Growth of Diamond on Si, SiC, Mo and Diamond Substrates for Heat Spreader, Optical Window, and Surface Graphitized Devices
Authors: Guo Linhai , Meng Lingxue , Li Chunlei , Yue Hangyu , Wang Hui , Tian Li 
Published: December 2024

Introduction

Diamond, owing to its exceptional impact resistance, high thermal conductivity, broad-spectrum optical transparency, elevated breakdown field strength, and superior carrier mobility, finds widespread application in diverse fields such as high-power laser windows, efficient heat spreader, and high-performance semiconductor devices.

Experimental

1. Heteroepitaxial Growth of Polycrystalline Diamond on Si, SiC, and Mo Substrates:

In this part, substrates employed include double-side polished 4H-polytype SiC, (100) single-side polished single-crystal Si wafers, and Mo substrates. Ultrasonic grinding of substrates using diamond powder ethanol suspension to promote the dispersion of nucleation seeds, followed by sequential ultrasonic cleaning in acetone, ethanol, and deionized water.

Subsequently, MPCVD was used for diamond growth, using hydrogen, methane, oxygen, and a mixture of hydrogen and nitrogen as reaction gases. Using different processes to produce efficient heat sinks and high-power laser windows.

2. Homoepitaxial Growth of Single-Crystal Diamond on Diamond Substrates:

In the homoepitaxial growth process, (100) double-side polished single-crystal diamond substrates are utilized. No need for ultrasonic grinding steps, cleaning steps are the same as above. Then use MPCVD for homogeneous epitaxial growth of single-crystal diamond.

3. Investigation of Graphitized Surface Devices on Diamond:

This study also investigated the graphitization devices on diamond surfaces. A high-temperature metal-catalyzed method is employed to promote surface graphitization of diamond. Firstly, polish and clean the diamond. Subsequently, a 300 nm thick nickel layer is deposited on the diamond surface. Rapid thermal annealing is then performed at 1300°C to form a highly conductive graphite layer. This process successfully prepares capacitor samples with a graphite-diamond-graphite three-layer structure.

Results and Discussion

1. High-Power Laser Window Plates:

The fabricated laser window plates utilize Si, SiC and Mo substrates, employing an ultra-low nitrogen growth process with a growth rate of approximately 3.7 µm/h. After double-sided polishing, the thickness reaches 1 mm. At room temperature, the transmittance at 10.6 µm wavelength approaches the theoretical maximum, reaching 67.9%. The overall thermal conductivity exceeds 1950 W/mK, approximating that of single-crystal diamond.

Raman spectroscopy and XRD results indicate that the primary component is polycrystalline diamond with a (110) crystallographic orientation. Laser testing results demonstrate that these window plates possess a high laser-induced damage threshold with a peak energy of 60 J/cm2 and a peak power of 12 MW/mm2, capable of withstanding high-power CO2 laser output.

2. Heat Spreader:

This study involves depositing diamond on Si and SiC substrates, forming diamond-based composite materials. This significantly improves the heat dissipation efficiency of the devices, bringing their performance closer to theoretical limits. The deposition process employs a low-nitrogen technique to balance growth rate and defect density, achieving a growth rate of approximately 5 µm/h.

Raman spectroscopy and XRD results indicate that the primary component is polycrystalline diamond with a (110) crystallographic orientation. Although the increased nitrogen content results in lower optical transmittance compared to optical window plates, the grain size can be increased from 124 nm to 22 μm through production process adjustments, thereby enhancing thermal conductivity.

Test results demonstrate that the thermal conductivity of the Si-diamond composite material reaches 450 W/mK,3 times that of single-crystal Si. The SiC-diamond composite material achieves a thermal conductivity of 500 W/mK,3.5 times that of SiC.

3. Single-Crystal Diamond:

The transmittance and thermal conductivity of single-crystal diamond grown by homoepitaxial growth are superior to those of polycrystalline diamond. However, limitations in size and growth conditions restrict its large-scale application in heat spreaders and optical windows. Single-crystal diamond is more suitable for manufacturing semiconductor devices such as diamond capacitors, Schottky diodes, and hydrogen-terminated MOSFETs.

The author has conducted research on doping of single-crystal diamond. Characterization through Raman spectroscopy, XRD, and TEM confirms that the primary component is single-crystal diamond with a (100) crystallographic orientation. The dislocation density is below 103/cm2, meeting the stringent requirements for semiconductor devices.

4. Graphite-Diamond-Graphite Capacitors:

Utilizing prepared diamond, graphite-diamond-graphite capacitors were fabricated. Measurements using a semiconductor parameter analyzer revealed a high capacitance value of 4 nF. TCAD simulation indicated a breakdown voltage as high as 1100 V. This research explores a novel method for diamond capacitor fabrication.

Conclusions

Diamond heteroepitaxial grown on Si, SiC, and Mo substrates has prepared high-power laser windows with 67.9% optical transmittance at 10.6 μm wavelength, and composite materials with thermal conductivity enhanced to 3-3.5 times that of the original materials. Homoepitaxially grown diamond on single-crystal diamond substrates exhibits a dislocation density below 10³/cm², meeting the requirements for semiconductor device substrates. Furthermore, this research has developed a novel diamond capacitor fabrication technique, yielding capacitors with a capacitance of 4 nF and a breakdown voltage of 1100 V.