Volume 32, Issue 10, 01 October 1961
Index of content:
- 2–5 AND 2–6 COMPOUNDS
32(1961); http://dx.doi.org/10.1063/1.1777050View Description Hide Description
The contribution of free electrons to the refractive index and extinction coefficient of Ga‐doped CdS has been measured in a series of samples with carrier concentrations ranging from about 1017 to 2×1019 electrons/cm3. The effective massm* for electrons near the bottom of the conduction band was calculated from the free electron contribution to the refractive index and from the carrier concentration as determined from the Hall coefficient. The result, m*=(0.22±0.01)me is in satisfactory agreement with previous studies by other workers. The magnitude and wavelength variation of the absorption coefficient observed with about 1017 carriers/cm3 are in fair agreement with theoretical results calculated for a polar‐mode lattice scattering mechanism. Although similar data for 2×1018 carriers/cm3 are in good quantitative agreement with the impurity scattering theory at room temperature, the absorption for high carrier concentration is observed to decrease with temperature, in contradiction with the theory for a nondegenerate population. A theory for the degenerate concentration range is needed. Reflection and transmission of radiation polarized parallel and perpendicular to the crystal C axis were studied for one sample. These data show (m ⊥*/m ∥*)=1.08±0.05.
32(1961); http://dx.doi.org/10.1063/1.1777051View Description Hide Description
The noncubic II–V semiconductors have been studied recently by several workers. A review will be given of the present situation. The energy gaps of these materials range from 0.13 to over 1 ev. Room temperature mobilities of 10–15 000 cm2/v sec have been observed. Anisotropy of electrical and optical properties have been reported for several of the compounds. For CdAs2 it has been possible to explain the anisotropy of Hall mobility by a simple energy band model.
32(1961); http://dx.doi.org/10.1063/1.1777052View Description Hide Description
HgSe single crystals are grown by zone melting. The compound crystallizes in the zinc‐blende structure and splits into (100) planes. For temperatures ranging from 90° to 500°K conductivity, Hall effect and thermoelectric power are measured; above 500°K evaporation of HgSe begins. The lowest carrier concentration of the crystals at 300°K is 3.5×1017 cm−3. Only n‐type conduction is found. The highest mobility at 300°K is 18 500 cm2/v sec. Magnetoresistance shows that the longitudinal effect is very small compared with the transverse. From the photo emf of the p‐n junction Se/HgSe crystal and from the absorption edge of layers the energy gap of 0.5 to 0.75 ev is obtained. Using the temf and the absorption an estimation of the effective mass leads to 0.04 to 0.07 m 0.
32(1961); http://dx.doi.org/10.1063/1.1777053View Description Hide Description
Edge emission in single crystals of ZnSe subjected to ultraviolet radiation at low temperatures has been examined in the temperature interval from 4.2° to 77°K. Two distinct edge emission spectra have been found indicating that two different types of single crystals exist. For type I crystals the edge emission spectrum at 4.2°K contains 10 lines located between 4400 A and 4800 A; at 77°K the emission spectrum contains two lines. For type II crystals the edge emission spectrum at 4.2°K contains 14 lines located between 4400 A and 4900 A; at 77°K the emission spectrum contains three lines, one of which is located at the fundamental absorption edge of the crystal. Both crystal emissions show evidence of phonon interaction with the ZnSe lattice and both emissions undergo significant reductions in intensity as the crystal temperature increases from 4.2° to 77°K.
32(1961); http://dx.doi.org/10.1063/1.1777054View Description Hide Description
HgTe1−x Se x solid solutions have been prepared, with x varying from 0 to 1. The samples are n type near x=1 and p type near x=0, but, due to the high electron‐to‐hole mobility ratio, electronic conudction is dominant in the range 100–400°K in all samples. The concentration of free electrons lies between 5·1016 and 3·1018 cm−3. Measurements of the Hall mobility μ H and magnetothermoelectric effect ΔQ show that, for Se‐rich samples, lattice scattering is dominant in the range 77–400°K and that, near room temperature, μ H ∝ T −2. For Te‐rich samples, lattice scattering is dominant in the range 200–400°K and, near room temperature, μ H ∝ T −1. Effective masses have been calculated and it is seen that the conduction band is not parabolic. The detailed band structure and the exact value of the mobility seem to depend little upon structural factors. For x=0.5 and x=0.9, the electron mobility can reach 12 000 cm2 v−1 sec−1 at 293°K and 30 000 cm2 v−1 sec−1 at 77°K.
32(1961); http://dx.doi.org/10.1063/1.1777055View Description Hide Description
A preliminary study has been made of the Righi‐Leduc effect in mercuric selenide (HgSe). The Righi‐Leduc magnetothermal effect is the thermal analog of the Hall effect wherein temperature plays the role of voltage and heat flow replaces electric current. The effect is particularly large in HgSe because of the coincidence of large electron mobilities (as high as 1.5 m2/v sec at 300°K) and low lattice thermal conductivity (about 0.02 w/°K cm at 300°K). Results are presented of measurements of the Righi‐Leduc coefficient S as a function of temperature, magnetic field strength, and electron concentration. Classically, for a one‐carrier material, S≅(κ E /κ)μ, where κ E is the electronic, κ is the total thermal conductivity, and μ is the electron mobility. This expression is in qualitative accord with the experimental results. At room temperature S ranged between 0.27 and 0.34 m2/v sec for samples that had between 55×1017 and 5.6×1017 electrons/cm3.
32(1961); http://dx.doi.org/10.1063/1.1777056View Description Hide Description
Single crystals of ZnSe have been prepared by the vapor growth technique and optical and electrical measurements on these crystals are reported. Analysis of the reststrahlen reflection peak gives 0.026 ev for the transverse optical phonon energy. The longitudinal optical phonon energy is 0.031 ev as calculated from the transverse phonon energy, the static dielectric constant, ε0=8.1±0.3, and the high‐frequency dielectric constant, ε∞=5.75±0.1. The effective ionic charge calculated from the Szigetti formula is 0.7±0.1. Exciton absorption peaks associated with the valence and conduction bands in the vicinity of Γ were observed at liquid hydrogen temperature with the principal peak at 2.81±0.01 ev. The exciton reduced mass 0.1 m 0 combined with the room temperature electron‐to‐hole mobility ratio of 12, obtained by preliminary transport measurements on n‐ and p‐type ZnSe gives tentative values of 0.1 m 0 and 0.6 m 0 for the electron and hole masses, respectively.
Reflectance was determined by various methods in the range 0.025 to 14.5 ev photon energy and was analyzed by the Kronig‐Kramers inversion method to obtain the optical constants in the 1 to 10 ev range. A number of peaks appear in the imaginary part of the dielectric constant.
The first set of peaks, 2.7 and 3.15 ev, are believed to be due to exciton and interband transitions at Γ with a spin‐orbit valence band splitting of 0.45 ev. The second set of peaks, 4.75 and 5.1 ev, are tentatively assigned to transitions at L with a spin‐orbit splitting of 0.35 ev. Other peaks are observed at higher energies.
32(1961); http://dx.doi.org/10.1063/1.1777057View Description Hide Description
A detailed analysis of Hall coefficient data obtained at temperatures between 77° and 350°K has been made for HgSe and HgSe0.5Te0.5 samples containing excess donor concentrations up to 1019 cm−3. On the basis of previous magnetoresistance, Seebeck coefficient, and reflectivity data, a spherically symmetric non‐quadratic conduction band exhibiting the ε(k) dependence described by Kane was adopted in making the analysis. Calculations based on a conventional two‐band model failed to give quantitative agreement with experiment, but good agreement was obtained on the basis of a model in which the conduction band and one valence band overlap in energy. Therefore the materials are semimetals rather than semiconductors. The best fit to the data was obtained with an overlap energy of 0.07 ev for both HgSe and HgSe0.5Te0.5, with hole density‐of‐states masses of 0.17 m 0 and 0.30 m 0, respectively. With increasing carrier concentration, the optical absorption edge for heavily doped HgSe exhibits a shift to higher energies which is characteristic of n‐type materials with low electron effective masses. Qualitatively, the optical data are consistent with a semimetalband model rather than with a semiconductormodel, since the interband absorption edge apparently occurs at photon energies less than the Fermi energy.