CONTROL OF ORDERING IN GAINP AND EFFECT ON BANDGAP ENERGY

GaxIn1-xP layers with x = 0.5 have been grown by atmospheric pressure organometallic vapor phase epitaxy on GaAs substrates with 10 micron w ide, [110]-oriented grooves produced photolithographically on the surf ace. The [110] steps and the misorientation produced at the edges of t he 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 maxim um angle to the (001) surface of between 10 and 16-degrees, single dom ains of the (111BAR) and (111BAR) variants of the Cu-Pt ordered struct ure are formed on the two sides of the groove. Shallow (less-than-or-e qual-to 0.25 mum deep) grooves, with maximum angles of <10-degrees, ar e 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 durin g growth, due to the change in shape of the groove during growth. For deep (1.5 mum) grooves, with maximum angles to the (001) plane of 35-d egrees, only a single variant is formed on each side of the groove. Ho wever, the domains are small, dispersed in a disordered matrix. For su bstrates with deep grooves on a GaAs substrate misoriented by 9-degree s toward the [110BAR] direction, an interesting and useful pattern is produced. One half of the groove is a single domain which shrinks in s ize as the growth proceeds. The other half of the groove, where the mi sorientation is larger, is disordered. Thus, every groove contains lar ge (>1 mum2 cross-sectional area and several mm long) regions of highl y ordered and completely disordered material separated by no more than a few microns. This allows a direct determination of the effect of or dering on the bandgap of the material using cathodoluminescence (CL) s pectroscopy. The 10K photoluminescence (PL) consists of three distinct peaks at 1.94, 1.88, and 1.84 eV. High resolution CL images reveal th at the peaks come from different regions of the sample. The high energ y peak comes from the disordered material and the low energy peak come s 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 first direct ver ification that ordering causes a reduction in bandgap of any III/V all oy. Decreasing the Ga0.5In0.5P growth rate from the normal 2.0 to 0.5 mu/h is found to enhance ordering in layers grown on planar GaAs subst rates. 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 pa ttern of fine lines lying in the (001) plane in the transmission elect ron 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.