SiGe Layer Thickness Research Articles

Interdiffusion at Si/SiGe interface analyzed by high-resolution X-ray diffraction

The interdiffusion at Si/SiGe interface with Ge mole fraction of 19.2%, and SiGe layer thickness of 283 nm was studied by high-resolution X-ray diffraction. The results showed that the obvious fringes of rocking curve obtained from pre-annealing sample faded out gradually and disappeared completely with increasing annealing temperature and prolonging annealing time, indicating that the interface was broadened gradually due to the interdiffusion. The angular separation between the SiGe and Si peaks gradually decreased, suggesting that a high temperature promoted the interdiffusion related to the strain relaxation and the change of Ge composition. The interdiffusivity was calculated from the decay of the integrated intensity of the SiGe peaks as a function of annealing time at different temperatures. After annealing the scattering distributions of Si substrate and SiGe film in both ω and 2 θ/ ω scans in (113) reciprocal space maps spread greatly from a very narrow in pre-annealing sample, indicating the formation of mosaic structure and the destruction of fully commensuration with the substrate.

Si/SiGe growth by low-energy plasma-enhanced chemical vapor deposition

Nowadays, microelectronic industry targets (in term of down-scaling and throughput) require some severe reduction of the SiGe epitaxial growth temperature or/and increase of the growth rate. A possible alternative to meet these requirements is low-energy plasma-enhanced chemical vapor deposition (LEPECVD). We have studied the deposition kinetics of silicon, silicon–germanium and germanium using LEPECVD. This new deposition technique offers promising advantages compared to thermally activated CVD such as low deposition temperature and high growth rate. Different regimes are observed depending on the growth temperature. High temperatures can be associated to a mix between thermally and plasma-activated deposition, whereas only plasma-assisted deposition occurs at low temperatures. Crystalline quality of the layers was checked through the mean of photoluminescence, which revealed no defects. A high growth rate (100 nm min −1) that can be achieved very easily with LEPECVD allows to grow quickly very thick layers. We have used this technique to grow step-graded thick SiGe layers which are almost fully relaxed. Those virtual substrates exhibited the well-known cross-hatch pattern, with RMS roughness from 2 to 10 nm for pure Ge layers.

Wafer bonding involving strain-relaxed SiGe

Wafer bonding of strained silicon (sSi) layer to oxidized Si (0 0 1) handle wafers is described in this paper. The sSi resides on two types of relaxed SiGe layers, a thick one (2 μm in thickness) grown on top of a compositionally graded buffer and a thin one (∼180 nm) for which He implantation and annealing are used to induce plastic strain relaxation by misfit dislocations. The thick SiGe layer shows a pronounced cross-hatch pattern resulting in a high surface micro-roughness (>1 nm). For direct bonding this roughness is reduced to <1 nm by the deposition of a chemical vapor deposition (CVD) silicon oxide followed by chemo-mechanical polishing (CMP). An outgassing process at 600 °C for 1 h in argon atmosphere was needed for the deposited CVD oxide prior to wafer bonding. Spontaneous and void-free bonded interfaces were obtained when both types of substrates were brought in contact with the oxidized handle wafers at room temperature. The bonding energies obtained after annealing at moderate temperatures (e.g. 500 °C) are sufficiently high to withstand further processing involved in layer transfer.

Growth of SiGe/Si superlattices on silicon-on-insulator substrates for multi-bridge channel field effect transistors

We have studied in reduced pressure chemical vapor deposition the growth kinetics of Si and Si0.8Ge0.2 on bulk Si(001) and on silicon-on-insulator (SOI) (100nm buried oxide/20nm Si over-layer) substrates. For this, we have grown miscellaneous Si/Si0.8Ge0.2 superlattices on both types of substrates that we have studied mainly in secondary ions mass spectrometry but also in X-ray diffraction. Systematic Si and SiGe growth rate decreases (together with a Ge concentration increase) occurred on SOI substrates as the stack thickness increased from zero to more than 100nm. Such phenomena are most likely associated to a decrease of the SOI surface temperature by 12–13°C compared to bulk. For Si, the growth rate on SOI increased back again towards the bulk value as the stack thickness neared two hundred nanometers. This is linked to a SOI surface temperature that went 5°C back up. Such a knowledge will be most useful to form in the near future regular superlattices on SOI substrates that will serve as the active regions of multi-bridge channel field effect transistors. Three periods Si/SiGe superlattices with either 20% or 31% of Ge and varying SiGe layer thickness were also grown on bulk Si(001) to study the critical thickness for plastic relaxation of the compressive strain that builds up in such stacks.

Surface roughness and dislocation distribution in compositionally graded relaxed SiGe buffer layer with inserted-strained Si layers

We investigate the surface roughness and dislocation distribution of compositionally graded relaxed SiGe buffer layers by inserting two tensile-strained Si layers. The 20nm thick strained Si layers, less than the critical thickness for dislocation formation, are inserted at 10 and 20% Ge content regions of the 1μm thick graded SiGe layer with a final Ge content of 30%. The surface immediately after growing the second strained Si layer on SiGe with 20% Ge content is found to be flat with about 1.1nm root-mean-square roughness. However, the crosshatched surface with the 7.8nm roughness develops during subsequent SiGe growths, which is slightly less than the 10.3nm value for SiGe without inserted Si layers. Another important issue of consideration is that inserting the strained Si layers leads to increased interaction among dislocations as shown by cross-sectional transmission electron microscopy. This study explores the possibility of using strained layers for achieving flat surfaces and illustrates the need for optimization when using this approach.

Characterization of Si1−xGex∕Si layers and depth profile of their heterobipolar transistor structures by high-resolution x-ray diffractometry and computer simulations

Using high-resolution x-ray diffractometry (HRXRD) and computer simulations, germanium content x, thickness T of pseudomorphic Si1−xGex layers, and Ge depth profile in their advanced heterobipolar transistor (HBT) structures grown on (001)Si substrates have been determined. High-resolution rocking curves (RCs) with well-resolved intensity oscillations have been recorded by using a double-crystal x-ray diffractometer in (+,−) geometry with CuKα1 radiation from a 12-kW rotating anode x-ray generator. The experimental RCs were simulated with the in-house developed computer program based on the semikinematical theory. In addition to the Ge depth profile, the thickness of SiGe layer, Si cap layer, and SiGe plateau layers of the HBT structures have been determined for the best fit of the experimental RCs with the theoretically simulated RCs. Due to the decay of intensity oscillations, in the case of partially relaxed layers, the values of germanium content x and relaxation R were calculated from the perpendicular and parallel (in-plane) lattice constants a⊥ and a‖ obtained by recording a pair of RCs from symmetric and asymmetric lattice planes. The simulation method using HRXRD has been described with the help of typical results obtained from samples of different structures. In this method the maximum error values of x and T are, respectively, ±0.5% and 1%. The error in the value of R is ±2% for partially relaxed samples. In the case of HBT structures, the maximum error in the value of thickness of SiGe and cap layers is ±1% and that of plateau layer is ±5%. The method is very convenient, faster, accurate, and nondestructive in comparison with other methods such as transmission electron microscopy and Secondary-ion-mass spectroscopy.

Strain relaxation and threading dislocation density in helium-implanted and annealed Si1−xGex/Si(100) heterostructures

Strain relaxation and threading dislocation densities in Si1−xGex (0.15&amp;lt;x&amp;lt;0.30) produced by He implantation and annealing have been investigated using x-ray diffraction and transmission electron microscopy. The degree of strain relaxation is very sensitive to the SiGe layer thickness; only small differences in strain relaxation are obtained when the helium dose and energy are varied over a relatively wide range. In contrast, the threading dislocation density is strongly influenced by the implantation dose and depth. A composite parameter, the He dose in the SiGe layer (He(SiGe)), calculated from He profiles simulated using the program Stopping and Range of Ions in Matter (SRIM2000), correlates well with the threading dislocation density. We have found that to achieve a low threading dislocation density, &amp;lt;5×107 cm−2, He(SiGe) must be less than 1×1015 cm−2.

Strained state of the layer system depending on the SiGe layer thickness by micro-Raman mapping

Micro-Raman scattering experiments, including Raman spectroscopy and microscopy, have been carried out to study the strain state of the Si 0.75Ge 0.25 films using the low-temperature Si (LT-Si) buffer layers. SiGe layers with different thickness of 1000, 3000 and 5000 Å on LT-Si were grown on Si (0 0 1) substrates by gas-source molecular beam epitaxy (MBE). The Raman-generated images have provided a direct picture of the strain state of the SiGe layer and Si substrate with SiGe thickness. The experimental results give us evidence that the change of strain state in the SiGe film and the Si substrate with the SiGe thickness is ascribed to the LT-Si buffer layer, which can effectively adjust the strain mismatch. This function is similar to compliant effect as a thin freestanding substrate.

Influence of He implantation conditions on strain relaxation and threading dislocation density in Si0.8Ge0.2 virtual substrates

ABSTRACTThe strain relaxation and threading dislocation density of He-implanted and annealed SiGe/Si heterostructures have been studied. For He doses above a threshold of 8×1015 cm−2, the degree of strain relaxation depends primarily on the SiGe layer thickness; a similar degree of strain relaxation is obtained when the He dose and energy are varied over a relatively wide range. In contrast, the threading dislocation density is strongly influenced by the implantation depth. There is a strong correlation between the parameter He(SiGe), the He dose in the SiGe layer calculated from He profiles simulated using the program Stopping and Range of Ions in Matter (SRIM), and the threading dislocation density. We find that to achieve a low threading dislocation density, &lt;5×107 cm−2, He(SiGe) must be less than 1015 cm−2. The strain relaxation mechanism is also discussed.

Fabrication of thick, high-quality strained SiGe layer on ultra-thin silicon-on-insulator and modeling of film strain

Fabrication of a thick strained SiGe layer on bulk silicon is hampered by the lattice mismatch and difference in the thermal expansion coefficients between Si and SiGe, and a high Ge content leads to severe strain in the SiGe film. When the thickness of the SiGe film is above a critical value (90 nm for 18% Ge), drastic deterioration of the film properties as well as dislocations will result. In comparison, a silicon-on-insulator (SOI) substrate with a thin top Si layer can mitigate the problems and so a thick SiGe layer with high Ge concentration can conceivably be synthesized. In the work reported here, a 110 nm thick high-quality strained Si 0.82Ge 0.18 layer was fabricated on an ultra-thin SOI substrate with a 30 nm top silicon layer using ultra-high vacuum chemical vapor deposition (UHVCVD). The thickness of the SiGe layer is larger than the critical thickness on bulk Si. Cross-sectional transmission electron microscopy (XTEM) reveals that the SiGe layer is dislocation-free and the atoms at the SiGe/Si interface are well aligned, even though X-ray diffraction (XRD) data indicate that the SiGe film is highly strained. The strain factors determined from the XRD and Raman results agree well.

Stress Metrology : The challenge for the next generation of engineered wafers

ABSTRACTRaman spectroscopy is a powerful and versatile technique for stress measurements in complex stacks of thin crystalline layers at macroscopic and microscopic scales. Using such a technique we show that thick SiGe layers epitaxially grown using graded buffer method are fully relaxed (&gt;95%) at a macroscopic scale but exhibit a small strain modulation at a microscopic scale. For the first time we report the results of Raman micro-mapping of stress distribution in SGOI wafers produced by Smart Cut™ technology. We conclude that Smart Cut™ is a unique method to manufacture the next generation of engineered wafers that can combine strained and/or relaxed SiGe alloys, Si and Ge films, while keeping their initial strain properties at both scales. It is important to develop Raman spectroscopy tool for in-line process control in fabrication of strained Silicon On Insulator (sSOI) wafers.

Optoelectronic properties of thick SiGe layers grown as small mesas by low pressure chemical vapor deposition

Arrays of Si0.80Ge0.20/Si(001) square mesas were epitaxially grown by low pressure chemical vapor deposition to optimize the light emission in the near infrared range. To study the influence of mesa size on light emission the current–voltage characteristics, the spectral photocurrent, and the electroluminescence of p-i-n structures were measured. While the plastic relaxation has a strong influence on the electroluminescence spectra, the current–voltage characteristics are only slightly changed. At low temperatures, a tunneling current was observed and its possible location is discussed. Due to the high SiGe thickness, both the contributions of the no-phonon and transversal optical phonon-assisted transitions to the photocurrent spectra could be observed. Direct evidence of the higher band gap of relaxed SiGe was obtained from electroluminescence studies.

Stress characterization of device layers and the underlying Si1-xGex virtual substrate with high-resolution micro-Raman spectroscopy

Silicon–germanium (Si–Ge) epitaxially grown mismatched heterostructures are becoming increasingly important for high-frequency microelectronics applications. One option under serious consideration is that of using Si–Ge virtual substrates, i.e., compositionally graded layers designed to accommodate the lattice mismatch between the underlying Si substrate and the overlying active epilayers(s). This assists in the prevention of misfit dislocations that can impact adversely on the active device regions. The stress in both device silicon cap layers and the underlying Si1−x Ge x virtual substrates is characterized with high-resolution micro-Raman spectroscopy (μRS). The device layers of the samples studied composed of a 7-nm thick silicon channel, a 6-nm thick SiGe layer and were capped with a 7-nm thick silicon layer. The device layers are grown over a 1-μm thick constant composition Si0.70Ge0.30 virtual substrate capping layer, and the Si-Ge virtual substrate is grown on a p+-type (0 0 1) silicon wafer with a thickness of about 500 μm. μRS measurement results with a 488-nm Ar+ visible laser source indicate that the Si0.70Ge0.30 capping layer at the virtual substrate is fully unstrained, while the top silicon cap layer is in extremely high tension. The use of a 325-nm HeCd UV laser for the μRS measurements, which probes only a very small depth into the Si cap layer (approximately 9 nm) confirms this high tensile stress is in the top silicon cap layer. The tensile stress in the top silicon cap layer is estimated to be as large as 2.4 GPa by analyzing the shift of the Si Raman peak with respect to the standard strain-free silicon sample. The measured stress value is almost equal to the theoretically predicted tensile stress that should exist in the fully strained Si cap layer. This implies that the Si cap layer remains strained in samples with this structure.

High resolution channeling contrast microscopy and channeling analysis of SiGe quantum well structures

High resolution channeling contrast microscopy (CCM) and channeling measurements were carried out to characterize SiGe quantum well structures on micron thick graded layers (i.e. virtual substrates). The virtual substrates were grown by gas source molecular beam epitaxy at a pressure of 10 −5 mbar and low pressure chemical vapor deposition at 10 −2 mbar on boron doped Si(0 0 1) substrates respectively. A homoepitaxial silicon buffer layer was grown prior to the deposition. The nominal structure is a 20 nm Si 0.75Ge 0.25 layer at the surface, followed by 10 nm pure Si, 500 nm Si 0.75Ge 0.25 and a 1000 nm thick graded SiGe (0–26%) layer. RBS was used to measure the depth profiles, and angular scans around the (1 0 0) axis were carried out to assess crystal and interface quality. CCM was used to acquire depth resolved images of micron-sized lateral inhomogenities (`cross-hatch') present on both samples.

Influence of cooling rate on the dislocations and related luminescence in LPE SiGe layers grown on Si (100) substrates

Thick relaxed SiGe layers have been grown using the liquid phase epitaxy (LPE) technique. The Ge contents in the layers varied from 3.4 to 12.9 at.%. The thickness of the layers was approximately 25–30 μm. The layers grown at low cooling rates and with low Ge contents had low threading dislocation densities. Etch pit density of the layers increased from 2×10 4 to 8×10 5 cm −2 with increasing cooling rates from 20 K/h to 500 K/h during the LPE process. Etch pit density of the SiGe layers increased from 8×10 4 to 7×10 6 cm −2 as the Ge content in the layers increased from 3.4 to 12.9 at.%. The good quality of the layers grown at low cooling rates has been demonstrated by the strong no-phonon and TO phonon near band gap luminescence of the layers. Strong increase in the intensities of D and T bands was observed in the layers having high Ge contents and grown at high cooling rates. The increase in the intensity of the dislocation related D and T band luminescence in the layers has been related to the increase in the dislocation density of the layers.

Nanoscale patterning of Si/SiGe heterostructures byelectron-beam lithography and selective wet-chemicaletching

We present a low-damage fabrication technique forlateral sub-100 nm patterning of Si (13.5 nm)/Si0.76Ge0.24(74 nm)/Si(001) heterostructures, which isbased on successive selective wet-chemical etching. An oxidelayer of 1-2 nm thickness on the Si top layer, which wasformed well below the growth temperature of theheterostructure, was used as a sacrificial layer for transferringthe resist pattern. Electron-beam lithography was done at 15 kVwith a scanning electron microscope equipped with afield-emitter source. Transfer of the resist pattern is done byselective wet-chemical etching of the (i) oxide, (ii) Si and(iii) SiGe layers. The selectivity of the anisotropic Sietchant (25% w/w aqueous solution of tetramethyl ammoniumhydroxide (TMAH) at 70 °C) is 20:1 for SiGe andbetter than 4200:1 for SiO2. The etch rate andselectivity of the SiGe etchant (buffered hydrofluoric acid,hydrogen peroxide and acetic acid) both depend on the waitingtime between mixing and use.Due to the high anisotropy of TMAH the minimum width of groovesetched into the Si layer along the [110] direction of about30 nm is mainly determined by the dimension of the oxide mask.Further transfer into the SiGe layer results in an undercut ofthe Si edge of about 70% of the etch depth, if the depth isless than the SiGe layer thickness. The profile of the SiGegrooves reveals a weak etching anisotropy. This patterningtechnique was applied to fabricate a geometric constriction inthe epitaxial layers realized as a broken groove with the gapranging from 15 to 300 nm width. Sufficiently long etching ofthe SiGe leads to complete underetching of the Si layer inthe gap. The resulting suspended Si bridge is mechanicallystable for 20 nm width and up to 360 nm length.

Formation of strained-silicon layer on thin relaxed-SiGe/SiO2/Si structure using SIMOX technology

The fabrication of a 20 nm strained-Si on a 360 nm relaxed-SiGe layer structure using SIMOX technology was successfully demonstrated for the first time. The thin relaxed-SiGe layer on SiO2 was obtained by the direct implantation of oxygen into the thick SiGe layer, and by annealing. It was found that hydrogen termination produced by HF treatment allows successful regrowth of strained-Si layer on the thin relaxed-SiGe layer by ultra-high vacuum chemical vapor deposition (UHV-CVD). Structure analyses such as secondary ion mass spectroscopy (SIMS), Rutherford back-scattering spectroscopy (RBS) analysis, and cross-sectional transmitting electron microscopic (TEM) revealed the perfection of the layer structure of Si/SiGe/SiO2. The fully strained-Si on the relaxed-SiGe layers was also confirmed by Raman spectroscopy. It was revealed that the combination of the SIMOX process and the regrowth on the SiGe layer provides an advanced layer structure including this strained-Si for future novel devices.

Defect distribution and morphology development of SiGe layers grown on Si(100) substrates by LPE

Thick relaxed SiGe layers having Ge content up to 13 at.% have been grown using a liquid phase epitaxy (LPE) technique. The thickness of the layers is around 20–30 μm. The strain relaxation and the resulting defect evolution during the growth of SiGe layers are found to be largely controlled by the growth kinetics involved in the LPE growth process. The cooling rate of the LPE process plays an important role in the supersaturation of solute materials in the growth process. A greater number of ridges and trenches are formed on the layer on using high cooling rates. The effect of cooling rate on the morphology development of the layers is significant for the layers having low Ge content. The wavelength of undulation of the SiGe layers decreases from 75 to 20 μm as the cooling rate during the LPE process is increased from 20 to 500 K/h. The etch pit density of the layers also increases from 2×10 4 to 8×10 5 cm −2 as the cooling rate is increased. Pyramidal growth results upon increasing the Ge content in the layer above 13 at.%.

High depth resolution Rutherford backscattering analysis of Si–Si 0.78Ge 0.22/(0 0 1)Si superlattices

Si 30 nm/Si 0.78Ge 0.22 5 nm superlattices were grown on (0 0 1) Si by solid source molecular beam epitaxy (MBE) at temperatures between 550°C and 810°C. Their structural properties were studied by high depth resolution RBS analysis. The sample grown at the highest temperature has the sharpest interfaces, correlating well with the observation by Raman spectroscopy of zone-folded acoustic modes. The Si and SiGe layer thickness and the Ge concentration were determined, and agree with sub-keV secondary ion mass spectroscopy (SIMS) results. Channelling shows a minimum yield better than 3%, denoting a good crystalline quality of the layers.

Electron Microscopic Studies of Co- and Ti-Germanosilicide Films Formed on SiGe Layers

AbstractCobalt films are sputter deposited on Si0.8Ge0.2 layers grown epitaxially on Si substrates. Compared with the silicidation on pure silicon, there is a retardation of the disilicide formation in the SiGe case. The conversion of the monosilicide into the disilicide strongly depends on the thickness of the initial SiGe layers. For the silicidation of titanium on poly-SiGe layers, the increase of the Ge content favours the nucleation of C54 titanium germanosilicide, hence helping the transition from the C49 to the C54 phase on the blanket layers. The thickness of the final C54 Ti-germanosilicide layers formed on narrow poly-Sil-×Gex runners is independent of the line width and the Ge concentration.