C. Paorici

Solution Growth and Characterisation of L-alanine Single Crystals

Single crystals of L-alanine have been grown from buffered aqueous solutions and characterised as to their optical quality via wavefront distortion analysis, electrooptical response and harmonic generation efficiency. Refraction indices as well as phase matching loci were found in satisfactory agreement with previously published data for crystals grown in non buffered solution. Estimates for the electro-optical response are reported for the first time. The experimentally observed crystal habit is discussed in terms of morphological importance (M.I.), which was found to be in disagreement with the predictions of crystallographic criteria based on the interplanar distance d hkl , on the periodic bond chains (PBC) and the attachment energy E att . This disagreement is tentatively attributed to the presence of impurities and non-appropriate supersaturation conditions.

A temperature variation method for the growth of chalcopyrite crystals by iodine vapour transport

Abstract A two-four fold increase in the size of some I-III-VI compounds crystals grown by closed-tube iodine vapour transport was obtained by means of a time-varying temperature profile procedure. The principle of the method consists in a gradual raising of the source temperature, which allows, first, a reduced primary nucleation, and, secondly, the avoidance of constitutional under-cooling during the first stages of the nucleus growth. The procedure is described for the growth of CuInS 2 , CuGaS 2 and AgIn 5 S 8 compounds.

Dislocation-free silicon-doped gallium arsenide grown by LEC procedure

Conditions have been found for growing GaAs single crystals, weighing 300–500 g with a silicon content in the (1–5) × 1018 atoms/cm3 range. By adding in a progressive way the silicon dopant to the growing crystal, the twinning formation could be avoided when proper initial silicon concentrations in the melt were chosen. This procedure also proved to be suitable for yielding single crystal ingots free of dislocation lines for about 90% of their length when their diameters reach 4.0–4.5 cm.

Chemical transport mechanisms in the Cd:Te:H:I system

The vapour phase chemical transport mechanisms in a closed tube are investigated for the Cd:Te:H:I system. The experimental results are compared with a proposed thermodynamic model. It is concluded that the closed-tube chemical transport mechanisms are mainly governed by (1) diffusive transport of the sublimation products of CdTe at low pressures, (2) thermal convection assisted diffusion and laminar flow of the gaseous species in an atmosphere of iodine [I2(g), I(g), HI(g)] at intermediate pressures, and (3) SLV processes at still higher pressures.

A new process for synthesizing high-purity stoichiometric cadmium telluride

In the frame of a research project aimed at preparing high-resistivity CdTe single crystals, the authors have developed as a first step, a new, low cost, fast technique which allows to synthesize from the elements polycrystalline CdTe with very low stoichiometric deviations and with a level of background impurities much lower than usually available in commercial materials. The material obtained by this technique as will be described and discussed, appears especially suitable for preparing source charges for physical vapour transport growth experiments.

Chemical-transport properties of the Cd:Te:H:Cl:N system

A method to grow CdTe single crystals by chemical transport in a closed tube has been described, where the transport agent is hydrochloric acid obtained by thermal dissociation of NH4Cl. It has been shown on a thermodynamical basis that the growth is possible only in a rather narrow range of temperature, total pressure and composition of the system. The formation of liquid tellurium prevents the occurrence of crystal growth out side of this narrow range.

Stoichiometric deviations and partial-pressure measurements in solid–vapour cadmium telluride system

Abstract In the frame of a research project aimed at developing a single-crystal-growth technology of cadmium telluride (CdTe) for electrooptics and photorefractive applications, the authors have implemented a laser absorption technique by which the partial pressure of the vapours (Te2(g), Cd(g)) over solid CdTe can be monitored as a function of temperature in the temperature range 500–900°C. By testing polycrystalline materials of different origins, this technique proved to be a reliable tool for selecting the most suitable polycrystalline CdTe to be used as a source in the physical vapour transport (PVT) growth of CdTe single crystals.

Closed-tube chemical-transport mechanisms in the Cd:Te:H:Cl:N system

Abstract The transport mechanism of solid CdTe in the vapor-solid Cd:Te:H:Cl:N system, in closed tubes, was experimentally and theoretically investigated as a function of the overall pressure, temperature, transport-agent concentration and geometry of the system. It was assumed that the chemical equilibrium is established in each zone of temperature, and that the transport rate only proceeds by ordinary diffusion, laminar flow and thermal convection. The conditions by which thermal convection can be reduced in the system were found. In the cases where the experimental results were compared with the theoretical previsions, a satisfactory agreement was found.

Solution crystal growth of urea and derivatives for nonlinear optical applications

Urea is an excellent nonlinear optical (NLO) medium. However, growth of large high-quality crystals for practical use still remains a challenge due to its unfavourable growth properties. Improved crystalline and optical quality could be expected in derivatives of urea. In this contribution we report on the growth of urea and its derivatives: monomethylurea (NMU), 1,1-dimethylurea (1,1-DMU), and 1,3-dimethylurea (1,3-DMU). Satisfactory quality and reasonable size of both urea and monomethylurea crystals have been achieved. Some evaluation of the quality of the grown crystals is reported. This work shows that NMU exhibits a better crystal habit than urea does, while preserving almost the same transparency window. This would suggest that NMU is a potentially promising material in nonlinear optical applications.

Growth of monomethylurea single crystals

Abstract We report on the crystal growth of monomethylurea, a biaxial organic compound potentially interesting as a ultraviolet non-linear optical medium. Large crystals of good quality have been obtained from methanol solution by the constant-volume decreasing-temperature method. Growth procedure and results are briefly discussed.