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High density aligned GaN nanorods are obtained on r-plane sapphire with substrate nitridation by Laser MBE. In contrast, a flat surface a-plane GaN epitaxial layer is achieved on r-plane sapphire without nitridation using GaN/AlN buffer by Laser MBE.

This report discusses the effect of post-oxidation processes and thickness of SnO2 thin films on their physical properties and CO gas sensing characteristics. SnO2 films have been grown by simple vacuum evaporation of Sn metal and post-oxidation process. The post-oxidation of Sn films was performed by two different methods, i.e., step-oxidation and direct-oxidation, at a maximum temperature of 850 °C. The thickness of Sn films was varied as 100, 200 and 500 nm. As characterized by x-ray diffraction technique, the SnO2 thin films have a tetragonal rutile structure with crystallite size of 21 and 27 nm for direct-oxidation and step-oxidation methods, respectively. The SnO2 films produced by both oxidation methods possess a porous, granular surface morphology as revealed by scanning electron microscopy. The band gap of SnO2 films calculated using ultraviolet–visible spectroscopy in absorption mode is found to be in the range of 3.36–3.59 eV. The CO sensing performance has been examined at various measurement temperatures up to 400 °C using calibrated CO gas. It is found that the CO sensitivity is significantly higher for the direct-oxidized SnO2 films as compared to the step-oxidized films. Also, thinner SnO2 film, i.e. 100 nm of thickness, obtained by direct-oxidation method is found to have the highest sensitivity of 70.7% against CO gas of 915 ppm concentration with a fast response time of 21 s. The effective CO sensing of thinner SnO2 film is understood based on the formation of smaller grains with the uniform surface distribution. The film is more selective towards CO gas as compared to NH3 and NO2 gases and shows a CO limit-of-detection of 10 ppm.

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 structural, optical, magnetic, and electrical properties of Co2CrAl Heusler alloy magnetic thin films grown on n-type silicon (100) substrate (n-Si) and glass substrate were studied. The films were deposited using DC magnetron sputtering. X-ray diffraction (XRD) analysis confirmed the polycrystalline nature of the films. The effect of grain size on transmittance was investigated. Magnetic measurements revealed the presence of magnetic ordering in the films. Partial densities of states (PDOS) of the Co2CrAl were calculated by density functional theory (DFT) methods using the Vienna Ab initio Simulation Package (VASP). Co2CrAl thin film deposited over a silicon substrate was investigated for I–V characteristics. The electrical behaviour confirmed the existence of a Co2CrAl/n-Si Schottky contact, which suggests a spin injection phenomenon from Co2CrAl to n-Si by tunnelling through the lowered Schottky barrier.

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.

In this study, we fabricated a planar Si/PEDOT: PSS heterojunction solar cell using three different solvents—ethylene glycol, acetonitrile, and dimethyl sulfoxide—to find the best one. The fabricated samples were characterized by diffuse reflectance spectroscopy, scanning electron microscopy, X-ray diffraction, and current–voltage. Diffused reflectance spectrum analysis showed reduced reflectance compared to the bare silicon wafers. The absorbance spectrum shows the change in absorption of the Si-coated PEDOT: PSS which was more than a 50% increase in the UV region, and for the EG sample, there was a 20% increase in the entire visible spectrum. This indicates that the solvent plays a major role in the bandgap between the Si and Si/PEDOT: PSS. Scanning electron microscope (SEM) was used to examine the surface morphology of Si/PEDOT: PSS as agglomerated, island-formed surfaces and carbon-layered Si-PEDOT: PSS. Cross-sectional images show the thickness of the PEDOT: PSS layer on the silicon wafer surface. The X-ray diffraction (XRD) pattern shows the characteristic peaks for silicon (69.5°), and Si/PEDOT: PSS shows a forbidden Si (200) peak at 32°. Current–voltage measurements have shown the characteristic diode curve for all fabricated cells. This characteristic diode curve indicated the presence of a heterojunction. Ethylene glycol-containing cells showed current output as 0.2 μA with Voc of 0.2 V.

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.

CSIR-NPL is the custodian of National standards of electrical and electronic parameters. These include DC parameters such as voltage, current and resistance; low frequency and high frequency impedance related quantities such as capacitance, inductance and AC resistance; AC/DC high voltage and AC high current; AC power and energy; and quantum standard which includes quantum hall resistance (QHR), quantum current (QC) and quantum nanophotonics (QN). The metrological traceability of the electrical and electronics parameters to SI units is derived from Josephson Voltage Standard (JVS), Quantum Hall Resistance (QHR) standard and frequency (time) standards; all of them are being maintained at CSIR-NPL with metrological precision at par with international standards. The traceability of the aforementioned parameters is disseminated through an unbroken chain of apex level calibrations and testing at par with international level to the industries and strategic sectors of the country to improve the quality of life, which in turn will lead to the inclusive growth of the country and economic development. R&D efforts on the development of quantum standards is a constant endeavour and continues to be at the forefront. Specifically, CSIR-NPL focuses on the development of quantum standards related to the unit of current (ampere), the unit of resistance (ohm) and quantum nanophotonics which aims for detection of few photons (or even single photons) using the novel concept of superconducting nanowire single photon detectors (SNSPD). Among these the research on QC needs special mention, as this will lead to realisation of the SI unit of electric current (ampere), the only unit out of the seven base units of SI system. The quantum current standard (QCS) realisation is bifurcated into two approaches, (i) based on the single electron tunnelling effect (SET) observable in semiconductor quantum dot (QD) structures and (ii) the quantum phase slip phenomenon (QPS) observable in superconducting nanowires of cross-sectional area of the order of coherence length of the system. The realisation of the resistance unit (ohm) is based on the quantum hall effect observable in semiconductor 2DEG structures such as GaAs/AlGaAs systems. Recently there has been tremendous evidence emerging for the use of monolayer graphene for the use of QHR metrology. CSIR-NPL also started the growth of “epitaxial graphene anchored on SiC” and have obtained encouraging results. The nano-photonics measurement research is also taken up actively for its applications to realize quantum standards for optical radiation and device fabrication.

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.

We report the successful growth of AlxGa1−xN (0 ≤ x ≤ 0.25) epitaxial films on c-plane sapphire substrate by using laser molecular beam epitaxy technique. The role of growth temperature on the Al incorporation and the structural, electronic and optical properties of the AlxGa1−xN epitaxial layers grown in the temperature range 500–700 °C have been systematically studied. The atomic force microscopy analysis shows that the grain size of AlxGa1−xN increases with increase in growth temperature and flat surface epilayers are obtained at ≥ 600 °C. The Al incorporation is confirmed with high resolution x-ray diffraction, x-ray photo electron microscopy and photoluminescence studies. It is observed that the growth temperature plays a critical role in determining the Al composition, which increases with increasing growth temperature. AlxGa1−xN layer with about 23% of Al composition is obtained on sapphire (0001) substrate at a growth temperature of 700 °C, which is about 100–150 °C lower than the conventional molecular beam epitaxy growth.

In this paper, we have reported a simple and efficient method for the synthesis of uniform, highly conducting single or few layer molybdenum disulfide (MoS2) on large scale. Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM) have been used for the confirmation of mono or few layered nature of the as-synthesized MoS2 sheets. X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) and Raman Spectroscopy have also been used to study the elemental, phase, and molecular composition of the sample. Optical properties of as-synthesized sample have been probed by measuring absorption and photoluminescence spectra which also compliment the formation of mono and few layers MoS2 Current-voltage (I-V ) characteristics of as-synthesized sample in the pellet form reveal that MoS2 sheets have an ohmic character and found to be highly conducting. Besides characterizing the as-synthesized sample, we have also proposed the mechanism and factors which play a decisive role in formation of high quality MoS2 sheets.

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.

This article discusses the selective self-assembly of semi-metal straight and branched nanorods on inert substrates. In particular, it describes antimony (Sb) nanorods and bismuth (Bi) nanobelts on inert substrates by physical vapor deposition in vacuum without using any catalyst and nanoscale template. After describing the experimental and drift correction procedures, the article reviews previous studies of semi-metal growth on inert substrates. It then measures the surface morphology and atomic structures of self-assembled Sb nanorods and Bi nanobelts using an in-situ scanning tunnelling microscope (STM) in ultrahigh vacuum (UHV). Based on these STM data, a mechanism for the self-assembly of straight and branched semi-metal nanorods is proposed.

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.

Using nanometer-resolution characterization techniques, we present a study of the local structural and electrical properties of grain boundaries (GBs) in polycrystalline high-κ (HK) dielectric and their role on the reliability of underlying interfacial layer (IL). A detailed understanding of this analysis requires characterization of HK/IL dielectrics with nanometer scale resolution. In this work, we present the impact of surface roughness, thickness and GBs containing high density of defects, in polycrystalline HfO 2 dielectric on the performance of underlying SiO x ( x ⩽ 2) IL using atomic force microscopy and simulation (device and statistical) results. Our results show SiO x IL beneath the GBs and thinner HfO 2 dielectric experiences enhanced electric field and is likely to trigger the breakdown of the SiO x IL.