Plasma‐enhanced chemical vapor deposition of 99.95% 28Si in form of nano‐ and polycrystals using silicon tetrafluoride precursor

The process of plasma chemical deposition of silicon was investigated from its tetrafluoride containing 99.99% of ²⁸Si isotope in the form of thin layer of nano‐crystalline silicon on silicon substrate and of thick layer of polycrystalline silicon on the inner surface of quartz reactor. The layers are characterized by the methods of X‐ray diffraction and Raman spectroscopy. Using the SIMS method the mechanism of isotopic dilution was investigated in the PECVD process (the content of ²⁸Si isotope in layers was 99.95‐99.98%). A necessity is indicated in thorough special preparation of the reactor for minimization of isotopic dilution in case of fabrication of silicon containing ≥99.9% of ²⁸Si. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and optical properties of vapour‐etching based porous silicon

The structural and optical properties of porous silicon (PS) layers prepared by Vapour‐Etching (VE) of moderately and heavily boron‐doped Si substrates are investigated. The VE technique produces rough PS layers that are essentially formed of interconnected cluster‐like structures. Optical investigations indicate that this surface roughness enables the PS layers to be used as antireflection coatings in silicon based devices. These optical characteristics are investigated by optical reflectivity and light scattering. The local chemical state and the microstructure of the PS layers are studied by electron energy loss spectroscopy (EELS) in transmission electron microscopy (TEM), and are correlated to the red photoluminescence (PL). TEM studies point out that the cluster‐like interconnected structures are composed of luminescent nanocrystallites. PL measurements display that both quantum confinement and surface passivation determine the electronic states of the silicon nanocrystallites. The complex dielectric function is calculated from the experimental single‐scattering distribution spectrum using a Kramers Kronig analysis. The first resonance peak in the imaginary part is observed at 2.3 eV; two other broadened features appear at 4.7 and 8.8 eV. The latter is generally related to an interface plasmon resulting from the silicon‐silicon oxide interface. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence in silicon rich oxide thin films under different thermal treatments

Photoluminescence (PL) was studied in silicon rich oxide (with the atomic percentage ranges of Si from 35% to 75%) thin film samples, fabricated by the plasma assisted CVD technique. A broad PL peak, blue-shifted from the bulk silicon band edge of ～1.1 eV, was observed. In one typical sample, the PL peak intensity shows a non-monotonic temperature dependence. This non-monotonic dependence was also observed in previous work by others and attributed to an energy splitting between the excitonic singlet and triplet levels in silicon nanocrystals, a consequence of quantum confinement effect. Finally, in more than 20 samples under different thermal treatments (with the annealing temperature range from 800 °C to 1100 °C), the wavelength of PL peak was observed to be pinned between ～900 and ～1000 nm, independent of thermal budget. This pinning effect, we believe, is probably due to the formation of oxygen-related interface states.     

Improving the performance of amorphous and crystalline silicon heterojunction solar cells by monitoring surface passivation

The influence of thermal annealing on the crystalline silicon surface passivating properties of selected amorphous silicon containing layer stacks (including intrinsic and doped films), as well as the correlation with silicon heterojunction solar cell performance has been investigated. All samples have been isochronally annealed for 1 h in an N₂ ambient at temperatures between 150 °C and 300 °C in incremental steps of 15 °C. For intrinsic films and intrinsic/n-type stacks, an improvement in passivation quality is observed up to 255 °C and 270 °C, respectively, and a deterioration at higher temperatures. For intrinsic/n-type a-Si:H layer stacks, a maximum minority carrier lifetime of 13.3 ms at an injection level of 10¹⁵ cm^? 3 has been measured. In contrast, for intrinsic/p-type a-Si:H layer stacks, a deterioration in passivation is observed upon annealing over the whole temperature range. Comparing the lifetime values and trends for the different layer stacks to the performance of the corresponding cells, it is inferred that the intrinsic/p-layer stack is limiting device performance. Furthermore, thermal annealing of p-type layers should be avoided entirely. We therefore propose an adapted processing sequence, leading to a substantial improvement in efficiency to 16.7%, well above the efficiency of 15.8% obtained with the ‘standard’ processing sequence.     

Crystallization of silicon thin films by current‐induced joule heating using a tungsten overcoat

A modified crystallization process using current‐induced joule heating under vacuum is presented. A thin layer of high temperature resistant tungsten was sputtered on the amorphous silicon as the conducting and annealing medium. The thin film thickness was measured by α‐stepper. The high current density provided effective means in crystallizing the amorphous silicon layer. The crystalline morphology was studied by scanning electron microscopy (SEM) after Secco‐etch, transmission electron microscopy (TEM), and x‐ray diffraction (XRD), under different annealing conditions. The grain size was controlled in the range of 0.1‐0.5 μm and could be increased with annealing time. No tungsten silicide was found. Some defects were formed due to electron‐migration effect near the electrodes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of rapid thermal annealing on Zn1‐xCdxO layers grown by radio‐frequency magnetron co‐sputtering

Zn_1‐xCd_xO layers were deposited on the sapphire substrate using the radio‐frequency magnetron co‐sputtering system. The grown Zn_1‐xCd_xO layers were carried out in the post‐annealing treatment for 1 min at the 800 °C oxygen‐ambient by the rapid thermal annealing (RTA) method. X‐ray diffraction (XRD) experiment shows that the Zn_1‐xCd_xO layers are changed from the single phase of the hexagonal structure at 0≤x ≤0.08 to the double phase of hexagonal‐and‐cubic structure at x =0.13. Thus, the maximum Cd‐composition ratio with the hexagonal structure was found out to be x =0.08. Also, the crystallinity of Zn_1‐xCd_xO layers at x =0.13 was remarkably improved by the RTA annealing treatment. This crystal quality improvement was thought to be associated with the relaxation of the compressive strain remaining in the Zn_1‐xCd_xO layers. Therefore, the results of XRD and transmittance lead that the crystal quality of the Zn_1‐xCd_xO layers forming the hexagonal ZnO phase is better than that forming the cubic CdO phase. Consequently, the reliable formation and the crystallinity of the Zn_1‐xCd_xO layers were achieved by using the RTA method of short‐time thermal‐annealing at the high temperature. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Heteroepitaxial growth of GaP on silicon

Epitaxial gallium phosphide layers have been grown on silicon substrates by the metal-organic process. This process involves the reaction between trimethylgallium (CH₃)₃Ga and PH₃ and gives a high density of nucleation sites on the silicon. The influence of the substrate orientation and of the deposition temperature on the crystallinity of the layers has been studied. Best results were obtained on (001) oriented substrate at a deposition temperature of 800°C. X-ray reflection topographs of the layers have revealed the formation of cracks extending along the [110] direction, which are explained by the lattice mismatch and the difference in thermal expansion coefficients. The cracking is asymmetric with the main direction parallel to [110]. The density of cracks can be reduced by a two stage epitaxy. The electrical properties of undoped and n-type doped layers have been assessed by Hall and C(V) measurements. It shows auto-doping with silicon coming from the substrate.     

Growth of boron nitride films on w‐AlN (0001), 4° off‐cut 4H‐SiC (0001), W (110) and Cr (110) substrates by Chemical Vapor Deposition

Boron Nitride is a promising group 13–group 15 compound material that exhibits various interesting properties like wide band gap, chemical stability, attractive mechanical properties and other. The growth behavior of this material has not been investigated in sufficient details to tailor properties of the resulting films. In this work we present the results on the growth of turbostratic boron nitride (t‐BN) thin films at a relatively high growth rate of 3 μm/h with the aim to investigate the potential use of boron trichloride in combination with ammonia as precursors for growth. Deposition experiments were conducted in a vertical cold wall high temperature chemical vapor deposition reactor in the temperature range 1000°C–1700°C depending on the substrate used. Templates of w‐AlN (0001), 4° off‐cut 4H‐SiC (0001), Cr (110) and W (110) were employed as substrates for the BN growth. As‐grown BN layers were characterized by Scanning Electron Microscopy, X‐Ray Diffraction, Electron Diffraction and Raman Spectroscopy. The results indicate that temperature and N/B ratio have a great influence on the crystallinity of the deposited films. For AlN and SiC substrates, a temperature of 1600°C and N/B ratio in range between 3 and 7.5 were identified as the best parameters for the growth of a 2 μm thick t‐BN layer with a spacing between basal planes of about 3.36 Å compare to the 3.33 Å spacing between basal planes of hexagonal or rhombohedral BN (h‐BN or r‐BN). For Cr and W substrates which have a lower mismatch with h‐BN (1 and 8.8 %), layers of t‐BN were deposited at much lower temperature (1000°C–1150°C) with a spacing between basal planes of 3.5 Å and morphology similar to that observed on SiC substrates. We obtained t‐BN layers with in plane strong disorder but out of plane orientation (c‐axis normal to the surface).     

Transformation of defect layer and zinc implant profile in silicon during thermal annealing

The transformation of the structure of a radiation-damaged silicon layer and profiles of implanted dopant Zn during thermal annealings has been investigated. The analysis was performed by Rutherford back-scattering spectroscopy, secondary-ion mass spectrometry, and high-resolution X-ray diffraction. It is established that the surface region of radiation-induced point defects (Frenkel pairs) 78 nm thick is formed in the implanted samples. A heat treatment at 400°C leads to the annealing of interstitial point defects and reduces the damaged-layer thickness to 56 nm. This layer may contain vacancy clusters or clusters of zinc-vacancy complexes. The segregation of implanted dopant Zn is observed near the maximum of its depth distribution. Annealing at 700°C leads to the almost complete recovery of the damaged layer. Two concentration peaks were formed in the sample, i.e., one near the substrate surface and the other near the peak of distribution of radiation-induced point defects. During the ion implantation of zinc and at subsequent stages of thermal annealing, zinc precipitated in the form of zinc silicide (of the ZnSiO₃ type).     

Structural,Raman, and photoluminescence properties of double‐shelled coaxial nanocables of In2O3 core with ZnO and AZO shells

We synthesized In₂O₃/ZnO/Al‐doped ZnO (AZO) core‐double shell nanowires, in which the inner shell (ZnO) and the outer shell (AZO) have been subsequently deposited on the core In₂O₃ nanowires. With their one‐dimensional morphology being preserved, the X‐ray diffraction (XRD), lattice‐resolved transmission electron microscopy (TEM) image, selected area electron diffraction, and Raman spectrum coincidentally revealed that the shell was comprised of hexagonal ZnO phase. In addition, TEM‐EDX investigation revealed the presence of Al elements in the shell region. The thermal annealing at 700 °C did not significantly change the nanowire morphology, however, the XRD spectrum indicated that the ZnO phase was crystallized by the annealing. PL spectrum of the 700 °C‐annealed In₂O₃/ZnO/AZO core‐double shell nanowires was comprised of three Gaussian bands at approximately 2.1 eV, 2.4 eV, and 3.0 eV, respectively. The integrated intensities of 2.1 eV‐, 2.4 eV‐, and 3.0 eV‐bands were decreased by the thermal annealing. This study will pave the road to the preparation and applicaition of double‐shelled nanowires. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some features of thermo‐activated structural transformation of biogenic and synthetic Mg‐containing apatite with β‐tricalcium‐magnesium phosphate formation

Temperature transformation of biogenic and synthetic Mg‐containing apatite with β‐tricalcium‐magnesium phosphate (β‐TCMP) formation was investigated by X‐ray diffraction. Samples were annealed in air at the temperature range from 600 to 1200 °C at intervals 100 °C and cooled down to room temperature. The appearance of β‐TCMP was ascertained in samples annealed at 800 °C. As revealed, the relative amount of β‐TCMP increases and Mg concentration in this phase decreases as the annealing temperature is raised. While this, the replacement degree of Ca by Mg in the β‐TCMP lattice is the annealing temperature function and does not depend either on sample origin (biogenic/synthetic) or on initial Mg concentration. The results of present work together with other investigation data testify to a high thermally activated mobility of Mg both in structure of Mg‐containing apatites and in the structure of β‐TCMP formed after thermal decomposition. Obtained data can be used for new biomaterials design with varied prolongation of Mg released into the living biological tissue. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and Electrical Properties of Cd Doped InSe Thin Films

Polycrystalline Cd doped InSe thin films were obtained by thermal co‐evaporation of alpha‐In₂Se₃ lumps and Cd onto glass substrates at a temperature of 150°C. The films were annealed at 150°C and 200°C. The films were found to contain around 46% In, 47% Se and 7% Cd in weight. The films exhibited p‐type conductivity. The results of conductivity measurements have revealed that thermionic emission and variable range hopping are the two dominant conduction mechanisms, in the temperature ranges of 320‐160 K and 150‐40 K respectively. It was observed that above 240 K mobility is limited by the scattering at the grain boundaries. As the temperature decreases, thermal lattice scattering followed by the ionized impurity scattering dominate as the two main mechanisms controlling the mobility. Acceptor to donor concentration ratio was found to be slightly increasing due to annealing.     

Photoelectrical properties of crystalline titanium dioxide thin films after thermo‐annealing

This paper reports the photoelectrical properties of sol gel derived titanium dioxide (TiO₂) thin films annealed at different temperatures (425‐900°C). The structure of the as‐grown film was found to be amorphous and it transforms to crystalline upon annealing. The trap levels are studied by thermally stimulated current (TSC) measurements. A single trap level with activation energy of 1.5 eV was identified. The steady state and transient photocurrent was measured and the results are discussed on the basis of structural transformation. The photocurrent was found to be maximum for the films annealed at 425°C and further it decreases with annealing at higher temperatures. The photoconduction parameters such as carrier lifetime, lifetime decay constant and photosensitivity were calculated and the results are discussed as a function of annealing temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

We report on the epitaxial growth of crystalline silicon films on (100) oriented crystalline silicon substrates by standard plasma enhanced chemical vapor deposition at 175 °C. Such unexpected epitaxial growth is discussed in the context of deposition processes of silicon thin films, based on silicon radicals and nanocrystals. Our results are supported by previous studies on plasma synthesis of silicon nanocrystals and point toward silicon nanocrystals being the most plausible building blocks for such epitaxial growth. The results lay the basis of a new approach for the obtaining of crystalline silicon thin films and open the path for transferring those epitaxial layers from c-Si wafers to low cost foreign substrates.     

Properties of non‐stoichiometric nitrogen doped LPCVD silicon thin films

The influence of nitrogen on the internal structure and so on the electrical properties of silicon thin films obtained by low‐pressure chemical vapor deposition (LPCVD) was studied using several investigation methods. We found by using Raman spectroscopy and SEM observations that a strong relationship exists between the structural order of the silicon matrix and the nitrogen ratio in film before and after thermal treatment. As a result of the high disorder caused by nitrogen on silicon network during the deposit phase of films, the crystallization phenomena in term of nucleation and crystalline growth were found to depend upon the nitrogen content. Resistivity measurements results show that electrical properties of NIDOS films depend significantly on structural properties. It was appeared that for high nitrogen content, the films tend to acquire an insulator behavior. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The impact of the nitridation process on the properties of the Si–SiO2 interface

A newly developed technique for the simultaneous measurement of the oxide–silicon interface properties and of minority carrier lifetime in the silicon volume was used for a systematic study of the nitridation process of oxide films.This technique is based on the surface recombination velocity measurements, and does not require the formation of a capacitor structure, so it is suitable for the measurement of as-grown interface properties. Oxides grown both in dry and in wet environments were prepared, and nitridation processes in N₂O and in NO were compared to N₂ annealing processes. The effect of nitridation temperature and duration were also studied, and processes of rapid thermal oxidation (RTO) and nitridation (RTN) were compared to conventional furnace nitridation processes. Surface recombination velocity was correlated with nitrogen concentration at the oxide–silicon interface obtained by secondary ion mass spectroscopy (SIMS) measurements. Surface recombination velocity (hence surface state density) decreases with increasing nitrogen pile-up at the oxide–silicon interface, indicating that in nitrided interfaces surface state density is limited by nitridation. NO treatments are much more effective than N₂O treatments in the formation of a nitrogen–rich interface layer and, as a consequence, in interface state reduction. X-ray photoelectron spectrometry (XPS) analyses were used to extend our correlation to very thin oxides (3 nm).     

Comparison of tin‐doped indium oxide films fabricated by spray pyrolysis and magnetron sputtering

A comparison of the properties between Tin‐doped Indium Oxide (ITO) films fabricated by sputtering and spray pyrolysis is presented. This analysis shows that the ITO films fabricated by DC magnetron sputtering in pure argon gas requires of a subsequent annealing for the improvement of their structural, electrical and optical properties, when they are compared to films fabricated by single‐stage spray pyrolysis process that includes a new approach. The optimum annealing temperature for ITO films sputtered at room temperature lies in the 300‐350 °C range. Under such conditions, the ITO sputtered films are slightly more resistive than the resistivity (2 × 10^‐4 Ω‐cm) shown by films sprayed at 480 °C using a solution with a 5 % of tin to indium ratio, and almost four times the worst value of the combination of transparency and conductivity determined by the value of the figure of merit (FOM). The sprayed films have a high value of the FOM, 2.9×10^‐2 Ω^‐1, which is comparable with the best published results.     

Growth,structure and optical characterization of Al‐doped ZnO nanoparticle thin films

Al‐doped ZnO nanoparticle thin films were prepared on glass substrate at the optimum temperature of (410±10) °C by spray pyrolysis technique using zinc nitrate as a precursor solution and aluminium chloride as a dopant. The dopant concentration (Al/Zn at%) was varied from 0 to 2 at%. Structural analysis of the films shows that all the films are of polycrystalline zinc oxide in nature, possessing hexagonal wurtzite structure. The films exhibit variation in peak intensities corresponding to (100), (002) and (101) reflection planes on Al‐doping. The crystallite size calculated by Scherrer formula has been found to be in the range of 35‐65 nm. The optical absorption study shows that the optical band gap in the Al‐doped films varies in the range of 3.11 – 3.22 eV. The width of localized states in the band gap estimated by the Urbach tail analysis has been found to be minimum in case of the 1 at% Al‐doped zinc oxide thin film. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of Zn‐doped talc in hydrothermal atmosphere

Zn‐doped talc were synthesized under hydrothermal conditions at constant reaction time and pressure of 160 hours and 2 kbar respectively, at three different temperatures (300, 500 and 650 °C) with pH‐values of 5 and 7. The starting materials and run products were characterized by X‐ray powder diffraction (XRPD), scanning electron microscopy with annexed energy‐dispersive spectrometry (SEM‐EDS), differential scanning calorimetry thermogravimetric analysis (DSC‐TG) and Fourier transform infrared (FT‐IR). The results showed that the temperature, pH‐value of the reaction mixture and amount of zinc in the starting mixture affected the growth of the Zn‐doped talc. When synthesized at low temperature talc presents low crystallinity, flawed morphology but higher content in zinc in its lattice. A thermal treatment at, or above 500 °C allowed a significant flaw reduction in talc morphology, a higher crystallinity and a lower content in dopant. When large amounts of Zn were present in the starting mixtures, Zn‐doped talc grew small in size and poorly shaped. The effect of Zn doping on some chemical/physical characteristics of the synthesized talc was also discussed.     

Flux growth and characterization of Ti‐ and Ni‐doped forsterite single crystals

Forsterite monocrystals doped with Ti and Ni were grown by the flux growth technique. A suitable mixture of flux (MoO₃, V₂O₅, Li₂CO₃) and nutrient was slowly cooled down to 750 °C from 1250 °C or 1350 °C. The crystals were then characterized by powder and single‐crystal X‐ray diffraction, scanning electron microscopy and differential scanning calorimetry (DSC). Variations observed in crystal size were attributed by both the varying experimental conditions in which they had been obtained, and to the amount of Ni substituted for Mg in the structure. High abundances of doped forsterite required a cooling rate of 1.8 K h^‐1. These synthetic, well‐characterized Ti and Ni doped forsterite crystals may have potential for exploitation in industrial fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)