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 ⁷⁴Ge crystals, irradiated with a high dose of neutron flux Φ (up to 4 × 10¹⁹ cm^-2). A series of heavily 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 Φ have been observed: (i) the dependence of the free electron concentration n = N_D - N_A on Φ, linear at small n, saturates at n > 3 × 10¹⁷ 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 formation of complexes of an As impurity with radiation damage at high Φ, which leads to a decrease of n.