Extremely narrow luminescence linewidth in GaAs single quantum wells by insertion of thin AlAs smoothing layers

We propose a new method to considerably reduce the overall growth interruption for high-quality GaAs single quantum wells during molecular beam epitaxy. The insertion of ultrathin AlAs smoothing layers at the constituent GaAs/Al _x Ga_1–x As heterointerfaces and growth interruptions of not more than 15 s yields an improvement of the luminescence linewidth (FWHM) to 0.56 meV for a 13 nm wide GaAs well and to a value as low as 0.195 meV for a 27 nm wide GaAs well. In addition, no Stokes shift between absorption and emission and no line splitting due to monolayer fluctuations in the well width is observed.     

Magnetic hyperfine fields in ultrathin Ni films on Cu(100)

The magnetic hyperfine field was measured at ¹¹¹In(¹¹¹Cd) probe atoms in ultrathin Ni films epitaxially grown on Cu(100) utilizing the perturbed -angular correlation (PAC) method. The behaviour of the hyperfine field as a function of temperature was studied for different film thicknesses ranging from 2 up to 10 monolayers. It was found that the strength of the hyperfine fields as well as the critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness. The orientation of the hyperfine field is discussed.     

Determination of in-depth thermal strain distribution in Molecular Beam Epitaxy GaAs on Si

In-depth stress distribution GaAs layers grown by Molecular Beam Epitaxy (MBE) on Si (001) has been studied by X-ray diffraction, photoluminescence and Raman spectroscopy. In order to determine the stress state at different distances to the interface GaAs/Si, layers of different thickness were prepared by chemical etching of the grown samples. We observe a non-uniform residual strain distribution through the GaAs on Si epilayer. Residual strain of thermal origin is larger in the highly defective region ( 0.4 m) near the GaAs/Si interface where we have found a non-elastic relation between measured in-plane (a ) and in growth direction (a ) lattice parameters. However, thermal strain is partially relaxed by formation of 10⁷ cm^–2 dislocations in the region of better crystalline quality near the external surface.     

Optical characterization of oxynitride films in the visible-ultraviolet range

Oxynitride optical properties in the visible-ultraviolet spectral range are very interesting, due to their use in electronic device manufacturing. This paper presents spectra of refractive index and extinction coefficient of oxynitride films deposited on silicon with different composition, as derived from spectroscopic ellipsometry measurements on the basis of an effective medium approach. These data evidence the presence of a Si-rich layer on the oxynitride/silicon interface. Electronic polarizability and energy gap of all compounds were evaluated. Moreover, absolute reflectance of the samples was derived from optical functions and compared with the measured value.     

Photon-induced reactions of No adsorbed on GaAs(110)

The interaction of cw laser radiation with NO adsorbed on GaAs(110) at 90 K has been studied over a wide range of wavelengths from 457 to 900 nm by high resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and laser induced desorption spectroscopy (LIDS). Adsorption of molecular NO is observed. By varying the incident laser power, it is found that desorption and dissociation of molecular NO are induced by a nonthermal process. By measuring the time profile and the power and wavelength dependence of the desorption signal, the observed desorption and dissociation of NO are attributed to interactions of the adsorbed NO with the photogenerated carriers which migrate to the surface.     

Resonant reflectance dips induced by coupled surface plasmon polaritons in thin metal-film/dielectric superlattices

The optical reflectance of metal films changes dramatically as the film thickness becomes thinner than the electron mean free path. We have developed a transfermatrix formalism for deducing the dispersion relations of the electromagnetic waves in infinite and semi-infinite metal-dielectric superlattices by taking into account the presence of the size effect and coupled plasmon waves. This work shows that the resonance frequency occurring at the reflecting dip increases while the bandwidth decreases as the thickness of the dielectric films increases. Reducing the values of p and q shifts the resonance frequency upward and yields multiple numbers of minimum reflectivity.     

Electrochemical oxidation of La2CuO4 thin films grown by molecular beam epitaxy

Thin insulating and c-axis oriented films of La₂CuO₄ are grown using a molecular beam epitaxy technique. Subsequently, these films are oxidized electrochemically using a 1N KOH solution. This approach is used to induce superconductivity, leading to a maximum T_c0 of 31 K,, measured both resistively and inductively. The surface morphology, lattice constants and the resistivity before and after the electrochemical treatment are compared.     

C60 growth on Si(100), GaSe(0001) and GeS(001)

C₆₀ films have been grown in ultra high vacuum on various crystalline substrates and the structure of the films has been investigated by low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS). The C₆₀ films form randomly oriented nanocrystals on Si(100), mesoscopic polycrystals on GaSe(0001) and microscopic single crystals on GeS(001). The vibrational structure of the C₆₀/substrate interfaces is analyzed in detail by HREELS carried out in the dipole and impact scattering regimes. It is shown that the epitaxy of C₆₀ on GeS(001) is induced by the weak van der Waals bonding and the peculiar corrugation of the substrate surface.     

Optical characterization of PLD grown nitrogen-doped TiO2 thin films

Nitrogen-doped TiO₂ thin films were prepared by pulsed laser deposition (PLD) by ablating metallic Ti target with pulses of 248 nm wavelength in reactive atmospheres of O₂/N₂ gas mixtures. The layers were characterized by UV-VIS spectrophotometry and variable angle spectroscopic ellipsometry with complementary profilometry for measuring the thickness of the films. Band gap and extinction coefficient values are presented for films deposited at different substrate temperatures and for varied N₂ content of the gas mixture. The shown tendencies are correlated to nitrogen incorporation into the TiO_2-xN_x layers. It is shown that layers of significantly increased visible extinction coefficient with band gap energy as low as 2.89 eV can be obtained. A method is also presented how the spectroscopic ellipsometric data should be evaluated in order to result reliable band gap values.     

Optical constants of thin CoSi2 films on silicon

The optical transmission of CoSi₂ films of thickness 2.6–15 nm is measured in the wavelength range 1–20 m. The optical constants are evaluated by taking into account multiple reflections in the film and by fitting a Drude model. The plasma frequency _p=5.4–7.6 eV is equivalent to a carrier density n _eff=3×10²² cm^–3 and one carrier per unit cell. The relaxation frequency of the plasma resonance assumes high values =2 eV near the interface to silicon and decreases into the bulk film over several nanometers. Films grown off-axis from the (111) Si orientation exhibit an enhanced relaxation frequency.     

Oxygen diffusion through thin Pt films on Si(100)

We have investigated the diffusion of oxygen through evaporated platinum films on Si(100) upon exposure to air using substrates covered with Pt films of spatially and continuously varying thickness (0–500 Å). Film compositions and morphologies before and after silicidation were characterized by modified crater edge profiling using scanning Auger microscopy, energy-dispersive X-ray microanalysis, scanning tunneling microscopy, and transmission electron microscopy. We find that oxygen diffuses through a Pt layer of up to 170 Å forming an oxide at the interface. In this thickness range, silicide formation during annealing is inhibited and is eventually stopped by the development of a continuous oxide layer. Since the platinum film consists of a continuous layer of nanometer-size crystallites, grain boundary diffusion of oxygen is the most probable way for oxygen incorporation. The diffusion constant is of the order of 10^–19 cm²/s with the precise value depending on the film morphology.     

Angle-resolved ultraviolet photoelectron spectroscopy of expitaxial films of vanadyl-phthalocyanine grown on alkali halides

The molecular orientation of Vanadyl-Phthalocyanine (VOPc) grown on alkali halides was studied by Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS). To avoid charging up of the specimen during ARUPS measurment, thin epitaxial films of alkali halides grown on GaAs (0 0 1) were used as substrates. Photoelectrons emitted from the highest occupied molecular orbital band of VOPc showed strong angular distribution leading to the conclusion that the molecules stay almost parallel on the surface. This was in accordance with the expectation based on the reflection high-energy electron diffraction analysis of the film.     

Aluminium oxide thin films prepared by plasma-enhanced chemical vapour deposition

Thin films of aluminium oxide have been deposited on glass, quartz, Si(100), steel, nickel, and aluminium by plasma-enhanced chemical vapour deposition (PECVD) using aluminium acetylacetonate (Al(acac)₃) as precursor. The deposits are hard (up to 2370 HK) and show good adherence to the substrates. The influence of various experimental parameters on deposition rate, film composition and hardness has been studied. The bias turned out to be the most effective parameter.On leave from Beijing Solar Energy Research Institute, Beijing, P.R. China     

Analysis of liquid surface films by pulsed laser photoacoustic spectroscopy

The photoacoustic generation of plane acoustic waves in strongly absorbing or opaque liquids by pulsed laser radiation is discussed both experimentally and theoretically. The regimes of a confined and a free surface of the liquid are considered. The model which takes the temporal shape of the laser pulses applied in the experiments into account, implies that spectroscopic studies are feasible with direct photoacoustic generation and detection also for opaque liquids. The experiments are performed with a tunable hybrid CO₂ laser and piezoelectric detection. For the first time liquid/liquid interfaces are studied by this technique. We demonstrate that the presence of an absorbing liquid film with a thickness of >1 m on the surface of another liquid amplifies the acoustic signal which is detected in the bottom liquid. The enhancement depends on the thickness and the optical and thermal properties of the film medium. The surface layer can be analyzed on the basis of the photoacoustic spectrum. It is also shown that this non-contact method is surface-film selective and should thus prove useful for pollution analysis of liquid surfaces.     

Modulated photoreflectance characterization of ion-implanted semiconductor wafers

Modulated PhotoReflectance (MPR) measurements on semiconductor wafers implanted with boron or silicon ions in the dose range 5×10¹⁰–5×10¹⁵ ions/cm² are presented. Correspondingly, a one-dimensional theoretical multilayer model is established. In the theory, as the implant dose is lower than a critical value, the variation of the MPR signal is contributed mainly by the implanted defects and damages. However, when the dose is above the critical dose, the change of the MPR signal is chiefly due to the formation and growth of an amorphous layer. The theoretical results are in good agreement with those of experiments.     

High-resolution nonlinear laser spectroscopy of exciton relaxation in GaAs quantum wells

This paper describes measurements of exciton relaxation in GaAs/AlGaAs quantum well structures based on high resolution nonlinear laser spectroscopy. The nonlinear optical measurements show that low energy excitons can be localized by monolayer disorder of the quantum well interface. We show that these excitons migrate between localization sites by phonon assisted migration, leading to spectral diffusion of the excitons. The frequency domain measurements give a direct measure of the quasi-equilibrium exciton spectral redistribution due to exciton energy relaxation, and the temperature dependence of the measured migration rates confirms recent theoretical predictions. The observed line shapes are interpreted based on solutions we obtain to modified Bloch equations which include the effects of spectral diffusion.     

High-resolution electron energy-loss spectroscopy at epitaxially grown GaAs(100)

High-Resolution Electron Energy-Loss Spectroscopy (HREELS) is shown to be a very sensitive tool to investigate the space-charge regime of n-respectively p-type semiconductors. The most simple model we applied to fit experimental spectra is based on a step-like distribution of free carriers with the Drude dielectric response function. In this case, the dispersion of surface plasmon excitations is neglected, but it is considered in the Thomas-Fermi and the Debye-Hückel models. We use these models to fit HREELS-spectra, obtained from heavily Si-doped GaAs(100), which was grown by Molecular Beam Epitaxy (MBE). A comparison shown that the Drude model overestimates both the free-carrier concentration and the plasmon damping factor. The use of a more realistic smooth free-carrier profile, obtained by the self-consistent solution of the Schrödinger and Poisson equations, leads to plasmon excitations with lower frequencies. Besides Ohmic damping, the calculations show that Landau damping should be incorporated in order to obtain a better fit, particularly at intermediate frequencies.     

Surface characterization and microstructure of ITO thin films at different annealing temperatures

In this study, the electron beam evaporation method is used to generate an indium tin oxide (ITO) thin film on a glass substrate at room temperature. The surface characteristics of this ITO thin film are then investigated by means of an AFM (atomic force microscopy) method. The influence of postgrowth thermal annealing on the microstructure and surface morphology of ITO thin films are also examined. The results demonstrate that the film annealed at higher annealing temperature (300 °C) has higher surface roughness, which is due to the aggregation of the native grains into larger clusters upon annealing. The fractal analysis reveals that the value of fractal dimension D_f falls within the range 2.16-2.20 depending upon the annealing temperatures and is calculated by the height-height correlation function.     

Growth and characterization of ultrathin GaP layer in a GaAs matrix by X-ray interference effect

An ultrathin two monolayers thick layer of GaP sandwiched within a GaAs matrix was grown by atomic layer molecular beam epitaxy (ALMBE). The X-ray interference effect (Pendellösung) was used to determine the structural parameters such as thickness, lattice parameter, chemical composition, and strain. Excellent agreement between the experimental rocking curve and the simulation using the dynamical theory of X-ray diffraction was found indicating the high quality of the sample. Analysis of the scans in symmetrical (004) and asymmetrical (224) reflections, sensitive to both perpendicular and parallel strain, shows that the GaP layer is coherent with the substrate, i.e., it is below the critical thickness in agreement with critical thickness theories. Despite the competition for incorporation between arsenic and phosphorus the experimental GaP thickness is found to be identical to the nominal growth value, demonstrating full incorporation of phosphorus when growing by ALMBE. No significant out-diffusion or segregation of P is observed.     

Epitaxial Properties of Co-Doped ZnO Thin Films Grown by Plasma Assisted Molecular Beam Epitaxy

High quality Co-doped ZnO thin films are grown on single crystalline Al2O3（0001） and ZnO（0001） substrates by oxygen plasma assisted molecular beam epitaxy at a relatively lower substrate temperature of 450℃. The epitaxial conditions are examined with in-situ reflection high energy electron diffraction （RHEED） and ex-situ high resolution x-ray diffraction （HRXRD）. The epitaxial thin films are single crystal at film thickness smaller than 500nm and nominal concentration of Co dopant up to 20%. It is indicated that the Co cation is incorporated into the ZnO matrix as Co^2＋ substituting Zn^2＋ ions. Atomic force microscopy shows smooth surfaces with rms roughness of 1.9 nm. Room-temperature magnetization measurements reveal that the Co-doped ZnO thin films are ferromagnetic with Curie temperatures Tc above room temperature.