【作者】 朱光耀；

【导师】 顾书林；

【作者基本信息】 南京大学 ， 微电子学与固体电子学， 2011， 博士

【摘要】 氧化锌是一种Ⅱ-Ⅵ族宽禁带化合物半导体材料,具有优越的光电性能,在蓝光、紫外光发光器件、激子型激光器、自旋电子器件等方面具有极大的开发和利用价值。MOCVD是目前最适合大规模制备ZnO薄膜的技术,具有外延面积大、可重复性强、组分控制精确、淀积速率高等优点。但MOCVD中包含有很多复杂的物理和化学过程,反应机理与生长动力学十分复杂,要想掌握这种技术,生长出优质的ZnO薄膜,就必须对这些规律有比较清楚深入的认识。本文通过计算机模拟计算,对MOCVD系统中生长ZnO中普遍存在的、严重影响薄膜结构与性质的气相预反应进行了深入的研究,提出了抑制气相预反应、提高薄膜质量的技术与方法。系统研究了蓝宝石衬底上ZnO缓冲层薄膜的生长规律,并对ZnO外延生长中的主要生长参数提出了优化方案与技术思路。取得的主要成果如下：1.建立了MOCVD的计算机三维模型,分析了ZnO生长过程中普遍存在的气相预反应的发生条件,发现气相预反应的发生强烈依赖于气体温度和反应气体的混合程度。计算显示反应室中气体流速、反应室高度、冷却水流量、衬底温度对反应区温度的分布具有较大的影响,而反应区温度的分布或气体的温度将决定金属有机源在空间的分解程度。当气体温度和混合度条件同时满足时,反应气体抵达衬底前的气相预反应就会发生。计算模拟了反应气源完全混合区域的空间分布范围,以及温度对气体分解速率的影响。通过对MOCVD技术参数与条件的研究,发现增加气体流量,缩短喷淋头到衬底的距离,增加冷却水流量等,可以有效的控制反应室中的温度分布,从而达到抑制气相预反应的目的。2.开展了DMZn和DEZn作为ZnO MOCVD生长的锌源的比较研究,采用四极质谱仪原位监测技术,研究分析了单一反应气体在反应室中随温度上升而发生分解反应的情况,揭示出不同锌源发生受热分解的温度区间。进一步对比了DEZn+O2和DMZn+t-BuOH两组反应气体在一定温度时的反应现象,发现前一组由于氧气强烈的氧化性,预反应速率较快,而后一组之间的预反应程度较弱。研究显示采用DMZn和t-BuOH作为反应源可以有效的避免气相预反应,为高质量ZnO薄膜的生长提供很好的条件。3.研究了以DMZn和t-BuOH为反应气源的ZnO MOCVD的生长与主要生长参数与工艺条件的关系。研究发现在MOCVD生长中加入H2气将有助于抑制杂质碳在ZnO的非故意掺入,但H2流量应该控制在一个较小的范围,否则会对ZnO产生强烈的腐蚀作用而导致材料质量的严重下降。研究表明采用较高的反应室压力有利于在生长的ZnO薄膜中获得光滑的表面形貌和较高的晶体质量,但薄膜的生长温度必须控制在一个合适的温度区间。合适、较高的生长温度将使生长表面原子由于较大的迁移能力从而获得较好的表面平整度,同时又不会因过高的生长温度导致反应物之间的气相预反应的恶化。另外研究也表明采取较低生长速率可以显著改善薄膜的表面平整度,而较厚的薄膜则可以明显改进缓冲层薄膜的外延质量,从而有助于在此基础上继续进行后续外延生长。更多还原

【Abstract】 Zinc Oxide(ZnO) is a II-VI compound direct-gap semiconductor with wurtzite structure. Because of its wide band gap of3.37eV at room temperature and large exciting binding energy of60meV, ZnO is a promising material for efficiency short-wavelength optoelectronic light-emitting devices and detectors. Metal-organic chemical vapor deposition (MOCVD) is a promising method to grow high-quality ZnO films nowadays, because of its large production scale, reproducibility, accurate control of components and high deposition rate. But lots of complicated physical and chemical changes occur in MOCVD system, and are still hard to understand for us. In order to grow high quality ZnO films through MOCVD, every physical and chemical process should be well studied.In this thesis, gas phase pre-reactions, which commonly exist in ZnO MOCVD process and deteriorate the structure and properties of films, has been studied from simulations and experiments in details. Solutions to suppress the gas phase pre-reaction and so to improve the quality of films have been proposed. Systematical Growth of ZnO buffer layers on sapphire substarte have been studied. Optimized growth parameters have been finally suggested for high qualiy ZnO growth. The results include:1.3D model of ZnO MOCVD has been established to investigate the behavior of the commonly existed gas phase pre-reactions, which are supposed to depend on the gas temperature and mixing content of reaction gases. Studies revealed that the flow rate of gas, the gap between the shower head and substrate, substrate temperature and convective heat transfer coefficient of cooling water have great impacts on the temperature distribution, which furtherly influences the pyrolysis of metal-oganics. When the gas temperature is high enough with the reaction gases fully mixed, the gas phase pre-reactions will occur before the reaction gases reach the substrate. Through the research of MOCVD parameters and conditions, larger gas flow rate, shortened distance between shower head and substrate, and increased cooling water flow rate are found to be effective ways to suppress the gas phase pre-reactions.2. Comparisons between DMZn and DEZn are made to determine a suitable Zn precursor for ZnO growth by MOCVD. Through in-situ quadrupole mass spectrometer mounted on MOCVD system, the dissociation behavior of the reaction precursors were studied at different substrate temperatures. The extent of gas phase pre-reactions are further studied under certain temperature between two groups of precursors, one using DEZn and O2, the other using DMZn and t-BuOH, as the zinc and oxygen precursors, respectively. The former group reacts violently because of the strong oxidability of O2, while the later group reacts moderately. The later group is then considered to be the preference precursors to grow high quality ZnO films with the gas phase pre-reactions suppressed.3. Using DMZn and t-BuOH as precursors, systematic ZnO growth by MOCVD method has been especially investigated. Growth experiments show hydrogen addition should be kept at a low content due to its strong etching effect on ZnO, although it could reduce carbon incorporation in ZnO films. Enhanced reactor pressure in the system may lead a smooth surface morphology and high quality of the film in unitary but the growth temperature should be carefully controlled. A proper growth temperature is found to be suitable for high quality ZnO growth with larger migration length of atoms on the growth surface and fully suppression of the gas phase pre-reactions. A lower growth rate is preferred to obtain smooth surface morphology, while a higher growth rate may be helpful for crystal quality, and may be of benefit to subsequent high temperature epilayer growth.更多还原