| Diamond is a new functional material with many excellent properties. It is the most important semiconductor, and has many applications. In the large-sized optical window area, CVD technique is the best method of producing diamond optical windows. In this thesis, optical-grade multi-crystalline diamond free-standing films were synthesized by MPCVD technique, and their growth properties, stress, impurities, and optical transmittance have been studied.Also, diamonds have the potential to become a new semiconductor by replacing silicon. Natural and HPHT diamonds can't fulfill the demands for their problems like size limitation, doping issue, and etc. So CVD technique becomes the most promising methond to synthesize large-sized single crystal diamonds. In this thesis, CVD homoepitaxial single-crystalline diamonds with high growth rate were deposited, and the nitrogen distribution in them has been investigated.The thesis includes three parts:I. Several main factors that affect the growth of diamond films, as well as the stress in them and impurity bonds on their surfaces, were studied and analyzed.1. During the process of deposition of diamond films by MPCVD, CH4 concentration affects their growth properties largely. The grow rate of the diamond films increases linearly, while their quality drops with increasing CH4 concentration. And 3% was confirmed to be the best CH4 concentration by compromising the film quality and its growth rate.2. Microwave power decides the color, shape and density of the plasma, and consequently influences the decomposition, initial energy and the ratio of different species in the reaction gases, and finally affects the growth of the films. With increasing microwave power, both their quality and growth rate increase. When the power is higher than 4500 w, their quality and growth rate becomes stable.3. The addition of oxygen largely improves the growth of diamond films, while keeping other conditions constant.4. The stress in the films has been investigated.①CH4 concentration has a big effect on the macro-stress in the films. When the CH4 concentration is 2% and 3%, the stress is tensile, and it becomes compressive with the CH4 concentration of 4%, and the compressive stress increases gradually with increasing CH4 concentration afterwards.②With the CH4 concentration increasing from 2% to 7%, the micro-stress in the films increases linearly.5. Impurity bonds on the surfaces of optical-grade diamond films have been studied by XPS, and a small amount of oxygen and nitrogen has been found, with their mainly existing states of C-O bond, C=O bond, O-C=O bond and O-N bond. And for the diamond films deposited without oxygen inlet, no nitrogen or oxygen bonded with carbon was found by XPS.II. Transparent free-standing diamond film, 50 mm in diameter and 300μm in thickness, was prepared. The optical properties of optical-grade multi-crystalline diamond films were alalysized, as well as their mechanical properties.1. The IR transmittance of large-sized transparent diamond thick film grown at a methane concentration of 2% is very high (about 70%), but the growth rate is low (only 1-2μm).2. The IR transmittance of large-sized transparent diamond thick film grown at a methane concentration of 4% is only about 60%, but the growth rate is high (7-8μm), which fulfills the real applications.3. The region with small grains near the nucleation side of the diamond film largely affected the film's IR transmittance, and polishing it away could further improve the IR transmittance of the diamond film.4. The IR transmittance of theΦ50 mm large-sized tranparent diamond film is the same at both the central and fringe region. Thus, it can be deduced that theΦ50 mm large-sized tranparent diamond film prepared under the conditions in this investigation has uniform optical quality.5. The films are transparent in visible region, and their UV absorption edge is around 225 nm (almost equals to the forbidden band width of single crystal diamonds), which makes it a promising material of producing UV detectors.6. The results of fracture strength measurements in a universal testing machine showed that their mechanical properties are close to the films produced by EA-CVD and DC-CVD, while worse than natural diamonds, which, however, has already fulfilled the demands of many applications, e.g. IR windows.III. The high rate growth process and optical properties of homoepitaxial CVD single-crystalline diamonds have been studied as well.1. High rate (>50μm) growth of homoepitaxial CVD single-crystalline diamonds has been achieved by adding a small amount of nitrogen in the reaction gases.2. The disappearance of void defect on the top surface of single-crystalline diamonds is discussed to be related to a filling-in mechanism.3. The results of PL spectrum showed that the nitrogen concentration is spatially inhomogeneous either on the top surface or in the bulk of the as-grown single-crystalline diamonds.4. The presence of N-distribution is attributed to the N-diffusion, resulting from the local growth temperatures changed during the high-rate deposition process. In addition, the formed nitrogen-vacancy centers play a crucial role in N-diffusion through the growing crystal.5. Based on the N-distribution observed in the as-grown crystals, we propose a simple method to distinguish natural diamonds and artificial CVD single-crystalline diamonds.
|
PDF Full Text Request