Erbium-doped silicon and germanium nanostructured building blocks

Figure 1. High resolution electron micrograph of Er−doped Si nanoparticles. Scale bar: 5nm. Inset: selected area electron diffraction pattern associated with the nanocrystals.

Figure 2. Near infrared light emission detected in Er³⁺-doped Si nanocrystals. PL: Photoluminescence.

Figure 3. High resolution transmission electron microscopy (TEM) image of a single Ge nanowire. Inset: scanning electron microscopy (SEM) image of an Er³⁺-doped Ge nanowire film.

To overcome these limitations, one approach has been to implant the erbium ions alongside silicon nanocrystals into an oxide matrix, where the erbium centers have a higher solubility, thus allowing the Si nanophase to act as a sensitizer that can absorb light more efficiently. While significantly improving the emission properties, this method, however, has the drawback of yielding a distribution of different structural Er³⁺environments that may be difficult to control.

Recently, our group developed an alternative preparation strategy that deliberately incorporates the erbium centers into the nanocrystal or nanowire during its synthesis. Our technique has the advantage of covalently trapping the Er³⁺ into the semiconductor with a targeted low dimensional geometry and greater control over its average location and concentration. We have recently focused our efforts on two different approaches to prepare nanoparticles for different optoelectronic components. The first produces a light source formed by introducing Er³⁺ into Si nanocrystals or Si or Ge nanowires that can emit near infrared light initially generated by energy transfer from the host semiconductor. The second is used to fabricate structures capable of modulating or guiding this emitted light along well−defined 1D geometries of the associated oxides (e.g. SiO₂ or GeO₂).

In the first approach, a volatile erbium precursor molecule is heated to relatively high temperatures (1000°C) in the presence of a reactive silicon source molecule such as very dilute disilane.^1–3 We have shown that relatively large amounts of Er³⁺ can be trapped inside semiconductor nanocrystals whose average diameter can be controlled by the duration of the heating event (see Figure 1). The location of the erbium also affects photophysical⁴ or structural^5,6 properties. If dispersed throughout the nanocrystal, Er³⁺ is excited by electron/hole pairs generated first in the Si phase (see Figure 2); however, if localized at the surface, then the erbium centers are directly excited,^4,7 thus reducing the role of the semiconductor. To produce crystalline Si or Ge emitters with different geometries, the reactions can be carried out in a heterogeneous environment, such as on a supported gold catalyst island that shapes the resulting nanostructure in the form of a wire.^8–10 This can be achieved using the so-called vapor-liquid solid reaction, as illustrated by the nanowires shown in Figure 3. With careful control of the different reactants, this approach can be exploited to produce complex structures, as for example Ge ‘core’ nanowires with a Si-rich shell and an Er³⁺ ‘sandwich’ layer. Given the role of SiGe alloys in new devices requiring fast switching speeds and lower power consumption, Er³⁺-doped structures of this composition are currently generating significant interest.

Figure 4. SEM image of aligned fibers of Er³⁺-doped SiO₂.

To produce complementary oxide waveguide/modulators, we have also developed a combined sol-gel condensation reaction route using suitable precursors in conjunction with high-voltage electrospinning methods. With this approach, we were able to easily fabricate 1D nanofibers of Er³⁺-doped silicon, germanium, or tin dioxide (see Figure 4). From a composition control standpoint, one of the main advantages of this fabrication method lies in the use of condensed phase reactions; reactants can be precisely weighed such that the Er³⁺concentrations in the host materials can be easily controlled by adjusting the ratios of the erbium precursor to a given Group IV oxide sol-gel precursor.

In the long-term, we seek to produce relatively large scale, ordered arrays of structurally compatible nanocrystals/nanowires whose emitted near infrared light can be generated (and possibly modulated) by the application of an electrical bias to the network. Initial experiments with aligned fibers of the Er³⁺-doped transparent conductor tin oxide (SnO₂) suggest that this platform can indeed be used for this purpose.

This work was supported by the Robert A. Welch Foundation and by the NSF. The collaborative assistance of Leandro Tessler (State Unversity of Campinas, Brazil), Clemens Burda (Case Western Reserve University), and Waldek Zerda (Texas Christian University) is also gratefully acknowledged.