The work described in this thesis is a study of imperfections in high purity, low dislocation content synthetic quartz single crystals which are grown on a commercial scale by the General Electric Company. Hydrogen and metal ion impurities in quartz were studied using low temperature transmission Fourier Transform Infrared Spectroscopy, (FTIR). Incorporation of impurities in quartz was investigated using crystals which were doped with Al, Fe, Cr, Co, Ti, P, Cu and K. Orientation of hydroxyl impurities was determined using polarised infrared spectroscopy. Deuterated quartz crystals were grown and studied as analogues to high purity crystals. X-ray diffraction topography was used to evaluate crystal quality and to perform a study of quartz which had been treated by electrodiffusion, or sweeping. Crystal lattice strain and sweeping damage was studied using double crystal topography at the Synchrotron Radiation Source, (SRS), at the Daresbury Laboratory. Metal ion impurities were sh own to incorporate interstitially into quartz crystals. The only exceptions were aluminium and iron, which incorporated substitutionally. All the impurities studied introduced hydrogen impurities such as hydroxyl ions and water molecules into quartz. A relationship between impurity concentration and hydrogen content was clearly indicated. The 3200 cm-1 broad absorption band characteristic of impure and fast grown synthetic quartz was strongly implicated as being due to interstitially incorporated water molecules. The major hydrogen impurities in quartz were sodium hydroxide molecules which were preferentially aligned along the X, Y and Z growth axes. Hydroxyl ions were not incorporated directly from the growth solution. Sweeping was shown to induce the formation of a negative space charge in treated quartz which gave rise to inhomogeneous lattice strain. The strain was gradually relieved upon prolonged exposure to x-rays. A mechanism for the production and relief of lattice strain has been proposed. Sweeping produced surface damage on mechanically polished crystals but not on those treated by chemical polishing using hydrofluoric acid. No further detrimental effects of sweeping were observed. Double crystal topographic studies revealed two possible bulk defects in Quartz. The first of these was a previously unreported planar defect parallel to the X-Y plane of quartz. The second was possibly a planar defect associated with the cellular growth cells in the (00.1) growth sector of quartz. The validity of these observations was put in doubt by the possibility of defect projection from the silicon monochromator which had been used in double crystal studies.

This thesis is primarily concerned with the characterisation of the native defects present in CdTe crystals grown from the vapour phase using two different methods. The principal characterisation techniques used were TEM, SEM and etching/optical microscopy. Crystal defects in bulk CdTe are of great importance since this material is in demand as a substrate for the epitaxial growth of Cd(_x)Hg(_1-x)Te for use in large area infra-red imaging devices. Large single crystals grown using the 'Durham' technique invariably contain first order twin boundaries lying on {111} planes and these are of the 'ortho'-type. Twin boundaries on other planes were characterised using the coincidence site lattice models of twin boundaries in sphalerite which were developed during this work. First order lateral twin boundaries lying on {511}-{111}, {112}-{112}, {001}-{221} and {110}-{114} and second order twin boundaries lying on {111}-{115} {114}-{114} and {22l}-{22l} are discussed. Twinning in CdTe is thought to be due to post-growth stress rather than to a growth phenomenon. The dislocations which are present in crystals grown by the 'Durham’ method are predominantly concentrated into well-formed arrays which are sometimes associated with other crystal defects. These arrays were shown by EBIC studies to be electrically active and the average sub-grain dimension was found to be ~ 150 µm. It is thought that the arrays form as a result of dislocation polygonisation during the latter stages of growth. Precipitates, which were identified as being comprised of Te, are found in the bulk of the crystals although they are more often seen on twin boundaries. The relationship between boundary type and the density of precipitates is discussed. Sub-grain boundaries and precipitates were also examined in a small number of CdTe crystals which had been grown by a modified 'Piper-Polich' technique. The differences between the defects in this material and those in CdTe crystals grown by the 'Durham' method are fully explained in terms of the differences in the crystal growth conditions employed in the two techniques. Networks of dislocations in Cd(_0.95) Zn(_0.0 5) Te crystals grown by the 'Durham' technique were also investigated. A mechanochemical polishing machine which was developed during the course of this work and which is capable of producing high quality chemically polished surfaces is described. Also the use of chemical etchants to determine the crystallographic polarity of {111} oriented surfaces is clarified. Important features of this work include; the development of coincidence site lattice models of first order lateral and second order twin boundaries in the sphalerite structure and the thorough characterisation of these boundaries in as-grown CdTe crystals, and the comparison of the sub-grain boundaries and precipitates present in material grown from the vapour phase by two different techniques.

DLTS, ODLTS and DLOS have been used to characterise the main deep level trapping centres in some II-VI semiconductors; these were single crystal CdS, (ZnCd)S, CdSe, CdTe and ZnS, and polycrystalline CdS films. Undoped, single crystal CdS contained four electron traps as detected by DLTS, at 0.29eV, 0.41eV, 0.61eV and 0.74eV below the conduction band (CB). The first two were observed in all samples and were due to native defects. The two states of highest energy were found only in material that had been annealed in S or Cd vapours. The 0.61ev level could be photoinduced by illumination at photon energies greater than about 1eV. It decayed in the dark with an activation energy of 0.25eV. The 0.61eV and 0.74eV centres were associated with electrically active extended defects (subgrain boundaries Such samples had dislocation densities of about 10(^10) cm(^-2). Copper was found to be a residual impurity in CdS. It produced two deep hole traps resulting from a crystal field splitting of the Cu d(^9) state. They were detected by ODLTS and DLOS and were found at 0.35eV and 1.lev above the valence band (VB).Introduction of the isoelectronic impurity tellurium into CdS induced a hole repulsive centre at 0.21eV above the VB. This is thought to be an inportant radiative recombination centre. The main electron trap in CdS at 0.41eV was found to shift to higher energy with incorporation of Zn. Replacement of 20% of the Cd with Zn shifted the energy to 0.63eV. The level appeared fixed to the VB and had a similar functional dependence on composition as the band gap. The activation energies of the copper centres observed in CdS remained unchanged with incorporation of Zn up to the composition (^Zn)0.45 (^cd)0.55(^s) showed that the crystal field splitting was constant and that these levels were also pinned to the VB. During the fabrication process of the (ZnCd)S/Cu(_2)S solar cell, a deep level was induced at about 1.2eV below the CB. This is thought to be a recombination centre and one of the contributory factors to the reduction observed in the current collection efficiency of these devices. Polycrystalline CdS films were prepared by silk screen printing (SP) and evaporation. The SP films were annealed at various times and temperatures to improve the crystallinity of the layers. At 640C for 1hr, deep states at 0.16eV and 0.48eV were detected. The levels disappeared when annealed at 670C-700C and a new level was observed at 0.13eV. CdS/Cu(_2)S heterojunctions were prepared on the material sintered at 670C; this induced a further trapping level at 1.1eV and one that was poorly resolved. Copper diffused into the CdS during the fabrication of the device so the states associated with copper were detected at 0.35eV and 1.1eV, The evaporated CdS layers showed that the defect signature was sensitive to the type of substrate. Using Ag instead of the usual SnO(_x), deep states were induced at 0.48eV and 0.98eV below the CB. These Ag-associated impurity centres prevent the indiffusion of Cu during the optimising heat treatment of the CdS/Cu(_2)S heterojunction. This maintains the stoichicmetry of the Cu(_2)S layer, thereby, preventing degradation of the devices. CdSe and copper doped CdSe were found to contain several important defect centres: a native sensitising centre (0.64eV from the VB), a class I recombination centre (0.9eV from the CB), a copper impurity centre (0.2eV from the CB) and two native defects (0.16eVand 0.45eV from the CB). n-type CdTe grown by the Piper-Polich technique contained6 electron traps at 0.15eV, 0.21eV, 0.40eV, 0.47eV, 0.53eV and 0.63eV. Their presence was shown to be dependent upon the method of growth of the crystal by comparing with material grown by other techniques. One or more of these states were thought to be due to extended defects or Te precipitates. Low resistivity ZnS contained two deep electron traps at 0.25eV and O.50eV as detected by DLTS. In addition DLOS showed the presence of four further states at 1.25eV, 1.37eV, 1.89eV and 2.19eV below the CB. The first two are thought to be the strong luminescence centres observed by other workers.

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