Design of quantum dot lattices in amorphous matrices by ion beam irradiation

We report on the highly controllable self-assembly of semiconductor quantum dots and metallic nanoparticles in a solid amorphous matrix, induced by ion beam irradiation of an amorphous multilayer. We demonstrate experimentally and theoretically a possibility to tune the basic structural properties of the quantum dots in a wide range. Furthermore, the sizes, distances, and arrangement type of the quantum dots follow simple equations dependent on the irradiation and the multilayer properties. We present a Monte Carlo model for the simulation and prediction of the structural properties of the materials formed by this method. The presented results enable engineering and simple production of functional materials or simple devices interesting for applications in nanotechnology.

Figure 1

(a) The initially amorphous multilayer (bilayer thickness $P$) is irradiated by an ion beam incident at angle ${\varphi }_{\mathrm{irr}}$ with respect to the film surface; the red arrows indicate the irradiation direction. The irradiation incites the formation of regularly ordered QDs (white spheres). Structural measurements were performed for parallel ($\left|\right|$) and perpendicular ($\perp$) configurations (indicated by gray arrows), showing the relation of the probing beam and the irradiation direction. (b) Description of the QD lattice formed by ion beam irradiation.

Figure 2

Structure of the irradiated and annealed multilayers of (Ge $+$ SiO${}_2$)/SiO${}_2$. (a)–(c) Examples of STEM cross sections of the same type of multilayers ($P=15$ nm) irradiated by a 3 MeV O${}^{3+}$ ion beam under various conditions indicated in the figure. The insets show the FT of the STEM images. The directions of ion irradiation and of the correlation of the dot positions are shown by white and red arrows, respectively. (d)–(f) The corresponding GISAXS maps measured in $\perp$ configuration. The insets show simulated GISAXS maps, obtained by fitting of the experimental data. The lateral intensity maxima (so-called Bragg spots) stemming from the in-plane correlation of the dot positions are indicated by yellow arrows.

Figure 3

Dependence of the QD arrangement and size parameters on the irradiation conditions: (a) correlation angle ${\varphi }_{\mathrm{irr}}$, (b) in-layer QD lattice parameter $a_0$, and (c) mean QD radius $R$. Theoretical values given by Eqs. (1, 2, 3) are shown as the black dotted line. The root mean square deviations of the measured values are shown by error bars. (d) Sketch of the change in the in-layer lattice parameter $a_0$ and effective width of ion track $w_{\mathrm{eff}}$ with the irradiation angle.

Figure 4

(a) Standard thermal annealing. The intrinsic separation of the QDs is $L_0$. (b) Effect of a single ion when the width of its track is comparable to $L_0$. A nucleation of a single QD within the track volume is possible. (c) Irradiation by a single ion if the ion track is broader than $L_0$; several QDs with the mean separation of $L_0$ can be created in the irradiated volume. (d) Influence of a high irradiation dose; the Ostwald ripening process takes place.

Figure 5

Time evolution of the temperature in the film during the ion passage through it. (a) Irradiation angle ${\varphi }_{\mathrm{irr}}={90}^{\circ }$, (b) ${\varphi }_{\mathrm{irr}}={60}^{\circ }$, (c) ${\varphi }_{\mathrm{irr}}={30}^{\circ }$. The time $t$ is indicated in each panel. Decreasing the irradiation angle causes increase of the effective width of the ion track (denoted by red arrows).

Figure 6

(a)–(c) Simulations of the structural properties of the films obtained from TEM cross sections presented in Figs. 2d, 2e, 2f. (d) Simulation of the QD arrangement for large dose and small irradiation angle under the influence of the Ostwald ripening illustrated in Fig. 4d. The irradiation and correlation directions are denoted by white and red arrows, respectively.

Figure 7

(a) Change in the QD size with the varying thickness of Ge-rich layers (dose $D_2$, ${\varphi }_{\mathrm{irr}}={60}^{\circ }$). The dependence following from Eq. (3) is shown by the blue dashed line. The layer thicknesses are corrected to include the effects of slightly different Ge:SiO${}_2$ molar ratios. (b) Optical absorbance of the films with various layer thicknesses, irradiated under the same conditions (dose $D_2$, ${\varphi }_{\mathrm{irr}}={60}^{\circ }$) producing Ge QDs of various sizes (indicated in the figure). The red arrow indicates the energy range of solar spectrum. (c) GISAXS map showing the self-assembly of Ni NPs in (Ni $+$ SiO${}_2$)/SiO${}_2$ multilayer (dose $D_2$, ${\varphi }_{\mathrm{irr}}={60}^{\circ }$).