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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 E2 (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 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.

We report the direct growth of self-aligned single crystalline GaN nanorod array on flexible Ta metal foil using laser molecular beam epitaxy. Scanning electron microscopy reveals the vertically aligned nanorods on Ta surface having diameters in the range of 60–80 nm. The nanorods show well-defined hexagonal facets and are quite uniformly distributed across the metal foil. Transmission electron microscopy shows single crystalline nature of the individual rods having c-axis oriented growth with wurtzite structure. Room temperature photoluminescence study exhibits a sharp, intense band-to-band emission without any deep-level bands indicating the excellent optical quality of the GaN nanorod array. X-ray photoemission spectroscopy to elucidate the electronic structure of the nanorods confirms Ga–N bonding and the calculated chemical composition turns out to be slightly Ga rich. Location of valence band maxima also suggests the n -type character of GaN nanorods. The photoelectrochemical water-splitting behaviour of the self-aligned GaN nanorod arrays on Ta foil has been investigated using 1 M oxalic acid as the electrolyte with AM 1.5 G simulated solar radiation under 1 Sun (100 mW/cm 2 ) conditions. The results demonstrate an effective way of fabricating well-aligned GaN nanorods on flexible metal foils for developing simple, relatively inexpensive, and flexible photo-electrodes for photocatalytic solar water-splitting applications.

Field emission and photo-detection properties of homoepitaxial GaN nanowall network structure grown on GaN template by laser molecular beam epitaxy technique are demonstrated. The GaN nanowall network has a tip-width of 8–12 nm and exhibited a band-edge photoluminescence emission at 3.53 eV. From the field-emission characterization, a very low turn-on field of 0.68 V/μm and a high emission current density of 2.22 mA/cm2 (at 2.38 V/μm) are observed. Photo-detection measurements were also performed on the GaN nanowall network structure under ultraviolet (UV) light irradiation using metal-semiconductor-metal device structure where, a photo-responsivity of 7.28 A/W is obtained at 3 V biasing.

We have studied the effect of growth temperature (550–700 °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 °C) favors a growth of dense, three-dimensional GaN islands whereas probe-shaped sparse GaN nanorods (NRs) are obtained at 700 °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 ∼9.6 × 108 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 °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.

Self-assembled AlGaN nanowall networks have been grown heteroepitaxially on sapphire (0001) substrate using laser molecular beam epitaxy (LMBE) technique. The effect of growth temperature on the formation of AlGaN nanowall network structure has been studied in the range of 500–700 °C. It is found that the growth of AlGaN under strong N-rich flux condition at a high growth temperature of 700 °C is conducive for the formation of self-assembled nanowall network. In-situ reflection high energy electron diffraction exhibits the three-dimensional growth of the AlGaN nanowall network structure oriented along c-axis. The nanowall width and pore size are measured to be 10–40 and 30–70 nm, respectively, by using field emission scanning electron microscopy. From room temperature photoluminescence measurement, a strong ultra-violet (UV) emission at about 3.52 eV due to band-to-band transition is obtained for the AlGaN nanowall structure with a high UV-to-yellow luminescence intensity ratio indicating a good optical quality. The grown AlGaN nanowall network is suitable for the applications in field emitters, photo-detectors and other nitride-based optoelectronic devices.

Self-aligned GaN nanorod assembly directly grown on metal foil substrates is very attractive for developing flexible devices. We have investigated the effect of growth temperature (500–700 °C) and laser repetition rate (10–30 Hz) on the formation, structure, and morphological and optical properties of GaN nanorods, grown on flexible Ta metal foil by laser molecular beam epitaxy. It is found that the growth temperature plays a critical role in the formation of vertically self-aligned growth of GaN nanorods. High-temperature growth at 700 °C yielded a highly dense and vertically aligned nanorod ensemble on Ta foil. The nanorod alignment is highly uniform across the sample and no tilt was observed irrespective of the polycrystalline nature of the Ta foil. The GaN nanorods are characterized by their hexagonal wurtzite structure and c-axis oriented growth. The effect of laser repetition rate on the growth of GaN at 700 °C was to alter only the dimension and areal density of GaN nanorods but did not affect the nanorod alignment. The grown GaN nanorod ensemble exhibits high optical quality with an intense and sharp near-band-edge emission in the UV region and negligible yellow band emission. The growth of GaN nanorods on Ta foil is observed to be self-driven without being influenced by the nature of the substrate.

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 AlN buffer layer growth parameters. High resolution x-ray rocking curve measurements indicate that the crystalline quality of GaN critically depends on the selection of AlN 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 ∼ 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 ∼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.

Low-dimensional semiconductor structures such as thin films, nanorods, nanowires and zero-dimensional (0-D) quantum dots or islands possess exotic electrical and optical properties compared to their bulk counterpart. Here, 1D nanorods draw a special attention due to their high aspect ratio for potential applications in the field of sensors and other nanoscale devices. Among various semiconductors, GaN having a direct wideband gap has stimulated a great deal of research interest due to the applications in the area of light-emitting diodes, solar cells, high-power electronics devices, laser diodes, ultraviolet photodetectors and water splitting, etc. Due to the advances in flexible or wearable optoelectronic devices, it is required to fabricate inorganic semiconductors hybrid devices directly on flexible substrates in near future. Here, we report the direct growth of various GaN nanostructures such as islands, thin films and nanorods on variety of flexible metal foils using laser-assisted molecular beam epitaxy (LMBE) technique and studied their structural and optical properties. Cubic and wurtzite mixed-phase GaN thin film and island structures have been obtained on the thin Cu and graphene/Cu metal foils under nitrogen-rich growth condition. Interestingly, the growth of high optical quality wurtzite GaN nanorods on bare and nitridated W foil is achieved at a low temperature of 600 °C. Vertically self-aligned GaN nanorods are successfully grown on flexible Ti metal foils at growth temperature of 650–700 °C by tuning the pre-nitridation condition of Ti foils. On the other hand, vertically well-oriented, high-density GaN nanorods have been achieved at 700 °C on bare Ta foil without any surface treatment. Raman spectroscopy, high-resolution X-ray diffraction and high-resolution transmission electron microscopy studies revealed the c-axis growth of high structural quality wurtzite GaN nanorods on these flexible metal foils. The photoluminescence spectroscopy measurements exhibit a near band edge emission around ~3.4 eV with a full width at half maximum value of ~100 meV for densely grown GaN nanorods. Our studies disclosed that various GaN nanostructures were grown directly on different flexible metal foils by tuning the surface treatment process by LMBE technique for developing futuristic flexible optoelectronics devices.

We report the band gap tuning and facilitated charge transport at perylenediimide (PDI)/GaN interface in organic–inorganic hybrid nanostructure system over flexible titanium (Ti) foil. Energy levels of the materials perfectly align and facilitate high efficiency charge transfer from electron rich n-GaN to electron deficient PDI molecules. Proper interface formation resulted in band gap tuning as well as facilitated electron transport as evident in I–V characteristics. Growth of PDI/GaN hybrid system with band gap tuning from ultra-violet to visible region and excellent electrical properties open up new paradigm for fabrication of efficient optoelectronics devices on flexible substrates.

We have grown GaN nanorods on thin Mo metal foil using laser molecular beam epitaxy (LMBE) at growth temperature of 700 °C and studied the effect of pre-nitridation parameters of Mo surface on GaN nanorods structural 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-taper GaN nanorods were grown on nitridated Mo foil underlow nitrogen plasma flux condition. The grown GaN nanorods are having porosity at the top side and tapered at the bottom side. The density and length of inverse-taper GaN nanorodsare ∼2.9 ×109cm-2 and 280-400 nm, respectively. In case of GaN growth on pre-nitridated Mo foil under high nitrogen plasma flux condition, the tapered GaN nanorods inclined with respect to the Mo foil surface normal were obtained on Mo foil. The tapered GaN nanorods density (3.47×109 cm−2) and length (340-670 nm) are higher than the inverse-taper GaN nanorods. The Raman spectroscopy measurements revealed that the GaN nanorods possess nearly stress-free wurtzite GaN structure. The room temperature photo luminescence spectroscopy measurements disclosed that the near band edge emission luminescence intensity is 3.9 times higher for the inverse-taper GaN nanorods than the tapered GaN nanorods. The prominent defect-related luminescence peak maxima of 2.28 eV was obtained for tapered GaN nanorods whereas 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.

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).

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.

Evolution of GaN nanostructure grown on Si (111) substrate has been studied systematically using laser molecular beam epitaxial process. The in-situ reflection high energy electron diffraction and ex-situ high resolution X-ray diffraction studies reveal that the GaN nanostructures have a hexagonal-wurtzite phase and grow along c-axis. The GaN morphology changes from compact granular layer to faceted pyramids to nanowall structure as a function of laser ablation frequency of the KrF excimer laser and radio frequency nitrogen plasma condition. It is observed that GaN nanowall structure is formed on Si (111) when grown under strong nitrogen-rich flux at a higher growth rate and growth temperature. The crystalline and optical quality of the GaN nanostructures significantly improved with increase of laser ablation frequency. The nanowall structure shows good optical emission properties with an intense, sharp near-band edge emission and a negligible deep band luminescence.

Vertical aligned GaN nanowall networks (NWN) have been grown on sapphire (0 0 0 1) substrates using laser assisted molecular beam epitaxy (LMBE) by tuning the sapphire pre-nitridation temperature (200–600 °C). Field emission scanning electron microscopy studies showed the formation of hexagonal shaped vertical GaN NWN with wall width of 8–20 nm on high temperature (600 °C) nitridated sapphire whereas flower shape granular GaN structure with grain size of 50–240 nm was obtained on low temperature (200 °C) nitridated sapphire. The crystalline properties of the GaN NWN have been studied using high resolution X-ray diffraction (HR-XRD). The results show that the full width at half maximum of the GaN(0 0 0 2) X-ray rocking curve has a relatively low value compared to previous reports of hetero-epitaxial grown GaN NWN on sapphire and other substrates. Raman spectroscopy measurements revealed the presence of compressive stress in film whereas tensile stress in GaN NWN structures which is also complimented by HR-XRD analysis. The room temperature photoluminescence spectroscopy measurement for NWN having tip width 8–20 nm showed a 94 meV blue-shift in near band edge emission, even though NWN experienced with tensile stress, which confirm the observation of quantum size effect. The hetero-epitaxial growth of porous GaN NWN with high crystalline quality holds promise for applications in field of nitride based sensors and the enhancement of light extraction efficiency in optoelectronics devices.

An enhanced self-powered near-ultraviolet photodetection phenomenon was observed in epitaxial gallium nitride (GaN) nanorods network grown on an intermediate layer of N:GaN on a nitridated HfO2(N:HfO2)/SiO2/p-Si substrate. The fabricated Au/GaN/N:GaN/N:HfO2/Ag heterostructure exhibited a giant change (OFF/ON ratio > 50 without applying any external electrical field) in its conductance when illuminated by a very weak (25 mW cm−2) near-UV monochromatic light with a low dark current (nearly 20 nA). The presented near-UV photodetector offers photoresponsivity of ∼2.4 mA W−1 at an applied voltage of 1 V. We observed an optically generated internal open circuit voltage of ∼155 mV and short circuit current ∼430 nA, which can be attributed to the quantum confinement of free charge carriers in the nanorod matrix. Interestingly, it also shows a negative capacitance after near-UV illumination. It has great potential as a self-powered UV photodetector and in metamaterial applications.

GaN epitaxial layers have been grown on sapphire (0001) substrate by laser molecular beam epitaxy technique using laser ablation of polycrystalline GaN solid and liquid Ga metal targets in the ambient of nitrogen plasma. In-situ reflection high energy electron diffraction and exsitu atomic force microscopy observations revealed that GaN growth using solid target yields rough surface under three-dimensional growth mode while a flat surface GaN is obtained using liquid Ga target. From X-ray rocking curve measurement, it is also observed that the GaN layer grown using solid GaN target has a relatively better structural quality. X-ray photoelectron spectroscopy confirmed Ga–N bond formation and near-stoichiometric composition of the GaN epilayers. The influence of threading dislocation density on the ultraviolet (UV) photoresponse properties of GaN layers have been studied using metal-semiconductor-metal (MSM) based device structure. It is found that the GaN MSM structure obtained using solid GaN target with lesser screw and dislocation densities exhibits a higher responsivity with fast response and recovery time.

Vertically well-aligned homo-epitaxial GaN nanowall network (NWN) was grown on metal organic chemical vapor deposited 3.5 μm thick GaN (0001) on c-sapphire by laser assisted molecular beam epitaxy (LMBE). The honeycomb GaN NWN with wall width of 8–12 nm and pore size of ∼100–200 nm were obtained at a low growth temperature of 600 °C. The optical properties measured by photoluminescence (PL) spectroscopy showed a blue-shift of ∼100 meV near band edge (NBE) emission for GaN NWN. In addition, temperature dependent (90–300 K) PL measurements of GaN NWN were investigated which follow the Varshni's temperature dependent energy gap relation. The structural and optical properties of pristine and wet-etched LMBE grown homo-epitaxial GaN thin film at 500 °C were also investigated and compared with the GaN NWN structures. X-ray photoelectron spectroscopy study confirms Ga rich nature of GaN nanowalls along with significant decrease in surface band bending in NWN compared to film. The KOH wet etched of GaN NWN showed that the wall width decreased to 6–10 nm and exhibited a blue-shift of 120 meV in the NBE in PL emission confirming the realization of quantum size effect in the GaN NWN structure. The low temperature grown GaN NWN structures can be integrated with light emitting diodes and solar cells for the enhancement of light extraction and device efficiency.

We have investigated the influence of growth temperature on the in-plane strain, structural, optical and mechanical properties of heteroepitaxially grown GaN layers on sapphire (0001) substrate by laser molecular beam epitaxy (LMBE) technique in the temperature range 500–700 °C. The GaN epitaxial layers are found to have a large in-plane compressive stress of about 1 GPa for low growth temperatures but the strain drastically reduced in the layer grown at 700 °C. The nature of the in-plane strain has been analyzed using high resolution x-ray diffraction, atomic force microscopy (AFM), Raman spectroscopy and photoluminescence (PL) measurements. From AFM, a change in GaN growth mode from grain to island is observed at the high growth temperature above 600 °C. A blue shift of 20–30 meV in near band edge PL emission line has been noticed for the GaN layers containing the large in-plane strain. These observations indicate that the in-plane strain in the GaN layers is dominated by a biaxial strain. Using nanoindentation, it is found that the indentation hardness and Young's modulus of the GaN layers increases with increasing growth temperature. The results disclose the critical role of growth mode in determining the in-plane strain and mechanical properties of the GaN layers grown by LMBE technique.

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 (~ 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 ~ 370 nm at the growth temperature of 700 °C. Nano-porous (NP)-GaN was obtained with pore sizes in the range of 70 ~ 110 nm having vertical height of ~ 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 (11–20) 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 ~ 358 mA/W at an applied bias of 1V. The photo-responsivity of NP-GaN MSM device is higher than that of GaN film (~ 36 mA/W) and NC-GaN (~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.

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 °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 (~2.42 × 109 cm−2) GaN NRs were grown on pre-nitridated Nb metal foil at growth temperature of 700 °C with length and diameter of ~550–640 nm and ~60–130 nm, respectively. The low growth temperature of 650 °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 ~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 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.

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 °C, 3–5 J/cm2and 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,~1.41 × 109 cm−2, 430–620 nm and 70–160 nm is achieved at 700 °C using laser energy density of ~5 J/cm2 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 (~3.42 × 109 cm−2 for 20 Hz and ~3.51 × 109 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 ~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.

Deep levels in unintentionally doped GaN epilayers grown on sapphire (0001) by ultra high vacuum pulsed laser deposition at 600 and 700 degrees C have been studied using photo-induced current transient spectroscopy (PICTS). The GaN epitaxial layers were deposited by laser ablating a hydride vapor phase epitaxy grown bulk GaN polycrystalline target in the ambient of r.f. nitrogen radicals. The activation energy of dark conductivity for these layers lies in the range 0.17-0.25 eV. An acceptor-like deep trap level is detected in the PICTS spectrum with activation energy of 1.32 eV. For the 700 degrees C grown GaN layer, an additional well-pronounced acceptor-like defect level near mid-gap at 1.52 eV has been observed. From the analysis of trap capture kinetics, both defect levels are identified to be extended 'bandlike' electronic states associated with extended defects such as dislocations present in GaN layers.

An ultra thin film (∼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 104 : 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.

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 (∼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.

Low dimensional structures such as two-dimensional (2D) nanowalls, 1D nanorods or nanowires and 0-D quantum dots of semiconductors exhibit different mechanical, electrical and optical properties compared to their bulk counterpart. Despite the promising properties of 1D and 0-D GaN nanostructures, they require complicated and expensive process to handle them individually for fabrication of specific devices. Here, 2D nanowall network draws a special attention due to their continuity in lateral direction and porous surfaces for fruitful applications in the field of nitride based sensors and other nano-scale devices. Among various semiconducting materials, a great attention has been given to the wide, direct band gap III-nitride semiconductors because of their applications in high efficient full color-spectrum light emitting diodes (LEDs), high power electronics devices and ultra-violet photo-detectors among others. This chapter describes the growth of 2D GaN nanowall network on GaN template and sapphire (0001) substrates using laser assisted molecular beam epitaxy (LMBE) technique. The GaN nanowalls of different dimensions were grown by laser ablation of a high purity polycrystalline GaN target in the presence of active r.f. nitrogen plasma. The wall width and pore size were controlled by tuning the laser frequency in the range 10–40 Hz. The structural, optical and electronic properties of the GaN nanowalls were investigated using various characterization techniques such as high resolution X-ray diffraction, field emission scanning electron microscopy, Raman spectroscopy, Rutherford backscattering spectroscopy, photoluminescence and X-ray photoemission spectroscopy. Surface morphology studies exhibited a GaN nanowall network formation of wall width in the range 10–30 nm and pore sizes of 90–180 nm. The photoluminescence spectroscopy measurements showed the optical emission related to GaN nanowalls with a blue shift of about 100 meV from the bulk GaN emission for thinner GaN nanowalls of width

We have grown AlGaN (0001) epitaxial layers on sapphire (0001) by using laser molecular beam epitaxy (LMBE) technique. The growth was carried out using laser ablation of AlxGa1-x liquid metal alloy under r.f. nitrogen plasma ambient. Before epilayer growth, the sapphire nitradation was performed at 700 °C using r.f nitrogen plasma followed by AlGaN layer growth. The in-situ reflection high energy electron diffraction (RHEED) was employed to monitor the substrate nitridation and AlGaN epitaxial growth. High resolution x-ray diffraction showed wurtzite hexagonal growth of AlGaN layer along c-axis. An absorption bandgap of 3.97 eV is obtained for the grown AlGaN layer indicating an Al composition of more than 20 %. Using ellipsometry, a refractive index (n) value of about 2.19 is obtained in the visible region.

GaN nanorods (NRs) and hollow nanocolumns (HNCs) were grown on flexible Ti foils using laser-assisted molecular beam epitaxy at a growth temperature of 700 °C. The shape, size and density of the GaN nanostructures were tuned by surface nitridation and AlN buffer layer growth temperature on a Ti foil. Sparse (∼ 5.5 × 108 cm−2) GaN NRs were obtained on the bare surface whereas dense (∼3.47 × 109 cm−2) GaN NRs were grown on the nitridated Ti foil. The shape of the GaN changed from NRs to HNCs by introducing an AlN buffer layer on nitridated Ti foil. Raman spectroscopy showed the grown GaN nanostructures have a wurtzite crystal structure. Room-temperature photoluminescence spectroscopy measurements show that the GaN nanostructures possess an intensive near band edge emission at ∼3.42 eV with a negligible defect-related peak. The growth of tunable GaN nanostructures on flexible metal foils is attractive for flexible optoelectronics and sensor devices.

Graphical abstractDisplay Omitted The real-time observation of faster degradation at GB sites of HfO2 dielectric.C-AFM results show enhanced defect generation rate at the GB sites.Preferential breakdown of SiOx IL occurs below the degraded GB region.A breakdown sequence in polycrystalline HfO2/SiOx dielectric stacks was proposed. Grain boundaries (GBs) in polycrystalline high-? (HK) dielectric films affect the performance and reliability metrics of HK based advanced metal-oxide-semiconductor (MOS) devices. A full understanding of device reliability can only be had with the knowledge of the detrimental role of GB in degradation and breakdown (BD) of polycrystalline HK/interfacial layer (IL) dielectric stacks. In this work, we present a nanoscale resolution study on how the polycrystalline microstructure affects the degradation and BD at GB sites of polycrystalline HfO2 in HfO2/SiOx (x≤2) dielectric stacks using conductive-atomic force microscopy (C-AFM), supported by a statistical failure distribution model and device level simulations. Results clearly show an enhanced trap generation and faster degradation of polycrystalline HfO2 gate dielectrics at GB sites as compared to the bulk (grain) regions implying shorter time-dependent dielectric breakdown (TDDB) lifetime at the GB sites. The SiOx IL below the degraded GB experiences a BD event when an enhanced electric field across the SiOx IL reaches the critical BD field, eventually triggering the overall BD of the HfO2/SiOx dielectric stack.

We have grown homoepitaxial GaN films on metal organic chemical vapor deposition (MOCVD) grown 3.5 µm thick GaN on sapphire (0001) substrate (GaN template) using an ultra-high vacuum (UHV) laser assisted molecular beam epitaxy (LMBE) system. The GaN films were grown by laser ablating a polycrystalline solid GaN target in the presence of active r.f. nitrogen plasma. The influence of laser repetition rates (10-30 Hz) on the surface morphology of homoepitaxial GaN layers have been studied using atomic force microscopy. It was found that GaN layer grown at 10 Hz shows a smooth surface with uniform grain size compared to the rough surface with irregular shape grains obtained at 30 Hz. The variation of surface roughness of the homoepitaxial GaN layer with and without wet chemical etching has been also studied and it was observed that the roughness of the film decreased after wet etching due to the curved structure/rough surface.

Generally, the GaN growth by conventional techniques like metal organic chemical vapor deposition (MOCVD) and plasma assisted- molecular beam epitaxy (PA-MBE) techniques employ a higher growth temperature (900–1000 °C in MOCVD and >720 °C in PA-MBE), in which, the probability of forming unwanted alloys or compounds with the substrate at interface will be quite high. To minimize the formation of undesirable interfacial compounds, a low temperature growth is favorable. Here, we have explored the possibility of low temperature growth of GaN layers on sapphire (0001) substrates using an ultra-high vacuum laser assisted molecular beam epitaxy (LMBE) system under different growth conditions. GaN epitaxial layers have been grown by laser ablating liquid Ga metal and polycrystalline solid GaN targets in the presence of active nitrogen environment supplied by radio frequency nitrogen plasma source. The structural and optical properties of the epitaxial GaN layers were characterized using reflection high energy electron diffraction, high resolution x-ray diffraction, atomic force microscopy, Raman spectroscopy, Rutherford Backscattering Spectroscopy, secondary ion mass spectroscopy and photoluminescence spectroscopy. The low temperature LMBE grown GaN layers showed high crystalline structures with a screw dislocation density in the range of 107 cm−2 as calculated from the x-ray rocking measurements along (0002) plane, which is the lowest value obtained so far using LMBE technique. A strong near band-edge photoluminescence emission has been obtained for the grown GaN layers at room temperature with a relatively weak yellow band emission. Our results indicate that the LMBE technique is capable of growing high quality III-nitride crystalline films at a relatively lower growth temperature compared to the conventional growth techniques.

The surface roughness and defect density of GaN epitaxial layers grown on c-plane sapphire substrate are investigated and found to be dependent on nitridation temperature. GaN epitaxial layers grown after nitridation of sapphire at 200°C have a higher defect density and higher surface roughness compared to the GaN layers grown at 646°C nitridation as confirmed by atomic force microscopy (AFM). The persistent photoconductivity (PPC) was observed in both samples and it was found to be decreasing with decreasing temperature in the range 150-300°C due to long carrier lifetime and high electron mobility at low temperature. The photoresponse of the GaN films grown in this study exhibit improved PPC due to their better surface morphology at 646°C nitrided sample. The point defects or extended microstructure defects limits the photocarrier lifetime and electron mobility at 200°C nitrided sample.

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.

Epitaxial thin GaN films (∼60 nm) have been grown on a-plane sapphire substrates at different growth temperatures (500–700 °C) using laser molecular beam epitaxy (LMBE). The effect of growth temperatures on the structural and optical properties of GaN layers grown on low temperature (LT) GaN buffer on prenitridated a-sapphire have been studied systematically. The in situ reflection high energy electron diffraction pattern revealed the three-dimensional epitaxial growth of GaN films on a-sapphire under the adopted growth conditions. The full width at half maximum (FWHM) value of x-ray rocking curves (XRCs) along GaN (0002) and (10-12) planes decreases with increasing growth temperature. The FWHM values of (0002) and (10-12) XRC for the 700 °C grown GaN film are 1.09° and 1.08°, respectively. Atomic force microscopy characterization showed that the grain size of GaN increases from 30–60 to 70–125 nm with the increase in growth temperature as GaN coalescence time is shorter at high temperature. The refractive index value for the dense GaN film grown at 600 °C is obtained to be ∼2.19 at the wavelength of 632 nm as deduced by spectroscopic ellipsometry. Photoluminescence spectroscopy confirmed that the epitaxial GaN layers grown on a-sapphire at 600–700 °C possess near band edge emission at ∼3.39 eV, close to bulk GaN. The GaN growth at 700 °C without a buffer still produced films with better crystalline and optical properties, but their surface morphology and coverage were inferior to those of the films grown with LT buffer. The results show that the growth temperature strongly influences the structural and optical quality of LMBE grown epitaxial GaN thin films on a-plane sapphire, and a growth temperature of >600 °C is necessary to achieve good quality GaN films.Epitaxial thin GaN films (∼60 nm) have been grown on a-plane sapphire substrates at different growth temperatures (500–700 °C) using laser molecular beam epitaxy (LMBE). The effect of growth temperatures on the structural and optical properties of GaN layers grown on low temperature (LT) GaN buffer on prenitridated a-sapphire have been studied systematically. The in situ reflection high energy electron diffraction pattern revealed the three-dimensional epitaxial growth of GaN films on a-sapphire under the adopted growth conditions. The full width at half maximum (FWHM) value of x-ray rocking curves (XRCs) along GaN (0002) and (10-12) planes decreases with increasing growth temperature. The FWHM values of (0002) and (10-12) XRC for the 700 °C grown GaN film are 1.09° and 1.08°, respectively. Atomic force microscopy characterization showed that the grain size of GaN increases from 30–60 to 70–125 nm with the increase in growth temperature as GaN coalescence time is shorter at high temperature. The refractiv...

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 HVPE 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 °C. The GaN film grown at 700 °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 pre-nitridation of sapphire at growth temperature 500 °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.

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 up to 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 °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 °C, which is about 150 °C lower than the conventional MBE growth.

Grain boundaries (GBs) in polycrystalline high-κ (HK) dielectric materials affect the electrical performance and reliability of advanced HK-based metal-oxide-semiconductor (MOS) devices. In this work, we present a localized study comparing the electrical conduction through grains and GBs for CeO 2 and HfO 2 -based HK dielectrics using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) at the nanometer scale, in conjunction with macroscopic MOS capacitor device level analysis. Nanoscale STM conduction analysis clearly reveals faster degradation at GB sites and their vulnerability to early percolation. Multi-layer HK dielectric stacks (capping of La 2 O 3 on CeO 2 and dual-layer ZrO 2 /HfO 2 ) are proposed as an effective technique to significantly enhance the time-dependent dielectric breakdown (TDDB) robustness of advanced HK metal gate (MG) stacks.

In this work, we use scanning tunneling microscopy (STM) to study the localized degradation, breakdown and post-breakdown of a high-к (HK) gate dielectric material, cerium oxide (CeO 2 ) deposited directly on a silicon substrate. The novelty of the study lies in analyzing the breakdown phenomenon from a macroscopic metal-oxide-semiconductor (MOS) capacitor level to a very localized nanoscale breakdown location. The physics of failure for these polycrystalline cerium oxide HK films is also discussed.

Epitaxial GaN films were grown on sapphire (0001) substrates by an ultra-high vacuum laser assisted molecular beam epitaxy (MBE) system using GaN solid target with laser energy density of ~3 J cm−2 at various growth conditions. The influence of growth temperature, layer thickness and growth rate on the structural properties of the GaN layers have been studied using high resolution x-ray diffraction, field emission scanning electron microscopy and scanning tunneling microscopy at room temperature. The epitaxial GaN layers grown at 700 °C exhibited good crystalline properties with a screw dislocation density of 3.1 × 108 cm−2 as calculated from the x-ray rocking curve measurements. The electronic properties such as core levels and valence band of GaN film were examined using x-ray photoelectron spectroscopy. Chemical composition of the GaN layer was determined using core level spectroscopy.

Grain boundaries in the polycrystalline microstructure of post-annealed high-κ (HK) dielectrics are a major limitation in the reliability of HK dielectrics used for advanced CMOS technologies. Another challenge in the field of HK dielectrics is to ensure higher drain drive current in CMOS, while maintaining low leakage current. In this work, the authors demonstrate enhanced performance of HfO2 and CeO2 dielectrics by incorporating lanthanum. The resulting stacks show promising dielectric characteristics with reduced leakage current and uniform (amorphous) crystal structure. The improved HK characteristics were shown to occur even over nanometer-length scales using scanning probe microscopy and transmission electron microscopy, in agreement with previous studies based on micron-scale device-level measurement. Published version

We report the effect of growth temperature on defect states of GaN epitaxial layers grown on 3.5 μm thick GaN epi-layer on sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The GaN samples grown at three different substrate temperatures at 730, 740 and 750 °C were characterized using atomic force microscopy and photoluminescence spectroscopy. The atomic force microscopy images of these samples show the presence of small surface and large hexagonal pits on the GaN film surfaces. The surface defect density of high temperature grown sample is smaller (4.0 × 108 cm−2 at 750 °C) than that of the low temperature grown sample (1.1 × 109 cm−2 at 730 °C). A correlation between growth temperature and concentration of deep centre defect states from photoluminescence spectra is also presented. The GaN film grown at 750 °C exhibits the lowest defect concentration which confirms that the growth temperature strongly influences the surface morphology and affects the optical properties of the GaN epitaxial films.

We use conducting atomic force microscopy (AFM) in ultra high vacuum to measure the thermoelectric power of Au, Pt, and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films. Tips coated with thick (1200 nm) Pt films or highly doped diamond film give reproducible data. The thermoelectric power of metal junctions formed with diamond tips is high but dominated by the diamond material thus making diamond tips of limited applicability in thermovoltage AFM. Pt coated tips on Au or Pt films gives small thermovoltage signal, making quantitative analysis of the thermopower on metal sample problematic. The thermovoltage AFM technique appears best suited to study organic thin films and the thermoelectric power of 1.5 nm and 2 nm thick PTCDA deposited on Au measured with Pt tips is −342 and −372 μV/K, respectively. The negative sign indicates that the lowest unoccupied molecular orbital level dominates electrical transport.

The presence of grain boundaries (GBs) in polycrystalline high-κ (HK) gate dielectric materials affects the electrical performance and reliability of advanced HK based metal-oxide-semiconductor devices. It is important to study the role of GB in stress-induced-leakage current (SILC) degradation and time-dependent dielectric breakdown of polycrystalline HK gate stacks. In this work, we present nanoscale localized electrical study and uniform stressing analysis comparing the electrical conduction properties at grain and GB locations for blanket cerium oxide (CeO2)-based HK thin films using scanning tunneling microscopy. The results clearly reveal higher SILC degradation rate at GB sites and their vulnerability to early percolation, supporting the phenomenon of GB-assisted HK gate dielectric degradation and breakdown.

One-dimensional (1D) semimetals (e.g., antimony and bismuth) are excellent thermoelectric materials. We demonstrate the self-assembly of 1D Sb nanowires on highly oriented pyrolytic graphite, along with structures of other dimensionality. Comparing with an Sb crystal in ambient condition, our scanning tunneling microscopy analysis indicates that these Sb nanowires have a compressed lattice structure, which is likely formed under the Laplace pressure that can be quite large in a nanostructure. The conditions for growing aligned semimetal nanowires exclusively are discussed.