Pulsed Laser Deposited GaN Film On Different Substrates

In recent years, Wurtzite structured Gallium nitride (GaN) has attracted much attention for fabrication of ultraviolet (UV), blue and green light emitting diodes (LEDs), laser diodes (LDs), UV-detectors and devices operating in high-temperature, high-frequency and high-power conditions. It is difficult to grow large-sized single crystals of GaN, so hetero epitaxy has been used for the growth of high quality GaN films on various substrates. Si substrates for growing GaN films have many advantages, it can be considered as an attractive substrate for the growth of GaN films. However, Single crystal GaN films on Si substrates have not been obtained because of the large lattice mismatch (17 %) and the large difference in thermal expansion coefficients (56 %) between GaN and Si. Buffer layers, such as AlN, SiC, ZnO, may improve the quality of GaN films.GaN thin films can be deposited by various methods, such as sputtering, metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and pulsed laser deposition (PLD), etc. PLD is a newly developed film growth technique. In this technique, high density laser ablates the target and produces plasma depositing on heated substrate in vacuum background. PLD has advantages in comparison with others methods such as lower temperature of film crystallization.In the research, we got the high quality of the GaN crystal films on Si and GaAs substrates by two step growth pattern. The GaN thin films were deposited by PLD at first and then annealed in NH3 ambient. The characteristics of GaN films grown on different growth conditions were studied by measurements of X-ray diffraction (XRD), atomic force microscopy(AFM),Fourier transform infrared spectroscopy(FTIR) and photoluminescence(PL) .We investigated the effect of the SiC buffer on GaN film keeping all the rest of the parameters unchanged when we deposited GaN films on Si substrate. Through analysis of the measured results, a conclusion was drawn that the SiC buffer improve the crystal quality and the surface morphology of the GaN films. It is found that the intensity of photoluminescence increases with the adoption of the SiC buffer.The samples were annealed in NH3 ambient for 15 minutes after deposition. We study the effects of annealing temperature to the GaN films which grow on the GaAs substrate. In the end we found that the films has good crystal quality and surface morphology when it was annealed at 950℃. The PL spectrum of the sample annealed at 950℃show a stronger emission at 353 nm and a weaker emission at 468nm. The stronger emission at 353 nm is attributed to the exciton emission from conduction band to valence band. The weaker emission at 468 nm is attributed to the defect level drive from nitrogen vacancy. and then study the reasons of the brushing off the GaN films which deposited on the GaAs substrate.