The electricity consumption in houses and commercial buildings generates about 18% of greenhouse gas emission. A critical issue of building energy consumption is heat and cooling loss through the window. Low-emissivity windows control thermal radiation through glass without decreasing the intensity of visible light. They are made up of optical filter coatings grown on a flat glass surface. Solar filters based on Ag/IZO multilayer films are grown and simulated on glass substrate. The targeted structure designs are grown by a sputtering system and characterized by scanning electron microscopy and x-ray diffraction techniques. To accurately simulate transmission spectrum, silver (Ag) and IZO optical constants were estimated by fitting ellipsometric data at different thicknesses. Transmission spectrum shows a good agreement among experiment and simulation. In addition, optical constant curves strongly show layer thickness dependence in both materials. In particular, the ultrathin Ag layer displays a percolation threshold in the vicinity of 15 nm, which leads to surface plasmon resonance with absorption at about 450 nm. These types of optical filter coatings would have potential applications as low-emission windows.
We report the observation of surface-enhanced Raman scattering (SERS) from a chemically etched ZnSe surface using 4-mercaptopyridine (4-MPy) as probe molecules. A thin film of ZnSe is grown by molecular beam epitaxy (MBE) and then etched using a strong acid. Protrusions of hemi-ellipsoidal nanoparticles are observed on the surface. Using the results of the Mie theory, we controlled the size of the nanoparticles to overlap significantly with maximum efficiency of near-field plasmon enhancement. In the Raman spectrum, we observe large enhancements of the a1, b1, and b2 modes when 4-MPy molecules are adsorbed on the surface using a 514.5 nm laser for excitation, indicating strong charge-transfer contributions. An enhancement factor of (2×106) is observed comparable to that of silver nanoparticles. We believe this large enhancement factor is an indication of the coupled contribution of several resonances. We propose that some combination of surface plasmon, charge transfer, band gap resonances are most likely the contributing factors in the observed Raman signal enhancement, since all three of these resonances lie close to the excitation wavelength.
Using surface enhanced Raman spectroscopy (SERS), we have observed bio-molecules at extremely low concentration, adsorbed on self-organized semiconductor quantum dots, grown by molecular beam epitaxy. Quantum dots have found application in the field of biosensors, and the performance of these devices depends critically on the properties of the surface features. It is therefore of interest to explore useful and versatile spectroscopic sensing technique such as SERS to determine these properties. The SERS technique employs rough substrates with structures in the nanometer range to enhance Raman signals from adsorbed species. This spectroscopy has a number of important advantages: sensitivity, selectivity and non-destructive detection. In addition to this, SERS enables the determination of detailed information about adsorbed species such as molecular structure and orientation, while greatly increasing the Raman cross section and suppressing fluorescence. We show that the Raman signal observed from various biologically important molecules can be enhanced by up to six orders of magnitude by means of surface enhancement caused by adhesion to self-organized CdSe/CdZnSeMg quantum dots grown by molecular beam epitaxy.
We have grown high quality lattice-matched ZnCdSe and ZnSeTe on InP. To optimize the interfaces, the initial growth temperature was lowered and an As flux was used during the thermal treatment of InP substrates prior to epitaxial growth. Under optimized condition, 2D nucleation was observed by reflection high energy electron diffraction (RHEED) throughout the entire growth. Photoluminescence (PL), photoreflectance (PR), transmission electron microscopy (TEM) were used to carry out the sample characterization. Low temperature PL spectra for ZnCdSe show a narrow excitonic emission. PR spectra from ZnCdSe samples also suggest very high quality layers. The ZnSeTe exhibits a strong defect level emission at energy close to band gap and very weak deep level emission. TEM study suggest that the interfaces are comparable to those obtained between ZnSe and GaAs. These results, combined with the new possibilities from these materials, make InP an attractive substrate for II-VI epitaxy.
Real-time control of epitaxial crystal growth is a necessity for the production of advanced materials in order to improve the yields of new generations of digital, RF, and optoelectronic devices. Process tolerances are becoming tighter in terms of both layer stoichiometry and layer thickness. The traditional grow-characterize-grow again technique that has served us well over past decades is no longer a production worthy method of ensuring that wafers grown after calibration meet the design specifications. The day to day drift in most epitaxial growth systems is often as great as the wafer specification window. In this paper we describe a spectroscopic ellipsometer based control system and present results obtained for GaAs-AlGaAs structures grown by organometallic molecular beam epitaxy.
The stimulated emission spectra of photoexcited quantum-well structures were measured at room temperature,
77 and 1.8K as a function of the illuminated stripe length and excitation intensity. Three
samples are examined: a multiple quantum well, and two superlattices (one of them is type-Il). Based
on these data, the gain spectra and gain saturation behavior were obtained using Agrawal's model for
the analysis. Both the spontaneous photoluminescence spectra and the stimulated emission spectra are
used in order to analyze the spectral shifts. Kronig-Penney model was used to approximate the energy
level diagram of our samples.