The epitaxial growth of gallium nitride and aluminum gallium nitride/gallium nitride heterostructure field effect transistors (HFET) on lithium gallate substrates .

摘要

In this work, an alternative approach for III-Nitride growth using lithium gallate (LiGaO₂) as a substrate, is examined. The growth of GaN and AlGaN/GaN HFET structure were developed with molecular beam epitaxy. Lithium gallate has the average in-plane (001) lattice mismatch to GaN of 0.9%. Critical steps for optimal growth are preparation of LGO substrate, nitridation and buffer growth, and III-V flux ratio. Growth of a GaN nucleation buffer layer of proper thickness under N-rich conditions after nitridation is necessary to achieve low background electron concentration in GaN active layer. For Si-doped GaN samples, highest mobility achieved was 526 cm2/Vs with an electron concentration of 7.5 × 1016 cm −3. A dislocation density of 6–7 × 108 cm−2 or lower is determined from mobility modeling. The anisotropic thermal expansion of LGO resulted in the asymmetric residual strain observed in 0.8 μm thick GaN on LGO. The electron mobility in an AlGaN/GaN HFET was measured to be 1365 cm2/Vs at 300 K with the sheet electron concentration of 7.7 × 1012 cm−2 , which is comparable to the average state of the art values. Thus, this first extensive study of GaN device structures on LGO shows the promise of this substrate for advanced device applications.