Feasibility study of plasma-assisted electroepitaxy for the growth of GaN layers and bulk crystals

There is a high level of commercial and scientific interest in nitride semiconductors. The group III-nitrides (AlN, GaN and InN and their solid solutions) are being used for amber, green, blue and white light emitting diodes, for blue/UV laser diodes and for high-power, high-frequency and high temperature electronic devices. Group III-nitride layers for device fabrication are grown by metal-organic vapour phase epitaxy (MOVPE), hydride vapour phase epitaxy (HVPE) and molecular beam epitaxy (MBE). One of the most severe problems hindering progress in the field of nitride technology is the rarity of suitable lattice-matched substrates onto which group III-nitride films can be grown. Group III-nitride layers are commonly grown on non-lattice matched sapphire, GaAs or SiC substrates. However, bulk GaN substrates, which are matched in lattice constant and thermal expansion properties to epitaxial nitride layers are needed for fabrication of the highest-quality GaN-based devices. Solution growth methods are normally used in the commercial production of standard III-V crystals with low dislocation density. It is well established that by using solution growth techniques, the crystallisation takes place very close to equilibrium. At present, solution growth methods are being actively explored to grow bulk GaN crystals. In order to achieve an efficient solution growth technique for GaN we need to fulfil 3 main requirements: 1) a high solubility of N in the Ga-based solution, 2) efficient transport of N and Ga to the growth interface and 3) prevent local supersaturation in the solution, which will otherwise result in spontaneous crystallisation. High quality low dislocation density bulk wurtzite GaN substrates can be grown from liquid Ga solutions. However, the solubility of N2 in liquid Ga is very low and it is difficult to obtain reasonable growth rates and therefore large area bulk GaN crystals are still not commercially available. Wurtzite GaN crystals have been synthesized by reacting gallium metal with atomic nitrogen produced by a microwave plasma source, which avoids the high equilibrium pressure needed for N2. Atomic nitrogen from an RF plasma source can be used to produce high concentrations of N in a Ga-based melt. Unfortunately, this high concentration of N only exists close to the surface of the metallic Ga and normally results in spontaneous crystallization of polycrystalline GaN on the surface of metallic gallium. In order to achieve an efficient epitaxial growth process one needs to develop a technique to transport the N species through the gallium melt to the growth surface and at the same time to minimize spontaneous nucleation. Liquid phase electroepitaxy (LPEE) is a crystal growth method, in which the layer growth is initiated and sustained by passing a direct electric current through the solution-substrate interface while the temperature of the overall system is maintained constant. An electric current passing through the LPEE growth cell causes four main effects: 1) electromigration of the solvents in the liquid metal solutions; 2) Peltier effect at hetero-interfaces: 3) Joule heating of the growth cell and 4) increased convection in the solution. Electromigration and Peltier cooling of the growth interface together produce the required concentration gradient to the growth interface. In this current application we are proposing to develop an entirely novel, inexpensive technique for the growth of high quality bulk GaN crystals with a low dislocation density - plasma assisted electroepitaxy (PAEE). We will combine advantages of the plasma process for producing high concentrations of N species in the Ga melt with the advantages of electroepitaxy in transferring these species from the Ga surface to the growth interface without spontaneous crystallisation on the surface or within the solution.