Optical and Electrical Properties of Boron and Potential Application

Abstract

The material investigated was mostly of the beta-rhombohe-dral form, and of the highest purity available at present. When necessary, single crystals or crystals having a slight mosaic structure were used. The surface structure of these crystals is shown, and an explanation given of the growth mechanism. Among the samples investigated there was a wide spread of impurity concentrations as determined by spectro chemical and vacuum fusion analysis. The spread in electrical properties was comparatively small, but there were few significant exceptions. The resistivity values were in the megohm-centimeter range; the Seebeck coefficient was approximately 500 μv/deg; the classical semiconductor contact phenomena, such as rectification and the photovoltaic effect, were not found. The variation among infrared absorption spectra is much larger than that of the electrical values, and the material could be grouped into four distinct classes, A, A′, B, B′, according to the spectra. Transitions between classes could be introduced by removal or introduction of impurities. Class A′ represents the most transparent material. An optical band gap of 1.56 ev and an absorption coefficient of only 10cm^-1 were found in the best material of this class, which was also the purest, with impurities at or below the 10 ppm range. Such material could be used for heat- and corrosion-resistant windows in the 1–8 μ region of the infrared. The high temperature coefficient of the material (ionization energies are approximately 0.6 ev at room temperature, and increase up tol.5evathigh temperatures) leads to applications in thermistors, etc. The main obstacle to other semiconductor applications seems to be the large concentration of traps in present-day boron, which becomes evident in the long relaxation times observed in photoconductivity and certain magnetic experiments.