SiGe Single Crystals Research Articles

Three-dimensional angle-resolved photoemission study of bulk SiGe single crystals

The composition- and structure-dependent electronic band structure of SiGe alloys facilitates a rich variety of its application in various fields. In this study, we performed high-resolution core-level and angle-resolved photoemission spectroscopy (ARPES) of unstrained SiGe single crystals with the (001) surface. Thermally cleaned surfaces of the single crystals of SiGe alloys exhibited double domain (2 × 1) reconstruction, wherein the asymmetric dimer was composed of Ge atoms, and Si atoms were located below the subsurface region. The compositional dependence of three valence bands and their constant energy contours were clearly resolved by three-dimensional ARPES measurements using the high-intensity synchrotron radiation. The valence bands of unstrained SiGe alloys could be well described by the composition-based interpolation of the band parameters of pristine Si and Ge crystals.

(Digital Presentation) SiGe Epitaxial Growth on Si Substrate Using Al-Ge Paste

Silicon-germanium (SiGe) has been considered as an important alternative material to silicon due to its high carrier mobility, low power consumption and excellent performance [1]. However, due to both material unique phase diagram with melting points separated by 376 °C, it is too difficult to realize a good bulk quality of polycrystalline-free SiGe single crystals with controlled Ge contents.In this work, crystalline thick epitaxial layers of SiGe with Ge contents exceeding 30% are fabricated by screen-printing technology on Si wafers. As illustrated in Figure 1, Aluminum-germanium (AlGe) paste with a mole ratio of 7:3 was prepared and screen-printed on p-type Cz-Si <111> substrate, dried at 100 °C for 10 minutes and then annealed in Ar ambient using IR image furnace at 900 °C for 5 minutes. The Al begins to melt at 660 °C, dissolving the silicon wafer surface and the Ge in the paste. The melted Al, Si and Ge form the liquid phase of the Al-Si-Ge, which when cooling process start, regrown epitaxially at the melt/Si interface to form the Al-doped SiGe crystalline layer.The formed SiGe was observed by scanning electron microscope (SEM), and energy dispersive X-ray spectrometry (EDX). As can be seen in the cross-sectional SEM image in Figure 2, SiGe layer of about 20mm was formed on the Si substrate surface. It is worth mentioning that the SiGe/Si interface is formed straight as the reaction stopped perfectly on façade <111> forming SiGe layer. EDX mapping of elements suggests that the Al in the Al-Si-Ge liquid phase formed at high temperature is segregated from the liquid to the Al-Ge paste layer during the cooling process and remains only as dopant in the SiGe layer due to its low solid solubility in Si and Ge. The Al-Ge paste residue layer will be etched chemically and further step of chemical mechanical polishing will be performed to improve the surface flatness of SiGe layer to be an epi-ready SiGe/Si substrate for different semiconductor devices applications.Reference[1] M. L. Lee and E. A. Fitzgerald, Strained Si/strained Ge dual-channel heterostructures on relaxed Si0.5Ge0.5 for symmetric mobility p-type and n-type metal-oxide-semiconductor field-effect transistors, Appl. Phys. Lett. 83, 4202 (2003). Figure 1

Pressure-sensitive liquid phase epitaxy of highly-doped n-type SiGe crystals for thermoelectric applications

Based on recent works, the most desirable high-temperature thermoelectric material would be highly-doped Si1−xGex crystals or films with sufficiently high Ge concentrations so that simultaneous enhancing the power factor and wave-engineering of phonons could be possible on the ballistic thermal conductor. However, available thin film deposition methods such as metal organic chemical vapor deposition, electron-beam evaporation, or sputtering are unable to produce highly-doped SiGe single crystals or thick films of high quality. To fabricate the desired material, we here employ liquid phase epitaxy to make highly-doped (up to 2% GaP doping) SiGe crystals with minimized concentration variations on Si (111) and (100) substrates. We find that growing Si1−xGex (x = 0.05~0.25) crystals from Ga solvents at relatively high vacuum pressure (0.1 torr) displays significant deviations from previous calculated phase diagram. Moreover, doping GaP into SiGe is found to affect the solubility of the system but not the resulting Ge concentration. We thus plot a new pressure-dependent phase diagram. We further demonstrate that the new pressure-induced liquid phase epitaxy technique can yield Si1−xGex crystals of much higher Ge concentrations (x > 0.8) than those grown by the conventional method.

Si1−xGex crystal growth by the floating zone method starting from SPS sintered feed rods – A segregation study

The availability of suitable feed rods for Si–Ge bulk crystal growth is known to be a limiting factor in floating zone growth and other growth techniques. In this work, three Si-rich SiGe single crystals were crystallized by an optical floating zone technique in a double ellipsoid mirror furnace. The feed rods were prepared by pre-synthesis in the Spark Plasma Sintering (SPS) process starting with powders of different compositions. In a detailed section the preparation method of consolidation by mechanical alloyed powders to feed rods will be given. Results from two growth experiments starting with uniform compositions with 11at% and 20at% germanium as well as a zone leveling experiment with a segmented feed rod consisting of a starting zone with 32at% Ge will be discussed. The latter experiment resulted in a crystal with nearly no axial segregation.

Seed Production and Melt Replenishment for the Czochralski Growth of Silicon Germanium

The silicon transport in a silicon germanium melt has been studied to address the issues of melt replenishment and seed production for the Czochralski growth of silicon germanium (SiGe) crystals. The growth of SiGe single crystals by the Czochralski method requires that the melt be replenished with silicon during the growth process due to the rejection of germanium into the melt during solidi cation. To facilitate the replenishment of the melt, an accurate knowledge of the dissolution rate of silicon into the melt and its transport through the melt is required. To address these issues, a number of experiments have been carried out on the dissolution of silicon into a germanium melt. Liquid phase di usion growth experiments were also conducted for insight into transport and as a possible method for seed crystal production. The experiments encompassed various temperatures, crucible geometries, crucible translation, and magnetic eld levels to determine optimum conditions for the most favorable dissolution rates and mass transport in the melt. Results have shown that replenishment from bottom of the crucible is most e ective due to the enhanced silicon transport by buoyancy. The application of magnetic elds may also provide an e ective mean to control the replenishment rate (mass transport rate) in the melt.

A Numerical Simulation Study on the Effects of Crucible Rotation and Magnetic Fields in Growth of SiGe by the Traveling Heater Method

A numerical simulation study was carried out to shed light on the effects of applied crucible rotation and static magnetic field during the traveling heater method growth of bulk SiGe single crystals. The simulation results show that the application of crucible rotation weakens the radial silicon concentration gradient due to the effect of centrifugal force. The effects of applied static magnetic field direction and strength on the concentration field in the melt were also studied. It was found that the simultaneous application of crucible rotation and static magnetic field is best to grow large crystals with uniform composition. An optimum combination of crucible rotation rates and applied magnetic field strengths is determined.

A numerical simulation study for the combined effect of static and rotating magnetic fields in liquid phase diffusion growth of SiGe

The article presents the results of a numerical simulation study conducted to examine the combined effect of static and rotating magnetic fields on the liquid phase diffusion (LPD) growth process of SiGe single crystals. The simulation results indicate that the combined use of static and rotating magnetic fields is more effective than using either a static or rotating field alone. The application of a combined field does not only suppress natural convection but also leads to better mixing in the solution and flatter growth interfaces. For the system considered here, the application of a static field of 0.3 T and a rotating field of 4.0 or 5.0 mT makes the growth interface almost flat.

Crystallographic investigation of homogeneous SiGe single crystals grown by the traveling liquidus-zone method

Crystallographic investigation of Si 0.5Ge 0.5 single crystals grown by the traveling liquidus-zone (TLZ) method was carried out using an X-ray diffraction method, a back-reflection Laue camera and X-ray rocking curve measurements. X-ray rocking curve of the Si 0.5Ge 0.5 crystals showed excellent crystallinity: full-width at half-maximum (FWHM) of the (4 4 0) diffraction was 0.009°, which is comparable to that of Si single crystal. Such high-quality Si 0.5Ge 0.5 bulk crystals were obtained for the first time and showed the superiority of the TLZ growth method for growing alloy bulk crystals.

A numerical simulation study for the effect of magnetic fields in liquid phase diffusion growth of SiGe single crystals

The article presents the results of a numerical simulation study conducted to examine the effect of static and rotating magnetic fields on the growth process of SiGe single crystals by liquid phase diffusion (LPD). A three-dimensional transient numerical simulation model was developed to observe the heat, mass, and momentum transfer characteristics of the SiGe solution. Simulation results indicate that the use of a static, vertical magnetic field in this growth setup is effective in suppressing natural convection in the solution. A stationary field intensity of 0.3 T is sufficient to provide significant suppression, above which the flow becomes numerically unstable. However, the vertical magnetic field does not provide the expected flattening in the growth interface. In the case of a rotating magnetic field (RMF), results show that the use of RMF is effective in providing sufficient mixing in the melt leading to more homogeneous SiGe crystals. In addition, RMF is also very beneficial for flattening the growth interface. At the 3.0 mT a RMF intensity level, the growth interface becomes almost flat.

Temperature dependence of electron and hole mobilities in heavily impurity-doped SiGe single crystals

Heavily impurity-doped single crystals of SixGe1−x alloy with the composition 0.84&amp;lt;x&amp;lt;1 and large-grained polycrystals with x=0.80 were grown by the Czochralski technique. The Hall-coefficient measurements of the electron and hole mobilities in the grown crystals were carried out in the temperature range of 300–1000K and compared with those in undoped SiGe. The Hall mobilities of electrons and holes in SiGe with a carrier concentration of 1019–1020cm−3 both show a Tn, n∼1, temperature dependence up to elevated temperatures. This indicates that the carrier transport process is mainly rate controlled by charged impurity scattering. In single-crystal SiGe free from grain-boundary effects, the hole mobility increases with decreasing Si content at least up to 0.84 and the electron mobility is greater than in Si and polycrystalline SiGe. These results suggest that scattering processes in alloy semiconductors are more complicated than previously thought.

Anisotropy of the Seebeck coefficient in Czochralski grown p-type SiGe single crystal

To discuss the possibility of improvement in thermoelectric properties of SiGe alloys, we examined anisotropy of the Seebeck coefficient of the p-type SiGe single crystals with orientation of 〈1 1 1〉 and 〈1 0 0〉 grown by Czochralski method. For measurement of the Seebeck coefficient, we developed an apparatus that is capable of measuring the Seebeck coefficient in the temperature range of 300–900 K. The Seebeck coefficient of the sample with 〈1 1 1〉 orientation was around 325–400 μV/K, while that of the sample with 〈1 0 0〉 orientation was around 450–530 μV/K. The difference was larger than experimental error, and appeared to attribute to the difference in crystallographic direction.

FTIR spectroscopy of high concentration Ge-doped Czochralski-Si

SiGe single crystals with different concentrations of doping Ge in CZSi were measured by FTIR together with the analysis by secondary ion mass spectroscopy and single-crystal X-ray diffraction. The results indicate that when Ge dopant is lower than 1.0wt%, the FTIR spectroscopy is nearly the same as that of CZSi. With increasing Ge, the spectroscopy becomes complex in the lower wave number range. At the wave number of 710cm−1 a new peak appears, which is considered to be related to Ge dopant. The new peak of the spectroscopy becomes clearer with an increase in Ge, possibly due to the Ge–C or Si–Ge–C system.

Compositional variation in Si-rich SiGe single crystals grown by multi-component zone melting method using Si seed and source crystals

The effect of the supply of depleted Si solute elements on the compositional variation in the Si-rich SiGe bulk crystals was studied using the method which was used to grow Ge-rich SiGe single crystals with a uniform composition. By selecting the proper pulling rate, we can obtain Si-rich Si 1− x Ge x bulk crystals with uniform composition of x=0.1 without using the supply mechanism of depleted Si solute elements. When the supply mechanism of Si solute elements was used, the initial composition in Si-rich SiGe crystals can be much more easily determined by controlling the growth temperature than that in Ge-rich crystals because the Si seed crystal is not melted down. The supply of Si solute elements is very effective to change the compositional variation even for Si-rich SiGe crystals.

Dynamics of kinks on dislocations in SiGe single crystals

The method of intermittent pulse loading is used for obtaining the dependences of the mean free path of individual dislocations in SiGe single crystals with various concentration of Ge (0–5.5 at. %) on the duration of loading pulses and time intervals between them. It is found that these dependences change qualitatively upon an increase in the Ge concentration. It is shown that the motion of dislocations in SiGe crystals under small shear stresses is characterized by a nonlinear drift of kinks and the formation of superkinks. A theory of the motion of dislocations under the action of intermittent pulse loading under the conditions of heterogeneous kink dynamics is developed. Extended quasi-one-dimensional defects repeating the shape of a part of a segment of a moving dislocation are discovered in SiGe crystals containing 0.96 at. % Ge. The mechanism of formation of such defects as the result of the shedding of a part of the impurity atmosphere by a dislocation segment during overcoming of a local obstacle is proposed.

Anodic oxidation as a low thermal budget process for passivation of SiGe

The oxide composition and the electronic behavior of anodically and thermally oxidized SiGe single crystals and epitaxial layers have been investigated. X-ray photoelectron spectroscopies show the existence of Si4+ and Ge4+ only in the anodic oxides, whereas the thermally grown oxide layers on SiGe consist of pure SiO2. Infrared spectroscopy reveals the existence of Ge as part of mixed oxide compounds like Si–O–Ge and not as GeO2 up to a Ge content of 26 at.% in the SiGe lattice. Ge segregation at the oxide/SiGe interface and relaxation phenomena of the strained SiGe lattice of epitaxial layers, which occur during thermal treatments, are completely suppressed using the anodic treatment. In addition, the defect concentration at the interfaces is reduced as obtained from photoluminescence and capacitance–voltage measurements as compared to thermally grown oxides. The amount of Ge in the lattice has an influence on the defect concentration.

X-ray and neutron scattering study of Si-rich Si-Ge single crystals

The local atomic environments of homogeneous Si-rich Si-Ge single crystals were studied using x-ray and neutron diffuse scattering. No evidence of either chemical short-range order or clustering was observed. Static atomic displacements, however, were clearly present and are consistent with an expansion of the lattice in the vicinity of the Ge atoms. The dispersion of the acoustic phonons was also measured using inelastic neutron scattering. The acoustic modes of the Si-Ge alloy were found to lie nearer those of pure Si than expected from the homologous relationship found between pure Si and pure Ge.

Bulk single crystal growth of silicon-germanium

Si-Ge single crystals up to 68 mm diameter and up to 17 at.% germanium were grown using a modified Czochralski technique. Pre-grown large diameter single crystal silicon seeds with various crystallographic orientations were used as templates for solidification to reduce cap crystallization time and to ensure single crystallinity at desired crystal diameters. A discussion is presented of the influences of seed preparation, crystal growing parameters and post-growth processing on the Si-Ge bulk single crystals produced using this new technique. The modified Czochralski technique described in this paper is ultimately intended for the manufacture of 100–200 mm diameter Si-Ge substrates.

Anodic passivation of SiGe

We have investigated the oxide composition and the electronic behavior of the interface of anodically and thermally oxidized SiGe single crystals and epitaxial layers. X-ray photoelectron spectroscopies show the existence of SiO2, GeO2 and possibly mixed oxide compounds such as SiOGe only in the anodic oxide, whereas the thermally grown oxide layers on SiGe samples consist of pure SiO2. In addition, the Ge enrichment at the interface and relaxation phenomena of the strained SiGe lattice of epitaxial layers, which occurs during thermal treatments, are completely reduced using the anodic treatment. Further, the defect concentration at the interface is reduced after the anodic oxidation as obtained from capacitance–voltage and photoluminescence measurements.

Graded X-ray Optics for Synchrotron Radiation Applications.

Using X-ray diffractometry and spectral measurements, the structure and properties of graded X-ray optical elements have been examined. Experimental and theoretical data on X-ray supermirrors, which were prepared by the magnetron sputtering technique using precise thickness control, are reported. Measurements on graded aperiodic Si(1-x)Ge(x) single crystals, which were grown by the Czochralski technique, are also presented. The lattice parameter of such a crystal changes almost linearly with increasing Ge concentration. The measurements indicate that Si(1-x)Ge(x) crystals with concentrations up to 7 at.% Ge can be grown with a quality comparable to that of pure Si crystals.

Czochralski growth of Si- and Ge-rich SiGe single crystals

Si 1 − x Ge x single crystals (0 < x < 0.15) with diameter up to 2 in were grown by conventional Czochralski technique. The method for the growth of Ge 1 − x Si x crystals using continuous feeding the melt with a number of Si rods was developed and the Ge 1 − x Si x crystals (0 < x < 0.3) were grown. The growth process both Si 1 − x Ge x and Ge 1 − x Si x crystals was carried out using the automated control system based on the crystal weighing. The peculiarities of weight control at the stage of conical part formation are considered.