The interest in semiconductor is due to the many opportunities offered by electrical doping and nano-structuration. Charge carriers (electrons and holes) can be introduced and manipulated by electric fields, using flexible configurations which combine bias voltage applied on gates and built-in interfaces. Here, the faster is the better. In semiconductors with a direct bandgap (such as III-Vs and II-VIs), optics further broadens the range of experimental tools and gives access to the spin states of the carriers.
By contrast, if magnetic systems have been widely used for information storage, this is because ferromagnetic domains are long lived. If the size is scaled down to the nanometer range, the anisotropy plays a crucial role in stabilizing the magnetization along well defined directions.
Can we introduce magnetic properties into semiconductor nanostructures ? One way is to dope the semiconductor with magnetic impurities : this is the concept of diluted magnetic semiconductors. In this case, a strong interaction exists between the spin of the impurity, and the carriers of the semiconductor. This gives rise to giant magneto-optical and magneto-electric effect.
Two extreme cases can be contemplated :
a single Mn impurity can be embedded in a quantum dot, and addressed optically through the carriers of the quantum dot.
ferromagnetism can be induced by the interaction between an ensemble of magnetic impurities and a hole gas. (Ga,Mn)As is the most studied system, and spintronics devices are designed and studied in several laboratories worldwide.
Our approach is to use the specific properties of II-VI semiconductors (easy insertion of Mn which is an isoelectronic impurity, easy access to optical spectroscopy, all properties of semiconductors present) to design and grow nanostructures which behave as model systems, and obtain new functions in the frame of spintronics or quantum manipulation.
Some of our present objectives are related to one question : can we use the confinement of carriers ? We illustrate this with two examples :
The single Mn atom in a quantum dot : speaking to a single spin with the language of nanoelectronics.
With a single Mn atom introduced in a II-VI quantum dot (Fig. 1a), the energy and polarization of the photon emitted or absorbed by the dot depends on the spin state of the S=5/2 magnetic atom : the exchange interaction between the confined electron-hole pair and the Mn spin splits the 2S+1=6 spin states of the Mn atom, leading to a 6-line optical spectrum for the quantum dot (Fig. 1b, top left). Laser excitation resonant with one of these optical transitions can be used to initialize the Mn spin and to probe its dynamics optically : the Mn atom behaves like an optically addressable long-lived spin-based memory. To go further, information processing using individual spins requires fast coherent control of a single spin and also tuning of the coupling between two spins. Preliminary steps are evidenced by the anticrossing appearing in the spectra of a quantum dot containing two Mn atoms (Fig. 1b, right and bottom) when the excitation intensity is increased.
An ensemble of magnetic impurities in a quantum dot : wavefunction and strain engineering of carrier-induced ferromagnetism.
At 3D (thick layers of (Ga,Mn)As and p-type (Zn,Mn)Te) and 2D ((Cd,Mn)Te quantum wells), the interaction between an ensemble of localized Mn spins and a gas of holes gives rise to ferromagnetism. The interest is twofold : the ferromagnetic character creates strong magneto-transport and magneto-optical properties, and ferromagnetic properties can be controlled by applying an electric field (Fig. 2a). One goal is now to move to 1D (nanowires) and 0D (quantum dots) in order to understand and optimize the role of confinement (can we increase the stability of the magnetic polaron by engineering the wavefunction of the carriers) and the anisotropy (can we enhanced the stability against reorientation by enhancing the anisotropy through strain engineering), as well as the coupling to the environment (surrounding spins and carriers, see Fig. 2b,c) and current-induced effects when inserting the quantum dot in a nanowire such as that in Fig. 2d.
Our research aims to fabricate, study, and manipulate different forms of magnetic polarons embedded in semiconductors nanowires. One challenge is to make a link between the quantum limit (single magnetic impurity, single carrier), and ferromagnetic-like system involving an ensemble of magnetic impurities and several carriers confined in a quantum dot, or a one dimensional hole gas. It may open also new routes for semiconductor structures embedding ferromagnetic elements, in the search for higher ordering temperatures by wavefunction engineering, for controlled anisotropy by strain engineering, and for strong magneto-electric effects.
The continuing decrease of the size of the structures used in semiconductor electronics and in magnetic information-storage devices has dramatically reduced the number of atoms necessary to process and store one bit of information : An individual magnetic atom would represent the ultimate size limit for storing and processing information. Towards this goal, we have demonstrated that an individual manganese atom embedded in a semiconductor quantum dot may act as a spin-based memory. Further, a pair of Mn atoms can act as a prototype of a pair of coupled memory units. We can exploit the optical absorption and emission of the quantum dot in order to initialize and to read out the spin state of the magnetic atoms. Under resonant optical excitation, we can enter the "strong coupling" regime where hybrid states of matter and the electromagnetic field are created, and this could be used for a coherent, optical “manipulation" of the Mn spin.
People : L. Besombes, H. Boukari, T. Clement, D. Ferrand, H. Mariette Former members : Y. Leger, L. Maingault, J. Bernos Contact : lucien.besombes grenoble.cnrs.fr Overview Our group has active research activities in optical and magnetic interactions in semiconductor quantum structures... > suite
People D. Ferrand, R. Giraud, D. Halley, H. Mariette, S. Marcet, W. Pacuski, E. Sarigiannidou, A. Titov Overview and results In II-VI and III-V diluted magnetic semiconductors (DMS), the spin carrier coupling between delocalized holes and localized spins is particularly efficient to induce... > suite