Ellipsometric studies of annealing of SiO2 layers during the formation of light-emitting Si nanocrystals in them

Annealing of SiO₂ layers with excessive Si leading to the formation of silicon nanocrystals capable of fluorescing in the visible region owing to quantum-dimensional limitations is studied by the ellipsometry method. Excessive Si was introduced in SiO₂ layers by ion implantation with an energy of 25 keV and a dose of 5× 10¹⁶ cm^?2. Isochronous (10³ s) annealings were carried out in a temperature interval of 200–1150°C with a step of 100°C. An LÉF-2 ellipsometer with a 70° angle of incidence at a wavelength of 632.8 nm was used for the measurements. Fluorescence excited by a nitrogen laser was monitored concurrently. It is found that variations in optical constants of the layers at each step of annealing over the entire temperature range studied are clearly detected by ellipsometry. Variations in optical parameters of excessive Si are calculated in the Bruggeman approximation. They are found to correspond to individual stages of the formation of nanoprecipitates revealed earlier by other techniques. Nanocrystals proper producing intense visible photoluminescence are formed at annealing temperatures of 1000°C and higher.     

Excimer laser crystallization of amorphous silicon on metallic substrate

An attempt has been made to achieve the crystallization of silicon thin film on metallic foils by long pulse duration excimer laser processing. Amorphous silicon thin films (100 nm) were deposited by radiofrequency magnetron sputtering on a commercial metallic alloy (N42-FeNi made of 41 % of Ni) coated by a tantalum nitride (TaN) layer. The TaN coating acts as a barrier layer, preventing the diffusion of metallic impurities in the silicon thin film during the laser annealing. An energy density threshold of 0.3 J?cm^?2, necessary for surface melting and crystallization of the amorphous silicon, was predicted by a numerical simulation of laser-induced phase transitions and witnessed by Raman analysis. Beyond this fluence, the melt depth increases with the intensification of energy density. A complete crystallization of the layer is achieved for an energy density of 0.9 J?cm^?2. Scanning electron microscopy unveils the nanostructuring of the silicon after laser irradiation, while cross-sectional transmission electron microscopy reveals the crystallites’ columnar growth.     

Surface morphology study of recrystallization dynamics of amorphous ZnO layers prepared on different substrates

Amorphous oxides-based devices are exposed, during fabrication, to different processing conditions affecting their properties. Zinc oxide is a prospective candidate for transparent amorphous oxides, but its structure is changing under the influence of temperature. We investigated surface recrystallization of amorphous zinc oxide layers deposited onto fused silica, sapphire and Si substrates by pulsed laser deposition. The prepared three series of layers had highly nonequilibrium phase structures. Using atomic force microscopy and scanning electron microscopy, the effect was studied of subsequent annealing at 200, 400, 600, 800 °C for 60 min upon the surface structural properties of the layers. The following parameters were analyzed: average roughness, RMS roughness and size of formed grains on selected places with 1 × 1 μm² area. Surface structural analysis revealed that annealing led to recrystallization of the prepared layers and roughening of the structural features on the surface. With increasing annealing temperature, the calculated parameters were increasing. The average surface roughness of zinc oxide layers annealed at 800 °C is three times higher than that of the layers annealed at lower temperatures for all substrates used. The process dynamics of thermally caused recrystallization of the layers was different for each of the substrates used.     

Behavior of antimony above solid solubility in silicon produced by implantation and laser annealing

Abstract

Antimony implantation into <111> silicon was carried out at RT with a dose of 4.5 × 10¹⁵ cm^?2, energy 75 keV. For the annealing of the sample pulses of a Q-switched ruby laser were used with energy density of ～ 1.5 Joule/cm² and duration of 15–20 nsec. Hall effect measurement was applied to determine the electrical activity of the layers. Lattice location and the depth profile of Sb was studied by RBS and channeling technique. Measurements show that after laser annealing Sb occupies mostly substitutional sites in Si with 84% electrical activity. It has been shown that after laser annealing the concentration of Sb in lattice sites is almost an order of magnitude higher than the limit of solid solubility. Isochron and isothermal annealing of these samples up to 1150°C was carried out to study the kinetics of reverse annealing of antimony.     

Oxygen and hydrogen accumulation at buried implantation-damage layers in hydrogen- and helium-implanted Czochralski silicon

Oxygen and hydrogen accumulations at buried implantation-damage layers were studied after post-implant-ation annealing of hydrogen- and helium-implanted Czochralski (Cz) silicon. Hydrogen implantation was carried out at energies E=180 keV and doses D=2.7×10¹⁶ cm^-2, and helium implantation at E=300 keV and D=10¹⁶ cm^-2. For comparison hydrogen implantation was also done into float-zone (Fz) silicon wafers. Post-implantation annealing at 1000 °C was done either in H₂ or N₂ atmosphere. Hydrogen and oxygen concentration profiles were measured by secondary ion mass spectroscopy (SIMS). It is shown that the ambient during annealing plays a significant role for the gettering of oxygen at buried implantation-damage layers in Cz Si. For both hydrogen and helium implantations, the buried defect layers act as rather effective getter centers for oxygen and hydrogen at appropriate conditions. The more efficient gettering of oxygen during post-implantation annealing in a hydrogen ambient can be attributed to a hydrogen-enhanced diffusion of oxygen towards the buried implantation-damage layers, where a fast oxygen accumulation occurs. Oxygen concentrations well above 10¹⁹ cm^-3 can be obtained. From the comparison of measurements on hydrogen-implanted Cz Si and Fz Si one can conclude that at the buried defect layers hydrogen is most probably trapped by voids and/or may be stable as immobile molecular hydrogen species. Therefore hydrogen accumulated at the defect layers, and is preserved even after high-temperature annealing at 1000 °C. Received: 3 July 2000 / Accepted: 11 July 2000 / Published online: 22 November 2000     

Oxidation of bismuth nanodroplets deposit on GaAs substrate

Bismuth nanodroplets on GaAs substrate were obtained by metalorganic vapor phase epitaxy (MOVPE). New products have been synthesized when Bi nanodroplets are heated under oxygen atmosphere. The oxidation process of Bi nanodroplets consists of a heating from the room temperature to different oxidation temperatures (350, 500, 600 ^°C) with a temperature rate of 14 ^°C/min. The annealing duration was fixed to 30 min. The presence of oxygen in the products was confirmed by energy dispersive X-ray (EDX) measurements using a scanning electron microscope (SEM). SEM images show that Bi microcomposites density decrease and their size increases with increasing annealing temperature. After X-ray diffraction analysis of the products no obvious peaks could be observed. The reflectance spectra of the products were studied in spectral domains ranged from 200 nm to 1100 nm. By fitting the reflectivity signal, we extracted the thickness of the products and their refractive index variation versus wavelength. The results show that the thickness of the samples increases with increasing annealing temperature. The photoluminescence (PL) spectra under excitation at 325 nm shows a broad emission centered at around 1.92 eV.     

pH-Dependent shape changes of water-soluble CdS nanoparticles

Annealing of silicon-carbon nanoparticles was performed in argon at atmospheric pressure to enable formation of silicon carbide nanomaterials and/or carbon structures. Three precursor powders with increasing crystallinity and annealing temperatures from 1,900 to 2,600 °C were used to gain information about the effect of precursor properties (e.g. amorphous vs. nanocrystalline, carbon content) and annealing temperature on the produced materials. Three structures were found after annealing, i.e. silicon carbide crystals, carbon sheets and spherical carbon particles. The produced SiC crystals consisted of several polytypes. Low annealing temperature and increasing crystallinity of the precursor promoted the formation of the 3C-SiC polytype. Raman analysis indicated the presence of single-layer, undoped graphene in the sheets. The spherical carbon particles consisted of curved carbon layers growing from the amorphous Si–C core and forming a ‘nanoflower’ with a diameter below 60 nm. To our knowledge, the formation of this kind of structures has not been reported previously. The core was visible in transmission electron microscopy analysis at the annealing temperature of 1,900 °C, decreased in size with increasing temperature and disappeared above an annealing temperature of 2,200 °C. With increasing crystallinity of the precursor material, fewer layers (~5 with the most crystalline precursor) were detected in the carbon nanoflowers. The method presented opens up the possibility to produce new carbon nanostructures whose properties can be controlled by changing the properties of the precursor material or by adjusting an annealing temperature.     

Properties of magnesium silicate doped with chromium in porous silicon

Forsterite doped with Cr⁴⁺ ions is prepared in silicon-based structures according to a simple technique. These structures are of interest due to the characteristic luminescence in the near-IR range. Forsterite is synthesized by impregnation of porous silicon layers on n⁺-Si and p⁺-Si substrates with subsequent annealing in air. A photoluminescence response at a wavelength of 1.15 μm is observed at room temperature in porous silicon layers doped with magnesium and chromium for which the optimum annealing temperature is close to 700°C. The photoluminescence spectrum of porous silicon on the p⁺-Si substrate contains a broad band at a wavelength of approximately 1.2 μm. This band does not depend on the annealing temperature and the magnesium and chromium content and is most likely associated with the presence of dislocations in silicon. The experimental EPR data and eletrical properties of the structures are discussed. It is found that layers of pure porous silicon and chromium-doped porous silicon on n⁺-Si subtructures exhibit indications of discrete electron tunneling.     

CW CO2-laser annealing of arsenic implanted silicon

CW CO₂-laser annealing of arsenic implanted silicon was investigated in comparison with thermal annealing. Ion channeling, ellipsometry, and Hall effect measurements were performed to characterize the annealed layers and a correlation among the different methods was made. The laser annealing was done with power densities of 100 to 640 W cm⁻² for 1 to 20 s. It was found that the lattice disorder produced during implantation can be completely annealed out by laser annealing with a power density of 500 W cm⁻² and the arsenic atoms are brought on lattice sites up to 96±2%. The maximum sheet carrier concentration of 6×10¹⁵ cm⁻² was obtained for 1×10¹⁶ cm⁻² implantation after laser annealing, which was up to 33% higher than that after thermal annealing at 600 to 900°C for 30 min.     

In situ RHEED analysis of epitaxial Gd2O3 thin films grown on Si (001)

Epitaxial Gd₂O₃ thin films were successfully grown on Si (001) substrates using a two-step approach by laser molecular-beam epitaxy. At the first step, a ～0.8 nm thin layer was deposited at the temperature of 200 ^°C as the buffer layer. Then the substrate temperature was increased to 650 ^°C and in situ annealing for 5 min, and a second Gd₂O₃ layer with a desired thickness was deposited. The whole growth process is monitored by in situ reflection high-energy electron diffraction (RHEED). In situ RHEED analysis of the growing film has revealed that the first Gd₂O₃ layer deposition and in situ annealing are the critical processes for the epitaxial growth of Gd₂O₃ film. The Gd₂O₃ film has a monoclinic phase characterized by X-ray diffraction. The high-resolution transmission electron microscopy image showed all the Gd₂O₃ layers have a little bending because of the stress. In addition, a 5–6 nm amorphous interfacial layer between the Gd₂O₃ film and Si substrate is due to the in situ high temperature annealing for a long time. The successful Gd₂O₃/Si epitaxial growth predicted a possibility to develop the new functional microelectronics devices.     

Effect of annealing time on the structural and ferromagnetic properties of the GaMnN thin films

GaMnN thin films were deposited on a sapphire (0001) substrate by using laser assisted molecular beam epitaxy. Subsequently, the samples were annealed in the ammonia ambience at 1000 °C for different time lengths. The crystalline quality was improved gradually, and the room temperature ferromagnetism of our samples becomes stronger with the increase of the annealing time within 25 min. The X-ray photoelectron spectra analysis confirmed that the Mn³⁺ concentration in the GaMnN films increased after annealing. The stronger ferromagnetism was observed in the sample with the higher Mn³⁺ concentration. However, too long annealing time, such as 35 min, will lead to the degradation of the crystalline quality and the decrease of Mn³⁺ concentration, which results in the weakened ferromagnetism. The optimal annealing time is 25 min at 1000 °C in our experiments. Finally, the origin of the room temperature ferromagnetism in our samples was discussed preliminarily.     

A low-temperature and short-annealing process for metal oxide thin film transistors using deep ultraviolet light for roll-to-roll processing

Solution-processed metal oxide semiconductors have superior electron mobility and stability than solution-processed organic semiconductors. However, their fabrication requires a very-high-temperature and long-time annealing process. In this study, we utilized deep ultraviolet (DUV) light to decrease both the temperature and time of the annealing process. High external energy is required to break the organic bonds in a metal oxide film, which is generally supplied by a high-temperature annealing process carried out for a long duration.Alternatively, the required high energy can be supplied more efficiently by irradiating the metal oxide film with DUV light for a shorter duration. In this work, we used DUV light whose peaks at 172 nm instead of the generally used mercury lamp, peaking at 254 and 185 nm. Owing to this difference, thin film transistors (TFTs) could be fabricated on silicon wafers at a lower temperature and shorter duration as compared to the conditions used in previous studies. Various conditions, such as the heating temperature, duration of DUV irradiation, and N₂ flow rate, were optimized to control the heating temperature so as to achieve a mobility of 4.44 cm²/V·s and on–off ratio of 2 × 10⁷, which are much higher than those of a transistor annealed at 300 °C for 30 min (mobility, 1.31 cm²/V·s and on–off ratio, 7 × 10⁵).     

Structural investigations of silicon nanostructures grown by self-organized island formation for photovoltaic applications

The self-organized growth of crystalline silicon nanodots and their structural characteristics are investigated. For the nanodot synthesis, thin amorphous silicon (a-Si) layers with different thicknesses have been deposited onto the ultrathin (2 nm) oxidized (111) surface of Si wafers by electron beam evaporation under ultrahigh vacuum conditions. The solid phase crystallization of the initial layer is induced by a subsequent in situ annealing step at 700 °C, which leads to the dewetting of the initial a-Si layer. This process results in the self-organized formation of highly crystalline Si nanodot islands. Scanning electron microscopy confirms that size, shape, and planar distribution of the nanodots depend on the thickness of the initial a-Si layer. Cross-sectional investigations reveal a single-crystalline structure of the nanodots. This characteristic is observed as long as the thickness of the initial a-Si layer remains under a certain threshold triggering coalescence. The underlying ultra-thin oxide is not structurally affected by the dewetting process. Furthermore, a method for the fabrication of close-packed stacks of nanodots is presented, in which each nanodot is covered by a 2 nm thick SiO₂ shell. The chemical composition of these ensembles exhibits an abrupt Si/SiO₂ interface with a low amount of suboxides. A minority charge carrier lifetime of 18 µs inside of the nanodots is determined.     

Thermal stability of amorphous SiOC/crystalline Fe composite

We examined the thermal stability of amorphous silicon oxycarbide (SiOC) and crystalline Fe composite by in situ and ex situ annealing. The Fe/SiOC multilayer thin films were grown via magnetron sputtering with controlled length scales on a surface-oxidized Si (100) substrate. These Fe/SiOC multilayers were in situ or ex situ annealed at temperature of 600 °C or lower. The thin multilayer sample (~10 nm) was observed to have a layer breakdown after 600 °C annealing. Diffusion starts from low groove angle triple junctions in Fe layers. In contrast, the thick multilayer structure (~70 nm) was found to be stable and an intermixed layer (Fe_xSi_yO_z) was observed after 600 °C annealing. The thickness of the intermixed layer does not vary as annealing time goes up. The results suggest that the Fe_xSi_yO_z layer can impede further Fe, Si and O diffusion, and assists in maintaining morphological stability.     

Thermoelectric effect in laser annealed printed nanocrystalline silicon layers

Nanocrystalline boron and phosphorus doped silicon particles were produced in a microwave reactor, collected, and dispersed in ethanol. Pulsed laser annealing of spin‐coated films of these particles resulted in p‐ and n‐type conductive layers on flexible substrates if a threshold laser energy density of 60 mJ/cm² was exceeded. The thermopower of the laser sintered layers exhibits a distinct maximum at a doping concen‐ tration around 10¹⁹ cm^–3 for both boron and phosphorus doping with an absolute value of the Seebeck coefficient of about 300 µV/K. Since the thermal conductivity of the layers is reduced by nearly the same factor compared to bulk crystalline silicon as the electrical conductivity, these results are promising for the application of such nanocrystalline layers in thin film thermoelectric devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of activation of Si29+ ion-implantation in GaAs on ohmic contact resistance and electrical performances of MESFETs

The active layers of Metal Semiconductor Field Effect Transistors (MESFETs) are obtained by Si²⁹⁺ ion implantation in GaAs. Implantation was done at 35 keV with a higher dose near the wafer surface for facilitating easier formation of ohmic contacts, and at 180 keV with a lower dose for obtaining the device channel. Post-implantation annealing was carried out in a rapid thermal processor for activating the implants. Very high activation levels of about 60% for the n⁺ GaAs layer, and 85% for the n-GaAs channel layer were achieved by annealing at 955 °C for 25 s. Activation was characterized using C–V profiling, secondary ion mass spectrometry and by electrical device data of fabricated MESFETs. We attempt an experimental correlation between the ohmic contact resistance (R _c) and activation of both the n⁺ and the channel layer. It was found that very high and simultaneous activation of the n⁺ and channel layers results in very low contact resistances. The conduction of source-drain current into the channel is easily facilitated due to reduction in the resistance of the transition region at the interface of n⁺-contact and n-channel layers.     

Determination of migration of ion-implanted Ar and Zn in silica by backscattering spectrometry

It is well known that the refractive indices of lots of materials can be modified by ion implantation, which is important for waveguide fabrication. In this work the effect of Ar and Zn ion implantation on silica layers was investigated by Rutherford Backscattering Spectrometry (RBS) and Spectroscopic Ellipsometry (SE). Silica layers produced by chemical vapour deposition technique on single crystal silicon wafers were implanted by Ar and Zn ions with a fluence of 1–2?×10¹⁶ Ar/cm² and 2.5?×10¹⁶ Zn/cm², respectively. The refractive indices of the implanted silica layers before and after annealing at 300°C and 600°C were determined by SE. The migration of the implanted element was studied by real-time RBS up to 500°C. It was found that the implanted Ar escapes from the sample at 300°C. Although the refractive indices of the Ar-implanted silica layers were increased compared to the as-grown samples, after the annealing this increase in the refractive indices vanished. In case of the Zn-implanted silica layer both the distribution of the Zn and the change in the refractive indices were found to be stable. Zn implantation seems to be an ideal choice for producing waveguides.     

Annealing characteristics of highly doped ion implanted phosphorus layers in silicon

A combination of sheet resistance, stripping and Hall effect measurements have been made on phosphorus layers implanted into silicon at 40 and 100 keV with doses between 1 × 10¹⁵ and 5 × 10¹⁶ atoms/cm². The implants were made at room temperature and 450°C. After annealing at 650°C, the profile of electrically active phosphorus following a high dose room temperature implant, was found to be flat topped with a concentration of approximately 5 × 10²⁰/cm³. Very little diffusion occurred when annealing to 850°C where the free electron concentration increased to approximately 1.5 × 10²¹/cm³. Highly doped channeled tails were found when implanting at 450°C along the 〈110〉 direction and damage was being continuously annealed out preventing the formation of an amorphous phase. The rapid diffusion of the profile into the bulk found when annealing between 650°C and 850°C was speculated to be due to the presence of a dense dislocation entanglement in these layers following a hot implant.     

Annealing behaviour of arsenic implants in silicon

Abstract

The annealing behaviour of 80 keV room temperature arsenic implants in silicon below the amorphization dose has been studied by comparing the physical profile and the electrical profiles following different isochronal anneals.

It is shown that the electrically active fraction, which is about 0.4 after 30 min annealing at 600°C, increases continuously until 100% electrical activation of the arsenic ions is reached at about 900°C.

The activation energy for the annealing process has been found equal to 0.4 eV. A tentative interpretation of the mechanism involved is given.

From the analysis of the physical profiles obtained after isochronal annealing, an effective diffusion coefficient at 900°C equal to 5 × 10^?16 cm² s^?1 has been calculated.     

Synthesis of α-In 2 Te 3 thin films from In/Te bilayer by Si ion irradiation

In this work, In/Te bilayer thin films were prepared using sequential thermal evaporation method and subsequently irradiated using swift heavy ions (SHIs) of 100 MeV silicon (Si) with different fluences (1×10¹³ to 5×10¹³/cm²). The inter-diffusion of In and Te layers was highly controlled by SHI irradiation and the In₂Te₃ formation capability was compared with that of the conventional annealing method. The structural as well as optical properties of a post-sintered SHI-irradiated In/Te bilayer were investigated using X-ray diffraction (XRD) measurements and UV–visible spectroscopy, respectively. We found that irradiated samples showed single-phase In₂Te₃ under post-annealed conditions at 150 °C unlike that prepared using the conventional thermal annealing method, which showed mixed phases under similar conditions. This confirms the effective inter-diffusion in bilayer films by SHI irradiation toward the formation of single-phase In₂Te₃. The estimated optical band gap energy was found to be 1.1±0.5 eV and strongly corroborated the XRD results. In addition, the estimated refractive index (n) value of the SHI-irradiated sample (～3.3) was higher than that of the sample obtained through the conventional annealing method (～2.8). This proves that SHI offers a highly compact nature even at low temperatures. This work has a wide scope for achieving single-phase alloyed films through bilayer mixing by SHI irradiation.