Synthesis of GaN nanowires by CVD method: effect of reaction temperature

GaN nanowires are fabricated on Si substrates by ammoniating Ga₂O₃/NiCl₂ thin films using chemical vapour deposition method. The influence of reaction temperature on microstructure, morphology and optical properties of GaN nanowires is characterised by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrophotometer, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence. The results demonstrate that the GaN nanowires are single crystalline and exhibit hexagonal wurtzite symmetry. The best crystalline quality was achieved for an reaction temperature of 1150°C for 15?min. The growth process follows vapour–liquid–solid mechanism and Ni plays an important role as the nucleation point and as a catalyst.     

化学气相沉积法制备GaN纳米线和纳米棒

采用浸渍法在未抛光的硅衬底上涂抹一层NiCl2薄膜,通过化学气相沉积法(CVD)制备出高质量的GaN纳米线和纳米棒.X射线衍射(XRD)、傅立叶红外吸收光谱(FTIR)、选区电子衍射(SAED)和高分辨透射电子显微镜(HRTEM)的分析结果表明,采用此方法得到了六方纤锌矿结构的GaN单晶纳米线.通过扫描电镜(SEM)观察发现纳米线的形貌,纳米线的直径在50～200nm之间,纳米棒的直径在200～800nm之间.     

Catalyst and its diameter dependent growth kinetics of CVD grown GaN nanowires

GaN nanowires were grown using chemical vapor deposition with controlled aspect ratio. The catalyst and catalyst-diameter dependent growth kinetics is investigated in detail. We first discuss gold catalyst diameter dependent growth kinetics and subsequently compare with nickel and palladium catalyst. For different diameters of gold catalyst there was hardly any variation in the length of the nanowires but for other catalysts with different diameter a strong length variation of the nanowires was observed. We calculated the critical diameter dependence on adatoms pressure inside the reactor and inside the catalytic particle. This gives an increasing trend in critical diameter as per the order gold, nickel and palladium for the current set of experimental conditions. Based on the critical diameter, with gold and nickel catalyst the nanowire growth was understood to be governed by limited surface diffusion of adatoms and by Gibbs–Thomson effect for the palladium catalyst.     

GaN nanowires: CVD synthesis and properties

The growth of GaN nanowires from Ga and NH₃ sources in the flow of Ar carrier gas using a chemical vapor deposition (CVD) system was systematically studied. The substrates used were Si(111) and Si(100). Fabricated nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). We investigated the influence of growth temperature, catalyst used, Ga amount, and the ratio of Ar and NH₃ flow rates on the morphology and properties of GaN nanowires. We found that the best results were obtained for a growth temperature of 950 °C. Optimal catalysts were Au and metallic Ni, while the use of nickel nitrate was found to lead to formation of SiO_x nanowire bunches in addition to GaN nanowires. For the optimal temperature and catalyst used, the influence of the Ga to N ratio on the nanowire growth was studied. It was found that different types of nanostructures are observed in relatively Ga-rich and in relatively N-rich conditions. Growth mechanisms of different types of nanowires, including the stacked-cone nanowires and the microscale structures formed by lateral growth under N-rich conditions, are discussed.     

AgCl nanoparticle nanowires fabricated by template method

A facile and effective template method was explored to prepare smooth AgCl nanowire arrays; moreover, by employing gelatin as a protector and stuffing, a novel nanoparticle nanowire arrays were successfully fabricated. Scanning electron microscopy and energy dispersive spectroscopy were applied to characterize the morphology and the composition of the two AgCl nanowires.     

The growth of GaN films by alternate source gas supply hot-mesh CVD method

Gallium nitride (GaN) films and Aluminium nitride (AlN) layers were deposited on SiC/Si (111) substrates by an alternating source gas supply or an intermittent supply of a source gas such as ammonia (NH₃), trimethylgallium (TMG) or trimethylaluminum (TMA) in a hot-mesh chemical vapor deposition (CVD) apparatus. The AlN layer was deposited as a buffer layer using NH₃ and TMA on a SiC layer grown by carbonization on Si substrates using propane (C₃H₈). GaN films were grown on an AlN layer by a reaction between NH_x radicals generated on a ruthenium (Ru) coated tungsten (W)-mesh and TMG molecules. An alternating source gas supply or an intermittent supply of one of the source gases during the film growth are expected to be effective for the suppression of gas phase reactions and for the enhancement of precursor migration on the substrate surface. By the intermittent supply of alkylmetal gas only during the growth of the AlN layer, the defect generation in the GaN films was reduced. GaN film growth by intermittent supply on an AlN buffer layer, however, did not lead to the improvement of the film quality.     

The controlled growth of GaN nanowires

This paper reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled. The approach is based on conventional metalorganic chemical vapor deposition using regular precursors and requires no additional metal catalyst. The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective growth mask that is patterned by interferometric lithography. It was found that use of a pulsed MOCVD process allowed the nanowire diameter to remain constant after the nanowires had emerged from the selective growth mask. Vertical GaN nanowire growth rates in excess of 2 mum/h were measured, while remarkably the diameter of each nanowire remained constant over the entire (micrometer) length of the nanowires. The paper reports transmission electron microscopy and photoluminescence data.     

One-step growth of curled GaN nanowires using chemical vapor deposition method

The studies of curled GaN nanowires grown on sapphire and silicon substrate using chemical vapor deposition method have been reported in this article. The mean diameters of the nanowires grown on sapphire and silicon were 108.1 nm and 98 nm respectively. A growth model was proposed to describe the growth of nanowires. X-ray diffraction pattern and Raman spectroscopy revealed that the nanowires were hexagonal wurtzite in structure. Gaussian fitting was done on photoluminescence spectra, which revealed two sub-bands that could be attributed to band emission and surface disorder caused by impurities. The absence of yellow luminescence signified undoped case and minimal shallow level defects.     

The initial stage of growth of self-induced GaN nanowires

The initial growth stage of self-induced GaN nanowires (NWs) on an AlN(0001)/Si(111) substrate is studied theoretically. Calculations are carried out within the model of Stranski-Krastanov quantum dot formation. The surface density of GaN islands is calculated, the formation of which precedes NW formation. GaN NW density is found as a function of gallium flux and deposition time for the case of molecular beam epitaxy growth.     

Gold nanowires fabricated by immersion plating

The growth mechanism of oriented Au nanowires fabricated by immersion plating was investigated. Both n-type crystal Si (c-Si) and amorphous Si (a-Si) with an electron-beam (E-beam) patterned resist nanotrench were immersed into the plating bath HAuCl(4)/HF. For the Au nanowires fabricated on c-Si, voids, nanograins, and clusters were observed at various plating conditions, time and temperature. The voids were often found in the center of the Au nanowires due to there being fewer nucleation sites on the c-Si surface. However, Au can easily nucleate on the surface of a-Si and form continuous Au nanowires with grain sizes about 10-50?nm. The resistivities of Au nanowires with width 105?nm fabricated on a-Si are about 4.4-6.5?μΩ?cm. After annealing at 200?°C for 30?min in N(2) ambient, the resistivities are lowered to about 3.0-3.9?μΩ?cm, measured in an atomic force microscope (AFM) in contact mode. The grain size of Au is in the range of ～50-100?nm. A scanning electron microscope (SEM) examination and grazing incident x-ray diffraction (GIXRD) analysis were also carried out to study the morphology and crystalline structure of the Au nanowires.     

Influence of co-catalyst on growth of carbon nanotubes using alcohol catalytic CVD method

This paper describes the influence of a co-catalyst on growth of carbon nanotubes (CNTs) by alcohol catalytic chemical vapour deposition (ACCVD) method. Silicon wafers covered with thermal oxide or polycrystalline diamond thin film were used as substrates. Ni thin film supported with Al, Cu or Ti was used as a catalyst. The films were deposited by pulsed laser deposition technique. Comparison of the various types of the co-catalyst (Al, Cu, Ti) leads to the conclusion that Cu co-catalyst is suitable for producing very thin single wall carbon nanotubes (SWCNTs) and combination of Al and Ni provide a good condition to the catalytic growth of CNTs. In addition, we observed also the influence of the various diffusion barriers (thermal oxide and polycrystalline diamond) on growth of CNTs. Prepared samples were analysed by Raman spectroscopy (RS) and scanning electron microscopy (SEM).     

Self-assembled GaN nanowires on diamond

We demonstrate the nucleation of self-assembled, epitaxial GaN nanowires (NWs) on (111) single-crystalline diamond without using a catalyst or buffer layer. The NWs show an excellent crystalline quality of the wurtzite crystal structure with m-plane faceting, a low defect density, and axial growth along the c-axis with N-face polarity, as shown by aberration corrected annular bright-field scanning transmission electron microscopy. X-ray diffraction confirms single domain growth with an in-plane epitaxial relationship of (10 ?10)(GaN) [parallel] (01 ?1)(Diamond) as well as some biaxial tensile strain induced by thermal expansion mismatch. In photoluminescence, a strong and sharp excitonic emission reveals excellent optical properties superior to state-of-the-art GaN NWs on silicon substrates. In combination with the high-quality diamond/NW interface, confirmed by high-resolution transmission electron microscopy measurements, these results underline the potential of p-type diamond/n-type nitride heterojunctions for efficient UV optoelectronic devices.     

Direct synthesis of SiC nanowires by multiple reaction VS growth

β-SiC nanowires were directly synthesized by heating single-crystal silicon wafer and graphite without metal catalysts. The diameter of SiC nanowires is in the range of 10–30 nm, and the length is up to a few millimeters. Two kinds of SiC nanowires, namely pure SiC nanowires and SiC/SiO₂ composite nanowires, formed at higher temperature (the holding stage) and lower temperature (the cooling stage), respectively. A multiple-reaction model was proposed to explain the formation of SiC nanowires.     

化学气相反应制备碳化硅纳米线

用硅粉、二氧化硅和石墨粉作原料，在无催化剂的条件下，在1400℃下用高温化学气相反应法制备了碳化硅纳米线，并用高分辨扫描电镜观察了所得碳化硅纳米线的形貌。所得碳化硅纳米线直径为100-500nm，长度可达几百微米。还提出了描述碳化硅纳米线的生长机理。     

Study on purification and tip-opening of CNTs fabricated by CVD

The purification effects of different methods including liquid oxidation by nitric acid and multi-step treatment on two raw CNT samples fabricated by CVD under different synthesis conditions were investigated. Opening effects of the purified CNTs were also explored. The results showed that the liquid oxidation by nitric acid has some effects on purification and opening of the CNTs, and that the effects vary with the CNT morphologies. When using multi-step treatment method, apparent effects of purification and opening on the CNTs are obtained.     

Nucleation and growth of GaN nanowires on Si(111) performed by molecular beam epitaxy

GaN nanowires (NWs) have been grown on Si(111) substrates by plasma-assisted molecular beam epitaxy (PAMBE). The nucleation process of GaN-NWs has been investigated in terms of nucleation density and wire evolution with time for a given set of growth parameters. The wire density increases rapidly with time and then saturates. The growth period until the nucleation of new nanowires is terminated can be defined as the nucleation stage. Coalescence of closely spaced nanowires reduces the density for long deposition times. The average size of the well-nucleated NWs shows linear time dependence in the nucleation stage. High-resolution transmission electron microscopy measurements of alternating GaN and AlN layers give valuable information about the length and radial growth rates for GaN and AlN in NWs.     

沉积条件对CVD碳纤维生长的影响

用CH4、H2或包含NH3的混合气体为反应气体,利用负偏压增强热丝化学气相沉积方法在沉积有过渡层(Ta或Ti)和催化剂层(NiFe)的Si衬底上制备碳纤维,并用扫描电子显微镜研究了它们的生长和结构,结果发现不同的沉积条件对碳纤维的生长和结构有很大的影响。在无辉光放电的条件下,衬底温度较低时碳纳米管或纤维生长困难;提高衬底温度,能够弯曲生长;在辉光放电的条件下,则呈现定向生长的特点。     

Nanostructured GaN on silicon fabricated by electrochemical and laser-induced etching

Nanostructured GaN layers have been fabricated by electrochemical and laser-induced etching (LIE) processes based on n-type GaN thin films grown on the Si (111) substrate with AlN buffer layers. The effect of varying current and laser power density on the morphology of the GaN layers is investigated. The etched samples exhibited a dramatic increase in photoluminescence intensity as compared to the as grown samples. The average diameter of the GaN crystallites was about 7-10 nm, as determined from the PL data The Raman spectra also displayed stronger intensity peaks, which were shifted and broadened as a function of etching parameters. A strong band at 522 cm^− 1 is from the Si (111) substrate, and a small band at 301 cm^− 1, due to the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 cm^− 1 and 568 cm^− 1due to E2 (low) and E2 (high) respectively.     

氨气流量对GaN纳米线生长及性能的影响

利用磁控溅射技术在Si衬底上沉积Ga2O3/Co薄膜,然后在不同氨气流量下于950℃退火15min。采用X射线衍射(XRD)、扫描电子显微镜(SEM)、傅立叶红外吸收(FTIR)光谱、高分辨透射电子显微镜(HRTEM)和光致发光谱(PL)对样品进行了分析表征。结果表明,氨气流量对GaN纳米线的生长及性能有很大影响。简单讨论了GaN纳米线的生长机理。     

Si基Au催化合成镁掺杂GaN纳米线

通过一种新奇的方法在硅衬底上成功地合成了掺杂镁的氮化镓纳米线,用金属镁粉末作为掺杂源,然后在900℃时于流动的氨气中进行氨化Ga2P3薄膜制备GaN纳米线.X射线衍射(XRD)、扫描电镜(SEM)、透射电子显微镜(TEM)、选区电子衍射(SAED)和能量弥散X射线谱(EDX)的分析结果表明,采用此方法得到的GaN纳米线为六方纤锌矿结构,纳米线的直径大约在60～100nm之间,纳米线的长约十几个微米.EDX分析表明纳米线掺杂了镁.室温下以325nm波长的光激发样品表面,发现由于镬的掺杂使GaN的发光峰有较大的蓝移.最后,简单讨论了GaN纳米线的生长机制.