Nanostructurale nature of the porous GaAs layer formed on p+-GaAs substrate by electrochemical anodization
A porous GaAs layer has been formed by electrochemical anodization in HF based solution on extremely doped p-type GaAs substrate. Porous nature of the elaborated sample has been evidenced by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Nanostructurale nature of the porous layer has been demonstrated by X-ray diffraction analysis (XRD) and confirmed by photoluminescence spectroscopy (PL).
Initiation and Formation of Porous GaAs
The present work deals with localized dissolution processes (pit and pore initiation and growth) of p- and n-type (100) GaAs. Pit and pore growth can be electrochemically initiated on both conduction types in chloride-containing solutions and leads after extended periods of time to the formation of a porous GaAs structure. In the case of p-type material, localized dissolution is only observed if a passivating film is present on the surface, otherwise, e.g., in acidic solutions, the material suffers from a uniform attack (electropolishing) which is independent of the anion present. In contrast, localized dissolution (pitting corrosion) and pore formation on n-type material can be triggered independent of the presence of an oxide film. This is explained in terms of the different current limiting factor for the differently doped materials (oxide film in the case of the p- and a space charge layer fin the case of the n-GaAs). The porous structure was characterized by scanning electron microscopy, energy dispersive X-ray analysis, and Auger electron spectroscopy, and consists mainly of GaAs. From scratch experiments it is clear that the pit initiation process is strongly influenced by surface defects. For n-type material, atomic force microscopy investigations show that light induced roughening ofmore » the order of several hundred nanometers occurs under nonpassivating conditions. This nanometer-scale roughening however does not affect the pitting process.« less
Transmission electron microscopy of lattice planes in human alveolar bone apatite crystals
SummaryPeriodic fringes corresponding to six different lattice planes have been observed in apatite crystals of human normal alveolar bone by transmission electron microscopy. Three of these sets of fringes have spacings less than 3.5 Å corresponding to the Scherzer resolution of the microscope used. The (0002) lattice plane of hydroxyapatite of 3.4 Å d-spacings, the
$$\left( {21\bar 11} \right)$$
 lattice plane with a d-spacing of 2.81 Å, and the
$$\left( {30\bar 30} \right)$$
 lattice plane with a d-spacing of 2.72 Å have been identified. The (0002) and
$$\left( {21\bar 21} \right)$$
 lattice planes have been observed for the first time in bone microcrystals. Some of the crystals studied were characterized by a mean width/thickness ratio of 6.91, typical of platelike habit, whereas observations of crystals aligned along the
$$\left\langle {1\bar 210} \right\rangle $$
 and
$$\left\langle {1\bar 211} \right\rangle $$
 directions showed a needlelike habit. The mean length of the bone apatite crystals was 470 Å. A dark line similar to the one observed in enamel and dentine crystals was also seen. The bone microcrystals observed have shown a high sensitivity to beam damage.