Control of ordering in GaInP and effect on bandgap energy

Ga_xIn_1-x P layers with x ≈ 0.5 have been grown by atmospheric pressure organometallic vapor phase epitaxy on GaAs substrates with 10 micron wide, 110]-oriented grooves produced photolithographically on the surface. The 110] steps and the misorientation produced at the edges of the grooves have been found to have important effects on the formation of the Cu-Pt ordered structure (ordering on {111} planes) in the GaInP layers during growth. In this work, the groove shape is demonstrated to be critically important. For the optimum groove shape, with a maximum angle to the (001) surface of between 10 and 16°, single domains of the (-111) and (1-11) variants of the Cu-Pt ordered structure are formed on the two sides of the groove. Shallow (≤0.25 μm deep) grooves, with maximum angles of <10°, are less effective. Within the large domains on each side of the groove, small domains of the other variant are observed. The boundary between the two domains is seen to wander laterally by a micron or more during growth, due to the change in shape of the groove during growth. For deep (1.5 μm) grooves, with maximum angles to the (001) plane of 35°, only a single variant is formed on each side of the groove. However, the domains are small, dispersed in a disordered matrix. For substrates with deep grooves on a GaAs substrate misoriented by 9° toward the -110] direction, an interesting and useful pattern is produced. One half of the groove is a single domain which shrinks in size as the growth proceeds. The other half of the groove, where the misorientation is larger, is disordered. Thus, every groove contains large (>1 μm² cross-sectional area and several mm long) regions of highly ordered and completely disordered material separated by no more than a few microns. This allows a direct determination of the effect of ordering on the bandgap of the material using cathodoluminescence (CL) spectroscopy. The 10K photoluminescence (PL) consists of three distinct peaks at 1.94, 1.88, and 1.84 eV. High resolution CL images reveal that the peaks come from different regions of the sample. The high energy peak comes from the disordered material and the low energy peak comes from the large ordered domains. Electron microprobe measurements of the solid composition demonstrate that the shift in emission energy is not due to changes in solid composition. This is the firstdirect verification that ordering causes a reduction in bandgap of any III/V alloy. Decreasing the Ga_0.5In_0.5P growth rate from the normal 2.0 to 0.5 μ/h is found to enhance ordering in layers grown on planar GaAs substrates. Transmission electron diffraction results show that the domain size also increases significantly. For material grown on exactly (001)-oriented substrates, a pronounced 001] streaking of the superlattice spots is observed. This is correlated with the presence of a dense pattern of fine lines lying in the (001) plane in the transmission electron micrographs. The PL of this highly ordered material consists of a single peak that shifts to higher energy by > 110 meV as the excitation intensity is increased by several orders of magnitude.