We have studied the crystalline, optical and photo-response properties of epitaxial GaN films grown on bare- and pre-nitridated a-sapphire substrates with different thicknesses of low temperature (LT) GaN buffer layer using laser-assisted molecular beam epitaxy (LMBE). It is found that the GaN grows epitaxially on a-sapphire along the c-axis and has low biaxial stress of ~ 0.3 GPa. Room-temperature photoluminescence spectroscopy disclosed the near band edge emission peak at ~ 3.4 eV with a line width of 100 ~ 110 meV for GaN grown on thin LT-GaN buffer, whereas an additional prominent blue luminescence emission is obtained for GaN film grown on thicker LT-GaN buffer. Metal–semiconductor-metal based ultra-violet (UV) photodetectors (PD) fabricated on GaN film grown on pre-nitridated sapphire with thin LT-GaN buffer showed a responsivity of ~ 1.35 A/W at 3 V applied bias. It is also observed that the PDs with thin LT-GaN buffer have nearly 5 times higher responsivity with a faster response than that with thick LT-GaN buffer. We found that the photoresponse characteristics of GaN-based UV PDs on a-sapphire are critically dependent on crystalline and optical defects.

We have grown various epitaxial GaN nanostructures on sapphire (11-20) substrates by tuning the buffer layer growth conditions in laser molecular beam epitaxy (LMBE) process. The pre-nitridation and buffer layer GaN growth at low temperature (LT) on sapphire (11-20) critically affect the surface morphology and structural properties. Granular GaN thin film (similar to 160 nm) was grown on pre-nitridated sapphire whereas nano-column (NC)-GaN was obtained on LT-GaN buffer layer on bare sapphire having a height of similar to 370 nm at the growth temperature of 700 degrees C. Nano-porous (NP)-GaN was obtained with pore sizes in the range of 70 similar to 110 nm having vertical height of similar to 560 nm under similar growth conditions on LT-GaN buffered pre-nitridated sapphire. In-situ reflection high energy electron diffraction, high-resolution x-ray diffraction and Raman spectroscopy measurements indicated the epitaxial growth of c-axis oriented, wurtzite crystalline GaN nanostructures on sapphire (11 similar to 20) substrate with nearly negligible biaxial stress (0.03-0.23 GPa). Further, metal-semiconductor-metal (MSM) ultra-violet (UV) photodetectors were fabricated on epitaxial GaN nanostructures. The photo responsivity studies revealed that the NP-GaN MSM device has a photoresponse of similar to 358 mA/W at an applied bias of 1V. The photo-responsivity of NP-GaN MSM device is higher than that of GaN film (similar to 36 mA/W) and NC-GaN (similar to 7 mA/W) which revealed the importance of shape and size of GaN nanostructures on the responsivity of UV-photodetector devices. These results demonstrate the capability of LMBE technique to grow different GaN nanostructures on sapphire (11-20) substrate by tuning buffer layer conditions for their application as UV-photodetectors.

The effect of flexible Ti metal foil surface modification and laser repetition rate in laser molecular beam epitaxy growth process on the evolution of GaN nanorods and their structural, electronic and optical properties has been investigated. The GaN nanostructures were grown on bare- and pre-nitridated Ti foil substrates at 700 °C for different laser repetition rates (10-30 Hz). It is found that the low repetition rate (10 Hz) promotes sparse growth of three-dimensional inverted-cone like GaN nanostructures on pre-nitridated Ti surface whereas the entire Ti foil substrate is nearly covered with film-like GaN consisting of large-sized grains for 30 Hz growth. In case of the GaN growth at 20 Hz, uniformly-aligned, dense (∼8 × 109 cm-2) GaN nanorods are successfully grown on pre-nitridated Ti foil whereas sparse vertical GaN nanorods have been obtained on bare Ti foil under similar growth conditions for both 20 and 30 Hz. X-ray photoemission spectroscopy (XPS) has been utilized to elucidate the electronic structure of GaN nanorods grown under various experimental conditions on Ti foil. It confirms Ga-N bonding in the grown structures, and the calculated chemical composition turns out to be Ga rich for the GaN nanorods grown on pre-nitridated Ti foil. For bare Ti substrates, a preferred reaction between Ti and N is noticed as compared to Ga and N leading to sparse growth of GaN nanorods. Hence, the nitridation of Ti foil is a prerequisite to achieve the growth of dense and aligned GaN nanorod arrays. The X-ray diffraction, high resolution transmission electron microscopy and Raman studies revealed the c-axis growth of wurtzite GaN nanorods on Ti metal foil with good crystallinity and structural quality. The photoluminescence spectroscopy showed that the dense GaN nanorod possesses a near band edge emission at 3.42 eV with a full width at half maximum of 98 meV at room temperature. The density-controlled growth of GaN nanorods on a flexible substrate with high structural and optical quality holds promise for potential applications in futuristic flexible GaN based optoelectronics and sensor devices.

Self-induced GaN nanorod (NR) growth on flexible niobium (Nb) metal foil has been reported using laser assisted molecular beam epitaxy (LMBE) technique. The role of growth temperature (600, 650 and 700 degrees C), laser repetition rate (10-30 Hz) and Nb foil pre-nitridation condition on the formation and properties of GaN NRs have been studied systematically. The dense (similar to 2.42 x 10(9) cm(-2)) GaN NRs were grown on pre-nitridated Nb metal foil at growth temperature of 700 degrees C with length and diameter of similar to 550-640 nm and similar to 60-130 nm, respectively. The low growth temperature of 650 degrees C or the high laser repetition rate of 30 Hz increases the radial growth rate of GaN NRs on Nb foil which results the coalescence of NRs. The size and density of GaN NRs on Nb metal foil could be controlled with tuning of LMBE growth parameters. The GaN NRs obtained on bare and nitridated Nb foil have excellent optical properties with an intense near band emission peak position at similar to 3.40 eV and a narrow linewidth of 90-100 meV. The controlled growth of GaN NRs with excellent optical quality on Nb foil offers the possibility of realization of futuristic high efficient flexible III-nitride nanostructure-based optoelectronic devices.

We report the direct growth of crystalline GaN on bare copper (Cu) and monolayer-graphene/Cu metal foils using laser molecular beam epitaxy technique at growth temperature of 700 °C. The surface morphology investigated with field emission scanning electron microscopy revealed that the size of GaN grains for film grown on bare Cu falls in range of 90 to 160 nm whereas large grains with size of ˜200 to 600 nm was obtained for GaN grown on graphene/Cu foil under similar growth condition. The transverse optical mode of cubic GaN and E 2 (high) phonon mode for wurtzite GaN phases were obtained on the GaN film grown on Cu and graphene/Cu metal foils as deduced by Raman spectroscopy. The photoluminescence (PL) spectroscopy studies showed that the near band edge emission peaks for GaN on Cu and graphene/Cu consist two major peaks at 3.26 and 3.4 eV, corresponding to cubic and wurtzite GaN, respectively. The Raman and PL studies disclosed that the mixed phase growth of GaN occurs on these foils and better structural and optical quality for GaN on graphene/Cu foil. The direct growth of GaN on two dimensional graphene on polycrystalline metal foils is beneficial various transferrable and flexible opto-electronics device applications.

The GaN nanoporous-film (NPF) and nanorods (NRs) were grown on sapphire (0001) using laserassisted molecular beam epitaxy (LMBE) technique by laser ablating solid GaN target at different laser energy density. The interconnected GaN NPF was grown at low laser energy density of ˜4 J/cm² whereas vertically aligned GaN NRs was obtained at high laser energy density of ˜7 J/cm². The pore size of the GaN NPF structure is in range of 40-120 nm. The GaN NRs possess hexagonal shape with six sidewall facets and truncated top facet. The length, width and density of GaN NRs are 600-900 nm, 150-250 nm and ˜2.5×107 cm-2, respectively. The X-ray rocking curve full width at half maximum values along GaN (0002) and (1012) planes for GaN NRs obtained to be 0.41 and 0.53°, respectively. The biaxial stress in hetero-epitaxially grown GaN was investigated with Raman spectroscopy and it was found that GaN NRs possesses a very low in-plane compressive biaxial stress of 0.09 GPa. The photoluminescence study exhibits a sharp band-to-band emission at 3.4 eV with a peak line width of 140 meV, signifying the good optical quality of the LMBE grown GaN NRs on sapphire (0001).

Direct growth of one-dimensional (1D) GaN nanostructures on metal foils is perceived as a promising approach for the realization of futuristic flexible opto-electronic devices. Optimization of growth process to achieve 1D GaN nanostructures on metal foils is a challenging task as the growth is sensitive to the nature of individual metals. Here, we report the laser molecular beam epitaxy growth of GaN nanorods by investigating the role of growth temperature, laser energy density and laser repetition rate in the range of 500-700 degrees C, 3-5 J/cm(2) and 10-30 Hz, respectively. A higher growth temperature is noticed to be very crucial in determining the formation of 1D GaN nanorod while the laser energy and repetition rate mostly control the nanorod density. GaN nanorod ensemble with the density, length and top cross-sectional width, respectively, similar to 1.41 x 10(9) cm(-2), 430-620 nm and 70-160 nm is achieved at 700 degrees C using laser energy density of similar to 5 J/cm(2) and repetition rate of 10 Hz. The shape of GaN nanorods is nearly same for different laser repetition rates (20-30 Hz) and the density of GaN nanorods increases with the laser repetition rate (similar to 3.42 x 10(9) cm(-2) for 20 Hz and similar to 3.51 x 10(9) cm(-2) for 30 Hz). X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy studies revealed the c-axis oriented growth of single crystalline wurtzite GaN nanorods on Mo foil. Room temperature photoluminescence spectroscopy exhibited a high intense near band edge emission at similar to 3.4 eV with negligible deep-bands. The controlled growth of self-assembled GaN nanostructures on polycrystalline metal foil with good structural and optical properties has the potential for developing III-nitride based bendable optoelectronic devices.

We studied the effect of nitridation condition of Mo foil on the properties of GaN nanorods grown by laser molecular beam epitaxy. It is found that randomly size-distributed three-dimensional GaN islands were grown on bare Mo foil at growth temperature of 700 °C. Upon nitridation of Mo foil with low nitrogen plasma flux, hexagonally-faceted inverse-tapered GaN nanorods were grown whereas tapered GaN nanorods were obtained for Mo foil nitridated under high nitrogen plasma flux. Using wet-chemical etching process, it is deduced that the inverse-tapered GaN nanorods have N-polarity while the tapered GaN nanorods have Ga-polarity. Optical analysis revealed that the inverse-tapered GaN nanorods have prominent near band edge (NBE) emission peak with negligible defect-related peaks whereas tapered GaN nanorods possess yellow luminescence peak along with NBE emission. The control of polarity of GaN nanords on flexible metal foils by tuning pre-nitridation condition is beneficial for futuristic nitride-based flexible opto-electronics devices.

We have grown GaN nanorods on thin Mo metal foil using laser molecular beam epitaxy (LMBE)at growth temperature of 700 degrees C and studied the effect ofpre-nitridationparameters of Mo surface on GaN nanorodsstructural and optical properties. It has been found that the tapering of GaN nanorods grown on Mo foil critically depends on the nitrogen plasma condition for pre-nitridation of Mo foil. The well-aligned and inverse-taperGaNnanorodswere grown on nitridated Mo foil underlow nitrogen plasma flux condition. Thegrown GaN nanorodsare havingporosity at the top side and tapered at the bottom side. The density and length of inverse-taper GaN nanorodsare similar to 2.9 x10(9)cm(-2)and 280-400 nm, respectively. In case of GaN growth onpre-nitridated Mo foil underhigh nitrogen plasma flux condition,the tapered GaN nanorodsinclined with respect to the Mo foil surface normal were obtained on Mo foil. The tapered GaN nanorods density (3.47 x 10(9) cm(-2)) and length(340-670 nm)arehigher than the inverse-taper GaN nanorods. The Raman spectroscopy measurements revealed that the GaN nanorodspossess nearly stress-freewurtziteGaN structure. The room temperaturephotoluminescence spectroscopy measurements disclosedthat the near band edge emission luminescence intensity is 3.9 times higherfor the inverse-taper GaN nanorodsthan the tapered GaN nanorods. The prominentdefectrelated luminescence peak maxima of 2.28 eVwas obtained for tapered GaN nanorodswhereas no defect related peak was observed for inverse-taper GaN nanorods. These observations revealed that the well-aligned inverse-tapered GaN nanorods possesses excellent optical properties compared to inclined dense tapered GaN nanorods.

We report on high-responsivity metal-semiconductor-metal (MSM) structure based ultraviolet (UV) GaN photodetectors fabricated on various GaN nanostructures such as porous nanocolumn network (PNCN), nanowall networks (NWNs), and granular and compact thin films. Different GaN nanostructures were hetero-epitaxially grown on c-sapphire using laser molecular beam epitaxy by tuning the AIN buffer layer growth parameters. High resolution x-ray rocking curve measurements indicate that the crystalline quality of GaN critically depends on the selection of AIN buffer layer growth conditions such as type of ablation target and growth temperature. The porous GaN nanostructures revealed a nearly stress-free wurtzite structure as deduced by Raman spectroscopy measurements. Room temperature photoluminescence spectroscopy showed that the GaN films possess a near band emission (NBE) peak at similar to 3.39 eV along with a broad yellow luminescence (YL) with maxima at 2.25 eV. For GaN PNCN and NWN structures, the NBE-to-YL emission ratio is more than an order higher compared to the GaN films. The fabricated MSM based UV-detector on GaN PNCN and NWN exhibited a high photo-responsivity of similar to 27.72 and 24.8 A/W, respectively, under 2 V applied bias at room temperature. The GaN PNCN and NWN nanostructures with excellent photo-responsivity and optical quality prove to be promising candidates for the fabrication of efficient UV photodetectors due to their continuity in lateral direction, high surface area-to-volume ratio and tailored surfaces. (C) 2018 Elsevier B.V. All rights reserved.

The optical properties of laser-assisted molecular beam epitaxy grown homo-epitaxialGaN nanowall networks (NWNs) were investigated using power dependent photoluminescence (PL) spectroscopy and compared with homo-epitaxial GaN thin film. The pore size and tip width of GaN NWN sample is ˜120–180 nm and 10–15 nm, respectively. The ultraviolet-visible spectroscopy study shows that the GaN NWNs have low optical light reflection and minimum Fabry-Perot cavity effect than GaN film. The room temperature PL spectroscopy reveals that the GaN NWNs possesses enhanced band gap of 3.51 eV with blue shift of 90 meV than the GaN film (3.42 eV). The excitation density dependent PL spectroscopy measurements reveal that the GaN NWNs nanowall and near band emission (NBE) peak position and its linewidth invariant. The intensity of NBE peak for GaN film and nanowalls varies linearly whereas NBE to defect related yellow luminescence peak intensity ratio shows a non-linear variation on the excitation density. The excitation density in PL measurements plays a key role when the sample quality compared on the basis of PL data.

The single crystalline GaN nanorods (NRs) have been grown on flexible tungsten (W) foil using laser molecular beam epitaxy (LMBE) at low growth temperature of 600 degrees C. The field emission scanning electron microscopy showed the growth of GaN NRs having length, width and density of 400 similar to 530 nm, 40-70 nm and 3.4 x 10(9) cm(-2), respectively. The high-resolution transmission electron microscopy reveals the c-axis oriented growth of single crystalline GaN NRs on W foil. The photoluminescence spectroscopy shows that the GaN NRs possesses a strong near band edge luminescence at 3.41 eV with a line-width of similar to 100 meV at room temperature which resembles the high optical quality of GaN nanorods grown on W foil. The possible mechanism of GaN nanords growth on W foil has been also discussed. The low temperature LMBE growth of single crystalline GaN NRs on flexible metal foil with high optical quality paves the way for the development of III-nitride based flexible opto-electronics devices.

We have studied the effect of growth temperature (550-700 degrees C) on the morphological, structural and optical properties of GaN nanostructures grown directly on flexible Ti metal foil by laser assisted molecular beam epitaxy (LMBE). The low growth temperature (550 degrees C) favors a growth of dense, three-dimensional GaN islands whereas probe-shaped sparse GaN nanorods (NRs) are obtained at 700 degrees C. The average length, diameter and density of GaN NRs were analyzed from field emission scanning electron microscopy images and were estimated to be 80 nm, 260 nm and similar to 9.6 x 10(8) cm(-2) respectively. The high-resolution transmission electron microscopy analysis confirmed that the GaN NRs have single crystalline structure over the entire length and had grown along c-axis. Raman spectroscopy studies revealed that the LMBE grown GaN nanostructures on Ti foil possess wurtzite structure with a low tensile stress (0.15-0.37 GPa). The intense near band edge emission peak appeared at 3.42 eV, similar to bulk GaN, with a smaller full width at half maximum of 100 meV for the sparse GaN NRs grown at 700 degrees C. Direct growth of GaN NRs at a relatively lower temperature on flexible metal foils with high structural and optical quality holds the promise for the fabrication of flexible GaN based futuristic optoelectronics devices.

Growth of high quality III-nitride films on high-k oxide/Si substrates is crucial for the fabrication of nitride based metal-oxide-semiconductor devices. Here, we report the effect of nitridation of HfO2 surface on the physical properties of gallium nitride (GaN) films grown on HfO2 (5 nm)/p-Si(100) substrates using laser molecular beam epitaxy (LMBE) technique. The grazing incidence x-ray diffraction (GIXRD) measurements showed the prominent GaN growth along (0002) direction. It is observed that the HfO2 nitridation significantly improves the surface coverage of GaN film with larger grains of size in the range 200 ∼ 250 nm. Room temperature photoluminescence (PL) spectroscopy exhibits a sharp, intense near band-edge emission line at 3.4 eV with negligible defect-related deep bands for the GaN film grown with HfO2 nitridation. Also, the current-voltage characteristics of Au/GaN Schottky contacts show a reduced leakage current for HfO2 nitridation. Thus, the surface nitridation of HfO2 is found vital to improve the quality of GaN films for the development of GaN-on-Si technology.

We have grown various GaN nanostructures such as three-dimensional islands, nanowalls and nanocolumns on c-plane sapphire substrates using laser assisted molecular beam epitaxy (LMBE) system. The shape of the GaN nanostructures was controlled by using different nucleation surfaces such as bare and nitridated sapphire with GaN or AlN buffer layers. The structural and surface morphological properties of grown GaN nanostructures were characterized by ex-situ high resolution x-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy. The symmetric x-ray rocking curve along GaN (0002) plane shows that the GaN grown on pre-nitridated sapphire with GaN or AlN buffer layer possesses good crystalline quality compared to sapphire without nitridation. The Raman spectroscopy measurements revealed the wurtzite phase for all the GaN nanostructures grown on c-sapphire.