Growth and Defect Formation Mechanisms of Silicon Carbide and Aluminum Nitride Thin Films by Gas-Source Molecular Beam Epitaxy.

Monocrystalline thin films and multilayered heterostructures of silicon carbide (SiC) and aluminum nitride (AlN) have been grown on 6H-SiC substrates by gas-source molecular beam epitaxy (GSMBE). Mechanisms of nucleation, growth and defect incorporation in these materials have been determined. Microstructural characterization of both on-axis and vicinal surfaces of 6H-SiC(0001) homoepitaxial layers grown by chemical vapor deposition (CVD) and subsequently annealed in ultra-high vacuum for 3-5 min. in the range of 1000-1150^circC has been investigated using scanning tunneling microscopy. Undulating steps on terraces having widths inversely proportional to the vicinal angle were observed. All steps were oriented along <1120> ; rather than the expected (1100), the latter of which is orthogonal to the vicinal direction. A model based on energetics was developed to explain the step directions. Control of the initiation and continuation of step flow growth of 6H-SiC(0001) at the step sites and island growth of 3C-SiC(111) on the terrace sites of the 6H(0001) epilayers was achieved by changing the rm C_2H _4/Si_2H_6 gas flow rate ratio. The (3 x 3) surface reconstruction, suggesting the presence of excess Si, was observed by in situ RHEED using the gas flow ratio of 1; this was considered to enhance the adatom mobilities to the steps and thereby promote step flow growth because of the lower surface energy. Adatom mobility was decreased using flow rate ratios greater than 1, resulting in island nucleation of 3C-SiC(111). Inversion domain boundaries (IDBs) were observed using contrast imaging and lattice imaging via transmission electron microscopy in 2H-AlN films grown on vicinal 6H -SiC(0001) surfaces. The origin of the IDBs is the incoherency in bilayer stacking between the two materials at the 6H steps. A very high quality AlN film having a thickness of ~1.5 nm was pseudomorphically deposited on the on-axis 6H-SiC(0001) surface. This structure was subsequently used as a template for the growth of either very smooth 2H-SiC or 3C-SiC films. Control regarding the choice of SiC polytype was again achieved by adjusting the rm C_2H_4/Si_2H _6 gas flow ratio. The ratios of 1 or 2 and 5 resulted in 3C- and 2H-SiC films, respectively. The use of the former two ratios causes 3C-SiC nucleation on the AlN terraces; whereas, the latter is believed to enhance step flow growth due to C-termination of the AlN surface. The underlying mechanisms and control of these mechanisms are still under investigation.