| 摘要: | 目前氮化鎵磊晶薄膜在矽基板的製程方式為以有機金屬氣相沉積(Metal-organic Chemical Vapor Deposition, MOCVD)生長單層或多層且很厚的氮化鋁薄膜於基板上作為緩衝層，本研究利用高功率脈衝磁控濺鍍(High Power Impulse Magnetron Sputtering，HiPIMS)的製程方式沉積100nm氮化鎵緩衝層在矽基板上，透過高能量且高電漿密度的方式生長氮化鎵緩衝層，再利用MOCVD沉積氮化鎵磊晶層，以氮化鎵緩衝層以改善晶格不匹配的問題。
在研究過程中，固定MOCVD磊晶層製程溫度為1095°C、壓力200mbar，以調整緩衝層參數為主，探討緩衝層的溫度、厚度、充放能時間對磊晶層的影響，也探討了氮電漿前處理能初步改善基板回蝕的問題，以及調整緩衝層氮氣氬氣比與磊晶層五三比；量測部分是使用由X射線繞射儀(X-ray Diffractometer，XRD)分析其結晶繞射(FWHM)與強度來判斷緩衝層及磊晶層晶格排列一致性與結晶品質，使用原子力顯微鏡(Atomic Force Microscopy ，AFM)分析粗糙度，並透過掃描式電子顯微鏡(Scanning Electron Microscopy，SEM)分析薄膜表面與側面結構，最後使用氫氧化鉀(KOH)蝕刻表面再使用表面輪廓儀(Alpha-Step)來做初步判斷極性。
最後可以發現加了氮化鎵緩衝層後可以讓MOCVD磊晶層從約10 μm的氮化鎵非連續膜晶粒改善成約2μm的氮化鎵連續膜。對矽基板做表面氮電漿前處理之後再鍍氮化鎵緩衝層，由SEM分析出，有氮電漿前處理之磊晶層由許多三角錐狀的晶柱變為六方晶柱，幫助氮化鎵生長磊晶層時更能有橫向生長的能力，XRD觀察到優化後的氮化鎵磊晶層半高寬從0.24 degree改善到0.18 degree，也從SEM觀察出表面形貌變得較連續；從KOH蝕刻後觀察SEM的表面形貌及蝕刻階高可以發現有氮極性的現象，因此未來必須再繼續解決表面平整之問題，進而提升氮化鎵直接磊晶在矽基板上的品質。
;Currently, the most common process method for GaN-on-Si epi layer gallium nitride (GaN) is a single-layered or multilayered thick aluminum nitride (AlN) buffer layer grown on Si substrate by using Metal-organic chemical vapor deposition (MOCVD). In this study, we employed high-power impulse magnetron sputtering (HiPIMS) to deposit 100-nm GaN buffer layer on silicon substrates under high energy and high density of plasma. Then, MOCVD has been used to deposit a GaN epitaxial layer. This approach aims to improve lattice mismatch problem.
In this research, the MOCVD epitaxial layer was maintained at a process temperature of 1095 °C and a pressure of 200 mbar. The primary focus was adjusting the buffer layer parameters, including process temperature, buffer-layer thickness, and discharge time of HiPIMS, to explore their effects on the epitaxial layer. Additionally, we investigated the improvement of substrate back-etching through nitrogen plasma pretreatment, as well as adjusting the Nitrogen-Argon ratio during the buffer layer process and the V/III ratio during the epitaxial layer process. For measurements, we utilized X-ray diffractometer (XRD) to analyze the Full Width at Half Maximum (FWHM) and intensity of the crystal diffraction, assessing the consistency of lattice alignment and the crystalline quality of the buffer and epitaxial layers. Atomic Force Microscopy (AFM) was employed to determine the surface roughness. Scanning Electron Microscopy (SEM) was used to examine the surface and cross-sectional structures of the films. Finally, Alpha-Step was used to determine polarity assessment after KOH etching.
The results revealed that incorporating the GaN buffer layer significantly improved the MOCVD-grown epitaxial layer, transforming it from approximately 10 μm GaN discontinuous film grains to a continuous film of about 2 μm. Before the GaN buffer layer process, a nitrogen plasma pretreatment on the silicon substrate was performed. It shows that the process with the nitrogen plasma pretreatment transformed the epitaxial layer from numerous triangular crystal columns to hexagonal crystal columns, enhancing the lateral growth of the GaN epitaxial layer. XRD results showed an improvement in the FWHM of the optimized GaN epitaxial layer from 0.24 degree to 0.18 degree. SEM analysis also confirmed the improved surface morphology. After KOH etching, SEM examination revealed a nitrogen polarity phenomenon. Consequently, the surface smoothness of the film has to be improved to further enhance the quality of direct GaN epitaxy on silicon substrates. |
