成長不同量子井數量於氮化鎵發光二極體之研究

Abstract

摘要

本論文中主要研究為變化不同量子井對數於氮化鎵發光二極體的表現，改變的對數為1、3、5、7、9、11、13元件樣品編號，分別為LED-P1、LED-P3、LED-P5、LED-P7、LED-P9、LED-11、LED-P13，探討不同量子井對數變化時光電特性和高電流效率下降(droop)改善。 利用光激發螢光光譜(Photo-Luminescence；PL)的分析，隨著量子井對數增加，其複合的機率也相對增加，所以PL強度也隨之增加，當量子井對數到LED-11時，PL強度呈現飽和的現象。於氮化鎵發光二極體元件光電特性LED-P1、LED-P3、LED-P5、LED-P7、LED-P9、LED-P11、LED-P13在操作電流60mA量測到的順向電壓分別為3.04V、2.92V、2.86V、2.89V、2.88V、2.89V、2.88V，量測可知量子井對數大於5對之後順向偏壓的差異並不明顯，其中LED-P5操作電壓比LED-P1低的原因為較多的量子井對數在靠近n型GaN有助於電流擴散，間接的抑制了電流擁擠效應。在逆向偏壓為-10V的量測條件時，發現漏電流有隨著對數增加越來越大的趨勢，原因可能是量子井對數過多導致薄膜品質變差，所以導致漏電流變大。 發光二極體元件於操作電流為60mA時，LED-P1、LED-P3、LED-P5、LED-P7、LED-P9、LED-11、LED-P13的光輸出功率(Light Output Power)分別為37.4mW、44.1mW、47.8mW、48.2mW、48.7mW、49.8mW、45.5mW，以LED-P1為標準之計算，LED-P11提升效率為15.2%，其原因為隨著量子井對數增加，使載子侷限能力變好，進而使複合機率增加，所以光輸出功率也跟著提升。其droop比較之結果是以60mA為基準點到200mA的區間所計算出來的下降效率，其LED-P1、LED-P3、LED-P5、LED-P7、LED-P9、LED-11、LED-P13的droop效率分別為，13.5%、13.1%、14.8%、12.9%、12.7%、11.2%、11%，其droop改善的可能原因是，隨著量子井數目增加使載子侷限效果變好及減少電子溢流的現象，進而達到降低droop效果。 藉由台大吳育任教授的模擬軟體matlab2012a，模擬不同量子井對數時的能帶圖，結果顯示在60mA和200mA的能帶圖，隨著對數增加能帶傾斜有變的越來越趨緩，這意味著量子井對數增加可抑制能帶傾斜改善了壓電場，使得發光層中的自由電子與電洞在位能井的空間分佈較為均勻，因此提高複合機率而達到亮度提升效果，進而抑制droop下降，其結果和實驗結果吻合。

Abstract

The objective of this study is to analyze the influence of GaN LEDs with different quantum well pairs. The changed number of MQW pairs are 1, 3, 5, 7, 9, 11, 13 (LED-P1, LED-P3, LED-P5, LED-P7, LED-P9, LED-11, and LED-P13). We aimed at finding out the optoelectronic characteristics during quantum well pairs changes, and improve efficiency droop at high current injection. By using photo-luminescence (PL) measurement, we discovered that as the quantum well pairs increases, the radiative recombination efficiency of the MQW pairs increases as well. The PL intensity is therefore increased, causing the PL intensity to reach saturation when the quantum well number reaches 11. It was found the forward voltages (at 60mA) of LED-P1, LED-P3, LED-P5, LED-P7, LED-P9, LED-P11, and LED-P13 are 3.04V, 2.92V, 2.86V, 2.89V, 2.88V, 2.89V, and 2.88V, respectively. When comparing LED-P1 and LED-P5, it was also found that more quantum well pairs around the n-GaN enhance the current spreading, which indirectly reduce the current crowding effect. When measured at a reverse voltage at -10V,it wad found that the leakage current increases as the MQW pair increase. The differences between the leakage current might have been caused by the low quality of the GaN film, which is related to the amount of quantum well pairs. The Light Output Power at 60mA injection current of LED-P1, LED-P3, LED-P5, LED-P7, LED-P9, LED-11, and LED-P13 are 37.4mW, 44.1mW, 47.8mW, 48.2mW, 48.7mW, 49.8mW, and 45.5mW, respectively. When using LED-P1 as the reference in calculation, the LOP of LED-P11 is more than 33.1%. The carrier confinement and radiative recombination improve as the quantum well pairs increase, thus increasing light output power. The droop comparison result derives from the calculation within 200mA using 60mA as the reference point. The droop efficiency of LED-P1, LED-P3, LED-P5, LED-P7, LED-P9, LED-11, and LED-P13 are13.5%, 13.1%, 14.8%, 12.9%, 12.7%, 11.2%, and 11%, respectively. The droop might have decreased since the carrier confinement have improved and the electron overflow has also decreased. We simulated the energy band diagram of each quantum well pairs, using the ‘matlab2012a’ software invented by Professor Wu from Taiwan University. It was found that the energy band diagrams of 60mA and 200mA, the energy band has become less slope as the MQW pairs increase. This means that the increase in quantum well MQW pairs may be able to prevent the energy band from tilting, thus reducing the piezoelectric field and making sure that the free electrons and holes are distributed in the well evenly. This results in higher radiative recombination and higher LOP, therefore preventing droop effect. It is in accordance with our experiment results.