Localized Donors in Gan: Spectroscopy Using Large Pressures

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Abstract
Properties of GaN and its alloys are strongly controlled by impurities and strain. Using large hydrostatic and biaxial pressure we identify the role of donor dopants and stress induced fields. The doping of Si and 0 as relevant representatives of group-IV and group-VI impurities are studied in Raman spectroscopy. For pressures above 20 GPa we find that oxygen induces a strongly localized gap state while Si continues to behave as a hydrogenic donor. Such a DX-like behavior of 0 indicates and corresponds to doping limitations in AIGaN alloys. The site specific (ON, SiGa) formation of a gap-state is attributed to bond strengths of the respective neighbors. In photoreflection of pseudomorphic GaInN we observe pronounced Franz-Keldysh oscillations corresponding to piezoelectric fields of 0.6 MV/cm. An observed redshift of the luminescence is found to originate in electric field induced tailstates. A reduced but similar effect is expected for GaN possibly explaining observations of persistent photoconductivity in a wide range of materials.