Vapor Liquid Solid Mode Research Articles

Nanowire-based telecom-band light-emitting diodes with efficient light extraction

Semiconductor nanowires grown by the gold-free, self-catalyzed, vapor–liquid–solid approach have great potential for realizing monolithically integrable light sources in the telecom band towards integrated optoelectronic circuits. However, light-emitting diodes based on such nanowires operating in the telecom band range remain challenging to achieve, due to difficulties in growing nanowires with suitable materials to cover the telecom band range, their geometrical shape to enhance light extraction efficiency, and their junction structure to suppress current leakage. Here we report telecom-band light-emitting diodes operating at room temperature using InP/InAs axial-heterostructure nanowires grown by self-catalyzed vapor–liquid–solid mode with efficient light extraction. Conically tapered nanowire structures with a single axial junction structure are advantageous for increasing light extraction and carrier injection efficiencies. We realize the structure by making use of a distinctive growth phenomenon in self-catalyzed vapor–liquid–solid growth; i.e. continuously tunable size of catalyst particles during growth.

Fabrication of star-shaped InP/GaInAs core-multishell nanowires by self-catalytic VLS mode

We successfully fabricated core-multishell nanowires using low-pressure metal organic vapor phase epitaxy (MOVPE) in self-catalytic vapor-liquid-solid (VLS) mode. The central core consists of an InP nanowire, and the shells are composed of alternating InP and GaInAs layers. The dependence of the shape and optical characteristics of the nanowires on the number of InP/GaInAs periods were investigated. By increasing the shell-periods, we observed a star-shaped cross section. The optical characteristics of the nanowires were investigated by room-temperature photoluminescence (PL) spectroscopy. The number of PL peaks was found to increase with increasing number of periods, and the peaks became redshifted.

Electrical and optical evaluation of n-type doping in InxGa(1−x)P nanowires

To harvest the benefits of III–V nanowires in optoelectronic devices, the development of ternary materials with controlled doping is needed. In this work, we performed a systematic study of n-type dopant incorporation in dense InxGa(1−x)P nanowire arrays using tetraethyl tin (TESn) and hydrogen sulfide (H2S) as dopant precursors. The morphology, crystal structure and material composition of the nanowires were characterized by use of scanning electron microscopy, transmission electron microscopy and energy dispersive x-ray analysis. To investigate the electrical properties, the nanowires were broken off from the substrate and mechanically transferred to thermally oxidized silicon substrates, after which electron beam lithography and metal evaporation were used to define electrical contacts to selected nanowires. Electrical characterization, including four-probe resistivity and Hall effect, as well as back-gated field effect measurements, is combined with photoluminescence spectroscopy to achieve a comprehensive evaluation of the carrier concentration in the doped nanowires. We measure a carrier concentration of ∼1 × 1016 cm−3 in nominally intrinsic nanowires, and the maximum doping level achieved by use of TESn and H2S as dopant precursors using our parameters is measured to be ∼2 × 1018 cm−3, and ∼1 × 1019 cm−3, respectively (by Hall effect measurements). Hence, both TESn and H2S are suitable precursors for a wide range of n-doping levels in InxGa(1−x)P nanowires needed for optoelectronic devices, grown via the vapor–liquid–solid mode.

Growth of Single-Crystalline Silicon Nanocone Arrays by Plasma Sputtering Reaction Deposition**Supported by the National Natural Science Foundation of China under Grant Nos 11261140327, 11325524, 11475102 and 11575057, the Chinese National Fusion Project for ITER under Grant Nos 2013GB112001, 2013GB107001 and 2014GB108000, the Tsinghua University Initiative Scientific Research Program, and the 221 Program.

Vertically aligned single-crystalline silicon nanocone (Si-NC) arrays are grown on nickel-coated silicon (100) substrates by a novel method i.e., abnormal glow-discharge plasma sputtering reaction deposition. The experimental results show that the inlet / ratio has great effects on the morphology of the grown Si-NC arrays. The characterization of the morphology, crystalline structure and composition of the grown Si-NCs indicates that the Si-NCs are grown epitaxially in the vapor–liquid–solid mode. The analyses of optical emission spectra further reveal that the inlet methane can promote the growth of Si-NCs by raising the plasma temperature and enhancing the ion-sputtering. The understanding of the growth mechanism of the Si-NC arrays will be helpful for fabrication of required Si-NC arrays.

Theoretical investigation about the optical characterization of cone‐shaped pin‐Si nanowire for top cell application

AbstractCone‐shaped semiconductor silicon nanowires (CS‐Si‐NWs) grown in vapor liquid solid mode are promising for the fabrication of low‐cost high‐performance solar cells because of their low processing cost and lower use of Si materials, as compared to planar devices. In this article, the effect of injected charge carriers on the refractive indices and extinction coefficient values in a cone‐shaped pin Si NW (CS‐pin‐Si NW) were considered. Then, the influence of top diameters and periods on the optical absorption was investigated using a finite difference time‐domain (FDTD) modeling method. The absorption increased when we decreased the period from 300 to 150 nm for a light wave with a wavelength of 700 nm. However, in the case of incident light at a wavelength of 500 nm, the absorption significantly increased by up to 100% and was found to be independent of the period. On the other hand, we varied the period and the top diameter of the NWs with a fixed bottom diameter. In this case, we found that the period did not significantly affect the absorption value. A high value of the short circuit current density of 19.5 mA/cm2 was found in the case of NWs with a top diameter of 110 nm and a period of 150 nm. Combined with the analysis of the ultimate photocurrents, an optimum geometric structure with a top diameter of 70 nm and a period of 150 nm for a CS‐pin‐Si NW‐based top cell for tandem solar cell applications was proposed.

Emission wavelength control of self-catalytic InP/GaInAs/InP core–multishell nanowire on InP substrate grown by metal organic vapor phase epitaxy

InP nanowires and InP/GaInAs/InP core–multishell nanowires were successfully grown on an InP(111)B substrate by low-pressure metal organic vapor phase epitaxy (MOVPE) using an indium catalyst. The self-catalytic vapor–liquid–solid (VLS) mode was used to obtain high-quality nanowires in which a deposited indium droplet acts as the catalyst instead of a metal particle, as in the case of the conventional VLS mode. InP core nanowire structures dependent on growth temperature and preheating temperature were obtained. InP/GaInAs/InP core–multishell nanowire structures, densities, and optical properties were investigated at various flow rates of trimethylindium (TMI) during the growth of InP core nanowires and the growth time of the GaInAs shell layer was also studied. The growth volume and density of nanowires were mainly dependent on growth temperature and preheating temperature, respectively. The height of nanowires was dependent on the TMI flow rate in the InP core nanowire growth, and the thickness of GaInAs shell layer was controlled by adjusting the growth time of the GaInAs shell layer. The photoluminescence (PL) intensity increased with increasing nanowire height and the peak wavelength was controlled by adjusting the thickness of the GaInAs shell layer.

Investigation of crystallinity and planar defects in the Si nanowires grown by vapor–liquid–solid mode using indium catalyst for solar cell applications

Stacking-fault-free and planar defect (twinning plane)-free In-catalyzed Si nanowires (NWs) are essential for carrier transport and nanoscale device applications. In this article, In-catalyzed, vertically aligned, and cone-shaped Si NWs on Si(111) were grown successfully, in the vapor–liquid–solid (VLS) mode. In particular, the influences of substrate temperature (TS) and cooling rate (ΔTS/Δt) on the formation of planar defects, twinning planes along the [112] direction, and stacking faults in Si NWs were investigated. When TS was decreased from 600 °C to room temperature at a rate of 100 °C/240 s after Si NW growth, twinning plane defects perpendicular to the substrate and along different segments of (111)-oriented Si NWs were observed. Finally, one simple model was proposed to explain the stacking fault formation as well as Si NW length limitation due to the In-nanoparticle (In-NP) migration, and root causes of the twinning plane defects in the Si-NWs.

Synthesis of single-crystalline NiCx nanorods by pulsed laser deposition accompanied by N2 annealing

By the method of pulsed laser deposition accompanied by annealing in N2 environment, single-crystalline NiCx nanorods (NCNRs) were first synthesized on C/Ni-coated quartz substrates. Field emission scanning electron microscopy images show that the densely and uniformly distributed NCNRs were synthesized on quartz substrates at the temperature of 1000–1250°C and the optimum annealing temperature for their growth is 1200°C. The results of transmission electron microscopy and selected area electron diffraction show that the grown NCNRs are single-crystalline hexagonal NiCx. The analyses of energy dispersive X-ray spectroscopy reveal that the NCNRs are grown by the Ni-leading vapor–liquid–solid mode. Transmission spectra demonstrate that the as-grown NCNR thin films can have about 98% visible-light transmittance and are candidates for transparent electrodes in thin film solar cells or photoelectric sensors.

Preheating temperature and growth temperature dependence of InP nanowires grown by self-catalytic VLS mode on InP substrate

InP nanowires were successfully grown on an InP(111)B substrate by low pressure metal organic vapor phase epitaxy (MOVPE) using an indium catalyst. The self-catalytic vapor–liquid–solid (VLS) mode was used to obtain high quality nanowires where a deposited indium droplet acts as the catalyst instead of a metal particle, as with the conventional VLS mode. The nanowire characteristics are discussed as a function of the preheating and growth temperatures used with this method. InP nanowires vertically aligned on (111) orientation were obtained and the nanowire density was dependent on the preheating temperature, while the total growth volume was mainly dependent on the growth temperature. From these results, the optimal growth conditions to produce InP nanowires with small diameter, high density, and lower dispersion have been determined.

Gold Contamination in VLS-Grown Si Nanowires: Multiwavelength Anomalous Diffraction Investigations

Silicon nanowires were grown by gold-catalyzed chemical vapor deposition in the vapor liquid solid mode. In this paper, we use grazing incidence X-ray diffraction and multiwavelength anomalous diffraction to investigate the presence of gold and its effects on silicon nanowire strain after different chemical and physical treatments. We especially focus on the efficiency of the thermal oxidation-oxide etching cycle to remove the gold contamination. Analysis reveals a decrease of the contamination and a core-shell like spatial distribution of gold, but also shows that this technique is not efficient enough to remove all traces of gold from the nanowires.

Catalyst-free synthesis of multi-walled carbon nanotubes from carbon spheres and its implications for the formation mechanism

A route to producing multi-walled carbon nanotubes (MWCNTs) was reported, in which polyacrylonitrile microspheres (PANMSs) were assembled into one-dimensional strings of carbon spheres and transformed to carbon nanotubes at 1000°C. It was found that the diameters of the MWCNTs are uniform and correlated with the size of the PANMSs used. Structures of intermediate products obtained by stopping the reaction have been examined by high-resolution transmission electron microscopy. The observations indicate that the formation of MWCNTs follows a different mechanism from the well-studied vapor–liquid–solid mode. It involves a direct self-assembly and solid-state structural transformation of PANMSs under the promotion of nitrogen atoms as shown by the X-ray photoelectron spectra of the resulting samples. On the basis of these observations, a sphere-string-tube mechanism was proposed for the MWCNT formation.

Confident methods for the evaluation of the hydrogen content in nanoporous carbon microfibers

Nanoporous carbon microfibers were grown by chemical vapor deposition in the vapor-liquid solid mode using different fluid hydrocarbons as precursors in different proportions. The as-grown samples were further treated in argon and hydrogen atmospheres at different pressure conditions and annealed at several temperatures in order to deduce the best conditions for the incorporation and re-incorporation of hydrogen into the microfibers through the nanopores. Since there are some discrepancies in the results on the hydrogen content obtained under vacuum conditions, in this work, we have measured the hydrogen content in the microfibers using several analytical methods in ambient conditions: surface tension, mass density, and Raman measurements. A discussion on the validity of the results obtained through the correlation between them is the purpose of the present work.

One-step growth of Si 3N 4 stem–branch featured nanostructures: Morphology control by VS and VLS mode

We report here one-step synthesis of Si 3N 4 nanodendrites by selectively applying a vapor–solid (VS) and vapor–liquid–solid (VLS) strategy via direct current arc discharge method. The resultant nanodendrites were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy and X-ray powder diffraction. The spine-shaped nanodendrites were generated by a noncatalytic growth following a VS mode. The uniform secondary nanowire branches were epitaxial grown from two side surfaces of the nanowire stems. The pine-shaped nanodendrites were obtained through a catalytic growth in a VLS process. These branch nanowires were unsystematically grown from the nanocone-like stems. The photoluminescence spectra of the nanodendrites show a strong white light emission around 400–750 nm, suggesting their potential applications in light and electron emission devices.

Temperature-controlled synthesis of Zn 2GeO 4 nanowires in a vapor–liquid–solid mode and their photoluminescence properties

Varying the heating temperature of a mixture of Zn and Ge powders, we have successfully prepared Zn 2GeO 4 nanowires. We suggest that the 900 °C-grown product was mainly grown via a vapor–liquid–solid process, whereas the 800 °C-grown product was dominated by a vapor–solid mechanism. For 900 °C-grown nanowires, the stem is mainly comprised rhombohedral Zn 2GeO 4 phase, whereas the tip is characterized by a particle hexagonal GeO 2 phase. For 800 °C-grown nanowires, both the stem and tip are mainly comprised hexagonal ZnO phase with some additional Zn 2GeO 4 phase. It was observed that variation of the growth temperature affected the photoluminescence spectra of the nanowires, leading to an overall intensification. We have discussed the possible mechanisms for the observed temperature-induced changes in the PL spectra.

GaP/GaAs1−xPx nanowires fabricated with modulated fluxes: A step towards the realization of superlattices in a single nanowire

Compositionally modulated GaP/GaAs1−xPx nanowires are grown by molecular beam epitaxy in the vapor liquid solid mode using Au as catalyst. By periodically supplying As, we achieve atomically flat and abrupt interfaces along the whole length of the nanowires. This is a step towards the realization of superlattices in nanowires.

Core-shell structure and unique faceting of Sn-catalyzed silicon nanowires

The potential of tin (Sn) as a catalyst for the vapor-liquid-solid (VLS) growth of silicon nanowires (SiNWs) has been relatively unexplored. Compared to the more commonly used catalysts (e.g., gold), Sn features a low eutectic point (at 235 °C) and a very low equilibrium Si solubility. These features make it possible to tune the supersaturation in the catalyst and the growth kinetics at the SiNW/catalyst interface. Here, we investigate the peculiarities of Sn-catalyzed VLS growth and show that the local growth kinetics, modified by the high supersaturation in the Sn catalyst, can lead to the formation of core-shell structures and sidewall faceting of the SiNWs. Challenges facing Sn-catalyzed growth of SiNWs in VLS mode are also discussed. We suggest that the unique features of Sn catalyst will help tailor SiNW structures to meet specific application needs.

Parallel-aligned GaAs nanowires with⟨110⟩ orientation laterally grown on [311]B substrates via the gold-catalyzed vapor–liquid–solidmode

We report parallel aligned GaAs nanowires (NWs) with⟨110⟩ orientation laterally grown on [311]B substrates via the vapor–liquid–solid mode anddemonstrate their controllability and growth mechanism. We control the size, density, andsite of the lateral NWs by using size- and density-selective Au colloidal particles and Au dotarrays defined by electron-beam lithography. The lateral NWs grow only along the [110]and directions and formation of the stable facets of (111)B and (001) on the sides of thelateral NWs is crucial for lateral NW growth. We clarify the growth mechanism bycomparing the growth results on [311]B, (311)A, and (001) substrates and the surfaceenergy change of lateral and freestanding NWs.

Structural, Compositional, and Optical Characterizations of Vertically Aligned AlAs/GaAs/GaP Heterostructure Nanowires Epitaxially Grown on Si Substrate

We structurally, compositionally, and optically characterize vertically aligned AlAs/GaAs/GaP heterostructured nanowires (NWs) grown on a Si substrate used for the integration of an optically active material into Si-based technology and its band-gap engineering. The NWs were grown using Au colloidal nanoparticles as catalysts via the vapor–liquid–solid mode. By alternately changing the source material between Ga and Al, we grew GaAs/AlAs/GaAs/AlAs/GaAs NWs with a well-controlled periodic structure and composition on a GaP segment, which was epitaxially grown on a Si substrate. No dislocations induced by the lattice mismatch were found in the GaAs segment of the NWs grown on the GaP segment despite a lattice mismatch of as large as 4%. This is because the NWs have a particular columnar structure with nanoscale diameters and can therefore relax laterally and accommodate a high strain. Stacking faults exist in zinc-blende-structured GaP and GaAs segments, while the AlAs segment has a pure wurtzite crystal structure without any stacking faults. It is found that the stacking fault in III–V NWs is significantly dependent on the stacking fault energy and ionicity. With increasing ionicity, stacking faults can be more easily introduced, and these NWs tend to have a wurtzite crystal structure. In addition, owing to the high surface nonradiative recombination rate resulting from the surface states on the GaAs NW surface, the excitonic emission of photoluminescence from the bare GaAs NW segment has a decay time of as short as 30 ps. With the growth of an AlGaAs capping layer and GaAs outer shell layer, the decay time of the excitonic emission increased 46-fold, indicating an excellent passivation effect on the GaAs segment surface.

Homojunction of Single-Crystalline ZnO Nanowire/Nanowall Network on c-Al2O3 Substrate Formed by Thermal Vapor Phase Epitaxy Method

Homojunctional ZnO nanowire/nanowall structures were synthesized using a vapor phase epitaxy method. The homojunctional ZnO nanowire/nanowall structures are vertically well aligned having a random honeycomb-like pattern, and the metal catalyst only exists on the ends of the ZnO nanowires. The ZnO nanowires and nanowall synthesized on c-Al2O3 in the vapor–liquid–solid mode form a single unit and are part of the same single crystal. We focused on the study of the growth mechanism and crystal structure of homojunctional ZnO nanowire/nanowall structures.

Growth temperature dependence of MBE-grown ZnSe Nanowires

The growth orientation and crystalline perfection of ZnSe nanowires (NWs) grown by the combination of molecular beam epitaxy (MBE) and vapor–liquid–solid (VLS) reaction was found dependent on the growth temperature. Transmission electron microscopy (TEM) observations reveal that the preferred growth orientation of NWs depends on both the size of NWs and the growth temperature. A growth temperature dependence of the thickness of the terminal growth zone under the metal/semiconductor interface is introduced to modify our previous proposed phenomenological model to explain these observations. Tapering growth (the size of the NWs becomes narrower towards the top) was observed for the samples grown at 400 °C, which can be attributed to the diffusion-induced VLS mode that can be activated at relatively low growth temperature but inhibited at higher growth temperature. A cubic to hexagonal phase transformation was observed on the tapered NWs and its formation mechanism was proposed.