D. Caputo et al., CHARACTERIZATION OF INTRINSIC A-SI-H IN P-I-N DEVICES BY CAPACITANCE MEASUREMENTS - THEORY AND EXPERIMENTS, Journal of applied physics, 76(6), 1994, pp. 3534-3541
A technique has been developed for characterizing the intrinsic layer
in a-Si:H p-i-n structures. The method is based on the measurement of
the differential capacitance under forward bias, i.e., with injection
of free carriers in the intrinsic layer. In these conditions, capacita
nce is extremely dependent on charges trapped in the band-gap defects.
Measurements were performed on several p-i-n samples, in a wide range
of frequencies and voltages. All the samples showed the same trend: C
apacitance diminished with increasing signal frequency and increased w
ith forward applied voltage. An analytical model explains the obtained
behavior. In particular, the model shows that the band-tail contribut
ion to capacitance decreases slowly with frequency, while deeper defec
ts are effective only below 100 Hz. At higher frequencies, trapping ph
enomena play a lesser and lesser role in the measurement while depleti
on charge becomes relevant and the measured capacitance tends to its a
symptotic junction value (i.e., the ratio dielectric constant/thicknes
s of the p-i-n). The model predicts a high sensitivity to defect distr
ibution in the gap of the intrinsic layer, thus allowing an effective
characterization of the semiconductor material as is in the actual dev
ice. Up to 10(17) cm(-3) defects have been detected via the capacitanc
e technique, presumably located in the portion of the intrinsic layer
closest to the p/i interface. The origin of such a large amount of def
ects can be related to the presence of inhomogeneities due to the inte
rfaces in the p-i-n device. These defects behave as free-carrier traps
, and do not affect the recombination mechanism. A picture of the elec
tronic defect distribution in the device material is finally proposed
which accounts for both midgap neutral dangling bonds and shallower ch
arged defects.