Letter pubs.acs.org/NanoLett

Selective Synthesis of Compound Semiconductor/Oxide Composite Nanowires Hideaki Hibi,† Masahito Yamaguchi,‡,⊥ Naoki Yamamoto,§ and Fumitaro Ishikawa*,†,∥ †

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Department of Electrical Engineering and Computer Science, Nagoya University, C3-1-631 Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan § Department of Physics, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo 152-8551, Japan ∥ Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan ‡

S Supporting Information *

ABSTRACT: Semiconductor/oxide composite nanowires (NWs) were synthesized by molecular beam epitaxial growth and subsequent wet oxidation. Nonselective and selective oxidation conditions applied to the GaAs/AlGaAs core−shell NWs grown on silicon substrates produced GaOx/AlGaOx and GaAs/AlGaOx NWs, respectively. The oxidized amorphous AlGaOx shell produced cathodoluminescence over a wide spectral range encompassing ultraviolet and visible wavelengths, possibly sourced from molecular species related to oxygen. The wire core was buried in the oxides when the diameter of the oxide shell increased, forming a planar structure. These composites are expected to pave the way to future electrical and optical functions for NWs. KEYWORDS: Nanowire, compound semiconductor, oxide, composite, luminescence

O

doping. Strain induced by a lattice mismatch between core and shell may be used to further alter the energy structure within the NW. Conversely, oxides displaying advantageous dielectric, thermal, and resistive properties, which cannot be achieved in semiconductors, make their combination with semiconductors appealing. Further, even in the amorphous state these materials present useful properties for developing advanced devices.20−22 For example, Ga2O3,23−25 In2O3,26 and Al2O327 NWs have been studied to control various functions, such as plasmonic and excitonic behaviors as well as field emitter array applications The AlGaAs/GaAs system was subjected to wet oxidation to integrate oxide functions into the semiconductor. The wellestablished selective oxidation is used for vertical-cavity surface emitting lasers,28−30 while the recent nonselective oxidation technique, which transforms entire semiconductor systems into oxides, can be exploited for optical waveguides.31,32 In addition to structural precision at the nanoscale, this technique enables selective reaction with the initial AlGaAs/GaAs semiconductor. Wet oxidation transforms the AlGaAs compound semiconductor into the amorphous oxide AlGaOx, resulting in high refractive index contrast between the materials before oxidation as well as current confinement because of the insulating nature of the oxides. Varying the original semiconductor compositions precisely tunes their properties such as

ne-dimensional composite nanowires (NWs) involving semiconductor, metal oxide, and organic materials have attracted extensive research efforts because of their numerous potential applications in nanoelectronics, catalysis, chemical sensing, and energy storage.1−8 Composite systems comprising various materials have produced high-performance electronic devices, such as metal oxide−semiconductor transistors.9 The combination of materials exhibiting contrasting properties is expected to provide superior functions that are not achievable from the individual materials. The introduction of III−V compound semiconductors into NWs provides dynamic control over the electronic band structure of these systems.1,10 These heterostructures can serve as charge carriers and light waveguides, making NWs attractive building blocks in nanoelectronics and nanophotonics.3,11−14 Moreover, advanced epitaxial techniques used to fabricate NWs can overcome the large mismatches between lattice constants and thermal coefficients of III−V semiconductors and Si, enabling the heteroepitaxy of these materials.15,16 The integration of semiconductor NWs with various Si substrates opens the door for large-scale integrated systems with enhanced electronic and optical properties.13,14,17,18 In addition to their usefulness and ease of fabrication, core−shell NWs present tunable properties and provide passivation for core material surfaces, resulting in an extensive range of high-performance device applications.19 For example, NWs can be capped with a larger bandgap material, either for passivation or modulation © XXXX American Chemical Society

Received: September 3, 2014

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refractive index.33 Controlling the extent of oxidation through selective and nonselective processes further increased the versatility of oxidized AlP/InP NWs involving native oxides34 and the reversible conversion of GaN/Ga2O3 NWs.35 AlGaAs/ GaAs NWs promise to play a major role in advanced electrical and optical applications.36−41 Here, an entirely buried semiconductor structure is synthesized, in which the GaAs/AlGaAs NW is surrounded by the oxides. This system is expected to benefit from passivation by the surrounding oxides and lead to future device applications because it easily forms top and bottom contact electrodes.42 When sufficiently buried by the low refractive index oxides, the structure itself may exhibit specific optical functions, such as the photonic crystal effect.43 Therefore, the combination of semiconductors and oxides within monolithically synthesized NWs and the burying of individual NWs in the coalesced oxide would have strong potential for future functional devices. In this report, semiconductor/oxide heterostructure NWs were fabricated by molecular beam epitaxy followed by wet oxidation of the compound semiconductor heterostructure system. Moreover, GaAs/AlGaAs heterostructure NWs were buried in the oxide shells by expanding the shell diameter to form planar structures that may find application in future devices. Experimental Details. GaAs/AlGaAs NWs were grown on phosphorus-doped n-type Si(111) substrates by molecular beam epitaxy using the Ga component as a catalyst.44−47 Epiready substrate surfaces were not treated prior to NW growth but were covered with a native oxide layer exhibiting a typical thickness of several nanometers. Al and Ga supplies were set to match a planar growth rate of 1.0 ML/s on a GaAs (001) substrate calibrated by the reflection of high-energy electron diffraction oscillation.48 This planar growth rate corresponds to an FAs/FGa atomic flux ratio of approximately 10 estimated from the stoichiometric conditions identified by the surface reconstruction transition between As-rich (2 × 4) and Garich (4 × 2) areas for the GaAs growth plane on GaAs (00l).49 Samples were heated to 570 °C using a constant As4 beam at an equivalent pressure of 1.7 × 10−5 mbar. The GaAs core was grown for 15 min under these temperature and As4 pressure conditions to initiate longitudinal wire growth. Next, the growth was interrupted for 15 min to crystallize the Ga catalyst, and the As4 pressure was increased to 3.3 × 10−5 mbar. Finally, Al and Ga were added, and the AlGaAs shell containing 80− 90% Al was allowed to grow for 30 min. During this period, the lateral growth became dominant, generating core−shell NWs. GaAs/AlGaAs heterostructure NWs were also buried in the oxide shells by expanding their shell diameters. Various Al0.9Ga0.1As shells differing in thickness were grown by changing the deposition time (30 min, 2 h, and 6 h). The wet oxidation was conducted for the NWs samples for 2 h in a furnace at 370 or 435 °C.33 Water was kept at 90 °C in a bubbler, and the extracted steam was transmitted into the furnace using N2 gas. The Al-rich AlGaAs shell selectively underwent oxidation because its oxidation rate differed significantly from that of the GaAs core.33 Structural characteristics of the NWs were investigated by scanning electron microscope (SEM) using a S-5200 instrument (Hitachi, Japan) and transmission electron microscope (TEM). The crosssectional analysis was conducted using a scanning transmission electron microscope (STEM, JEM-ARM200F, JEOL, Japan) operating at 200 kV equipped with an energy dispersive X-ray (EDX) spectroscopy imaging system (JED-2300T, JEOL, Japan). For the TEM measurements, finely sliced samples

displaying thicknesses smaller than 200 nm were fabricated using a focused ion beam system (FEI Strata DB 235). STEM images were obtained in bright-field (BF) and high-angle annular dark-field (HAADF) modes. The crystallinity was investigated by electron diffraction using a TEM (Hitachi H9000UHR-II) operating at 300 kV in the ⟨112⟩ azimuth direction. The optical characteristics of the NWs were investigated by cathodoluminescence (CL) measurements using a STEM (JEM-2100F, JEOL, Japan) combined with a light detection system, operating at an acceleration voltage of 200 kV. To characterize their optical properties, individual NWs were transferred to a carbon-coated supporting film. The cathodoluminescence spectra were obtained by subtracting the background spectrum of the surrounding area from the raw data. The diameter and beam current of the electron beam was 1 nm and 1 nA, respectively.50−52 All measurements were performed at room temperature. Results and Discussion. Wet Oxidation of Core−Shell GaAs/AlGaAs Nanowires. The synthesized GaAs/AlGaAs core−shell NWs underwent wet oxidation for 2 h, as shown in Figure 1. The wires were selectively or nonselectively

Figure 1. Temperature-dependence of the wet oxidation procedure.

oxidized by varying the oxidation temperatures between 370 and 435 °C because GaAs core and Al-rich AlGaAs shell displayed large differences in their oxidation rates.33 At 370 °C, the selective oxidation solely affected the AlGaAs shell while preserving the GaAs core. Conversely, under nonselective conditions (above 435 °C), the entire was oxidized to produce GaOx/AlGaOx NWs. Figure 2a shows a BF-STEM image of a single GaAs/AlGaAs core−shell NW after oxidation at 435 °C. The GaAs core and the AlGaAs shell were grown for 15 and 30 min, respectively. The wire was slightly bent because of the slicing step required during sample preparation. The wire consists of a 100 nm diameter GaAs core surrounded by a wide (approximately 100 nm) AlGaAs shell. Small crystals observed on the sample surface coalesced with the NW bases but exhibited negligible effects on the NW upper portion. HAADF-STEM and EDX elemental mapping images (Figure 2b−d) showed strong intensities for Ga and O2 at the core. Conversely, Al and O2 presented strong intensities in the shell. Arsenic generated a weak signal inside the wires, except at the bottom, where its intensity approached the detection limit. This indicates that the wire was oxidized by exchanging As for O2 throughout the structure, preserving the doping-level As at a density of approximately 1019 cm−3. The oxidation progressed inward on the wires and the upper portion of the surface crystals exposed to air, consistent with the formation of core− B

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Figure 2. (a) BF-STEM image of a single GaAs/AlGaAs core−shell NW after oxidation at 435 °C. (b−d) HAADF-STEM and EDX elemental mapping images for the areas delimited by the dashed squares in (a). The intensity of As is enhanced because the signal is close to the EDX detection limit.

Figure 3. (a) HAADF and EDX elemental mapping images of NW after oxidation at 370 °C. (b) Diffraction patterns of wire core (left) and shell (right) at the indicated spots.

oxidation. Conversely, Al and O2 exhibited strong intensities at the shell. This demonstrates that the shell was selectively oxidized at 370 °C to provide the GaAs/AlGaOx heterostructure NW. The diffraction pattern of the shell presented a diffused ringlike pattern (Figure 3b), consistent with its amorphous nature. In contrast, the core showed a clear diffraction spot, indicative of the crystalline structure of GaAs. Therefore, wet oxidation selectively produced a semiconductor/amorphous oxide NW at 370 °C. Overall, this approach provided oxide/semiconductor or oxide/oxide heterostructure NW by adjusting the process conditions.

shell GaOx/AlGaOx heterostructures. Note that NWs were found easier to oxidize than the surface crystals. Under the same reaction conditions, GaAs did not undergo oxidation on planar samples.33 However, exposing a large surface area to the oxidizing steam enhanced the oxidation. Therefore, to selectively oxidize AlGaAs over GaAs, the reaction was performed at a lower temperature. Figure 3a shows the HAADF-STEM and EDX elemental mapping images for the same NW as above but oxidized at a reduced temperature of 370 °C. Unlike for the NW oxidized at 435 °C, As and Ga showed strong intensities at the core upon C

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Figure 4. Cathodoluminescence of GaAs/AlGaOx NWs. (a) Backscattered electron image; (b) panchromatic CL image; (c) cathodoluminescence spectra acquired from the 1 × 1 μm2 area involving the NW; monochromatic cathodoluminescence image at (d) 342 nm and (e) 423 nm. Enlarged images of the area delineated by the red square in (b) are shown at (f) 342 nm and (g) 423 nm; (h) spectral mapping of line scanning across the nanowire (indicated by the white line in (b)). Warmer colors in the color-imaged intensity plots indicate stronger intensity. Regions of lowest and highest (saturation) intensity are shown in black and white, respectively.

Optical Characteristics of GaAs/AlGaOx Heterostructure. To elucidate the optical properties of the composite NW series, we performed cathodoluminescence measurement of the GaAs/AlGaOx heterostructure. Specifically, we investigated GaAs/Al0.9Ga0.1As NWs after oxidation at 370 °C (see Figure 2). Results are summarized in Figure 4a−h. Two NWs were transferred from the original Si substrate to a carbon coated supporting film (Figure 4a). The contrast in the NW in Figure 4b (the panchromatic CL image) reflects the core−shell structure of the NWs. Figure 4c shows the spectrum acquired from the 1 × 1 μm2 area involving the NW. The spectrum exhibits a sharp peak at 340 nm and a broad peak extending from approximately 400 to 700 nm, probably constituting several overlapped peaks. This spectrum is similar to previously reported spectra of Al2O3 nanoparticles.53−56 As seen in Figure 4d−g, the shell layer emitted strong luminescence at 342 and 423 nm. Furthermore, the observed emissions were localized (Figure 4h), which is consistent with luminescence sourced from nanostructured oxides. This suggests the formation of shell oxides with luminescent properties. In the literature, the observed spectral range varies from violet to a green-yellow regime,53−56 possibly reflecting the different arrangements of molecules responsible for the luminescence.53 In our case, luminescence is observed from ultraviolet (300 nm) to nearinfrared (700 nm) wavelengths, including the broadened tail of the spectrum. This broad emission might be sourced from the small Ga constituent within the amorphous AlGaOx shell, as recognized in Figure 3a, and the wide nanoscale particle size distribution of the shell.55 Notably, the spectral features, especially the broad peak in the visible range, vary among the NWs, being strongest at approximately 530 nm (a green wavelength) in some cases (see Supporting Information). On the other hand, the GaAs peak around 850 nm was not

observed. Negligible GaAs luminescence is expected from the weak nature of the unpassivated 100 nm GaAs core.37,57 In the present study, the passivation effect is inhibited by the nonoptimized structure and oxidation conditions. Instead, the optical quality of the core GaAs can be degraded by the irradiation of oxygen or water vapor during the oxidation process, corresponding to the GaAs core/AlGaOx shell interface in our case. The luminescence efficiency of the core GaAs could be improved by additional passivation such as an AlGaAs multishell passivation layer with low Al concentration.37 Optimizing the luminescence efficiency of the proposed GaAs-oxide composite system is beyond the scope of this study, but combining the optical characteristics of oxides and compound semiconductor GaAs could be considered in future work. Synthesis of Buried GaAs/AlGaAs Heterostructure Nanowires. Next, the synthesis of buried GaAs/AlGaAs heterostructure NWs was attempted. As schematically depicted in Figure 6a, free-standing NWs are expected to be buried under other shells if their shell diameters expand upon growth time increase. Three series of samples were synthesized at AlGaAs shell growth times of 30 min, 2 h, and 6 h under identical temperature and As4 pressure conditions. These samples were subjected to wet oxidation at 370 °C to oxidize the shells while keeping the cores intact and provide semiconductor NWs buried in an AlGaOx shell. When the entire structure is sufficiently buried by low refractive index oxides (Figure 5a), the structure itself may exhibit attractive optical properties, such as photonic crystal effects, because of the refractive index contrast between the scale of NWs and the oxides with their separations equaling several hundreds of nanometers.43,58 Figure 5b−g shows planar and cross-sectional SEM images of NWs obtained for shell growth times of 30 min, 2 h, and 6 h. D

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Figure 5. (a) Formation procedure of GaAs NWs buried in an oxide layer. Planar SEM images of NWs obtained after different shell growth times: (b) 30 min, (c) 2 h, and (d) 6 h. Cross-sectional SEM images of NWs obtained after different shell growth times: (e) 30 min, (f) 2 h, and (g) 6 h.

preserved single-crystalline areas containing 90 nm diameter GaAs NWs (Figure 6c). The EDX mapping of this rhomboidal region revealed that oxygen showed strong intensity in the upper portion, but As was predominant in the bottom portion (Figure 6d), suggesting that the As initially present in the AlGaAs shell was exchanged for oxygen by wet oxidation at 370 °C. Considering the finely sliced nature of the sample, these results indicate that wet oxidation was suitable for this region and affected the 5 μm uppermost portion of the sample. This oxidation preserved the free-standing GaAs/AlGaAs core−shell NWs within the structure (Figure 6c,e). The strong oxide intensity observed in the rhomboidal region (Figure 6d) suggest that the oxidation may have progressed from the surface, grain boundaries, and voids. Consequently, AlGaOx oxides can confine the AlGaAs shell-surrounded GaAs core. EDX images revealed that crystalline GaAs/AlGaAs core−shell heterostructures were preserved within the system. A more precise structural control over surface, boundary, and void configurations may open the door to advanced functions relying on electrical and optical confinement. Conclusion. In summary, semiconductor/oxide composite NWs were fabricated by selective or nonselective wet oxidation of GaAs/AlGaAs core−shell NWs grown on a Si substrate. Varying oxidation conditions produced GaAs/AlGaOx and GaOx/AlGaOx core−shell heterostructure NWs. The GaAs semiconductor core after oxidation kept its single crystallinity. Conversely, the obtained oxides showed amorphous character-

These images clearly indicated that NW diameters expanded with increasing shell growth time. The rate of this diameter increase amounted to approximately 500 nm/h. Moreover, neighboring wires coalesced; the entire structure became growingly buried. The sample obtained after 6 h of shell growth displayed a complex structure consisting of disordered wires with voids at individual NW boundaries (Figures 5d and 5g). This may stem from adatom flux shadowing by the adjacent wires, resulting in tapered side facets.54 Wire tips adopted a disordered arrangement while maintaining their triangular shape, resulting in a top surface roughness exceeding several micrometers. These results provide a trend for the coalescence process of neighboring NWs to produce fully buried structures. Figure 6ac shows postoxidation cross-sectional TEM images on the sample composed of coalesced wires (Figure 5d,g). The images presented a quarried part of the entire sample because the thickness of the finely sliced sample was smaller than 200 nm for the measurement. Grain boundaries, which may result from the complex NW growth and coalescence at large shell width, were observed (Figure 6, dashed lines). Specifically, rhomboidal regions, which displayed heights and widths of approximately 8 and 1.5 μm, respectively, were detected within the buried structure. These regions may be considered as initial GaAs/AlGaAs core−shell NWs unaffected by adjacent NWs. Their structure may stem from the shadowing of the adatoms observed during the growth of dense and thick NWs.59 In addition, the lower portion of the rhomboidal region exhibited E

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Figure 6. (a−c) Cross-sectional TEM images of the sample exhibiting coalesced shells after oxidation. Dashed lines are eye guides indicating interfaces between different crystallites within the structure. (b,c) Enlarged images of areas marked by the blue rectangles. (d) Summarized EDX images of the area delimited by the orange box in (a). (e) Summarized EDX images of the area delimited by the orange box in (c). The image shows a quarried part of the entire sample because the thickness of the finely sliced sample was below 200 nm for the measurement.



ACKNOWLEDGMENTS This work was partly supported by KAKENHI (No. 23686004) from the Japan Society for the Promotion of Science and a research scholarship from the Kurata Memorial Hitachi Science and Technology Foundation and the Murata Science Foundation.

istics. The oxidized amorphous AlGaOx shell exhibited cathodoluminescence over a wide spectral range (ultraviolet to visible wavelengths), possibly sourced from molecular-like species related to oxygen. By increasing the diameter of the oxide shell, the GaAs NW cores were buried in the oxides to form a planar structure. The control of surface, boundary, and void configurations is expected to pave the way to new optoelectronic device applications involving electrical and optical confinement as well as easy electrode contact.





ASSOCIATED CONTENT

S Supporting Information *

Synthesis of core−shell GaAs/AlGaAs nanowires: the initial nonoxidized GaAs/AlGaAs core−shell nanowires fabricated by molecular beam epitaxial growth showed a straight sidewall with a clear core−shell architecture throughout their structure, validating the presented discussions. Presented are schematic diagrams of the NW growth mechanism, bird’s eye view and planar SEM images of NWs containing an Al0.8Ga0.2As shell, and enlarged cross-sectional and side-view SEM images of a typical NW. The cathodoluminescence characteristics of another NW obtained from the same sample as Figure 4 are also shown. This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. ⊥ Deceased. F

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oxide composite nanowires.

Semiconductor/oxide composite nanowires (NWs) were synthesized by molecular beam epitaxial growth and subsequent wet oxidation. Nonselective and selec...
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