以氫化物氣相磊晶法開發氮化鎵基板研究

Abstract

由於缺乏晶格匹配的基板，三族氮化物元件只能採用異質磊晶的方式，利用晶格常數不同的材料來當作基板，因此有缺陷密度過高，以及因為熱膨脹係數不同造成晶體彎曲甚至破裂的問題。對於低電流密度的元件（例如LED），這些問題的影響不大，但是對於高電流密度元件（例如藍光LD）、高功率元件或UV元件而言，影響就相當可觀，若有單晶三族氮化物基板來進行同質磊晶，不但可以降低晶格常數不匹配所造成的缺陷密度、熱膨脹係數不同所造成的彎曲；更可大幅簡化元件的製程及提昇元件的可靠度。未來在光電元件產業的發展上，單晶三族氮化物基板必然具有舉足輕重的地位。對於國內學術界而言，若有單晶三族氮化物基板（GaN、AlN等）作為研究基礎，在相關領域的研究上等於直接躍遷至不同的競爭平台上。 本論文的研究方向在於使用氫化物氣相磊晶法成長氮化鎵厚膜，其中最大的問題在於如何克服因熱膨脹係數不同所產生的熱應力所導致的晶片破裂，我們運用了一些ELOG所衍生的方法及溫度梯度成長方法來克服應力的問題，並獲得良好的成效。經由氫化物氣相磊晶所成長的厚膜再利用雷射剝離的方法將厚膜從原生基板上分開，而得到獨立氮化鎵基板。我們分別使用了掃描式電子顯微鏡，陰極螢光，光致螢光，拉曼光譜，霍爾量測，高解析X光，原子力顯微鏡，微分干涉差光學顯微鏡等工具來量測氮化鎵基板的特性。除此之外，以氫化物氣相磊晶所成長的氮化鎵化學特性也經由濕式化學蝕刻法加以研究，其不同晶面的活化能也分別經由實驗值計算出來。 總之，厚度超過300微米的透明氮化鎵厚膜已經開發出來，其差排密度大約介於10^6到10^7cm^-2之間，霍爾電阻大約是0.09ohm-cm，未摻雜的載子濃度大約是1.6x10^17 cm^-3，X光搖擺曲線的半高寬大約是150秒。在成長厚度增加以及磊晶條件最佳化後，這些特性都可以再改善。 單晶三族氮化物基板將是推動下一代三族氮化物元件發展的關鍵載具，希望經由本論文研究計劃後，未來可以穩定提供高品質的單晶三族氮化物基板給其他學界單位進行研究使用，以對國內的光電發展盡一份心力。

Due to the lack of lattice-matched substrates, current nitride base devices are mainly hetero-epitaxially grown on lattice mismatched substrates. The large mismatches in lattice constants between substrates and epitaxial films cause high defects densities. Furthermore, differences in thermal expansion coefficients between substrates and epitaxial films induce large warpage or bowing of the epi-layers. While these disadvantages may not limit the performance of low current density devices, such as LEDs, they will certainly severely affect high current density devices, such as blue laser diodes and high power electronic devices. The availability of freestanding GaN substrates will provide the overgrown GaN epi-layers the advantages of reduced defects, less bowing, improved reliability, as well as greatly simplified device process afterwards. It is believed that free-standing nitride substrates will play vital roles in the next stage of GaN and InAlGaN devices development. In this thesis, we used the HVPE system to study the GaN wafer fabrication. Several methods for preventing the crack of GaN thick-film during the HVPE epitaxy process were used in this thesis, including the epitaxial lateral overgrowth, air-bridged, dot air-bridged, and temperature ramping technique. These methods can effectively preventing the crack of GaN thick-film in HVPE growth process. After the GaN thick-film prepared, the laser lift-off technique was used to separate the GaN thick-film from original substrate. The freestanding GaN thick-film was obtained after these processes. The characteristics of freestanding GaN were analysised by SEM, CL, Raman spectroscopy, PL, Hall measurement, HRXRD, AFM, and Normaski OM. Furthermore, the chemical properties of HVPE GaN were also studied by wet chemical etching. The activation energy was calculated of different crystal facets of GaN. Overall, the thickness more than 300 micron freestanding GaN was obtained by these strain reduced methods. The color is transparent and the dislocation density is in the range of 10^6 to 10^7 cm^-2. The resistivity is about 0.09ohm-cm. The undoped carrier concentration is about 1.6x10^17 cm^-3. The FWHM of HRXRD rocking curve is about 150 aresec. The performance can be improved after increasing the thickness and optimizing the growth conditions. The project of this thesis is dedicated to provide free-standing nitride substrates to other researchers in Taiwan in the future. It is hoped that we can hence provide a major new vehicle for our domestic research on nitride devices.