Highly c-axis oriented growth of GaN film on sapphire (0001) by laser molecular beam epitaxy using HVPE grown GaN bulk target

Growth temperature dependant surface morphology and crystalline properties of the epitaxial GaN layers grown on pre-nitridated sapphire (0001) substrates by laser molecular beam epitaxy (LMBE) were investigated in the range of 500-750 degrees C. The grown GaN films were characterized using high resolution x-ray diffraction, atomic force microscopy (AFM), micro-Raman spectroscopy, and secondary ion mass spectroscopy (SIMS). The x-ray rocking curve full width at a half maximum (FWHM) value for (0002) reflection dramatically decreased from 1582 arc sec to 153 arc sec when the growth temperature was increased from 500 degrees C to 600 degrees C and the value further decreased with increase of growth temperature up to 720 degrees C. A highly c-axis oriented GaN epitaxial film was obtained at 720 degrees C with a (0002) plane rocking curve FWHM value as low as 102 arc sec. From AFM studies, it is observed that the GaN grain size also increased with increasing growth temperature and flat, large lateral grains of size 200-300 nm was obtained for the film grown at 720 degrees C. The micro-Raman spectroscopy studies also exhibited the high-quality wurtzite nature of GaN film grown on sapphire at 720 degrees C. The SIMS measurements revealed a non-traceable amount of background oxygen impurity in the grown GaN films. The results show that the growth temperature strongly influences the surface morphology and crystalline quality of the epitaxial GaN films on sapphire grown by LMBE

Structural, optical and electronic properties of homoepitaxial GaN nanowalls grown on GaN template by laser molecular beam epitaxy

We have grown homoepitaxial GaN nanowall networks on GaN template using an ultra-high vacuum laser assisted molecular beam epitaxy system by ablating solid GaN target under a constant r.f. nitrogen plasma ambient. The effect of laser repetition rate in the range of 10 to 30 Hz on the structural properties of the GaN nanostructures has been studied using high resolution X-ray diffraction, field emission scanning electron microscopy and Raman spectroscopy. The variation of the laser repetition rate affected the tip width and pore size of the nanowall networks. The z-profile Raman spectroscopy measurements revealed the GaN nanowall network retained the same strain present in the GaN template. The optical properties of these GaN nanowall networks have been studied using photoluminescence and ultrafast spectroscopy and an enhancement of optical band gap has been observed for the nanowalls having a tip width of 10-15 nm due to the quantum carrier confinement effect at the wall edges. The electronic structure of the GaN nanowall networks has been studied using X-ray photoemission spectroscopy and it has been compared to the GaN template. The calculated Ga/N ratio is largest (similar to 2) for the GaN nanowall network grown at 30 Hz. Surface band bending decreases for the nanowall network with the lowest tip width. The homoepitaxial growth of porous GaN nanowall networks holds promise for the design of nitride based sensor devices

Photoconductivity and photo-detection response of multiferroic bismuth iron oxide

We report visible light detection with in-plane BiFeO3 (BFO) thin films grown on pre-patterned inter-digital electrodes. In-plane configured BFO film displayed photocurrents with a 40:1 photo-to-dark-current ratio and improved photo-sensing ability for >15000 s (4 hrs) under small bias voltage (42V). Nearly sixty percent of the photo-induced charge carriers decay in 1.0 s and follow a double-exponential decay model. At 373 K the effect of light does not significantly increase the dark current, probably due to reduced mobility. Sub-bandgap weak monochromatic light (1 mw/cm2) shows one fold increase in photo-charge carriers.Comment: 18 pages, 7 figure

Microstructural image based convolutional neural networks for efficient prediction of full-field stress maps in short fiber polymer composites

The increased demand for superior materials has highlighted the need of investigating the mechanical properties of composites to achieve enhanced constitutive relationships. Fiber-reinforced polymer composites have emerged as an integral part of materials development with tailored mechanical properties. However, the complexity and heterogeneity of such composites make it considerably more challenging to have precise quantification of properties and attain an optimal design of structures through experimental and computational approaches. In order to avoid the complex, cumbersome, and labor-intensive experimental and numerical modeling approaches, a machine learning (ML) model is proposed here such that it takes the microstructural image as input with a different range of Young's modulus of carbon fibers and neat epoxy, and obtains output as visualization of the stress component S11 (principal stress in the x-direction). For obtaining the training data of the ML model, a short carbon fiber-filled specimen under quasi-static tension is modeled based on 2D Representative Area Element (RAE) using finite element analysis. The composite is inclusive of short carbon fibers with an aspect ratio of 7.5 that are infilled in the epoxy systems at various random orientations and positions generated using the Simple Sequential Inhibition (SSI) process. The study reveals that the pix2pix deep learning Convolutional Neural Network (CNN) model is robust enough to predict the stress fields in the composite for a given arrangement of short fibers filled in epoxy over the specified range of Young's modulus with high accuracy. The CNN model achieves a correlation score of about 0.999 and L2 norm of less than 0.005 for a majority of the samples in the design spectrum, indicating excellent prediction capability. In this paper, we have focused on the stage-wise chronological development of the CNN model with optimized performance for predicting the full-field stress maps of the fiber-reinforced composite specimens. The development of such a robust and efficient algorithm would significantly reduce the amount of time and cost required to study and design new composite materials through the elimination of numerical inputs by direct microstructural images.</p

Light assisted irreversible resistive switching in ultra thin hafnium oxide

An ultra thin film (similar to 5 nm) high-k Hafnium oxide dielectric, grown on a doped p-Si(100) substrate by the atomic layer deposition technique has been investigated for resistive and capacitive switching with and without illumination of light. As grown samples illustrate small non-switching leakage current under high applied electric fields and probe frequencies and trap charge assisted counter-clockwise capacitance-voltage behavior. A unique resistance switching was observed under illumination of 15-60 mW light. In the first cycle, the light assisted switching provide a 10(4) : 1 resistance ratio, which diminishes in the next cycle onward, which may be due to irreversible charge injection in the oxide layers. The band offset and band match-up energy diagram for the charge carriers responsible for resistive switching and charge trapping near the interface have been demonstrated under the application of a bias electric field and light

Laser Molecular Beam Epitaxy Growth of GaN layer on Sapphire (0001) under various process conditions

We have grown high quality epitaxial GaN films on sapphire (0001) substrates using a ultra-high vacuum laser molecular beam epitaxy (MBE) system at different growth temperatures, deposition rate and nitrogen species. The HYPE grown GaN solid target was ablated at laser energy density 5 J/cm2 with laser frequency 5 Hz (low flux) and 10 Hz (high flux) in presence of r.f. nitrogen plasma. Structural properties of the epitaxial GaN films were characterized using high resolution x-ray diffraction, atomic force microscopy (AFM), and photoluminescence spectroscopy (PL). At high flux, the full width at half maximum (FWHM) of x-ray diffraction rocking curve of GaN (0002) peak decreases with increasing growth temperature from 500 to 720 degrees C. The GaN film grown at 700 degrees C with low flux shows a large FWHM (368 arc sec) with small grain sizes in comparison to the GaN film grown with high flux (FWHM: 110 arc sec). We have also studied the effect of high pressure nitrogen ambient during ablation of GaN target for growth of GaN films on sapphire with and without prenitridation of sapphire at growth temperature 500 degrees C. The typical PL measurement on the GaN film grown on sapphire using laser MBE system shows the high quality of GaN film with minimum defects. The obtained results suggest that the present growth technique could be an alternative for fabrication of high quality GaN based devices

Low Temperature Growth of GaN Epitaxial Layer on Sapphire (0001) Substrate by Laser Molecular Beam Epitaxy Technique

GaN epitaxial layers have been grown on sapphire (0001) substrate by laser molecular beam epitaxy. The Ga and N fluxes have been optimized for a good quality, smooth surface GaN layer growth by suitably adjusting the laser power and frequency. It is found that the moderate laser energy with high frequency upto 45 Hz yields more uniform Ga flux for the growth. Similar to conventional MBE, the N-rich growth condition produced rough surface GaN layers while flat surface GaN was obtained under slightly Ga-rich condition. The effect of growth temperature in the range 300-750 degrees C on the structural properties of the grown GaN layers has been studied. The (0002) plane x-ray rocking curve full width at half maximum (FWHM) of GaN epilayers has been found to decrease dramatically with increasing growth temperature. A narrow x-ray rocking curve value of about 245 arcsec has been achieved for GaN (0002) plane reflection for the epilayers grown in the range of 500-600 degrees C, which is about 150 degrees C lower than the conventional MBE growth

Nucleation and growth of aluminum on an inert substrate of graphite

10.1088/0953-8984/20/22/225002Journal of Physics Condensed Matter2022-JCOM

Low Temperature Growth of GaN Epitaxial Layers on Sapphire (0001) by Pulsed Laser Deposition Using Liquid Gallium Target

Smooth surface GaN epilayers have been grown on sapphire (0001) by pulsed laser ablation of liquid Ga target under radio frequency assisted atomic nitrogen ambient. Laser energy density, laser frequency and substrate-to-target distance were adjusted to optimize the Ga flux for a fixed nitrogen plasma condition. From in-situ reflection high energy electron diffraction and atomic force microscopy analyses, it is found that the Ga/N flux ratio plays a crucial role in determining the growth mode and that only the Ga-rich condition promotes a two dimensional GaN growth. Also, the increase of Ga flux by increasing the laser energy density above 8 J/cm(2) is observed to produce Ga droplets on the grown GaN surface even if the growth front is excess with nitrogen flux. Instead, the increase of laser frequency (up to 50 Hz, in this study) is preferable. The effect of growth temperature on the crystalline quality of GaN layers has also been investigated in the range from 300 to 700 degrees C. Highly c-axis oriented GaN layers, with a (0002) plane X-ray rocking curve full width at half maximum of 245 arcsec, have been obtained at 500-600 degrees C, which is about 150 degrees C lower than the conventional MBE growth