氏名　ナジャホフ　ヒクメット
学位の種類　博士（工学）
学位記番号　博甲第238号
学位授与の日付　平成14年3月25日
学位論文題目　Optical processes in Ce and Eu co-doped and separately doped CaGa2S4　（CeおよびEu同時添加・単独添加CaGa2S4における光学過程）
論文審査委員
　主査　教授　打木　久雄
　副査　教授　上林　利夫
　副査　教授　野坂　芳雄
　副査　助教授　内富　直隆
　副査　助教授　安井　寛治

• 論文目次
• 論文要旨

［平成13（2001）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

Chapter I:　Introduction　p.1
1.1　General features of luminescence in solid　p.1
1.2　Rare-earth ions as luminescent centers　p.4
1.3　Historical survey of sulfide phosphors doped with rare-earth ions　p.4
1.4　Motivation and purpose　p.6
1.5　Crystallographic structure of CaGa2S4and energy level formations of Ce and Eu ions　p.7
1.6　Chapter construction　p.9
Chapter II:　Experimental　p.10
2.1　Introduction　p.10
2.2　Sample preparation　p.11
2.3　X-ray diffraction and analyses of dopant concentrations　p.12
2.4　PL and PLE measurements　p.12
2.5　Time decay measurements　p.12
2.6　Measurements of the afterglow of the Eu emission　p.13
2.6.1　TTL measurements　p.13
2.6.2　TL measurements　p.14
2.6.3　Infrared stimulation measurements　p.14
2.7　Measurements of the temperature dependence of the Eu emission　p.14
Chapter III:　Energy transfer from Ce to Eu ions　p.19
3.1　Introduction　p.19
3.2　Results　p.19
3.2.1　Crystallographic investigation and analyses of dopant concentrations　p.19
3.2.2　PL and PLE spectra of co-doped and mixture samples　p.19
3.2.3　Time decay curves　p.21
3.3　Analysis and discussion　p.22
3.3.1　PL spectra of co-doped and mixture samples　p.22
a）　Re-absorption effect of the Ce emission　p.22
b）　Resonance energy transfer from Ce to Eu ions　p.24
3.3.2　Estimation of energy transfer rate　p.25
3.3.3　Concentration dependence of energy transfer rate　p.26
3.4　Conclusion　p.28
Chapter IV:　Thermoluminescence　p.38
4.1　Introduction　p.38
4.2　Experimental results and analysis of activation energies　p.39
4.2.1　Temperature quenching of the Eu emission in CaGa2S4　p.39
4.2.2　Transient thermoluminescence （TTL）　p.39
4.2.3　Thermoluminescence （TL）　p.40
4.2.4　Infrared stimulation　p.41
4.3　Discussion　p.41
4.3.1　Temperature quenching of the Eu emission　p.41
4.3.2　Difference between TTL and TL data　p.43
4.3.3　Two activation energies of the Eu afterglow and infrared stimulation　p.43
4.3.4　Model with one trap level　p.44
4.3.5　Theoretical formulation and comparison with the experiment　p.45
4.4　Conclusion　p.49
Chapter V:　Ultraviolet emission from Ce-doped CaGa2S4　p.61
5.1　Introduction　p.61
5.2　Results　p.61
5.2.1　PL spectra of CaGa2S4:0.20%Ce at different temperatures　p.61
5.2.2　PL and PLE spectra of the UV emission at 10K　p.61
5.2.3　Temperature quenching of the UV emission　p.62
5.2.4　PLE spectra of the blue Ce emission at various temperatures　p.63
5.2.5　Decay time constant of the UV emission　p.63
5.3　Analyses and discussions　p.63
5.3.1　Temperature dependencies of the blue and the UV emissions　p.63
5.3.2　Energy level diagram and configuration coordinate description of the Ce ion　p.64
5.3.3　Estimation of optical gain for the UV emission　p.65
5.4　Conclusion　p.67
Chapter VI:　Application feasibility　p.73
6.1　Introduction　p.73
6.2　Phosphor application of co-doped and separately doped samples　p.73
6.3　Flat panel display application　p.75
6.4　Laser application of co-doped samples　p.77
Chapter VII:　Summary　p.80
References　p.83
Acknowledgments　p.87
List of publications　p.88
Appendices
A. Relationship between optical and thermal activation of the electron from the trap　p.A1
B. Equation for the TL intensity　p.A1

Ce or Eu doped CaGa2S4 attracts some attention from the viewpoint of applications to phosphors, flat panel displays and lasers. Ce incorporation in the material gives an efficient emission in the blue to green region of the spectrum, while Eu incorporation does in the yellow to orange region. The main subject of this research is to investigate the effect of co-doping of these two elements for the first time, since there exists a possibility of resonance type energy transfer from Ce to Eu ions due to strong overlap oftheir elllission and excitation spectra. From practical point of view the energy transfer process may increase a possibility for this material to be used as a good candidate for the applications mentioned above. As expected this energy transfer process has been confirmed to exist. The confirmation of this process was done based on the analyses of Ce emission intensities, selective excitation of Ce ions and decay time measurements of Ce emissions in co-doped and Ce doped samples. The rate of the energy transfer process was determined from time decay measurements of Ce emission in variously co-doped samples under pulsed dye laser excitation The decay time constant of the Ce-doped sample was found to be ～24 ns. The decay time constant reduces with increasing dopant concentration and reaches ～8 ns for 4 at.% Ce and 4 at.% Eu co-doped sample. The transfer rate determined from the time decay measurements favorably compares with the result of intensity analyses of the Ce emission which takes into account re-absorption effect by the Eu ions. All co-doped samples were found to exhibit long afterglow of the Eu emission having a peak energy of ～2.20 eV. Analyses of measured transient thermoluminescence curves showed two activation energies, ～0.2 eV and ～0.94 eV for most co-doped samples. These two activation energies are shown to be explainable with the model which includes' one trap level with two activation processes, and hole release from the terminal state of the Eu ion. Experimentally observed thermoluminescence curves are shown to be well reproduced by the theoretically formulated model which takes into account the above hole release process. Some discussions are given for the relation of the effect of Ce and Eu co-doping on the applications mentioned above,
In relatively lightly Ce doped CaGa2S4, a new emission band was found to appear in the ultraviolet region （～3.25 eV） of the spectrum for the first time. This emission is assigned to radiative transition from the highest level of 5d configuration to the ground state configuration of the Ce ion. The emission exhibits temperature quenching and almost vanishes at room temperature. This temperature quenching process was found to co-exist with a thermally activated contribution from the excitation of the ultraviolet emission to the blue Ce emission in the same sample. These two processes are found to be favorably explainable in terms of configuration coordinate description of the Ce ion. Discussion is also given on the possibility of utilization of this emission to an UV laser.