Semiconductor nanowires for ultimate magnetic objects

Magnetic semiconductor quantum dots are very promising building blocks for the development of future nanoscale spintronics devices. The coupling of confined carriers with a few localized magnetic ions results in the formation of a local ferromagnetic order, called magnetic polaron, which acts as an ultimate ferromagnetic object whose magnetic moment is controlled by one or a few 0D quantum confined carriers. 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 most flexible structure for studying magnetic polarons is a magnetic quantum dot inserted in a core-shell nanowire: (i) the size and shape and the Mn density are determined by the growth conditions, and (ii) a proper choice of the shell allows tailoring the strain in the dot and achieving electrical doping. Then the challenge is to measure all the characteristics of the quantum dot and the properties of the magnetic polaron on the same single quantum dot.

Fig. 1: Scheme and EDX map of ZnTe nanowires grown by MBE on a ZnTe buffer layer, under Te-rich (a) and stoichiometric (b) growth conditions. Both types of nanowires contain a CdTe quantum dot as revealed by the EDX images.

Although, it is likely that actual devices would be based on GaAs or Ge-Si for their better compatibility with existing technology, II-VI ZnTe based NWs are particularly promising as offering a large range of potentialities for fundamental studies: (i) it guarantees a well-controlled insertion of Mn atoms from the single spin limit up to large concentrations, independently of any electrical doping; (ii) ZnTe can be heavily p type doped which allows us to use holes – the type of carriers that is strongly coupled to magnetic impurities in diluted magnetic semiconductors; (iii) (Cd,Mn)Te magnetic quantum dots can be inserted in a ZnTe nanowire.

Fig.1 shows an example of the extensive work done to control the molecular beam epitaxy (MBE) mechanisms of NW growth [1]. It shows scanning electron microscope (SEM) and X ray energy dispersive spectroscopy (EDX) images of ZnTe nanowires containing an insertion of CdTe, grown under different values of the II/VI flux ratio. Under Te rich growth condition, zinc-blende cone-shaped nanowires (a) are obtained as a result of a large lateral overgrowth when the nanowire length exceeds the adatom sidewall diffusion length. Under stoichiometric growth conditions, wurtzite nanowires (b) are also obtained with a cylinder shape due to the absence of lateral overgrowth.

Fig. 2: (a) Scheme of a ZnTe nanowire containing a (Cd,Mn)Te magnetic quantum dot deposited on a 50 nm thick Si3N4 membrane. The SEM image of the nanowire is superimposed with the cathodoluminescence image. (b) High resolution TEM-EDX image of the same nanowire revealing the QD geometry. (c) Magnetic moment of the dot deduced from magnetooptical spectroscopy.

Fig. 2 shows combined physical studies performed on a ZnTe-(Zn,Mg)Te core-shell nanowire containing a (Cd,Mn)Te quantum dot. Nanowires were transferred individually onto a home-made, 50-nm thick, Si3N4-based membrane using the nano-manipulator of a ZEISS Cross Beam NVision 40. This allows us to perform, on the same nanowire, a wide range of experiments including (a) scanning electron cathodoluminescence, (b) high resolution EDX map, and (c) magneto-optical spectroscopy of the quantum dot [2]. The presence of the dot is first revealed by the localized cathodoluminescence profile (superimposed to the SEM image). As shown by the EDX map, the chosen growth parameters lead to the insertion of a 10 x 10 x 20 nm (Cd,Mn)Te dot elongated along the nanowire axis and to the formation of a (Zn,Mg)Te outer shell. The spin properties have been investigated by studying micro-photoluminescence under magnetic field. The emission line which results from the recombination of electron-hole pairs (excitons) optically injected into the dot, exhibits a spectral shift: it results from the large exchange interaction between the confined carriers and localized Mn spins and it is proportional to the Mn magnetic moment. The Mn magnetic moment at saturation (about 1e4 µB) allows us to determine the Mn content of the dot (about 2% in this dot). This demonstrates for the first time the feasibility of a quantitative study of the polaron formation in magnetic dots in which the Mn content, geometry and strain can be controlled independently. This work is financially supported by the ANR program Magwires and by the Institut Universitaire de France.

[1] “Structure and Morphology in Diffusion-Driven Growth of Nanowires: The Case of ZnTe”, P. Rueda-Fonseca, E. Bellet-Amalric, R. Vigliaturo , M. den Hertog , Y. Genuist , R. André , E. Robin, A. Artioli, P. Stepanov , D. Ferrand , K. Kheng, S. Tatarenko and J. Cibert, Nano Lett. 14, 1877 (2014).

[2] “Optical properties of single ZnTe nanowires grown at low temperature”, A. Artioli, P. Rueda-Fonseca, P. Stepanov, E. Bellet- Amalric, M.D. Hertog, C. Bougerol, Y. Genuist, F. Donatini, R. André, G. Nogues, K. Kheng, S. Tatarenko, D. Ferrand, J. Cibert, Appl. Phys. Lett. 103, 222106 (2013).

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