【摘要】 利用电子束蒸发方法制得ITO-Y₂O-3-ZnS:Tm³⁺-Y₂O₃-Al结构的薄膜可以得到蓝色电致发光。本文首次报导了ZnS:Tm³⁺薄膜与ZnS:Er³⁺、ZnS:Tb³⁺薄膜电致发光的激发过程有所不同。Tm³⁺离子的激发可以通过某些杂质中心到Tm³⁺离子的能量传递来实现。更多还原

【Abstract】 This paper first reports that the mechanism of electroluminescence (EL) excitation in ZnS:Tm thin film (TF) is different from that in ZnS:Er and ZnS:Tb TF. It appears that excitation energy transfers from some impure centers to Tm ions.The ZnS:Tm TF with ITO-Y2O3-ZnS:Tm -Y2O3-A1 structure is made by electron-beam evaporation. During the evaporation the vacuum pressure, the evaporation rate of ZnS:Tm layer, and the substrate temperature are 5×10-5 torr, 200A/min,and 160℃-200℃ respectively. The optimum Tm3+ ion concentration is approximately 2% by weight and the thickness of ZnS: Tm layer is in the range from 400nm to 600nm.The emission spectra of EL in ZnS:Tm TF depend on the dopant concentration as shown in Fig. 6. At low concentration (3×10-4 by weight) there is one broad band emission (from 390nm to 600nm). It is attributed by some impure centers. At high concentration (3×10-2 by weight) this band is suppressed and the spectra of ZnS:Tm TFEL show two groups of characteristic lines of Tm ion. One is the blue emission around 480nm lab-led I1 and the other is infrared around 800nm denoted by I2. I2 can be readily excited by 514nm (with the photo energy is about 1500cm-1 below the state 1G4, i.e. the state 1G4 can not be populated), and its spectrum structure is very similar to that of EL as show in Fig.5. On the other hand, the EL decay behaviors of I1 and I2 emission are quite different from each other. These indicate that I2 corresponds to the transition 3F4→3H6 instead of the transition 1G4→3J5.It is well-known that the EL of RE doped TF is generated by impact of hot electrons, therefore the ratio of the emission intensity between the short wavelength and long wavelength should increase with applied voltage such as in Fig.3(b) in ZnS:Er3+ TFEL. This does not happen, however, in ZnS:Tm as shown jn Fig,3(a), The excitation spectra of I1,I2 emission in ZnS:Tm TF is different from that of the 542nm emission which is the transition 5D4→7F5 in ZnS:Tb TF. In the former case there exist clearly excitation band around 340nm but in ZnS:Tb TF there is nothing there except the excitation peaks at 380nm and 486nm which can be attributed to excitation from ground lever to 5D3 and 5D4, as shown in Fig.8 (b,c and d). I1,I2 and the broad band of impure centers have similar excitation spectra around 340nm, as show in Fig.8(a, b,c). This indicates that energy might transfer from the broad band center to Tm ions.Time-resolved spectra confirms this hypothesis. ZnS:Tm TF is excited by voltage pulse of width 1μs. Time delayed EL spectra using a 0.2ns boxcar gate is shown in Fig.11. At 0.2μ’s delay the spectrum is dominated by the broad luminescence band. The broad band decays rapidly in 5μs while the intensities of I1, I2, in the contrary, rise. It shows clearly that the energy transfers from the centers which emit the broad band to the Tm ions.It can be concluded that the excitation mechanism of EL of ZnS:Tm3+ TF is different from that of ZnS:Er3+ and ZnS:Tb3+ . In the latter case, Er3+ or Tb3+ ions are excited by impact of hot electron. In the case of ZnS:Tm TF there are some broad band emission centers which absorb energy from the electric field and transfer it to the Tm3+ ions. The characteristics of these broad band emitting centers are being discussed.更多还原