Measurements
Electron Spin Resonance and Transient Photocurrent ... - JuSER
Electron Spin Resonance and Transient Photocurrent ... - JuSER
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Abstract<br />
The electronic properties of microcrystalline silicon (µc-Si:H) films have been<br />
studied using electron spin resonance (ESR), transient photocurrent time-of-flight<br />
(TOF) techniques, and electrical conductivity measurements. Structural properties<br />
were determined by Raman spectroscopy. A wide range of structure compositions,<br />
from highly crystalline films with no discernable amorphous content, to<br />
predominantly amorphous films with no crystalline phase contributions, was investigated.<br />
Models and possible explanations concerning the nature and energetic<br />
distribution of electronic defects as a function of film composition are discussed.<br />
It is shown that the spin density N S in µc-Si:H films is linked strongly to the<br />
structure composition of the material. The highest N S is always found for material<br />
with the highest crystalline volume fraction. With increasing amorphous content,<br />
N S decreases, which is attributed to increasing hydrogen content and improved<br />
termination of dangling bonds. Moreover, the amorphous phase content, incorporated<br />
between the crystalline columns, appears to act as a passivation layer,<br />
leading to more effective termination of unsatisfied bonds at the column boundaries.<br />
Both reversible and irreversible changes in the ESR signal and dark conductivity<br />
due to atmospheric effects are found in µc-Si:H. These are closely connected to<br />
the structure composition, in particular the active surface area. The porous structure<br />
of highly crystalline material facilitates in-diffusion of atmospheric gases,<br />
which strongly affects the character and/or density of surface states. Two contributing<br />
processes have been identified, namely adsorption and oxidation. Both<br />
processes lead to an increase of N S . In the case of adsorption the increase is<br />
identified as arising from changes of the db2 resonance (g=2.0052), while the intensity<br />
of the db1 resonance (g=2.0043) remains constant. With increasing amorphous<br />
content the magnitude of both adsorption and oxidation induced changes<br />
decreases, which may be linked to the greater compactness of such films.<br />
<strong>Measurements</strong> on n-type µc-Si:H films were used as a probe of the density of<br />
gap states, confirming that the spin density N S is related to the density of defects.<br />
The results confirm that for a wide range of structural compositions, the doping<br />
induced Fermi level shift in µc-Si:H is governed by compensation of defect states,<br />
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