Preparation and analysis of nano diamond films by

Preparation and analysis of nano diamond films by MPCVD

Abstract: very smooth, dense and uniform nano diamond films were successfully prepared on glass substrates by MPCVD. The deposition process is divided into two steps: nucleation, ch4/h2=3%; Growth, o2/ch4/h2=0.3 ∶ 3 ∶ 100; During deposition, the working pressure is maintained at 4.0kpa and the substrate temperature is 500 ℃. Raman, transmission electron microscopy, infrared spectroscopy, surface profiler and other tests show that the film is composed of nano diamond grains, the maximum grain size is less than 100nm, and the nucleation density is greater than 1011/cm2. The lattice constant of the nucleation surface grain is large, indicating that there are many defects, the surface roughness is less than 2nm, it is completely transparent in the visible light region, and the infrared optical properties are close to the theoretical value of diamond single crystal

Keywords: MPCVD; Nano diamond film; Raman spectrum; Infrared spectroscopy

1 introduction

because diamond has extremely excellent optical properties, the highest hardness, thermal conductivity and excellent chemical stability, chemical vapor deposition diamond film optical coating has broad application prospects and very important significance. However, for optical coatings, in addition to good adhesion with the optical substrate, the flatness of the coating surface is also very important. Otherwise, the transmittance will be greatly reduced due to surface scattering, which will seriously affect its application. For diamond film coating, the surface after deposition is generally very rough, which can not meet the application requirements; At the same time, because the optical coating is generally very thin, coupled with the extremely high hardness, corrosion resistance and low adhesion of diamond, the surface of diamond coating cannot be reprocessed. Therefore, direct deposition of smooth and uniform diamond films is very important for optical applications. The key of application is to obtain diamond films with very fine grains, even nano grains

there have been some reports of successful preparation of nano diamond films [1 ~ 4]; Many authors have studied methods to improve nucleation density [5 ~ 8]; However, the report of depositing nano diamond coating on glass substrate under high working pressure (several kPa) has not been seen

in this paper, the author successfully deposited nano diamond films on optical glass substrates by MPCVD method, and characterized and analyzed their microstructure, structure, surface state and optical properties

2 experimental method

the equipment used for depositing diamond film is a quartz bell jar microwave plasma equipment developed by our laboratory. The structural diagram of the deposition chamber is shown in Figure 1. The maximum power of the microwave source is 800W. The glass substrate has a softening point of 829 ℃, a refractive index of 1.55, and an expansion coefficient of 4.6 ~ 5.2 ×/℃, special glass with density of 2.7g/cm3. The preparation process of diamond film is as follows: firstly, the glass substrate is properly ground with diamond powder with particle size of 0.5mm, then nucleated in microwave plasma atmosphere with c/h ratio of 3% for 2h, and finally grown for 2.5h under the condition of adding 0.2% O2; In the whole deposition process, the pressure of the deposition chamber is 4.0kpa and the substrate temperature is 500 ℃

the diamond films were analyzed by Raman spectroscopy, transmission electron microscopy, surface profiler and infrared spectroscopy

fig. 1 Schematic diagram of MPCVD system

3 experimental results and discussion

Figure 2 shows the photo of the diamond coating obtained by the above method. The substrate is 3cm diameter optical glass. It can be seen that the coating is completely transparent and smooth, and there are evenly distributed interference fringes from the center to the edge of the coating. This is due to the thin coating and the temperature difference between the center and the edge during the film forming process, resulting in uneven film thickness. At the same time, the deformation of glass substrate is also one of the possible reasons

fig. 2 samples of diamond coated optical glass

Figure 3 shows the Raman spectrum of typical samples. From the figure, we can see the obvious diamond characteristic peak at 1328/cm, with a half peak width of 15/cm. At the same time, in a wide range around 1500/cm, there is a high spectral intensity, indicating that there is a non diamond phase in the film. This is the characteristic of diamond film deposited at low temperature. Some authors have pointed out that the Raman scattering sensitivity of non diamond phase is about 50 times that of diamond phase, and the smaller the diamond grain is, the higher the composition of non diamond phase is, at the same time, the 1332/cm peak shifts to low wavenumber, widening, weakening and asymmetry [9 ~ 12]; Therefore, it can be determined from this figure that the coating is mainly composed of diamond, and the grain size of diamond is very small

fig. 3 Raman spectrum of diamond film on g the scientific research investment of the company has reached 259million euros. Last

Fig. 4 is a TEM photo of a typical diamond film close to the nucleation surface. Figure 4A shows the bright field image. It can be seen that the coating grain is very fine,

dense and uniform, and the average size is about 30nm. It can be inferred that the nucleation density is above 1011/cm2. Figure 4B shows the selected area electron diffraction pattern obtained from the same region, and its plane spacing, crystal plane index and corresponding lattice constant are listed in Table 1. It can be seen that it is a typical face centered cubic diamond structure, and the calculated lattice constant value is a = 0 3647nm。

Fig. 4 TEM images of diamond film close to the nucleation side

(a) plan2view TEM image ； (b) Electron diffraction pattern

Fig. 5 is a TEM photo of a typical diamond film close to the growth surface. Figure 5A shows the bright field image, which shows that the coating grains are dense, uniform and fine, and the average grain size is less than 100nm, which is larger than the grains near the nucleation surface shown in Figure 4. Figure 5B shows the polycrystalline diffraction pattern obtained from the same region, and the plane spacing of the corresponding crystal plane index is also listed in Table 1. The calculated lattice constant is a = 0 3557nm, which is slightly smaller than the lattice constant of diamond grains near the nucleation surface. The nucleation surface has large lattice distortion. Figure 5C shows the bright field image of larger grains with higher magnification, which can be seen with obvious twins, stacking faults and other characteristics

fig. 5 TEM ima the control accuracy of the former often fails to meet the requirements, while the latter is generally used in the constant pressure system, with large power consumption and accurate Ge reading s of diamond film close to the growth side

(a) plan2view TEM image; (b) electron diffraction pattern ； (c) Plan2view TEM image at higher magnetism

in addition, due to the nucleation pretreatment, 0 five μ M diamond powder, while the average grain size of the nucleation surface is only 30nm,

this shows that in the nucleation process, it is mainly the defects on the substrate surface, rather than the seed crystal, that play the role of nucleation. With the extension of deposition time, the grain size is gradually increasing

fig. 6 IR spectrum of nanocrystalline diamond films on glass substrate

the test results of surface profiler show that at 0 Within 1mm, the curve is completely straight, test 400 μ m、200 μ m、100 μ The unevenness within the range of M is 3 7nm、1. 4nm、1. 0nm, after deducting factors such as inconsistent substrate thickness, it can be considered that the surface roughness is 2 Within 0nm, the coating is very smooth, dense and uniform. Measured from the steps on the edge of the sample, the coating thickness is about 0 six μ m。

Figure 6 shows the infrared transmission spectrum of diamond film in the wavenumber range of 4000 ~ 15000/cm. It can be seen from the figure that there is an extreme transmittance due to the interference effect caused by multiple reflections in the film. The curve is divided into two sections because the measurement is completed in two times. Due to the error caused by replacing the grating, the two measurement curves cannot be smoothly connected. It can be seen from Figure 6 that the maximum transmittance of diamond coating in this area is 81%, and the average is close to 78%

4 conclusion

smooth, uniform and dense diamond coatings were successfully prepared on glass substrates by MPCVD. Raman and TEM analysis show that the coating has complete diamond phase characteristics, fine grains, and typical nanocrystalline morphology. The nucleation density is 1011/cm2. Without any analysis, the average grain size is less than 100nm, and the lattice constant of the diamond film close to the nucleation surface is large, indicating that there are many defects and distortions in its lattice structure, and the surface roughness of the diamond coating is less than 2nm; Infrared spectrum analysis shows that the diamond coating has good optical properties

this project has been supported by the national "863" and the Optical Physics Laboratory of the Institute of physics, Chinese Academy of Sciences. Thank you here

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