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In view of the fact, the poor stability and inadequate chemical properties of conductive polymers inhibit their use in gas sensor applications. Aside from this, the elegant two-dimensional (2D) polymeric graphitic carbon nitride (g-C3N4) have attracted numerous attention for the room temperature (RT) gas sensing applications due to its intrinsic porous structure and the lone pair electrons on nitrogen atoms. Herein, we report surface acoustic wave (SAW) NO2 gas sensor using 2D g-C3N4 nanosheets deposited on langasite (LGS). Further, the thickness effect on gas sensing performances were manifested and it was noticed that 220 nm thickness of g-C3N4 coated LGS SAW sensor showed better sensing performances. The g-C3N4/LGS SAW sensor showed a significant negative differential frequency shift (Δf) of ~3.1 kHz at room temperature (RT~27 oC) for 100 ppm of NO2 gas with a quick response/recovery times (42/22 s). Furthermore, our experimental results demonstrated that the effect of temperature remarkably influences the NO2 sensing responses and it was escalated from ~3.1 kHz to ~23.3 kHz in the temperature range of 27 oC to 200 oC for 100 ppm of NO2. Also, the device showed immense sensitivity towards NO2 (100 ppm) under various relative humidity (RH) conditions (20-80%) at RT. Besides, the g-C3N4/LGS SAW sensor showed a low detection limit (~158 ppb), excellent long-term stability, high sensitivity, and selectivity to NO2 gas, which could be ascribed to the enhancement in mass loading effect resulting by the absorption of NO2 gas molecules. Finally, with help of resistive gas sensors, we have deliberately discussed the underlying mechanism associated with the 2D g-C3N4 nanosheets deposited LGS SAW device.

Although excellent milestones of III-nitrides in optoelectronic devices have been achieved, the focus on the optimization of their geometrical structure for multiple applications is very rare. To address this issue, we exclusively designed a prototype device to enhance the photoconversion efficiency and gas interaction capabilities of GaN nanorods (NRs) grown on a V-grooved Si(100) substrate with Si(111) facets for photodetector and gas sensor applications. Photoluminescence studies have demonstrated an increased surface-to-volume ratio and light trapping for GaN NRs grown on V-grooved Si(111). GaN NRs on V-grooved Si(100) with Si(111) facets exhibited high photodetection performance in terms of photoresponsivity (217 mA/cm2), detectivity (3 × 1013 Jones), and external quantum efficiency (2.73 × 105%) compared to GaN NRs grown on plain Si(111). Owing to the robust interconnection between NRs and a high surface-to-volume ratio, the GaN NRs grown on V-grooved Si(100) with Si(111) facets probed for NO2 detection with the assistance of photonic energy. The photo-assisted sensing makes it possible to detect NO2 gas at the ppb level at room temperature, resulting in significant power reduction. The device showed high selectivity to NO2 against other target gases, such as NO, H2S, H2, NH3, and CO. The device showed excellent long-term stability at room temperature; the humidity effect on the device performance was also examined. The excellent device performance was due to the following: (i) benefited from the V-grooved Si structure, GaN NRs significantly trapped the incident light, which promoted high photocurrent conversion efficiency and (ii) GaN NRs grown on V-grooved Si(100) with Si(111) facets increased the surface-to-volume ratio and thus improved the gas interaction with a better diffusion ratio and high light trapping, which resulted in increased response/recovery times. These results represent an important forward step in prototype devices for multiple applications in materials research.

Although metal nanoparticles (NPs) have been widely reported, Au NPs functionalized reduced graphene oxide (rGO)/GaN nanorods (NRs) for multi-functional applications are rarely discussed. The rGO is a well known transparent electrode and has been considering an alternative electrode to ITO in the current optoelectronic community. In this work, Au NPs functionalized rGO (Au@rGO)/GaN NRs hybrid structure probed for photodetector and CO gas sensing applications. The hybrid structure was characterized by scanning electron microscopy, transmission electron microscope, current-voltage characteristics, photo conductivity, and gas sensor measurements. The Au@rGO/GaN NRs showed higher photoresponsivity (λ = 382 nm, 516 nm) compared to rGO/GaN NRs at room temperature. The rising and falling times of Au@rGO/GaN NRs are faster than that of rGO/GaN NRs. The hybrid structure Au@rGO/GaN NRs exhibited high CO gas response compared to rGO/GaN NRs at room temperature (∼38% to the 20 ppm). Au NPs played an important role in terms of electronic and chemical changes in the hybrid structure for improving both photodetectors the CO gas response. Such a multi-functional hybrid device is an interest of various room temperature applications.

The surface states, poor carrier life, and other native defects in GaN nanorods (NRs) limit their utilization in high-speed and large-gain ultraviolet (UV) photodetection applications. Making a hybrid structure is one of the finest strategies to overcome such impediments. In this work, a polypyrrole (Ppy)-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/GaN NRs hybrid structure is introduced for self-powered UV photodetection applications. This hybrid structure yields high photodetection performance, while pristine GaN NRs showed negligible photodetection properties. The ability of the photodetector is further boosted by functionalizing the hybrid structure with Ag nanowires (NWs). The Ag NWs-functionalized hybrid structure exhibited a responsivity of 3.1 × 103 (A/W), detectivity of 3.19 × 1014 Jones, and external quantum efficiency of 1.06 × 106 (%) under a UV illumination of λ = 382 nm. This high photoresponse is due to the huge photon absorption rising from the localized surface plasmonic effect of a Ag NWs network. Also, the Ag NWs significantly improved the rising and falling times, which were noted to be 0.20 and 0.21 s, respectively. The model band diagram was proposed with the assistance of X-ray photoelectron spectroscopy to explore the origin of the superior performance of the Ag NWs-decorated Ppy-PEDOT:PSS/GaN NRs photodetector. The proposed hybrid structure seems to be a promising candidate for the development of high-performance UV photodetectors.

Recently, III-nitride semiconductor nanostructures, especially InGaN/GaN quantum well nanorods (NRs), have been established as a promising material of choice for nanoscale optoelectronics and photoelectrochemical (PEC) water-splitting applications. Due to the large number of surface states, III-nitride NRs suffer from low quantum efficiency. Therefore, control of the surface states is necessary to improve device performance in real-time applications. In this work, we investigated the effect of hydrogen plasma treatment on the optical properties of InGaN/GaN single-quantum-well (SQW) NRs. The low-temperature photoluminescence (PL) studies revealed that yellow and green emissions overlapped and the yellow band is more dominant in the pristine InGaN/GaN SQW NRs. However, the emission corresponding to yellow luminescence was strongly suppressed and the green emission is more intensified in hydrogenated InGaN/GaN SQW NRs. Furthermore, the time-resolved PL spectroscopy studies revealed that the carrier lifetimes of hydrogenated InGaN/GaN SQW NRs are relatively short compared to the pristine InGaN/GaN SQW, indicating the effective reduction of non-radiative centers. From the PEC measurement, the photocurrent density of hydrogenated InGaN/GaN SQW NRs in the H2SO4 solution is found to be 5 mA cm-2 at -0.48 V versus reversible hydrogen electrode, which is 3.5-fold larger than that of pristine ones. These findings shed new light on the significance of surface treatment on the optical properties and thus nanostructured photoelectrodes for PEC applications.

In this work, reduced graphene oxide (rGO)/GaN nanorods (NRs) hybrid structure based sensors for hydrogen (H2) and hydrogen sulfide (H2S) gases has been demonstrated at room temperature. The morphological, elemental, and structural analyses were carried out by using scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The electrical characterization of rGO/GaN NRs hybrid structures showed good rectifying behavior compared to pristine GaN NRs. The H2 and H2S gas sensing measurements at different gas concentrations revealed that the rGO/GaN NRs exhibit superior sensing properties compared to pristine GaN NRs. In order to find the gas sensing mechanism of rGO/GaN NRs, hybrid structure sensor is also tested with NOx gas. Our experimental results revealed that the rGO/GaN NRs are good candidates for selective detection of H2S gas. The rGO/GaN NRs showed remarkably improved response under ultra-violet (λ = 365 nm) illumination, the photogenerated carriers could be responsible for increasing response of the gas sensor at 30 °C under UV illumination. In addition, humidity test of the rGO/GaN NRs sensor was also conducted in this work. Our experimental results suggested that decorating GaN NRs with solution-processable rGO is one of the effective ways to enhance the response of GaN nanostructures.

Although the physicochemical properties of graphene oxide-Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (GO-PEDOT:PSS) nanocomposite has widely reported, the urgency of this nanocomposite in the surface acoustic wave (SAW) technology still remains at the infancy stage. In this work, a high response SAW NO2 gas sensor was developed by depositing the GO-PEDOT:PSS nanocomposite on piezoelectric LGS substrate. The pristine LGS SAW sensor has showed a resonant center frequency of 136.10 MHz and it was shifted to 135.78 MHz for the GO-PEDOT:PSS coated LGS SAW sensor. The observed shift in resonance frequency likely due to the mass loading effect of GO-PEDOT:PSS nanocomposite interface. However, under the gas ambient the GO-PEDOT:PSS/LGS SAW sensor exhibited a significant negative differential frequency shift (Δf) of 5.89 kHz to100 ppm of NO2 gas with a good cycling stability, excellent sensitivity and a low detection limit (∼175 ppb) at room temperature (RT), while the pristine GO/LGS and PEDOT:PSS/LGS SAW sensors showed the Δf of about 0.9 kHz and 2.1 kHz, respectively. In addition, under the various relative humidity conditions at RT for 100 ppm of NO2, the GO-PEDOT:PSS/LGS SAW device showed a dominant Δf. Our experimental results divulged that the effect of mass loading is the responsible underlying mechanism for the superior NO2 gas sensing performances. In turn, the predominant mechanism between the chemical interface and NO2 gas molecules on GO-PEDOT:PSS/LGS SAW device has been deliberately discussed in detailed by comparing with chemo-resistive type gas sensing properties These intriguing results indicate that the significance of GO-PEDOT:PSS has a synergetic effect in SAW sensor applications.

Although gallium nitride (GaN) nanostructures are auspicious for photocatalytic activity, geometrical optimization has paid much attention for a significant light trapping in photoelectrochemical applications. To minimize the optical losses, we designed a prototype V-groove textured Si (100) with (111) facets, and GaN nanorods (NRs) were grown over a prototype substrate using plasma-assisted molecular beam epitaxy. The photocurrent density of V-groove textured GaN NRs in the NaOH electrolyte is found to be 801.62 μA/cm2 at 1.14 V vs reversible hydrogen electrode, which was 2.1-fold larger than that of GaN NRs on plain Si (111). Using this prototype V-groove textured Si(100) with (111) facets, a significant light can be trapped and modulated into GaN NRs. Furthermore, the heterostructure between GaN NRs and V-groove textured Si stimulates effective charge separation and transportation. These results represent an important forward step in solar photoelectrolysis.

Understanding the metal/semiconductor interface is very significant for real-time optoelectronic device applications. In particular, the presence of interface states and other defects is detrimental to photodetector applications. In this study, the electrical transport properties of a pristine gallium nitride (GaN) nanorod (NR)-based Schottky diode are demonstrated at different temperatures by current–voltage characteristics in the range of 200–360 K. An enhancement in the Schottky barrier height (0.65 eV for hydrogen-passivated GaN NRs compared to 0.56 eV for pristine ones) is noticed. The effect of deep traps residing within the forbidden gap of GaN NRs is investigated using deep-level transient spectroscopy. Two deep defects are found at E C − 0.19 eV and E C − 0.31 eV in pristine GaN NRs; the E C − 0.31 eV defect peak is attributed to V Ga or nitrogen interstitials. After hydrogenation the peak at E C − 0.31 eV is suppressed and that at E C − 0.19 eV remains unchanged. The hydrogenated GaN NRs show a high photoresponse, which is nearly 2.83 times higher than that of pristine GaN NRs. The hydrogenated GaN NRs exhibit a photoresponsivity of 4.7 × 10−3 A W−1 and detectivity of 1.24 × 1010 Jones under UV illumination of λ = 382 nm. The enhanced performance is attributed to the deep defect passivation by hydrogenation along with the surface-state-free interface between the GaN NRs and metal contacts. The experimental results demonstrate the significance of hydrogen treatment use in the fabrication of GaN-based optoelectronic devices.

Coupling of graphene and its derivatives with organic moieties is one of the promising strategies to enhance the gas sensing performance. A novel ternary nanocomposite of p-Phenylenediamine-graphene oxide (PGO)/InGaN nanorods (NRs) probed as chemiresistive gas sensor for carbon monoxide (CO) in ppm concentrations. The InGaN NRs were grown by molecular beam epitaxy, and a drop-casting method was employed to decorate the PGO composite on InGaN NRs. The sensor devices consisting of PGO/InGaN NRs with interdigitated electrodes have been fabricated, and their sensing characteristics for CO were studied. PGO composite provided a significant decrease in the electrical resistance of the ternary nanocomposite of PGO/InGaN at the operating temperature (27 °C – 100 °C). The sensor device based on PGO/InGaN NRs exhibited a higher response than the GO/InGaN NRs. The PGO/InGaN NRs device exhibited a maximum sensing response of ∼131 % at a CO concentration of 100 ppm at an operation temperature of 100 °C, which is 2.7 times higher than the GO/InGaN NRs sensor response. Also, the response (∼93.58%) was remarkably improved under UV illumination at room temperature. The combined effect of active cites in GO and the increased electron density at the nitrogen atom in the amine groups of the PGO are responsible for the excellent performance

We demonstrate the evolution of GaN films on novel langasite (LGS) substrate by plasma-assisted molecular beam epitaxy, and assessed the quality of grown GaN film by comparing the experimental results obtained using LGS, sapphire and silicon (Si) substrates. To study the substrate effect, X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and photoluminescence (PL) spectra were used to characterize the microstructure and stress states in GaN films. Wet etching of GaN films in KOH solution revealed that the films deposited on GaN/LGS, AlN/sapphire and AlN/Si substrates possess Ga-polarity, while the film deposited on GaN/sapphire possess N-polarity. XRD, Raman and PL analysis demonstrated that a compressive stress exist in the films grown on GaN/LGS, AlN/sapphire, and GaN/sapphire substrates, while a tensile stress appears on AlN/Si substrate. Comparative analysis showed the growth of nearly stress-free GaN films on LGS substrate due to the very small lattice mismatch (~3.2%) and thermal expansion coefficient difference (~7.5%). The results presented here will hopefully provide a new framework for the further development of high performance III-nitride-related devices using GaN/LGS heteroepitaxy.

Gallium-nitride (GaN) nanorods (NRs) were selectively grown on patterned Ti/Si(111) substrates by using plasma-assisted molecular beam epitaxy. The effects of the growth temperature and the Ga flux on the optical properties of the GaN NRs were investigated. Both the length and the diameter of the GaN NRs decreased with increasing growth temperature whereas the diameter of the GaN NRs remained constant and the length increased with increasing Ga flux. The photoluminescence spectra of GaN NRs grown under different experimental conditions exhibited emission peaks corresponding to neutral-donor-bound excitons (D O X), donor-acceptor pair (DAP), and yellow luminescence (YL) bands at 3.477, 3.26, and 2.25 eV, respectively. Results demonstrate that the variation in the diameters of the GaN NRs has a greater influence on the defect concentration than the lengths of the NRs.

Deep level transient spectroscopy has been used to characterize the deep level traps in Ga- and N-polarity GaN films grown by plasma assisted molecular beam epitaxy on Si(111) substrate. The two deep level traps at Ec−Et∼0.21 eV (E1) and Ec−Et∼0.48 eV (E2) have been detected in Ga-polarity GaN. The E1 level commonly observed deep trap related to the nitrogen vacancy in GaN. It is found that, the E2 level exhibits logarithmic capture kinetic behavior and substantially increases its capture cross section from 10−15 to 10−12 cm2 by employing different pulse width ranging from 5 ms to 35 ms. Such behavior of E2 trap with filling pulse length attributes that, the trap is originated from threading dislocations. In case of N-polarity GaN, we observed two deep level traps with activation energies of Ec−Et∼0.53 eV (E3) and Ec−Et∼0.89 eV (E4). The estimated capture cross-sections (σS) for these defects were found to be ∼2.51×10−15 cm2 and ∼5.21×10−16 cm2 respectively. The E3 and E4 are nitrogen antisite point defect and extended defect related to dislocation deep levels, results from growth on N-polarity crystal structure.

We demonstrated the coupling of localized surface plasmons and surface plasmon polaritons modes in a system composed of a metallic particle chain separated from a thin metallic film. The results showed that: (1) the thickness of the metallic particles buried in the dielectric space, (2) the positions of the particles influence the level of interaction between localized surface plasmons and surface plasmon polaritons modes. Meanwhile, the positions of the particles and the thickness of the metallic particles control the electromagnetic enhancement and influence the electric field distributions in this system. This kind of system has a very promising candidate for biosensing and surface enhanced spectroscopy applications.

Thin metallic nanowires and stripes are highly promising candidates for plasmonic waveguides in photonic and electronic devices. We observed light from one end of a gold stripe, following excitation of plasmons at the other end of the stripe, with almost no light emitted along the direction of the stripe, and compared the propagation capacity of different size of stripes. We measured how the polarization of the incident light affected the emitted light intensity through changing the polarization of the incident light. The stripes were synthesized by lithographic fabrication technology. The results will be important for the development of photonic or electronic devices and systems.

The strain-relief and structural properties of GaSb films with thin AlSb islands and thick AlSb buffer layers grown on GaAs (0 0 1) substrate at low temperature (LT) by molecular beam epitaxy are investigated by atomic force microscopy and transmission electron microscopy. The strain arising from depositing the buffer layer onto the GaAs substrate was relieved by a periodic array of the 90° misfit dislocations with the Burgers vector of 1/2a 〈1 1 0〉 for the thin AlSb islands buffer, but by both 60° and 90° misfit dislocations for the thick rough-and-flat AlSb buffer. The 90°-misfit dislocation array of AlSb/GaAs interface had an average spacing of 4.80 nm. The mean roughness of the GaSb film on the thin AlSb islands buffer layer was found to be less than 1 nm. These results clearly demonstrate that the presence of a thin, LT AlSb islands initiation layer is very useful for improving the quality of GaSb crystals grown on GaAs substrates.

We investigate the width, spacing, and number dependencies of an Al+-implanted field limiting ring (FLR) and the influence of an Al+-implanted internal ring on the breakdown voltage characteristics of a silicon carbide (SiC) Schottky barrier diode (SBD). SiC-SBDs having an Al+-implanted guard ring-assisted FLR surrounding a Schottky contact edge and an internal ring inside a Schottky contact were fabricated via ion implantation, annealing, photolithography, and metal lift-off techniques. The breakdown voltage characteristics of the SiC-SBD are enhanced by incorporation of the guard ring-assisted FLR at the Schottky contact edge and the internal ring at Schottky contact region. The breakdown voltage characteristics of the guard ring-assisted FLR SiC-SBDs with an internal ring are significantly dependent on the width, spacing, and number of FLRs. The breakdown voltage is improved as either the FLR width and FLR number increase or the FLR spacing decreases. 1650 V maximal breakdown voltage, corresponding to 82% ideal breakdown voltage, is observed with seven FLRs having 5 µm width and 1 µm spacing. The results indicate that a greater number of FLRs, as well as increased FLR width and decreased spacing can effectively suppress electric field crowding at the periphery of the Schottky contact edge, resulting in higher breakdown voltage. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The dependence of the size and the strain in the double-stacked InAs/GaAs quantum dot (QD) was investigated by using the reflection high-energy electron diffraction pattern, transmission election microscopy (TEM), high-resolution X-ray diffraction (HRXRD), and photoluminescence (PL) measurements. The results of the TEM images and the reciprocal spacer mapping, obtained from the HRXRD measurements, showed that the microstructures of the InAs QDs were significantly affected by the strain-controlled InAs layer thickness, and the PL spectra showed that the peak intensity and position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band of the InAs QDs strongly depended on the thickness of the InAs layer embedded in the GaAs barrier. The present results can help improve the precise control of the size and the density in the InAs/GaAs multiple-stacked QD arrays.

In order to improve GaN-based light-emitting diode (LED) performance, a sapphire-etched vertical-electrode nitride semiconductor (SEVENS) LED is fabricated by a chemical wet etching technique. The light-output power, heat dissipation, and reverse electrostatic discharge (ESD) characteristics of SEVENS-LED are improved compared to those of the conventional NiAu lateral-electrode (LE) GaN-based LED formed on the sapphire substrate. The 20 mA light-output power and reverse ESD pulse amplitude of SEVENS-LED are approximately 4.5 mW and 2000 V, respectively. The improved LED performance is attributed to changing a lateral electrode to a vertical electrode although the sapphire substrate is not transferred to a conducting substrate. The device fabrication method will be useful for providing us high-brightness and high-power GaN-based LEDs for future solid-state general lighting applications. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements were used to investigate the dependences of the formation process and the strain on the As/In ratio and the substrate temperature of InAs quantum dots (QDs) grown on GaAs substrates by using molecular beam epitaxy. The thickness of the InAs wetting layer and the shape and the size of the InAs QDs were significantly affected by the As/In ratio and the substrate temperature. The strains in the InAs layer and the GaAs substrate were studied by using RHEED patterns. The magnitude in strain of the InAs QDs formed at a low substrate temperature was larger than that in InAs QDs grown at high substrate temperature. The present results can help to improve the understanding of the formation process and the strain effect in InAs QDs.

We investigated the Si-doped InAs quantum dots (QDs) grown by molecular beam epitaxy and the annealing effects on the QD size distribution through photoluminescence (PL) spectroscopy. A double-peak feature in PL was observed from as-grown InAs QDs with Si-doping, and excitation intensity dependence of PL indicated that the double-peak feature is related to the ground-state emission from InAs QDs with bimodal size distribution. The PL spectrum from Si-doped InAs QDs subjected to annealing treatment at 800°C in nitrogen ambient showed three additional PL peaks and blue-shift of the double-peak feature observed from as-grown sample. The excitation-intensity-dependent PL and consideration of thermal stability of carriers through temperature-dependent PL measurement demonstrated that three additional peaks come from the InAs QDs with three new branches of QD occurring during annealing process.

We report on the light emitting of InN/InGaN/GaN dot-in-a-nanorods heterostructure grown on Si(111) substrates using plasma-assisted molecular beam epitaxy. Sharp and isolated single exciton emission line in the near infrared spectral range was observed.

The structural and optical properties of height-controlled InAs quantum dots (QDs) have been investigated by transmission electron microscopy (TEM) and photoluminescence (PL). By depositing 1.4 nm In0.15Ga0.85As and a 1 monolayer (ML) InAs layer with different periods on 3 ML InAs QDs, the height of InAs QDs was systematically controlled with similar lateral size. In TEM images, the indication of dislocations due to the large strain, which can be easily seen in large QDs, is not observed even for the QD sample with the highest aspect ratio (height/width). The PL peak position is shifted toward the longer wavelength with an increase in the aspect ratio of QDs. As the aspect ratio is increased, the full width at half maximum in PL measured at 10 K is decreased from 71 to 34 meV indicating that the inhomogeneous broadening caused by the fluctuation in QD size, especially the height, is significantly reduced.

We experimentally investigate the dependences of the surface plasmon polarization (SPP) in the Ag nanowires on the polarizations of the excitation light and the emission light with a wavelength of excitation light 750nm. We find that the excitation and transmission efficiency change obviously with the polarization of incident light. However, the emission light is always linearly polarized light with an unchanged polarization direction. For the nanowires synthesized using a self-seeding process, their ends are axisymmetric. When the direction of the incident light is parallel to these Ag nanowires, the excitation and transmission efficiency of SPP are high. Conversely, when the polarization direction of the incident light is perpendicular to the wire axis, it is very low. For the nanowires with asymmetric ends, when the direction of the incident light has an angle with respect to the Ag nanowires, the excitation and transmission efficiency of SPP are high. While the polarization direction of emission light is always parallel to the wire axis which means that the polarization of the emission light does not depend on the polarization direction of the incident light. The polarization characteristics of the SPP in the thin Ag nanowires can realize the nanoscale manipulation of the intensity and polarization.