C. D. Lee

Morphology and effects of hydrogen etching of porous SiC

The morphology of the porous network in porous SiC has been studied. It has been found that pore formation starts with a few pores on the surface and then the porous network grows in a V-shaped branched structure below the surface. The hydrogen etching rates of porous and nonporous SiC have been measured. Etch rates of porous and nonporous wafers of various miscuts are found to be equal within a factor of two, indicating that the rate-limiting step in the etching process arises from the supply of active etching species from the gas phase. The porous SiC etches slightly faster than the nonporous SiC, which is interpreted simply in terms of the reduced average density of the porous material.

Recent developments in surface studies of GaN and AlN

Recent developments in the structural analysis of GaN and AlN surfaces are reviewed, and the implications of these structures for molecular beam epitaxial growth are discussed. The GaN(0001), AlN(0001), and GaN(101¯0) surfaces are all found to be terminated by metallic layers containing approximately one bilayer of Ga or Al atoms. However, in contrast to GaN(0001) where the Ga bilayer exists in an incommensurate, fluid-like state at room temperature, the metallic layers for AlN(0001) and GaN(101¯0) form large-unit-cell commensurate structures with static atomic arrangements. Small amounts of H on the GaN(0001) surface leads to facet formation on the surface, whereas larger amounts of H produce a 2×2 surface arrangement that displaces the Ga bilayer. A possible model for the H-terminated GaN(101¯1) surface is introduced and first-principles total energy calculations employing a finite temperature thermodynamics approach are employed to determine the conditions in which it could be stable.

Morphology and surface reconstructions of GaN(1100) surfaces

GaN is grown by plasma-assisted molecular-beam epitaxy on ZnO(1100) substrates. Well-oriented (1100) GaN surfaces are obtained, and (1101) oriented facets are also observed. On the GaN(1100) surfaces under Ga-rich conditions, a surface reconstruction with approximate symmetry of “4×5” is found. A model is proposed in which this reconstruction consists of ⩾2 monolayers of Ga terminating the GaN surface.

Separation of contributions to spin valve interlayer exchange coupling field by temperature dependent coupling field measurements

In this work, interlayer exchange coupling fields of spin valve samples have been measured as a function of temperature, and fit to a temperature dependent combination of RKKY and Neel coupling fields. The RKKY coupling strength is assumed proportional to the form (T/T0)/sinh(T/T0), where T is temperature and T0 is characteristic temperature. [N. Persat and A. Dinia, Phys. Rev. B 56, 2676 (1997)] This allows the RKKY coupling and Neel coupling field to be separated quantitatively. The results of such an analysis on various CoFe/Cu/CoFe spin valve structures allow the extraction of a roughness parameter from the Neel model and the T0 parameter from the RKKY model. The measured roughness on the top surface was generally 2–3 times greater than the value obtained from the Neel analysis. The extracted T0 parameter was one order of magnitude smaller than that measured for bulk Cu by the de Hass–van Alphen effect. [N. Persat and A. Dinia, Phys. Rev. B. 56, 2676 (1997); B. Lengeler and W. R. Wampler, Phys. Rev. B...

Plasma-assisted molecular beam epitaxy of GaN on porous SiC substrates with varying porosity

We have grown GaN on porous SiC substrates and studied the effect of substrate porosity on the overgrown film quality in terms of defect structure and density and film strain. The growth was performed by plasma-assisted molecular beam epitaxy. The GaN films were characterized by x-ray diffraction, transmission electron microscopy (TEM), and wafer curvature measurements by surface profilometry. TEM images show that the GaN film grown on porous substrates contains open tubes and a relatively low dislocation density in regions between tubes. We discuss various growth mechanisms that can lead to these defect features in the GaN film. However, we do not find any overall improvement in the x-ray rocking curve full width at half maximum of the GaN films grown on porous substrates compared to those on nonporous substrates. It was found that the GaN films grown on porous SiC were significantly more strain relaxed compared to those grown on nonporous substrate. We attribute this strain relaxation in part to the obs...

Surface morphology of GaN surfaces during molecular beam epitaxy

The reconstruction and surface morphology of gallium nitride (0001) and

Growth of Gan on Porous Sic Substrates by Plasma-Assisted Molecular Beam Epitaxy

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Silicon on GaN(0001) and (0001) Surfaces

surfaces are studied using scanning probe microscopy and reflection high energy electron diffraction. Results for bare GaN surfaces are summarized, and changes in the surface structure and morphology due to codeposition of indium or magnesium during growth are discussed.

Analysis of angular dependent resistance measurements on IrMn-based spin valves using a finite pinning model

We have explored the growth of GaN on porous SiC substrates by plasma-assisted molecular beam epitaxy. The porous 4H- and 6H-SiC(0001) substrates used in this study contain 10 to 100-nm sized pores and a thin skin layer at the surface. This skin layer was partially removed prior to the growth by H-etching. Transmission electron microscopy (TEM) observations indicate that the epitaxial GaN growth initiates from the surface areas between pores, and the exposed surface pores tend to extend into GaN as open tubes and trap Ga droplets. Plan-view TEM observations indicate that the GaN layers grown on porous substrates contain fewer dislocations than layers grown on non-porous substrates by roughly a factor of two. The GaN layers grown on a porous SiC substrate were also found to be mechanically more relaxed than those grown on non-porous substrates; electron diffraction patterns indicate that the former are free of misfit strain or are even in tension after cooling to room temperature.

Growth and Surface Reconstructions of AlN(0001) Films

Surface reconstructions and adatom kinetics of silicon on GaN(0001) and (0001) surfaces are studied by scanning tunneling microscopy, electron diffraction, and first-principles calculations. For silicon coverage near 0.5 monolayer, a 2×2 structure is observed, and is interpreted in terms of a model consisting of a Ga adatom on a monolayer of 3 Ga+1 Si and a SiGa atom in the third layer. For higher silicon coverage, disordered 2×2 domains and “1×1” domains are found to coexist. After annealing above 300 °C the “1×1” regions become dominant and a 4×4 structure is seen near step edges. It is concluded that the silicon adatoms tend to reside in subsurface sites on the Ga-polar surface. Surface morphology in the presence of Si is smooth for the (0001) surface but rough for the (0001) surface. This difference is attributed to the presence of multiple Ga surface layers in the former case, which enhance surface diffusivities.