雷射濺鍍法透過改變氧壓成長不同濃度之M面氧化鎂鋅薄膜達成調整能隙之效果

Abstract

因為氧化鋅在(0001)極性(polar)方向存在內建電場，使得其應用在氧化鋅量子阱(quantum well)時，有能帶傾斜的現象，我們稱此現象為量子侷限史塔克效應(Quantum-confine Stark effect)。此現象會造成發光波長紅移，以及電子電洞複合率下降。為了避免此現象產生，許多團隊成長非極性方向氧化鋅量子阱。而摻鎂氧化鋅常在氧化鋅量子阱中扮演能障(barrier)之角色。能障越大能使氧化鋅量子阱發光波段藍移。而透過高濃度鎂摻雜使得能隙變寬。但是鎂摻雜太多會使部分結構為形成氧化鎂岩鹽型晶體結構(rock-salt structure)，使得應用在纖鋅礦結構(wurtzite structure)的氧化鋅量子阱中產生晶格不匹配的問題。我們希望能成長出高濃度鎂摻雜之纖鋅礦結構氧化鎂鋅。 在此論文中我們利用雷射濺鍍法成長(101 ̅0)的氧化鎂鋅薄膜在(303 ̅0)的藍寶石基板上。透過控制腔體氧壓來改變摻鎂濃度，和利用光致發光光譜檢測鎂的摻雜濃度。而我們成功地在高真空(10-7 torr)的環境下長出～22%的摻鎂氧化鋅;合金的位能異動(alloy fluctuation)會使得摻鎂氧化鋅的半高全寬變寬。從變溫的光致發光光譜中，研究摻雜不同鎂成分的氧化鎂鋅薄膜其激子(exciton)與縱光學聲子(longitudinal phonon)耦合的強度變化以及激子的束縛能(binding energy)變化。我們發現摻鎂氧化鋅激子與縱光學聲子耦合強度較純氧化鋅強。且在高濃度摻雜時會產生較多的結構紊亂(structural disorder)使得束縛能會隨濃度增加而下降。從X光繞射光譜檢測當氧化鎂鋅濃度為22%時仍為(101 ̅0)方向成長，並且沒有氧化鎂岩鹽型晶體結構產生。利用拉曼散射光譜研究氧化鎂鋅合金在不同濃度摻雜下晶格震動模態變化情形來計算應力大小進而觀察晶格常數的變化。Z軸受到壓應力的影響使得晶格常數c變短。假設在體積不變的情況下，X軸會受到張應力的影響使得晶格常數b變長。

The ZnO built-in electric field along the c-orientation makes the energy band tilting. This phenomenon is called the quantum confinement Stark effect (QCSE). The QCSE leads the electric field to separate the electrons and holes present in the quantum wells (QWs) after excitation and to reduce the overlap of their respective wave functions, in turn reducing the exciton binding energy and red shift of the optical emission. Some groups choose non-polar planes to avoid the QCSE. Based on ZnO/ZnMgO finite QW, the quantized ground state of the finite well shows increasing with reducing the well width and with increasing the potential barrier V0. We choose to tune barrier height by varying the Mg concentration to enhance the quantum confinement. However, the larger Mg dopant would make the phase separation of different crystal structures of ZnMgO (hexagonal) and MgO (rock-salt structure). To keep lattice match in ZnO/Zn1-xMgxO quantum wells, we maintain the structure of Zn1-xMgxO as the hexagonal. In this thesis, we have successfully grown ZnMgO films with various Mg dopants by controlling O2 ambient in pulsed laser deposition from 8% and 15% Mg contents ZnMgO targets. From the photoluminescence (PL) spectrum measurement, we obtained the largest band gap of 3.783eV, corresponding to ~22% Mg content, grown under high vacuum (10-7 torr). The alloy fluctuation by Mg dopant in ZnO causes spectral width broadening of near-bandedge emission. Determined from the temperature dependent PL, the coupling strengths of exciton-phonon with the E2low, A1(LO) and E1 (LO) modes show increasing as increasing Mg content, and the exciton binding energy decreases with increasing Mg dopant caused by the disorders. Furthermore, we didn’t observe extra domain and phase separation in the X-ray diffraction even for x reaching 0.22. The lattice vibration study from Raman measurement reveals that peak shift of E2high mode with x shows m-direction tensile strain (b constant increases) and c-direction compress strain (c constant decreases).