SiGe Alloy Layers Research Articles

Germanium Nanocrystal Properties from Photoluminescence

Morphological information has been obtained using the strong near-infrared photoluminescence emitted by germanium (Ge) nanocrystals (NCs) coherently imbedded in SiGe alloy layers, grown by molecular beam epitaxy on Si substrates. The emission spectra are analyzed for the effects of strain, carrier confinement, and disorder over a wide range of Ge concentrations in the surrounding SiGe medium. This analysis provided significant insight into the properties of the Ge nanocrystals, including their size and shape. We also discuss the mechanisms leading to the high quantum efficiency observed for emission from the Ge nanocrystals at low temperatures. We indicate how direct gap behavior might be achieved for Ge NCs lattice matched within dilute Ge1-ySny alloys, where tensile strain would be present in the NCs in all three directions.

Effect of a Low-Temperature Si Underlayer on the Photoluminescence of Misfit Dislocations in Si(001)Si1 − xGe x Heterostructures with Ge-Rich Alloy Layers

The effect a low-temperature Si underlayer has on the photoluminescence of misfit dislocations in Si(001)Si1 – xGe x heterostructures with high Ge contents in a SiGe alloy layer is investigated. It is found that the introduction of this layer prevents the formation of dislocations in the alloy layer.

Fabrication and characterization of SiGe coaxial quantum wells on ordered Si nanopillars

Controlled SiGe coaxial quantum wells (CQWs) on periodic Si(001) nanopillars in a large area are explored systematically. The periodic SiGe CQW nanopillars are fabricated by a combination of nanosphere lithography, metal assisted chemical etching and epitaxial growth. The period, the radius, the height, the composition and the thickness of the SiGe alloy layer can all be intentionally modified. Considerably enhanced photoluminescence (PL) from the SiGe CQW nanopillars is observed, which is composed of four peaks. Such PL features are explained by the coupling between the spontaneous emissions of the SiGe CQW and the Mie resonant modes of the nanopillars, which can be further improved by optimizing the structural parameters of the SiGe CQW and the nanopillars. Our results demonstrate a feasible route to obtaining controlled SiGe CQW nanopillars, which have potential applications in optoelectronic devices.

Similarity of Stranski-Krastanow growth of Ge/Si and SiGe/Si (001)

This study investigates the onset of islanding (Stranski-Krastanow transition) in strained pure germanium (Ge) and dilute silicon-germanium (SiGe) alloy layers grown by chemical vapour deposition on Si(001) substrates. Integration of compositional profiles is compared to a novel method for quantification of X-ray maps acquired in cross-sectional scanning transmission electron microscopy, together with simulations of surface segregation of Ge. We show that Si1−xGex alloys for germanium concentrations x ≤ 0.27 grow two-dimensionally and stay flat up to considerable layer thicknesses, while layers with concentrations in the range 0.28 < x ≤ 1 form islands after deposition of ∼3.0/x monolayers (=quarter unit cells in the diamond lattice, ML). The uncertainty in the amount of deposited material for pure Ge is ±(0.2–0.3) ML. Modelling shows that of the amount of germanium deposited, 0.7 ML segregate towards the free surface so that only ∼2.3/x ML are directly incorporated in the layer within a few nanometres, in good agreement with our measurements. For pure Ge (x = 1), this thickness is smaller than most values quoted in the literature, which we attribute to the high sensitivity of our method to fractional monolayer changes in the effective chemical width of such thin layers.

Synthesis of SiGe layered structure in single crystalline Ge substrate by low energy Si ion implantation

Nanometer-thickness SiGe alloy layers were synthesized by direct Si ion implantation in Ge (100) wafers at different fluences followed by high temperature annealing. The cross-sectional transmission electron microscopy and secondary ion mass spectrometry reveal the formation of a thin Si-rich crystalline layer in the near-surface region. The micro-Raman spectroscopy and X-ray reflectivity techniques were used to determine the composition and strain in SiGe alloy layers. The photoluminescence measurements of the annealed samples showed a broad emission, peaking around 500 nm. The peak intensity is, however, dependent on the bombarding fluence.

Defect-Free Single-Crystal SiGe: A New Material from Nanomembrane Strain Engineering

Many important materials cannot be grown as single crystals in bulk form because strain destroys long-range crystallinity. Among them, alloys of group IV semiconductors, specifically SiGe alloys, have significant technological value. Using nanomembrane strain engineering methods, we demonstrate the fabrication of fully elastically relaxed Si(1-x)Ge(x) nanomembranes (NMs) for use as growth substrates for new materials. To do so, we grow defect-free, uniformly and elastically strained SiGe layers on Si substrates and release the SiGe layers to allow them to relax this strain completely as free-standing NMs. These SiGe NMs are transferred to new hosts and bonded there. We confirm the high structural quality of these new materials and demonstrate their use as substrates for technologically relevant epitaxial films by growing strained-Si layers and thick, lattice-matched SiGe alloy layers on them.

The effects of low temperature buffer layer on the growth of pure Ge on Si(001)

We investigated the effects of low temperature (LT) Ge buffer layers on the two-step Ge growth by varying the thickness of buffer layers. Whereas the two-step Ge layers using thin (< 40 nm) Ge buffer layers were roughened due to the formation of SiGe alloy, pure and flat Ge layers were grown by using thick (> 50 nm) LT Ge buffer layers. The lowest threading dislocation density of 1.2 × 10 6 cm − 2 was obtained when 80-nm-thick LT Ge buffer layer was used. We concluded that the minimum thickness of buffer layer was required to grow uniform two-step Ge layers on Si and its quality was subject to the thickness of buffer layer.

Effect of Germanium content and strain on the formation of extended defects in ion implanted Silicon/Germanium

We studied the evolution of extended defects in relaxed and strained Si and SiGe structures after an amorphising implant. The investigated structures included three relaxed SiGe alloy layers with various Ge contents (20, 35 and 50 at.%), a 40 nm-thick tensely strained Si layer and a 40 nm-thick compressively strained Si 0.8Ge 0.2 layer. Concerning the compositional effects, we found that the increase of Ge concentration in relaxed SiGe structures leads to: (i) an overall decrease of the defect stability and to (ii) an enhanced {311}-to-loops transformation. As for the strain effects, it is found that: (i) Tensile strain (in Si) retards the transformation of {311} defects into loops; (ii) compressive strain (in SiGe) enhances the transformation of {311}s into loops; (iii) in all cases, the overall defect stability is not strongly modified in the presence of strain. The observed results are discussed in terms of the various mechanisms involved, including the increase of the interstitial diffusivity in relaxed SiGe alloys (with respect to Si) and the strain effects on both interstitial equilibrium concentration and defect formation energy.

Measurement of phonon pressure coefficients for a precise determination of deformation potentials in SiGe alloys

AbstractFor an effective use of Raman scattering as strain characterization tool in SiGe nanostructures a precise knowledge of the phonon deformation potentials (DPs) is strictly necessary. The optical phonon DPs can be determined by means of Raman scattering measurements from the cleaved edge of a biaxially strained SiGe alloy layer grown pseudomorphically on silicon and subsequently capped. Due to uncertainties in the literature values of the unstrained phonon frequencies it turns out that the desired degree of accuracy is only attained by complementing the Raman measurements from the edge with that of the hydrostatic pressure coefficient of the optical phonons. For that purpose we have grown by molecular beam epitaxy up to seven partially strained Si1–x Gex alloys on Si spanning the entire compositional range and measured the dependence on hydrostatic pressure of the frequency of the Ge‐like, Si‐like and mixed Si–Ge optical modes. After correcting for the pressure dependent biaxial stress induced by the Si substrate on the alloy layer and taking into account the dependence on alloy composition of the bulk modulus we obtain a fairly constant value of the Grüneisen parameter around 1.0 for all three optical modes in the whole range of Ge contents. We also determined the strain shift coefficients for the three modes, which are essentially independent of Ge content between 0.4 and 1. Our results are in very good agreement with recentcalculations of the SiGe phonon deformation potentials using a modified Keating model, which settles the longstanding issue about the large discrepancies between results from different experiments. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Strained Si films grown by chemical vapor deposition of trisilane on Ge buffered Si(100)

Relaxed Ge buffer layers were grown on Si substrates using a recently developed chemical vapor deposition (CVD) approach that combines digermylmethane and digermane precursors. Ultrathin Si films were deposited on these buffer layers at 420 °C by CVD using trisilane as a precursor. The Si films were studied with a variety of experimental techniques, with emphasis on Raman spectroscopy. The analysis of the Raman results shows that a thin (< 1 nm), fully strained Si–Ge alloy layer is formed at the Si–Ge interface. Pure Si grows on this transitional alloy with a strain that approximately follows the predictions from a simple equilibrium strain theory. These results are significant for Ge-based Metal-Oxide-Semiconductor applications that require a thin Si-layer to isolate the Ge channel from the high permittivity oxide.

High-speed strained-single-crystal-silicon thin-film transistors on flexible polymers

We fabricate thin-film transistors (TFTs) on both strained and unstrained single-crystal Si membranes transferred to flexible-polymer substrates. The active layer is transferred from the starting silicon on insulator (SOI) using a simple, fast, and reliable dry-printing method. When a multilayer Si∕SiGe∕Si structure is pseudomorphically grown on SOI and the buried oxide is selectively removed, strained Si with a negligible density of dislocations is achieved via elastic strain sharing between the SiGe alloy layer and the Si layers. Both the drain current and the transconductance of TFTs fabricated on this strained Si∕SiGe∕Si membrane after its transfer to the flexible polymer are much higher than of TFTs fabricated on the unstrained-Si counterpart.

Ballistic phonon transport and self-heating effects in strained-silicon transistors

In this manuscript, different aspects of nanoscale thermal transport in strained silicon transistors will be addressed. The two-dimensional Boltzmann transport equations for phonons in Si and SiGe alloy layers, along with the acoustic mismatch model for the interface, are used to capture the sub-continuum heat conduction effects in the device. It is shown that the lateral thermal conductivity of a 10-nm strained-Si layer grown on the SiGe underlayer can vary from 14 to 20W/m-K, depending on the interface specularity parameter. The resulting temperature distribution in the device is used to predict the impact of self-heating on performance of future generations of strained-Si devices. The analysis shows that the merits of strained-Si technology can be suppressed by excessive self-heating; therefore, additional considerations in the design of these devices need to be taken into account.

Physico-chemical and electrical properties of rapid thermal oxides on Ge-rich SiGe heterolayers

Rapid thermal oxidation of high-Ge content (Ge-rich) Si 1− x Ge x ( x = 0.85) layers in dry O 2 ambient has been investigated. High-resolution X-ray diffraction (HRXRD) and strain-sensitive two-dimensional reciprocal space mapping X-ray diffractometry (2D-RSM) are employed to investigate strain relaxation and composition of as-grown SiGe alloy layers. Characterizations of ultra thin oxides (∼6–8 nm) have been performed using Fourier transform infrared spectroscopy (FTIR) and high-resolution X-ray photoelectron spectroscopy (HRXPS). Formation of mixed oxide i.e., (SiO 2 + GeO 2) and pile-up of Ge at the oxide/Si 1− x Ge x interface have been observed. Enhancement in Ge segregation and reduction of oxide thickness with increasing oxidation temperature are reported. Interface properties and leakage current behavior of the rapid thermal oxides have been studied by capacitance–voltage (C–V) and current–voltage (J–V) techniques using metal-oxide-semiconductor capacitor (MOSCAP) structures and the results are reported.

HRTEM observation on microstructures in the Ge-implanted silicon wafers re-processed by solid phase epitaxy

Solid-phase epitaxy combined with Ge implantation has been employed to process silicon wafer. The resulting microstructural details have been examined by high-resolution transmission electron microscopy. It is observed that a thin layer of SiGe alloy forms around the projected ion range. Some dislocations as half loops extend into the substrate itself instead of the top part above the alloy layer, so the completeness in the top silicon has less destruction. It is assumed that the gradient profile of Ge composition under the SiGe alloy layer may be the primary cause. Within the SiGe alloy layer, there appear special nanoclusters. The distribution and the number of these nanoclusters are closely correlated with the implant energy and dose. It is revealed by Fourier transform analysis that the lattice within the nanoclusters is modulated with a period of four times the (1 1 1) plane spacing in the regular Si lattice. It is argued that the lattice-modulated characteristics should result from Ge ordering instead of Moiré interference.

Pure-edge dislocation network for strain-relaxed SiGe∕Si(001) systems

We have grown strain-relaxed SiGe layers on Si(001) substrates with a pure-edge dislocation network buried at the heterointerface and analyzed dislocation morphology depending on growth conditions. The process employed here consists of pure-Ge film growth on Si(001) and subsequent high temperature annealing for solid-phase intermixing of the Ge film and Si deposited on the top to form a SiGe alloy layer. Transmission electron microscopy revealed morphological changes of shorter pure-edge dislocation segments initially formed at the Ge∕Si interface into a network structure consisting of longer and regularly spaced dislocations during post-deposition annealing. The dislocation network was explicitly preserved even after the intermixing of Si and Ge and predominantly contributed to in-plane strain relaxation of the SiGe layer. Applicability of the pure-edge dislocation network to strain-relaxed SiGe buffer layers on Si(001) substrates is discussed.

Discontinuous tracks in relaxed SiGe alloy layers: Formation and thermal evolution

Strain-relaxed epitaxial Si 1− x Ge x alloy layers partially doped with arsenic were irradiated with U 238 ions of energy 0.8–2.7 GeV in the electronic stopping regime. Using transmission electron microscopy, clear evidence is found that in some of the Si 1− x Ge x alloy layers latent tracks are created. Depending on the composition of the Si 1− x Ge x alloy and on the doping level of As, the tracks exhibit a more or less discontinuous character. Larger GeAs precipitations are observed in supersaturated alloy layers. We also find indications that the velocity of the projectile ions plays an important role. Additional thermal treatment results in gradual annealing of track-related defects at low temperature (200–400 °C) and in the formation of GeAs precipitates at high (850 °C) temperature.

High PVCR Si/Si 1−xGe x DW RTD formed with new triple-layer buffer

A new thin triple-layer buffer consisting of three SiGe alloy layers is proposed for Si/Si 1− x Ge x resonant tunneling diodes (RTDs). The triple-layer buffer is compared to our previously proposed thin double-layer buffer with x being stepwise increased. The triple-layer buffer is formed by inserting an ultra-thin high Ge composition SiGe alloy layer between Si substrate and the thin double-layer buffer. By insertion of the ultra-thin high Ge composition SiGe alloy layer, the triple-layer buffer is more relaxed; however, the relaxation rate is limited to around 50% under our experimental conditions. Although the triple-layer buffer has still strain and the barrier height is relatively low, a double-well RTD using this buffer exhibits a peak-to-valley current ratio (PVCR) of as high as ∼50 000, which is far larger than that of standard III–V RTDs. The results also indicate that the proposed buffer leads to a high crystalline quality quantum well structure and that inelastic electron scattering in the Si/Si 1− x Ge x RTD is inherently low.

Fast nondestructive technique to determine the content of components in a strain-compensated crystalline ternary alloy

The x-ray Bragg diffraction intensity profile for a model strain-compensated structure consisting of a thin SiGe alloy layer grown on a thick Si substrate is derived using a Laplace transform interpretation of the kinematical approximation of x-ray diffraction theory. It is shown that in the case of fully strain-compensated crystals a simplified x-ray phase-retrieval technique can be applied to determine the alloy composition from this x-ray diffraction data. An experimental intensity profile from an almost perfectly unstrained SiGe:C/Si structure is analyzed using this method.

Nanovoids in MBE-grown SiGe alloys implantedin situwithGe+ions

Spherically shaped voids, of nanometer size, are observed in molecular-beam epitaxially grown SiGe alloy layers implanted in-situ at elevated temperature with low-energy Ge ions, followed by thermal treatments. The voids are exclusively assembled in the narrow, implanted band. The voids only appear in the layers after a heat treatment at a temperature higher than 700 \ifmmode^\circ\else\textdegree\fi{}C, and they are stable up to 900 \ifmmode^\circ\else\textdegree\fi{}C. Arsenic ion implantation at similar conditions does not give rise to void formation but to regular interstitial dislocation loops. The nucleation stage of the voids is accompanied by a strong photoluminescence-yield enhancement in the range of 1.4--1.55 \ensuremath{\mu}m, originating from the strained SiGe alloy layer which contain vacancy clusters or small voids.

Formation and Properties of Epitaxial CoSi2 Layers on p-Si0.83Ge0.17/p-Si(001) using a Si Capping Layer by Metal-Organic Chemical Vapor Deposition

Epitaxial CoSi2 layers were in-situ grown using a sacrificial Si capping layer on p-Si0.83Ge0.17/p-Si(001) by metal organic vapor deposition (MOCVD) at 600°C using cyclopentadienyl cobalt, Co(η5-C5H5)(CO)2. The grown epitaxial CoSi2 layers remain thermally stable at the annealing temperature, TA≤800°C. The CoSi2/p-Si0.83Ge0.17/p-Si(001) samples annealed at TA≤850°C showed the sheet resistance values as low as \\cong5.6 Ω/□. At TA=900°C, the observed increase in the sheet resistance value to \\cong14.7 Ω/□ was attributed to the formation of CoSi phase with higher resistivity resulting from the direct interaction of Co with the SiGe alloy layer. The effective Schottky barrier heights (φBp), measured for the first time, ranged from 0.103 to 0.194 eV at the reverse bias of -5–0 V.