The authors report the successful development of a method stabilising the fundamental transverse mode of a vertical-cavity surface-emitting laser using a low-temperature-deposited amorphous GaAs (a-GaAs) layer of a high refractive index. For a device of 10 mu m diameter, stable fundamental mode emission has been observed over a wide range of current. Results are attributed to the antiguide effect of the a-GaAs-buried structure.