Near band-edge optical properties of cubic GaN with and without carbon doping
We report the results of studying the optical properties of cubic GaN thin films with photoluminescence and photoluminescence excitation spectroscopies. The films are deposited by plasma-assisted molecular beam epitaxy on GaAs (001) substrates, with and without intentional doping with carbon atoms (p-type doping). The evolution of the optical spectra of the C-doped samples is consistent with a picture in which carbon enters into N-vacancies at low concentrations, producing a marked improvement in the crystalline properties of the material. At higher concentrations it begins to form complexes, possibly due to interstitial occupation. The temperature dependence on the absorption edge of the doped material is also measured and is analyzed with standard theoretical models.
Temperature-dependent optical band gap of the metastable zinc-blende structure beta -GaN.
The temperature-dependent (10--300 K) optical band gap [ital E][sub 0]([ital T]) of the epitaxial metastable zinc-blende-structure [beta]-GaN(001)4[times]1 has been determined by modulated photoreflectance and used to interpret low-temperature photoluminescence spectra. [ital E][sub 0] in [beta]-GaN was found to vary from 3.302[plus minus]0.004 eV at 10 K to 3.231[plus minus]0.008 eV at 300 K with a temperature dependence given by [ital E][sub 0]([ital T]) =3.302--6.697[times]10[sup [minus]4][ital T][sup 2]/([ital T]+600) eV. The spin-orbit splitting [Delta][sub 0] in the valence band was determined to be 17[plus minus]1 meV. The oscillations in the photoreflectance spectra were very sharp with a broadening parameter [Gamma] of only 10 meV at 10 K. The dominant transition observed in temperature-dependent photoluminescence was attributed to radiative recombination between a shallow donor, at [congruent]11 meV below the conduction-band edge and the valence band.
Economics of General-Purpose Assembly Robots
Summary A study of the comparative economics of various automatic assembly systems using general-purpose assembly robots is described. In the study, use is made of a “typical” candidate assembly whose profile is obtained from statistical analysis of actual products submitted by interested companies. It is shown that the choice of assembly system and the various parts presentation methods depends on three main factors, namely: the annual production volume per shift, the number of parts in the product and the equipment payback period.