Quantum confinement in layer-by-layer deposited colloidal HgTe nanocrystals determined by spectroscopic ellipsometry

We report spectroscopic ellipsometry studies in the energy range of 0.5-5 eV on samples of 1-10 bilayers of polymer and HgTe nanocrystals, which exhibit strong transitions at higher critical points in the dispersion relation. We show that the dispersion relation for nanocrystals can be modelled with the same concepts for critical points as used in semiconductor bulk optics. We find an energy shift of up to 0.4 eV of the critical points to higher energies compared to the HgTe bulk properties, caused by quantum confinement in the nanocrystals, which increases with decreasing nanocrystal size.     

Determination of size and concentration of copper nanoparticles dispersed in glasses using spectroscopic ellipsometry

For the generation of particular optical properties the melt of a commercially manufactured glass is doped with copper compounds. The glass obtained is opaque black at the usual thickness and looks dark red after making it into bulbs of incandescent lamps. It is generally assumed that copper particles cause this colouring. A proof in a spectrophotometric way fails due to the very high absorbance even for a sample thickness below 20 μm. It will be shown that in these cases spectroscopic ellipsometry is a suitable method of investigation. The pseudo-optical constants of this material were determined as a function of wavelength in the range from 350 nm to 700 nm by ellipsometric measurements. They can be reproduced very well by those of a model that consists of a roughness layer situated on a substrate of glass containing spherical copper particles with a Gaussian size distribution with =6.5 nm and σ=0.24 and a volume concentration of 2.4×10^-3. For this modelling the dielectric function of the roughness layer was approximated by Bruggeman effective-medium theory and that of the copper-containing glass substrate was calculated on the basis of the theory of Gans and Happel. The results were verified by transmission electron microscope investigations. Received: 1 July 2001 / Published online: 10 October 2001     

A spectroscopic ellipsometry study on the variation of the optical constants of tin-doped indium oxide thin films during crystallization

In this paper, the variation of the optical constants of tin-doped indium oxide thin films during thermal treatment was explored using spectroscopic ellipsometry based on appropriate analysis models combining a Drude absorption edge and Lorentz oscillators. It was found that the refractive indices and the extinction coefficients show different behaviors depending on depth, thermal treatment time and temperature. The optical constants varied more abruptly in the lower part of the films, which confirms the model that crystallization starts from the film-substrate interface. Hall measurement showed that the significant increase in the extinction coefficients in the near infrared range is due to the increased number of free electrons.     

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq3) by spectroscopic ellipsometry

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq₃) have been studied in the spectral range from 1.55 eV to 5 eV using spectroscopic ellipsometry. The samples have been deposited by thermal evaporation on glass substrates. Optical functions of Alq₃ deposited on unheated substrates and on substrates kept at 100 °C have been determined. The optical functions have been modeled using point-to-point fitting, with the conventional oscillator model and modified oscillator model. It has been found that point-to-point fitting gives the best agreement with the experimental data, and that the modified oscillator model yields better agreement with the experimental data than the conventional oscillator model. Received: 3 September 2001 / Accepted: 22 March 2002 / Published online: 5 July 2002 RID="*" ID="*"Corresponding author. Fax: +852-2559/8738, E-mail: dalek@eee.hku.hk     

Optical determination of interface roughness in multilayered semiconductor structures

Received: 20 September 1998     

Optical properties of Al-doped ZnO thin films by ellipsometry

Al-doped ZnO thin films (AZO) were prepared on Si (1 0 0) substrates by using sub-molecule doping technique. The Al content was controlled by varying Al sputtering time. The as-prepared samples were annealed in vacuum chamber at 800 °C for 30 min. From the XRD observations, it is found that all films exhibit only the (0 0 2) peak, suggesting that they have c-axis preferred orientation. The average transmittance of the visible light is above 80%. Spectroscopic ellipsometry was used to extract the optical constants of the films. The absorption coefficient and the energy gap were then calculated. The results show that the absorption edge initially blue-shifts and then red-shifts with increase of Al content.     

Microstructural and optical investigations of sol-gel derived ferroelectric BaTiO3 nanocrystalline films determined by spectroscopic ellipsometry

Ferroelectric BaTiO₃ nanocrystalline films (BTNFs) with the crystalline sizes of about 30 nm were grown on Pt/Ti/SiO₂/Si substrates by a modified sol-gel method. Spectroscopic ellipsometry (SE) was used to characterize the films in the photon energy range of 1.5-5.0 eV with a five-phase layered model (air/surface rough layer/BaTiO₃/interface layer/Pt). The optical properties in the transparent and absorption regions have been investigated with the Forouhi-Bloomer dispersion relation. With the aid of the structural and dielectric function models, the microstructure and electronic structure of the BTNFs can be readily obtained. It was found that the refractive index reaches the value of 2.20 in the transparent region. Based on the Sellmeier dispersion analysis, the single-oscillator energy is about 4.7 eV for the BTNFs. The long wavelength refractive index n(0) can be estimated to about 2.00 at zero point. The direct optical band gap energy approaches approximately 4.2 eV and Urbach band tail energy is 262±2 and 268±1 meV respectively with increasing crystalline size. A higher band gap observed can be owing to the known quantum confinement effect in the nanocrystalline formation and different fraction of amorphous and crystalline components. The theoretical analysis based on the effective mass approximation theory is well used to explain these experimental data.     

Optical constants of 3,4,9,10-perylenetetracarboxylic dianhydride films on silicon and gallium arsenide studied by spectroscopic ellipsometry

The optical constants of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films grown by organic molecular beam deposition on Si and GaAs substrates were determined in the spectral range from 300 nm to 1700 nm. All PTCDA layers deposited at room temperature with a low deposition rate of about 0.2 nm/min are uniaxial and strongly optically anisotropic. For the layers on Si a refractive index of 2.21 is derived in the substrate plane at 830 nm. The out-of-plane refractive index has a much lower value of 1.58. A similar anisotropy is observed for PTCDA layers on GaAs. The altogether lower refractive indices of 2.03 and 1.54, however, indicate a lower density of the films, which can be explained by the film structure. Received: 6 November 2000 / Accepted: 10 August 2001 / Published online: 17 October 2001     

Observation of new critical point in InxAl1−xAs alloy using spectroscopic ellipsometry

Using a spectroscopic ellipsometry, pseudodielectric functions 〈?〉 of In_xAl_1−xAs ternary alloy films (x = 0.43, 0.62, 0.75, and 1.00) from 0.74 to 6.48 eV were determined. Fast in-situ chemical etching to effectively remove surface overlayers using charge-coupled device detector and to avoid the reoxidation of the surface of films prior to the ellipsometric spectrum measurement was performed. At the high energy region, an additional critical point structure which is interpreted as the E′₁ transition from the band structure calculation of the linear augmented Slater-type orbital method was reported.     

Evolution of microcrystalline growth pattern by ultraviolet spectroscopic ellipsometry on Si:H films prepared by Hot-Wire CVD

Ultra-violet spectroscopic ellipsometry has been applied successfully to determine the evolution of μc-Si:H film structure, including incubation layer, bulk layer and the growth zone and surface over-layer, their individual thickness and composition. In view of the availability of a significantly high atomic H density in H₂-diluted SiH₄ ensemble in Hot-Wire CVD, microcrystallization seems to be achieved easily at a low substrate temperature. Atomic H induced etching at the growing network has been identified as instrumental in controlling the microcrystallization in Si:H. Sharp elimination of the incubation layer as well as surface roughness and significant improvement in the overall crystallinity was obtained on increase in H₂ dilution to SiH₄. A H₂ dilution limited sharp transition from an amorphous dominated (a+μc)-Si mixed phase to a virtually amorphous free μc-Si phase has been identified. However, enhanced atomic H reactivity at the growth zone beyond the attainment of the amorphous free bulk and the surface layer induces porosity in the network and a gradual deviation from crystallinity.     

Optical spectra in the region of exciton resonances in quantum wells and quantum dots of In0.3Ga0.7As/GaAs heterostructures

The transparency, reflection and luminescence spectra of In_0.3Ga_0.7As structures with 8 nm thickness and quantum wells limited by the barrier layer GaAs of a 9 nm (upper layer) and 100 nm (bottom layer) thickness had been studied in the region of photon energy 0.5–1.6 eV. Lines associated with the transitions hh,lh1-e1(1s,2s,3s), hh2,lh2-e2(1s,2s,3s), hh1,lh1-e2(1s) and hh3,lh3-e3(1s) had been revealed in reflection spectra. The shapes of the reflection and transparency lines had been calculated using a single oscillator model of dispersion relations and the Kramers–Kronig integrals. The binding energy of hh,lh1-e1 excitons, the effective mass m_hh^∗ and m_lh^∗ and the damping factor for the optical transitions to QW and QD had been determined. The lifetime of charge carriers on quantum dots varies in the range of 0.04–0.1 ps, while the radiative lifetime of excitons in quantum wells in the considered structure is around 2 ps.     

Use of optical anisotropy for study of ultrafast phase transformations at solid surfaces

A new method of crystalline-order detection in highly absorbing anisotropic crystals is worked out and is demonstrated experimentally on a monocrystal Zn. The method is based on partial transformation of incident p-polarized electromagnetic wave into s-polarizedreflected wave due to optical anisotropy. The method is applicable to anisotropic metals (for example, Zn, Ti, Cd) and makes it possible to follow changes of crystalline structure in thin (10–100 nm) surface layers. It must be emphasized, that the method permits the detection of changes of the long-range order, whereas most of the conventional methods provide information on changes of the short-range order, which need not be changed on melting and amorphization for certain crystals. Using picosecond laser pump pulse (time duration ≈1 ps) and streak camera “Agat”, surface melting and evaporation of Zn are studied. By means of measurement of time dependencies of s- and p-components of a reflected probe pulse (time duration ≈500 ps) the dynamics of melting and evaporation of a surface layer was studied at various flows of energy laser pump pulse. The characteristic time of disappearance of the long-range order is <3 ps. The crystal structure is restored through 100–300 ps after action of a pump pulse. The theoretical analysis of experimental results was performed. Estimations, based on the proposed model, are in satisfactory agreement with the experimental results. Pump-probe experiments with time resolution higher than 3 ps are in progress. Received: 27 November 1998 / Accepted: 21 April 1999 / Published online: 27 October 1999     

The ultimate in real-time ellipsometry: Multichannel Mueller matrix spectroscopy

A review of the techniques and applications of multichannel ellipsometry in the dual-rotating-compensator configuration is given. This ellipsometric approach has been established as the ultimate in real-time, single-spot optical measurement, as it determines the entire 16-element Mueller matrix of a sample over a wide spectral range (up to 1.7-5.3 eV) from raw data collected over a single optical period of 0.25 s. The sequence of optical elements for this ellipsometer is denoted PC_1rSC_2rA, where P, S, and A represent the polarizer, sample, and analyzer. C_1r and C_2r represent two MgF₂ rotating compensators, either biplates or monoplates that rotate synchronously at frequencies of ω₁ = 5ω and ω₂ = 3ω, where π/ω is the fundamental optical period. Previous high-speed Mueller matrix measurements with this instrument have been performed on uniform, weakly anisotropic samples such as (110) Si, in which case one can extract the bulk isotropic and near-surface anisotropic optical responses simultaneously. In such an application, the instrument is operated at its precision/accuracy limits. Here, ex situ results on a strongly anisotropic, locally biaxial film are presented that demonstrate instrument capabilities for real-time analysis of such films during fabrication or modification. In addition, the use of the instrument as a real-time probe to extract surface roughness evolution on three different in-plane scales for an isotropic film surface is demonstrated for the first time.     

Slow light propagation via quantum coherence in a semiconductor quantum well

With the Schrödinger equations, we investigate the low-intensity light pulse propagation through a semiconductor quantum wells. Through studying the dispersion and absorption properties of the weak probe field, it is shown that slow light propagation is observed in this system. From the view point of practical purpose, it is more advantageous than its corresponding atomic system. Such investigation of slow light propagation may lead to important practical applications in semiconductor quantum information.     

Optical bistability in a triple semiconductor quantum well structure with tunnelling-induced interference

We study the behavior of optical bistability (OB) in a triple semiconductor quantum well structure with tunnelling-induced interference, where the system is driven coherently by the probe laser inside the unidirectional ring cavity. The results show that we are able to control efficiently the bistable threshold intensity and the hysteresis loop by tuning the parameters of the system such as laser frequency and tunnelling-induced frequency splitting. This investigation can be used for the development of new types of nanoelectronic devices for realizing switching process.     

Kinetic effects in recombination of optical excitations in disordered quantum heterostructures: Theory and experiment

An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.     

Optical and AFM study of electrostatically assembled films of CdS and ZnS colloid nanoparticles

CdS and ZnS semiconducting colloid nanoparticles coated with the organic shell, containing either SO₃⁻ or NH₂⁺ groups, were prepared using the aqueous phase synthesis. The multilayer films of CdS (or ZnS) were deposited onto glass, quartz and silicon substrates using the technique of electrostatic self-assembly. The films produced were characterized with UV-vis spectroscopy, spectroscopic ellipsometry and atomic force microscopy. A substantial blue shift of the main absorption band with respect to the bulk materials was found for both CdS and ZnS films. The Efros equation in the effective mass approximation (EMA) theoretical model allowed the evaluation of the nanoparticle radius of 1.8 nm, which corresponds well to the ellipsometry results. AFM shows the formation of larger aggregates of nanoparticles on solid surfaces.     

Current-in-plane giant magnetoresistance: The effect of interface roughness and spin-depolarization due to the proximity of a buffer layer

We report on proximity effects of a Au buffer layer on the current-in-plane giant magnetoresistance effect (CIP-GMR) in high-quality, epitaxial Fe/Cr/Fe(001) trilayers. The lower Fe layer is grown in the shape of a wedge and allows simultaneous preparation of 24 GMR stripe-elements with different lower Fe thicknesses in the range from 13 to 14.5 ML. The layer-by-layer growth mode in combination with the small thickness variation gives rise to: (i) well-controlled roughness changes from stripe to stripe as confirmed by reflection high-energy electron diffraction (RHEED), and (ii) to a varying influence of the underlying Au buffer. The oscillatory roughness variation along the wedge yields an oscillatory GMR behavior as a function of Fe thickness and confirms the previous result that slightly increased interface roughness causes a higher GMR ratio. The proximity of the Au buffer to the GMR trilayer results in an increase of the GMR ratio with increasing Fe thickness. The latter effect is explained by spin-depolarization at the Fe/Au interface and in the bulk of the Au buffer.     

Dipole-allowed optical absorption in a parabolic quantum dot with two electrons

Dipole-allowed optical absorption in a parabolic quantum dot with two electrons are studied by using the exact diagonalization techniques and the compact density-matrix approach. Numerical results are presented for typical GaAs parabolic quantum dots. The results show that the total optical absorption coefficient of two electrons in quantum dot is about five times smaller than that of one electron in quantum dot.