J. Woods

I-V characteristics of carbon black-loaded crystalline polyethylene

The I-V characteristics of carbon black-loaded crystalline polyethylene were investigated at room temperature and at few degrees above the melting temperature, Tm. The material was chemically cross-linked using silane. It had a large carbon black content of 28 parts per hundred by weight and displayed a strong positive temperature coefficient of resistance (PTCR) effect with a resistivity jump of four orders of magnitude. Logarithmic current-voltage plots were found to be linear with unity slopes at temperatures above and below Tm, indicating good ohmic behaviour. This is in contrast with previous theories which explain the PTCR effect on the basis of electron tunnelling as the current conduction mechanism. A new model capable of explaining both the PTCR effect and the steep reduction in resistance above Tm is presented, according to which the PTCR effect is the result of the co-operative effect of changes in crystallinity and volume expansion.

Preparation of ZnO films by spray pyrolysis

Decomposition of zinc acetylacetonate monohydrate (Zn(acac) 2 .H 2 O) to produce undoped ZnO films by spray pyrolysis has been investigated between 100 and 400°C. Spray trials were conducted in both dry conditions and with excess water. Films with enhanced adhesion, optical transmission and conductivity were deposited above 200°C. With excess water this temperature correlated with a change in reaction mechanism from intermolecular and intramolecular at T 200°C. Water enhanced the reactant utilisation and yielded films with lower resistivity. Use of lamps to heat the spray served only to decrease surface roughness. XRD assessment of preferred orientation and grain size is also presented. Annealing in a reducing atmosphere reduced the resistivity of the films by three orders of magnitude, the lowest obtained being ∼10 -4 Ω.m.

Growth of ZnSe single crystals from the vapour phase

Abstract The growth of single crystals of ZnSe with centimetre dimensions is described. The basic method employed is that in which ZnSe is sublimed down a temperature gradient as the growth capsule is pulled through the furnace. Crystals have been grown at 1250–1300°C in vacuum, and at 1100–1150°C in controlled partial pressures of zinc and selenium. When excess zinc is used with this latter method, the transport rate is high if the zinc is placed directly in the reservoir initially. However, if the zinc is mixed with the charge, transport is almost negligible. An X-ray photographic method is described which allows the (111) and ( 1 1 1 ) faces of suitably prepared slices to be distinguished. Some luminescent and photoconductive properties are also reported.

The growth of CdS in sealed silica capsules

Using emission and mass spectrographic techniques it has been demonstrated that carbon monoxide is evolved from a silica capsule when it is sealed-off from a vacuum system. Consequently crystals of CdS grown by sublimation down a temperature gradient in sealed, evacuated silica capsules were in fact grown in the presence of carbon monoxide at a pressure of the order of 1 Torr. At elevated temperatures the CO reacts with the CdS to form CS2 and COS. The variability in the quantity of CO evolved during seal-off accounts for the observed variations in the quantity of material transported in different growth runs. Experiments are also reported which demonstrate that silica tubing is porous to certain metallic ions when it is held at a temperature near 1150°C. Because of this effect many crystals grown in this laboratory have, in the past, been accidentally contaminated with copper.

The absolute determination of CdTe crystal polarity

Abstract The absolute and discriminatory methods of identifying the {111} polar surfaces of CdTe are reviewed and the division of authors between the conventions of Warekois (1962) and Fewster (1983) is noted. In this work electron microdiffraction patterns were recorded which support the findings of Fewster and we note that the Te{ 1 1 1 } surface of CdTe is the best for CdTe or (Hg,Cd)Te epitaxy. Finally, reliable unambiguous polarity revealing etchants are recommended for CdTe.

The effect of CdTe substrate orientation on the Movpe growth of CdxHg1−xTe

Abstract Layers of Cd x Hg 1− x Te have been grown by MOVPE on CdTe substrates oriented on /t 111t B and on the planes 2° off /t 100t towards (110). Their structure has been investigated using the combined techniques of RHEED and scanning electron microscopy. While the surfaces of layers grown on /t 111t substrates were smoother than those deposited on the samples cut 2° off /t 100t and also avoided the problem of lamella twins in the CdTe substrate, they invariably exhibited a twinned grain structure. Consequently it is concluded that for the purpose of device applications, particularly when conduction mechanisms in the plane of the layers are involved, growth on substrates cut 2° off /t 100t may be preferable.

The growth and defect structure of single crystals of zinc selenide and zinc sulpho-selenide

Abstract Single crystals of zinc selenide and solid solutions of zinc sulpho-selenide with sulphur compositions up to 60% have been grown successfully by a vapour phase technique. The growth interface of all boules examined lies within 20° of a {111} non-metal face of the cubic crystals. Examination in transmission in the electron microscope shows that the crystals of zinc selenide contain large regions of excellent crystallinity. The major defects are long thin twins, which etching studies have revealed to be ortho-twins. In the mixed crystals there are many more thinner twins, and intrinsic stacking faults are frequently observed. A mechanism which relies on the preferential growth rates of {111} faces might account for the incidence of some of the ortho-twins. However the intrinsic character of all the stacking faults observed in the mixed crystals suggests that these defects owe their origin to post-growth stress. Such defects would have a high stability in a material with a large ionic component in its bonding. In the more covalent zinc selenide, stress is relieved by the passage of numerous partial dislocations over successive {111} planes adjacent to intrinsic stacking faults, which leads to the faults widening into ortho-twins.

Growth of single crystals of cadmium sulphide

Abstract A method is described in which CdS crystals can be grown in controlled atmospheres of the constituent elements. The growth rate and growth direction are strongly influenced by the atmosphere, as is the incidence of macroscopic voids and internal strain in the crystals. These and other problems are discussed. The photoconductive and luminescent properties of the crystals are described briefly. It is shown that although growth in high partial pressures of sulphur introduces numerous acceptor levels ∼0.16 eV above the valence band, they are automatically compensated by the introduction of an equivalent concentration of shallow donors.

Growth of single crystals of zinc selenide from the vapour phase

Abstract More than one hundred crystals of zinc selenide have been grown from the vapour phase using a technique in which the material is sublimed down a temperature gradient in a sealed tube which is connected to a reservoir containing either zinc or selenium. It is found experimentally that maximum transport occurs when the temperature of the resevoir is adjusted to give a vapour pressure of the element in the reservoir equal to that of the same element above the compound under Pmin conditions. It is demonstrated that in these circumstances the vapour above the subliming charge is very nearly stoichiometric. The composition of the vapour over the growing interface is less stoichiometric and is rich in that element which is contained in the reservoir. Using the ideas developed by Faktor et al. [6–9] it is shown that a calculated temperature difference of the order of 20°C is required to provide the observed transport rate. The experimental temperature difference is also of the order of 20°C. The conclusion is that the growth of zinc selenide is limited by the transport processes which occur in the growth capsule.

Structural properties of crystals of CdTe grown from the vapour phase

Abstract The structural defects in large single crystals of CdTe grown from the vapour phase have been investigated using transmission electron microscopy and etching techniques. Twin bands, sub-grain boundaries and presipitates are the major features described.