多孔硅形成过程中的晶态转变

Transformation of Crystalline State during Porous SiliconFormation

采用电化学阳极氧化法,对p型单晶硅施加30 mA/cm^2的恒定电流密度,在40%氢氟酸溶液中和氮气保护气氛下分别极化1、3和5 min,制备不同生长阶段的多孔硅样品,通过表面形貌观测及微观结构表征,获得从单晶硅到多孔硅形成过程所涉及的各种晶态组成的相对比例。结果表明:不同极化时间制得的单晶硅表面均形成了海绵状均匀分布的纳米孔洞结构。在多孔硅的形成过程中,单晶硅结构发生了显著的晶态转变和晶粒尺寸变化,导致大单晶、纳米晶和无定形态并存,晶粒直径从初期的1.41 nm减小到并保持在0.65 nm,而晶态的转变和晶粒尺寸的变化被认为与晶格畸变程度有关。

The porous silicon samples were fabricated on the p–type single crystal silicon wafers in 40% hydrofluoric acid solution under a nitrogen atmosphere by applying a constant current density of 30 mA/cm2 for 1, 3 and 5 min, respectively. The surface morphologies and microstructures of the freshly prepared porous silicon layers were investigated. The relative amounts of large single crystal, nanocrystalline and amorphous silicon related to the extent of crystallinity transformation and the change of crystallite diameter involved during porous silicon formation were evaluated via multi–peak fitting Raman spectra. The results reveal that the sponge–like nanoporous structures are formed on the silicon surfaces at different polarization time. The apparent changes in crystal structure and crystallite diameter occur during the porous silicon formation, leading to the coexistence of large single crystal, nanocrystalline and amorphous silicon. The crystallite diameter reduces to 0.65 nm from 1.41 nm for the initial growth. The degree of lattice distortion could be contributed to the crystallinity transformation and crystallite diameter variation.