OBSERVATION OF QUASI-DIRECT TRANSITIONS IN IN1-XGAXP/GRADED GAP(0.58-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.75) ALLOYS NEAR THE GAMMA-X(1) CROSSOVER

We used room-temperature ellipsometry to study the quasidirect (no-phonon) transitions of disordered In1-xGaxP/graded GaP near crossover compositions (0.64≤x≤0.75) for the Γ1c and X1c conduction-band minima. Careful study of sample microstructures by transmission electron microscopy revealed no ordering. We identify the observed transitions at (Γ15v-X1c), (Γ15v-L1c), and (Γ15v-X3c). The enhanced amplitude of the quasidirect transitions is attributed to band mixing between Γ and L or X points in k space. This observation is corroborated by low-temperature photoluminescence and time-decay experiments. A two-level anticrossing behavior of the Γ1c and X1c gaps is observed in photoluminescence with Γ-X mixing energy VΓ-X12 meV induced by random-alloy disorder. © 1995 The American Physical Society.