Self-organized growth of alloy superlattices

Patterning in nature typically occurs through self-organization, and intere st has developed recently in the use of such spontaneous processes to fabri cate periodically structured materials at the nanometre scale. For example, ordered arrays of semiconductor 'quantum dot' particles (superlattices) ha ve been created by deposition from a suspension(1), or by self-organization of diffusing atoms on surfaces(2) or in sequentially grown stacked layers( 3). The spontaneous formation of layered structures in epitaxial growth has also been reported, and attributed to the process of spinodal decompositio n(4,5). Yet highly ordered layered superlattices, developed for application s in optoelectronics (and in future perhaps for thermoelectrics(6)), are cr eated 'by hand' through the sequential deposition of two different material s. Here we show that superlattices can appear spontaneously during crystal growth of an alloy, as appear spontaneously a consequence of the distributi on of strain at surface step sites. When a strained alloy grows by 'step fl ow', the surface steps form periodic bunches(7). We find that the resulting modulated strain field biases the incorporation of the respective alloy co mponents at different steps in the bunch, leading to segregation and superl attice formation. We also present experimental observations (X-ray diffract ion and electron microscopy) of a silicon-germanium alloy grown on silicon, which show clear evidence for the formation of such a self-organized struc ture.