Shallow Donor Thermionic Emitter

Abstract

A possible approach to the development of a low work function thermionic emitter involves the introduction of shallow donor states into a matrix having a low electron affinity. It is the intent of this paper to explore the concept of a shallow donor emitter from both a general point of view and with specific application to barium oxide as the host lattice. The problem is discussed in terms of the single-donor model with consideration given the various characteristics which would be required for a practical matrix-additive system. Although the electron affinity of BaO has been estimated to be as low as 0.6 eV, the ordinary oxygen vacancy donor present is a deep level with an ionization energy of about 1.4 eV; it yields a work function of 1.4 to 1.5 eV at 1000°K. By comparison, a work function of 0.83 eV would be expected for shallow donor BaO at this temperature. The substitution of a tripositive rare-earth ion for a ${\mathrm{Ba}}^{\text{2+}}$ ion in the lattice is presented as a possible mechanism for the incorporation of an impurity donor center. Some speculation is offered concerning the ionization energy of this type of donor as well as the associated activation process. An attempt was made to observe the donor behavior of several rare-earth ion additives in BaO and also SrO by studying the temperature dependence of the effective work function. Data were secured for La, Gd, Nd, Er, and Eu in BaO, and Eu and Sm in SrO at analytical concentrations of from 0.01 to 0.05 mole % While no lowering of the work function is reported, it is clear that no definitive interpretation of a negative result can be made until certain other experimental information becomes available, particularly the solubility and oxidation state of the additive ion in the matrix crystal.