SiGe Growth Research Articles

Facet investigation in selective epitaxial growth of Si and SiGe on (001) Si for optoelectronic devices

The facet evolution and growth rate of Si on {hkl} facets were investigated in the temperature range 700–850 °C using multilayer structures with thick Si and very thin SiGe markers prepared by selective epitaxial growth using low pressure chemical vapor deposition. The most stable facet, observed at all temperatures, is the high index plane {113}. It is the dominant facet up to the top of all mesas (∼1μm thick). Even at the corners of square dots steep {113} facets developed [angle of 72° with the (001) plane]. Several facets reported here are observed for the first time. In the 〈110〉 zone it is the {119} facet, while in the 〈100〉 zone two facets with Miller indices h≠k≠l≠h, the {018} and {0 1 12}, are found. The measured growth rate relationship is R111<R110<R113<R018<R119<R0112<R001. The activation energy of the growth rate Rhkl on the facet is roughly equal to the activation energy for growth on (001), which implies that the same step involved in the deposition limits the growth rate on (001) and on {hkl} surfaces (at least for the {111} and {113} facets). Only the {113} and {110} facets extend and grow from the beginning of the epitaxy up to the top. The first demonstration of photoluminescence from SiGe quantum wells grown on {113} Si is given.

Kinetics and dynamics of Si GSMBE studied by reflectance anisotropy spectroscopy

Kinetic and dynamic information from semiconductor surfaces is of great importance in the understanding of growth processes in molecular beam epitaxy. Surface related information, such as coverage and composition, is often measured with techniques such as temperature programmed desorption under conditions away from those encountered in a realistic growth environment. If the adsorbate/precursor has a strong influence on the electronic structure of the surface it is possible to make in situ and more direct measurements using optical techniques. Reflectance anisotropy spectroscopy (RAS) has been shown to be sensitive to morphological, structural and chemical changes on the surface and is well-suited for this role. Structural changes on the Si(001) surface resulting from epitaxial growth were shown to be the origin of RA oscillations observed during layer by layer growth. Hydrogen produced on the surface from the decomposition of hydride precursors is very important in the growth of Si and SiGe, since it can prevent further adsorption of hydrides through site blocking, behave as a surfactant and change the spectroscopic RA response. We can use the single reconstruction domain produced on vicinal Si(001) surfaces to explore surface-hydride–hydrogen interactions. We have made isothermal measurements during adsorption and desorption cycles and compared the results with various models whose validity under the condition studied is discussed.

New insights on SiGe growth instabilities

In this work we investigate the influence of the Si substrate orientation on the growth instability of strained Si1−xGex heterostructures. The work mainly consists in atomic force microscopy and grazing incidence x-ray diffraction analyses of the Si1−xGex layers deposited by gas source molecular beam epitaxy on vicinal Si substrates tilted from (001) to (111) surfaces. The major result is that the two- to three-dimensional growth transition is dramatically affected by the orientation of the substrate but also by the equilibrium shape of silicon. For instance, we evidence the layer by layer growth of Si1−xGex on Si (111) in contrast to the nucleation of three-dimensional islands on 2° off Si (111) in the same experimental conditions. We systematically verify that the homoepitaxial growth of unstressed Si on vicinal Si (111) consists in a regular array of single steps. Therefore, we propose that the stress induced by the heteroepitaxial growth destabilizes the regular step train by reducing the repulsive elastic interaction between steps, and induces step-bunching. The presence of close-spaced steps and the metastability of the vicinal surfaces increase the tendency towards instable growth and result to earlier development of bunching. Despite the accompanying increase of surface area, the development of low-energy facets balances the surface free energy excess. In all cases, step-bunching instability is a kinetic pathway towards the faceted equilibrium state. Long annealing treatment of the strained metastable Si1−xGex layers confirms this last point.

Study of GeSi alloy deposition on Ge substrate by very low-pressure chemical vapor deposition

We have studied the GeSi alloy deposition on Ge substrate by rapid thermal process, very low-pressure CVD method. The growth rate of the GeSi alloy increases with increasing the Si composition at a proper temperature. The high substrate temperature will cause the Si fraction and the GeSi growth rate to increase. Kinetics study of very low-pressure chemical vapor deposition has been used to discuss these results.

Loading Effects During Low-Temperature Seg Of Si And SiGe

AbstractSelective Epitaxial Growth (SEG) of Si and SiGe suffers from pattern sensitivity. The growth rate and layer composition change with the pattern and the window size. In relation to growth at atmospheric pressure, the sensitivity to the window size is suppressed at reduced pressure, whereas some growth rate effects of a more global nature become more visible. The Si growth rate decreases when the area of exposed silicon on the wafer and the susceptor decreases. SiGe shows the opposite behavior: its growth rate increases with decreasing silicon area. Another difference between Si and SiGe is the range over which the loading effects are active. The influence of a large silicon area on the Si growth rate can be felt inches away, whereas the SiGe growth rate is affected over a much shorter distance. In common epi reactors the wafer rests on a susceptor, which extends beyond the wafer, exposing a large Si-coated surface area around the circumference of the wafer. Consequently, the Si growth rate varies unacceptably across the wafer. A sacrificial polysilicon layer has been successfully applied to improve the growth rate uniformity across the wafer.

Fabrication of SiGe quantum dots on a Si(100) surface

This paper reports a method to produce SiGe quantum dots on a Si(100) surface, which is based on the selective adsorption effect of hydride molecules on a partially hydrogen-terminated Si(100) surface. It is shown that etching of Si(100) surfaces for a limited time in ammonium fluoride $({\mathrm{NH}}_{4}\mathrm{F})$ solution initially produces an atomically flat and dihydride-terminated surface and that further etching leads to the formation of microscopic (111) facets which are regularly distributed along the surface. Hydrogen atoms are found to desorb completely from dihydride sites at $\ensuremath{\sim}400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ while those from monohydrides remain stable up to $650\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Hence, we show that in the temperature range of $400--650\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, SiGe growth occurs only on the sites that were previously terminated by dihydrides, i.e., free of hydrogen. We demonstrate that SiGe dots formed by using this approach are much smaller in size $(\ensuremath{\sim}400 \mathrm{\AA{}})$ and more uniform than dots formed by the strain-induced growth mode transition.

Interface dislocations forming during epitaxial growth of GeSi on (111) Si substrates at high temperatures

During high-temperature epitaxial growth of GeSi layers on (111) Si substrates the relaxation of misfit stresses produces regular misfit dislocation networks in the GeSi/Si interface. In these networks the misfit dislocations dissociate into Shockley partial dislocations with Burgers vectors parallel to the interface. The dissociation leaves behind intrinsic and extrinsic stacking faults, which cover a major fraction of the GeSi/Si interface. Observations on the early stages of layer growth indicate that the GeSi grows layer by layer, and that the critical layer thickness for misfit stress relaxation corresponds to the thickness expected from the energy criterion of Matthews and Blakeslee. Experimental observations of misfit dislocations forming during cyclic heating of as-grown GeSi/Si layers reveal the mechanisms by which misfit dislocations with Burgers vectors parallel to the GeSi interface can form.

Selective Si and SiGe epitaxial heterostructures grown using an industrial low-pressure chemical vapor deposition module

Low-temperature epitaxial growth of Si and Si1−xGex (referred to as SiGe, hereafter) has been obtained using an industrial, 200 mm, single wafer chemical vapor deposition module operating at reduced pressure. Epitaxial Si and heteroepitaxial SiGe deposition with Ge content ⩽30% have been studied for buried channel applications in (PMOSFET) devices or as base for heterojunction bipolar transistors (HBTs). The dependence of Si and SiGe deposition rates on filling ratio and exposed windows and their evolution with the addition of HCl to the gas mixture are investigated. In contrast to selective Si growth where the global loading effect decreases slowly with temperature, the growth rate of SiGe at low temperature is strongly dependent on the oxide coverage. The addition of HCl into the gas mixture allows minimizing the dependence of the SiGe growth rate on both oxide coverage and window size. The effect of the addition of HCl on Ge and dopants incorporation is investigated on bare and/or device wafers. Results on facet formation and orientations are also presented for selective Si and SiGe growths. Finally, we report basic electrical results on selective Si epitaxial and SiGe heteroepitaxial structures, which have been integrated in PMOSFET and HBT devices.

SiH 4 and GeH 4 chemical vapor deposition of GeSi/Ge heterostructures

The deposition of GeSi alloys on Ge substrate by Rapid Thermal Process, Very Low Pressure CVD method has been studied. The growth rate of the GeSi alloy increases as the Si atoms are incorporated into the GeSi alloy at a proper temperature. The high substrate temperature will cause the Si fraction and the GeSi growth rate to increase.

Process Improvements in the Selective Epitaxial Growth of Si1 − xGex / Si Strained Layers in a Conventional Hot‐Wall LPCVD System

Three process considerations were identified as essential for the selective epitaxial growth (SEG) of SiGe in hot‐wall tubular low pressure chemical vapor deposition reactors. A thermodynamic analysis of the system revealed that the hydrogen bake condition for SiGe SEG had to be modified from that for Si SEG to eliminate outgassing of residual Ge, which adversely affected the initial growth surface. A two‐step hydrogen bake was developed to eliminate the problem. A high temperature bake, carried out before wafer loading, removed residual Ge deposited on the reactor wall from the previous run. After wafer loading, removed residual Ge deposited on the reactor wall from the previous run. After wafer loading and purge, a lower temperature bake then removed the native oxide. Second, a selectively grown Si buffer layer improved the initial growth surface for later SiGe SEG. The Si buffer layer covered the remaining native oxide, which the previous hydrogen bake did not totally remove, and reduced the defects at the interface. Finally, a small flow of Si source gas during the temperature ramp down period helped to keep the buffer layer surface clean prior to SiGe SEG. All three steps proved to be essential in obtaining SiGe SEG of higher quality.

Growth of single-crystal Si, Ge and SiGe layers using plasma-assisted CVD

The results of a preliminary study on the growth of epitaxial Si, hetero-epitaxial Ge and Si/Si x Ge 1− x multilayer structures on (100) Si substrates using r.f. plasma-assisted chemical vapour deposition are described. Silane and germane were used as the precursor gases. The lowest temperature for homo-epitaxial Si growth was 600 °C and epitaxy was critically dependent on an in-situ hydrogen plasma clean of the substrate. Ge and SiGe growth was carried out following the growth of an initial Si epitaxial layer, also at the lowest temperature of 600 °C. Successful Ge epitaxy required a continuously graded Si x Ge 1− x “buffer layer” (with x decreasing from 1 to 0), on top of the initial Si epilayer. Si/Si x Ge 1− x superlattice structures having in excess of 100 alternating layers of Si and Si x Ge 1− x with x≈0.6 and good layer uniformity, periodicity and abruptness have been demonstrated.

Defect-free strain relaxation in locally MBE-grown SiGe heterostructures

Defect-free SiGe layers and virtual substrates are of great interest for SiGe heterostructures and device processing. To overcome the problems arising from strain-induced defects, local growth of Si and SiGe on a micrometer scale with ultra high vacuum compatible shadow masks can be used. The geometrical shape of locally grown SiGe mesa structures was investigated by scanning electron microscopy. This reveals a facet formation like in silicon, which can be explained by a self-organized growth mechanism. The defect density can be observed by transmission electron microscopy analysis. A decreasing defect density for decreasing mesa width is found. Growth exceeding the critical layer thickness for uniformly deposited layers is possible for locally grown mesa structures. This effect can be explained by suppressed nucleation and suppressed multiplication of defects. MicroRaman spectroscopy measurements of these local structures can be used to investigate the strain relaxation. For locally grown SiGe on a (001) Si substrate the formation of dislocations is suppressed resulting in increasing strain. In contrast to this locally grown SiGe on local Si buffer layers is defect free, but reveal strain relaxation for micron dimensions. This effect of defect-free elastic relaxation offers a new method to fabricate virtual substrates, with buffer thicknesses on a nanometer scale. © 1997 Elsevier Science S.A. All rights reserved.

Single-wafer Si and SiGe processes for advanced ULSI technologies

Low thermal budget as well as new industrially processable materials will be key issues in future advanced CMOS (≤0.18 μm) and bipolar technologies. Low-temperature deposition of Si and Si 1− x Ge x (referred to as SiGe hereafter) has been performed using an industrial, 200 mm, single-wafer CVD module operating at reduced pressure. Epitaxial Si and heteroepitaxial SiGe growth has been studied for buried channel applications in CMOS devices or as a base for heterojunction bipolar transistors (HBTs), as well as polycrystalline SiGe with a high Ge content as a potential alternative material for poly-gate stacks. The dependence of the deposition rate on the filling ratio and window size, and its evolution with the addition of HCl to the gas mixture is investigated for selectively grown SiGe layers. The pattern dependence of the Ge content is also presented. Preliminary results on poly-SiGe films with Ge contents up to 100% are shown and discussed.

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.

UHV-CVD heteroepitaxial growth of Si 1− xGe x alloys on Si(100) using silane and germane

In this paper, we investigate the growth rate and strain relaxation of Si 1− x Ge x layers grown on Si substrates by UHV-CVD. The Si 1− x Ge x growth rate is found to exhibit two different behaviors as a function of growth temperature. Above the temperature corresponding to hydrogen desorption ( T H), the SiGe growth rate first decreases with x, and finally becomes almost independent of x at higher values of x. Below T H, the SiGe growth rate first increases with x, then shows a maximum and finally becomes almost independent of x at higher values of x. Using in-situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy and photoluminescence, we have clearly identified two distinct mechanisms for strain relaxation as a function of Ge content: at x=0.15 strain is relaxed by nucleation of misfit dislocations while the film surface remains smooth throughout growth and relaxation process. The introduction of dislocations is found to occur before lattice relaxation. At x=0.22, strain is first relaxed via formation of coherent islands before reaching the equilibrium critical thickness for dislocation nucleation. We also show that in-situ RHEED is a powerful technique to probe the evolution of film morphology in an ultra high vacuum chemical vapour deposition system. © 1997 Elsevier Science S.A. All rights reserved.

Selective epitaxial growth of Si1−xGex/Si strained-layers in a tubular hot-wall low pressure chemical vapor deposition system

Selective epitaxial growth (SEG) of SiGe on patterned-oxide silicon substrates, using a tubular hot-wall low pressure chemical vapor deposition (LPCVD) system, has been demonstrated. This conventional LPCVD system was proposed as a low cost alternative for SiGe epitaxial growth. Dichlorosilane (SiH2Cl2) and germane (GeH4) were used as the reactant gases with hydrogen as a carrier gas, with no addition of HCl needed to achieve selectivity in quality epitaxial growth of SiGe. Nomarski microscopy showed good selectivity with no nucleation occurring on the SiO2 areas. A low defect silicon buffer layer grown under SEG conditions was key in obtaining high-quality growth. Cross-sectional transmission electron microscopy showed that the SiGe strained layers grown at 700 °C, 750 °C, and 800 °C were of high quality.

CALCULATION OF CRITICAL LAYER THICKNESS BY TAKING INTO ACCOUNT THE THERMAL STRAIN IN Si1-xGex /Si STRAIN LAYER HETEROSTRUCTURES

Effect of thermal strain on growth of SiGe strained layer on Si was analysed.Formulae for critical layer thickness were obtained by considering thermal strain in energy balance under the assumption of screw dislocation energy density equal to the sum of areal strain energy density and thermal strain energy density.The relationship between the thermal expansion coefficient associated with thermal strain and Ge content x was linear.The results are in good agreement with the experimental results reported.

Selective Si/SiGe Heterostructures for Advanced CMOS and BiCMOS Technologies

ABSTRACTNew device architectures for advanced Integrated Circuits (IC) have been studied within the frame of our GRESSI Program through the technological development of the well known but less implemented low temperature selective epitaxial Si and SiGe films. High performance IC manufacturing requirements, such as large diameter wafer uniformity, reproducibility, throughput and reliability can be met by commercial integrated processing, single wafer cluster tools.Using an industrially available CVD single wafer reactor, special care has been taken to integrate these films into advanced CMOS and BiCMOS processes with a minimum of drawbacks. Indeed, the Si and SiGe growth rate dependence on filling ratio as well as full selectivity are major issues. The loading effect as a function of temperature, gas mixture, Ge and dopant incorporation has been studied and optimized for these applications.Finally, device results are compared between conventional and developed architectures showing real improvements with minimum process modifications.

Epitaxial growth of SiGe on Al 2O 3 using Si 2H 6 gas and Ge solid source molecular beam epitaxy

SiGe films were epitaxially grown on sapphire (0001), (011̄2) and γ- Al 2 O 3(111) Si(111) using Si 2H 6 gas and Ge solid source molecular beam epitaxy (Si 2H 6Ge MBE) for the first time. The orientation relationship between SiGe and Al 2O 3 was investigated, by reflection high-energy electron diffraction, as a function of the growth temperature. Epitaxial growth of SiGe films on Al 2O 3 was possible at low temperatures in comparison with the epitaxial growth of Si. From X-ray photoelectron spectroscopy, it is considered that the low-temperature growth of SiGe was achieved by the suppression of surface reaction by-products during the initial growth, because the continuous layer of SiGe was grown earlier due to short incubation times and a high growth rate. High nucleation densities and the high Ge contents of SiGe films at the heterointerface were observed and is considered to be due to the difference of the initial growth between Si 2H 6 gas and Ge atoms.

Adsorption mechanisms of Si 2H 6 and GeH 4 on Si(100)2 × 1 surfaces

We studied the adsorption mechanisms of Si 2H 6 and GeH 4 on Si(100)2 × 1 surfaces by investigating the gas-exposure dependence of surface-hydrogen coverage at room temperature. In Si 2H 6 adsorption there are two distinct adsorption mechanisms depending on surface-hydrogen coverage. At low coverage below 1 monolayer, dissociative adsorption of Si 2H 6 to dangling bonds (Si 2H 6 + 2db → 2SiH 3(ad)) is rate-limiting, while adsorption to a dimer bond (Si 2H 6(ad) + SiSi → SiH 3(ad)) dominates at high coverage (ad and db denote an adsorbate and a dangling bond, respectively). Adsorption of GeH 4 is characterized as four-site adsorption, which is independent of surface-hydrogen coverage. Based on these results, we discuss a possible adsorption model of SiGe growth using Si 2H 6 and GeH 4.