TEM Study On Diamond Films Deposited By MPCVD

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:â‘ a relationship of Si/SiC/D isfound at Si tip indicating an epitaxial nucleation on the SiC interlayer;â‘¡grooves formed bySi tips served as non-epitaxial nucleation sits;â‘¢a strained groove on the Si surface servedas an epitaxial nucleation site;â‘£a 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.