Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI $^{125}$Te$^+$ donors implanted into natural Si at depths of 20 and 300 nm. We examine spin activation yield, relaxation ($T_1$) and coherence times ($T_2$) and show how a zero-field 3.5 GHz `clock transition extends spin coherence times to over 1 ms and narrows the inhomogeneous spin linewidth to 0.6 MHz. We show that surface band-bending can be used to ionise Te to spin-active Te$^+$ state, and that coherence times of near-surface donors are comparable to the bulk. We demonstrate initialization protocols using optical illumination to generate excess Te$^+$. These results show that $^{125}$Te$^+$ is a promising system for silicon-based spin qubits and ensemble quantum memories.
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