Defect-free Single Crystals Research Articles

A comparative study of the electroreduction of polycrystalline and single crystal CdS electrodes: implications for h 2 evolution in colloidal suspensions

The cathodic decomposition of CdS, both poly crystalline and single crystal, has been studied by electrolyte electroreflectance, cyclic voltammetry and impedance measurements. With a defect-free single crystal, CdS electroreduction (surface deposition of Cd 0 from lattice Cd 2+ ions) has a very high overpotential (1.5–2.0 V). However, after photocorrosion or with polycrystalline electrodes the overpotential is reduced to 100–200 mV due to the accumulation of lattice defects at the electrode surface; reaction is thus possible even at potentials positive of the flatband. Simultaneous with Cd° deposition, the energy levels of the semiconductor are shifted with respect to the electrolyte by 400 mV, conduction band electrons gaining energy to reduce water. Implications for hydrogen evolution at CdS particles are discussed.

The theoretical and experimental fundamentals of decreasing dislocations in melt grown GaAs and InP

Abstract The availability of high quality substrate material is a prerequisite to future advances in the emerging high-speed electronic and photonic device/IC technologies. Consequently, significant progress has been made recently in reducing the dislocation densities in the bulk growth of GaAs and InP. We shall review the evidence implicating thermal-stress induced slip as the predominant mode of dislocation generation in these compounds. Then, the quasi-steady state heat transfer/thermal stress theory for defect generation in LEC growth together with its key predictions are outlined. Particular emphasis is placed on the degree of impurity hardening (reflected by an increase in the critical resolved shear stress) and ambient temperature gradient reduction as they relate to suppressing dislocation formation. We conclude by discussing the currently used experimental approaches to grow virtually defect-free single crystals, explain their shortcomings, and examine the outlook for future improvements.

Relationship Between Structure and Properties for Donor-Doped and for Acceptor-Doped Polyacetylene

Abstract Two basic structural types are experimentally derived for polyacetylene complexes: channel structures (Na, K, Rb, and Cs) and layered structures (iodine and a variety of other acceptors). Observed variants for the layered structures include first-stage and higher-stage complexes, with either complete or fractional occupation of dopant on the dopant-containing planes. A third structural type, in which dopant is statistically distributed, is predicted up to high dopant levels for lithium dopant. These structural results are used to explain observed properties and to predict properties obtainable for nearly defect-free single crystals. Discussions will include the structural origin of electrochemical discharge curves, major increases in conductivity on annealing K-doped and Rb-doped polyacetylene, the extremely high thermal stability of alkali metal channel complexes, and predicted mechanicals and transport properties for single-crystal complexes.

The single crystal growth and characterization of indium phosphide

The authors have grown dislocation-free and defect-free InP single crystals, doped with sulphur or zinc, by means of the liquid encapsulation Czochralski technique. The perfection of the crystals was studied by transmission X-ray topography and preferential etching. By optimizing the growth conditions, in particular the temperature gradient in the crystal, the generation of dislocations could be suppressed at doping levels as low as 2×10 18 cm -3 in sulphur-doped crystals and 1×10 18 cm -3 in zinc-doped crystals. Via chemical analysis of the first-to-freeze portions of the crystals the effective distribution coefficients of sulphur and zinc were calculated and compared with earlier published data. In one sulphur-doped crystal unusual inclusions (“grappes”) were detected at a density of 2×10 4 cm -3. They were analyzed by preferential etching, scanning electron microscopy and energy dispersive X-ray analysis and were found to consist of a central core surrounded by dislocation loop arrays in the four 〈110〉 directions. The mechanism underlying the formation of these inclusions is discussed.

Introductory lecture: solid-state polymerization

Two different approaches to the problem of how to synthesize macroscopic and nearly defect-free single crystals of polymers are reviewed with the aim of clarifying the underlying physico-chemical principles and of defining the scope of such methods for the further development of the solid-state physics of polymers. The two methods are (i) the topochemical polymerization of diacetylenes and (ii) the simultaneous polymerization and crystallization of trioxane close to the ceiling-equilibrium. Polymers with a backbone of conjugated multiple carbon bonds are formed in the first case via a reaction which proceeds without nucleation phenomena such that the single-crystal nature of a polymerizing crystal is always retained up to quantitative conversion.Single crystals of poly(oxymethylene) grow in the second case following a BCF-type of crystal growth, the growth features being controlled by the nature and the concentration of the catalyst employed.

X-ray topographic study of defects in KH 2PO 4 single crystals and their relation with impurity segregation

The occurrence of crystallographic defects in solution grown potassium dihydrogenphosphate single crystals, KDP, has been studied by means of X-ray transmission topography. The characteristics of the dislocations which are found can be predicted on a theoretical basis. They are generated at the seed but their numbers depend on the growth conditions. The occurrence of other defects such as growth bands and the “ghost”, can be correlated with the impurity concentration in both the prismatic and pyramidal facets. It is shown that the entrance of the M 3+ impurities in the prismatic facets severely influences the perfection of the pyramidal facets. It is finally concluded that defect-free single crystals can be achieved only if they do not taper.

Nucleation of martensite in small particles

The nucleation of martensite is studied in defect-free single crystals of iron and iron-nickel over a size range of 0.002–0.2 μ, where the particles are still transparent to 100 kV electrons. The particles are first grown as a continuous precipitate in a copper matrix and then extracted by an electro-polishing technique. By varying the nickel content in the starting alloy, the particles (when extracted) could be observedin different stages of transformation. Particles of high nickel content remained fully austenitic and defect-free when extracted. Particles of low or zero nickel content transformed to single crystal twinned martensite when extracted. Particles of an intermediate nickel content exhibited only partial transformation. In the latter case thin plates of martensite grew into the crystal from the surface until stopped by the elastic restraints of the austenite. It thus appears that further growth of the nucleus is inhibited until it can generate stresses sufficient to nucleate a twinning dislocation.