Low Ge Content Research Articles

Defect bands in a-Si–Ge:H alloys with low Ge content

We have studied a series of optimized glow discharge a-Si 1− x Ge x :H films with Ge content 0.02⩽ x⩽0.2. The drive level capacitance profiling method indicated defect densities < 5×10 15 cm −3 in these samples. Modulated photocurrent measurements detected two defect bands at 0.68±0.05 and 0.79±0.05 eV, compared to the single band normally observed in pure a-Si:H. The magnitudes of these bands vary with Ge content and with the state of light-induced degradation. Based on electron spin resonance (ESR) measurements on matched samples we have identified these defect bands as Si and Ge neutral dangling bonds. The Ge dangling bond concentration was found to be larger in the annealed state for all samples, even at 2 at.% Ge. However, in the light-induced degraded state the density of Si dangling bonds was larger for all the alloys in this series.

Defect related photoluminescence of SiGe/Si heterostructures grown by APCVD

Heterostructures of Si1−xGex/Si with Ge content x from 14 to 67%, grown on Si by atmospheric pressure chemical vapor deposition (APCVD) at 650 or 700°C, have been studied by photoluminescence (PL) spectroscopy, Normarski microscopy after Schimmel etching, atomic force microscopy (AFM) and secondary ion mass spectroscopy (SIMS). For SiGe grown by APCVD with a low Ge content (<40%), strain relaxation is achieved by misfit dislocation formation. For high Ge content (>40%), the relaxation mode is influenced by the deposition temperature, where the energetically most favorable mechanism will dominate. At 700°C, the SiGe is relaxed by 3D island formation and the PL alloy band can be correlated to island structuring of the surface. At 650°C, the SiGe is relaxed by misfit dislocations due to the increased density of born-in point defects at low temperatures.

Local structure of bulk amorphous and crystalline(GaSb)1−x(Ge2)x

The local structure of bulk amorphous and crystalline $(\mathrm{GaSb}{)}_{1\ensuremath{-}x}({\mathrm{Ge}}_{2}{)}_{x}$ produced by solid-state amorphization and subsequent crystallization of amorphous samples, respectively, have been studied using extended x-ray absorption fine structure. Measurements have been made at the Ga and Ge K edges for $c\ensuremath{-}(\mathrm{GaSb}{)}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x},$ and at all K absorption edges for $\mathrm{a}\ensuremath{-}(\mathrm{GaSb}{)}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}.$ All samples are found to be fourfold coordinated with a local structure close to the $c\ensuremath{-}\mathrm{GaSb}$ or the $c\ensuremath{-}\mathrm{Ge}$ phase. The partial distances ${R}_{\mathrm{G}\mathrm{a}\ensuremath{-}\mathrm{S}\mathrm{b}}{,R}_{\mathrm{G}\mathrm{e}\ensuremath{-}\mathrm{S}\mathrm{b}}{,R}_{\mathrm{G}\mathrm{e}\ensuremath{-}\mathrm{G}\mathrm{a}}{,R}_{\mathrm{G}\mathrm{a}\ensuremath{-}\mathrm{G}\mathrm{e}}{,R}_{\mathrm{S}\mathrm{b}\ensuremath{-}\mathrm{G}\mathrm{a}},$ and ${R}_{\mathrm{S}\mathrm{b}\ensuremath{-}\mathrm{G}\mathrm{e}}$ are shown to be independent of Ge concentration and defined by covalent radii of the components in both materials. The nature of local ordering in the amorphous materials is such that Ge seems to substitute mainly for Sb at lower Ge concentration $(x&lt;30\ensuremath{-}40$ at. %), aiding relief of stress inserted during amorphization of the material. Crystalline $(\mathrm{GaSb}{)}_{1\ensuremath{-}x}({\mathrm{Ge}}_{2}{)}_{x}$ seems to contain a phase with around 4 Ga/Ge neighbors around Ge along with Ge randomly substituting on Ga and Sb sites in a solid solution. Debye-Waller broadening of amorphous and crystalline $a\ensuremath{-}(\mathrm{GaSb}{)}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ are of the same order of magnitude, suggesting chemical disorder in the crystalline material.

Diffusion of gold in relaxed Si–Ge epi-layers

The diffusion of implanted 195Au in relaxed, low-dislocation-density Si1−yGey epi-layers has been measured as a function of temperature (700–950°C) and composition (0⩽y⩽0.24) by means of a radiotracer technique, where serial sectioning was done by precision grinding. In all cases, the diffusion takes place by the kick-out mechanism, which is self-interstitial-controlled at low Ge contents, low temperatures, or close to the epi-layer surface, but gold-interstitial-controlled otherwise. An interpretation is presented in which a novel mechanism of alloy-retarded substitutional–interstitial diffusion plays a major rôle.

The Ge-Hg (Germanium-Mercury) System

No reports of direct alloying of Ge and Hg could be found. Our knowledge of the Ge-Hg system is based mainly on numerous electroanalytical experiments performed at room or moderately elevated temperatures. Ge and Hg most likely form a simple eutectic system with a likelihood of immiscibility in the liquid state. Of course, this could be observed only at constraint pressure, which would be required to keep Hg in the liquid state. The eutectic point is expected to be at a temperature very near the melting point of Hg and at a very low Ge content, probably on the order of 10–8 at.% Ge. No formation of intermetallic compounds in the system has been found, and mutual solid solubilities were observed to be very small. The provisionally assessed Ge-Hg phase diagram at 0.101 MPa is presented in Fig. 1.

Mutual solubility of Si and Ge in the Al-rich amorphous and crystalline phases

Rapidly solidified alloys of the Al-Si-TM and Al-GeTM (TM-transition metals) systems obtained by melt spinning technique were found to have wide compositional ranges for the formation of the amorphous single phase [1–5]. Moreover, such compositional ranges in the Al-Ge-TM ternary alloys are relatively close to that observed in the Al-Si-TM alloys [2, 3]. Mechanical and electrical properties of these alloys change greatly with composition [3]. Quaternary Al-Si-Fe-TM (TM= 3d transition metals) system alloys were studied recently [6, 7]. One of the widest composition ranges in the direction of Si concentration axis from 16 to 45 at % Si was obtained for Al86−x Six Fe10Cr4 alloys. The microstructure of rapidly solidified Al-Si-TM [8] and Si-Al-TM [9] alloys studied recently was found to change from homogeneously amorphous to mixed structure consisting of amorphous phase and nanocrystalline Fd3m Ge-Si solid solution by the Ge addition. However, as it will be shown in the present paper, at sufficiently low total Si and Ge concentration both these elements are dissolvable in the Al-rich amorphous phase. Ingots of Al-Si-Ge-Fe-Cr alloys were prepared by arc melting the mixture of Al (99.99%), Si (99.9999%), Ge (99.9999%), Fe (99.9%) and Cr (99.9%) in an argon atmosphere. From these alloys, ribbon samples of about 0.015–0.02 mm in thickness and 0.9 mm in width were prepared by rapid solidification of the melt on a single copper roller at the wheel surface velocity of 41.9 m s−1. The structure of ribbon samples was examined by X-ray diffraction with monochromatic CuKα radiation. Transmission electron microscopy (TEM) was carried out using a JEM 2010 microscope operating at 200 kV. Samples for TEM were polished electrolytically in a solution of 10 vol % perchloric acid and 90 vol % methanol at 208–213 K. Crystallization temperature and heat of crystallization were examined by differential scanning calorimetry (DSC) at a heating rate of 0.67 K s−1. Contrary to rapidly solidified Si-based and Ge-based alloys in which replacing of Si by Ge and Ge by Si, respectively, causes precipitation of nanocrystalline Ge particles, Al61Si25−x Gex Fe10Cr4 alloys form an amorphous phase at different x values from 0 to 25 as shown in Fig. 1. It should be mentioned, that the mixed amorphous+ nanocrystalline Ge structure also forms in Albased alloys by replacement of Al by Ge and increment of the total Si+Ge content [8]. Some characteristic features of Al61Si25−x Gex Fe10Cr4 alloys should be described. The split of the first halo peak slightly seen in the alloys containing 0–15% Ge becomes significant at higher Ge content and the left shoulder of the first halo peak marked by an arrow in Fig. 1 appears. This effect is commonly observed in Al-Ge-TM alloys and results from phase separation of the amorphous phase into Aland Ge-rich phases over a short range [2, 3]. The brightand dark-field TEM images of the Al61Si5Ge20Fe10Cr4 alloy shown in Fig. 2 exhibit modulated contrast that was observed in Al-Ge-TM alloys [2, 3] whereas the structure of the Al-Si-Fe-Cr amorphous alloys studied recently [7] is homogeneous. Although crystallization temperature very slightly changes with increase in Ge content the shape of the DSC curves shown in Fig. 3 varies greatly. Phase composition in the fully crystallized state also changes significantly in alloys containing more than 15 at % Ge. Two heat effects seen in the DSC curve of the Al61Si25Fe10Cr4 alloy (see Fig. 3) are related to the precipitation of the Fd3m Si and a quaternary (Fe,Cr)4Si4Al13 phase with a cubic structure of a= 1.260 nm [10]. The addition of 5 at % Ge causes the appearance of the small left shoulder of the second exothermic peak. During the crystallization process Ge dissolves in Si and the quaternary (Fe,Cr)4Si4Al13 phase replacing Si atoms and forming a substitutional solid solution. Lattice parameters of Si and (Fe,Cr)4Si4Al13 phase increase and the diffraction peaks relevant to these phases shift to lower 2θ angle because the atomic radius of Ge (0.123 nm) is higher than that of Si (0.117 nm) as well as the corresponding lattice parameter of pure crystalline Ge is higher than that of Si. Some unidentified

Ion assisted MBE growth of SiGe nanostructures

The bombardment of thin SiGe buffers with 1-keV Si + ions during molecular beam epitaxial growth is a possible way for the injection of point defects in order to promote the relaxation and to reduce the dislocation density. For this purpose, the e-beam evaporator was optimized by increasing the emission current and decreasing the energy of the impinging electrons to create a high density Si + ion flux in our MBE system. To the isolated substrate holder a potential up to several kilovolts can be applied to direct, focus and accelerate Si + ions. A high efficiency Ge effusion cell ensures stable and controllable Ge fluxes for growth rates up to 2.5 Å/s. Under these conditions, several sets of thin SiGe layers (65–300 nm) containing from 23 to 100% of Ge were grown and investigated comparatively with reference samples deposited without ions at 650°C. By the ‘ion growth program’, after the deposition of Si buffers, SiGe layers were grown in three stages. The first part of the layer (e.g. 1/8 of the nominal thickness) and the last one (e.g. 5/8 of the thickness) were grown without ion bombardment. The second part (e.g. 2/8 of the thickness) was deposited under 1-keV accelerated Si + ion bombardment. Ge content was kept constant during all three stages. Sharp interfaces and uniform Ge profiles were shown by SIMS. Strain relaxation in the thicker layers is nearly 100% as proven by XRD. In thin pseudomorphic layers with low Ge content, a bombardment may result in nucleation of stacking faults shown by TEM. AFM and preferential chemical etching of relaxed ion bombarded layers have shown higher surface smoothness and a reduction of etch pit densities.

Defect distribution and morphology development of SiGe layers grown on Si(100) substrates by LPE

Thick relaxed SiGe layers having Ge content up to 13 at.% have been grown using a liquid phase epitaxy (LPE) technique. The thickness of the layers is around 20–30 μm. The strain relaxation and the resulting defect evolution during the growth of SiGe layers are found to be largely controlled by the growth kinetics involved in the LPE growth process. The cooling rate of the LPE process plays an important role in the supersaturation of solute materials in the growth process. A greater number of ridges and trenches are formed on the layer on using high cooling rates. The effect of cooling rate on the morphology development of the layers is significant for the layers having low Ge content. The wavelength of undulation of the SiGe layers decreases from 75 to 20 μm as the cooling rate during the LPE process is increased from 20 to 500 K/h. The etch pit density of the layers also increases from 2×10 4 to 8×10 5 cm −2 as the cooling rate is increased. Pyramidal growth results upon increasing the Ge content in the layer above 13 at.%.

Si 1− xGe x gradient crystals: a new monochromator material for hard X-rays

The diffraction properties of a large Czochralski grown Si 1−x Ge x (0.02≤x≤0.07) gradient crystal have been studied by means of a triple-crystal diffractometer using 120 keV synchrotron radiation. Over a crystal length of 40 mm a homogeneous variation of the lattice parameter by 1.7×10 −3 has been observed. The diffraction pattern measured in transmission geometry at low Ge content showed a FWHM of 14.8 arcsec at a peak reflectivity of 96% disregarding absorption. With increasing Ge content the FWHM of the diffraction pattern increases up to 49 arcsec with the peak reflectivity decreasing to a value of 72%. Taking absorption into account the effective reflectivity for 120 keV photons varies between 68% and 50%, which is surprisingly high. This material opens up new opportunities of optimising optics in synchrotron radiation instrumentation, it is expected that it will also be of advantage in neutron scattering experiments.

Combined high resolution X-ray diffraction and EXAFS studies of Si (1− x) Ge x heterostructures

In this paper, we compare the structural information obtained by measuring X-ray diffraction reciprocal space maps and by EXAFS analysis on two Si (1− x) Ge x alloys with low (LC) and high (HC) Ge concentration. From the analysis of the X-ray diffraction maps we determined the alloy lattice parameters for the cubic cell ( a=5.58 Å) of the fully relaxed HC sample and for the tetragonal cell ( a=5.43 Å, c=5.46 Å) of the fully strained LC sample and from these values the Ge alloy content (HC: x=0.66±0.04, LC: x=0.048±0.003). The EXAFS data were taken at the Ge k-edge for incident wave polarization directions either parallel or perpendicular to the surface plane. The satisfactory correlation found between the Ge coordination ratio and the Ge–Si and Ge–Ge distances, derived by best-fitting the EXAFS curves, and the X-ray diffraction structural parameters indicates that the combination of the two techniques is a unique approach to achieve a thorough characterization of the structural and chemical properties of lattice mismatched heterostructures.

Chemical Classification of Iron Meteorites: XII. New Members of the Magmatic Groups

Data are reported for thirty iron meteorites that are members of the magmatic groups, for three main group pallasites, one anomalous mesosiderite, and for three ungrouped irons and an ungrouped pallasite that are similar to IIIAB irons in their Ni, Ga, and Ge contents. The set includes four observed falls (11% of iron falls) Ban Rong Du, Chisenga, Nyaung and Sterlitamak, and Zaisho, one of two known pallasite falls. Two of the ungrouped irons (Ban Rong Du and Mount Howe 88403) and the ungrouped pallasite Yamato 8451, although having Ni, Ga, and Ge contents in the same general range as IIIAB, have very different contents of Co and exhibit significant differences for several other elements; they are clearly not related to IIIAB or to its little sister, group IIIE. A fourth ungrouped iron, Tres Castillos, chiefly differs from IIIAB in terms of its low Ga and high Ge contents; its Ga/Ge ratio is 35% higher than that of any other IIIAB iron. We report data on four new IIAB irons, all falling within established fields; the Bilibino iron is somewhat unusual, having a low Ir content (0.12 μg/g) and a structure altered by reheating. The IVA irons are also typical. One, Albion, may be a mislabeled specimen of Gibeon; another, Page City, exhibits large cracks (up to 3 cm). The Chaunskij anomalous mesosiderite has exceptionally high Ni and very low Ir concentrations. Two of three new main group pallasites are anomalous; Pecora Escarpment 91004 has an Ir content above the normal range, and Zaisho has an exceptionally high Fa content in the olivine.

Excitonic transitions in (GaAs)1−x(Ge2)x/GaAs multilayers grown by magnetron sputtering

We report on excitonic transitions observed at room temperature in multilayers of (GaAs)1−x(Ge2)x/GaAs grown by magnetron sputtering. We attribute the observation of the exciton absorption at room temperature to confinement effects on the bound electron–hole pairs inside the Ge-rich layers of the (GaAs)1−x(Ge2)x alloys. From secondary ions mass spectroscopy these layers are measured to be 50, 33, and 30 nm in full width at half maximum (FWHM), for three different samples. These layers alternate with (GaAs)1−x(Ge2)x alloy layers of very low Ge concentration that can be considered of plain GaAs, ∼360 nm in thickness for two samples and 240 nm for a third one. The observed transition energies of the exciton are very close to ≈1 eV both at 300 and 4 K. A calculation of exciton energy-level spacing and transition probabilities provides acceptable values for the exciton transition energies observed, considering a triangular quantum well as an approximation to the Ge profile in the confinement layers. The observed excitonic FWHM remain nearly constant at all temperatures examined (300–4 K).

Evolution of magnetism and superconductivity in CeCu2(Si1−xGex)2

We have determined the magnetic phase diagram of CeCu${}_{2}$(Si${}_{1\ensuremath{-}x}$Ge${}_{x})$${}_{2}$ using specific heat, susceptibility, and resistivity measurements and additional thermal expansion and muon spin resonance experiments. The system evolves continuously from the heavy-fermion state at $x=0$ to an antiferromagnetically (AF) ordered state at $x=1$, though the magnetic structure undergoes significant modifications. The results strongly suggest that the AF ordering emerges from the $A$ phase of pure CeCu${}_{2}$Si${}_{2}$. The phase diagram can be divided into three regions: Low Ge content $(x&lt;~0.2)$ leads to an enhancement of the $A$ phase of pure CeCu${}_{2}$Si${}_{2}$ and to a moderate decrease of the Kondo temperature. Despite this increase of the magnetic character, we could observe superconductivity up to $x=0.1$. At large Ge content $(x&gt;0.5)$ the behavior resembles that of pure CeCu${}_{2}$Ge${}_{2}$. The intermediate region is characterized by the appearance of a second magnetic transition below ${T}_{N},$ which seems to be of first-order type. The appearance of this transition leads to a well-defined critical point at $x=0.3$ and suggests the possibility of another critical point at $x\ensuremath{\simeq}0.5$.

A deep level transient spectroscopic study of boron-ion-implanted single quantum wells

The defects induced by boron-ion implantation with a relatively low dosage of in single quantum wells are studied by deep level transient spectroscopy (DLTS). For low Ge content x, a defect level at an energy of 0.52 eV above the silicon valence band edge was found in the well region and its boundaries. For samples with higher Ge content, such that the strain is released, an electron trap rather than is formed by the ion implantation. Rapid thermal annealing at removes most of the defects induced by the ion implantation without changing the properties of the quantum well.

Investigation on solid phase epitaxy of deposited SiGe film on a Si substrate

The solid phase hetero-epitaxy of amorphous SiGe on a Si(100) substrate has been investigated. The initial SiGe was deposited by ion beam sputtering at room temperature from a composite target of Si and Ge wafers. The solid phase epitaxial SiGe layer was formed through a boron diffusion, thermal annealing and oxidation process, respectively. The boron diffusion was carried out at 970 °C and then 1065 °C for redistribution, thermal annealing was carried out at 1000 °C in a N 2 ambient, and oxidation at 90 °C in steam, dry O 2 and steam ambient subsequently. Auger electron spectroscopy, X-ray diffraction, scanning electron microscopy, Rutherford backscattering spectroscopy/channelling, cross-sectional transmission electron microscopy, and Raman Spectroscopy were used to characterize the SiGe film. The experimental results indicate that through a boron diffusion, Ge atoms diffuse considerably generating a SiGe layer with a lower Ge concentration than that of the as-deposited film, while the thermal annealing makes the amorphous SiGe layer crystallize, for the oxidation of the a-SiGe/Si structure, a newly formed SiGe/Si heterostructure with a higher Ge concentration than that or the as-deposited film has been found due to the selective oxidation of Si at the applied temperature.

Ge nanocrystals with a sharp size distribution: A detailed study of the crystallization of a-Si 1- xO xGe y alloy films

Thin amorphous Si 1− x O x Ge y alloy films were prepared by dc magnetron sputtering. We found that the as-prepared films contain amorphous Ge clusters with small size distributions. A low Ge content of the films is connected with very small Ge clusters in the as-prepared films and with regions where Ge atoms are densely spread in the matrix. Films with a larger Ge content show an increasing number of larger clusters. These clusters are closed together, thus producing dimeres and trimeres. Different annealing procedures resulted in different modes of crystallization. A crystallization of the as-prepared clusters is found after an annealing process with crystal diameters slightly greater than the cluster size. With annealing step by step inside the diffraction system we demonstrated the growth process including the final recrystallization of the aggregates.

Roughening of SiGe layers grown with gas-source MBE: dependence on Ge concentration and growth temperature

We have investigated the roughening of SiGe layers grown from disilane and germane on Si(001) using gas-source molecular beam epitaxy (GSMBE). Si 1 − x Ge x layers were grown in the temperature range of 500–650°C while the Ge concentration x was varied between 0.06–0.28. Photoluminescence spectroscopy (PL) was used to calibrate the Ge concentration in SiGe layers. During growth the SiGe surfaces were inspected with reflection high-energy electron diffraction (RHEED) and after deposition with scanning electron microscopy (SEM). At low temperatures and Ge concentrations we found smooth (2D) layers. However, for high temperatures and Ge concentrations we obtained rough {118} and {113} faceted layers (3D). Comparison with solid-source MBE (SSMBE) results (Bean et al. [J. Vac. Sci. Technol. A 2 (1984) 436]) reveal that 3D growth in GSMBE is observed for lower Ge concentrations and temperatures. This difference can be explained by the much lower growth rates used in our experiments. At higher growth rates the surface atom diffusion length is reduced and the development of the thermodynamically favourable 3D phase is suppressed.

Enhanced Step Waviness on SiGe(001)-(2 x 1) Surfaces under Tensile Strain.

Scanning tunneling microscopy measurements of thermally cleaned, low Ge content SiGe(001) films under an $\ensuremath{\sim}1%$ biaxial tensile strain show large amplitude, quasiperiodic, thermally stable ${S}_{B}$-step undulations alternating with extremely straight ${S}_{A}$ steps on extended areas wherever the local step separation $L&gt;20$ nm. The wavelength $\ensuremath{\lambda}$ scales roughly with ${L}^{1/2}$ for $20&lt;L&lt;120$ nm, and reaches a limiting value between 100 and 200 nm on larger terraces. The measured behavior suggests that the wavy steps are stabilized by surface stress relaxation effects which are enhanced by tensile strain.

The double nature of the 3.1 eV emission in silica and in Ge-doped silica

The 3.1 eV photoluminescence emission ('beta' band) has been studied both in natural silica and in Ge-doped silica in the temperature range 80-475 K under UV and VUV excitation. A detailed analysis of the beta band, as a function of temperature and excitation energy in the 5 eV region, gives evidence for a double structure of this emission. A first component peaks at 3.05 eV at 300 K and increases its intensity as the temperature rises up to 380 K. A second one has a peak at 3.21 eV at room temperature and its emission intensity is constant up to 300 K. The two beta bands may be due to two different modifications of a Ge-related centre. However the presence of the beta emission in silica obtained from fused quartz with low Ge content would indicate that other impurities could favour the presence of the beta band as well.

Improved Ambipolar Diffusion Length in a-Si1-xGex:H Alloys for Multi-Junction Solar Cells

ABSTRACTTo prepare hydrogenated amorphous silicon-germanium alloys as low gap material for multi-junction solar cells in plasma enhanced chemical vapour deposition, the well established concept of strong dilution of the process gases with hydrogen has been used. Two different regimes of alloying were found: for low Ge content (x &lt; 0.40) we observe material with low defect density, small Urbach energy and high values of the ambipolar diffusion length. In the regime of high Ge content (x &gt; 0.40) the defect densities and Urbach energies are high and the values of the ambipolar diffusion length low. The transition is accompanied by the appearance of a low-temperature peak in hydrogen effusion experiments indicating a void rich film structure. Material from just above and below the transition zone is used in pin solar cells leading to a much enhanced red response compared with a-Si:H cells. The differences seen in the material quality are mirrored in the solar cell properties. By carefully adjusting the active layer thickness material with low diffusion length shows also reasonable solar cell performance.