NEC (United Kingdom)

The electrical resistivities, Hall coefficients and thermoelectric powers of undoped, Si-doped and Mn-modified CrSi 2 crystals have been measured over the temperature range from 90°K to 1100°K. The undoped crystal is a p -type semiconductor with the hole concentration of about 4×10 20 /cm 3 . The hole concentration is decreased by doping silicon or manganese. Manganese atoms act as donors. Specimens heavily doped with manganese are n -type at low temperatures and change to p -type at high temperatures. Analysis of the experimental results leads to the following conclusions: (1) the forbidden energy gap=0.35 eV, (2) the ratio of the electron mobility to the hole mobility≃0.01 and (3) the density of state effective mass of electrons≃7 m 0 and that of holes≃5 m 0 . Assuming that the effective masses and the mobility ratio do not vary with temperature, the temperature dependence of the thermoelectric powers of the undoped and doped crystals can be explained satisfactorily.

The coupled Boltzmann equations are used to develop a theory of the electron-phonon interaction in strong electric fields which provoke the stimulated emission of phonons. As to the electron-phonon interaction three cases are considered : (i) the interaction through the piezo-electric coupling, (ii) the interaction through the deformation potential and (iii) the interaction through the polar scattering. An estimation is made of the time till the onset of the current decay to saturation after applying the voltage pulse in semiconducting CdS. The dependence of the decay time upon the electron concentration is also investigated. The theoretical results are in semi-quantitative agreement with experiments. The possibility of current saturation by stimulated phonons in Ge is examined. The conclusion is that the occurrence of such phenomenon in Ge is highly impossible except at very low temperature. A study is also made of the possibility of the current instability in GaAs by the electron-phonon interaction through the polar scattering. The result is not conclusive, because the know ledge of the distribution function of electrons in polar crystal in a hot electron state is not sufficient enough to permit an accurate estimate of the critical drift velocity. However, it is found that the situation in GaAs is quite different from that in CdS. In the latter case the phonon with small wave number (q~10 5 /cm) is mainly amplified in the narrow Cerenkov cone, and when their number becomes enormous, the current begins to decrease. On the other hand, in the case of GaAs, only the phonons with much larger wave number (q~l0 7 /cm) would be amplified and they can reduce the current, even when their number is small.

KCuF3 is one of the typical example of the Heisenberg type one-dimensional antiferromagnet with S=1/2. It has a broad maximum in susceptibility curve at about 243°K but the antiferromagnetic ordering temperature is only 20°K (or 38°K). Recent experimental results on crystallogaphic, susceptibility, specific heat, neutron diffraction, ERS and NMR measurements etc. are summarised and more recent NMR studies of the dynamics of this system will be reported in greater detail. All these results are strongly characteristic of one-dimensionality of this spin system.

The substrate leakage current vs. gate voltage characteristics of MOS-FET was examined over a wide range of device parameters and measurement conditions. With the increase in the gate voltage, the substrate current increases until it reaches a maximum value. Then it decreases to the value of the generation-recombination current. The substrate current has a high value at low measurement temperatures, high drain voltages, high impurity concentrations of silicon substrates, thin gate-oxide thicknesses and a large drain current. These experimental results were semi-quantitatively explained on the basis of a model in which the substrate current is caused by the first-order impact ionization of the carriers within the pinched-off region. The observed increase of the substrate current is mainly dominated by an increase of the drain current, and the decrease of the substrate current is mainly dominated by a decrease of the impact ionization coefficient.

A phase diagram of the system MnSi-Si containing Si in the range from 45.00 to 57.00 wt% is determined by the methods of X-ray, metallographic and thermal analyses and also by the electrical measurements. It is found that the intermetallic compound in this system is MnSi 1.72 . This compound has the maximum melting point of about 1145°C in the neighboring composition range. A single crystal of MnSi 1.72 is obtained by the Czochralski method.

The electrical resistivity, Hall coefficient and thermoelectric power have been measured over the temperature range from 4.2 to 800°K on Co 1- x Mn x Si(\(x{\lesssim}0.06\)), Co 1- x Cr x Si(\(x{\lesssim}0.04\)) and, Co 1- x Cr x Si(\(x{\lesssim}0.10\)) solid solutions. The specimen is n -type when x =0. With increasing x , the specimens of Co 1- x Ni x Si become more n -type, while in the specimens of Co 1- x Mn x Si and Co 1- x Cr x Si, the contribution of hole conduction in-creases. The experimental data are qualitatively interpreted in terms of the overlapping band structure scheme, which is appropriate for Co 1- x Fe x Si solid solutions. It is concluded that the replacement of cobalt with manganese or chromium produces approximately one hole per atom, while the replacement of cobalt with nickel produces approximately one electron per atom in the solid solutions.

The electron-pair model, proposed for superconductivity by Bardeen, Cooper and Schrieffer, is discussed in the configuration space. The wave function for the system of electron-pairs is taken as follows,

Based on effective mass theory, broadening of Landau levels in two dimensional electron gas is obtained bar taking account of multiple scattering due to impurities. Using the diagram technique, one particle Green's function is derived and it is shown that the physical origin of the level broadening is characteristic of two dimensional electron gas. Using the Green's function, the notches occuring in capacitance-voltage characteristics at low temperatures are qualitatively discussed in terms of the broadening of Landau levels. The implication of this broadening of Landau levels is discussed in relation to transport phenomena of two dimensional electron gas in strong magnetic fields.

The substitution of a small amount of Co 2+ and Ge 4+ for Fe 3+ in yttrium iron garnet causes remarkable changes of the magnetic anisotropy constants and Δ H . The magnetic anisotropy constants K 1 and K 2 of Y 3 Fe 5- x Co x /2 Ge x /2 O 12 depend strongly on temperature, and K 1 is linear to the concentration of cobalt at a given temperature, where x =0, 0.0042, 0.017, 0.048. The effect of cobalt substitution is similar to the case of magnetite containing cobalt, and can be accounted for by Slonczewski's theory with the value of the parameter |αλ|=100 cm -1 . The line width Δ H increases with the increase in cobalt concentration.

A trial is made to clarify the cause of the high resistance region at the interface between the GaAs substrate and the epitaxial layer grown by the Ga/AsCl 3 /H 2 process. Layers were grown under conditions (a) the temperature of the substrate is changed during the growth run and (b) excess arsenic vapor is introduced at the beginning of the growth. In layers grown under high arsenic vapor pressure, the high resistance region does not exist and instead a very thin low resistance region appears at the interface. Such phenomena are qualitatively interpreted as due to the deviation of arsenic vapor pressure from that in the steady state condition in the reaction system. Hall measurement on the high resistance region at different temperatures shows that the high resistance is due to deep acceptors whose energy level is approximately 0.5 eV from the valence band edge and which are inferred to be related with arsenic vacancy.

Doped and undoped GaAs films have been grown epitaxially on semi-insulating or heavily doped n-type (100) plane substrates by the AsCl 3 /H 2 vapor transport method. Doped GaAs single crystals were used as source materials for doped films, and Ga source was used for undoped films. The carrier-concentration of films grown from Te-doped source material varies from 2 to 11 times that of the source GaAs, depending on the growth condition. The carrier-concentration of the films grown from Sn-doped source material agrees with that of the source GaAs, without depending on the temperature difference between the source and substrate. Ge is doped into films as donors, but Si is not doped at all. The carrier-concentration profiles of the films grown on heavily Te or S-doped substrates are strongly affected by autodoping from the substrates. While in the case of Si or Sn-doped substrates autodoping is not observed.

Transverse magnetoconductance of two-dimensional electron gas in strong magnetic field is calculated on the basis of the model and approximation used in a previous paper. Calculating two particle Green's function consistently with the approximation for one particle Green's function yields transverse magnetoconductance free from divergence. Strong energy dependence of one particle spectral density leads to some characteristic features of two-dimensional electron gas in strong magnetic fields. Transverse magnetoconductance calculated as a function of gate voltage exhibits broadening of Landau levels and is sharply cut off in midgap regions between Landau levels. These results are discussed in connection with the experiment of Fowler, Fang, Howard and Stiles. Within the model and approximation of the present paper, splittings of Landau levels are also obtained in strong magnetic field.

Physical, chemical and electrical properties of Si rich silicon nitride (SiN) film deposited by using the SiH4–NH3 system have been studied. By changing the volume ratio of NH3 to SiH4 from 100 to 1/30, samples with varying excess Si content within SiN films can be obtained. The current conduction mechanism of the SiN film can be represented by the superposition of Poole-Frenkel current, field ionization current and hopping current as reported by Sze in the Si3N4 films without Si excess. Energy levels of trapping centers responsible for Poole-Frenkel current become shallow while their densities remain nearly constant as a function of the Si content. From the dependence of the shift characteristics of C–V curves, it has been deduced that the maximum charge storage within the SiN film is controlled by current conduction through the film.

For the purpose of obtaining ZnS single crystals the growth from several inorganic salts was examined. Among these salts KCl was most adequate. Using this as flux needle-like crystals as large as 1 mm in diameter and 10 mm in length were obtained as well as thin flat crystals of about 4 mm in diameter. Most crystals were of wurtzite structure. In needle-like crystals c-axis was parallel to the needle-axis and in flat crystals it was vertical to the flat surface. The content of potassium in these crystals was determined to lie between 0.1 to 0.4 atomic percent.

The influence of various device parameters on the characteristics of MNOS (metal-nitride-oxide-silicon) memory devices has been examined. Three types of charge storage are found to exist according to the thickness of SiO2 film and the specific conductivity of Si3N4 film; injection type, ion drift type and mixed type (where only negative charges are stored at the SiO2-Si3N4 interface under both polarities of bias voltages). Furthermore, even if the thickness of Si3N4 film is changed with other parameters kept constant, no change of charge storage types is observed. It is considered that there are two discharging mechanisms, namely, the one where stored charges near the interface between SiO2 and Si3N4 films flow through SiO2 film into silicon and the other one where the stored charges flow into bulk region of Si3N4 film.

The raised bog sediments that have been continuously accumulated over time represent the most suitable natural object which enables us to reconstruct Late Glacial and Holocene vegetation and palaeoclimates. Bog peat consists of organic carbon formed in situ. It contains moss, plant fragments and microfossils that are necessary for the study of palaeovegetation and palaeoclimate. However, a successful study of palaeoenvironment can be carried out on the basis of investigation of a great quantity of samples along the whole peatbog thickness. In the present paper, the authors present the results of palynological, botanical investigations and radiocarbon dating of 31 peat samples taken from the raised bog Velikoye, located in the eastern part of Kaliningrad Region. The data obtained have enabled us to reconstruct the palaeovegetation, reveal the evolution of the bog and determine rate of peat formation at different evolutional stages over the last 7500 cal BP.

For the purpose of obtaining ZnS single crystals the growth from several inorganic salts was examined. Among these salts KCl was most adequate. Using this as flux needle-like crystals as large as 1 mm in diameter and 10 mm in length were obtained as well as thin flat crystals of about 4 mm in diameter. Most crystals were of wurtzite structure. In needle-like crystals c-axis was parallel to the needle-axis and in flat crystals it was vertical to the flat surface. The content of potassium in these crystals was determined to lie between 0.1 to 0.4 atomic percent.

Ruby crystals were grown by the Czochralski technique by using an induction-heated iridium crucible in various atmospheres. The crystals are grown along directions of the a - and c -axes and of 60° from the c -axis. The crystal on the whole is free from residual stresses, in contrast to the remarkable and irregular stresses usually found in crystals grown by the Verneuil technique. Laser oscillation were observed in the pulled crystals.

Experimental studies have been made of the zinc diffusion into GaAs Crystals containing dislocations of 0 to 10 8 cm -2 at various arsenic vapor pressures. The diffused layers were observed by means of chemical etching and an infrared microscope. For the diffusion without arsenic, (1) the fast diffusion takes place in the presence of dislocations, and (2) a large number of dislocations, proportional to the square of the zinc concentration, are induced in the diffused layers. For the diffusion under high arsenic vapor pressure, (3) generation of new dislocations is suppressed in the shallow diffusion layers, while in layer deeper than 40∼60 µ the dislocation density is suddenly increased, accompanied by fast diffusion, thereby resulting doubly stepped diffusion front, (4) the rate of the ordinary substitutional diffusion is comparatively lowered, and (5) the existence of ZnAs 2 is detected in the diffused layer. The observed fast diffusion along the dislocations is qualitatively interpreted in terms of an induced dislocation model.

The electronic properties of MNOS diodes consisting of vapor deposited Si3N4 and thermally grown SiO2 films are studied. The diodes exhibiting injection or ion drift type hysteresis are prepared using n- or p-type substrate. It is found that ion drift type MNOS diodes utilizing n-type substrate exhibits a new type of C-V and I-V characteristics, while the other kinds of diodes exhibit similar characteristics as those of MNS diodes. This new type of characteristics are interpreted in terms of single carrier (electron) transport while the other ones are interpreted in terms of electron and hole transport throughout the diodes. Combining these findings with the general concept of charge transport and storage (CTS) model, a two carrier CTS model which gives a qualitative but unified understanding of shift characteristics of flat band voltage as well as C-V and I-V curves associated with MNOS diodes is proposed.