Page 145 - Organic Electronics in Sensors and Biotechnology

122 Cha pte r F o u r

480
–80
450 –81 470
–82 460
430 –83 450
Z ·f (kΩ · Hz) 410 –84 Phase (deg) Capacitance (pF) 440
–85
390 –86 430
–87
420
370 –88
410
–89
350 –90 400
1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 –40 –30 –20 –10 0 10 20 30 40 50
Frequency (Hz) Voltage (V)
Measurement time 28 h
Measurement time 160 s
FIGURE 4.3 Left: IS of an MPP MIS structure at depletion (−40 V) (symbols–
measured, lines–modeled) (fi t parameters: contact resistance R = 99 Ω, inductivity
c
L = 10 nH; capacitive layer 1: C = 391 pF, R = 10 GΩ; p = 0.98%, R(0) = 10.1
1 1 1 1
kΩ; capacitive layer 2: C = 5.7 nF, R = 704 Ω, see text). Right: C(V) curve of the
2 2
same structure at two different measurement speeds (black curve: 875 mV/s, gray
curve: 1.4 mV/s)

curve is mirrored at the reverse point indicates that slow voltages
dependent charging effects play a role. This is discussed in greater
detail in Sec. 4.2.
The second device is a pentacene MIS structure with BCB (benzo-
cyclobutene) as dielectric. An analysis with impedance spectroscopy
reveals the layer structure of the device. The IS is measured from
accumulation ( −6 V) to depletion (5 V). They can be fit by four layers:
two layers have a constant capacitance over the entire bias voltage
range and hence are attributed to the dielectric. However, to fit the IS
one of the two layers is modeled by a Young element. The Young
characteristic of this layer is due to the semiconductor-insulator inter-
face with the contact.
The capacitances of the remaining two layers have an opposite
dependence on the bias voltage. However, the sum of the two serial
capacitances is constant. This indicates that the two layers correspond
to the depletion and bulk layers of the semiconductor. The depletion
layer is also modeled by a Young element, which accounts for the
interface to the semiconductive bulk layer.
From the capacitances of the four layers, the device area, and
the dielectric constants of pentacene (~3) and BCB (3.78), the layer
thicknesses can be derived. As can be seen in Fig. 4.4, the bulk and
depletion thicknesses add up to the thickness of the thermally grown
pentacene layer (~50 nm); the thicknesses of the two BCB layers are in
total 86 nm. This fits well to the thickness values obtained from direct
measurements (oscillating crystal: ~45 nm pentacene, profilometer:
~85 nm BCB).