We describe an imaging polarimeter for high sensitivity measurements of solar polarisation signals in the wavelength range from 300 nm to 1 mum. At higher wavelengths the system is limited by the wavelength cut-off of the silicon CCD sensor used. To the blue the limitation arises from the atmospheric cut-off around 310 nm. The complete system is a modified version of the Zurich Imaging Polarimeter ZIMPOL II which has been equipped with a special CCD sensor. The CCD combines for the first time a so-called open electrode structure with on-chip demodulation. The concept as well as the detailed design of the instrument are presented. Examples of observations are shown and interpreted in order to experimentally evaluate the performance of the system.

While the diffusion mechanism on the Ga sublattice of GaAs is fairly well understood, the diffusion on the As sublattice is still under discussion. In this contribution the use of spectrally resolved and imaging cathodoluminescence (CL) for investigating diffusion phenomena is reported. Interdiffusion experiments with GaAsP/GaAs and GaAsSb/GaAs superlattice samples and in-diffusion experiments of phosphorus and antimony in GaAs were performed at various temperatures and arsenic pressures. The interdiffusion coefficient could be deduced from the energy shift of the superlattice luminescence bands. A comparison with conventional SIMS investigation of the superlattices shows that spectral CL is a simple and reliable alternative to SIMS for the investigation of interdiffusion in luminescing superlattices. Imaging CL studies of P in-diffusion in GaAs show that a high phosphorus partial pressure during diffusion causes the formation of a GaAsP phase, leading to a high density of dislocations which disturb the in-diffusion process. The formation of dislocations may be prevented by using a phosphorus partial pressure below 0.08 bar and an arsenic partial pressure above 0.1 bar. The As partial pressure dependence of the obtained diffusivities combined with results from other diffusion studies clearly demonstrate that P and Sb diffusion as well as arsenic self-diffusion on the As sublattice of GaAs is dominated by As self-interstitials.

Nominally undoped GaAsSb/GaAs superlattices were annealed at temperatures between 900 and 1100 °C in a closed quartz ampoule. A strong dependence of the interdiffusion coefficients in the GaAs/GaAsSb superlattices on the arsenic vapor pressure was observed by two independent methods: secondary ion mass spectroscopy, and high-resolution x-ray diffraction using dynamic calculations to extract interdiffusion coefficients. The interdiffusion coefficient was low in the Ga-rich regime where an Arrhenius like dependence with an activation energy of 1.5±0.4 eV and a preexponential factor of 7.1×10−12 cm2 s−1 was found. For the As-rich regime the activation energy was 2.0±0.1 eV and the preexponential factor 7.8×10−9 cm2 s−1.

Interdiffusion experiments with GaAsP/GaAs and GaAsSb/GaAs superlattice samples were performed at various temperatures and arsenic vapor pressures. From the depth-concentration profiles effective diffusion coefficients were calculated. The dependence of these effective diffusion coefficients on the ambient arsenic pressure led to the conclusion that the interdiffusion process is governed by a substitutional-interstitial diffusion mechanism. The good agreement of the effective diffusion coefficients of the GaAsP/GaAs and GaAsSb/GaAs samples with each other and the agreement with arsenic self-diffusion data from the literature is an indication that phosphorus and antimony have good tracer properties to investigate arsenic self diffusion. Comparing our results with sulfur in-diffusion experiments from the literature we conclude that the kick-out mechanism governs self-diffusion on the arsenic sublattice in GaAs. Our results are in contradiction to arsenic self-diffusion experiments which indicated a vacancy mechanism.

Interdiffusion coefficients on the group V sublattice of GaAs were determined in GaAsP/GaAs and GaAsSb/GaAs superlattices. Strained GaAs0.86P0.14/GaAs, GaAs0.8P0.2/GaAs0.975P0.025 and GaAs0.98Sb0.02/GaAs superlattices were annealed between 850 °C and 1100 °C under different arsenic vapor pressures. The diffusion coefficient was measured by secondary ion mass spectroscopy and cathodoluminescence spectroscopy. The interdiffusion coefficient was higher under arsenic-rich conditions than under gallium-rich conditions, pointing to an interstitial-substitutional type of diffusion mechanism.

This paper reviews recent developments in our understanding of self- and impurity diffusion processes in gallium arsenide with special emphasis on incorporating recent gallium isotope diffusion data. Specific diffusion mechanisms for carbon, phosphorus, antimony, and sulfur, which are all substitutionally dissolved on the arsenic sublattice are suggested.

Interdiffusion experiments in GaAs/GaAsP and GaAs/GaAsSb superlattices have been performed at various temperatures and As gas pressures. The analysis of the arsenic pressure dependence of the effective diffusion coefficient revealed that a substitutional-interstitial diffusion mechanism governs the interdiffusion process. Computer simulations were performed to study the profile shape of the annealed samples and the As pressure dependence of the effective diffusion coefficient. Combining these observations it was found that the Frank-Turnbull diffusion mechanism governs the interdiffusion for the GaAs/GaAsP and most likely for the GaAs/GaAsSb superlattice samples. The As pressure dependence of the effective diffusion coefficients measured in interdiffusion experiments is opposite to the published pressure dependence measured in As and P in-diffusion experiments. In principle, the apparently contradicting in- and out-diffusion behavior can be reconciled in terms of a diffusion model which involves arsenic vacancies, fast diffusing arsenic vacancy - phosphorus interstitial complexes and fast diffusing phosphorus interstitials (or the corresponding antimony related defects).

Silicon diffusion was carried out from a thin (50 nm) sputtered film into undoped semi-insulating and Te- or Zn-doped LEC-GaAs at 900 °C for 5 h under various As pressures. Secondary ion mass spectroscopy and spreading resistance technique were used to characterize the Si indiffusion profiles. Lattice defects in highly Si-doped diffusion region were examined as a function of post-diffusion heat treatments (first at 700 °C for 15 min and second at 1000 °C for 30 min) by using transmission electron microscopy of plan-view and cross-sectional samples. Two types of defect were observed in the diffusion region: (i) perfect prismatic loops on {110} planes and of the interstitial type and (ii) Frank faulted loops on {111} planes, also of the interstitial type. A model for the defect formation and the role of Si in the defect generation are discussed in terms of a negative temperature dependence of thermal equilibrium concentrations of V Ga 3− , which are assumed to mediate the Si diffusion under high n-doping conditions. Cathodoluminescence spectra measured at 4 K and 77 K were obtained from the diffusion layer. Si diffusion affects the band-gap luminescence and generates two deep-level emission bands in the 0.9–1.3 eV spectral region. It is suggested that these deep levels are associated with some diffusion-induced defects and defect complexes.

The combination of the German Vacuum Tower Telescope and the prototype of ZIMPOL I (Zurich Imaging Stokes Polarimeter I), a novel, very sensitive imaging polarimeter, has resulted in the first spectra of solar intra-network (IN) fields in circular polarization. The sensitivity in terms of flux density is 0.7 Mx/cm2. While magnetic fields in solar spots, pores, plages, and in the network have predominantly kG field strengths, the magnetic field strength of the IN flux is a controversial subject due to the absence of direct measurements. We first summarize the current ideas on IN fields and examine previous arguments for their field strength. Our measurements of the magnetic line ratio formed between the amplitudes of the Stokes V profiles of Fe I 5247.1 angstrom and Fe I 5250.2 angstrom are consistent with a field strength well below 1 kG. Since the sensitivity of the magnetic line ratio becomes low for small field strengths, we can only set an upper limit on the field strength of IN fields of 500 G at the level of line formation with a probability of 68% and an upper limit of 1 kG with a probability of 95%. We emphasize that these are the first observations of a magnetic line ratio of unity near disk center.

Diamond indentations have been carried out on Zn-diffused GaAs p(+)n junctions on (100) oriented material. Electron-beam induced current (EBIC) investigations revealed the well-known dislocation slip bands in <110> directions. Scanning deep level transient spectroscopy (SDLTS) imaging proved a deformation-induced point-defect level at E(V) + 0.5 eV, which is preferentially concentrated in the dislocation-free regions between the slip bands rather than within the slip bands. Monochromatic cathodoluminescence (CL) imaging at 10K using different wavelengths revealed only the dislocation-induced recombination activity but not any point-defect luminescence corresponding to the 0.5 eV level found by SDLTS.

From a thin (50 nm) sputtered film silicon is diffused into undoped semi-insulating and Zn-doped LEC-GaAs in two annealing steps - at 900 degrees C for 5 h and afterwards at 700 degrees C for 15 min - under various As pressures. SIMS was used to characterize the silicon diffusion profiles. TEM was carried out on cross-sectional and planar samples to investigate the Si-diffusion induced defect formation. Depth-dependent CL spectra were obtained at helium temperatures on cleaved faces and correlated with the TEM and SIMS results.