Growth and characterization of ZnO

Abstract

ZnO films grown on different substrates by molecular beam epitaxy have been investigated in this work. The substrates, composed of silicon, sapphire and quartz glass, were applied in this research. Silicon and sapphire are single crystal substrates, whereas the quartz glass is an amorphous one. In this work, we emphasized mainly on the study of the properties of ZnO films grown on quartz glass substrates. The characteristics of the grown samples were measured by scanning electron microscope (SEM), x-ray diffraction (XRD), ex-situ atomic force microscopy (AFM), 10K photoluminescence (PL) and hall measurement. The investigated thickness of ZnO film grown on glass with the growth rate of 7 nm/s was about 800 nm. After the ZnO film growth, the surface-crack problem appeared due to the very high difference of the thermal expansion coefficient between glass and ZnO (5.5x10[superscript -7] K[superscript -1] and 4.75x10[superscript -6] K[superscript -1], respectively). To avoid this problem, MgO-buffer-layer growth is necessary. However, the growth rate of ZnO film approximately reduced to 3.4 nm/s when MgO-buffer-layer thickness was varied from 130 to 170 nm. The ZnO growth temperature was fixed at 530 ํC and the amount of deposited zinc pressure and oxygen plasma flow rate were 4.0x10[superscript -6] Torr and 1.07 sccm, respectively. We found that no cracks was observed for the samples with MgO-buffer-layer thickness less than 170 nm, while the surface cracks appeared with the buffer layer thickness of 170 nm or more. However, the structural and optical properties of the samples with the buffer layer thinner than 130 nm were not improved, even though no cracks appeared, and also the electrical property was not good. For the samples with the buffer layer from 130 to 170 nm, the PL intensity was much improvement, approximately 20-time improvement. Anyway, no c-orientation existed. This experiment indicated that the optimum buffer layer thickness was around 130-150 nm. Finally, via increasing zinc amount (5.0x10[superscript -6]) or increasing temperature (600 ํC) during ZnO film growth, the structural property with the very good optical property and no cracks can be improved