The effects of various parameters (i.e. temperature, pulse duration, magnetic field center) on the relaxation and coherence times are also explored. Results of this work provide new understanding of the spin properties of Na in Si clathrates and provides useful insights into the potential use of Na in Si clathrate as a qubit material. This work was supported by National Science Foundation award #2114569.