Al x Ga1-x N(0≦x≦0.4) films grown on sapphires are found to be composed of many mosaic crystallites with various orientations by means of double-crystal X-ray diffractometry. This fluctuation of orientation can be considerably reduced and the surface morphology (smoothness and uniformity) of the film can be remarkably improved by the preceding deposition of the AlN buffer layers. A phenomenological model, which can explain the effect of the AlN buffer layer on the surface morphology and the crystalline quality of AlGaN films, is proposed.

Nanoporous silica thin films with low dielectric constants were deposited by gas evaporation of SiO2 nanoparticles in an argon atmosphere. With increasing gas pressure during the evaporation, the dielectric constant decreases, while the porosity increases. The correlation between the dielectric constant and porosity is well modeled by a serial connection of two capacitors, one with air and the other with SiO2 as the dielectric medium. This suggests that the dielectric constant of the nanoporous silica thin film using the gas evaporation technique is more effectively lowered by forming “uniformly” distributed voids of closed gaps than those of the nanoporous silica films with pores extending from the back to front surface. Therefore, the former nanoporous silica thin film requires less porosity to obtain a low dielectric constant and is regarded as an ideal low-k material.

Imperfection (cracking, stress) in (0001)GaN/(OOOIja-AI203 grown by using the metalorganic chemical. vapor deposition (MOCVD) or chemical vapor deposition (CVD) method is investigated. First, the epitaxial relation is confirmed by using the oscillating-crystal method. Subsequently, imperfections, especially cracks, are observed in detail by optical microscopy and x-ray diffraction topography. Cracking occurs at the layer thickness larger than 13!-lm in the crystals having epitaxial layers of good quality, while it does not appear even at the thickness of30 !-lm in crystals including inferior layers. Finally, stress in the J.ayer and in the substrate are determined by measuring the warp of the wafer. Compressive stress in the good epitaxial. layer is 1. 6 X 10 9 dyne cm -2, and in this case, cracking occurs at the maximum tensil.e stress in the substrate less than 3 X lOS dyne cm -2. However, cracking does not occur even at the compressive stress of 3 X 10 9 dyne cm -2 in the inferior inyer and at the tensile stress of l. X 10 9 dyne cm -2 in the substrate. From these facts, a possible mechanism of cracking is proposed. During the epitaxial growth, microcracks occur in the substrate at the interface owing to the lattice mismatching. During the cooling, macrocracks which can be observed are grown from the already existing microcracks owing to the thermal stress. In the latter course, the cracking in the substrate must be simultaneously propagated to the epitaxial layer.

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The structural properties of InN films grown on (0001) sapphire (α-Al2O3) substrates are investigated by reflection high-energy electron-diffraction and x-ray-diffraction methods. The epitaxial relationship between the InN layer and the α-Al2O3 substrate is revealed to be (0001)InN //(0001)α-Al2O3 with [101̄0]InN//[101̄0] α-Al2O3, which is different from that of AlN and GaN epitaxial films. The x-ray rocking curve measurements prove that the crystalline quality of as-grown InN films is not so good, while it can be significantly improved by thermal annealing at temperatures of 450–500 °C within 10 min in nitrogen ambient. The improvement of quality is ascribed to the rearrangement of mosaic crystallites in the films through the movement of their boundaries during a short period of time.