High-temperature electron-hole liquid and dynamics of Fermi excitons in a ${\mathrm{In}}_{0.65}{\mathrm{Al}}_{0.35}\mathrm{As}∕{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum dot array

State-filling effects of the exciton in a ${\mathrm{In}}_{0.65}{\mathrm{Al}}_{0.35}\mathrm{As}∕{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum dot array are observed by quantum dot array photoluminescence at a sample temperature of $77\mathrm{K}$. The exciton emission at low excitation density is dominated by the radiative recombination of the states in the $s$ shell and at high excitation density the emission mainly results from the radiative recombination of the exciton state in the $p$ shell. The spectral interval between the states in the $s$ and $p$ shells is about $30–40\mathrm{meV}$. The time resolved photoluminescence shows that the decay time of exciton states in the $p$ shell is longer than that of exciton states in the $s$ shell, and the emission intensity of the exciton state in the $p$ shell is superlinearly dependent on excitation density. Furthermore, electron-hole liquid in the quantum dot array is observed at $77\mathrm{K}$, which is a much higher temperature than that in bulk. The emission peak of the recombination of electron-hole liquid has an about $200\mathrm{meV}$ redshift from the exciton fluorescence. Two excitation density-dependent emission peaks at 1.56 and $1.59\mathrm{eV}$ are observed, respectively, which result from quantum confinement effects in QDs. The emission intensity of electron-hole liquid is directly proportional to the cubic of excitation densities and its decay time decreases significantly at the high excitation density.

Figure 1

(a) Atomic force micrograph image of an ${\mathrm{In}}_{0.65}{\mathrm{Al}}_{0.35}\mathrm{As}∕{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum dot array. (b) Cross-sectional transmission electron microscopy image of the active region of an ${\mathrm{In}}_{0.65}{\mathrm{Al}}_{0.35}\mathrm{As}∕{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum-dot array.

Figure 2

Photoluminescence spectra of exciton in an ${\mathrm{In}}_{0.65}{\mathrm{Al}}_{0.35}\mathrm{As}∕{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum dot array for different excitation density at $77\mathrm{K}$. The spectra are normalized and vertically offset for clarity. (a) State-filling spectra on quantum dots. The spectra from the lowest to the uppermost are recorded, respectively, at excitation densities of $P_{\mathrm{ex}}=0.29$, 1.24, 1.92, 2.43, 4.68, 15.2, 36.5, 59.6, 243, 355, 365, 737, 1065, and $1420\mathrm{W}∕{\mathrm{cm}}^2$. (b) A sketch of quantum dot energy levels distributing exciton and multiexcitons. Spin orientations are sketched with arrows. (c) Spectral peak energy of the exciton emission (●) as a function of the logarithm of the excitation density $\left(\ln P_{\mathrm{ex}}\right)$. The dot line is a guide.

Figure 3

The excitation density dependence of the integrated luminescence of exciton (●) and EHL (▴) in an ${\mathrm{In}}_{0.65}{\mathrm{Al}}_{0.35}\mathrm{As}∕{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum dot array. The solid line is guidance of nonlinear dependence.

Figure 4

Decay times of a single exciton state in the $s$ shell (solid line) and in the $p$ shell (dash line), and decay time of multiexciton states in the $p$ shell (dot line) as well. The spectra are normalized for clarity.

Figure 5

(a) Time-resolved photoluminescence spectrum of ${\mathrm{In}}_{0.65}{\mathrm{Al}}_{0.35}\mathrm{As}∕{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum dot array at the excitation density of $1065\mathrm{W}∕{\mathrm{cm}}^2$ and $77\mathrm{K}$. Exciton: the exciton emission band; EHL: electron-hole liquid emission band with the peak energy of $1.59\mathrm{eV}$. (b) The spectra show time-resolved luminescence spectra of EHL at times of 109, 217, 327, 826, $954\mathrm{ps}$ after excitation. The spectra are normalized to the emission intensity of EHL and are offset for clarity. (c) The spectra are illustrated for different excitation densities of (a) 243, (b) 355, (c) 365, (d) 737, (e) 1065, and (f) $1420\mathrm{W}∕{\mathrm{cm}}^2$. The spectra are normalized to the emission intensity of exciton.

Figure 6

Decay times of EHL bands at lower excitation density of $355\mathrm{W}∕{\mathrm{cm}}^2$ (dash line) and at a higher excitation density of $1120\mathrm{W}∕{\mathrm{cm}}^2$ (solid line). The spectra are normalized for clarity.