Measurements

Chapter 5
N-Type Doped µc-Si:H
In section 4 results of intrinsic films of µc-Si:H with a systematic variation of material
structure ranging from highly crystalline to amorphous growth were shown.
ESR measurements have been used to determine the spin density N S . However,
from these investigations it is not clear how far N S correlates with the defect density
in the material and if the spin density N S is a measure of the real defect density
N DB . This is the subject of the following section.
For this purpose, material prepared with different SC=[SiH 4 ]/([H 2 ]+[SiH 4 ])
and phosphorous doping levels PC =[PH 3 ]/([PH 3 ]+[SiH 4 ]) will be studied. The
silane concentration was varied in the range from SC = 2% to 8%, resulting in
structure compositions comparable to those studied in Chapter 4. The doping
concentrations PC of 1, 5, and 10 ppm were chosen to be of the order of the
intrinsic spin density N S (see Fig. 4.5 (a)). To study effects of doping on the
position of the Fermi level and the occupation of defect states, electrical dark
conductivity σ D and ESR measurements have been performed.
5.1 Structure Characterization
In order to obtain a measure of the crystalline volume content, Raman spectra were
recorded on both glass and aluminum substrates. The results are summarized in
Fig. 5.1 (a), showing the Raman intensity ratio IC
RS plotted versus the silane concentration
SC. For doping concentrations in the range of 1−10 ppm the transition
from highly crystalline to predominantly amorphous growth can be observed. In
the highly crystalline growth regime between SC=2% and 5%, the Raman intensity
ratio decreases only slightly from IC
RS =0.85 to 0.74 and the spectra are
dominated by the crystalline signal. Above SC=5% the transition to amorphous
growth can be observed. An increasing amorphous phase contribution results in
a fairly steep decrease of IC
RS between SC=5% and 7%. For silane concentra-
51