磷化鋁鎵銦發光二極體之反射鏡特性研究

Abstract

本論文研究磷化鋁鎵銦AlGaInP四元化合物發光二極體(Light Emitting Diodes)的壓合製程結構特性，使用不同反射鏡材料、不同蒸鍍速率下之光電性表現差異。在研究中，我們首先測試在Ag、Au、Al、Pt四種反射鏡材料下之光電特性表現，結果顯示反射率越高之金屬可獲得較高反射率(在四元LED紅光630nm下)，所以具有最高反射率之Ag有最佳的LED出光效率。其中Al在回火前的反射率雖高於Pt，但因製程中包含高溫回火設計，Al在此高溫回火後會有氧化問題致使反射率降低許多，除了嚴重影響到LED出光效率之外，也因此影響到順向電壓之表現，隨著電流增加，電壓上昇之幅度更大，這也讓LED在大電流下之外部量子效率(EQE)明顯下降。 另外，針對不同蒸鍍速率下的反射鏡作特性研究，實驗設計三種不同的蒸鍍速率0.5 、1.5、 3.0 Å/s，結果顯示，蒸鍍速率與表面粗糙度呈反比，隨著蒸鍍速率變高，則表面粗糙度也會跟著變較平，因而得到越高之反射率；除了上述兩項觀察之外，在LED光電性之變化方面，因為前述所提鍍率不同所造成之表面粗糙度差異，所反映出來的反射率變化，也就是影響亮度的主要因素，鍍率高可得到較高的亮度表現。在使用Au反射鏡之LED操作在20mA電流下，最高鍍率3.0Å/s比最低鍍率0.5Å/s所得之輸出功率約增加了9.76%，使用Ag反射鏡之LED操作在20mA電流下，所得之3.0Å/s輸出功率增加約9.41%。 同時，也觀察到了在鍍率越高之條件下，會有隨著電流增加而致使LED之外部量子效率明顯下降的情形，研究結果顯示由於鍍率越高，反而會有電壓上昇的問題，這也致使外部量子效率下降(Efficiency Droop)更趨明顯。主要原因來自於電子束蒸鍍過程之加速電壓所影響，高鍍率之材料鍍在ITO表面時，因為加速電壓較高而使蒸發之原子有較大之動能，此動能讓原子在接觸ITO表面時會有類似撞擊之情形，這會對ITO表面造成某種程度的傷害。

In this thesis, we researched the electrical & optical characteristics of different reflective materials and different deposition rates for AlGaInP Red LEDs with bonding process. We compared the electrical-optical performance of different reflective materials in this research. There are four materials chose, Ag, Au, Al, Pt. The result showed that Ag had the best optical LED performance, followed by Au and Pt. Al was the worst one. A high temperature alloy treatment was designed in the LED process. Then oxidation issue occurs for Al. So that reflectivity of Al turned to low, even that was higher than Pt before alloy. Not only output power, but also the variation of forward voltage was influenced. Increasing ratio of forward voltage was even higher while increasing the driving current, especially at high current condition. Furthermore we do researches on surface morphology of each material, three different deposition rates designed to observe the different surface roughness. The result shows that with the deposition rate becomes high, the surface roughness will change to be a smooth one, low deposition rate will get a rougher surface morphology. Deposition rate and the surface roughness are inversely proportional. Surface roughness will affect the performance of reflectivity. If we compare the reflectivity with deposition rate, the higher deposition rate will have the higher reflectivity. On the contrary, the lower deposition rate will have the lower reflectivity. In our researches, except these two observations that we discussed above, we also observe the variation of electrical-optical characteristics. Higher output power can be obtained by higher deposition rate; oppositely, lower deposition rate leads to lower output power, because of surface roughness. In our experiments, the maximum deposition rate is 3.0Å/s, the minimum deposition rate is 0.5Å/s. For LED with Au material, we can have a 9.76% enhancement with injection current 20mA. For LED with Ag material, the enhancement is 9.41%. We also observed that the variations of the external quantum efficiency by XRD analysis. While increasing the deposition rate, the grain size of reflective mirror becomes larger. But ITO surface will been damaged by the evaporated atoms with higher dynamic energy that accelerated by electron beam voltage in higher deposition rate. This result leads to that forward voltage became larger at higher injection current. External quantum efficiency droop occurred more obviously. In our experiments, efficiency droop begins when the current is higher than about 150mA with a 10mil*10mil chip size.