Composition of Ge+ and Si+ implanted SiO2/Si layers: Role of oxides in nanocluster formation

X-ray photoelectron spectroscopy (XPS) has been used to examine the atomic content of implanted SiO₂/Si layers. In particular, an XPS analysis permits to identify elemental Ge and Si, as well as GeO₂ precipitations in SiO₂ matrices. The XPS results reveal valuable information not only about the formation mechanism of Ge and Si nanoclusters but also on the annealing kinetics of SiO₂ whose properties are known to be significantly altered during the process of ion implantation and subsequent annealing. The composition of ion beam-modified SiO₂ layers strongly depends on the annealing temperature. With respect to germanium implanted samples a possibility of Ge nanocrystals formation appears at high (above 1000 °C) annealing temperatures. It has been shown that an intermediate step in the Ge oxide formation is necessary for the creation of Ge nanoclusters. Additionally, the presence of a subsurface zone GeO_x (about 100 nm thick) predicted in kinetic three-dimensional lattice simulations has been confirmed. In the case of Si⁺ implanted samples substoichiometric silicon oxide lines in the XPS spectra of a SiO₂ layer for all samples have been observed. No evidence of a line connected to the Si–Si bonding has been observed even at the highest annealing temperatures, at which only stoichiometric SiO₂ has been detected.     

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO₂ nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiO_x (a-SiO_x) films with 0 < x < 1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 °C. The series resistance through Ag/[Si-nanodots in SiO₂]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiO_x films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.     

Direct-PECVD a-SiOx:H/a-SiNx:H 叠层系统对多晶硅电池的钝化研究

本文中应用全部PECVD工艺沉积a-SiOx:H、a-SiNx:H,和a-SiOx:H/a-SiNx:H叠层系统,比较了不同钝化膜对多晶硅太阳能电池发射极和背面钝化效果,应用FGA、RTP等热处理方法对钝化膜进行处理,重点讨论了FGA(Forming gas annealing)温度和时间的长短对钝化的影响.结果表明:低温FGA对只有单面钝化膜的硅片钝化效果不明显,而在800 ℃下FGA有明显作用,而且钝化效果随着时间的增加呈现出先增大后减小然后再增大的现象;退火后降温环境中是否有H和降温时间对钝化效果有很大的影响,但是对于双面膜无论FGA温度高低对钝化都有帮助,文中对上述现象做了合理的解释.最后利用双面叠层钝化膜经过FGA处理后得到的多晶硅片的少子寿命达到14.2 μs,比镀膜之前的3.0 μs提高了11.2 μs,使多晶硅太阳能电池暗电压 Voc达到630 mV.     

Influence of hydrogen on SiO2 thick film deposited by PECVD and FHD for silica optical waveguide

Silicon dioxide (SiO₂) thick films have been deposited by plasma enhanced chemical vapor deposition (PECVD) and flame hydrolysis deposition (FHD). PECVD SiO₂ films were obtained at low temperatures (<350 °C) by the decomposition of the appropriate mixture of (SiH₄+N₂O) gases under suitable rf power and N₂O/SiH₄ ratio. For low N₂O/SiH₄ ratio, a refractive index(n) value close to 1.50 is obtained. The deposition rate increased with the increase of rf power. FHD SiO₂ films were produced by the flame hydrolysis reaction of halide materials such as SiCl₄, POCl₃ and BCl₃ in an oxy‐hydrogen torch. The porous SiO₂ layer, under the POCl₃/BCl₃ ratio deposition condition, has to be consolidated by annealing at around 1300 °C.     

Analysis of ambient gas influence on silicon layers crystallized by means of LPE

Epitaxial growth of thin layers from the liquid phase can occur with the use of solutions saturated under different ambient gases. Most often this process takes place in a vacuum or gaseous atmosphere of hydrogen or argon. As the experimental data show, the morphology of crystallized layers is determined by the ambient type in which the process occurs.The cohesion energy responsible for epitaxial lateral deposition processes on the substrate surface depends on the surface free energy which is a measure of attraction of the solution atoms by substrate atoms. In the case of crystallization of an epitaxial lateral layer of Si on a substrate partially masked with dielectric, the chemical potentials of atoms in the neighboring phases (determining the interface evolution) are not without influence on the relaxation velocity of the saturated liquid phase, and on the horizontal and vertical growth rate.The aim of the investigation was to analyze experimentally the influence of the ambient gases used during the LPE growth on the cohesion of the Sn–Si solution with substrates applied for the lateral epitaxial growth of Si layers. This work presents comparative temperature analysis of the wetting angle of such surfaces as Si, SiO₂ and SiN_x by the Sn–Si solution.     

Optical investigations of germanium nanoclusters – Rich SiO2 layers produced by ion beam synthesis

In this work, refractive index and extinction coefficient spectra of germanium nanoclusters – rich SiO₂ layers have been determined using variable angle spectroscopic ellipsometry (VASE) in the 250–1000 nm range. The samples were produced by Ge⁺ ion implantation into SiO₂ layers on Si substrates and subsequent annealing at temperatures from 700 to 1100 °C. It is known from previous investigations of similar samples that the Ge nanoclusterization process starts already at 800 °C and spherical Ge nanocrystallites 5–8 nm in diameter are observed in the SiO₂ layers after annealing for 1 h at even higher temperatures of 1000–1100 °C. Rutherford backscattering spectrometry (RBS) was employed to measure the Ge atom concentration depth profiles in the studied samples. The RBS results helped us choose realistic models for the VASE analysis which were necessary for a proper interpretation of the VASE data. It has been found that the refraction index value for the SiO₂/Si layer increases after Ge implantation. This effect can be explained by a defect-dependent compaction of ion-bombarded layers. A band’s tail in the extinction coefficient spectra for all the samples is observed which originates from a strong ultraviolet absorption band at 6.8 eV due to a Germanium Oxygen-Deficiency Center (GeODC) and/or a Ge-E’center in SiO₂. The annealing process results in the emergence of weaker extinction coefficient bands in the 400–600 nm region, associated with direct band-to-band transitions in Ge nanostructures. Transformation of these bands, including their blue-shift with the increasing annealing temperature could be explained via a quantum-confinement mechanism, by size and structural changes in Ge nanostructures.     

Enhanced luminescence from Si quantum dots/SiO2 multilayers by hydrogen annealing

Si quantum dots/SiO₂ multilayers were prepared by annealing a-Si:H/SiO₂ stacked structures at 1100 °C . Photo- and electro-luminescence band around 750 nm can be observed from Si QDs/SiO₂ multilayers due to the recombination of electron-hole pairs in Si QDs/SiO₂ interfaces. The electro-luminescence intensity was obviously enhanced after post hydrogen annealing at 400 °C. Electron spin resonance measurements were used to characterize the change of the defect states after hydrogen annealing. It is found that there exists a-centers (g value = 2.006), which is related to the Si dangling bonds in Si QDs in our samples. Hydrogen annealing can significantly reduce non-luminescent a-centers and enhance the electro-luminescence intensity consequently.     

Structure and properties of hydrogenated amorphous silicon carbide thin films deposited by PECVD

a-Si_1?xC_x:H films are deposited by RF plasma enhanced chemical vapor deposition (PECVD) at different RF powers with hydrogen-diluted silane and methane mixture as reactive gases. The structure and properties of the thin films are measured by infrared spectroscope (IR), Raman scattering spectroscope and ultra violet–visible transmission spectroscope (UV–vis), respectively. Results show that the optical band gap of the a-Si_1?xC_x:H thin films increases with increasing Si–C bond fraction. It can be easily controlled through controlling Si–C bond formed by modulating deposition power. At low deposition power, the bond configuration of the a-Si_1?xC_x:H thin film is more disordered owing to the distinct different bond lengths and bond strengths between Si and C atoms. At a too high deposition power, it becomes still high disordered due to dangling bonds appearing in the a-Si_1?xC_x:H thin film. The low disordered bond configuration appears in the thin film deposited with moderate deposition power density of about 2.5 W/cm².     

Bonding rearrangement in amorphous silicon nitrides deposited by hot-wire chemical vapor deposition upon thermal annealing

The bonding rearrangement upon thermal annealing of amorphous silicon nitride (a-SiN_x:H) films deposited by hot-wire chemical vapor deposition was studied. A wide range of N/Si atom ratio between 0.5 and 1.6 was obtained for the a-SiN_x:H sample series by varying the source gases ratio only. Evolutions of Si–N, Si–H and N–H bonds upon annealing were found to depend strongly on the N/Si atom ratio of the films. According to the above observations, we propose possible reaction pathways for bonding rearrangement in a-SiN_x:H with different N/Si ratios.     

Growth and optical characterisation of multilayers of InGaN quantum dots

We report on the growth (using metal-organic vapour phase epitaxy) and optical characterisation of single and multiple layers of InGaN quantum dots (QDs), which were formed by annealing InGaN epilayers at the growth temperature in nitrogen. The size and density of the nanostructures have been found to be fairly similar for uncapped single and three layer QD samples if the GaN barriers between the dot layers are grown at the same temperature as the InGaN epilayer. The distribution of nanostructure heights of the final QD layer of three is wider and is centred around a larger size if the GaN barriers are grown at two temperatures (first a thin layer at the dot growth temperature, then a thicker layer at a higher temperature). Micro-photoluminescence studies at 4.2 K of capped samples have confirmed the QD nature of the capped nanostructures by the observation of sharp emission peaks with full width at half maximum limited by the resolution of the spectrometer. We have also observed much more QD emission per unit area in a sample with three QD layers, than in a sample with a single QD layer, as expected.     

Influence of amorphous buffer layers on the crystallinity of sputter-deposited undoped ZnO films

Undoped ZnO films were deposited by radio frequency (RF) magnetron sputtering on amorphous buffer layers such as SiO_x, SiO_xN_y, and SiN_x prepared by plasma enhanced chemical vapor deposition (PECVD) for dielectric layer in thin film transistor (TFT) application. ZnO was also deposited directly on glass and quartz substrate for comparison. It was found that continuous films were formed in the thickness up to 10 nm on all buffer layers. The crystallinity of ZnO films was improved in the order on quartz>SiO_x >SiO_xN_y>glass>SiN_x according to the investigated intensities of (0 0 2) XRD peaks. The crystallite sizes of ZnO were in the order of SiO_x～glass >SiN_x. Stable XRD parameters of ZnO thin films were obtained to the thickness from 40 to 100 nm grown on SiO_x insulator for TFT application. Investigation of the ZnO thin films by atomic force microscope (AFM) revealed that grain size and roughness obtained on SiN_x were larger than those on SiO_x and glass. Hence, both nucleation and crystallinity of sputtered ZnO thin films remarkably depended on amorphous buffer layers.     

Effects of passivation and postannealing on the photoluminescence properties of MgO/SiO2 core‐shell nanorods

MgO nanorods were grown by the thermal evaporation of Mg₃N₂ powders on the Si (100) substrate coated with a gold thin film. The MgO nanorods grown on the Si (100) substrate were a few tens of nanometers in diameter and up to a few hundreds of micrometers in length. MgO/SiO₂ core‐shell nanorods were also fabricated by the sputter‐deposition of SiO₂onto the MgO nanorods. Transmission electron microscopy (TEM) and X–ray diffraction (XRD) analysis results indicated that the cores and shells of the annealed core‐shell nanorods were a face‐centered cubic‐type single crystal MgO and amorphous SiO₂, respectively. The photoluminescence (PL) spectroscopy analysis results showed that SiO₂ coating slightly decreased the PL emission intensity of the MgO nanorods. The PL emission of the MgO/SiO₂ core‐shell nanorods was, however, found to be considerably enhanced by thermal annealing and strongly depends on the annealing atmosphere. The PL emission of the MgO/SiO₂ core‐shell nanorods was substantially enhanced in intensity by annealing in a reducing atmosphere, whereas it was slightly enhanced by annealing in an oxidative atmosphere. The origin of the PL enhancement by annealing in a reducing atmosphere is discussed with the aid of energy‐dispersive X‐ray spectroscopy analyses. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Isochronal Studies of α‐Si:H Crystallization and the Performance of it's Schottky Barriers

Hydrogenated amorphous silicon films (α‐Si:H) were crystallized employing a metal induced crystalline (MIC) technique. Structural changes during annealing these films at 300 °C for different periods (0‐300 minutes) were obtained by XRD. Al was used as a metal induced crystalline for α‐Si:H produced by ultra high vacuum (UHV) plasma enhanced chemical vapor deposition (PECVD). XRD shows that crystallization of the interacted α‐Si:H film underneath Al initiates at 300 °C for 15 minutes. A complete crystallization was obtained after annealing for 60 minutes. A gold dot was evaporated onto α‐Si:H films, which annealed for different periods to form Schottky barriers. Electrical properties of Au/α‐Si:H were calculated such as the ideality factor, n, barrier height, Φ_B, donor concentration, N_D, and the diffusion voltage, V_d, as a function of the annealing time of α‐Si:H films. All these parameters were carried out through the current voltage characteristics (J‐V) and the capacitance voltage measurements (C‐V). The results were presented a discussed on the basis of XRD performance and the thermionic emission theory.     

Trap-limited diffusion of hydrogen in precursor derived amorphous Si–B–C–N-ceramics

The tracer diffusion of hydrogen is studied in precursor derived amorphous Si–C–N and Si–B–C–N ceramics using deuterium as a tracer and secondary ion mass spectrometry (SIMS). Since the amorphous ceramics are separated in carbon rich phases (amorphous carbon and amorphous C(BN)_x, respectively) and silicon rich phases (amorphous Si₃N₄ and amorphous Si_3+(1/4)xC_xN_4−x, respectively) we additionally measured the diffusivities of hydrogen in amorphous carbon, in amorphous SiC and in amorphous C–B–N films. The silicon rich phases are identified as diffusion paths for hydrogen in the precursor derived ceramics. Diffusion of hydrogen in these materials is explained with a trap limited diffusion mechanism with a single trap level. We found activation enthalpies of about 2 eV for the precursor derived ceramics, where the activation enthalpy is the sum of a migration enthalpy and a binding enthalpy. The low values for the pre-exponential factors of less than 10⁻⁷ m²/s can be explained with an appropriate expression for the entropy factor.     

Photoluminescence of Si nanocrystallites in different types of matrices

This paper presents the comparative investigation of photoluminescence (PL) and its temperature dependence for rf-magnetron co-sputtered Si-enriched SiO_x systems and amorphous Si films prepared by hot-wire CVD method with Si nanocrystallites of different sizes. It is shown that PL spectra of Si–SiO_x films consist of the five PL bands peaked at 1.30, 1.50, 1.76, 2.05 and 2.32 eV. Amorphous Si films with Si nanocrystallites are characterized by three PL bands only peaked at 1.35, 1.50 and 1.76 eV. The peak position of the 1.50 eV PL band shifts with the change of Si quantum dot sizes and it is attributed to exciton recombination inside of Si quantum dots. The nature of four other PL bands is discussed as well.     

Advanced deposition phase diagrams for guiding Si:H-based multijunction solar cells

Phase diagrams have been established to describe very high frequency (vhf) plasma-enhanced chemical vapor deposition (PECVD) of intrinsic hydrogenated silicon (Si:H) and silicon–germanium alloy (Si_1?xGe_x:H) thin films on crystalline Si substrates that have been over-deposited with n-type amorphous Si:H (a-Si:H). The Si:H and Si_1?xGe_x:H films are prepared under conditions used for the top and middle i-layers of high efficiency triple-junction a-Si:H-based n–i–p solar cells. Identical n/i cell structures were co-deposited in this study on textured (stainless steel)/Ag/ZnO which serve as substrate/back-reflectors in order to relate the phase diagrams to the performance parameters of single-junction solar cells. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si_1?xGe_x:H solar cells are obtained when the i-layers are prepared under previously-described maximal H₂ dilution conditions.     

Structural and electrical characterization of ALCVD ZrO2 thin films on silicon

We report on the structural and electrical properties of ZrO₂ thin layers grown on Si by atomic layer chemical vapour deposition. Atomic force microscopy, X-ray diffraction, X-ray reflectivity and time-of-flight secondary ion mass spectrometry have been used to characterize as-grown and annealed samples. High frequency capacitance-voltage measurements have been performed to determine the capacitance of the gate dielectric stack. The ZrO₂ film is found to be polycrystalline. Electrical and structural data suggest a coherent picture of film modification upon annealing.     

Structure and photoelectrical properties of SiO2/Si/SiO2 single quantum wells prepared under ultrahigh vacuum conditions

SiO₂/Si/SiO₂ single quantum wells (QWs) were prepared under ultrahigh vacuum conditions in order to study their structural, chemical and photoelectrical properties with respect to a possible application in photovoltaic devices. Amorphous silicon (a-Si) layers (thickness <10 nm) were deposited onto quartz glass (SiO₂) substrates and subsequently oxidized with neutral atomic oxygen at moderate temperatures of 600 °C. Under these conditions, the formation of suboxides is mostly suppressed and abrupt Si/SiO₂ interfaces are obtained. Crystallization of a-Si QWs requires temperatures as high as 1000 °C resulting in a nanocrystalline structure with a small amorphous fraction. The spectral dependence of the internal quantum efficiency of photoconductivity correlates well with the nanocrystalline structure and yields mobility lifetime products of <10^?7 cm² V^?1. This rather low value points towards a strong influence of Si/SiO₂ interface states on the carrier mobility and the carrier lifetime in Si QWs. Electronic passivation of interface states by subsequent hydrogen treatment in forming gas enhances the internal quantum efficiency by nearly one order of magnitude.     

Comparative study of electroluminescence from annealed amorphous SiC single layer and amorphous Si/SiC multilayers

Si quantum dots (Si QDs) films were prepared by annealing amorphous SiC single layer and amorphous Si/SiC multilayers fabricated in plasma enhanced chemical vapor deposition system. The microstructures were characterized by Raman spectroscopy as well as Fourier transforms infrared spectroscopy for both samples. It was found that Si QDs with average size of 2.7 nm were formed after annealing and the electroluminescence (EL) band centered at 650 nm can be observed at room temperature. The EL intensity from the Si/SiC multilayers was obviously improved by one order of magnitude and the corresponding EL band width was reduced compared with that from SiC single layer film. The improved electroluminescence behavior can be attributed to the formation of the denser Si QDs, good size distribution and the strong confinement effect of carriers in multilayerd structures.     

A comparison of structures and properties of SiNx and SiOx films prepared by PECVD

In this paper, amorphous silicon nitride (SiNx) and silicon oxide (SiOx) films were prepared by plasma-enhanced chemical vapor deposition (PECVD). The chemical structures and physical properties of the resulting materials were investigated and compared. Results reveal that SiOx films match better with Si substrate, whereas SiNx films exhibit larger refractive index, moderate optical band gap, and higher Young's modulus and film hardness. In overall, SiNx films are more suitable to serve as supporting and insulating materials for VOx-based infrared microbolometers. The difference in the physical properties is attributed to various bonding configurations for the resulting materials. Finally, several methods for evaluating the residual stresses in amorphous thin films were discussed.