Room-temperature light-emitting diodes with Ge islands

Abstract

There is an increasing interest in Si-based optoelectronics using Si 1− x Ge x nanostructures due to the possibility of their integration with the Si technology. To overcome the problem of the indirect character of SiGe one is looking for possibilities to increase the contribution of the radiative recombination to the emission. One possible approach involves self-organised growth of lattice-mismatched layers. In the present paper, p–i–n structures, using one layer with Ge islands and which emit in the near infrared up to room temperature were fabricated. The self-organised growth of Ge was performed at 700°C with a small coverage (9 ML) so as to avoid plastic relaxation of the islands, but with a high growth rate (0.3 ML/s) which leads to the formation of a broad bimodal island distribution (small- and medium-sized islands). The diode structure including the Ge islands was deposited in the form of mesas using selective epitaxial growth by low-pressure chemical vapour deposition. The mesa areas were varied with the aim of demonstrating the influence of size distribution of the islands on the light emission. At low current density the emission is dominated by islands with smaller band gap (larger valence band offset) while at higher currents emission from islands with larger band gap takes place. From the comparison of single diodes with arrays of small-area diodes with the same total area it is found that the arrays emit three times more light due to the lower total number of deep traps in each diode.