J. C. Moore

Comparative study of the (0001) and (0001) surfaces of ZnO

The authors compare the surface and optical properties of the Zn-polar (0001) and O-polar (0001¯) surfaces of bulk ZnO samples. For optical characterization, steady-state photoluminescence using a He–Cd laser was measured at 15 and 300K. At room temperature, the (0001¯) surface demonstrates nearly double the near-band-edge emission intensity seen for the (0001) surface. Using scanning Kelvin probe microscopy, the authors have measured surface contact potentials of 0.39±0.05 and 0.50±0.05V for the (0001) and (0001¯) surfaces, respectively. The resulting small difference in band bending for these two surfaces indicates that charge transfer between the surfaces is not a dominant stabilizing mechanism. Conductive atomic force microscopy studies show enhanced reverse-bias conduction in localized regions on the (0001¯) vs (0001) surface. The differences in surface conduction and band bending between the two polar surfaces can be attributed to their chemical interactions with hydrogen and water in the ambient.

Sublimation behavior of SiO2 from low- and high-index silicon surfaces

The authors have used atomic force microscopy to investigate the sublimation behavior of 100-nm-thick oxide layers from the low-index Si(001) and Si(111) surfaces, as well as the stable, high-index Si(113) and Si(5 5 12) surfaces. Similar to previous thin-film (<50nm) sublimation studies, high vacuum annealing results in the formation of circular voids that grow laterally with annealing time (1–6min, 1150–1350°C). The depth of these voids is ∼200nm, or twice the thickness of the original oxide film, which is consistent with the thermal decomposition of SiO2 to form volatile SiO. There are subtle morphological differences, however, between the voids formed on the different surface orientations. Line profiles of the bottom Si surfaces inside the voids indicate flat terraces for the (111) and (113) orientations and sloped conic sections for the (001) and (5 5 12) orientations, indicating that the latter surfaces are less stable with respect to step formation during oxide sublimation. At the centers of voids,...

Effects of Hydrogen on the Morphology and Electrical Properties of GaN grown by Plasma-assisted Molecular-Beam Epitaxy

We study the effect of introducing hydrogen gas through the rf-plasma source during plasma-assisted molecular-beam epitaxy of GaN(0001). The well-known smooth-to-rough transition that occurs for this surface as a function of decreasing Ga flux in the absence of H is found to persist even with H present, although the critical Ga flux for this transition increases. Under Ga-rich conditions, the presence of hydrogen is found to induce step bunching (facetting) on the surface. Conductive atomic force microscopy reveals that leakage current through dislocation cores is significantly reduced when hydrogen is present during the growth.

Local electronic and optical behaviors of a-plane GaN grown via epitaxial lateral overgrowth

Conductive atomic force microscopy and near-field optical microscopy (NSOM) were used to study the morphology, conduction, and optical properties of a-plane GaN films grown via epitaxial lateral overgrowth (ELO) by metal organic chemical vapor deposition. The AFM images for the coalesced ELO films show undulations, where the window regions appear as depressions with a high density of surface pits. At reverse bias below 12V, very low uniform conduction (2pA) is seen in the window regions. Above 20V, a lower-quality sample shows localized sites inside the window regions with significant leakage, indicating a correlation between the presence of surface pits and leakage sites. Room temperature NSOM studies explicitly showed enhanced optical quality in the wing regions of the overgrown GaN due to a reduced density of dislocations, with the wings and the windows clearly discernible from near-field photoluminescence mapping.

Scanning tunneling microscopy studies of the Cu:Si(5 5 12) system

The growth of Cu on the stable Si(5 5 12) clean surface has been studied as a function of coverage and temperature using scanning tunneling microscopy. Similar to previously studied group IB metals, Cu produces overlayer “nanowires” at lower growth temperature ( 500 °C), however, the underlying Si surface is disrupted and Cu induces faceting to the nearby (113) plane. At coverages above approximately 0.5 ML, the surface rearranges to form sawtooth facets composed of wide (113) planes opposed by narrow (111) segments. The (113) planes show a Cu-induced 2×2 surface reconstruction that incorporates a large number of domain boundaries. We have also studied the O2 reactivity of the Cu-induced (113)/(111) sawtooths. At temperatures above 650 °C, the sawtooths are gradually etched away to produce trapezoidal islands. The density of these islands decreases with increasing temperature, providing a possible route f...

Carrier relaxation and stimulated emission in ZnO nanorods grown by catalyst-assisted vapor transport on various substrates

ZnO nanorods were grown by catalyst-assisted vapor phase transport on Si(001), GaN(0001)/c-Al2O3, and bulk ZnO(0001) substrates. Morphology studies as well as X-ray diffraction and transmission electron microscopy showed that ZnO nanorods grew mostly perpendicular to the GaN(0001) and ZnO(0001) substrate surface, whereas a more random directional distribution was found for nanorods on Si(001). Integral optical properties of fabricated nanorods were studied by steady-state photoluminescence and time-resolved photoluminescence. Stimulated emission was observed from ZnO nanorods on GaN(0001)/c-Al2O3 substrates, most likely due to their vertical orientation. Near-field scanning optical microscopy was applied to investigate luminescent properties of individual rods. Raman spectroscopy revealed biaxial compressive strain in the nanorod samples grown on Si(001). Conductive atomic force microscopy showed that nanorods are electrically isolated from each other. I-V spectra of individual nanorods were measured.

AFM and CAFM studies of ELO GaN films

The techniques of atomic force microscopy (AFM) and conductive AFM (CAFM) have been used to study the morphology and conduction properties of a-plane GaN films grown via epitaxial lateral overgrowth (ELO). Four GaN samples were prepared using metal organic chemical vapor deposition (MOCVD) with slightly different growth conditions. In AFM images, the coalesced ELO films show undulations, where the window regions appear as depressions with a higher defect density than surrounding areas. At reverse bias above 20 V, lower quality samples show localized leakage defect sites inside the window regions, whereas higher quality samples show no localized leakage. This behavior is consistent with previous observations on non-ELO samples where significantly enhanced localized leakage occurs at voltages above 15 V. Surface oxidation was also observed, where continuous scanning at reverse bias results in decreased conduction. This CAFM study confirms that ELO-grown GaN samples show enhanced reverse-bias leakage inside window regions where a higher defect density is present.

Scanning tunneling microscopy studies of oxide growth and etching on Si(5 5 12)

We have used scanning tunneling microscopy to study how the Si(5 5 12) surface morphology evolves when exposed to oxygen at elevated temperatures, in particular when both oxide nucleation and etching occur simultaneously. This study includes results for sample temperatures of 650 to 750 °C at O2 pressures of ∼5×10−7 Torr and exposures of 50 to 600 L. It is already known that the Si(001) surface is significantly disrupted in this transition regime by etch pits and islands caused by etching around oxide-induced pinning sites. For the high-index Si(5 5 12) surface, no etch pits are found on the terraces, but pyramidal or linear islands are observed on terraces and along step edges. Both types of islands incorporate (113) facets, indicating an enhanced stability of this orientation against etching. The absence of etch pits and the presence of well-defined islands qualitatively distinguish the etching behavior of this high-index surface from its low-index counterparts.