Structural and optical properties of lateral composition modulation in (GaP)_n/(InP)_n short-period superlattice

The structural and optical properties of lateral composition modulation (LCM) in (InP)_n/(GaP)_n short-period superlattice grown by molecular beam epitaxy were studied with transmission electron microscopy (TEM) and photoluminescence (PL) at the growth temperature (T_g) of 425 and 490 for n = 1, 1.7, and 2. LCM occurs only in a [1 -1 0] direction at T_g = 490 °C for n = 1 and 2. On the contrary, LCM occurs both in [1 -1 0] and [1 1 0] directions, parallel to [1 0 0] direction, at T_g ≥ 425 °C for n = 1.7. This is due to the stronger induction of LCM in tensile strain (∼ -10% for n = 1.7) than in compressive strain (∼ 6% for n = 1 and 2). The 9 K-PL measurements show that the LCM experiences the reduction of bandgap up to ∼ 345 meV as both n and T_g increase. This is the best data ever reported so far. The origin of bandgap shrinkage is mainly attributed to LCM along with the contribution of CuPt-type ordering.