| As a new functional material, diamond film has great prospect of applying inhigh-tech fields, such as microelectronics, photoelectronics, and mechanics,owing to their excellent mechanical, optical, thermal, and electrical properties.Natural diamond, however, is too expensive to meet the demands in manyapplications. So the techniques of synthetic diamond attract much attention, andhave been developed quickly. It is well known that the small scale of the diamondprepared by HPHT method limits its applications, while CVD method cansynthesize large scale diamond film which extends its application fields. Thedevelopment trends in researching and producing CVD diamond films are highgrowth rate, large scale, and high quality. Both the domestic and foreignresearchers put their emphasis on two aspects in researching diamond films. Thefirst aspect is to develop the equipments and techniques to produce high-quality,large-scale, and uniform diamond films with high growth rate;the other one is tostudy of their applications.In order to further improve and optimize the growth techniques of large-scalediamond thick films, and, accordingly, produce them to meet different demands aslong as reducing the cost to promote the process of their industrial production,large-scale, high-quality, and uniform diamond thick films were prepared byEA-CVD method, and the influence of nitrogen doping on their growthcharacteristics and the existing states of nitrogen impurities were systematicallystudied in this paper. The main work of this paper includes three parts, and thedetails are given below.1.Large-scale high-quality diamond thick films were prepared by optimizing thegrowth parameters, and were then characterized by SEM and Raman spectrum.In the process of depositing large-scale diamond films, the nonuniformdistribution of the plasma and the substrate temperature leads to thenonuniformity of the thickness and the quality of the films. Since the arrangementof the filaments directly affects the nonuniformity the plasma's intensity and thesubstrate temperature, it is important to adjust the filaments to make them parallelwith the substrate. And two effective methods of reducing the micro-crack in thefilms are reducing the temperature fluctuation of the substrate, and selecting a lowsubstrate temperature. It can be seen from the SEM images of the cross-section ofthe diamond films, the grains grow cylindrically along the growth direction.Methane flow rate directly influence the growth characteristics of the films.When it is 4sccm, growth steps are evident on the surface of the films. With themethane flow rate of 6sccm, the morphology becomes "cauliflower-like", and(100) faces can be easily observed on the "cauliflowers". As it rises to 8sccm, the"cauliflower" becomes spherical, and the crystal shape deteriorates.Through the above research work and the optimization of the growthparameters, we prepared large-scale uniform diamond thick films with its scale ofΦ100mm×1.4mm, and its thickness nonuniformity of less than 5%, and itsgrowth rate of 5μm. The results of SEM and Raman spectra show that thediamond thick films exhibit good crystallinity and high quality.2.Study of the influence of nitrogen doping on the growth characteristics of thediamond films.Nitrogen concentration in the reacting gas directly affects the growth rate ofthe films. When the substrate temperature is 950℃, the growth rate of the filmsfirst rises and then drops with the increase of the nitrogen flow rate. With thesubstrate temperature of 800℃, the growth rate decreases with the increase of thenitrogen concentration.At the substrate temperature of both 950 ℃ and 800℃, the surfacemorphology of the films changes evidently with different nitrogen flow rates. At950℃, when the nitrogen flow rate is 4sccm, the crystal faces are mainly (111)and (110) faces, whereas they are rough and the twins as well as secondarynucleation increase. As the nitrogen flow rate rises to 8 and 12sccm, themorphology become "cauliflower-like", and (100) faces can be easily observedthe "cauliflowers", and the grains are incompact, which indicates the decrease ofthe crystallinity. At 800℃, after the addition of nitrogen in the reacting ambient,the crystal has nice flat facets, and the grains have clear edges, and the twins aswell as secondary nucleation decrease, which indicates the evident improvementof the crystallinity.The results of Raman spectra show that at 950℃, the concentration ofamorphous carbon increases with the increase of the nitrogen flow rate, and thequality of the diamond films drops, and at 800℃, the films exhibit good qualitywith relatively low concentration of nondiamond phase at different nitrogen flowrates.The influence of nitrogen flow rate on the internal stress in the films wasstudied. The results show that the stress in the films deposited at 950℃ and 800℃ is tensile, and the macro-stress changes irregularly, while the micro-stress firstdecreases and then increases with the increase of the nitrogen flow rate.3.The existing states of nitrogen in the films are analyzed by SEM, Raman, XPS,and EPR.The analysis of the Raman spectra shows that there are [N-V]-1 and [N-V]0 inthe diamond films, and the content of [N-V]0 decreases with the increase of thenitrogen flow rate, while at 800℃ with nitrogen flow rate of 16sccm, thecharacteristic peak of [N-V]0 is not observed. The analysis and calculation of theEPR spectra show that at 950℃, Ns0 content first decreases and then increasesalong with the increase of the nitrogen flow rate, while at 800℃, it increases withthe increase of the nitrogen flow rate. XPS results show that at 950℃, the contentof sp2 C-C bond and sp2 C-N bond increase, and the content of sp3 C-C bonddecreases with the increase of the nitrogen flow rate. At 800 ℃ , thenitrogen-doped diamond films are mainly comprised of sp3 C-C bond and smallquantity of C-O bond, while the content of sp2 C-C bond and sp2 C-N bond arevery low, and they change irregularly at different nitrogen flow rates. The resultsconfirm that in the nitrogen-doped diamond films, the nitrogen impurities mainlyexist in the forms of [N-V]-1, [N-V]0, and Ns0.As has been mentioned above, large-scale diamond thick diamond films ofΦ100mm×1.4mm were successfully prepared by EA-CVD method with theoptimization of the growth parameters. At different nitrogen flow rates in thereacting ambient, we systematically study of the influence of nitrogen on thegrowth rate, crystallinity, quality, and internal stress of the diamond films. Theresults show that the addition of nitrogen in the reacting ambient largely affectsthe films' growth, and in the nitrogen-doped diamond films, the nitrogenimpurities mainly exist in the forms of [N-V]-1, [N-V]0, and Ns0.
|
PDF Full Text Request