Quantum-confined Stark effect (QCSE) of vertically-stacked quantum dots (VSQDs) is investigated by modulated electroabsorption (EA) spectroscopy. Clear EA spectra are observed by a careful experimental setup with high signal-to-noise (S/N) ratio.
The QCSE of single-stack quantum dots (QDs) is reported to be negligible since the height of the QDs along the vertical electric field is quite small. In this study, enhanced QCSE of vertically-stacked QDs (VSQDs) is proposed and experimentally demonstrated. In the VSQDs, the QDs are naturally aligned in a vertical column and individual electronic wave functions of the QDs are coupled with each other. Then, the effective length of quantum confinement along the field direction is much increased and the Stark shift is expected to be greatly enhanced.
To survey basic properties of QDs, photoluminescence (PL) is inspected at various temperatures. Among the QD wafers, KRISS wafer exhibits well-resolved ground- and excited-state PL peaks, which survive up to room temperature. Whereas, the PLs of HY and SS wafers do not show distinct excited states and the PL peaks continuously shift to shorter wavelengths to become very broad as the pumping is increased. Even for QDs with apparently similar growth conditions, the PL characteristics such as the peak position, the width, and luminescence efficiency, are very different for each wafer.
To examine the QCSE, the modulated EA spectra of HY and SS wafers are measured at 80 K and at 295 K. To vividly observe the Stark shift, modulation technique with the lock-in detection is indispensable, and the proper choice of the optical detector is crucial. The S/N ratio of the realized experimental setup is better than $1×10^{5}$, which is sufficient for this study. The observed EA spectra of HY10 and HY6 wafers are almost identical except in their amplitudes, and reveal two redshifting absorption peaks.
By numerical fitting it is discovered that all the measured PL curves can be decomposed into several Gaussian peaks of same widths, which means that the peaks are to be attributed to the ground, first-excited, and second-excited QD states. With the fitting result, the two absorption peaks in the EA spectra can be successfully matched to the decomposed PL peaks. Interestingly, it is found that the absorption peaks in the EA spectra correspond to the QD,s second-excited and to the wetting-layer states. No signals are detected for other states. The Stark shifts of the second-excited states of HY6 and of HY10 wafers are measured to be -5.9 meV and - 9.5 meV with the field change of $-5×10^4$ V/cm and $-1×10^5$ V/cm, respectively, which is about 6 times larger than that of single-stack QDs with similar dimensions.
