Studies on Optical Properties of ZnTe/ZnSe semiconductor nanostructure

早期因為硒化鋅碲系統在發光二極體方面有廣泛的應用及由於等價束縛激子(isoelectronic bound excitons)所造成的光學特性而引起廣大的討論，這系統的光激發冷光(photoluminescence)光譜都歸因於束縛在等價中心(isoelectronic centers)的激子再結合放出的光子能量，而主要掌控這光學特性的等價中心是指有兩到三個或更多的碲原子束縛在鋅原子上，最近有許多研究指出原本認為由於等價束縛激子產生的硒化碲/硒化鋅的光激發冷光光譜是因為量子侷限激子(quantum-confined excitons)和等價束縛激子所共同產生的。本文中，我們利用有機金屬化學氣相沉積法磊晶成長了ZnTe-doped和Te-doped 這兩種樣品，根據時間解析光激發冷光(time-resolved photoluminescence)及改變激發功率的光激發冷光光譜(power-dependent photoluminescence)的結果分析，這兩種樣品所發出的綠光帶有著不同的峰值能量而且是來自不同的發光機制，在Te-doped樣品裡，我們發現綠光和藍光帶是不隨激發功率而變化，而且有較短的衰減時間，然而在ZnTe-doped 樣品裡，綠光帶隨著激發功率增加而且有藍移的現象而且有著比Te-doped樣品的綠光帶較長的衰減時間，因此我們推論在Te-doped成長樣品裡，綠光帶是來自Ten>2等價中心的發光；而在ZnTe-doped樣品裡，証實了綠光帶是來自硒化碲/硒化鋅第二型量子點結構所產生，從這些光譜結果証實了在ZnSeTe系統中，綠光帶有兩種不同的發光機制，而我們的成長方法可以選擇其中任何一種的發光機制。ZnSeTe systems have been of great interest both for their applications in light emitting devices and for interesting optical properties related to isoeletronic bound excitons (IBE). The photoluminescence (PL) of these systems is usually attributed to excitons bound to isoelectronic centers (ICs). The centers that dominate the optical properties are so-called Te2 and Ten≧3 ICs, where two and three or more Te atoms are bound to the same Zn atom. More recently, the PL of these ZnSeTe systems always have been attributed to the coexistence of the isoelectronic bound excitons and the quantum-confined excitons. In this thesis, ZnSe doped with Te and doped with ZnTe were grown by using metal organic chemical vapor deposition. The green band emissions obtained from these two different samples not only have different peak energies but are of different origins. For Te-doped sample, the PL emission band is laser power-independent and the lifetime of the PL emission is short, on the other hand, for the ZnTe-doped sample, the energy of the green emission band is laser power-dependent and the PL lifetime is much longer than the Te-doped sample. For the Te-doped sample, the green band emission is attributed to the emission from Te isoelectronic centers, and for the ZnTe-doped sample, the green band emission is attributed to the emission from ZnTe quantum dots. The results demonstrate that there are two different mechanisms for the green band emission in ZnSeTe system and our growth method provide a way of selecting one of the two mechanisms.Acknowledgement..........................................Ⅰbstract (Chinese).......................................Ⅱbstract (English).......................................Ⅲ. Introduction...........................................1.1 Ⅱ-Ⅵ Compound Semiconductors.........................1.2 Low-Dimensional Ⅱ-Ⅵ Semiconductor Structures........3. Experimental Techniques and Setup.....................13.1 Experimental Techniques..............................13.1.1 Basic Concept of MOCVD.............................13.1.2 Photoluminescence..................................16.1.3 Time-Resolved Photoluminescence....................28.2 Experimental Setup...................................31.2.1 Photoluminescence..................................31.2.2 Time-Resolved Photoluminescence....................32. Theoretical Models....................................36.1 Isoelectronic Centers................................36.2 Electronic States in Low-Dimensional Semiconductors..37.2.1 Type-Ⅰ Quantum Structure..........................37.2.2 Type-Ⅱ Quantum Structure..........................40.3 Quantum Confined Effect..............................43. Optical Properties of ZnSe doped with Te and dope with ZnTe.....................................................46.1 Sample Preparation...................................46.2 Formation of type-Ⅱ Quantum Dots....................49. Conclusions...........................................6