Alternative substrates for gallium nitride epitaxy and devices: Laterally overgrown gallium nitride and silicon(111)

Gallium nitride films grown on sapphire or silicon carbide using the conventional ‘two-step’ technique typically exhibit threading dislocations on the order of ∼109 cm−2, which are detrimental to device performance. In addition, sapphire and silicon carbide substrates are expensive and available only in limited size (2–3 inch diameter). This work addresses both issues by evaluating the properties of GaN films synthesized by lateral epitaxial overgrowth (LEO) and conventional growth on sapphire and Si(111) substrates.; LEO consists of partially masking a previously-grown seed layer and performing a subsequent regrowth such that the regrown features extend over the masked areas. Under favorable conditions the threading dislocations originating from the seed material are blocked by the mask material or redirected by the growing facets. In this work dislocation densities as low as ∼106 cm−2 were observed in the laterally-overgrown areas. The overgrown features exhibited well-defined facets ((0001), {lcub}11¯01{rcub}, {lcub}112¯0{rcub}, {lcub}112¯1{rcub}, {lcub}112¯2{rcub}), the persistence of which depended on the orientation of the mask as well as on the growth conditions. The relationship between the morphology of the LEO stripes and the growth conditions (temperature, pressure, ammonia and trimethylgallium partial pressures) was characterized for LEO on GaN/sapphire substrates. A qualitative model of the growth mechanisms was presented based on the microscopic structure of the growing surfaces. Microstructural characterization revealed a crystallographic tilt between the seed and the LEO region, which resulted in the formation of dislocations above the mask edge and at the junction plane of adjacent stripes. GaN stripes laterally overgrown on AlN/Si(111) exhibited similar properties. However, chemical interactions between the substrate and the precursors caused morphological degradation, which could be avoided by using a thick (≥180 nm) AlN buffer layer. In addition, thermal expansion mismatch resulted in film cracking.; Unpatterned GaN films were grown on Si(111) using two nucleation techniques. Films grown using a thin (∼25 nm) AlN buffer exhibited moderate dislocation densities (∼109 cm−2) and small residual tensile stress (∼500 MPa). Films grown using a thick (0.8 μm) AlN-to-GaN graded buffer had higher dislocation densities (1010–10 11 cm−2) but exhibited net compressive stress (∼260 MPa). Heterojunction field-effect transistors based on the graded-buffer process showed well-defined characteristics and high saturated current and transconductance (525 mA/mm and 100 mS/mm, respectively).