Defect Control in Semiconductors 

Abstract: There has been a long dispute on the question, whether electron and hole transport in photoconducting organic molecular crystals is correctly described by incoherent hopping or by coherent band model concepts. An experimental access to the decisive low temperature transport behaviour has been precluded so far by the presence of traces of chemical impurities which act as trapping centers. This contribution elaborates on the nature and origin of defects, on their trapping influences, and on sensitive and selective methods for their characterization. With analytical methods for trap spectroscopy and efficient methods for trace impurity removal at hand it became possible to reach the liq. He temperature regime with time of flight charge carrier mobility measurements. High, band-like mobilities in conjunction with strongly nonlinear (sub-Ohmic) velocity — field dependences were discovered.

Abstract: X-ray topography is a powerful non-destructive method for the direct observation of defects in nearly perfect crystals. Organic molecular crystals, consisting of ‘light’ atoms, exhibit low X-ray absorption and scattering power. Thus ‘thick’ crystal plates (up to 3mm for CuKα radiation) can be studied, and defects appear by broad kinematical contrast. The ‘image widths’ of dislocation lines are usually > 20 μ, leading to limited spatial resolution of the method. Typical defects observed in nearly perfect organic crystals are inclusions, growth striations, faulted growth-sector boundaries, grown-in and glide dislocations. The determination of Burgers vectors of dislocations is discussed and demonstrated. Topographs of benzil, benzophenone, salol and 2,3-dimethylnaphthalene crystals grown from solution, from undercooled melt, by the Bridgman method or by the Czochralski technique are presented.

Abstract: We have performed a photoluminescence study of Si 1−x Ge x alloys with compositions in the whole range 0≤x≤1. The samples are grown by liquid phase epitaxy on silicon substrates. Due to the lattice mismatch between the two materials, the 1–5 μm thick layers exhibit a high dislocation density. We detect strong photoluminescence bands in the samples with properties similar to the D bands in silicon. The energetic position of these bands depends on the composition χ. From the different behavior of the bands in the alloys, we are able to identify the recombination processes.

Abstract: Two new deep traps, ED1 at 0.68 eV below the conduction band and HD1 at 0.37 eV above the valence band, have been revealed to appear as a result of plastic deformation in the deep-level transient spectroscopy spectra of n-and p-type GaAs crystals, respectively. A systematic study of these traps, including their capture and emission characteristics, provides evidence for their association with the core states of dislocations. Possible types of dislocations responsible for the traps are discussed.