Heavily Ga-doped Germanium Layers Produced by Ion Implantation and Flash Lamp Annealing - Structure and Electrical Activation

Heavily p-type doped Ge layers were fabricated by 100 keV Ga implantation and subsequent flash lamp annealing for 3 ms in the temperature range between 700°C and 900°C. For comparison some samples were annealed in a rapid thermal processor for 60 s. Ga fluences of 2x10¹⁵cm^-2, 6x10¹⁵cm^-2 and 2x10¹⁶cm^-2 were chosen in order to achieve Ga peak concentrations ranging from values slightly below the equilibrium solid solubility limit of 4.9x10²⁰ cm^-3 up to 3.5x10²¹ cm^-3 which corresponds to a maximum Ga content of about 8 at-%. The structure of the doped layer and the Ga distribution were investigated by Rutherford backscattering spectrometry in combination with ion channelling, cross-sectional electron microscopy and secondary ion mass spectrometry. Temperature dependent Hall effect measurements were carried out in order to determine the electrical properties of the Ga doped layers. It is shown that by flash lamp annealing Ga diffusion into the bulk can be completely avoided and the Ga loss by outdiffusion from the surface is reduced. The lowest sheet resistances of 36 Ohm/sq. was achieved for the medium Ga concentration annealed at 900°C. The best Ga activation values are 73%, 60% and 24% for the three Ga fluences under investigation. The Ga activation is correlated with the layer regrowth. Incomplete epitaxial regrowth as observed in some samples leads to lower activation.