therefore goes in an undesired direction although it is conceivable for the electrons liberated during oxidation to be removed from the crystal it is nevertheless particularly easily practicable and therefore advantageous to use the present method of assisting the oxidation by applying thermal energy and an electric field to the crystal furthermore an embodiment of the method according to the present invention can be used particularly advantageously in combination with the method described in de 10 300 080 a1 in this case foreign atoms are initially selectively introduced into the crystal by doping prior to carrying out the method of assisted oxidation thus the foreign atoms are doping elements it is also advantageous that the assisted oxidation according to the present invention can convert the doping elements not just to any arbitrary valency state but to the lowest possible valency state that can be achieved with the means used it is further significant that the oxidation state reached does not show any absorption as also described in de 10 300 080 a1 it is advantageous if the doping elements used for increasing the dark conductivity are extrinsic ions in particular iron ions in a concentration of more than 1 u00d7 10 25 m u2212 3 to be able to carry out embodiments of the method in a particularly effective and simple manner it is advantageous to place the crystal between two electrodes such as between two metal electrodes which are connected to a voltage source one of the electrodes can take the form of a corona electrode which is not in contact with the crystal the corona electrode for instance being.