| 摘要: | 在本論文中，氮化物材料是以有機金屬氣相沉積法 (metal-organicvapor phase epitaxy, MOVPE)所成長的並研究其氮化物光電半導體材料特性，主要是針對氮化物系列的發光二極體元件和太陽能電池之光電特性進行研究。首先，我們利用氮化鎵/氮化鎂(GaN/MgxNy)緩衝層(buffer layer)來減少錯位 (dislocation)的產生，從X-ray及霍爾量測分析可知使用氮化鎵/氮化鎂緩衝層確實可以提升氮化鎵薄膜的品質，其錯位缺陷密度可有效地從傳統使用低溫氮化鎵緩衝層~4.5 × 108 cm-2降低一倍到2.2 × 108 cm-2，在藍光發光二極體方面，使用氮化鎵/氮化鎂緩衝層可降低元件的逆向飽和電流，在逆偏-20 V量測下，其元件漏電流可從2.1 × 10-5 A改善到4.53 × 10-6 A，在順向20 mA電流注入下，對於元件光輸出功率可從原本的5.36 mW提升到5.97 mW，提升幅度為12%。 在發光二極體方面，在成長傳統單一層量子井主動層之前，先成長一層氮化銦鎵(In0.08Ga0.92N)層，此氮化銦鎵(In0.08Ga0.92N)層可有效地促進隨後成長之主動層的銦自聚形成，提供較大的載子侷限效應 localized state effect)，跟未使用氮化銦鎵(In0.08Ga0.92N)層的發光二極體的光輸出功率比較，可從原本的2.9 mW提升到6.6 mW，其發光二極體光輸出功率提升為2.2倍。接著，我們針對藍光發光二極體在大電流注入下其內部量子效率下降問題作探討，本實驗提出使用氮化銦/氮化鎵多層式成長法作為主動層，可有效的改善從量子井與量子能障間所產生的缺陷、提升量子井與量子能障間的接面品質、提升載子的侷限效應、降低極化場的應力，在注入電流密度500 A/cm2下，跟傳統多層式量子井結構比較其發光功率可從71.0 mW提到到89.4 mW，提升幅度為26%，在注入電流密度500 A/cm2下，其傳統多層式量子井結構發光二極體之量子效率與其峰值之差異為47%，使用氮化銦/氮化鎵多層式結構可改善到與其峰值之差異為21%。 在氮化鎵太陽能電池應用方面，我們利用多層量子井結構(multiplequantum wells, MQW)作為吸收層，與傳統的厚膜氮化銦鎵吸收層比較起來，多層量子井結構的太陽能電池可避免厚膜的高銦含量的氮化銦鎵相分離問題，提升太陽能電池之填充因子(fill factor, FF)，此外，利用多層量子井結構可控制其太陽能電池之開路電壓和短路電流特性;在改善太陽能電池光電特性方面，我們利用AZO當作穿透層(transparent layer)，可以有效增加短路電流密度，可從原本的0.343 mA/cm2提升到0.473 mA/cm2，整體轉換效 率可從原本的0.26 %提升到0.39 %，最佳化條件的太陽能電池之開路電壓1.30 V、短路電流密度0.473 mA/cm2、填充因子0.630和轉換效率為0.39 %成功的研製。 In this dissertation, the growth and characterization of multiple MgxNy/GaN buffer layers and InGaN/GaN blue light-emitting diodes have been studied. The improvement of both LED light output power and efficiency droop are investigated. In addition, we also discuss the optoelectronic characteristics of nitride-based solar cells with MQW absorption layer. The primary results obtained in this dissertation are summarized as follow: (a) It was found that the GaN grown on MgxNy buffer layer showed lower dislocation density (~2.2 × 108 cm-2), higher carrier mobility (~630 cm2/Vs), lower background carrier concentration (~5.1× 1016 cm-3) and narrower FWHM of (002) (~228 arcsec) and (102) (~248 arcsec) in DCXRD, as compared with conventional GaN grown on low-temperature (LT) GaN buffer layer. The blue LEDs grown on 12-pairs MgxNy (120 sec)/GaN buffer layers could also reduce the reverse leakage current and enhance the LED output power from 5.36 mW to 5.97 mW at a 20 mA current injection, as compared with conventional LEDs. (b) For GaN-based LEDs, nitride-based asymmetric two-step LEDs with a In0.08Ga0.92N shallow step was proposed and fabricated. By inserting an In0.08Ga0.92N shallow step, it was found that LED output powers can be improved from 2.9 mW to 6.6 mW under a 20 mA current injection. The improvement of output power could be attributed the fact that the significant carrier localization effect in the asymmetric two-step LEDs can lead to higher IQE. Under high injection current density, LEDs with InN/GaN multilayer wells (MLWs) structure can improve both the light output power and efficiency droop, compared to the conventional InGaN MQW LED. Under an injection current density of 500 A/cm2, we can enhance LED output power from 71.0 mW to 89.4 mW, compared to conventional LED. As compared to the EQE at an injection current density of peak value, the EQE values at an injection current of 500 A/cm2 are approximately reduced by 47 % and 21 % for the conventional LED and InN/GaN MLWs LED, respectively. These improvements could be attributed to the InN/GaN MLWs can significantly reduce the threading dislocations generated from active region, improve the interfacial quality between QW and QB, enhance the localized state and release strain in InGaN layer grown on GaN. (c) For GaN-based solar cells, The MQW structure should be able to maintain the material quality of high-indium-content InGaN alloys, leading to better device performance than devices with a single InGaN layer as the active layer. The short-circuit current density (JSC) and open-circuit voltage (VOC) can be modulated by different arrangement of blue and green QW in MQW absorption layer. The optimal electrical characteristics of solar cell with JSC = 0.473 mA/cm2, VOC = 1.30 V, fill factor (FF) = 0.630 and conversion efficiency = 0.39 % with AZO transparent contact layer (TCL) can be obtained. |
