以微波輔助化學浴沉積法成長氧化鋅磊晶之研究

Abstract

微波輔助化學浴沉積法(microwave-assisted chemical bath deposition, MWCBD)是一種新的快速合成 ZnO 單晶奈米柱的方法，本研究使用MWCBD，以硝酸鋅六結晶水(Zn(NO3)2∙6H2O)當作鋅離子源，六亞甲基四胺(HMT, C6H12N4)作為酸鹼緩衝劑，在低於 100 °C 溫度條件下沉積，基板為晶格常數匹配程度與 ZnO 相近的 GaN， 探討成長高品質之 ZnO 磊晶薄膜所需之條件，特別是用檸檬酸鈉(Na3C6H5O7)改質劑在微波環境中對成長氧化鋅晶柱的影響，並且探討溫度、濃度、時間對在使用微波加熱的水溶液 ZnO 析出以及 pH值的影響。 從溶液外觀的顏色特徵觀察結果發現，當溫度越高時水溶液的顏色就越純白並且產生更多的 ZnO 粉末，pH 值的變化則是約降低為5.5；而濃度越高時，製程結束之後的 ZnO 粉末就越多，水溶液的 pH值也更低。 在 ZnO 磊晶方面，從掃描式電子顯微鏡之觀察，可以確定 MWCBD 在加入檸檬酸鈉後，有助於成長出連續膜磊晶； ZnO 膜厚約為 1 m，X 光繞射 –scan 量測得到(0004)面的半高寬為 860 arcsec，約為 GaN 半高寬的兩倍，而 ZnO (303 ̅2)面的半高寬則為 1288 arcsec，略高於 GaN 之 1078 arcsec，因此 ZnO 成核過程中所形成的螺旋差排是影響 ZnO 結晶品質的因子。檸檬酸鈉濃度為 0.04 mM 時，對晶柱側向成長即有明顯之效果，ZnO 晶柱的高寬比約 0.83 左右，平均成膜速率 ~ 1 m/h。

Microwave-assisted chemical bath deposition (MWCBD) is a newly developed method for rapid synthesis of single-crystalline ZnO nanorods. In this study, MWCBD was used to synthesize ZnO with high-quality epitaxial ZnO film, using hexahydrate zinc nitrate (Zn(NO3)2∙6H2O) as the Zn2+ source, hexamethylenetetramine (HMT, C6H12N4) as pH buffer, GaN/sapphire as the substrate with small lattice mismatch with ZnO, at temperature less than 100°C. Also, the effect of sodium citrate (Na3C6H5O7) as the capping agent on lateral growth of ZnO rod for continuous film formation has been explored based on the evolution of the measured aspect ratio of height-to-width in the microwave environment. Additionally, the effects of temperature, concentration, and time on pH change of aqueous solutions with precipitation of ZnO powders and during microwave heating were evaluated. When the heating temperature is increased, the color of the aqueous solution as seen from the visual appearance becomes more pure white as more precipitated ZnO powders were produced and the pH value decreases from 6.8 to about 5.5. Increasing the concentration of zinc nitrate precursor results in the higher quantity of ZnO powders and reduction of the pH value of the aqueous solution. In the study of ZnO epitaxy on GaN, scanning electron microscopy observations in top view and cross-section show that thin film of epitaxial ZnO in thickness of about 1 μm can be effectively grown on GaN/sapphire with MWCBD by adding sodium citrate to the solution to enhance the lateral growth of ZnO rods with coalescence. It is found that to reach an aspect ratio of height to width of 0.83, the concentration of sodium citrate is required to increase to 0.04 mM which can still have an average growth rate of ~ 1 m/h. The film quality as characterized by x-ray diffraction in –scan shows a full width at half maximum of the (0004) rocking curve in 860 arcsec, which is twice as high as substrate of GaN, whereas it is 1288 arcsec for (303 ̅2) FWHM slightly increased from 1078 arcsec for GaN, suggesting that ZnO film quality is mainly affected by screw dislocations formed in rod nucleation.