H.N. Wanka

Amorphous and microcrystalline silicon by hot wire chemical vapor deposition

Amorphous hydrogenated silicon (a‐Si:H) was deposited by SiH4 decomposition on a hot tungsten filament. The substrate temperature was held at 400 °C for all samples, maintaining conditions where material combining a low defect density and a low hydrogen content is obtained. A systematic study of the effects of gas pressure, substrate‐to‐filament distance, and filament temperature on film properties is presented, allowing insight into the growth condition required for this material as well as the significance of secondary gas phase reactions. Material of good optoelectronic quality is obtained at high growth rates. The stability with respect to light degradation was compared to typical plasma deposited films. Conditions for the transition from amorphous to microcrystalline films, observed under gas phase dilution with hydrogen, were investigated. By in situ ellipsometry and atomic force microscopy the nucleation and film morphology were shown to be significantly different from those for plasma‐chemical vap...

High silicon etch rates by hot filament generated atomic hydrogen

The etching of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon by hot tungsten filament generated atomic hydrogen has been investigated. Room-temperature etch rates of 27 A for amorphous and 20 A for microcrystalline silicon have been achieved. Boron doping decreases the etch rate, whereas phosphorus doping does not affect it. No surface roughening occurs, even for the highest a-Si:H etch rates. In the initial phase of the etch process, however, a bond structure modification arises close to the surface. An increase of microcrystalline silicon etch rates towards the substrate/film interface reflects the coalescence of the microcrystalline nuclei. Hot filament atomic hydrogen etching provides high etch rates of amorphous and polycrystalline silicon with a high selectivity against metals and thermal oxide. Due to its simple setup and control, this kind of hydrogen etching is very interesting for applications in semiconductor technology where F- or Cl-etchants are to be avoided.

Electronic and optical properties of hot-wire-deposited microcrystalline silicon

Abstract We conducted a study on undoped and doped hydrogenated microcrystalline silicon ( μ c-Si) samples deposited by hot-wire chemical vapor deposition for determination of the structural, electronic and optical properties. Light scattering, investigated by total and specular transmission and reflection, as well as angular resolved measurements, results mainly from the surface but with an intrinsic contribution from the interior. The optical properties resemble that of monocrystalline Si: the refractive index in the visible, the reflectance peaks in the near ultraviolet which may only appear after surface polishing, and the absorption coefficient which is larger than in monocrystalline silicon and varies with sample thicknesses. Depending on the doping level, the dark conductivity prefactor and activation energy exhibit either normal or anti-Meyer–Neldel rule behavior. The mobility-lifetime product from steady state photoconductivity strongly depends on the position of the Fermi energy with a minimum for low p-type doping, suggesting the importance of information on the Fermi energy if the mobility-lifetime product is given as an indicator for material quality of microcrystalline Si.

Characterization and Optimization of The Tco/a-Si:H(,B) Interface for Solar Cells by In-Situ Ellipsometry and Sims/XPS Depth Profiling

The chemical reactions at the surface of transparent conductive oxides (SnO 2 , ITO and ZnO) have been studied in silane and hydrogen plasmas by in-situ ellipsometry and by SIMS as well as XPS depth profiling. SIMS and XPS of the interface reveal an increasing amount of metallic phases upon lowering a-Si:H growth rates (controlled by plasma power), indicating that the ion and radical impact is more than compensated by protecting the surface by a rapidly growing a-Si:H film. Hence, optical transmission of TCO films as well as the efficiency of solar cells can be improved if the first few nanometers of the p-layer are grown at higher rates. Comparing a-Si:H deposition on top of different TCOs, reduction effects on ITO and SnO 2 have been detected whereas ZnO appeared to be chemically stable. Therefore an additional shielding of the SnO 2 surface by a thin ZnO layer has been investigated in greater detail. Small amounts of H are detected close to the ZnO surface by SIMS after hydrogen plasma treatment, but no significant changes occur to the optical and electrical properties. In-situ ellipsometry indicates that a ZnO layer as thin as 20 nm completely protects SnO 2 from being reduced to metallic phases. This provides for shielding of textured TCOs, and hence rising solar cell efficiencies, too. Regarding light trapping efficiency we additionally investigated the smoothing of initial TCO texture when growing a-Si:H on top by combining atomic force microscopy and spectroscopie ellipsometry.

Growth of a-Si:H on transparent conductive oxides for solar cell applications

Abstract Different concepts for optimizing the transparent conductive oxide (TCO)/p-interface in hydrogenated amorphous silicon (a-Si:H) based solar cells have been studied in order to avoid the segregation of metal layers, and hence considerable reductions in short circuit current. We analysed structural and chemical changes which occur at the surface of transparent conductive oxides — TCOs (SnO 2 , ZnO, and Indium Tin Oxide — ITO) in silane, hydrogen and C0 2 plasmas. We also used a-SiO:H instead of a-SiC:H in the p-doped layer. In-situ ellipsometry and SIMS/XPS depth profiling show that room temperature as well as fast deposition easily overcome all detrimental effects. TCO deterioration by ion and radical bombardement at high deposition rates is more than compensated if the surface is protected by a rapidly growing a-Si:H film. Using ZnO as a TCO, or as a 20 nm buffer layer only, completely suppresses metal formation. In-situ ellipsometry in conjunction with atomic force microscopy reveals significant changes in surface morphology, namely filling of the TCO-texture during deposition, which is of crucial importance for light trapping in solar cells.

Electronic Properties of Hot-Wire Deposited Nanocrystalline Silicon

We compare the electronic properties of nanocrystalline silicon from hot-wire chemical vapor deposition in a high-vacuum and an ultra-high-vacuum deposition system, employing W and Ta as filament material. From the constant photocurrent method we identify a band gap around 1.15 eV while, in contrast, a Tauc plot from optical transmission data guides to a wide band gap above 1.9 eV. The sudden change-over from nanocrystalline to amorphous structure in a hydrogen dilution series is also find in the dark and photoconductivity measurements. The samples show a metastability effect in the dark conductivity upon annealing in vacuum with an increase in the dark conductivity, with the large dark conductivity decreasing slowly after the annealing cycle when the cryostat is flushed with air. We identify larger values for the mobility-lifetime products, which corresponds to the smaller defect density shoulder in constant photocur- rent spectra, for the ultra-high-vacuum deposited material compared to the high-vacuun counterpart.

CO2 plasma treatment of tin oxides

Abstract By combining in situ ellipsometry and XPS depth profiling, the effects of a CO 2 plasma treatment on tin oxides have been investigated. Tin oxide, used as the transparent front contact in hydrogenated amorphous silicon (a-Si:H) solar cells, is chemically reduced by the impact of the silane plasma during the preparation of a-Si:H. In this report a detailed study will be presented how tin oxides can be protected by a specific CO 2 plasma treatment which prevents this reduction process. In situ ellipsometry proves that no reduction occurs during the deposition of a-Si:H on tin oxides after a pre-treatment by CO 2 plasma. Moreover these pre-treated tin oxides are resistant to a pure hydrogen plasma, which causes significant chemical reduction of untreated ones and substantially deteriorates their optical transmission. XPS analysis of CO 2 treated tin oxides reveals the formation of a SiO 2 layer, since silicon is chemically transported from the reactor walls to the surface of the substrate by means of the plasma. XPS depth profiling of a-Si:H/SiO 2 /SnO 2 interfaces confirms the interpretation of the in situ ellipsometry measurements. With the appropriate setting of CO 2 plasma parameters, no metallic tin is formed at the a-Si:H/SnO 2 interface. Employing inadequate parameters, however, leads to a reduction of the tin oxide, and to the formation of metallic tin at the interface. The investigation of these different situations convincingly demonstrates how in situ ellipsometry and XPS depth profiling are able to provide complementary information on this chemical reduction process. The deposition of the samples has been monitored in real time by ellipsometry, while the erosion of these layers by sputtering and simultaneous XPS analyses quantitatively reveals their chemical composition.

Investigation of the Surface Morphology of a-Si:H by Atomic Force Microscopy and In-Situ Ellipsometry

Combining real-time ellipsometry and atomic force microscopy (AFM) the growth of hydrogenated amorphous silicon (a-Si:H), deposited on crystalline silicon wafers with a native oxide layer on top and on fused silica from a dc glow discharge, has been studied from initial nucleation to the final morphology. By in-situ ellipsometry we detected the evolution of morphology changes. The surface structure has been determined by AFM in the nucleation phase and in the subsequent growth stage. During nucleation on crystalline silicon only few (about 40 per 1μm) flat islands of a-Si:H (up to 4 nm high and up to 50 nm in diameter) grow with a strongly enhanced rate compared to bulk deposition. Once the crystalline surface has completely been covered by a-Si:H, the fast deposition of these islands stops and further surface structures, comparable with the initial ones, start to grow gradually until a homogeneous final roughness up to 5nm high is formed. Nucleation of a-Si:H on fused silica yields densely distributed nuclei (up to 1.5 nm high and up to 25 nm in diameter), indicating a shorter surface diffusion length on this substrate compared to the growth on silicon wafers. The ongoing film deposition, however, finally results in a morphology comparable to the one of a-Si:H grown on crystalline silicon. Using hydrogen dilution we found that the final roughness is affected by the dilution ratio; furthermore infrared spectroscopy reveals the surface structure to be correlated with the hydrogen content of the a-Si:H films.

Microcrystalline silicon from very high frequency plasma deposition and hot-wire CVD for 'micromorph' tandem solar cells

The authors have grown microcrystalline silicon from a glow discharge at very high frequencies of 55 MHz and 170 MHz with high hydrogen dilution, and also, at more than 10 times higher growth rates, similar films by hot-wire chemical vapor deposition. Both kinds of materials have extensively been characterized and compared in terms of structural, optical and electronic properties, which greatly improve by deposition in a multi- instead of a single-chamber system. Incorporation of these different materials into p-i-n solar cells results in open circuit voltages of about 400 mV as long as the doped layers are microcrystalline and rise to more than 870 mV if amorphous p- and n-layers are used. Quantum efficiencies and fill factors are still poor but leave room for further improvement, as clearly demonstrated by a remarkable reverse bias quantum efficiency gain.

Flexible Micro-Photodiode Array as a Subretinal Implant

M.B. Schubert, A. Hierzenberger, H.N. Wanka, M. Graf*, H.G. Graf*,W. Nisch°Institute of Physical Electronics, University of Stuttgart,Pfaffenwaldring 47, D-70569 Stuttgart, Germany; *Institute forMicroelectronics Stuttgart, Allmandring 30a, D-70569 Stuttgart, Germany;°Natural and Medical Science Institute at Univ. of Tubingen,Eberhardstr. 29, D-72762 Reutlingen, Germany The development of an ultrathin, flexible Micro-Photodiode Array aimsto replace degenerated photoreceptors in the human eye to re-establisha certain amount of vision. For realizing devices according tobiocompatible requirements, a broad technological approach has beenchosen which in addition to crystalline silicon microelectronics alsoincludes amorphous silicon photodiodes for subretinal implantation.1. IntroductionSeveral types of diseases destroy theouter retinal layers and, in the finalstage, people become blind. Anambitious neurotechnology programwas started in Germany in 1995,pursuing the goal of developing aretinal implant. Two research teamshave chosen fundamentally differentapproaches to solve this problem,namely the subretinal [1] and theepiretinal [2] one. We report on thesubretinal approach here, which aimsat directly replacing degeneratedphotoreceptors with a Micro-Photodiode Array (MPDA) in thesubretinal space of the eye. TheseMPDAs have to be flexible, in order