Oxide and Nitride Films for Tunable Capacitors and HEMTs

A report describes research on thin oxide films intended for use as variable-high-permittivity dielectrics in advanced tunable capacitors and on thin nitride films as starting materials for advanced high-electron- mobility transistors (HEMTs). In this research, a custom molecular-beam-epitaxy system was used to grow thin films of TiO₂, SrTiO₃, Ba₁-_xSr_xTiO₃, MgO, and Al_xGa₁-_xN (where 0 ≤ x ≤ 1). Growth parameters and properties of the films were investigated with a view toward developing processes for fabricating HEMTs using epitaxy of Al_xGa₁-_xN, processes for fabricating varactors using epitaxy of Ba₁-_xSr_xTiO₃, and processes in which integration of Ba₁-_xSr_xTiO₃ onto Al_xGa₁-_xN templates would be facilitated by use of MgO buffer layers. The films were characterized, variously, in situ by reflected-high-energy-electron diffraction, source-flux monitoring, and/or residual-gas analysis. The films were characterized, variously, ex situ by atomic-force microscopy, x-ray rocking-curve measurements, high-resolution x-ray diffraction, Rutherford backscattering, cross-section transmission electron microscopy and diffraction, and/or radio-frequency-loss metrology. In addition, the Al_xGa₁-_xN films were characterized in situ by multi-beam optical stress-sensor analysis.