Contact resistance of interface between the channel layers and various S/D electrodes was investigated by transmission line method. Different electrodes such as Ti/Au, a-IZO, and multilayer of a-IGZO/Ag/a-IGZO were compared in terms of contact resistance, using the transmission line model. The a-IGZO TFTs with a-IGZO/Ag/a-IGZO of S/D electrodes showed good performance and low contact resistance due to the homo-junction with channel layer.
1. INTRODUCTION
Amorphous oxide thin film transistors (TFTs) | have advantages of high mobility and low temperature in device fabrication. Since they also have high transparency in the visible range, they were regarded as transparent TFTs (TTFTs)
[1
-
3]
. Amorphous InGaZnO (a-IGZO) is regarded as channel layer of TTFT due their good performance, transparency, and stability
[4]
. There are many studies of channel layer as well as source and drain (S/ D) electrodes of TTFTs, because the S/D electrodes have an effect on the electrical performance and transparency of TTFTs. Transparent conductive oxides (TCOs) are suitable for the S/D electrodes of TTFTs due to their high optical bandgap (> 3.5 eV), good electrical conductivity, and high optical transparency of 80% in visible-infrared. Particularly, amorphous indium zinc oxide (a-IZO) was used for S/D electrode of ZnO TTFTs, due to high transparency and lower resistivity. Although there have been many researches on S/D electrodes of ZnO based TFTs, regarding their transparency, resistivity, and contact problem with channel layer, the relationship between the stability of ZnO based TFTs under the various stress and S/D electrodes is still unknown
[5
-
8]
.
In this paper, the influence of S/D electrodes on contact resistance of a-IGZO TFTs has been investigated by transmission line method (TLM). The change in S/D electrode has an effect on interfacial traps and energy barrier between the channel layer and S/D electrodes. The different contact resistance in electrical performance of a-IGZO TFTs, depending on S/D electrode, has been studied using TLM. .
2. EXPERIMENTAL
The a-IGZO TFTs were fabricated by using a thermally grown SiO
2
thin film on highly p-type Si (001) substrate as both substrate and gate electrode. The a-IGZO layer (50 nm) was deposited using rf magnetron sputtering with the channel width (250 μm) and channel length (L
ch
) ranging from 100 μm to 650 μm, as shown in
Fig. 1
. All devices were annealed at 350℃ for 1 h in N2 ambient. Then, the different electrode was deposited and patterned using lift-off method. The Ti (10 nm) and Au (60 nm) as S/D electrodes was deposited by e-beam and thermal evaporation, respectively. The a-IZO electrodes and a-IGZO layer of a-IGZO/Ag/a-IGZO electrodes were grown by rf magnetron sputter method, and the Ag was deposited using the thermal evaporation.
Figure 2
shows optical microscope image of real TLM patterned devices. Transfer characteristics are measured at a drain voltage VDS of 5.1 V with a single-sweep gate voltage V
GS
mode from -20 to 40 V. The Vth is defined as the V
GS
value, where a drain current IDS of 5 nA flows.
Schematic diagram of a-IGZO TFT with TLM patterned electrodes.
Optical microscope image of a-IGZO TFT with TLM patterned electrodes.
3. RESULTS AND DISCUSSION
Figure 3
shows the transfer curves of a-IGZO TFTs depending on L
ch
with (a) Ti /Au electrodes, (b) a-IZO electrodes, and (c) aIGZO/ Ag/ a-IGZO electrodes. The inset shows output characteristics of a-IGZO TFTs measured at V
GS
= 15 V with different L
chs
. As L
ch
increases, the off current (I
off
) and subthreshold swing (SS) were increased and the on current (I
on
) was decreased, since the resistivity of channel and total trap density were increased. It is observed that the electrical properties of a-IGZO TFTs were different depending on the contact between the a-IGZO and S/D electrode. In the case of the device with Ti/Au electrode, the on current was higher but the value of SS was also higher than those of a-IZO electrode. However, the a-IGZO TFT with Oxide/Metal/Oxide (OMO) electrode showed good performance in both respects.
Transfer curves of a-IGZO TFTs depending on Lch with (a) Ti / Au electrodes, (b) a-IZO electrodes, and (c) a-IGZO/Ag/a-IGZO electrodes at VDS = 5 V. The inset shows output characteristics of a-IGZO TFTs measured at VGS = 15 V with different Lchs.
In order to investigate the specific contact resistance with various electrodes, the transfer line method (TLM) was used. Total resistance (R
tot
) was determined from the slope of the linear region of the output characteristics by using Equation (1)
[9
,
10]
.
where L
ch
is the separation of the electrodes, and W is the electrode width fixed at 250 μm, as shown in
Fig. 4
. R
tot
is a function of the contact resistances (R
SD
) of the two contacts and the sheet resistance (R
SH
) of the semiconducting layer outside the contact, and R
SD
and R
SH
were obtained from the y-axis intercept and slope the R
tot
- L
ch
plot of
Fig. 5
, respectively.
Explanation and equation of transmission line method to derive total resistance and channel resistance.
Plot of Rtot vs. Lch for TFTs with different S/D electrodes.
Figure 5
shows total resistance variation as a function of channel length depending on various electrodes. The R
SD
with Ti /Au was higher than that with the a-IZO, and IGZO/Ag/IGZO electrodes and the ratio of R
SD
/R
SH
were also higher. This is mainly due to low energy barrier between the channel and S/D electrodes, so the oxide electrodes were easy for injecting the carriers into channel layer. Therefore, the IGZO/Ag/IGZO electrodes showed low contact resistance for S/D electrodes of a-IGZO TFTs.
4. CONCLUSIONS
In conclusion, different S/D electrodes of the a-IGZO TFTs have been compared by TLM. The electrical properties and stability of a-IGZO TFT showed a different trend with S/D electrodes. The a-IGZO TFTs with a-IGZO/Ag/a-IGZO of S/D electrodes had a good performance and a low contact resistance due to the homo-junction with channel layer. It is clearly observed that the R
SD
with Ti /Au was higher than that with the a-IZO and IGZO/Ag/IGZO electrodes, and the ratio of R
SD
/R
SH
was also higher. This is mainly due to low energy barrier between the channel and S/D electrodes, so the oxide electrodes were easy for injecting the carriers into channel layer. Therefore, the IGZO/Ag/IGZO electrodes showed low contact resistance for S/D electrodes of a-IGZO TFTs.
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