© (2006) AIP

The rewards for finding an efficient way of getting light from silicon are enormous: as well as a reduction in cost, the fabrication of silicon devices, driven by the electronics industry, is far more advanced than that of other semiconductors. As pure silicon is intrinsically such a poor light generator, scientists are forced to take a more lateral approach. One possibility, among a whole plethora suggested so far, is the incorporation of silicon’s close relation, germanium, and researchers in Japan have now found a way to increase the amount of detectable light this is able to emit1.

When a thin layer of germanium is grown on a silicon substrate, it clusters into small islands, or quantum dots, that can trap electrons, confining them in all three dimensions. This is analogous to the trapping of electrons in atoms, in fact, quantum dots are often referred to as ‘artificial atoms’, and can produce photons in much the same way — by electrons moving between energy levels. In addition, they can emit at the important telecommunications wavelengths of 1.3 µm and 1.55 µm. The problem is that these photons are emitted in random directions, severely limiting the amount of useful light.

J. S. Xia and his colleagues have increased the amount of detectable light from germanium quantum dots by surrounding them with a photonic crystal1. Reflections from the periodic surfaces that make up a photonic crystal prevent light from escaping in the wrong direction, forcing the emission out through the top of the structure. This is already a very common way of improving more conventional semiconductor devices such as InGaAs quantum dots, but the germanium dots have one very important advantage — they emit light even at room temperature, giving them an edge when it comes to practical, mass-produced devices.