On the origin of visible photoluminescence in nanometer-size Ge crystallites

Abstract

We have studied the origin of visible photoluminescence of Ge nanocrystals in SiO2 glassy matrix. Spectroscopic analyses of Ge nanocrystals indicate that the room-temperature photoluminescence comes from Ge nanocrystals of diameter of 4 nm or less. High-resolution electron microscopic studies imply that the structure of Ge nanocrystals of diameter ≤4 nm differs from the diamond structure. These data suggest that new nanostructure crystalline Ge having a character of direct optical transition exhibits the visible photoluminescence.

The role of quantum-confined excitons vs defects in the visible luminescence of SiO2 films containing Ge nanocrystals

“…2, however, exhibit no appreciable shift in peak luminescence energy, similar to other reports on SiO 2 , containing Ge nanocrystals. [2][3][4][5][6] To our knowledge, no experimental observation of large PL peak energy shifts, comparable to those predicted by calculations, has been reported for Ge nanocrystals. The reported values 4 of PL peak energy shift are an order of magnitude smaller than what has been predicted 8 for radiative recombination of quantum-confined excitons.…”

“…The principal debate regarding the origin of luminescence seems to be whether or not the luminescence mechanism can be attributed to the radiative recombination of quantumconfined electron/hole pairs ͑or excitons͒ in Ge quantum dots. [2][3][4][5][6] In this letter, we report results of experiments on Ge nanocrystals in SiO 2 made by ion implantation which indicate that the dominant process responsible for visible PL observed from SiO 2 containing Ge nanocrystals is not the radiative recombination of quantum-confined excitons in Ge nanocrystals. Instead, evidence from PL peak energy and lifetime measurements as a function of nanocrystal size, temperature dependent PL measurements, studies on luminescence from Xe ϩ -implanted thermal SiO 2 , and studies on the effect of hydrogen and deuterium on PL all strongly suggest that the visible PL is primarily due to luminescent defect centers in the matrix.…”

“…For Ge, most reports of visible PL from low-dimensional structures have involved Ge nanocrystals embedded in SiO 2 . [2][3][4][5][6] Three principle methods have been employed to synthesize Ge nanocrystals in SiO 2 :rf cosputtering of Ge and SiO 2 , 2-4 hydrothermal reduction of Si x Ge 1Ϫx O 2 , 5 and Ge ion implantation. 6 For all methods of synthesis, similar visible PL spectra have been observed.…”

The Selective Growth of Silicon Nanowires and Their Optical Activation

“…From this level, a rapid relaxation occurs to the 4 I 11/2 level with the subsequent emission of 0.98 µm photons or with a relaxation to the metastable level 4 I 13/2 and emission of photons at 1534 nm. For GeNWs, the recombination of the electrons with holes in the valence band gives the Ge nc light emission at the range of 0.9-2.3 eV (Kanemitsu et al, 1992;de Azevedo et al, 2005). Other wavelength besides 1.5 eV, such as 0.9 eV, could also couple well with the metastable 4 I 13/2 level of the Er manifold and emission of photons at 1534 nm (as shown in Figure 3.4B).…”

Photoluminescence of nanocrystals embedded in oxide matrices

“…A common method for preparing these systems is by alternate deposition of oxides and Ge, followed by an annealing process to increase the diffusion of nanocrystalites (NC) into the adjacent layers 4,6,7,8,9 . This process leads also to homogenize the sizes and spacial dispersion of NCs.…”