Growth of Ge-rich SixGe1-x single crystal with uniform composition (x=0.02) on a compositionally graded crystal for use as GaAs solar cells

An improved growth technology was developed to grow Ge-rich SiGe crystals on Ge seeds on the basis of the Multi-component zone melting method. The purpose of growing such a crystal is the preparation of exactly lattice-matched GaAs/SiGe heterostructures for higher-efficiency tandem cells than GaAs/Ge cells. In this method, Si solute elements are continuously supplied from the Si source crystal, and the compositional profile in the SiGe crystal is controlled by the pulling rate. We were successful in preparing a SixGe1-x single crystal with a uniform composition (x = 0.02) following a graded composition (from x = 0 to 0.02). The size of the uniform crystal is 2 cm in length and 1.5 cm in diameter. The combined structure of uniform SiGe/graded SiGe/Ge has two expected merits. A lattice-matched GaAs epitaxial layer can be grown on the SiGe uniform crystal, and the graded composition from Ge (x = 0) to uniform SiGe (x = 0.02) widens the range of absorbed wavelengths of the solar spectrum. (C) 1999 Elsevier Science B.V. All rights reserved.