Dielectric Effects in Laser-Crystallised Polycrystalline Silicon Thin Film Transistors
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INTRODUCTION Both amorphous silicon (a-Si) and polycrystalline silicon (poly-Si) thin film transistors (TFTs)
have become essential components for use in large area electronic applications, such as twodimensional imaging and flat panel active-matrix displays. In these applications it is important to study and understand the transient response of the TFT current-voltage characteristics. These were first reported by Van Berkel et al and Powell 1.2 who considered the dynamic behaviour of the drain current of an a-Si TFT upon application of a gate voltage. Measurements carried out on a-Si TFTs showed three basic regimes: between Ips and Is transient effects are observed attributed to carrier trapping in the a-Si; a region between 1 and 100s where current is independent of time; and beyond I00s where current decreases due to defect creation in the a-Si 1. Inthe poly-Si TFTs studied here the transient could not be split into different regimes intime. However measurements carried out at different gate voltages showed a definite change in transient behaviour dependent on the magnitude of the gate bias. EXPERIMENTAL DETAILS The TFTs used in this work were inverted gate staggered devices. Since the devices were designed to be integrated onto the same substrate as a-Si TFTs (see Fig 1) the gate dielectric had to be specially selected to ensure good TFT characteristics (see Fig 2). The gate dielectric, consisting of a nitride film and an oxide film was formed on a gate electrode by plasma enhanced chemical vapour 707 Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materials Research Society
deposition. Subsequently a 70nm a-Si film was deposited at 250 0C. The laser de-hydrogenation and
crystallisation were then performed using a three step irradiation of increasing laser energy density4. Isd (A)
1E-3
.
1E-64
S D
S D
-
-1
E-7~ I E-8
rPa-SH
SN •
"•
Poly-SiTFT
a .ate ....
1E-9
_1E-11.
-v-l10-5 15
a-i.H F7 F1 Pdysi a-Si:HTFT
ov)15 220 10 0 go•5 Vg Mv
W-60prm, L-15Jm Vsd=10V
Fig 2 : Typical transfer characteristic
Fig 1: Device structure of typical TFT
Current decay measurements were taken using two Keithley source measure units (SMU) linked to a personal computer (PC). The gate pulse was provided by one SMU while the other supplied the drain voltage and measured the subsequent source drain current. Aprogramme was written to digitise the current values of the SMU and take measurements , every second out to 8000 seconds, of the sourcedrain current so that a plot of the decay of the source-drain current was produced. For ease of interpretation all transient data was normalised to one of the decays and then offset slightly for clarity. Unless otherwise stated all our devices had a nominal channel length of 15pm. Typical values of the field effect mobility are inthe range 30-60cm2 V1 s-1 ,with threshold voltages inthe range 1-2 Volts. RESULTS The transient behaviour of the source-drain current of the laser-crystallised poly-Si TFTs is shown in figure 3. Of particular interest is the transient behaviour at a gate v
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