氮化鎵材料發光二極體與面射型雷射之製作與特性研究

Abstract

由於氮化鎵基材發光元件可廣泛的應用於如指示燈、各種照明、光儲存等領域，因此自1960年代以來氮化鎵相關材料成為世界上各研發團體的重要研究課題之ㄧ，且在此數十年間氮化鎵材料與元件特性上有屢有重大的發現與進展。在本研究論文中，我們製作氮化鎵發光二極體(LED)於金屬基板上與包含兩介電質反射鏡的氮化鎵面射型雷射(VCSEL)，並研究分析此二元件之主要特性。 首先我們製作大發光面積 (1x1 mm2) 藍光發光二極體於銅基板上。利用金屬接合與雷射剝離製程，成長於sapphire 基板上之藍光發光二極體可以被轉移至銅基板上。因具有良好的電流分佈，此大發光面積LED不需要透明電極便可以提供均勻的發光圖形。且因銅基板為良好的導電與導熱材料，此LED可以操作高至1安培且其光輸出功率呈線性增加。以在晶圓上(未切割及封裝)量測方式，此LED 在1安培輸入電流的操作下，光輸出功率為240 mW。 而當單一發光二極體的面積增大並於高電流操作時，則需要考慮電流聚集效應(current crowding)對於元件光電特性之影響。依實驗結果得到的電流擴散長度為400 □m，我們針對銅基板上的大發光面積LED設計了不同的電極圖形以研究電極圖形對於元件特性之影響。實驗結果顯示，設計良好的電極圖形可以提供均勻的電流分佈而讓LED有均勻的發光圖形。在1安培操作電流下，含有良好電極圖形之LED的光輸出功率為含有簡單圓形電極之LED的4倍。 另一方面，我們也提出了含有上下兩介電質反射鏡之氮化鎵面射型雷射結構。利用雷射剝技術並沉積成長介電質反射鏡，我們可以製作一含有氮化鎵與氮化銦鎵(GaN/InGaN)多量子井(MQW)的面射型雷射結構。在室溫下以一雷射(波長為355 nm)為激發光源，在光激發操作下，研究其發光特性。此氮化鎵面射型雷射的Q 系數 (quality factor) 為518，雷射波長為415.9 nm，雷射頻譜之半高寬為0.25 nm。由頻譜半高寬的減小、雷射頻譜之極化率與在發光孔徑中的雷射輻射圖形等結果，明確地顯示垂直雷射的行為。此氮化鎵面射型雷射於室溫下之臨界條件為270 nJ 且特徵溫度為278 K，自發輻射係數約為1x10-3。 我們也利用 Hakki-Paoli 方法量測並計算此面射型雷射之增益特性。在室溫臨界條件時，光增益為2.9x103 cm-1。在不同操作溫度下，我們發現越低溫時，光增益隨著注入載子數目增加而增加的速率越大。線寬增加係數 (linewidth enhancement factor) 在室溫下為2.8，其數值隨著操作溫度降而降低，溫度為80 K時，線寬增加係數為0.6。 此外，我們也觀察到當激發光源的能量增加時，在面射型雷射之發光孔徑中會有數個相對於不同輻射波長的雷射亮點。利用微螢光激發(micro-PL)方式量測發現在雷射發光孔徑中有螢光強度不均勻的現象。此一結果可以歸因於銦含量在MQWs中的不均勻分佈所造成。而銦含量的不均勻也造成在一個發光孔徑中所觀察到的多個雷射亮點。 最後，我們提出一個雙介電質反射鏡電激發操作氮化鎵面射型雷射的結構，並討論此電激發面射型雷射結構的製程與結果。

Due to the widespread applications such as indicators, back lighting, ambient lighting, display, optical storage, optical communication, etc., the GaN-based light emitting devices are widely investigated and have many remarkable breakthroughs in performance. In this study, the fabrication of GaN-based light emitting diodes (LEDs) on a metal substrate and vertical-cavity surface-emitting lasers (VCSELs) with two dielectric mirrors are demonstrated. The characteristics and performance of the LEDs and VCSELs are analysed and characterized. We demonstrated a large emission-area (1x1 mm2) blue LEDs on copper substrate. The LEDs structure grown on sapphire substrate by metal organic chemical vapor deposition system was transferred onto a Cu substrate using bonding and laser lift off techniques. The large emission-area LEDs showed a uniform light-emission pattern over entire defined mesa area without the transparent contact layer on the n-type GaN. The operating current of the LEDs can be driven up to 1000 mA with continuously increasing light output-power. Under wafer-level measurement, the light output-power is 240 mW with a driving current of 1000 mA. As the area of a single LED chip is scaled up, the current crowding effect under high current operation should be considered, while the current spreading length in the LEDs on Cu is about 400 μm. For optimizing the n-electrode pattern, we observed and reported the effect of n-electrode patterns on the optical characteristics of the large emission-area. The light emitting patterns showed an obvious current crowding effect in the LEDs with a simple circular n-electrode pattern. The LEDs with well-designed n-electrode pattern showed a uniform distribution of light emission and a higher output power due to uniform current spreading. Under 1000 mA operation, the LEDs with a well-designed n-electrode showed about a 4-fold increase in the light output power over the LEDs with a simple circular n-electrode pattern. On the other hand, we also proposed a GaN-bsed VCSEL structure consists of InGaN/GaN MQWs and two dielectric DBRs with high reflectivity. The GaN-based cavity including MQWs was gown on a sapphire substrate. Then the grown cavity was embedded by two dielectric DBRs and transferred onto a silica substrate. The laser emission characteristics of a GaN-based vertical-cavity surface-emitting laser with two dielectric distributed Bragg reflectors were investigated under optically pumped operation at room temperature. The quality factor of the VCSEL is 518, indicating a good interfacial layer quality of the structure. The laser emits emission wavelength at 415.9 nm with a linewidth of 0.25 nm. The measurement results, including the linewidth reduction, degree of polarization of 70% and emission images confined inside apertures, clearly indicate a vertical lasing action. The laser has a threshold pumping energy of 270 nJ at room temperature and a characteristic temperature of 278 K. The VCSEL has a high spontaneous emission factor of about 1x10-3. Meanwhile, Hakki-Paoli method was applied to analyze the temperature dependent optical gain and linewidth enhancement factor of the VCSELs. Due to multiple cavity modes in the structure, the optical gain can be obtained by measuring the photoluminescence spectra below the threshold condition. At room temperature, the optical gain of 2.9x103 cm-1 was estimated at the threshold condition with a carrier density of 6.5x1019 cm-3. Under different ambient temperature, it is found that the gain increases more rapidly as a function of the injected carrier density at lower temperature. The linewidth enhancement factor (α-factor) shows dependence on the wavelength and was smaller at shorter wavelength. The α-factor at room temperature was estimated as 2.8 and decreased to as low as 0.6 at 80K. The characterization of temperature dependent gain and α-factor provides further understanding in operation of the GaN-based VCSEL. The laser emission patterns show single and multiple emission spots spatially with different emission spectra under different pumping conditions. μ-PL intensity mapping indicated that the nonuniform PL emission intensity across the VCSEL aperture. The CL and TEM images of the as-grown cavity used in the VCSEL also showed the inhomogeneity of indium composition in the InGaN/GaN MQWs. The inhomogeneous material gain distribution resulting in nonuniform PL emission could be due to the fluctuated indium composition in the MQW active layers and result in multiple emission spots in an aperture. In the final chapter, we presented a two dielectric DBRs VCSEL structure for current injection operation. The issues of modifying the fabrication and design for the proposed VCSELs structure were also characterized and discussed.