We have found that arsenic implanted into SiO2segregates at high temperatures into As‐rich spherical inclusions of 50–500 A˚ in diameter, provided that there is no free oxygen in the SiO2and the initial As concentration exceeds 1 at. %. The phase separation suppresses the diffusion of arsenic, even at temperatures as high as 1400 °C. We have discovered, however, that the As‐rich inclusions can be easily moved in a temperature gradient. They migrate towards the heat source at a rate of 2300 A˚/h in a gradient of 0.14 °C/&mgr;m, at 1405 °C, permitting their efficient removal from the oxide and into silicon.