| 摘要: | 改善氮化銦鎵/氮化鎵多重量子井發光二極體效率衰退效應與降低操作電壓已經成為近年來熱門研究的議題。由於壓電極化場之不匹配造成極化片電荷存在於氮化銦鎵/氮化鎵之界面進而影響電子的傳輸，此現象在發光二極體之效率衰退效應中扮演著重要的因素。在一般利用化學氣相沉積沿著[0001]方向所成長之傳統鎵面(或稱作一般極化方向)的氮化銦鎵多重量子井發光二極體結構中，極化效應在最後一個氮化鎵位能障所產生的電場會加速電子傳輸而使得大量的電子溢流至氮化鋁鎵電子阻擋層。同時，壓電極化場的不匹配也使得氮化鎵與氮化鋁鎵電子阻擋層界面的價電帶之間產生釘狀的位能障，使之減少了由P型氮化鎵區注入主動區的電洞濃度而降低了在量子井的載子輻射複合效率。在這個研究中，吾人利用SiLENSe模擬軟體研究了氮面(或稱作反向極化方向)單一量子井發光二極體的光電特性。與傳統的鎵面發光二極體結構相比，氮面發光二極體有更高的內部量子效率且有較低的操作電壓，並可藉由降低溢流電流來改善效率衰退的效應，而這些特性都藉由了在氮化銦鎵量子井中的反向壓電極化電場而呈現出來。
除此之外，此研究也藉由加入一層氮化鋁鎵電子阻擋層至發光二極體中使之在氮化鎵與氮化鋁鎵的介面產生了二維電洞氣來成功地降低了元件操作電壓。由模擬結果顯示，與未使用氮化鋁鎵的電子阻擋層相比，當吾人使用12% 鋁含量的氮化鋁鎵電子阻擋層時，其操作電壓可由2.93伏特降至2.74伏特。另外一方面，吾人也針對量子井的周期數對氮面發光二極體的影響作討論，其結果顯示氮面的發光二極體與傳統鎵面的發光二極體相比在大電流密度下有著較大的串聯電阻，這是因為在量子井氮化銦鎵與氮化鎵介面的反向極化電場所產生的位能障會阻擋電子與電洞在能帶中的傳輸，故使得量子井周期數越多則串聯阻值也會隨之越大。
然而，為了達到氮面發光二極體的物理特性，本實驗利用化學氣相沉積系統的再成長方式開發了具有反向壓電極化電場特性的P型氮化鎵於下方的發光二極體。首先，為了保護P型氮化鎵的歐姆接觸區域，吾人沉積厚度50奈米的二氧化矽於P型氮化鎵的表面，接著再使用黃光微影製程去定義反轉的發光二極體結構之再成長區域於P型氮化鎵表面，其成長結構與順序分別為700奈米的P型氮化鎵，五個週期的氮化銦鎵與氮化鎵多重量子井結構和600奈米的N型氮化鎵。實驗結果顯示，與傳統P型氮化鎵於上方的發光二極體結構相比，在73電流密度下，此實驗成功的將輸出功率從2.35 毫瓦改善至2.64毫瓦並且沒有效率衰退的現象。這個研究顯示了P型氮化鎵於下方的發光二極體在未來仍具有很大的發展淺力且值得更進一步的去開發與探討。
Reducing the efficiency droop and turn on voltage of InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) have been the subjects under intensive investigations lately. Piezoelectric polarization mismatch, which induces polarization sheet charge at InGaN/GaN interfaces and impacts electron transport, is considered an important factor in the efficiency roll-off behavior of these LEDs. For typical Ga-face InGaN MQW LEDs grown along the [0001] orientation by metal-organic chemical vapor deposition, also referred as “normal polarization,” the polarization-induced electric field in the last barrier layer is responsible for the enhanced electron spillover toward the electron blocking layer (EBL). Meanwhile, the piezoelectric polarization mismatch between the EBL and GaN creates a potential spike in the valence band that blocks the injection of holes from p-GaN to MQW and reduces radiative recombination in MQW. In this work, the optical and electrical characteristics of N-face single quantum well LEDs are investigated by SiLENSe simulation software, also referred to as “reversed polarization”. Compared to its Ga-face counterparts, N-face LEDs exhibit higher internal quantum efficiency and lower turn-on voltage as well as less droop effect due to the reduced electron spillover. These properties can be well accounted for by the reversed piezoelectric field in the InGaN quantum well.
Besides, the operating voltage of these LEDs is reduced when an AlGaN electron blocking layer (EBL) is added to the LED structure. This is attributed to the formation of a 2-dimensional hole gas at the GaN/AlGaN interface. Simulation shows that the use of an Al0.12Ga0.88N EBL results in an operating voltage of 2.74 V, compared to 2.93 V for the LED without an EBL. Otherwise, the varied quantum well periods of N-face LEDs were also discussed, the series resistance increasing much higher than Ga-face LEDs at high current density, it is attributed to the reversed polarization field to induce the higher potential than conventional Ga-face LEDs at InGaN/GaN interface that impedes electrons and holes to transportation in the MQW when we increasing the quantum periods.
However, in order to achieve the same physical property of N-face LEDs, the p-side down LEDs with a reversed polarization field is developed by regrowth method using metal-organic chemical vapor deposition (MOCVD). The experiment firstly deposited 50 nm thick SiO2 on p-type GaN surface to protect the p-type GaN contact region. The LED structure regrowth region is then defined by photolithography following an inversed LED structure growth, whish’s sequence is a 700 nm p-type GaN, five period InGaN/GaN MQWs and a 600 nm n-type GaN layer. As a result, comparing with p-side up LED structure, the output power is successful improved from 2.35 mW to 2.64 mW under 73 A/cm2 and show the no droop effect. This work indicates that the p-side down devices are very promising and deserve further development in the future. |
