Limitations of the direct-indirect transition on In1− x Ga x P1− z As z heterojunctions

Because In1−xGaxP1−zAsz is the highest-energy direct-gap alloy employed in laser diodes, we attempt to establish where the direct-indirect transition (xc+0.516yc=1.235, 77 °K) hinders or possibly even limits the use of this sytem in large-barrier (ΔE∼250 meV) double-heterojunction (DH) structures. The behavior at 300 and 77 °K of the I-V characteristics of single-heterojunction and homojunction structures employing high Ga composition (x?0.72, z≲0.01) wide-gap In1−xGaxP1−zAsz layers, lattice matched on GaAs1−yPy substrates, show that the nature of the direct-indirect transition of the ternary boundary In1−xGaxP (x∼xc, y=1 or z=0) acts as a limiting factor on the DH diode and laser performance of the quaternary system. As in GaAs1−yPy, donor states associated with subsidiary minima (X) cause a premature onset of the direct-indirect transition, and limit the growth of lattice-matched large-barrier In1−xGaxP1−zAsz (ΔE∼250 meV) DH lasers, with all layers direct, to substrates of composition y≲0.40.