Electron Microscope contrast mechanisms of doped semiconductor layers

Abstract

With the development of crystal growth techniques such as molecular beam epitaxy, it is now possible to fabricate modulation-doped superlattices consisting of alternating, ultra-thin layers of n- and/or p-type material abruptly separated by undoped material. At sufficiently high dopant concentrations it has been found that these atomically abrupt layers can be readily imaged in the transmission electron microscope. This study is concerned with the visibility of boron and arsenic-doped layers in silicon from which a generalized interpretation of the TEM contrast of doped layers can be obtained.Fig.l is a cross-sectional, two-beam dark field image of a B-doped superlattice consisting of five three-layer sets. Each three-layer set was grown using a different substrate temperature in the range 600 to 800 °C; the boron occupies substitutional positions in the Si lattice at the lower temperatures while at the higher temperatures the boron has precipitated in stable clusters (∽3nm) as observed by direct lattice imaging. The maximum concentration of boron in all layers is ≈ 0.7 at.% which is well above the solid solubility limit at these temperatures. There are several important contrast features to note in Fig. 1. First, in the thicker areas of the foil where anomalous absorption effects are dominant, the B-doped layers appear dark relative to the Si matrix; the same is true in bright field.