Long-wavelength, confined optical phonons in InAs nanowires probed by Raman spectroscopy (original) (raw)
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Structural characterization of GaAs and InAs nanowires by means of Raman spectroscopy
Journal of Applied Physics, 2008
We report Raman studies of GaAs and InAs nanowires ͑NWs͒ grown on SiO 2 and GaAs surfaces by means of catalyst-assisted molecular beam epitaxy. We have investigated several tens of NWs grown using either Mn or Au as a catalyst. The LO and TO phonon lines of the NWs showed an energy downshift and a broadening as compared to the lines usually observed in the corresponding bulk materials. A doublet is sometimes observed in the LO region due to the observation of a signal attributed to the surface optical ͑SO͒ phonon. The energy position of the SO phonon agrees with the values expected considering the section diameter of the NWs. LO and TO downshifts are due to the presence of structural defects within the NWs. The larger the energy downshift, the smaller the dimension of the defect-free regions. The results demonstrate that different catalysts provide wires with comparable crystal quality. The measurements also point out that differences in defect density can be found in wires coming from the same batch indicating that a substantial effort will be needed to obtain high homogeneities of the NW quality.
Applied Physics A, 2006
Raman scattering is shown to be an effective probe of optical and surface optical phonons in highly crystalline semiconducting nanowires (SNWs). We show that the confinement model of Richter et al. well describes the nanowire diameter dependence of the asymmetric broadening of the onephonon band in Si nanowires observed at ∼ 520 cm −1. We also show that the use of high laser flux (∼ 0.1 mW/µm 2) leads to a second mechanism that can asymmetrically broaden the 520 cm −1 Raman band. This broadening has nothing to do with confinement, and can qualitatively be understood in terms of inhomogeneous laser heating. A model is presented that supports this explanation. The production of SNWs via the vapor-liquidsolid growth mechanism leads, in many cases, to an instability in the nanowire diameter or cross-sectional area. In the second part of this review, we show that this instability activates the surface optical (SO) phonon Raman scattering. Examples of this phenomenon are shown for GaP and ZnS nanowires. The former and latter have, respectively, cylindrical and rectangular cross sections. We show that the cross-sectional shape of the nanowire is important for a quantitative analysis of these SO modes.
Internal field induced enhancement and effect of resonance in Raman scattering of InAs nanowires
Solid State Communications, 2013
An internal field induced resonant intensity enhancement of Raman scattering of phonon excitations in InAs nanowires is reported. The experimental observation is in good agreement with the simulated results for the scattering of light under varying incident wavelengths, originating from the enhanced internal electric field in an infinite dielectric cylinder. Our analysis demonstrates the combined effect of the first higher lying direct band gap energy (E 1 ) and the refractive index of the InAs nanowires in the internal field induced resonant Raman scattering. Furthermore, the difference in the relative contribution of electro-optic effect and deformation potential in Raman scattering of nanowires and bulk InAs over a range of excitation energies is discussed by comparing the intensity ratio of their LO and TO phonon modes.
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.
Raman-active phonon line profiles in semiconducting nanowires
Vibrational Spectroscopy, 2006
Results of Raman scattering investigations of optical phonons confined in the cross section of small diameter Si nanowires are discussed. Using low excitation intensity at 514.53 nm to study Si as a prototypical material, we first demonstrate that the outcome of the phonon confinement is an asymmetric broadening and downshifting of the LO-TO phonon line observed at 520 cm À1 in the bulk. The effect is important in Si wires with diameter d < 10 nm. We find good agreement between our data and an early theory due to Richter et al., provided we introduce an additional factor that sets the phonon confinement length. Furthermore, we examine the effects on the 520 cm À1 lineshape from increasing the laser power density in the tight microRaman focal spot size ($1 mm). We find that a second asymmetric line broadening mechanism is also present that can be identified with an inhomogeneous temperature distribution set up in the nanowires. This distribution is driven by the Gaussian spatial intensity of the laser beam in the focal plane of the microRaman instrument. Using the thermal properties of the phonons in Si, we can explain semi-quantitatively the complex changes in the lineshape that we observe in both small and large diameter Si nanowires, i.e., with or without phonon confinement as an active consideration.
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]
Probing Phonons in Nonpolar Semiconducting Nanowires with Raman Spectroscopy
Journal of Nanotechnology, 2012
We present recent developments in Raman probe of confined optical and acoustic phonons in nonpolar semiconducting nanowires, with emphasis on Si and Ge. First, a review of the theoretical spatial correlation phenomenological model widely used to explain the downshift and asymmetric broadening to lower energies observed in the Raman profile is given. Second, we discuss the influence of local inhomogeneous laser heating and its interplay with phonon confinement on Si and Ge Raman line shape. Finally, acoustic phonon confinement, its effect on thermal conductivity, and factors that lead to phonon damping are discussed in light of their broad implications on nanodevice fabrication.
Effects of dangling bonds and diameter on the electronic and optical properties of InAs nanowires
In this article we explore the effects of dangling bonds and diameter on the electronic properties of the wurtzite InAs nanowires (NWs) using the density functional theory. The NWs are confined in the hexagonal supercell and are simulated in the [0001] direction. The calculations have been carried out by applying the periodic boundary conditions along the NW axis, i.e., z-Cartesian coordinate, providing enough vacuum to isolate the system from its neighbors. The optical properties of a material are directly related to the band-gap; therefore a relationship between the band-gap and diameter of the nanowires is obtained by using two models, where the band-gap for a larger diameter NW can be estimated. The results of these models are compared with each other and the effects of the dangling bonds on the band-gaps are also investigated. The band-gap of the nanowires decreases and the dangling bond ratio increases with the increase in the diameter of the nanowire, and hence we expect that for large diameter nanowires the band-gap will approach the band gap of the bulk material. An interesting feature of the shift in the band-gap from indirect to direct, i.e. optically inactive to active, is also observed in these NWs with the increase in the diameter.