xGex Layers Research Articles

Refractive Index Variation of Magnetron-Sputtered a-Si1−xGex by “One-Sample Concept” Combinatory

Gradient a-Si1−xGex layers have been deposited by ”one-sample concept” combinatorial direct current (DC) magnetron sputtering onto one-inch-long Si slabs. Characterizations by electron microscopy, ion beam analysis and ellipsometry show that the layers are amorphous with a uniform thickness, small roughness and compositions from x = 0 to x = 1 changing linearly with the lateral position. By focused-beam mapping ellipsometry, we show that the optical constants also vary linearly with the lateral position, implying that the optical constants are linear functions of the composition. Both the refractive index and the extinction coefficient can be varied in a broad range for a large spectral region. The precise control and the knowledge of layer properties as a function of composition is of primary importance in many applications from solar cells to sensors.

Kirkendall void formation in reverse step graded Si1−xGex/Ge/Si(001) virtual substrates

Formation of Kirkendall voids is demonstrated in the Ge underlayer of reverse step graded Si1−xGex/Ge buffer layers grown on Si(001) using reduced pressure chemical vapour deposition (RP-CVD). This phenomenon is seen when the constant composition Si1−xGex layer is grown at high temperatures and for x ≤ 0.7. The density and size of the spherical voids can be tuned by changing Ge content in the Si1−xGex and other growth parameters.

Quantitative HAADF STEM of SiGe in presence of amorphous surface layers from FIB preparation

The chemical composition of four Si1−xGex layers grown on silicon was determined from quantitative scanning transmission electron microscopy (STEM). The chemical analysis was performed by a comparison of the high-angle annular dark field (HAADF) intensity with multislice simulations. It could be shown that amorphous surface layers originating from the preparation process by focused-ion beam (FIB) at 30 kV have a strong influence on the quantification: the local specimen thickness is overestimated by approximately a factor of two, and the germanium concentration is substantially underestimated. By means of simulations, the effect of amorphous surface layers on the HAADF intensity of crystalline silicon and germanium is investigated. Based on these simulations, a method is developed to analyze the experimental HAADF-STEM images by taking the influence of the amorphous layers into account which is done by a reduction of the intensities by multiplication with a constant factor. This suggested modified HAADF analysis gives germanium concentrations which are in agreement with the nominal values. The same TEM lamella was treated with low-voltage ion milling which removed the amorphous surface layers completely. The results from subsequent quantitative HAADF analyses are in agreement with the nominal concentrations which validates the applicability of the used frozen-lattice based multislice simulations to describe the HAADF scattering of Si1−xGex in STEM.

Effect of composition in Si1−xGex seed layer on the solid phase crystallization of ultrathin amorphous silicon layer

The effect of top silicon-germanium (Si1−xGex) layers on the crystallization of underlying amorphous Si (a-Si) films was investigated for various Ge compositions in Si1−xGex. Si and Si1−xGex films were sequentially deposited on thermal SiO2 by using SiH4 and GeH4 source gases in reduced pressure chemical vapor deposition. The fraction and the microstructure of underlying crystallized Si were characterized by plan-view transmission electron microscopy. Scanning nanobeam diffraction was performed to investigate the grain size of crystallized Si films. The crystallization of the underlying a-Si thin film in a-Si1−xGex/Si bilayer stack gradually increased with the increasing Ge composition of the top Si1−xGex layer. The grain size of underlying crystallized Si films showed a dependence on the Ge composition of top Si1−xGex. For Si1−xGex with 22% Ge, microtwins were observed to be the dominant defect, while stacking faults were dominant for Si1−xGex with 47% Ge. The Ge composition and the thickness of Si1−xGex are important factors to obtain the optimized crystallization time and grain size of ultrathin a-Si with a reduced density of intragrain defects.

Effect of atomic silicon and germanium beams on the growth kinetics of Si1 – x Ge x layers in Si–GeH4 molecular beam epitaxy

The steady-state kinetics of growth of Si1 – xGex layers in one version of hybrid molecular beam epitaxy with a molecular germane source and a sublimating silicon bar has been studied. It has been demonstrated that the processes of capture of hydride molecule radicals by the epitaxial surface and their subsequent decomposition must be taken into account in the analysis of growth kinetics. The comparison of experimental data with the results of kinetic analysis has revealed a close match between them. At low germane pressures \({P_{Ge{H_4}}}\) < 0.5 mTorr, the nature of the growth process is defined exclusively by the specifics of interaction between the molecular beam of Ge monohydride with the growth surface. The influence of the atomic Ge beam from the Si source only manifests at germane pressures exceeding 1 mTorr. Under these conditions, the fluxes of Ge and Si atoms from the sublimating Si source become equal, and the concentration of germyl molecules on the surface reaches saturation. The observed increase in the \({V_{Ge{H_3}}}\) parameter is associated with the activating influence of the flux of silicon atoms from the sublimating source on the decomposition of molecules.

Influence of precursor gas on SiGe epitaxial material quality in terms of structural and electrical defects

We have investigated the structural and electrical properties of n-type doped Si1−xGex epitaxial layers (x = 24–26%) grown by chemical vapor deposition with conventional [SiH2Cl2 (DCS)/GeH4] and high-order (Si2H6/Ge2H6) precursor combinations. X-ray diffraction, atomic force microscopy, and deep-level transient spectroscopy (DLTS) measurements were performed for characterization. The crystalline properties and surface morphology of the Si1−xGex layer with Si2H6/Ge2H6 grown at temperatures as low as 550 °C show good structural quality similar to that with DCS/GeH4 grown at 615 °C. On the other hand, in terms of electrical properties, the DLTS measurement reveals the existence of vacancy-related complexes in the as-grown layer with Si2H6/Ge2H6. We found that post-deposition annealing at 200 °C for the Si1−xGex epitaxial layer is effective for annihilating vacancy-related defects with densities down to as low as that of conventional precursors.

Epitaxially Grown Monoisotopic Si, Ge, and Si1–x Ge x Alloy Layers: Production and Some Properties

The technology of the growth of Si, Ge, and Si1–xGex layers by molecular-beam epitaxy with the use of a sublimation source of monoisotopic 30Si or 28Si and/or gas sources of monogermane 74GeH4 is demonstrated. All of the epitaxial layers are of high crystal quality. The secondary-ion mass spectroscopy data and Raman data suggest the high isotopic purity and structural perfection of the 30Si, 28Si, 74Ge, and 30Si1–x74Gex layers. The 30Si layers doped with Er exhibit an efficient photoluminescence signal.

Work Function Measurement of Silicon Germanium Heterostructures Combining Kelvin Force Microscopy and X-ray Photoelectron Emission Microscopy

Work function in Si1–xGex heterostructures with Ge content in the 6% to 49% range was studied with high energy resolution by combining Kelvin force microscopy and X-ray photoelectron emission microscopy. Although the two methods are based on distinct physical mechanisms, we show that both techniques give the same work function differences between each Si1–xGex layer, as small as 20 meV. To detect such small work function differences, we put in evidence the necessity of preparing the Si1–xGex sample surface with polishing, HF etching and Ar+ sputtering. Such surface preparation allows, in principle, to reduce the deleterious influence of surface states, coming for instance from carbon atoms or native oxide, on quantitative work function extraction. We show in this paper that even after such a sample surface preparation, a strong band bending can be present, which causes a contrast inversion on the surface of the material and yields an artificially lower surface work function with respect to theoretical val...

Phase analysis and thermal stability of thin films synthesized via solid state reaction of Ni with Si1 − xGex substrate

The products of the solid state reaction involving ultra-thin Ni film (6nm) and Si1−xGex layers (Ge 25 and 55at.%), were analysed using sheet resistance (Rs), glancing angle X-ray diffraction (GIXRD), scanning electron and atomic force microscopy (SEM, AFM) techniques. The reaction was carried out via rapid thermal process (RTP) annealing using two different steps (RTP1 and RTP2) while applying a selective etch (SE) in between them. The intermediate and the end reaction products resulting after RTP1 and RTP2 were found to be dependent on the Ge content, forming Ni-rich silicide (Ni2Si) and NiSi on Si75Ge25, while Ni-rich germanide (Ni5Ge3) and NiGe were obtained by using Si45Ge55. Though the onset of intermediate Ni-rich silicide or germanide phase formation occurs at similar RTP1 temperature (275°C), the reaction completion to yield low resistive phase NiSi or NiGe phase results at different RTP2 temperatures (400°C vs 350°C). Based on the volume expansion, a resistivity value of 25μΩcm was obtained for the synthesized NiGe (12nm) and NiSi (14nm) layers. Independent of the phases obtained, the films were found to be closed and homogeneous and exhibit rms roughness of 0.5–0.8nm as evidenced by SEM and AFM analysis. Thermal stability studies carried out on NiSi and NiGe thin films, post RTP1/RTP2, show the latter phase to have limited stability and result in Rs degradation starting already at 475°C due to phase decomposition.

Selective epitaxy growth of Si1−xGex layers for MOSFETs and FinFETs

This article reviews the selective epitaxy growth of intrinsic, B- and C-doped SiGe layers on recessed (or flat) exposed Si areas for MOSFETs as well as on Si-fins for FinFETs. A detailed empirical model for the growth, integration issues including epitaxy quality, selectivity, dopant incorporation, and pattern dependency (or loading effect) is presented.

In-situ phosphorous-doped SiGe layer on Si substrate by reactive thermal chemical vapor deposition at low temperature

We have studied in-situ phosphorus-doping characteristics in Ge-rich Si1−xGex (0.95≤x≤1) epitaxially grown on Si substrate by Reactive Thermal Chemical Vapor Deposition system at 350°C using GeF4 and Si2H6. The impact of PH3 in-situ doping on the growth kinetics, micro-structure and electric properties of Si1−xGex layers is investigated. The results indicate that the growth rate of Si1−xGex layers drops when the doping ratio (PH3/GeF4) is increased up to 750ppm. Correspondingly, the surface morphology as well as the crystal quality of Si1−xGex layers starts to deteriorate. The electrical measurements show that an excellent doping efficiency is achieved in the case of an acceptable surface morphology and crystalline quality. These results provide the promise for future n+/p diode fabrication by Reactive Thermal Chemical Vapor Deposition at low temperature.

Electrical characteristics of amorphous Si:H/crystalline Si0.3Ge0.7 heterojunction solar cells grown on compositionally graded buffer layers

We examined the influence of the processing temperature on hydrogenated amorphous Si/crystalline Si0.3Ge0.7 heterojunction solar cells with a bandgap of 1.0eV, which were grown on Si(001) by solid-source molecular beam epitaxy. Stepwise, compositionally graded buffer Si1−xGex layers were applied to prepare Si0.3Ge0.7 films with low dislocation density. We studied the influence of the annealing temperature applied at the buffer layer on the diode characteristics. The diode factor was minimized to 1.45 at 800°C, and the saturation current density monotonically decreased from 7.5×10−4A/cm2 for an annealing temperature of 650°C to 5.0×10−5A/cm2 for that of 900°C, as a result of the promotion of dislocation annihilation. Consequently, the Si0.3Ge0.7 heterojunction solar cell showed a maximum conversion efficiency of 1.50% when the annealing temperature was 850°C. We also studied the influence of the growth temperature of the p-Si0.3Ge0.7 active layer (absorber) on the cell performance. The short circuit current density increased from 16.03mA/cm2 for 520°C to 20.95mA/cm2 for 600°C. This result corresponded to an improvement of quantum efficiency response in the longer wavelength region due to the reduction of crystal defects caused by oxygen contamination. However, the Si0.3Ge0.7 active layer processed at a higher temperature showed an accumulation of threading dislocations and roughening of the surface, resulting in the degradation of both the open circuit voltage and the fill factor.

Effect of Sn on crystallinity and electronic property of low temperature grown polycrystalline-Si1−x−yGexSny layers on SiO2

We examined the formation of polycrystalline-Si1−x−yGexSny layers on SiO2 by the solid phase crystallization method. We investigated the impact of Sn incorporation on the polycrystallization, crystallinity, and electrical property of Si1−xGex layers. We found that the polycrystallization time of Si1−x−yGexSny decreases with increasing in the Sn content in the annealing at 500°C. No Sn precipitation is observed after the crystallization of Si1−x−yGexSny layer with an Sn content of 2%, while severe Sn precipitation is found in the sample with an Sn content of 10%. Moreover, larger grains of polycrystal can be obtained by the incorporation of 2%-Sn with comparison to that without Sn. The enlargement of polycrystalline grain of Si1−x−yGexSny improves in the hole mobility to 74cm2/Vs.

Formation of Er‐germanosilicide films on strained Si1−xGex with different Ge contents

Er‐germanosilicide [Er(Si1−zGez)2−y] films were formed on strained Si1−xGex layers with different Ge contents (x = 0.14 and 0.28), and their dependency of formation kinetics on the Ge concentrations was investigated while minimizing the oxygen‐incorporation effect using a TaN capping layer. When a ~30 nm‐thick Er film was used, uniform Er(Si1−zGez)2−y films were obtained at an annealing temperature of around 600 °C without their agglomeration and also the strain relaxation of the remaining Si1−xGex layer. The increase in the Ge concentration of the reacting Si1−xGex layer enhanced the formation of the crystalline Er(Si1−zGez)2−y film. Furthermore, some enrichment of the Ge content within the Er(Si1−zGez)2−y film occurred in comparison with that in the initial and remaining Si1−xGex layers.

Hetero-epitaxial growth of stoichiometry tunable Si1−xGexfilm via a low temperature aluminium-induced solid phase epitaxy (AI-SPE) process

A novel process of aluminum-induced solid-phase epitaxy (AI-SPE) is used to fabricate a hetero-epitaxial Si1−xGex (epi-SiGe) film on an sc-Si (100) substrate at relatively low temperatures (lower than 450 °C) using a 1/4 × 4′′ wafer. The stoichiometry of Ge in the epi-SiGe film can be easily tuned in the AI-SPE process by controlling the annealing temperature and the Ge fraction (x) of the initial a-Si1−xGex layer. The stoichiometry and epitaxial relationship of the epi-SiGe film were verified by grazing incidence X-ray diffraction (GI-XRD) and transmission electron microscopy (TEM). The AI-SPE mechanism is directly verified by in situ heating TEM. Based on the experimental results, it is concluded that the AI-SPE mechanism for the epi-SiGe film can be divided into four steps: (a) initial stage; (b) formation of a-Si1−xGex free atoms and diffusion along Al grain boundaries to the sc-(100) substrate surface; (c) nucleation of crystalline Si1−xGex at the surface of the sc-Si (100) substrate; and (d) crystal growth and layer exchange.

Simulation study of Ge/Si heterostructure solar cells yielding improved open-circuit voltage and quantum efficiency

The performance of crystalline Si solar cells with type-II Ge/Si heterostructures is numerically investigated using a simple p-Si/Si1−xGex/n-Si model with carrier recombination at the Si1−xGex layer. From an analysis of the open-circuit voltage (Voc), we find that carrier recombination at the Si1−xGex layer dominates the solar cell performance, and further suppression of this behavior is therefore the key to improving Voc. Furthermore, we discuss the device structure required to improve Voc and the quantum efficiency and find that the thin p-Si layer should be located on the front side to improve quantum efficiency in the visible region.

Thin, relaxed Si1−xGex virtual substrates on Si grown using C-doped Ge buffers

Ultrathin C-doped Ge (Ge:C) buffers are used as effective buffer layers to grow thin and smooth Si1−xGex relaxed virtual substrates on Si over a wide range of Ge content (x = 0.23, 0.38, 0.50, 0.65, 1) by ultrahigh vacuum chemical vapor deposition. High degree of relaxation (≥90%) in thin Si1−xGex films (∼50 nm thick) is demonstrated using this approach without additional annealing. Raman data suggest that the Ge:C buffers are still under compressive stress with the subsequent Si1−xGex layer growth. A low threading dislocation density of the order of ∼6 × 104 cm−2 is obtained from the relaxed 500-nm Si0.77Ge0.23 film grown using this method.

Thin SiGe virtual substrates for Ge heterostructures integration on silicon

The possibility to reduce the thickness of the SiGe virtual substrate, required for the integration of Ge heterostructures on Si, without heavily affecting the crystal quality is becoming fundamental in several applications. In this work, we present 1 μm thick Si1−xGex buffers (with x &amp;gt; 0.7) having different designs which could be suitable for applications requiring a thin virtual substrate. The rationale is to reduce the lattice mismatch at the interface with the Si substrate by introducing composition steps and/or partial grading. The relatively low growth temperature (475 °C) makes this approach appealing for complementary metal-oxide-semiconductor integration. For all the investigated designs, a reduction of the threading dislocation density compared to constant composition Si1−xGex layers was observed. The best buffer in terms of defects reduction was used as a virtual substrate for the deposition of a Ge/SiGe multiple quantum well structure. Room temperature optical absorption and photoluminescence analysis performed on nominally identical quantum wells grown on both a thick graded virtual substrate and the selected thin buffer demonstrates a comparable optical quality, confirming the effectiveness of the proposed approach.

Thin-film Si1−xGex HIT solar cells

We fabricated silicon–germanium (Si1−xGex) based HIT solar cells with x=0, 0.25, 0.41 and 0.56 in order to quantify the effect of germanium fraction on key solar cell performance parameters. The p-type absorber layer consists of 2 and 4μm Si1−xGex layer grown on p+ silicon substrate using a graded buffer layer to reduce the threading dislocation density. The emitter is n+ amorphous-Si. A thin strained-Si layer is grown on the c-Si1−xGex layer prior to a-Si deposition and is believed to improve a-Si–H/c-Si1−xGex interface quality. The short-circuit current (Jsc) increases, from ∼14mA/cm2 for Si cells to 21mA/cm2 for Si0.44Ge0.56 cells with 2μm-thick active layers, while open-circuit voltage decreases. The spectral response of the Si1−xGex solar cells improves due to a reduction in absorption depth and smaller band-gap associated with the higher germanium fractions.

Si1 − x Ge x /Si heterostructures grown by molecular-beam epitaxy on silicon-on-sapphire substrates

The growth of heterostructures with Si1 − xGex layers on \(\left( {1\bar 102} \right)\) sapphire substrates by molecular-beam epitaxy with a silicon sublimation source and a germanium gas source (GeH4) is reported. The systematic study of the influence of substrate temperature and thickness of the silicon buffer layer shows that the optimal conditions for growing epitaxial Si1 − xGex layers are provided at a temperature of TS = 375–400°C. There are significant differences in the orientations of Si1 − xGex layers, depending on the thickness d of the Si buffer layer: the preferred orientations are (100) at d ≥ 100 nm and (110) for thinner layers. Heterostructures with thick (∼1 μm) Si1 − xGex layers, doped with erbium atoms, exhibit intense photoluminescence at λ = 1.54 μm.