Enhanced field emission from Si doped nanocrystalline AlN thin films

Si doped and undoped nanocrystalline aluminum nitride thin films were deposited on various substrates by direct current sputtering technique. X-ray diffraction analysis confirmed the formation of phase pure hexagonal aluminum nitride with a single peak corresponding to (1 0 0) reflection of AlN with lattice constants, a = 0.3114 nm and c = 0.4986 nm. Energy dispersive analysis of X-rays confirmed the presence of Si in the doped AlN films. Atomic force microscopic studies showed that the average particle size of the film prepared at substrate temperature 200 °C was 9.5 nm, but when 5 at.% Si was incorporated the average particle size increased to ∼21 nm. Field emission study indicated that, with increasing Si doping concentration, the emission characteristics have been improved. The turn-on field (E_to) was 15.0 (±0.7) V/μm, 8.0 (±0.4) V/μm and 7.8 (±0.5) V/μm for undoped, 3 at.% and 5 at.% Si doped AlN films respectively and the maximum current density of 0.27 μA/cm² has been observed for 5 at.% Si doped nanocrystalline AlN film. It was also found that the dielectric properties were highly dependent on Si doping.     

Chemical vapor deposition of ZrxTi1−xO2 and HfxTi1−xO2 thin films using the composite anhydrous nitrate precursors

Zr-Ti and Hf-Ti composite nitrates were successfully developed as single-source precursors for the chemical vapor deposition (CVD) of Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ thin films. The Zr-Ti nitrate can be assumed as a solid solution of the individual Zr and Ti nitrates, and the Zr/Ti molar ratio in the deposited Zr_xTi_1−xO₂ films is consistent with that in the precursor. The Hf-Ti nitrate appears to be a mixture of the Hf and Ti nitrates and the composition of the deposited Hf_xTi_1−xO₂ films depends remarkably on the heating time of precursor. Both Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ films exhibit trade-off properties between band gap and dielectric constant. The obtained results suggest that Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ films are promising candidates for gate dielectric application to improve the scalability and reduce the leakage current of the future complementary metal-oxide-semiconductor (CMOS) devices.     

Field emission from a-GaN films deposited on Si (100)

Amorphous gallium nitride (a-GaN) films have been deposited on Si (100) substrates using ion-assisted deposition. The deposited films were characterised by X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD confirms the amorphous nature of the films and AFM showed nanostructures in the films. The field electron emission from the film was obtained in a probe-hole field emission microscope, and the current-voltage (I-V) characteristics were studied. The corresponding Fowler-Nordheim (F-N) plots showed a linear behaviour. A current density of 0.1 A/cm² has been obtained for 1.2 V/μm electric field. The field emission current-time (I-t), curves were recorded at a current level of 500 nA for 3 h. The field emission behaviour is compared with that of crystalline GaN as reported in literature.     

Microstructure and interfacial properties of HfO2-Al2O3 nanolaminate films

High-k HfO₂-Al₂O₃ composite gate dielectric thin films on Si(1 0 0) have been deposited by means of magnetron sputtering. The microstructure and interfacial characteristics of the HfO₂-Al₂O₃ films have been investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry (SE). Analysis by XRD has confirmed that an amorphous structure of the HfO₂-Al₂O₃ composite films is maintained up to an annealing temperature of 800 °C, which is much higher than that of pure HfO₂ thin films. FTIR characterization indicates that the growth of the interfacial SiO₂ layer is effectively suppressed when the annealing temperature is as low as 800 °C, which is also confirmed by spectroscopy ellipsometry measurement. These results clearly show that the crystallization temperature of the nanolaminate HfO₂-Al₂O₃ composite films has been increased compared to pure HfO₂ films. Al₂O₃ as a passivation barrier for HfO₂ high-k dielectrics prevents oxygen diffusion and the interfacial layer growth effectively.     

Structural, dielectric and pyroelectric studies of Pb1−XCaXTiO3 thin films

Thin films of Pb_1−xCa_xTiO₃ [x=0.20, 0.24 and 0.28] have been prepared on ITO coated Corning glass substrates by sol gel technique. The perovskite phase of PCT films is formed at 650 °C with a polycrystalline tetragonal structure. The tetragonal factor (c/a) decreases with increasing Ca concentration. Dielectric, pyroelectric and ferroelectric studies have been carried out on these films. The effects of introduction of Ca ion in PbTiO₃ have also been discussed.     

Efficient field emission from coiled carbon nano/microfiber on copper substrate by dc-PECVD

Crystalline coiled carbon nano/micro fibers in thin film form have been synthesized via direct current plasma enhanced chemical vapor deposition (PECVD) on copper substrates with acetylene as a carbon precursor at 10 mbar pressure and 750 °C substrate temperature. The as-prepared samples were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). XRD pattern as well as selected area electron diffraction (SAED) pattern showed that the samples were crystalline in nature. SEM and HRTEM studies showed that as synthesized coiled carbon fibers are having average diameter ∼100 nm and are several micrometers in length. The as-prepared samples showed moderately good electron field emission properties with a turn-on field as low as 1.96 V/μm for an inter-electrode distance 220 μm. The variation of field emission properties with inter-electrode distance has been studied in detail. The field emission properties of the coiled carbon fibrous thin films are compared with that of crystalline multiwalled carbon nanotubes and other carbon nanostructures.     

Band bending and band alignment at HfO2/HfSixOy/Si interfaces

Band bending and band alignment at HfO₂/SiO₂/Si and HfO₂/Hf/SiO₂/Si interfaces were investigated using X-ray photoelectron spectroscopy. After Hf-metal pre-deposition, a 0.55 eV band bending in Si and a 1.80 eV binding energy decrease for Hf 4f and O 1s of HfO₂ were observed. This was attributed to the introduction of negative space charges at interface by Hf pre-deposition. Band bending decrease and synchronous binding energy increases of O 1s and Hf 4f for HfO₂ were observed during initial Ar⁺ sputtering of the Hf pre-deposited sample. This was interpreted through the neutralization of negative space charges by sputtering-induced oxygen vacancies.     

Field-electron emission from polyimide-ablated films

Polyimide-ablated film was deposited by using pulsed laser ablation of a polyimide target, and field-electron emission from the film was observed for the first time. The turn-on field of the polyimide-ablated film is 12 V/μm. The current density is 0.725 mA/cm², and the emission sites density is on the order of 10⁶/cm² at the applied field of 24 V/μm. The field-electron emission measurements indicate that this kind of film could be a new cold cathode material. It is suggested that the graphite-like clusters contained in the film play an important role in the field-electron emission. Received: 2 February 2000 / Accepted: 13 March 2000 / Published online: 9 August 2000     

AlNxOy thin films deposited by DC reactive magnetron sputtering

AlN_xO_y thin films were produced by DC reactive magnetron sputtering, using an atmosphere of argon and a reactive gas mixture of nitrogen and oxygen, for a wide range of partial pressures of reactive gas. During the deposition, the discharge current was kept constant and the discharge parameters were monitored. The deposition rate, chemical composition, morphology, structure and electrical resistivity of the coatings are strongly correlated with discharge parameters. Varying the reactive gas mixture partial pressure, the film properties change gradually from metallic-like films, for low reactive gas partial pressures, to stoichiometric amorphous Al₂O₃ insulator films, at high pressures. For intermediate reactive gas pressures, sub-stoichiometric AlN_xO_y films were obtained, with the electrical resistivity of the films increasing with the non-metallic/metallic ratio.     

Modelling and Optimization for Deposition of SiOxNy Films by Radio-Frequency Reactive Sputtering

SiOxNy films are deposited by reactive sputtering from a Si target in Ar/O2/N2 atmospheres. In order to achieve the control of film composition and to keep a high deposition rate at the same time, a new sputtering model based on Berg＇s work is provided for the condition of double reactive gases. Analysis based on this model shows that the deposition process can easily enter the target-poisoning mode when the preset gas flow （N2 in this work） is too high, and the film composition will change from nitrogen-rich to SiO2-like with the increase of oxygen supply while keeping the N2 supply constant. The modelling results are confirmed in the deposition process of SiOxNy. Target self-bias voltages during sputtering are measured to characterize the different sputtering modes. FTIR-spectra and dielectric measurements are used to testify the model prediction of composition. Finally, an optimized sputtering condition is selected with the O2/N2 flow ratio varying from 0 to I and N2 supply fixed at I sccm. Average deposition rate of 17nm/min is obtained under this selected condition, which has suggested the model validity and potential for industry applications.     

Structural, optical properties and band gap alignments of ZrOxNy thin films on Si (1 0 0) by radio frequency sputtering at different deposition temperatures

ZrO_xN_y thin films have been prepared by radio frequency magnetron sputtering at various substrate temperatures. The effect of substrate temperature on structural, optical properties and energy-band alignments of as-deposited ZrO_xN_y thin films are investigated. Atomic force microscopy results indicate the decreased root-mean-square (rms) values with substrate temperature. Fourier transform infrared spectroscopy spectra indicate that an interfacial layer has been formed between Si substrate and ZrO_xN_y thin films during deposition. X-ray photoelectron spectroscopy and spectroscopy ellipsometry (SE) results indicate the increased nitrogen incorporation in ZrO_xN_y thin films and therefore, the decreased optical band gap (E_g) values as a result of the increased valence-band maximum and lowered conduction-band minimum.     

Preparation of composition-controlled silicon oxynitride films by sputtering; deposition mechanism,and optical and surface properties

Silicon oxynitride films have been grown on silicon by current-controlled reactive sputtering. The content of oxygen in the films could be well controlled by regulating the sputtering current under the reactive gas of Ar+ N₂ with an oxygen content of around 3%. The atomic ratio of oxygen to nitrogen in the silicon oxynitride film became larger with increasing sputtering current. It has been found that electron irradiation of the silicon substrate induces adsorption of oxygen and nitrogen. The degree of oxygen adsorption was about ten times larger than that of nitrogen. This phenomenon is a key mechanism in controlling the film composition. The adsorptive mechanism might be explained by the phenomenon of surface activation by the electron bombardment. Utilizing this technique, wettability by germanium of silicon oxynitride films could be controlled by varying their oxygen and nitrogen contents. A better wetting condition was obtained from films with large atomic ratio of nitrogen to oxygen in the silicon oxynitride film.     

Field emission from ZnS nanorods synthesized by radio frequency magnetron sputtering technique

The field emission property of zinc sulphides nanorods synthesized in the thin film form on Si substrates has been studied. It is seen that ZnS nanorod thin films showed good field emission properties with a low-macroscopic turn-on field (2.9-6.3 V/μm). ZnS nanorods were synthesized by using radio frequency magnetron sputtering of a polycrystalline prefabricated ZnS target at a relatively higher pressure (10⁻¹ mbar) and at a lower substrate temperature (233-273 K) without using any catalyst. Transmission electron microscopic image showed the formation of ZnS nanorods with high aspect ratio (>60). The field emission data were analysed using Fowler-Nordhiem theory and the nearly straight-line nature of the F-N plots confirmed cold field emission of electrons. It was also found that the turn-on field decreased with the decrease of nanorod's diameters. The optical properties of the ZnS nanorods were also studied. From the measurements of transmittance of the films deposited on glass substrates, the direct allowed bandgap values have been calculated and they were in the range 3.83-4.03 eV. The thickness of the films was ∼600 nm.     

Photocatalytic activity of dc magnetron sputter deposited amorphous TiO2 thin films

For photocatalytic thin film applications TiO₂ is one of the most important materials. The most studied TiO₂ crystal phase is anatase, though also rutile and brookite show good photoactivity. Usually anatase or a mixture of rutile and anatase is applied for powder or thin film catalysts. It has been claimed that amorphous films do not exhibit any or only a very low photocatalytic activity.We have deposited amorphous thin films by dc magnetron sputtering from sub-stoichiometric TiO_2−x targets. The coatings are transparent and show a photocatalytic activity half of that of a thin layer of spin-coated reference photocatalyst powder. Annealing the thin films to yield anatase crystallization more than doubles their photocatalytic activity. At the same film thickness these thin films show the same activity as a commercially available photocatalytic coating.The dependence of the photocatalytic activity on deposition parameters like gas pressure and sputter power is discussed. A decrease in film density, as deduced from the refractive index and the microstructure, resulted in an increase in photocatalytic activity. Film thickness has a marked influence on the photocatalytic activity, showing a strong increase up to 300-400 nm, followed by a much shallower slope.     

Epitaxial growth of CeO2 on (100) InP using reactive r.f. magnetron sputtering

The epitaxial growth of CeO₂ thin films has been realized on (100) InP substrates using reactive r.f. magnetron sputtering. Oxide films were nucleated in the presence of molecular hydrogen (4% H₂/Ar sputtering gas) in order to reduce the native oxide formation on the InP surface, which interferes with CeO₂ epitaxy. A metal cerium target was used as the cation source, with water vapor serving as the oxidizing species. Epitaxial films were sputter-deposited at a substrate temperature of 550 °C in a H₂O vapor pressure of approximately 10^-3 Torr. Crystallinity of the oxide films was examined using θ–2θ X-ray diffraction, ω-rocking curves, and in-plane φ-scans. The best results were obtained when the initial nucleation layer was deposited with P(H₂O)<10^-5 Torr, followed by deposition at P(H₂O)=10^-3 Torr. The epitaxial growth of CeO₂ on InP could prove enabling in efforts to integrate functional oxides with InP-based optoelectronic and microwave technologies. Received: 20 February 20002 / Accepted: 21 February 2002 / Published online: 19 July 2002     

Influence of thermal treatment of low dielectric constant SiOC(H) films using MTES/O2 deposited by PECVD

Low dielectric constant SiOC(H) films are deposited on p-type Si(100) substrates by plasma enhanced chemical vapor deposition (PECVD) using methyltriethoxysilane (MTES, C₇H₁₈O₃Si) and oxygen gas as precursors. The SiOC(H) films are deposited at room temperature, 100, 200, 300 and 400 °C and then annealed at 100, 200, 300 and 400 °C temperatures for 30 min in vacuum. The influence of deposition temperature and annealing on SiOC(H) films are investigated. Film thickness and refractive index are measured by field emission scanning electron microscopy and ellipsometry, respectively. Chemical bonding characteristics of as-deposited and annealed films are investigated by Fourier transform infrared (FTIR) spectroscopy in the absorbance mode. As more carbon atoms are incorporated into the SiOC(H) films, both film density and refractive index are decreased due to nano pore structure of the film. In the SiOC(H) film, CH₃ group as an end group is introduced into OSiO network, thereby reducing the density to decrease the dielectric constant thereof. The dielectric constant of SiOC(H) film is evaluated by C-V measurements using metal-insulator-semiconductor (MIS), Al/SiOC(H)/p-Si structure and it is found to be as low as 2.2 for annealed samples deposited at 400 °C.     

Reduced dielectric loss and leakage current in CaCu3Ti4O12/SiO2/CaCu3Ti4O12 multilayered films

The CaCu₃Ti₄O₁₂/SiO₂/CaCu₃Ti₄O₁₂ (CCTO/SiO₂/CCTO) multilayered films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition method. It has been demonstrated that the dielectric loss and the leakage current density were significantly reduced with the increase of the SiO₂ layer thickness, accompanied with a decrease of the dielectric constant. The CCTO film with a 20 nm SiO₂ layer showed a dielectric loss of 0.065 at 100 kHz and the leakage current density of 6×10⁻⁷ A/cm² at 100 kV/cm, which were much lower than those of the single layer CCTO films. The improvement of the electric properties is ascribed to two reasons: one is the improved crystallinity; the other is the reduced free carriers in the multilayered films.     

Anisotropic in-plane misfit strains dependence of phase diagrams and dielectric behavior in epitaxial Pb(Zr1−xTix)O3 thin films

A phenomenological Landau–Devonshire thermodynamic theory is used to describe the effects of anisotropic in-plane misfit strains on equilibrium polarization states and dielectric properties of single domain epitaxial Pb(Zr_1−xTi_x)O₃ thin films grown on dissimilar orthorhombic substrates. Compared with the “isotropic in-plane misfit strains-temperature” phase diagrams, the characteristic features of “misfit strain-misfit strain” and “misfit strain-temperature” phase diagrams under the circumstance of strain anisotropy are the presence of four different phases (a^′$a^{\prime }$, a^″$a^{″}$, a^′c$a^{\prime }c$, and a^″c$a^{″}c$) and the direct 90° polarization switching between c   phase and a^′$a^{\prime }$ phase (or a^″$a^{″}$ phase), between a^′$a^{\prime }$ phase and a^″$a^{″}$ phase. The misfit strain dependence of polarization components, the small-signal dielectric responses and the tunabilities at room temperature are also calculated. We find that the phase diagrams and dielectric properties largely depend on anisotropic in-plane misfit strains as well. Moreover, the strain anisotropy will lead to the polarization and dielectric anisotropy.     

Growth of Y-junction bamboo-shaped CNx nanotubes on GaAs substrate using single feedstock

Nitrogen-doped Y-junction bamboo-shaped carbon nanotubes were synthesized by chemical vapor deposition of monoethanolamine/ferrocene mixture on GaAs substrate at 950 °C. The use of monoethanolamine as the C/N feedstock simplifies the experimental arrangement by producing ammonia during the growth process. The structure, morphology and graphitization of as-grown nitrogen-doped carbon nanotubes (CN_x) were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy analysis. TEM analysis indicates that nanotubes have a bamboo-like structure. The nitrogen concentration on as-grown CN_x nanotube was found to be 7.8 at.% by X-ray photoelectron spectroscopy (XPS) analysis. XPS analysis also indicated that there are two different types of nitrogen atoms (pyridinic and graphitic) in these materials. The possible growth mechanism of formation of Y-junction CN_x nanotubes was briefly discussed. Field emission measurement suggested that as-grown CN_x nanotubes are excellent emitters with turn-on and threshold fields of 1.6 and 2.63 V/μm, respectively. The result indicated that monoethanolamine proves to be an advantageous precursor to synthesize Y-junction nitrogen-doped carbon nanotubes and such nanotubes might be an effective material to fabricate various field emission devices.     

Microstructure and dielectric properties of La2O3 doped amorphous SiO2 films as gate dielectric material

As a potential gate dielectric material, the La₂O₃ doped SiO₂ (LSO, the mole ratio is about 1:5) films were fabricated on n-Si (0 0 1) substrates by using pulsed laser deposition technique. By virtue of several measurements, the microstructure and electrical properties of the LSO films were characterized. The LSO films keep the amorphous state up to a high annealing temperature of 800 °C. From HRTEM and XPS results, these La atoms of the LSO films do not react with silicon substrate to form any La-compound at interfacial layer. However, these O atoms of the LSO films diffuse from the film toward the silicon substrate so as to form a SiO₂ interfacial layer. The thickness of SiO₂ layer is only about two atomic layers. A possible explanation for interfacial reaction has been proposed. The scanning electron microscope image shows the surface of the amorphous LSO film very flat. The LSO film shows a dielectric constant of 12.8 at 1 MHz. For the LSO film with thickness of 3 nm, a small equivalent oxide thickness of 1.2 nm is obtained. The leakage current density of the LSO film is 1.54 × 10⁻⁴ A/cm² at a gate bias voltage of 1 V.