MPCVD preparation of diamond technology

 

Keywords: MPCVD diamond; preparation technology; crystal defects; double substrate structure

 

0 Introduction

Currently, among the main synthesis methods for single-crystal diamond, the relatively mature technologies are high-pressure high-temperature (HPHT) and microwave plasma chemical vapor deposition (MPCVD). Although the high-pressure high-temperature method has a simple preparation process and a fast growth rate of single-crystal diamond, its equipment is unstable, which easily leads to the inability to continuously grow large-sized single-crystal diamonds for a long time. In addition, some impurities will be added during the synthesis of single-crystal diamonds, which is not conducive to the growth of high-quality single-crystal diamonds . The homoepitaxial growth method of single-crystal diamond by microwave plasma chemical vapor deposition (MPCVD) is currently the most commonly used method and has received widespread attention from scholars at home and abroad. Compared with other methods, the MPCVD method for preparing single-crystal diamonds has the advantages of large electron kinetic energy, wide stable working pressure range, high ionization degree, no electrode contamination, and long-term stable operation .

 

 

1. Influence of Dual-Substrate Structure on Single-Crystal Diamond Growth

The experimental apparatus used was a novel MPCVD device independently developed by Xia Yuhao et al.  A dual-substrate structure was introduced into the traditional coupled resonant cavity. The operating frequency was 2.45 GHz, and the maximum output power was 1.6 kW. The schematic diagram is shown in Figure 1.

 

Figure 1 Principle of microwave resonant cavities with different structures

 

After growth, the influence of the dual-substrate structure on plasma emission spectrum and single-crystal diamond growth under the same parameters was analyzed using Raman spectroscopy, scanning electron microscopy, emission spectroscopy, and other equipment.

 

Compared with the traditional single-substrate structure, under the same deposition parameters, the plasma spheres generated by the dual-substrate structure were smaller in volume and had higher power density. The intensity and density of C2 groups and H groups in the plasma were also much higher than those of the single-substrate structure.

 

Compared with the single-substrate structure, the single-crystal diamond grown under the dual-substrate structure under high methane content conditions had a smoother and more even surface morphology, higher crystallinity, fewer internal defects, and smaller characteristic peak shift in the diamond. The growth rate of single-crystal diamond grown in a dual-substrate structure increases significantly with increasing methane volume fraction, reaching up to 24 μm/h, making it suitable for growing thick films in high methane content.

 

 

2. Process of Diamond Film Preparation by MPCVD

Although diamond thin film deposition technology has been extensively studied, the high-quality, high-rate, and low-cost growth of diamond films under numerous process parameters has always been the goal pursued by the industry. High-quality diamond films grown under optimized conditions not only have low production costs, but also achieve a qualitative leap in applications in the fields of electronics and energy.

 

HUANG et al. explored the fastest growth rate of diamond films under different chamber pressures, methane volume fractions, and microwave power.

 

Jiang Caiyi studied the correlation between substrate temperature, reaction chamber pressure, and methane volume fraction on the purity and growth rate of diamond films.

 

The nucleation of diamond films is not only closely related to factors such as substrate material type and surface pretreatment method, but is also greatly affected by process parameters such as substrate temperature, chamber pressure, and methane volume fraction. Lower substrate temperature is conducive to nucleation, but too low a temperature will lead to a slow nucleation rate and poor uniformity; increasing the methane volume fraction can promote nucleation, but too high a volume fraction will lead to a decrease in diamond purity [7-9]. Li Sijia et al. [10] studied the effects of substrate temperature, cavity pressure, and methane volume fraction on the quality of diamond films through experiments, and developed the optimal process conditions, obtaining the following results:

(1) When the temperature is too low, there are fewer excited-state hydrogens, the diamond film growth rate is slow, and it is not conducive to the growth of the diamond film phase; when the temperature is too high, the diamond film grows rapidly, but the crystal quality is poor and it is easy to graphitize.

(2) When the pressure is too low, the ion spheres are dispersed, the growth rate is slow, and the hydrogen atom etching ability is insufficient, resulting in poor diamond film quality; when the pressure is too high, the growth rate is faster, but at this time the plasma spheres are more concentrated and the volume fraction of excited-state hydrogen atoms is higher, which will lead to an increase in diamond defects and a decrease in quality.

(3) When the methane volume fraction is too low, the volume fraction of carbon-containing active groups is low and the growth rate is slow, but the diamond quality is high; when the methane volume fraction is too high, the volume fraction of carbon-containing active groups is high and the growth rate is fast, but the diamond quality is poor. It is evident that both excessively high and excessively low methane volume fractions are detrimental to the formation of high-quality diamond films [11-12].

 

3. The Influence of Temperature on Defects in Homoetheroepitaxy Single-Crystal Diamond

MPCVD, as a commonly used diamond deposition method, has advantages such as electrodeless discharge, fast growth rate, and low product impurities, making it an ideal diamond growth method [13]. However, MPCVD has stringent requirements on growth parameters and the quality of single-crystal diamond, and the grown single-crystal diamond still contains defects and impurities, which have a significant impact on its performance [14]. Therefore, reducing defects in single-crystal diamond is of positive significance for improving its performance and promoting its application in electronic devices.

 

Diamond defects are mainly classified into three types: dislocations, twins, and stacking faults. Stacking faults and micro-thick grains often exist on the (111) crystal plane, and the possibility of metamorphism on the (111) plane is much greater than that on the (100) crystal plane.

 

Studies have found that stacking faults mostly occur on the (111) crystal plane and are distributed near the grain boundaries; the causes of defects in single-crystal diamond may be defects in the seed crystal itself, impurities in the gas source, impurities in the cavity, or inconsistencies in the experimental parameters used . Therefore, TALLAIRE et al.  used H₂/O₂ plasma to etch the surface defects of single-crystal diamond for a long time before growth to reduce defects.

 

WANG et al. [17] found that the transformation from (111) to (100) facets was related to methane volume concentration and temperature, and that increasing the methane volume concentration could promote the transformation from (111) to (100) facets.

Yan Lei et al. [18] found in their study of the effect of I(C2)/I(Hα) on the quality of diamond growth that the lower the ratio, the better the quality of the grown diamond. This is because C2, as a precursor of diamond, directly participates in the homoepitaxial growth process of diamond; while Hα preferentially etches the non-diamond phase, so the higher the Hα group content, the better the quality of the grown diamond. 

 

4. Conclusion

4.1 Compared with the single-substrate structure, the single-crystal diamond grown under high methane content conditions with the dual-substrate structure has a smoother and more even surface morphology, higher crystallinity, and fewer internal defects. 4.2 The growth rate of the diamond film is positively correlated with the substrate temperature, cavity pressure, and methane volume fraction.

4.3 Single-crystal diamond deposited at 740℃ will generate large compressive stress, eventually leading to surface cracks; the taper defects of diamond deposited at 780℃ and 820℃ will shrink to some extent after growth; the defect area of ​​the (111) plane of single-crystal diamond deposited at 860℃ will increase.

 

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