氢化非晶碳薄膜的光致发光线型及其激发能量依赖关系

LINESHAPE AND EXCITATION ENERGY DEPENDENCE OF PL SPECTRA OF HYDROGENATEDAMORPHOUS CARBON

本文报道了氢化非晶碳薄膜在2.9—4.5eV光激发下的发光谱。它的光致发光谱是无结构的不对称宽带,半宽度约为0.8eV。在低于3.56eV的光激发下,谱带的峰值能量随激发能量的降低明显红移。在安德森带结构和指数分布的带尾态密度的基础上,考虑了尾态中粒子的两种跃迁过程,实验的PL谱就可得到解释。并用这个简单模型计算了这种材料的光致发光谱特征。

Hydrogenated amorphous carbon films (a-C:H) were prepared by r.f. glow discharge deposition from C6H5CH3. The PL spectra and the excitation energy dependence of the spectra were measured at room temperature using monochromatic light of 2.9-4.5eV for studying electronic structure. The PL spectra show featureless and very broad band (FWHM is about 0.8eV).From the plot that the peak energy EPL versus excitation energy Ex data,we identified two approximately linear regions, one with constant EPL at high Ex, and the other with a slope of 0.25. By assuming the exponential density of localized states of band tails,considering two types of transitions of the carriers,the PL of a-C:H can be explained.One transition is that carriers make nonradiative transition from shallower to deeper states to lower energy or from deeper to shallower states because of thermal excitation among the localized states of a band tail.Another transition is recombination among the localized states of different tails. The rates of two types of transitions are dependent on degree of localization of initial and final states. The deeplier state trapped, the less is the rate. Based on this model, the very broad band of the PL spectra is due to distribution of carriers on some energy region of broad band tail states.The red shift of the spectra with decreasing Ex, excitation energy at sub-gap energy, can be obtained easily from distribution of carriers shifting to the deeper tail states. By the model we calculate the shape of the spectra and its changes with Ex.