Effective band gap inhomogeneity and piezoelectric field in InGaN/GaN multiquantum well structures

The emission mechanisms of strained In_xGa_1-xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, F_PZ, and L exceed the valence band discontinuity, ΔE_V. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that F_PZ×L exceeds the conduction band discontinuity ΔE_C, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius a_B. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and F_PZ, produces a confined electron-hole pair whose wave functions are still overlapped (quantized excitons) provided that L<a_B.