Makoto Sakata

Anisotropic galvanomagnetic and thermoelectric properties of n-type Bi2Te3 single crystal with the composition of a useful thermoelectric cooling material

Abstract A single crystal of n -type Bi 2 Te 2..85 Se 0.15 with the composition of a useful thermoelectric cooling material was grown by the Bridgman technique. Components of the resistivity tensor, Hall tensor and magnetoresistivity tensor were measured over the temperature range from 77 to 300 K, and the results were analyzed with the six-valley model. The scattering parameter r , the reduced Fermi energy and the Lorentz number were computed by the numerical analysis of the Fermi-Dirac integral with an arbitrary real index, r increases monotonically with increasing temperature and the values are −0.72 and −0.57 at 90 and 270 K, respectively. The carrier density has a constant value of 1.0 × 10 25 m −3 over the temperature range from 90 to 270 K; however, above 270 K it increases with increasing temperature under the influence of the intrinsic region and has the somewhat larger value of 1.07 × 10 25 m −3 at 300 K. The temperature dependence of two components of the lattice thermal conductivity tensor can be explained in terms of the Umklapp process and electron-phonon scattering.

Crystal growth of manganese silicide, MnSi∼1.73 and semiconducting properties of Mn15Si26

Boules of the most silicon-rich silicide of manganese, MnSi∼1.73, were grown by the Bridgman method in the composition range from 62.96 to 63.64 at% Si. There are plate-like MnSi precipitates parallel to thec-plane of Mn15Si26 matrix in all boules except at their upper and lower ends. The amount of the MnSi precipitates was about 2 vol%. Measurements of electrical resistivity, Hall coefficient, and thermoelectric power of the boules were made in the temperature range from 77 to 1200 K. Thermoelectronic properties of Mn15Si26 were estimated on the basis of the distribution state of MnSi precipitates in the boule by assuming that metallic MnSi and semiconducting Mn15Si26 alternately crystallized in lamellae. Although the resistivities and thermoelectric powers measured in thec- anda-axis directions of Mn15Si26 showed anisotropy, the former were proportional to exp (4073/T) in the intrinsic region in both directions. The Hall coefficients showed isotropy over the whole temperature range. The degenerate holeconcentration of Mn15Si26 was determined to be 2.1×1027 m−3. Hall mobilities of Mn15Si26 in both thec- anda-axis directions obey aT−3/2-law above 600K. The Hall coefficient calculated from this relationship was in good agreement with the one found for Mn15Si26 in the intrinsic region. The effective-hole masses of Mn15Si26 along thec- anda-axis were found to be, respectively, 15 and 11 times larger than those of a free electron. Assuming that acoustic lattice scattering is dominant and that carriers obey Fermi-Dirac statistics, the temperature dependence of the calculated thermoelectric power was in reasonable agreement with the one estimated for Mn15Si26 in the temperature range from 400 to 1200K.

Valence-band parameters and hole mobility of Ge-Si alloys-theory

Using the Lawaetz method the authors have estimated the k.p band parameters and determined the band structure near the valence-band edge of the Ge-Si alloy system. They have also obtained the coupling constants between holes and phonons by Wileys method. Using these two kinds of parameters they have calculated the time for relaxation of holes due to the lattice scattering, where they have taken into account the nonpolar optical phonon, the impurity scattering due to ionised and neutral centres and also the alloy scattering. The intervalence-band interactions are shown to produce the complicated temperature dependence of the hole mobility.

Striations and crystal structure of the matrix in the MnSiSi alloy system

Alloys in the MnSiue5f8Si system with striations in the crystal grains were prepared by arc melting. The crystal structure of the matrix and the orientation relationships between the striations and the matrix of alloys with 62.5–63.8 at.% Si were examined by metallographic observation, X-ray diffraction and the electron microprobe X-ray analyser technique. The striations in all the alloys were plate-like MnSi precipitates and were parallel to the c plane of the matrix. MnSi precipitates were observed even in alloys with free silicon and were stable in alloys annealed at 1100 °C for 100 h. The matrix of all the alloys was Mn15Si26. The lattice parameters of the alloys with 62.5–63.8 at.% Si were constant with a0 = 5.525A and c0 = 65.44A. There were no specific orientation relationships between the striations and the matrix.

Study on the formation of Fe1 − xMnxSi2 from the sintered FeSi-Fe2Si5 eutectic alloy doped with manganese

Abstract The formation and growth of β-FeSi 2 from the sintered eutectic alloy composed of e-FeSi and α-Fe 2 Si 5 doped with manganese has been studied by measuring the electrical resisitivity θ and thermoelectric power Q as well as by X-ray analysis for annealing temperatures from 1073 to 1173 K. The fully annealed specimen of (1 − x ) FeSi 2− x MnSi 2 formed a single β phase in the range 0 ⩽ x ⩽ 0.108, and the lattice parameters of the b and c axes linearly increased with x up to x = 0.108, whereas that of the a axis was independent of x . The ρ and Q values increased with the amount of the β phase, and two peaks with a notch between them were found on the curves of ρ and Q vs . isochronal annealing temperature. The temperatures of the peaks and the notch for the specimen with a manganese concentration of x = 0.01 were 1115, 1135 and 1128 K, decreasing with increasing x . X-ray analysis showed that two different reaction mechanisms operate on the β-phase formation, as for undoped FeSi 2 . One is a two-stage transformation consisting of the disproportionate reaction α → β + Si and the subsidiary reaction Si + e → β below the notch temperature T c ; the other is a simple transformation, i.e. the peritectoid reaction e + → β above T c . The optimum annealing temperature for the formation of the β phase was found to be at the peak of ρ or Q , where the β phase was formed through the two-stage reaction.

Thermal emission rates and capture cross sections of majority carriers at vanadium centers in silicon

Abstract The thermal emission rates and capture cross sections of majority carriers on the vandium associated centers in the depletion region of reverse biased silicon p-n junctions have been measured by the dark capacitance transient method. The three vanduim associated levels observed, two donor levels and a deep acceptor level, belong to the same vandium center. Least square fits of the emission data give the following emission rates; enlt = 1.047 × 106T2 exp [−0.179±0.004 eV/kT], en0t = 3.55 × 107T2 exp [−0.426±0.004 eV/kT] and ep-2t = 1.514 × 106T2 exp [−0.450±0.003 eV/kT]. The activation energy of the hole emission rate at the lower donor level is about 0.1 eV larger than the equilibrium thermal activation energy. The capture cross sections are σn0 = 3 × 10−17cm2 and σp0 = 8 × 10−16cm2 for the electron capture process at the deep acceptor level and the hole capture process at the upper donor level, respectively. The hole capture cross section on the lower donor level (σp-1) depends significantly on temperature. The large temperature dependence of the hole capture cross section can be expected due to the nonradiative multiphonon emission process.

Crystal growth and characterization of MnSi∼1.73

Abstract Boules having a composition which is most silicon-rich among the silicides of manganese MnSi∼1.73 were grown by the Bridgman method using starting materials of Mn-Si alloys within a composition range between 62.5 to 63.8 at% Si. In this composition range, three kinds of boule could be obtained as follows: The first one had only one region of a crystalline Mn15Si26 with the MnSi precipitations, the second one had two regions composed of a large crystalline Mn15Si26 with the MnSi precipitations and a thin polycrystal at the upper end, and the third one had two regions composed of a large crystalline Mn15Si26 with MnSi precipitations and a small crystalline MnSi at the lower end. Plate-like MnSi precipitations were always observed in the Mn15Si26 matrix of the first boule contained 63.2 at% Si. These precipitations were distributed parallel to the c-plane of the Mn15Si26 compound, where the volume ratio of MnSi to Mn15Si26 was 0.02. The second boules which are Si-rich compared with the first boule were composed of a narrow region of three phases of Mn15Si26, MnSi and Si at the upper end. The third boule of Si-deficient with respect to the first boule was composed of a small single crystal of MnSi at the lower end. In the largest middle portions of all the boules, however, the manganese concentration and the distribution state of MnSi precipitations were corresponding to the ones of the first boule. The amount of manganese in the boules and the lattice parameters of Mn15Si26 showed no difference between the as-grown and annealed boules and were independent of the compositions in starting materials and the growth rate.

Energy levels and degeneracy ratios for magnesium in n-type silicon

The energy levels and degeneracy ratios of magnesium in n-type silicon have been determined by Hall effect measurements with the least square method. Magnesium ions appear to occupy two different sites and show different electrical properties. The first is amphoteric and exhibits an acceptor level at Ec − 0.115 eV (±0.002 eV), degeneracy ratio γI− = 2.5 as well as a donor level at Ec − 0.40 eV (±0.01 eV), γIII+ = 1. The second exhibits a donor level at Ec − 0.227 eV (±0.004 eV), degeneracy ratio γII+ =12.5. The physical nature of these Mg associated site is unknown.

Energy levels and degeneracy ratios for chromium in silicon

Abstract The energy levels and the degeneracy ratios for chromium in silicon have been determined by the Hall coefficients which were measured by the van der Pauw method. Using the curve fitting method for carrier concentration based on the charge balance equation with the root mean square deviation, the analysis shows that chromium in silicon gives rise to two donor levels. The energy levels of the upper and lower donors are located at Ec-0.226(±0.010)eV and Ev+0.128(±0.005)eV, and their degeneracy ratios are 1 3 and 1 4 , respectivel

Magnetophonon Resonance in Epitaxial n-InP in High Pulsed Magnetic Fields

The transverse magnetophonon resonance in high purity epitaxial n-InP was investigated at temperatures between 123 and 270 K. High pulsed magnetic fields up to 40 T were applied to observe the fundamental resonance field. The temperature dependence of the magnetophonon effective mass m 0 * at the bandedge was precisely determined through the analysis of the resonant field positions including the peak for N =1. The determined value of m 0 * is 0.079 m 0 at 270 K and 0.084 m 0 at 143 K. The magnetic field dependence of the damping factor was found to be ( tilde{gamma}propto B^{2/3}) in magnetic fields higher than 10 T. The temperature dependence of the damping factor was also obtained.