Letter pubs.acs.org/NanoLett
Rectifying Single GaAsSb Nanowire Devices Based on Self-Induced Compositional Gradients Junghwan Huh,† Hoyeol Yun,‡ Dong-Chul Kim,*,†,§ A. Mazid Munshi,†,§ Dasa L. Dheeraj,†,§ Hanne Kauko,∥ Antonius T. J. van Helvoort,∥ SangWook Lee,‡ Bjørn-Ove Fimland,† and Helge Weman*,† †
Department of Electronics and Telecommunications, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway ‡ School of Physics, Konkuk University, Seoul 143-701, Korea § CrayoNano AS, Otto Nielsens vei 12, NO-7052, Trondheim, Norway ∥ Department of Physics, Norwegian University of Science and Technology (NTNU), NO-7491, Trondheim, Norway S Supporting Information *
ABSTRACT: Device configurations that enable a unidirectional propagation of carriers in a semiconductor are fundamental components for electronic and optoelectronic applications. To realize such devices, however, it is generally required to have complex processes to make p−n or Schottky junctions. Here we report on a unidirectional propagation effect due to a self-induced compositional variation in GaAsSb nanowires (NWs). The individual GaAsSb NWs exhibit a highly reproducible rectifying behavior, where the rectifying direction is determined by the NW growth direction. Combining the results from confocal micro-Raman spectroscopy, electron microscopy, and electrical measurements, the origin of the rectifying behavior is found to be associated with a self-induced variation of the Sb and the carrier concentrations in the NW. To demonstrate the usefulness of these GaAsSb NWs for device applications, NW-based photodetectors and logic circuits have been made. KEYWORDS: Self-catalyzed GaAsSb nanowires, self-induced compositional variation, rectifying behavior, molecular beam epitaxy, photodetectors, logic circuits
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and optoelectronic properties, whereas applying them in devices has not been reported yet. In the case of bulk and thin film Sb-based III−V semiconductors, the electronic and optoelectronic properties are strongly dependent on the Sb concentration and Sb-related point defects such as antisite defects and vacancies.15,16 Thus, it is important to understand and control both local variations of the Sb concentration as well as Sb-related point defects introduced during the NW synthesis in order to develop reliable Sb-based NW devices. Here, we demonstrate that single GaAsSb NWs can have a consistent self-induced variation in the Sb concentration and carrier density that results in a specific and reproducible rectifying behavior. Such intrinsic rectifying behavior can be utilized to achieve various functional nanodevices despite a simple configuration. The highly reproducible and reliable rectifying behavior is observed in GaAsSb NW devices with two identical electrodes where the direction of the rectifying behavior is given by the NW growth direction. Finally, we
emiconductor nanowires (NWs) have demonstrated unique capabilities for nanoscale electronic1 and optoelectronic devices2 and solar cells.3 To realize and further improve such NW-based devices, however, it is generally required to have complex growth structures and device fabrication processes. For example, axial4 or radial5 NW doping, asymmetric contacts,6 or gate structures7 are needed to fabricate for example p−n junction diode, Schottky diode, or field effect transistor (FET) NW devices. Such complex fabrication routes hamper NW device developments, limiting the realization of potential applications. In addition, NW device processing is in its infancy compared to that of conventional bulk or thin film materials. Therefore, easy and simple processes in the growth and fabrication of NW devices should be considered.8 Among various semiconductor NW materials, Sb-based ternary III−V NWs have received a great deal of attention in recent years for their potential in a variety of applications, such as field effect transistors (FETs),9,10 tunnel diodes,11 and optical detectors.12 However, reports on GaAsSb NWs have been mostly concerned with NW growth13,14 and detailed studies aimed at simultaneously understanding their electronic © XXXX American Chemical Society
Received: January 8, 2015 Revised: April 23, 2015
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DOI: 10.1021/acs.nanolett.5b00089 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters show that the GaAsSb NWs can serve as simple and effective building blocks in both optoelectronic and electronic devices by demonstrating photodetectors and OR/AND logic gates, respectively. Morphology of the GaAsSb NWs. The GaAsSb NWs were grown on Si(111) substrates by self-catalyzed vapor− liquid−solid (VLS) technique using molecular beam epitaxy (MBE) (see Methods section). The morphology and the crystallographic properties of the GaAsSb NWs were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses (Figure 1). The average diameter and length of the NWs are estimated to be around 180 nm and 2.5 μm, respectively, without any facet changes or tapering along the NW axis (Figure 1a,b). Dark-field TEM and selected area electron diffraction (Figure 1c,d) reveal that the GaAsSb NWs have a pure (i.e., twin-free) zinc blende crystal structure with [111]B as the growth direction, consistent with previous reports on Au-assisted GaAsSb NWs grown on GaAs substrates13 and self-catalyzed GaAsSb NWs grown on Si substrates.14,17
Figure 2. Rectifying behavior of GaAsSb NW devices. (a) Representative current−voltage (I−V) characteristics of a single GaAsSb NW device showing a rectifying behavior. (b) SEM image of the GaAsSb NW device with the Ga droplet marked by a dashed circle. The scale bar is 1 μm. (c) Schematic of the single GaAsSb NW device. The GaAsSb NW is placed on a SiO2/p+2-Si substrate and contacted with two identical metal electrodes. (d) A histogram of the current rectification ratio (IForward/IReverse) at ±3 V for 21 different NW devices. The distribution is fitted with a Gaussian curve.
to those obtained in heterojunction NW devices,18 radial p−n junction NW devices,19 and graphene diodes.20 To confirm the consistency of the rectifying behavior, 21 NW-devices were measured. All NW devices showed similar rectifying behaviors with the directionality determined by the orientation of the NW. Figure 2d summarizes the rectification ratio at a voltage range of ±3 V for all the measured NW devices. These results clearly show the highly reproducible and consistent rectifying characteristics in these intrinsically undoped GaAsSb NWs. The rectifying I−V characteristics are further investigated by considering the effect of the Ga-catalyst and the effect of contact annealing, which might have an influence on the rectifying behavior of the NW devices (Supporting Information Section 1, Figure S1). It is found that the NW orientationdependent rectifying behavior is not determined by such external factors (i.e., the position of the contacts, the contact quality, the NW top contact including the Ga droplet or not), but attributed to an intrinsic property of these GaAsSb NWs. Variations of Sb and Carrier Concentrations in the GaAsSb NW. To understand the origin of the intriguing electrical characteristics in terms of local structural and electronic properties along the NW, confocal micro-Raman spectroscopy was employed. A plot of Raman spectra measured along a single GaAsSb NW displays the optical phonon modes corresponding to GaSb-like transverse optical (TO), GaAs-like TO, and GaAs-like longitudinal optical (LO) modes (Figure 3a). These plots appear similar to a recent report for a series of GaAsSb NWs with varying Sb concentrations.17 All phonon modes show a red shift along the NW from the base to the top, as shown in Figure 3c. Analysis of the shift of Raman peaks in the GaAs1−ySby alloy system with the variation of the Sb mole fraction (y)17,21,22 indicates an increase of the average Sb mole fraction from the NW base to the NW top. Based on previous reports on GaAs1−ySby thin films22 and NWs,17 the average Sb mole fraction of the NW base (position 9 in Figure 3c) is estimated to be ∼30%, whereas that of the NW top (position 1 in Figure 3c) is ∼37%. The variation of the Sb mole fraction along the NWs is consistently confirmed by energy dispersive
Figure 1. Morphological and structural characterizations of GaAsSb NWs. (a) Tilted-view (20° tilt angle) SEM image of the self-catalyzed GaAsSb NWs grown on a Si(111) substrate. (b) Higher magnification SEM image of a single GaAsSb NW showing the presence of a Ga droplet at the NW tip and the hexagonal side-facets of the NW. (c) (002) Dark-field TEM image of a GaAsSb NW taken along the ⟨11̅0⟩ zone axis showing that the entire NW is free from structural defects. (d) Selected-area electron diffraction pattern taken from the middle part of the GaAsSb NW in (c), showing a twin-free zinc blende crystal phase.
Rectifying Behavior in Single GaAsSb NW Devices. Figure 2a shows representative I−V characteristics of a single GaAsSb NW device, and the SEM image of the device is shown in Figure 2b. The I−V characteristics of individual GaAsSb NW devices exhibit excellent and consistent rectifying behaviors in spite of using symmetric and identical contact electrodes. The direction of the current rectification is the same in all studied NWs. Here the forward (reverse) bias in the NW device denotes that a positive (negative) voltage is applied to the electrode near the NW base, while the other electrode near the NW top (close to the Ga droplet) is connected to ground (Figure 2c). The current rectification ratio (IForward/IReverse) is as high as ∼300 at a voltage range of ±1 V, which is comparable B
DOI: 10.1021/acs.nanolett.5b00089 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. Confocal micro-Raman spectroscopy for the estimation of Sb and carrier concentrations in the GaAsSb NWs. (a) Raman spectra of a GaAsSb NW, showing three main phonon modes corresponding to GaSb-like TO, GaAs-like TO, and GaAs-like LO. The Raman spectra were taken at different positions along the NW, as indicated in panel b. The distance between the measured positions is approximately 200 nm. (c) Raman peak shifts of the GaSb-like TO, GaAs-like TO, and GaAs-like LO phonon modes as a function of the measurement position. (d) Raman intensity ratio (GaAs-like LO/GaAs-like TO) plot as a function of measurement positions along the NW growth direction. Inset in (d): two-dimensional Raman mapping of the GaAs-like LO/GaAs-like TO intensity ratio for the GaAsSb NW. Error bars show the standard deviation based on five NWs.
radial variations are caused by an As for Sb exchange at the NW surface during growth causing outward diffusion of Sb or/and radial GaAs overgrowth.23,30 For the GaAsSb NWs in the present study, only As−Sb exchange at the NW surface during growth resulting in an outward diffusion of Sb (and a corresponding inward diffusion of As) needs to be considered. The NW sidewalls near the base will have endured the longest exposure to As−Sb exchange at the surface, thus causing more outward Sb diffusion and lower Sb concentration near the surface at the NW base as compared to at the NW top (Figure S2). The radial Sb diffusion process leads to the formation of Sb vacancies. Such Sb vacancies can convert into other point defects, which act as acceptors.15 Therefore, the acceptor density near the NW surface is expected to increase from the top to the bottom of the NW, consistent with the result of a higher carrier density at the NW bottom compared to at the NW top as indicated by the Raman spectra (Figure 3d). Influence of the Compositional Variation in GaAsSb NWs on Their Electrical Characteristics. To investigate the effect of the self-induced radial Sb compositional variation in the GaAsSb NW on its electrical properties, the spatial dependence of the conductance along the NW is measured using four contacts along the NW, as shown in Figure 4a and b. If we consider that the radial Sb diffusion process leads to an increase in the density of acceptors along the NW from the top to the bottom of the NW, then the conductance near the NW base should be higher than near the NW top. This is confirmed by I−V measurements, as shown in Figure 4b and Figure S5. From these electrical measurements, it can be concluded that the metal/NW contact near the NW top is more rectifying than the metal/NW contact near the NW base, as seen from Figure
X-ray spectroscopy (EDX) point scans along the NWs (Figure S2a and b), although with a lower estimated Sb mole fraction (∼20% at the NW base and ∼30% at the NW top).23 All studied NWs, in the here presented batch and GaAsSb NWs grown under other conditions (Supporting Information, Section 2), exhibit such decrease of the Sb mole fraction along the NW from top to base. Since the LO phonon mode in polar semiconductor materials is screened by charge carriers, giving rise to a reduction of the LO mode intensity,24,25 the intensity ratio between the LO and TO phonon modes (ILO/ITO) is inversely proportional to the local carrier concentration at the position probed by the laser. The GaAs-like TO phonon mode is used as a reference intensity as the TO mode intensity is both proportional to the volume probed26 and independent of the surface space charge layer.25 It is found that the ILO/ITO ratio increases from the NW base to the NW top (Figure 3d), indicating that the carrier concentration decreases along the NW from the base to the top. Self-Induced Compositional Gradient in the GaAsSb NW. Compositional inhomogeneity in ternary III−V semiconductor NWs has been reported previously in the literature. For example, axial variation of the composition has been observed in InGaAs27 and AlGaAs NWs.28 However, these reports have not dealt with Sb-based ternary III−V NWs and the underlying cause for the inhomogeneity nor used the effect constructively in device applications. The above-reported cases are different compared to the here studied GaAsSb NWs with a self-induced Sb variation formed during the NW growth process. Recently, it has been proposed that axial variations in the Sb concentration are caused by a reservoir effect29 and that C
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Furthermore, the estimated hole concentration from the fourprobe resistivity measurement also makes it appropriate to apply the TFE model.24,31,32 Figure 4c shows a plot of the current as a function of the bias voltage in the reverse bias configuration of the GaAsSb NW device (between contact A and B). From the slope of the plot, the acceptor concentration (Na) under reverse bias is calculated (Supporting Information, Section 4). The average acceptor concentration for six NWs is ∼2.4 × 1017 cm−3 at the metal/ NW contact near the top of the NW (Figure 4d). For the six analyzed NWs, the E00’s are in the order of kBT, indicating that the main transport mechanism is a TFE generated current at high reverse-biased condition.33,34 To further investigate the spatial distribution in carrier concentration along the NW, the acceptor concentration is estimated with a higher spatial resolution using the correlation between the Raman intensity of the LO phonon mode and the surface depletion layer width.25,35 Figure 4e shows the spatial dependence of the acceptor concentration estimated by using this correlation (Supporting Information, Section 5). It is found that the estimated acceptor concentration increases exponentially along the NW from the top to the base. The trend in all these different data sets indicates that there is a gradient in the acceptor concentration along the NW (i.e., higher at the base than at the top). Origin of the Rectifying Behavior in the GaAsSb NW. All results support a self-induced Sb compositional variation and indicate that the carrier and Sb concentrations are gradually changed along the NW. This indicates that the rectifying behavior is attributed to the asymmetric Schottky contacts formed between the NW and the metal electrodes near the base and the top of the NW. First, we consider the Schottky barrier width at both contacts. The variation of acceptor concentration along the NW leads to a difference in the depletion layer width at the Schottky contacts since the depletion width is inversely proportional to the carrier concentration.36 Therefore, the depletion layer at the Schottky contact near the top of the NW is wider than that at the contact near the base of the NW (Figure 5) and thus limits the hole current through the NW.
Figure 4. Spatial variation of electrical conductance along the GaAsSb NW. (a) SEM image of a GaAsSb NW device with four identical metal electrodes (Pt/Ti/Pt/Au). The Ga droplet is marked by a dashed circle, and the scale bar is 500 nm. (b) I−V characteristics for three different NW segments along the NW. The NW device segment near the top of the NW (A−B) shows a stronger rectifying behavior and has a lower conductivity than the segments nearer to the NW base (B−C and C−D). The inset shows the I−V characteristic of the NW device segment A−B with an enlarged y-axis. (c) Current as a function of reverse bias voltage for the NW device segment near the top of the NW (A−B). The line is fitted according to the thermionic field emission (TFE) model. (d) Histograms of the acceptor concentration Na near the NW top extracted from the TFE model for six different GaAsSb NW devices. (e) Spatial dependence of the acceptor concentration extracted from the Raman spectra in Figure 3. In b and c, the measurement configuration is the same as depicted in Figure S5.
4b and Figure S5. In other words, the I−V characteristics show a stronger rectifying behavior near the NW top compared to near the NW base. These results imply that the Schottky contact barrier varies between the top and the base of the NW. To estimate the average carrier concentration in the GaAsSb NW, a four-probe resistivity measurement have been carried out. The four-probe resistivity measurement on the GaAsSb NW shows an average resistivity of ∼0.14 Ωcm for the B−C segment (Figure 4a). Using an estimated effective field-effect hole mobility of 11.4 cm2/(V s) (Supporting Information, Section 3) gives an average hole concentration of ∼3.9 × 1018 cm−3. In a GaAs with a carrier concentration higher than 1017 cm−3, it can be assumed that thermionic field emission (TFE) becomes the dominant transport mechanism.24,31,32 Furthermore, the Padovani−Stratton parameter (E00) is estimated to be ∼14.3 meV. Considering that the dominant transport is due to TFE when E00 is in the order of kBT,33,34 this indicates that TFE is the main transport mechanism in the GaAsSb NW device. Within the device model for the GaAsSb NW (Supporting Information, Section 3), most of the voltage drop in the NW occurs near the top of the NW (on the reverse-biased contact), and the top part of the NW thus determines the limit on the total current that can pass through the whole NW. For a reverse-biased Schottky contact, the TFE generated current can become significant, especially in low dimensional systems.34
Figure 5. Model for the origin of the rectifying behavior in the GaAsSb NW. Schematic band diagrams for the interfaces between the metal contacts and the GaAsSb NW, near the NW top (left) and near the NW base (right), respectively. Blue horizontal lines represent surface pinning defect states. D
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Figure 6. Photodetector and logic devices based on the GaAsSb NW. (a) I−V curves of a GaAsSb NW device in dark and under 532 nm laser illumination at intensities of 20.7, 44.6, 83.2, 133, 223, and 316 mW/cm2. Inset: The laser intensity dependence of the photocurrent (Iphoto − Idark) measured at a reverse bias of −3 V. (b) Time resolved photoresponse plots of the NW device under laser illumination (532 nm, 173 mW/cm2) at reverse bias (−3 V).
dependence of the photocurrent amplitude on the laser intensity at a voltage of −3 V is shown in the inset of Figure 6a. From the photoresponse curves (Figure 6b), the rise time (τr) and the decay time (τd) under reverse bias conditions (−3 V) are estimated to be 32 and 136 ms, respectively. Although the GaAsSb NW photodetectors show a slow response compared with the GaAs/AlGaAs NW,43 its response/decay times are comparable to other nanostructure-based photodetectors (Table S1). As shown in Figure 6b, the GaAsSb NWbased photodetector shows a persistent photocurrent. Considering surface states and/or vacancies to contribute to the persistent photocurrent,44 the persistent photocurrent observed in the GaAsSb NW can be attributed to the Sb-related defect. Overall, these GaAsSb NWs with a simple device processing routine show a decent performance in terms of a photodetector, compared with other nanostructure-based photodetectors (Supporting Information, Section 8). This indicates that GaAsSb NWs can be a promising candidate for future nanoscale photodetectors. In conclusion, we have demonstrated a reliable and consistent high-performance self-induced rectifying behavior in GaAsSb NWs grown by a self-catalyzed VLS technique on Si substrates using MBE. The combination of confocal microRaman spectroscopy, electron microscopy, and electrical measurement reveals that the rectifying behavior is attributed to the asymmetric distribution in the acceptor concentration induced by radial Sb out-diffusion during the NW growth. Finally, we have demonstrated that these GaAsSb NWs can serve as photodetectors and logic circuits with a rudimental design and without the need of any complex fabrication processes. Since a gold-free NW growth technique has been used in this work, these GaAsSb NWs are compatible with Si complementary metal oxide semiconductor (CMOS) technology. Moreover, the unique self-induced rectifying behavior offers a promising and practical route over conventional device configurations, making GaAsSb NWs attractive building blocks for a range of integrated electronic/optoelectronic devices. Methods. GaAsSb NW Growth. A solid-source Varian GEN II Modular MBE system equipped with a Veeco dual filament Ga source and Veeco valved crackers with As and Sb, allowing adjustment of the proportion of dimers and tetramers, was used for the growth of the NWs. Si(111) substrates covered with fresh native oxide were loaded into the MBE system for the
Second, we discuss the Schottky barrier height in the GaAsSb NW. It is well-known that Sb-related defects at the surface give rise to electronic states that pin the Fermi level nearer the valence band edge.37−39 Therefore, the higher acceptor concentration near the surface of the NW base plays a dominant role for the surface Fermi level pinning, while the same effect leads to a weaker Fermi level pinning near the top of the NW (Figure 5). If the rectifying behavior in the GaAsSb NWs is associated with the surface Fermi level shift induced by the Sb-related point defects, the rectifying behavior should evolve along the NW, which is consistent with what is observed in Figure 4b and Figure S5. It is thus the Schottky contact near the NW top that dominates the rectifying behavior due to a combination of the wider depletion layer, the weaker Fermi level pinning, and a higher barrier height than at the contact near the NW base. In addition, considering the Schottky barrier lowering due to the image force (Supporting Information, Section 6), the effective barrier height of the contact near the NW base becomes even smaller than if this is neglected, as shown in Figure 5. Example Device: Photodetectors Based on the GaAsSb NW. The self-induced rectifying behavior of GaAsSb NWs can be exploited to realize NW-based photodetectors and logic circuits (Supporting Information, Section 7) with a simple design and a simple fabrication process. Figure 6a shows typical I−V characteristics of a GaAsSb NW photodetector in dark and under light illumination. Photoresponsivity (Rλ) is an important figure of merit for a photodetector. Rλ is defined as the ratio of the photocurrent (Iphoto) and the illumination power on the active area of the photodetector: Rλ =
Iphoto P×S
(1)
where P is the incident power density and S is the effective area of the photodetector. Rλ acquired at reverse bias (−3 V) under an illumination intensity of 20.7 mW/cm2 is estimated to be around 1463 A/W. This value is 48 000 times higher than for a recently reported GaAs NW-based photodetector,40 is more than 2500 times higher than for a recently reported GaAs/ AlGaAs core−shell NW-based photodetector,41 and is comparable to the state-of-the-art nanostructure-based photodetectors (Table S1). The high responsivity of the GaAsSb NW-based photodetector can be attributed to the higher local electric field for a reverse biased Schottky barrier.6,42 The E
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NW growth. The self-catalyzed GaAsSb NW growth was initiated by supplying all the fluxes simultaneously at a substrate temperature of 640 °C and continued for a duration of 40 min. The temperature of the Ga source was preset to achieve a nominal planar growth rate of 2 Å/s on a GaAs(001) substrate as calibrated by reflection high-energy electron diffraction. The beam equivalent pressures of the As4 and Sb2 fluxes used were 4.3 × 10−6 and 1.2 × 10−6 Torr, respectively. The growth was terminated by simultaneously stopping all the fluxes and immediately ramping down the substrate temperature. For NWs grown under different conditions, see Supporting Information, Section 2. Device Fabrication and Characterization. The synthesized GaAsSb NWs were suspended in isopropyl alcohol (IPA) by sonication. The dispersed NWs in the solution were then dropped onto a heavily doped p-type Si substrate with a thermally grown SiO2 (300 nm thick) layer. The heavily doped Si layer was used as a back gate electrode. The probe pads were fabricated using conventional photolithography process. To fabricate individual NW devices, electron-beam lithography (Elphy Plus, Raith) and e-beam evaporation (AJA Inc.) were applied. Prior to metal contact evaporation (Pt/Ti/Pt/Au, 5/ 10/10/200 nm), an oxygen plasma, and a diluted HCl solution were used to improve the contact quality between the NW surface and the metal electrode. Rapid thermal annealing (RTA) was carried out under N2 ambience at 400 °C for 30 s. The electrical characteristics of the individual NW devices were investigated at room temperature using a semiconductor characterization system (SCS) parameter analyzer (Keithley 4200) and a probe station (Cascade Microtech). The morphologies of the synthesized NWs and NW-based devices were studied using a field emission gun scanning electron microscope (FEG-SEM, Hitachi 4300). Raman Spectroscopy. Confocal micro-Raman spectra were collected from a spectrometer (Alpha 300R WiTec) equipped with a ×100 microscope objective (N.A. = 0.90) and a piezo stage, which enables to scan along the NW with a precision of 10 nm. All Raman spectra were obtained in a backscattering configuration. A 532 nm laser was used for the excitation of the individual GaAsSb NWs. The power and the spot diameter of the incident laser were about 0.14 mW and 0.3 μm, respectively, giving a power density of ∼50 kW/cm2. The polarization direction of the incident laser beam was perpendicular to the NW axis. All measured Raman spectra were fitted with Lorentzian functions. Transmission Electron Microscopy (TEM). For the TEM studies, the NWs were scraped off the substrate with a diamond scriber, dispersed in IPA and transferred to a graphene-coated 2000 mesh Cu-grid (Graphene Laboratories Inc.). Conventional TEM and high-angle annular dark field scanning TEM (HAADF STEM) were performed on a Phillips CM30 and a JEOL 2010F (Cs = 1 mm, probe size ∼0.2 nm). Both TEMs were operated at 200 kV. Energy dispersive X-ray spectroscopy (EDX) point scans were performed on the JEOL 2010F in the STEM mode, with a probe size of 1 nm using a Si drift detector (Oxford Instruments, solid angle 0.23 sr). Photocurrent Measurement. Individual GaAsSb NW devices were mounted in a Janis ST-500 optical cryostat with a Mitutoyo 50× objective lens (N.A. = 0.65). Photocurrent measurements were carried out at room temperature in ambient air using a source meter (Keithely 2636A). A 532 nm laser was used for the linearly polarized laser excitation.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details and additional data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b00089.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
The experiments were conceived and designed by J.H. and D.C.K. Devices were fabricated by J.H. Optoelectronic measurements were carried out by J.H. with help from D.-C.K. Y.H. performed the Raman spectroscopy under S.W.L.’s supervision. A.M.M. and D.L.D. carried out the NW growths. H.K. carried out compositional analysis by TEM and A.M.M. carried out structural analysis by TEM and SEM. A.T.J.v.H., S.W.L., B.O.F., and H.W. contributed in interpretation of the results. H.W. supervised the project. J.H. prepared the manuscript with all authors discussing and commenting. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the Research Council of Norway (FORNY program (Grant 217566), FRINATEK program (Grant 214235)), and NANO2021 program (Grant 228758) and for the support from the Norwegian Micro- and Nano-Fabrication Facility, NorFab (197411/V30). H.Y. and S.W.L. were supported through BSR (2012R1A2A2A01045496), and the framework of international cooperation programs (2012K2A1A2032569) by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. The authors would like to thank Ho-Kyun Jang for the 3D graphics.
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REFERENCES
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DOI: 10.1021/acs.nanolett.5b00089 Nano Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.nanolett.5b00089 Nano Lett. XXXX, XXX, XXX−XXX
