Low Ge Content Research Articles

Growth and photoluminescence of self-assembled islands obtained during the deposition of Ge on a strained SiGe layer

The growth and photoluminescence of Ge(Si) self-assembled islands on strained Si 1-x Ge x layers (0 < x < 20%) has been investigated. Both island size and density increase when the Ge content in the predeposited Si 1-x Ge x alloy increases. The increased island density is associated with an enhanced surface roughness after SiGe deposition. The increased island size is attributed to enhanced Si intermixing and to a wetting layer thickness reduction. The latter is caused by accumulation of elastic strain energy in the SiGe layer. The increase in both island size and density leads to a strong interaction between neighboring islands. The island-island interaction results in island self-ordering, which is analyzed using a surface autocorrelation function. The lower Ge content in the islands accounts for the observed blueshift of the island related photoluminescence signal. We observe a room temperature photoluminescence signal for Ge islands grown on a predeposited SiGe alloy which is higher than for Ge islands grown on Si(001). This result is explained by an increased island density, which provides an efficient way to capture charge carriers in the islands.

Transition in growth mode by competing strain relaxation mechanisms: Surfactant mediated epitaxy of SiGe alloys on Si

Surfactant mediated epitaxy of Si(1−x)Gex alloys on Si(111) can, besides technological importance, clarify the influence of the lattice mismatch during surfactant mediated heteroepitaxial growth. For low Ge concentration we find an immediate layer-by-layer growth, whereas at high Ge concentration a roughening transition followed by relaxation of the lattice mismatch in a periodic dislocation network is preferred. This behavior can be explained by comparing the dislocation nucleation rate on a smooth surface with the island nucleation rate on a pseudomorphic film.

Introduction of Si∕SiO2 interface states by annealing Ge-implanted films

Nanocrystals embedded in SiO2 films are the subject of a number of recent works, mainly because of their potential usefulness in the fabrication of optoelectronic devices and nanocrystal memory structures. One interesting method for the fabrication of such nanocrystals is the ion implantation of segregating species into SiO2 films followed by heat treatment in order to induce nanocrystal formation. This method is both relatively simple and also compatible with the current MOS (metal-oxide-semiconductor) device technology. An unintentional effect can occur during the fabrication of nanocrystals using this method, namely a significant diffusion of the implanted species during annealing, away from the regions with the highest concentration. The Si∕SiO2 interface can be exposed to this diffusion flux. This can result in an altered interface and have a significant influence on electronic devices. Here, we report on ion implantation of Ge into SiO2 on Si followed by annealing under conditions, resulting in Ge accumulation at the Si∕SiO2 interface as determined by secondary-ion mass spectroscopy analysis, transmission electron microscopy with energy dispersive analysis of x-rays, and Rutherford backscattering spectrometry. The accumulation of Ge at the Si∕SiO2 interface has also been reported before. The resulting effect on the electronic structure of the interface is a priori unknown. We have fabricated MOS capacitors on the sample structures and their capacitance-voltage characteristics were measured and analyzed. We measure an interface state density around 1×1012cm−2, which is high compared to standard Si MOS devices. We discuss the results in terms of the previous electrical measurements on Ge-oxide interfaces and SiGe interfaces, which also can yield a high interface state density. The specific conditions we report result in a sufficiently low Ge concentration that nanocrystals are not segregated in the SiO2 film, while Ge still accumulates at the Si∕SiO2 interface after annealing.

Bleaching of optical activity induced by UV laser exposure in natural silica

We report experimental data on two types of natural silica, differing in their OH content, irradiated with UV photons (4.66eV) from a pulsed Nd:YAG laser. Irradiation induces a reduction of the absorption band at 5.12eV and of the associated emissions at 3.14eV and 4.28eV, ascribed to twofold coordinated Ge(Ge) centers pre-existing in our samples. The bleaching is mainly due to the post-irradiation conversion of Ge into the paramagnetic H(II) center via trapping of a H atom. Comparison with literature data points out the peculiarities of silica with a low Ge concentration as regards UV-induced transformations.

The Effect of Ge Incorporation on the Brønsted Acidity of ZSM‐5

The effect of the isomorphous substitution of some of the Si atoms in ZSM-5 by Ge atoms on the Brønsted acid strength has been investigated by i) DFT calculations on cluster models of the formula ((HO)3SiO)3-Al-O(H)-T-(OSi(OH)3)3, with T=Si or Ge, and ((HO)3SiO)3-Al-O(H)-Si-(OGe(OH)3)(OSi(OH)3)2, ii) a 31P NMR study of zeolite samples contacted with trimethyl phosphine oxide probe molecules and iii) a X-ray photoelectron spectroscopy (XPS) study of ZSM-5 and Ge-ZSM-5 samples. The calculations reveal that the effect of Ge incorporation on the framework acidity strongly depends on the degree of substitution and on the exact T-atom positions that are occupied by Ge. High Ge concentrations allow for enhanced stabilisation of the deprotonated Ge-ZSM-5 through structural relaxation, resulting in a slightly higher acidity as compared to ZSM-5. This structural relaxation is not achievable in Ge-ZSM-5 with a low Ge content, which therefore has a slightly lower acidity than ZSM-5. The NMR study indicates no difference between the Brønsted acidity of ZSM-5(47) and Ge(0.09)ZSM-5(36). Instead, evidence for the presence of a substantial amount of Ge-OH groups in the Ge-containing samples was obtained from the NMR results, which is consistent with earlier FTIR studies. The XPS results do not point to an effect of Ge on the framework acidity of ZSM-5(47), instead, the results can be best interpreted by assuming the presence of additional Ge-OH and Si-OH groups near the surface of the Ge(0.08)ZSM-5(47) sample.

Magnetic behavior of (Al,Ge)65CuMn quaternary alloy quasicrystals

We report the results of field and temperature dependent magnetization measurements on (Al,Ge)65CuMn quasicrystalline samples with ∼22 and 24 at. % Mn. The Ge content is also kept ∼25 at. %. The data for these samples are then compared with the data taken on samples with low Ge content (∼10 and 0 at. %). Al40Ge25Cu10Mn25, a composition very close to one of the alloys studied here, is reported [Tsai et al., Jpn. J. Appl. Phys., 27, L2252 (1988)] in literature as a ferromagnetic, icosahedral QC with large TC. The detailed magnetic studies described here prove that the magnetism observed in these quaternary alloys is highly disordered. The two important observations made in our studies are (i) observation of short-range ferromagnetism in samples with high Ge and high Mn content, up to very high fields (H=5 T) and (ii) oscillatory magnetization (or existence of multiple minima in the M-T curves) observed for the same samples for H⩾1 kOe.

Matrix effects in SIMS depth profiles of SiGe relaxed buffer layers

The combined use of Rutherford backscattering spectrometry and secondary ion mass spectroscopy allowed a complete characterization of a set of SiGe relaxed buffer layers grown by low-energy plasma-enhanced chemical vapor deposition. The Ge contents for the top SiGe constant composition layers have been obtained by RBS. Matrix effects have been studied by using monoatomic and biatomic ions as well as low and high energy O 2 + and Cs + primary beam ions. We show that matrix effects are suppressed when an O 2 + primary beam ion source is used at 3 keV, and when detecting with 30 Si + and 70 Ge + secondary ions for Ge contents <0.47. For higher Ge contents a better compromise is achieved with Cs + bombardment at 14.5 keV when detecting with 74 Ge 76 Ge − secondary ions. The procedure allows to extract the Ge concentration profiles with good accuracy even at very high depths and at very low Ge concentrations.

Effect of germanium concentration and tunnel oxide thickness on nanocrystal formation and charge storage/retention characteristics of a trilayer memory structure

The effect of germanium concentration and the rapid thermal oxide (RTO) layer thickness on the nanocrystal formation and charge storage/retention capability of a trilayer metal–insulator–semiconductor device was studied. We found that the RTO and the capping oxide layers were not totally effective in confining the Ge nanocrystals in the middle layer when a pure Ge middle layer was used for the formation of nanocrystals. From the transmission electron microscopy and secondary ion mass spectroscopy results, a significant diffusion of Ge atoms through the RTO and into the silicon substrate was observed when the RTO layer thickness was reduced to 2.5 nm. This resulted in no (or very few) nanocrystals formed in the system. For devices with a Ge+SiO2 cosputtered middle layer (i.e., lower Ge concentration), even though a higher charge storage capability was obtained from devices with a thinner RTO layer, the charge retention capability was poorer as compared to devices with a thicker RTO layer.

Analysis of x-ray diffraction as a probe of interdiffusion in Si/SiGe heterostructures

We investigate numerical simulations that utilize a nonlinear interdiffusion solver and dynamical x-ray diffraction calculations to predict the local composition evolution in low Ge concentration Si/SiGe superlattices and their diffraction patterns during annealing. Superlattice satellite peak decay rates are compared with experimentally measured values and simulated diffraction patterns are matched directly to data with good success. The simulations are used to test the sensitivity of x-ray diffraction to various uncertainties commonly encountered when measuring interdiffusion at Si/SiGe interfaces. It is found that the most serious errors result from variations in the Ge content across the surface of the wafer. For example, the resolution limit of most experimental techniques used to measure Ge concentration in a SiGe film is ±1 at. %, for a film with 11% mean Ge concentration annealed for 5 h at 870 °C, this level of error will cause the observed interdiffusivity values to deviate by −25% or +50%. The simulations are further used to show that for Si/SiGe interdiffusion, superlattice diffraction produces valid measurements when applied to 004 superlattice satellite peaks and square wave composition modulations even though it is only exactly applicable to satellite peaks about 000 reflections and to sinusoidal composition modulations. Finally, we show that proper interpretation of x-ray scattering data to extract Si/SiGe interdiffusivity values must account for the strong dependence of the interdiffusivity on Ge concentration.

Reduced pressure chemical vapour deposition of Si/Si1−x−yGexCy heterostructures using a chlorinated chemistry

We have studied in reduced-pressure chemical vapour deposition the influence of the growth temperature on the interstitial–substitutional carbon incorporation ratio in SiGeC using dichlorosilane as the Si gas precursor. X-ray diffraction and secondary ion mass spectrometry were used in order to determine the substitutional and the total (interstitial + substitutional) carbon concentrations in the layers. Si/Si1−yCy tensile-strained heterostructures were grown at 650 °C. The highest substitutional carbon concentration incorporated is around 0.75% (with a large amount of interstitial C atoms (also 0.75%), however). Low Ge concentration Si1−x−yGexCy layers have also been grown. We have managed through the increase of the SiCH6 mass flow to tailor the Si1−x−yGexCy layer strain from compressive to tensile. The highest substitutional carbon concentrations obtained are 1.82% (0.95%) for growth temperatures of 600 °C (650 °C). Finally, compressively strained high Ge concentration Si1−x−yGexCy layers with up to 3.20% (2.49% and 1.41%) in substitutional carbon atoms were grown at 550 °C (600 °C and 650 °C). The growth temperature does not seem to impact upon the total carbon concentration CT in these layers. Its dependence on the F(SiCH6)/(F(SiH2Cl2) + F(GeH4)) mass flows ratio is indeed rather well fitted using the following relationship: CT1.4/(1 − CT) = q(F(SiCH6)/(F(SiH2Cl2) + F(GeH4)), with q in between 0.38 and 0.34 for the 550–650 °C temperature range. However, the more the temperature increases, the more the substitutional/interstitial carbon incorporation ratio is reduced.

Properties of polymorphous silicon–germanium alloys deposited under high hydrogen dilution and at high pressure

Hydrogenated polymorphous silicon–germanium samples with Ge content x up to 0.35 have been deposited in a plasma-enhanced chemical vapor deposition system from silane and germane precursors highly diluted in hydrogen and in a high pressure range. The optical, defect-related, and transport properties of both types of carriers have been studied using a set of complementary techniques before light soaking, after light soaking, and after subsequent annealing. Before light soaking, we observe a small deterioration of the electron transport properties with increasing Ge content. This is related to the widening of the conduction band tail and to the increase of a deep defect band with emission energies at about 0.4 eV below the conduction transport path, which is attributed to Ge dangling bonds. The hole transport properties are much less sensitive to the incorporation of Ge, which we attribute to the unchanged Urbach tail parameter. Light soaking results in a deterioration of the transport properties due to an increase of the density of states, the relative effect being less pronounced for the highest Ge contents due to the higher native defect concentrations. Again, the electron transport properties are more affected than the hole transport properties, which appear quite stable against light soaking. Annealing at 460 K following light soaking is found to restore the initial state more easily in the low Ge content samples. Compared to samples deposited under more conventional conditions (no or less hydrogen dilution and low pressure) the striking features of our alloys are that (i) they exhibit an enhanced stability of the transport properties against light soaking and (ii) the characteristic time of the light-soaking kinetics is shorter. These features reveal the polymorphous character of our alloys, related to a specific hydrogen microstructure observed by infrared spectroscopy and hydrogen effusion, which indicate that the amorphous matrix exhibits an improved order like in hydrogenated polymorphous silicon.

A study of the oxidation behavior and the postannealing effect in a graded SiGe/Si heterostructure

A study on the dry thermal oxidation of a graded SiGe layer was performed. To reduce the Ge pileup effect during the thermal oxidation, the SiGe layer was deposited with much lower Ge content near the free surface than near the SiGe/Si heterointerface. After dry thermal oxidation at 900°C, the Ge composition in the pileup layer was significantly reduced and strain relaxation by defect formation was prevented due to the graded Ge distribution. To homogenize the Ge distribution between the pileup layer and remaining SiGe layer, the oxidized layers were postannealed. The homogenization is significantly enhanced by strain-induced diffusion, and it was confirmed by uphill diffusion of Ge. This result can propose an alternative oxidation method of strained SiGe/Si heterostructures.

Device structure and electrical characteristics of strained-Si-on-insulator (strained-SOI) MOSFETs

Strained-Si CMOS is an attractive device structure to be able to relax several fundamental limitations of CMOS scaling, because of high electron and hole mobility and compatibility with Si CMOS standard processing. In this paper, we present a new device structure including strained-Si channel, strained-SOI MOSFET, applicable to CMOS under sub-100 nm technology nodes. The main feature of this device is that thin strained-Si channel/relaxed SiGe heterostructures are formed on buried oxides. The principle and the advantages are described in detail. The strained-SOI structure has been successfully fabricated by combining the SIMOX technology with regrowth of strained Si films. It is demonstrated that strained-SOI n- and p-channel MOSFETs have mobilities 1.6 and 1.3 times higher than conventional Si MOSFETs, respectively. We also present a novel technique for fabricating ultra-thin SiGe-on-insulator (SGOI) virtual substrates with high Ge content by high temperature oxidation of SGOI with lower Ge content. A 16-nm-SGOI substrate having a Ge content as high as 57% has been successfully fabricated.

MBE grown Si/SiGe undulating layer superlattices for infrared light detection

We have grown Si/Si 0.61Ge 0.39 and Si/Si 0.55Ge 0.45 superlattices by molecular beam epitaxy (MBE) and fabricated vertical metal-semiconductor–metal (MSM) detectors. Analysis by Rutherford backscattering yielded the actual Ge content of the layers. Transmission electron microscopy revealed undulating layers with vertically ordered Ge-rich regions. A MSM detector fabricated from the superlattice with the higher Ge content showed a quantum efficiency of 5.2% for the wavelength 1.3 μm, and 1% for 1.55 μm, while the material with lower Ge concentration did not absorb light at 1.55 μm.

Alloy fluctuations in Si 1− xGe x crystals

We have introduced S donors into SiGe alloys as probe atoms for a spectroscopic investigation of the local environment of these donors. Even for low Ge content ( x=2.4%), we already observe strongly broadened lines for the intra-centre transitions, whereas the onset of the photo-ionisation stays at approximately the same energy, in contrast to expectations from a linear extrapolation of the impurity level from values of the pure compounds or from deformation potentials. We thus conclude that the S centres seen are located in the neighbourhood of significantly lower Ge content suggesting that Ge clusters may exist with much higher Ge content which, however, cannot be resolved in the photo-conductivity spectra. This finding, together with the observed line broadening demonstrates strong, non-statistical fluctuations in the alloy distribution.

Si1−xGexgrowth instabilities on vicinal Si(001) substrates: Kinetic vs. strain-induced effects

A comparative study of kinetically and thermodynamically driven instabilities on vicinal Si(001) surfaces during overgrowth with ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ is reported. We mapped out a wide range of the multidimensional growth parameter space and found, in contrast to previous reports, no evidence for strain-induced step bunching. At low Ge concentrations strain is insufficient to promote strain-induced step bunching, and the modified surface kinetics in the presence of segregated Ge leads to a smoother rather than rougher morphology. High Ge concentrations around 50% could be expected to provide enough strain, but near equilibrium hut cluster formation is the more effective strain-relaxation mechanism. We found the characteristically rippled step-bunching morphology only in a kinetically limited growth regime, where strain is of limited relevance, and in experiments where the SiGe layers replicate an underlying ripple morphology.

Formation of SiGe on Insulator Structure and Approach to Obtain Highly Strained Si Layer for MOSFETs

The formation of a SiGe layer on insulators, which can be realized by applying the separation by implanted oxygen (SIMOX) technique to SiGe layers, is essential for fabricating strained silicon on insulator (SOI) metal oxide semiconductor field effect transistors (MOSFETs). In this study, the SIMOX process for SiGe films is examined in terms of Ge diffusion during SIMOX annealing and the annealing temperature. It is found that the SIMOX annealing at temperature above 1300°C is necessary to realize uniform buried oxides, even though the melting point of SiGe crystal decreases with the Ge content. Ge diffusion during high-temperature annealing must also be taken into account when preparing the SiGe layer for SIMOX. These facts indicate that the realization of a SiGe layer on buried oxides is difficult with the simple SIMOX process for SiGe crystal, particularly so in the case of high Ge content. In order to overcome this problem, the double layer SiGe structure on an insulator is proposed and the effectiveness of this structure on the increase of strain in Si is verified experimentally. The strain relaxation of the SiGe layer with higher Ge content, which is grown on the SiGe layer with lower Ge content, is observed with expanding of the under-layer.

Structure and Characteristics of Strained-Si-On-Insulator (Strained-SOI) MOSFETs

Abstrct:Strained-Si MOSFET is an attractive device structure to be able to relax several fundamental limitations of CMOS scaling, because of high electron and hole mobility and compatibility with Si CMOS standard processing. In this paper, we present a new device structure including strained-Si channel, strained-SOI MOSFET, applicable to CMOS under sub-100 nm technology nodes. The main feature of this device is that thin strained-Si channel/relaxed SiGe hetero-structures are formed on buried oxides. The principle and the advantages are described in detail. The strained-SOI MOSFETs, whose electron and hole mobility is 1.6 and 1.3 times, respectively, higher than in conventional MOSFETs, have successfully been fabricated by combining the SIMOX technology with re-growth of strained Si films. We also present novel fabrication techniques to realize ultra-thin SiGe-on-Insulator (SGOI) virtual substrates with high Ge content, including Ge condensation due to oxidation of SGOI with lower Ge content. Strained-Si/SGOI structures with total thickness of 21 nm and Ge content of 56 % have been fabricated by oxidizing SiGe films on conventional SOI substrates and re-growing strained-Si films.

Kinetic vs. strain-induced growth instabilities on vicinal Si(001) substrates

The vicinal Si(001) surfaces are known to exhibit both kinetically and thermodynamically driven instabilities during overgrowth with Si or SiGe. Here, we present a comparative study that allows the discrimination of kinetic effects and strain-induced equilibrium effects. Under kinetic growth conditions, layers with low Ge content become smoother than their homoepitaxial Si counterparts. In contrast, for layers with high Ge content, hut cluster formation is identified as the dominant mechanism of the epilayer to relax strain. We find no evidence for one-dimensional strain-induced step bunching on small angle miscut substrates. Our results strongly suggest that kinetic or kinetically limited processes rather than thermodynamic effects determine the morphology of Si/SiGe interfaces at 550°C.

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.