Single-Crystalline Si Nanotip Array Fabricated Using Si-Based Porous Anodic Alumina Template

Si-based SiO₂ nanoislands can be obtained by anodizing Si-based aluminum film in a 0.3 M sulfuric acid solution under a constant voltage of 25 V, with the help of monitoring current-time curve. Nanoislands were observed at the bottoms of nanochannels of Si-based porous anodic alumina membrane. The morphology and composition of this nanoisland array were characterized using atomic force microscope (AFM), x-ray photoelectron spectroscopy (XPS), and Fourier transfer infrared spectrum. The results indicate that the nanoislands consist of silicon dioxide, while the 25-nm height of the SiO₂ nanoisland can be determined by XPS depth profile. According to the growth model of the SiO₂ nanoislands, a Si nanotip exists at each triple point of the SiO₂ nanoisland array. After a series of experiments, we established appropriate experimental conditions to completely remove the surface SiO₂ nanoislands and to exposure the underneath single-crystalline Si nanotips, which have the same crystal orientation as that of the starting wafer. Obviously, the density and ordering of the Si nanotips inherit those of previous SiO₂ nanoislands. AFM measurement demonstrates that these single-crystalline Si nanotips with a very sharp end have uniform bottom diameters of 20-30 nm and heights of ~20 nm. Most importantly, this array shows an excellent field emission property with a low turn-on field of 8.5 V/μm (defined as the electric field to extract a current density of 10 μA/cm²) and a homogenous emission area. The obtained Fowler-Nordheim plot is linearly dependent, indicating that the emission current arises from the quantum tunneling effect. Based on our experimental results, we can roughly estimate the field enhancement factor (β) to be as large as 1100 for current single-crystalline Si nanotip array. The favorite field emission property of the single-crystalline Si nanotip array is considered to be intimately connected with its unique geometrical structure, perfect surface composition, and good contact with underlying Si substrate. This kind of Si nanotip arrays can be expected to have important applications in nanoelectronic devices.