Current—Voltage Characteristics by Image Photometry in a Field-Ion Microscope

Current—voltage characteristics of a FIM were determined indirectly through total image photometry in order to avoid known difficulties with ion current measurements. Relative phosphor screen efficiencies as a function of ion energy were calibrated separately. The total ion current under isothermal conditions is found to depend more sensitively upon the temperature than the T−1 power predicted by the present theory. The total ion current remarkably depends also on the tip temperature when the gas temperature is kept constant. Both temperature effects indicate that only a fraction of the gas molecules attracted to the tip by polarization contributes to the ion current. From the emitter cone-angle dependence of the slope in the imaging range of the log I—log V plot and from the time-lag effect after changing the emitter temperature it is concluded that the initial arrival of molecules at the emitter shank contributes significantly to the image gas supply. Image photometry can be used as a simple means to monitor the tip temperature with sufficient accuracy for most practical purposes.