Self-assembled Al nanoparticles on Si and fused silica, and their application for Si solar cells.

This paper presents a novel method for the self-assembly of aluminum nanoparticles on Si and fused silica. Due to high reactivity with oxygen, ex-vacuo annealing of thin deposited metal films, a method used extensively with other metals, does not work with aluminum. In the present experiment this problem was overcome by annealing the samples in-vacuo in the deposition chamber. Aluminum was thermally evaporated onto substrates at elevated temperatures (200-400 ° C) and annealed for 60 min without breaking the vacuum. It is shown that at 300 and 400 ° C the average particle size can be controlled by adjusting the amount of evaporated aluminum. Particle diameters ranging from 20 to 130 nm are demonstrated. These particles support localized surface plasmon resonances, a property that can be utilized for enhancing the efficiency of thin Si solar cells. This is explored here, and an increase in external quantum efficiency of up to 15% in a thin-film Si solar cell is demonstrated.

Vertical charge-carrier transport in Si nanocrystal/SiO2 multilayer structures.

Charge-carrier transport in multilayer structures of Si nanocrystals (NCs) embedded in a SiO(2) matrix grown by magnetron sputtering has been investigated. The presence of two types of Si NCs with different diameters after post-growth annealing is concluded from transmission-electron microscopy and photoluminescence measurements. Based on the electric field and temperature dependences of capacitance and resistivity, it is established that the carrier transport is best described by a combination of phonon-assisted and direct tunneling mechanisms. Poole-Frenkel tunneling seems to be a less suitable mechanism to explain the vertical carrier transport due to the very high values of refractive indices obtained within this model. The possibility to more effectively collect charge carriers generated by light in structures having Si NCs of different size is discussed.

Interstitial-mediated diffusion in germanium under proton irradiation.

We report experiments on the impact of 2.5 MeV proton irradiation on self-diffusion and dopant diffusion in germanium (Ge). Self-diffusion under irradiation reveals an unusual depth independent broadening of the Ge isotope multilayer structure. This behavior and the observed enhanced diffusion of B and retarded diffusion of P demonstrates that an interstitial-mediated diffusion process dominates in Ge under irradiation. This fundamental finding opens up unique ways to suppress vacancy-mediated diffusion in Ge and to solve the donor deactivation problem that hinders the fabrication of Ge-based nanoelectronic devices.

Rare earth ions and Ge nanocrystals in SiO(2).

Experimental studies of Ge nanocrystals embedded in SiO(2) films doped with Er and Yb deposited by rf-magnetron sputtering are presented. Although inter-band photoluminescence (PL) from the Ge nanocrystals is not observed, it is nevertheless found that the presence of Ge nanocrystals is crucial for obtaining light emission from Er(3+) and Yb(3+). For both kinds of rare earth ions, the intensity of the related PL line has a maximum after heat treatment at 800 °C, and the PL excitation spectra for the two cases are very similar. This suggests that the presence and the structure of the nanocrystals are important for the efficiency of PL from Er(3+) and Yb(3+). Experiments performed with multilayer structures of Ge nanocrystals and SiO(2) show that the optically active rare earth ions are located in the SiO(2) layers, and not inside the Ge nanocrystals. The mechanism of energy transfer from Ge nanocrystals to the rare earth ions is found to be non-optical.

Ge self-diffusion in epitaxial Si(1)-(x)Ge(x) layers.

Diffusion coefficients and activation energies have been determined for Ge diffusion in strain-relaxed Si(1)-(x)Ge(x) with x = 0.00, 0.10, 0.20, 0.30, 0.40, and 0.50. The activation energy drops from 4.7 eV in Si and Si(0.90)Ge(0.10) to 3.2 eV at x = 0.50. This value compares with the literature value for Ge self-diffusion in Ge, suggesting Ge-like diffusion already at x approximately equal to 0.5. The effect of strain on the diffusion was also studied showing a decrease in diffusion coefficient and an increase in activation energy upon going from compressive over relaxed to tensile strain.

Structure and low-temperature thermal relaxation of ion-implanted germanium.

The structure of implantation-induced damage in Ge has been investigated using high resolution extended X-ray absorption fine structure spectroscopy (EXAFS). EXAFS data analysis was performed with the Cumulant Method. For the crystalline-to-amorphous transformation, a progressive increase in bond-length was observed without the presence of an asymmetry in interatomic distance distribution (RDF). Beyond the amorphization threshold the RDF was dose dependent and asymmetric, where the bond-length and asymmetry increased as functions of ion dose. Such an effect was attributed to the formation of three- and five-fold coordinated atoms within the amorphous phase. Low-temperature thermal annealing resulted in structural relaxation of the amorphous phase as evidenced by a reduction in the centroid, asymmetry and width of the RDF, as consistent with a reduction in the fraction of non four-fold coordinated atoms. The results have been compared to other EXAFS studies of amorphous Ge, and it is suggested that the range of bond-lengths reported therein is related to the sample preparation method and state of relaxation.