Structural characterization of GaAs and InAs nanowires by means of Raman spectroscopy (original) (raw)
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Surface optical phonons in GaAs nanowires grown by Ga-assisted chemical beam epitaxy
Journal of Applied Physics, 2014
Surface optical (SO) phonons were studied by Raman spectroscopy in GaAs nanowires (NWs) grown by Ga-assisted chemical beam epitaxy on oxidized Si(111) substrates. NW diameters and lengths ranging between 40 and 65 nm and between 0.3 and 1.3 lm, respectively, were observed under different growth conditions. The analysis of the Raman peak shape associated to either longitudinal or surface optical modes gave important information about the crystal quality of grown NWs. Phonon confinement model was used to calculate the density of defects as a function of the NW diameter resulting in values between 0.02 and 0.03 defects/nm, indicating the high uniformity obtained on NWs cross section size during growth. SO mode shows frequency downshifting as NW diameter decreases, this shift being sensitive to NW sidewall oxidation. The wavevector necessary to activate SO phonon was used to estimate the NW facet roughness responsible for SO shift. V
Nanotechnology, 2008
Gallium arsenide nanowires were synthesized by gallium-assisted molecular beam epitaxy. By varying the growth time, nanowires with diameters ranging from 30 to 160 nm were obtained. Raman spectra of the nanowires ensembles were measured. The small line width of the optical phonon modes agree with an excellent crystalline quality. A surface phonon mode was also revealed, as a shoulder at lower frequencies of the longitudinal optical mode. In agreement with the theory, the surface mode shifts to lower wave numbers when the diameter of the nanowires is decreased or the environment dielectric constant increased. PACS numbers: 62.23.Hj; 63.22.Gh; 81.15.Hi; 81.16.Dn In the past decade the field of semiconducting nanowires has developed significantly mostly due to the fact that these systems with unique geometry offer great possibilities for further development of optic and electronic devices 1,2 . Equally important, they offer numerous possibilities for studying exciting physical phenomena arising from carrier confinement and/or the large surface-to-volume ration 3 . However, the growth of nanowires free of structural defects and contaminants is still one of the key issues. The vapor-liquid-solid growth method is one of the most common technique, in which typically gold is used as a catalyst for the nucleation and growth of the nanowires 4 . It is widely known that gold introduces deep level traps in the semiconductor band gap that hinder the optoelectronic properties of the material 5 . By avoiding the use of gold, the properties of the grown nanowires improve significantly. Recently, catalyst-free synthesis of III-V nanowires has been demonstrated by both MOCVD and MBE 6,7 . From the two techniques, with MBE it is possible to obtain materials with an extremely high purity and good structural properties. Raman spectroscopy as a non destructive characterization tool is extensively applied for characterization of low dimensional systems such as nanowires and nanocrystals, as it provides valuable information on the structural properties . In this letter we present a systematic Raman spectroscopy study of GaAs nanowires grown by MBE without the use of external catalyst for the growth. Nanowires with diameters ranging from 30 nm up to 160 nm were grown. The underlaying motivation for investigation of nanowires with a broad range of diameters was to correlate the features in the Raman spectrum with the change of the surface to volume ratio. The nanowires were grown in a GEN II MBE system. For the growth we have used (001) GaAs wafers covered with a thin layer (approximately 35 nm) of sputtered SiO 2 . In order to ensure clean surface, prior to growth the SiO 2 thin films were etched down to 10 nm by a diluted buffered HF solution. After the etching the substrates were blown dry with nitrogen and immediately transferred in the MBE system. Prior to growth, the substrates were heated to a temperature of 650 • C in order to desorb any remnant molecules on the surface. Then, the temperature was lowered to the growth temperature of 630 • C. For the growth we have used As beam equivalent pressure (BEP) of 2×10 −6 mbar and Ga growth rate of 0.25Å/s, which gives respectively longitudinal and radial growth rates of 2Å/s and 0.07Å/s 10 . The nominal thickness of deposited GaAs was varied for different samples in order to synthesize nanowires with different average diameter. In this way, we have prepared samples with diameters ranging from 30 nm up to 160 nm. Each sample was characterized with rather narrow diameter distribution below 10%. Transmission electron microscopy (TEM) analysis on the grown wires showed that the wires grow in the (111) B growth direction and have a hexagonal cross section with side facets belonging to the {110} crystalline family 11 . AFM measurements on single nanowires have also shown that the side facets exhibit very small roughness. The Raman measurements were performed at room temperature by using the 488 nm line from Ar + laser. A microscope objective (50x) focused the laser on the sample with a spot of several micrometers in size. The same lens collected the scattered light to a triple DILORXY spectrometer and was further analyzed with a nitrogen cooled Si CCD. The measurements were realized with low excitation power (0.5 mW), with the purpose of avoiding the heating of the sample, which can produce asymmetric broadening and down shift of the Raman peaks. The sample temperature stayed always below 120 • C, as shown by Stokes/Antistokes ratio measurements. The wires were mechanically removed from the GaAs substrate and transferred on clean (001) Si pieces by friction between the substrates. The transferred wires were partially oriented along the sliding direction, as shown in the scanning electron micrograph presented in a). The inset shows the hexagonal cross section of the wires. Since the laser was focused on a spot with diameter of several microns, we estimate that only several nanowires were probed during each measurement. The scattering geometry is presented on . The arXiv:0804.3713v1 [cond-mat.mtrl-sci]
Micro-Raman spectroscopy for the detection of stacking fault density in InAs and GaAs nanowires
Physical Review B, 2017
We investigate the relation between crystal stacking faults in individual wurtzite InAs and GaAs nanowires and the intensity of the forbidden longitudinal optical (LO) phonon mode in the Raman spectra. Micro-Raman spectroscopy and transmission electron microscopy are combined on the same individual nanowires to evaluate the LO mode intensity as a function of the stacking fault density. A clear increase in the LO mode intensity was observed when the stacking fault density was increased. Our results confirm the utility of Raman spectroscopy as a powerful tool for detecting crystal defects in nanowires.
Nanotechnology, 2016
Any device exposed to ambient conditions will be prone to oxidation. This may be of particular importance for semiconductor nanowires because of the high surface-to-volume ratio and only little is known about the consequences of oxidation for these systems. Here, we study the properties of indium arsenide nanowires which were locally oxidized using a focused laser beam. Polarization dependent micro-Raman measurements confirmed the presence of crystalline arsenic, and transmission electron microscopy diffraction showed the presence of indium oxide. The surface dependence of the oxidation was investigated in branched nanowires grown along the [ ] 0001 and [¯] 0110 wurtzite crystal directions exhibiting different surface facets. The oxidation did not occur at the [¯] 0110 direction. The origin of this selectivity is discussed in terms transition state kinetics of the free surfaces of the different crystal families of the facets and numerical simulations of the laser induced heating.
Lineshape analysis of Raman scattering from LO and SO phonons in III-V nanowires
Journal of Applied Physics, 2009
Micro-Raman spectroscopy is employed to study the phonon confinement in Au-and Mn-catalyzed GaAs and InAs nanowires. The phonon confinement model is used to fit the LO phonon peaks, which also takes into account the contribution to the asymmetry of the line shape due to the presence of surface optical ͑SO͒ phonons and structural defects. This also allows us to determine the correlation lengths in these wires, that is the average distance between defects and the defect density in these nanowires. Influence of these defects on the SO phonon is also investigated. A good agreement between the experimental results and the calculations for the SO phonon mode by using the dielectric continuum model is also obtained.
Long-wavelength, confined optical phonons in InAs nanowires probed by Raman spectroscopy
European Physical Journal B, 2011
Strongly confined nano-systems, such as one-dimensional nanowires, feature deviations in their structural, electronic and optical properties from the corresponding bulk. In this work, we investigate the behavior of long-wavelength, optical phonons in vertical arrays of InAs nanowires by Raman spectroscopy. We attribute the main changes in the spectral features to thermal anharmonicity, due to temperature effects, and rule out the contribution of quantum confinement and Fano resonances. We also observe the appearance of surface optical modes, whose details allow for a quantitative, independent estimation of the nanowire diameter. The results shed light onto the mechanisms of lineshape change in low-dimensional InAs nanostructures, and are useful to help tailoring their electronic and vibrational properties for novel functionalities.
Surface optical phonon propagation in defect modulated nanowires
Journal of Applied Physics
Planar defects, such as stacking faults and twins, are the most common defects in III-V semiconductor nanowires. Here we report on the effect of surface perturbation caused by twin planes on surface optical (SO) phonon modes. Self-catalyzed GaAs nanowires with varying planar defect density were grown by molecular beam epitaxy and investigated by Raman spectroscopy and transmission electron microscopy (TEM). SO phonon peaks have been detected, and the corresponding spatial period along the nanowire axis were measured to be 1.47 lm (60.47 lm) and 446 nm (635 nm) for wires with twin densities of about 0.6 (60.2) and 2.2 (60.18) per micron. For the wires with extremely high density of twins, no SO phonon peaks were detected. TEM analysis of the wires reveal that the average distance between the defects are in good agreement with the SO phonon spatial period determined by Raman spectroscopy.
Nanotechnology, 2015
In this study, the effect of substrate orientation on the structural quality of Au-catalyzed epitaxial GaAs nanowires grown by a molecular beam epitaxy reactor has been investigated. It was found that the substrate orientations can be used to manipulate the nanowire catalyst composition and the catalyst surface energy and, therefore, to alter the structural quality of GaAs nanowires grown on different substrates. Defect-free wurtzite-structured GaAs nanowires grown on the GaAs (110) substrate have been achieved under our growth conditions.