Production and characterization of nanoscale structures

In an innovative use of scanning tunnelling microscopy (STM), the Gallagher group is writing nanometer-scale aluminum features on silicon by pinning aluminum with the electron beam from the STM; this research is relevant to quantum-limited electronics. Using the technique of single molecule confocal microscopy, the Nesbitt and Gallagher groups are investigating quantum dots, which are semiconductors intermediate in size between single molecules and the condensed phase. The fluorescence from a single CdSe dot as a function of time, shown in Figure 1, exhibits a digital "on/off" emissive behavior, in which periods of emission are followed by periods during which the dot no longer emits light. Such studies provide insight into the evolution of electronic and optical properties of matter with sample size. Applications of such materials range from nanoscale electronics to biological fluorescent labeling. Figure 1. Fluorescence trajectory of a quantum dot.

As the need for more complex semiconductors increases, so does the need for precise control over growth processes. In the deposition of germanium on silicon (100), nanodots grow spontaneously because of the mismatch of lattice spacings. Recent experiments in the Leone group show that the growth of these nanodots can be controlled by using small amounts of an arsenic "surfactant" during the molecular beam epitaxy deposition. Figure 2 shows atomic force microscopy (AFM) images of the germanium nanodots for different arsenic coverages, ranging from 0 to 1 monolayer. Figure 2. AFM images of spontaneous growth of nanodots.

Self-organized quantum dots of InGaAs grown on a GaAs surface are being developed for use in near-infrared (NIR) lasers. Using a pump-probe technique, the Cundiff group is studying both carrier capture into the dots and relaxation within the dots. The dots are fabricated in a waveguide. A femtosecond NIR probe pulse is coupled into the waveguide. Carriers are injected into the dots using a 750 nm pump pulse. By varying the delay between the pump and probe pulses, as shown in Figure 3, relaxation is studied. Figure 3. Pump-probe results for InGaAs quantum dots. Variation with temperature reveals the role of phonons in carrier relaxation.  [D]

Investigation of film fabrication

The Gallagher group is investigating the plasma processing of films used for solar panels. A laser is combined with STM both to detect particles formed in the plasma above the film during fabrication and to correlate these particles with film defects. This work has the potential to enhance the efficiency of photovoltaic devices. The Leone group is developing a new infrared (IR) near-field optical microscope to interrogate photoresist polymer films with wavelength tunability. This microscope combines the high spatial resolution of visible near-field scanning optical microscopy (NSOM) with the vibrational selectivity of IR spectroscopy. A new method for producing IR-transparent probe tips, shown in Figure 4, results in a 3-orders-of-magnitude improvement in throughput over conventional methods. Figure 4. Scanning electron micrograph (SEM) of a new NSOM tip used for mapping submicrometer chemical modification (left) and the infrared transmittance of this tip (right) at relevant infrared wavelengths.  [D]

Using this technique of IR NSOM, the Leone group is developing photoresist metrology. Figure 5 shows both infrared (left) and topographic (right) images of a photoresist sample, for two IR wavelengths (top and bottom). By tuning the IR light source, molecular group specificity is achieved. Thus latent image metrology provides the means to study the evolution of chemical modifications to a photoresist polymer throughout microlithographic processing. In another approach to film fabrication, the Leone group is using lasers to produce translationally fast atoms and molecules for deposition and etching. Specific studies include the growth of cobalt disilicide and the etching of silicon by chlorine. This work shows that kinetic-energy-enhanced chlorine can induce etching of room temperature silicon, a process potentially important for the achievement of damage-free etching. Figure 5. IR (left) and topographic (right) images of a photoresist sample for two different IR wavelengths (2.8 µm top, 2.95 µm bottom). The different contrast observed for the two wavelengths demonstrates chemical specificity.

Investigation of surface processes

The investigation of surface processes on films used in ultra-miniature electronic devices requires detection techniques with high spatial resolution. In a modification of traditional NSOM, which has used probe tips constructed of metal-coated optical fibers, the Nesbitt and Gallagher groups are developing apertureless NSOM, which results in a spatial resolution of 2 nm to 3 nm, an order-of-magnitude improvement. A combination of NSOM with both STM and lasers enables both identification of surface spectral features and time-resolved interrogation of fast processes on surfaces. Figure 6 shows the remarkable sensitivity of this technique. The sub-diffraction-limited shadow seen in the upper fluorescence image is caused by the AFM tip blocking the molecular fluorescence. These images represent a world record for the highest spatial resolution achieved to date with near-field excitation and fluorescence detection. In another study, the Cornell group is working on a novel method to cool semiconductor materials by optical pumping with lasers. In this technique, a laser, tuned to the band gap, excites an electron, giving it just enough energy to attain the band gap but not enough to have any kinetic energy. Subsequent equilibration of this electron causes heat to flow from the semiconductor into the electron. Thus, upon recombination, a photon is emitted with higher energy than the excitation photon, leaving behind a cooler semiconductor. The achievement of such optical refrigeration of solid-state materials is expected to have impact on the efficiency of both light-emitting diodes and semiconductor lasers. In a third study, the Cundiff group is using the technique of surface second-harmonic generation (SSHG) to characterize semiconductor-dielectric interfaces. This interface-sensitive technique enables in situ probing of the roughness of an interface. As the sizes of semiconductors shrink, the effect of any interface roughness becomes increasingly detrimental to the semiconductor performance, making this research especially relevant to the electronics industry. Moreover, since the physical processes underlying the effect of interface roughness on the SSHG signal remain a mystery, another goal of this research program is the elucidation of these physical processes.

Figure 6. Apertureless AFM/NSOM fluorescence image (above) of a dye-doped polystyrene nanosphere (~80 nm) and the corresponding AFM image.

Interaction of ultra-short laser pulses (10-100 femtoseconds) with solid-state materials

In another program, the Cundiff group is developing a dynamic pump-probe spectroscopic technique to investigate ultra-fast carrier relaxation in semiconductors. Figure 7 shows a transient four-wave mixing (TFWM) signal from an indium-galium arsenide (InGaAs) multiple quantum well as a function of the delay between two incident laser pulses. Figure 7. Transient four-wave mixing signal from a multiple quantum well.  [D]

Resources

Scanned probe microscopy (SPM) is heavily represented within the catalog of surface science methods now used by the Quantum Physics Division, which includes scanning tunneling microscopy (STM), atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM). Quantum Physics Division scientists use SPM to diagnose deposited and etched surfaces and to elucidate the associated surface chemistry on the atomic scale. Another use of SPM is to modify surfaces, producing nanostructures whose properties display many quantum characteristics usually associated with individual molecules. A new W. M. Keck Optical Measurement Laboratory at JILA provides Quantum Physics Division scientists with a suite of general-purpose optical measurement and characterization instruments, including a scanning electron microscope (SEM) and an atomic force microscope (AFM). It also provides both nanofabrication facilities, such as vacuum deposition, and optical polishing instruments. In addition, the Quantum Physics Division instrument shop and special techniques laboratory are well equipped with glass blowing and optical coating facilities.

For technical information or questions, please contact us: Eric Cornell 303-492-6281 fax: 303-492-5235 Cornell@Jila.Colorado.Edu Steven Cundiff 303-492-7858 fax: 303-735-0101 CundiffS@Jila.Colorado.Edu Alan Gallagher 303-492-7841 fax: 303-492-5235 Alang@Jila.Colorado.Edu Stephen Leone 303-492-5128 fax: 303-492-5504 Srl@Jila.Colorado.Edu David Nesbitt 303-492-8857 fax: 303-735-1424 Djn@Jila.Colorado.Edu NIST is an agency of the U.S. Department of Commerce. Online: May 2000   -   Last update: August 2007