Efficient dipole-dipole coupling of Mott-Wannier and Frenkel excitons in (Ga,In)N quantum well/polyfluorene semiconductor heterostructures

We investigate interactions between Mott-Wannier (MW) and Frenkel excitons in a family of hybrid structures consisting of thin organic (polyfluorene) films placed in close proximity (systematically adjusted by GaN cap layer thickness) to single inorganic $[(\mathrm{Ga},\mathrm{In})\mathrm{N}∕\mathrm{Ga}\mathrm{N}]$ quantum wells (QWs). Characterization of the QW structures using Rutherford backscattering spectrometry and atomic force microscopy allows direct measurement of the thickness and the morphology of the GaN cap layers. Time-resolved photoluminescence experiments in the $8–75\mathrm{K}$ temperature range confirm our earlier demonstration that nonradiative energy transfer can occur between inorganic and organic semiconductors. We assign the transfer mechanism to resonant Förster (dipole-dipole) coupling between MW exciton energy donors and Frenkel exciton energy acceptors and at $15\mathrm{K}$ we find transfer efficiencies of up to 43%. The dependence of the energy transfer rate on the distance $R$ between the inorganic QW donor dipole and organic film acceptor dipole indicates that a plane-plane interaction, characterized by a $1∕R^2$ variation, best describes the situation found in our structures.

Figure 1

Measured and simulated RBS spectra from samples A (lower panel) and B (top panel) at near-normal and grazing incidence. The vertical lines through the maxima of the indium scattering peaks emphasize the shift in peak position as the angle of incidence changes.

Figure 2

Height-scaled AFM image (a) and deflection-scaled AFM image (b) of as-grown sample C, illustrating the morphology of the GaN cap. The full tone range corresponds to a height difference of $3\mathrm{nm}$.

Figure 3

Low temperature PL spectra obtained from all three QW samples with nominally equal thickness and composition. Note that the integrated intensities are similar.

Figure 4

Low temperature PL spectra obtained from the hybrid structures. With decreasing cap thickness the QW emission decreases while the polymer emission increases consistent with nonradiative energy transfer.

Figure 5

Comparison of the (normalized) PL decay curves detected at the QW peak with (gray points) and without (black points) the F8DP overlayer for (a) sample A, (b) B, and (c) C. The black solid lines correspond to the biexponential fitting curves and the excitation laser pulse. (d) The dependence of the normalized Förster rate $(k_F∕k_{\mathrm{QW}})$ on the measured cap thickness $d$ together with a quadratic fit [using Eq. (4) with $P=2$].

Figure 6

Förster efficiency deduced from the time resolved data of Fig. 4 for sample C (thinnest cap sample) over the $8–75\mathrm{K}$ range. There is a clear upward trend in efficiency with increasing temperature.