Photoelectric Emission and Contact Potentials of Semiconductors

In spherical photo-tubes with interchangeable emitters, energy distributions of external photoelectrons from the semiconductors, Te, Ge, and B, were compared with those of several metals. Contact potentials were determined from the saturation points of current-voltage characteristics. For the semiconductors, as contrasted with metals of the same work function, there was a pronounced sparsity of electrons with energies near the Einstein maximum. In general form, the distributions could be described by the expression, N (, E) dE () E (h---E) ^mdEbased on simplified assumptions analogous to those previously applied in the case of metals. Here is the work function; is the energy difference between the Fermi level and the top of the occupied band of energy states; m is a parameter depending both on the form of this band and on the energy dependence of the photoelectric excitation probability; q is a slowly varying function of. The photoelectric properties of the semiconductors could also be specified conveniently by plotting measured values of N (, E) E as functions of h--E. Spectral distributions of the photoelectric yields varied more rapidly with than those for metals and were not measurable unless h exceeded by several tenths of an electron volt. Near the Einstein maximum, energy distributions deviated from the simple relation given above. The experiments failed to disclose sharply defined stopping potentials corresponding to upper edges of occupied bands of energy states in the semiconductors. Thus m and were not defined uniquely. Typical values of m obtained both from energy and spectral measurements were 32 and 2; illustrative values of were 0. 10 to 0. 18 ev for evaporated Te (4. 76 ev), 0. 2 to 0. 3 for evaporated Ge (4. 8), and 0. 3 to 0. 5 for pyrolytic films of B (4. 6). Thermoelectric measurements showed that all samples were P type. For Te the results were in agreement with available data on electrical properties. Using the above values of, upper limits were found for photo-currents originating in the "forbidden" zones (as defined by E>h--. For the surface states that were assumed as possible sources of these currents, estimated densities were of the order of magnitude, 10^12 cm^-2.