Raman Spectroscopic Characterizations of Self-Catalyzed InP/InAs/InP One-Dimensional Nanostructures on InP(111)B Substrate using a Simple Substrate-Tilting Method
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We report optical phonon vibration modes in ensembles of self-catalyzed InP/InAs/InP multi core-shell one-dimensional nanostructures (nanopillars and nanocones) grown on InP(111)B substrates using liquid indium droplets as a catalyst via metal-organic chemical vapor deposition. We characterized the Raman vibration modes of InAs E1(TO), InAs A1(TO), InAs E1(LO), InP E1(TO), InP A1(LO), and InP E1(LO) from the ensemble of as-grown nanostructures. We also identified second-order Raman vibration modes, associated with InP E1(2TO), E1(LO+TO), and E1(2LO), in the InP/InAs/InP core-shell nanopillars and nanocones. Raman spectra of InP/InAs/InP nanopillars showed redshift and broadening of LO modes at low-frequency branches of InAs and InP. Due to the polar nature in groups III–V nanowires, we observed strong frequency splitting between InAs E1(TO) and InAs A1(LO) in InP/InAs/InP nanocones. The Raman resonance intensities of InP and InAs LO modes are found to be changed linearly with an excitation power. By tilting the substrate relative to the incoming laser beam, we observed strong suppression of low-frequency branch of InP and InAs LO phonon vibrations from InP/InAs/InP nanocones. The integrated intensity ratio of InP E1(TO)/E1(LO) for both nanostructures is almost constant at 0-degree tilt, but the ratio of the nanocones is dramatically increased at 30-degree tilt. Our results suggest that Raman spectroscopy characterization with a simple substrate tilting method can provide new insights into non-destructive characterization of the shape, structure, and composition of the as-grown nanostructures for the wafer-scale growth and integration processing of groups III–V semiconducting hetero-nanostructures into nanoelectronics and photonics applications.
KeywordsSelf-catalyst Nanowires Vapor-liquid-solid process MOCVD Raman spectroscopy InP InAs
Longitudinal optical phonon
Metal-organic chemical vapor deposition
Scanning electron microscope
Transverse optical phonon
Wurzite crystal structure
Zinc blende crystal structure
Semiconducting heterostructure nanowires have received considerable attention over the past decade . A variety of material combinations have been synthesized both in core-shell [2, 3, 4, 5] and superlattice [6, 7, 8] and alloy nanowires [9, 10]. InP-InAs nanowire [11, 12, 13] is one of such combinations with potential applications in light-emitting diodes , single-photon source , photodetectors , and heterojunction transistors  due to its band gap tunability, high carrier mobility, and large breakdown field [18, 19]. The performance of any of these devices depends on the optical and electronic properties of nanoscale semiconductors, which in turn vary critically with the crystallinity, morphology, and composition of the nanowires [20, 21]. Among a suite of available characterization tools, Raman spectroscopy is a non-destructive technique that can provide insights into the effects of shape, structure, and composition of semiconductor structures (i.e., thin films , nanowires , and quantum dots ) on physical properties (i.e., phonon confinement and surface optical phonon modes [25, 26]). Polarization-dependent Raman scattering measurements on single semiconducting nanowires revealed that highly anisotropic shapes of nanowires have angular dependences of Raman active modes and scattered intensities (i.e., Si , GaAs , InAs [29, 30], GaP [31, 32], ZnO , GaN ). Recent advances of Raman spectroscopy technique further achieved the single-molecule level sensitivity of Raman signals through the exploitation of near-field surface resonances [35, 36] using engineered substrates with roughened metal-coated two-dimensional surface (i.e., metal nanoparticle–decorated substrate ) or in the form of zero-dimensional metal particles (i.e., core-shell nanoparticles ). By tuning the shell thickness, core size, and materials of core-shell nanoparticles, this technique can find extensive applications in chemical sensing and imaging, thermal therapy, nanophotonics, plasmon-induced photocatalysis, plasmon-enhanced signal amplifications, and fluorescence [35, 36, 38, 39]. However, Raman spectroscopic characterization of the self-catalyzed growth of one-dimensional hetero-nanostructures has not been extensively studied yet. The variations in analytical parameters (i.e., peak positions, line width, and intensities) of obtained Raman spectra can explain the scientific details of the composition, chemical environment, and crystalline/amorphous in nanostructured materials . Non-destructive optical characterization on as-grown samples would provide useful information to understand their novel chemical and physical properties of unique one-dimensional hetero-nanostructures.
In this Letter, we present the results from Raman spectroscopic studies of self-catalyzed InP/InAs/InP multi core-shell nanopillars and nanocones with their strong dependencies of the Raman vibration modes and intensities on the morphology, crystal structure, and scattering geometry of the one-dimensional nanostructures.
The as-grown samples analyzed in this experiment comprised vertically oriented nanostructures, grown on InP(111)B substrate. After deposition, we examined the morphology of as-grown nanostructures using a FEI NOVA 230 field emission SEM at an acceleration voltage of 5 kV. From the SEM images, we measured the average height and base diameter of over 30 individual nanostructures. Raman spectra of the as-grown samples, ensembles of InP/InAs/InP nanopillars or nanocones, were measured in backscattering geometry with confocal configuration using a Renishaw InVia Raman spectrometer. In order to avoid any Raman scope-induced physical damages on the as-grown nanostructures, a substrate tilting angle was limited up to 35 degrees. In this system, the incident laser wavelength is 514.5 nm and excitation power can be varied between 5 and 25 mW. The laser beam was focused through a microscope to a spot size of approximately 1 μm in diameter. The spectra were characterized with a resolution of 0.5 cm−1. All spectra were collected in air, at room temperature, and are calibrated to the reference Si peak arising from the substrate (520.1 cm−1). All the Raman spectra were fit with symmetric Gaussian-Lorentzian functions to extract the parameters of interest.
Results and Discussion
Figure 1 shows typical morphologies of InP nanopillars, InP/InAs/InP nanopillars, and InP/InAs/InP nanocones grown on InP(111)B substrates. InP/InAs multi core-shell nanostructures are grown in the temperature range from 320 to 400 °C. All nanostructures grow vertically and straight in the <111>B direction with slight tapering. The pillars are low profile due to the two competitive growth modes, vapor-liquid-solid and vapor phase epitaxy, that are active at a relatively high growth temperature of 400 °C [13, 41]. The nanopillars are 150 nm in base diameter and up to 250 nm in height while the nanocones are 50 nm in base diameter and up to 2 μm in height. Detailed structural characterizations are described in .
All the spectra from InP(111)B substrate and InP/InAs/InP nanocones exhibit two distinct peaks at 303.7 cm−1 and at 344.5 cm−1 which are assigned to be TO and LO phonon vibration modes in ZB InP bulk system, respectively. The Raman spectra for InP nanopillars in backscattering geometry mode revealed the two phonon modes at 303.8 cm−1 and 343.0 cm−1, which are consistent with the InP E1(TO) and InP E1(LO) modes for WZ structures, respectively. Interestingly, InP/InAs/InP nanopillars exhibit a noticeable enhancement and broadening of the LO band, which is not seen from InP bulk. The Raman spectra of InP/InAs/InP nanopillars at 303.8 cm−1 and 341.7 cm−1 are identified to be InP E1(TO) and InP A1(LO) modes, respectively. It is known that the LO modes are more sensitive to the Raman resonance due to the Frölich interaction .
The Raman peaks located at 218 cm−1 and 241 cm−1 are assigned to the first-order E1(TO) and E1(LO) modes of zinc blende InAs [46, 47] in Fig. 2. The Raman intensities of the InAs peaks in InP/InAs/InP nanostructures are lower than those of the InAs(111)B reference, indicating that both nanopillar and nanocones are either core-shell or InPAs alloy structures [13, 42]. Interestingly, the redshifts of InAs E1(LO) and InAs A1(LO) peaks compared with InAs bulk crystal with significant broadening are found in InP/InAs/InP nanopillars (see Additional file 1: Figure S2). Material size and shape (i.e., sub nanometers) can lead to a redshift and broadening of the LO Raman line  due to the relaxation at Г (q = 0) point governed by the selection rule . In particular, the InAs A1(LO) Raman active mode confirms that WZ crystal phases are dominant in the InP/InAs/InP nanopillars  and our results are consistent with other reports [29, 50].
Raman vibration modes of InP and InAs nanostructures on InP(111)B substrate
X, L, Г
We have presented the experimental results of Raman spectroscopy performed on one-dimensional self-catalyzed InP/InAs/InP multi core-shell nanopillars and nanocones on InP(111)B substrates. The measurements are performed by varying laser power and substrate tilt angle under the fixed backscattered geometry of the Raman spectroscopy system. The InP/InAs/InP multi core-shell nanostructures exhibited the Raman resonance peaks of InAs E1(TO), InAs A1(TO), InAs E1(LO), InP E1(TO), InP A1(LO), and InP E1(LO). Contrary to the reference single-crystal InAs(111)B and InP(111)B substrates, the InP/InAs/InP nanostructure bundles revealed the unique 2nd harmonic Raman interaction modes: InP E1(2TO), InP E1(LO+TO), InP E1(2LO). The InP and InP/InAs/InP nanopillars showed the redshift and broadening of LO modes. Strong splitting between InAs E1(TO) and InAs A1(LO) are observed in InP/InAs/InP nanocones. We also found that the intensities of LO and TO modes are dependent linearly on an excitation power and the changes in the integrated intensity ratio of TO over LO modes are almost constant. By tilting a substrate, however, we observed a strong suppression at the low-frequency branches of the InAs LO and InP LO phonon vibrations from the InP/InAs/InP nanocone bundles, where the intensity ratio of InP TO/LO for nanopillars and nanocones is approximately 1.3 and 2.3, respectively. Our work provides new insight into the non-destructive characterization of groups III–V semiconducting hetero-nanostructures with a simple substrate tilting method.
The authors are grateful to Prof. Robert F. Hicks (UCLA) for providing access to the MOCVD reactor and Prof. Suneel Kodambaka (UCLA) for the useful discussion and the financial support. The authors would also like to thank Prof. Yang Yang (UCLA) for providing access to micro-Raman spectroscopy systems.
JHP conceived and designed the project, performed the synthesis and characterizations of nanostructures (MOCVD growth, SEM, EDS, and Raman spectroscopy), analyzed the data, and wrote the manuscript. CHC helped in the interpretation of the spectroscopic data and co-wrote the manuscript. Both authors read and approved the final manuscript.
JHP acknowledges the financial supports from the National Science Foundation (NSF-CMMI Grant No. 0926412) and Princeton Catalysis Initiative. CHC acknowledges the financial support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant No. NRF-2019R1F1A1058917).
The authors declare that they have no competing interests.
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