Amorphous SiGe Alloys Research Articles

Dual crystallization modes of sputter-deposited amorphous SiGe films

The crystallization behavior of sputter-deposited films of amorphous Si (a-Si) and SiGe alloys (a-SiGe) induced by electron irradiation at room temperature and by thermal annealing was investigated by in situ transmission electron microscopy. On electron irradiation at room temperature, extremely rapid crystallization, so-called explosive crystallization, occurred at a higher electron flux but not at a lower electron flux. On in situ thermal annealing, explosive crystallization occurred preferentially and partially at low temperatures in Ge-rich a-SixGe100−x for x < 50 but not for x > 50. These results indicate that the increase of Si content in a-SiGe prevents the occurrence of explosive crystallization. We previously proposed that explosive crystallization can occur in pristine a-Ge films via the interface of a liquid-like, high-density amorphous state at the growth front. An increase in the instability of this high-density amorphous state caused by the increase of Si in a-SiGe apparently gives rise to the suppression of explosive crystallization.

Low Thermal Conductivity of Bulk Amorphous Si1−xGe x Containing Nano-Sized Crystalline Particles Synthesized by Ball-Milling Process

Amorphous Si0.65Ge0.35 powder containing a small amount of nano-sized crystalline particles was synthesized by means of the mechanical alloying process. Hot pressing for 24 h under the pressure of 400 MPa at 823 K, which is below the crystallization temperature, allowed us to obtain bulk amorphous Si-Ge alloy containing a small amount of nanocrystals. The thermal conductivity of the prepared bulk amorphous Si-Ge alloy was extremely low, showing a magnitude of less than 1.35 Wm−1 K−1 over the entire temperature range from 300 K to 700 K. The sound velocity of longitudinal and transverse waves for the bulk amorphous Si0.65Ge0.35 were measured, and the resulting values were 5841 m/s and 2840 m/s, respectively. The estimated mean free path of phonons was kept at the very small value of ∼ 4.2 nm, which was mainly due to the strong scattering limit of phonons in association with the amorphous structure.

Secondary growth mechanism of SiGe islands deposited on a mixed-phase microcrystalline Si by ion beam co-sputtering

We discuss the SiGe island co-sputtering deposition on a microcrystalline silicon (μc-Si) buffer layer and the secondary island growth based on this pre-SiGe island layer. The growth phenomenon of SiGe islands on crystalline silicon (c-Si) is also investigated for comparison. The pre-SiGe layer grown on μc-Si exhibits a mixed-phase structure, including SiGe islands and amorphous SiGe (a-SiGe) alloy, while the layer deposited on c-Si shows a single-phase island structure. The preferential growth and Ostwald ripening growth are shown to be the secondary growth mechanism of SiGe islands on μc-Si and c-Si, respectively. This difference may result from the effect of amorphous phase Si (AP-Si) in μc-Si on the island growth. In addition, the Si-Ge intermixing behavior of the secondary-grown islands on μc-Si is interpreted by constructing the model of lateral atomic migration, while this behavior on c-Si is ascribed to traditional uphill atomic diffusion. It is found that the aspect ratios of the preferential-grown super islands are higher than those of the Ostwald-ripening ones. The lower lateral growth rate of super islands due to the lower surface energy of AP-Si on the μc-Si buffer layer for the non-wetting of Ge at 700 °C and the stronger Si-Ge intermixing effect at 730 °C may be responsible for this aspect ratio difference.

Morphological evolution of self-assembled SiGe islands based on a mixed-phase pre-SiGe island layer grown by ion beam sputtering deposition

We present a study of the evolution of self-assembled SiGe islands grown on a microcrystalline Si (μc-Si)-based mixed-phase pre-SiGe island layer. Using atomic force microscopy, lots of new short islands with smaller diameters, high islands with transition dome (TD) shape, and super islands with ∼20nm in height are observed after the Ge layer deposition. This anomalous experimental finding is well clarified by a model of regeneration and secondary growth. It is found that the density of the super islands increases rapidly at the initial stage when the Ge coverage exceeds ∼1.2nm. This is essentially the result of the selective aggregation of Ge adatoms on the pre-SiGe island layer. The Ge content and stored strain in SiGe islands calculated based on Raman spectra decrease with the increase of the Ge coverage. It is demonstrated that the chemical potential difference-induced lateral atomic migration (LAM) from amorphous SiGe alloy into SiGe islands can be responsible for this phenomenon. The LAM also leads to the formation of very large dome islands. Finally, the overlap behavior of neighbor islands in the sample with 2.2nm-thick Ge layer is explained by the combined action of denser nucleation centers, faster growth rates of super islands in lateral direction, and coarsening of neighbor small islands.

Pressure-induced transformations in amorphous Si-Ge alloy

The pressure behavior of an amorphous Si-rich SiGe alloy ($\ensuremath{\alpha}$-Si${}_{x}$Ge${}_{1\ensuremath{-}x}$, $x=0.75$) has been investigated up to about 30 GPa, by a combination of Raman spectroscopy, x-ray absorption spectroscopy, and x-ray diffraction measurements. The trends of microscopic structural properties and of the Raman-active phonon modes are presented in the whole pressure range. Nucleation of nanocrystalline alloy particles and metallization have been observed above 12 GPa, with a range of about 2 GPa of coexistence of amorphous and crystalline phases. Transformations from the amorphous tetrahedral, to the crystalline tetragonal ($\ensuremath{\beta}$-Sn) and to the simple hexagonal structures have been observed around 13.8 and 21.8 GPa. The recovered sample upon depressurization, below about 4 GPa, shows a local structure similar to the as-deposited one. Inhomogeneities of the amorphous texture at the nanometric scale, probed by high-resolution transmission electron microscopy, indicate that the recovered amorphous sample has a different ordering at this scale, and therefore the transformations can not be considered fully reversible. The role of disordered grain boundaries at high pressure and the possible presence of a high-density amorphous phase are discussed.

Synthesis of amorphous Si–Ge alloys using microwave energy

Amorphous silicon–germanium alloys with 70% Si and 30% Ge have been formed by rapid heating in a pure E-field in a single mode 2.45 GHz microwave cavity. The synthesis of amorphous Si–Ge alloy by all conventional processing is difficult. This polarized microwave process provides another striking example of its unique abilities: producing bulk amorphous Si–Ge alloys. The microwave fabricated amorphous Si0.7Ge0.3 alloy composition exhibited relatively high Seebeck coefficient with high potential of its application in high efficient thermoelectric materials.

Strain-stabilized solid phase epitaxy of Si–Ge on Si

We compare solid phase epitaxial growth of amorphous Si–Ge alloys created by Ge ion implantation into Si with and without the imposition of 0.5GPa of externally applied biaxial tensile stress. External loading stabilizes the growth front against roughening, resulting in a doubling of the maximum reported Ge concentration for stable growth to 14at.%. The externally applied stress appears to superpose with the intrinsic compositional stress and indicates a threshold of approximately 0.6GPa for interface breakdown. This principle is expected to be applicable to expanding the composition range for stable growth of other semiconductor alloy combinations by other growth techniques.

Research Trends

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Microstructure of amorphous and microcrystalline Si and SiGe alloys using X-rays and neutrons

This review describes recent experimental applications of small-angle X-ray (SAXS) and neutron scattering, as well as X-ray diffraction (XRD), for the determination of the microstructure of relevant solar cell thin film materials based on hydrogenated amorphous Si and SiGe alloys. Due to the recent trend toward use of amorphous Si and SiGe materials prepared near the onset of microcrystallinity, this review also describes the use of XRD to detect and determine the role of medium-range order and partial microcrystallinity in solar cell behavior. Some recent results from SAXS and XRD applied to microcrystalline Si are also summarized.

Low-temperature conduction and electron interaction in amorphous Si-Ge films

Abstract We have measured electron spin resonance, optical absorption and low-temperature conduction in amorphous Si-Ge alloy (SixGe1−x) films and studied their dependences on the Si concentration x in SixGe1−x. The density of states and the Coulomb gap have been determined from the transport analysis related to hopping conductions in the alloy films. Thermal washing out of the Coulomb gap has been observed in the alloy films. The coherence length related to the hopping and the density of states determined have been almost independent of the Si concentration x for amorphous SixGe1−x. It is expected that these parameters must be fairly intrinsic in structural disordered tetrahedral semiconductors.

Segregation, clustering, and suppression of phase separation in amorphous silicon–germanium alloys

We study the problem of decomposition in amorphous Si–Ge alloys. Our Monte Carlo (MC) simulations show that phase separation is totally suppressed in the amorphous phase. Still, in the short-range structure of the network there is a tendency for clustering of homopolar bonds, as this relieves strain energy. The strain field due to the amorphous/crystalline interface produces compositional modulations in the alloy.

Fabrication of luminescent ge nanocrystals started from unlayered hydrogenated amorphous SiGe films or hydrogenated amorphous Si/hydrogenated amorphous Ge multilayers

Nanocrystalline Ge embedded in SiOx matrix is fabricated by oxidizing hydrogenated amorphous SiGe alloys or hydrogenated amorphous Si/hydrogenated amorphous Ge multilayers. The structures before and after oxidation are systematically investigated. Visible light emission was observed from both samples. The luminescence peak is located at 2.2 eV which is independent of the starting materials. Compared to the luminescence from unlayered samples, the photoluminescence spectrum from multilayered samples has a narrower band width, which can be attributed to the uniform size distribution. The light emission origin is also discussed briefly and a mechanism different from the quantum size effect is suggested.

Role of atomic hydrogen for the formation of network structure of amorphous SiGe alloy: comparison of theoretical predictions and experimental results

Abstract The influence of systematic variation of hydrogen dilution and rf power density and their inter correlation on the optoelectronic and structural properties as well as defect densities of amorphous silicon germanium (a-SiGe:H) alloy deposited in ultra high vacuum (≈10 −9 Torr) plasma enhanced chemical vapor deposition (PECVD) system have been investigated. It has been observed that low hydrogen dilution of source gases (silane and germane) during the growth of films improves the resulting properties whereas high dilution has the opposite effect. The optimal value of hydrogen dilution for relaxed network structure of a-SiGe:H alloy is very much linked with rf power density. The variation of film properties with plasma condition during its growth nicely corroborate Street's hypothesis on optimal growth condition for amorphous silicon and its alloys, based on hydrogen density of states (HDOS). This work also indicates that metastable hydrogen (H ∗ ) has a vital role to transport energy to the matrix for its requisite structural relaxation apart from its chemical activity.

Temperature Dependence of the Photoconductivity and the Near Absence of Light-Induced Defects in a-SixGe1-x:H

AbstractThe photoconductivities ap of glow discharge deposited a-Ge:H and amorphous Si-Ge alloys prepared in different laboratories were measured between 4.2K and 300K and compared with σp(T) of intrinsic and compensated a-Si:H films. The alloys as well as a-Ge:H do not exhibit thermal quenching of σp(T) at elevated temperatures which suggests that the valence and conduction band tails have similar widths. Remarkable is the near absence of light-induced metastable defects in the alloys as well as in a-Ge:H even after prolonged exposures at low temperatures.

Structural analysis of thermally oxidized amorphous Si 1-xGe x layers

The study of the oxidation process of amorphous SiGe alloys is reported from the microstructural characteristics of the obtained oxides by wet oxidation at 700 °C during 30 minutes. The analysis has been carried out by TEM, Raman, FTIR and XPS measurements which reveal that the whole layers are partially oxidized, (Si,Ge)O y, without Ge pile-up. The deduced suboxides concentrations show the preferential silicon oxidation in spite of the simultaneous oxidation of Si and Ge. Likewise, a comparison with the oxidation of the polycrystalline alloys is performed to stand out the role of the Si and Ge diffusivities.

Optoelectronic Properties of Amorphous SiGec Alloys

ABSTRACTWe describe the optoelectronic characteristics of hydrogenated amorphous silicon germanium carbon (a.Si1-x-yGexCy:H) alloys prepared by plasma deposition from SiH4/GeH4/CH4/H2 gas mixtures. a-Si1-x-yGexCy:H is a homogeneous random alloy having a variable optical gap depending on composition, with properties similar to those of amorphous Si-Ge alloys of the same optical gap but with improved thermal stability. Calculations show that if the ratio of Ge/C atomic fractions is 8.2, the average bond length matches that of unalloyed amorphous a-Si:H with the possibility of reduced defect densities at heterointerfaces. After light-soaking with high intensity white light, a sample having a 1.3 eV optical gap exhibited no Staebler-Wronski change in its properties.

Independent control of spin density and hydrogen-bonding configuration in glow-discharge-hydrogenated Si-Ge alloys using a cathode-heating method

The number density of dangling bonds (spin density) and the hydrogen content as well as the hydrogen-bonding configuration were independently controlled in the glow-discharge hydrogenated amorphous Si-Ge alloys using a cathode-heating technique. A tentative mechanism is presented in terms of the top surface process as well as the growth zone.

Deposition mechanism of hydrogenated amorphous SiGe Films

The deposition rates for hydrogenated amorphous silicon, hydrogenated amorphous germanium and hydrogenated amorphous SiGe alloys, prepared by the plasma chemical vapour deposition method, are presented as functions of the substrate temperature. On the basis of the experimental results the behaviour of hydrogen on the growing surface is discussed quantitatively using a simplified model. The selection of long-lifetime radicals (such as SiH 3 and GeH 3) and the hydrogen-coverage factor are stressed as two key factors for controlling not only the deposition process but also the structural and electronic properties of the resultant alloy films.

Extended x-ray-absorption fine-structure study of hydrogenated amorphous silicon-germanium alloys. I. Analysis based on spherical waves of photoelectrons

In conventional EXAFS (extended x-ray-absorption fine-structure) analyses, reliable structures are obtained with different values of absorption-edge energy ${E}_{0}$ for different neighboring atoms. It is shown in this study that the Ge K-edge EXAFS resulting from the Ge-Ge and Ge-Si bonds in hydrogenated amorphous Si-Ge alloys can be excellently explained by a unique ${E}_{0}$ value provided that a newly developed formula based on the spherical wave functions of photoelectrons is employed. The ${E}_{0}$ value is just at the steepest point in the experimental edge-jump curve. With the conventional formula the adjusted ${E}_{0}$ values for the Ge-Ge and Ge-Si bonds differ by 7 eV at maximum, and in addition they deviate by 3 eV at least from the steepest point.

Extended x-ray-absorption fine-structure study of hydrogenated amorphous silicon-germanium alloys. II. Dependence of bond length and coordination on composition.

The local structure of hydrogenated amorphous Si-Ge alloys has been determined using the extended x-ray-absorption fine-structure (EXAFS) technique. Numerical computations and comparison with experimental Ge K-edge EXAFS have shown that Ge-Ge and Ge-Si bond lengths are constant throughout the entire compositional range studied and equal to 2.46 and 2.41 A\r{}, respectively. The Ge-Si bond length is found to be close to the average value of the bond lengths for both Ge and Si crystals. A study of the coordination around the Ge atoms has revealed that Ge and Si atoms are randomly mixed in the Ge concentration range below 40 at. %. In films with high Ge content, the coordination of Ge atoms surrounding Ge atoms is 15--20 % higher than that expected for a random mixture of Ge and Si atoms, suggesting a structural inhomogeneity in Ge-rich alloys.