Chapter 4 Results and Discussions
4.2 A preparation by making Use of SiO 2 Substrate
Part A: Morphology Evolution
4.2-1 The Relationship between the Deposition Temperature to the Grain Size
Figure 4. 1 AFM images of 60-nm-thick pentacene deposited at a fixed flux rate of 0.5Å/sec on a 200-nm-thick SiO2 substrate without surface treatment at various deposition temperatures. The focus position is between the source/drain electrodes.
(A)T =17℃, R=10.273nm. (B) T =30℃, R=9.610nm. (C) T =50℃, R=8.976nm. (D) T =70℃, R=6.680nm. (E) T =90℃, R=7.105nm. (F) T =110℃, R=8.570nm.
All the images are 3×3μm2 except the bigger two which are 5×5μm2. (R: roughness presented by root-mean-square form)
The AFM images of Figure 4. 1 display the surface morphology of 60-nm-thick pentacene deposited on bare SiO2 substrate at 17℃, 30℃, 50℃, 70℃, 90℃, and 110℃ respectively. The rms surface roughness of each film varied in the range of 6-10nm. Note that it merely forms stable film until the substrate is heated at 120℃
which is probably due to the equal rate of adsorption and desorption.
According to the evolution of morphology, the grains show a progress enlargement conspicuously as the substrate temperature goes up. Since the grain boundaries behave as a barrier against carrier hopping [11], it is believed that larger grain sizes, which mean less grain boundaries, should be more conductive.
Part B: I-V Characteristics
4.2-2 The Mobility Calculation
In the following section, we will extract the mobility (μ) from the saturation regime of the field-effect transistor model.
At higher VD, which means –VD > –(VG-VT), ID tends to saturate, and is approximately determined by the equation:
)2
μ . It is important to note that this equation is only valid when μ is
constant. Thus, despite of the most widespread use, it should be treated as an approximate value.
4.2-3 The Transfer Characteristics
the OTFTs, which were fabricated at
-60 -40 -20 0 20 40 60 80
Figure 4. 2 The transfer characteristics of
various deposition temperatures with bare SiO2 substrate, obtained under VD=-60V.
10 30 50 70 90
Figure 4. 3 The results of the mobility calculation from the saturation regime of the modeling of field-effect transistor. Circle: the average obtained from the maximum tangent in the plot of ID versus VG.
The relationship of the hole mobility to deposition temperature is revealed in Figure 4. 3
Figure 4. 2 T
√-ID) plots in Figure 4. 2 exhibits two gradations at higher tem
G T
art C: Crystal Structure
. For the device without surface treatment, the average mobility goes up from 0.17 to 0.41 cm2/Vsec by deposition temperature elevated. It demonstrates that the higher the temperature is, the better the device performance becomes.
However, the corresponding transfer characteristics of the OTFTs as shown in present a threshold voltage (V ) shift which moves toward a more positive bias with the higher deposition temperature. The early turn on at higher temperature may result from the higher trap density [25] or more dipoles in the insulator/pentacene interface.
In addition, the slop of the (
perature seriously: a slow slope nearby the turn on point while a steep one away from it. It is well known that the mobility in some OTFTs is gate bias dependent.
Strictly speaking, it is more relative to the (V -V ). If we extract each mobility from the secant to obtain the average value, the device prepared at 90℃ will not perform outstandingly as before. Hence, the mobility estimated from the FET model might be overestimated for the device fabricated at high temperature.
P
4.2-4 The Phase Transition
ctra of pentacene structures on various conditions.
Figure 4. 4 illustrates the XRD spe
There are two sets of diffraction peaks in the spectra: (00l’) is for 2θ of a multiple of 5.7 degrees, and (00l) is for a multiple of 6.2 degrees.
By Bragg’s law:2dsinθ =nλ , the set, (00l’), is determined with vertical periodicity of 15.4Å, while the other set, (00l), reveals the d-spacing of 14.4Å. The set of (00l’) has been identified as “thin-film phase”, and (00l) is so-called “bulk phase.”
[6] [25]
In terms of the full widths at half maximum, the films deposited through substrate heating show slightly higher crystalline quality than the ones deposited at RT.
We ascribe the formation of higher order to annealing during pentacene deposition.
However, as indicated by the rectangle of Figure 4. 4 not only thin-film phase but also bulk phase grows with deposition temperature. Growth of both phases might bring about incoherence between phase boundaries.
5 10 15 20 25
(004) (004') (003)
(003') (002)
(002') (001)
(001')
90oC 70oC 50oC 17oC
log(Intensity) (a.u.)
2
θ(deg.)
Figure 4. 4 X-ray diffractograms using CuKα (λ=1.54 Å). The samples were prepared by the pentacene deposited at a fixed flux rate of 0.5Å/sec on a 200-nm-thick SiO2
substrate at various deposition temperatures.
Note that the fatty peak at 2θ of 13.5 degrees belongs to SiO2, not to pentacene.
4.2-5 Conclusion
In summary, we have prepared pentacene-based TFTs, which were deposited on SiO2 gate dielectrics at various substrate temperatures. The AFM images show that average grain size enlarges as the substrate temperature increases. It is supposed that
more grain boundaries could hamper carrier transport.
However, the XRD spectra indicate that an incoherence between two phases occurs in spite of the high order growth in thin film phase. Even if we only utilize substrate heating to enlarge grains, other factors would disturb this effect. Too many variables would make the analysis less powerful.
Furthermore, the VT shift is a thorny problem in mobility estimation especially.
The outcome, the better performance due to the higher deposition temperature, seems to jump to a conclusion.
In order to make sense of the XRD spectra and to reduce the VT shift, it is necessary to inhibit the phase transition and trap density at higher temperature. There is a possible method to come to this objective: to modify the substrates [26]. It might relax the strain of phase transition and simultaneously alter the interfacial states.
In the following sections, we have tried two types of surface treatments:
1,1,1,3,3,3- hexamethyldisilazane [27] and poly(α- methylstyrene) [28]. We attempt to find out the relationship of electrical properties to grains without disturbance such as VT shift and phase transition.
Si