Undoped Si Research Articles

Effects of stress on the dielectric function of strained pseudomorphic Si1−xGex alloys from 0 to 75% Ge grown on Si (001)

The dielectric function of bi-axially strained, intrinsic, and pseudomorphic Si1−xGex alloys was measured at room temperature using spectroscopic ellipsometry from 0.74 eV to 5.06 eV. Un-doped Si1−xGex with germanium compositions ranging from 0 to 75% was grown on Si (001) using chemical vapor deposition. High resolution x-ray diffraction was used to confirm sample composition, thickness, and strain. X-ray relaxation scans showed that all the samples were fully strained. All the alloy films in this study have low values of surface roughness, which allowed determination of the dielectric function. The presence of strain in the Si1−xGex alloys clearly altered the dielectric response. The bi-axial stress induced shift of the E1 and El + Δ1 critical point energies of pseudomorphic alloys can be described by the elastic response to the strain based on k*p theory [Lange et al., J. Appl. Phys. 80, 4578 (1996)]. Although the critical point energies of the alloys having higher germanium concentration showed deviations from the large shear approximation, the strain induced shift in critical point energy and the relative intensities of E1 and El + Δ1 were reasonably well described by the full elastic theory.

Mechanism of thermal conductivity suppression in doped silicon studied with nonequilibrium molecular dynamics

We examined the underlying mechanisms for thermal conductivity suppression in crystalline silicon by substitutional doping with different elements ($X$ $=$ boron, aluminum, phosphorus, and arsenic). In particular, the relative effects of doping-induced mass disorder, bond disorder, and lattice strain were assessed using nonequilibrium molecular dynamics simulations. Stillinger--Weber potential parameters for Si-$X$ interatomic interactions were optimized by fitting to relevant atomic forces from first-principles calculations. We first calculated the thermal conductivity variation of B-doped Si as a function of dopant concentration; the result shows excellent agreement with existing experimental data, indicating the reliability of our force-field-based simulations. At the dopant concentration of about 5 \ifmmode\times\else\texttimes\fi{} 10${}^{20}$ cm${}^{\ensuremath{-}3}$, the Si thermal conductivity value is predicted to be reduced from 137 W/mK at 300 K in undoped Si to 18/39/57/78 W/mK in As/B/P/Al-doped Si. Our study demonstrates that the mass disorder effect is primarily responsible for the thermal conductivity suppression in the As- and B-doped cases, whereas the bond disorder contribution is found to be more important than the mass disorder contribution in the Al- and P-doped cases; for all these systems, the lattice strain effect turns out to play a minor role in the reduction of lattice thermal conductivity.

2 MeV e‐irradiation UHVEM study on the impact of O and Ge doping on {113}‐defect formation in Si

AbstractFirst results are presented of a study on the impact of O and Ge doping on {113}‐defect formation in Si using in situ 2 MeV e‐irradiation in a high voltage electron microscope. Comparison with undoped Si, reveals a suppression of {113}‐defect formation due to O and Ge. The observations are discussed in the frame of a theoretical model assuming quasi‐chemical reactions to describe the intrinsic point defect behaviour during irradiation as a function of irradiation temperature and specimen thickness. It is shown that for irradiations at high temperatures, intrinsic point defect diffusivity values derived from crystal growth and metal diffusion experiments can be used to explain the observations semi‐quantitatively.

Rapid Epitaxial Growth of Si and SiGe Mono-Crystalline Films on Silicon-on-Glass Substrates by Reactive CVD Using SiH4, GeH4, and F2

ABSTRACTWe have succeeded in the rapid epitaxial growth of Si, Ge, and SiGe films on Si substrates below 670 ºC by reactive CVD utilizing the spontaneous exothermic reaction between SiH4, GeH4, and F2. Mono-crystalline SiGe epitaxial films with Ge composition ranging from 0.1 to 1.0 have been successfully grown by reactive CVD for the first time.This technique has also been successfully applied to the growth of these films on silicon-on-glass substrates by a 20 - 50 ºC increase of the heating temperature. Over 10 μm thick epitaxial films at 3 nm/s growth rate are obtained. The etch pit density of the 5.2 μm-thick Si0.5Ge0.5 film is as low as 5 x 106 cm-2 on top. Mobilities of the undoped SiGe and Si films are 180 to 550 cm2/Vs, confirming the good crystallinity of the epitaxial films.

Interface trap density metrology from sub-threshold transport in highly scaled undoped Si n-FinFETs

Channel conductance measurements can be used as a tool to study thermally activated electron transport in the sub-threshold region of state-of-art FinFETs. Together with theoretical tight-binding (TB) calculations, this technique can be used to understand the dependence of the source-to-channel barrier height (Eb) and the active channel area (Saa) on three important parameters: (i) the gate bias (Vgs), (ii) the temperature, and (iii) the FinFET cross-section size. The quantitative difference between experimental and theoretical values that we observe can be attributed to the interface traps present in these FinFETs. Therefore, based on the difference between measured and calculated values of (i) Saa and (ii) |∂Eb/∂Vgs| (channel to gate coupling), two new methods of interface trap density (Dit) metrology are outlined. These two methods are shown to be very consistent and reliable, thereby opening new ways of analyzing in situ state-of-the-art multi-gate FETs down to the few nanometer width limit. Furthermore, theoretical investigation of the spatial current density reveals volume inversion in thinner FinFETs near the threshold voltage.

Solid-phase epitaxy of undoped amorphous silicon by in-situ postannealing

The solid phase epitaxy (SPE) of undoped amorphous Si (a-Si) deposited on SiO2 patterned Si(001) wafers by reduced pressure chemical vapor deposition (RPCVD) using a H2–Si2H6 gas system was investigated. The SPE was performed by applying in-situ postannealing directly after deposition process. By transmission electron microscopy (TEM) and scanning electron microscopy, we studied the lateral SPE (L-SPE) length on sidewall and mask for various postannealing times, temperatures and a-Si thicknesses. We observed an increase in L-SPE growth for longer postannealing times, temperatures and larger Si thicknesses on mask. TEM defect studies revealed that by SPE crystallized epi-Si exhibits a higher defect density on the mask than at the inside of the mask window. By introducing SiO2-cap on the sample with 180nm Si thickness following postannealing at 570°C for 5 h, the crystallization of up to 450nm epi-Si from a-Si is achieved. We demonstrated the possibility to use this technique for SiGe:C heterojunction bipolar transistor (HBT) base layer stack to crystallize Si-buffer layer to widen the monocrystalline region around the bipolar window and to improve base link resistivity of the HBT.

Fabrication of discrete gallium nanoislands on the surface of a Si(001) substrate using afocused ion beam

A gallium focused ion beam (FIB) has been used to implant Ga at specificsites on the surface of undoped Si(001) substrates. Upon annealing at600 °C, discrete nanoscale surface islands form within the FIB patternedregions when the total Ga ion dose, or fluence, is greater than1.0 × 1016 ions cm − 2. The number of islands depends on the size of the irradiated regionand a single island can be achieved for a FIB milled region that is100 nm × 100 nm.The average sizes of the islands were found to range from 24.5 nm when exposed to a total ion dose of1.2 × 1016 ions cm − 2 to 45 nm fora dose of 3.0 × 1016 ions cm − 2. We have confirmed that these surface islands are metallic Ga by performing aselective chemical etch that removes the islands and by transmission electronmicroscopy characterization. These patterned Ga surface templates could serve asnucleation sites for the lateral arrangement of discrete quantum dot structures.

Dopant Modulated Li Insertion in Si for Battery Anodes: Theory and Experiment

We examine the effects of p-type and n-type dopants on the lithiation of crystalline Si as related to Li-ion batteries. In situ Raman spectroscopy and electrochemistry are used to investigate two crystallographic faces, (100) and (111), for boron (B) and phosphorus (P) dopants, to monitor the insertion of Li and the associated transition to amorphous Si. Density functional theory calculations are used to investigate the lithiation of doped and undoped crystalline Si bulk and surface models. The experimental and computational results suggest that lithiation voltages are different for P-doped and B-doped Si. The B-doped surfaces are found to insert Li at higher voltages than P- and undoped surfaces but result in less Li insertion. These results provide an understanding of the effects on dopants on the lithiation of silicon which may ultimately aid in the development of alternate anode materials for Li-ion batteries.

Size- and doping-dependent time-resolved photoluminescence of doped Si nanocrystals

Time-resolved photoluminescence (PL) has been studied for B- and Sb-dopedSi nanocrystals (NCs) fabricated by ion beam sputtering and annealing.For B-doped Si NCs, the PL intensity as well as the PL lifetime (τPL) increases asNC size (d) varies from 1.5 to 2.6 nm, similar to the case for undoped Si NCs, but with further increase ofd, they decrease, possibly resulting from the increase of optically less active NCswith the increase of NCs containing more dopants. The PL intensity andτPL monotonically decrease with increasing doping concentration (nD), irrespective of doping element. Si NCs show smallerτPL in B doping than in Sb doping over the full range ofnD. The sharp decrease in PL intensity, accompanied by the gradual decrease inτPL for thehigher nD of Sb, may be attributed to Auger recombination due to the presence of Sb inside Si NCs.The higher PL quench rate by Sb compared to B could be attributed to better ionization ofSb dopants in Si NCs.

Direct Measurement of Individual Deep Traps in Single Silicon Nanowires

The potential of the metal nanocatalyst to contaminate vapor-liquid-solid (VLS) grown semiconductor nanowires has been a long-standing concern, since the most common catalyst material, Au, is known to induce deep gap states in several semiconductors. Here we use Kelvin probe force microscopy to image individual deep acceptor type trapping centers in single undoped Si nanowires grown with an Au catalyst. The switching between occupied and empty trap states is reversibly controlled by the back-gate potential in a nanowire transistor. The trap energy level, i.e., E(C) - E(T) = 0.65 ± 0.1 eV was extracted and the concentration was estimated to be ∼2 × 10(16) cm(-3). The energy and concentration are consistent with traps resulting from the unintentional incorporation of Au atoms during the VLS growth.

25-nm Gate Length nMOSFET With Steep Channel Profiles Utilizing Carbon-Doped Silicon Layers (A P-Type Dopant Confinement Layer)

Steep channel profiles of scaled transistors are a promising approach for advancing transistor generation in bulk complementary metal-oxide-semiconductor (MOS). In this paper, a carbon-doped Si (Si:C) layer under an undoped Si layer is proposed to form steep p-type channel profiles in n-channel MOS field-effect transistors (nMOSFETs) due to extremely low diffusivity of boron and indium in Si:C layers. This structure with low channel impurity improves mobility and suppresses threshold voltage (VTH) variation. Both items are essential for aggressively scaled MOSFETs with a gate length less than 25 nm. We demonstrated well-controlled, high-performance, and low VTH variability nMOSFETs with a Si:C-Si epitaxial channel structure.

Interface Trap Density Metrology of State-of-the-Art Undoped Si n-FinFETs

The presence of interface states at the MOS interface is a well-known cause of device degradation. This is particularly true for ultrascaled FinFET geometries where the presence of a few traps can strongly influence the device behavior. Typical methods for interface trap density (D_it) measurements are not performed on ultimate devices but on custom-designed structures. We present the first set of methods that allow direct estimation of D_it in state-of-the-art FinFETs, addressing a critical industry need.

High rate performance of virus enabled 3D n-type Si anodes for lithium-ion batteries

A patterned 3D Si anode is fabricated by physical vapor deposition of n-type Si on a self-assembled TMV1cys-structured nickel current collector. The combination of the large surface area conferred by the virus-enabled 3D Ni/TMV1cys current collector with the high electric conductivity of n-type Si rods results in excellent cyclic stability and rate capability for the core-shell n-type Si/Ni/TMV1cys anodes. Electrochemical impedance spectroscopy reveals that the high electronic conductivity of n-type Si significantly reduces charge transfer resistance, thus even at high current densities the capacity of the n-type Si is increased to almost 630mAh/g compared to undoped Si.

Raman characterization of the structural evolution in amorphous and partially nanocrystalline hydrogenated silicon thin films prepared by PECVD

AbstractHydrogenated silicon (Si:H) thin films were obtained by plasma‐enhanced chemical vapor deposition (PECVD). Raman spectroscopy was used to investigate the structural evolution in phosphor‐doped n‐type amorphous hydrogenated silicon thin films, which were prepared under different substrate temperatures and gas pressures. Meanwhile, the effect of nitrogen doping on the structure of P‐doped thin films was also investigated by Raman spectroscopy. Moreover, the transition from the amorphous state to the nanocrystalline state of undoped Si:H films deposited through low argon dilution was studied by Raman spectroscopy, X‐ray diffraction, and transmission electron microscopy. The results show that Raman spectroscopy can sensitively detect the structural evolution in hydrogenated silicon thin films deposited under different conditions in a PECVD system. Copyright © 2010 John Wiley & Sons, Ltd.

X-ray photoelectron spectroscopy of Sm 3+-doped CaO–MgO–Al 2O 3–SiO 2 glasses and glass ceramics

Sm 3+ doped CaO–MgO–Al 2O 3–SiO 2 glass and glass ceramics have been prepared. The diopside crystal (CaMgSi 2O 6) was identified in the glass ceramics by X-ray diffraction analysis. X-ray photoelectron spectra of the glass and glass ceramics were measured by a monochromatised Al-Kα XPS instrument. Sm 3d core level spectra for the Sm doped samples showed that Sm ions are predominantly in the Sm (III) state in glass and glass ceramics. The O 1s core spectra could be fitted by summing the contributions from bridging oxygen (BO) and non bridging oxygen (NBO) for samarium undoped glass, BO, NBO and Si–O–Sm for the doped glass. The O 1s XPS spectrum of undoped glass ceramics was curve fitted with BO and NBO in glass phase, as well as SiOSi, SiOMg and SiOCa in diopside. In addition to the five components above mentioned, SiOSm in diopside also appeared in O 1s XPS spectra of samarium doped glass ceramics. According to the fitting results, we demonstrate that the Sm 2O 3 exist in glass network as a glass modifier. After heat treatment, nearly all the Sm 3+ existed in diopside phase as the substitution for Ca 2+.

Growth of amorphous silica nanowires using nickel silicide catalyst by a thermal annealing process

We report on the growth of NiSi 2-catalyzed amorphous SiO 2 nanowires by rapid-thermal-annealing of Ni(40 nm)/poly-Si(60 nm)/SiO 2(110 nm)/undoped Si substrate structures at 900 °C in N 2 ambient. The diameter of the nanowires is dependent on the diameter of the NiSi 2 catalyst particles; the former is about 16–45% smaller than the later. Considering the presence of the nanoparticles located at the tip of the nanowires, the growth behavior of the a-SiO 2 nanowires is described in terms of the generation of SiO vapor and the VLS mechanism.

Temperature-dependent photoluminescence of arsenic-doped Si nanocrystals

The characteristics of temperature-dependent photoluminescence (PL) from Si nanocrystals and effects of arsenic-doping (As-doping) were investigated. The Si nanocrystals on a p-type Si substrate were prepared by low pressure chemical vapor deposition and post-deposition thermal oxidation. The As-doping process was carried out using the gas-phase-doping technique. Temperature-dependent PL from Si nanocrystals exhibited considerable differences between samples with/without As-doping. Phase transition between electron–hole liquid and free exciton was observed in the undoped Si nanocrystals, leading to the increase in PL intensity with temperature less than 50 K. Electron emission from As-doped Si nanocrystals to the p-Si substrate was responsible for the significant increase in PL intensity with temperature greater than 50 K. Characteristics of light emission from Si nanocrystals will facilitate the development of silicon-based nanoscaled light-emitting devices.

Size shift of XPS lines observed from PbS nanocrystals

AbstractCrystals of PbS with different sizes were obtained by chemical deposition from aqueous solution (chemical bath deposition technique) onto undoped Si (100) substrate. The morphology and size of crystals were analyzed by high resolution scanning electron microscopy. The fine structure of Pb 4f and S 2p photoelectron lines was measured by X‐ray photoelectron spectroscopy with monochromatic Al Kα source (hν = 1486.6 eV). The phenomenon of size shift, i.e. growth of binding energies for both donor Pb 4f and acceptor S 2p photoelectron lines with decreasing nanocrystals' size, was found and quantified as a function of nanocrystals' size. The size shift values of the Pb 4f5/2 and 4f7/2 lines were determined for nanocrystals with a size from 20 to 250 nm. The interband and intraband electronic transitions were investigated by high resolution electron energy losses spectroscopy. It was shown that width and spatial anisotropy of the band gap depend on the size of PbS nanocrystals. The dependence of size shift from nanocrystals' size, the nature of this phenomenon, and a role of changes in fine structure of electronic states near the band gap are discussed in the report. Copyright © 2010 John Wiley & Sons, Ltd.

Structural evolution and epitaxial stabilisation of pulsed laser deposited Sm 0.55Nd 0.45NiO 3 solid solution nanostructured films on undoped Si (1 0 0) and NdGaO 3 substrates

We report on the effects of substrate, ambient oxygen pressure and deposition time on the crystal structure, and morphology of Sm 0.55Nd 0.45NiO 3 solid solution nanostructured films synthesized by pulsed-laser deposition. In each film the structure was found to be consistent with a perovskite structure with preferential planes growth and reveals a strong orientation along the orthorhombic (2 1 0) plane of the perovskite subcell for the film deposited on NdGaO 3 where highly crystalline films were obtained within 15 min deposition time with a low surface roughness of 8.79 nm. Similar structure is observed on Si (1 0 0) substrate only at O 2 pressure of 0.4 mbar. The surface morphology of the different samples shows a net dense film structure with several droplets population. The nano-scaled droplets are in general spherical in shape; a detailed analysis indicates that the laser ablation of this nickelate family is governed to a certain extent by a heat transfer phenomenon.

Effect of target–substrate separation on HgCdTe films formed by pulsed laser deposition

HgCdTe thin films were deposited on Si (1 1 1) substrates at different target–substrate separations by pulsed laser deposition (PLD). The results show that the kinetic energy of the incident particles, and the thickness, crystallinity, elemental composition and surface morphology of the HgCdTe thin films are affected by the change of the target–substrate separation. The HgCdTe films prepared using Nd:YAG laser show cubic phase. The films prepared by PLD on the undoped Si substrates can form SiTe 4 between the HgCdTe film and the Si substrate at small target–substrate separation distance. The formation of interfacial Si–Te may affect the quality of PLD HgCdTe thin films on Si substrates.