【作者】 刘燕燕；

【导师】 张庆瑜； BAUER-GROSSE；

【作者基本信息】 大连理工大学 ， 等离子体物理， 2007， 博士

【摘要】 金刚石的各种优良性能吸引了众多科学研究者的目光，而低气压金刚石薄膜沉积的成功使得越来越多的学者对它产生了兴趣。金刚石薄膜的制备技术正逐步提高，对金刚石薄膜异质成核和生长机制的理解正在不断深入。但是实际的多晶金刚石薄膜的制备结果与制备电子器件所需的单晶金刚石薄膜之间存在着很大差距，对于金刚石薄膜制备中的各方面问题都需要更加深入地研究。本文对在微波等离子体化学气相沉积（MPCVD）系统中制备的金刚石薄膜样品进行了电子显微学分析和Raman分析。所用的金刚石薄膜样品采用分步法制备。首先，金刚石薄膜的成核阶段采用偏压增强成核（BEN）的方式，即在C-H（CH₄+H₂）等离子体中，对基片施加一定的负偏压沉积2～3 min；然后，在C-H等离子体中无偏压生长30 min，以促进金刚石晶粒的长大；最后，分别在C-H、C-H-N和C-O-H等离子体中生长10 h，以获得不同等离子体中合成的金刚石薄膜样品。上述三个阶段分别定义为：成核过程、核长大过程和薄膜生长过程。利用扫描电子显微镜（SEM）、透射电子显微镜（TEM）和Raman光谱对不同阶段的样品进行了表征，对金刚石在Si基体表面的异质成核和在不同反应气体中的生长行为及缺陷特征进行了研究。1、金刚石异质外延成核过程分析。首先对BEN过程后的基体表面形貌进行了分析，发现在Si基体的表面同时存在金刚石核、SiC核和非晶碳（a-C）层，并且观察到了由溅射Si原子再次沉积形成的Si尖包。然后对经过生长过程的样品中各种金刚石的成核位置分析，发现金刚石在Si基体表面的异质成核具有以下特征：①金刚石（D）在Si尖包处成核，具有Si／SiC／D外延特征；②金刚石在Si表面的凹坑处成核，具有Si／D非外延特征；③金刚石在具有应力的Si表面成核，具有Si／D直接外延特征；④金刚石在具有应力的Si表面成核，具有Si／D非外延特征；⑤金刚石在非晶碳（a-C）中成核，具有随机取向的成核特征。并根据以上的观察结果，讨论了金刚石异质成核的物理机制。2、金刚石核长大过程分析。对不同参数下C-H等离子体中无偏压生长30 min的金刚石形貌和微观结构进行了分析。SEM形貌分析显示，随着基体温度的增加，金刚石晶粒的密度增大，晶粒尺寸减小，晶粒越容易沿{111}面生长。TEM分析表明，金刚石在生长初期，特别是在Si基体上成核位置附近产生较多孪晶、层错、位错等缺陷，生长后期缺陷变少。此外，我们还观察到了一种新的纳米级的缺陷——位错环，这种位错环的形成应与空位的聚集有关。3、C-H等离子体体系合成金刚石薄膜的微观结构特征分析。SEM分析显示，薄膜沉积不均匀，特别是边缘处，晶粒密度、尺寸和形貌与中心处有较大区别。我们通过控制样品减薄的方法，制备了不同生长阶段的TEM平面样品和截面样品，分析了薄膜微观结构和缺陷特征。TEM分析表明，对于在C-H等离子体体系中生长的金刚石薄膜，我们所发现的位错环普遍存在于金刚石晶粒的各个生长阶段，并趋于排布在表面能较低的{111}面。薄膜以{111}面生长的晶粒为主，{100}面生长的晶粒较少；在{111}面生长的晶粒中，各个{111}面上均存在层错和位错环，多数晶粒中有一个主要的孪生／层错方向；{100}面生长的晶粒体积较小，只有纳米尺寸的位错环缺陷，基本没有面缺陷存在。我们根据空位聚集的观点，讨论了位错环的形成机制。4、C-H-N等离子体体系合成金刚石薄膜的微观结构特征。在C-H等离子体体系中加入微量（180 ppm）的N₂以后，使得金刚石薄膜的生长速度增加，薄膜的中心区域呈现为{100}高织构取向（HOD）生长，晶粒的{111}面变得粗糙。TEM分析显示，{111}生长区域的面缺陷密度显著增加，形成大量的微孪晶／层错，C-H等离子体体系中广泛存在的位错环仅出现在金刚石晶核的生长阶段，N₂的微量掺杂导致位错环几乎全部消失。参考相关文献中有关N掺杂的光谱实验结果，我们讨论了N掺杂在抑制位错环形成中的作用及其物理机制。此外，在样品的边缘区域，观察到了一种6H结构的多型金刚石。5、C-O-H等离子体体系合成金刚石薄膜的微观结构特征。以CO+H₂为反应气体的C-O-H等离子体体系中生长的金刚石薄膜，生长速度较慢、晶粒尺寸较小、薄膜致密。在金刚石晶核生长初期形成较多的面缺陷，导致晶粒在薄膜表面形成平行孪晶或者近五次对称的形貌。样品中心处，晶粒内的层错和微孪晶密度较低，有少量的位错环存在。金刚石薄膜的生长速度较慢和缺陷较少均与O对于薄膜中非sp³键的物质具有强烈的刻蚀作用有关。但样品的边缘处的晶粒内部，位错环的密度相对较大，这是由于边缘处等离子体密度产生变化的原因。此外，在样品的边缘处，我们发现了一种由碳组成的特殊晶体相，与金刚石的相关相（如多型金刚石）明显不同。6、金刚石薄膜的Raman光谱分析。分别采用波长为325和632.8nm的激光作激发源，对C-H、C-H-N和C-O-H等离子体体系合成的金刚石薄膜进行了Raman谱分析。研究结果显示，除1333 cm^-1附近出现较强的金刚石特征峰以外，各种等离子体体系合成的金刚石薄膜均在1580 cm^-1附近出现了较弱的漫散峰，说明薄膜中均存在一定比例非sp³键，可能与薄膜中的面缺陷和位错环有关。显微Raman光谱分析表明，同一样品不同区域具有相似的Raman光谱特征，但边缘处晶粒团簇的Raman光谱峰明显锐化。此外，显微Raman光谱中存在较多的缺陷荧光。根据这些缺陷荧光的特点，讨论了不同等离子体体系合成的金刚石薄膜中缺陷荧光的起源。更多还原

【Abstract】 Diamond attracts many scientists because of its excellent properties. More and moreinterests focus on it since it was successfully synthesized in low pressure vapor. Thetechnique of preparing diamond film is improving. The mechanisms of diamond nucleationon hetero-substrates and of the growth of diamond film are better and better understood. Butthe best result that we have got is the polycrystalline highly oriented diamond （HOD） film,which is far from a single crystal diamond film demanded in practically using. So, it needsmore experimental and theoretical studies to better understand the mechanisms in thesynthesis of diamond film.In this thesis, diamond films synthesized in microwave plasma chemical vapordeposition （MPCVD） system were analyzed by electron microscopy and Raman spectroscopy.The films were prepared step by step. First, a bias enhanced nucleation （BEN） process wasapplied in C-H （CH₄+H₂） atmosphere for 2 and 3 minutes; second, a nuclei growth processwas applied also in C-H atmosphere for 30 min; third, a film growth process was applied inC-H, C-H-N or C-O-H atmosphere for 10 h. Scanning electron microscope （SEM）,transmission electron microscope （TEM） and Raman spectroscope were used to characterizedthe samples.1. The nucleation stage. Firstly, substrates after the BEN process were analyzed anddiamond nuclei and SiC were deposited as well as amorphous carbon （a-C）. There are also Sitips redeposited by etching Si atoms in the BEN process. Secondly, interface of the Si anddiamond films both after the nuclei growth process and the film growth process wereinvestigated. Several diamond nucleation sites were found:①a relationship of Si/SiC/D isfound at Si tip indicating an epitaxial nucleation on the SiC interlayer;②grooves formed bySi tips served as non-epitaxial nucleation sits;③a strained groove on the Si surface servedas an epitaxial nucleation site;④a strained bump at the Si surface served as a non-epitaxialnucleation site;⑤diamond nucleation in a-C indicated random nucleation site in a-C. Fromthese nucleation sites, the habit of the diamond nucleation on hetero-substrate Si wasconcluded.2. The nuclei growth process. Samples of different deposition parameters were analyzed.SEM morphologies showed a small grain size and a {111}-facet tendency when the substratetemperature was high. TEM analysis showed there were twins, stacking faults （SFs） anddislocations at the primary growth stage. A new nanometer defect was observed byplanar-view TEM observation, which was considered as dislocation loop associated withvacancy aggregation. 3. Diamond film growth in C-H atmosphere. TEM analysis showed that dislocationloops always existed at both {100} and {111} growth sectors during growth in CH₄ and H₂gas mixture. These loops tended to locate at the low-surface-energy {111} planes. Themechanism of the formation of the loops was suggested based on the vacancy aggregation. Atthe surface of the film, most grains were {111} growth with parallel twins or SFs. Besides,there were partial SFs on every {111} planes. {100} growth grains were few and of smallvolume. They were nearly free from planar defects. SEM images showed the film was notvery uniform because of edge effect and the nonuniformity of the plasma. Amorphous layerwas found between the Si substrate and the diamond film.4. Diamond film growth in a C-H-N atmosphere. Before the film growth, the depositionparameters were the same as in the C-H atmosphere. In the film growth process, a very smallamount of N₂ （about 180 ppm） was added. The growth rate increased after N₂ added and thefilm was HOD film at the center region. SEM morphologies showed the grains with veryrough {111} growth sectors around a smooth {100} growth square. TEM analysis showed theplanar defects at {111} growth sectors increasing compared to that in C-H atmosphere. At thenuclei growth process, dislocation loops existed in a C-H atmosphere; after adding N₂, loopsdisappeared. The disappearance of the dislocation loops was explained by the formation ofN-V centers which lowered the activity of the vacancy and prevented them from aggregation.Near the edge, polytype diamond was found. The amorphous layer was much thinner than thatin the C-H atmosphere and vanished at the center.5. Diamond film growth in a C-O-H atmosphere. Before the film growth, the depositionparameters also were the same as in the C-H atmosphere; during the film growth, CO and H₂gas mixture was used. In the C-O-H atmosphere, the growth rate of the diamond film was low.The film was compact and the grain size was small. Planar defects as well as dislocation loopswere less than in C-H atmosphere. That’s because O has an effectively etching ability to thesp² bond of carbon. Near the edge, more dislocation loops were found in the grains. Anon-diamond carbon structure was found near the edge. There was no obvious amorphouslayer in the sample.6. Raman analysis of the growth process. All the Raman spectra of three kinds ofdiamond films from 325 nm wavelength showed a strong diamond peak and a diffuse peak ofG peak. A micro Raman spectroscopy also was applied with a wavelength of 632.8 nm.Spectra from different regions of a same sample had similar contour. Luminescence of pointdefects was also detected in micro Raman spectra. The point defects were analyzed accordingto the luminescence and their distribution in different films was discussed.更多还原