Aligned GaN Research Articles

Modelling the photoemission characteristics of parallel aligned (Al)GaN nanowall arrays assisted by built-in/external field

The photoemission and electron collection properties of nanostructure photoemitter based on parallel aligned (Al)GaN nanowall arrays are investigated. In general, the photoelectric conversion efficiency of intrinsic GaN NWAs exhibit a significant improvement compared with planar photoemitter. Assisted by the built-in field induced by gradient decreasing Al component along axial direction of the nanowall, the proportion of emitting photoelectrons on each surface of a nanowall is redistributed and the collected photocurrent is obviously enhanced yet with a negative effect against quantum efficiency and emitted photocurrent. External field incorporation can tune the electron emitting path and improve the collection efficiency, but accompanied with a further decline in emitting photocurrent and quantum efficiency. For a certain configuration with nanowall height of 1 µm and spacing length of 1 µm, the optimum field intensity for emitting and collected photocurrent are respectively 0.3 and 0.5 V/µm. The alteration of photocurrent varied with field intensity dominated by the photoelectrons from the top surface and irradiated side face of the nanowall.

In Situ Growth of Leakage-Free Direct-Bridging GaN Nanowires: Application to Gas Sensors for Long-Term Stability, Low Power Consumption, and Sub-ppb Detection Limit.

Direct-bridge growth of aligned GaN nanowires (NWs) over the trench of GaN-coated sapphire substrate was realized in which the issues of parasitic deposition and resultant bypass current were resolved by combining the novel shadowing effect of the deep trench with the surface-passivation effect of the SiO2 coating. Due to the robust connection and the absence of a contact barrier in bridging NWs, the intrinsic sensing properties of the NW itself can be obtained. For the first time, the gas-sensing properties (e.g., NO2) of the bridging GaN NWs were studied. With the assistance of UV light, the detection limit was improved from 4.5 to 0.5 ppb at room temperature, and the corresponding response time was reduced from 518 to 18 s. This kind of sensor is promising for high sensitivity (detection of less than parts per billion), low power consumption (capable of room-temperature operation), high stability (variation in resistance of <0.8% during 240 days), and in situ monolithic integration.

A highly sensitive label-free electrochemical immunosensor based on an aligned GaN nanowires array/polydopamine heterointerface modified with Au nanoparticles.

Aligned GaN nanowire arrays show great potential not only in optoelectronic devices, but also in sensitive biosensor applications, owing to their excellent chemical stability and biocompatibility, as well as high electron mobility and surface-to-volume ratio. However, to construct electrochemical immunosensors, proper surface modification of GaN nanowires, which can enable efficient charge transfer and provide large densities of immobilization sites for antibodies to anchor, is still challenging. Herein we demonstrate a highly sensitive label-free electrochemical immunosensing platform based on the integration of polydopamine (PDA) on a GaN nanowire surface. The PDA polymer was self-assembled on GaN nanowire surfaces via organic polymerization. The interface dipole layer generated at the GaN nanowire array/PDA polymer heterointerface enabled efficient charge transfer. The aligned GaN nanowire array/PDA hybrids were further modified with gold nanoparticles for subsequent covalent binding of antibodies. The fabricated immunosensor yielded a wide linear range between 0.01 and 100 ng ml-1 and a detection limit as low as 0.003 ng ml-1 for the detection of alpha-fetoprotein (AFP). The immunosensor showed good selectivity, reproducibility, and stability and was utilized in human serum samples for AFP detection. This work demonstrates the superiority of taking advantage of a nanowire array configuration and a semiconductor/polymer heterointerface in an immunosensing platform for sensitivity enhancement.

Effect of nitridation temperature on formation and properties of GaN nanowall networks on sapphire (0 0 0 1) grown by laser MBE

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.

Controlled growth of aligned GaN nanostructures: from nanowires and needles to micro-rods on a single substrate

Growth of aligned GaN nanostructures by tuning the substrate distance and the growth pressure.

Alignment control and atomically-scaled heteroepitaxial interface study of GaN nanowires.

Well-aligned GaN nanowires are promising candidates for building high-performance optoelectronic nanodevices. In this work, we demonstrate the epitaxial growth of well-aligned GaN nanowires on a [0001]-oriented sapphire substrate in a simple catalyst-assisted chemical vapor deposition process and their alignment control. It is found that the ammonia flux plays a key role in dominating the initial nucleation of GaN nanocrystals and their orientation. Typically, significant improvement of the GaN nanowire alignment can be realized at a low NH3 flow rate. X-ray diffraction and cross-sectional scanning electron microscopy studies further verified the preferential orientation of GaN nanowires along the [0001] direction. The growth mechanism of GaN nanowire arrays is also well studied based on cross-sectional high-resolution transmission electron microscopy (HRTEM) characterization and it is observed that GaN nanowires have good epitaxial growth on the sapphire substrate following the crystallographic relationship between (0001)GaN∥(0001)sapphire and (101[combining macron]0)GaN∥(112[combining macron]0)sapphire. Most importantly, periodic misfit dislocations are also experimentally observed in the interface region due to the large lattice mismatch between the GaN nanowire and the sapphire substrate, and the formation of such dislocations will favor the release of structural strain in GaN nanowires. HRTEM analysis also finds the existence of "type I" stacking faults and voids inside the GaN nanowires. Optical investigation suggests that the GaN nanowire arrays have strong emission in the UV range, suggesting their crystalline nature and chemical purity. The achievement of aligned GaN nanowires will further promote the wide applications of GaN nanostructures toward diverse high-performance optoelectronic nanodevices including nano-LEDs, photovoltaic cells, photodetectors etc.

Growth and characterization of horizontal GaN wires on silicon

We report the growth of in-plane GaN wires on silicon by metalorganic chemical vapor deposition. Triangular-shaped GaN microwires with semi-polar sidewalls are observed to grow on top of a GaN/Si template patterned with nano-porous SiO2. With a length-to-thickness ratio ∼200, the GaN wires are well aligned along the three equivalent 〈112¯0〉 directions. Micro-Raman measurements indicate negligible stress and a low defect density inside the wires. Stacking faults were found to be the only defect type in the GaN wire by cross-sectional transmission electron microscopy. The GaN wires exhibited high conductivity, and the resistivity was 20–30 mΩ cm, regardless of the wire thickness. With proper heterostructure and doping design, these highly aligned GaN wires are promising for photonic and electronic applications monolithically integrated on silicon.

Morphological control over nonpolar GaN nanostructures on (1 0 0) γ-LiAlO2 substrate

We present the fabrication of aligned nonpolar GaN nanostructures on closely lattice-matched Au coated (100) γ-LiAlO2 substrate by chemical vapor deposition. The morphologies and structural properties of GaN nanostructures were characterized by scanning electron microscopy and x-ray diffraction. Vertical standing nanostructures with different cross-sections including triangle, trapezoid and pentagon were controllably synthesized by varying the important growth parameter of atmosphere gas flows. It is suggested that the ratio of N2 and H2 atmosphere gas flows plays an important role in determining the nanostructure morphologies, due to the anisotropic hydrogen reduction effects.

Ferromagnetism in undoped One-dimensional GaN Nanowires

We report an intrinsic ferromagnetism in vertical aligned GaN nanowires (NW) fabricated by molecular beam epitaxy without any external catalyst. The magnetization saturates at ∼0.75 × emu/gm with the applied field of 3000 Oe for the NWs grown under the low-Gallium flux of 2.4 × 10−8 mbar. Despite a drop in saturation magnetization, narrow hysteresis loop remains intact regardless of Gallium flux. Magnetization in vertical standing GaN NWs is consistent with the spectral analysis of low-temperature photoluminescence pertaining to Ga-vacancies associated structural defects at the nanoscale.

The synthesis route and the growth mechanism of aligned GaN nanobelts.

Crystallographically aligned GaN nanobelt arrays were synthesized through a hybrid process of Au-assisted nucleation followed by non-Au-assisted anisotropy vapor-solid growth on a (100) γ-LiAlO2 substrate. To the best of our knowledge, this is the first report on aligned GaN nanobelts.

Yellow Luminescence of Polar and Nonpolar GaN Nanowires on r-Plane Sapphire by Metal Organic Chemical Vapor Deposition

We have grown horizontal oriented, high growth rate, well-aligned polar (0001) single crystalline GaN nanowires and high-density and highly aligned GaN nonpolar (11-20) nanowires on r-plane substrates by metal organic chemical vapor deposition. It can be found that the polar nanowires showed a strong yellow luminescence (YL) intensity compared with the nonpolar nanowires. The different trends of the incorporation of carbon in the polar, nonpolar, and semipolar GaN associated with the atom bonding structure were discussed and proved by energy-dispersive X-ray spectroscopy, suggesting that C-involved defects are the origin responsible for the YL in GaN nanowires.

Growth of GaN Columns and Microstructures on (111) Si by Using Ge Layer via Metal Organic Chemical Vapor Deposition

We demonstrate the successful growth of GaN columnar and microdisk structures on a (111) Ge/Si substrate via metal organic chemical vapor deposition. X-ray diffraction and energy disperse spectroscopy results reveal that the well aligned GaN nanocolumnar structures are grown on a Si substrate by using the Ge layer. The relatively higher growth temperature induces a change in surface morphology of GaN from columnar to microdisk structures. The possible growth mechanism of columnar structured GaN was considered from the Ga-metal/Ge eutectic materials system.

Nucleation control for the growth of vertically aligned GaN nanowires

Aligned GaN nanowire arrays have high potentials for applications in future electronic and optoelectronic devices. In this study, the growth of GaN nanowire arrays with high degree of vertical alignment was attempted by plasma-enhanced CVD on the c-plane GaN substrate. We found that the lattice matching between the substrate and the nanowire is essential for the growth of vertically aligned GaN nanowires. In addition, the initial nucleation process is also found to play a key role in creating the high-quality homoepitaxy at the nanowire-substrate interface. By controlling the nucleation stage, the growth of highly aligned vertical GaN nanowire arrays can be achieved. The reasons for the observed effects are discussed.

Ultralow nonalloyed Ohmic contact resistance to self aligned N-polar GaN high electron mobility transistors by In(Ga)N regrowth

Ultralow Ohmic contact resistance and a self-aligned device structure are necessary to reduce the effect of parasitic elements and obtain higher ft and fmax in high electron mobility transistors (HEMTs). N-polar (0001¯) GaN HEMTs, offer a natural advantage over Ga-polar HEMTs, in terms of contact resistance since the contact is not made through a high band gap material [Al(Ga)N]. In this work, we extend the advantage by making use of polarization induced three-dimensional electron-gas through regrowth of graded InGaN and thin InN cap in the contact regions by plasma (molecular beam epitaxy), to obtain an ultralow Ohmic contact resistance of 27 Ω μm to a GaN 2DEG.

Synthesis of vertically oriented GaN nanowires on a LiAlO2 substrate via chemical vapor deposition

Vertically oriented nanowires (NWs) of single-crystalline wurtzite GaN have been fabricated on γ-LiAlO2 (100) substrate coated with a Au layer, via a chemical vapor deposition process at 1000 °C using gallium and ammonia as source materials. The GaN NWs grow along the nonpolar [10\( \bar 1 \)0] direction with steeply tapering tips, and have triangular cross-sections with widths of 50–100 nm and lengths of up to several microns. The GaN NWs are formed by a vapor-liquid-solid growth mechanism and the tapering tips are attributed to the temperature decrease in the final stage of the synthesis process. The aligned GaN NWs show blue-yellow emission originating from defect levels, residual impurities or surface states of the GaN NWs, and have potential applications in nanotechnology.

Well-Aligned Single-Crystalline GaN Nanocolumns and Their Field Emission Properties

Well-aligned GaN nanocolumns on silicon substrate were fabricated by simple and low-cost chemical vapor deposition without any catalyst. The structure and morphology of the as-synthesized GaN nanocolumns were characterized by X-ray diffraction and scanning and transmission electron microscopies. The aligned GaN nanocolumn arrays exhibited excellent field emission properties with a low turn-on field of 2.6 V/μm (0.01 mA/cm2) and high stability at room temperature, which is sufficient for applications of field emission displays and vacuum nanoelectronic devices. The room-temperature photoluminescence emission with a strong peak at 369 nm indicates that the well-aligned GaN nanocolumns have potential application in light-emitting nanodevices.

Improvement in aligned GaN nanowire growth using submonolayer Ni catalyst films

We report a route to ultrahigh-density and highly aligned single-crystalline GaN nanowires on sapphire by employing ultrathin Ni catalyst films with submonolayer thickness. The nanowire density and the degree of alignment were found to be highly sensitive to changes in the Ni catalyst film thickness below 1nm, a regime rarely explored in catalyzed nanowire growth before. For submonolayer Ni films on sapphire, high activation energy for Ni diffusion on sapphire surface is attributed to the formation of high-density and ultrasmall Ni islands with a narrow size distribution, which in turn leads to high-density and highly aligned GaN nanowires.

Silicon-Induced Strain Relaxation and Enhanced Gallium Surfactant Effects on Gallium Nitride Island Shaping

The self-organization of large-scale uniform aligned three-dimensional GaN nanoislands with triangular (0001) and distinct sidewall faceting has been realized by metal organic vapor-phase epitaxy on in situ Si-rich SiNx nanoislands patterned c-sapphire substrates. We find that the GaN island shaping is closely related to the SiNx pretreatment chemistry. It is suggested that enhanced surface Ga surfactant effects and compressive strain relaxation caused by site exchanges between excess Si and subsurface Ga atoms are responsible for the distinct triangular island shaping with large lateral size, smooth sidewall facets, and sharp triangle corners. Photoluminescence studies also show Si-doping-induced compressive strain relaxation and improved crystalline qualities for triangular GaN islands grown with the Si-rich SiNx pretreatment.

Epitaxial growth of aligned GaN nanowires and nanobridges

AbstractHomo‐epitaxialy grown aligned GaN nanowires were prepared on crystalline GaN mesas. The GaN nanowires showed preferential growth along the 〈10$ \bar 1 $0〉 direction (m‐axis direction). By using selectively positioned and crystallographically well defined GaN epitaxial lateral overgrowth (ELO) mesas as substrate, we obtained horizontally aligned GaN nanowires, in comb‐like arrays and hexagonal network interconnecting the ELO mesas. Preliminary testing of the nanomechanical behavior of horizontal nanowires is reported. Combination of ELO with nanowire synthesis is expected to provide a new paradigm for nano‐electronic and electromechanical devices. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Synthesis and Characterization of One-Dimensional GaN Nanostructures Prepared via Halide Vapor-Phase Epitaxy

High-quality one-dimensional GaN nanorods and nanowires were synthesized on Ni-coated c-plan sapphire substrate using halide vapor-phase epitaxy (HVPE). Their structure and optical properties were investigated by X-ray diffraction, scanning and transmission electron microscopy, and photoluminescence techniques. Full substrate coverage of densely packed, uniform, straight and aligned one-dimensional GaN nanowires with a diameter of 80nm were grown at 700~900℃. The X-ray diffraction patterns, transmission electron microscopic image, and selective area electron diffraction patterns indicate that the one-dimensional GaN nanostructures are a pure single crystalline and preferentially oriented in the [001] direction. We observed high optical quality of GaN nanowires by photoluminescence analysis.