U. Kroll

Hydrogen in amorphous and microcrystalline silicon films prepared by hydrogen dilution

Hydrogen incorporation in silicon layers prepared by plasma‐enhanced chemical‐vapor deposition using silane dilution by hydrogen has been studied by infrared spectroscopy (IR) and elastic recoil detection analysis (ERDA). The large range of silane dilution investigated can be divided into an amorphous and a microcrystalline zone. These two zones are separated by a narrow transition zone at a dilution level of 7.5%; here, the structure of the material cannot be clearly identified. The films in/near the amorphous/microcrystalline transition zone show a considerably enhanced hydrogen incorporation. Moreover, comparison of IR and ERDA and film stress measurements suggests that these layers contain a substantial amount of molecular hydrogen probably trapped in microvoids. In this particular case the determination of the total H content by IR spectroscopy leads to substantial errors. At silane concentrations below 6%, the hydrogen content decreases sharply and the material becomes progressively microcrystalline...

On the Way Towards High Efficiency Thin Film Silicon Solar Cells by the “Micromorph” Concept

Note: IMT-NE Number: 222 Reference PV-LAB-CONF-1996-008 Record created on 2009-02-10, modified on 2017-05-10

Device grade microcrystalline silicon owing to reduced oxygen contamination

As‐deposited undoped microcrystalline silicon (μc‐Si:H) has in general a pronounced n‐type behavior. Such a material is therefore often not appropriate for use in devices, such as p‐i‐n diodes, as an active, absorbing i layer or as channel material for thin‐film transistors. In recent work, on p‐i‐n solar cells, this disturbing n‐type character had been successfully compensated by the ‘‘microdoping’’ technique. In the present letter, it is shown that this n‐type behavior is mainly linked to oxygen impurities; therefore, one can replace the technologically delicate microdoping technique by a purification method, that is much easier to handle. This results in a reduction of oxygen impurities by two orders of magnitude; it has, furthermore a pronounced impact on the electrical properties of μc‐Si:H films and on device performance, as well. Additionally, these results prove that the unwanted donor‐like states within μc‐Si:H are mainly due to extrinsic impurities and not to structural native defects.

Frequency-Effects in Silane Plasmas for Plasma Enhanced Chemical Vapor-Deposition

It is now generally recognized that the excitation frequency is an important parameter in radio‐frequency (rf) plasma‐assisted deposition. Very‐high‐frequency (VHF) silane plasmas (50–100 MHz) have been shown to produce high quality amorphous silicon films up to 20 A/s [H. Curtins, N. Wyrsch, M. Favre, and A. V. Shah, Plasma Chem. Plasma Processing 7, 267 (1987)], and therefore the aim of this work is to compare the VHF range with the 13.56 MHz industrial frequency in the same reactor. The principal diagnostics used are electrical measurements and a charge coupled device camera for spatially resolved plasma‐induced emission with Abel inversion of the plasma image. We present a comparative study of key discharge parameters such as deposition rates, plasma uniformity, ion impact energy, power transfer efficiency, and powder formation for the rf range 13–70 MHz.

Evolution of the Microstructure in Microcrystalline Silicon Prepared by Very High Frequency Glow-Discharge using Hydrogen Dilution

A series of samples was deposited by very high frequency glow discharge in a plasma of silane diluted in hydrogen in concentrations SiH4/(SiH4+H2) varying from 100% to 1.25%. For silane concentrations below 8.4%, a phase transition between amorphous and microcrystalline silicon occurs. Microcrystalline silicon has been characterized by transmission electron microscopy (TEM) and x-ray diffraction. The medium-resolution TEM observations show that below the transition, the microstructure of microcrystalline silicon varies in a complex way, showing a large variety of different growth structures. For the sample close to the phase transition, one observes elongated nanocrystals of silicon embedded in an amorphous matrix followed at intermediate dilution by dendritic growth, and, finally, at very high dilution level, one observes columnar growth. X-ray diffraction data evidence a (220) crystallographic texture; the comparison of the grain sizes as evaluated from TEM observations and those determined using Scherr...

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 1016/cm3 could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 A/s for microcrystalline silicon could be achieved.

Nanostructured three-dimensional thin film silicon solar cells with very high efficiency potential

We report on the experimental realization of amorphous/microcrystalline silicon tandem solar cells (Micromorph) based on our three-dimensional design. An enhancement is reached in the short-circuit current by 40%, with an excellent open-circuit voltage of 1.41V and a fill factor of 72%. We have used nanoholes or microholes dry etched into the ZnO front contact layer. Monte Carlo optical modeling shows that stable efficiency of amorphous silicon p-i-n solar cells in over 12% range is possible. For the Micromorph cells, efficiency over 15% with the thickness of amorphous Si below 200 nm and of microcrystalline Si around 500 nm is possible.

High Rate Growth of Microcrystalline Silicon by VHF-GD at High Pressure

Microcrystalline silicon growth using very high frequency-glow discharge PECVD has been studied under conditions of high pressure and high VHF-power conditions. Hereby, the influence of the total gas flow and the silane concentration on the deposition rate has been investigated. Deposition rates of over 25 A/s have been achieved at relatively low total gas flows of 100 sccm. These high-rate films show device-grade quality with respect to subband gap absorption and microcrystalline structure. Dark conductivity measurements reveal midgap character and transmission electron microscopy investigations confirm a highly crystalline microstructure from the bottom to the top of the μc-Si:H films. These high-rate μc-Si:H layers are very interesting candidates for solar cell and other devices.

From amorphous to microcrystalline silicon films prepared by / hydrogen dilution using the VHF 70 MHz GD technique

Abstract The amorphous and microcrystalline silicon films have been prepared by hydrogen dilution from pure silane to silane concentrations ≥1.25%. At silane concentrations of less than 10%, a transition from the amorphous phase to the microcrystalline phase can be observed. X-ray diffraction spectroscopy indicates a preferential growth of the crystallites in the [220] direction. Additionally, the transition into the microcrystalline regime is accompanied by a shrinking of the optical gap, a reduction in hydrogen content and by a modified trend of the deposition rate. The observed changes in the infrared absorption modes indicate modifications in the hydrogen bonding and can be correlated with results known from monocrystalline silicon. Close to the transition zone, but still in the amorphous regime, the hydrogen content is increased, whereas the microstructure parameter reaches its smallest value. Precisely these films have a 0.06 eV higher optical gap and a reduced defect density by a factor of 4 as compared to a-Si:H layers prepared from pure silane.

Influences of a high excitation frequency (70 MHz) in the glow discharge technique on the process plasma and the properties of hydrogenated amorphous silicon

Hydrogenated amorphous silicon has been prepared at a plasma excitation frequency in the very‐high‐frequency band at 70 MHz with the glow discharge technique at substrate temperatures between 280 and 50 °C. The structural properties have been studied using hydrogen evolution, elastic recoil detection analysis, and infrared spectroscopy. The films were further characterized by dark and photoconductivity and by photothermal deflection spectroscopy. With respect to films prepared at the conventional frequency of 13.56 MHz considerable differences concerning the electronic and structural properties are observed as the substrate temperature is decreased from 280 to 50 °C. Down to a substrate temperature of 150 °C the electronic film properties change only a little and the total hydrogen content cH and the degree of microstructure that can be directly correlated to cH increase only moderately. Below 150 °C the electronic properties deteriorate in the usual manner but still the total hydrogen content does not ex...