Growth of Low dislocation density diamond substrates – CROISADD

The increasing worldwide demand in electric energy requires the use of efficient power converters in order to control and dispatch the large electrical flows between production plants and loads with a minimum loss. Military applications can also benefit from more efficient power switches. Indeed, current weapon systems must constantly improve their performance in terms of launching and mobility and one of the keys to success relies in our ability to develop solutions to a controlled release of very high instantaneous energy (repetitive or not). Transient or on impulse, this energy release is usually ensured by common components such as capacitors, but also involves switching components which are more precisely the scope of this project. Traditional wide bandgap semiconductors such as GaN or SiC are good candidates to fulfil this task and intense research efforts have been devoted to the improvement of their synthesis and properties. The first power-devices based on these materials are now on the way to be commercialized.
Due to its outstanding and unrivalled properties, CVD diamond is the ultimate large bandgap semiconductor that could drastically push the performance of the actual power electronic devices. The improvements of plasma assisted CVD techniques has led to the routine synthesis of intrinsic or p-doped diamond layers, but the use of this material in electronics has been largely compromised by the lack of thick defect-free crystals. Moreover poor reproducibility is believed to be mostly due to extended defects such as dislocations that have a catastrophic effect on breakdown voltages and carrier mobilities. Obtaining in a reproducible way, diamond crystals that are virtually free of dislocations constitutes one of the essential issues which, if addressed, could open the path to high-performance diamond based power devices. This is the main objective of the present project.
Dislocations in CVD single crystals are typically in the range of 10E5 to 10E6 cm-2 and have two main origins: (i) dislocations already present in the substrates that extend into the CVD layer, (ii) dislocations generated at the early stages of the epitaxial growth, near the substrate or every time the growth is stopped and resumed. These dislocations inevitably propagate in the direction of growth (threading dislocations) and emerge at the surface of the layer where they sometimes generate growth features (hillocks, unepitaxial crystals).
This project intends to provide a better understanding of the mechanisms of dislocations formation and propagation in the diamond crystal and their interaction with the growth front, with the final objective to grow defect-free crystals. Our approach involves an iterative process based on: the preparation of substrates surface by etching, texturing or masking, followed by the deposition of CVD diamond films which will then be thoroughly characterised using relevant techniques such as TEM, birefringence, cathodoluminescence or selective etching in order to assess the dislocation types and densities. Appropriate treatments and growth conditions will therefore either inhibit the formation of new dislocations, or ensure that existing dislocations will be blocked or bent over and will not further propagate into the crystal lattice. This work is based on strategies that have proved to be efficient for other semiconductors such as GaN and that will be used for the first time for diamond synthesis. The electronic properties of the best diamond crystals synthesized in this project will then be evaluated by time-of-flight measurements and the possibility of using such crystals as substrates for further growths will be considered.
This project will bring together partners that are expert in large bandgap semiconductor growth and their characterizations (LSPM, LMGP, GEMaC) and dislocation characterization by TEM (CEMES). It will ensure the real breakthrough that is needed to develop an electronic-grade-diamond technology sector.