氮化銦鎵/氮化鎵量子井聯合能態密度的變化及V型結構對元件特性的影響

Abstract

由於III-V族氮化鎵具有較寬的材料能隙，再搭配銦及鋁成份的調變，其能隙可從0.7eV到6.2eV，涵蓋了所有可見光的部分。因此III-V族氮化物適合作為發光二極體的材料，近年來更被廣泛的應用在指示燈、顯示器的背光源及照明上，然而受限其磊晶基板的選擇，目前較為普遍使用的藍寶石基板與氮化鎵材料存在著極大的晶格不匹配差異，因此氮化鎵磊晶結構易形成密度高達108~1010cm-2的貫穿式差排。貫穿式差排一般被認為是非幅射複合中心，會造成元件的發光效率下降，然而實際上氮化鎵藍光二極體的發光效率並沒有顯著的下降，這些特徵被認為可能來自銦原子群聚現象所產生的影響，並持續受到矚目。 本研究是利用有機金屬氣相磊晶機台成長氮化鎵發光二極體元件並針對其複合機制進行討論。我們引入一套自發性幅射光譜模型可用來描述磷化鋁鎵銦及氮化銦鎵發光二極體之螢光光譜，並可直接透過元件之螢光光譜，得到其量子井之聯合能態密度分佈，此方法所取得之聯合能態密度分佈與光電流量測所得到的吸收光譜相符。進一步的利用聯合能態密度分佈分析不同波長之氮化物發光二極體(紫光、藍光、綠光)隨電流變化的影響，發現波長隨電流增加所產生之藍移現象與聯合能態密度的變化息息相關。該現象可以解釋為當外加電流時，量子井的極化現象受到額外注入的載子而產生屏蔽效應，進而產生波長藍移。而低能態之聯合能態密度則是反應出銦原子群聚所形成之局部能態的現象，並導致較寬的螢光光譜。 另外，透過變溫光激螢光及時間解析螢光光譜分析載子在氮化物發光二極體中之動態行為。研究發現幅射複合的位置主要發生在銦聚集所形成之局部低能態，此局部低能態可以有效的捕捉載子防止載子擴散至貫穿式差排的位置產生非幅射複合使得發光效率下降。 最後我們研究V型結構於量子井對氮化物發光二極體元件的影響。V型結構是透過貫穿式差排所形成，其側邊所形成之較窄的多重量子井結構可提供較大的能隙，防止一般量子井中的載子與貫穿式差排產生非幅射複合。研究並指出，透過磊晶填平V型結構可形成電流阻擋層，進一步抑制電流的注入貫穿式差排，因此氮化物發光二極體可透過此磊晶結構的調整大幅的改善其發光效率。

The wide range of energy gaps in group-III nitride-based materials allows for adjustment of the direct band gap energies from 0.7eV to 6.2eV by varying the compositions of In and Al contents, including all visible energy. Therefore, among the many commercial applications of nitride-based light-emitting diodes (LEDs) include indicator lights, backlight displays and light bulbs. However, these devices typically have a high density of dislocations (108–1010cm-2) that thread through active regions, due to the significant lattice mismatch between nitride epilayers and sapphire substrates. Although being identified as efficient nonradiative centers, threading dislocations do not significantly degrade the performances of LEDs. This feature is recognized as the effects of self-formed In-rich regions in InGaN quantum wells and continues to draw considerable attention, thus warranting further study. This work studies the recombination mechanism of InGaN LEDs grown by metal organic chemical vapor deposition. An analytical method is also developed for determining the optical joint densities of states of AlGaInP LEDs and InGaN LEDs. Capable of extracting the optical joint density of states, the proposed method can be obtained from photoluminescence (PL) or electroluminescence spectrum of the devices, results of which correlate with the absorption spectrum evaluated by photo current measurements. Furthermore, the optical joint densities of states of three InGaN LEDs with different emission wavelengths (violet, blue and green) operated at various currents are examined. Experimental results indicate that the blueshift of the emissions with an increasing current is related to the variation in optical joint densities of states and can be attributed to the carrier screening of quantum-confined Stark effect induced by the piezoelectric field. Tails at the low-energy end of the density of states, corresponding to localized states, is found, and these tails broaden the spectra of the devices. Moreover, the carrier dynamic process of InGaN LEDs is investigated by using temperature dependence of PL and time-resolved PL. Experimental results indicate that the internal high efficiency of InGaN LEDs is mainly related to the spontaneously formed In-rich regions. The electron-hole pairs injected into wells typically drift to low-energy locations and then recombine radiatively rather than diffuse to dislocations. Furthermore, the extent to which V-shaped structures affect the internal quantum efficiency of nitride LEDs is investigated. The V-shaped structures are initiated at threading dislocations, subsequently enclosing them. Since quantum wells grown on the sidewalls of V-shaped structures are thin, electron and hole transition energy is higher than that of normal wells due to the quantum confinement effect. This increased energy functions as a potential barrier, preventing carriers in normal wells from diffusing into dislocations. Moreover, our sample contains an unintentionally formed current blocking mechanism, suppressing the current density in the region around dislocations. Since both possible paths have been blocked, electrically injected electron-hole pairs cannot generally reach the dislocations, reducing drawbacks associated with threading dislocations in nitride LEDs.