Optical Investigations of Symmetry Breaking in GaInP 2
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OPTICAL
INVESTIGATIONS
OF SYMMETRY BREAKING IN
GaInP 2
A. MASCARENHAS, SARAH KURTZ, A. KIBBLER, and J.M. OLSON Solar Energy Research Institute, 1617 Cole Boulevard, Golden,
CO 80401
ABSTRACT We present experimental evidence for the spontaneous breaking of cubic symmetry in the band structure of films of Ga0 5 2 1n 0 . 4 8 P grown by organomeltallic vapor phase epitaxy on (100) GaAs substrates. We show how this effect is related to the spontaneous ordering of the alloy, and its correlation with the anomalous lowering of the band-gap observed in these films. INTRODUCTION Spontaneous long-range ordering into the CuAuI, CuPt and chalcopyrite structures has recently been observed in several normally disordered isovalent III-V alloys AxBlxC.[1] In the case of the alloy CaInP 2 grown by organo-metallic vapor phase epitaxy (OHVPE) on (001) GaAs substrates electron diffraction studies reveal ordering of the cations on the group-III sublattice along (1111 or (111], two of the four -type directions (see Fig. 1) (2-5). A mechanism for this spontaneous long-range ordering has been proposed [91. This mechanism consists of : (1) The alignment of Ca and In atoms into a series of alternate (110J direction Ga atom lines within each (001) plane, which is caused by the anisotropic site occupation affinity for column-III atoms because of their large bond-length difference, and the asymmetry in the direction for the dangling bonds. (2) The in-phase alignment of two Ca lines belonging to adjacent (001) ordered planes, which is caused by the selective settling of Ca atoms on the (11)B micro-facets of [110] step arrays. Thus, although bulk CamnP 2 is metastable [101 %Jith respect to phase separation, the long-range ordering observed during epitaxial growth is a result of surface thermodynamic effects rather than bulk thermodynamic effects 111). It has been observed that even at a fixed composition, the band gap of OMVPE grown CaInP varies with the reactor growth temperature, reaching a minimum at about 670°C 16-81. Excellent qualitative correlation between the degree of sublattice ordering and the lowering of the band gap in GamnP 2 has been reported [2]. However, there has been no conclusive evidence that the band gap lowering is a result of spontaneous ordering. In the ordered alloy the size of the Brillouin zone is halved due to the doubling of the unit cell. As a result of this, the conduction band state at the L point is zone-folded back to the r point. This new state has the same symmetry as the conduction band at rF and lies just above it. The repulsion between these like-symnetry states results in a lowering of the band-gap of the ordered alloy [1). In addition, the reduced symmetry should result in a crystal field splitting of the valence band, but this has not been previously observed. RESULTS We now present experimental evidence for the spontaneous breakdown of cubic symmetry in the band structure of spontaneously ordered films of Gan3 2 Ino 4 8 P grown by OMVPE on (001) GaAs substrates, misoriented 2° toward (0il). 6ur re
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