Surfactant Effects on Ordering in GaInP Grown by OMVPE
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because it promises a new degree of freedom in the "design" of semiconductor alloys for specific applications. The properties of a semiconductor alloy, notably the bandgap energy, are found to be determined not only by the alloy composition, but also by the atomic arrangement of the constituent atoms in the solid. For Ga 0 .521n0.48P lattice matched to GaAs grown by liquid phase epitaxy (LPE) the placement of the Ga and In atoms on the group III sublattice is typically random, so the low temperature bandgap energy is fixed at approximately 2 eV. For alloys grown from the vapor, the arrangement of the Ga and In atoms is typically not random. A spontaneous ordering process occurs at the surface during growth, most often producing a modulation of the Ga/In ratio with a periodicity of twice the inter planar spacing along a direction in the lattice resulting in materials having the CuPt ordered structure[l]. The bandgap energy of partially CuPt ordered material obtained spontaneously during organometallic vapor phase epitaxial (OMVPE) growth has been shown to be lower than for disordered material by up to 160 meV[2,3]. 201 Mat. Res. Soc. Symp. Proc. Vol. 583 © 2000 Materials Research Society
Ordering is also of considerable technological importance, mainly due to the reduction in bandgap energy in ordered alloys. It has already proven to be important for GaInP solar cells[4], light emitting diodes[5], and visible lasers[6]. All of these devices require the control of CuPt ordering during OMVPE growth to control the bandgap energy. The ability to control the order parameter by variations in the system parameters during growth presents the opportunity to produce heterostructures that are useful for photon and carrier confinement and for quantum wells, with no change in solid composition at the interface[7, 8]. Ordering is also potentially useful as a means of reducing the energy band gaps of Sb-containing alloys for use in infrared photonic devices[9]. Equally important is the fundamental significance of the ordering, since studies show that ordering is intimately connected with the surface structure. Thus, it presents a useful "window" through which information about the surface processes occurring during growth can be obtained. This is because the thermodynamic driving force for CuPt ordering is due to the (2x4)-like reconstruction of the (001) surface[1,3,10]. The [110] rows of [110 ] P dimers on the surface drive the CuPt ordering. Optical techniques, such as surface photo absorption (SPA) can be used to measure the concentration of these P dimers and, indirectly, the surface reconstruction during growth[3, 11 ]. Using this technique the relationship between the degree of order and the concentration of the [I10 ] P dimers, characteristic of the (2x4) reconstructed surface, was demonstrated as the growth process was changed by variations in the substrate temperature, the flow rate of the group V precursor, the growth rate, and the substrate misorientation from (001)[3]. Perhaps the most vivid demonstration of the rel
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