Depth Profiles Of Si Research Articles

Silicon diffusion in AlN

In this study, we investigate the diffusion of Si donors in AlN. Amorphous Si1−xNx sputtered on the surface of bulk AlN with low dislocation density is used as a Si source. The diffusion experiments are conducted through isochronal and isothermal annealing in a protective N2 atmosphere at temperatures between 1500 and 1700 °C. The Si depth profiles measured by secondary ion mass spectrometry exhibit a convex box-like shape with a steep diffusion front. These concentration profiles are best described with a diffusion coefficient that depends on the square of local Si concentration. From the characteristic box-shaped Si profiles, we conclude that diffusion of Si in AlN is mediated by singly negatively charged dopant–vacancy pairs SiAlVAl−. The strong concentration dependence of Si diffusion is due to the electric field associated with the incorporation of Si donors (SiAl+1) on substitutional Al lattice sites and reflects that Si is fully electrically active at diffusion temperature. The experimentally obtained extrinsic Si diffusion coefficient is reduced to intrinsic doping conditions. The temperature dependence of Si diffusion for intrinsic conditions is described by an activation enthalpy of (10.34±0.32)eV and a pre-exponential factor of 235−203+1485cm2s−1. The migration enthalpy of the donor–vacancy pair SiAlVAl− is estimated to be around 3.5 eV. This estimation is based on the activation enthalpy of the transport capacity of SiAlVAl− and theoretical results concerning the formation energy of negatively charged vacancies on Al-sites in AlN.

28Si(p,p′γ)28Si nuclear reaction in the detection and depth profiling of Si in materials

A proton induced γ-ray emission method for depth profiling Si in the surface regions of materials is described. The method utilizes the resonance at 3098 keV in 28Si(p,p′γ)28Si nuclear reaction and involves the measurement of the characteristic 1778 keV γ-rays. The yield curves of the γ-rays are constructed in 3.0–3.8 MeV proton energy region using different Si targets to determine the analytical capabilities of the resonance. The resonance has a width of ∼12 keV (depth resolution: ∼600 nm in Si), a large probing depth (∼11 μm in Si) and a detection sensitivity of ∼0.7 at.%. A procedure for quantification based on Breit-Wigner formula that encompasses corrections for off-resonance cross-sections is also outlined. Depth profiling of Si in silicon carbide crystals and also in a silicon nitride film with <10% combined uncertainty shows that this method is well suited for probing Si in silicon bearing materials.

Study of interface reaction in a B4C/Cr mirror at elevated temperature using soft X-ray reflectivity.

Boron carbide is a prominent material for high-brilliance synchrotron optics as it remains stable up to very high temperatures. The present study shows a significant change taking place at 550°C in the buried interface region formed between the Cr adhesive layer and the native oxide layer present on the silicon substrate. An in situ annealing study is carried out at the Indus-1 Reflectivity beamline from room temperature to 550°C (100°C steps). The studied sample is a mirror-like boron carbide thin film of 400 Å thickness deposited with an adhesive layer of 20 Å Cr on a silicon substrate. The corresponding changes in the film structure are recorded using angle-dependent soft X-ray reflectivity measurements carried out in the region of the boron K-edge after each annealing temperature. Analyses performed using the Parratt recursive formalism reveal that the top boron carbide layer remains intact but interface reactions take place in the buried Cr-SiO2 region. After 300°C the Cr layer diffuses towards the substrate. At higher temperatures of 500°C and 550°C the Cr reacts with the native oxide layer and tends to form a low-density compound of chromium oxysilicide (CrSiOx). Depth profiling of Si and Cr distributions obtained from secondary ion mass spectroscopy measurements corroborate the layer model obtained from the soft X-ray reflectivity analyses. Details of the interface reaction taking place near the substrate region of boron carbide/Cr sample are discussed.

Chemical and morphological characterization of photoactive SiOx films electrodeposited on Pt substrate

SiOx films electrodeposited on platinum (Pt) substrate were shown to exhibit n-type photoactivity while placed both in aqueous and organic solutions. The films were obtained via potentiostatic deposition at potential values ranging from −2.25 to −2.75V. The mechanism of the electrodeposition involved reactions with the used electrolyte and with traces of water as sources of oxygen and hydrogen, as interpreted from Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The photoactivity of the deposits deceased at potential −2.75V. The highest observed photocurrent was ca. 60μA/cm2 and it was registered in organic solution. The band gap energies determined by XPS technique were approximately 2.5, 2.3 and 3.7eV for films electrodeposited at −2.25, −2.5 and −2.75V, respectively. The depth profiles of Si, O and H in the films were registered by glow discharge optical emission spectroscopy (GD-OES), which showed that the film thickness was ca. 0.4μm. The n-type photoactivity was associated with oxygen evolution from the aqueous solution (0.1M HClO4, pH1), whereas the time instability as a function of time observed for the photocurrent was associated with morphological changes of the films as seen by scanning electron microscopy (SEM) and their oxidation detected by XPS and FTIR spectroscopies.

Spectroscopic characterization and photoactivity of SiO x -based films electrochemically grown on Cu surfaces

Electrodeposited SiO x electrodes were shown to be photoactive and exhibit n- and p-type effects for electrodes placed in aqueous and organic solutions, respectively. As seen by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron (XPS) spectroscopy, the mechanism of the electrodeposition included reactions with the used electrolyte as well as with traces of water as sources of oxygen and hydrogen. The lowest band gap energy (E g) of the films of approximately 1.6 eV was observed for the film electrodeposited at −2.5 V in comparison to 1.9 eV for the films obtained at −2.25 and −2.75 V. The depth profiles of Si and O in the films were registered by XPS, secondary ion mass spectrometry (SIMS), and glow discharge optical emission spectroscopy (GD-OES), which showed that Si and O were relatively uniformly distributed across the entire layer of the film. The n-type photoactivity was associated with the evolution of oxygen from the aqueous solution, and the p-type was attributed to the reductive deterioration of the amorphous SiO x deposit and simultaneous photodecomposition of the electrolyte.

Suppression effect of silicon (Si) on Er3+ 1.54μm excitation in ZnO thin films

We have investigated the photoluminescence (PL) characteristics of ZnO:Er thin films on Si (100) single crystal and SiO2-on-silicon (SiO2) substrates, synthesized by radio frequency magnetron sputtering. Rutherford backscattering/channeling spectrometry (RBS), X-ray diffraction (XRD) and atomic force microscope (AFM) were used to analyze the properties of thin films. The diffusion depth profiles of Si were determined by second ion mass spectrometry (SIMS). Infrared spectra were obtained from the spectrometer and related instruments. Compared with the results at room temperature (RT), PL (1.54μm) intensity increased when samples were annealed at 250°C and decreased when at 550°C. A new peak at 1.15μm from silicon (Si) appeared in 550°C samples. The Si dopants in ZnO film, either through the diffusion of Si from the substrate or ambient, directly absorbed the energy of pumping light and resulted in the suppression of Er3+ 1.54μm excitation. Furthermore, the energy transmission efficiency between Si and Er3+ was very low when compared with silicon nanocrystal (Si-NC). Both made the PL (1.54μm) intensity decrease. All the data in experiments proved the negative effects of Si dopants on PL at 1.54μm. And further research is going on.

Effective work function control of metal inserted poly-Si electrodes on HfSiO dielectrics by in-situ oxygen treatment of metal surface

We fabricated novel poly-Si/TiN/HfSiO/SiO2 gate stacks by using an in-situ oxygen treatment process of a TiN electrode surface in order to suppress Si diffusion from poly-Si upper layer during post deposition annealing (PDA). The Si depth profile after PDA showed that no Si diffused into oxidized stacks due to the formation of TiON layers as a Si diffusion barrier. Furthermore, a high effective work function (EWF) of 4.94 eV was obtained from the oxidized stack even after PDA at 1000 °C. This high EWF is due not only to Si diffusion barrier formation but also to oxygen vacancy compensation by diffused-oxygen from the TiON layer. However, we observed significant growth of interfacial SiO2 after high temperature annealing. These results indicate a trade-off relationship between EWF control and equivalent oxide thickness (EOT) scaling, and imply that an additional method for EWF modulation is required for scaled high-k devices.

Impact of Si Diffusion Barrier Layer Formed on TiN Surface by In Situ Oxygen Treatment Process for Advanced Gate-First Metal/High-k Stacks

We investigated the formation of Si barrier layer by an in-situ oxygen treatment process of TiN electrode surface before poly-Si deposition. The Si depth profile after post deposition annealing (PDA) showed that no Si diffused into oxidized stacks due to the formation of TiON layers as a Si-diffusion barrier. Moreover, a high effective work function (EWF) of 4.94 eV was obtained from the oxidized stack even after PDA at 1000oC. However, we observed significant growth of interfacial SiO2 after high temperature annealing. These results indicate a trade-off relationship between EWF control and equivalent oxide thickness (EOT) scaling, and imply that an additional method for EWF modulation is required for scaled high-k devices.

Tree-mycorrhiza symbiosis accelerate mineral weathering: Evidences from nanometer-scale elemental fluxes at the hypha–mineral interface

In soils, mycorrhiza (microscopic fungal hypha) living in symbiosis with plant roots are the biological interface by which plants obtain, from rocks and organic matter, the nutrients necessary for their growth and maintenance. Despite their central role in soils, the mechanism and kinetics of mineral alteration by mycorrhiza are poorly constrained quantitatively. Here, we report in situ quantification of weathering rates from a mineral substrate, (0 0 1) basal plane of biotite, by a surface-bound hypha of Paxillus involutus, grown in association with the root system of a Scots pine, Pinus sylvestris. Four thin-sections were extracted by focused ion beam (FIB) milling along a single hypha grown over the biotite surface. Depth-profile of Si, O, K, Mg, Fe and Al concentrations were performed at the hypha–biotite interface by scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX). Large removals of K (50–65%), Mg (55–75%), Fe (80–85%) and Al (75–85%) were observed in the topmost 40 nm of biotite underneath the hypha while Si and O are preserved throughout the depth-profile. A quantitative model of alteration at the hypha-scale was developed based on solid-state diffusion fluxes of elements into the hypha and the break-down/mineralogical re-arrangement of biotite. A strong acidification was also observed with hypha bound to the biotite surface reaching pH < 4.6. When consistently compared with the abiotic biotite dissolution, we conclude that the surface-bound mycorrhiza accelerate the biotite alteration kinetics between pH 3.5 and 5.8 to ∼0.04 μmol biotite m −2 h −1. Our current work reaffirms that fungal mineral alteration is a process that combines our previously documented bio-mechanical forcing with the μm-scale acidification mediated by surface-bound hypha and a subsequent chemical element removal due to the fungal action. As such, our study presents a first kinetic framework for mycorrhizal alteration at the hypha-scale under close-to-natural experimental conditions.

Theoretical Studies for Si and C Emission into SiC Layer during Oxidation

To understand the structure of SiC–oxide interface more in detail, we propose a profiling theory of Si and C emission into SiC layer during oxidation. Simulations of the depth profiles of Si and C interstitials results in the structures analogous with those observed from a spectroscopic ellipsometry. To determine the diffusivities of Si and C interstitials, we performed capacitance–voltage measurements for examining the re-distribution profiles of nitrogen after oxidation and compared between observed and calculated profile. The calculated nitrogen profiles showed good fits to the observed ones in the case of self-diffusivity of C interstitials magnified by several 10 times for literature value. Finally, we discuss the validity of the proposed theory.

Model Calculations of SiC Oxide Growth Rate at Various Oxidation Temperatures Based on the Silicon and Carbon Emission Model

We have tried to apply the oxidation model of SiC proposed previously, termed ‘Si-C emission model’, to the oxide growth rate at various oxidation temperatures. We have found that the model well reproduces the oxide thickness dependences of oxide growth rates for all of the temperatures measured for both of the SiC Si- and C-faces. We have estimated the temperature dependence of oxide depth profiles of Si and C interstitials by using the Si-C emission model, and discussed the structure at/near the SiC–oxide interface.

Model Calculation of SiC Oxide Growth Rate Based on the Silicon and Carbon Emission Model

We proposed a kinetic model for SiC oxidation, named ‘silicon and carbon (Si-C) emission model’, taking into account the emission of Si and C atoms from the SiC–oxide interface, which suppresses the oxidation rate at the interface. Based on the model, we calculated oxide growth rates for SiC (0001) Si- and (000–1) C-face and found that the calculated values exhibit good fits to the measured ones in the entire oxide thickness range for both faces. We also calculated depth profiles of Si and C interstitials and oxidants, and discussed the oxidation mechanism of SiC as well as the difference in the oxidation process of Si-face and C-face.

PL and XPS depth profiling of Si/Al 2O 3 co-sputtered films and evidence of the formation of silicon nanocrystals

We have studied the correlation between light emission and structural properties of silicon nanocrystals formed in Al 2O 3 matrix by co-sputtering followed by an annealing in the temperature range of 900–1100 °C. X-ray diffraction (XRD) experiments revealed the nanocrystal formation which is a function of Si sputtering power and annealing temperature. The chemical structure and atomic compositions of Si, Al, O and their variations with depth from the sample surface have been investigated by X-ray photoelectron spectroscopy (XPS). Formation of Si nanocrystals is identified from the presence of Si–Si bonds whose strength increases with the annealing temperature. However, Si atoms resting in the region close to the surface of the sample is found to be oxidized. Photoluminescence (PL) spectroscopy is employed as a function of Si power, annealing temperature and sample depth. The origin of various PL peaks is discussed in terms of defect formation in Al 2O 3, emission from nanocrystals and interfaces. Defect related peaks are attributed to Cr 3+, Ti 3+ and F centers which are commonly observed in Al 2O 3. The emission from the Si nanocrystals is clearly identified in the wavelength range of 700–900 nm. The variation of this emission with the depth is in good correlation with the depth profile of Si–Si bonds as determined by the XPS analysis.

Interlayer analysis of HfO 2/SiO 2/Si by SIMS and HRBS

The thickness of an interlayer between HfO 2 and Si substrate was estimated by SIMS and high-resolution Rutherford backscattering spectrometry (HRBS). The model sample that changes the interlayer thickness was prepared to find an appropriate measurement condition of SIMS. The thickness of the interlayer can be estimated by the depth profile of Si − measured by 250 eV Cs + primary ion and incidence angle of 45°. The thickness of the interlayer was also estimated by HRBS spectrum measured by 450 keV He + and the scattering angle of 52°. The thickness of the SiO 2 layer estimated by SIMS and HRBS shows a good linear correlation.

Electrical Activation and Carrier Compensation in Si and Mg Implanted GaN by Scanning Capacitance Microscopy

We studied the effect of the annealing temperatures (in the 1100-1200°C temperature range) on the electrical activation of ion implanted Si and Mg in GaN. Si + ions were implanted at multiple energies (from 80keV to 180keV) and with a total fluence up to 27x10 14 cm -2 in heteroepitaxial GaN films on sapphire. Some samples were subjected to conventional low ramp rate processes at 1100°C and 1200°C in N 2 ambient. A fast ramp rate pre-annealing in N 2 ambient was carried out on other samples before those low-ramp rate processes at 1100°C and 1200°C. The calibrated Scanning Capacitance Microscopy (SCM) was applied to determine the active (i.e. substitutional) Si depth profile. The rapid pre-annealing process leads to an increase from 36% to 63% in the substitutional Si dose. The Mg ions were implanted with energy of 50keV and fluence of 5x10 14 cm -2 in n' GaN layer on sapphire. Two fast ramp rate annealing process were performed. However, only an annealing at 1200°C for 30s in N 2 allows to observe, by means of SCM, the formation of a p layer.

Molecular beam epitaxy and doping of AlN at high growth temperatures

The growth of AlN on c-plane sapphire substrates is reported for a growth temperature of 1100 °C in a radio frequency plasma assisted molecular beam epitaxy system. The high growth temperature in combination with Al-rich growth conditions resulted in a high crystalline quality (FWHM of the 0002 x-ray reflection of 650″) and low O incorporation. Furthermore, the incorporation of Si and Mg as n- and p-type dopants, respectively, has been studied for these growth conditions. For Si doping the corresponding cell temperature was varied between 1300 and 1350 °C. Secondary ion mass spectrometry (SIMS) showed a homogeneous Si depth profile up to a concentration of 7 × 1020 cm−3, but the Si doped layers remained highly resistive. Incorporation of Mg was observed only at a low growth temperature of 830 °C; at higher growth temperatures SIMS revealed a strong surface segregation effect while the amount of Mg incorporated into the AlN layer remained below the detection limit. The build-up of a Mg accumulation layer at the growth surface due to segregation was found to cause a significant reduction in growth rate.

ALD Deposited Al&lt;sub&gt;2&lt;/sub&gt; O&lt;sub&gt;3&lt;/sub&gt; Films on 6H-SiC(0001) after Annealing in Hydrogen Atmosphere

Atomic Layer Deposited Al2O3 films on hydrogen-terminated 6H-SiC(0001) were annealed in hydrogen atmosphere and characterized by admittance spectroscopy measurement and photoelectron spectroscopy (PES). The resultant density of interface trap (Dit) from admittance spectroscopy measurement is reduced near mid gap, but increases strongly towards the conduction band edge. Systematic PES measurements show that hydrogen annealing introduces Si4+ as a new component besides Si0 and Si+. Using different electron escape depths for photon electrons, depth profiling of Si in its different oxidation states was performed. The result indicates the formation of a top SiO2 layer and a rougher interfacial layer containing more Si+ and Si4+ which could be responsible for the strong increase of Dit just below the conduction band edge.

Deconvolution analysis of dopant depth profile of Si at AlGaAs/GaAs interface using Al composition profile as reference

A novel method of deconvolution without using a reference sample was investigated for dopant depth profiling of Si in the AlGaAs/GaAs epilayers. The true Si distribution was numerically analyzed by deconvolution using a depth resolution function (DRF) obtained from an Al composition profile. The composition analysis by thickness fringe-transmission electron microscopy (CAT-TEM) revealed that the Al composition changed rapidly at the AlGaAs/GaAs interface and followed an ideal step function. The DRF of Al was, thus, derived by differentiating the measured Al profile. Deconvolution of the measured Si profile was performed by least-squares fitting of the convolved Si profile with the measured profile, under the assumption that a true Si distribution was in a Gaussian function. The variance σ of the true distribution was found to be 1.9 nm after deconvolution. Simultaneously, decay length of the Si profile decreased by almost one-third compared to that before deconvolution. The deconvolved Si profile agreed well with the carrier profile of a high electron mobility transistor (HEMT).

Deconvolution analysis of dopant depth profile of Si at AlGaAs/GaAs interface using Al composition profile as reference

Deconvolution analysis of dopant depth profile of Si at AlGaAs/GaAs interface using Al composition profile as reference

Nanocrystal formation in Si implanted thin SiO 2 layers under the influence of an absorbing interface

Kinetic 3D lattice Monte Carlo studies are presented on Si nanocrystal (NC) formation by phase separation in 1 keV Si + implanted thin SiO 2 films. The simulation start from Si depth profiles calculated using the dynamic, high-fluence binary collision code TRIDYN. From the initial Si supersaturation, NCs are found to form either by nucleation, growth and Ostwald ripening at low Si concentrations. Or at higher concentrations, non-spherical, elongated Si structures form by spinodal decomposition, which spheroidize by interface minimization during longer annealing. In both cases, the close SiO 2/Si interface is a strong sink for diffusing Si atoms. The NCs align above a thin NC free oxide layer at the SiO 2/Si interface. Hence, the width of this zone denuded of NCs has just the right thickness for NC charging by direct electron tunneling, which is crucial for non-volatile memory applications. Moreover, the competition of Ostwald ripening and Si loss to the interface leads at low Si concentrations (nucleation regime) to a constant width of the denuded zone and a constant mean NC size over a long period of annealing.