X-Ray Interference Measurements of Ultrathin Semiconductor Layers
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X-RAY INTERFERENCE MEASUREMENTS OF ULTRATHIN SEMICONDUCTOR LAYERS C.R. Wie State University of New York at Buffalo, Dept. of Electrical and Computer Engineering, Bonner Hall, Amherst, New York 14260
ABSTRACT We present various x-ray diffraction phenomena from semiconductor hetero-epitaxial layers. Each of these phenomena gives useful information on the layers. Knowing what to look for in the x-ray rocking curve (XRC) can make this nondestructive technique a very powerful tool for characterization of a few A-several g.tm thick layers We discuss the use of individual Bragg peak, diffraction fringe, and interference structure to obtain layer information. We particularly emphasize the use of x-ray interference in studying buried strained quantum well or quantum barrier layers. We present experimental rocking curves of an AlGaAs/GaAs double heterojunction laser structure and GaInAs/GaAs strained layer superlattices in both and orientations.
INTRODUCTION X-ray rocking curve (XRC) analysis is commonly used to characterize semiconductor hetero-epitaxial layers and superlattices",2 . Information can be readily and nondestructively obtained on layer thickness, lattice mismatch, layer composition, and interface roughness. Even though XRC is a widely used technique for semiconductor heterostructures, its complete capability, especially for samples with ultrathin layers, is not yet fully recognized. There are several ways of using XRC to obtain layer parameters: individual Bragg peak position, diffraction fringe peaks and their modulation, interference fringe structure, and superlattice as a diffraction grating. The objective of this paper is to review various diffraction phenomena from hetero-epitaxial samples and show that the interference structure in XRC is an excellent tool for characterizing ultrathin layers (quantum well or quantum barrier) and the active layer in a double heterojunction laser structure.
XRC METHODS 1. Bragg Peak Position For layers with thickness greater than about 0.1 tm, measurement of the position of layer peak, relative to the substrate peak, gives the most simple way of measuring the lattice mismatch and layer composition 3 . This method, Mat. Res. Soc. Symp. Proc. Vol. 145. @1989 Materials Research Society
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however, is limited to a certain minimum layer-thickness either by the peak diffraction intensity which should be above the background, or by the peak-shifting effect. This peak-shifting effect is described in a paper in the present proceedings 4 . The minimum intensity requirement limits this method to layers of a few 100 A. Limitation due to the peak-shifting effect 6 is important for the thin layer which is at a small lattice mismatch with the subtrate or with a thick layer. For a single hetero-epitaxial layer on a substrate, 5 a 5 % error occurs in peak position due to this effect at the layer thickness of h = 2.3 (1-v) as/[(I+ v) (af - a,)] A where v is the Poisson's ratio for the layer, as is the substrate lattice constant, and af is the layer lattice constant. Above this thickne
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