Research On Single Crystal Diamond Mosaic Growth Using HPHT Diamond Substrates

With large transmittance in wide wavelength range,super hardness,excellent thermal conductivity and thermal stability as well as 5.47eV band gap energy,diamond receives extensive attentions from many different fields known as "Ultimate semiconductor material".However,due to the limitation of the growth mechanism and equipment ability,it is still difficult to grow inch size high quality single crystal diamond with low cost and high repeatability,which restricts its further development for the application as semiconductor material.At present,the technical routes for the inch size diamond single crystal growth lie in two approaches,silicon heteroepitaxy growth and diamond mosaic growth.The former one uses microwave plasma chemical vapor deposition(MPCVD)technology to achieve diamond heteroepitaxy growth on inch size silicon wafer as substrate.The key to improve the diamond quality is to promote the crystal grain boundary movement and reduce the diamond dislocation density.The dialnond mosaic growth is method that arranging several single diamond chips as one substrate,to growth a large single crystal diamond layer by integrating lateral epitaxial growth from different substrate edges in MPCVD technology.In this dissertation,we first analyzed the technical details and limitations of diamond mosaic growth with "Clone" technology.Then we brought up a new technical routine of diamond mosaic growth combining the High Pressure High Temperature(HPHT)technology and MPCVD technology.In this technical routine,high pressure MOCVD method was applied to grow single mosaic layer on substrates prepared by several HPHT diamond chips.After systematically studying the growth mechanism and processing parameter,we have successfully fabricated high quality inch size MPCVD single crystal diamond layer with no inner boundaries.The main results are described as follows:1.Growth and characterization of HPHT single crystal diamond substrateFinite element simulation was applied to analyze the single crystal diamond growing mechanism in the high pressure and high temperature environment.By measuring the system and cubic cavity dimensions,analyzing the thermo conductivity and electrical resistance of the key materials,segmentation of finite element and ideal boundary setting and division,we simulated the HPHT diamond growth environment and focused the study on the temperature field distribution.The size of the heater and functional parts were changed to describe the influence on the temperature distribution which was important to the growth rate of radial and axial direction.High quality centimeter size HPHT single crystal diamond was grown based on the finite element simulation optimized result.Laser cutting,mechanical lapping and fine polishing were followed to prepared large size HPHT diamond chips with low surface roughness for the MPCVD growth.Raman Spectroscope,X-ray diffractometer and heat conductivity test were applied to study the substrate crystal quality.2.Growth and characterization of MPCVD single crystal diamond layerThe influence factors for the MPCVD diamond single crystal growth were discussed,including the chamber pressure,substrate temperature,microwave power and methane concentration.The processing parameter of the MPCVD growth under 300torr pressure was studied for high quality single crystal diamond homoepitaxy growth.Laser confocal microscope was applied to observe the layer surface morphology on the homoepitaxy area and lateral growth area.Raman Spectroscopy and x-ray diffraction techology were used to assess the internal stress and crystal quality.The result indicated the lateral growth area show a little lower quality than the homoepitaxy area,which proved the possibility for the mosaic joint combination.A crystal lattice visual structural analysis was applied to describe the step flow rotation that originated from the temperature driven concentration difference of the CH3 and C2H2 ion charged particles on the diamond center and edge.MPCVD diamond based Metal Semiconductor Field-Effect transistor(MESFET)was prepared.The test result proved that the transistor was an enhancement semiconductor device.The saturation current was 6?A,threshold voltage was-0.7V,and on-off ratio was 105.The MESFET showed repeatable and strong responses for the ultraviolet ray with 210nm wavelength and the rate of light and dark was 105.All these results revealed the MPCVD diamond was in a high quality and able to meet the requirement for the semiconductor application.3.Growth and characterization of inch size single crystal diamond mosaic layerSingle crystal diamond mosaic growth was achieved by four pieces HPHT diamond chips with dimensions of 5mmx5mm as an entire substrate.After MPCVD growth,the mosaic sample became a solid block with 11.75mmx 11.75mm in size and showed no poly-crystalline and joint edges.Laser Confocal microscope was applied to observe the mosaic surface after the growth of 24 hours and 48 hours and analyze the rule of surface step movement.Raman scanning image was used to analyze the internal stress distribution in the substrate joint area.It was found that mosaic layer in the center junction possessed high crystal quality and the junction stress field moved from one substrate to another,which matched the microscopic observation.Combing the result of both Laser Confocal Microscope and X ray,it proved that mosaic surface step flow direction had nothing to do with the off-axis direction of the original substrates.Furthermore,the surface height distribution proved that the surface step flow generation and movement was driven by the height difference of the entire mosaic surface.The mechanism of the junction interface formation and the mosaic surface step morphology transformation was proposed and the substrates preparation detail was summarized.Finally,we used four pieces of single diamond substrates in 10mm×10mm to achieve the preparation of inch size mosaic layer with high quality and no clear joint boundary.