Tem and Cathodoluminescence investigations of CdS grown by different techniques

Abstract

This work presents the results of an investigation of cadmium sulphide with various characterisation techniques such as TEM- Cathodoluminescence, SEM, XRD, PL and optical transmittance. A model based on a convolution method is proposed to predict CL signal emission. It was assumed that the CL can originate from electron and X-ray sources. Measurement of CL signal and noise over a wide range of magnifications showed that signal-to-noise is too low when STEM is operating at higher magnifications than 3000 times. This may lead to some extent to some CL imaging limitations. The origin of this phenomenon is discussed. A study of the suitability of a combined transmission electron microscope (ТЕМ) / Cathodoluminescence (CL) imaging and spectroscopy apparatus for investigations of CdS is presented. Photoluminescence (PL) was used to evaluate the effect of the Ar+ and I+ ion beam thinning used in ТЕМ specimen preparation of CdS: a minor increase in yellow emission (594 nm) resulted. However, excitation of luminescence spectra m the ТЕМ had a quenching effect on red luminescence (734 nm), this being considered due to the high excitation density compared to that in PL. Significant electron beam damage to the CdS single crystal could be avoided by using scanning transmission electron microscope (STEM) illumination in preference to the conventional ТЕМ mode. Dislocation images were correlated with contrast in the STEM-CL imaging mode. The potential of the apparatus to make further direct correlations of CL images with diffraction contrast ТЕМ imaging was assessed using the Rose visibility criterion. Finally, optical and structural properties of thin films CdS grown by chemical bath deposition and metal organic chemical deposition were investigated. For both growth methods, ТЕМ diffraction pattern confirm that the crystallographic structure of CdS was hexagonal, with a strong selected (002) orientation. It was found that either air annealing or CdCI(_2) treatment on CBD CdS increased modestly the grain size of 6 nm. While for MOCVD CdS, the growth temperature and the II/VI play an important role in obtaining large grain. Hence, sample grown at 330 c with a II/VI of 0.78 revealed a better crystalline quality due to the presence of the excitonic bound centred around 2.55 eV and exhibited the largest grains of about 97 nm.