Nitrogen-passivation effects of Si substrates on the properties of ZnO epitaxial layers grown by using plasma-assisted molecular beam epitaxy

ZnO epilayers were grown on pretreated Si (100) substrates by using a N2 plasma and plasma-assisted molecular beam epitaxy (PA-MBE). The pretreatment for the surfaces of the Si substrates was conducted at different temperature in the range from 100 to 700 .deg. C before the growth. High resolution X-ray diffraction (HR-XRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements were carried out to investigate the effects of the pretreatment on the properties of the ZnO epilayers. All samples show the typical XRD patterns, AFM images, and PL emission peaks for ZnO. A higher intensity and a narrower full width at half maximum (FWHM) of the XRD (002) diffraction peak are observed from the ZnO epilayers grown on the pretreated substrates. The residual stress of the ZnO epilayers is relaxed, and the average grain size is gradually increased as the pretreatment temperature is increased to 300 .deg. C. The luminescent properties of the ZnO epilayers grown on the Si pretreated at a pretreatment temperature of 100 .deg. C are enhanced. However, the ZnO epilayers grown on the Si pretreated at the temperature of 300 .deg. C show anomalous PL behaviors. With further increases in the pretreatment temperature up to 700 .deg. C, the nitrogen passivation efficiency of the substrate surface is degraded. Therefore, a pretreatment temperature below 300 .deg. C during the pretreatment process is the most suitable for obtaining high-quality ZnO epilayers with good luminescence performance.