Study Of Growth, Annealing Of MgZnO Films And MgZnO-based Light Emitting Devices Fabricated By MOCVD

ZnO has recently attracted considerable attention due to its favorable properties such asthe wider band gap (3.37eV) at room temperature, the large binding energy of excitons (60meV), hith c-axis oriented and the wurtzite structure. ZnO has immensity develop space at surface acoustic wave devices, light emitting diodes and laser systems, photodetectors and gas sensor, solar cells etc. Although MgO has rock-salt crystal structure, the ion radii of Mg²⁺ (0.57(?)) and Zn²⁺ (0.60(?)) are so similar that Mg²⁺ can replace the location of Zn²⁺ in ZnO film partially with fairly small lattice mismatch (0.1%). MgZnO has the similar properties to ZnO. Furthermore, the band gap of wurtzite structure MgZnO can be adjusted from 3.3 to 4.0eV due to the wider band gap of MgO (7.7eV). A variety of deposition techniques has been employed to prepare MgZnO thin films such as pulsed laser deposition, sputtering, molecule beam epitaxy, metal-organic chemical vapor deposition (MOCVD), ultrasonic spray pyrolysis, and so on. Among them, MOCVD offers high throughput due to higher growth rates and hence is promising for industrial applications.In this thesis, high quality MgZnO films were grown on c-plane sapphire substrate in optimized growth condition by MOCVD. The effects of anneal in N₂, O₂, and vacuum on the properties of MgZnO film were discussed detailed. P-MgZnO films were obtained on GaAs substrate by As diffusion. MgZnO-based p-n homojunction and p-MgZnO/n-ZnO heterojunction light emitting devices were also fabricated on the basis of this method. The characteristics of the two series of devices were investigated.MgZnO film were grown on sapphire substrate by MOCVD. The effects of the growth temperature on the properties of the films were analyzed. The results showed that the crystal and optical qualities and surface morphology of MgZnO film grown at 610℃were all best. Under lower temperature, the growth speed of MgZnO was rapid. A great deal of gas atoms were adsorbed by the surface. It affected the mobilities of atoms, which resulted in the crystal quality degrading. Under higher temperature, because MgZnO film had decomposed before the reaction happened, the speed decreased with the increasing of the temperature. Note that the flux of oxygen had an important effect on the structural properties and surface morphology of MgZnO film. The qualities of MgZnO film became best when the flux attained 200sccm. The thicknesses of ZnO buffer layers were also adjusted by controlling deposition time. When the thickness was 20nm, the intensity of 002 diffraction peak in XRD pattern was the strongest. And the peak value began to reduce when the thickness increases continually. This phenomenon was maybe caused by the decrease of the nucleation density of MgZnO nuclei. It implied the buffer layer thickness of 20nm was optimized. We also fabricated MgZnO film on the GaAs substrate by MOCVD, and obtained the optimized growth conditions. The properties of MgZnO film grown on the sapphire substrate were much better than that grown on the GaAs substrate. The reasons were attributed to As diffusion from substrate and the large lattice mismatch between the GaAs substrate and MgZnO film.The properties of MgZnO film were annealed under different conditions. The results showed that anneal in N₂ had little effect on the properties of the film. The crystal quality of the films annealed in oxygen, however, had been improved greatly. The PL spectra showed that the deep level emission peak became stronger after the films were annealed in oxygen attributing to the ratio of oxygen and Zn defects in the film. It was showed that the deep level emission intensity was related to the concentration of V_Zn and O_Zn defects. MgZnO thin films were annealed in vacuum at different temperatures for 1h. The UV emission peak was blue shifted in the photoluminescence spectra and a dramatic shift of (002) diffraction peak to higher angle was observed in X-ray diffraction pattern with the increasing anneal temperature. It suggested the band gap and the lattice parameter of MgZnO had been affected by anneal in vacuum. Furthermore, the structure of the film became sparser due to anneal in vacuum. From the X-ray photoelectron spectroscopy and ICP of the MgZnO film, we can find that the anneal temperature have an effect on the content of each element in MgZnO quantificationally. In addition, the content of Mg in MgZnO varied slighter than that of Zn and O with the anneal temperature increased. The above phenomena indicated that anneal in vacuum could slightly adjust the percentage of Mg indirectly in MgZnO film and offer a good idea in MgZnO devices facture.P-MgZnO films were prepared by As diffusion from GaAs substates. The state of As existed in MgZnO films strongly affected the conductions of the film. The unannealed films exhibited n-type conduction owing to most As existed as As-O bond. After anneal, many As-O bond had broken and the As existed as As_Zn-2V_Zn form. The complex acted as shallow acceptors, so films present p-type conduction.On the basis of p-MgZnO fabrication, MgZnO-based homojunction devices had been fabricated on p-GaAs by MOCVD. Theâ… -â…¤curve of it exhibited typical rectifying behavior of p-n diode with the forward turn-on voltage of 3.5V. The crystal quality degraded when the As was diffused into the film in the XRD profile. A strong emission band from 2.3eV to 2.8eV can be observed attributing to the deep-level transition. Comparing to the EL spectra of ZnO LED, several peaks from 2.1eV to 2.3eV can be found and there was a blue shift in several peaks from 2.4eV to 2.8eV in that of MgZnO LED. They were maybe the main differences between MgZnO and ZnO LEDs. When the injection current reached 180mA, a peak located 3.31eV can be measured, which resulted from the relative near band emission. The effects of variation of the Mg composition on the properties of the LEDs were researched. The near-band emission peaks in the photoluminescence and the deep-level emission peaks in the electroluminescence spectra were both blueshifted attributing to the broadish band gap. The full-width at half-maximum value and the intensity of the DLE peak were also affected by the increasing defect density.MgZnO/ZnO p-n heterojunction light emitting devices were grown on GaAs substrate by MOCVD. Theâ… -â…¤characteristics showed a diode characteristic between the n-ZnO and p-MgZnO layers with a threshold voltage of 3.6V. The room temperature measurements, such as the HALL, XRD, PL and EL spectra were carried out. The EL spectra of the devices under different injection current all showed strong broad DLE peaks attributing to the deep-level transition. However, the ultraviolet peak related to carrier combination can not be found all along. It was resulted from the great difference of electrical properties between n-ZnO and p-MgZnO film. The lower carrier mobility of p-MgZnO film resulted in the carriers combining only in p-MgZnO film. The crystal and optical qualities of p-MgZnO film, however, were not good at all, which affected the carrier combinations certainly.