Erbium-doped silicon and germanium nanostructured building blocks

New strategies have been developed for producing crystalline nanostructures that emit strongly in the near infrared for nanoscale optoelectronic platforms.
25 October 2007
Jeffery Coffer
Optoelectronics, based on the idea of using photons to transfer information, represents one appealing strategy to overcome the limitations of conventional device platforms. Yet silicon, the semiconductor material of choice on which billions of research and development dollars have been spent, does not even emit light in crystalline form, which is a requirement for its use in optoelectronic devices. One interesting alternative is to introduce an optically active center into the silicon host. In this context, Er3+ has generated significant interest. However, the ion is not very soluble in crystalline Si where it prefers to form clusters. Further, at high concentrations, it tends to undergo energy transfer with itself (self-quenching) or absorb an additional photon to move to even higher energy excited states. All of these effects translate into significantly reduced emission in the desired near infrared region (∼1.5μm).

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 structural5,6 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.

Jeffery Coffer
Department of Chemistry
Texas Christian University
Fort Worth, TX

Jeff Coffer is Professor of Chemistry and Chair of the Chemistry Department at Texas Christian University. His current research interests include the development of new semiconducting nanomaterials relevant to biomaterial applications.

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