Electronic, optical, and structural properties of (In,Ga)As/GaP quantum dots

We study in detail self-assembled (In,Ga)As quantum dots grown on GaP substrate from the structural, theoretical, and optical points of view. Single quantum dot morphology is first determined at the atomic level using plane-view scanning tunneling microscopy. Unusual $C_2$ symmetry properties with high-index $\left\{136\right\}$ facets are demonstrated for small quantum dots, whereas the apparent shape of the quantum dots approximately exhibits $C_{2v}$ symmetry, with the appearance of low-index $\left\{111\right\}$ facets when the quantum dot ripens. This is interpreted as a consequence of the competition between strain and surface energy during quantum dot formation. Electronic properties are then simulated using both k·p and tight-binding models. The indium content and geometry of the quantum dots are found to have a strong influence on the transition type (direct-indirect). Finally, temperature-dependent optical properties of quantum dots are analyzed between 10 and 375 K. Photoluminescence and time-resolved photoluminescence studies show a clear proximity of two different types of optical transitions. Supported by the theoretical calculations, these transitions are interpreted as a competition between conduction band states in the $X$ and Γ valleys.

Figure 1

(a) 800×800-nm${}^2$ AFM image of uncapped (In,Ga)As QDs, and (b) 800×800-nm${}^2$ STM image ($U_t$ = −3.27 V; $I_t$ = 0.04 nA) of (In,Ga)As QDs, transferred to the STM chamber with amorphous As capping layer. The QD morphology has not changed during the transfer.

Figure 2

(a) 60×60-nm${}^2$ STM plane view ($U_t$ = −2.87 V; $I_t$ = 0.01 nA) of (In,Ga)As QDs not ripened. A detailed analysis of the image allows the identification of the facets in the inset. The symmetry of the crystal is not conserved in (b). The facet analysis for ripened QDs shows low-index facets appearing with underlying crystal symmetry.

Figure 3

Band alignment and band structure of bulk In${}_x$Ga${}_{1-x}$As biaxially strained on GaP at 0 K.

Figure 4

(a) 75×75-nm${}^2$ STM 3D plane view of (In,Ga)As QDs showing the different size of QDs. (b) Cone-shaped QD geometry for the eight-band k·p calculations.

Figure 5

First electronic levels calculated in the Γ, $X_{\mathrm{XY}}$, and $L$ valleys for an In${}_{0.3}$Ga${}_{0.7}$As/GaP QW as a function of the thickness by the TB model. The first electronic level in the Γ valley calculated by the k·p method in shown by the solid line.

Figure 6

Band alignment and band structure of In${}_x$Ga${}_{1-x}$As QDs for the four geometries defined in Fig. 4b. The first electronic level in the Γ valley and the first hole level are calculated by the k·p method. The first electronic levels in the $X$ and in the $L$ valleys are calculated by the TB method. All calculations are performed at 0 K.

Figure 7

First direct transition energy and first indirect transition energy calculated for QD with (a) geometry C and (b) geometry D at 0 K. The red dash-dotted line shows the direct transition calculated by Fukami et al.[14] for a square pyramid shape with a 4-nm height and a 20-nm width at 300 K.

Figure 8

Temperature-dependent PL spectra of (In,Ga)As/GaP QDs. The red thin lines show the fit of the two transitions by two Gaussian peaks.

Figure 9

Ratio between the integrated intensities related to the two HE and LE transitions as a function of $1/k_{B}T$. The red line shows a linear fit.

Figure 10

Normalized PL spectra as a function of excitation density at 12 K. The arrow represents the increasing of excitation density with the following values: 80, 180, 370, and 800 mW cm${}^{-2}$; and 1.7, 3.4, 7.4, 17.8, 37, and 80 W cm${}^{-2}$. The insert shows the energy of the PL peak as a function of the cubic root of excitation density. The line is a guide to the eye.

Figure 11

Streak camera image of time-resolved PL at 10 K for an excitation density of (a) LP = 70 W cm${}^{-2}$ and (b) HP = 4000 W cm${}^{-2}$. The white horizontal dashed line marks the instant of the laser pulse.

Figure 12

PL dynamics at selected energies ($E_{\mathrm{LE}}$ = 1.76 eV and $E_{\mathrm{HE}}$ = 1.86 eV) for two power densities (LP =70 W cm${}^{-2}$ and HP = 4000 W cm${}^{-2}$). Red lines show biexponential fits.