Optical Imaging of Electrical Carrier Injection into Individual InAs Quantum Dots

We image the micro-electroluminescence (EL) spectra of self-assembled InAs quantum dots (QDs) embedded in the intrinsic region of a GaAs $p\mathrm{\text{−}}i\mathrm{\text{−}}n$ diode and demonstrate optical detection of carrier injection into a single QD. Tunneling of electrons and holes into the QDs at bias voltages below the flat-band condition leads to a spectrum of sharp EL lines from a small number of bright spots on the diode surface, characteristic of emission from individual QDs. We explain this behavior in terms of Coulomb interaction effects and the selective excitation of a small number of QDs within the ensemble due to preferential tunneling paths for carriers.

Figure 1

(a) EL spectra of the diode for a series of bias voltages $U$. Inset: schematic diagram of the resonant injection of electrons and holes into a QD. (b) Color scale image of the normalized EL intensity as a function of photon energy and bias. The dashed lines represent the conditions $eU=h\nu$, $eU=h\nu +100\mathrm{meV}$ and $eU=h\nu +160\mathrm{meV}$. (c) and (d) $\mu \mathrm{EL}$ spectra showing the fragmentation of the continuous EL spectrum into a sharp emission line.

Figure 2

Maps of the maximum surface $\mu \mathrm{EL}$ intensity for a range of applied biases. For a given bias the EL spectrum is recorded for a grid of positions. The images show the maximum intensity of each spectrum as a function of the position on the diode. The number at the bottom right of each image denotes the factor by which the EL intensity is scaled compared to the first image.

Figure 3

(a) Overlay image of several $\mu \mathrm{EL}$ intensity maps at different photon energies, given by arrows of the same color in (b). The symbols P1-P7 mark the position of the maximum EL intensity at this energy. Features with intensities below the threshold of 10% of the maximum in a scan are omitted for clarity. (b) EL spectra recorded for $U=1.372\mathrm{V}$ at the positions P1-P7 given in (a). The EL spectra are offset for clarity.

Figure 4

(a) Peak intensity of the L1, L4, and L5 lines as a function of bias. The arrows show the energy separation of two resonances from the energy of the L1 line. (b) Energy shift of several EL lines as a function of bias. P5, P6, and P7 correspond to the positions on the mesa given in Fig. 3a.