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We use Raman scattering to study the spatially-resolved strain and stress in a complex zinc blende GaAs/GaP heterostructured nanowire which contains both axial and radial interfaces. The nanowires are grown by metal-organic chemical vapor deposition in the  direction with Au nano particles as catalysts, High spatial resolution Raman scans along the nanowires show the GaAs/GaP interface is clearly identifiable. We interpret the phonon energy shifts in each material as one approaches the interface.
Recently, Fickenscher et al.  have shown that, in a core-multi-shell structure where a GaAs quantum well is embedded into an AlGaAs shell wrapped around a  oriented GaAs nanowire, the electron and hole ground states are strongly confined to the corners of the hexagonally symmetric quantum well. Thus this confinement defines quantum wires which run along the length of the nanowires along its corners. Here we review single nanowire photoluminescence measurements which show the significant confinement energy of the excitons. For well widths larger than 5 nm, optical transitions between electron and hole excited states can be seen in excitation spectra, while for widths less than 5 nm only the ground state optical transitions are observed. For well widths smaller than 5 nm, high resolution spatially resolved photoluminescence measurements show directly the appearance of localized states. Single nanowire spectra from the 4 nm QWT sample display ultranarrow emission lines on the high energy side of the luminescence band. Spatially-resolved PL images show that these quantum dots are localized randomly along the length of the wire.
We study the photocurrent from photoexcited charged carriers excited with lasers of energy both above and below the energy gap in CdS nanostructures. We observe non-linear photocurrents in CdS nanosheet devices in the metal-semiconductor-metal configuration with Schottky contacts for sub-band gap excitations. Analysis of two-photon absorption dominated photocurrents reveals a nonlinear coefficient of β = 2 cm/GW for these nanosheet devices, which is comparable to those of bulk CdS. We demonstrate the use of the photocurrent polarization measurements to determine the orientation of atoms in the nanosheet.
We demonstrate the newly developed technique Photomodulated Rayleigh Scattering spectroscopy in order to probe the electronic band structure of single semiconductor nanowires. We show that both the electronic transition energies and nanowire diameter can be measured simultaneously and with high accuracy in a single non-destructive measurement. We demonstrate our results for zincblende GaAs as well as wurtzite InP nanowires where we probed the band gaps and transition energies at both room and low temperatures. This technique should advance the study of optical properties of single nanowires as well as other types of nanostructures.
In the spirit of Frege's gripping opener in “Ueber Sinnund Bedeutung”, one can equally well say that the concept of existence challenges reflection; for how can one deny that Pegasus exists without presuming existence? After all, such claims can be informative, for they could be false. Consequently, one might argue, they must say something about something. Thus, they succeed in being, in a back-handed, paradoxical way, existence statements of a sort. This problem is very old; it is Plato's problem of non-being. Frege's solution to the problem is also well known.
We report on single dot photoluminescence imaging and spectroscopy at B=0T on magnetically doped CdMnTe self-assembled quantum dots with average Mn concentration of several percent. Quasi-resonant excitation with circularly polarized laser leads to formation of magnetic polarons with magnetization induced by the laser light. In this case all quantum dots are polarized in the same direction. In contrast, when the dots are populated using above the barrier excitation, with randomly polarized excitons, the resultant magnetization is random and varies from dot to dot. These experiments demonstrate a way to control the magnetization of magnetically doped quantum dots by means of light excitation. In addition, they point towards extremely long spin memory times in these structures, reaching hundreds of microseconds, making CdMnTe quantum dots promising candidates for local magnetic field sources on the nanoscale.
Raman scattering experiments on high quality ZnGeP2 single crystals grown by the seeded horizontal dynamic gradient technique have been carried out. Polarized Raman spectra were obtained in the backscattering geometry at both room and low temperatures for several crystal orientations and compared with group theoretical predictions. Raman spectra from as-grown and annealed samples display distinctive differences which were explored by utilizing two different excitation wavelengths: 514.3 nm and 632.8 nm; the observed differences are attributed to a surface interdiffuasion effect.
Compositional interdiffusion in Al0.3 Ga0.7 As/GaAs superlattices induced by Si focused ion beam implantation and subsequent rapid thermal annealing is modeled using a set of diffusion equations which take into account the dynamics of the vacancy spatial profile. The inclusion of a new phenomenological term, which depends on the time derivative of the vacancy concentration spatial profile, provides good agreeement with experiment.
Compositional interdiffusion in Al0.3Ga0.7As/GaAs superlattice structures with equal 3.5 nm barrier and well widths induced by Si focused ion beam implantation and subsequent rapid thermal annealing has been modeled. A strong depth dependence of the mixing process is observed at a Si++ energy of 100 keV and at a dose of 1×1014 cm−2; this depth dependence is modeled by considering the second derivative of the vacancy profile. That is the maximum in the vacancy injection generated by the transient vacancy concentration gradient. We have included the dynamics of the spatial vacancy profile in the model and find good agreement with experimental results.
Interdiffusion across the well/barrier interfaces modifies the subband structure in AlGaAs/GaAs single quantum well (QW) structures. We have investigated the interrelated changes in both confinement energy of the subband states and the composition dependence of the bandgap energy in the QW, both of which are a strong function of the initial well width. Higher order transitions are found to be more sensitive than the ground state transitions to interdiffusion especially during the early stages of interdiffusion. These calculations model the experimental measurements (photoluminescence and photoreflectance) which are used to characterize interdiffused QW structures.
Photon scanning tunneling microscopy (PSTM) has been used to obtain effective refractive indices of optical channel waveguide structures. The local evanescent field intensity associated with the propagation modes of optical channel waveguides are measured at two different wavelengths. Both a tapered optical fiber tip and a semiconductor heterostructure tip are employed for detection. Local values of effective refractive index are measured for both TE and TM polarizations and compared to model calculations.
Temporally and spatially resolved photoluminescence has been used to study patterned structures prepared by focused ion beam (FIB) implantation of multiple quantum wells (MQWs) followed by rapid thermal annealing (RTA). Exciton lifetimes at different positions across the interface between implanted and unimplanted regions suggest that the interface between these two regions is of good quality. Anisotropic exciton diffusion is also observed, suggesting that these structures provide lateral confinement for excitons.
Raman scattering has been used to characterize ultrathin films of β-SiC, ranging in thickness from 38 nm to 240 nm. These films were prepared on the surface of a <100> Si substrate by a carbonization process at a temperature of 1300°C. In each case, the LO phonon near 970 cm−1 and the TO phonon near 795 cm−1 are observed, indicating the formation of β-SiC crystal. The Raman linewidths and peak positions indicate evidence of nonuniform stress and disorder. The Raman intensity of the TO phonon is nearly twice the intensity of the LO phonon measured both with and without the Si substrate, which indicates that the crystal growth was not entirely confined in the <100> direction.
Raman and both cw and time-resolved photoluminescence (PL) spectroscopy have been used to characterize 3.5nm/3.5nm Al0.30Ga0.70 As/GaAs multiple quantum well structures that have been patterned by focused ion beam (FIB) implantation followed by rapid thermal annealing (RTA). Raman scattering is used to characterize a FIB-delineated optical waveguide structure by identifying the appropriate RTA conditions that provide for removal of ion implantation induced damage in order to produce compostionally mixed regions that possess crystalline order. Spatially and temporally resolved PL spectra provide information on the degree of compositional mixing, exciton lifetime, and lateral transport. Anisotropic diffusion of excitons is observed in samples patterned with parallel line structures.
Raman and photoluminescence (PL) spectra have been used to characterize A10.3Ga0.7As/GaAs multiple quantum well (MQW) structures that have been patterned by focused ion beam (FIB) implantation followed by rapid thermal annealing (RTA). Microprobe Raman scattering is used to identify the appropriate RTA and FIB implantation conditions that provide for removal of implantation-induced damage and for compositional intermixing. FIB patterned wire-like structures are characterized by spatially resolved PL spectra.
Optical channel waveguiding in AlGaAs multiple quantum well structures formed by compositional mixing implemented by focused ion beam (FIB) implantation is demonstrated. To achieve selective mixing, Si is FIB implanted with a dose of 5×1014 cm−2 followed by RTA at 950°C for 10 s. Raman microprobe spectra are used to characterize the lateral variation of mixing. Propagation loss in a channel waveguide is measured. Measurement of the waveguide mode field distribution allows for the determination of changes in refractive index due to mixing and an approximate mixing depth.
The performance and reliability of personal computers, workstations and other electronic products depend on the effective soldering of electronic components to printed wiring oards (PWBs). The copper surfaces of PWBs are frequently treated with benzotriazole (or similar organic complexes) to preserve solderability by preventing copper oxide formation. However, new assembly process techniques and more complex processes may degrade protective organo-copper surface complexes and allow copper oxidation to occur, thus inhibiting subsequent solder operations. This study uses Auger electron spectroscopy (AES) in conjunction with meniscograph wettability results to determine the effects of processing conditions on the solderability of PWB surfaces. Effects are characterized for aging up to 19 months; Infrared (IR) reflow in air and nitrogen; cleaning; and temperature cycles associated with adhesive or encapsulant cure. Surface compositions, oxide thicknesses, and solderability measurements are correlated to the above process steps. For example, IR reflow in air increases oxide thickness from ∼10 Å to ∼150Å (relative to sputtering rates in Ta2O5 with an attendant increase in the meniscograph time-to-neutral-buoyancy from <2 seconds to >10 seconds, relative to unprocessed PWBs. Such fundamental information serves as an invaluable complement to standard phenomenological observations of defective solder joints, and can aid in guiding processing decisions for improved yield and reliability.
Recent results from our laboratory on the characterization of optical waveguides are reviewed. In particular, two means of experimental characterization that provide spatially local information are presented. Raman microprobe spectroscopy is used to explore the role of stress in a GaAlAs channel waveguide and to characterize the nature of impurity induced compositional mixing in a multiple quantum well structure suitable for optical waveguiding. Finally, photon tunneling microscopy is shown to probe the optical waveguide evanescent field and thus the local surface and index variations in an optical waveguide.