Thermal activation of low-density Ga implanted in Ge

Recent results have shown that quantum dots in Ge/SiGe heterostructures have high hole mobilities, long spin lifetimes, tunable g-factors, and strong spin-orbit coupling providing an intriguing alternative to silicon. In contrast to silicon, the potential for using dopants, such as Ga, as qubits in pure Ge is relatively unexplored. Further, an optical interface with dopant-spin-qubits is an exciting avenue for state readout through spin-selective bound exciton transitions, as has been observed in silicon. However, a thorough analysis of implanted Ga activation in Ge is lacking in the literature and optimal dopant density and activation are necessary for bulk optical spin detection. Here, we use spreading resistance profiling and secondary ion mass spectrometry to derive ion activation in bulk Ge implanted with Ga at 175 keV as a function of anneal temperature (T_a) and implant fluence. We show that activation is maximized near T_a = 650 ^oC and declines for T_a > 650 ^oC. Additionally, we show activation increases with implant fluence from 10% for 6x10¹⁰ cm^-2 to about 65% for 6x10¹² cm^-2. Our results guide the fabrication process towards realizing practical optical experiments and potential qubit devices.