[Show abstract][Hide abstract] ABSTRACT: Irradiance dependence of excitonic nonlinear absorption in Cadmium Sulfide (CdS) nanocrystals has been studied by using Z-scan method with nanosecond laser pulses. The wavelength dependence of nonlinear absorption has also been measured near the excitonic transition of 1S(e)-1S3/2(h). We observe the saturable absorption, which can be described by a third-order and a fifth-order nonlinear process for both 3.0-nm-sized and 2.3-nm-sized CdS nanocrystals. The experimental results show that the excitonic nonlinear absorption of CdS nanocrystals is greatly enhanced with decreasing particle size. A two-level model is utilized to explain both irradiance and wavelength dependence of the excitonic nonlinearity. Comment: 24 pages, 6 figures
[Show abstract][Hide abstract] ABSTRACT: We report on the fabrication of domain-reversed structures in LiNbO3 by means of direct electron beam lithography at room temperature without any static bias. The LiNbO3 crystals were chemically etched after the exposure of electron beam and then, the patterns of domain inversion were characterized by atomic force microscopy (AFM). In our experiment, an interesting phenomenon occurred when the electron beam wrote a one-dimensional (1-D) grating on the negative c-face: a two-dimensional (2-D) dotted array was observed on the positive c- face, which is significant for its potential to produce 2-D and three-dimensional photonic crystals. Furthermore, we also obtained 2-D ferroelectric domain inversion in the whole LiNbO3 crystal by writing the 2-D square pattern on the negative c-face. Such a structure may be utilized to fabricate 2-D nonlinear photonic crystal. AFM demonstrates that a 2-D domain-reversed structure has been achieved not only on the negative c-face of the crystal, but also across the whole thickness of the crystal. Comment: 17 pages, 4 figures
[Show abstract][Hide abstract] ABSTRACT: The rough interface between a silicide film and a Si substrate induces large junction leakage currents in Si-based devices. In this work, we demonstrate that micro-Raman imaging can be used to characterize the interface roughness, without any special sample preparation. Pure metal (Ni and Ti) thin films and the subsequently formed silicide thin films after annealing (NiSi and TiSi2) were investigated using the intensity of the Si Raman band at 520 cm-1. Uniformity results of the pure metal films are in good agreement with those obtained using global four-point-probe measurement. In comparison, the signals obtained from the interfaces between Si and NiSi are nonuniform with large variations. The intensity of the major peak of NiSi at 214 cm-1 shows a point-to-point correlation with the intensity of the Si peak at 520 cm-1, which reveals that the local grain orientation affects the diffusion velocity of Ni atoms and the growth of NiSi. Images of Ti-silicides show that the interface of the C49 TiSi2 phase is much smoother than that of the C54 TiSi2 phase which is due to different formation mechanisms. Simulation was carried out based on two interfacial models, a two-step interface and a sinusoidal interface, to understand the correlation between the average attenuation of the Si Raman signal and the film thickness.
No preview · Article · Mar 2005 · Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society
[Show abstract][Hide abstract] ABSTRACT: Amorphous boron nitride (a- BN ) was produced by ball milling of hexagonal BN (h- BN ) . Turbostratic BN (t- BN ) and mesographite BN (m- BN ) with different degrees of three-dimensional order ( DTDO ) were subsequently prepared by annealing the a- BN at different temperatures. The photoluminescence ( PL ) of the BN samples with different structures and DTDOs was investigated in the visible region from 460 to 800 nm . The h -, a -, and t- BN samples, which have completely ordered, disordered and two-dimensionally ordered structures, respectively, emitted weak continuums. The m- BN samples with three-dimensionally ordered structures gave discrete PL bands, which we attributed to the presence of N<sub>B</sub> antisites or a complex formed by an Fe atom and a B vacancy in the BN layer. The intensity and energy of the PL bands changed greatly with DTDO and could be tuned by varying the annealing temperature. The strongest PL bands at 593 and 612 nm were observed in one of the m- BN samples produced at an annealing temperature of 1100 ° C </formu-
la>, and their intensity was about 1000 times the intensity of h- BN . Unintentional Fe contamination did not affect the PL nature of the m- BN samples but increased the PL intensities of some bands greatly.
No preview · Article · Sep 2004 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Raman spectroscopy was used to investigate excimer laser annealing and thickness determination of amorphous silicon (a-Si) layers which are less than 20 nm thick. The a-Si layers were produced on silicon (Si) substrates using Si+ ion implantation with an energy of 10 keV and a dose of 1 × 1015 cm−2. Excimer laser annealing was applied to re-crystallize the a-Si layers. The dependence of re-crystallization on laser fluence was investigated using Raman spectroscopy. A threshold laser fluence of 0.4 J cm−2 was required to re-crystallize the a-Si layers. In Raman spectroscopy, the Raman intensity shows a periodical variation with a period of 90° as a function of the angle between the Si orientation and the laser polarization. Based on this phenomenon, a method to determine nanoscale a-Si film thickness was proposed in two ways. One way was carried out without sample rotation to determine the a-Si thickness provided that the reference c-Si and a-Si/c-Si samples are in the same crystal orientation. The other way was carried out with sample rotation to determine the a-Si thickness without knowing the crystal orientation beforehand.
[Show abstract][Hide abstract] ABSTRACT: Ge nanocrystals (nc-Ge) embedded in silicon oxide thin films have been fabricated using rf magnetron co-sputtering deposition and post-growth thermal annealing method. Raman scattering and photoluminscence (PL) measurements have been used to characterize their crystallization and light emission properties. The Ge crystallinity and nanocrystal size obtained using a three-peak fitting method based on a phonon confinement model have been found to increase with the increase of annealing temperature. The observed blue photoluminescence band located at ∼3.1 eV and a weaker band at ∼2.4 eV under the excitation of 325 nm laser did not show significant size dependence while their relative intensities were related to the addition of H2 or O2 into the sputtering ambient. The photoluminescence mechanism is discussed.
No preview · Article · Feb 2004 · Materials Science and Engineering B
[Show abstract][Hide abstract] ABSTRACT: The combination of near-field scanning optical microscopy and Raman spectroscopy provides chemical/structural specific information with nanometer spatial resolution, which are critically important for a wide range of applications, including the study of Si devices, nanodevices, quantum dots, single molecules of biological samples. In this paper, we describe our near-field Raman study using apertureless probes. Our system has two important features, critical to practical applications. (1) The near-field Raman enhancement was achieved by Ag coating of the metal probes, without any preparation of the sample, and (2) while all other apertureless near-field Raman systems were constructed in transmission mode, our system works in the reflection mode, making near-field Raman study a reality for any samples. We have obtained the first 1D Raman mapping of a real Si device with 1s exposure time. This is a very significant development in near-field scanning Raman microscopy as it is the first demonstration that this technique can be used for imaging purpose because of the short integration time. In addition, the metal tips used in our set-up can be utilized to make simultaneous AFM and electrical mappings such as resistance and capacitance that are critical parameters for device applications.
[Show abstract][Hide abstract] ABSTRACT: Half-metallic CrO2 powder compact with rod-shaped nanoparticles was studied by micro-Raman scattering in the presence of an external magnetic field at room temperature (300 K). In the low-field region (H ≤ 250 mT), the frequency and intensity of the Eg mode, an internal phonon mode of CrO2, increase dramatically with increase in the magnetic field, while the corresponding linewidth decreases. The above parameters become constant when the CrO2 powder enters the saturation state at higher magnetic field. The pronounced anomalies of the Raman phonon parameters under a low magnetic field are attributed to the spin–phonon coupling enhanced by the magnetic ordering, which is induced by the external magnetic field.
Preview · Article · Mar 2003 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: Tip characteristics play an important role in the resolution and sensitivity of scanning probe microscopy. Extensive efforts have been devoted to tip fabrication. Most of the research is focused on scanning tunneling microscopy applications, which require sharp and short tips. Long tips that can be bent into cantilevered tips have great potential in atomic force microscopy/apertureless near-field scanning optical microscopy applications. However, the fabrication of such tips has been rarely reported. The present work is carried out with the aim of optimizing the conditions suitable for fabricating long and sharp tungsten tips. Besides topography, optical, and spectroscopic information, electrical and magnetic measurements can also be carried out with such tips obtained with the recipe reported in this article. The long tips also make it possible to measure deep grooves/trenches.
No preview · Article · Aug 2002 · Review of Scientific Instruments
[Show abstract][Hide abstract] ABSTRACT: A MgB2 powder was mechanically milled for different times. X-ray diffraction indicated the formation of an intermediate phase with an unknown structure before formation of amorphous phase. The intermediate structure was superconducting below 18 K, while the amorphous phase was non-superconducting till 4.2 K. After heat treatment at 700 °C, all the mechanically milled samples formed back into the hexagonal MgB2 structure with Tc = 39 K. X-ray and Raman investigations suggested that the intermediate structure was metastable with a lattice compression, which was probably associated with the reduction in transition temperature.
No preview · Article · Jun 2002 · physica status solidi (a)
[Show abstract][Hide abstract] ABSTRACT: Raman scattering study of a dilute GaAsN epitaxy layer was carried out at variable temperature and pressure. The localization due to the presence of the N atoms is responsible for the small correlation length in the GaAsN alloy, which is also evident from the broadening and asymmetry of the LO mode of the GaAs-like Raman band. The temperature dependence of the correlation length was analyzed. Nitrogen-induced localization also has a strong influence on the pressure dependence of the Born's effective dynamic charge e*.
No preview · Article · Dec 2001 · Materials Science in Semiconductor Processing
[Show abstract][Hide abstract] ABSTRACT: Near-field scanning optical microscope (NSOM) has the potential to become a very important tool for material characterization due to its ability to investigate the structure and micro- environment of materials in nano-scale by performing spectroscopy as well as topographic mapping. However, near-field Raman results have been rarely reported although Raman spectra are unique in chemical and structural identification. This is due to the fact that Raman signal is intrinsically weak (less than 1 in 10 7 photons) and the laser power emerging from tip is extremely low (typically 100 nW) because of the low optical throughput of metal coated fiber tips. The long integration time (typically 10 minutes per spectrum) required for collecting good quality Raman spectra makes it impractical to construct a Raman image through this conventional method. In this paper, we report an integration of NSOM and Raman spectrometer using an apertureless configuration, in which the laser is focused onto the sample through a microscope objective and Raman signal is collected by the same objective. This is similar to the conventional micro- Raman except that a metal tip is brought into the laser spot on sample surface to enhance the Raman signal through surface enhanced Raman scattering (SERS). Raman enhancement of 10 4 times has been achieved and Raman mapping on real silicon devices has been realized with 1 second exposure time. Furthermore, the reflection scattering geometry employed in our experiments allows the study of any sample without specific sample preparation, unlike the conventional SERS which needs coating samples with metal or growing sample on metal surface.
[Show abstract][Hide abstract] ABSTRACT: High-energy mechanical milling of spinel NiFe2O4 leads to the formation of a disordered wustite-like structure. Cluster glass behavior was found in the Mössbauer study. The investigation suggested ferrimagnetic clusters in an antiferromagnetic matrix. The ferrimagnetic and antiferromagnetic exchange coupling results in a strong uni-directional anisotropy and a coercivity of over 10kOe after magnetic cooling.
No preview · Article · Jun 2000 · Solid State Communications