Epitaxial Growth Of GaN Thin Films By Laser Molecular Beam Epitaxy

Gallium Nitride(GaN)is an exchemical semiconductor material,and it has been regarded as one of the most promising materials for light emitting,high temperature microelectronic,high power and high frequency electronic deices.Because it's advantages of stability of chemical properities,large band gap and high electron mobility.The GaN materials has been mainly epitaxially grown on foreign substrates,such as sapphire(Al2O3)and Si,buffer layers can improve the quality of GaN films because of the large lattice mismatch and the large difference in thermal expansion coefficients between GaN and Al2O3,Si substrates.In this paper,GaN films were deposited by the laser molecular beam epitaxy technique(L-MBE).The paper is consisted of two parts.In the first part,GaN films was deposited on Al2O3(0001)substrate and AIN as buffer layers.Effects of different energy of buffer,depositon time on the crystalline and surface qualities were discussed.The 'band structure of GaN is simulated by Materials Studio software.The second parts,GaN were deposited on Si(100)substrate and TiN/A1N as buffer layers,and influences of the different energy on the optical properties of GaN were analyzed.The results of our work are as follows:1.GaN films was deposited on Al2O3(0001)substrate and AIN as buffer layers.The orthogonal experimtents indicate that the suitable laser energy for depositing GaN at 150mJ/p.With the increasing of the deposition time the crystallinity and surface morphology of GaN films improve first and then decreases.At the deposition time of 0.5h,the crystalline qualities are bad and the surface of GaN films has particulate matter,as for the lattice mismatch between AIN and GaN.The best crystalline and surface qualities is obtained at the deposition time of 1h,GaN films exhibit the layer-by-layer growth mode and the hexagonal structure with the single(0002)preferred orientation.At the deposition time of 1.5h,GaN can't crystallize because of the unbalance of the Ga and N atomic ratio during the deposition.2.The Uv-vis optical performance of GaN films was analyzed,and the experiment value of band gap of GaN is 3.39eV.According to the first principle,the energy band structure of GaN is simulated by MS software.The calculated bandgap of hexagonal GaN is 1.89eV smaller than the experiment value of 3.28eV,due to the maximum estimation of Ga3d state energy during the calculation,which may cause the interaction between Ga3d and N2p has increased,the valence band bandwidth was enlarge and the band gap was lower,so the calculated value is smaller than the experiment value.3.The research shown that when the laser energy growth GaN films on Si(100)substrate was 220mJ/p,the partial pressure of N2 was 0.1Pa,the substrate temperature was about 750?,the cubic GaN thin films would be obtained and the crystalline quality of the crystal influences its optical properties.the photolu minescence spectrum is blue shift because of the GaN glow.4.The experiment showed that it was hard to deposit GaN films with high quality in Si substrate by L-MBE system,so we give some measures to improve the preparation process.Firstly,using electric spark assisted L-MBE to increase the degree of nitrogen ionization so that sufficient nitrogen ions are gain in the reaction chamer.Secondly,it is better to use NH3 as the working gas and it can improve the nitriding effect.