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The use of composites are being increased day by day. There many types of composites like mud building bricks, concrete and reinforced concrete, fiberglass, natural composites. Natural composites are the one which are made of plant derived fiber with polymeric matrix. The natural fiber consists of sisal, jute, hemp, banana leaf fibers, bamboo, wheat straw, coir fiber, grass reeds, wood fiber, rice husk, wheat, prosopis julifora, oats, cane and many other fibrous materials. They have a vast application and are eco-friendly. Natural fiber reduces both cost and density when compared to other types of fibers which are available. The alternative source of synthetic fibers are natural fibers which are renewable in nature, cost effective and also causes no harm to the environment. In this the uses and applications of prosopis juliflora and coir fiber natural composites are discussed.

Thin films of AlN and GaN were grown by pulsed laser deposition on c-plane sapphire substrates. It is demonstrated that the surface morphology and the structure of layers can actively be controlled by adjusting the nitrogen partial pressure during the growth. The observed trends in the structural quality of the thin films can be attributed to the changes in the surface diffusion of ad-atoms. The surface diffusion of ad-atoms can be influenced by the collisions between the nitrogen gas molecules and the activated atoms, which can reduce the kinetic energy of activated atoms and increase the rate of formation of immobile surface dimers. Through these nitrogen pressure-related effects, thin films with microstructure ranging from crystalline to amorphous can be produced. Our results on the growth of GaN and AlN thin films indicate that this pressure-assisted growth procedure is generally applicable to design the surface morphology of III-nitride thin films. A minimal surface root means square roughness of 0.7 nm for amorphous AlN is obtained, which compares well with the substrate roughness of 0.5 nm.

Thin films of AlN and GaN were grown by pulsed laser deposition on c-plane sapphire substrates. It is demonstrated that the structure and surface morphology of layers can actively be controlled by adjusting the nitrogen partial pressure during the growth. The observed trends in the structural quality of the thin films can be attributed to the changes in the surface diffusion of adatoms. It is argued that the surface diffusion of adatoms can be influenced by the collisions between the nitrogen gas molecules and the activated atoms which can reduce the kinetic energy of activated atoms and increase the rate of formation of immobile surface dimers. Through these nitrogen pressure related effects, thin films with microstructure ranging from crystalline to amorphous can be produced. The observed similar impact of nitrogen pressure on the growth of GaN and AlN thin films indicates that a pressure assisted growth procedure is generally applicable to design the surface morphology of group III-nitride thin films. A minimal surface root mean square roughness of 0.7 nm for amorphous AlN is obtained which compares well with the substrate roughness of 0.5 nm. Rutherford backscattering spectroscopy of thin films of GaN and AlN showed a large incorporation of oxygen which was found to reduce the lattice constants of GaN and AlN.

Transmission electron microscopy was employed to study the effect of N/Ga flux ratio in the growth of GaN buffer layers on the structure of GaN epitaxial layers grown by molecular-beamepitaxy (MBE) on sapphire. The dislocation density in GaN layers was found to increase from 1×1010 to 6×1010 cm−2 with increase of the nitrogen flux from 5 to 35 sccm during the growth of the GaN buffer layer with otherwise the same growth conditions. All GaN layers were found to contain inversion domain boundaries (IDBs) originated at the interface with sapphire and propagated up to the layer surface. Formation of IDBs was often associated with specific defects at the interface with the substrate. Dislocation generation and annihilation were shown to be mainly growth-related processes and, hence, can be controlled by the growth conditions, especially during the first growth stages. The decrease of electron Hall mobility and the simultaneous increase of the intensity of “green” luminescence with increasing dislocation density suggest that dislocation-related deep levels are created in the bandgap.

We report on growth of GaN on Germanium as an alternative substrate material. The GaN films were deposited on Ge(001) substrates by plasma-assisted molecular beam epitaxy. Atomic force microscopy, x-ray diffraction, photoluminescence, and Raman spectroscopy were used to characterize the structural and optical properties of the films. We observed that the Ga/N ratio plays a crucial role in determining the phase purity and crystal quality. Under N-rich conditions the films were phase-mixed, containing cubic and hexagonal GaN, while in the Ga-rich regime they were purily hexagonal. The latter samples show bandedge luminescence with linewidths as small as 31 meV at low temperatures.

In this study, the causes of biaxial and hydrostatic stress components in epitaxially grown thin GaN films on sapphire are analyzed. It is observed that growth by Molecular Beam Epitaxy (MBE) and by Metal Organic Chemical Vapor Deposition (MOCVD) are governed by very similar physical principles. Differences in the absolute stress values are mainly due to the difference in growth temperature. It is argued that in the case of MOCVD growth the onset of plasticity for higher growth temperatures is responsible for a larger stress relaxation in the buffer layer. It is further found that either process can result in highly off-stoichiometric GaN layers, as manifested by the large variations in the a and c lattice parameters caused by intrinsic point defects.

The paper describes the influence of strain on the optical quality of GaN films grown by MBE on c-plane sapphire. The photoluminescence (PL) line width of the donor-bound exciton can be designed to be as narrow as 1.2 meV by actively utilizing hydrostatic and biaxial stress components. Unstrained p-type Mg-doped GaN films exhibit comparably narrow near band edge transitions. A sharp PL line at 3.261 eV in some of our films is identified as the donor bound exciton of the cubic phase. The formation of these cubic inclusions can be stimulated by a high III/V flux ratio at the growth temperature of T = 725°C. The PL spectrum of an InGaN multi quantum well structure is significantly broadened compared with the spectra of single quantum well structures. Combination of PL and TEM indicates that this effect relates to a progressive increase of the quantum well widths and their spacing along the growth direction. It is argued that strain affects the growth rate and the incorporation of Indium into the quantum well structures.

This study analyzes the impact of most common impurities and dopants on the c lattice parameter for thin films of Gallium Nitride (GaN) deposited on basal plane sapphire. Both Mg (∼1017 cm-3) and Zn (∼3 × 1020 cm-3) doping were found to expand the c lattice parameter as much as +0.38% and +0.62%, respectively. On the contrary, Oxygen up to concentrations 9 1021 cm-3 is shown to replace N in GaN thin films reducing the c parameter only by a small amount. Incorporation of Si leads to a large decrease of the c parameter which can not be attributed to the different size of Ga and Si atom. It is suggested that doping alters the film stoichiometry by a predicted Fermi level dependence of defect formation energies. The impact of stoichiometry on c lattice parameter and the effect of hydrostatic strain on resistivity in undoped and doped GaN is discussed.

GaN films were grown on sapphire substrates at temperatures below 725 °C utilizing a Constricted Glow Discharge plasma source. A three dimensional growth mode is observed at such low growth temperatures resulting in films that are composed of individual but oriented grains. The strain that originates from the growth on the lattice mismatched substrate with a different thermal expansion coefficient is utilized to influence the thin film growth. The strain can be largely altered by the growth of suitable buffer layers. Thereby, optical and structural film properties can be engineered. It is argued that the surface diffusion of Ga ad-atoms is affected by engineering the strain. Alternatively, surface diffusion can be influenced by surfactants. It is demonstrated that the use of bismuth as a surfactant allows to modify the surface morphology of the GaN films that reflects the size of the grains in the films. The results suggest that a substantial increase of the oriented grain sizes in the films is possible while maintaining a low growth temperature.

The stress in GaN thin films grown on sapphire is shown to be determined by lattice mismatch, by differences in thermal-expansion-coefficients and by the incorporation of point defects. It can be controlled by the buffer layer thickness, the buffer layer growth temperature, the V/III flux ratio, and by doping. It is argued that a Fermi-level dependence of defect formation energies affects the material stoichiometry and thereby lattice constants and stresses. We observed that stress relaxation occurred if the stresses exceeded a critical compressive or tensile stress value. The stress changes materials properties. As an example, it is demonstrated that the electron Hall mobility in GaN:Si can be increased with constant electron carrier concentration if large compressive stress is present.

GaN films were grown on sapphire substrates at temperatures below 1000 K utilizing a Hollow Anode nitrogen ion source. A Ga flux limited growth rate of ~ 0.5 µm/h is demonstrated. Active utilization of strain and the assistance of a nitrogen partial pressure during buffer layer growth are found to be crucial issues that can improve the film quality. The best films exhibit a full width at half maximum of the x-ray rocking curves of 80 arcsec and 1.85 meV for the excitonic photoluminescence measured at 4 K. A Volmer-Weber three dimensional growth mode and the spontaneous formation of cubic GaN inclusions in the hexagonal matrix are observed in the investigated growth temperature range. It is argued that this growth mode contributes to a limitation of the carrier mobility in these films that did not exceed 120 cm2/Vs though a minimum carrier concentration of ~ 1015 cm−3 was achieved.

A KrF (248 nm) excimer laser with a 38 ns pulse width was used to study pulsed laser annealing of AIN/GaN bi-layers and dopant activation of Mg-implanted GaN thin films. For the AIN/GaN bi-layers, cathodoluminescence (CL) showed an increase in the intensity of the GaN band-edge peak at 3.47 eV after pulsed laser annealing at an energy density of 2000 mJ/cm2. Rutherford backscattering spectrometry of a Mg-implanted A1N (75 nm thick)/GaN (1.0 μm thick) thin-film heterostructure showed a 20% reduction of the 4He+ backscattering yield after laser annealing at an energy density of 400 mJ/cm2. CL measurements revealed a 410 nm emission peak indicating the incorporation of Mg after laser processing.