以有機金屬氣相沉積法從事氮化鎵奈米柱生長和接合再生長以及其特性研究

Abstract

在本論文中，我們驗證了使用奈米柱接合再生長製作高品質氮化鎵基板的可行性。我們首先使用分子束磊晶法所成長的奈米柱上利用有機金屬氣相沉積法從事奈米柱之接合再生長。在掃描式電子顯微鏡的俯視影像中，我們可以觀察到接合的表面形貌以及一些六角形結構。從陰極射線螢光的橫截面影像當中，我們可以發現在接合再生長層有比奈米柱層更強的發光。而從陰極射線螢光的俯視影像中我們可以發現此些較強的發光主要是來自於表面的六角形結構。光激發螢光的量測結果顯示接合再生長層的發光效率甚至比高品質且未摻雜的氮化鎵薄膜高。而表面六角形結構造成了大約數個奈米的表面粗糙度。 除了使用分子束磊晶法來成長奈米柱之外，我們使用奈米壓印製作有週期性孔洞的基板，並於其上使用有機金屬氣相沉積法作流量調變來成長周期性與一致性高的奈米柱。在完成接合再生長之後，雖然在陰極射線螢光的量測結果中可以發現在接合再生長層中有數十微米大小的結構，但從原子力顯微鏡的量測中，邊長5微米的正方形影像可測到約0.411奈米的表面粗糙度，該數值大約為直接成長於藍寶石基板上的氮化鎵薄膜的一半(控制組樣品)。根據原子力顯微鏡以及深度解析的X光繞射量測結果，在接合再生長層的表面區域的差排密度約為107 cm-2，此結果約比控制組樣品低了一個數量級，比一般用來製作發光二極體的氮化鎵基板低了二到三個數量級。側向區塊大小也變為2.7微米，約為控制組樣品的三倍。接合再生長的樣品其在室溫的光激發螢光強度與低溫的強度比值也為控制組樣品的六倍。雖然奈米柱幾乎已釋放了所有應變，但在接合再生長完之後又會產生大約0.66 GPa的應力。 我們也進一步探討接合再生長的品質與奈米柱橫截面大小以及間距之間的關係。一般而言，橫截面以及間距較小的奈米柱接合再生長之後品質會較好，此處描述的品質包括較低的線差排密度以及較大的側向區塊大小。由穿透式電子顯微鏡的量測結果看來，奈米柱中的線差排密度取決於用來做奈米柱生長的圖案孔洞大小，而且在奈米柱開始接合再生長時，線差排的形成也與奈米柱的間距有關。雖然在接合再生長層的底部線差排密度與奈米柱直徑以及間距大小幾乎沒有相關性，但在表面部分線差排密度與奈米柱的大小有明顯的關係。我們固定奈米柱直徑與間距的比例，在不同的奈米柱直徑以及間距的接合再生長樣品中，奈米柱直徑以及間距最小的樣品具有最低的線差排密度、最大的側向區塊大小、最高的光激發螢光效率。並且所有接合再生長的樣品其表面的光學和晶體特性都比氮化鎵基板優越。 我們也在接合再生長的基板上成長量子井以及發光二極體結構。我們可以觀察到藍光和綠光的量子井和發光二極體發光的提升。尤其是藍光發光二極體有接近80%輸出強度的提升。在發光二極體的應用上，與綠光發光二極體比較起來，藍光發光二極體更能因為連結生長基板中的線差排密度減少而有更高的發光效率的提升。

In this dissertation, the fabrication of high-quality GaN template is demonstrated by using nanocolumn (NC) coalescence overgrowth. First, the NC coalescence overgrowth by metalorganic chemical vapor deposition (MOCVD) is performed on NCs grown by molecular beam epitaxy (MBE). Plan-view scanning electron microscopy (SEM) shows coalesced surface morphology, although hexagonal structures are still visible in the images. The cross-section cathodoluminescence (CL) image shows more efficient emission in the overgrowth layer than from the NC layer. The plan-view CL image demonstrates that the emitted light is mainly from the hexagonal structures. The photoluminescence measurement result indicates that the emission efficiency of the overgrown layer is even higher than that of an un-doped GaN thin film of high quality. The presence of hexagonal structures correlates to surface roughness values in the range of several nm. Besides the NCs grown by MBE, the MOCVD growth method of flow-rate modulation is applied to grow periodic and uniform NCs on the templates patterned with nanoimprint lithography. After NC coalescence overgrowth, although domain structures of a tens-micron scale in the overgrown layer can be identified with cathodoluminescence measurement, from atomic force microscopy (AFM) measurement, the surface roughness of the overgrown layer in an area of 5um*5um is as small as 0.411 nm, which is only one-half that of the high-quality GaN thin-film template directly grown on sapphire substrate (the control sample). Based on the AFM and depth-dependent X-ray diffraction measurements, near the surface of the overgrown layer, the dislocation density is reduced to the order of 107 cm-2, which is one order of magnitude lower than that of the control sample and 2-3 orders of magnitude lower than those of ordinary GaN templates for fabricating light-emitting diode. Also, the lateral domain size, reaching a level of ~2.7 um, becomes three times larger than the control sample. Meanwhile, the ratio of photoluminescence intensity at room temperature over that at low temperature of the overgrown sample is at least six times higher than that of the control sample. Although the strain in nanocolumns is almost completely released, a stress of ~0.66 GPa is rebuilt when the coalescence overgrowth is implemented. Furthermore, the overgrowth quality dependence on NC cross section size and NC spacing size is studied in detail. Generally, a smaller NC dimension and spacing size lead to higher overgrowth quality, including lower threading dislocation (TD) density and larger lateral domain size. From the measurement results of cross-section transmission electron microscopy (TEM), it is found that the TD density in an NC depends on the patterned hole size for NC growth. Also, the TD formation at the beginning of coalescence overgrowth is related to the NC spacing size. Although the TD density at the bottom of the overgrown layer is weakly dependent on NC and spacing sizes, at its top surface, the TD density strongly relies on NC size. Among the overgrowth samples of different NC diameters and spacing sizes with a fixed NC diameter/spacing ratio, the one with the smallest size and spacing leads to the lowest TD density, the largest lateral domain size, and the highest photoluminescence efficiency. Also, the optical and crystal qualities at the surfaces of all the overgrowth samples are superior to those of a GaN template. Finally, the QW and LED structures are grown on the overgrown templates. The emission enhancement results of blue and green-emitting InGaN/GaN QW and LED structures based on NC growth and coalescence overgrowth are presented. Significant enhancements (up to ~80 % output intensity increase in a blue LED) are demonstrated. For LED application, the TD density reduction in an overgrown GaN template can more effectively enhance the emission efficiency of a blue LED, when compared with a green LED.