Study of porous silicon morphologies for electron transport

Field emitter devices are being developed for the gigatron, a high-efficiency, high frequency and high power microwave source. One approach being investigated is porous silicon, where a dense matrix of nanoscopic pores are galvanically etched into a silicon surface. In the present paper pore morphologies were used to characterize these materials. Using of scanning electron microscope (SEM) and transmission electron microscope (TEM) images of both N-type and P-type porous layers, it is found that pores propagate along the <100> crystallographic direction, perpendicular to the surface of (100) silicon. Distinct morphologies were observed systematically near the surface, in the main bulk and near the bottom of N-type (100) silicon lift-off samples. It is seen that the pores are not cylindrical but exhibit more or less approximately square cross sections. X-ray diffraction spectra and electron diffraction patterns verified that bulk porous silicon is still a single crystal. In addition, a scanning tunnelling microscope (STM) and an atomic force microscope (AFM) were successfully applied to image the 40 Å gold film structure which was coated upon a cooled porous silicon layer. By associating the morphology study with the measured emitting current density of the oxidized porous silicon field emission triode (OPSFET), techniques for the surface treatment of porous silicon will be optimized