Ignatius S. T. Tsong

Stress evolution during metalorganic chemical vapor deposition of GaN

The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.

Atomic structures of 6HSiC (0001) and (0001̄) surfaces

Abstract We have studied the reconstructions of the 6HSiC (0001) and (0001) surfaces after annealing at 850–950°C under a Si flux using scanning tunneling microscopy (STM). On both the Si-terminated (0001) and C-terminated (0001) surfaces, we observed a (3 × 3) reconstruction after annealing at 850°C, which changed to a (√3 × √3) reconstruction after further annealing at 950°C. We propose a model for the (3 × 3) surface with a 4 9 ML adatom-coverage. The (√3 × √3) surface has a 1 3 ML adato observations are consistent with previous LEED results. We also observed a new (9 × 9) reconstruction on the (0001) surface after further annealing the (3 × 3) surface at 900°C under a Si flux. When the (0001) surface was flashed to 1150°C without a Si flux, a graphitized surface with a (6 × 6) reconstruction was formed. The empty-state and filled-state images showed a contrast reversal. When Si was deposited in the (6 × 6) surface, the polarity of the contrast reversal was reversed, confirming the validity of our previous electronic structure calculations for this surface.

Brittle-ductile relaxation kinetics of strained AlGaN/GaN heterostructures

The authors have directly measured the stress evolution during metal organic chemical vapor deposition of AlGaN/GaN heterostructures on sapphire. In situ stress measurements were correlated with ex situ microstructural analysis to directly determine a critical thickness for cracking and the subsequent relaxation kinetics of tensile-strained Al{sub x}Ga{sub 1{minus}x}N on GaN. Cracks appear to initiate the formation of misfit dislocations at the AlGaN/GaN interface, which account for the majority of the strain relaxation.

Scanning tunneling microscopy and spectroscopy of cubic β-SiC(111) surfaces

Abstract Scanning tunneling microscopy (STM) images show a 6 × 6 reconstruction of the β-SiC(111) surface annealed at 1150–1200°C. Contrast reversal is observed as tunneling voltage bias is reversed. Spectroscopic I / V data indicate the presence of a graphite layer on the top surface. A model of the surface is proposed where an incommensurate graphite monolayer is grown over a (1 × 1) Si-terminated β-SiC(111) surface. This model helps to explain the discrepancy between the 6√3 × 6√3 geometry observed by LEED and the 6 × 6 geometry observed by STM on the same surface.

Etching of Si(111)‐(7×7) and Si(100)‐(2×1) surfaces by atomic hydrogen

We present scanning tunneling microscopy (STM) studies of the etching behavior of the surfaces of Si(111)‐(7×7) and Si(100)‐(2×1) by atomic hydrogen. Etching proceeds via the formation of volatile SiH4 through a two‐step mechanism in which the H atoms react with SiH2 and SiH3 species on the surface at room temperature. By measuring the area of etch pits in the STM images taken under the same flux conditions, the etching of Si(100) is determined to be nearly three times faster than that of Si(111) because intermediate hydride products are more readily stabilized on the (100) surface.

Obsidian Dating and East African Archeology

New experimental procedures have made it possible to establish specific hydration rates for the numerous compositional types of obsidian to be found at archeological sites in Kenya. Two rates are applied to artifacts from the Prospect Farm site, revealing a history of occupation extending back 120,000 years.

Obsidian Hydration Dating: A Coming of Age

Publisher Summary This chapter discusses the dating of obsidian artifacts, which is based on the fact that a freshly made surface of obsidian will adsorb water from its surroundings to form a measurable hydration layer. This layer is not visible to the unaided eye and should not be confused with the patina that develops on many materials as a result of chemical weathering. The surface of obsidian has a strong affinity for water, as is shown by the fact that the vapor pressure of the adsorption continues until the surface is saturated with a layer of water molecules. These water molecules then slowly diffuse into the body of the obsidian. Obsidian dating, unlike most dating methods currently available to archaeologists, has many applications. The diversity of its application is partly because of the fact that age determination is made directly upon the artifact. Archaeologists in Africa, the Arctic, Eastern Europe, Oceania, and Mesoamerica have submitted specimens for analysis. Their critical evaluation of the dating efforts would ultimately decide whether the technique has truly come of age or whether the analytical breakthroughs of the late 1970s through the early 1980s represent but another step in a much larger process.

Electronic structures of polar and nonpolar GaN surfaces

Abstract GaN has a large ionicity. The polarity of its surface may have a substantial influence on its electronic structure, in particular its work function. Our first-principles calculations for several polar and nonpolar surfaces suggest that the polar surfaces are semimetallic, while the nonpolar surfaces are semiconducting. The polar surfaces have an array of dipole layers that result in a substantial increase or decrease of the work function depending on whether the surface is terminated by the N or Ga layer. The nonpolar surfaces have tilted surface NGa bonds, which result in an outward dipole layer that increases the work function.

Growth of self-assembled GaN quantum dots via the vapor–liquid–solid mechanism

Self-assembled nanometer-scale GaN quantum dots were fabricated on 6H–SiC(0001) substrates via the formation of Ga liquid droplets and their subsequent nitridation with a supersonic gas source seeded with NH3 molecules. The entire process was observed and controlled in situ and in real time in a low-energy electron microscope. The microstructure of the quantum dots was studied by high-resolution cross-sectional transmission electron microscopy illustrating the perfectly coherent wurtzite structure of GaN quantum dots with 5 nm base width. Spatially resolved cathodoluminescence spectra yield the characteristic band edge emission near 3.48 eV for larger size GaN dots.

InGaN/GaN multiple-quantum-well light-emitting diodes grown on Si(111) substrates with ZrB2(0001) buffer layers

Multiple-quantum-well light-emitting diode (LED) structures of InGaN/GaN were grown by metalorganic chemical vapor deposition on Si(111) substrates via ZrB2(0001) buffer layers and a GaN template comprising composite AlxGa1-xN (where x lies in the range from 0 to 1) transition layers to minimize cracking due to thermal expansion mismatch between Si and GaN. Photoluminescence and electroluminescence results from the LED structures compared favorably with similar measurements obtained on identical LED structures grown on sapphire substrates. However, in spite of all the precautions taken, cracking was still present in the LED structures. Scanning electron microscopy and transmission electron microscopy in plan-view and cross-section geometries were conducted on the LED structures to examine the presence and the influence of various defects such as microvoids, micropipes, and threading dislocations on the mechanism of cracking. Our results suggest that the crack network propagates from microvoids on the surf...