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The possible dislocations and slip systems in the wurtzite structure are the same as in hcp structure [1]. The Burgers vectors of these dislocations are . The dislocations can lie on either the (0001) basal plane or prism planes. The dislocations lie on pyramidal planes. TEM studies have revealed that there are predominately three types of dislocations in a wurtzite GaN epilayer which has not been grown by selective overgrowth (e. g. [2, 3]). The majority of the dislocations are threading dislocations with Burgers vector which appear randomly in the epilayer, they result from the growth errors during the growth process. The other two types of dislocation are halflpops with a [0001] or Burgers vector. The [0001] dislocation half-loop lies on the prism plane and the dislocation half-loop lies on the (0001) basal plane which usually appears near the epilayer/substrate interface.

The performance of III-V nitride-based microelectronic and optoelectronic devices relates directly to the micro structure of these materials. Selective lateral overgrowth has been exploited to produce GaN heteroepitaxial films with low defect density [1]. Si is a promising substrate due to its low cost, large size, and the potential for the intergration of GaN-based optoelectronic devices with Si-based electronics. It is also possible to produce high-quality GaN material for devices using lateral overgrowth on a Si substrate [2]. At present, only limited information on the defect structure in GaN heteroepitaxial films grown by selective lateral growth is available, especially those grown on Si substrate. Recent work [3] on GaN pyramids grown on (111) Si substrates by this method has shown that in the center, or core, of the GaN pyramid (at and above the window area) dislocations thread through the pyramid nearly perpendicular to the substrate surface and the dislocation density is quite high.

Crystallization of intergranular glass films can influence the performance of ceramic materials [1]. The crystallization of anorthite glass films has been studied in situ in the transmission electron microscope (TEM) [2]. The films are grown by pulsed-laser deposition (PLD) on sapphire (α-AI2O3) substrates. These substrates can be pre-thinned to electron transparency, so that contamination or microstructural damage of the TEM specimen due to the thinning process can be eliminated. This process also allows the microstructure of the glass films to be characterized before crystallization; the evolution of specific features of the film can then be tracked through repeated heat treatments. Prior to the crystallization of the anorthite glass films, observations confirmed that all the glass films deposited by PLD are indeed amorphous. Crystallization experiments were performed in a Philips CM30 TEM equipped with a Gatan single-tilt heating holder. Each specimen was heated in the microscope to a temperature of about 1200 °C at a heating rate of about 30 °C/min. and held at this temperature for 20 minutes.

Silicate glasses are the most common constituents of intergranular phases which can be found in liquid-phase sintered ceramics [1]. Silicates are known to influence the structure of ceramic interfaces which, in turn, frequently affect mechanical properties of ceramic materials [2]. In earlier studies of silicate glasses on single-crystal alumina Ramamurthy et al [3] and Mallamaci [4] showed that the morphology of dewetted glass films and the mechanism of devitrification depend on the crystallographic orientation of the substrate surface. In continuation of these studies, results are presented on the dewetting behavior of monticellite (CaMgSi04) in contact with the (OOl)-oriented surface of single-crystal MgO. Due to the simplicity of sample preparation and availability of 3- dimensional topographic information, Atomic Force Microscopy (AFM) was used for surface characterization. These AFM results are complemented by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) data on the chemical composition and the structure of the glass-substrate interface.

III-V nitrides grown on (0001) sapphire substrates often contain a very high density of defects; this high density is usually attributed to the poor lattice and thermal match between the nitride and the sapphire [1]. There is thus a need to understand the defect structures in these materials. In the present study, a microstructural characterization of MBE-grown GaN films on (0001) sapphire using various TEM techniques was performed to evaluate defect structures in this material. This paper will outline some of the different types of defects found in these films and suggest some possible mechanisms by which they were formed.

The properties of aluminum nitride (A1N) make it an interesting material for applications in the microelectronics industry. These properties include: a large bandgap, good thermal conductivity, high-temperature stability and chemical inertness. Additionally, thin A1N films are used as buffer layers for the growth of GaN on alumina. In the present study, thin-films of A1N have been grown on (000l)-oriented α-Al2O3 through the use of an electric field. At elevated temperatures an electric field applied across a normally insulating oxide material will typically induce a flux of ions that carry current. In the case of Al2O3, since the more mobile species are the Al3+ cations, by applying an electric-field across the material, with an appropriate electrode material, a flux of 3+ cations can be induced toward the cathode surface (negative electrode).