Ge Flux Research Articles

Micropipe healing in SiC wafers by liquid-phase epitaxy in Si–Ge melts

Liquid-phase epitaxy (LPE) of silicon carbide (SiC) was shown to be an effective technique to overgrow micropipe defects (MP) in SiC wafers prepared by the physical vapour transport (PVT) technique. Growth close to thermodynamic equilibrium, e.g. low supersaturation, provides a favourable condition for effective MP healing. Therefore, the aim of this work was to grow epitaxial layers from strongly diluted Si-based solutions. Using the method of horizontal dipping, the dependence of MP elimination efficiency on Si–Ge flux composition and on the crystallographic orientation (on- and off-axis) of the SiC wafer was investigated. High-quality single crystalline SiC layers of a thickness up to 10μm were grown with the growth rate of 0.5μm/h. On off-oriented wafers, stepped growth morphology was observed independent of the melt composition. Micropipes with the diameter below 5 μm were closed with an efficiency of about 80%. SEM investigations as well as inspection under reflected/transmitted light did not show any specific distortion of the growth morphology at the micropipe healing place.

Desorption of Arsenic Species during the Surfactant Enhanced Growth of Ge on Si(100)

The desorption and scattering of As4 and As2 species from the surface during the growth of germanium films on Si(100) with continuous As4 deposition is monitored using laser ionization time-of-flight mass spectrometry. A significant increase in the flux of As2 from the surface is observed when the Ge flux is admitted to the surface. Upon discontinuation of the germanium growth process, the As4 and As2 signal levels remain at this increased level. A comprehensive study of the desorption fluxes of As4, As2, and As species from both Ge(100) and Si(100) substrates was performed as a function of substrate temperature and incident As4 flux to determine the kinetics of desorption of the different species from the substrates. The behavior of the As2 desorbed fluxes as a function of surface temperature is qualitatively different on the Ge(100) and Si(100) substrates. The results indicate that the catalytic cracking of As4 to As2 is more effective on Ge(100) compared to Si(100) at substrate temperatures between 800...

Modelled glacial and non-glacial HCO 3−, Si and Ge fluxes since the LGM: little potential for impact on atmospheric CO 2 concentrations and a potential proxy of continental chemical erosion, the marine Ge/Si ratio

The runoff and riverine fluxes of HCO 3 −, Si and Ge that arise from chemical erosion in non-glaciated terrain, are modelled at six time steps from the Last Glacial Maximum (LGM) to the present day. The fluxes that arise from the Great Ice Sheets are also modelled. Terrestrial HCO 3 − fluxes decrease during deglaciation, largely because of the reduction in the area of the continental shelves as sea level rises. The HCO 3 − fluxes, and the inferred consumption of atmospheric CO 2 are used as inputs to a carbon cycle model that estimates their impact on atmospheric CO 2 concentrations ( atmsCO 2). A maximum perturbation of atmsCO 2 by ∼5.5 ppm is calculated. The impact of solutes from glaciated terrain is small in comparison to those from non-glaciated terrain. Little variation in terrestrial Si and Ge fluxes is calculated (<10%). However, the global average riverine Ge/Si ratio may be significantly perturbed if the glacial Ge/Si ratio is high. At present, variations in terrestrial chemical erosion appear to have only a reduced impact on atmsCO 2, and only little influence on the global Si and Ge cycle and marine Ge/Si ratios during deglaciation.

Proof of kinetic influence in Ge nanowire formation on Si(1 1 3)

Based on scanning tunneling microscopy observations, we have investigated the formation and self-stabilization of Ge nanowires on Si(1 1 3) during Ge deposition. Under zero Ge flux, we observed a shape transition from nanowires to dot-like islands by annealing at 430°C, which is a favorable temperature for nanowire formation during deposition. The nanowires are, therefore, metastable and are formed under a kinetically limited growth condition. We find that the strain of the nanowire is relaxed anisotropically. During growth the nanowire shape is effectively self-stabilizing, which leads to elongated growth of the islands.

Scanning tunneling microscopy tip shape imaging by “shadowing”: Monitoring of in situ tip preparation

The shape of the scanning tunneling microscope (STM) tip was imaged with nanometer resolution using “shadow images.” When a Ge molecular beam is evaporated from the side onto a Si sample while the STM tip is stationary in tunneling position close to the sample surface, the tip shades part of the sample surface. Subsequent imaging of this shadow area with the STM results in contrast between areas on the sample where Ge was evaporated, and areas where the Ge flux was impeded by the tip. This method was applied to monitor the tip shape after in situ tip preparation processes, like sputtering and heating of the tip.

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.

Ge islands on Si(111) at coverages near the transition from two-dimensional to three-dimensional growth

We use scanning reflection electron microscopy (SREM) to study three-dimensional Ge island formation on the Si(111) surface at coverages near the transition from two-dimensional to three-dimensional growth under Ge fluxes between 0.001 and 0.02 BL/s and at temperatures between 380 and 690°C. The SREM images clearly show that the formation of flat three-dimensional islands with large lateral dimensions is dominant at higher temperatures. An average critical island size of about nine atoms was obtained from the flux dependence of island density. The existence of such large size critical islands is supported by the intermediate formation of the second Ge bilayer before island nucleation, which then decomposes, and by the rapid saturation of island density. From the temperature dependence of island density the activation energy for a Ge atom to detach from the islands to form an adatom was estimated to be ∼1.1 eV.

Low-temperature growth properties of Si 1− xGe x by disilane and solid-Ge molecular beam epitaxy

Low temperature (⩽500°C) growth properties of Si 1− x Ge x by disilane and solid-Ge molecular beam epitaxy have been studied with an emphasis on surface morphology and growth kinetics. It is found that low-temperature growth (<500°C) is in layer-by-layer mode and atomically-smooth surfaces have been obtained in as-grown samples with large Ge composition (>0.5). Ge composition dependence on substrate temperature, Ge cell temperature and disilane flow rate have been investigated. It is found that in low-temperature growth (⩽500°C) and under large disilane flux, Ge composition increases with the increase of Ge flux and further increase of Ge flux leads to the saturation of Ge composition. Similar compositional dependence has been found at different growth temperatures. The saturated composition increases with the decrease of substrate temperature. The results can be explained if H desorption is assumed to occur from both Si and Ge monohydrides without diffusional exchange and the presence of Ge enhances H desorption on a Si site.

Hall factor and drift mobility for hole transport in strained Si1−xGex alloys

Hole transport in B-doped strained Si1−xGex has been studied using Hall measurements for boron concentrations from 2×1018 to 7.5×1018 cm−3 with Ge content 0⩽x⩽0.36. By keeping the B flux constant during the molecular beam epitaxy growth of sets of samples and only varying the Si and Ge fluxes, we were able to prepare samples for an accurate determination of the Hall factor based on using the established relationship between B-doping concentration and resistivity for pure Si. It was found that the Hall factor drops considerably when the Ge content is increased. Determined Hall factor values are compared with calculated values taking into account the full valence band structure and various scattering mechanisms. The hole drift mobility has been derived from our measured Hall mobility using the determined Hall factor for the corresponding Ge content. We find that, depending on the doping concentration, the drift mobility can be higher for strained layers containing Ge. The grown layers were also characterized using x-ray diffraction, where the Ge contents and layer thicknesses could be confirmed. Temperature dependent measurements from 50 K to room temperature have also been made.

Study of interaction between incident silicon and germanium fluxes and SiO2 layer using solid-source molecular beam epitaxy

The dependence on the substrate temperature and the flux rates of the behavior of impinging elemental Si and Ge fluxes on a SiO2 surface, including SiO2 etching and the deposition of polycrystalline Si and SiGe films, was investigated by using the solid source molecular beam epitaxy (MBE). Flux rates of the source beams and a substrate temperature were in the range of 1 to 5×1013 atoms/cm2 s and 710–810 °C, respectively. Under these experimental conditions, the Ge flux was not individually effective to etch the SiO2, but contributed to etching the oxide layer with an accompanying Si flux. The critical flux of Si for SiO2 etching at 740 °C was determined to be about 1×1013 atoms/cm2 s.

Ge growth on Si using atomic hydrogen as a surfactant

We have examined the effect of adsorbed atomic hydrogen (H) on the evolution of Ge films on Si(001) and (111) substrates in solid-source molecular-beam epitaxy. The H flux was supplied separately from the Ge flux. By cross-sectional high-resolution transmission electron microscopy it was observed that H acted as a surfactant during growth, suppressing island formation of Ge on both substrates. The effect of the H surfactant on variation of the growth mode is also discussed.

Silica and germanium in Pacific Ocean hydrothermal vents and plumes

Dissolved silica (Si) and inorganic germanium (Ge) concentrations were measured in hydrothermal fluids from black smoker vents on the East Pacific Rise (21°N EPR) and the Southern Juan de Fuca Ridge (45°N SJdFR: North and South Cleft Sites, Axial Volcano). These typically display end-member concentrations ranging from 16 to 23 mM (Si) and 150 to 280 nM (Ge), and end-member Ge/Si ratios clustering between 8 and 14 × 10 −6, more than 10-fold greater than the ratio entering the ocean via rivers (0.54 × 10 −6) and being recycled in seawater (0.7 × 10 −6). ‘Excess’ concentrations of dissolved Si and Ge above oceanic background are observed in mid-water hydrothermal plumes over mid-ocean ridge (MOR) spreading centers on the Southern EPR (SEPR) (10°–20°S) and the SJdFR. The largest Si and Ge concentration anomalies occur over the North Cleft Segment of the SJdFR. These are a factor of three greater than anomalies over the SEPR (10°–20°S). Excess Ge correlates with excess 3He in plumes at a Ge/ 3He molar ratio of about 1 × 10 4, approximately the same ratio as in black smokers. These observations, combined with low abundances of Ge in Fe Mn-rich metalliferous sediments, suggest that Ge (and Si) behave conservatively in mid-ocean ridge hydrothermal plumes. A simple ocean Si and Ge balance, constrained by the global river silica flux and Ge/Si ratios in hydrothermal vents, rivers and biogenic silica, suggests that the global hydrothermal silica flux is about 1–4 × 10 11 mole yr −1, much lower than that estimated from 3He. Either (1) 70–80% of the Ge flux to the ocean is removed in as-yet undiscovered sinks (not opal), or (2) only 10% of the mantle to ocean 3He and heat fluxes is associated with MOR hydrothermal convection through the 350°C isotherm (90% is off-ridge), or (3) the oceanic Ge/Si, 3He/ (and 87Sr 86Sr) balances today are far from steady-state.

High resolution X-ray diffraction studies of semiconductor superlattices

Recent years have seen the development and application of high resolution X-ray diffraction techniques in order to non-destructively assess the structural and compositional quality of complex epitaxial semiconductor structures. The most widely used technique is double crystal diffraction, which is used to record the complex multipeaked rocking curves associated with current multilayer epitaxial structures. However, effects of material quality on this diffraction data can only be quantified and understood by the application of a versatile theoretical rocking curve simulation package which is capable of simulating the effects of small deviations from the ideal material structure. The use of both theory and experiment to identify and solve growth induced materials problems in semiconductor superlattice structures is demonstrated with a number of examples. In Si 1- x Ge x /Si superlattices grown by molecular beam epitaxy a detailed analysis of the asymmetric broadening of superlattice reflections enables this effect to be directly attributed to a slight drift of 1.25% in the Ge flux during the growth run. In In 1- x Ga x As/InP superlattices grown by MOCVD the exact nature of the growth system improvements necessary to sharpen the layer interfaces are identified and in MBE grown InSb/CdTe superlattices the presence and nature of thin, highly strained interfacial layers is assessed. The limitations of the double crystal system are outlined along with the advantages of moving towards a multiple crystal diffraction system.

Double-crystal x-ray diffraction from Si1−xGex/Si superlattices: Quantification of peak broadening effects

Double-crystal x-ray diffraction has been successfully used to study dimensional and compositional variations in Si1−x Gex /Si superlattices. For most samples studied an adequate agreement between theoretical and experimental diffraction profiles is obtained by using a theoretical model (kinematical) which assumes uniform superlattices with abrupt interfaces. In some samples, however, the superlattice-related diffraction peaks reveal an asymmetric broadening which is shown by comparison with dynamical diffraction simulations to be due to a grading in layer compositions and thicknesses throughout the superlattice stack. Detailed analysis of the diffraction data identified a gradual drift of 1.25% in the Ge flux during molecular beam epitaxy (MBE) growth. The MBE system parameters were modified to compensate for this effect and allow the growth of more structurally perfect strained-layer superlattices.

Molecular beam epitaxial growth of II–V semiconductor Zn 3As 2 and II–IV–V chalcopyrite ZnGeAs 2

The first molecular beam epitaxial growth of the II–V semiconducting compound Zn 3As 2 and mixed crystals of chalcopyrite ZnGeAs 2 and Ge is reported. Zn 3As 2 can be grown on InP or GaAs at 300 through 360°C. A small mismatch with InP leads to commensurate single domain growth of the tetragonal Zn 3As 2. At room temperature, the unintentionally doped Zn 3As 2 has a hole concentration of 5×10 18 cm -3, a hole mobility of 43 cm 2/V.s, and an absorption edge at 0.99 eV. Low temperature data suggest that the lowest acceptor level is very close in energy to the valence band maxima located at k ≠ 0, while the k = 0 energy of the uppermost valence band is slightly lower. Single crystalline layers of Ge-ZnGeAs 2 alloys are obtained on GaAs by using a substrate temperature of 380°C. A very high Zn: Ge flux ratio is required for the growth to compensate for the disproportionately small sticking coefficient of Zn relative to that of Ge. The lowest Ge/Zn ratio obtained in the epitaxial layer is ⋍ 2. This is achieved by using a very low Ge flux corresponding to a growth rate of 50 nm/h. The epitaxial layer has zinc-blende structure due to the high Ge content.

Growth, nucleation, and electrical properties of molecular beam epitaxially grown, As-doped Ge on Si substrates

Epitaxial Ge is grown on (100)Si substrates by molecular beam epitaxy (MBE). The effect of various MBE growth conditions on both the nucleation and morphology of Ge grown on Si is studied by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and reflection high-energy electron diffraction (RHEED). These studies indicate that, at the substrate temperatures examined (300–600 °C), heteroepitaxy of Ge on Si favors three-dimensional growth, which is enhanced by both higher growth temperatures and substrate preparation techniques that leave residual surface contamination. Heavily doped n+ Ge layers are obtained using an elemental As source. The electrical properties of these films are evaluated by Hall–van der Pauw measurements. Growth temperatures of 250 °C and optimum As:Ge flux ratios yield electron concentrations as high as 2.5×1020 cm−3. Secondary ion mass spectroscopy (SIMS) and Hall effect data show that for As concentrations which exceed this optimum level, a decrease in both the electron concentration and drift mobility is observed, indicating the presence of electrically inactive As.

Germanium incorporation in heavily doped molecular beam epitaxy grown GaAs:Ge

Germanium is normally an n-type dopant in MBE GaAs but compensation and p-type action has been observed under certain conditions. Studies have been made of the growth conditions under which p+ and n+ layers are obtained with heavy Ge fluxes during MBE GaAs growth on GaAs substrates at 570 °C. Both p+ and n+ dopings in excess of 1020 cm−3 have been observed. The production of p-type layers is favored by As4/Ga ratios of 2–3:1 and high Ge fluxes. For As4/Ga ratios of 4–5:1 and Ge fluxes corresponding to dopings of the order of mid-1018 cm−3 or less, the layers grown are n-type. For As4/Ga ratios greater than 6:1, the layers may be n+ type with concentrations that are 1020 cm−3 or higher as given by capacitance measurements or van der Pauw measurements. Photoluminescence studies at 4 K for 1020 cm−3 doped layers show a broadband with a maximum at 1.27–1.33 eV for n+ layers, and for p+ layers a broadband luminescence with evidence of peaking at about 1.38 and 1.35 eV. Transmission electron microscope studies show almost no tendency for the formation of a second phase or microprecipitation in the most heavily doped (2×1020 cm−3 p and n) specimens. Finally, the paper considers the possible role of the species GeGa, GeAs, (GeGaVGa), and (GeAsVGa) and other complexes in the doping effects observed.

Dependence of the electrical characteristics of heavily Ge-doped GaAs on molecular beam epitaxy growth parameters

Ge incorporation into molecular beam epitaxial GaAs as a function of substrate temperature (350–620 °C), Ge flux (109–1014 cm−2 sec−1) and of As4/Ga flux ratio (10/1) has been studied. The free-electron (7.4×1019 cm−3) and free-hole (3.0×1020 cm−3) concentrations in GaAs were the highest obtained to date. Tentative incorportion mechanisms for Ge in GaAs are also presented.

Abstract: Photovoltaic effect in obliquely deposited germanium films

If germanium is obliquely deposited at relatively high pressures, an inplane photovoltaic effect can produce voltages as high as 3000 V. Reproducibility of the effect within a single laboratory and among various laboratories has been poor. No satisfactory explanation of the phenomenon has been offered. We find that the partial pressure of oxygen during deposition and the angle of deposition are both of first−order importance in determining both the magnitude and the polarity of the photovoltage. Increasing the partial pressure of oxygen PO2 increases the magnitude of the photovoltage. Decreasing the angle of incident Ge flux from normal (90°) to 15° increases the magnitude of the photovoltage. There is a polarity reversal as PO2 goes from 10−5 Torr to 10−4 Torr. Photovoltage magnitudes increase slowly as average thickness of the film increases. ESCA studies indicate that for highest photovoltages, the film must be made up of two distinct constituents that are (or are associated with) Ge and GeO2. These new results, along with previous findings, are consistent with a structural model for the film which appears to provide the basis for an understanding of the phenomena.