xGex Films Research Articles

Analysis and characterization of native oxide growth on epitaxial Si1−xGex films after a chemical clean

Ellipsometry, contact angle goniometry, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) are used to study native oxide growth on SiGe films (with Ge content of 0%, 20%, 40%, and 100%) after a chemical clean. Ellipsometry suggests that the presence of Ge affects the initial oxide thickness right after the clean but it does not affect the rate of native oxide growth. Roughness of SiGe samples as measured by AFM does not appear to be affected by the native oxide growth or the Ge content in the film. The decrease in advancing (and receding) contact angles of SiGe samples after the chemical clean is apparently the result of both increasing oxide thickness and oxide solid phase composition. XPS results suggest that increasing Ge content in the film increases the oxidation of SiGe surface atoms. The chemical shifts in the Si 2p and Ge 3d spectra suggest that both Si and Ge react with oxygen to form SiO2 and GeO2. Such data suggests that contact angle measurements could be a rapid method to determine the state and passivation characteristics of a silicon substrate surface as a function of time; however, such a technique would not be as effective for SiGe films, the chemical composition of their native oxides of which would also change as a function of time.

Pulsed KrF laser annealing of Ni/Si0.76Ge0.24 films

Pulsed KrF laser annealing can suppress the island structure formation and Ge segregation associated with the interfacial reactions of Ni/Si0.76Ge0.24. For the Ni/Si0.76Ge0.24 films annealed at an energy density of 0.1–0.3 J/cm2 nickel germanosilicide associated with the amorphous overlayer was formed, while at energy densities above 0.4 J/cm2 cellular structures of Ge-deficient Si1−xGex islands surrounded by Ni(Si1−xGex)2 due to the constitutional supercooling occurred. For the continuous Ni(Si1−xGex) films grown at 200 °C, subsequent laser annealing at a higher energy density of 0.6–1.0 J/cm2 caused transformation into homogeneous Ni(Si0.76Ge0.24)2 films without island structure and Ge deficiency which readily appeared on furnace annealing at temperatures above 400 °C. At energy densities above 1.6 J/cm2 the same cellular structures as described above were also noted.

Biaxial moduli of coherent Si1−xGex films on Si (001)

The biaxial moduli of coherently strained Si1−xGex thin films have been determined over the composition range x=0.15–0.6 from independent measurements of film stress and strain. Our results indicate that use of the rule of mixtures to determine the strained-alloy elastic constants from the bulk values for pure Si and Ge is valid, and that higher-order elastic effects are relatively unimportant over this composition range.

Effects of water vapor and chlorine on the epitaxial growth of Si1−xGex films by chemical vapor deposition: Thermodynamic analysis

The effects of water vapor and chlorine on the Si1−xGex epitaxial growth are studied through thermodynamic analyses. Since Ge does not form chemical species with oxygen at an appreciable level, the water vapor and oxygen background level requirements for Si1−xGex epitaxial growth are mostly similar to the ones for Si growth. At large excesses of Ge gas phase sources and low system water vapor level, however, the critical temperature above which oxide-free epitaxial growth occurs decreases with increasing system pressure. Analyses of the results obtained suggest that there are two mechanisms for the removal of SiO2(s) from the surface: one is by forming volatile SiO, and the other is by forming gas phase H2O. The former one appears to dominate for most conditions studied; the latter one appears to dominate when the water vapor level of the system is very low and/or small amounts of the Si gaseous source are present. The boundary between etching and deposition of Si1−xGex films is examined for all Ge solid phase compositions. The etch/deposition boundaries for pure Si and pure Ge show opposite trends with increasing chlorine concentrations, because of the different affinities of Si and Ge to form chlorinated species. The chlorine concentration needed to reach etching increases with increasing system temperature for Si, whereas it decreases with increasing temperature for Ge. It is found that the formation of SiCl4 is favored over GeCl2 at low temperatures, while the opposite happens at higher temperatures. The etch/deposition boundary for Si1−xGex thin films is found to form a saddlelike contour because of the interaction of the different behaviors of Si and Ge. Overall, calculated results are seen to be in good agreement with available experimental results.

Strain relaxation and defect formation in heteroepitaxial Si1−xGex films via surface roughening induced by controlled annealing experiments

Mechanisms of strain relaxation and defect formation during surface roughening in Si1−xGex films grown epitaxially on (100)Si substrates have been investigated by controlled annealing experiments. Epitaxial films 10 nm in thickness and containing 18% Ge, which are subcritical with respect to the formation of misfit dislocations, show strain relaxation through surface roughening on annealing at 850 °C, where surface grooves are aligned along 〈100〉 directions. Other films with 22% Ge and supercritical thicknesses have also been studied, where surface grooves are aligned along 〈110〉 directions.

Unusual strain relaxation in SiGe/Si heterostructures

Si 1−x Ge x films (x=0.22) epitaxially grown by ion beam-sputter deposition on (001) Si substrates were subjected to rapid and conventional thermal annealings at different temperatures. Strain measurements carried out by means of high-resolution x-ray diffraction exhibited strongly nonmonotonous strain dependencies on the annealing time. We observed short-time and long-time relaxation modes with activation energies of 4.6 and 1.3 eV, respectively, and unexpectedly, an additional mode of strain recovery at intermediate time durations with an activation energy of 1.6 eV. This behavior was attributed to processes that involve {113} two-dimensional defects, i.e., agglomerates of interstitials, which were identified by means of transmission electron microscopy.

Schottky Barrier Height of Ti on Strained Layer Si/Si1–xGex Films

The Schottky barrier height of Ti on p-type Si1–xGex/Si and Si have been investigated-in the temperature range 110–250 K using the current voltage technique. Rutherford Backscattering Spectroscopy (RBS) and X-ray Diffraction (XRD) were used to characterize the reaction between Ti and SiGe alloys.

MeV ion implantation induced damage in relaxed Si1−xGex

The damage produced by implanting, at room temperature, 3-μm-thick relaxed Si1−xGex alloys of high crystalline quality with 2 MeV Si+ ions has been studied as a function of Ge content (x=0.04, 0.13, 0.24, or 0.36) and Si dose in the dose range 1010–2×1015 cm−2. The accumulation of damage with increasing dose has been investigated by Rutherford backscattering spectrometry, optical reflectivity depth profiling, and transmission electron microscopy. An enhanced level of damage, and a strong decrease in the critical dose for the formation of a buried amorphous layer in Si1−xGex is observed with increasing x. Electron paramagnetic resonance studies show that the dominant defects produced by the implantation are Si and Ge dangling bonds in amorphouslike zones of structure similar to a-Si1−xGex films of the same x, and that the effect of increasing the ion dose is primarily to increase the volume fraction of material present in this form until a continuous amorphous layer is formed. A comparative study of the optically determined damage in the alloys with the use of a damage model indicates a significant increase in the primary production of amorphous nuclei in the alloys of Ge content x>0.04.

Ultrathin Oxides on Strained Epitaxial Si1–xGex Films at Low Temperature

Integration issues of SiGe strained layers in Si-technology-based integrated circuits are addressed. The present status of the silicon dioxide formation on SiGe films is reviewed. Results of our studies on the low temperature (150–200°C) growth of ultrathin oxides using microwave plasma (in both O2 and N2O ambient), compositional analysis, electrical characterization and interfacial properties of the oxides are presented. Hole confinement in a Gas Source Molecular Beam Epitaxy (GSMBE) grown Si0.74Ge0.26/Si quantum well on silicon using (i) computer simulation and (ii) C-V measurements are also demonstrated.

The stability of Si1−xGex strained layers on small-area trench-isolated silicon

The combined effects of isolation stress, active area size, and SiGe misfit strain on dislocation generation in an advanced SiGe heterojunction bipolar transistor (HBT) process were studied. Eight-inch wafers were patterned with polysilicon-filled deep, and oxide-filled shallow trench isolation similar to that used in IBM's analog SiGe HBT technology. Half of the wafers were subjected to an additional stress-producing oxidation prior to SiGe growth. Si1−xGex films containing 0, 5.5, 9, and 13 at.% Ge were grown epitaxially by ultrahigh vacuum chemical vapor deposition (UHV CVD). The films were of constant thickness with an intrinsic Si cap. Some samples received an additional relaxation anneal following deposition. After the growth and anneal cycles, the dislocation density was determined by transmission electron microscopy (TEM). On nonstressed samples, no dislocations were observed in the device areas, even at Ge concentrations which are not stable to misfit dislocation generation in blanket form. This small area effect has been observed on patterned substrates that do not have functional device isolation. On the stressed-isolation wafers, the compressive stress from the oxidation of the trench sidewalls was found to intensify stress in the SiGe films, and to lower the critical strain at which misfit dislocations appeared. In large active areas on these wafers, two distinct dislocation regions were observed. Defects at the edge resembled those caused by isolation stress, while the defects in the center were more typical of the misfit dislocations associated with lattice-mismatch epitaxial films. It is clear that isolation stress must be minimized when fabricating integrated circuits using SiGe epitaxial films. It is also evident that SiGe films grown on nonstressed isolation exhibit the same increase in critical thickness with decreasing lateral dimension that has been observed on much simpler patterned substrates.

In-Situ TEM Observations of Surface Roughening and Defect Formation in Lattice Mismatched Heteroepitaxial Thin Films

ABSTRACTThis paper focuses on in-situ transmission electron microscopy observations of surface roughening and defect formation in heteroepitaxial Sil−xGex thin films. Annealing experiments have been carried out in-situ in the microscope under a high vacuum environment. We comment on the sample preparation procedure for in-situ TEM experiments and explain the importance of having a sufficiently thick sample to have the stress state in the film unaltered. Experimental results of in-situ surface roughening are presented for suberitically and supercritically thick Sil−xGex films. We found that, in a vacuum environment, the kinetics of surface roughening and the resulting surface morphology are much different than in a hydrogen environment.

Structural characterization of ordered SiGe films grown on Ge(100) and Si(100) substrates

We observe the formation of an ordered structure in Si1−xGex films grown on Ge(100) substrates, as well as on Si(100) substrates, by molecular beam epitaxy. The structural characterization of these ordered films is performed. The degree of order in the films is quantitatively measured using x-ray diffraction. The dependence of the degree of order on Ge composition is similar between films on Ge(100) and Si(100) substrates. By careful x-ray diffraction analysis, we find that the degree of order is not equivalent in variants.

Effects of Germanium on Grain size and surface roughness of the solid phase crystallized polycrystalline Si1−xGex films

AbstractSi1−xGex (x≤0.5) films were deposited by using Si2H4 and GeH4 source gases in a low pressure chemical vapor deposition (LPCVD). The deposition temperature was varied from 375 °C for Si0.5Ge0.5 to 450 °C for Si film in order to deposit amorphous Si1−xGex films and the deposition pressure was about 1 Torr. The grain size of polycrystalline Si1−xGex films made by solid phase crystallization (SPC) decreases with germanium content in the films, and the activation energy obtained from the dependence of grain size on annealing temperature was 0.4 eV for Si and 0.45 eV for Si0 69Ge0.31. From obtaining a similar activation energy irrespective of Ge content in the film, the decrease of grain size with germanium content is attributed to the difference of the asdeposited film conditions. The surface roughness of Si1−xGex films investigated by atomic force microscope (AFM) increases with germanium content in the film before and after SPC. For instance, the root-mean square (rms) values of the surface roughness of the as-deposited Si and Si0.5Ge0.5 films were 2.3 and 17 Å, while those values were increased to 2.6 and 41 Å, respectively, after SPC. In order to reduce the surface roughness of Si0.5Ge0.5 film, we have deposited a thin Si capping layer on top of the Si1−xGex layer and identified that this capping layer effectively reduces the increase of surface roughness after SPC.

Addendum: ‘‘Influence of misfit dislocations on the surface morphology of Si1−xGex films’’ [Appl. Phys. Lett. 66, 724 (1995)

The omission of a key reference (Ref. 2) in the authors’ earlier publication (Ref. 1) is noted. The major conclusion of (Ref. 2) was the same as that of the authors’ in (Ref. 1), namely, that surface steps generated by dislocation glide play an important role in the morphology in Si1−xGex films. (AIP)

Crystallization of amorphous hydrogenated Si1−xGex films

The incubation time t0 of the crystallization of intrinsic and P, B-doped amorphous silicon-germanium (a-Si1–xGex: H) layers deposited on SiO2/Si(100) substrates is studied as a function of temperature and composition using in situ transmission electron microscopy (TEM). The temperature dependence of t0 follows an Arrhenius behavior t0 = t0* exp (W/kT) with activation energy W and prefactor t0* depending on x. The dependences of grain growth, nucleation rate as well as of the morphology of the polycrystalline films on temperature, doping, and composition are estimated. Doping SiGe films with B decreases grain growth rate during crystallization. Independent of the Ge content, such films stay nanocrystalline (grain size ≪10 nm) up to an annealing temperature of 800°C. During crystallization decomposition within SiGe films is observed. Furthermore, metal-induced crystallization in the Al/a-Si1–xGex:H structures is demonstrated. Die Inkubationszeit t0, die bei der Rekristallisation von intrinsischen bzw. P- und B-dotierten amorphen Silizium-Germanium-Schichten (a-Si1–xGex:H) auf SiO2/Si(100)-Substraten auftritt, wird in Abhängigkeit von der Temperatur und Zusammensetzung mittels in situ Transmissionselektronenmikroskopie (TEM) untersucht. Die Temperaturabhängigkeit von t0 folgt einer Arrhenius-Kurve der Form t0 = t0* exp (W/kT), wobei die Aktivierungsenergie W und die Zeitkonstante t0* von der Konzentration x abhängen. Durch in situ TEM-Experimente wird die Abhängigkeit des Kornwachstums und der Keimbildungsrate, aber auch der Morphologie der polykristallinen Schichten von Temperatur, Dotierung und Komposition ermittelt. So führt eine B-Dotierung z. B. zu einer Verringerung der Wachstumsrate. Unabhängig vom Ge-Gehalt solcher Schichten erreichen die Kristallite nur eine Größe von ≪ 10 nm bei Temperaturen bis 800°C. Im Rahmen solcher Rekristallisationen treten auch Dekompositionen auf. Darüber hinaus läßt sich eine Rekristallisation durch Metall-Phasen beeinflussen, wie am System Al/a-Si1–xGex:H gezeigt wird.

Evolution of strain relaxation in compositionally graded Si1−xGex films on Si(001)

High-resolution x-ray reciprocal space mapping was employed to determine the in-depth strain distribution of Si1−xGex films with linear composition gradings between 4.2% and 15% Ge per μm, and thicknesses between 0.4 and 1.7 μm. The variation of grading and thickness parameters of the samples provides a complete picture of the overall relaxation behavior of linearly graded epilayers. The x-ray data show a top layer of grading-dependent residual strain whereas the lower parts of the films are completely and/or partly relaxed with respect to the Si substrate.

Low Pressure Chemical Vapor Deposition of Si1 − xGex Films on SiO2: Characterization and Modeling

Deposition of silicon‐germanium thin films in a hot wall tubular low‐pressure chemical vapor deposition (LPCVD) furnace using and at 100 mTorr with temperatures ranging from 297 to 650°C has been studied. The deposited films range from pure Si to pure Ge. The deposition rate behavior is presented. The apparent activation energy of deposition rate of Ge from in reaction‐rate limited regime was found to be 0.91 eV, and for the deposition of Si from was found to be 1.7 eV. The relation between the mole fraction in the gas phase during deposition and Ge atomic fraction in the film is described. A model based on the sticking probability of Si and Ge precursors on the surface is proposed to describe this relation.

Micromorphology of Ge and Si1−xGex layers grown on Si(001) by solution epitaxy and the negative-differential conductivity on the Ge layer

The micromorphology and the electronic structure of heteroepitaxial Ge and Si1−xGex layers grown pseudomorphically by solution epitaxy on Si(001) has been investigated by Auger electron spectroscopy as well as scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. A pure Ge film of 4–5 monolayer thickness typically repeats the structure of the Si substrate surface. A significant decrease of tunneling current at a sample voltage of 1.5 V is observed in areas of 0.5 nm diameter between Ge dimer rows. This decrease is due to a negative-differential conductivity at a tunnel diode configuration consisting of a surface defect structure of the Ge layer and the STM tungsten tip. On Si1−xGex films a strong Ge enrichment was found due to segregation during growth. The high Ge surface concentration on Si1−xGex results in additional surface strain fields and trench formation in the uppermost monolayer.

Polycrystalline silicon–germanium films on oxide using plasma-enhanced very-low-pressure chemical vapor deposition

Si1−xGex thin films on oxide-coated Si substrates have been formed by plasma-enhanced very-low-pressure chemical vapor deposition. Two modes of deposition, thermal and plasma enhanced, were studied using SiH4 and GeH4 at temperatures ≤600 °C. In both cases, growth rates and grain sizes increase with Ge content, and the polycrystalline-to-amorphous transition temperature is lower for Si1−xGex than Si. Compared to thermal growth of polycrystalline Si1−xGex, plasma-enhanced deposition of poly-Si1−xGex promotes higher growth rates, smaller grain sizes, and direct deposition onto oxide, as well as improved structural properties such as a smoother surface and a more columnar grain texture. Plasma deposition of amorphous-Si1−xGex films followed by a crystallization anneal at 600 °C results in an even smoother surface morphology with grain sizes enhanced by an order of magnitude compared to plasma-deposited poly-Si1−xGex films.

Effect of Si+ and Ge+ ions on the growth of Si1−xGex films on Si(001) using potential-enhanced molecular-beam epitaxy

The effect of Si+ and Ge+ source ions on surface morphology and strain relaxation of Si1−xGex (0.25≤x≤1.0) film on Si (001) substrate was investigated using potential-enhanced molecular beam epitaxy. The growth temperature ranged from 450 to 710 °C. The applied potential to the substrate was varied between −2.0 and 1.5 kV. The acceleration of a small fraction of source ions toward the substrate suppressed three-dimensional nucleation mode, and dramatically improved the surface morphology. Contrary to the negative potential, the application of a positive potential did not contribute to improving the surface smoothness of Ge on Si. For the growth of Si0.5Ge0.5 film, the surface morphology was degraded further by applying a positive potential of 1.5 kV. Si0.75Ge0.25 films grown with a negative potential of −1.6 kV relieved the strain at the much earlier stage of heteroepitaxial growth than that of conventional molecular-beam epitaxy.