ON THE DOPING OF ISOTOPICALLY CONTROLLED GERMANIUM BY NUCLEAR-TRANSMUTATION WITH A HIGH-CONCENTRATION OF SHALLOW DONOR IMPURITIES

Experimental results are presented of the neutron transmutation doping (NTD) of isotopically controlled Ge-74 crystals, irradiated with a hi gh dose of neutron flux Phi (up to 4 x 10(19) cm(-2)). A series of hea vily doped samples of n-Ge:As with small compensation ratio K = N-A/N- D was obtained, Two novel effects connected with high neutron dose Phi have been observed: (i) the dependence of the free electron concentra tion n = N-D - N-A on Phi, linear at small n, saturates at n > 3 x 10( 17) cm(-3); (ii) if the NTD procedure is applied to samples previously highly doped by As, the final value of n even decreases (a 'negative doping'). These effects are explained by the enhancement of the format ion of complexes of an As impurity with radiation damage at high Phi, which leads to a decrease of n.