Behavior of germanium ion-implanted into SiO<Subscript>2</Subscript> near the bonding interface of a silicon-on-insulator structure

The properties of germanium implanted into the SiO₂ layers in the vicinity of the bonding interface of silicon-on-insulator structures are studied. It is shown that, under conditions of high-temperature (1100°C) annealing, germanium nanocrystals are not formed, while the implanted Ge atoms segregate at the Si/SiO₂ bonding interface. It is established that, in this case, Ge atoms are found at sites that are coherent with the lattice of the top silicon layer. In this situation, the main type of traps is the positive-charge traps; their effect is interpreted in the context of an increase in the surface-state density due to the formation of weaker Ge-O bonds. It is found that the slope of the drain-gate characteristics of the back MIS transistors increases; this increase is attributed to an increased mobility of holes due to the contribution of an intermediate germanium layer formed at the Si/SiO₂ interface.     

Interface states and deep-level centers in silicon-on-insulator structures

Deep-level centers in a split-off silicon layer and trap levels were studied by deep-level transient spectroscopy both at the Si/SiO₂ interface obtained by direct bonding and also at the Si(substrate)/〈thermal SiO₂〉 interface in the silicon-on-insulator structures formed by bonding the silicon wafers and delaminating one of the wafers using hydrogen implantation. It is shown that the Si/〈thermal SiO₂〉 interface in a silicon-on-insulator structure has a continuous spectrum of trap states, which is close to that for classical metal-insulator-semiconductor structures. The distribution of states in the upper half of the band gap for the bonded Si/SiO₂ interface is characterized by a relatively narrow band of states within the range from E _c −0.17 eV to E _c −0.36 eV. Furthermore, two centers with levels at E _c −0.39 eV and E _c −0.58 eV are observed in the split-off silicon layer; these centers are concentrated in a surface layer with the thickness of up to 0.21 μm and are supposedly related to residual postimplantation defects. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 8, 2001, pp. 948–953. Original Russian Text Copyright ? 2001 by Antonova, Stano, Nikolaev, Naumova, Popov, Skuratov.     

The charge accumulation in an insulator and the states at interfaces of silicon-on-insulator structures as a result of irradiation with electrons and gamma-ray photons

The accumulation of charge in an insulator and the states at interfaces in silicon-on-insulator structures irradiated with 2.5-MeV electrons and 662-keV gamma-ray photons were studied. It was found that an additional positive charge appears in the buried insulator of the structures as a result of irradiation. The concentration of hole traps generated by radiation in the oxide is higher at the boundary with substrate than at the bonding interface between a split-off silicon layer and oxide. It is shown that the presence of even a weak built-in field in the structures (F?5×10³ V/cm) gives rise to efficient separation of charge carriers. There is no generation of additional states at the Si/SiO₂ interfaces in the silicon-on-insulator structures for both irradiation types, although this generation is observed in the initial thermal oxide.     

Lateral growth of monocrystalline Ge on silicon oxide by ultrahigh vacuum chemical vapor deposition

The lateral growth of Ge on, both, chemically and thermally formed silicon oxide layers, from nanoscale silicon seed is studied. We have developed a method using standard local oxidation of silicon to create well-localized nanoscale silicon seeds that enable to grow Ge on thick thermal silicon oxide. The germanium growth starts selectively from the silicon seed lines and proceeds by wetting the SiO₂ layer. Analysis by high-resolution transmission electron microscopy have shown that Ge layers grown above silicon oxide were perfectly monocrystalline and are free of defect. The only detected defects are situated at the Ge/Si interface.     

Anodization of nanoscale Si layers in silicon-on-insulator structures

Anodization of silicon-on-insulator (SOI) layers is studied as a function of the SOI layer thickness in the range from 5 to 500 nm. It is found that thinning of the silicon film to less than 100 nm is accompanied by a sharp decrease in the anodization rate. For SOI films thinner than or on the order of magnitude of ∼10 nm, the limiting thickness of the oxidized silicon layer is 0.4 nm. It is shown that the main cause of a decrease in the anode current efficiency during oxidation of nanoscale Si layers is an increase in the resistance of the silicon active layer, which limits the hole current in the film plane and, hence, the number of silicon cations coming to the SiO₂/electrolyte interface for their subsequent oxidation.     

Anomalous distribution of germanium implanted into a SOI dielectric layer after the annealing of radiation defects

The germanium-distribution profile is investigated in a Si/SiO₂/Si structure after the implantation of ⁷⁴Ge into SiO₂ dielectric layer, bonding with the Si device layer, and high-temperature annealing. The anomalously high transport and accumulation of ⁷⁴Ge atoms near the SiO₂/Si interface far from the bonded boundary is found. The observed ⁷⁴Ge distribution is beyond the framework of the existing model of diffusion of Ge in Si and SiO₂ after postimplantation annealing. A modified model of diffusion of Ge atoms near the Si/SiO₂ interface qualitatively explaining the observed features is proposed.     

Transformation of interface states in silicon-on-insulator structures under annealing in hydrogen atmosphere

Changes induced by annealing the spectrum of states on a Si/SiO₂ interface obtained by direct bonding and on a Si(substrate)/〈thermal SiO₂〉 interface in silicon-on-insulator (SOI) structures were investigated by charge-related deep-level transient spectroscopy. The structures were formed by bonding silicon wafers and slicing one of the wafers along a plane weakened by hydrogen implantation. The SOI structures were annealed at 430°C for 15 min in hydrogen, which corresponded to the conventional mode of passivation of the Si/SiO₂-interface states. The passivation of interface states by hydrogen was shown to take place for the Si/〈thermal SiO₂〉 interface, as a result of which the density of traps substantially decreased, and the continuous spectrum of states was replaced by a band of states in the energy range E _c=0.1–0.35 eV within the entire band. For the traps on the bonded Si/SiO₂ interface, the transformation of the centers occurs; namely, a shift of the energy-state band is observed from E _c=0.17–0.36 to 0.08–0.22 eV. The trapping cross section decreases by about an order of magnitude, and the density of traps observed increases slightly.     

Traps with near-midgap energies at the bonded Si/SiO<Subscript>2</Subscript> interface in silicon-on-insulator structures

Capture centers (traps) are studied in silicon-on-insulator (SOI) structures obtained by bonding and hydrogen-induced stratification. These centers are located at the Si/SiO₂ interface and in the bulk of the split-off Si layer. The parameters of the centers were determined using charge deep-level transient spectroscopy (Q-DLTS) with scanning over the rate window at fixed temperatures. Such a method allows one to study the traps near the Si midgap at temperatures near 295 K. It is shown that the density of traps with a continuous energy spectrum, which are located at the bonded Si/SiO₂ interface, decreases by more than four orders of magnitude at the mid-gap compared with the peak density observed at the activation energy E _a ≈0.2–0.3 eV. The capture centers are also found in the split-off Si layer of the fabricated SOI structures. Their activation energy at room temperature is E _a =0.53 eV, the capture cross section is 10^?19 cm², and the concentration is (0.7–1.7)×10¹³ cm^?3. It is assumed that these capture centers are related to deep bulk levels induced by electrically active impurities (defects) in the split-off Si layer close to the Si/SiO₂ interface.     

Silicon-on-insulator structures with a nitrogenated buried SiO<Subscript>2</Subscript> layer: Preparation and properties

Electrical properties of silicon-on-insulator (SOI) structures with buried SiO₂ layer implanted with nitrogen ions are studied in relation to the dose and energy of N⁺ ions. It is shown that implantation of nitrogen ions with doses >3 × 10¹⁵ cm⁻² and an energy of 40 keV brings about a decrease in the fixed positive charge in the oxide and a decrease in the density of surface stares by a factor of 2. An enhancement of the effect can be attained by lowering the energy of nitrogen ions. The obtained results are accounted for by interaction of nitrogen atoms with excess silicon atoms near the Si/SiO₂ interface; by removal of Si-Si bonds, which are traps of positive charges; and by saturation of dangling bonds at the bonding interface of the SOI structure.     

Surface texture of single-crystal silicon oxidized under a thin V<Subscript>2</Subscript>O<Subscript>5</Subscript> layer

The process of surface texturing of single-crystal silicon oxidized under a V₂O₅ layer is studied. Intense silicon oxidation at the Si–V₂O₅ interface begins at a temperature of 903 K which is 200 K below than upon silicon thermal oxidation in an oxygen atmosphere. A silicon dioxide layer 30–50 nm thick with SiO₂ inclusions in silicon depth up to 400 nm is formed at the V₂O₅–Si interface. The diffusion coefficient of atomic oxygen through the silicon-dioxide layer at 903 K is determined (D ≥ 2 × 10^–15 cm² s^–1). A model of low-temperature silicon oxidation, based on atomic oxygen diffusion from V₂O₅ through the SiO₂ layer to silicon, and SiO _x precipitate formation in silicon is proposed. After removing the V₂O₅ and silicon-dioxide layers, texture is formed on the silicon surface, which intensely scatters light in the wavelength range of 300–550 nm and is important in the texturing of the front and rear surfaces of solar cells.     

Growth and characterization of germanium and boron doped silicon epitaxial films

The epitaxial growth and characterization of in-situ germanium and boron (Ge/B) doped Si epitaxial films is described. As indicated by secondary ion mass spectroscopy and spreading resistance measurements, the total and electrically activated B concentrations are essentially identical and independent of Ge incorporation. The B and Ge concentrations are uniformly distributed in these Ge/B doped films. A slight enhancement of Hall mobility is obtained, possibly due to the stress relief induced by Ge counterdoping. Carrier conduction in these films is due to the activated B with an activation energy of 0.04 eV as revealed by conductivity versus temperature measurements. Ge atoms appear to be isoelectronic with Si atoms in these films. A slight degradation of minority carrier diffusion length is observed. Electrical characterization of PN diodes on these Ge/B doped films do not reveal any anomaly. SiO₂ on these Ge/B doped films has similar oxide fixed charge density, interface state density and dielectric breakdown strength compared to silicon dioxide on boron doped epitaxial films. Electron injection reveals a different transport mechanism of the SiO₂ grown on these Ge/B doped films.     

Gate-first Germanium nMOSFET with CVD HfO/sub 2/ gate dielectric and silicon surface passivation

A gate-first self-aligned Ge n-channel MOSFET (nMOSFET) with chemical vapor deposited (CVD) high-/spl kappa/ gate dielectric HfO/sub 2/ was demonstrated. By tuning the thickness of the ultrathin silicon-passivation layer on top of the germanium, it is found that increasing the silicon thickness helps to reduce the hysteresis, fixed charge in the gate dielectric, and interface trap density at the oxide/semiconductor interface. About 61% improvement in peak electron mobility of the Ge nMOSFET with a thick silicon-passivation layer over the CVD HfO/sub 2//Si system was achieved.     

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.     

X-ray-emission study of the structure of Si:H layers formed by low-energy hydrogen-ion implantation

Amorphous silicon layers formed by implantation of 24-keV hydrogen ions into SiO₂/Si and Si with doses of 2.7×10¹⁷ and 2.1×10¹⁷ cm^?2, respectively, were studied using ultrasoft X-ray emission spectroscopy with variations in the energy of excitation electrons. It is ascertained that the surface silicon layer with a thickness as large as 150–200 nm is amorphized as a result of implantation. Implantation of hydrogen ions into silicon coated with an oxide layer brings about the formation of a hydrogenated silicon layer, which is highly stable thermally.     

The effect of NO<Subscript>2</Subscript> adsorption on optical and electrical properties of porous silicon layers

The Raman spectra and current-voltage characteristics of porous silicon layers are studied before and after exposure to NO₂. It is shown that spherical nanocrystallites with the diameter of approximately 6–8 nm are present in the samples’ structure. The effect of NO₂ brings about a decrease in the resistance of porous Si by two-three orders of magnitude. An increase in the conductance of the structures at gas concentrations as high as 2000 ppm and a drastic decrease in this conductance if the concentration exceeds the above value are observed. This effect is explained in the context of the model that implies the formation of additional defects of the type of dangling silicon bonds at the Si/SiO₂ interface as a result of oxidation of the porous silicon surface. These defects are traps for holes and reduce the increase in the hole concentration.     

X-ray reflectivity of silicon on insulator wafers

The ability of X-ray reflectivity to analyse different silicon on insulator structures is underlined. The standard geometry with first reflection occurring at the surface gives information about the thickness, roughness, and density of the layers. Deeply buried interfaces, i.e. in between thick wafers, are analysed with a non-standard geometry (the first reflection occurs at the buried interface) and with a high-energy radiation. These two methods are, respectively, illustrated by the reflectivity measurements of (SiO₂/Si/SiO₂|bulk Si) and (bulk Si/thermal SiO₂|native SiO₂/bulk Si) bonded structures, and are explained in the framework of kinematic theory of X-ray reflectivity.     

Nitrogen bonding configurations near the oxynitride/silicon interface after oxynitridation in N2O ambient of a thin SiO2 gate

The identification of the bonding environments and their progressive modifications upon reaching the oxynitride/silicon interface, in a SiO₂/SiO_xN_y/Si structure, have been investigated by means of X-ray photoemission spectroscopy (XPS). The SiO₂ film was grown at 850 °C by means of a mixed dry-steam process, followed by a 60 min, 950 °C furnace oxynitridation in N₂O gas. A depth profile analysis was carried out by a progressive chemical etching procedure, reaching a residual oxide thickness of about 1.2 nm. XPS analysis of the Si 2p and N 1s photoelectron peaks pointed out that the chemistry of the oxynitride layer is a rather complex one. Four different nitrogen bonding environments were envisaged. Both the overall nitrogen content, which rises up to 2.5%, and its bonding configurations are progressively changing while moving towards the silicon interface.     

The effects of gamma irradiation on low-pressure chemical-vapor-deposited silicon dioxide metal-oxide-silicon structures

The effects of gamma irradiation on as-deposited, oxygen-annealed, and dual-dielectric gate (undoped polysilicon/oxide) low-pressure chemical-vapor-deposited (LPCVD) silicon dioxide (SiO₂) metal-oxide-silicon (MOS) structures were investigated. As-deposited LPCVD SiO₂ MOS structures exhibit the largest shift in flatband voltage with gamma irradiation. This is most likely due to the large number of bulk oxide traps resulting from the nonstochiometric nature of as-deposited LPCVD SiO₂. Dual-dielectric (undoped polysilicon/annealed LPCVD SiO₂) MOS structures exhibit the smallest shift in flatband voltage and increase in interface state density compared to as-deposited and oxygen-annealed LPCVD SiO₂ MOS structures. The interface state density of dual-dielectric MOS structures increases from 5 × 10¹⁰ eV cm⁻² to 2–3 × 10¹¹ eV cm⁻² after irradiation to a gamma total dose level of 1 Mrads(Si). This result suggests that the recombination of atomic hydrogen atoms with silicon dangling bonds, either along grain boundaries or in crystallites of the undoped polysilicon layer in dual-dielectric (undoped polysilicon/annealed LPCVD SiO₂) MOS structures, probably reduces the number of atomic hydrogen atoms reaching the Si/SiO₂ interface to generate interface states.     

Optical properties of germanium monolayers on silicon

Photoluminescence and Raman spectra of thin germanium layers grown on silicon at a low temperature (250°C) have been studied. In structures of this kind, in contrast to those grown at high temperatures, luminescence from quantum wells is observed at germanium layer thicknesses exceeding ～9 monolayers (ML). With the development of misfit dislocations, the luminescence lines of quantum wells are shifted to higher energies and transverse optical (TO) phonons involved in the luminescence are confined to a quasi-2D germanium layer. Introduction of an additional relaxed Si_0.95Ge_0.05 layer into the multilayer Ge/Si structure leads to a substantial rise in the intensity and narrowing of the luminescence line associated with quantum dots (to 24 meV), which points to their significant ordering.     

Germanium on sapphire by wafer bonding

This paper describes the creation of a germanium on sapphire platform, via wafer bonding technology, for system-on-a-chip applications. Similar thermal coefficients of expansion between germanium (5.8 × 10^?6 K^?1) and sapphire (5 × 10^?6 K^?1) make the bonding of germanium to sapphire a reality. Germanium directly bonded to sapphire results in microvoid generation during post bond annealing. Inclusion of an interface layer such as silicon dioxide layer by plasma enhanced chemical vapour deposition, prior to bonding, results in a microvoid free bond interface after annealing. Grinding and polishing of the subsequent germanium layer has been achieved leaving a thick germanium on sapphire (GeOS) substrate. Submicron GeOS layers have also been achieved with hydrogen/helium co-implantation and layer transfer. Circular geometry transistors exhibiting a field effect mobility of 890 cm²/V s have been fabricated onto the thick germanium on sapphire layer.