以新型元件結構提升發光二極體性能

Abstract

在本論文中，我們展示了幾個新穎的發光二極體元件結構，用來提高發光二極體的發光效率和調變頻寬。首先，為瞭解表面光柵在垂直式發光二極體上用以提高光萃取效率的最佳化光柵週期，我們建構勞埃德干涉搭配光輔助電化學蝕刻技術在垂直式發光二極體上製作可變化週期的表面光柵。我們發現較小光柵週期的垂直式發光二極體其光萃取效率較高，而與表面粗糙化的垂直式發光二極體相比，光柵週期小於2微米的的垂直式發光二極體具有較高的光萃取效率。第二，證明在p型氮化鎵和晶圓鍵合金屬介面製作銀奈米光柵結構來產生表面電漿子耦合效果，可增強垂直式發光二極體的發光強度、產生極化輸出並降低效率滑落效應。基於金屬光柵的反射頻譜量測和數值模擬結果，發現在金屬光柵頂部附近產生侷域表面電漿子共振，在表面電漿子耦合量子井中扮演著主要角色。若我們再於n型氮化鎵上增加表面光柵，可更進一步增強發光強度、更進一步增加輸出極化率並更進一步抑制效率滑落效應。第三，在p型氮化鎵和週期排列或隨機分佈的銀奈米顆粒間插入折射率較氮化鎵低的介電質中間層，可藍移表面電漿子共振波長而增強藍光發光二極體的表面電漿子耦合效應。沒有介電質中間層時，表面銀奈米顆粒產生表面電漿子耦合發效果，可增強內部量子效率、發光二極體發光強度、減少效率滑落效應並加速調變響應。加入介電質中間層後，可使表面電漿子耦合效應增強，導致所有上述因素進一步增強。雖然規則排列的銀奈米顆粒可以有波長較專一性的侷域表面電漿子共振，然而表面電漿子耦合效率取決於侷域表面電漿子在量子井發光波段的共振強度。第四，我們比較發光二極體在不同元件平台大小和有無表面電漿子耦合下的調變頻寬，在發光二極體300到60微米的方形平台，表面電漿子耦合下可有效增加載子的衰退率，導致在注入電流密度範圍從139到1667 A/cm2下可以增加調變頻寬約44-48%。第五，於不同的佔空比和注入電流下，我們比較發光二極體在四種不同基板結構的發光行為，包含成長在藍寶石基板上的側向式發光二極體、晶圓鍵合於矽基板的垂直式發光二極體、銀膠黏著在金屬平板的可撓式的發光二極體以及銀膠黏著在彎曲金屬表面的可撓式發光二極體。在不同注入佔空比下，發光行為的不同主要決定於熱效應造成在氮化鎵層應力的改變，當氮化鎵層無法緊密接合基板，發光二極體所產生的熱造成強烈的拉伸應力，導致量子侷限史塔克效應變弱，因此發光效率增強，這樣的行為在彎曲的發光二極體更顯著。第六，我們比較只有介面層和整層氧化鎵鋅熱退火下接觸電阻率的不同結果。我們使用電漿輔助分子束磊晶成長氧化鎵鋅，不論在p型或n型氮化鎵上，只有介面層熱退火時的接觸電阻皆較小。我們將只有介面層熱退火的氧化鎵鋅分別成長在側向式及垂直式的發光二極體上，得到較低的元件電阻、較高的發光效率以及較低的效率滑落效應。

Several novel device structures of light-emitting diode (LED) for enhancing LED emission efficiency and modulation bandwidth are demonstrated in this dissertation. First, to understand the optimum grating period in forming a surface grating on a vertical light-emitting diode (VLED), we construct a Llyod’s interferometer within photoelectrochemical (PEC) electrolyte (KOH) to fabricate surface gratings of various periods on VLEDs for comparing their light extraction efficiencies. The comparisons of VLED characterizations show that among those grating VLEDs, the light extraction is more effective in a VLED of a smaller grating period. Compared with VLEDs of rough surfaces, the grating VLEDs of short grating periods (<2 m) have the higher light extraction efficiencies. Second, the enhancement of output intensity, the generation of polarized output, and the reduction of the efficiency droop effect in a surface plasmon (SP) coupled VLED with an Ag nano-grating structure located between the p-GaN layer and the wafer bonding metal for inducing SP coupling with the InGaN/GaN quantum wells (QWs) are demonstrated. Based on the reflection measurement from the metal grating structure and the numerical simulation result, it is found that the localized surface plasmon (LSP) resonance induced around the metal grating crest plays the major role in the SP-QW coupling process. By adding a surface grating structure to the SP-coupled vertical LED on the n-GaN side, the output intensity is further enhanced, the output polarization ratio is further increased, and the efficiency droop effect is further suppressed. Third, the enhanced SP coupling effects in a blue LED with regularly patterned (REG) and randomly distributed (RAN) surface Ag nanoparticles (NPs) on a dielectric interlayer (DI) of a lower refractive index overgrown on p-GaN are demonstrated. Without a DI, the surface Ag NPs induced SP coupling with the QWs in the LED can lead to the increases of internal quantum efficiency (IQE) and LED output intensity, the reduction of the efficiency droop effect, and the enhancement of modulation response. By adding a DI, the SP coupling effect is enhanced, resulting in the further improvements of all the aforementioned factors. Although the REG Ag NPs can produce stronger collective LSP resonance with a narrower spectral width, the SP coupling effect depends mainly on the LSP resonance strength at the QW emission wavelength. Fourth, the modulation bandwidths of the LEDs of different mesa sizes with and without SP coupling effect are compared. Due to the significant increase of carrier decay rate, within the size range of LED square-mesa from 60 through 300 micron and the injected current-density range from 139 through 1667 A/cm2, the SP coupling can lead to the enhancement of modulation bandwidth by 44-48 %, independent of the variations of LED mesa size or injected current level. The increases of the RC time constants in the samples with SP coupling are attributed to the increases of their device resistance levels when the Ag nanoparticles and GaZnO dielectric interlayer are added to the LED surface for effectively inducing SP coupling. Fifth, the emission behaviors of four LEDs of different substrate structures, including a lateral LED grown on sapphire, a vertical LED wafer-bonded onto Si (111), a bendable LED Ag-epoxied onto a flat metal, and another bendable LED Ag-epoxied onto a metal of a curved surface, under different duty cycles of current injection are compared. Their different variation trends of emission behavior with injection duty cycle are attributed to the different thermally-induced strain conditions in the epitaxial layers, which are controlled by their substrate structures, in increasing injection duty cycle or current level. The results of Raman scattering measurements during LED operation show that a stronger tensile strain is generated under heating for reducing the quantum-confined Stark effect and hence increasing emission efficiency when the epitaxial layer is not tightly bonded onto a hard substrate. Such a behavior is particularly stronger when the epitaxial layer is bent. Sixth, to identify the individually optimized thermal annealing conditions for reducing the contact resistivity between highly Ga-doped ZnO (GaZnO) and doped-GaN and for improving the electrical and optical properties of GaZnO, the effects of the junction-layer (JL) and whole-layer (WL) thermal annealing processes of GaZnO at various temperatures are compared. GaZnO is grown with plasma-assisted molecular beam epitaxy. The JL-annealing process always results in lower contact resistivity on either p-GaN or n-GaN at any annealing temperature. Lateral and VLEDs with GaZnO layers on the top are fabricated to show that the LED samples with the JL-annealing process have lower device resistance levels, higher emission efficiencies, and weaker efficiency droop effects, when compared to those with the WL-annealing process.