ZnS:Ho^(3+)和ZnS:HoF_3交流电致发光薄膜发光特性的研究

A STUDY ON EL SPECTRA OF ZnS THIN FILM DOPED WITH Ho3 + AND HoF3

利用高真空条件,分舟蒸发制备ZnS:Ho^(3+)和ZnS:HoF_3绿色交流电致发光薄膜屏。讨论了ZnS:Ho^(3+)及ZnS:HoF_3薄膜的发光特性和光谱差异。确定其激发机理为热电子直接碰撞激发,以及660nm的发射在低浓度下来自于~5F_3→~5I_7的跃迁。并认为在两种材料中,由于热电子在外电场中的加速过程中受到不同散射中心的散射,改变了热电子的统计分布,从而直接引起两种材料发射光谱的明显差异。

In present paper, we study the difference of electroluminescence (EL) spectra of ZnS:Ho3+ and ZnS:HoF3 ACEL thin films. ZnS:Ho3+ and ZnS: HoF3 thin films are deposited by two separete boats in vacuum.The mechanism of EL excitation in ZnS:Ho3 + is discussed. The ratio of shorter wavelength emission intensity (490mn) to longer one (550nm) increa= ses with applied voltage, shown as in Fig.3, we suggest that the EL in ZnS thin film doped with Ho3+ is generated by impact excitation of hot electrons. According to the EL decay behaviors of blue emission around 490nm (labled Ib) and red emission around 660 nm (labled Id ) , as shown in Fig. 4 and Table 1. it is indicated that the emissions of Ib and Id are the transitions from the same excited state 5F3 when the concentration of Ho3+ ions is lower.The EL spectra of ZnS:Ho3+ and ZnS:HoF3 thin films are shown in F'ig. 1 . It appears that the emissions of 660nm and 490nm in ZnS:Ho3+ are much stronger than that in ZnS:HoF3 , i. e . the emissions from higher excited state is decreased in ZnS:HoF3. It is well known that the electrons accelerated by electrical field in semiconductor should be scattered by impurity centres. This processes will restrict the electrons to get higher kinetic energies from electrical field. In ZnS:Ho3 + thin film, the electrons are scattered by Yukawa potential V = . The scattering probability iswhere nI is impurity concentration , m* is effective mass , v is velocity of hot electrons.In ZnS:HoF3 thin film, Ho3+ and F- form a dipole, the electrons arescattered by the dipole field of V = , where d is the distance from F- to Ho3+ . The scattering probability isto take v=1×10m/s, d = 2.7A, m* = 0.38m,, e = 8.3, the result isPH0-F/PH = 5The scattering probability of Ho-F centre in ZnS:HoF3 thin film is much larger than that of Ho8+ centre in ZnS:Ho3+ thin film. Therefore, the electrons are uneasy to get higher kinetic energy and then to impact excite centre to higher excited state in ZnS:HoF3 thin film. It makes the transitions from higher excited state decreased in ZnS:HoF3 thin film, as show in Fig,1.