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Nanosilicon photonics

Silicon-based nanostructures enable a wide range of cheaper, faster, and more efficient devices.
16 June 2010, SPIE Newsroom. DOI: 10.1117/2.1201005.002949

The electronic evolution (‘faster, smaller, cheaper’) pushes electronics toward their physical limitations. Some of the stumbling blocks (such as frequency scaling) have forced development of new architectures (e.g., multicore chips), while others (including power consumption) need a paradigm shift. Photonics and, in particular, silicon photonics could overcome many of these limitations by providing high bandwidth and high speed at low cost.1,2 Photonics already dominates electronics in both short- and long-haul telecommunications and is rapidly gaining pace in the interconnect sector (e.g., rack-to-rack and server-to-server). A further push is provided by developments in nanotechnology,3 where devices are engineered at scales on which photon propagation and photon-matter interactions are affected by nanostructuring. Silicon nanostructuring is most promising, since it can significantly affect photonics. CMOS processing can add novel functionalities to silicon photonics that are otherwise not achievable (e.g., based on employing silicon quantum dots4 or related to photon generation,5 photon switching,6 or photon-electron conversion7).

Silicon quantum dots, also known as silicon nanocrystals (Si-ncs), are formed by a thermally induced partial phase separation of silicon-rich silicon oxide. LEDs can be realized if the latter is used as the gate oxide of a metal-oxide-silicon (MOS) capacitor. We recently fabricated a multilayer MOS LED with band-gap-engineered Si-nc sizes.5 Each layer is formed by nearly monodisperse nanocrystals with sizes that gradually decrease toward the multilayer's center: the larger Si-ncs exhibit greater electrical conductivity and facilitate bipolar injection of carriers into smaller Si-ncs, where electron-hole pairs recombine more efficiently: see Figure 1(a). This resulted in the highest ever reported light-emission efficiency for a silicon-based LED: see Figure 1(b). Operation of the same LED in reverse-bias configuration enables photodetection. Thus, a single MOS LED can be used as both transmitter and receiver in a bidirectional optical link—see Figure 2(a)—i.e., this way we can fabricate all-silicon transceivers.8 Figure 2(c) shows the system's power efficiency: η = (PPD / PS) × 100%, where PPD and PS are, respectively, the power generated at the receiver (polarized at 4V) and that injected at the source (based on a voltage sweep). Even with poor optical coupling between devices, one can obtain efficiencies greater than 0.3%. The inset in Figure 2(c) shows an example of the integrity of a transistor-transistor-logic signal transmitted through the link. (We limited the frequency to 1MHz.)

Figure 1. (a) Graded-band-gap LED. (b) Electro-optical characterization of a LED with different silicon-nanocrystal (Si-nc) sizes (black curves) and multilayer structure (red curve). Inset: Power efficiency. SiO2: Silicon dioxide. SRO: Silicon (Si)-rich oxide.

Figure 2. (a) Schematic of the transceiver. (b) Photo of a wafer with several devices linked via optical fibers. Two identical devices are used as source and receiver. The optical link uses visible multimode fiber optics. (c) Conversion power efficiency as a function of emitter current. Inset: Time response of the transceiver for a frequency of 1MHz. Poly-n: N-type doped polysilicon.

We also exploited the strong optical Kerr nonlinearity of Si-ncs.9 A pump laser coupled to a ring resonator (formed by a silicon-slot waveguide activated by Si-ncs) enables optical switching of a signal beam from a bus waveguide coupled to the ring resonator:6 see Figure 3(a). The pump beam changes the nonlinear refractive index of the Si-ncs in the ring resonator, which in turn shifts the ring resonance frequency: see Figure 3(b). The signal beam, if initially tuned to a ring resonance, is driven out of resonance by the ring resonator's resonance redshift, and we observe an increase in signal transmission: see Figure 3(b). Because of the nearly instantaneous response of the Kerr nonlinearity, we achieve extremely fast optical switching, which is only limited by the pump beam's pulse duration: see Figure 3(c).

Figure 3. (a) Schematic of the waveguide and microring. The cyan layer outlines the nanosilicon-rich active volume. (b) Schematic of the switching mechanism. The red and cyan arrows indicate the pump and continuous-wave signal beams, respectively. In the unperturbed state (blue line), the probe's transmission (T) is low because of intrinsic ring losses. Pumping at the (m+1)thmode (red line), achieves a redshift. The probe beam is then temporarily out of resonance and completely transmitted along the waveguide. λ: Wavelength. (c) Experimental data of the optical switching achieved. Pulse full width at half maximum: ~20ps.

Another interesting application of Si-ncs is in photovoltaics, where we can use Si-ncs to increase the power-conversion efficiency of single-crystalline solar cells.10 Figure 4 shows one concept of particular interest. We used a cell based on a standard p-i-n silicon junction where Si-ncs are formed in the intrinsic layer. We obtained a superlinear enhancement of the photocurrent when both IR and visible light illuminate the cell simultaneously. As explanation, we propose that secondary carriers are generated in the Si-ncs because of an inverse-Auger effect from Si-nc interfacial states. These states, which are emptied by secondary-carrier generation, are refilled by IR radiation. As a result, the short-circuit current increases by almost 10%.

Figure 4. (a) Qualitative sketch of the internal-multiplication mechanism (secondary-carrier generation) in our Si-nc solar cell. (b) Photocurrent enhancement (ΔI) caused by simultaneous IR and visible (vis.)-light illumination (dash-dotted line) and photocurrent under only IR illumination (square dots) as a function of applied bias. SRON: Si-rich oxynitride.

In summary, we have discussed recent progress in silicon-based nanophotonics. Our next steps, which include optimization of existing structures and design of innovative photonic devices exploiting the novel properties and capabilities of nano-silicon photonics, will contribute to further widening of silicon-photonics applications, broadening their scope to well beyond the field of optical interconnects.

This research was supported by grants from Intel and the European Commission (grants FP7-224312 HELIOS, FP6-IST-NMP-017158 PHOLOGIC, and FP7-248909).

Lorenzo Pavesi, Paolo Bettotti
Department of Physics
Università degli Studi di Trento
Trento, Italy

Paolo Bettotti is a postdoctoral researcher with a focus on nanophotonics. His expertise spans characterization of structural and spectroscopic materials and modeling of complex photonic structures. He has authored more than 20 articles in refereed journals.