Deposition of device quality silicon nitride with ultra high deposition rate (> 7 nm/s) using hot-wire CVD

The application of hot-wire (HW) CVD deposited silicon nitride (SiN_x) as passivating anti-reflection coating on multicrystalline silicon (mc-Si) solar cells is investigated. The highest efficiency reached is 15.7% for SiN_x layers with an N/Si ratio of 1.20 and a high mass density of 2.9 g/cm³. These cell efficiencies are comparable to the reference cells with optimized plasma enhanced (PE) CVD SiN_x even though a very high deposition rate of 3 nm/s is used. Layer characterization showed that the N/Si ratio in the layers determines the structure of the deposited films. And since the volume concentration of Si-atoms in the deposited films is found to be independent of the N/Si ratio the structure of the films is determined by the quantity of incorporated nitrogen. It is found that the process pressure greatly enhances the efficiency of the ammonia decomposition, presumably caused by the higher partial pressure of atomic hydrogen. With this knowledge we increased the deposition rate to a very high 7 nm/s for device quality SiN_x films, much faster than commercial deposition techniques offer [S. von Aichberger, Photon Int. 3 (2004) 40].     

SiNx/a-SiCx:H passivation layers for p- and n-type crystalline silicon wafers

In this work we present a detailed investigation of Si surface passivation obtained by a PECVD double dielectric layer, composed of intrinsic hydrogenated amorphous silicon-carbon (a-SiC_x:H), followed by a silicon nitride (SiN_x). The double layers have been deposited on p- and n-type of mono- and multi-crystalline silicon wafers. IR spectra have been carried out to evaluate the structure of a-SiC_x:H layers on monocrystalline wafers. The passivation effects have been studied performing the following measurements: the photoconductance decay, to measure contactlessly the effective lifetime of passived mono and multi Si wafers; the capacitance voltage profile of Al/SiN_x/Si, Al/a-SiC_x:H/Si and Al/SiN_x/a-SiC_x:H/Si MIS structures, to estimate the field effect at the dielectric/silicon interface and individuate the passivation mechanism on silicon surfaces. It has been found that the mechanism of the surface passivation depends on the doping type of the silicon wafer. Indeed from C-V measurements it has been realized that the great amount of positive charge within the SiN_x is able to promote an inversion layer if it is deposited on a-SiC_x:H/Si p-type and an accumulation if it is grown on a-SiC_x:H/Si n-type.     

Highly Porous Silicon Membranes Fabricated from Silicon Nitride/Silicon Stacks

Nanopore formation in silicon films has previously been demonstrated using rapid thermal crystallization of ultrathin (15 nm) amorphous Si films sandwiched between nm‐thick SiO₂ layers. In this work, the silicon dioxide barrier layers are replaced with silicon nitride, resulting in nanoporous silicon films with unprecedented pore density and novel morphology. Four different thin film stack systems including silicon nitride/silicon/silicon nitride (NSN), silicon dioxide/silicon/silicon nitride (OSN), silicon nitride/silicon/silicon dioxide (NSO), and silicon dioxide/silicon/silicon dioxide (OSO) are tested under different annealing temperatures. Generally the pore size, pore density, and porosity positively correlate with the annealing temperature for all four systems. The NSN system yields substantially higher porosity and pore density than the OSO system, with the OSN and NSO stack characteristics fallings between these extremes. The higher porosity of the Si membrane in the NSN stack is primarily due to the pore formation enhancement in the Si film. It is hypothesized that this could result from the interfacial energy difference between the silicon/silicon nitride and silicon/silicon dioxide, which influences the Si crystallization process.     

气体流量比对反应溅射Si3N4薄膜的影响

利用射频磁控反应溅射法，以高纯Si为靶材，高纯N2气为反应气体，在Si衬底上制备出了Si3N4薄膜，研究了气体流量比对薄膜质量的影响。结果表明，薄膜的沉积速率主要与气体的流量比有关，随着气体流量比的增加，沉积速率下降，靶面的溅射由金属模式过渡到氮化物模式；薄膜中N／Si的原子比增加；红外吸收谱的Si—N键的振动峰向标准峰逼近。     

Electrical and structural properties of TaSiN electrode for phase change random access memory

Characteristics of tantalum silicon nitride (TaSiN) thin films have been investigated as an electrode material for Ge₂Sb₂Te₅ chalcogenide phase change material. The films were deposited by co-sputtering system in which the ratio of tantalum nitride to silicon was controlled by the plasma power on each target. The TaSiN films showed tunable resistivity from 260 to 560 μΩ cm with increasing Si content. From the evaluation of PRAM cell structures consisting of the TaSiN and the Ge₂Sb₂Te₅, we found that the SET voltages are nicely correlated with the resistivity of the TaSiN. Moreover, the sensing margin (resistance ratio: R_SET/R_RESET) turned out to be good for practical application.     

Nitridation enhancing effect of ZrO2 on silicon powder

The nitridation behavior of silicon powder with added Zr compounds was studied in order to assess the catalytic effect of zirconium on the formation of reaction bonded silicon nitride, using high purity silicon powder and monoclinic zirconia as starting materials. Thermogravimetric analysis revealed that the addition of ZrO₂ to Si powder reduced the temperature of the main nitridation reaction, and increased the amount of silicon converted to silicon nitride at a given temperature. On the other hand, the nitridation rate at higher temperatures (1380-1400 °C) indicated similar values for both pure Si and Si with ZrO₂ additions.     

Quantitative Auger electron spectroscopy of SiC

The article is devoted to quantitative Auger electron spectroscopy (AES) of silicon carbide (SiC). It compares the results obtained by various procedures applied to LVV and KLL Auger peaks of silicon (Si) for determining the Auger current. We have found that reliable results for the content of Si in SiC can be achieved by the method in which the Auger current is determined as the area below the Auger peak in the direct N(E) spectrum after subtracting the background of inelastically scattered Auger electrons of the given peak and of the so-called true secondary electrons. The achieved results have also shown that bombardment of SiC by Ar⁺ ions with energy 1 keV at an angle of 68° with respect to the surface normal while the sample rotates at 6 rpm leads to Si depletion of SiC in comparison with its stoichiometric composition.     

RF PE CVD deposition of amorphous a-SixNy:H layers for application in solar cells

The most promising technologies of silicon nitride hydrogenated layers for application in solar cells are based on plasma-chemical methods. In this paper, results concerning radio-frequency plasma-enhanced chemical vapour deposition (RF PE CVD) method applied in the technology of a-Si_xN_y:H layers on glass and polycrystalline silicon are presented. For the purpose of this research the series of the samples have been deposited and subjected to optical and structural studies. The layers have been grown from silane and ammonia used as gaseous precursors. A special attention has been paid to the choice of a gas flow ratio [NH₃]/[SiH₄]. The results show that it has a significant influence on the atomic-level structure, chemical composition and optical properties of the layers.     

Structure analysis of aluminium silicon manganese nitride precipitates formed in grain-oriented electrical steels

We report a detailed structural and chemical characterisation of aluminium silicon manganese nitrides that act as grain growth inhibitors in industrially processed grain-oriented (GO) electrical steels. The compounds are characterised using energy dispersive X-ray spectrometry (EDX) and energy filtered transmission electron microscopy (EFTEM), while their crystal structures are analysed using X-ray diffraction (XRD) and TEM in electron diffraction (ED), dark-field, high-resolution and automated crystallographic orientation mapping (ACOM) modes. The chemical bonding character is determined using electron energy loss spectroscopy (EELS). Despite the wide variation in composition, all the precipitates exhibit a hexagonal close-packed (h.c.p.) crystal structure and lattice parameters of aluminium nitride. The EDX measurement of ~ 900 stoichiometrically different precipitates indicates intermediate structures between pure aluminium nitride and pure silicon manganese nitride, with a constant Si/Mn atomic ratio of ~ 4. It is demonstrated that aluminium and silicon are interchangeably precipitated with the same local arrangement, while both Mn^2 + and Mn^3 + are incorporated in the h.c.p. silicon nitride interstitial sites. The oxidation of the silicon manganese nitrides most likely originates from the incorporation of oxygen during the decarburisation annealing process, thus creating extended planar defects such as stacking faults and inversion domain boundaries. The chemical composition of the inhibitors may be written as (AlN)_x(SiMn_0.25N_yO_z)_1 − x with x ranging from 0 to 1.     

The microstructures of silicon nitride ceramics during hot-pressing transformations

The microstructure of silicon nitride hot-pressed with a magnesium oxide additive has been studied by transmission electron microscopy. This includes material at various stages in a hot-pressing process: the initial (～ 90%α) silicon nitride powder; specimens partially densified and partially transformed from α-silicon “nitride” (Si_11.5N₁₅O_0.5) to β-silicon nitride (Si₃N₄); and almost fully dense and fully transformed β-Si₃N₄. The observations substantiate a solid/liquid/solid transformation mechanism, whereby Si and N are transported from α grains through a silicate liquid phase to nucleation sites for β at α/liquid interfaces or to β grains nucleated homogeneously in the liquid phase.     

Nitrogen ion induced nitridation of Si(111) surface: Energy and fluence dependence

We present the surface modification of Si(111) into silicon nitride by exposure to energetic N₂⁺ ions. In-situ UHV experiments have been performed to optimize the energy and fluence of the N₂⁺ ions to form silicon nitride at room temperature (RT) and characterized in-situ by X-ray photoelectron spectroscopy. We have used N₂⁺ ion beams in the energy range of 0.2–5.0 keV of different fluence to induce surface reactions, which lead to the formation of Si_xN_y on the Si(111) surface. The XPS core level spectra of Si(2p) and N(1s) have been deconvoluted into different oxidation states to extract qualitative information, while survey scans have been used for quantifying of the silicon nitride formation, valence band spectra show that as the N₂⁺ ion fluence increases, there is an increase in the band gap. The secondary electron emission spectra region of photoemission is used to evaluate the change in the work function during the nitridation process. The results show that surface nitridation initially increases rapidly with ion fluence and then saturates.     

Properties of silicon nitride films prepared by plasma-enhanced chemical vapour deposition of SiH4-N2 mixtures

The refractive indices and IR absorption spectra are measured for silicon nitride films plasma deposited from SiH₄-N₂-H₂ gas mixtures. The composition of the film (N:Si ratio) is derived from the value of the refractive index and the concentration of bonded hydrogen as Si-H and N-H in the film is estimated from the absorption intensities in the IR spectrum. The optimum deposition conditions for giving excellent insulating silicon nitride films are confirmed to be same as the conditions for giving films with stoichoimetric composition and the lowest amount of incorporated hydrogen.     

Interface instability of r.f. sputtered silicon nitride films on silicon

Silicon nitride films have been deposited on sputter-etched silicon by r.f. sputtering. The quality of the sputtered nitride films was demonstrated by infrared spectrum analysis and measurement of the refractive index and dielectric constants. The interface instability of the Si₃N₄Si structure was investigated by MNS (metal-nitride-silicon) techniques. The C-V measurements revealed that Si injection (injection and/or extraction from the silicon side) is the dominating factor causing trapping instability in the MNS devices and that the application of positive or negative bias voltages is equally effective in producing Si injection. Little ion movement or polarization effect was observed under bias-and-temperature stresses. C-V traces were also plotted at liquid nitrogen temperature and the effect of Si injection was found to be the same as at room temperature. The commonly used tunneling model is employed to account for the temperature-insensitive instability at the Si₃N₄Si interface.     

Electronic properties of silicon nanocrystallites obtained by SiOx (x<2) annealing

Nanocrystal-based devices are possible candidates for future electronics. In this context, we have studied the electronic properties of Si nanocrystals (nc-Si) embedded in a SiO₂ matrix. This work is devoted to the characterization of nc-Si by means of morphological, optical and electrical techniques.SiO_x (x<2) layers are deposited by low-pressure chemical vapor deposition (LPCVD). Morphological measurements have shown that the as-deposited layers are homogeneous and that thermal annealing induces a precipitation of the excess silicon into nanocrystallites. Photoluminescence (PL) measurements show a large emission spectrum around 1.5 eV in agreement with the literature results for confined levels in 5-nm Si dots.Current–voltage (I–V) measurements recorded at different temperatures on metal-oxide-semiconductor (MOS) capacitors with nc-Si are analysed in terms of structural and electronic modifications induced by the annealing. The as-deposited SiO_x layers show a Poole–Frenkel conduction behavior. In annealed samples, it appears that the current follows a hopping transport mechanism. This is understood as a direct tunneling from dots to dots.Finally, it is shown that the annealed SiO_x is able to store carriers and is therefore a good candidate for non-volatile memory applications. Charging curves are presented and discussed with a model previously validated on MOS structures with silicon dots obtained by pure silane deposition.     

Fabrication of epitaxial III-nitride cantilevers on silicon (1 1 1) substrates

The molecular beam epitaxy of AlGaN/GaN epilayers on silicon (1 1 1) using an aluminum nitride buffer layer, and subsequent fabrication of free standing III-nitride cantilevers on Si(1 1 1) has been investigated. Transmission electron microscopy (TEM) of cross-section samples reveals a columnar structure consisting of the hexagonal gallium nitride polytype. Selected area diffraction indicates an epitaxial relationship between the gallium nitride and silicon substrate which is described by GaN[0 0 0 1]//Si[1 1 1] and GaN(1 1 0 0)//Si(1 1 1). Imaging of the electronic structure of an AlGaN/GaN interface has been investigated by mapping the variation in the plasmon frequency using an electron energy loss spectrometer on a dedicated scanning transmission electron microscope. Cantilevers were fabricated using a combination of etching processes. Nitride etch rates during inductively coupled plasma dry etch processing using a Cl₂/Ar plasma etchant were obtained by monitoring the optical reflectivity of the nitride films in situ. A peak GaN etch rate of 250 nm/min was measured, the etch rate was found to be strongly dependent on the d.c. self-bias. Thin beams of GaN having a length of 7 μm and 0.7 μm thickness, were fabricated and mechanically released from Si(1 1 1) substrates using a combination of two dry ICP etch processes, using Cl₂/Ar and CF₄/Ar/O₂ chemistries, and a potassium hydroxide (KOH) aqueous wet etch.     

Epitaxy of semipolar GaN on a Si(001) substrate with a SiC buffer layer

A new method of synthesis of semipolar gallium nitride on a silicon substrate using the technology of solid-phase epitaxy of 3C-SiC nanocrystals has been suggested. It has been demonstrated that application of buffer layers of 3C-SiC and AlN enables one to form epitaxial layers of semipolar gallium nitride with layer deviation from the polar position of the c axis of a wurtzite crystal by an angle of 48°–51° at the minimal half-width of the X-ray diffraction rocking curve (ω_θ) ～ 24′. The observed bend of a cylindrical character in the structure of GaN/AlN/3C-SiC(001) is explained by the anisotropic deformation of semipolar GaN on silicon.     

Silicon nitride layers of various n-content: Technology, properties and structure

Two series of amorphous silicon nitride layers (a-SiN_x:H) were formed with Radio Frequency Chemical Vapor Deposition method (13.56 MHz) from a NH₃/SiH₄ gas mixture: the first one on Si (001) and the second on glass. The deposition process was repeated at various [NH₃]/[SiH₄] ratios, while the other parameters (pressure, plasma generator power, substrate temperature, total gas flow, and time) were kept constant. It has been confirmed in optical measurements that the refractive indexes decrease for the layers obtained at increasing [NH₃]/[SiH₄] ratios. Simultaneously, the position of the band assigned to Si-H stretching vibrations (at about 2100 cm^− 1) shifts towards higher frequencies. The observed dependencies were applied in evaluation of nitrogen and hydrogen contents in the respective layers. It has been shown that when [NH₃]/[SiH₄] increases from 0 (no silane flow) to 0.2 then the a-SiN_x:H layers of x = [N]/[Si] increasing between 0 and nearly 1.4 may be obtained. The obtained layers have the refractive indexes higher than 2.1 and lower than 2.7 which make them good materials for antireflective coatings on crystalline and multicrystalline silicon solar cells.     

Formation and photoluminescence of Si nanocrystals in controlled multilayer structure comprising of Si-rich nitride and ultrathin silicon nitride barrier layers

The effect of ultrathin silicon nitride (Si₃N₄) barrier layers on the formation and photoluminescence (PL) of Si nanocrystals (NCs) in Si-rich nitride (SRN)/Si₃N₄ multilayer structure was investigated. The layered structures composed of alternating layers of SRN and Si₃N₄ were prepared using magnetron sputtering followed by a different high temperature annealing. The formation of uniformly sized Si NCs was confirmed by the transmission electron microscopy and X-ray diffraction measurements. In particular, the 1 nm thick Si₃N₄ barrier layers was found to be sufficient in restraining the growth of Si NCs within the SRN layers upon high annealing processes. Moreover, X-ray photoelectron spectroscopy spectra shown that films subjected to post-anneal processes were not oxidized during the annealing. X-ray reflection measurements revealed that high annealing process induced low variation in the multilayer structure where the 1 nm Si₃N₄ layers act as good diffusion barriers to inhibit inter-diffusion between SRN layers. The PL emission observed was shown to be originated from the quantum confinement of Si NCs in the SRN. Furthermore, the blue shift of PL peaks accompanied by improved PL intensity after annealing process could be attributed to the effect of improved crystallization as well as nitride passivation in the films. Such multilayer structure should be advantageous for photovoltaic applications as the ultrathin barrier layer allow better electrical conductivity while still able to confine the growth of desired Si NC size for bandgap engineering.     

Investigation of charges carrier density in phosphorus and boron doped SiNx:H layers for crystalline silicon solar cells

Dielectric layers are of major importance in crystalline silicon solar cells processing, especially as anti-reflection coatings and for surface passivation purposes. In this paper we investigate the fixed charge densities (Q_fix) and the effective lifetimes (τ_eff) of phosphorus (P) and boron (B) doped silicon nitride layers deposited by plasma-enhanced chemical vapour deposition. P-doped layers exhibit a higher τ_eff than standard undoped layers. In contrast, B-doped layers exhibit lower τ_eff. A strong Q_fix decrease is to be seen when increasing the P content within the film. Based on numerical simulations we also demonstrate that the passivation obtained with P- and B-doped layers are limited by the interface states rather than by the fixed charges.     

Surface passivation of silicon by rf magnetron-sputtered silicon nitride films

Si-rich silicon nitride (SiN_x(:H)) films are deposited on single crystalline p-type silicon to investigate their properties as a passivation and antireflection coating for solar cells. The SiN_x(:H) films were reactively sputtered from an intrinsic Si-target in an Ar/N₂/H₂ rf (13.56 MHz) magnetron plasma at substrate temperatures from 150°C to 350°C. The optical band gap of Si-rich SiN_x(:H) becomes lower than 3 eV which was determined from spectral data of the complex refractive index. Infrared spectra show a strong Si–H band in Si-rich films. The effective surface recombination velocity (SRV) was calculated from the effective life time in SiN_x(:H) covered p-Si wafers by the microwave detected photoconductivity decay (MW PCD) technique. Very low values for the effective SRV of about 60 cm/s were determined. The low values of the effective SRV are due to field-effect passivation. The field-effect passivation of the SiN_x(:H)/Si contact is explained with the model of a hetero junction.