Evidence for correlated hole distribution in neutron-transmutation-doped isotopically controlled germanium

We report on low-temperature infrared-absorption spectroscopy studies of compensated p-type Ge(Ga,As) samples with varying doping compensation ratios. Previous difficulties in preparing appropriate samples are overcome by neutron-transmutation doping of high-purity, isotopically controlled germanium composed exclusively of Ge-70 and Ge-74, viz. Ge-70(x) Ge-74(1-x). With this technique, we have produced a series of crystals with compensation ratios between 0.082 and 0.87, while maintaining the net-acceptor concentration [Ga]-[As] constant at 5 x 10(14) cm(-3). The observed excitation lines of Ga accepters broaden linearly with the ionized impurity concentration due to the quadrupole interactions between Ga bound holes and the electric-field gradient. Experimental linewidths are quantitatively compared with existing theories of electric-field broadening developed in the context of donor transitions. We find excellent agreement with the theory based on the correlated distribution of ionized impurity centers.