Germanium (Ge) nanocrystals have been synthesized by annealing co-sputtered SiO2–Ge samples in N2 or forming gas (90% N2+10% H2) at temperatures ranging from 700 to 1000 °C. We concluded that the annealing ambient, temperature and Ge concentration have a significant influence on the formation and evolution of the nanocrystals. We showed that a careful selective etching of the annealed samples in hydrofluoric acid solution enabled the embedded Ge nanocrystals to be liberated from the SiO2 matrix. From the Raman results of the as-grown and the liberated nanocrystals, we established that the nanocrystals generally experienced compressive stress in the oxide matrix and the evolution of these stress states was intimately linked to the distribution, density, size and quality of the Ge nanocrystals.

Influence of reductant and germanium concentration on the growth and stress development of germanium nanocrystals in silicon oxide matrix

In this article, the work on Ge nanocrystals embedded in dielectric films formed by phase separation from supersaturated solid solutions is reviewed. Different methods to synthesize supersaturated solid solutions are covered, e.g., magnetron sputtering, ion implantation, and chemical vapor deposition. The phase separation is activated by subsequent high temperature annealing. Important parameters that influence the formation and properties of the Ge nanocrystals are discussed. Various matrix materials like SiO2, Al2O3, HfO2, HfAlOx, Lu2O3, ZrO2, TaZrOx, and Si3N4 are reported in the literature. The influence of the matrix on the formation and properties of the Ge nanocrystals is addressed in this review. Ge nanocrystals are investigated for applications such as charge storage nodes in nonvolatile memory devices or as silicon technology compatible light emitters. A key to establish these applications seems to be embedding the Ge nanocrystals in still amorphous matrices. This could help to avoid grain boundaries that act as leakage paths for electrical charges (unfavorable for nonvolatile memories) and facilitate defects, which could act as recombination centers (unfavorable for light emitters). A further important point is the synthesis of size and position controlled Ge nanocrystals. Both aspects are reviewed for Ge nanocrystals embedded in the mentioned matrix materials.

A Review on Ge Nanocrystals Embedded in SiO2 and High‐k Dielectrics

We report the controlled self-seeded growth of highly crystalline Ge nanowires, in the absence of conventional metal seed catalysts, using a variety of oligosilylgermane precursors and mixtures of germane and silane compounds (Ge:Si ratios between 1:4 and 1:1). The nanowires produced were encased in an amorphous shell of material derived from the precursors, which acted to isolate the Ge seed particles from which the nanowires were nucleated. The mode diameter and size distribution of the nanowires were found to increase as the growth temperature and Ge content in the precursors increased. Specifically, a model was developed to describe the main stages of self-seeded Ge nanowire growth (nucleation, coalescence, and Ostwald ripening) from the oligosilylgermane precursors and, in conjunction with TEM analysis, a mechanism of growth was proposed.

Self-Seeded Growth of Germanium Nanowires: Coalescence and Ostwald Ripening

Ge nanoclusters (NCs), synthesized by ion implantation and annealing up to 900 °C, result small (∼2 nm) and amorphous in Si3N4, crystalline and much larger in SiO2. The NCs ripening and crystallization kinetics in Si3N4 is retarded by larger interfacial energy and lower diffusivity of Ge in comparison to SiO2. Ge NCs absorb light more efficiently when embedded in Si3N4 than in SiO2. A significant effect of the barrier height on absorption was evidenced, in agreement with effective mass theory predictions. The smaller bandgap of Ge NCs embedded in Si3N4 and their closeness is promising features for light harvesting applications.

Matrix role in Ge nanoclusters embedded in Si3N4 or SiO2

The authors demonstrate from their Raman and transmission electron microscopy results that the annealing ambient, temperature, and duration play a significant role in the formation, the evolution, and the crystallinity of Ge nanocrystals in silicon oxide matrix. With careful etching experiments, the authors established that the compressive stress experienced by the nanocrystals in the oxide matrix is intimately linked to the density, the size, and the quality of the Ge nanocrystals.

Stress development of germanium nanocrystals in silicon oxide matrix

The growth and formation of Ge nanocrystals embedded into a silicon oxide (SiO2) system synthesized by furnace annealing have been studied based on the Ge content of co-sputtered Ge–SiO2 films. The influence of a silicon oxide diffusion barrier inserted between the co-sputtered film and the substrate on the formation of the Ge nanocrystals is also examined. We found that the effect of Si reduction on the Ge oxides plays an important role in the synthesis of Ge nanocrystals in co-sputtered Ge plus SiO2 samples with high Ge oxide concentration (80%) relative to the elemental Ge concentration (20%). The oxide diffusion barrier influenced the distribution of the nanocrystals by preventing Ge from diffusing into the substrate such that nanocrystals were formed throughout the film. When the Ge concentration was much higher (75%) than the Ge oxide concentration, the contribution from the effect of Si reduction on the Ge oxides in the formation of the nanocrystals is not significant. Again, the oxide diffusion barrier was very effective in preventing Ge atoms from diffusing into the Si substrate, and significant coarsening of the nanoclusters was observed in these samples.

Effect of germanium concentration and oxide diffusion barrier on the formation and distribution of germanium nanocrystals in silicon oxide matrix

Growth and evolution of germanium (Ge) nanocrystals embedded into a silicon oxide (SiO2) system have been studied based on the Ge content of co-sputtered Ge-SiO2 films using transmission electron microscopy (TEM) and Xray photoelectron spectroscopy (XPS). It was found that when the proportion of Ge relative to Ge oxide is 20%, TEM showed that annealing the samples at 800C for 60 min resulted in the formation of a denuded region between the silicon/silicon oxide (Si/SiO2) interface and a band of Ge nanocrystals towards the surface of the film. By introducing a 20-nm thick thermal oxide barrier on top of the silicon (Si) substrate on which the film is deposited, no denuded region in the bulk of this sample is observed. It is proposed that this barrier is effective in reducing both Ge diffusion into the Si substrate and Si diffusion from the substrate into the film. Si diffusing from the Si substrate reduces the Ge oxide into Ge which can subsequently diffuse into the Si substrate. However, the oxide barrier is able to confine the Ge within the oxide matrix so that the denuded region in the bulk of the film cannot form. However the reduction in diffusion should be more significant for Ge as its diffusion coefficient is lower than Si due to its larger size. It is suggested that the denuded region consists of amorphous Ge diffusing towards the Si/SiO2 interface. When the Ge content is increased to slightly more than 70%, TEM showed that Ge nanocrysyals formed after annealing at 800C for only 30 min for samples with and without the oxide barrier. There is no denuded region between the Ge nanocrystals band and the Si/SiO2 interface for both samples but it was observed that coarsening effects were more prominent in the film deposited on top of the oxide barrier. The reduction effect of Si on Ge oxide should not play a significant role in these samples as the Ge content is high..

/pdf/tem-study-on-the-evolution-of-ge-nanocrystals-in-si-oxide-2c1ncrqim8.pdf

TEM Study on the Evolution of Ge Nanocrystals in Si Oxide Matrix as a Function of Ge Concentration and the Si Reduction Process

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Effect of germanium concentration and oxide diffusion barrier on the formation and distribution of germanium nanocrystals in silicon oxide matrix

TEM Study on the Evolution of Ge Nanocrystals in Si Oxide Matrix as a Function of Ge Concentration and the Si Reduction Process