Sb-Mediated Tuning of Growth- and Exciton Dynamics in Entirely Catalyst-Free GaAsSb Nanowires

Vapor-liquid-solid (VLS) growth is the mainstream method in realizing advanced semiconductor nanowires (NWs), as widely applied to many III-V compounds. It is exclusively explored also for antimony (Sb) compounds, such as the relevant GaAsSb-based NW materials, although morphological inhomogeneities, phase segregation, and limitations in the supersaturation due to Sb strongly inhibit their growth dynamics. Fundamental advances are now reported here via entirely catalyst-free GaAsSb NWs, where particularly the Sb-mediated effects on the NW growth dynamics and physical properties are investigated in this novel growth regime. Remarkably, depending on GaAsSb composition and nature of the growth surface, both surfactant and anti-surfactant action is found, as seen by transitions between growth acceleration and deceleration characteristics. For threshold Sb-contents up to 3–4%, adatom diffusion lengths are increased ≈sevenfold compared to Sb-free GaAs NWs, evidencing the significant surfactant effect. Furthermore, microstructural analysis reveals unique Sb-mediated transitions in compositional structure, as well as substantial reduction in twin defect density, ≈tenfold over only small compositional range (1.5–6% Sb), exhibiting much larger dynamics as found in VLS-type GaAsSb NWs. The effect of such extended twin-free domains is corroborated by ≈threefold increases in exciton lifetime (≈4.5 ns) due to enlarged electron-hole pair separation in these phase-pure NWs.     

Multiple quantum well AlGaAs nanowires

This letter reports on the growth, structure, and luminescent properties of individual multiple quantum well (MQW) AlGaAs nanowires (NWs). The composition modulations (MQWs) are obtained by alternating the elemental flux of Al and Ga during the molecular beam epitaxy growth of the AlGaAs wire on GaAs (111)B substrates. Transmission electron microscopy and energy dispersive X-ray spectroscopy performed on individual NWs are consistent with a configuration composed of conical segments stacked along the NW axis. Microphotoluminescence measurements and confocal microscopy showed enhanced light emission from the MQW NWs as compared to nonsegmented NWs due to carrier confinement and sidewall passivation.     

Peculiarities of the MBE growth of nanowhiskers on GaAs(100) substrates

We have studied the formation of nanowhiskers (NWs) by molecular beam epitaxy (MBE) on GaAs(100) substrates. The MBE growth of NWs exhibits two stages (initial and developed) and leads to the formation of NWs with surface morphology of two types (nucleation and intergrowth). The stage of developed growth is characterized by the predominant formation of intergrown NWs oriented in the 〈111〉B direction, having (110) habit (including the NW tip surface) and hexagonal cross sections with a transverse size within 50–300 nm. It was found that the transverse size of a hexagonal NW may significantly differ from that of an Au-GaAs melt droplet. The ratio of longitudinal and transverse dimensions of intergrown NWs can be on the order of 150 and above. When the transverse size of NWs exceeds a certain value (about 200 nm), the crystal length exhibits a slight decrease. The existence of two types of morphology is indicative of inhomogeneous character of the NW growth on a GaAs(100) surface, which depends on the catalyst droplet size, effective thickness of the deposited GaAs layer, and the growth temperature.     

A Mask Based on a Si Epitaxial Layer for the Self-Catalytic Nanowire Growth on GaAs(111)B and GaAs(100) Substrates

Technical Physics Letters - GaAs nanowires (NWs) were generated on the surface of GaAs(111)B and GaAs(100) substrates from molecular fluxes by the self-catalytic growth method. A mask for NW growth...     

Growth of InAs quantum dots on GaAs nanowires by metal organic chemical vapor deposition

InAs quantum dots (QDs) are grown epitaxially on Au-catalyst-grown GaAs nanowires (NWs) by metal organic chemical vapor deposition (MOCVD). These QDs are about 10-30 nm in diameter and several nanometers high, formed on the {112} side facets of the GaAs NWs. The QDs are very dense at the base of the NW and gradually sparser toward the top until disappearing at a distance of about 2 μm from the base. It can be concluded that these QDs are formed by adatom diffusion from the substrate as well as the sidewalls of the NWs. The critical diameter of the GaAs NW that is enough to form InAs QDs is between 120 and 160 nm according to incomplete statistics. We also find that these QDs exhibit zinc blende (ZB) structure that is consistent with that of the GaAs NW and their edges are faceted along particular surfaces. This hybrid structure may pave the way for the development of future nanowire-based optoelectronic devices.     

Self-directed growth of AlGaAs core-shell nanowires for visible light applications

Al0.37Ga0.63As nanowires (NWs) were grown in a molecular beam epitaxy system on GaAs(111)B substrates. Micro-photoluminescence measurements and energy dispersive X-ray spectroscopy indicated a core--shell structure and Al composition gradient along the NW axis, producing a potential minimum for carrier confinement. The core--shell structure formed during growth as a consequence of the different Al and Ga adatom diffusion lengths.     

High‐Responsivity Photodetection by a Self‐Catalyzed Phase‐Pure p‐GaAs Nanowire

Defects are detrimental for optoelectronics devices, such as stacking faults can form carrier‐transportation barriers, and foreign impurities (Au) with deep‐energy levels can form carrier traps and nonradiative recombination centers. Here, self‐catalyzed p‐type GaAs nanowires (NWs) with a pure zinc blende (ZB) structure are first developed, and then a photodetector made from these NWs is fabricated. Due to the absence of stacking faults and suppression of large amount of defects with deep energy levels, the photodetector exhibits room‐temperature high photoresponsivity of 1.45 × 10⁵ A W^?1 and excellent specific detectivity (D*) up to 1.48 × 10¹⁴ Jones for a low‐intensity light signal of wavelength 632.8 nm, which outperforms previously reported NW‐based photodetectors. These results demonstrate these self‐catalyzed pure‐ZB GaAs NWs to be promising candidates for optoelectronics applications.     

High crystal quality wurtzite-zinc blende heterostructures in metal-organic vapor phase epitaxy-grown GaAs nanowires

We have prepared GaAs wurtzite (WZ)-zinc blende (ZB) nanowire heterostructures by Au particle-assisted metal-organic vapor phase epitaxy (MOVPE) growth. Superior crystal quality of both the transition region between WZ and ZB and of the individual segments themselves was found for WZ-ZB single heterostructures. Pure crystal phases were achieved and the ZB segments were found to be free of any stacking defects, whereas the WZ sections showed a maximum stacking fault density of 20 μm⁻¹. The hexagonal cross-sectional wires are terminated by -type side facets for the WZ segment and predominantly {110}-type side facets for the ZB part of the wire. A diameter increase occurred after the transition from WZ to ZB. Additionally, facets of the -type as well as downwards-directed overgrowth of the WZ segments were formed at the WZ to ZB transition to compensate for the observed diameter increase and facet rotation. In the case of WZ-ZB multiple heterostructures, we observed slightly higher densities of stacking faults and twin planes compared to single heterostructures.      

Optical gain of 1.3 /spl mu/m GaAsSbN/GaAs quantum well lasers

Optical gain of 1.3 mum GaAsSbN/GaAs quantum well (QW) structure is investigated using the multiband effective mass theory. The results are compared with those of 1.3 mum InGaNAs/GaAs and GaAsSb/GaAs QW structures. The optical gain of the GaAsSbN/GaAs QW structure is found to be similar to that of the InGaAsN/GaAs QW structure. In contrast, GaAsSbN/GaAs and InGaNAs/GaAs QW structures show significantly larger optical gain than the GaAsSb/GaAs QW structure. This is mainly attributed to the fact that the former has a larger optical matrix element than the latter. In addition, GaAsSbN/GaAs and InGaNAs/GaAs QW structures have much smaller threshold current density than the GaAsSb/GaAs QW structure. This is because the Auger recombination current density gives dominant contribution to the threshold current density and the former has smaller threshold carrier density than the latter. On the contrary, the threshold current density of the GaAsSbN/GaAs QW structure is shown to be similar to that of the InGaAsN/GaAs QW structure     

Control of the crystal structure of InAs nanowires by tuning contributions of adatom diffusion

The dependence of crystal structure on contributions of adatom diffusion (ADD) and precursor direct impingement (DIM) was investigated for vapor-liquid-solid growth of InAs nanowires (NWs). The ADD contributions from the sidewalls and substrate surface can be changed by using GaAs NWs of different length as the basis for growing InAs NWs. We found that pure zinc-blende structure is favored when DIM contributions dominate. Moreover, without changing the NW diameter or growth parameters (such as temperature or V/III ratio), a transition from zinc-blende to wurtzite structure can be realized by increasing the ADD contributions. A nucleation model is proposed in which ADD and DIM contributions play different roles in determining the location and phase of the nucleus.     

Direct observation of a noncatalytic growth regime for GaAs nanowires

We identify a new noncatalytic growth regime for molecular beam epitaxially grown GaAs nanowires (NWs) that may provide a route toward axial heterostructures with discrete material boundaries and atomically sharp doping profiles. Upon increase of the As/Ga flux ratio, the growth mode of self-induced GaAs NWs on SiO(2)-masked Si(111) is found to exhibit a surprising discontinuous transition in morphology and aspect ratio. For effective As/Ga ratios <1, in situ reflection high-energy electron diffraction measurements reveal clear NW growth delay due to formation of liquid Ga droplets since the growth proceeds via the vapor-liquid-solid mechanism. In contrast, for effective As/Ga ratios >1 an immediate onset of NW growth is observed indicating a transition to droplet-free, facet-driven selective area growth with low vertical growth rates. Distinctly different microstructures, facet formation and either the presence or absence of Ga droplets at the apex of NWs, are further elucidated by transmission electron microscopy. The results show that the growth mode transition is caused by an abrupt change from As- to Ga-limited conditions at the (111)-oriented NW growth front, allowing precise tuning of the dominant growth mode.     

Strain accommodation in Ga-assisted GaAs nanowires grown on silicon (111)

We study the mechanism of lattice parameter accommodation and the structure of GaAs nanowires (NWs) grown on Si(111) substrates using the Ga-assisted growth mode in molecular beam epitaxy. These nanowires grow preferentially in the zincblende structure, but contain inclusions of wurtzite at the base. By means of grazing incidence x-ray diffraction and high-resolution transmission electron microscopy of the NW-substrate interface, we show that the lattice mismatch between the NW and the substrate is released immediately after the beginning of NW growth through the inclusion of misfit dislocations, and no pseudomorphic growth is obtained for NW diameters down to 10 nm. NWs with a diameter above 100 nm exhibit a rough interface towards the substrate, preventing complete plastic relaxation. Consequently, these NWs exhibit a residual compressive strain at their bottom. In contrast, NWs with a diameter of 50 nm and below are completely relaxed because the interface is smooth.     

GaAs:Mn nanowires grown by molecular beam epitaxy of (Ga,Mn)as at MnAs segregation conditions

GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs phase separation. Their density is proportional to the density of catalyzing MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate temperature. After deposition corresponding to a 200 nm thick (Ga,Mn)As layer the nanowires are around 700 nm long. Their shapes are tapered, with typical diameters around 30 nm at the base and 7 nm at the tip. The wires grow along the 111 direction, i.e., along the surface normal on GaAs(111)B and inclined on GaAs(001). In the latter case they tend to form branches. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the nanowires combine one-dimensional properties with the magnetic properties of (Ga,Mn)As and provide natural, self-assembled structures for nanospintronics.     

Orientation-dependent phase separation of GaAsSb epilayers grown by gas-source molecular-beam epitaxy

This work describes a regular solution model that considers the free energy of the surface monolayer to explain the orientation-dependent phase separation in GaAsSb. In the proposed model, only the interaction between the second nearest-neighboring atoms sitting on the same monolayer contributes to the interaction parameter. Consequently, the parameter reduces to Ω/2 and Ω/3 for (111)B GaAsSb and (100) GaAsSb, where Ω denotes the parameter of bulk GaAsSb. By including the strain effect, the proposed model thoroughly elucidates the immiscibility behavior of (111)B GaAsSb and (100) GaAsSb.     

Rate-limiting mechanisms in high-temperature growth of catalyst-free InAs nanowires with large thermal stability

We identify the entire growth parameter space and rate-limiting mechanisms in non-catalytic InAs nanowires (NWs) grown by molecular beam epitaxy. Surprisingly huge growth temperature ranges are found with maximum temperatures close to ~600°C upon dramatic increase of V/III ratio, exceeding by far the typical growth temperature range for catalyst-assisted InAs NWs. Based on quantitative in situ line-of-sight quadrupole mass spectrometry, we determine the rate-limiting factors in high-temperature InAs NW growth by directly monitoring the critical desorption and thermal decomposition processes of InAs NWs. Both under dynamic (growth) and static (no growth, ultra-high vacuum) conditions the (111)-oriented InAs NWs evidence excellent thermal stability at elevated temperatures even under negligible supersaturation. The rate-limiting factor for InAs NW growth is hence dominated by In desorption from the substrate surface. Closer investigation of the group-III and group-V flux dependences on growth rate reveals two apparent growth regimes, an As-rich and an In-rich regime defined by the effective As/In flux ratio, and maximum achievable growth rates of > 6 μm h(-1). The unique features of high-T growth and excellent thermal stability provide the opportunity for operation of InAs-based NW materials under caustic environment and further allow access to temperature regimes suitable for alloying non-catalytic InAs NWs with GaAs.     

Correlated micro-photoluminescence and electron microscopy studies of the same individual heterostructured semiconductor nanowires

To correlate optical properties to structural characteristics, we developed a robust strategy for characterizing the same individual heterostructured semiconductor nanowires (NWs) by alternating low temperature micro-photoluminescence (μ-PL), low voltage scanning (transmission) electron microscopy and conventional transmission electron microscopy. The NWs used in this work were wurtzite GaAs core with zinc blende GaAsSb axial insert and AlGaAs radial shell grown by molecular beam epitaxy. The series of experiments demonstrated that high energy (200 kV) electrons are detrimental for the optical properties, whereas medium energy (5-30 kV) electrons do not affect the PL response. Thus, such medium energy electrons can be used to select NWs for correlated optical-structural studies prior to μ-PL or in NW device processing. The correlation between the three main μ-PL bands and crystal phases of different compositions, present in this heterostructure, is demonstrated for selected NWs. The positions where a NW fractures during specimen preparation can considerably affect the PL spectra of the NW. The effects of crystal-phase variations and lattice defects on the optical properties are discussed. The established strategy can be applied to other nanosized electro-optical materials, and other characterization tools can be incorporated into this routine.     

Effect of carbon tetrabromide on the morphology of GaAs nanowires

Carbon is a commonly used p-type dopant in planar III-V semiconductors, however its use in nanowire (NW) growth has been much less reported. In this work we show that the morphology of gold assisted GaAs NWs can be strongly modified by the presence of CBr(4) vapor during growth by metalorganic vapor phase epitaxy. GaAs NWs were grown under conditions which result in strong tapering and lateral growth at low growth temperatures by the use of triethylgallium (TEGa) instead of the more usual precursor, trimethylgallium (TMGa). Under these conditions, NWs grown in the presence of CBr(4) exhibit higher axial and lower radial growth rates, and negligible tapering compared with NWs grown in the absence of CBr(4) under the same conditions. We attribute this primarily to the suppression of the 2d growth rate by CBr(4), which enhances the axial growth rate of the nanowires. NWs grown with CBr(4) show stacking-fault-free zincblende structure, while the NWs grown without CBr(4) show a high density of stacking faults. This work underlines the striking effects which precursor chemistry can have on nanowire morphology.     

Facile synthesis and growth mechanism of Ni-catalyzed GaAs nanowires on non-crystalline substrates

GaAs nanowires (NWs) have been extensively explored for next generation electronics, photonics and photovoltaics due to their direct bandgap and excellent carrier mobility. Typically, these NWs are grown epitaxially on crystalline substrates, which could limit potential applications requiring high growth yield to be printable or transferable on amorphous and flexible substrates. Here, utilizing Ni as a catalytic seed, we successfully demonstrate the synthesis of highly crystalline, stoichiometric and dense GaAs NWs on amorphous SiO(2) substrates. Notably, the NWs are found to grow via the vapor-solid-solid (VSS) mechanism with non-spherical NiGa catalytic tips and low defect densities while exhibiting a narrow distribution of diameter (21.0 ± 3.9 nm) uniformly along the entire length of the NW (>10 μm). The NWs are then configured into field-effect transistors showing impressive electrical characteristics with I(ON)/I(OFF) > 10(3), which further demonstrates the purity and crystal quality of NWs obtained with this simple synthesis technique, compared to the conventional MBE or MOCVD grown GaAs NWs.     

Self-assembly of conductive Cu nanowires on Si(111)'5 × 5 '-Cu surface

Upon room-temperature deposition onto a Cu/Si(111)'5 × 5' surface in ultra-high vacuum, Cu?atoms migrate over extended distances to become trapped at the step edges, where they form Cu?nanowires (NWs). The formed NWs are 20-80?nm wide, 1-3?nm high and characterized by a resistivity of ～8?μΩ?cm. The surface conductance of the NW array is anisotropic, with the conductivity along the NWs being about three times greater than that in the perpendicular direction. Using a similar growth technique, not only the straight NWs but also other types of NW-based structures (e.g. nanorings) can be fabricated.     

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.