Deposition of Zinc Selenide by Atomic Layer Epitaxy for Multilayer X-Ray Optics
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DEPOSITION OF ZINC SELENIDE BY ATOMIC LAYER EPITAXY FOR MULTILAYER X-RAY OPTICS J.K. Shurtleff, D.D. AlIred, R.T. Perkins and J.M Thorne Center for X-ray Imaging, Brigham Young University, Provo, Utah, 84602 ABSTRACT Thin film deposition techniques currently being used to produce multilayer x-ray optics (MXOs) have difficulty producing smooth, uniform multilayers with d-spacings less than about twelve angstroms. We are investigating atomic layer epitaxy (ALE) as an alternative to these techniques. ALE is a chemical vapor deposition technique which deposits an The atomic layer of material during each cycle of the deposition process. thickness of a film deposited by ALE depends only on the number of cycles. Multilayers deposited by ALE should be smooth and uniform with precise d-spacings which makes ALE an excellent technique for producing multilayer x-ray optics. We have designed and built an ALE system and we have used this system to deposit ZnSe using diethyl zinc and hydrogen selenide. MULTILAYER X-RAY OPTICS Multilayer x-ray optics (MXOs) are designed so that x rays which are diffracted from each interface of the multilayer constructively interfere to give a large total reflectivity. The maximum reflectivity is obtained when the interfaces between the layer pairs are abrupt and the layers are uniform and smooth with precise d-spacings, as depicted in Figure 1.
d
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Figure 1. a. A good multilayer x-ray mirror which gives the maximum reflectivity. b. A poor multilayer x-ray mirror. Layers are rough and nonuniform so that the diffracted x rays do not constructively interfere. Mat. Res. Soc. Symp. Proc. Vol. 161. @1990 Materials Research Society
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ATOMIC LAYER EPITAXY ALE was developed at the Lohja Corporation in Finland for producing large-area thin film electroluminescent displays [1]. It is currently being used to produce quantum well devices [2]. The ability of ALE to produce high quality thin films has prompted a great deal of research on ALE in the electronics industry. We are the first research group to apply ALE to the production of MXOs. ALE is a thin film deposition technique which deposits an atomic layer of material during each cycle of the deposition process. The thickness of the deposited film depends only on the total number of cycles and is independent of other process parameters so that the thickness of a
film deposited by ALE can be controlled to within one atomic layer [3-5]. ALE is possible due to the difference in bond energies of chemisorbed atoms and physisorbed atoms. Chemisorption vs Physisorption A chemisorbed atom shares electrons with atoms of the previous layer to form strong chemical bonds. A physisorbed atom is attracted to atoms of the previous layer by weak dipole-dipole interactions or Van der Waals forces. Covalent bonds are typically ten times stronger than Van der Waals forces, as depicted in Figure 3.
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R (A)
Figure 3. Interaction energy as a function of internuclear separation. a. Chemisorbed atoms form a strong chemical bond with atoms of the previous layer.
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