Investigation of GaN-based light-emitting diodes prepared on high aspect ratio patterned sapphire with sputtered AlN nucleation layer
|關鍵字:||高深寬比圖案化藍寶石基板;high aspect ratio patterned sapphire|
|摘要:||本論文成功的應用濺鍍氮化鋁緩衝層成長高品質氮化鎵薄膜於高深寛比圖案化藍寶石基板(High aspect ratio pattern sapphire substrate, HARPSS )，分別討論一般MOCVD成長的氮化鋁緩衝層和濺鍍氮化鋁緩衝層所成長氮化鎵薄膜於高深寛比圖案化藍寶石基板上的薄膜品質、晶格面和磊晶成長機制；並且應用濺鍍氮化鋁緩衝層成長氮化鎵藍光發光二極體結構於圖案化藍寶石基板，分別討論使用濺鍍氮化鋁緩衝層成長的發光二極體與使用傳統MOVCD成長氮化鋁緩衝層所成長的發光二極體在光電特性之差異。
在氮化鎵薄膜成長於不同深寬比的圖案化藍寶石基板材料分析方面，透過掃描式電子顯微鏡(SEM) 分析發現使用一般MOCVD氮化鋁緩衝層所成長的氮化鎵薄膜與藍寶石基板之接合面有明顯之空隙(Void)，而使用濺鍍氮化鋁緩衝層所成長的氮化薄膜則無此現象。利用高解析度X-光繞射儀(X-ray)量測氮化鎵薄膜，可以觀察出使用濺鍍氮化鋁緩衝層成長氮化鎵薄膜於高深寛比圖案化藍寶石基板上，其氮化鎵薄膜的薄膜品質是優於使用一般傳統MOCVD氮化鋁緩衝層成長的氮化鎵薄膜。此外，利用穿透式電子顯微鏡(TEM)進一步檢視其氮化鎵薄膜的晶格結構，發現使用一般傳統MOCVD氮化鋁緩衝層所成長的氮化鎵薄膜於圖案化基板的圖案斜面上的氮化鎵晶格結構是閃鋅結構(zinc blend structure)，其它的成長區域的氮化鎵晶格結構是傳統纖鋅結構(wurtzite structure)；不同晶格面的區域相接出形成缺陷。而使用濺鍍氮化鋁緩衝層成長氮化鎵薄膜的晶格結構則是全為纖鋅結構。
在氮化鎵發光二極體之光電特性探討方面，我們利用濺鍍氮化鋁緩衝層成長氮化鎵發光二極體結構於不同深寬比的圖案化藍寶石基板，同時使用一般MOCVD氮化鋁緩衝層成長氮化鎵發光二極體結構於不同深寬比的圖案化藍寶石基板做為比較的試片。定義使用一般MOVCD氮化鋁緩衝層成長氮化鎵二極體結構於標準尺寸圖案化藍寶石基板為LED I、使用濺鍍氮化鋁緩衝層成長氮化鎵二極體結構於標準尺寸圖案化藍寶石基板為LED II、使用一般MOCVD氮化鋁緩衝層成長氮化鎵二極體結構於高深寬比之圖案化藍寶石基板為LED III和使用濺鍍氮化鋁緩衝層成長氮化鎵二極體結構於高深寬比之圖案化藍寶石基板為LED IV。利用變功率光激螢光光譜分析(PL)和變溫(20K-300K)光激螢光光譜分析發光二極體樣品，其LED I、LED II、LED III 和LED IV的相對內部量子效率是26.1%、26.7%、20.4%和28.1%。從量測顯示出使用濺鍍氮化鋁緩衝層成長氮化鎵發光二極體於圖案化藍寶石基板上的相對內部量子效率皆大於使用一般MOCVD氮化鋁緩衝層成長於圖案化藍寶石基板的氮化鎵發光二極體相對內部量子效率。成長在標準尺寸圖案化藍寶石基板上的發光二極體結構的相對內部量子效率是接近，而成長在高深寬比圖案化藍寶石基板上的發光二極體結構，使用濺鍍氮化鋁緩衝層成長氮化鎵發光二極體的內部量子效率比使用MOCVD氮化鋁緩衝層成長氮化鎵發光二極體提昇37.7%；而且從300 μm × 300 μm 尺寸的LED元件量測結果顯示出，LED元件在20 mA操作電流下，LED I、LED II、LED III 和LED IV的操作電壓( Vf )分別是3.20V、3.19V、3.21V和3.15V；光輸出功率分別為14.87 mW、14.99 mW、11.51 mW與16.81 mW。從X-ray量測氮化鎵薄膜、變溫光激螢光光譜量測分析LED樣品的內部量子效率和量測LED元件的光電特性結果，可以觀察出: 1.減少圖案化基板的(0001) c-plane面積比例，會增加使用傳統MOCVD氮化鋁緩衝層成長氮化鎵薄膜的磊晶接合困難度，提高薄膜中的缺陷，進而降低LED元件效率；2.藉由使用濺鍍氮化鋁緩衝層厚度均勻性佳和覆蓋率高之優點，可以有效提昇氮化鎵薄膜成長於高深寬比圖案化基板的薄膜品質；3.量測使用濺鍍氮化鋁緩衝層成長不同深寬比圖案化藍寶石基板的LED元件，在薄膜中無void和內部量子效率接近的情況下，進行比較出光萃取效率，觀察出和標準尺寸圖案化藍寶石基板相比，其使用高深寬比圖案化藍寶石基板是可以提高7%的光萃取效率。|
In the study, we prepared pattern sapphire substrate (PSS) and high aspect ratio pattern sapphire substrate (HARPSS) with an aspect ratio of 4 and 8, respectively. The characteristics of GaN-based materials grown on PSS and HARPSS with sputtered AlN nucleation layer by metal-organic chemical vapor deposition (MOCVD) have been studied. The 5-µm-thick un-doped GaN (u-GaN) layers were grown on PSS and HARPSS with sputtered AlN nucleation layer, respectively. The GaN-based LED structures were also successfully grown on PSS and HARPSS with sputtered AlN nucleation layer, respectively. For comparison, the u-GaN and GaN-based LED structures grown on PSS and HARPSS with conventional MOCVD-grown AlN nucleation layer were also prepared, respectively. The primary measurement results obtained in this study are summarized as follows: It was found that the irregular voids are observed near the interface of the GaN epitaxial layer between cone surfaces of pattern in the GaN with MOCVD-grown AlN nucleation layer. It was also found that the u-GaN grown on PSS and HARSS with sputtered AlN nucleation layer reduced dislocation density by full width at half maximum (FWHM) of X-ray ω-rocking curve of GaN (102) and TEM measurement. In addition, the nature of GaN crystals was verified by TEM. The wurtzite structure is the most common crystal structure of the GaN epitaxial layer prepared on the c-plane sapphire substrate. The other was zinc blend structure. According to the TEM measurements, two types of GaN crystal structure were observed on GaN grown on HARPSS. The zinc blend structure was observed on the cone surface of HARPSS by using the conventional MOCVD-grown AlN nucleation layer. However, the crystal structure of the GaN epitaxial layer with the sputtered AlN nucleation layer from cone surfaces of HARPSS was still wurzite structure. It was revealed the pure wurzite structure GaN prepared on HARPSS could be obtained by using sputtered AlN nucleation layer. The GaN-based LED structure with PSS and conventional MOCVD-grown AlN nucleation layer (i.e., LED I), with PSS and sputtered AlN nucleation layer (i.e., LED II), with HARPSS and conventional MOCVD-grown AlN nucleation layer (i.e., LED III), with HAPSS and sputtered AlN nucleation layer (i.e., LED IV) were prepared via MOCVD. Power dependent photoluminescence analysis and variable temperature (20 K to 300 K) photoluminescence analysis were carried out. The internal quantum efficiencies (IQE) were 26.1%, 26.7%, 20.4%, and 28.1% for LED I, LED II, LED III and LED IV, respectively. The internal quantum efficiency of GaN-based LED prepared on HARPSS with sputtered AlN nucleation layer was increased by 37.7% compared with conventional MOCVD grown AlN nucleation layer. The optical-electrical properties of GaN-based LED devices were measured. With 20mA current injection, it was found that forward voltages were 3.20, 3.20, 3.21 and 3.15 V for LED I, LED II, LED III and LED IV, respectively. Under 20mA current injection, the optical output powers were 14.87, 14.99, 11.51, and 16.81 mW for LED I, LED II, LED III and LED IV, respectively. The LED structure and PSS in LED I and LED II are identical; thus, the LEE in LED I and LED II are the same. Compare LED I with LED II, it was found that we can enhance the 20-mA output power by 0.8%. Compared with LED III, the 46 % enhancement in 20-mA output power of LED IV should be attributed to the sputtered AlN nucleation layer significantly reduced the dislocation density in GaN prepared on HARPSS; thus, enhanced the IQE. With sputtered AlN nucleation in LED structure, the light extraction of HARPSS was improved by 7% compared that of PSS.
|Appears in Collections:||Thesis|