Evidence of energy enhancement in photoluminescence emission from Ge nanocrystals

Abstract

We report on the large photoluminescence emission (PL) obtained in germanium nanocrystals of about 100 nm, in porous deposition. Investigating Raman and PL spectra in such closely spaced agglomerates, we discovered that photoluminescence emission can be obtained with energy yield enhancement with respect to the laser beam used for the Raman detection (He-Ne laser, λ = 632.8 nm). We demonstrate that such crystalline germanium nanoparticles can undergo a local high temperature increase with consequent stimulation of photoluminescence in the visible range. With respect to the bulk crystal, we found: i) a high gain of Raman yield larger than a factor 10; ii) photoluminescence emission increased up to a factor 100 between 750 K and 520 K; iii) radiation emission with energy yield enhancement with respect to the red laser light. A simple explanation is discussed in the light of the Ge specific energy bands both of electrons and phonons. This result is of particular importance for applications in optoelectronic devices.