yGexCy Layers Research Articles

Raman and Fourier transform infrared study of substitutional carbon incorporation in rapid thermal chemical vapor deposited Si1−x−yGexCy on (1 0 0) Si

We report on a detailed study of the dependence of the vibrational modes in rapid thermal chemical vapor deposited Si1−x−yGexCy films on the substitutional carbon concentration. Si1−x−yGexCy films were investigated using Raman and infrared spectroscopy with x varying in the range of 10%–16% and y in the range of 0%–1.8%. The introduction of C into thin SiGe layers reduces the average lattice constant. It has been shown that the integrated infrared intensity of the Si–C peak and the ratio of both the Raman integrated and peak intensities of the Si–C peak (at ∼605 cm−1) to the Si–Si peak of SiGeC layer, increase linearly with C content and are independent of the Ge content. This leads to the conclusion that infrared absorption and Raman scattering data can be used to determine the fraction of substitutional carbon content in Si1−x−yGexCy layers with a Ge content of up to 16%. It is also shown that the intensity ratio of the carbon satellite peak to the local carbon mode increases linearly with C content up to a C level of 1.8%. This confirms a conclusion of an increase in the probability of creating third-nearest-neighbor pairs with increasing carbon content, as derived from theoretical calculations.

Substitutional C effect on generation lifetime in MBE-grown SiGeC layers

This paper investigates the effect of substitutional carbon on generation lifetime in pseudomorphically strained Si1−x−yGexCy (x ∼ 0.1, y < 0.006) layers grown by gas source molecular beam epitaxy (GSMBE). The electrical assessment consists of capacitance–voltage and capacitance transient techniques on metal-oxide-semiconductor (MOS) capacitors based on Si/SiGeC heterostructures. The carrier lifetime study shows microsecond range lifetimes for substitutional carbon contents, [Cs] < 0.5%. The charge confinement in the potential well, formed at the SiGeC/Si heterointerface, is found to have less effect on generation lifetime than the substitutional to interstitial carbon ratio in these alloys. Samples with substitutional carbon concentrations above 0.5% exhibit a sharp decrease of generation lifetime into the nanosecond range, attributed to an increased ratio of interstitial carbon. Si0.8826Ge0.116C0.0014 and Si0.8809Ge0.116C0.0031 layers grown by GSMBE can be used in high-performance heterojunction bipolar transistors (HBTs).

Strain relaxation in thin SiGe epilayers doped with carbon

The possibility of obtaining a heterosystem consisting of the upper partially strained and lower relaxed sublayers by stepwise in situ doping of SiGe layers by carbon is considered. The properties of the as-grown and annealed samples are studied by means of Raman spectroscopy and atomic force microscopy. The degree of strain relaxation in the as-grown layers was derived from the Raman spectra. The Si0.7−yGe0.3Cy layer was shown to be stable to annealing up to 700°C. Moreover, the strain relaxes during annealing not homogenously over the whole structure but in a layer-by-layer way. The segregation of carbon atoms is observed for as-grown Si1−x−yGexCy layers in the near-substrate regions.

Electrical and materials characterization of GSMBE grown Si1−x−yGexCy layers for heterojunction bipolar transistor applications

This paper reports on detailed materials and electrical characterization of strain-compensated Si1−x−yGexCy (x ⩽ 11%, y ⩽ 1%) layers synthesized by gas source molecular beam epitaxy (GSMBE). Materials assessment included the application of spectroscopic ellipsometry (SE), x-ray diffraction (XRD) and high-resolution cross-section transmission electron microscopy (HRXTEM). An almost fully strain-compensated Si0.88Ge0.11C0.01 alloy with high substitutional carbon incorporation (XRD) and well-defined structure with coherent interfaces (HRXTEM) was produced. An increase of C content (up to 1.6%) resulted in a tensile strained SiGeC alloy and degraded electrical characteristics. The minority-carrier lifetime is a parameter strongly dependent on mid-gap energy band levels, which determine the leakage currents in heterojunction bipolar transistors, and it was assessed here using the capacitance-transient technique. Metal–oxide–semiconductor (MOS) test capacitors were fabricated on Si/SiGe/Si and Si/SiGeC/Si structures using low-temperature plasma oxidation for dielectric growth. The strain-free Si0.88Ge0.11C0.01 layers exhibit a minority-carrier lifetime, τg, of ∼0.1–0.2 µs which constitutes degradation of the carrier lifetime by two orders of magnitude compared to control SiGe layers. The obtained range of τg is nevertheless sufficient for the successful application of epitaxial strain-compensated SiGeC layers in high performance heterojunction bipolar transistors (HBTs).

Methods to reduce the loading effect in selective and non-selective epitaxial growth of sigec layers

Various methods to reduce both global and local loading effect during non-selective and selective epitaxial growth of Si1−x−yGexCy (0.09≤x≤0.28 and 0≤y≤0.01) layers have been proposed. Evaluation of the proposed solutions for issues such as defect generation and the possibility for integration in device structures have been performed. The key point in these methods is based on reduction of surface diffusion of the adsorbed species on the oxide. In non-selective epitaxy, this was achieved by introducing a thin silicon polycrystalline seed layer on the oxide prior to Si1−x−yGexCy deposition. The thickness of this seed layer had a crucial role on both the global and local loading effect, and also on the epitaxial quality. Higher carbon content (y≥0.006) in Si1−x−yGexCy layers had no noticeable influence on the loading effect, however, the defect density was clearly increased in these layers. In selective epitaxy case, introducing square polycrystalline Si stripes around the oxide openings acting as diffusion barriers have reduced the loading effect effectively. Meanwhile, using Si nitride stripes showed no visible effect on Si1−x−yGexCy layer profile. Further decrease in loading effect can be performed by increasing the HCl partial pressure during epitaxy. Chemical–mechanical polishing (CMP) was performed to remove the polycrystalline stripe on the oxide.

The effect of carbon and germanium on phase transformation of nickel on Si1−x−yGexCy epitaxial layers

The influence of carbon and germanium on phase transformation and sheet resistance of Ni on epitaxially grown Si1−x−yGexCy (0⩽x⩽0.24 and 0⩽y⩽0.01) layers annealed in a temperature range of 360 to 900 °C has been investigated. The role of strain relaxation or compensation in the reaction of Ni on Si1−x−yGexCy layers due to Ge or C out-diffusion to the underlying layer during the phase transformation has also been investigated. The formed NiSiGe layers were crystalline, with strong (020)/(013) growth orientation in the direction, but the thermal stability decreased rapidly with increasing Ge amount due to agglomeration. However, this thermal behavior was shifted to higher annealing temperatures when carbon was incorporated in the SiGe layers. A carbon accumulation at the interface of NiSiGeC/SiGeC has been observed even at low-temperature annealing, which is suggested to retard the phase transformation and agglomeration of Ni/SiGeC system.

Reduced pressure chemical vapour deposition of Si/Si1−x−yGexCy heterostructures using a chlorinated chemistry

We have studied in reduced-pressure chemical vapour deposition the influence of the growth temperature on the interstitial–substitutional carbon incorporation ratio in SiGeC using dichlorosilane as the Si gas precursor. X-ray diffraction and secondary ion mass spectrometry were used in order to determine the substitutional and the total (interstitial + substitutional) carbon concentrations in the layers. Si/Si1−yCy tensile-strained heterostructures were grown at 650 °C. The highest substitutional carbon concentration incorporated is around 0.75% (with a large amount of interstitial C atoms (also 0.75%), however). Low Ge concentration Si1−x−yGexCy layers have also been grown. We have managed through the increase of the SiCH6 mass flow to tailor the Si1−x−yGexCy layer strain from compressive to tensile. The highest substitutional carbon concentrations obtained are 1.82% (0.95%) for growth temperatures of 600 °C (650 °C). Finally, compressively strained high Ge concentration Si1−x−yGexCy layers with up to 3.20% (2.49% and 1.41%) in substitutional carbon atoms were grown at 550 °C (600 °C and 650 °C). The growth temperature does not seem to impact upon the total carbon concentration CT in these layers. Its dependence on the F(SiCH6)/(F(SiH2Cl2) + F(GeH4)) mass flows ratio is indeed rather well fitted using the following relationship: CT1.4/(1 − CT) = q(F(SiCH6)/(F(SiH2Cl2) + F(GeH4)), with q in between 0.38 and 0.34 for the 550–650 °C temperature range. However, the more the temperature increases, the more the substitutional/interstitial carbon incorporation ratio is reduced.

Growth temperature dependence of substitutional carbon incorporation in SiGeC/Si heterostructures

We have studied in reduced-pressure chemical vapor deposition the influence of the growth temperature on the interstitial-substitutional carbon incorporation ratio in SiGeC. Si1−yCy as well as high- and low-Ge concentration Si1−x−yGexCy layers were grown at 550, 600, and 650 °C. X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS) were used in order to determine the substitutional and the total (interstitial+substitutional) carbon concentrations. As far as high-Ge concentration Si1−x−yGexCy layers are concerned, compressively strained layers with up to 1.74% (1.26% and 0.62%) in substitutional carbon atoms were grown at 550 °C (600 and 650 °C). Low-Ge concentration Si1−x−yGexCy layers have also been grown. We have managed through the increase of the SiCH6 mass flow to tailor the Si1−x−yGexCy layer strain from compressive to tensile. The highest substitutional carbon concentrations obtained are 1.51% (1.21% and 0.92%) for growth temperatures of 550 °C (600 and 650 °C). Si1−yCy/Si tensile-strained heterostructures were also grown. The highest substitutional carbon concentration incorporated is around 1.44% (1.12% and 0.83%) at 550 °C (600 and 650 °C). Varying the Si1−x−yGexCy and Si1−yCy layers growth temperature does not seem to impact upon the total carbon incorporation. However, by comparing SIMS and XRD data, we showed that the more the temperature increases, the more the substitutional/interstitial carbon incorporation ratio is reduced.

Reduced pressure chemical vapor deposition of Si1−x−yGexCy/Si and Si1−yCy/Si heterostructures

We have grown by reduced pressure chemical vapor deposition Si1−x−yGexCy/Si and Si1−yCy/Si heterostructures for electrical purposes. The incorporation of substitutional carbon atoms into Si is expected to play a double role. On the one hand, by creating a carrier confinement in the channel region of metal–oxide–semiconductor transistors, the compressive strained Si1−x−yGexCy and the tensile strained Si1−yCy layers can be used to improve transport properties. On the other hand, the addition of carbon atoms can compensate the compressive strain induced by large amounts of Ge. As far as high Ge concentration Si1−x−yGexCy layers are concerned (Ge=18 at. %), x-ray diffraction (XRD) measurements show the progressive shift of the Si1−x−yGexCy peak towards the Si substrate peak, evidencing the strain compensation due to the incorporation of carbon atoms into the substitutional sites of the SiGe matrix. Carbon incorporation results in a decrease of the growth rate, from 10 nm/min (for substitutional C=0.46 at. %) down to 9 nm/min (when substitutional C=1.26 at. %). High quality Si1−x−yGexCy layers with up to 1.26 at. % of substitutional carbon atoms were grown. Transmission electron microscopy imaging and the comparison between secondary ion mass spectrometry (SIMS) and XRD results revealed that a further increase of the SiCH6 flow leads to an increase of the carbon incorporation into interstitial sites, which translates into a rapid deterioration of the Si1−x−yGexCy layer. Slightly compressive-strained Si1−x−yGexCy layers (Ge=4 at. %) have also been grown. Through the increase of the SiCH6 flow we have managed to tailor the Si1−x−yGexCy layer strain from compressive to tensile. The Si1−x−yGexCy growth rate drops from 3.4 nm/min down to 3.0 nm/min when the substitutional carbon concentration increases from 0.4 up to 1.21 at. %. Finally, Si1−yCy/Si tensile-strained heterostructures were grown. Smaller growth rates (1.8 nm/min) for Si1−yCy than for Si1−x−yGexCy are obtained. SIMS and XRD measurements indicate that the highest carbon concentration Si1−yCy layer (substitutional C=1.12 at. %) also contains many interstitial carbon atoms (interstitial C=0.45 at. %).

Hall mobilities in B-doped strained Si1−xGex and Si1−x−yGexCy layers grown by ultrahigh vacuum chemical vapor deposition

Hall mobilities in a temperature range of 80–300 K have been measured in fully strained Si1−xGex and partially strain-compensated p-type Si1−x−yGexCy alloy layers grown on Si (100) by ultrahigh vacuum chemical vapor deposition. The effect of the addition of C on strain compensation of Si1−xGex films has been studied by high-resolution x-ray diffraction analysis. The Hall hole mobility is found to increase with decreasing compensative strain or effective Ge content in the layer throughout the studied temperature range. The effect of a Si-cap layer on the hole mobility of Si1−x−yGexCy film has been investigated.

Electrical properties of rapid thermal oxides on Si1−x−yGexCy films

The electrical characteristics of rapid thermal oxides on Si1−x−yGexCy layers are reported. X-ray photoelectron spectroscopy results indicate segregation of Ge at the SiO2/Si1−x−yGexCy interface, a thin GeO2 layer at the oxide surface, and elemental Ge at the interface and in the oxide. The interface state density of the samples ranges from 3.0×1011 to 3.6×1012 eV−1 cm−2. All the samples show electron trapping behavior and the trap generation rate decreases with increasing C concentration. The charge-to-breakdown value and the oxide breakdown field are higher for Si0.887Ge0.113 than for Si1−x−yGexCy samples, and these values decrease with increasing C concentration.

Doped vs. undoped Si1−x−yGexCy layers in sub-100 nm vertical p-channel MOSFETs

By introducing Si1−x−yGexCy layers in vertical p-channel MOSFETs, boron diffusion from the source/drain regions into the channel region has been suppressed to enable sub-100 nm scaling. The trade-off of doped and undoped Si1−x−yGexCy layers has been studied. Due to detrimental effects from oxide grown on Si1−x−yGexCy layers, doped Si1−x−yGexCy diffusion barriers are preferable, and have allowed vertical p-channel MOSFETs with channel lengths down to 25 nm to be demonstrated. No excess leakage current due to the Si1−x−yGexCy layers is observed.

Oxidation of Si1−x−yGexCy strained layers grown on Si: kinetics and interface properties

We have investigated the thermal oxidation of strained Si1−xGex and Si1−x−yGexCy layers and the influence of the thermal process on the structure of the layers. Using XPS and SIMS depth profiles, we have found a germanium pile-up in the epitaxial layer near the oxide-layer interface. Using transmission electron microscopy (TEM), we have observed that crystallinity is well conserved, but additionally, we have found SiC precipitates. A qualitative model for the oxidation of this kind of binary and ternary alloys is presented. The model is based on the strain development of the samples and depends on germanium and carbon compositions and on the temperature of the process.

Optical characterization of thermally oxidized Si1−x−yGexCy layers

The oxidation kinetics and the optical properties of the new SiO2/Si1−x−yGexCy system (grown epitaxially on Si) have been studied in detail by ellipsometry (EL) and infrared spectroscopy (FTIR). While EL is straightforwardly applied to silicon, the thermal oxidation process affects the structure and refractive index of the underlying non-oxidized Si1−x−yGexCy. Thus, to determine the oxidation kinetics, part of the oxide must be etched-away to monitor the variation of the refractive index of the Si1−x−yGexCy layer after oxidation. The EL data has been interpreted as having a Ge-rich superficial layer and β-SiC precipitates embedded in a SiGeC matrix. This model have allowed us to correlate the EL results with the characterization performed by SIMS and TEM (Ge pile-up at the interface with the oxide and β-SiC nanoprecipitates in the bulk of the semiconductor). These changes are driven by rejection of Ge from the oxidation front and carbon leaving its substitutional site (Cs). The inverse evolution of the Cs and β-SiC content has been quantified by monitoring their IR bands at 607 and 820 cm−1, respectively. Accurate results on dry thermal oxidation kinetics of Si1−x−yGexCy in the thin oxide regime are presented.

Substitutional carbon reduction in SiGeC alloys grown by rapid thermal chemical vapor deposition

The substitutional carbon reduction in Si1−x−yGexCy strained layers, annealed at high temperatures, increases the compressive strain in the originally strain-compensated alloys. From the rocking curve simulation, the maximum amount of carbon reduction was below 0.9% for the various samples which were annealed below 1000 °C in the nitrogen flow. The interstitial silicon injection by thermal oxidation of the Si cap on the Si1−x−yGexCy layer enhances the reduction of substitutional carbon to a concentration of 1.3%. Oxidation of Si1−x−yGexCy alloys yields a Ge-enriched Si1−xGex layer with the Ge concentration larger than the initial content, and the formation of 3C silicon carbide precipitate is observed by the Fourier transform infrared spectroscopy.

Suppression of boron penetration by polycrystalline Si1−x−yGexCy in metal–oxide–semiconductor structures

A polycrystalline Si1−x−yGexCy layer grown by rapid thermal chemical vapor deposition has been used as a thin intermediate layer on top of the thin gate oxide in a metal–oxide–semiconductor structure with boron-doped polycrystalline silicon as the gate electrode. Although boron readily penetrated through the Si1−x−yGexCy and accumulated in it, boron penetration across the gate oxide into the substrate was greatly suppressed compared to that in structures without Si1−x−yGexCy layer. Our work suggests that the addition of carbon reduced the chemical potential of boron in polycrystalline Si1−x−yGexCy, compared to that in polycrystalline silicon or polycrystalline Si1−xGex. We have also observed no gate depletion effects with the use of poly-Si1−x−yGexCy layers in both p+ and n+ gate doping.

Complete suppression of boron transient-enhanced diffusion and oxidation-enhanced diffusion in silicon using localized substitutional carbon incorporation

In this letter, we show the ability, through introduction of a thin Si1−x−yGexCy layer, to eliminate the enhancement of enhanced boron diffusion in silicon due to an oxidizing surface or ion implant damage. This reduction of diffusion is accomplished through a low-temperature-grown thin epitaxial Si1−x−yGexCy layer which completely filters out excess interstitials introduced by oxidation or ion implant damage. We also quantify the oxidation-enhanced diffusion (OED) and transient-enhanced diffusion (TED) dependence on substitutional carbon level, and further report both the observation of carbon TED and OED, and its dependence on carbon levels.

Band-gap changes and band offsets for ternary Si1−x−yGexCy alloys on Si(001)

An estimation for the band offsets and the fundamental band gap will be presented for Si1−x−yGexCy alloys tensile or compressive strained on Si(001). This estimation considers both the band lineup at the interface of two different materials as well as the strain effects. Unknown material parameters have been adjusted to obtain the best agreement with experimental results for tensile strained Si1−yCy layers. The obtained results agree very well with the first experimental data for the effect of C on band-structure properties in Si1−x−yGexCy. For a completely strain-compensated (cubic) Si1−x−yGexCy layer, we predict significant “Ge effects” (smaller gap than Si, valence-band offset to Si) with values depending on the Ge content.

Thermal stability and electrical properties of Zr/Si1−x−yGexCy contacts after rapid thermal annealing

In this work, we have investigated the reaction between Zr and SiGeC alloys. Annealings have been performed in a rapid thermal annealing (RTA) furnace at temperatures ranging from 400 to 800 °C for 5 min. The reaction of the metal with the alloy has been investigated by x-ray diffraction and Rutherford backscattering spectrometry. Four crystal x-ray diffraction was performed to measure the residual strain in the epilayer. The analyses indicate that the C49-Zr(Si1−xGex)2 is the final phase of the reaction. For all compositions examined (from 0% up to 33% of Ge), the C49 film has the same Ge content as in the as-deposited Si1−x−yGexCy layer and no Ge segregation has occured. In addition, this thermal treatment leads to only a small strain relaxation in the unreacted epilayer. The presence of C does not modify the reaction and it prevents any strain relaxation. Schottky barrier height measurements have been performed on p-type layers. RTA leads to a slight decrease of the barrier without any degradation of the contact. The C49 film presents a resistivity of about 80 μΩ cm. These results indicate that Zr may be a good candidate for contacts on IV–IV alloys in terms of thermal stability.

Stabilizing the surface morphology of Si1−x−yGexCy/Si heterostructures grown by molecular beam epitaxy through the use of a silicon-carbide source

Si 1−x−y Ge x C y / Si superlattices were grown by solid-source molecular beam epitaxy using silicon carbide as a source of C. Samples consisting of alternating layers of nominally 25 nm Si1−x−yGexCy and 35 nm Si for 10 periods were characterized by high-resolution x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry to determine strain, thickness, and composition. C resonance backscattering and secondary ion mass spectrometries were used to measure the total C concentration in the Si1−x−yGexCy layers, allowing for an accurate determination of the substitutional C fraction to be made as a function of growth rate for fixed Ge and substitutional C compositions. For C concentrations close to 1%, high-quality layers were obtained without the use of Sb-surfactant mediation. These samples were found to be structurally perfect to a level consistent with cross-sectional TEM (&amp;lt;107 defects/cm2) and showed considerably improved homogeneity as compared with similar structures grown using graphite as the source for C. For higher Ge and C concentrations, Sb-surfactant mediation was found to be required to stabilize the surface morphology. The maximum value of substitutional C concentration, above which excessive generation of stacking fault defects caused polycrystalline and/or amorphous growth, was found to be approximately 2.4% in samples containing between 25 and 30% Ge. The fraction of substitutional C was found to decrease from roughly 60% by a factor of 0.86 as the Si1−x−yGexCy growth rate increased from 0.1 to 1.0 nm/s.