LETTERS TO THE EDITORX-ray diffraction studies have revealed no readily apparent differences in the crystal structure of the two materials characterized by curves A and B. Similarly, annealing below the melting point does not affect the position of the limit of absorption. It would therefore seem that the transmission between 3.2 and 7.0 microns is caused by some impurity which is removed by zone refining. Although this proposal is somewhat unusual, there is other evidence to support it. Material, whose transmission is at first of the type in curve A, can be converted to material similar to B by sufficient zone refining. In addition, by sampling the material between zone refining cycles, the long-wavelength limit of absorption was observed to progress from 3.2 to 7.0 microns. Curve C is the transmission curve of a 0.007-inch slice of such a partially refined sample. Its resistivity was 2.5X10~4 ohm cm. Similarly, if a heavily refined bar is examined along its length, it is observed that the limit of absorption moves to shorter wavelengths as one proceeds from the pure to the impure end of the bar. Thus, it appears that the intrinsic limit of absorption lies at 7.0 microns, and the anomalous transmission from 3.2 to 7.0 microns is caused by an impurity with distribution coefficient less than unity.Selective doping should reveal the nature of the impurity responsible for this behavior, and experiments have been performed in which material with an intrinsic limit of absorption at 7.0 microns is doped with lead, nickel, arsenic, excess indium, and excess antimony. These five agents were chosen since chemical analysis has shown them to be the most abundant impurities in the 3.2micron material. Doping with up to 0.05 percent lead, arsenic, indium, or antimony produced no observable shift in the longwavelength limit of absorption. However, an equal amount of nickel caused a shift of about one micron. By adding 0.1 percent of nickel, the long-wavelength limit has been moved to 5 microns. Thus it seems that nickel is at least partially responsible for the anomalous transmission betw r een 3 and 7 microns.If a single crystal of InSb with a long-wavelength limit of absorption at 7.0 microns at room temperature is cooled to 77°K, the long-wavelength limit shifts to 4.5 microns coriesponding to an optical gap of 0.28 ev. Assuming that the change in energy gap is essentially linear over this temperature range, this corresponds to a temperature coefficient for the energy gap of -4X10~4 ev/degree K. This coefficient is in excellent agreement with the electrical properties of InSb if the effective mass of the charge carriers is assumed to be 0.083. 4 We wish to thank R. F. C. Cummings who assisted with the experimental measurements.
The conductivity of liquid selenium has been measured in the temperature range 200°–500°C. The resistivity was expressed by log10ρ=A+(B/T). Average values for different selenium lots and melts of A and B were −3.81 and 5850. The maximum deviations from the averages were 10 percent and 3.4 percent respectively. The resistivity was a function of temperature alone. Various non-metal impurities Cl2, I2, P lowered the resistivities and produced different values of B in different temperature ranges. Mercury addition caused no change in either A or B although that metal greatly influences the resistance of solid hexagonal selenium. Melts doped with Cl2 or Br2, along with Hg exhibited behaviors different from those with single additions. It was concluded that selenium is an ideal semiconductor in the range of measurement.
The thermoelectric power of liquid selenium has been measured in the range 250–500°C. Values of activation energies calculated from the temperature dependences of thermoelectric powers and resistivities agree. This activation energy taken as the intrinsic activation energy (or the energy to break the Se–Se bond) is 2.31 ev calculated from data given by Henkels. The mobility of holes in the intrinsic region exceeds that of electrons and has a value (8.4×106)/T32 cm2/volt sec, assuming electronic mass. As a further check of the electronic nature of the conduction in the liquid and of theories of conduction in the hexagonal form, preliminary measurements of the thermoelectric power of pure selenium doped with small percentages of As, Sb, and Bi up to 4 percent were made. In the intrinsic region the specimens exhibited p type conduction, but at lower temperatures the sign of the thermoelectric powers reversed and values became quite large but negative. The crossover point depended qualitatively on the amount of impurity in the correct manner. The n type conduction is tentatively attributed to donor levels introduced by the normal trivalence of Group Vb elements. Two p electrons are considered bound in the selenium chain, the third ionized at high temperature.
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