High temperature oxidation behavior of CrN/AlN superlattice films

The oxidation behavior of CrN/AlN superlattice films with different bilayer periods (Λ), Al/(Cr + Al) ratios, and crystal structures of the AlN layer was investigated. The films were deposited using a pulsed dc closed field unbalanced magnetron sputtering system. The oxidation tests were carried out in the ambient air at elevated temperatures from 700 to 1100 °C for 1 h. The changes in the crystal phase, microstructure and hardness of the films after the oxidation tests were characterized using X-ray diffraction, scanning electron microscopy and nanoindentation, respectively. When both CrN and AlN layers were in the NaCl cubic structure, the film with Λ = 3.8 nm and an Al/(Cr + Al) ratio of 0.6 exhibited a superior oxidation resistance than the film with Λ = 12.4 nm and an Al/(Cr + Al) ratio of 0.19. The film with Λ = 3.8 nm maintained the nanolayered structure with an oxidation temperature up to 1000 °C by the protection of a thin and dense X-ray amorphous oxide layer. In contrast, when the AlN layers were in the Wurzite hexagonal structure, the film with Λ = 22.5 nm and an Al/(Cr + Al) ratio of 0.67 exhibited poor oxidation resistance. The film lost the superlattice structure at 800 °C and was completely oxidized at 1000 °C due to the formation of a porous crystalline oxide layer on the surface.     

Oxidation of the surface of a thin amorphous silicon film

The oxidation of clean crystalline silicon surfaces is self-limiting at moderate oxygen pressures (10^− 5 Pa) and temperatures (500 °C), forming 0.7-0.8 nm thick oxide layers. This study looks at the oxidation of a surface of a thin amorphous silicon film to establish if a similar mechanism is active in this case. We have devised a special experimental procedure to check the oxidation mechanism of thin amorphous silicon films. For the spectroscopic investigations we used photoemission with synchrotron radiation with the highest possible surface sensitivity and resolution. This permits a detailed decomposition of the Si 2p spectral details, using a mathematical decomposition procedure. The results clearly show that the oxidation mechanism of the surface of an amorphous silicon film under similar conditions is severely hindered compared to cases of crystalline substrates, indicating less reactivity at the surface and less transport of oxygen into the amorphous material.     

Performance enhancement of organic thin-film transistors with improved copper phthalocyanine crystallization by inserting ultrathin pentacene buffer

Organic thin-film transistors (OTFTs) with high crystallization copper phthalocyanine (CuPc) active layers were fabricated by inserting an ultrathin pentacene buffer layer between the dielectric and CuPc layers. Comparing with the OTFTs without a pentacene buffer layer, the charge carrier mobility of the OTFT with a buffer layer presented a much higher value of ~ 0.20 cm²/V s. Meanwhile, by investigating the morphology of the CuPc active layer with an ultrathin pentacene buffer layer through scanning electron microscopy and X-ray diffraction, the high crystallization of the CuPc film with a larger grain size and less grain boundaries can be observed. As a result, the resistance of the conducting channel was decreased, leading to a performance improvement of the OTFTs.     

Tensile strain engineering of Si thin films using porous Si substrates

Highly tensile strained (up to 2.2%) thin monocrystalline silicon (mc-Si) films were fabricated by a simple and low-cost method based on the in-plane expansion of meso-porous silicon (PS) substrates upon low temperature oxidation. To control the film thickness below 100 nm, an original “two wafer” technique was employed during the porosification process. This method enables the fabrication of a 60 nm thick mc-Si films on 250 μm thick meso-porous silicon substrates over areas as large as 2 in. with a surface roughness and cleanliness comparable to that of standard Si wafers. Crack-free 60 nm thick Si films can be strained up to 1.2% by controlled low temperature oxidation of the PS substrate. Structural and strain analysis of the PS/mc-Si structures performed by transmission electron microscopy and micro-Raman scattering spectroscopy are reported.     

Highly c-axis oriented AlN layers grown on c-plane sapphire substrates by radio-frequency sputter epitaxy at 1080 °C

Aluminum nitride (AlN) single-crystalline layers were grown on c-plane sapphire substrates by radio-frequency magnetron sputter epitaxy using N₂/Ar mixture ambient gas and 5-N grade Al target. The crystalline structures of the AlN layers depending on substrate temperature and N₂ composition ratio in ambient gas, were predominantly studied. The crystalline quality of the AlN layer was improved by elevating substrate temperature, and the full-widths at half-maximum (FWHMs) of X-ray rocking curves (XRC) for both symmetric and asymmetric planes of AlN layers grown at N₂ composition ratio of around 25%, became low. The FWHMs of XRC for (0002) diffraction of the AlN layers grown at 1080 °C, were less than 20 arcsec. The surface root-mean-square roughness of such highly c-axis oriented AlN layer was determined by atomic force microscopy, and was increased from 0.6 nm to 1.3 nm when AlN layer thickness was varied from 0.15 to 0.7 μm.     

Effect of annealing on the characteristics of Au/Cr bilayer films grown on glass

Au layers with thickness of about 110 nm were sputter-deposited on unheated glass substrates coated with a Cr layer about 20 nm thick. The chamber was evacuated to a pressure of 2 Pa and then sputtering was carried out at Ar pressure of 4 Pa. The Au/Cr bilayer films were annealed in a vacuum of 5×10⁻⁴ Pa at 170°C, 180°C, 200°C and 250°C for from 5 to 120 min, respectively. Atomic force microscopy was used to observe the structural characteristic of the bilayer films. Auger electron spectroscopy was used to analyze the composition inside the Au layers. The sheet resistance of the films was measured using the four-point probe technique. The grain size of the bilayer film gradually increases with an increase in annealing temperature while its average surface roughness ranging from 4.5 to 6.8 nm does not show any systematic change with annealing temperature and time. No impurities such as carbon, nitrogen and oxygen are detected inside all of the Au layers. When the annealing temperature reaches 200°C and the annealing time exceeds 30 min, chromium atoms markedly diffuse into the Au layer. Furthermore, for the bilayer films annealed at 250°C, chromium atoms have markedly diffused into the Au layer even for annealing time of 5 min. Regardless of the increase in grain size of the Au layer, the diffusion of chromium atoms into the Au layer causes an increase in the resistivity of the bilayer film.     

Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays

A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiO_x) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiO_x film was less than 0.05 g/m²/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m² day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiO_x by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10^− 4 ohm cm.     

Influence of thermal oxidation on the interfacial properties of ultrathin strained silicon layers

In this work we examine the influence of thermal oxidation on the electrical characteristics of ultra-thin strained silicon layers grown on relaxed Si_0.78Ge_0.22 substrates under moderate to high thermal budget conditions in N₂O ambient at 800 °C. The results reveal the presence of a large density of interfacial traps which depends on the oxidation process. As long as the strained silicon layer remains between the growing oxide and the underlying Si_0.78Ge_0.22 layer, the density of interface traps increases with increasing oxidation time. When the oxidation process consumes the s-Si layer the interface state density undergoes a significant reduction of the order of 40%. This experimental evidence signifies that the strained silicon-Si_0.78Ge_0.22 interface is a major source of the measured interfacial defects. This situation can be detected only when the front SiO₂-strained silicon interface and the rear strained silicon-Si_0.78Ge_0.22 interface are in close proximity, i.e. within a distance of 5 nm or less. Finally, the influence of the material quality deterioration—as a result of the thermal treatment—to the interfacial properties of the structure is discussed.     

Deposition and structural properties of RF magnetron-sputtered ZnO thin films on Pt/Ti/SiNx/Si substrate for FBAR device

This work investigates high-quality bottom electrode and piezoelectric film used in a thin-film bulk acoustic resonator (TFBAR) device. The titanium (Ti) seeding layer and platinum (Pt) bottom electrode were deposited on silicon substrates by DC sputtering using a dual-gun system. Zinc oxide (ZnO) was then deposited onto the Pt bottom electrode by RF magnetron sputtering. Field-emission scanning electron microscopy (SEM), atom force microscopy (AFM) and the four-point probe method showed that the Pt bottom electrode deposited on the Ti seeding layer exhibited favorable characteristics, such as a crystallite size of less than 10 nm, a surface roughness of 0.69 nm and a sheet resistance of 2.27 Ω/□. The ZnO thin film with a highly c-axis-preferred orientation (FWHM = 0.28°) and a roughness of 6.22 nm was investigated by X-ray diffraction (XRD) and AFM analysis, respectively. The bottom electrode with a low resistance and the highly crystalline ZnO thin film will contribute significantly to the favorable characteristics of the FBAR devices.     

Deposition of Al-doped ZnO thin-films with radio frequency magnetron sputtering for a source/drain electrode for pentacene thin-film transistor

Al-doped ZnO thin-films were deposited with the radio frequency magnetron sputtering technique at various temperatures and sputtering powers for a source/drain electrode in the pentacene thin-film transistor. With the increase in the deposition temperature and the decrease in the radio frequency sputtering power, the crystallinity was increased and the surface roughness was decreased, which lead to the decrease in the electrical resistivity of the film. Al-doped ZnO film deposited at 200 °C and sputtering power of 50 W showed a low resistivity (9.73 × 10⁴ μΩcm), high crystallinity, low roughness and uniform surface morphology. The pentacene thin-film transistor fabricated with Al-doped ZnO film as a source/drain electrode showed a device performance, (mobility: 7.89 × 10^− 3 cm²/Vs and on/off ratio: ~ 5 × 10⁴) which is comparable with an indium tin oxide electrode grown at room temperature.     

The role of the Zn buffer layers in the structural and photoluminescence properties of ZnO films on Zn buffer layers deposited by RF magnetron sputtering

Highly c-axis oriented ZnO thin films were grown on Si (100) substrates with Zn buffer layers. Effects of the Zn buffer layer thickness on the structural and optical qualities of ZnO thin films were investigated for the ZnO films with the buffer layers 90, 110, and 130 nm thick using X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analysis techniques. It was confirmed that the quality of a ZnO thin film deposited by RF magnetron sputtering was substantially improved by using a Zn buffer layer. The highest ZnO film quality was obtained with a Zn buffer layer 110 nm thick. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.     

Hot wire CVD deposition of nanocrystalline silicon solar cells on rough substrates

In silicon thin film solar cell technology, frequently rough or textured substrates are used to scatter the light and enhance its absorption. The important issue of the influence of substrate roughness on silicon nanocrystal growth has been investigated through a series of nc-Si:H single junction p-i-n solar cells containing i-layers deposited with Hot-wire CVD. It is shown that silicon grown on the surface of an unoptimized rough substrate contains structural defects, which deteriorate solar cell performance. By introducing parameter v, voids/substrate area ratio, we could define a criterion for the morphology of light trapping substrates for thin film silicon solar cells: a preferred substrate should have a v value of less than around 1 × 10^- 6, correlated to a substrate surface rms value of lower than around 50 nm. Our Ag/ZnO substrates with rms roughness less than this value typically do not contain microvalleys with opening angles smaller than ~ 110°, resulting in solar cells with improved output performance. We suggest a void-formation model based on selective etching of strained Si-Si atoms due to the collision of growing silicon film surface near the valleys of the substrate.     

Experiments and theory on pentacene in the thin film phase: structural, electronic, transport properties, and gas response to oxygen, nitrogen, and ambient air

We investigated the morphological, structural, electronic, and transport properties of pentacene thin films grown by vacuum thermal evaporation on different inert substrates at room temperature. The results of our atomic force microscopy (AFM), X-ray diffraction and scanning tunnelling microscopy (STM) analysis show a structure in the so called “thin film phase” with 1-2 μm sized grains. Atomic terraces are clearly evidenced with AFM and give an inter-planar spacing of 1.54 nm corresponding to the (001) distance. The Scanning Tunneling Spectroscopy measurements show an HOMO-LUMO gap of 2.2 eV. After vacuum thermal evaporation on patterned substrates with different inter-electrodes distances, we have performed in situ measurements of the electrical response of such thin films. We found for these films a resistivity of ρ = 4.7 ± 0.2 · 10⁴ Ω m, that is an order of magnitude lower than the value reported to date in literature for single crystals of pentacene. This value is not affected by the presence of grain boundaries. The resistivity is further reduced by a factor 8.9 ± 0.7, 14 ± 1, 2.3 ± 0.3 upon exposure to oxygen, nitrogen and ambient air, respectively. In addition density functional theory calculations have been performed to investigate the electronic structure of pentacene in this specific phase, focusing on the effects on the relevant electronic properties of the relative orientation of the molecules within the crystalline unit cell, so far experimentally unknown. Our results show that the energy bandwidth and band-gap are crucially affected by the molecular stacking. Furthermore, by comparing our theoretical spectra with the scanning tunneling spectroscopy (STS) measurements, we propose a molecular arrangement that gives a good agreement with experiments as far as the relevant orbitals are concerned. For this polymorph, we find a HOMO and LUMO bandwidth of ≈ 0.7 eV and ≈ 0.8 eV, respectively, which are significantly larger than those obtained for the pentacene bulk-phase and are consistent with the larger conductivity experimentally observed in pentacene thin films.     

Investigation of the tribological behavior and its relationship to the microstructure and mechanical properties of a-SiC:H films elaborated by low frequency plasma assisted chemical vapor deposition

Amorphous hydrogenated silicon carbide (a-SiC:H) coatings are promising candidates for tribological applications in the mechanical and aeronautical industries. Alternately high values of hardness H (15 < H < 32 GPa) and elastic modulus E contribute to their good wear resistance as well as to a low friction coefficient. The latter has been found to vary in the range 0.1 < μ < 0.65, depending upon the microstructure of the layers. The roughness of the films determined by atomic force microscopy is in all cases low (Ra ~ 5 nm). Comparisons between the tests carried out in air and those performed under vacuum conditions point to a substantial role of the adhesive part of the friction coefficient in vacuum. They also highlight the role played by the transfer layer between the film and the pin in producing a low friction coefficient for several coatings. This transfer layer consists chiefly of silicon and oxygen (O/Si ~ 2), whilst low quantities of carbon are also present.     

AES investigation of the stainless steel surface oxidized in plasma

Auger electron spectroscopy (AES) depth profiling was used to study the oxidation phenomena of AISI316L stainless steel during treatment with oxygen plasma. Samples were exposed to low-pressure RF plasma with a high dissociation degree, so that the flux of oxygen atoms onto the sample surface exceeded 10²⁴ m⁻² s⁻¹. A set of samples was oxidized 4 min at different temperatures up to 1300 K during plasma treatment. AES measurements showed that the oxide film thickness increased with the increasing temperature. The thickness of the oxide film on the samples oxidized in plasma at 300 K was nearly the same as for the untreated sample. The thickness of the oxide film of the samples which were oxidized at 1000 K was about 170 nm and it consisted of iron oxide. The thickest oxide film of about 350 nm was found on the samples heated in oxygen plasma to 1300 K. Depth profiling showed the uppermost layer of manganese oxide, followed by a mixture of chromium oxide and iron oxide. The scanning electron microscope analyses showed a dramatic increase of the surface roughness.     

Porous silicon oxide sacrificial layers deposited by pulsed-direct current magnetron sputtering for microelectromechanical systems

The development of silicon oxide layers with high etch rates to be used as sacrificial layers in surface micromachining for microsystems fabrication poses a great technological challenge. In this work, we have investigated the possibility of obtaining easily removable silicon oxide layers by pulsed-direct current (DC) magnetron reactive sputtering. We have carried out a comprehensive study of the influence of the deposition parameters (total pressure and gas composition) on the composition, residual stress and lateral etch rate in fluoride wet solutions of the films. This study has allowed to determine the sputtering conditions to deposit, at high rates (up to 0.1 μm/min), silicon oxide films with excellent characteristics for their use as sacrificial layers. Films with roughness around 5 nm rms, residual stress below 100 MPa and very high lateral etch rate (up to 5 μm/min), around 70 times higher than for thermal silicon oxide, have been achieved. The structural characteristics of these easily removable silicon oxide layers have been assessed by infrared spectroscopy and atomic force microscopy, which have revealed that the films exhibit a porous structure, related to very specific sputter conditions. Finally, the viability of these films has been demonstrated by using them as sacrificial layer in the fabrication process of AlN-based microresonators.     

Oxidation resistance of thin boron carbo-nitride films on Ge(100) and Ge nanowires

Chemical vapor deposition of thin (< 10 nm) films of amorphous boron carbo-nitride (BC_0.7N_0.08, or BCN) on Ge(100) and Ge nanowire (GeNW) surfaces was studied to determine the ability of BCN to prevent oxidation of Ge. X-ray photoelectron spectroscopy was used to track Ge oxidation of BCN-covered Ge(100) upon exposure to ambient, 50 °C deionized water, and a 250 °C atomic layer deposition HfO₂ process. BCN overlayers incorporate O immediately upon ambient or water exposure, but it is limited to 15% O uptake. If the BCN layer is continuous, the underlying Ge(100) surface is not oxidized despite the incorporation of O into BCN. The minimum continuous BCN film thickness that prevents Ge(100) oxidation is ~ 4 nm. Thinner films (≤ 3.2 nm) permitted Ge(100) oxidation in each of the oxidizing environments studied. GeNWs with a 5.7 nm BCN coating were resistant to oxidation for at least 5 months of ambient exposure. High resolution transmission electron microscopy images of HfO₂/BCN/Ge(100) cross-sections and BCN-coated GeNWs reveal clean, abrupt BCN-Ge(100) interfaces.     

Correlation between thermal fatigue and thermomechanical properties during the oxidation of multilayered TiSiN nanocomposite coatings synthesized by a hybrid physical/chemical vapour deposition process

TiSiN and TiSiAlN coatings were deposited on M2 steel by a hybrid physical/chemical vapour deposition process. SiH₄ was used as precursor for Si, while metals were brought by arc evaporation. This hybrid process allowed us to control the silicon enrichment along the coating thickness. Both films were synthesized applying a serrated silane partial pressure during deposition, leading to a multilayered structure with a 700 nm period. X-ray diffraction analyses showed only TiN peaks, whose width revealed a mean grain size below 10 nm. The multilayer structure and the nanometric size of the grains in layers containing a high Si content were observed by cross-section microscopy in transmission mode. Mechanical properties were improved compared to both TiN and SiN_x references, in relation to the nanocomposite microstructure of layers enriched in silicon. The oxidation behaviour was assessed by thermogravimetric analyses. The oxidation resistance was studied in isothermal, dynamic as well as cycling (10-cycle runs 25-800-25 °C) conditions. The multilayered nanocomposite TiSiN film exhibited a high durability in terms of mechanical and oxidation behaviours. Thermal cycling experiments revealed its high resistance which seems to result from a synergy between the shield effect of the SiN_x network — that would limit the oxidation process — and the intrinsic “deformability” of TiN layers — that would withstand the volume modifications of the substrate due to temperature variations. A further addition of aluminium, without significantly affecting the mechanical properties, contributes to the improvement of the oxidation resistance thanks to the formation of the expected outer refractory alumina layer.     

Double-gate organic thin-film transistor using a photosensitive polymer as the dielectric layer

Dichromated poly (vinyl alcohol) (DCPVA) has been proposed for protecting the pentacene active layer from organic solvents such as PGMEA. DCPVA is water-soluble, and the damage to the pentacene active layer can be minimized compared to other materials dissolved in organic solvents. In this study, the dielectric properties and leakage current density of DCPVA gate dielectric samples with different ammonium dichromate (ADC) concentrations and exposure times were obtained from 6.73 to 10.5 and 6.37 × 10^− 5 to 4.87 × 10^− 8 (A/cm²) respectively. Also the FTIR spectroscopy was used to determine the cross-linked DCPVA films based on the relative intensity of the vibration bands of chromium(VI) and the OH group. According to FTIR, the performance of double-gate OTFTs based on blending ADC:PVA ratio of 0.25:1 film as dielectric layers of double-gate OTFT devices to exclude the acute influence of large gate leakage current caused by excess hydroxyl groups and residual Cr⁶⁺ in dielectric layer on device performance.By analyzing the electrical performance, it was found that the fabricating method of the top-dielectric layer has a great effect on the performance of double-gate OTFTs. SEM images of pentacene microstructure show significant cracks on the top of the pentacene layer due to the swelling effect of the DCPVA polymer after post-baking. Therefore, we used vacuum drying to form photosensitive DCPVA as the top-dielectric layer of a double-gate OTFT circuit array. The use of top-dielectric and electrode layers on the top of pentacene significantly increased the performance of double-gate OTFTs. In summary, we eventually demonstrate that double-gate OTFTs using a low-temperature solution process exhibited an equivalent mobility of 0.53 cm²/Vs and on-off ratio of 8 × 10³.     

Instantaneous cleaning of silicon substrates by mesoplasma for high-rate and low-temperature epitaxy

Homoepitaxial films having properties identical to films deposited on the conventional wet-cleaned substrates have been achieved even in the absence of any substrate pre-treatment through the mesoplasma CVD. X-ray photoelectron spectroscopy reveals that the native oxide layer is effectively removed by exposure of the bare silicon substrates to the Ar-H₂ plasma at exposure times as short as 2 s and temperatures less than 100 °C. Although an exposure to the Ar-H₂ plasma is accompanied by an anisotropic island formation resulting in an increase in rms roughness (~ 5 nm) of the surface, addition of as little as 2 sccm SiH₄ into the plasma reduces the roughness greatly. The absence of Si-H_x peaks in the FTIR spectrum and uniform concentration distribution of H and O atoms across the growth interface observed by SIMS analysis indicate that minimal damage was induced in the silicon film by the hydrogen while attaining high yield of surface cleaning. These suggest that surface interaction with Ar-H₂ plasma at the mesoplasma condition supportively facilitate lateral growth at low temperature in the way of instantaneous surface cleaning and anisotropic Si etching structure favorable for incorporation of the atoms comprising the Si nanoclusters as growth precursors.