Frank Hamelmann

Metal oxide/silicon oxide multilayer with smooth interfaces produced by in situ controlled plasma-enhanced MOCVD

Molybdenum oxide/silicon oxide and tungsten oxide/silicon oxide: multilayer with 24 periods and a period thickness of 9.2 nm were fabricated with plasma-enhanced MOCVD. The layer thickness was controlled by an in situ soft X-ray reflectivity measurement. For the deposition of the SiO2 layers, a new silicon organic precursor, pentamethylcyclopentadienyldisilane (Me5C5Si2H5) was used in an O-2 remote: plasma process. The high quality of multilayer interfaces was observed by cross-section transmission electron microscopy (TEM), the interface toughness was measured by hard X-ray reflectivity and diffuse scattering at grazing incidence experiments to be about 0.1 nm. Auger electron spectroscopy (AES) gives the information, that the silicon oxide is practically carbon free, and the carbon content of the metal oxides is low (<5%)

Carbon buffer layers for smoothing superpolished glass surfaces as substrates for molybdenum/silicon multilayer soft-x-ray mirrors

Zerodur and BK7 glass substrates (developed by Fa. Glaswerke Schott, D-55014 Mainz, Germany) from Carl Zeiss Oberkochen polished to a standard surface roughness of varsigma = 0.8 nm rms were coated with a C layer by electron-beam evaporation in the UHV. The roughness of the C-layer surfaces is reduced to 0.6 nm rms. A normal-incidence reflectance of 50% at a wavelength of 13 nm was measured for a Mo/Si multilayer soft-x-ray mirror with 30 double layers (N = 30) deposited onto the BK7/C substrate, whereas a similar Mo/Si multilayer (N = 30) evaporated directly onto the bare BK7 surface turned out to show a reflectance of only 42%.

Hydrogen Diffusion in Zinc Oxide Thin Films

Hydrogen diffusion in zinc oxide thin films was studied by secondary ion mass spectrometry (SIMS) measurements, investigating the spreading of implanted deuterium profiles by annealing. By effusion measurements of implanted rare gases He and Ne the microstructure of the material was characterized. While for material prepared by low pressure chemical vapour deposition an interconnected void structure and a predominant diffusion of molecular hydrogen was found, sputter-deposited ZnO films showed a more compact structure and long range diffu- sion of atomic hydrogen. Hydrogen diffusion energies of 1.8 - 2 eV, i.e. higher than reported in literature were found. The results are discussed in terms of a H diffusion model analogous to the model applied for hydrogen diffusion in hydrogenated amorphous and microcrystalline silicon.

Design of Schottky Contacts for Optimum Performance of Thin-Film Silicon Solar Cells

A full-scale model combining TCAD simulations with circuit modeling of p-type window layers in thin-film silicon solar cells is validated by experiment. The results demonstrate that Schottky contacts with a barrier height greater than 0.5 eV cause a kink in the simulated J-V characteristics that represents a considerable reduction of the open-circuit voltage when thermionic emission is the dominant transport mechanism. An optimum cell can be designed by facilitating tunneling mechanism through the Schottky barrier via an adjustment of the doping concentration of the semiconductor or by introducing thin μc-Si:H(p) in order to lower the barrier height between ZnO and a-Si:H(p). In such a case, however, the narrower bandgap of μc-Si:H compared with that of a-Si:H, and the misalignment of the energy bands between μc-Si:H(p) and a-Si:H(i), also compromise the short-circuit current and open-circuit voltage of the cell, respectively. Optimum interfaces for our 15-nm window layer are found when a combined μc-Si:H(p)/a-Si:H(p) with a 4/11-nm thickness ratio is used. A general circuit model for solar cells that accounts for the effects of nonohmic contacts is demonstrated. The ideality factor “n2 ” of the Schottky junction of the contact indicates the transport mechanism at the interface with values less than 0.5 eV having no impact on the cell performance.

Transport mechanisms and effective Schottky barrier height of ZnO/a-Si:H and ZnO/μc-Si:H heterojunction solar cells

The impact of boron doping on the p-layer of thin film silicon solar cells is assessed by measuring the effective Schottky barrier height of ZnO/a-Si:H and ZnO/μc-Si:H heterojunctions. A deviation from ideal diode characteristics is revealed by an increase of ideality factor with doping concentration. Higher current densities and lower effective Schottky barriers are evaluated for higher doping levels, resulting in increasingly Ohmic behaviour. This is attributed to an enhancement of tunneling through a thinner depletion region, as supported by computer simulations. Extracted barriers are in the range of 0.7–1 eV for the heterojunctions with rectifying behaviour.

Modelling of infrared optical constants for polycrystalline low pressure chemical vapour deposition ZnO:B films

Doped zinc oxide films are of high interest in thin film solar cell technology for application as transparent conducting oxide. Rapid and detailed characterisation of ZnO thin film properties is required for quality control and optimisation of the deposited films. In the present work, a new model of dielectric functions based on the effective medium approximation (EMA) is developed and is applied for characterisation of polycrystalline boron doped zinc oxide (ZnO:B) films, deposited by low pressure chemical vapour deposition (LPCVD) technique onto glass substrates. The model takes into account that polycrystalline ZnO is considered to consist of crystal grains surrounded by depletion layers. Using this model and Fourier Transform Infrared Spectroscopy (FTIR) performed in reflection configuration over a wide mid-infrared spectral region (from 2 μm up to 25 μm), the properties of depletion layer and the bulk of the grains in ZnO can be rapidly characterised in detail, and the volume fraction of the depletio...

Photoemission microscopy with microspot-XPS by use of undulator radiation and a high-throughput multilayer monochromator at BESSY

Abstract We present a new experiment for photoelectron microspectroscopy by use of undulator radiation, which has been set up at the beamline U2 at the Berlin electron storage ring BESSY 1. This approach employs a non-imaging simulated hemispherical electron energy analyser attached to an imaging photoemission electron microscope (FOCUS IS-PEEM) with integrated microarea selector. The photoemission microscope exhibits a lateral resolution of 25 nm (with 4.9 eV UV-excitation), while the resolution with incident synchrotron radiation in the soft X-ray range is about 100–120 nm (mainly due to chromatic aberrations). Photoemission microscopy as well as photoelectron microspectroscopy of selected areas on the sample surface were performed by using the third harmonic of the direct undulator beam which was monochromatized and refocused by a two-element multilayer optic in the 70–95 eV energy range. The multilayer monochromator operating at near-normal incidence consists of a concave spherical multilayer mirror ( r =650 mm) and a plane multilayer grating (blazed grating 1221 L/mm, blaze angle 0.8 deg). Both elements were coated with a Mo/Si multilayer of equal d -spacing (20 doublelayers, d =10.5 nm) to enhance the reflectivity for EUV radiation at near-normal incidence angles. The characterization of the individual optical components by EUV reflectometry shows a peak reflectance of about 47% at a photon energy of 95 eV in the case of the focusing multilayer mirror while the first order diffraction efficiency of the multilayer blaze grating was measured to be up to 32%. The evaluation of the photoelectron spectra measured with this set-up displays that the spectral resolution of the incident radiation is better than 0.7 eV, while it is about 2–4 eV in the case of a two-multilayer-mirror configuration. Analysis of the surface topography and the chemical composition of inhomogeneities of thin evaporated layers on a mesoscopic scale are the main applications of this experiment.

Thermal stability of W1−xSix/Si multilayers under rapid thermal annealing

W1−xSix/Si multilayers (MLs) (x⩽0.66) were deposited onto oxidized Si substrates, heat treated by rapid thermal (RTA) and standard furnace annealing up to 1000 °C for 30 s and 25 min, respectively, and analyzed by various x-ray techniques and Rutherford backscattering spectrometry. W1−xSix/Si MLs are more stable the higher the value of x because the driving force for interdiffusion is suppressed by the doping; the temperature for complete interdiffusion increases from 500 to 850 °C as x increases from 0 to 0.66. The as-deposited MLs were amorphous. Their thermal stability increases with increasing x. The interface roughness is independent of x but increases with increasing RTA temperature. The reflectivity of W1−xSix/Si MLs is lower than that of W/Si because of lower optical contrast.

Thermal stability of W1-xSix/Si multilayer reflective coatings under high intensity excimer laser pulses

Abstract UHV e-beam evaporated W 1 − x Si x Si multilayers (MLs) with four different compositions (x = 0, 0.33, 0.5 and 0.66) were XeCl laser irradiated at different fluences F ≤ 0.6 J cm−2 with number of pulses N ≤ 100. The samples were analyzed by soft X-ray reflectivity, X-ray diffraction and resistometry measurements. It was shown, that the hardness of the samples against laser irradiation increases with increasing x up to x = 0.5, then goes down again. This behavior is explained by the interplay of the suppression of the interdiffusion at the interfaces of MLs with increasing content of Si in W and the gradual decrease of the refractory nature of W when it is diluted by Si.

W/Si multilayers deposited by hot-filament MOCVD

Abstract W/Si multilayers with eight double layers (double layer spacing d=20 nm) were deposited on Si [100] substrates using hot-filament (or hot-wire) metal organic chemical vapor deposition (MOCVD). The process was performed in a stainless steel reactor with a tungsten filament at a temperature of 1000°C and a substrate temperature of 190°C. The film thickness and growth was controlled by an in situ soft X-ray reflectivity measurement. The multilayers were characterized by cross-section transmission electron microscopy (XTEM) and sputter auger electron spectroscopy (AES). The results are compared to W/Si bilayers, which were deposited without a hot-filament at higher substrate temperatures (500–670°C).