Size and environment dependence of surface phonon modes of gallium arsenide nanowires as measured by Raman spectroscopy (original) (raw)

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]