以光致電化學蝕刻技術製作氮化物發光元件

Abstract

在本論文中，我們驗證利用光致電化學蝕刻技術製作高品質之氮化物發光元件。我們首先製作一系列的氮化鎵材料之光子晶體薄膜結構，包含完美光子晶體及三種典型的光子晶體缺陷結構(H1, L3, H2)。光子晶體之形成為在氮化鎵磊晶層中製作不等邊三角形排列之圓形孔洞，薄膜結構則是利用能隙選擇性之光致電化學蝕刻技術所製作。然後，我們在室溫下利用光激發的方式研究這些光子晶體結構之光激發共振輻射模態特徵。其中，我們發現所有的光子晶體缺陷結構之光激發雷射的波長都非常靠近完美光子晶體結構之光激發雷射的波長。雖然這些光激發雷射擁有不同的光激發臨界能量，不同的頻譜寬度，不同的共振腔品質因子及不同的偏振程度，但是其偏振方向幾乎一致。其相似的雷射特徵可能由於製作缺陷結構後所產生極窄的部份能隙所導致。此外，雷射的偏振方向是由於光子晶體之不等邊三角形排列所致。其中一個光子晶體缺陷結構(H2)之光激發雷射的激發臨界值為0.82 mJ/cm2，共振腔之品質因子為1743，偏振程度為25.4，其雷射特性優於其他發表過的類似結構。 此外，我們利用光致電化學蝕刻配合相位式干涉技術，在氮化物發光二極體之平台周圍製作表面光柵結構，有效提升光汲取效率。表面光柵結構之形成乃是由光致電化學蝕刻在空間上不同之蝕刻速率之分布所造成。而光致電化學蝕刻速率乃是將所使用的雷射光穿透相位式光罩，雷射經干涉後在樣品表面形成之光強度分布所控制。此蝕刻技術並未影響發光元件之電特性表現，然而發光二極體卻由於表面光柵結構所產生的繞射效應提升高於43%的光輸出。 另外，我們還利用光致電化學蝕刻技術將側向磊晶接合方式所成長的氮化物發光二極體結構成功地從藍寶石基板剝離並製作成垂直型發光二極體。而這種藍寶石剝離技術仰賴側向磊晶接合成長時在二氧化矽條狀結構上方所形成的孔洞，可使光致電化學之蝕刻溶液得以接觸在二氧化矽結構上方之氮化鎵，而藍寶石上之氮化鎵預長層也需夠薄以使照射的光源能被二氧化矽結構上方之氮化鎵所吸收。剝離機制為光致電化學蝕刻由二氧化矽結構上方非常薄之氮化鎵層開始，之後往側向磊晶接合成長之開窗區域擴展，最後得以完全將氮化鎵磊晶層及藍寶石基板分離。我們並展示以此方式所製作之垂直型發光二極體之特性。

In this dissertation, the fabrications of high-quality nitride-based light-emitting devices by using photoelectrochemical (PEC) etching technique are demonstrated. First, the implementation of a series of optically-pumped GaN photonic crystal (PhC) membrane laser at room temperature is demonstrated. The photonic crystal is composed of a scalene triangular arrangement of circular holes in GaN. Three defect structures are fabricated for comparing their lasing characteristics with those of perfect PhC. It is observed that all the lasing defect modes have the lasing wavelengths very close to the band-edge modes in the perfect PhC structure. Although those lasing modes, including band-edge and defect modes, have different optical pump thresholds, different lasing spectral widths, different quality factors (Q factors), and different polarization ratios, all their polarization distributions show the maxima in the directions around one of the hole arrangement axes. The similar lasing characteristics between the band-edge and defect modes are attributed to the existence of extremely narrow partial band gaps for forming the defect modes. Also, the oriented polarization properties are due to the scalene triangle PhC structure. In one of the defect lasing modes, the lasing threshold is as low as 0.82 mJ/cm2, the cavity Q factor is as large as 1743, and the polarization ratio is as large as 25.4. Such output parameters represent generally superior lasing behaviors when compared with previously reported implementations of similar laser structures. Besides, the enhancement of light extraction by fabricating a surface grating structure around the mesa of a nitride-based light-emitting diode (LED) with an approach combining PEC wet etching and phase mask interferometry is demonstrated. The PEC etching rate is controlled by the intensity of illuminating UV light, which is spatially modulated by the fringe pattern of phase mask interferometry, for forming the grating structure. Without affecting the resistance characteristics of the device, the diffraction of such a grating structure leads to LED output enhancement by >43 % on either the top or bottom side. In addition, the method of sapphire substrate liftoff for the fabrication of vertical nitride-based blue LED, based on the combination of the PEC etching and epitaxial lateral overgrowth (ELOG) techniques, is demonstrated. This method relies on the formation of connected voids during the ELOG process on a GaN template such that PEC electrolyte can approach the GaN portions above the SiO2 masks. Also, the GaN template must be thin enough for the illuminating ultraviolet light to reach the GaN portions above the SiO2 masks. It is shown that PEC etching starts from a very thin layer of GaN right above a SiO2 mask. It then extends into the window regions of ELOG to completely separate GaN from sapphire. The performances of a vertical LED are illustrated.