Pulsed Laser Atom Probe Analysis of Stoichiometry Variations in GaAlAs
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C.R.M.GROVENOR,A.CEREZO and G.D.W.SMITH. Department of vletallurgy and Science of Materials,Parks Road,OXFORD,UK. ABSTRACT The recent development of Pulsed Laser Atom Probe (PLAP) analysis has allowed routine analysis of the composition of a wide range of semiconducting materials. This paper presents results on the analysis of stoichiometry variations in MOCVD 6aAlAs layers demonstrating that accurate analysis of aluminium concentration fluctuations can be achieved with this technique. INTRODUCTION Pulsed Laser Atom Probe (PLAP) analysis is a relatively new technique for the study of semiconducting materials. Atom Probe Field Ion Microscopy [Ii was developed for the study of metallic materials, and Kellogg and Tsong [2] were the first to add a laser to an Atom Probe system to allow the analysis of insulating and semiconducting materials. PLAP analysis permits chemical information to be obtained from very small volumes of a sample (in the form of a finely pointed needle) by the extremely gradual field evaporation of the surface. The evaporated ions are individually identified by time-of-flight mass spectrometry. A full description of the application of this technique to semiconductor materials can be found in a recent publication [j]. Here it is sufficient to remark that the composition of tiny volumes of a semiconductor material (a cylinder about 2nm in diameter and a few atomic spacings in depth) can be achieved easily in the PLAP facility. A number of publications have presented composition data on III-V semiconducting compounds obtained in conventional voltage pulsed Atom Probe equipment [4-6]. These experiments failed to obtain the correct stoichiometry of GaAs and GaP specimens, however the analysis conditions were varied. Only very recently has a satisfactory explanation for this failure been proposed, and a somewhat awkward method for obtaining stoichiometrically correct analyses been put forward [7]. Wehave recently demonstrated that it is simple to obtain the correct stoichiometry in samples of binary compound semiconductors using the PLAP technique [8,9]. In order to demonstrate the potential of the PLAP in situations where the inherent very high spatial resolution can be used to best advantage we have now extended our work to investigate ternary III-V materials in which significant composition variations may occur. There are two obvious ways in which these composition variations may arise; spinodal decomposition, and fluctuations in conditions during epitaxial growth processes. Fluctuations in composition can obviously have a dramatic effect on the electronic nature of the material, changing the local bandgap, carrier mobility and optical properties. Studies of the composition fluctuations in III-V compounds have been extensively reported. Most recently electron optical techniques have been applied to the investigation of spinodal decomposition reactions [10,11]. The very high spatial resolution available in a TEM or STEM can be used to image the spinodal composition fluctuations, but even with very care
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