Er implanted Si (Er:Si) is a promising platform for quantum networking applications, but a major obstacle is the formation of multiple Er centres. Here, we have developed an annealing technique, referred to as deep cooling (DC), where the quench rate is ~1000 °C/s, compared to ~100 °C/s for standard rapid thermal annealing (RTA). We implanted Si with Er and O (each 1019 cm-3) and annealed by DC or RTA. From crystal field analysis of photoluminescence spectra and comparisons to extended X-ray fine structure (EXAFS) measurements, we show that all know Er centres in Er:Si can be arranged in a sequence ranging from entirely Si coordinated, to mixed Si and O coordination, to entirely O coordinated. By changing only the quench rate, the Er centre changes from entirely O coordinated to mixed Si and O coordination. For quantum networking applications, having only the Si coordinated Er centre is essential, and our high quench rate annealing process can push the centre formation towards the required greater Si coordination.