A method of producing particles containing metal oxide for use in semiconductor devices includes the steps of heating metal-containing particles in a flame produced by a mixture of oxygen and a fuel component comprising at least one combustible gas selected from hydrogen and hydrocarbons, the oxygen being present in the mixture in a proportion of not less than 10 mole% below, and not more than 60 mole% above, a stoichiometric amount relative to the fuel component, so as to oxidize metal in at least an outer shell of the particles; cooling the oxidized particles by feeding them into a liquid or sublimable solid medium; collecting the cooled oxidized particles; and providing a distance between entry of the particles into the flame and collection of the particles of at least 300mm. In this manner, such particles may be oxidized so as to provide a shell of metal oxide material which leaving unoxidized a core of metal. A semiconductive layer of such particles on a substrate may be formed by feeding, to a hot zone, such preoxidized metal-containing particles; heating the metal-containing particles in the hot zone to render the particles at least partially molten; and depositing the particles in the at least partially molten state onto the substrate. The above oxidation process may be employed to provide metal oxide particles in which different respective metals having different respective valencies are present in different respective molar proportions. The valencies and molar proportions may be selected so as to provide n- or p-type semiconductor layers.