Bandgap Anomaly, Atomic Ordering, and Their Applications

In 1916 a group of Russian chemists—Kurnakov and his colleagues—discovered that slowly cooled CuxAu1 − xmetal alloys had anomalously low electrical resistivities at simple compositions of CuAu and Cu3Au. Nine years later in 1925, Swedish physicists Johansson and Linde found by x-ray-diffraction experiments that the alloys had ordered structures on the face-centered-cubic lattice, now called CuAu I-type and AuCu3-type. Two years later, Johansson and Linde discovered CuPt-type ordering in Cu0.5Pt0.5 alloy by noticing a similar anomaly in their electrical-resistivity measurements for CuxPt1 − x. These were among the events at the dawn of physical metallurgy and study on order-disorder transformations. Many years later in 1985, the first observation of ordering in semiconductor alloys was made in AlGaAs grown on GaAs(110); the type was CuAu I. CuPt-type ordering was observed in SiGe grown on Si(001). Since the (110) orientation of the substrate for AlGaAs was clumsy for practical purposes and the degree of ordering in SiGe was extremely low, these phenomena seemed to have little relevance in practical applications. Other observations were made such as famatinite-type ordering in GalnAs and chalcopyrite-type ordering in GaAsSb. These observations however were quite rare, with only isolated cases reported.