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 compensati on 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 cr ystals with compensation ratios between 0.082 and 0.87, while maintain ing the net-acceptor concentration [Ga]-[As] constant at 5 x 10(14) cm (-3). The observed excitation lines of Ga accepters broaden linearly w ith the ionized impurity concentration due to the quadrupole interacti ons between Ga bound holes and the electric-field gradient. Experiment al linewidths are quantitatively compared with existing theories of el ectric-field broadening developed in the context of donor transitions. We find excellent agreement with the theory based on the correlated d istribution of ionized impurity centers.