Ge Desorption Research Articles

Influence of silicon dangling bonds on germanium thermal diffusion within SiO2 glass

We study the influence of silicon dangling bonds on germanium thermal diffusion within silicon oxide and fused silica substrates heated to high temperatures. By using scanning electron microscopy and Rutherford backscattering spectroscopy, we determine that the lower mobility of Ge found within SiO2/Si films can be associated with the presence of unsaturated SiOx chemical bonds. Comparative measurements obtained by x-ray photoelectron spectroscopy show that 10% of silicon dangling bonds can reduce Ge desorption by 80%. Thus, the decrease of the silicon oxidation state yields a greater thermal stability of Ge inside SiO2 glass, which could enable to considerably extend the performance of Ge-based devices above 1300 K.

Control of the Ge nanocrystal synthesis by co-implantation of Si+

The synthesis of Ge nanocrystals (Ge-nc) prepared by 74Ge+ implantation into fused silica followed by co-implantation of Si+ has been investigated for annealing temperatures varying between 850 and 1150 °C. By limiting the thermal diffusion of Ge, co-implanting Si reduces the Ge desorption and affects the growth of Ge-nc, through a Ge trapping mechanism involving the formation of Ge-Si chemical bonds. This is supported by Raman analysis, providing information regarding the material composition for a large variety of fabrication parameters, as well as high resolution scanning electron microscopy imaging, indicating that the average dimension of the synthesized Ge-nc decreases for increasing doses of co-implanted Si. From the spectral analysis of Raman measurements, a systematic evolution of the Ge-Ge, Ge-Si, and Si-Si bond concentrations is characterized as a function of the co-implantation fluences. Two different regimes are clearly identified for each annealing temperature. The first is associated with a linear increase of the residual Ge content with respect to the co-implanted Si, having a slope of ∼1, independent of the annealing temperature. Here, the nucleation of pure Ge-nc and Ge-nc containing Si impurities occurs at similar rates, for co-implanted Si fluences generally lower than the dose of implanted Ge. The second regime occurs for greater co-implantation fluence thresholds that depend on the annealing temperature. It is related to the saturation of the Ge trapping efficiency. In this regime, the formation of Si-Ge bonds dominates, sufficiently reducing the diffusion of Ge to prevent the formation of pure Ge-nc. In addition to limiting the unwanted and critical Ge desorption effects, Si co-implantation is a promising technique for precisely controlling the Ge-nc density, diameter, and uniformity at nanoscale dimensions, parameters which cannot be solely set from the local Ge concentration and/or the annealing parameters due to the high thermal diffusivity of Ge.

Influence of annealing conditions on the formation of regular lattices of voids and Ge quantum dots in an amorphous alumina matrix

In this work, the influence of air pressure during the annealing of Ge quantum dot (QD) lattices embedded in an amorphous Al2O3 matrix on the structural, morphological and compositional properties of the film is studied. The formation of a regularly ordered void lattice after performing a thermal annealing process is explored. Our results show that both the Ge desorption from the film and the regular ordering of the QDs are very sensitive to the annealing parameters. The conditions for the formation of a void lattice, a crystalline Ge QD lattice and a disordered QD lattice are presented. The observed effects are explained in terms of oxygen interaction with the Ge present in the film.

Nanocavities and germanium nanocrystals produced by Ge ion implantation in fused silica

High-resolution SEM images of germanium nanocrystals (Ge-nc) synthesized by ion implantation in fused silica samples annealed at temperatures below and above the melting point of Ge show a strong size-selective depth-distribution of nanostructures, as evidenced by the correlation between the dimension of the observed objects and the local concentration of implanted Ge measured by Rutherford backscattering spectroscopy (RBS). Whereas the Ge-nc nucleation seems to obey the Ostwald ripening process in samples annealed below 900 °C, Ge desorption effects, non-uniform in depth, in conjunction with the formation of large and spherical nanocavities, become dominant for annealing performed above the solid–liquid phase transition of Ge. Measurements for different annealing times at 1050 °C show two distinct processes in the Ge desorption dynamics: the first is related to direct Ge outgassing effects during the nucleation of Ge-nc, which occurs within the first minutes of the thermal annealing, while the second is due to the release of Ge from Ge-nc, associated with the formation of nanocavities. The formation rate of these nanocavities is more efficient at greater depth than in the vicinity of the sample surface. It appears to be strongly dependent on the local concentration of defects, responsible for the reduction of the Ge diffusion, and to be related to the breaking of Ge–O and Si–Ge bonds at the Ge-nc/SiO2 interface.

Thermal desorption of Ge native oxides and loss of Ge from the surface

The authors have identified oxidation and desorption processes of Ge native oxide by chemical bonding states measured by X-ray photoemission spectroscopy. Ge oxidation occurs at the temperatures of 450–500 °C in an oxidizing ambient. Ge desorption in nitrogen ambient is observed at the temperatures of 500–550 °C, which is higher than the oxidation temperature by 50 °C. Combined oxidation and desorption processes proceed subsequently and cause a loss of Ge from the surface when Ge is annealed in oxidizing ambient at a temperature higher than desorption temperature. The surface loss is avoided when Ge is annealed with SiO 2 cap layer in an identical annealing condition.

Kinetics of Ge diffusion, desorption and pit formation dynamics during annealing of Si0.8Ge0.2/Si(001) virtual substrates

Thermal stability of Si(0.8)Ge(0.2)/Si(001) virtual substrates (VS) is studied as a function of annealing temperature and time by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Two regimes describing different Ge behavior are observed when the Si(0.8)Ge(0.2) VS is annealed. Heating the substrate from room temperature to 500 degrees C results in some degree of Ge segregation. The surface morphology however remains relatively smooth and there is no formation of 3D islands on the surface. Above 500 degrees C, Ge is preferentially lost from the surface and microscopic pits with edges aligned along 110 azimuth are formed. As temperature increases, Ge% decreases and the size of pits also increases. The decrease in Ge% and the formation of holes at the surface are attributed to Ge desorption from the surface. A kinetic model involving diffusion and desorption processes is proposed to describe the Ge behavior and pits formation in this regime.

10.1007/s11451-008-1032-1

The growth of Ge thin films on the surface of a textured predominantly (100)-oriented tungsten ribbon is studied by thermal desorption spectrometry at different substrate temperatures over a wide range of coverages. The mechanism of growth of the Ge films at T = 300 K is similar to a layer-by-layer mechanism. For T > 300 K, the films grow through the Stranski-Krastanov mechanism, according to which the completion of the monolayer coverage is followed by the formation of three-dimensional crystallites; as a result, the desorption kinetics changes. For small coverages (i.e., in the absence of lateral interactions), the activation energy of Ge desorption from W(100) is E = 4.9 ± 0.2 eV. In a monolayer, this activation energy decreases to E = 3.9 ± 0.2 eV due to the repulsive lateral interactions. The energy of pairwise lateral interactions is determined to be ω = 0.3 eV.

Energetics of Ge nucleation on SiO 2 and implications for selective epitaxial growth

We have measured the time evolution of Ge nucleation density on SiO 2 over a temperature range of 673–973 K and deposition rates from 5.1 × 10 13 atoms/cm 2 s (5 ML/min) to 6.9 × 10 14 atoms/cm 2 s (65 ML/min) during molecular beam epitaxy. The governing equations from mean-field theory that describe surface energetics and saturation nucleation density are used to determine the size and binding energy of the critical Ge nucleus and the activation energy for Ge surface diffusion on SiO 2. The critical nucleus size is found to be a single Ge atom over substrate temperatures from 673 to 773 K, whereas a three-atom nucleus is found to be the critical size over substrate temperatures from 773 to 973 K. We have previously reported 0.44 ± 0.03 eV for the Ge desorption activation energy from SiO 2. This value, in conjunction with the saturation nucleation density as a function of substrate temperature, is used to determine that the activation energy for surface diffusion is 0.24 ± 0.05 eV, and the binding energy of the three-atom nucleus is 3.7 ± 0.1 eV. The values of the activation energy for desorption and surface diffusion are in good agreement with previous experiments of metals and semiconductors on insulating substrates. The small desorption and surface diffusion activation barriers predict that selective growth occurring on window-patterned samples is by direct impingement of Ge onto Si and ready desorption of Ge from SiO 2. This prediction is confirmed by the small integral condensation coefficient for Ge on SiO 2 and two key observations of nucleation behavior on the window-patterned samples. The first observation is the lack of nucleation exclusion zones around the windows, and second is the independence of the random Ge nucleation density on patterned versus unpatterned oxide surfaces. We also present the Ge nucleation density as a function of substrate temperature and deposition rate to demarcate selective growth conditions for Ge on Si with a window-patterned SiO 2 mask.

Growth of germanium thin films on the W(100) surface

The growth of Ge thin films on the surface of a textured predominantly (100)-oriented tungsten ribbon is studied by thermal desorption spectrometry at different substrate temperatures over a wide range of coverages. The mechanism of growth of the Ge films at T = 300 K is similar to a layer-by-layer mechanism. For T > 300 K, the films grow through the Stranski-Krastanov mechanism, according to which the completion of the monolayer coverage is followed by the formation of three-dimensional crystallites; as a result, the desorption kinetics changes. For small coverages (i.e., in the absence of lateral interactions), the activation energy of Ge desorption from W(100) is E = 4.9 ± 0.2 eV. In a monolayer, this activation energy decreases to E = 3.9 ± 0.2 eV due to the repulsive lateral interactions. The energy of pairwise lateral interactions is determined to be ω = 0.3 eV.

Investigation of Volmer-Weber growth mode kinetics for germanium nanoparticles on hafnia

Germanium nanoparticle deposition kinetics were investigated on extended HfO2 surfaces and HfO2 confined within features etched into a 12.5nm SiO2 film. Electron beam lithography was used to pattern square features from 200nmto100μm in width. Hot wire chemical vapor deposition produced Ge nanoparticles on exposed HfO2 surfaces at 0.06–4.2ML∕min fluxes and 700–775K surface temperatures. Using mean-field nucleation descriptions of particle density versus flux and temperature, a noninteger critical cluster nuclei size between 0.27 and 0.4 was found, while the critical cluster formation activation energy of 0.4–0.6eV was estimated assuming a critical cluster size of zero for studies on extended, unpatterned HfO2. Restricting the HfO2 deposition area decreases nanoparticle density compared to the density found on extended surfaces. The density decrease is due to Ge desorption through adatom contact with the silica perimeter, which depletes the system of Ge adatoms that can undergo nucleation. Comparable activation energies for Ge desorption, surface diffusion, and cluster formation obscure the change with temperature an individual process rate has on nanoparticle growth characteristics as the feature size changes.

Bidimensional intercalation of Ge between SiC(0001) and a heteroepitaxial graphite top layer

High temperature annealing of $4\mathrm{H}\text{\ensuremath{-}}$ or $6\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}(0001)$ crystals is well known to desorb Si from the surface and to generate a C-rich $(6\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}6\ensuremath{\surd}3)\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$ $(6\ensuremath{\surd}3)$ reconstruction explained as a graphite monolayer in heteroepitaxial registry with the substrate. Ge deposition at room temperature and in the monolayer range on this graphitized reconstruction results in Ge islands. Using a number of surface techniques, we follow subsequent Ge morphology evolutions as a function of isochronal post-annealing treatments at increasing temperatures. In a particular temperature window Ge reacts with the substrate by diffusion under the graphite planes and wets the Si-terminated SiC surface. In spite of this bidimensional insertion of a Ge layer, the epitaxial relationship between the SiC substrate and the graphite is maintained as shown by very clear graphite-$(1\ifmmode\times\else\texttimes\fi{}1)$ LEED or RHEED patterns. They denote extended and well-ordered graphite planes at the surface of a graphite/$\mathrm{Ge}∕\mathrm{Si}\mathrm{C}$ heterostructure. XPS analyses reveal a complete passivation of the intercalated Ge layer against oxidation by the overlying graphite sheets. Moreover, drastic spectroscopic changes on the bulk-SiC $\mathrm{Si}\phantom{\rule{0.2em}{0ex}}2p$ and $\mathrm{C}\phantom{\rule{0.2em}{0ex}}1s$ core levels are observed, depending on whether $\text{graphite}(6\ensuremath{\surd}3)∕\mathrm{Si}\mathrm{C}$ or $\text{graphite}(1\ifmmode\times\else\texttimes\fi{}1)∕\mathrm{Ge}∕\mathrm{Si}\mathrm{C}$ terminations are analyzed. In the latter case, the observed core level splitting of the bulk components is interpreted by a significant upward band bending $(\ensuremath{\sim}1.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ of the $n$-doped SiC, making this second interface to act as a Schottky barrier. Above $1300\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, a delayed Ge desorption takes place that allows the graphite sheets to re-form in their initial $6\ensuremath{\surd}3$ form, i.e., without Ge and with flatter bands.

Influence of the surface-termination of hexagonal SiC(0 0 0 1) on the temperature dependences of Ge growth modes and desorption

With the aim of comparing initial Ge adsorption and desorption modes on different surface terminations of 4H–SiC(0 0 0 1) faces, 3 × 3, √3×√3R30° (R3) and 6√3×6√3R30° (6R3) reconstructions, of decreasing Si surface richness, have been prepared by standard surface preparation procedures. They are controlled by reflection high energy electron diffraction (RHEED), low energy electron diffraction and photoemission. One monolayer of Ge has been deposited similarly at room temperature on each of these three surfaces, followed by the same set of isochronal heatings at increasing temperatures up to complete Ge desorption. At each step of heating, the structural and chemical status of the Ge ad-layer has been probed. Marked differences between the Si- (3 × 3 and R3) and C-rich (6R3) terminations have been obtained. Ge wetting layers are only obtained up to 400 °C on 3 × 3 and R3 surfaces in the form of a 4 × 4 reconstruction. The wetting is more complete on the R3 surface, whose atomic structure is the closest to an ideally Si-terminated 1 × 1 SiC surface. At higher temperatures, the wetting layer stage transiets in Ge polycrystallites followed by the unexpected appearance on the 3 × 3 surface of a more ordered Si island structure. It denotes a Si clustering of the initial Si 3 × 3 excess, induced by the presence of Ge. A phase separation mechanism between Si and Ge prevails therefore over alloying by Ge supply onto a such Si-terminated 3 × 3 surface. Conversely, no wetting is obtained on the 6R3 surface and island formation of exclusively pure Ge takes place already at low temperature. These islands exhibit a better epitaxial relationship characterized by Ge(1 1 1)//SiC(0 0 0 1) and Ge〈1 1 −2〉//SiC〈1 −1 0 0〉, ascertained by a clear RHEED spot pattern. The absence of any Ge–C bond signature in the X-ray photoelectron spectroscopy Ge core lines indicates a dominant island nucleation on heterogeneous regions of the surface denuded by the 6R3 graphite pavings. Owing to the used annealing cycles, the deposited Ge amount desorbs on the three surfaces at differentiated temperatures ranging from 950 to 1200 °C. These differences probably reflect the varying morphologies formed at lower temperature on the different surfaces. Considering all these results, the use of imperfect 6R3 surfaces appears to be suited to promote the formation of pure and coherent Ge islands on SiC.