We report an experimental study of photocarrier lifetime, transport, and excitation spectra in silicon-on-insulator doped with sulfur far above thermodynamic saturation. The spectral dependence of photocurrent in coplanar structures is consistent with photocarrier generation throughout the hyperdoped and undoped sub-layers, limited by collection of holes transported along the undoped layer. Holes photoexcited in the hyperdoped layer are able to diffuse to the undoped layer, implying (μτ)h ∼ 5 × 10−9 cm2/V. Although high absorptance of hyperdoped silicon is observed from 1200 to 2000 nm in transmission experiments, the number of collected electrons per absorbed photon is 10−4 of the above-bandgap response of the device, consistent with (μτ)e < 1 × 10−7cm2/V.

We report nonlinear optical investigation of green emission GaInN/GaN multi-quantum structures grown along c- and m-axes on sapphire and bulk GaN substrates, respectively. Under intense pulsed photo excitation, we observed strong superluminescence near the lasing condition in c-plane grown quantum well structures with full width at half maximum of 6 nm. The superluminescence couples out of the edge of the sample in a mode pattern consistent with gain in a high mode of the waveguide. The wavelength of the superluminescence is 474 nm. The threshold intensity of superluminescence was found to be 156 kW/cm2. Increasing pump intensity leads to a strong photoluminescence blueshift as large as 380 meV in samples grown along the c-axis on sapphire substrate, while under the same excitation conditions, the blue shift for the m-axis grown structure on bulk GaN substrate is less than 10 meV. The large emission blueshift is hereby attributed to the internal piezoelectric field in the c-axis grown structure. We observe a gain value of 20 cm-1 together with internal absorption losses of 2.3 – 6.0 cm-1 for the superluminescent samples.

We report experimental measurements and analysis of HOMO-LUMO transition photoluminescence and photoinduced bleaching magnitude and decay kinetics in lead sulfide quantum dots in glass. We find that the radiative lifetime is independent of temperature from 77K to 500K. The lifetime of the HOMO-LUMO state decreases dramatically for T>300K, indicating thermally activated nonradiative recombination. The excitation decay time is much greater than the radiative lifetime and increases monotonically with decreasing temperature with decay times of 2 μs at room temperature and 4-10 μs at 77K.

Glow discharge amorphous hydrogenated silicon (a-Si:H) prepared at near room temperature typically results in an inhomogeneous morphology that is undesirable for a number of thin film applications. The most commonly observed features of this include columnar morphology and surface roughness. This usually results from anodic deposition, where substrates are placed on the grounded electrode. We have discovered that placing substrates on the RF-powered electrode (referred to as cathodic deposition) offers a much wider processing range for homogenous growth than anodic growth. We have also found that the magnitude of the surface roughness and the bulk void fraction of both anodic and cathodic a-Si:H thin films processed at low-temperatures is proportional to ∼D/F, where D is the surface diffusivity and F, the adatom flux, though anodic and cathodic deposition affect these global parameters differently. Surface processes unique to cathodic deposition can enhance adatom surface diffusion, while diffusion during anodic deposition is fixed and cannot attain homogeneous growth at high adatom fluxes. Processing a-Si:H on the cathode, associated with enhanced adatom surface diffusion, allows for homogeneous growth even at high deposition rates that has benefits for a number of applications.

We have found that traps can dominate important relaxation processes in PbS nanoparticles prepared by precipitation in borosilicate glass. The primary photoluminescence peak energy falls about 50 meV below the lowest absorption peak energy and has a long decay time of ∼4 μs. Photoinduced bleaching of the lowest absorption peak also has a long decay lifetime of ∼2 μs. Photoinduced bleaching also exhibits a surprisingly long rise time of hundreds of nanoseconds. Such long-lifetime effects must be attributed to multiple traps.

Surface topography of a-Si:H thin films, deposited at 75°C by Plasma-enhanced Chemical Vapor Deposition (PECVD) has been examined using helium/silane feedstock mixtures under different substrate bias conditions. Notable differences in the surface roughness evolution are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Smooth and apparently featureless surfaces result from deposition on RF powered surfaces, upon which a self-bias induces high-energy ion bombardment. Rougher surfaces result from films deposited on electrically grounded surfaces. These anodic films show that after a transition period, surface roughness grows linearly with processing time, exhibiting mounded type growth as evidenced by 2-D power spectral density functions of surface height measurements. Linear growth in roughness has been predicted for shadow growth models assuming film precursor sticking coefficients of one and random angle approach of film precursor species. Growth of this nature has not been reported before in a-Si:H studies, which usually assume directional deposition conditions and sticking coefficients less than unity – occurring even at low processing temperatures.

Optical waveguides are being explored for on-chip purposes to overcome the speed limitations of electrical interconnects. Passive optical components like waveguides and vertical outcouplers are important components in such schemes. In this study we fabricate planar waveguides with integrated vertical micro-mirrors using standard Back End of the Line silicon (BEOL) CMOS based processes. Around 1.6 μm of a hybrid alkoxy siloxane polymer with a refractive index of ∼ 1.50 at the intended wavelength of 830 nm is used as the core and plasma deposited silicon oxide with a refractive index of ∼ 1.46 is used as the cladding. The angular face in the polymer waveguide that would function as the mirror surface was fabricated by a pattern transfer method which involves transferring the angle in a template to the waveguide using anisotropic reactive ion etching. The sidewall angle realized in a positive resist on patterning was used as the angle template. Exposure and development conditions were adjusted for Shipley® S1813 photoresist to generate a sidewall angle of ∼ 65°. The anisotropic Reactive Ion Etching (RIE) was done using a CF4/O2 plasma chemistry. A gas composition of 50/50 CF4/O2 was chosen in order to minimize the etch related roughness of the polymer and the photoresist. The metallization of the mirror faces was done using a self-aligned maskless technique which ensures metal deposition only on the angular face and also eliminates a lithography step.

We have fabricated PbS particles in glass of average size from 1.2 nm to 3 nm. This allows us to tune the energy of the lowest electron-hole pair transition from 1.7 eV to 0.8 eV respectively. Optical absorption spectra show a pronounced quantum peak, implying a narrow size distribution. We have performed pump-probe spectroscopy where the probe is cw and the pump is nanosecond; this allows us to investigate a wide time scale. We have observed bleaching of the lowest quantum peak with lifetimes of ∼2 microseconds. We interpret this in terms of trapped carriers. Phenomenological description of observed effect is also presented.

Surface or interface roughness can impact optical, electronic, and MEMS applications of thin a-Si:H films. Deposition at lower temperatures can be advantageous for some applications of a-Si:H, but lower temperature deposition generally leads to rougher films. We have found that the evolution of surface roughness growth can be modified substantially by ion bombardment due to the self-bias of the plasma during Plasma-Enhanced Chemical Vapor Deposition (PECVD). Notable differences in the surface roughness evolution and deposition rate are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Suppression of surface roughness growth of a-Si:H can be achieved under conditions of relatively high ion bombardment even at deposition temperatures as low as 75 C. Atomic force microscopy (AFM) was used to measure the relative surface roughness profile as a function of deposition time. Analysis of the power spectral density of the roughness yielded important statistical surface parameter information. Based on these observations, insight is given into growth mechanisms under the two deposition conditions.

Pressure tuned photoluminescence and Raman scattering were employed to study the structural phase transition from wurtzite to rocksalt structure of CdSxSei1-x nanocrystals embedded in a glass matrix as a composite material and in the colloidal form. In both the composite and the colloidal form the increased phase stability of the wurtzite to rocksalt structure is reversible and size dependent. In contrast the phase transition in bulk semiconductors of cadmium-sulfide is sharp but for the nanocrystals it is broad and occurs at higher pressures. A size dependent study of the phase stability was conducted and a correlation between size and increase in phase stability was definitely established. For the nanocrystals in the composite form the difference in surface tension in both phases allowed for a model that predicted the tendencies observed in optical spectra. For the colloidal nanocrystals a model based on surface tension and volume effects (defects) gave good agreement in explaining the observations. The above results are discussed with respect to surface tension and deep level defects and their pressure dependence.

The structural and optoelectronic properties of polycrystalline CdS films, fabricated by three different methods, are compared to one another for the purpose of preparing CdTe/CdS solar cells. The three methods were: alternated spraying of cation and anion solution at room temperature, spray pyrolysis, and chemical bath deposition. We studied the surface morphology and crystal quality and texture by scanning electron and optical microscopy and x-ray diffraction. All films had a well-developed wurtzite structure. Films grown by the alternated-spray method and the chemical bath method consist of randomly-oriented crystallites with dimensions <0.5 microns. Annealing at 400°C increases the crystallite size slightly. Films which were grown by pyrolysis at substrate temperatures from 400°C to 500°C were oriented in the <002> direction. For growth by pyrolysis at 500°C, the surface is rough on a lateral scale of 0.1 to 0.3 microns.

Intensity, temperature and frequency-dependent photoabsorption spectroscopy are used to probe trap states in CdSo.44Seo.56 nanoparticles in glass. The photoabsorption signal from 6.2 nm radius particles is found to be a combination of two effects: bleaching, due to phase space filling, and an electric-field effect. Frequency-dependent photoabsorption data show that the electric-field effect is caused by a long-lived (r=30 /µs at T=293 K) trap state and that the bleaching occurs on a faster time scale. The trap activation energy obtained from the temperature dependence of the lifetime is 0.23 ±0.06 eV. Only bleaching is observed in the smaller nanocrystallites.

We present photoluminescence excitation spectra for glasses doped with CdSxSe(1-x). The peak of the emission spectrum is blue shifted toward the bandgap as excitation energy flux is increased. Total photoluminescence emission intensity is observed to be significantly enhanced and blue shifted by exciting with photon energies greater than ∼4 eV. A simple model of the nanoparticle band structure is presented to account for the observed spectra.

The optical and vibrational properties of small CdSe and CdS particles embedded in a glass matrix and as a colloid have been studied as a function of pressure up to 90 kbar using Raman scattering and photoluminescence techniques. We will discuss the use of high pressure optical spectroscopy techniques, where the sample is contained in a diamond anvil.cell with optical access, to study the nature of the electronic states in semiconductor nanocrystals. Raman scattering is employed to establish the enhanced pressure stability of the wurtzite phase in the II-VI nanocrystalline composites. Photolurninescence is used to study the energies of electronic states. The wurtzite to rocksalt phase transition behavior in the nanocrystallite systems is quite different from that in the bulk material. This different behavior is attributed to a large number of defects (vacancies) in the nanocrystallite system. This work demonstrates that the main defects in the CdS glass composites are cadmium vacancies; while in CdSe Selenium vacancies exist in the conduction band. The pressure dependence of the Huang-Rhys parameter, characteristic for the strength of the electron-phonon coupling, will also be discussed for the nanocrystalline samples versus the bulk material.

On the basis of Raman, photoluminescence, and absorption studies of porous and nanoparticle silicon we propose that the strong luminescence in porous silicon results from strained silicon quantum dots. A silicon nanoparticle is a special Jahn-Teller system induced by extended electron states rather than localized state. Thus Raman scattering and photoluminescence in porous silicon are multi-phonon assisted free electronic transition processes, all observed anomalous properties of porous silicon can be clearly explained by using this strained quantum dot model.

We have used Resonant Raman scattering induced by pressure tuning to study the phase transition and electronic states of bulk and 60±20 Å colloidal microcrystallite CdS. The experimental results show that bulk CdS undergoes a well-defined first order phase transition at 27 kbar and that the intensity of the Raman scattering increases sharply when the level of the intermediate state (bound exciton I2) is close to the photon energy. After the phase transition no Raman scattering and photoluminescence can be observed. However, the phase transition in the colloidal CdS is quite different from the bulk CdS and the complete phase transition occurs above 60 kbar. Both bulk and colloidal CdS reverse to the original wurtzite phase after releasing the pressure.