K. Jagannadham

Epitaxial growth of AlN thin films on silicon (111) substrates by pulsed laser deposition

Aluminum nitride thin films have been grown epitaxially on Si(111) substrates, for the first time, by pulsed laser ablation of sintered AlN target. The influence of process parameters such as laser energy density, substrate temperature, pulse repetition rate, nitrogen partial pressure, etc. on epitaxial growth has been investigated to obtain high quality AlN films. These films were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, x‐ray diffraction (Θ and ω scans) technique, high resolution transmission electron microscopy, and scanning electron microscopy. The films deposited at laser energy density in the range of 2–3 J/cm2, substrate temperature of 750 °C, and base pressure of 3×10−7 Torr are single phase and highly oriented along c axis normal to the Si(111) planes. The results of x‐ray diffraction and electron microscopy on these films clearly show the epitaxial growth of the AlN films with an orientational relationship of AlN[0001] ∥ Si[111] and AlN[2110] ∥ Si[011]. Th...

Epitaxial growth in large‐lattice‐mismatch systems

Epitaxial growth in the TiN/Si and TiN/GaAs metal‐semiconductor systems with a large lattice mismatch was investigated. The orientation relationships have been found to be 〈001〉TiN∥〈001〉Si for TiN growth on Si(001) and [001]TiN∥[110]GaAs and [110]TiN∥[110]GaAs for TiN growth on GaAs(001). The epitaxial growth is characterized by domain epitaxial orientation relationships with m lattice constants of epilayer matching with n of the substrate and with a small residual domain mismatch present in the epilayer. This residual mismatch is responsible for a coherent strain energy. The magnitude of compression of Ti—N bond in the first atomic layer, contributing to the chemical free energy during the initial stages of growth, is found to be a very important factor in determining the orientation relationship. This result was used to explain the differences in the orientation relationships between TiN/Si and TiN/GaAs systems. The various energy terms associated with the domain epitaxial growth are evaluated to illu...

Structural characteristics of AlN films deposited by pulsed laser deposition and reactive magnetron sputtering: A comparative study

Aluminum nitride films have been deposited on Si(111) substrates at different substrate temperatures using two techniques; pulsed laser deposition or reactive magnetron sputtering. The films deposited by either of the techniques have been characterized by x-ray diffraction and transmission electron microscopy to determine the crystalline quality, grain size, and epitaxial growth relation with respect to the substrate. The bonding characteristics and the residual stresses present in the films have been evaluated using Raman and Fourier transform infrared spectroscopy. Secondary ion mass spectrometry has been performed to determine the nitrogen stoichiometry and the presence of impurities such as oxygen and silicon. The adhesion strength of the AlN films to the silicon substrate and the wear resistance have been determined by scratch test and a specially designed microscopic wear test. A comparison of the different characteristic features associated with the AlN films deposited by pulsed laser deposition or...

Pulsed laser deposition of Bi2Te3-based thermoelectric thin films

Thin films of p-type Bi0.5Sb1.5Te3, n-type Bi2Te2.7Se0.3, and n-type (Bi2Te3)90(Sb2Te3)5(Sb2Se3)5 (with 0.13 wt % SbI3) were deposited on substrates of mica and aluminum nitride (on silicon) using pulsed laser ablation at substrate temperatures between 300 °C to 500 °C. The films were characterized using x-ray diffraction and transmission electron microscopy for crystalline quality and epitaxial growth on the substrates. The surface morphology and microstructure were examined using scanning electron microscopy. X-ray mapping and energy-dispersive spectroscopy were performed to determine nonstoichiometry in the composition and homogeneity. The quality of the films, in terms of stoichiometric composition and crystal perfection, was studied as a function of growth temperature and laser fluence. The values of the Seebeck coefficient, electrical resistivity, and Hall mobility in the thin films were measured and compared with those in the bulk. Thermoelectric figure of merit of the films was evaluated from the ...

CHARACTERISTICS OF TITANIUM NITRIDE FILMS GROWN BY PULSED LASER DEPOSITION

Laser physical vapor deposition (LPVD) has been used to grow titanium nitride films on hydrogen-terminated silicon(100) substrates at deposition temperatures ranging from room temperature to 600 °C. A pulsed KrF excimer laser (λ = 248 nm, τ = 25 ns) was used with the deposition chamber maintained at a base pressure of 10 −7 Torr prior to deposition. Different properties of the films were investigated by x-ray diffraction, Auger electron spectroscopy, Raman spectroscopy, optical, scanning, and high resolution transmission electron microscopy, and measurement of electrical resistivity. When the substrate temperature was low (at and below 500 °C), oxygen atoms from the residual gases were incorporated in the films. The microstructures and resistivities of TiN films were found to be strongly dependent on the temperature of the silicon substrates. The TiN films deposited at 600 °C were oxygen-free, as observed from Auger analysis, and the room temperature resistivity was found to be 14–15 μΩ-cm. Raman spectroscopy of the films showed that the nitrogen-related optical phonon peak increased with deposition temperature in comparison with the titanium-related acoustic peak. Transmission electron microscopy and x-ray diffraction analyses showed that the films were polycrystalline at low temperature with grain size ranging from 300–600 A, depending on the temperature of the substrate. At 600 °C, the films were found to be single crystals with occasional presence of dislocation loops. The spacing of Moire fringes in TiN/Si samples deposited at 600 °C established the nearly periodic elastic strain field extending into the TiN and Si at the interface. Although there exists a large misfit between TiN and Si (24.6%), the epitaxial growth of TiN films on Si(100) substrates was explained by means of domain-matched epitaxy with a 4-to-3 match in unit cells for TiN/Si structure, giving rise to a residual lattice misfit of only 4%.

Pulsed laser deposition of titanium nitride and diamond-like carbon films on polymers

We have investigated the deposition of titanium nitride (TiN) and diamond-like carbon (DLC ) films on polymethylmethacrylate (PMMA) substrates using pulsed laser deposition (PLD) technique. The TiN and diamond-like films were deposited by laser ablation (KrF excimer laser λ = 248 nm, pulse duration τ∼25 × 10−9 s, energy density ∼2−15J/cm2) of TiN and graphite targets, respectively, at room temperature. These films were characterized by transmission electron microscopy, scanning electron microscopy, x-ray diffraction, Auger electron spectroscopy, UV-visible absorption spectroscopy, and Raman spectroscopy. The TiN films were smooth and found to be polycrystalline with average grain size of 120Å. The diamond-like carbon films were amorphous with a characteristic Raman peak at 1550 cm−1. The TiN films are highly adherent to the polymer substrates as compare to DLC films. The adhesion strength of DLC films on polymers was increased by interposing thin TiN layer (200Å) on polymers byin-situ pulsed laser deposition. The DLC films were found to be amorphous with good adhesion to TiN/PMMA substrates.

DIAMOND-CERAMIC COMPOSITE TOOL COATINGS

We have developed multilayer composite diamond coatings with improved adhesion and wear resistance on WC(Co) tool substrates. The coatings consist of a first layer of discontinuous diamond crystallites that are anchored to the WC(Co) substrate by an interposing layer of ceramic films. These films consist of TiC, TiN, SiC, Si 3 N 4 or WC deposited to provide a conformal coverage on the first layer of diamond. A second or final layer of continuous diamond film is deposited to provide the cutting edge of the tool. The diamond film in the composite layers is deposited by hot filament chemical vapor deposition (HFCVD) and the interposing layer is deposited by laser physical vapor deposition (LPVD). The different parameters associated with the deposition of diamond and interposing layers are optimized to improve the adhesion and wear resistance. We have studied the adhesion characteristics by indentation tests in which the critical load for peeling of the diamond films is determined. Adhesion and wear resistance of the films are also tested using an overlap polishing on diamond paste with 5–6 μm particle size. The diamond and interposing layers in the composite are characterized by scanning electron microscopy and Raman spectroscopy. Results of improvement in adhesion and wear resistance are correlated with the quality of the diamond film and the interposing layer. Better accommodation of thermal stresses and strains in the composite layers has been shown to be responsible for improvement in the adhesion and wear resistance of the composite diamond films.

Formation of epitaxial and textured platinum films on ceramics-(100) MgO single crystals by pulsed laser deposition

We have investigated the formation of textured and epitaxial metallic films on (100) MgO single crystals substrates (lattice constant a=4.21 A) as a function of deposition temperature during pulsed laser ablation. Platinum (a=3.92 A) films on MgO with lattice misfit of 7.4% were found to grow epitaxially in the temperature range 500–700 °C. Three‐dimensional x‐ray diffraction results (theta, phi, and chi scans) show 〈100〉 epitaxy with the alignment of all three cube axes. Rutherford backscattering and channeling measurements on a film deposited at 700 °C showed a minimum yield of 2.2%, which is very close to the defect‐free single crystal value. In the temperature range 200–500 °C both 〈100〉 and 〈111〉 textures were observed. The 〈111〉 oriented (normal to the surface) films were random in the plane of the substrate, whereas 〈100〉 crystallites were epitaxial. Below 200 °C, only 〈111〉 crystallites were observed. The 〈111〉 texture of platinum films is also observed when grown on amorphous substrates such as S...

Electrical conductivity of copper–graphene composite films synthesized by electrochemical deposition with exfoliated graphene platelets

Films of graphene/copper composite in copper matrix were deposited on copper foil using an aqueous electrolyte solution of 0.2 M CuSO4 containing graphene oxide suspension at a low current density of 1.75 mA cm−2. Graphene oxide is reduced by further heating the samples in flowing hydrogen atmosphere maintained at 20 Torr and 400 °C for 3 h. The composite samples with different thickness, between 365 and 515 μm, deposited on a Cu foil of thickness 135 μm were characterized for graphene structure, morphology, and distribution. Electrical resistivity and temperature coefficient of electrical resistance of the samples at 300 K were measured using a four-probe method. The results were used to determine the electrical resistivity and temperature coefficient of resistance of the composite layers. The volume fraction and resistivity of graphene were evaluated using effective mean field analysis of the resistivity and temperature coefficient of resistance of the composite films. The results illustrate that the re...

Heteroepitaxial structures of SrTiO3/TiN on Si(100) by in situ pulsed laser deposition

High‐quality ceramics based heteroepitaxial structures of oxide‐nitride‐semiconductors, i.e., SrTiO3/TiN/Si(100) have been fabricated by in situ pulsed laser deposition. The dependence of substrate temperature and oxygen partial pressure on the crystalline quality of the SrTiO3 films on Si with epitaxial TiN template has been examined. We found that epitaxial growth occurs on TiN/Si(100) above 500 °C, initially at a reduced O2 pressure (10−6 Torr), and followed by a deposition in the range of 5–10×10−4 Torr. X‐ray diffraction (Θ, ω, and Φ scans) and transmission electron microscope (TEM) results revealed an excellent alignment of SrTiO3 and TiN films on Si(100) with a cube‐on‐cube epitaxy. Rutherford backscattering and ion channeling results show a channeling minimum yield (χmin) of ∼13% for the SrTiO3 films. High‐resolution TEM results on the SrTiO3/TiN interface show that the epitaxial SrTiO3 film is separated from the TiN by an uniform 80–90 A crystalline interposing layer presumably of TiNxO1−x (oxy‐n...