Dislocation Engineering Enables Light-Emitting Silicon

In the new technique, boron is implanted into silicon both as a dopant to form a p-n junction, and as a means of introducing dislocation loops. The dislocation loops introduce a local strain field, which modifies the band structure and provides spatial confinement of the charge carriers. It is this spatial confinement that allows room-temperature electroluminescence (EL) at the band edge.

Because of its indirect bandgap, silicon is fundamentally a poor emitter of light. The main reason for this, Homewood explained, is that fast nonradiative recombination routes dominate the slower radiative route. Nonradiative recombination is the result of diffusion of carriers to point defects in the silicon where efficient nonradiative recombination then occurs, even in high-quality silicon. The trick to creating radiative emissions — or light — is to prevent carrier diffusion and recombination.

	1. The current-voltage plot for the light-emitting device measured at room temperature. Inset is a diagram of the device. (Source: University of Surrey)

To minimize the number of process steps in the device described, boron implantation has been used both to introduce the dislocation loop array and as the p-type dopant to form a p-n junction in an n-type silicon substrate. However, another implant species such as the host silicon could be used to form the dislocations, so that the dislocation engineering and subsequent doping to form the p-n junction can be achieved independently.

The device operates as a conventional light-emitting diode under forward bias. A simple diagram of the device is shown in the inset to Figure 1. The device reported here was made by implanting boron at a dose of 1 × 10¹⁵ cm^-2 at an energy of 30 keV. The sample was subsequently annealed in a nitrogen atmosphere for 20 min at 1000°C to form the dislocation loop array and activate the boron dopants.

	2. The integrated electroluminescent intensity as a function of applied forward voltage is shown for several temperatures. (Source: University of Surrey)

Light from the device was focused into a conventional spectrometer and collected. Electroluminescence measurements were then taken from 80 K to above room temperature. The onset of electroluminescence was observed as the diode turned on under forward bias. No electroluminescence was observed under reverse bias. The integrated electroluminescence intensity as a function of applied forward voltage is shown in Figure 2 for several temperatures.