背面氧化銦錫層對鏡面基板發光二極體光電特性的影響

Abstract

中文摘要 近年來，發現在AlGaInP鏡面基板發光二極體的反射器與分佈式電極間加上ITO層可使發光二極體的光輸出功率更為提升。本篇論文分析研究此ITO層對反射率與電流分佈的影響，藉以了解加入ITO層提升發光二極體之物理機制。 我們發現ITO層會吸收反射光，進而使反射器的反射率下降；當實驗波長為631 nm時，膜厚268 nm的ITO層穿透率為88%，並使反射器的反射率降低了16%。因此ITO層對反射率並沒有幫助。 在電流散佈方面，我們在主動層下方結構製作比較了四種不同的介面結構：GaP/Au, GaP/ITO/Au, GaP/p+-GaAs/ITO/Au, 以及GaP/p+-GaAs/Au。我們發現增加接觸電阻有助於主動層上方的電流側向散佈，並可提升光輸出效率。但其缺點是會使光輸出飽和時的注入電流下降。這四種結構的接觸電阻以GaP/ITO/Au結構為最大；而GaP/p+-GaAs/ITO/Au最小。因此以這兩種結構構成散佈式接觸電極層時，可利用前者加強電流的散佈，利用後者降低電流路徑上的電阻。實驗顯示這種結構具有最高的光輸出功率。 本論文的分析結果顯示ITO層並不能提升反射率，其作用在於使GaP/ITO接面的電阻提高，強化散佈接觸電極的作用以提升LED的光輸出功率。

Abstract Sandwiching an ITO layer in between the metal reflector and the dispersive electrode of mirror-substrate (MS) type AlGaInP LEDs has become a standard procedure in industry to enhance the light power. The mechanism of this improvement, which is not well understood yet, is the research subject of this study. We first study the effect of ITO layer on the reflectivity of the reflector by comparing reflectors with and without ITO layer. The result shows that the reflectors with ITO layer have lower reflectivity, which is due to the absorption of extra ITO layer. Our measurement shows that an ITO layer with a thickness of 268 nm has a transmission of 88%, which lowers the reflectivity by 16%, when the incident light is at 631 nm. We then study the effect of ITO layer on the performance of the dispersive electrodes. We compare LEDs with different structures in between the GaP window layer and the Au mirror. Four structures, GaP/Au, GaP/ITO/Au, GaP/p+-GaAs/ITO/Au, and GaP/p+-GaAs/Au, were investigated. Results from transmission-line measurement show that GaP/ITO/Au and GaP/p+-GaAs/ITO/Au has the highest and lowest contact resistance, respectively. In addition, LED with higher contact resistance has better current spreading which leads to higher quantum efficiency at small current region. However, higher contact resistance also results in severer Joule heating, leading to output saturation at lower injection current. Based on this knowledge, a patterned dispersive electrode structure consisting of GaP/ITO/Au structure for the better lateral current spreading and GaP/p+-GaAs/ITO/Au structure for lower resistance along the vertical current path is the best combination to achieve the best light output. Experimental result shows that such device outperforms a control device without ITO layer in saturation output power by 15%. In summary, this work shows that ITO layer slightly degrades the reflectivity of the back-side reflector but significantly improve the function of the dispersive electrode in MS-type AlGaInP LEDs.