A.W. Brinkman

The effect of Mn on the positive temperature coefficient of resistance characteristics of donor doped BaTiO3 ceramics

BaTiO3 positive temperature coefficient of resistance (PTCR) specimens were made from commercial BaTiO3 which was mixed with 0.35 mol.u2009% of Ho2O3 to make it semiconducting and 0.07 mol.u2009% of MnCO3. After sintering at 1320u2009°C the samples were annealed in batches at 1220u2009°C for various periods between 0 and 5 h. The effect of Mn was studied by making direct comparison with previous results obtained from Mn free but otherwise identical specimens. Room‐temperature dielectric measurements in the audio and radio frequency ranges revealed that Mn had a negligible effect on the grain bulk resistance. Mn was found to result in an increase in the minimum and maximum values of the resistivity, the temperature at which the resistivity is a maximum and the slope of the resistivity‐temperature characteristic in the transition region, all of which were attributed to an enhancement of the potential barrier at the grain boundaries. The acceptor state energy of the Mn‐doped samples was found to be ∼1.4 eV, while a lower va...

The influence of Mn on the grain‐boundary potential barrier characteristics of donor‐doped BaTiO3 ceramics

Two compositions of BaTiO3 positive temperature coefficient of resistance ceramics, prepared identically except for the fact that a small addition of Mn (0.04u2009at.u2009%) was made to one of them, were studied. The samples were sintered simultaneously in air at 1400u2009°C for 1 h and then annealed at 1200u2009° for 5 h, using a muffle furnace. Room‐temperature dielectric measurements in the audio‐ and radio‐frequency ranges confirmed that Mn has a negligible effect on the bulk resistance. Arrhenius plots of resistivity vs 1/[Te’m(T)] were found to give straight lines for Tc<T<Tmax (where e’m is the relative permittivity of the specimen measured at a constant frequency of 30 kHz, Tc is the ferroelectric transition temperature, Tmax is the temperature corresponding to the maximum in resistivity, and T is the absolute temperature), in accordance with the well‐known Heywang model. The height of the potential barriers at different temperatures, as calculated from the slopes of these plots, were found to increase by about 4...

Sintering behaviour of zinc stannate

On etudie la formation de stannate de zinc par reaction a letat solide a haute temperature entre loxyde detain (IV) et loxyde de zinc. On examine le comportement au frittage du compose obtenu

Electrical properties of ZnxCd1-xSe

The variation in bandgap energy with composition was determined for single crystals of ZnxCd1-xSe at 300 and 90 K. Au-ZnxCd1-xSe (x<0.45) diodes were fabricated and used to determine the dependence of barrier height and uncompensated donor density on composition. Deep levels were also investigated in these diodes using photocapacitance, which revealed the presence of two dominant levels with activation energies of 0.55–0.6 and 1.14–1.16 eV (referred to the valence band edge) that were independent of the composition.

A comparison of the structure of CdTe and (Hg, Cd)Te layers grown by MOVPE on {111}A and {111}B CdTe substrates

Abstract This paper presents a comparison of the structural properties of layers of CdTe and (Hg, Cd)Te grown onto {111}A and {111}B CdTe substrates as determined by the combined techniques of RHEED, SEM and cross-sectional TEM.

The effects of donor dopant concentration on the grain boundary layer characteristics in n‐doped BaTiO3 ceramics

Positive temperature coefficient of resistance BaTiO3 specimens containing different donor dopant concentrations of Ho ranging from 0.05 to 1.8 at.u2009% were investigated. The intergranular barrier layer capacitance per unit area, C’L, measured at a constant frequency of 30 kHz at both 40 and 160u2009°C was found to be proportional to the donor concentration up to 0.55 at.u2009%, but then began to decrease as the donor concentration was increased beyond this. This indicated that both the density of acceptor states at the grain surfaces, Ns, and the relative permittivity eL of the material within the barrier layer were not affected by donor impurity concentrations below 0.55 at.u2009% Ho. However, above this level of Ho concentration, the decrease in C’L appears to be related mainly to an increase in the value of Ns although it is possible that there were changes in eL. Initially both the maximum resistance and the room‐temperature resistance (normalized per grain boundary per unit area), ρ’max and ρcold, respectively, w...

Growth and characterisation of ZnTe and ZnTeCdTe superlattices on GaAs substrates

Abstract Epitaxial layers of ZnTe and CdTeue5f8ZnTe superlattices have been grown by atmospheric pressure MOVPE on {;100}; GaAs substrates and characterised by RHEED, SEM and TEM. Although they are of high crystal quality, the ZnTe layers exhibit a characteristic ridged morphology due to anisotropic facetting. These layers are found by TEM to contain a high density of misfit dislocations extending throughout a 2 μm thick layer. In contrast, CdTeue5f8ZnTe superlattices grown under the same conditions are shown to have a much improved surface morphology but with a characteristic assymetric rectangular network of slip traces and cracks. These are attributed to the thickness of the CdTe layers and the high growth temperature causing the strain in the system to be largely relieved.

ZnTe and CdTe:ZnTe superlattices grown by MOVPE

Epitaxial layers of ZnTe and CdTe:ZnTe superlattices have been grown by MOVPE on {100} and {⦶111}B GaAs substrates at temperatures down to 300°C using di-isopropyl telluride. In contrast to growth at 410°C using di-ethyl telluride, these layers exhibit excellent morphology. Using RHEED, however, it has been possible to show that the morphology remains better on the {⦶111}B. ZnTe has also been grown on {100} GaSb.

Current transport mechanisms in epitaxial CdS/CdTe heterojunctions

Abstract Epitaxial n-CdS/p-CdTe heterojunctions have been fabricated by the vacuum evaporation of CdS onto phosphorus doped {⦶111} B CdTe substrates. Analysis of the current-voltage and capacitance-voltage characteristics suggests that electrical transport across the junction is dominated by a multi-step tunneling process. Although open circuit voltage and short circuit current values were high, the overall photovoltaic conversion efficiency of the devices was restricted to ≈ 6%, because of the relatively high resistivity of the CdTe substrates.

Growth of HgTe-ZnTe strained layer superlattices by MOVPE

Abstract HgTe-ZnTe strained layer superlattices have been grown by MOVPE on { 111 }B CdTe, { 111 }B GaAs and {100} GaAs substrates. It was found that the presence of the Zn precursor, dimethyl zinc (DMZn), seriously affected the growth of HgTe and it was necessary to ensure complete flushing of the reactor between ZnTe and HgTe phases of the growth cycle. The superlattices were characterised by a combination of SEM, EDX, RHEED, XTEM and infra-red transmission spectroscopy.