R. D. Vispute

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...

High quality epitaxial aluminum nitride layers on sapphire by pulsed laser deposition

We have grown high quality epitaxial AlN layers on sapphire substrates by pulsed laser ablation of a stoichiometric AlN target. The AlN films deposited at 800 °C and laser energy densities in the range of 2–3 J/cm2 were found to be epitaxial with the c axis normal to the Al2O3(0001) surface. The x‐ray rocking curve of epitaxial AlN films yielded a full width at half maximum of 0.21°. The selected area electron diffraction patterns and high resolution transmission electron microscopy also revealed that the films were epitaxial with an orientational relationship of AlN[0001]∥Al2O3[0001] and in‐plain alignment of AlN[1210]∥Al2O3[0110] and AlN[1010]∥Al2O3[2110]. This is equivalent to 30° rotation in the basal plane of the AlN film with respect to the sapphire substrate. The absorption edge measured by ultraviolet‐visible spectroscopy for the epitaxial AlN film was sharp and the band gap was found to be 6.1 eV. The electrical resistivity of the films was about 5–6×1013 ohm cm with a breakdown field of 5×1...

High quality optoelectronic grade epitaxial AlN films on α-Al2O3, Si and 6H-SiC by pulsed laser deposition

Abstract AlN is one of the most important optoelectronic materials in the wide band gap III–V semiconductors because of its wide and tunable energy band gap in conjunction with other nitrides, high thermal conductivity, doping capabilities, and high hardness. The proposed optoelectronic devices require high quality epitaxial films on various substrates. Here, we present our recent work on the fabrication of high quality epitaxial AlN films on Al2O3(0001), Si(111) and 6H-SiC(0001) by pulsed laser deposition (PLD). The PLD is a nonequilibrium technique where thin film growth temperature can be reduced by more than 250–350°C and epitaxial films comparable in quality to MOCVD (equilibrium technique) obtained. The laser fluence and the substrate temperature were found to be crucial processing parameters for the formation of high quality monocrystalline AlN films. The AlN films deposited above 750°C and laser energy densities of 2–3 J cm−2 were found to be epitaxial with c-axis normal to substrate plane. The X-ray rocking curve of epitaxial films on sapphire and SiC yielded full-width-at-half-maximum of ∼0.06–0.07°. The transmission electron microscopy also revealed that the films were epitaxial with an orientational relationship of AlN[0001]‖Al2O3[0001], AlN[0001]‖Si[111], AlN[0001]‖SiC[0001] and in-plane alignment of AlN[1210]‖ Al2O3[0110], AlN[1010]‖Al2O3[2110], AlN[2110]‖Si[011] and AlN[0110]‖SiC[0110]. The optical absorption edge measured by UV-visible spectroscopy for the epitaxial AlN film on sapphire was sharp and the band gap was found to be 6.1 eV. The electrical resistivity of the films was about 5–6×1013 Ω cm−1 with a breakdown field of 5×106 V cm−1.

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.

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...

Pulsed laser deposition and characterization of epitaxial Cu/TiN/Si(100) heterostructures

Three‐dimensional epitaxial Cu/TiN/Si(100) heterostructures have been grown by pulsed laser deposition using a single chamber, in situ processing method. The epitaxial TiN layers on Si(100) were grown at 600 °C and epitaxial Cu layers on TiN/Si(100) in the temperature range 200–600 °C using optimized laser parameters. These structures were characterized using three‐axis x‐ray diffraction (Θ, Φ, Ψ scans) technique and high‐resolution transmission electron microscopy. The results clearly indicate cube‐on‐cube epitaxial alignment along the three axes, i.e., 〈100〉Cu∥〈100〉TiN∥〈100〉Si. The Cu/TiN and TiN/Si interfaces were found to be quite sharp without any indication of interfacial reaction. The growth mechanism of copper on TiN was found to be three dimensional, with the size of island varying from 0.3 to 1.5 μm. We discuss domain matching epitaxy as a mechanism of growth in these large lattice mismatch systems, where three lattice constants of Si(5.43 A) match with four of TiN(4.24 A) and seven units of Cu(...

Influence of oxygen background pressure on crystalline quality of SrTiO3 films grown on MgO by pulsed laser deposition

We have systematically investigated the effect of oxygen partial pressure (PO2) on the crystalline quality of SrTiO3 films grown on MgO (001) substrates using pulsed laser deposition and established optimized conditions for the growth of high-quality epitaxial films. The crystalline quality is found to improve significantly in the O2 pressure range of 0.5–1 mTorr, compared to the films deposited at higher pressures of 10–100 mTorr. The x-ray diffraction rocking curves for the films grown at PO2 of 1 mTorr and 100 mTorr yielded full width at half-maximum (FWHM) of 0.7° and 1.4°, respectively. The in-plane x-ray φ scans showed epitaxial cube-on-cube alignment of the films. Channeling yields χmin were found to be <5% for the 1 mTorr films and ∼14% for 100 mTorr films. Thermal annealing of the SrTiO3 films in oxygen further improves the quality, and the 1 mTorr films give FWHM of 0.13° and χmin of 1.7%. In-plane misorientations of the annealed SrTiO3 films calculated using results of transmission electron mic...

Field emission from amorphous diamond coated Mo tip emitters by pulsed laser deposition

Previous studies have shown that carbon films deposited on needle-shaped Si emitters by filtered cathodic arc are amorphous with a high sp2 content. These results can be ascribed to the poor thermal conductivity inherent to this geometry. Our present studies overcome this difficulty by depositing amorphous diamond films on Mo tip emitters by pulsed laser deposition. Monitoring films were grown on sapphire substrates and appeared transparent with a resistivity greater than 1×106 Ω cm, showing a typical amorphous diamond nature. Electron energy loss spectroscopy showed that the sp3 content of the film was 50% at the apex of the tip and 30% at the shank, which was lower than on planar substrates. High resolution transmission electron microscopy images revealed that the film at the apex was much denser than that at the shank, but both film showed a nano-columnar microstructure. Selected area electron diffraction confirmed that the films were amorphous in nature. Field emission from coated Mo tip emitters show...

Misfit dislocations in low-temperature grown Ge/Si heterostructures

Abstract We have investigated the initial stages of two-dimensional epitaxial growth of Ge on (001)Si substrates using pulsed laser deposition. The Ge films grow epitaxially above 170°C and switched from two- to three-dimensional growth mode above 350°C. The evolution of the dislocation network as a function of film thickness was studied by high-resolution transmission electron microscopy (TEM) in the films grown at 300°C and at relatively high growth rates to suppress the three-dimensional nucleation and obtain low rate of inelastic misfit relaxation. The relaxation process begins with generation of 60° slip dislocations with a strongly non-equilibrium inhomogeneous distribution which seems to be controlled by additional stress resulting from random surface undulations. When the film is just 8 nm thick, 90% of misfit stress is already relaxed by mostly 60° dislocations which demonstrate a distinct tendency of pairing of parallel dislocations from different glide planes. As the thickness of the film incre...