Interface trap densities in the upper half of the (0001) 4H-SiC bandgap for unpassivated samples and samples passivated with NO and NO + H2.
The planar semiconductor-dielectric interface is the key to a successful MOSFET technology and has played the essential role in the silicon revolution. Wide-band gap materials have presented a challenge to achieve the same degree of interface perfection as silicon, although considerable progress is underway.
The SiO2/SiC interface is of particular scientific interest in this development because of its close relationship to silicon, both in processing and structure.
Silicon carbide itself provides an intriguing scientific platform for understanding such interfaces due to the availability of many poly-types with different band-gaps, access to different crystal faces, polar and non-polar, and with a fabrication process that is similar to the well-studied silicon structure.
Thus Systematic use of chemical modification and processing, combined with a careful analysis of interfacial structure, results in significant progress in reducing defects and improving device properties.
Size tunable fluorescence emission from CdSe/ZnS core shell nanocrystals
The introduction of nanocrystalline materials into opto-electronic devices such as light emitting diodes, photo-detectors, blue lasers etc. necessitates the understanding of the reliability of these devices under harsh radiation environments. The radiation tolerance in these nanocrystalline materials can be significantly different from bulk due to large surface contributions and quantum size effects.
One of the most ideal nanocrystalline materials is CdSe quantum dots with excellent size contributions and optical properties.
Absorption and photoluminescence spectrum of a CdSe nanocrystal
Our objective is to experimentally ascertain the cross-section for damage for a given size of a CdSe nanocrystal. We have measured the proton irradiation effects on the optical properties of these CdSe nanocrystals. We employ a drop casting procedure to make thin films of these nanocrystals, synthesized by the colloidal technique. A Fluorolog spectrophotometer is used to gather the luminescence information before and after irradiation from these films.
Rutherford Backscattering Spectrometry (RBS) approach is adopted to verify the absence of sputtering of organics from the surface of the nanocrystals due to proton beam bombardment. The cross-section for damage per nanocrystal is experimentally determined and compared with a theoretical model.
Scanning Electron Micrograph of a collection of VO2 nanoparticles
Vanadium dioxide is a fascinating material which undergoes an abrupt semiconductor to metal transition (SMT) at the convenient temperature of 70oC. We, and others, have demonstrated that the critical temperature may be varied with the appropriate dopants and dimensionality. Since the transition implies great changes in the optical and electrical properties of the material, this makes it suitable for optoelectronic applications like memory devices, optical limiters...
We use a Pulsed Laser Deposition (PLD) setup to synthesize the oxide. We use Rutherford Backscattering (RBS) methods to inspect the stoichiometry, infrared lasers to study the optical properties and various electrical instruments to measure the resistance behavior as a function of temperature.
Our main objective is to understand how the properties of the phase transition change with the thickness of the vanadium dioxide films and how much it is affected by the size of the nanocrystalline domains.
Direct-current conductivity vs reciprocal temperature for unoriented V2O3 crystal
We have recently developed an interest in vanadium sesquioxide (V2O3)--another compound that exhibits a first-order metal-to-insulator phase transition. At room temperature, V2O3 is a "poor" paramagnetic metal, which abruptly switches to a "poor" antiferromagnetic insulator at 150 K (-123°C); changes up to eight decades in the resistivity of bulk V2O3 samples have previously been reported.
Our initial goal is to synthesize thin films of stoichiometric V2O3 using the PLD system, and to perform electrical and optical measurements as a function of temperature.
We intend to fabricate regular as well as random arrays of V2O3 nanoparticles, and hope to uncover new physical effects that arise from finite sizes and collective interactions.
The study of dynamics of bond-selective vibrational excitation of hydrogen in solids is one of our major interests. Knowledge of the rates and pathways of energy flow is critical for a complete understanding of solid state matter. The vibrational lifetime of hydrogen related bonds in semiconductors can influence the device reliability. Previous studies predicted extraordinary variations of a factor of ~100 in vibrational decay rates. Such large variations are unusual in solids and motivate a deeper understanding. A major innovation in this study is to elucidate the respective roles of phonon and electronic channels in determining the population lifetime and the role of dephasing on the collective dynamics of the system. This study would provide an underpinning to the exciting vision of wavelength-specific materials modification. Special equipment:
In-situ FTIR and In-situ ultra-fast pump-probe spectroscopy after low temperature (6K) implantation.
Hydrogen passivation on silicon surfaces is a bottleneck in low temperature Si homoepitaxy. We employ infrared laser radiation from the Vanderbilt Free-Electron Laser (FEL) to desorb hydrogen from a Si(111) surface at a temperature below 300C where thermal effects are minimal. The vibrational mode of the Si-H bond is excited by the infrared laser to enhance hydrogen desorption. Since their vibrational energies are different at terrace sites and step sites, this provides a way to selectively desorb hydrogen atoms from different locations on the Si surface, and allows site-selective epitaxial growth of Si.
Currently we examine the dependence of hydrogen desorption on the wavelength and polarization of the FEL. We find that the coupling between the laser and the Si-H bonds shows a resonant effect at the energy of the vibrational mode at 2087/cm. Our ultimate goal is to develop a new selective crystal growth method, based on this mechanism, for fabrication of complicated and robust nanostructures in harsh environment.
©Aksimentiev A. et al., Biophys. J., 2004, 87, 2086
We intend to investigate the dynamics of fluid flow through nanopores to gain a better understanding of the effect of the electric double layer (EDL) on fluid flow at these small dimensions. Nanopores are fabricated in silicon wafers using the focused ion beam (FIB) microscope in conjunction with regular silicon processing techniques.
These nanopores will then be used to study the conductance properties of electrolyte solutions and colloidal suspensions at these small dimensions. Understanding fluid flow through nanopores will enable us to design single-molecule devices such as single-strand DNA sequencers as shown in the figure below.
The figure shows a possible experimental setup and expected results for such a device.
This project demonstrates the feasibility of using silicon based planar heterostructure technology as a substrate for realizing zero-, one-, two- and three-dimensional nanostructures with applications in the area of nanofluidics.
The Figure shows an example of a nano-porous silicon membrane 100 nm thick that was produced in a “top down” approach using a Focused Ion Beam Microscope to sputter an array of 42 nm diameter holes through the thickness of the membrane.
These membranes can function in a nanofluidic device as an addressable ion channel array.
Studies of the optical properties of materials in the nano-scale is a powerful tool to analyze, control, and predict the structure and composition of nano-materials.
We employ several approaches using volume high-sensitive spectroscopy and microscopic methods, involving scanning near-field optical microscopy (SNOM). Scanning methods (nano-Raman and other spectroscopic measurements through the SNOM fiber tip) allow to provide surface spectroscopic mapping with nano-scale lateral resolution.
Different quantum selection rules for optical excitation and emission in materials in the near-field zone of the fiber probe show new aspects of the material structure.