VALENCE-BAND PROPERTIES OF RELAXED GE1-XCX ALLOYS

We report hole effective mass calculations of Ge1-xCx alloys. A 16 x 1 6 Hamiltonian matrix constructed from the linear combination of atomic orbitals with spin-orbit interaction terms is used for the calculatio ns. The properties of alloys are calculated under the virtual crystal approximation. The 1 meV constant energy surface below the valence ban d edge is used to determine the nominal hole effective masses. Calcula tions are carried out by taking the diamond split-off energy Delta Es- o(C) as 0 and 6 meV, respectively. In both cases, the light hole band results of Ge1-xCx alloys agree to within less than 1%. The effective masses of light hole increase monotonically from 0.078 m(0) (for pure Ge) to 0.19 m(0) (for pure C) while the non-parabolicity drops rather monotonically. The heavy hole effective masses of the alloys show a hi ghly non-linear dependence on the carbon content (x). The results in b oth cases are indistinguishable from x=0.0 to about x=0.9; it decrease s slightly from x=0.0 to x=0.5 and increases slowly from x=0.5 to x=0. 9. The values increase for x > 0.9. With Delta Es-o(C)=0 meV, there is an abrupt increase by a factor of two from x=0.97 to x=1.0 to a value of 0.89 m(0). For Delta Es-o(C)=6 meV, a similar trend is found with a lower value of 0.45 nz, at x=1.0. The non-parabolicity of the heavy hole masses increases monotonically from x=0.0 to x=0.99, and nearly d isappears for pure diamond for Delta Es-o(C)=0 meV, while a monotonic increase of non-parabolicity is found for Delta Es-o(C)=6 meV from pur e Ge to pure C. The interaction between the split-off hole band and th e heavy hole band is proposed for the anomalous behavior of the heavy hole effective masses of GeC alloys. (C) 1998 Elsevier Science S.A.