xGex Alloys Research Articles

Structure and Electronic Properties of the A‐center in Si1−xGex Alloys: Insight from the Heyd–Scuseria–Ernzerhof Hybrid Functional Calculations

This study investigates the structure and electronic properties of the A‐center in Si1−xGex alloys (x = 0, 0.25, 0.50, 0.75, and 1), using the Heyd–Scuseria–Ernzerhof hybrid functional. It is found that the most stable structures form SiOSi bonds, with the other two atoms surrounding the A‐center being Ge atoms, which form a long bond between them. The behavior of the O atoms is significantly influenced by the Ge content. As it increases, the position of the O atom shifts relative to the surrounding atoms, thereby affecting the bond lengths and angles. The formation energy of the A‐center first decreases with increasing Ge content up to 0.75, which enhances the defect production; however, then it decreases from 0.75 to 1, thus suppressing defect production. The calculations are consistent with the experimental data, showing that the distance of the defect level from the conduction band minimum increases with Ge content, supporting the observed enthalpy changes in electron ionization.

Direct bandgap quantum wells in hexagonal Silicon Germanium

Silicon is indisputably the most advanced material for scalable electronics, but it is a poor choice as a light source for photonic applications, due to its indirect band gap. The recently developed hexagonal Si1−xGex semiconductor features a direct bandgap at least for x > 0.65, and the realization of quantum heterostructures would unlock new opportunities for advanced optoelectronic devices based on the SiGe system. Here, we demonstrate the synthesis and characterization of direct bandgap quantum wells realized in the hexagonal Si1−xGex system. Photoluminescence experiments on hex-Ge/Si0.2Ge0.8 quantum wells demonstrate quantum confinement in the hex-Ge segment with type-I band alignment, showing light emission up to room temperature. Moreover, the tuning range of the quantum well emission energy can be extended using hexagonal Si1−xGex/Si1−yGey quantum wells with additional Si in the well. These experimental findings are supported with ab initio bandstructure calculations. A direct bandgap with type-I band alignment is pivotal for the development of novel low-dimensional light emitting devices based on hexagonal Si1−xGex alloys, which have been out of reach for this material system until now.

Impact of surface reflectivity on the ultra-fast laser melting of silicon-germanium alloys

Ultraviolet nanosecond laser annealing (LA) is a powerful tool where strongly confined heating and melting are desirable. In semiconductor technologies the importance of LA increases with the increasing complexity of the proposed integration schemes. Optimizing the LA process along with the experimental design is challenging, especially when complex 3D nanostructured systems with various shapes and phases are involved. Within this context, reliable simulations of laser melting are required for optimizing the process parameters while reducing the number of experimental tests. This gives rise to a virtual Design of Experiments (DoE). Si1−xGex alloys are nowadays used for their compatibility with silicon devices enabling to engineer properties such as strain, carrier mobilities and bandgap. In this work, the laser melting process of relaxed and strained Si1−xGex is simulated with a finite element method/phase field approach. Particularly, we calibrated the dielectric functions of the alloy for its crystalline and liquid phase using experimental data. We highlighted the importance of reproducing the exact reflectivity of the interface between air and the material in its different aggregation states, to correctly mimic the process. We indirectly discovered intriguing features on the optical behaviour of melt silicon-germanium.

Tuning the Disconnected Magnetic Phase by Regulating the Lattice Distortion in FeSi1−xGex Alloys

Tuning the Disconnected Magnetic Phase by Regulating the Lattice Distortion in FeSi1−xGex Alloys

Segregation-induced formation of Ge nanocrystals in silicon oxide

The investigation of initial stage of Si1 – xGex alloy deposition and clarification of Ge nanocrystal formation mechanism has been carried out. It was found that at the initial stages of growing layers of Si1 – xGex alloys, the density of island nuclei Si1 – xGex increases by a factor of 2.5–3.4 compared to the density of polycrystalline silicon islands (from 1.07 ⋅ 1011 to 1.90 ⋅ 1011 cm–2 and from 3.1 ⋅ 1010 to 4.3 ⋅ 1010 cm–2 respectively). A decrease in the thickness of the layer corresponding to the end of the induction period and the formation of a continuous Si1 – xGex layer to 8–10 nm (for polycrystalline silicon, the thickness of a similar layer is approximately 22 nm) has been established. It is shown that the Ge nanocrystal formation is occurred by segregationist pushback of Ge atoms by the SiO2 /Si1 – xGex oxidation front and oxidation through grain boundaries during oxidation of Si1 – xGex thin layers, produced by chemical vapor deposition. The MOS structure with array of Ge nanocrystal, which has the hysteresis capacitance characteristics of 1.7–1.8 V and leakage current density from 1.5 ⋅ 10–16 to 2.2 ⋅ 10–16 A/µm2 was obtained.

5.5 MeV Electron Irradiation-Induced Transformation of Minority Carrier Traps in p-Type Si and Si1−xGex Alloys

Minority carrier traps play an important role in the performance and radiation hardness of the radiation detectors operating in a harsh environment of particle accelerators, such as the up-graded sensors of the high-luminosity hadron collider (HL-HC) at CERN. It is anticipated that the sensors of the upgraded strip tracker will be based on the p-type silicon doped with boron. In this work, minority carrier traps in p-type silicon (Si) and silicon–germanium (Si1−xGex) alloys induced by 5.5 MeV electron irradiation were investigated by combining various modes of deep-level transient spectroscopy (DLTS) and pulsed technique of barrier evaluation using linearly increasing voltage (BELIV). These investigations were addressed to reveal the dominant radiation defects, the dopant activity transforms under local strain, as well as reactions with interstitial impurities and mechanisms of acceptor removal in p-type silicon (Si) and silicon–germanium (SiGe) alloys, in order to ground technological ways for radiation hardening of the advanced particle detectors. The prevailing defects of interstitial boron–oxygen (BiOi) and the vacancy–oxygen (VO) complexes, as well as the vacancy clusters, were identified using the values of activation energy reported in the literature. The activation energy shift of the radiation-induced traps with content of Ge was clarified in all the examined types of Si1−xGex (with x= 0–0.05) materials.

Dendritic Growth in Si1−xGex Melts

We investigated the types of dendrites grown in Si1−xGex (0 < x < 1) melts, and also investigated the initiation of dendrite growth during unidirectional growth of Si1-xGex alloys. Si1−xGex (0 < x < 1) is a semiconductor alloy with a completely miscible-type binary phase diagram. Therefore, Si1−xGex alloys are promising for use as epitaxial substrates for electronic devices owing to the fact that their band gap and lattice constant can be tuned by selecting the proper composition, and also for thermoelectric applications at elevated temperatures. On the other hand, regarding the fundamentals of solidification, some phenomena during the solidification process have not been clarified completely. Dendrite growth is a well-known phenomenon, which appears during the solidification processes of various materials. However, the details of dendrite growth in Si1−xGex (0 < x < 1) melts have not yet been reported. We attempted to observe dendritic growth in Si1−xGex (0 < x < 1) melts over a wide range of composition by an in situ observation technique. It was found that twin-related dendrites appear in Si1−xGex (0 < x < 1) melts. It was also found that faceted dendrites can be grown in directional solidification before instability of the crystal/melt interface occurs, when a growing crystal contains parallel twin boundaries.

Toward accurate electronic, optical, and vibrational properties of hexagonal Si, Ge, and Si1−xGex alloys from first-principle simulations

A new hexagonal phase of Si1−xGex alloys have been successfully synthesized through efforts in recent reports. Utilizing the combined first-principle calculations and special quasi-random model, we precisely investigated the structural, electronic, optical, and vibrational properties of hexagonal Si and Ge and disordered hexagonal Si1−xGex random alloys. We found a large negative deviation between the calculated lattice constants within the revised Perdew–Burke–Ernzerhof for solids functional and the linear fitting results. The electronic structures obtained by using the Tran–Blaha modified Becke–Johnson exchange potential confirm that hexagonal Si1−xGex (x > 0.625) alloys present direct bandgaps. Through solving the Bethe–Salpeter equation, the linear optical spectra of hexagonal Si and Ge are demonstrated. We reveal that the peaks of complex dielectric functions are redshifted with the addition of Ge atoms. Also, the real and imaginary parts exhibit strong anisotropy, which makes hexagonal Si1−xGex alloys potentially useful as nonlinear crystals. The transition is allowed in the infrared region for the hexagonal Si1−xGex (x > 0.625) alloys, and the linear optical spectra can be continuously tuned over a wide range of frequency with Ge addition in the infrared region. Furthermore, density-functional perturbation theory calculations were carried out to predict the off-resonance Raman activity. The results suggest that the vibrational modes of the Si–Si bond exhibit a strong dependency on the compositions, which provides a useful way to identify the most probable atomic configurations of hexagonal Si1–xGex alloys in future experiments.

Low-temperature hall effect and martensitic transition temperatures in magnetocaloric Ni50Mn35Sb15−xGex (x = 0, 1, 3) alloys

The field dependence of Hall resistivity ρH and magnetization M of the magnetocaloric Ni50Mn35Sb15−xGex (x = 0, 1, 3) alloys were measured at T = 4.2 K and in magnetic fields of up to 70 kOe. The martensitic transitions temperatures, i.e., the martensite start temperature MS, martensite finish temperature MF, austenite start AS and austenite finish AF temperatures, were obtained from the magnetization temperature dependence that measured from 4.2 to 350 K in a field of 1 kOe. It was observed that the martensitic transitions temperatures correlate strongly both with the valence electron concentration e/a and with the electronic transport characteristics, which are the coefficients of normal R0 and anomalous RS Hall effect and the concentration of charge carriers n. Apparently, similar correlations should be observed in other magnetocaloric compounds that could be used to study martensitic transitions.

Effect of H2S pre-annealing treatment on interfacial and electrical properties of HfO2/Si1−xGex (x = 0–0.3)

To understand the effect of H2S pre-annealing treatment on a Si1−xGex alloy film, the interfacial and electrical characteristics of atomic-layer-deposited HfO2/Si1−xGex were studied while varying the Ge concentration (x value) from 0 to 0.3.

Behavior of Lithium Donors in Bulk Single-Crystal Isotopically Pure 28Si1 –x72Gex Alloys

The behavior of lithium donors in bulk single-crystal isotopically pure Si1 – xGex alloys (x = 0.0039–0.05) enriched in spinless 28Si and 72Ge isotopes (99.998 and 99.984%, respectively) is investigated by electron spin resonance. The fine structure of the spectrum of electrons localized at Li donors is studied at temperatures of T = 3.5–30 K in the expectation that, much like with pure silicon, enriching Si1 – xGex alloys in spinless isotopes (28SiisoGe, where iso = 70, 72, 74, 76) makes it possible to attain a higher resolution in the spectra of electron spin resonance. The results demonstrate that, despite the irregular arrangement of Ge atoms in the lattice of the 28Si1 – x72Gex alloy, the presence of local distortions in their vicinity, and the broadening of the electron-spin-resonance lines of donor electrons due to random strains, the lines of Li donors observed in the spectra of isotopically pure 28Si1 – x72Gex single crystals with x = 0.39, 1.2, and 2.9 at. % are narrower than the corresponding lines in the spectra of similar crystals with the natural composition of silicon and germanium isotopes. Similarly to the case of silicon crystals, this allowed us to study for the first time the angular dependences of the line positions in the electron-spin-resonance spectra of lithium in Si1 – xGex for different values of x.

Behavior of Phosphorus Donors in Bulk Single-Crystal Monoisotopic 28Si1 – x72Gex Alloys

The behavior of phosphorus donors in bulk single-crystal monoisotopic Si1 – xGex alloys (x = 0.0039–0.05) enriched by zero-spin isotopes 28Si (99.998%) and 72Ge (99.984%) by the electron-spin-resonance method is investigated. The hyperfine structure of the donor-electron spectrum giving information on the density of the donor wave function in the ground state on the 31P nucleus (I = 1/2) as well as the temperature dependences of the spin-relaxation rate (T = 3.5–30 K), which make it possible to analyze the longitudinal component T1 relaxation mechanism and magnitude of the valley–orbit splitting of the donor state, are investigated. Interest in these investigations is also caused by the fact that the Si1 – xGex alloy enriched by zero-spin isotopes (28SiisoGe, iso = 70, 72, 74, 76) is a poorly known material when compared with 28Si. The irregular arrangement of germanium atoms in the lattice of the SiGe solid solution and local distortions formed by it can level isotopic effects under isotopic enrichment. However, despite broadening of the lines of the electron-spin resonance of donor electrons due to random deformations formed by dissolved germanium atoms in silicon, narrower lines of the spectra of the electron-spin resonance are observed in isotopically pure Si1 – xGex single crystals at x = 0.39, 1.2, 2.9 at % when compared with similar crystals with the natural composition of silicon and germanium isotopes.

Generation of empirical pseudopotentials for transport applications and their application to group IV materials

Empirical pseudopotentials (EPs) allow for accurate and efficient modeling of atomistic electron transport. Unfortunately, EPs are available only for a few materials and atomic configurations. Furthermore, EPs for nanostructures have historically been described using a variety of different parameterized forms. To compete with more general first-principles methods, we propose an automated workflow to generate EPs of a general form for any material and atomistic configuration. In particular, we focus on the generation of EPs for electron transport calculations, i.e., we provide an EP that accurately reproduces a reference band structure. To demonstrate the applicability of the proposed method, we generate the EPs to reproduce the band structure for bulk Si, Ge, 3C–SiC (zinc-blende polytype), 4H–SiC (hexagonal polytype), diamond, and hydrogen terminated ⟨100⟩ oriented Si and Ge thin films, calculated using first principles. In addition, using the generated EPs, along with the virtual crystal approximation, we demonstrate that our method reproduces accurately the band structure related properties of Si1−xGex alloy as a function of Ge mole fraction, x. As an application of our generated EPs, we perform ballistic quantum transport simulations of extremely scaled (≈0.6 nm wide), hydrogen terminated, ⟨100⟩ oriented Ge and Si gate-all-around nanowire field-effect transistors and compare their transfer characteristics.

Strain compensation by relieving defects in SiGe channel for FinFET technologies

Current generations of FinFET devices are incorporating SiGe alloys as stressor material in channel regions in order to enhance hole mobility, drive current and channel conductivity. However, the presence of SiGe gives rise to new issues to be controlled during the device fabrication process, such as the strain retention or defectivity control, as they may seriously impact the quality of the strained SiGe channel and so the final device performance. The present work addresses the study of defect formation during the optimized integration of SiGe FinFETs for 10 nm technology nodes, aimed at determining the Ge threshold content for the nucleation of defects. Due to the relevance of atomistic models in determining the mechanisms and nature of defect formation, molecular dynamics (MD) simulations have been performed to emulate the FinFET fabrication process. The channel region is generated by removing a portion of the total volume of the Si substrate, further refilled with Si1 − xGex alloy to form the co-integrated FinFETs. After deposition, the presence of Ge induces a lattice distortion which is expected to be relieved by defect formation. Samples are annealed varying the Ge fraction, allowing to determine that threshold Ge content for the nucleation of defects is x = 0.27. MD provides also the nature of the formed defects, which have been suggested to be twinning developed at {111} planes and 60 ° misfit dislocations. Simulation results have been compared to experimental observations, both in good agreement.

Anomalous low energy phonon dispersion in bulk silicon-germanium observed by inelastic x-ray scattering

We report on an anomalous mode distinct from both optical and acoustic modes in phonon dispersion curves of bulk Si1−xGex alloy with x taking the values of 0.16, 0.32, 0.45, and 0.72. The anomalous mode at approximately 13 meV was observed directly using inelastic x-ray scattering along the Γ–X ([00q]) direction. The phonon dispersion relations of the anomalous mode indicate that there was no momentum dependence, similar to those of the longitudinal and transverse optical modes (Ge–Ge, Si–Ge, and Si–Si modes). In contrast to the acoustic and optical phonon modes, the energy of the anomalous mode shows no Ge fraction dependence. The molecular dynamics simulation corroborates that the Ge–Ge pairs or Ge atom clusters, which are surrounded by Si atoms, provide the anomalous mode, which is unique to the alloy structure. It has been suggested that such a localized vibration mode with no propagation significantly affects the acoustic modes, leading to low thermal conductivity in the SiGe alloy.

Phonon dispersion of bulk Ge-rich SiGe: inelastic X-ray scattering studies

Group IV semiconductors, such as SiGe with a higher Ge fraction, are attracting attention as next-generation materials for high-mobility channel or thermal conductivity devices. Thus, we investigated the phonon dispersions of a Si1–xGex (x = 0.72) alloy crystal by the inelastic X-ray scattering (IXS) method. In this method, measurement with an X-ray beam with a small spot size of 75 μm and resolution of millielectronvolts is possible due to the synchrotron technique. The longitudinal and transverse phonon spectra of optical and acoustic modes were measured from the Γ to the X point in the phonon energy band (0–70 meV) with millielectronvolt energy resolution. SiGe showed dispersion characteristics similar to those of Si and Ge crystals. It was confirmed that the IXS method is useful for obtaining properties of phonon dispersion.

Comparison of quantitative analyses using SIMS, atom probe tomography, and femtosecond laser ablation inductively coupled plasma mass spectrometry with Si1−XGeX and Fe1−X NiX binary alloys

Due to their electrical and physical properties, Si1−XGeX materials are widely used in microelectronic devices. In particular, the Ge component found within Si1−XGeX compounds is important for enhancing carrier mobility and altering the lattice constant of metal-oxide-semiconductor field-effect transistors. In this study, magnetic sector secondary ion mass spectrometry (magnetic sector SIMS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to determine the accurate concentrations of major compositions present within binary alloy samples. However, quantitative SIMS analysis is limited by the matrix effect, which influences the sputter yield of an element in a compound and alters the secondary ionization yields. Quantitative deviations that were due to the matrix effect were reduced by using Cs cluster ions (MCs+ and MCs2+) instead of elemental ions; the SIMS results using the elements were, therefore, compared with those using MCs+ and MCs2+ cluster ions. In the case of Fe1−XNiX alloys that have a less matrix effect compared to Si1−XGeX alloys, both the Cs primary ion beam (Cs+) and an oxygen primary ion beam (O2+) were used to measure the Fe1−XNiX compositions. The quantitative results from the two different primary ion beams were then compared to understand the ionization process. Deviations in the quantitative values gained with the O2+ beam were lower than those obtained using the Cs+ primary ions, meaning that using oxygen as the primary ion improves the accuracy in quantifying Fe1−XNiX compounds. Other reliable tools for analysis such as atom probe tomography and femtosecond laser ablation inductively coupled plasma mass spectrometry were also used in the quantitative analysis, with results that were consistent with the most accurate results obtained using magnetic sector SIMS and ToF-SIMS.

Identification of the donor and acceptor states of the bond-centered hydrogen–carbon pair in Si and diluted SiGe alloys

The electrical and structural properties of two levels (E90 and H180) in diluted n- and p-type Si1 − xGex alloys (0 ≤ x ≤ 0.070) are investigated by high-resolution Laplace deep level transient spectroscopy measurements and first-principles calculations. By exploiting the presence of Ge atoms close to a substitutional C atom, we show that E90 and H180 belong to the same C–H pair (labeled CH1BC) with H in a bond-centered configuration (C—HBC—Si). The relative energies of the various configurations of the CH pair are calculated, and the complete vibrational spectra in the lowest-energy structures for each charge state are predicted.

First principles study of structural and magnetic properties in Fe100−xGex alloys

Using density functional theory, the structural, magnetic and electronic properties for Fe100−xGex (x = 12.5–25 at.%) with A2, B2, D03, L12, and D019 structures were studied. Based on the total energy calculation, the phase diagram was plotted as a function of Ge concentration. In agreement with the experimental results, the following sequences of phase transitions were observed: A2→B2→D03 (12.5 ≤ x < 18.75 at.%), A2→B2→D019→ D03 (x = 18.75 at.%), and A2 → B2 → D019 → L12 → D03 (x ≥ 21.875 at.%). It was shown that D019 and L12 structures in the range of 24 ≤ x ≤ 25.6 at.% demonstrate a complex competitive behavior between ferromagnetic and antiferromagnetic magnetic exchange interactions.

First-principle calculation of the pressure-induced variation in the formation energy of iron defect in Si1−xGex alloy

We report on the systematic first-principle calculations of the formation volumes of a single Fe (neutral iron) in Si1−xGex at various Ge compositions (x = 4.7–20.3%). The formation volume was defined as the derivative of the Fe formation energy with respect to the pressure within 0–0.8 GPa. Interestingly, the formation volume was found to be equivalent to the difference between the volumes of the relaxed bulk and defective structures. The formation volume versus ‘x’ exhibited a nonlinear pattern with regions of Fe-induced volume contraction (within x 50%, i.e., Ge-rich region). These results explain the reported observation that Fe prefers Si-rich environment and that Ge does not favor to be a nearest neighbor to Fe.