One of the most important requirements for a spin-based quantum computer [1] is a long spin coherence time in comparison to time necessary for computations. To prolong the quantum superposition, the spin must be isolated from its environment, however this is done at the expense of ability to control the system. Donors implanted in silicon have been demonstrated to offer a solution for easy qubit manipulation [2,3], with coherence times of several seconds [4]. This is typically achieved in doped bulk crystals of silicon. On the other hand, implanting donors very close to the surface (at the depth of hundreds of nanometres) is highly attractive for functional quantum technologies. However, structural defects arising during implantation may limit the number of active spins and shorten their coherence time. Some structural damage could be repaired during annealing [5], but essential parameters for this process are not well studied.
Here, we search for optimal annealing parameters of tellurium donors at the silicon surface. We test samples of different implantation depth by measuring their coherence times and active spin concentrations using electron spin resonance. We base our donor spin concentration estimates on reference sample of silicon doped with phosphorus.