GaN Wires Research Articles

The influence of Ga source and substrate position on the growth of low dimensional GaN wires by chemical vapour deposition

This paper presents the investigation of low dimensional GaN structures synthesized from Ni-catalyzed chemical vapour deposition (CVD) method under two different conditions, i.e. Ga source and substrate position. Comparative studies based on the morphological, structural and optical characteristics of synthesized GaN wires were carried out in this work. The variations of morphological and dimensional aspects of the GaN wires were attributed to the position of Ga precursor and substrates. These factors were found to be able to influence the degree of supersaturation of gaseous reactants, which is essential in the growth of GaN wires by vapour–liquid–solid (VLS) mechanism. The synthesized GaN wires typically were found to have diameters ranging 35–80nm (nanowires) and 0.4–1.3μm (microwires), respectively, with length up to several ten of microns. X-ray diffraction (XRD) results indicated that the grown GaN wires were hexagonal wurzite phase. Ultraviolet (UV) and blue emissions were observed from photoluminescence (PL) measurements. Raman spectra displayed asymmetrical and broadened bands which could be ascribed to the size effect, surface disorder and internal strain of the synthesized GaN wires.

Single-wire photodetectors based on InGaN/GaN radial quantum wells in GaN wires grown by catalyst-free metal-organic vapor phase epitaxy

We present a letter on single-wire photodetectors based on radial n-i-n multiquantum well (QW) junctions. The devices are realized from GaN wires grown by catalyst-free metalorganic vapor phase epitaxy coated at their top by five nonpolar In0.16Ga0.84N/GaN undoped radial QWs, and are sensitive to light with energy E>2.6 eV. Their photoconductive gain is as high as 2×103. The scanning photocurrent microscopy maps evidence that the detector response is localized at the extremity containing the QWs for both below (at λ=488 nm) and above GaN band gap (at λ=244 nm) excitation. This confirms that the device operates as a radial n-i-n junction.

On the polarity of GaN micro- and nanowires epitaxially grown on sapphire (0001) and Si(111) substrates by metal organic vapor phase epitaxy and ammonia-molecular beam epitaxy

The polarity of GaN micro- and nanowires grown epitaxially by metal organic vapor phase epitaxy on sapphire substrates and by molecular-beam epitaxy, using ammonia as a nitrogen source, on sapphire and silicon substrates has been investigated. On Al2O3(0001), whatever the growth technique employed, the GaN wires show a mixture of Ga and N polarities. On Si(111), the wires grown by ammonia-molecular beam epitaxy are almost entirely Ga-polar (around 90%) and do not show inversion domains. These results can be understood in terms of the growth conditions employed during the nucleation stage.

Homoepitaxial growth of catalyst-free GaN wires on N-polar substrates

The shape of c-oriented GaN nanostructures is found to be directly related to the crystal polarity. As evidenced by convergent beam electron diffraction applied to GaN nanostructures grown by metal-organic vapor phase epitaxy on c-sapphire substrates: wires grown on nitridated sapphire have the N-polarity ([0001¯]) whereas pyramidal crystals have Ga-polarity ([0001]). In the case of homoepitaxy, the GaN wires can be directly selected using N-polar GaN freestanding substrates and exhibit good optical properties. A schematic representation of the kinetic Wulff’s plot points out the effect of surface polarity.

Molecular dynamics prediction of thermal conductivity of GaN films and wires at realistic length scales

Recent molecular dynamics simulation methods have enabled thermal conductivity of bulk materials to be estimated. In these simulations, periodic boundary conditions are used to extend the system dimensions to the thermodynamic limit. Such a strategy cannot be used for nanostructures with finite dimensions which are typically much larger than it is possible to simulate directly. To bridge the length scales between the simulated and the actual nanostructures, we perform large-scale molecular dynamics calculations of thermal conductivities at different system dimensions to examine a recently developed conductivity vs. dimension scaling theory for both film and wire configurations. We demonstrate that by an appropriate application of the scaling law, reliable interpolations can be used to accurately predict thermal conductivity of films and wires as a function of film thickness or wire radius at realistic length scales from molecular dynamics simulations. We apply this method to predict thermal conductivities for GaN wurtzite nanostructures.

Violet electroluminescence from p-GaN thin film/n-GaN nanowire homojunction

The difficulty associated with the precise positioning of nanowires has been one of the most significant issues hindering nanoelectronic integration. In this paper, we employed dielectrophoretic force to manipulate n-type GaN nano- and microwires onto a p-type GaN thin film to form a pristine p-n homojunction. The GaN wires were attracted to the n-type Ohmic metal in a direction parallel to the electric field, which was consistent with our simulation results. Violet electroluminescence emanated from the point of the n-GaN wire in contact with the p-GaN thin film. This p-n homojunction device displayed forward conduction above 6–9 V and current rectifying behavior down to a −20 V reverse bias. The current-voltage characteristics are distinctive of a p-n homojunction formed without deleterious damage or contamination.

Self-assembled growth of catalyst-free GaN wires by metal–organic vapour phaseepitaxy

A catalyst-free method for growing self-assembled GaN wires onc-plane sapphire substrates by metal–organic vapour phase epitaxyis developed. This approach, based on in situ deposition of a thinSiNx layer (∼2 nm), enablesepitaxial growth of c-oriented wires with 200–1500 nm diameters and a largelength/diameter ratio(>100)on c-plane sapphire substrate. Detailed study of the growth mechanisms shows that a combinationof key parameters is necessary to obtain vertical growth. In particular, the duration of theSiNx deposition prior to the wire growth is critical for controlling the epitaxywith the substrate. The GaN seed nucleation time determines the meansize diameter and structural quality, and a high Si-dopant concentrationpromotes vertical growth. Such GaN wires exhibit UV-light emission centred at∼350 nm and a weakyellow band (∼550 nm) at low temperature.

VLS growth of GaN nanowires on various substrates

We have studied the vapour–liquid–solid (VLS) growth of free-standing GaN wires on various III–V substrates using metalorganic vapour phase epitaxy. The low-temperature deposition including in situ droplet formation was investigated applying 1.1-dimethylhydrazine and trimethylgallium. In particular, the surface structure of BP and GaN intermediate layers on the growth of GaN wires is discussed. Furthermore, the GaN growth was examined with a special focus on the influence of triethylboron on the droplet formation and wire growth. Additionally, the Au-initiated VLS growth on c-plane Al 2O 3 is reported, where NH 3 and triethylgallium have been used at usual growth conditions. Depending on the growth conditions used, the wires have hexagonal (h-GaN) wurtzite or cubic (c-GaN) zinc blende structure. The resulting nanowire diameters range from 50 to 300 nm and they are up to 2 μm in length.

Interface and Wetting Layer Effect on the Catalyst‐Free Nucleation and Growth of GaN Nanowires

To avoid catalyst-induced contaminations that might alter the electronic properties of the material, catalyst-free growth is preferable. However, the nucleation and growth mechanisms of GaN wires in the catalyst-free procedure are still under debate. Two mechanisms are usually invoked for the nucleation and the growth of NWs. One is based on the Ga-droplet formation followed by the well-established vapor–liquid–solid mechanism, [13] and the other is based on small GaN clusters as nucleation seeds and a vapor–solid growth process. [12] In the present work the formation of crystalline GaN nanoclusters as possible NW precursors in catalyst-free plasma-assisted MBE (PAMBE) growth is studied by high-resolution transmission electron microscopy (HRTEM) imaging. Details of the interface between the GaN layer and the substrate are investigated and discussed in connection with the mechanism of catalyst-free NW growth. GaN NWs were grown at 7808C by PAMBE without the use of catalysts, on clean Si(111) and oxidized Si(100) substrates, according to the procedure described elsewhere. [6] The GaN NWs were investigated by HRTEM using a JEOL 3000F

Synthesis of GaN nanowires by Tb catalysis

Rare earth metal seed Tb was employed as catalyst for the growth of GaN wires. GaN nanowires were synthesized successfully through ammoniating Ga 2O 3/Tb films sputtered on Si(1 1 1) substrates. The samples characterization by X-ray diffraction and Fourier transform infrared indicated that the nanowires are constituted of hexagonal wurtzite GaN. Scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy showed that the samples are single-crystal GaN nanowire structures. The growth mechanism of the GaN nanowires is discussed.

Fabrication of one‐dimensional “trenched” GaN nanowires and their interconnections

AbstractIn this work, a new approach of nanowire fabrication utilizing selective growth of planar GaN wires embedded into the insulating matrix is reported. The selective growth has been released by means of a Six Ny mask and a metal‐organic chemical vapour deposition. The main advantage of the investigated fabrication process is a planar geometry of the wires allowing the use of conventional interconnection techniques such as optical lithography. Design, fabrication technology and interconnection geometry of some early examples are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Assembly of ordered carbon shells on GaN nanowires

In situ annealing experiments on individual group III-nitride nanowires (NWs) in a transmission electron microscope demonstrate the encapsulation of GaN wires in crystalline carbon shells in the presence of C at moderate temperatures. The complete encapsulation of GaN wires in carbon shells can be achieved when small indium metal clusters are introduced on the wire surface. No encapsulation is observed on pure GaN wires under the same conditions. The observations suggest a general processing route for the formation of semiconductor core/C-shell NW structures based on surface decoration with small metal clusters.

Spontaneously grown GaN and AlGaN nanowires

We have identified crystal growth conditions in gas-source molecular beam epitaxy (MBE) that lead to spontaneous formation of GaN nanowires with high aspect ratio on Si (1 1 1) substrates. The nanowires were oriented along the GaN c-axis and normal to the substrate surface. Unlike in many other reports of GaN nanowire growth, no metal catalysts were used. Low growth rates at substrate temperatures near 820 °C were combined with high nitrogen flux (partially dissociated with RF plasma excitation) to form well-separated GaN wires with diameters from 50 to 250 nm in diameter and lengths ranging from 2 to 7 μm. The nanowires grew out of an irregular matrix layer containing deep faceted holes. X-ray diffraction indicated that the wires were fully relaxed and aligned to the silicon substrate. The growth morphology was strongly affected by the presence of Al and Be. The changes suggest that surface diffusion is a primary driving force in the growth of GaN nanowires with MBE.

Fabrication of nanoscale structures of InGaN by MOCVD lateral overgrowth

Nanoscale InGaN quantum wires and quantum dots (QDs) have been realized on the top of micron size of GaN dots and wires by lateral overgrowth with Si mask using metal-organic chemical vapor deposition (MOCVD). Patterning of the mask was carried out by focused ions beam method to form the stripe and square windows in Si covered MOCVD-GaN substrate. The density, sizes and positions of InGaN nano-structures have been artificially designed and controlled. The characteristics of InGaN nano-structures have been studied by scanning electron microscopy, transmission electron microscopy and cathodoluminescence (CL). A lot of dislocations were found in the interface between underlying and regrown GaN due to damage of ion sputtering. However, the propagation of dislocations was changed the direction parallel to the c plane in the region of laterally overgrown GaN, and no dislocations in the top of GaN dots and wires were observed. Compared to band-gap emission from InGaN/GaN multi-quantum well, the blue-shift of emission peak in InGaN QDs is observed by CL, which suggest that the carriers in InGaN QDs are three-dimensionally confined. It also demonstrates that InGaN structures grown on the top of GaN dots and wires with micron sizes are nanometric scale.