利用非等向性蝕刻氮化鎵與藍寶石基板介面製作高效率紫外光發光二極體

Abstract

我們提出於氮化鎵和藍寶石基板介面製作倒六角型金字塔結構，以提高紫外光發光二極體的效率。該倒六角形金字塔結構係由非等向性化學蝕刻而成。該紫外光發光二極體，歷經磊晶中斷、蝕刻後再成長的過程。我們分別從TEM剖面圖及XRD中，觀察及量測到較少的穿透缺陷與較窄的晶體繞射半高寬。我們提出了多階層側向磊晶(ML-ELOG)機制以解釋穿透缺陷減少的原因。該樣品在20毫安培的注入電流下，其電激發光之軸向光發光強度相較於參考片提升了120%，而積分強度則提升了85%。 我們透過光追跡模擬及遠場光學實驗，以佐證紫外光發光二極體具倒六角型金字塔結構下之量測結果。此具倒六角型金字塔結構之發光二極體，可藉由氮化鎵-空氣-藍寶石基板間的折射率差異與幾何結構，使光線得以重新導正至軸向光因而使視角減小。此倒金自塔結構可有效提高發光效率，並可藉由部份的應力釋放及多階層側向磊晶機制以減少氮化鎵中穿透缺陷來提高磊晶的晶體品質。 此外，我們可利用該此倒金自塔結構於介面造成的脆弱特性，近一步使用熱壓法去除藍寶石基板以製作thin-GaN發光二極體。實際上，我們亦已大面積地利用該結構以機械式剝離法剝離藍寶石基板。

We report the fabrication and study of high efficient UV light emitting diodes (UVLED) with inverted pyramid microstructures at GaN-sapphire interface. The HIP structures were created by anisotropic chemical wet etching. The UV LED was fabricated by growth interrupt, chemical etching, and regrowth processes. The as grown sample showed reduction of dislocation density in the cross-section TEM images and narrow XRD rocking curves. The ML-ELOG mechanism might elucidate the reason for threading dislocation reduction. The electroluminescent output power at normal direction was enhanced by 120% at 20 mA operating current and the output power integrated over all directions was improved by 85% compared to a reference sample. The ray tracing simulation and three-dimension far field pattern experimental results of UVLED with HIP GaN/air structure are reported. The HIP structure could reduce the view angle, which redirects the escape light employed by optical geometric property and refraction index difference within GaN/air/Sapphire. The UVLED with inverted pyramid structures have significantly enhanced the light extraction efficiency and at the same time also improved the crystal quality by partially relieving the strain and reducing the dislocation defects in ML-ELOG regrowth process. Besides, we might employ the structure weakness near the interface, to remove to sapphire substrate with thermal bonding process for fabricate thin GaN LED. Indeed we have had successfully remove sapphire substrate by mechanical lift-off to develop the large area lift-off by HIP structure.