B. Schroder

Deposition of device quality a-Si:H films with the hot-wire technique

Abstract High quality hydrogenated amorphous silicon (a-Si:H) films have been deposited using thermal decomposition of silane (SiH 4 ) at a heated tungsten filament. Deposition rates up to 8 A/s were achieved for device quality material. The stability of the films deposited at substrate temperatures above 400 °C has been observed to be superior to that of device quality glow discharge material. A very low hydrogen content and a small structural disorder (low Urbach energy) of the bulk material can explain this higher stability. Successful phosphine doping was performed and an efficiency of η = 4.3% was achieved for a solar cell with hot wire (HW) deposited i- and n + -layers.

Density of glow discharge amorphous silicon films determined by spectroscopic ellipsometry

The dielectric function of thin‐film hydrogenated amorphous silicon (a‐Si:H) grown on fused silica at different substrate temperatures has been investigated by spectroscopic ellipsometry. An improved interpretation of the ellipsometric data is based on a tetrahedron model that takes into account the influence of hydrogen incorporation in the amorphous network. It is shown that the film density can be derived from an accurate data interpretation, whereas the maximum value of the imaginary part of the dielectric function e2max and the void volume fraction are not proportional to the density of a‐Si:H films.

In situ ellipsometric study of the influence of powder formation on the initial growth of glow discharge a−Si:H

Abstract Even under standard preparation conditions for amorphous hydrogenated silicon (a−Si:H) by r.f. glow discharge decomposition of silane, powder formation can occur leading to an initial transient behaviour of silane discharges. Phase-modulated real-time in situ ellipsometry is used to study the influence of the discharge behaviour on the initial stage of growth of a−Si:H after ignition and subsequent to intentional discharge interruptions. A clear correlation between the powder-determined time dependences of the discharge parameters pressure and self-bias voltage and of the ellipsometric angles is confirmed. In the discharge regime where this initial transient behaviour due to powder formation is observed, the corresponding Δ - ψ trajectories can be attributed to the initial growth of less dense material. The time-dependent Δ signal of the growing film is found to be very sensitive even to minor discharge perturbations. The influence of controlled temporal variations of preparation parameters on the growing a−Si:H film can be simultaneously monitored by real-time in situ ellipsometry. This offers an opportunity for interface optimization in a−Si:H structures.

Effective nitrogen doping of a—Ge: H films prepared by DC-magnetron sputtering

Abstract Nitrogen incorporation below 2 at% into magnetron sputtered a—Ge: H increases the dark conductivity from 3·10-6(Ωcm)-1 to 3·10-1(Ωcm)-1 at maximum. The density of defect states detected by PDS increases proportional to the square root of nitrogen partial pressure. It is suggested that nitrogen-hydrogen complexes acting as donor states are responsible for this doping effect. Comparative investigations on nitrogen and phosphorus doped a—Ge: H show a higher doping efficiency, larger maximum dark conductivity and stronger structural disorder for nitrogen doping performed under identical preparation conditions.

Influence of powder formation in a silane discharge on a-Si:H film growth monitored by in situ ellipsometry

Abstract Certain discharge regimes in the silane glow-discharge deposition of amorphous hydrogenated silicon (a-Si:H) are influenced by the formation of polysilane powder. Using kinetic and spectroscopic in situ ellipsometry we found that powder formation not only influences the initial film growth after discharge ignition leading to the deposition of a layer of less dense material but also is responsible for a reduction of bulk film density. Modifications of the ignition procedure enable a reduction of these disturbing effects of powder formation. The present results hint at a possible extension of the powder-free discharge regime under “soft start” conditions.

The influence of the film-substrate interface on the defect density and other properties of sputter-deposited amorphous hydrogenated silicon

The properties of sputter-deposited amorphous hydrogenated silicon have been found to vary considerably as a function of the film thickness for d >

Photoconductivity spectroscopy (CPM) on a-Ge:H at low temperatures

Abstract The constant photoconductivity measurement technique (CPM) is applied to high quality hydrogenated amorphous germanium (a-Ge:H) samples at temperatures between 300 and 130K. Considering the temperature dependence of the Urbach energy Eo an upper limit of 30meV is found for the contribution of the structural disorder to Eo in a-Ge:H. From CPM measurements at temperatures below 170K the defect density can be successfully determined. A comparison of CPM and PDS results reveals a strong influence of surface defects on the total defect density of high quality a-Ge:H.

Effect of "soft start" i-layer deposition on the performance and stability of a-Si:H p-i-n solar cells

In this study, the authors investigate the effect of an ignition procedure for the i-layer deposition on cell performance and stability. Starting with low RF power (soft start), the hydrogen concentration and the defect density at the p-i interface is reduced leading to an increase of the conversion efficiency. This is due to the reduction of particle formation in the discharge and of its detrimental effects on initial film growth under soft start conditions. Light soaking experiments performed with solar cells and intrinsic films simultaneously deposited indicate that the relative degradation of the efficiency is mainly determined by the bulk i-layer rather than by the p-i interface region properties. However, due to the increase of efficiency in the initial state, an improvement of the efficiency after degradation could be achieved by this p-i interface optimization.

Correlation between improved stability and microstructural properties of a-Si:H and a-Ge:H

In this paper we report on comparative investigations concerning the stability of glow discharge and hot wire deposited a-Si:H films. Using spectroscopic ellipsometry measurements and a data interpretation based on a tetrahedron model we investigated the microstructural properties of the films. We found that the stability of a-Si:H films is significantly increased by a deposition process guided in a way that high quality material can be grown with high density and low hydrogen content. The a-Si:H results are confirmed by similar results obtained on sp-a-Ge:H. For the first time we have shown that metastability is not connected with the existence of surface like (SiH/sub 2/, (SiH)/sub x/) hydrogen bonding configurations.

The saturation behaviour of metastable defect creation in a-Ge:H investigated by keV-electron irradiation

We report on the stability of differently prepared high quality a-Ge:H using keV-electron irradiation as a fast and strong degradation technique for the creation of metastable defects. The influence of film properties on the saturation defect density is carefully studied and a chemical equilibrium model developed for a-Si:H is successfully applied to a-Ge:H. This allows to calculate the saturation defect density of a-Ge:H from its initial film properties. Analysing the chemical equilibrium the influence of specific parameters like hydrogen content and Urbach energy on the saturation defect density after keV-electron irradiation is examined and proposals for the improvement of the stability of a-Ge:H against metastable defect creation are made and experimentally confirmed. The experiments reveal no indication for a principally different stability behaviour of a-Ge:H compared to a-Si:H.<<ETX>>