Recently, the authors reported defect-free, InGaAs-GaAsP multiquantum well modulators operating at 1015nm, but with an unusual dual-peak spectra that resulted in slight degradation in performance. In the Letter, a similar sample grown under reactor gas overpressure that has normal spectra is reported. The dual-peak spectra are therefore attributed to microscopic domain formation that the gas overpressure inhibits by reducing surface atom mobility. The operation has also been extended to 1.06 μm, the Nd: YAG wavelength.