Chemically sensitive atomic resolution images of semiconductor interfaces have been achieved at ORNL using a new Z-contrast imaging technique and a high resolution scanning transmission electron microscope (STEM). Conventional high resolution electron microscopy is based on the phase contrast method in which diffracted beams transmitted through the sample are recombined onto the microscope screen to produce an image of a crystal lattice. Such images have inherently weak chemical sensitivity and are extremely sensitive to sample thickness, localized strain fields, and precise lens defocus, often making the location of an interface impossible to determine. The new imaging method overcomes these problems by scanning an electron probe of atomic dimensions across the sample, replacing phase contrast with atomic number or Z-contrast by detecting electrons scattered through large angles. The resulting atomic resolution image exhibits a contrast determined primarily by the atomic species with greatly reduced sensitivity to sample thickness, localized strain fields, and lens defocus. Using this technique, we have shown that interfaces in epitaxial Ge on Si grown by oxidation of Ge-implanted Si are atomically sharp (see fig.1), whereas interfaces in sixGe1-x/Si strained layer superlattices grown by MBE are chemically diffuse over two unit cells (~10Å) (fig. 2).
This reasearch provides for the first time an imaging technique for studying the atomic structure of interfaces in the structurally similar semiconductors. Such systems are currently of great technological and research interest and include strained layer superlattices which can provide unique optical and electrical properties. Interface sharpness and smoothness are critical to these properties and can now be studies directly. The Z-contrast technique is ideally suited to studying heterostructures such as the "artificial crystals" SimGen made by alternating m layers of pure Si with n layers of Ge. Z-contrast STEM can also be used to image dopant distributions in Si at atomic resolution, including "δ-doped" materials consisting of very thin layers of high dopant concentration and dopant segregation at defects and interfaces such as at an advancing solid phase epitaxial growth front. The ability to study these effects at atomic resolution represents a significant advance in high resolution electron microscopy.