具氮化鎵與氮化鋁布拉格反射鏡之氮化鎵面射型雷射之研究

Abstract

過去幾十年來由於氮化鎵材料具有直接能隙與强鍵結力等優點，因此吸引許多學界與業界的目光並且廣泛的製作成各種光電元件，如發光二極體、雷射二極體與光偵測器等，並且常被應用於日常生活中的照明、光儲存、平面顯示及生物科技中。本論文旨在設計並製作具混合式布拉格反射鏡之電激發氮化鎵垂直型共振腔面射型雷射與具氮化鎵與氮化鋁布拉格反射鏡之光激發氮化鎵光子晶體面射型雷射。 首先，我們成功的製作出室溫電激發氮化鎵材料之垂直型共振腔面射型雷射。本雷射結構具有一組氮化鋁與氮化鎵布拉格反射鏡、七倍的光學波長與一組氧化物反射鏡。為了增加載子的復合效率與減少光之損耗，我們在七倍的光學波長共振腔中穿插一層氮化鋁鎵電子阻擋層與一層約30 nm的極薄ITO薄膜層，兩層結構均位於多重量子井的上方。本雷射在室溫所量測出來的臨界電流為9.7mA，相對應之臨界電流密度約12.4kA/cm2，其輻射波長為412nm且半高寬約0.5nm。極化程度與雷射發散角分別為55%與8度。在CCD影像中，本雷射光點約為2μm，原因在於InGaN材料本身的不均勻特性所導致。此外，自發輻射耦合效率約為5*10-3，此結果與一般的邊射型雷射相比約高達100倍之高，代表其效率被微共振腔結構大幅提高。 其次，我們也成功的製作出具有單一雷射模態與較小雷射發散角之光激發氮化鎵光子晶體面射型雷射。藉由設計不同週期的光子晶體結構(190nm-300nm)，這些光子晶體雷射元件出射之雷射波長範圍可從395nm調變到425nm。利用平面波展開法模擬其TE能帶圖與布拉格理論，本雷射結構所激發出的雷射波長所相對應之正規化頻率正好相對於TE能帶圖三個能帶邊界(Γ1，K2，M3)，而從極化方向也可證實雷射模態確實存在此三個能帶邊緣模態。從變角度光致發光系統的量測中，我們得到雷射模態Γ1，K2及M3的雷射出射角度，相對於垂直方向，分別為0˚, 29˚及59.5˚，其雷射發散角分別為(1.2˚, 2.5˚, 2.2˚)。此外，Γ1模態的雷射臨界操作功率均小於K2與M3模態，藉由二維耦合波模型所計算之雷射耦合係數，可推測此結果可歸因於Γ1模態具有最大的雷射耦合係數。 由於氮化鎵垂直型共振腔面射型雷射與光子晶體面射型雷射具有許多優越的光學特性，我們相信此結構可以應用在可見光及藍紫光雷射等高功率、大範圍及多用途之光電元件中。

For several decades, GaN-based material has been attracted much attention of academia and industry and widely used in several optoelectronic devices due to its wide direct bandgap and strong binding energy, such as light emitting diodes, laser diodes, and photon-detectors which can be applied in lighting, optical storage, display, and biotechnology. The thesis is focus on the design and fabrication of the electrical pumped GaN-based vertical cavity surface emitting lasers (VCSEL) with hybrid distributed Bragg reflectors and the optical pumped GaN-based photonic crystal surface emitting lasers (PCSEL) with AlN/GaN distributed Bragg reflectors. First, we demonstrated a electrical pumped CW laser action on GaN-based VCSEL at room temperature. The laser structure consists of a 10-pair Ta2O5/SiO2 distributed Bragg reflector (DBR), a 7λ-thick optical cavity, and a 29-pairs AlN/GaN DBR. To enhance the carrier coupling efficiency, an AlGaN electron blocking layer was inserted above the multi-quantum wells region, and a thinner ITO layer (30nm) was used in the 7λ cavity. The laser has a threshold current 9.7mA corresponding to threshold current density of about 12.4kA/cm2. The lasing wavelength is located at 412nm with a linewidth of about 0.5nm. The degree of polarization and the laser beam divergence angle are about 55% and 80, respectively. In the CCD image, the laser spot size is about 2μm contributed by the InGaN inhomogeneous characteristics. Besides, the spontaneous emission coupling factor is about 5*10-3. The value is 100 times larger than the commercial edge emission lasers means the efficiency is substantially enhanced by the micro cavity structure. Second, we successfully fabricated the optical pumped GaN-based photonic crystal surface emitting lasers (PCSEL) with single mode emission and small divergence angle. By designing different photonic crystal lattice constants from 190nm to 300nm, different PCSELs with lasing wavelength ranging from 395nm to 425nm were achieved. According to the plane-wave expansion method and satisfied Bragg theory, the different lasing wavelengths can be attributed to the different band-edge modes, such as Γ1, K2, and M3. This result can be further confirmed by the polarization directions of these three band edge modes (Γ1，K2，M3). Moreover, from the angular-resolved μ-PL measurements, we can obtain the lasing angles of these modes normally from the sample surface are 0˚, 29˚, and 59.5˚, and the laser beam divergence angles are 1.2˚, 2.5˚, and 2.2˚. Furthermore, the threshold pumping power of Γ1 mode is smaller than that of K2 and M3 modes. This result can be attributed to the largest coupling coefficient calculated by the 2-D couple wave theory. Due to these optical and electrical advantages of GaN-based VCSEL and PCSEL, we believe these laser devices can be applied visible light and UV light lasers in high output power and multiple purposes of optoelectronic devices.