Buried interfaces studied by photoelectron diffraction

Angle- and energy-scanned photoelectron diffraction data can be used to investigate structures below surfaces. The modulations in photoelectron intensity result from diffraction of the emitted electron wave at neighbor atoms. In the past, scanned-energy photoelectron diffraction had been mainly used to determine the adsorption site of molecules at surfaces. Recent data show, however, that the technique can also be employed to obtain information about the upper substrate layer(s). At low kinetic energies, backward scattering is strong and in scanned-angle photoelectron diffraction the recorded patterns result from backward- and multiple-scattering effects. For a structural analysis, the intensity modulations have to be compared with the results for simulations performed for model clusters. As an example, recent angle-scanned photoelectron diffraction patterns recorded for the technologically important silicon oxide/silicon interface were compared with simulations. At the Si(001) surface orientation, the interface is extended over a few layers, whereas at the Si(111) surface orientation the transition is rather abrupt and occurs within one or two layers. Received: 9 September 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-231/755-3657, E-mail: carsten.westphal@physik.uni-dortmund.de     

Green/blue light emission and chemical feature of nanocrystalline silicon embedded in silicon-oxide thin film

2 in the range of annealing temperatures used. The PL intensity, weight of the Si⁴⁺ states, and the volume fraction of Si nanocrystals exhibit a large increase as T_a>750 °C. The PL peak position is independent of the annealing temperature (T_a). From our observations, the green/blue light emission is related to the defects. Received: 10 July 1996/Accepted: 2 April 1997     

Effect of the growth conditions on infrared upconversion efficiency of CaS: Eu, Sm thin films

High-quality electron-trapping thin films CaS: Eu, Sm with red light output have been successfully deposited on quartz and single-crystal silicon substrates by electron beam evaporation (EBE) and radio frequency (RF) magnetron sputtering in vacuum and H₂S partial pressures. Infrared upconversion efficiency of CaS: Eu, Sm thin films under different growth conditions has been investigated by using ultrashort infrared (IR) laser pulse with 20 ps (full width at half-maximum (FWHM)). The results show that upconversion efficiency of CaS: Eu, Sm thin films depends strongly on growth conditions in spite of the existence of “exhaustion” phenomena. Microstructures identified by X-ray diffraction (XRD) indicate that crystallinity of CaS films relies on both substrate materials and growth conditions. The stoichiometric composition of CaS films was measured by secondary-ion mass spectrometry (SIMS). The post-annealing process was found to promote grain growth and activate strong luminescence so that it could obviously improve upconversion efficiency of CaS: Eu, Sm films, even though it had negative influence on transmittance and spatial resolution of these films. Received: 5 June 2000 / Accepted: 7 June 2000 / Published online: 23 August 2000     

Adherent SiC coatings on Ni-Cr alloys with a composition-graded intermediate layer

Received: 8 September 1997/Accepted: 7 January 1998     

Phthalocyanine-C60 composites as improved photoreceptor materials?

2 -Pc, in contact with C₆₀ is studied using photoelectron spectroscopy with ultraviolet and X-ray radiation (UPS and XPS, respectively), X-ray absorption near edge spectroscopy (XANES), and optical transmission spectroscopy. A possible improvement of the charge-carrier generation efficiency, which is essential for the performance as photoreceptor material, is thereby found for these materials upon doping with C₆₀. No ground-state charge transfer is detected for the Pcs in contact with C₆₀. The effect of an enhanced photoconductivity is demonstrated for τ-H₂-Pc when it is doped by 5% C₆₀. Received: 16 March 1998/Accepted: 16 March 1998     

Transparent conductive electrodes for electrochromic devices: A review

This paper reviews the optical and electrical performance of thin films that are useful as transparent electrodes in electrochromic devices. The properties of certain heavily doped wide-bandgap semiconductor oxides (especially In₂O₃:Sn) and of certain coinage metal films are discussed.     

Effect of intrinsic-gain fluctuations on quantum noise of phosphor materials used in medical X-ray imaging

The quality of a medical image depends, among other parameters, on quantum noise. Quantum noise is affected by the fluctuations in the number of optical quanta produced within the phosphor, per absorbed X-ray (i.e. phosphor intrinsic-gain fluctuations). This effect is considered by means of a factor, called in this study intrinsic-gain noise factor, IGNF(E). In existing theoretical models of quantum noise, the corresponding factor is taken to be equal to one. In this paper, an expression that accounts for the coefficient of variation of the phosphor intrinsic gain is introduced. This expression takes into account the process of electron–hole pair conversion to optical photons and the frequency distribution function of the emitted optical photon energy. Subsequently IGNF(E) is expressed in terms of this coefficient of variation. IGNF(E) has been calculated for several phosphors and for various energies. For all medical X-ray energies studied, phosphors that exhibit a high relative fluctuation of emitted optical photon energy, IGNF(E) exceeds by 2% to over 17% the corresponding factor of the existing theoretical models of quantum noise. Received: 25 March 1999 / Accepted: 29 March 1999 / Published online: 14 July 1999     

Generation of optical contrast in insulating materials by irradiation with focused ion beams

The optical properties of crystalline insulating materials can markedly be modified by irradiation with energetic ions. The exposed areas of such materials absorb and reflect light more strongly than non-irradiated ones. In this way, optical contrast is created. With modern equipment, ion beams of sufficient intensity can be focused to submicron dimensions. Thus, both analog and digital information can be recorded with pixel densities of Gbit/cm² to Tbit/cm². In particular, crystalline films of group-IV elements of the periodic system, such as Si, SiC and C_D (diamond), are best suited for this novel ionographic process. The physical foundations of this technology and the resulting properties of the recorded data will be discussed. Received: 11 July 2000 / Accepted: 13 July 2000 / Published online: 13 September 2000     

Influence of molecular adsorbate layers on the optical spectra of small metal particles

2 , N₂O, CO₂ or N₂ on the particle surface. Depending on the molecules to which the clusters are exposed, different changes of the surface plasmon resonance in the spectra can be identified. The most essential experimental results are as follows. First, changes of the optical spectra are found if the clusters are covered by less than a molecular monolayer. Secondly, the observed variations of the spectra allow to distinguish between physisorption and chemisorption, i.e. characterize the strength of the surface chemical bond. Third, in sharp contrast to the usual shift of the plasmon frequency to longer wavelengths by change of the dielectric surrounding, a blue shift of the resonance has been observed upon dosage of CO₂. Finally, diffusion of adsorbate molecules into the bulk of the particles can be identified. Received: 11 September 1997     

Characterization of p-n junctions and surface-states on silicon devices by photoemission electron microscopy

Received: 22 November 1996/Accepted: 17 December 1996     

Structure, refractive-index dispersion and the optical absorption edge of chemically deposited ZnxCd(1-x)S thin films

Zinc cadmium sulfide, Zn_xCd_(1-x)S, thin films have been deposited by a simple and inexpensive chemical bath deposition method from an aqueous medium using thiourea as a sulfide-ion source. The structure of the deposited films has been characterized by X-ray diffraction and transmission electron microscopy. It was observed from X-ray diffraction that the as-deposited films were amorphous in nature. However Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h show a crystalline structure with a small scattering volume. The obtained results were confirmed throughout the transmission electron microscopy and the corresponding electron-diffraction patterns. The optical constants of Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h in the compositional range 0≤x≤1 were estimated using transmission and reflection spectra in the wavelength range 300–2500 nm. The band gap varies non-linearly with the value of x. The dependence of the refractive index on the wavelength obeys the single-oscillation model, from which the dispersion parameters and the high-frequency dielectric constant were determined. A graphical representation of the surface and volume energy-loss functions was also given. Received: 23 February 2001 / Accepted: 26 February 2001 / Published online: 27 June 2001     

Optical studies on the deposition of carbon nitride films by laser ablation

2 radicals when the 355 and 1064 nm outputs of a Nd:YAG laser were applied. While for the 532 nm ablation, a relatively higher concentration of excited atomic carbon was obtained. Different Raman and FTIR spectral features were observed from the deposited films with different ablation wavelengths. The 532 nm laser ablation is proposed for the synthesis of high quality carbon nitride films. Received: 16 October 1996 / Accepted: 11 April 1997     

Entropy as a measure of the performance of phosphor materials used in medical imaging radiation detectors

In information theory, entropy expresses the information gain obtained after detection of a signal concerning the state of a parameter of interest. In this study, entropy has been expressed in terms of physical quantities (emitted optical fluence and MTF) related to the imaging performance of phosphor materials, which are employed in medical imaging radiation detectors. Four phosphor materials, used in the form of laboratory-prepared fluorescent layers (screens), were compared on the basis of their entropy performance. Measurements were performed using 30- and 80-kVp X-ray beams often employed in X-ray imaging. Results showed that phosphor materials with high density and effective atomic number exhibit high entropy performance, especially at the higher X-ray tube voltage of 80 kVp. Entropy values are also affected by the type of activator, which determines the intrinsic X-ray-to-light conversion efficiency, and the spectrum of emitted light. The proximity of the incident X-ray quanta energy to the energy of the K-shell threshold for photoelectric absorption is an additional important factor which increases entropy. This effect was more apparent in the performance of yttrium-based phosphors at the lower voltage of 30 kVp. Received: 7 January 2000 / Accepted: 28 March 2000 / Published online: 23 August 2000     

Morphology, bond saturation and reconstruction of hexagonal SiC surfaces

Received: 21 March 1997     

SEM investigation of pillared microstructures formed by electrochemical anodization

Two methods of multistep electrochemical processing of aluminum films during porous anodic oxide films formation were developed to improve the morphology of pillared microstructures. Analysis of experimental data shows that the developed methods allow us to obtain the ordered pillared microstructures with high aspect ratio (≥4), radius 15 to 35 nm, and pillar density (3.0to8.5)×10⁹ cm^-2. High reproducibility of physical and morphological parameters of the structured layers for large-area samples is achieved. Such pillared matrices can be used for manufacturing of the microelements based on the mesoscopic and quantum effects: solar cells, controlled and uncontrolled emitter matrices for flat panel displays and emission cathodes, cathode luminescent displays, functional screens and polarizors for optoelectronics, etc. Received: 22 June 1998/Accepted: 24 June 1998     

Interplay of surface morphology, strain relief, and surface stress during surfactant mediated epitaxy of Ge on Si

Lattice-mismatch-induced surface or film stress has significant influence on the morphology of heteroepitaxial films. This is demonstrated using Sb surfactant-mediated epitaxy of Ge on Si(111). The surfactant forces a two-dimensional growth of a continous Ge film instead of islanding. Two qualitatively different growth regimes are observed. Elastic relaxation: Prior to the generation of strain-relieving defects the Ge film grows pseudomorphically with the Si lattice constant and is under strong compressive stress. The Ge film relieves strain by forming a rough surface on a nm scale which allows partial elastic relaxation towards the Ge bulk lattice constant. The unfavorable increase of surface area is outbalanced by the large decrease of strain energy. The change of film stress and surface morphology is monitored in situ during deposition at elevated temperature with surface stress-induced optical deflection and high-resolution spot profile analysis low-energy electron diffraction. Plastic relaxation: After a critical thickness the generation of dislocations is initiated. The rough phase acts as a nucleation center for dislocations. On Si(111) those misfit dislocations are arranged in a threefold quasi periodic array at the interface that accommodate exactly the different lattice constants of Ge and Si. Received: 1 April 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

The effects of annealing on the structure and magnetic properties of CoNi patterned nanowire arrays

NiCo alloy nanowire arrays were fabricated by using an electrochemical method in a porous anodic aluminum oxide (PAO) template. X-ray-diffraction results showed that there was a preferred orientation in the CoNi nanowire arrays with Ni content ranging from 0.2 to 0.8 in the as-electro-deposited state, while a random orientation was observed after the sample was heat treated in an argon atmosphere. When the shape anisotropy is very high in the sample, it is found that the squareness (Mr/Ms) of the hysteresis loops for the samples is only about 0.6 before annealing, but increases to about 0.9 after annealing. All the results are discussed qualitatively and an explanation for the magnetic reversal mechanism in terms of a localized magnetization model is given. As the CoNi alloy nanowire arrays have high bit densities, they can be used as perpendicular magnetic recording media of high-density quantum magnetic disks. Received: 8 January 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001     

Evaluating Zn2SiO4:Mn phosphor for use in medical imaging radiation detectors

2 SiO₄:Mn phosphor was evaluated for use in radiation detectors of medical imaging systems. Zn₂SiO₄:Mn was used in the form of laboratory-prepared fluorescent layers (screens) with coating weights from 18 to 150 mg/cm². The phosphor was excited to luminescence by low-energy X-raysusing X-raytube voltages ranging from 15 to 50 kVp. The number of emitted optical photons per incident X-rayquantum was thus determined for various X-rayenergies and phosphor coating weights. The optical emission spectrum was also measured and it was used to evaluate the spectral compatibility of Zn₂SiO₄:Mn with radiographic films, photocathodes and the Si photodiode. Finally, phosphor optical properties were estimated by fitting a theoretical model to experimental data. Results showed that Zn₂SiO₄:Mn is more efficient for low-energy X-rays. Its intrinsic conversion efficiency was found equal to 0.08, which is comparable to that of actually used phosphors. Zn₂SiO₄:Mn was also adequately compatible with orthochromatic films and the ES-20 photocathode, thus being appropriate for low-voltage radiography and fluoroscopy. Received: 31 July 1998/Accepted: 3 August 1998     

Ion-beam synthesis and growth mechanism of diamond-like materials

Opposite to most other deposition methods, the dominating nucleation and growth mechanism during ion-beam deposition of energetic ions in the range between 10 eV and 10 keV occurs in a region of a few nanometers below the surface of the growing film. This process is called ‘subplantation’ – emphasizing the implantation of ions into a subsurface region. Ordering and phase formation is a result of the interaction of the deposited ions with the solid state that takes place within the short time scale of femto- and picoseconds. This extreme non-equilibrium process can result in metastable amorphous or crystalline structures. This review will present several examples of the influence of the deposition parameters on the properties of diamond-like materials synthesized using mass-selected ion-beam deposition. Furthermore, several existing models of the deposition process will be presented and critically discussed. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-551/39-4493, E-mail: carsten.ronning@phys.uni-goettingen.de     

Core-level and valence-band characteristics of carbon nitride films with high nitrogen content

Carbon nitride films with high nitrogen content were prepared by reactive pulsed-laser deposition at nitrogen partial pressures varying from 0.1 to 20.0 Torr. It was found that the nitrogen content in the films first increases with increase of the nitrogen pressure, reaches a maximum of 46 at. % at 5.0 Torr, and then decreases to 37 at. % at 20.0 Torr. The almost pure carbon nitride films were systematically characterized by using X-ray photoelectron spectroscopy (XPS) concerning the core-level and valence-band structures. Some fingerprint information, which shows the role of nitrogen in controlling the electronic structure of carbon nitride films, was found based on the XPS studies. With enhancing the nitrogen incorporation, both the binding energy and the peak intensity of the core-level and the valence-band spectra vary systematically as a function of nitrogen content in the films. Received: 26 June 2000 / Accepted: 26 June 2000 / Published online: 20 September 2000