ARTICLE pubs.acs.org/JPCC
Composition and Phase Tuned InGaAs Alloy Nanowires Chan Su Jung, Han Sung Kim, Gyeong Bok Jung, Kang Jun Gong, Yong Jae Cho, So Young Jang, Chang Hyun Kim, Chi-Woo Lee, and Jeunghee Park* Department of Chemistry, Korea University, Jochiwon 339-700, Korea
bS Supporting Information ABSTRACT: InxGa1xAs (0 e x e 1) alloy nanowires (average diameter = 50 nm) were synthesized at 800 C with complete composition tuning by the thermal evaporation of GaAs/InAs powders. X-ray diffraction and Raman spectroscopy confirmed the complete composition tuning over the whole range. They exhibit exclusively a superlattice structure composed of zinc blende phase twinned octahedral slice segments having alternating orientations along the axial [111] direction and wurtzite phase twin planes. When the mole fraction (x) approaches 0.5, the period of the twinned superlattice structures becomes shorter, showing a controlled wurtzitezinc blende polytypism. At x = 0.5, the wurtzite phase is dominant with the shortest superlattice periodicity (∼2 nm). The smaller diameter consistently induces shorter periodic superlattice structures. This unique polytypism shows that the incorporation of In (or Ga) and the smaller diameter promotes the crystallization of the nanowires in the wurtzite phase. These InxGa1xAs nanowires produce an efficient THz emission, showing minimized carrier mobility at x = 0.5, where the superlattice stacking faults are maximized.
1. INTRODUCTION Since the discovery of carbon nanotubes (CNTs), a tremendous amount of research has been conducted on the synthesis and utilization of one-dimensional (1D) nanostructures (nanorods, nanowires, nanobelts, etc.) as well-defined building blocks for future nanodevices using “bottom-up” approaches.1,2 Various potential applications, such as high-performance optoelectronic devices, field-effect transistors, logic circuits, nonvolatile memories, and biosensors, were successfully demonstrated in certain pioneering works.35 Recently, the designing of various typed 1D heteronanostructures, such as coaxial coreshells, axially composition-modulated superlattices, and multicomposition alloy nanowires, has gained much interest, due to their unique and fascinating optoelectronic properties.612 In particular, alloy nanowires offer the advantage of a tunable band gap, which can be achieved by adjusting the relative composition.11,12 The synthesis of alloy nanowires in a controlled manner would, therefore, broaden the application range to more advanced and integrated nanoscaled optoelectronic devices. As one of the important ternary alloy semiconductors, the InxGa1xAs ternary alloy is of great interest, because its band gap (Eg) can be tuned in the range from the near-infrared (NIR) to IR region (0.36 e Eg e 1.42 eV). InxGa1xAs alloy nanowires could be used in a wide range of potential applications, such as IR emission lasers and detectors and photovoltaics or solar cells.1321 In an effort to synthesize InGaAs NWs, there have been a number of reports on metalorganic chemical vapor deposition (MOCVD) techniques.2229 However, due to their complex growth process, there remains considerable difficulty in obtaining a uniform composition along the whole NW along with tuning r 2011 American Chemical Society
across the entire compositional range from x = 0 to 1. Therefore, developing new methods of synthesizing InGaAs NWs that allow the complete control of not only their composition but also their morphology and crystal structure uniformity, is still quite a challenging subject. Herein, we synthesized InxGa1xAs (0 e x e 1) alloy NWs with completely tuned compositions by a simple thermal evaporation method. The composition tuning was successfully achieved by changing the ratio of the GaAs and InAs powder sources. Their crystal and electronic structures as a function of the composition were thoroughly examined using high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. We found a unique wurtzitezinc blende (WZZB) polytypic superlattice structure that can be controlled by their composition and diameter. For the first time, we detected a terahertz (THz) emission (in the range of 02 THz) from these InGaAs NWs. THz emission spectroscopy is a powerful technique for studying ultrafast charge carrier dynamics in semiconductors. The THz emission spectra of the InGaAs bulk were investigated through theoretical and experimental approaches, but not for the NWs.30,31 The analysis of the THz radiation of the present NWs can provide insight into the carrier mobility of semiconductor nanostructures.
Received: October 11, 2010 Revised: March 28, 2011 Published: April 06, 2011 7843
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Figure 1. (a) SEM micrograph of high-density In0.3Ga0.7As NWs homogeneously grown on the substrate (scale bar = 5 μm). (b) TEM image revealing their general morphology (average diameter = 50 nm). The scale bar represents 100 nm. (c) HRTEM images of a selected NW revealing its twinned superlattice structure (scale bar = 20 nm for the left image and 2 nm for the magnified image on the right). The distance between the adjacent (111) planes is 3.3 Å. FFT images of (i)(iii) correspond to those of the separated twin segments and the twin planes (insets). The zone axis is indexed as [011]. (d) HRTEM images of the 30 turn morphology and their corresponding FFT images (inset), showing a single-crystalline nature at the zone axis of [112]. (e) HRTEM images of the 60 turn morphology and their corresponding FFT images (insets, zone axis = [101]), showing its twinned superlattice structure. (f) TEM images for selected InxGa1xAs NWs, where x = 0.4, 0.5, and 0.6, showing the progressive development of stacking faults as x approaches 0.5. The scale bar represents 20 nm.
2. EXPERIMENTAL SECTION GaAs (99.999%, Sigma-Aldrich) and InAs (99.99%, SigmaAldrich) powders were placed separately in two quartz boats loaded inside a quartz tube reactor. A silicon substrate, on which a 3 nm thick Au film was deposited, was positioned at a distance of 10 cm away from the GaAs/InAs powder source. Argon gas was continuously flowed at a rate of 500 sccm during the synthesis. The temperature of the powder sources was set to 1000 C and that of the substrate was approximately 800 C. The composition was controlled using the ratio of GaAs and InAs powders. The structure and composition of products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700), field-emission transmission electron microscopy (TEM, FEI TECNAI G2 200 kV), high-voltage TEM (HVEM, JEOL JEM ARM 1300S, 1.25 MV), and energy-dispersive X-ray fluorescence spectroscopy (EDX).
High-resolution XRD patterns were obtained using the 8C2 and 3C2 beamlines of the Pohang Light Source (PLS) with monochromatic radiation (λ = 1.54520 Å). Synchrotron XPS measurements were performed at the U7 beamline of the PLS. XPS (ESCALAB 250, VG Scientifics) using a photon energy of 1486.6 eV (Al KR) was also employed to investigate the electronic states. The Raman spectra were measured using the 514.5 nm line of an argon ion laser. The THz emission (at room temperature) from the as-grown NW samples (on the Si substrates with an area of 5 mm 5 mm) was measured in reflection geometry, using a Ti:sapphire laser as a light source, operating at a wavelength of 800 nm with a repetition rate of 76 MHz and a pulse width of 190 fs.32 The polarization was parallel to the plane of incidence for best pump absorption, using a half-wave plate. The laser beam passes through 7844
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Figure 2. (a) TEM image revealing the general morphology of the In0.4Ga0.6As NW. The scale bar represents 100 nm. (b) The zigzagged morphology of the selected NW (diameter = 60 nm) changes to a (c) straight one upon consecutive 30 tilts around the wire axis. The scale bar represents 20 nm. The lattice-resolved images (scale bar = 2 nm) of the 0- and 30-turn morphologies and their corresponding FFT images are shown in the insets. The zone axis is indexed as [011] for the 0 turn and [112] for the 30 turn. The distance between the adjacent (111) planes is 3.4 Å. At the [011] zone axis, FFT images (i) and (iii) correspond to those of the twin segments, and (ii) corresponds to the twin plane region. (d) TEM images of the In0.4Ga0.6As NWs having diameters of 40 and 120 nm, revealing the diameter-dependence period of the twin segments. The scale bar represents 20 nm.
a half-wave plate, after being reflected by mirrors, and is split into a pump beam and a probe beam by a beam splitter. The pump beam goes through a chopper, while the probe beam is delayed with respect to the pump beam by means of a scanning optical delay line. A variable attenuator is used for the pump beam to vary the excitation power. The laser spot size is approximately 3 mm at the surface of the sample. The THz radiation from the surface of the sample oriented at an angle of incidence of 45 is collected and focused onto the detector using a pair of parabolic mirrors. The detection antenna was a Hertzian dipole antenna, which had a gap of 5 μm on low-temperature-grown GaAs (LT-GaAs). The optical power (probe beam) incident on the LT-GaAs receiver is 5 mW.
3. RESULTS AND DISCUSSION We synthesized a whole composition range of InxGa1x As alloy NW samples with high yield. The growth mechanism of the present NWs follows the typical vaporliquidsolid (VLS) mechanism that makes use of Au catalytic nanoparticles (NPs). The average diameter of the NWs is 50 nm over all the composition range. The diameter distribution of each sample is plotted in the Supporting Information, Figure S1. The XRD was used to determine the mole fraction of In element (x) in each sample. Figure 1a shows an SEM micrograph of the highdensity In0.3Ga0.7As NWs grown on the substrates. The TEM image reveals their straight and smooth surface (Figure 1b). The high-resolution TEM (HRTEM) image of a selected NW
(diameter = 50 nm) reveal its ZB phase twinned segments having various lengths in the range of 1080 nm with an average value of 50 nm (Figure 1c). It shows a d spacing between neighboring (111) planes of ca. 3.3 Å, which is consistent with the value calculated by Vegard’s rule using the lattice parameters of bulk GaAs (JCPDS Card No. 80-0016; a = 5.654 Å) and InAs (JCPDS Card No. 73-1984; a = 6.036 Å). Insets show its fast Fourier transform (FFT) images: (i) and (iii) for the individual segment and (ii) for the twin plane region of the segments. The two zigzagged segments share the [111] and [111] spots at the zone axis of [011]; that is, they have common (111) twin planes. We turned the TEM grid holder in order to tilt the NW around the axial direction. Figure 1d corresponds to the TEM image at a tilt of 30, showing the single-crystalline feature without any twin planes. The zone axis becomes [112], where the twinned spots cannot be seen (inset). After further tilting the NW by 30, the straight surface turns into a zigzagged one (Figure 1e). At this [101] zone axis, the alternating orientation of the twin segments appears, as shown in the twin spots of the (i)(iii) regions (insets). Upon each 30 sequential rotation, the surface morphology changes from zigzagged to straight and then zigzagged again. The [111] axial direction remains the same for each rotation. This unique sequential change in the morphology and FFT images upon the tilts indicates that the NW consisted of the ZB phase octahedral slice segments having alternate orientations along the axial [111] direction, although the lengths of the segments are not uniform.33 Figure 1f shows the TEM images of the typical 7845
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Figure 3. (a) HRTEM images (scale bar = 20 nm for the left image and 2 nm for the magnified image on the right) of a selected In0.5Ga0.5As NW (diameter = 40 nm), showing the superlattice structure having a periodicity of 1.7 nm (marked in white lines). The zone axis of the SAED pattern (inset) is indexed as WZ [2110] (marked in green color) and ZB [011] (marked in red/yellow color). (b) When tilted by 30, the TEM image and its corresponding SAED pattern (inset) show a single-crystalline WZ [1100] or ZB [112] phase. (c) The 60 turn recovers the stacking faults and two sets of SAED patterns corresponding to the WZ [1210] and ZB [101] zone axes. The distance between the adjacent WZ (002) (or ZB (111)) planes is 3.4 Å. (d) TEM image of another In0.5Ga0.5As NW (diameter = 90 nm) for 0 and 30 turns, and the zone axis of the SAED pattern is indexed as WZ [2110]/ZB [011] and WZ [0110] (or ZB [112]), respectively. The scale bar represents 20 nm.
InxGa1xAs NWs (diameter = 60 nm), where x is 0.4, 0.5, and 0.6. When the In mole fraction (x) approaches 0.5, more stacking faults develop perpendicular to the wire axis, producing the shorter periodic twinned superlattice structure. The respective average length of the segments is 20, 2, and 50 nm. The nearly same length is observed for x = 0.3 and 0.6. Negligible segments appear at the ranges of x e 0.2 and x g 0.7. The average value of the segment length as a function of diameter and composition is summarized in the Supporting Information, Table S1. Figure 2a shows that nearly all of the In0.4Ga0.6As NWs have uniform stacking faults with long-range order along the wire axis. The TEM images show sequential zigzagstraight morphologies for each 30 turn (Figure 2b,c). The twin segments appear alternately with a period of 1040 nm (avg = 20 nm), which is shorter than that of the In0.3Ga0.7As ones (50 nm). The zone axis is [011] for the 0 turn and [112] for the 30 turn (insets). The alternating orientation of the twin segments is evident at the zone axis of [011], where the two segments have common (111) twin planes. The distance between the adjacent (111) planes is 3.4 Å. We examined the TEM images of the other diameter NWs in the same composition and found that, as the diameter increases from 40 to 120 nm, the average length of the twin segments increases from 15 to 40 nm, as shown in Figure 2d. Figure 3a shows the TEM images of the In0.5Ga0.5As NWs (diameter = 40 nm), revealing bright and dark striped regions whose period is as short as ∼2 nm. New WZ phase diffraction spots (marked in green color) overlap weak ZB phase twin spots
(marked in red/yellow color). It shows a d spacing between neighboring WZ (002) planes of 3.4 Å, which is consistent with the value of In0.5Ga0.5As calculated by Vegard’s rule using the lattice parameters of bulk WZ GaAs (JCPDS Card No. 80-0003; a = 3.914 Å, c = 6.441 Å) and WZ InAs (a = 4.27 Å; c = 7.02 Å).34 The zone axis of the corresponding selected-area ED (SAED) pattern is indexed as [2110] for the WZ phase and [011] for the ZB phase. The density of the planar defects (twins and/or stacking faults) increases significantly along the WZ Æ0001æ (or ZB Æ111æ) directions, following the mixing of the two phases. The two basic WZ [0002] ED spots were subdivided into five equal parts; the superlattices have a periodicity of 1.7 nm, which coincides with five times the (002) planes. This means that four (001) planes of the WZ structure and one (111) plane of the ZB structure are stacked alternately in the growth direction. This indicates that the WZ phase is dominant with ∼80%. Figure 3b corresponds to the TEM image at a tilt of 30, showing no stacking faults. The zone axis of the SAED pattern becomes WZ [1100] (or ZB [112]) (inset). When the NW is tilted by 60, the stripe morphology appears again with two sets of SAED patterns corresponding to the WZ [1210] and ZB [101] zone axes (Figure 3c). For the larger diameter NWs (90 nm), which we found in the same composition sample, the TEM images at various tilt angles also exhibited the same change in their morphology and ED pattern (Figure 3d). The EDX elemental mapping and line scanning for selected In0.1Ga0.9As, In0.5Ga0.5As, and In0.8Ga0.2As NWs are shown in 7846
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Figure 4. STEM image and EDX elemental mapping of Ga, In, As, and O for selected (a) In0.1Ga0.9As (scale bar = 30 nm), (b) In0.5Ga0.5As (scale bar = 20 nm), and (c) In0.8Ga0.2As NWs (scale bar = 50 nm) and their corresponding EDX line-scanning profiles. The outerlayers consisted of Ga and O elements.
Figure 5. (a) Full-range XRD patterns taken from the InxGa1xAs NWs and (b) the magnified scaled (111) peaks. The data points (open circles) of the (111) peaks are resolved into two peaks (dotted or dashed lines) using a Voigt function, corresponding to the GaAs-like (blue) and InAs-like (red) composition phases. The black line represents the sum of the resolved bands.
Figure 4. Their corresponding high-angle annular dark-field (HAADF) scanning TEM (STEM) images are also shown. They are usually sheathed with amorphous outerlayers (thickness < 10 nm), forming a coreshell structure. In and As elements are distributed at the core part, while Ga element is distributed over the whole of the coreshell parts. The distribution of oxygen (O) element indicates that the shell parts consisted of amorphous
gallium oxide. The thickness of these gallium oxide outerlayers decreases as x increases. The preference for gallium oxide outerlayers would be due to the higher binding energy of the smaller radius Ga3þ ion than the In3þ ion toward the O2 anions. The EDX data of an individual nanowire for each of the samples are shown in the Supporting Information, Figure S2. The fine-scanned Ga and In 2p XPS peaks confirm the existence of gallium oxide outerlayers sheathing the GaInAs NWs (Supporting Information, Figure S3). Because of these gallium oxide outerlayers, the composition of each sample was determined using the XRD, as described in the following paragraph. Figure 5a shows the high-resolution XRD pattern of the InxGa1xAs NWs. The peaks of the GaAs and InAs NWs exactly match those of the bulk ZB GaAs and InAs, respectively. As the composition varies, the diffraction angle shifts continuously from that of GaAs (InAs) to that of InAs (GaAs). Figure 5b displays the (111) peak in a magnified scale. As x increases, the peak becomes broader and then narrower, showing increasing-todecreasing behavior. As shown in the TEM images, the incorporation of heteroatoms (Ga or In) induced a disordered crystalline phase, such as twinned superlattice structures with a higher periodicity, which increases the XRD peak width. The largest peak broadening at x = 0.5 is well correlated with the maximum lattice disorder that is produced at this composition, due to the period of the twin segments being the shortest. To calculate exactly the lattice constant and composition, we resolved the asymmetric (111) peak into two bands corresponding to the Ga-rich and In-rich compositions (hereafter referred to as “GaAs-like” and “InAs-like”, respectively) by means of a Voigt function. The observed band can be considered as the product of a Lorentzian natural line-shape function and a Gaussian instrument function. The composition (x1 and x2) of two components was calculated using Vegard’s rule. From the composition and 7847
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Figure 6. (a) Fourier transform frequency-domain spectra of the InxGa1xAs (x = 0, 0.05, 0.3, 0.5, 0.8, and 1) NWs. (b) Peak intensity of the frequencydomain THz emission spectrum as a function of the In mole fraction (x).
the fraction (f1 and f2) of the resolved bands, we estimated the composition (x) of each sample, using the mean equation x = x1f1 þ x2f2 (see the Supporting Information, Table S2). The Raman spectroscopy measurements were carried out with the 514.5 nm line of an Ar ion laser. We confirmed full-range composition tuning of the InxGa1xAs NWs, as shown in the Supporting Information, Figure S4. The present GaAs and InAs NWs (with average diameters of 50 nm) have ZB phase structures and, in general, contain negligible stacking faults. As they formed the InGaAs alloy, they exhibit noticeably the twinned superlattice structures, in which a twin plane in a ZB NW consisted of a monolayer of the WZ phase. Twin planes and, more generally, planar stacking faults are commonly found along the [111] growth direction of ZB IIIV NWs grown by the VLS mechanism. The formation of WZZB polytypic superlattices in the GaP, GaAs, InP, and InAs NWs has been extensively investigated in a number of publications, where dependence on temperature, diameter, V/III reactant ratio, dopant, and substrate was considered.3551 Theoretical models have identified the basic driving force for WZ formation as its lower surface energy of the parallel side facets and the interface energies at the solidliquidvapor three phase lines.52,53 The more frequent formation of the WZ phase layers (as the twin planes) in the smaller diameter InGaAs NWs, is consistent with other IIIV NWs synthesized by MOCVD.3551 Akiyama et al. calculated the relative stability between WZ and ZB phases and showed the WZ phase NW stabilized for a small diameter, due to their lower surface energies of the {110} side facets.52 Later theoretical models based on nucleation predict that WZ nucleation is favored at high liquid supersaturation, producing preferentially the smaller diameter NWs.53 Bulk GaAs and InAs have a ZB phase, because the free energy is lower for this structure than those with the WZ phase.54 Therefore, we expect that the larger diameter induces the formation of the thermodynamically stable ZB phase InGaAs. However, due to a subtle WZZB energy difference (i.e., 12.02 meV/atom for GaAs and 5.2 meV/atom for InAs) and their atomically identical ((0001) and ({111} facets, the thermodynamically unstable WZ phase becomes allowed for the smaller diameter NWs, due to the lower surface energies.52 Remarkably, as x approaches 0.5, the length of the ZB phase twin segments significantly decreases. At x = 0.5, the major phase transforms from ZB to WZ. Here, we tentatively suggest a possibility as follows. The incorporation of In (Ga) into the GaAs (or InAs) lattice would produce the disorders at the atomic
level, which leads the WZ phase NW growth that can reduce their surface energies. The maximized atomically disorders at the same mole fraction (x = 0.5) would drive more favorably the crystallization through the WZ phase. Further theoretical studies will be required to provide the mechanism behind this energy change of the crystal structure upon the composition tuning. Kim et al. synthesized InxGa1xAs (x = 00.23) NWs at 450 C using MOCVD and showed that a large x value results in a higher growth rate and larger base diameter, due to the fact that the diffusion length is longer for In than for Ga.22 On the contrary, Sato et al. reported that a higher Ga/In ratio induces a higher growth rate and smaller diameter.25 They explained that, at a high growth temperature (650 C), the In atoms are easily evaporated from the SiO2 mask, while the Ga atoms are incorporated into the NWs. Unfortunately, our InxGa1xAs NWs are not vertically grown on the substrates so that their growth rate cannot be directly determined from the length. More studies need to reveal the correlation between the growth rate and the composition in the future. The time-domain THz emission from the as-grown NWs was measured at room temperature by irradiating them with 800 nm fs Tisapphire laser pulses at maximum power (450 mW). There is a negligible THz emission from the Si substrates under our experimental conditions. Because the InGaAs NWs are oriented randomly on the substrates, it is difficult to obtain the relative quantification of the THz emission from such uneven surface samples. Therefore, we carefully prepared 56 samples (5 mm 5 mm area) that keep the same doping level and same density/morphology NWs and obtained the averaged value as presented here. Figure 6a shows the corresponding Fourier transform frequency-domain THz emission spectrum (02 THz) from the as-grown InxGa1xAs NWs. The intensity and width of the THz emission depend strongly on the composition. In particular, the In0.5Ga0.5As NWs show the weakest and narrowest THz emission. Figure 6b plots the THz peak intensity as a function of x, showing a decrease in the range of 0 e x e 0.5, a minimum at x = 0.5, and an increase thereafter. The semiconducting nanostructures were suggested for use as efficient THz radiation emitters, due to the collective oscillations of the conductive electrons confined to the surface of the nanostructure, or so-called localized surface plasmons (LSPs).32,5557 The THz emission of all of these InGaAs NWs would be produced by such an LSP enhanced field. It is commonly understood that the THz emission from a semiconductor surface originates from either nonlinear optical rectification or ultrafast 7848
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The Journal of Physical Chemistry C surface surge currents.5862 In the surge-current model, the acceleration of the photoexcited carriers by the intrinsic surface-depletion field or the photo-Dember field causes the THz emission. For bulk GaAs, the THz emission usually originates from the surface-depletion field and optical rectification.58 The THz emission of the InAs was attributed to the photo-Dember field and optical rectification.61,62 There are theoretical and experimental studies showing that the THz emission intensity of InxGa1xAs increases with increasing In fraction, due to the increase in the carrier mobility.30,31 Ko et al. reported that the THz emission from InxGa1xAs (0.35 < x < 1) polycrystalline films increases with increasing x.31 They suggested that the surface-depletion field is dominant in the range of 0.35 e e 0.57, whereas optical rectification is dominant in the range of 0.57 e x < 1. The THz emission intensity due to optical rectification increases with polycrystal grain size. In the present InxGa1-xAs NWs, the THz emission increases in the range of 0.5 e x e 1, which is consistent with previous studies on the bulk crystal.30,31 If the optical rectification dominates in this composition range, the enhancement of the THz emission with increasing x would be due to the increased fraction of the ZB phase segments. According to the surfacedepletion field mechanism, which can be applicable to the range of 0 e x e 0.5, the intensity of the THz emission comes from the accelerated carriers driven by the conduction band bending at the surface and, thus, is proportional to the carrier mobility. The carrier mobility will decrease as the crystalline disorder increases. It is known that stacking faults of twinned superlattices can significantly affect the electronic properties.6366 The wave function is discontinuous at a stacking fault, which leads to the reduced mobility of the charge carriers. Therefore, the minimum THz intensity at x = 0.5 results from the more significant crystalline disorder due to the shortest periodic superlattice structures, which could be distinctive from the case of the bulk. On the other hand, the lower intensity in the range of 0 e x e 0.5 (compared to that in the range of 0.5 e x e 1) can be explained by the more significant oxide outerlayers. We experimentally demonstrated for the first time that the superlattice structure reduces the carrier mobility by means of the THz emission.
4. CONCLUSION Completely composition tuned InxGa1xAs (0 e x e 1) NWs (average diameter = 50 nm) were grown at 800 C by the thermal evaporation of GaAs/InAs powders. The TEM and ED patterns at sequential tilt angles revealed their WZZB polytypic superlattice structure that consists of ZB phase twinned octahedral slice segments having alternating orientations along the axial [111] direction and WZ phase twin planes. As x approaches 0.5, the period of the twinned superlattice structures becomes shorter, showing a unique, controlled WZZB polytypism. At x = 0.5, the major phase transforms from ZB to WZ, with the shortest superlattice periodicity of 2 nm. This WZZB polytypism indicates that the incorporation of In (or Ga) of GaAs (or InAs) NWs promotes the crystallization of the nanowires in the WZ phase. The smaller diameter induces the shorter periodic superlattice structures, which is consistent with other IIIV NWs synthesized by MOCVD. XRD and Raman spectroscopy confirmed the complete composition tuning over the whole range and the maximization of the crystalline defects at x = 0.5. Upon their excitation by 800 nm fs Tisapphire laser pulses, the LSP enhanced field
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allows these InxGa1xAs NWs to produce an efficient THz emission. The THz emission of the InxGa1xAs NWs exhibits a minimum intensity at x = 0.5, corresponding to the lowest carrier mobility. This strong correlation with the twinned superlattice structures and mobility would provide insight into the effects of the structural defects on the nanodevices.
’ ASSOCIATED CONTENT
bS
Supporting Information. Segment length, XRD peak analysis, diameter distribution, EDX, XPS, and Raman spectroscopy data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This study was supported by the NRF (2009-82528; 20100029164), ITRC (2008-C1090-0804-0013), and WCU program (R31-10035). The HVEM and XPS (Pusan) measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH. ’ REFERENCES (1) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435–445. (2) Yang, P.; Yan, R.; Fardy, M. Nano Lett. 2010, 10, 1529–1536. (3) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K.-H.; Lieber, C. M. Science 2001, 294, 1313–1317. (4) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617–620. (5) Patolsky, F.; Zheng, G.; Hayden, O.; Lakadamyali, M.; Zhuang, X.; Lieber, C. M. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 14017–14022. (6) Bj€ork, M. T.; Ohlson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Nano Lett. 2002, 2, 87–89. (7) Wu, Y.; Fan, R.; Yang, P. Nano Lett. 2002, 2, 83–86. (8) Qian, F.; Li, Y.; Gradecak, S.; Wang, D.; Barrelet, C. J.; Lieber, C. M. Nano Lett. 2004, 4, 1975–1979. (9) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M. Nature 2002, 420, 57–61. (10) Xiang, J.; Lu, W.; Hu, Y.; Wu, Y.; Yan, H.; Lieber, C. M. Nature 2006, 441, 489–493. (11) Kuykendall, T.; Ulrich, P.; Aloni, S.; Yang, P. Nat. Mater. 2007, 6, 951–956. (12) Lim, S. K.; Tambe, M. J.; Brewster, M. M.; Silvija, G. Nano Lett. 2008, 8, 1386–1392. (13) Geels, R. S.; Corzine, S. W.; Coldren, L. A. IEEE J. Quantum Electron. 1991, 27, 1359–1367. (14) Chang-Hasnain, C. J.; Harbison, J. P.; Hasnain, G.; Von Lehmen, A. C.; Florez, L. T.; Stoffel, N. G. IEEE J. Quantum Electron. 1991, 27, 1402–1409. :: (15) Unl€u, M. S.; Strite, S. J. Appl. Phys. 1995, 78, 607–639. (16) Huffaker, D. L.; Park, G.; Zou, Z.; Shchekin, O. B.; Deppe, D. G. Appl. Phys. Lett. 1998, 73, 2564–2566. (17) Ribordy, G.; Gautier, J.-D.; Zbinden, H.; Gisin, N. Appl. Opt. 1998, 37, 2272–2277. (18) Kim, S.; Mohseni, H.; Erdtmann, M.; Michel, E.; Jelen, C.; Razeghi, M. Appl. Phys. Lett. 1998, 73, 963–965. 7849
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