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HELIOS results: Silicon nanocrystals for light emission and detection

Within the frame of HELIOS, the University of Trento (Nanoscience Laboratory) demonstrated a bidirectional optical link between Si-nc based light-emitting diodes

The results have been published by A. Marconi et al. “ High power efficiency in Si-nc/SiO2 multilayer light emitting devices by bipolar direct tunneling” Applied Physics Letters 94, 221110 (2009) and will be presented at ECIO conference, 7-9 April 2010, Cambridge, UK

Silicon based light emitting and detecting devices are of crucial interest for CMOS photonics. The HELIOS consortium has demonstrated a bidirectional optical link between Si-nc based light-emitting diodes fabricated in a standard CMOS fabrication line. Two devices, which can work both as emitter or as receiver, are linked together via a visible multimode fiber. An high coupling efficiency was found which makes the system suitable for a large number of applications.

The device structure is a metal-oxide-semiconductor (MOS) capacitor. Alternating layers of stoichiometric SiO2 and silicon rich silicon oxide (SRO) were grown on p-type Si wafer by PECVD. After a thermal treatment, silicon nanocrystals are formed. This is used as the gate oxide in capacitors (Fig. 1).

Result Apr10_1+2

Figure 1. Scheme of the setup (left) and actual photo of a wafer with several devices linked via optical fiber (right). Two identical devices are used as source and as receiver. The optical link is formed with a visible multimode fiber. In light blue the oxide surrounding the devices.

The Fig. 2 shows the Current-Voltage characteristics under dark and under illumination. The dots present on the I-V curve, in forward bias, represents the voltages where electroluminescence is measured. In this region the I-V characteristic is not affected by the illumination while in reverse bias the characteristics differ due to the contribution of the photogenerated current.

Result Apr10_3+4

Figure 2. (left) Current-Voltage characteristics under dark (blue) and under illumination (red). In forward bias the dots refer to the EL shown in the inset. In reverse bias, it is shown the photocurrent. (right) Optical power density versus injected current when the device is forward bias. Three multilayer configurations are reported, best results are achieved for the graded gap structures. Inset: power efficiency versus current density.

The Fig. 3 shows the power efficiency of the system (ratio between the power generated at the receiver and the power injected at the source), when the receiver was polarized at 4V and a voltage sweep on the source was done. From this figure it is clear how much efficient is the electroluminescence process at low injection and the effect on the transceiver system under test. Even with a bad optical coupling between devices, it is possible to obtain an efficiency greater than 0.3%. On the right hand side of the figure, an example of the integrity of a TTL signal transmitted through the link is shown. Frequency is limited at 1 MHz.

Result Apr10_5+6

Figure 3. Power conversion efficiency versus source current. Time response behavior of the transceiver: the frequency is 1 MHz.

We acknowledge FBK-Trento for the wafer processing and collaboration.