Volume 32, Issue 10, 01 October 1961
Index of content:
- OXIDES: SIC; BI2TE3
32(1961); http://dx.doi.org/10.1063/1.1777041View Description Hide Description
The electrical conductivity and optical absorption data for many compounds of the transition metals suggest that there is a trend from ionic insulators, to metals, to covalent semiconductors as the overlap of atomic orbitals is increased. In this paper it is shown that a correlation exists between the electrical behavior of insulators and metals and the magnitude of the overlap integrals S(dεdε) and S(dεpπ).
32(1961); http://dx.doi.org/10.1063/1.1777042View Description Hide Description
This paper reviews the work which has been carried out in these Laboratories during the past two years on single crystals of bismuth telluride and its alloys.
The combination of experiments on Faraday rotation with those performed previously on galvanomagnetic effects has established that there are 3‐ and 6‐valley band structures associated with p‐ and n‐type Bi2Te3, respectively. However, observations of the anisotropy ratio for the electrical conductivity and of the galvanomagnetic coefficients for heavily doped n‐type material have shown that the shape of the equal‐energy surfaces is dependent on carrier concentration. Similar conclusions have been drawn from the behavior of the Seebeck coefficient at low temperatures.
Measurements on Bi2Te3 at low temperatures and on alloys of Bi2Te3 have shown that the lattice thermal conductivity is particularly sensitive to the substitution of atoms of I, Se, or S for those of Te. It has also been shown that the anisotropy ratio for the lattice thermal conductivity is almost the same for the alloys as for pure Bi2Te3.
32(1961); http://dx.doi.org/10.1063/1.1777043View Description Hide Description
It is shown that the simple ``hopping model'' for the transport processes of mixed valency semiconductors is inadequate for impurity concentrations [similar or greaterthan]1%. In particular, it is necessary to redefine (1) the number of free charge carriers, and (2) the density of available states because of the dominant role played by the impurities in the high concentration range.
32(1961); http://dx.doi.org/10.1063/1.1777044View Description Hide Description
The low‐lying eigenstates of the ``large polarons'' have been calculated by several authors for arbitrary strengths of the electron lattice interaction α. However, if α > 1 the large polaron picture becomes questionable, since for finite temperatures the polaron eigenstates may be strongly affected by the presence of thermal phonons; then a new approach to the polaron theory is applicable which takes into account the atomicity of the lattice and the presence of thermal phonons and which results in the ``small polaron'' picture. The eigenstates of small polarons depend on T. If the eigenstates form a band, the bandwidth is a function of T, and the eigenstates near the band extremum can be expressed in terms of a T‐dependent effective mass. From measurements of the high‐ and low‐frequency dielectric constants, of the temperature dependence of the Seebeck coefficient, and of the electronic mobility, it appears that the eigenstates of the electronic charge carriers in Ce–S semiconductors may be adequately described by the small polaron picture.
32(1961); http://dx.doi.org/10.1063/1.1777045View Description Hide Description
A review is made of the work on reduced and ``doped'' rutile performed since the appearance of Grant's survey article in the Reviews of Modern Physics (1958). Measurements of electrical and optical properties, and of electron spin resonance spectra are discussed. A model of electronic bound states and conduction levels is suggested that is compatible with the results of these experiments. There is strong evidence that the defects in reduced rutile are interstitial Ti3+ ions. At very low temperatures, nearly all electrons are self‐trapped on cation sites (polarons). As the temperature increases, some of these trapped electrons will be excited into the conduction band. The activation energy for this process is approximately 0.007 ev below 50°K, and about one order of magnitude higher around room temperature. It is concluded that conduction takes place in a narrow 3d band associated with Ti ions; the effective mass at the bottom of this band is ∼25m 0. If one assumes that the polaron binding energy can be described with a hydrogenic model, one calculates an effective dielectric constant close to the static value. This result is at variance with the commonly accepted ideas concerning electron lattice coupling.
32(1961); http://dx.doi.org/10.1063/1.1777046View Description Hide Description
A Heitler‐London approach to electrical conductivity is proposed in order to discuss conduction in semiconductors with incomplete d shells, in which the mobility of carriers is very low. The physical conditions which make the hopping motion of the electrons predominant is examined in detail.
32(1961); http://dx.doi.org/10.1063/1.1777047View Description Hide Description
The p, T, x diagram of the Sn‐S system was determined especially in the region of the compound SnS. The pressure of S2 in equilibrium with SnS and a liquid phase was found to extend over several decades up to 25‐mm Hg at the ``Sn‐rich'' side, whereas at the ``S‐rich'' side the S2 pressures in equilibrium with solid SnS and a liquid phase lie between 25‐mm Hg and 100‐mm Hg. It was shown that the existence region of solid SnS very probably lies entirely at the excess sulfur side. The hole mobility in a plane perpendicular to the c axis, ≈90 cm2/v sec at room temperature, was proportional to T −2,2 for higher temperatures. The mobility in the direction of the c axis was about five times smaller. Reversible annealing effects were found for temperatures above 200°C which could be explained by assuming association of neutral Sn vacancies. Absorption measurements showed that the edge absorption is due to indirect transitions. The bandgap was 1.08 ev at 300°K and 1.115 ev at 77°K. Interband transitions in the valence band were also found. The effective charge of the atoms (e*=0.7e 0) and the effective masses of the holes in the three principal crystal directions (ma *=mb *=0.20m 0; mc *≈m 0) were determined from reflection measurements in the infrared. From these values and the value for the density of states mass obtained by means of the Seebeck effect (md *≥0.95m 0), the number of equivalent maxima of the valence band was found to be at least four.
32(1961); http://dx.doi.org/10.1063/1.1777048View Description Hide Description
Measurements of Hall effect and resistivity up to 1300°K on p‐type hexagonal SiC showed an acceptor level for aluminium of 0.27 ev at zero donor concentration and a not yet identified acceptor level of 0.39 ev. The spin multiplicity of this unknown center appears to be four times smaller than that of the aluminium center, so that we may conclude that this unknown center in non‐ionized state has paired electrons. Taking a temperature dependence of the level depths proportional to that of the bandgap, the density‐of‐states effective mass of the holes amounts to 0.59 m 0. The Hall mobility shows at high temperatures the same temperature dependence as that ascribed to scattering of holes by optical phonons. Assuming that optical phonons really come into effect, the behavior of the Hall mobility in the temperature range from 1300° to 300°K can be explained taking also into account the effect of scattering by acoustical phonons and charged impurities. By a study of I–V characteristics of grown junctions in αSiC and also by applying Roosbroeck‐Shockley's theory to the spectral distribution of the p‐n luminescence under forward bias, inhomogeneities were found over the junction area. By means of pyrolysis of gaseous compounds of Si and C pure crystals (4×2×2 mm3) of ``cubic'' βSiC were obtained. With the aid of polarized light the existence of a skeleton of a hexagonal twinning system was found in these crystals, the cubic SiC filling up the pores of this skeleton structure.
32(1961); http://dx.doi.org/10.1063/1.1777049View Description Hide Description
The ESR spectra of oxygen‐deficient and doped rutile have been investigated at liquid helium temperatures. Niobium and tantalum are shown to give rise to the donors Nb 4+ and Ta 4+ rather than Ti3+. The spectrum of reduced rutile depends on the method of reduction. Reasons for this are discussed. Under certain circumstances, involving hydrogen reduction, a particularly simple spectrum is observed. This degenerates and is replaced by a single line as the temperature is raised. A similar effect is obtained by increasing the concentration of centers. Possible assignments are discussed. Vacuum reduction results in distinctly different spectra which persist to higher temperatures. Resistivity measurements indicate a higher activation energy, by a factor of three, for vacuum‐reduced samples.