Structural and morphological properties of GaN buffer layers grown by ammonia molecular beam epitaxy on SiC substrates for AlGaN/GaN high electron mobility transistors

Abstract

The impact of growth conditions on the surface morphology and structural properties of ammonia molecular beam epitaxy GaN buffers layers on SiC substrates was investigated. The threading dislocation (TD) density was found to decrease with decreasing NH3:Ga flux ratio, which corresponded to an increase in surface roughness and reduction in residual compressive lattice mismatch stress. Furthermore, the dislocation density and compressive stress decreased for increasing buffer thickness. TD inclination was proposed to account for these observations. Optimized surface morphologies were realized at high NH3:Ga flux ratios and were characterized by monolayer-high steps, spiral hillocks, and pyramidal mounds, with rms roughness of ∼1.0 nm over 2×2 μm2 atomic force microscopy images. Smooth surface morphologies were realized over a large range of growth temperatures and fluxes, and growth rates of up to 1 μm/h were achieved. TD densities in the buffers as low as 3×109 cm−2 were demonstrated. These buffers were highly insulating and were used in recently reported AlGaN/GaN HEMTs with power densities of >11 W/mm at 4 and 10 GHz.

Gate current analysis of AlGaN/GaN on silicon heterojunction transistors at the nanoscale

“…10,16 They are systematically ignored as they vanished for thickness larger than 0.7 lm, again when the screw dislocation induced spiral growth turns to kinetic roughening. However, these nanopipes present in the bulk of the GaN layer may enable highly conductive path when the vertical current is sufficiently high.…”

Nanoscale conductive pattern of the homoepitaxial AlGaN/GaN transistor

“…As shown in figure 2, nanopipes are clearly a visible morphology feature on the HEMT surface. The presence of nanopipes is commonly reported in the initial stages of the heteroepitaxial growth of GaN (with a density of ∼10 8 cm −2 and a diameter of 100-200 nm) regardless of the growth method and the substrate used [35,36]. However, it appears that the reduced lattice mismatch of the homoepitaxial growth mitigates the kinetic roughening process and, therefore, the nanopipes are visible even after 1 μm thick of MBE growth.…”

Growth kinetics and electronic properties of unintentionally doped semi-insulating GaN on SiC and high-resistivity GaN on sapphire grown by ammonia molecular-beam epitaxy

“…2͑b͒ yields 3D columnar growth and inferior quality, and thus rarely applied in practical growth. 13 Such observation may give insight into the corre- lation between the different surface kinetics ͑adsorption, cracking, desorption, and incorporation͒ and the mechanisms behind the different growth modes and the resultant electrical properties.…”

“…[10][11][12] There have been also several reports of high resistivity UID GaN on grown on SiC or sapphire substrates with the ammonia-MBE technique. 13,14 Again, no detailed study of electrical properties and mechanisms analysis has been reported. In fact, high-resistivity UID GaN has been routinely grown on silicon wafers by ammonia-MBE, as part of the GaN HEMT-on-silicon recipe developed by a group in France.…”