Growth of anisotropic Ge quantum dot lattices in alumina matrix

Figure 1. Structural properties of the films. (a), (b) GISAXS maps measured for parallel and perpendicular direction of the probing x-ray beam with respect to the main direction of the adparticle diffusion (the insets show the corresponding simulations), and (c), (d) the corresponding TEM images of the films cross-sections. (e) GISAXS map of a film deposited under standard isotropic conditions. (f) Structure of the anisotropic QD lattice. It is distorted along the y axis with respect to the ideal BCT lattice, which is shown by light symbols. (g) and (h) Simulations of the QD ordering found by GISAXS analysis. [M. Buljan et al. J. Appl. Cryst. 46, 709 (2013)].


Figure 2. Electrical properties of the films. (a) Geometry used for the electrical conductivity measurements (above), and frequency-dependent measurements of the electrical conductivity for the perpendicular (┴) and parallel (||) geometry (below). (b) A three-dimensional model of the QD arrangement and separations. The shortest edge-to-edge distances between QDs (in nm) are indicated. (c) and (d) The most probable path of the charge carriers for the perpendicular and parallel geometries, respectively. Only two characteristic layers (A and B) are shown for clarity; the consideration is valid for the entire multilayer stack. [M. Buljan et al. J. Appl. Cryst. 46, 709 (2013)].

An anisotropic lattice of Ge quantum dots embedded in amorphous alumina was produced by magnetron sputtering deposition. A specific deposition geometry with oblique incidence of Ge and Al2O3 adparticles was used to achieve the anisotropy. The observed quantum dot ordering is explained by a combination of directional diffusion of Ge and Al2O3 adparticles and a shadowing process which occurs during deposition as a result of the specific surface morphology. The prepared material shows a strong anisotropy of the electrical conductivity in different directions parallel to the sample surface.

Simple processes for the preparation of regularly ordered lattices of semiconductor quantum dots (QDs) embedded in dielectric amorphous matrices play an important role in various nanotechnology applications. Of particular interest are QD lattices with properties that differ significantly in different directions parallel to the material surface.

In our past work we have analysed the preparation and ordering properties of isotropic Ge QD lattices in different amorphous matrices produced by magnetron sputtering deposition. The regularity in the QD positions was achieved by a self-assembly process during the films growth, which is based on surface morphology influence on Ge nucleation places. However, our previous depositions were all performed using a standard substrate stage which rotates during the deposition. This ensures the homogeneity of the films, but the regular ordering of QDs appears in domains, randomly rotated around the surface normal.

In our most recent work we examined the ordering in ten period (Ge+Al2O3)/Al2O3 multilayer prepared also by magnetron sputtering deposition but under specific deposition geometry. We used small sputtering targets, a close distance between the substrate and targets, and a fixed substrate stage (held at a temperature of 573 K) during deposition. Under such conditions, the Ge and Al2O3 adparticles coming to the substrate from the sputtering targets have a preferential diffusion direction and a non-vanishing in-plane (tangential) component of their velocities. The growth of the Ge QDs and alumina matrix is not isotropic under such conditions due to the combination of directional diffusion of adparticles and shadowing effects caused by Ge QDs.

The result of such deposition is formation of an anisotropic lattice of Ge QDs embedded in amorphous alumina matrix. The details of the QD ordering properties in these films were investigated at the SAXS beamline of the synchrotron Elettra. Different directions of the probing x-ray beam with respect to the preferential diffusion direction of adparticles were used. Two specific cases, i.e. grazing incidence small angle x-ray scattering (GISAXS) maps taken with the x-ray beam set parallel and perpendicular (|| and ┴) to the diffusion direction are demonstrated in Fig. 1a and Fig. 1b. The different arrangement of Bragg spots in them demonstrates clearly the anisotropy in the material structure. The anisotropy occurs along the diffusion direction of the Al2O3 adparticles. The anisotropy is also nicely visible in the TEM cross-sections of the film (Fig. 1c and Fig. 1d), where the correlation direction of the QD ordering is different for the parallel and perpendicular cross-sections. The GISAXS map of the film deposited under standard, isotropic conditions and using of rotational stage (Fig. 1e) is symmetrical and looks the same for all probing x-ray directions due to the existence of randomly distributed domains. The numerical analysis of the GISAXS maps shows, that the resulting ordering in the system is a distorted body centred tetragonal (BCT) lattice, tilted toward alumina target, which is schematically illustrated in Fig. 1f and Fig. 1h.

The prepared materials show also a strong anisotropy in their electrical properties. The conductivities measured in directions parallel and perpendicular to the main diffusion direction differ by an order of magnitude (Fig. 2a). Such a big difference is explained by the structural properties of the films. More precisely, the conductivity in QD-based materials is highly influenced by the hoping probability between neighbouring QDs, and it decreases exponentially with the QD separation. The QD separations for various directions, found by GISAXS analysis, are shown in Fig. 2b. Due to difference in QD separations along the paths used for conductivity measurements (Fig. 2c and Fig. 2d), the large anisotropy in the electrical transport properties occurs.

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Growth of a three‐dimensional anisotropic lattice of Ge quantum dots in an amorphous alumina matrix;
M. Buljan, O. Roshchupkina, A. Šantić, V. Holý, C. Baehtz, A. Mücklich, L. Horák, V. Valeš, N. Radić, S. Bernstorff and J. Grenzer; J. Appl. Cryst. 46, 709-715 (2013).

Last Updated on Tuesday, 14 May 2019 17:05