Application of a Position Sensitive Atom Probe to the Analysis of the Chemistry and Morphology of Multi-Quantum Well Int
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APPLICATION OF A POSITION SENSITIVE ATOM PROBE TO THE ANALYSIS OF THE CHEMISTRY AND MORPHOLOGY OF MULTI-QUANTUM WELL INTERFACES. ALFRED CEREZO, J.ALEX LIDDLE, CHRIS R.M.GROVENOR, ANDREW G. NORMAN AND GEORGE D.W.SMITH Department of Metallurgy and Science of Materials, Oxford University, Parks Road, Oxford, OXl 3PH, UK. ABSTRACT A position sensitive detector has been added to the Oxford atom probe facility, allowing the microchemistry of field ion specimens to be analysed with excellent chemical specificity and a lateral and depth resolution of better than 0.5nm. This paper presents some recent results obtained with this equipment on the chemistry and morphology of interfaces in multi-quantum well samples, illustrating the power of the technique in obtaining very detailed information on microstructural features with dimensions less than Inm. INTRODUCTION It is important to be able to analyse the microstructure of low dimensional semiconductor materials in as much detail as possible because it is believed that features like interface roughness and chemical diffuseness control for instance the optoelectronic properties of multi-quantum well samples [1]. The analysis of the chemical abruptness and morphological smoothness of interfaces in multi-quantum well structures is extremely difficult because chemical analysis with subnanometer resolution is required if interfaces between the wells and the buffer layers are to be investigated in any detail. Conventional microanalysis techniques like SIMS or analytical transmission electron microscopy have been used with only limited success to study interfacial chemistry because the individual layers are often only 5-10nm thick, and because of ion beam surface roughening in SIMS and beam spreading effects in TEM samples of finite thickness. In Oxford we have for several years been developing the technique of Pulsed Laser Atom Probe (PLAP) microanalysis for the study of semiconductor materials. This requires the development of both special specimen preparation techniques [2] and an understanding of the laser assisted field evaporation process [3]. In a PLAP experiment ions field evaporated from the sample surface are identified in a time-of-flight mass spectrometer [4]. The lateral resolution is defined by an aperture, and is of the order of 2nm; the depth resolution is better than 0.5nm, and is a result of the surface specificity of the field evaporation process. We have been able to obtain chemical analyses from a wide range of semiconductor materials [3,5], and from oxide layers on semiconductor surfaces [6]. More recently, we have designed and constructed a position-sensitive detector which when mounted at the end of a short time-of-flight mass spectrometer identifies both the chemical identity of evaporated ions and the position on the sample surface from which they were evaporated [7,8]. This unique instrument allows the quantitative identification of composition variations within an area some 15nm across. This paper reports results obtained using both PLAP and POsition Mat.
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