Physics of Amorphous Silicon–Carbon Alloys

The correlation between the density of dangling-bond defects and the slope of the Urbach tail in hydrogenated amorphous silicon is examined. It is shown that this correlation can be explained quantitatively by a spontaneous decay of the weakest bonding orbitals into non-bonding defects during deposition or annealing of a sample and that the same correlation holds for all types of disorder affecting the slope of the Urbach edge. The temperature dependence of the defect density as well as the creation of metastable defects are discussed, and quantitative expressions are derived which can be used to estimate the quality and the stability of a given sample on the basis of the slope of its Urbach tail alone. Possible ways for future improvement of the material are indicated.

The defect density in amorphous silicon

The sub‐band‐gap optical absorption spectra of high‐quality hydrogenated amorphous silicon (a‐Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not. For bulk defect states (both as‐deposited and light‐induced), the integrated subgap absorption is approximately twice as large for PDS as for CPM. Similarly, the conversion factor relating integrated subgap absorption with neutral dangling bond density is twice as large for CPM as PDS. This factor of 2 results from CPM seeing only transitions from below midgap into the conduction band while PDS sees transitions from the valence band into states above midgap as well.

Photothermal and photoconductive determination of surface and bulk defect densities in amorphous silicon films

In amorphous/crystalline silicon heterojunction solar cells, optical losses can be mitigated by replacing the amorphous silicon films by wider bandgap amorphous silicon oxide layers. In this article, we use stacks of intrinsic amorphous silicon and amorphous silicon oxide as front intrinsic buffer layers and show that this increases the short-circuit current density by up to 0.43 mA/cm2 due to less reflection and a higher transparency at short wavelengths. Additionally, high open-circuit voltages can be maintained, thanks to good interface passivation. However, we find that the gain in current is more than offset by losses in fill factor. Aided by device simulations, we link these losses to impeded carrier collection fundamentally caused by the increased valence band offset at the amorphous/crystalline interface. Despite this, carrier extraction can be improved by raising the temperature; we find that cells with amorphous silicon oxide window layers show an even lower temperature coefficient than referenc...

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Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells

The known properties of nitrogen as an impurity in, and as an alloy element of, the germanium network are reviewed in this article. Amorphous and crystalline germanium–nitrogen alloys are interesting materials with potential applications for protective coatings and window layers for solar conversion devices. They may also act as effective diffusion masks for III-V electronic devices. The existing data are compared with similar properties of other group IV nitrides, in particular with silicon nitride. To a certain extent, the general picture mirrors the one found in Si–N systems, as expected from the similar valence structure of both elemental semiconductors. However, important differences appear in the deposition methods and alloy composition, the optical properties of as grown films, and the electrical behavior of nitrogen-doped amorphous layers. Structural studies are reviewed, including band structure calculations and the energies of nitrogen-related defects, which are compared with experimental data. ...

Nitrogen in germanium

Visible light emission at room temperature from partially oxidized amorphous silicon

Disorder and defects in sputtered a-SiH from subgap absorption measurements

Angular distortions and the exponential absorption edge of silicon-rich a-SixNyHz alloys

The derivation of a density of states g(E) in the gap from best fit to an experimental optical α(hν) spectrum below the Urbach tail and the assignment of the integrated number of absorbing defects NR to ESR active centres are discussed. The calculated α(hν) are shown to be relatively unsensitive to the gap DOS profile. Photothermal Deflection Spectroscopy (PDS) measurements on glow-discharge (GD), sputtered (SP) and post-hydrogenated CVD aSi:H films show that in a few cases NR as deduced from PDS by the standard procedure cannot be correlated to the intensity of the ESR line.

E. Bustarret

Papers

Visible light emission at room temperature from partially oxidized amorphous silicon

Disorder and defects in sputtered a-SiH from subgap absorption measurements

Angular distortions and the exponential absorption edge of silicon-rich a-SixNyHz alloys

On the interpretation of sub-bandgap optical absorption in a-Si:H