yGexCy Alloys Research Articles

Monte Carlo simulation of high-energy transport of electrons and holes in bulk SiGeC alloys

High-energy transport properties of charge carriers in ternary random Si1−x−yGexCy alloys are investigated using Full-Band Monte Carlo simulations. Models for scattering mechanisms include phonon scattering, impact ionization and alloy scattering. Phonon scattering rates are wave-vector dependent and calculated consistently with the Full-Band structure. Impact ionization rates are modeled using analytical Keldysh formulas fitted to previously reported ab initio results. We derive a model for alloy scattering rate specific to ternary random alloys. It involves 2 effective alloy potentials which are independently calibrated on experimental mobility measurements. Presented Monte Carlo simulation results are shown to be in very good agreement with a variety of high-energy transport measurements, including drift velocities, impact ionization coefficients and quantum yields. Effects of alloy composition on the electrical characteristics of Si1−x-yGexCy alloys are investigated.

Effects of stoichiometries on the positron distribution in Si1–x–yGexCy alloys

The effect of alloying on the positron distribution in Si1–x–yGexCy alloys is reported. Our computations of the thermalized positron densities are based on the independent particle method coupled with the empirical pseudopotential method to derive the positron and electron wave functions, respectively. Initial results show a clear symmetrical positron charge distribution relative to the bond centre, a quite similar situation that was found in the IV groups. Since the minimum band gaps were found to be very sensitive to the carbon as well as the germanium concentration variations, this has an immediate consequence on the positron densities, where we find a deviation from linearity. This illustrative result was an indication of the alloying effect in this system (bowing effect).

Fabrication and characterization of C implantation standards for Si1−x−yGexCy alloys

Known amounts of carbon were implanted into a set of Si1−xGex alloy films (0<x<0.35) to provide quantification standards for C composition measurements of Si1−x−yGexCy alloys by secondary ion mass spectrometry. The implanted doses were fixed to within ±2%, the thicknesses of implanted films were measured to within ±1% using high-resolution electron microscopy, and the Ge concentrations were determined to within ±0.5% using Rutherford backscattering spectroscopy. For Si:Ge ratios in the range Si66Ge34 to Si91Ge9, the relative sensitivity factor for carbon with respect to silicon, and for carbon relative to germanium, both decreased substantially with increasing Ge content.

Fermi level position at metal Si1−x−yGexCy interfaces

In this work, we have investigated the Schottky barrier heights on n- and p-type Si1−x−yGexCy alloys with Zr, Ti, W, Ni and Pt as metals (ΦBn and ΦBp, respectively). Contacts on Si1−xGex alloys showed various behaviors depending on the metal work function Φm. For low-Φm metals (Zr, Ti), ΦBn increases with x, while ΦBp(x) decreases. For higher Φm metals (Pt), ΦBn strongly decreases with x. In the particular case of W (intermediate Φm value), ΦBp follows exactly the decrease of the SiGe band gap with x, while ΦBn remains constant. Nevertheless, whatever the metal, the reduction of the sum ΦBn+ΦBp gives the band-gap variation as a function of x, and the Fermi level is located at the same position for both n and p-type layers. A weaker effect of Φm on the Schottky barrier heights is observed compared to pure Si: the position of the Fermi level tends to remain in the range 0.60–0.65 eV below the conduction band, as soon as Ge is adding in Si. W contacts on Si1−x−yGexCy alloys evidenced the strong effect of C on ΦBn and ΦBp. The variations of ΦBn(y) or ΦBp(y) cannot be correlated to the band gap. In addition, the position of the Fermi level at the interface depends on the type of the alloy. Nevertheless, as in the case of the binary alloy SiGe, a weaker dependence on Φm compared to that observed for pure Si is shown. High values of the ideality factor with increasing the C content may evidence the presence of interfacial inhomogeneities, which could be correlated to C short range order. The present results have been compared to existing published results.

Optical constants and ellipsometric thickness determination of strained Si1−xGex:C layers on Si (100) and related heterostructures

The complex dielectric functions ε(ω) from 0.75 to 6.6 eV of pseudomorphically strained Si1−xGex (0<x<0.275) and Si1−x−yGexCy (x≈0.21, 0<y<0.013) alloys grown on Si (001) were determined using spectroscopic ellipsometry. Our rotating-analyzer instrument uses a computer-controlled MgF2 Berek waveplate as a compensator to improve the accuracy of the ellipsometric angles, particularly below 3 eV. By performing a least-squares analysis of the raw data, taken at three angles of incidence, we obtain the thicknesses of the alloy and the native oxide cap as well as ε(ω) for the alloy, which is parametrized using a semiempirical oscillator model. Differences between our data and those in the literature are due to differences in strain conditions and/or the improved accuracy of our instrument employing a compensator, which allows the determination of the native oxide thickness from measurements at long wavelengths. We apply our dielectric functions to analyze a variety of group-IV heterostructures and find good agreement with high-resolution x-ray diffractometry. The optical constants reported here are important, because they allow thickness and composition measurements using automated inline spectroscopic ellipsometers and reflectometers.

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.

Substitutional carbon incorporation during Si1−x−yGexCy growth on Si(100) by molecular-beam epitaxy: Dependence on germanium and carbon

We present results on the kinetics of substitutional carbon incorporation during growth of Si1−x−yGexCy alloys on silicon (100) using solid-source molecular-beam epitaxy. Substitutional carbon concentration decreases with increasing germanium content for samples grown at 400 °C with the same carbon flux and growth rate. The reduction in substitutional carbon concentration is small at low carbon flux, increasing significantly at higher carbon fluxes. The results indicate that the effect of either Ge or C concentration can dominate the substitutional C incorporation, depending on the total C concentration range.

MBE growth and properties of supersaturated, carbon-containing silicon/germanium alloys on Si(001)

The growth and properties of Si1−yCy and Si1−x−yGexCy alloys pseudomorphically strained on Si(001) will be reviewed. Although the bulk solubility of carbon in silicon is small, epitaxial layers with more than 1% C can be fabricated by molecular beam epitaxy and various chemical vapor deposition techniques. One of the most crucial questions is the relation between substitutional and interstitial carbon incorporation, which has a large impact on the electrical and optical properties of these layers. The carbon substitutionality (fraction of substitutional incorporated carbon atoms) is strongly influenced by the growth conditions, such as growth temperature and Si growth rate. The mechanical and structural properties, and the influence of C atoms on band structure and charge carrier properties will be discussed. Further, we will show how lower carbon concentrations can influence dopant diffusion, without affecting strain and band alignment. We demonstrated that suppressed boron diffusion in carbon-rich epitaxial layers can be used to increase the performance of SiGe heterojunction bipolar transistors (HBTs). We demonstrate epitaxially grown SiGe:C HBTs with static and dynamic performance suitable for high frequency applications. Compared to SiGe technologies, the addition of carbon provides significantly greater flexibility in process design and offers wider latitude in process margins. The physical mechanism for suppressed boron diffusion in carbon-rich Si and SiGe (about 0.1% carbon) is an undersaturation of Si self-interstitials due to outdiffusion of carbon.

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.

Carbon incorporation in SiGeC alloys grown by ultrahigh vacuum chemical vapor deposition

We report on incorporation of carbon in Si1−x−yGexCy alloys by ultrahigh vacuum chemical vapor deposition and on thermal relaxation properties of Si1−x−yGexCy alloys with low carbon levels. Si1−x−yGexCy alloys have been grown at temperatures between 550 and 650 °C using silane, germane and methylsilane as precursor gases. For levels of less than 1% total C the layers are of excellent quality. The total carbon level was found to be independent of the Ge fraction and growth temperature. However, the Ge fraction in the alloys was observed to increase when carbon was added to the alloys, suggesting that C alters the sticking probabilities of silane and germane. We also studied the thermal stability of Si1−x−yGexCy alloys with low levels of carbon and found that adding even 0.2% C significantly improves the thermal stability when compared to SiGe alloys of similar strain and thickness.

Thermal stability of Si/Si1−x−yGexCy/Si quantum wells grown by rapid thermal chemical vapor deposition

The thermal stability of Si/Si1−x−yGexCy/Si quantum wells was studied by high resolution x-ray diffraction, Fourier transform infrared spectroscopy, and defect etching. There are different pathways of strain relaxation in this material system, depending on the annealing temperature. The lattice structure of Si1−x−yGexCy was as stable as the Si1−xGex alloys at an annealing temperature of 800 °C for 2 h. At an annealing temperature of 900 °C for 2 h, the structures of both Si1−x−yGexCy and Si1−xGex started to relax. The addition of C enhanced the Ge outdiffusion in Si1−x−yGexCy, compared to that of Si1−xGex. For the annealing temperatures of 950 and 1000 °C for 2 h, the Si1−xGex continued to relax with the decrease of strain in the quantum wells, but the Si1−x−yGexCy relaxed with the increase of the strain due to the formation of SiC precipitates. Misfit dislocation formation was observed in the Si1−x−yGexCy alloys with initial thicknesses below the critical thickness after annealing at 1000 °C for 2 h. This relaxation is probably caused by the SiC precipitation, since SiC precipitates can reduce the strain compensation and, therefore, decrease the critical thickness.

Substitutional Ge in 3C–SiC

The incorporation of substitutional Ge into 3C–SiC alloys is studied theoretically with an anharmonic Keating model specifically adapted to the computation of the structural properties and the lattice dynamics of Si1−x−yGexCy alloys. Basic energy calculations show that the substitution of Si by Ge is more probable than the substitution of C by Ge in the zinc-blende silicon carbide crystal. If Ge replaces only Si, then the lattice parameter equals (0.43593±0.00002)+(0.000337±0.000002)y, where y stands for the Ge content. Hence, Vegard’s law is not applicable. The alloy is characterized by a distinct phonon spectrum whose maximum peak position in cm−1 is best described by the exponential decay (243±1)+(27±2)exp[−y/(7.5±1.2)] up to the zinc-blende GeC compound.

Strain modification in thin Si1−x−yGexCy alloys on (100) Si for formation of high density and uniformly sized quantum dots

The effects of alloying C with Ge and Si and varying the C/Ge ratio during the growth of very thin layers of the ternary alloy SiGeC grown on Si (100) substrates and the resulting strain modification on self-assembled and self-organized quantum dots are examined. During coherent islanded growth, where dislocations are not formed yet to relieve the strain, higher strain energy produced by greater lattice mismatch acts to reduce the island size, increase the density of islands, and significantly narrow the distribution of island sizes to nearly uniformly sized quantum dots. Strain energy can also control the critical thickness for dislocation generation within the three-dimensional islands, which then limits the maximum height which coherent islands can achieve. After the islands relax by misfit dislocations, the island sizes increase and the island size distribution becomes broader with the increase of misfit and strain. The optimal growth for a high density of uniform coherent islands occurred for the Si0.49Ge0.48C0.03 alloy composition grown on (100) Si, at a growth temperature of 600 °C, with an average thickness of 5 nm, resulting in a narrow size distribution (about 42 nm diameter) and high density (about 2×1010 dots/cm2) of quantum dots.

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.

Band gap and heterojunction discontinuities of pseudomorphic Si1−x−yGexCy alloy layers on Si(001)

We present a theoretical study of the minimum band gap of the pseudomorphic Si1−x−yGexCy ([C]⩽9%) alloy layers grown on Si(001). We also investigate the valence-band offset and conduction-band offset at the strained Si1−x−yGexCy/Si(001) heterointerfaces, in the framework of the average bond energy theory in conjunction with the deformation potential method. Self-consistent calculations are based on the local density functional theory, ab initio pseudopotentials and the virtual-crystal approximation. Our results show the correct tendency and order of magnitude compared with most of the theoretical and experimental data. It is encouraging to find that the tendencies of the minimum band gap and band offsets with the alloy composition and lattice mismatch are changed suddenly at the critical point due to the difference of the strain properties at the two sides of zero lattice mismatch. Our results also indicate that it is possible to obtain a larger conduction-band offset of the Si1−x−yGexCy/Si(001) heterostructure than that of the Si1−xGex/Si(001) heterostructure which offers a new prospect for the development of heterostructure devices compatible with Si integrated circuit technology.

Carbon self-organization in the ternary Si1−x−yGexCy alloy

This article demonstrates for the first time the possible self-ordering of carbon in Si1−x−yGexCy thin films pseudomorphically grown on silicon. Germanium and carbon atomic distributions have been studied for a C-rich Si0.9−yGe0.1Cy heterostructure using high-resolution transmission electron microscopy (HRTEM), high-resolution x-ray diffraction, Raman spectrometry, and secondary ion mass spectrometry (SIMS). HRTEM images show the spontaneous formation of carbon-rich tilted sublattices and local germanium fluctuations, despite constant growth parameters. X-ray diffraction confirms this thin sublayers formation. A complementary insight into local ordering effects around C is obtained by Raman spectroscopy. A new model for perpendicular lattice parameter reduction is proposed. It involves C atoms mostly in third-nearest-neighbor positions and the local formation of a distorted CSi3 graphitic arrangement. In these C-rich sublayers, the perpendicular lattice mismatch to silicon is as low as −0.014. This aperiodic structure remains highly distorted and a statistical description of these strain fluctuations is detailed. The atomistic configuration of these δ layers indicates the likely contribution of surface steps during the growth, while SIMS measurements hint at the probable involvement of carbon interstitials to explain this ordering. For technological applications, this self-organization of carbon is promising for the ultrashallow junction challenge. These carbon-rich embedded layers can be considered as quantum wells, etch stops or very thin barriers against transient enhanced diffusion.

Contacts on Si1−x−yGexCy alloys: Electrical properties and thermal stability

In this work, we have investigated the reactions between Zr and SiGeC alloys using rapid thermal annealing. The interactions of the metal films with the Si1−x−yGexCy alloys have been investigated by using sheet resistance measurements, Rutherford backscattering (RBS), energy dispersive spectroscopy (EDS), and x-ray diffraction. The morphologies of surfaces and interfaces were examined using atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM). The analyses indicated that the C49–Zr(Si1−zGez)2 phase is the final phase of the reaction. The ternary alloy even annealed at temperature as high as 800 °C indicates the same Ge index x as in the as-deposited Si1−xGex layer. We did not observe any Ge segregation after annealing. The results indicate that Zr may be a good candidate for contacts on IV–IV alloys in terms of thermal stability. In addition, we have studied the electrical properties of Schottky contacts to SiGeC alloys. We have shown that the dependence of the SBH on the metal work function is less pronounced for the alloys than for pure Si. The addition of Ge in the binary alloys always leads to a decrease of the SBH to p type. The incorporation of C results in a large increase of the SBHs and provides the possibility to get field effect on p-type IV–IV alloys by using Schottky contacts.

Size distribution of SiGeC quantum dots grown on Si(311) and Si(001) surfaces

Quantum dots of Si1−x−yGexCy alloys with high Ge contents were grown on Si(311) and Si(001) substrates by solid source molecular beam epitaxy and were measured by atomic force microscopy. The quantum dot layers had a nominal thickness (equivalent two-dimensional) of 4 nm. The smallest quantum dots occurred for the composition Si0.09Ge0.9C0.01 on Si (311), and had a 40 nm mean diameter, an 8 nm mean height, and a density of 3.3×1010 cm−2. Quantum dots on Si(001) were larger and had less regular spacing than quantum dots on Si(311) with the same composition. Carbon decreased both the mean size and spacing of SiGe quantum dots and the ratio of size deviation to mean diameter. The presence of small uniform quantum dots for particular compositions is attributed to a reduction in the surface migration of adatoms due to decreased atomic surface diffusivity. These results suggest that quantum dot organization is controlled by composition, substrate orientation, strain, and surface diffusion.

Charge transport in strained Si1−yCy and Si1−x−yGexCy alloys on Si(001)

We have investigated the temperature dependencies of charge carrier densities and Hall mobilities in tensile strained Si1−yCy and in compressively strained Si1−x−yGexCy layers. In both cases, the measured charge carrier densities at room temperature are not affected substantially by the addition of a small concentration of carbon (<1%) under identical growth conditions and dopant fluxes. The measured Hall mobilities monotonically decrease with increasing carbon content for electrons in Si1−yCy, and for holes in Si1−x−yGexCy, respectively. Our results indicate that electrically active defects are formed with the addition of carbon. These defects are presumably connected with carbon/Si interstitials or other C-related complexes. It seems to be difficult to attribute the formation of those electrically active defects solely to contaminations originating from the used carbon evaporation source. We observed that donor- and acceptor-like defects are formed in Si1−yCy as well as in Si1−x−yGexCy layers with roughly a constant ratio, independent of source temperature.

Si 1−x−y Ge x C y alloy band structures by linear combination of atomic orbitals

We have applied a virtual crystal approximation to the linear combination of atomic orbitals method to calculate critical point energies of unstrained Si1−x−yGexCy alloys spanning the composition parameter space. Additionally, we have calculated the band structure across the Brillouin zone for a series of alloy compositions. We found the band energies had significant bowing departures from linearity throughout the system. In some cases, the energy band gap was not monotonically dependent on composition. Our theoretical results are compared with recent experimental results, and good agreement was found overall.