Investigation of porous silicon morphology for electron emission applications

The morphology and composition of porous silicon (PS) of high porosity (80%), realized on flat and on emitter covered surfaces, were investigated in order to establish the optimal preparation conditions for electron emission applications. Scanning force microscopy (SFM) images reveal a variable number of hillocks of different sizes, depending on silicon type, randomly distributed on the Si surface. A great number of cracks are present on the high porosity PS layers thicker than 2–3 μm realized on flat Si surfaces, but the average hillocks size is more uniform compared to the case of PS on emitters. A higher density of hillocks was observed on PS/p-Si comparatively to the case of PS/n-Si. A good correlation between the PS morphology and field emission characteristics was obtained. A maximum value of 2.5 μA of the emission current was measured on PS layers prepared on p-type Si flat surface for applied voltage Va=100 V. Secondary ion mass spectrometry (SIMS) analyses indicate a high degree of PS oxidation when kept in atmospheric conditions after preparation. The Fowler–Nordheim behavior of the emission characteristics occurs only after native oxide breakdown.