InGaN-based light-emitting diodes with an embedded conical air-voids structure

The conical air-void structure of an InGaN light-emitting diode (LEDs) was formed at the GaN/sapphire interface to increase the light extraction efficiency. The fabrication process of the conical air-void structure consisted of a dry process and a crystallographic wet etching process on an undoped GaN layer, followed by a re-growth process for the InGaN LED structure. A higher light output power (1.54 times) and a small divergent angle (120o) were observed, at a 20mA operation current, on the treated LED structure when compared to a standard LED without the conical air-void structure. In this electroluminescence spectrum, the emission intensity and the peak wavelength varied periodically by corresponding to the conical air-void patterns that were measured through a 100nm-optical-aperture fiber probe. The conical air-void structure reduced the compressed strain at the GaN/sapphire interface by inducing the wavelength blueshift phenomenon and the higher internal quantum efficiency of the photoluminescence spectra for the treated LED structure

An AlN Sacrificial Buffer Layer Inserted into the InGaN Light Emitting Diode on Patterned Sapphire Substrate for a Chemical Lift-Off Process

在本論文中，我們利用氮化鋁犧牲緩衝層與兩種圖案化的藍寶石基板應用於氮化銦鎵發光二極體之化學基板剝離技術。 第一種氮化銦鎵發光二極體是成長在有金字塔形狀的藍寶石基板上。在發光二極體的磊晶成長完之後，可以發現在圖案化藍寶石基板和氮化鎵磊晶層的介面之間有空氣空孔，氫氧化鉀溶液藉由這個空氣空孔蝕刻氮化鋁緩衝層，增加側向蝕刻速率。從被剝離100微米寬度的發光二極體晶粒上，可以發現氮化鋁緩衝層的側向蝕刻速率是每分鐘10微米。在被剝離的發光二極體晶粒表面，可以發現從藍寶石基板轉印過來的三角形孔洞及六角形的空氣空孔。在顯微光激螢光的光譜中，可以發現被剝離的發光二極體晶粒比還沒被剝離的發光二極體晶粒的光激螢光波峰位置的波長較短。這個現象是因為在氮化鎵磊晶層和藍寶石基板之間的應力釋放所造成的。 第二種氮化銦鎵發光二極體是成長在有截面三角形條狀的藍寶石基板上。在被剝離的氮化鎵磊晶上，可以發現有兩個穩定晶面終止的V形狀溝槽。在顯微光激螢光的光譜中，我們也可以發現被剝離的發光二極體晶粒比還沒被剝離的發光二極體晶粒的光激螢光波峰位置的波長較短。還沒被剝離之前的氮化鎵磊晶層所激發出來的光激螢光最高強度的波長位置在445.8奈米，而被剝離最高強度的波長位置在440.7奈米。我們可以利用化學剝離的方式來製造出獨立的發光二極體，發光的波段在453奈米。 我們可以利用氮化鋁的緩衝層當作犧牲層，再利用熱的氫氧化鉀蝕刻這層犧牲層以達到化學剝離的目的。這種方式可以取代傳統的雷射剝離方式來製造垂直式的發光二極體。 我們也利用這種化學蝕刻技術側向蝕刻氮化銦鎵發光二極體。這個氮化銦鎵的發光二極體是成長在有截面三角形條狀的藍寶石基板上。當氮化鎵磊晶層成長在圖案化的藍寶石基板後，可以發現在V形的條狀空氣空孔溝槽有較高的側蝕速率。在由下往上的氮化鎵N晶面濕式蝕刻後，形成了氮化鎵穩定晶面{10(11) ¯} 。我們發現經過溼式蝕刻的發光二極體的亮度提升65%且有較小的發散角。形成在氮化鎵和圖案化藍寶石基板介面之間的菱形空氣空孔提供一個較高的光取出路徑。We use chemical lift-off technology to lift-off GaN epilayer grown on AlN sacrificial from two kinds of pattern sapphire. The first type is InGaN-based light-emitting diodes (LEDs) grown on triangle-shaped patterned sapphire substrates. After the epitaxial growth, an air-void structure was observed at the patterned region on the sapphire substrate that provided an empty space to increase the lateral etching rate of the AlN buffer layer. The lateral etching rate of the AlN buffer layer was calculated at 10μm/min for the 100-μm-width LED chip that was lifted off from the sapphire substrate. A triangular-shaped hole structure and a hexagonal-shaped air-void structure were observed on the lift-off GaN surface that was transferred from the patterned sapphire substrate. Comparing to the LED/sapphire structure, a peak wavelength blueshift phenomenon of the micro-photoluminescence spectra was observed on the lifted off LED chip caused by the release of a compressive strain at the GaN/sapphire substrate interface. The second type is an epitaxial layer of an InGaN light-emitting diode (LED) structure grown a truncated-triangle-striped patterned-sapphire substrate. A crystallographic stable and terminated V-shaped GaN grooved pattern was observed on the lift-off GaN surface. The peak wavelength blueshift phenomenon of the micro-photoluminescence spectrum was also observed on the lift-off LED epitaxial layer (440.7 nm) compared with the LED/sapphire structure (445.8 nm). The free-standing LED epitaxial layer with a 453nm electroluminescence emission spectrum was realized through a CLO process The chemical lift-off process was achieved by using an AlN buffer layer as a sacrificial layer in a hot potassium hydroxide solution, which has the potential to replace the traditional laser lift-off process for vertical LED applications. We also use this chemical etching technology to lateral etch InGaN light-emitting diodes (LED) which were grown on a truncated-triangle-striped patterned sapphire substrate. After growing a GaN layer on the patterned-sapphire substrate, it was observed that a higher lateral growth process formed a V-shaped-striped air-void structure. After a bottom-up N-face wet etching process on a GaN layer, the stable crystallographic etching planes were formed as the GaN {10(11) ¯} planes. We found out that treated LED structures had 65% light enhancements and smaller divergent angles. A rhombus-like air-void structure formed at GaN/patterned-sapphire interface provided a high light extraction process.Contents 中文摘要 i Abstract ii Chapter 1 Introduction 1 1-1 The history of LED 1 1-2 White LED 3 1-3 Vertical LED and lift off technology 4 1-4 Motivation of the investigation 5 Chapter 2 The Principle for both LED and sapphire and the mechanism of wet etching 6 2-1 Light-emitting diodes light generation principle 6 2-2 The crystal plane of sapphire 8 2-3 Wet etching of GaN and AlN 11 2-3-1 AlN 11 2-3-2 GaN 13 Chapter 3 Experiment 15 3-1 Sample prepare 15 3-2 Micro-Photoluminescence (μ-PL) 23 3-3 Electroluminescence (EL) 25 3-4 Light-Output Power 25 3-5 Far-Field Radiation Patterns 25 3-6 Light-Intensity Profiles 26 Chapter 4 Results and Discussion 27 4-1 An AlN Sacrificial Buffer Layer Inserted into the GaN/Patterned Sapphire Substrate or a Chemical Lift-Off Process 27 4-1-1 Secondary Electron Micrograph (SEM) Analysis 27 4-1-2 Micro-Photoluminescence Spectroscopy (μ-PL) 31 4-2 Chemical Lift-Off Process for Blue Light-Emitting Diodes 34 4-2-1 Optical Microscopy Images Analysis 34 4-2-2 Cold Field Emission Scanning Electron microscopy (FE-SEM) 36 4-2-3 Micro-Photoluminescence (μ-PL) Analysis 40 4-2-4 Electroluminescence (EL) Analysis 40 4-2-5 Current-Voltage (I-V) Characteristics & Light-Output Power Analysis 41 4-2-6 Light-Intensity Profiles 41 4-3 Enhanced the Light Extraction Efficiency of an InGaN Light Emitting Diodes with an Embedded Rhombus-Like Air-Void Structure 43 4-3-1 Optical Microscopy Images Analysis 43 4-3-2 Secondary Electron Micrograph (SEM) Analysis 45 4-3-3 Light-Intensity Profiles 47 4-3-4 Micro-Photoluminescence (μ-PL) Analysis 51 4-3-5 Electroluminescence (EL) Analysis 53 4-3-6 Current-Voltage (I-V) Characteristics & Light-Output Power Analysis 54 4-3-7 Far-Field Radiation Patterns 54 Chapter 5 Conclusions and Prospects 58 5-1 The Conclusions of the Experiment 58 5-2 Future work 60 Reference 61 Appendix 65   Figure Captions Fig. 1- 1 The visible spectrum LEDs evolution of luminous efficiency versus time and other light sources luminous efficiency (referred from Schubert [9]). 2 Fig. 2- 1 The energy versus crystal momentum diagram (k) of direct bangap and indirect bandgap material. 7 Fig. 2- 2 (a) The hexagonal and rhombohedral elementary structure of the sapphire: (1) octahedral hollows; (2) aluminum ions. (b) The organization of Al3+ (black circles) and octahedral hollows (small light circles) between two layers of O2- (large light circles) in the basal plane (referred from Elena R. Dobrovinskaya et al. [22]). 10 Fig. 2- 3 The diagrams depict the polarity selective etching mechanism of the N-polar GaN film viewed along [1 ¯1 ¯20] direction. (a) Each surface nitrogen atom has a negatively charged dangling bond; (b) chemical adsorption of hydroxide ions; (c) formation of Ga2O1-x oxides; (d) Ga2O1-x oxides dissolving (referred from Li et al. [26]). 14 Fig. 3- 1 Schematic of the experimental flow 29 Fig. 3- 2 Sapphire fabrication process of sample [I] 30 Fig. 3- 3 Sample [I] LED fabrication process 31 Fig. 3- 4 Sample [II] sapphire fabrication process 32 Fig. 3- 5 Sample [II] LED etching process 33 Fig. 3- 6 Rhombus-like air-void structure LED etching process 33 Fig. 3- 7 The μ-PL measure system 35 Fig. 3- 8 The far-field radiation patterns system 37 Fig. 4- 1 (a, b) The air-void structures are observed at the top of the patterned sapphire substrate shown in these two cross-section SEM micrographs. (c, d) The lateral wet etching process occurs at the AlN buffer layer located at the GaN/sapphire interface. (e) The schematic diagram and process of the InGaN-based LED structure with the lateral etching channel at GaN/sapphire interface is shown here. 40 Fig. 4- 2 (a, b) The SEM micrographs of the lift-off epitaxial layer are observed on the adhesive tape. The SEM micrographs: (c) top view and (d) 45° bird’s eye view, of the lift-off epitaxial layer of the flat GaN surface at the chip center and of the cone-shaped GaN structure around the central region. (e, f) Here are the flat GaN surface and bottom epitaxial layer with the triangular patterns and the hexagonal shaped air-voids. 41 Fig. 4- 3 (a) The peak wavelength and intensity of the PL spectra across the 100-μm-width mesa region are measured. (b) The μ-PL spectra of the partial laterally etched LED mesa structure are measured from the mesa center to the mesa edge at room temperature. The PL spectra of the lift-off epitaxial layer are also measured at the mesa center region. 44 Fig. 4- 4 The etching process of the AlN buffer layer etching in 80℃ KOH solution (a) 0 min (b) 3 min (c) 6 min (d) 9min (e) 15min. 46 Fig. 4- 5 (a)(b) An LED epitaxial layer was grown on the patterned sapphire substrate with V-shaped air channels. (c) The stable V-shaped GaN planes were formed through a bottom-up crystallographic etching process. (d) Cross-sectional SEM image of the lift-off GaN layer. (e) Schematic diagram of the LED structure for the CLO process. 49 Fig. 4- 6 (a) Lift-off LED chips on an adhesive tape. (b) The individual LED chips defined through the laser scribing process were lifted off from the patterned sapphire. (c, d) V-shaped striped-line patterns are observed on the lift-off GaN surface. 50 Fig. 4- 7 (a) The μ-PL emission spectra of both LED samples are measured where the laser spot is focused on the p-type GaN:Mg layer. (b) The peak wavelengths of the EL spectra are measured by varying the injection current. (c) The current–voltage (I–V) characteristics and the light output power as a function of the operating current are measured here. The light emitting images of the lift-off LED epitaxial layer are observed at (d) 1mA and (e) 5mA operating currents. 53 Fig. 4- 8 Optical microscope images with front- and back -light illumination. (a, b) ST-LED, (c, d) VA-LED, (e, f, g, h) RA-LED. (a, c, e, f) at 10X, (b, d, g, h) at 100X. 55 Fig. 4- 9 The SEM micrographics of (a) the VA-LED and (b) the RA-LED (c) the ST-LED were shown. (d) The schematic diagram of the RA-LED structure is shown here. 57 Fig. 4- 10 The light-intensity profiles of (a) (b) the RA-LED, (c) (d) the VA-LED, (e) (f) the ST-LED structures were analyzed by a beam profiler at a 20mA operating current. 60 Fig. 4- 11 Line-scan light intensities of the all the LED structures 61 Fig. 4- 12 The μ-PL emission spectra of all the LED samples were measured where the laser spot was focused on the mesa edge region. 63 Fig. 4- 13 The EL emission wavelength is measured at about 454nm for all LED structures at a 20mA operating current. 64 Fig. 4- 14 (a) Current–Voltage (I–V) characteristics (b) For all LED samples, the lightoutput power as a function of the operating current were measured here. 67 Fig. 4- 15 Far-field radiation patterns of all LED samples were analyzed by the divergent angle measurements. 68 Table Captions Table 1 GaN and AlN etching results at 25℃ in acid and base solutions 2