Research Topics

　

          size effect

          shape effect

          surface phonons

          SWNT Chem-FET

          NW Chemical Sensors

          Vibrational modes

          Electronic modes

　

Surface optical phonons in III-V, II-VI semiconducting nanowires

  Qihua Xiong

　

In a static model of lattices, the position of atoms is fixed. However, in a lattice dynamics model, the atoms are treated as objects with finite masses capable of motion (vibration). Phonons are therefore quanta of collective lattice vibrations. Phonons can be created or annihilated. Since phonons are bosons, they obey the Boson-Einstein statistics. Phonon is one of the fundamental concepts in condensed matter physics to understand not only the fundamental properties of condensed matter materials (e.g., heat capacity, resistivity, etc.) but also some advanced topics in condensed matter physics (e.g., superconductivity). Lattice dynamics is to find the normal modes of lattice vibrations. The relation between energy of the phonons (or frequencies) and the wave vectors is called phonon dispersion. For example, the phonon dispersion is like the following picture for a di-atomic one-dimensional lattice.

If we consider a three dimensional lattice, the branches of phonons (either longitudinal/transverse acoustic or optic, LA, TA, LO, TO) depends on the number of atoms in a primitive unit cell. For instance, if there are N atoms in a primitive unit cell, there are 3 branches of acoustic phonons and 3(N-1) optic phonons. The exact phonon dispersion curve is also dependent on the characteristic of the interaction between atoms, e.g., polar or non polar, and the crystal structures, e.g., body-centered cubic (bcc) or face-centered cubic (fcc). Here two examples of phonon dispersion curves are given for a diamond structure (diamond and silicon, Fig.1 and Fig.2), zinc-blende structure (GaAs, Fig.3) and wurtzite structure (GaN, Fig.4). In non-polar semiconductors (like Si and C), the LO and TO at zone center have the same frequencies. However, for polar semiconductors (like, III-Vs, II-VIs), the LO and TO frequencies split at the zone center because of long-range dipolar interaction. In semiconducting nanowires, compared with bulk semiconductors, Prof. Mahan recently worked out that the dipole sums of long-range dipolar interaction gives rise to another important splitting of the Raman-active LO and TO phonons at the zone center. The dipole sums that determine the LO and TO phonon frequencies in the nanowires are actually sensitive to the aspect ratio, i.e., the ratio between the length and diameter. This effect is normally referred as "Shape Effect". 

Fig.1 Schematic phonon dispersion curve in diamond

　

Fig.2 Phonon dispersion curves in Silicon

Fig.3 Phonon dispersion curves in GaAs (zinc-blende structure)

Fig.4 Phonon dispersion curves in GaN (wurtzite structure)

SWNT-FETs for Chemical Sensors 

Awnish Gupta

Isolated nanotubes on Si/SiO2 substrate are grown by CVD process using CH4 as a precursor gas. Iron catalyst particles are prepared by dipping the substrate in Fe(III)NO3 solution (Hafner et al). Nanotubes are grown at 800-1100 oC. These nanotubes samples are characterized by RAMAN as well as AFM to relate the growth conditions with the sample (diameter, length and density).

　

Fig 1. AFM Image of substrate after nanotubes growth (left), Micro-Raman at one sample point shows 2 sharp peaks in RBM, which corresponds to .7 nm & 1.1 nm diameters.

　

AFM statistics shows bi-average diameters of .8 nm and 1.3 nm and an average length of 4 mm for this particular sample grown at 900 oC. A good control over length, diameter and the density of nanotubes has been achieved by varying the growth conditions.

 SWNT-FETs are made by putting metal contacts (Ti/Au) on a nanotubes and using highly doped silicon as a back gate. Measurements on these SWNT-FETs are in progress

　

　

　

　

　

　

InN and GaN nanowires and their integration in III-nitride one-dimensional heterostructures

Humberto R. Guti└rrez

　

     In the last decade, group III-nitrides have received a lot of attention due to their very attractive optical and electrical properties for opto-electronic devices such as blue-green semiconductor lasers, light detectors and high electron mobility transistors (HEMT). Particularly, InN has very interesting transport properties such as mobility and saturation velocity higher than GaN and GaAs. It has been shown that the excellent transport properties of InN do not change across a wide range of temperature and doping concentration. For this reason InN, when combined with other nitride compounds such as GaN and AlN, offers potential advantages for high frequency devices, either as heterostructure or ternary alloys. Furthermore, changing the composition of ternary alloys such as InGaN and InAlN, a wide range of wavelength (from visible to ultraviolet) can be scanned in light emitting devices. Intense research has been done in InN-based thin film devices. However the large lattice mismatch between InN and GaN or AlN can be an obstacle for heterostructure-based device quality. In this sense free-standing nanowires-based one-dimensional heterostructures can be an alternative to reduce the formation of crystalline defects. In our laboratory we work on the synthesis, doping and characterization of InN and GaN nanowires and their integration in III-nitride one-dimensional heterostructures. The growth of InN nanowires takes place within a CVD reactor placed in a three-zone furnace. Different metal nanoparticles are used as catalysts to promote the vapor-liquid-solid growth mechanism. Our approach permits flexibility for controlling the sources, doping and substrates temperature independently