Room-temperature light-emitting diodes with Ge islands

There is an increasing interest in Si-based optoelectronics using Si1-xGex nanostructures due to the possibility of their integration with the Si tech nology. To overcome the problem of the indirect character of SiGe one is lo oking for possibilities to increase the contribution of the radiative recom bination to the emission. One possible approach involves self-organised gro wth of lattice-mismatched layers. In the present paper, p-i-n structures, u sing one layer with Ge islands and which emit in the near infrared up to ro om temperature were fabricated. The self-organised growth of Ge was perform ed at 700 degreesC with a small coverage (9 ML) so as to avoid plastic rela xation of the islands, but with a high growth rate (0.3 ML/s) which leads t o the formation of a broad bimodal island distribution (small- and medium-s ized islands). The diode structure including the Ge islands was deposited i n the form of mesas using selective epitaxial growth by low-pressure chemic al vapour deposition. The mesa areas were Varied with the aim of demonstrat ing 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 fro m islands with larger band gap takes place. From the comparison of single d iodes 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. (C) 2001 Published by Elsevier Science Ltd.