Evidence from EELS of oxygen in the nucleation layer of a MBE grown III-NHEMT

The presence of oxygen throughout the nominally AlN nucleation layer of a R F assisted MBE grown III-N HEMT was revealed upon examination by Electron E nergy Loss Spectroscopy (EELS) in a Scanning Transmission Electron Microsco pe (STEM). The nucleation layer generates the correct polarity (gallium fac e) required for producing a piezoelectric induced high mobility two dimensi onal electron gas at the AlGaN/GaN heterojunction. Only AlN or AlGaN nuclea tion layers have provided gallium face polarity in RF assisted MBE grown II I-N's on sapphire. The sample was grown at Cornell University in a Varian G enII MBE using an EPI Uni-Bulb nitrogen plasma source. The nucleation layer was examined in the Cornell University STEM using Annular Dark Field (ADF) imaging and Parallel Electron Energy Loss Spectroscopy (PEELS). Bright Fie ld TEM reveals a relatively crystallographically sharp interface, while the PEELS reveal a chemically diffuse interface. PEELS of the nitrogen and oxy gen K-edges at approximately 5-Angstrom steps across the GaN/AlN/sapphire i nterfaces reveals the presence of oxygen in the AlN nucleation layer. The g radient suggests that the oxygen has diffused into the nucleation region fr om the sapphire substrate forming this oxygen containing AlN layer. Based o n energy loss near edge structure (ELNES), oxygen is in octahedral intersti tial sites in the AlN and Al is both tetrahedrally and octahedrally coordin ated in the oxygen rich region of the AlN.