Optical Spectroscopy on Non-Magnetic and Semimagnetic Single Quantum Dots in External Fields

Abstract

We demonstrate the ability to control the eigenstates in single quantum dots by applying welldefined external fields. Electric fields oriented in-plane as well as perpendicular to the disc-shaped dots allow a modification of the spatial part of the excitonic wavefunction, giving access to the charge distribution in the dot. In contrast, magnetic fields modify the spin part of the wavefunction, resulting in a Zeeman splitting and a diamagnetic shift of the photoluminescence emission. We used the unique property of semimagnetic quantum dots to tailor the effective g-factor, i.e. the sensitivity of the eigenstates to external magnetic fields, by about two orders of magnitude simply by varying the Mn concentration in the dots.Introduction Semiconductor single quantum dots (SQDs) are often designated as 'artificial atoms', because their discrete energy level structure very much resembles that of 'real' atoms. In contrast to atoms, however, the eigenstates in SQDs can be modified by varying the extension or the composition of the quantum dots. For that reason, semiconductor SQDs represent a new class of materials with size dependent electronic, optical or even magnetic properties. This has caused a huge interest in such systems both from a basic physics point of view as well as due to fascinating device concepts like single electron devices, single photon sources or quantum computing. In this context, two topics are of strong interest: (i) the ability to control the occupation of SQDs with a discrete number of electrons and/or holes and (ii) the feasibility to manipulate the SQD eigenstates in a well-defined manner by using external electric or magnetic fields.The most convenient technique to vary the SQD occupation with electron-hole pairs is optical excitation. Simply by changing the excitation power, SQDs can be occupied by either one, two or even more electron-hole pairs and indeed, lots of recent publications have been devoted to optical studies on single excitons, biexcitons or multiexcitons in both, III-V [1-5] and II-VI [6-11] SQDs. In order to achieve a SQD occupation with an unequal number of electrons and holes, one may use above-barrier optical

Quantum-confined Stark effect in a single InGaN quantum dot under a lateral electric field

“…This is in line with previous measurements on epitaxial arsenide-based III/V QDs and selenide-based II/VI QDs, which showed redshifts in exciton recombination energies of up to ⌬E = 1.1 meV. [7][8][9] This behavior can be readily understood in terms of the QCSE. The electric field forces the electron and hole to opposite sides of the QD and band tilting leads to a reduction in the band gap and thus to a redshift of the PL.…”

“…The application of the external field produces an energy shift in the exciton recombination energy of up to 5.4 meV, much greater than the shifts reported in arsenide-based III/V QDs ͑up to 0.5 meV͒ and selenide-based II/VI QDs ͑up to 1.1 meV͒. [7][8][9] This shows that the effect of the lateral electric field is not negated by the presence of the strong internal field in a perpendicular direction…”

“…7 This is in contrast to CdSe QDs, for which only a quadratic energy shift with an electric field is observed, the permanent dipole moment contribution being seen to be negligible. 8,9 Nitride-based III/V QD nanomaterials ͑e.g., InGaN and GaN͒ ͑Refs. 10-12͒ possess interesting electronic properties such as built-in electric fields and high exciton binding energies that are not found in the widely studied arsenide-based III/V family of QDs.…”

“…1͑c͒, the intensity of the PL peak drops from a relative value of 1.0 at 0 V to 0.4 at 10 V, in accordance with observations made in other epitaxial QD systems. [7][8][9] The application of the electric field results in a reduction of the oscillator strength because of the decreased electron-hole wave function overlap. An additional process which may also reduce the PL intensity is field-induced carrier tunneling out of the QD.…”

Quantum dot photoluminescence as charge probe for plasma exposed surfaces

“…Similarly, Stark-shifted energy states of an 'artificial atom', i.e. of a quantum dot (QD), can in principle be used for electric field measurements near a surface since their discrete energy level structure is also subject to electric-field-induced shifts and alterations [18].…”