Electrical properties of crystalline boron

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Shaw, William
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Physics and Astronomy
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The boron crystals used in this study were prepared by reducing boron tribromide with hydrogen upon contact with a hot (1500°C) tantalum filament. The largest single crystals weighed less than ten micrograms and were a few hundred microns in length. The high purity of the specimens was verified spectrographically and their crystalline nature was verified by means of Laue photographs;Micromanipulative techniques for mounting very small crystals with several electrical contacts were developed. Microscopic single crystals of boron were mounted with as many as five spring loaded tungsten probes. Measurements were made of voltage current characteristics, resistivity, Hall and thermoelectric effects as functions of temperature. Qualitative studies were made of rectification, forming and photoconductivity;The voltage current characteristics of crystalline boron were explained in terms of joule heating. The resistivity of crystals typical of the majority of those tested was found to be approximately 1.7 x 106 ohm cm at 25°C. The resistivity of a typical crystal decreased by a factor of almost 1010 between the temperatures of 200°K and 1000°K. From resistance data obtained in the temperature range between 700°K and 1000°K the intrinsic energy gap Eg between the filled and conduction bands was computed to be about 1.55 ev;Hall, thermoelectric and rectification studies showed the purest specimens of crystalline boron were predominately p-type. It was found that heat treatment could convert typical p-type boron crystals into relatively low resistivity n-type crystals. Ball effect measurements were made on one such heat treated crystal and on another low resistivity n-type crystal selected on the basis of qualitative thermoelectric measurements. From these Hall effect data the mobility of electrons in these particular crystals of boron was calculated to be about 0.7 cm2/volt sec. From Hall effect data obtained on a typical crystal the mobility of holes was calculated to exceed that of electrons by about 0.2 cm2/volt sec. The effective masses of the carriers were calculated to exceed ten electron masses and the mean free paths of the carriers to be between one and one hundred minimum interatomic distances.

Thu Jan 01 00:00:00 UTC 1953