Electric field broadening of gallium acceptor states in compensated Ge:Ga,As

We report on low-temperature infrared absorption spectroscopy studies of p-type Ge:Ga,As samples with varying doping compensation ratios. Previous difficulties in fabricating appropriate samples are overcome by applying the neutron transmutation doping technique to high purity germanium of isotopically controlled composition with Ge-70 and Ge-74. 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 5x10(14)cm(-3). The observed Ga impurity absorption peaks 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 to existing theories of electric field broadening for donor 1s-2p transitions. We find excellent agreement with the theory which is based on the correlated distribution of ionized impurity centers.