Low-temperature preparation of phosphorus doped μc-Si:H thin films by low-frequency inductively coupled plasma assisted chemical vapor deposition

The phosphorus doped n-type hydrogenated microcrystalline silicon (n-μc-Si:H) thin films are prepared, at the two low substrate temperatures of room temperature and 200 °C, through a low-frequency inductively coupled plasma assisted chemical vapor deposition. The effect of the substrate temperature on the structural properties of the thin films, such as the X-ray Diffraction (XRD) patterns and the Raman spectra, is studied. The XRD measurements show that the diffraction orientations of the thin films present an obvious change when the radio frequency power is increased from 1300 W to 2300 W. The Raman spectra of the thin films deposited at room temperature unambiguously present a phase transition from the amorphous structure to microcrystalline structure whereas no structural phase transition is observed for the thin films deposited at 200 °C. The effect of the substrate temperature on the crystalline volume fraction of the thin films presents a large difference for the radio frequency power in the range of 1300 W-1700 W, while the difference becomes small when the power is increased from 1700 W to 2300 W. The deposition rate and the radio frequency power-sheet resistance curve of the thin films deposited at room temperature are obviously different from those of the thin films prepared at 200 °C. It is attributed to the joint effect of the radio frequency power and substrate temperature on the doping concentration. The electron energy distribution function of the species in the chamber is mainly distributed in a low energy range.     

Greatly enhanced infrared normal spectral emissivity of microstructured silicon using a femtosecond laser

The infrared normal spectral emissivity of microstructured silicon prepared by femtosecond laser irradiation in SF₆ was measured for the wavelength range 2.5 μm to 25 μm. Greatly enhanced emissivity compared to that of flat silicon was observed over the entire wavelength range. For a sample with 13-14 μm high spikes, the emissivity at a temperature of 100 °C is approximately 0.96. The emissivity decreases slightly in the wavelength region above 8 μm, but remains higher than 0.9 over most of the measured wavelength range. Also the average emissivity is less than Nextel- Velvet-811-21 Coating, it can be used stably at more wide temperatures from 100 °C to 400 °C. These results show the potential for microstructured silicon to be used as a flat blackbody source or silicon-based pyroelectric and microbolometer devices.     

Advances in silicon carbide X-ray detectors

The latest advances in SiC X-ray detectors are presented: a pixel detector coupled to a custom ultra low noise CMOS preamplifier has been characterized at room and high temperature. An equivalent noise energy (ENE) of 113 eV FWHM, corresponding to 6.1 electrons r.m.s., has been achieved with the detector/front-end system operating at +30 °C. A Fano factor of F=0.10 has been estimated from the ⁵⁵Fe spectrum. When the system is heated up to +100 °C, the measured ENE is 163 eV FWHM (8.9 electrons r.m.s.). It is determined that both at room and at high temperature the performance are fully limited by the noise of the front-end electronics. It is also presented the capability of SiC detectors to operate in environments under unstable temperature conditions without any apparatus for temperature stabilization; it has been proved that a SiC detector can acquire high resolution X-ray spectra without spectral line degradation while the system temperature changes between +30 and +75 °C.     

Thickness dependence of the amorphous-cubic and cubic-hexagonal phase transition temperatures of GeSbTe thin films on silicon nitride

The crystallization temperature of GeSbTe thin films with thicknesses between 11 and 87 nm on silicon nitride was studied through resistance versus temperature measurements. The amorphous-cubic phase transition occurs at ~ 150 °C for all films thicknesses, whereas the cubic-hexagonal phase transition temperature increases with film thickness, from ~ 200 °C for the 20 nm film to ~ 250 °C for the 87 nm film. The cubic-hexagonal transition occurs gradually for the 11 nm film. Implications for phase-change memory devices are discussed.     

Influence of magnetic annealing on high-frequency magnetic properties of FeCoNd films

FeCoNd thin film with thickness of 166 nm has been fabricated on silicon (1 1 1) substrates by magnetron co-sputtering and annealed for one hour under magnetic field at different temperatures (T_a) from 200 °C to 700 °C. The As-deposited and annealed FeCoNd film samples at T_a ≤ 500 °C were amorphous while the ones obtained at T_a ≥ 600 °C were crystallized. We found that the perpendicular anisotropy field gradually decreases as the annealing temperature increases from room temperature to 300 °C. A well induced in-plane uniaxial anisotropy is achieved at the annealing temperature between 400 and 600 °C. The variation of the dynamic magnetic properties of annealed FeCoNd films can be well explained by the Landau-Lifshitz equation with the variation of the anisotropy field re-distribution and the damping constant upon magnetic annealing. The magnetic annealing might be a powerful post treatment method for high frequency application of magnetic thin films.     

Temperature dependent microstructure of MTES modified hydrophobic silica aerogels

Hydrophobic silica aerogels were prepared using a single step sol-gel process followed by ethanol supercritical drying. Using tetraethoxysilane (TEOS) as a precursor and ammonium hydroxide as a catalyst the aerogel surface was chemically modified with methyltriethoxysilane (MTES). A MTES/TEOS molar ratio of 0.5 (M5) was used. The microstructure of the surface modified aerogels was evaluated as a function of heat treatment temperature over a range of 200-500 °C. The thermal stability was analyzed by simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC) and the microstructure was evaluated by physisorption analysis (BET) and scanning electron microscopy (SEM). The chemical composition and hydrophobicity/hydrophilicity of the aerogels were investigated by Fourier Transform-Infrared (FT-IR) spectroscopy. The M5 aerogels, which were initially hydrophobic, exhibited partial hydrophilicity at treatments above 244.5 °C and complete hydrophilicity above 429.9 °C. The surface area of the aerogels ranged from 776.65-850.20 m²/g. Pore size increased after heat treatment, ranging from 16.25 to 18.52 nm vs. an initial pore size of 14.71 nm. The maximum pore size of 18.52 nm was found at the lowest heat treatment temperature (~ 200 °C). Heat treatment had a mixed effect on the pore volume, as pore volumes decreased at lower treatments (~ 200-400 °C) and increased at higher heat treatments (~ 450-500 °C) relative to the untreated aerogels. With initial heat treatment the Si-CH₃ group began to oxidize to Si-OH. Aerogels heated above 429.9 °C exhibited hydroxyl polymerization leading to aerogels with large particles and a dense microstructure.     

Plasma oxidation of Al thin films on Si substrates

In this study, Al thin films deposited on silicon wafers by direct current magnetron sputtering were oxidized under radio frequency 13.56 MHz O₂ plasma at temperatures up to 550 °C. During oxidation, plasma powers as well as oxidation temperature and time were varied to investigate the oxidation behavior of the Al films. X-ray photoelectron spectroscopy and Auger electron spectroscopy results show that the apparent alumina could be observed after O₂ plasma treatment with powers above 200 W as well as at temperatures above 250 °C. However, no alumina increment could be discerned after individual either heat treatment at 550 °C or plasma treatment at room temperature. The thickness of alumina layers increased remarkably with plasma power and could reach about 60 nm when undergone 400 W O₂ plasma treatment at 550 °C for 2 h. Moreover, the thickness of alumina increased parabolically with time during plasma oxidation aided by thermal treatment. The deduced activation energy of such plasma oxidation was 19.1 ± 0.5 kJ/mol.     

Electronic properties of n-ZnO(Al)/p-Si heterojunction prepared by dc magnetron sputtering

The electronic properties of the interface between p-type Si and Al-doped ZnO have been investigated. Films of ZnO(Al) with a thickness of 300 nm were deposited at room temperature by dc magnetron sputtering and subsequently subjected to heat treatment in air in the temperature range 100-400 °C. Current-voltage (I-V), capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements were used to characterize the electrical properties of the heterostructure. The I-V measurements show a diode-like behavior with a rectification of ~ 3-4 orders of magnitude. However, annealing above 200 °C gives rise to a pronounced recombination/generation current in the depletion region, which correlates with an increase of the carrier concentration close to the interface and indicates defect formation. Indeed, DLTS reveals the presence of two prominent defect states, one at 0.38 eV above the valence band edge (E_v), and the other, formed during the heat treatment above 250 °C, around E_v + 0.43 eV, which is consistent with the I-V and C-V data.     

The carbonization of thin polyaniline films

Polyaniline films on silicon and ceramic supports were prepared in situ during the oxidative polymerization of aniline. The films were heated up to 500 °C in an inert nitrogen atmosphere. The changes in molecular structure during the carbonization have been studied by infrared spectroscopy and Raman scattering using 514, 633 and 785 nm laser excitation lines. The transformation from polyaniline salt to the base form has been detected above 100 °C. The conversion to nitrogen-containing carbon-like material followed above 200 °C. The molecular structure of the films produced during heating to 500 °C contains crosslinked phenazine-like and oxidized quinonoid units. The aniline oligomers deposited on the support in the early stages of aniline oxidation are stable during heating as it has been observed by resonance Raman scattering using 785 nm laser excitation line. The water contact angles changed after carbonization, and the films became more hydrophilic as carbonization progressed.     

Microwave-crystallization of amorphous silicon film using carbon-overcoat as susceptor

Crystallization of amorphous silicon (a-Si:H) film is extremely important in many aspects of electronic devices and has been heavily explored. We demonstrate that microwave irradiation, 200 W, is able to fast-crystallize a-Si:H film using as susceptor carbon-overcoat which contains graphite and carbon nano-tube. X-ray diffraction and Raman spectra reveal that nearly full crystallization is reached within 90 s. Microwave absorption by the carbon-overcoat generates thermal energy which heats up a-Si:H film to a threshold temperature 440 ± 10 °C required for initiation of microwave crystallization. Dielectric properties of a-Si:H film facilitate its self-heating and nucleation of Si crystallites at above the threshold temperature. This method is extendable to fast-crystallize a-Si:H film on a remote and large-area basis.     

Substrate temperature effect on the SiC passivation layer synthesized by an RF magnetron sputtering method

This paper describes amorphous silicon carbide (a-SiC) film as an alternative material to silicon nitride (SiN) and silicon oxide (SiO₂) for the passivation layer of solar cells. We deposited the film on p-type silicon (100) wafers and glass substrates by RF magnetron sputtering using a SiC (99%) target. Structural and optical properties of the films were investigated according to the process temperature (room temperature, 300 °C, 400 °C, 500 °C and 600 °C). The structural properties were analyzed by Raman microscopy and XPS (X-ray Photoelectron Spectroscopy). The XPS showed that the content of SiC in the film is increased when the substrate temperature is higher. The optical properties of the films were examined by UV-visible spectroscopy and Ellipsometer. The optical characteristic measurement showed that the lowest refractive index of the film is 2.65. Also, using carrier lifetime measurement, we investigated the performance of SiC as the passivation layer. At the substrate temperature of 600 °C, we obtained a highest carrier lifetime of 7.5 μs.     

Highly conductive p-type nanocrystalline silicon films deposited by RF-PECVD using silane and trimethylboron mixtures at high pressure

In this paper we present a study of boron-doped nc-Si:H films prepared by PECVD at high deposition pressure (≥4 mbar), high plasma power and low substrate temperature (≤200 °C) using trimethylboron (TMB) as a dopant gas. The influence of deposition parameters on electrical, structural and optical properties is investigated. We determine the deposition conditions that lead to the formation of p-type nanocrystalline silicon thin films with very high crystallinity, high value of dark conductivity (>7 (Ω cm)⁻¹) and high optical band gap (≥1.7 eV). Modeling of ellipsometry spectra reveals that the film growth mechanism should proceed through a sub-surface layer mechanism that leads to silicon crystallization.The obtained films are very good candidates for application in amorphous and nanocrystalline silicon solar cells as a p-type window layer.     

Comparative study of resistivity characteristics between transparent conducting AZO and GZO thin films for use at high temperatures

This paper compares in detail the resistivity behavior of transparent conducting Al-doped and Ga-doped ZnO (AZO and GZO) thin films for use in an air environment at high temperatures. AZO and GZO thin films with thicknesses in the range from approximately 30 to 100 nm were prepared on glass substrates at a temperature of 200 °C by rf superimposed dc or conventional dc magnetron sputtering deposition, pulsed laser deposition or vacuum arc plasma evaporation techniques. In heat-resistance tests, the resistivity was measured both before and after heat tests for 30 min in air at a temperature up to 400 °C. The resistivity stability of AZO thin films was found to be always lower than that of GZO thin films prepared with the same thickness under the same deposition conditions, regardless of the deposition technique. However, the resistivity of all AZO and GZO thin films prepared with a thickness above approximately 100 nm was stable when heat tested at a temperature up to approximately 250 °C. It was found that the resistivity stability in both GZO and AZO thin films is dominated by different mechanisms determined by whether the thickness is below or above approximately 50 nm. With thicknesses above approximately 100 nm, the increase in resistivity found in GZO and AZO films after heat testing at a temperature up to 400 °C exhibited different characteristics that resulted from a variation in the behavior of Hall mobility.     

Post-annealing behavior of Ni/Au Schottky contact on non-polar a-plane GaN

The effects of annealing on the performance of Schottky devices on a-plane GaN/r-plane sapphire were investigated. The results show that the post-anneal Schottky barrier height (SBH) increased with increasing annealing temperature, reaching a peak increase of 43% at 500 °C. A further increase in the anneal temperature above 500 °C degraded the SBH. The ideality factor displayed a weak dependence on post-annealing temperature until rising dramatically at a post-annealing temperature of 600 °C. The degradation at 600 °C post-annealing temperature can be attributed to the formation of Nickel-Gallides. In-situ current-voltage characteristics obtained between 15 °C and 165 °C revealed that both the ideality factor and SBH were stable up to 165 °C with increasing in-situ measurement temperature.     

Microwave synthesis of phase-pure, fine silicon carbide powder

Fine, monophasic silicon carbide powder has been synthesized by direct solid-state reaction of its constituents namely silicon and carbon in a 2.45 GHz microwave field. Optimum parameters for the silicon carbide phase formation have been determined by varying reaction time and reaction temperature. The powders have been characterized for their particle size, surface area, phase composition (X-ray diffraction) and morphology (scanning electron microscope). Formation of phase-pure silicon carbide can be achieved at 1300 °C in less than 5 min of microwave exposure, resulting in sub-micron-sized particles. The free energy values for Si + C → SiC reaction were calculated for different temperatures and by comparing them with the experimental results, it was determined that phase-pure silicon carbide can be achieved at around 1135 °C.     

Interfacial solid phase reactions in cobalt/aluminum oxide/silicon(001) system

Auger electron spectroscopy, secondary neutral mass spectrometry and high-resolution transmission electron microscopy were used to assess the chemical, morphological and structural modifications after annealing of cobalt/aluminum oxide/silicon(001) hetero-structure. The results show that the aluminum oxide forms a diffusion barrier for temperatures lower than 200 °C. Beyond this temperature, cobalt atoms diffuse in the silicon region without apparent modification of the barrier. At 340 °C, the asymmetric diffusion could be explained by the formation of an AlCoO complex oxide playing the role of a diffusion barrier for Si atoms.     

Void-free strong bonding of surface activated silicon wafers from room temperature to annealing at 600 °C

In order to investigate the high temperature application of surface activated silicon/silicon wafer bonding, the wafers were bonded at room temperature and annealed up to 600 °C followed by optical, electrical, mechanical and nanostructure characterization of the interface. Void-free interface with high bonding strength was observed that was independent of the annealing temperature. The bonding strength was as high as 20 MPa. The normalized interfacial current density was increased with the increase in the annealing temperature. A thin interfacial amorphous layer with a thickness of 8.3 nm was found before annealing, which was diminished at 600 °C. A correlation between the current density and nanostructure of the interface was observed as a function of the annealing temperature. The high quality silicon/silicon bonding indicates its potential use not only in low temperature microelectronic applications, but also in high temperature harsh environments.     

Effect of annealing temperature on ZnO:Al/p-Si heterojunctions

Al-doped, zinc oxide (ZnO:Al) films with a 1.2 at.% Al concentration were deposited on p-type silicon wafers using a sol-gel dip coating technique to produce a ZnO:Al/p-Si heterojunction. Following deposition and subsequent drying processes, the films were annealed in vacuum at five different temperatures between 550 and 900 °C for 1 h. The resistivity of the films decreased with increasing annealing temperature, and an annealing temperature of 700 °C provided controlled current flow through the ZnO:Al/p-Si heterojunction up to 20 V. The ZnO:Al film deposited on a p-type silicon wafer with 1.2 at.% Al concentration was concluded to have the potential for use in electronic devices as a diode after annealing at 700 °C.     

Effect of substrate on processing of multi-gun sputter deposited, near-stoichiometric Ni2MnGa thin films

Near-stoichiometric Ni₂MnGa thin films were sputter deposited with a multi-gun sputter deposition system onto sapphire, silicon dioxide and silicon substrates and exposed to heat treatments in vacuum. The multi-gun setup was proven to be feasible for switching compositions quickly and reliably. Using chemical, morphological, magnetic and structural characterisation methods the effects of the different substrates on the Ni₂MnGa film properties as a function of heat treatment temperature were studied: sapphire and silicon dioxide provided a metallurgically inert substrate for Ni₂MnGa thin films and resulted in films showing room temperature magnetizations of up to  ~ 350 kA/m and austenitic/martensitic structures upon heat treatments at 700 °C. The highest mechanical stability of Ni₂MnGa occurred on sapphire substrates, due to the closest match of the thermal expansion coefficients. Silicon substrates led to silicide formation for heating temperatures of 550 °C and above, leading to the loss of ferromagnetism and the austenite/martensite structure in the films.     

Anomalous behavior in ZnMgO thin films deposited by sol-gel method

The magnesium doped zinc oxide is a promising optical material to enhance the luminescence for possible application in solid state lighting. Magnesium doped zinc oxide thin films (Zn_0.85Mg_0.15O) were deposited by sol-gel route on p-type silicon and annealed at different temperatures in oxygen environment for an hour. The doping of magnesium in zinc oxide was confirmed by X-Ray diffraction and the samples were found to have wurtzite crystal structure with (002) preferred orientation. The films were characterized by Hall-effect, atomic force microscopy, UV-VIS spectroscopy, photoluminescence (PL) and work function measurements. The different studies exhibited an anomalous behavior for the film annealed at 900 °C. The Hall effect, work function measurements and UV-VIS spectroscopy indicated that the resistivity, work function and optical band gap increased as a function of annealing temperature (from 300 °C to 700 °C) however these parameters were found to decrease for the films annealed above 700 °C. The particle size increased with the annealing but for the samples annealed at 900 °C, the shape of the grains changed and became elongated like fibers as observed by the atomic force microscopy. The PL measurements displayed the existence of oxygen vacancies defects for the samples annealed at and above 600 °C. The possible mechanism for this anomaly has been discussed in this work.