Properties of Tellurium based II-VI semiconducting materials

Opto-electronic devices operating as radiation detectors in the infra-red region of the electromagnetic spectrum are currently of interest. By operating in the infra-red, particularly in the 8-12 µm wavelength range, it is possible to detect the infra-red radiation emitted by objects at ordinary temperatures and so to image in darkness. Furthermore, at such wavelengths, vision is also possible in mist, fog or smoke. Semiconducting materials which have an energy gap corresponding to the photon energy of the radiation of interest are suitable for fabricating such devices. The growth and characterisation of two such materials both formed from elements in groups IIB and VIA of the periodic table and generally refered to as II-VI compounds, forms the subject matter of this thesis. The first of these materials is the ternary compound mercury cadmium telluride ((Hg,Cd)Te). This is a well established infra-red material and was grown for this work in thin film form by Metal Organic Vapour Phase Epitaxy (MOVPE) using the Inter- diffused Multilayer Process (IMP). The resulting layers were characterised optically and electrically and were shown to be of excellent compositional uniformity, an important consideration for infra-red devices, but to contain extremely high acceptor concentrations in the as grown state. These high acceptor concentrations were attributed to mercury vacancies present due to the inherent weakness of the material. Fitting of the electrical data obtained from p-type samples using a multicarrier/multilayer transport model suggested that the mercury vacancy concentrations were also highly non-uniform. A more novel alternative to (Hg(_1)Cd)Te is the HgTe:ZnTe superlattice system. By forming a superlattice from the two constituent binary compounds, rather than alloy, quantum confinement and strain effects may, in principle, be used to tailor the optical and electronic properties to some extent independently of the composition. The resulting material may also be structurally more stable than an equivalent alloy. Here the development of a thermal MOVPE growth process for this superlattice system is described and it is shown that such superlattices may be preferable to the equivalent alloys as they are easier to grow by MOVPE. Initial structural and optical studies and theoretical calculations have confirmed the suitability of MOVPE for the growth of this superlattice system and its applicability for infra-red applications

The PHENIX Experiment at RHIC

The physics emphases of the PHENIX collaboration and the design and current status of the PHENIX detector are discussed. The plan of the collaboration for making the most effective use of the available luminosity in the first years of RHIC operation is also presented.Comment: 5 pages, 1 figure. Further details of the PHENIX physics program available at http://www.rhic.bnl.gov/phenix

Gendering the careers of young professionals: some early findings from a longitudinal study. in Organizing/theorizing: developments in organization theory and practice

Wonders whether companies actually have employees best interests at heart across physical, mental and spiritual spheres. Posits that most organizations ignore their workforce – not even, in many cases, describing workers as assets! Describes many studies to back up this claim in theis work based on the 2002 Employment Research Unit Annual Conference, in Cardiff, Wales

Twinning in vapour-grown, large volume Cd1-xZnxTe crystals

The onset of twinning from View the MathML source to View the MathML source in large volume Cd1−xZnx Te crystals, grown by vapour transport on View the MathML source, often referred to as (211)B, oriented GaAs seeds, has been investigated using X-ray diffraction imaging (X-ray topography). Twinning is not associated with strains at the GaAs/CdTe interface as the initial growth was always in View the MathML source orientation. Nor is twinning related to lattice strains associated with injection of Zn subsequent to initial nucleation and growth of pure CdTe as in both cases twinning occurred after growth of several mm length of Cd1−xZnxTe. While in both cases examined, there was a region of disturbed growth prior to the twinning transition, in neither crystal does this strain appear to have nucleated the twinning process. In both cases, un-twinned material remained after twinning was observed, the scale of the resulting twin boundaries being sub-micron. Simultaneous twinning across the whole sample surface was observed in one sample, whereas in the other, twinning was nucleated at different points and times in the growth

Structural and optical properties of oxygen doped single crystal ZnTe grown by multi-tube physical vapour transport

Bulk single crystals of zinc telluride up to 10 mm thick have been grown by the Multi-Tube Physical Vapour Transport technique and doped, in-situ during growth, with oxygen. Following hetero-epitaxial nucleation and buffer growth on 100 mm diameter GaAs seed wafers, oxygen was introduced to the quartz growth envelope, using nitrous oxide as a precursor, via a novel gas injection system. Mass spectra from a residual gas analyser sampling the gases exiting the growth envelope indicated that the nitrous oxide had been cracked at the operating temperature of the furnace releasing oxygen into the growth region. The structural perfection of the grown crystals was assessed by synchrotron based X-ray diffraction measurements and found to be extremely high, improving significantly with distance from the seed. Rocking curve widths, measured over a 4 mm × 7 mm area, as low as 20 arcsec were observed. No evidence was found for a reduction in crystalline quality resulting from the incorporation of oxygen. Luminescence studies (4–300 K) showed strong red luminescence at 680 nm persisting up to room temperature indicating that oxygen had been incorporated substitutionally onto tellurium sites. This material is highly transparent at the red emission wavelength with absorption coefficients of approximately 2 cm−1. Under alpha radiation from a 241Am source, scintillation was observed from the doped material with approximately 12,700 photons/MeV and a full width at half height maximum of 27%. The material is a potential candidate for large volume scintillation based radiation detectors