Silicon Photonics: Fundamentals and Devices

6.6 Impurities in Silicon

The role of isoelectronic impurities, like N, Be, and so on, in GaP is well known. These impurities bind excitons in the indirect-gap semiconductors, and the dissociation of bound excitons gives rise to the characteristic light emission from GaP or its alloy Ga_1−xAs_xP. This approach has been undertaken to fabricate LEDs with silicon, by doping the materials with rare-earth impurities, in particular Er 31, carbon complexes 32, and sulphur–oxygen complexes 33. In the following paragraphs, we first describe briefly the results of introducing isoelectronic impurities. Then a detailed discussion of the energy levels and transition in Er-doped silicon and an alloy is given. The work related to Er-doped Si and SiGe is then described, and finally the devices using Er and other rare-earth elements are presented.

6.6.1 Isoelectronic Impurities

Isoelectronic centers are created in silicon by using electrically neutral dopants like isovalent elements C, Ge, Sn, N, Be, S, and Se. The optical properties of all these elements in Si and Si_1−xGe_x have been studied. The isoelectronic impurities bind free excitons in Si, and due to spatial confinement of the particles there is an enhancement of the recombination. In all these studies, a luminescence EQE as large as 5% has been observed and a lifetime greater than 1 ms has been reported. However, PL intensity and lifetime decrease rapidly with a rise in temperature, due to the dissociation of excitons by thermal energy. In ...