SiGe Growth Research Articles

Compliant effect of low-temperature Si buffer for SiGe growth

Relaxed SiGe attracted much interest due to the applications for strained Si/SiGe high electron mobility transistor, metal-oxide-semiconductor field-effect transistor, heterojunction bipolar transistor, and other devices. High-quality relaxed SiGe templates, especially those with a low threading dislocation density and smooth surface, are critical for device performance. In this work, SiGe films on low-temperature Si buffer layers were grown by solid-source molecular-beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, and photoluminescence spectroscopy. It was demonstrated that, with the proper growth temperature and Si buffer thickness, the low-temperature Si buffer became tensily strained and reduced the lattice mismatch between the SiGe and the Si buffer layer. This performance is similar to that of the compliant substrate: a thin substrate that shares the mismatch strain in heteroepitaxy. Due to the smaller mismatch, misfit dislocation and threading dislocation densities were lower.

Effect of Ge on the P doping in Si gas-source molecular beam epitaxy using Si2H6 and PH3

The effect of Ge on the P doping has been investigated by the in-situ observation of surface P in the P-doped Si growth using Si2H6 and PH3. If the P-doped Si growth is performed without Ge, PH3 adsorption reaches saturation at the higher P-doping concentrations exceeding 3×1018/cm3. With Ge on the growing surface, Ge diminishes the self-limiting behavior of the PH3 adsorption on the Si surface and enhances the PH3 adsorption. The enhancement of the P doping in the SiGe growth is caused by an increase of surface P coverage due to the enhancement of PH3 adsorption.

Low-dislocation relaxed SiGe grown on an effective compliant substrate

An effective compliant substrate was successfully fabricated for growth of high quality relaxed SiGe templates. The compliant substrate was fabricated by synthesizing a 20% B2O3 concentration borosilicate glass in the silicon on insulator wafers through boron and oxygen implantation followed by high temperature annealing. Substrates with 5%, 10% and 20% B2O3 were used for 150 nm Si0.75Ge0.25 epitaxy. Double-axis x-ray diffraction measurements determined the relaxation and composition of the Si1−xGex layers. Cross-sectional transmission electron microscopy was used to observe the lattice of the SiGe epilayer and the Si substrate, dislocation density and distribution. Raman spectros-copy was combined with step etching to measure the samples. For 20% BSG sample, the strain in the thin Si layer was calculated from the Raman shift and it matched the results from DAXRD very well. The density of threading dislocation on the surface of 500 nm Si0.75Ge0.25 layers was 2×104 cm−2 for the sample on the 20% borosilicate glass substrate. This method is promising to prepare effective compliant substrate for low-dislocation relaxed SiGe growth.

Effect of low-temperature SiGe interlayer on the growth of relaxed SiGe

A low-temperature Si 0.8Ge 0.2 (LT-Si 0.8Ge 0.2) interlayer was grown at 500°C to improve the relaxed Si 0.8Ge 0.2 surface and reduce the dislocation density in it, which was confirmed by the change of reflective high-energy electron diffraction (RHEED) pattern from spotty to streaky and etch pits counts. For the same extent of strain, the threading dislocation density was reduced from 8×10 7 cm −2 in the latter to 2×10 6 cm −2 in the former.

The influence of thermoelectromagnetic convection (TEMC) on the Bridgman growth of semiconductors

Application of a low-intensity axial magnetic field can promote significant convection during Bridgman growth of GeSi when resident thermoelectric currents at the growth interface are large due to the difference of thermoelectric powers of the melt and of the crystal and the tangential temperature gradient at the interface. Thermoelectromagnetic convection (TEMC) in the GeSi melt is characterized by a meridional flow driven by the rotation of the fluid due to the azimuthal Lorentz force from currents in the radial direction concentrated near the interface and an axial magnetic field. In this work, we developed a computational model to study convection of the GeSi melt in varying g and magnetic field intensity levels.

Gas-source molecular beam epitaxy of SiGe virtual substrates: I. Growth kinetics and doping

We have studied the growth by gas-source molecular beam epitaxy (GS-MBE) of SiGe virtual substrates. We have first determined the relationship existing between the Ge concentration in SiGe thick films and the gas phase ratio of disilane and germane, and its behaviour versus growth temperature. We find that Si atoms are 4.6 times more likely to be incorporated than Ge atoms at 550 °C. This incorporation probability decreases as the growth temperature increases, following a thermally activated law with a 0.082-0.126 eV characteristic energy. The dependence of SiGe growth rate on substrate temperatures has a cross-over point at approximately 8% of Ge, above which the growth rate decreases significantly as the temperature increases. Otherwise, we show what p-type or n-type doping levels are typically achievable in SiGe virtual substrates, and the influence diluted diborane and arsine have on the growth kinetics of SiGe. Additionally, we demonstrate that the `pre-build-up/flash-off' technique originally proposed by Iyer et al for solid-source MBE (1981 J. Appl. Phys. 52 5608) yields abrupt arsenic doping profiles in GS-MBE.

Optical second harmonic generation studies of epitaxial growth of Si and SiGe

Optical second harmonic generation (SHG) from Si surfaces has the advantage of surface sensitivity and can be applied in-situ during epitaxial growth process such as gas source molecular beam epitaxy (GSMBE) or chemical vapour depositions (CVD). This enables a number of processes occurring on the surface during epitaxial growth to be investigated in real time. Two processes are of particular interest in the epitxial growth of Si and SiGe structures: (1) oscillatory processes on the surface due to layer-by-layer growth for growth rate measurements and (2) surface segregation of Ge during formation of Si/SiGe heterojunctions. In this paper, we report on the oscillatory SHG response from the Si(0 0 1) surface during homo-epitaxy of Si and variations of SHG response across the SiGe/Si heterojunction during GSMBE growth. The oscillatory response is attributed to the domain coverage variation during layer-by-layer growth analogous to the origin of reflectance anisotropy oscillations. The changes in SHG during formation of SiGe/Si heterojunction is attributed to the gradual variation of surface Ge concentration due to segregation of Ge. The contribution from buried interfaces is shown to be negligible at the photon energy used.

Doping during low-temperature growth of materials for n–p–n Si/SiGe/Si heterojuction bipolar transistor by gas source molecular beam epitaxy

In situ doping for growth of n–p–n Si/SiGe/Si heterojuction bipolar transistor (HBT) structural materials in Si gas source molecular beam epitaxy is investigated. We studied high n-type doping kinetics in Si growth using disilane and phosphine, and p-type doping in SiGe growth using disilane, soild-Ge, and diborane with an emphasis on the effect of Ge on B incorporation. Based on these results, in situ growth of n–p–n Si/SiGe/Si HBT device structure is demonstrated with designed structural and carrier profiles, as verified from characterizations by X-ray diffraction, and spreading resistance profiling analysis.

Selective versus non-selective growth of Si and SiGe

Selective epitaxial growth of Si and SiGe at low temperatures and reduced pressure in a single-wafer CVD reactor has been characterized with respect to its sensitivity to pattern changes. Small variations in the surface ratio of nitride (or oxide) and exposed Si cause relatively large deviations in the growth rates. The Si and SiGe growth rates are affected in opposite ways. Even worse are pattern discontinuities across the area to be deposited, unavoidable at the edge of the wafer. These discontinuities are detrimental to the deposition uniformity and are active over a long range. A sacrificial poly layer has been successfully applied to overcome these problems. An alternative solution is the deposition of a continuous Si layer which grows epitaxially on the Si in the windows and polycrystalline on the nitride or oxide fields. This offers the same advantages in suppressing the loading effects, albeit that the specific features of selective growth are lost. A limited comparison of the two growing methods in terms of process robustness was made. There is no clear winner, each method has its own share of problems.

A numerical investigation of the effect of thermoelectromagnetic convection (TEMC) on the Bridgman growth of Ge 1− xSi x

Thermoelectric currents at the growth interface of GeSi during Bridgman growth are shown to promote convection when a low-intensity axial magnetic field is applied. TEMC, typically, is characterized by a meridional flow driven by the rotation of the fluid; meridional convection alters the composition of the melt, and shape of the growth interface substantially. TEMC effect is more important in micro-gravity environment than the terrestrial one, and can be used to control convection during directional solidification of GeSi. In this work, we report on the numerical simulation of the effect of TEMC on the growth of GeSi.

Enabling technologies for forming and contacting shallow junctions in Si: HF-vapor cleaning and selective epitaxial growth of Si and SiGe

Future generation devices with critical dimensions of less than 130 nm will have source/drain areas with junction depths of less than about 70 nm and a sheet resistance of around 3 Ω/sq. Conventional technologies used to form and contact such shallow and low resistance source/drain areas are concluded to no longer be feasible in manufacturing. Elevated source/drain technology is shown to be very attractive for manufacturing sub-130 nm devices. In this article we describe two critical processes to form such elevated source/drains. First, a novel HF-vapor clean chemistry for native oxide removal is described. The etch chemistry uses acetic acid vapor as a catalyst to initiate and control etching with HF vapor. Excellent repeatability and selectivity are achieved. Second, in situ doped selective epitaxial growth (SEG) of Si and SiGe is addressed. The advantages of adding Ge to the epitaxial film are discussed. Issues like microloading and facet formation are also discussed and are demonstrated as solvable. Vacuum integration of the above two mentioned processes eliminates the need for a high temperature H2 bake. The elimination of the H2 bake and the addition of Ge to enable SEG at lower temperatures are demonstrated to substantially decrease the thermal budget, increase throughput, and eliminate queue time in the factory. These improvements make elevated source/drain technology technically and economically feasible for the manufacturing of 130–70 nm devices.

Mechanisms determining three-dimensional SiGe lsland density on Si(001)

Thin, coherently strained, films of SiGe were deposited on Si(001) in the Stranski-Krastanow (SK) growth mode to form small, faceted, dislocation-free-three-dimensional (3D) islands. The number density of these islands was determined as functions of SiGe alloy composition, growth rate, and substrate temperature during growth. From these experiments, the classical model of 3D island nucleation and growth yields an approximate activation energy for diffusion of Ge dimers on a Ge covered Si(001) surface of 0.70 eV. The dependence of the 3D-island number density on growth rate cannot be understood without modifying the classical model to account for the wetting layer present in SK systems. Heteroepitaxial strain is not included in the classical model of island nucleation and growth. A simple linear elastic model that fits the data is developed that predicts the island number density is proportional to the inverse square of the Ge mole fraction in the alloy plus a constant.

X-ray photoelectron spectra of low temperature plasma anodized Si0.84Ge0.16 alloy on Si(100): Implications for SiGe oxidation kinetics and oxide electrical properties

The material properties of low-temperature plasma-grown oxide on gas-source molecular beam epitaxial Si0.84Ge0.16 have been investigated. X-ray photoelectron spectra studies show that plasma anodization leads to no segregation of Ge species during thin oxide growth with the onset of partial segregation occurring for thicker oxides. Depth profiling shows that the plasma oxide is stoichiometric in form with the exception of a small percentage of Ge atoms left in their unoxidized state. The density of these Ge atoms agrees with that measured in previous electron trapping studies. In addition, oxide growth rate enhancement of SiGe is observed. These phenomena are explained using a qualitative model for the mechanism of oxide growth of SiGe which is consistent with published results for SiGe: oxides grown with other systems.

Step Bunching during SiGe Growth on Vicinal Si(111) Surfaces

AbstractWe investigate step bunching during SiGe growth on vicinal Si(111) surfaces. Step bunching occurs irrespective of the misorientation angle and direction of the vicinal surface, the growth temperature, and the Ge concentration. At 550°C, the average number of the steps in the bunch increases with the Ge concentration. After growth of 10-nm-thick SiGe layers, twodimensional islands are formed on the terraces, which indicates that the terrace width has already been saturated. Therefore, the terrace width is mainly determined by the diffusion length of the adatom. The average number of steps in the bunch increases with the Ge concentration because the diffusion length increases with the Ge concentration. The diffusion length also increases with the temperature. So the higher the temperature is, the larger the step bunch becomes.

Selective and non-selective growth of self-aligned SiGe HBT structures by LPCVD epitaxy

Silicon-germanium (SiGe) heterojunction bipolar transistors (HBT) have become increasingly important for high speed applications. Novel device structures are often required to fully exploit the advantages from incorporation of a heterojunction. In this work, a growth technique is described which uses both selective and non-selective growth of Si and SiGe to produce an advanced SiGe HBT structure. The surface morphology of the material grown is examined using Nomarski contrast optical microscopy and scanning electron microscopy (SEM), and the surface of the epitaxial areas appears smooth with a low defect density. The growth surface is reasonably planar, as needed for further device processing, which suggests the required thicknesses of both selective and non-selective epitaxy were achieved. The Ge and B profiles of the material are measured using secondary ion mass spectroscopy (SIMS), and the layer thicknesses are found to meet the device specification. The crystallinity and defects in the material are examined by transmission electron microscopy (TEM). The material produced is shown to be suitable for fabrication into the proposed device.

Highly regular self-organization of step bunches during growth of SiGe on Si(113)

We have studied the structural properties of highly periodic arrays of terrace steps in a Si/SiGe multilayer grown on a miscut Si(113) substrate by atomic force microscopy, x-ray reflection and high resolution x-ray diffraction. The data reveal a regular array of step bunches with vertical correlation within the multilayer and periodic surface steps extending over lengths of several tens of microns. The (113)-faceted terraces have a lateral period of about 360 nm which is locally modulated due to a long-range waviness of the surface.

Dependence of SiGe growth instability on Si substrate orientation

We present an experimental study of the influence of the Si substrate orientation on the growth modes and relaxation mechanisms of Si 1− x Ge x for x ranging between 0 and 1 (misfit m up to 4%). At low misfits ( m<1%), strain is relaxed on both Si(001) and (111) orientations by the well-known tetragonal distortion. The latter is about twice as large on (001) as on (111), in good agreement with linear elastic theory. In this low stress regime, Si 1− x Ge x /Si(001) heterostructures display surface morphology fluctuations due to the kinetic roughening phenomenon. With increasing thickness, this surface roughness orders into small isotropic undulations of large wavelength. At larger misfits (1%< m<2.7%), in the regime of medium stress, the surface morphology dramatically changes with the substrate orientation. While a growth instability leads to undulation of Si 0.7Ge 0.3(001), the Si 0.7Ge 0.3(111) remains completely flat and strained. This indicates a total inhibition of the growth instability on Si(111), whatever the elastic strain energy. We suggest, in agreement with recent theoretical models, that the development of the growth instability is related also to atomic step interactions. This is consistent with our CBED, GIXRD and HREM analyses which demonstrate a negligible elastic relaxation of the Si 0.7Ge 0.3(001) in-plane parameter. Consequently, the elastic relaxation of the undulated layer currently invoked in the literature cannot be the single underlying mechanism for the development of the growth instability in Si 1− x Ge x epilayers.

Modeling growth in SiGe gas-source molecular beam epitaxy using Si2H6 and GeH4

We have modeled SiGe growth with gas-source molecular beam epitaxy using Si2H6 and GeH4. The model is described by a combination of Si2H6 and GeH4 adsorptions and hydrogen desorption. The Si2H6 and GeH4 adsorptions are two-site and four-site adsorption processes respectively. The hydrogen desorption is a first order reaction. The model allows predictions of substrate-temperature dependencies of SiGe growth rate and surface-hydrogen coverage in the wide temperature range of 450–700°C.

Surface smoothing induced by epitaxial Si capping of rough and strained Ge or Si 1− xGe x morphologies: a RHEED and TEM study

The same Si/Ge/Si or Si/Si 1− x Ge x /Si structures grown at 400°C on Si(0 0 1) are compared, either in real time by reflection high-energy electron diffraction (RHEED) or on the final product by transmission electron microscopy (TEM). This allows us to follow interface morphology variations during Si re-growth upon Ge containing layers of various Ge thicknesses or alloy x fractions. As shown by the passage from spotty to streaky RHEED patterns and by specular beam intensity oscillation evolutions, the surfaces roughen systematically during strained Ge or Si 1− x Ge x (SiGe) growth and smooth rapidly during subsequent growth of 4 to 6 Si monolayers, at least in the elastic hut-clustering islanding range with {1 0 5} facets. With the help of TEM examinations, a coherent picture may be proposed for these surface smoothing observations: (i) A dominant mechanism in form of a quick Si surface diffusion occurring initially on the Ge-strained surfaces. It ensures a heteregeneous Si accumulation towards the places of minimized misfit, i.e., in the troughs of the Ge or SiGe morphologies. (ii) A slower Ge diffusion (as occuring on Si) depleting the emerging island crests and contributing to an overall Ge surface termination (Ge surface segregation) and to a complementary island smoothing. The latter mechanism, only important at low growth kinetics, favours the formation of alloyed interfaces as a by-product of the island smoothing and lateral intermixing. At high Si growth kinetics the former mechanism prevails leading to better preserved island morphologies and final interfaces appearing chemically more abrupt but less flat.

Role of strain in dopant surface segregation during Si and SiGe growth by molecular beam epitaxy

Dopant surface segregation during molecular beam epitaxy (MBE) growth is a serious problem for controlling the doping profiles. To understand the segregation mechanism is essential. In this study, we report the B segregation ratio values, determined using concentration transient analysis based on secondary ion mass spectrometry (SIMS) measurements, for Si and SiGe, respectively. For a comparison, segregation ratio calculations based on a simplified two-site exchange model were performed. It is found that the surface segregation effects of B and Ge during MBE Si growth are interconnected, where the lattice strain likely plays an important role.