Charge control in InP/(Ga,In)P single quantum dots embedded in Schottky diodes

We demonstrate control by applied electric field of the charge states in single self-assembled InP quantum dots placed in GaInP Schottky structures grown by metalorganic vapor phase epitaxy. This has been enabled by growth optimization leading to suppression of formation of large dots uncontrollably accumulating charge. Using bias- and polarization-dependent micro-photoluminescence, we identify the exciton multiparticle states and carry out a systematic study of the neutral exciton state dipole moment and polarizability. This analysis allows for the characterization of the exciton wave-function properties at the single-dot level for this type of quantum dot. Photocurrent measurements allow further characterization of exciton properties by electrical means, opening new possibilities for resonant excitation studies for such systems.

Figure 1

$\mu$PL spectra of $\mathrm{InP}/\mathrm{GaInP}$ QD ensembles measured at temperature 10 K with a HeNe laser ($E_{\mathrm{HeNe}}=1.96$ eV). Spectra for samples with $d_{\mathrm{InP}}$ varying from $1.65$ Å to $4.4$ Å are shown for laser excitation powers: (a) $P=5\mu \mathrm{W}$ and (b) $P=0.15\mu \mathrm{W}$. (c) Spectrum of a QD ensemble grown with $d_{\mathrm{InP}}=11$ Å measured at $P=15\mu \mathrm{W}$.

Figure 2

(a) Single QD $\mu$PL measured in sample B as a function of the bias applied between the $n$ and Schottky contacts. The inset illustrates Schottky-diode layer structure of samples A and B under the reverse-bias condition. (b) Linear-polarization-resolved PL measured at $-2.55$ V for the $X_0$ and $X^{-1}$ lines shown on (a). The black and red lines represent polarization parallel to the [110] and $\left[1\overline{1}0\right]$ crystallographic directions, respectively. (c) Negatively charged exciton ($X^{-1}$) binding energy for samples A (black dots) and B (empty squares) against $E_0$, the energy of the neutral exciton at zero field. (d) Distribution of the $X^{-1}$ binding energy. (e) Neutral-exciton energy $E_{X0}$ as a function of the applied electric field $F_z$. Inset shows the values of $p$ and $\beta$ obtained from the fit to the data (solid curve).

Figure 3

(a) Permanent dipole moment $p$ and (b) polarizability $\beta$ plotted against the neutral-exciton energy $E_0$ for samples A (dots) and B (empty squares). In (a), the inset shows the distribution of the electron-hole separation $r$. In (b), the inset shows the extent of the hole wave function along the z direction ($L_{h,z}$) for sample A. (c) Permanent dipole moment $p$ against polarizability $\beta$ for samples A and B. Solid lines are linear fits to the data.

Figure 4

Photocurrent (PC) of a neutral exciton state $X_0$ measured in a single InP QD. The central panel shows a 3D plot with PC spectra. Each PC spectrum is measured at a fixed excitation-laser energy $E_{LD}$ by tuning the $X_0$ exciton energy using bias (see text for details). The top panel shows a 2D plot with amplitudes of the PC signal measured for different laser excitation energies $E_{LD}$, where the parabolic Stark shift is clearly observed. Arrows connect the parts of the parabola in the 2D plot and the corresponding PC peaks observed in the 3D plot.