Optimization of hot electron emitters

A Metal-Insulator-Semiconductor (MIS) structure may constitute the foundation for the realization of hot electron emitters. A theoretical description of the heterostructure is presented with a particular reference to emission. The derived parameters are the basis for the design of real devices; these are fabricated through cleanroom technologies. The fabrication sequence is illustrated. The experimental results from standard IV measurements and emission measurements taken in UHV environment are compared with the theoretical predictions and show good agree- ment. The possibility of emitting electrons under gas pressures up to 1 bar was observed and exploited for a qualitative characterization of the devices. A model for the transport of electrons through a gas, originally due to Townsend (1928), is modified in order to estimate the transmission probability of electrons from the device to the collector where emission is measured. Measurements of emission as a function of the accelerating electric field and of the pressure are taken and compared to the model. Various combinations of metals are tested in order to maximize the emission and the efficiency of the devices. The deposition of 2 Å of titanium (Ti) as wetting layer and of 100 Å of gold (Au) produced the best results: an emission of 5µA/cm2 and an efficiency of 3.4 × 10−4 is reached at 6V. The possibility of enhancing the emission current by lowering the work function of the metal is demonstrated: the dramatic effects of the covering metals on the electrical properties of the devices due to the deterioration of the oxide, however, does not allow to produce real improvements. The durability of the devices is investigated. The results support the project of substituting the traditional electron sources of a mass spectrometer with hot electron emitters. These have the advantage not to generate heat and outgassing in a UHV chamber. The spectra measured with these devices and with thermionic filaments are compared: the good agreement between the two measurements confirms the possibility of employing hot electron emitters in mass spectrometry.