Ordering effects on optical transitions in Ga_xIn_1−xP/(Al_0.66Ga_0.34)_yIn_1−yP quantum wells studied by photoluminescence and reflectivity spectroscopy

Optical transitions are investigated systematically by excitation-density dependent photoluminescence (PL), magneto-PL, and reflectivity spectroscopies in ordered Ga_xIn_1−xP/(Al_0.66Ga_0.34)_yIn_1−yP quantum-well (QW) structures with different strain, substrate orientation, and capping layer. Blueshift and narrowing of the PL peak are observed as the excitation density increases in the range of ≲40 W/cm² and approach nearly constant values as the excitation gets higher. They are affected by strain, substrate and doping of the capping layer. A larger blueshift corresponds to a larger stable line width of the PL peak measured at a high excitation level. In the magneto-PL measurements, the excitonic feature of the PL transition is clearly identified. The Stokes shift of the first peak in the reflectivity spectrum relative to the corresponding PL peak is also affected by strain, substrate misorientation, and doping of the capping layer. With the observations, (i) the assumptions are checked to be unreasonable of the spatially type-II band alignment and the band-tail states in the case of QW’s. (ii) The existence of domain distribution and order-induced piezoelectric field is verified and the scattering of the domain distribution (Δη) and the magnitude of the piezoelectric field (epe,⊥) are estimated. (iii) The Δη is obviously larger in the QW’s than that determined previously in GaInP2 bulk alloy. It takes a large value for the sample with (001) GaAs substrate and a small value for that with 6° off toward [111]B misaligned (001) GaAs substrate. (iv) The e_pe,⊥ is significantly smaller in the QW’s than the theoretical prediction for the GaInP₂ bulk alloy. It is relatively high for the sample with the (001) GaAs substrate, and is obviously enhanced by the Zn-doped GaP capping layer. The results clarify that the optical transitions can be well interpreted with the combination of the Δη and e_pe,⊥ in the QW’s. A model is hence established in understanding the optical phenomena based on the concepts of the domain distribution and the piezoelectric field.