The Poly-Si Thin Film Property

After discussing about the basic factors, in this section, we will focus on the poly-Si thin film properties in detail. Because this is a n-type TFT, in the density of states (DOS) of the thin film, the main influence on induced carriers’ numbers is acceptor-like state region which is composed by two states which are acceptor-like deep state and acceptor-like tail state. In the Slivaco software, the deep state is assumption as a Gaussian distribution and the tail state is assumption as an exponential distribution. There are two parameters can adjust these two states. One is NGA which specifies the total density of acceptor-like states in a Gaussian distribution, and unit is cm^-3. The other is NTA which specifies the density of acceptor-like states in the tail distribution at the conduction band edge, and unit is cm^-3/eV. Therefore, we can through varying the NGA and NTA to respectively adjust the deep state and tail

state in the acceptor-like state of the DOS.

First of all, we fix the NGA which is 6×10¹⁷ cm^-3, and modify the NTA as 1.12×10¹⁷, 1.12×10¹⁸, and 1.12×10¹⁹ cm^-3/eV. The major simulation parameters are shown in Table V. The DOS is shown in Fig.3-5. Again, we change the LDD doping concentration with above three kinds of the DOS to analysis the influence on ΔL, which is shown in Fig.3-6. However, except the effect of doping concentration, the various NTA values seem to only make little variations.

Then, the NTA and NGA exchange for each other. In this time, we fix the NTA

which is 1.12×10²⁰ cm^-3/eV, and modify the NGA as 1×10¹⁷, 3×10¹⁷, 6×10¹⁷ and 1.2×10¹⁸ cm^-3. The major simulation parameters are shown in Table V. The simulation result is shown in Fig.3-7. The DOS is shown in Fig.3-8. Although the NGA are different, the effects of doping concentration are all the same： As increasing the doping concentration, ΔL will be decreased. While exceeding the turning points , ΔL will drop swiftly. Nevertheless, four kinds of NGA values have four respective critical points. In other words, the critical point and NGA have specific correlation.

According to the mechanism of ΔL explained in the preceding sections, we believe the extended current path from gate channel in LDD region is related to the resistivity of poly-Si thin film.

Fig.3-5 The density of states (DOS). NGA is fixed in 6x10¹⁷ cm^-3. (a) NTA=1.12x10¹⁹ cm^-3 , (b) NTA=1.12x10²⁰ cm^-3 , (c) NTA=1.12x10²¹ cm^-3

N_GA=6×10¹⁷cm^-3 N_TA=1.12×10²¹cm^-3/eV N_GA=6×10¹⁷cm^-3 N_TA=1.12×10²⁰cm^-3/eV N_GA=6×10¹⁷cm^-3 N_TA=1.12×10¹⁹cm^-3/eV

( a )

( b )

( c )

Fig.3-6 The influence of thin film property NTA on the channel extension effect obtained from Silvaco ATLAS simulation with parameters listed in Table V.

Fig.3-7 The influence of thin film property NGA on the channel extension effect obtained from Silvaco ATLAS simulation with parameters listed in Table V.

V_GS= 15V

Fig.3-8 The density of states (DOS). NTA is fixed in 1.12x10²⁰ cm^-3. (a) NGA=1x10¹⁷ cm^-3, (b) NGA=3x10¹⁷ cm^-3, (c) NGA=6x10¹⁷ cm^-3 (d) NGA=1.2x10¹⁸ cm^-3.

The correlation between resistivity and doping concentration is referred in Seto’s model [17]. Consider the depletion of the grain of poly-Si thin film, the correlation curve is shown in Fig.3-9 which also has a specific turning point at

N_GA=3×10¹⁷cm^-3 N_TA=1.12×10²⁰cm^-3/eV

N_GA=6×10¹⁷cm^-3 N_TA=1.12×10²⁰cm^-3/eV

N_GA=1.2×10¹⁸cm^-3 N_TA=1.12×10²⁰cm^-3/eV

( a ) ( b )

( c ) ( d )

N_GA=1×10¹⁷cm^-3 N_TA=1.12×10²⁰cm^-3/eV

N = Q_t / L (1)

where N is the doping concentration and unit is cm^-3.

Qt is the deep level trap density of state and unit is cm^-2.

L is the average grain length of poly-Si thin film and unit is cm.

Fig.3-9 The correlation between resistivity and doping concentration. [17]

Compared to our study,

GA GA

t N

L L N L

N =Q = × = (2)

Therefore, when the LDD doping concentration (N) is equal to NGA, ΔL should be going to drop rapidly. This assumption is matched perfectly with our simulation results in Fig.3-7.

Now, because of the critical point N = NGA, the Fig.3-7 can be divided into two regions to discuss. The two regions are the left-hand side N < NGA, and the right-hand side N > NGA. We transfer the diagram from Fig.3-7 into Fig.3-10. Vary the NGA from 1×10¹⁷ cm^-3 to 1.2×10¹⁸ cm^-3 with respect to different doping concentrations (N) which are 1×10¹⁷, 1.2×10¹⁸, and 4×10¹⁸ cm^-3. In this figure, the bottom curve is N >

NGA, the top curve is N < NGA, and the central curve is cross the N = NGA point.

Fig.3-10 The influence of LDD doping concentration and NGA on the channel extension effect obtained from Silvaco ATLAS simulation with parameters listed in Table V.

resistivity and ΔL is increased conspicuously. In brief, the better thin film quality has fewer defects and higher conductivity, therefore has shorter extended channel length.

While N < NGA, the minimum NGA has the best thin film quality and the smallest resistivity, however, it also has the maximum ΔL in our simulation. This result is contrary to our earlier inferences. Clearly, there have other mechanisms which we do not realize to involve in.

Finally, we will additionally study the effect of thin film property on different temperatures. The major simulation parameters are shown in Table VI. In the section 3.1.3, we have proposed that increasing the surrounding temperature reduces the ΔL.

In the different thin film properties, this phenomenon is also existence. The results are shown in Fig.3-11 and Fig.3-12. The slope means the various degree of ΔL with respect to different temperatures.

While we adjust the NGA, the slopes are corresponding changed but the changes do not exist specific trends. In spite of the influence of the NGA on slope does not exist consistency, the NTA has different effect on slope. As the NTA is arisen, the decay of ΔL is also raised. The slope is more negative. These results we deduce that the increasing NTA causes the free carriers which is induced by gate voltage become fewer, so the influence of temperature will be more conspicuously. But why the various NGA does not have the same trend, the reasons we are still researching, now.

Fig.3-11 The influence of temperature and NGA on the channel extension effect obtained from Silvaco ATLAS simulation with parameters listed in Table VI. (a) VGS=3V (b) VGS=9V (c) VGS=15V

V_GS= 3V N_TA= 1.12x10²⁰

Temperature ( K )

220 240 260 280 300 320 340 360 380

Δ L ( μm )

220 240 260 280 300 320 340 360 380

Δ L ( μm )

220 240 260 280 300 320 340 360 380

Δ L ( μm )

Fig.3-12 The influence of temperature and NTA on the channel extension effect obtained from Silvaco ATLAS simulation with parameters listed in Table VI. (a) VGS=3V (b) VGS=9V (c) VGS=15V

V_GS= 15V N_GA= 6x10¹⁷

Temperature ( K )

220 240 260 280 300 320 340 360 380

Δ L ( μm )

220 240 260 280 300 320 340 360 380

Δ L ( μm )

220 240 260 280 300 320 340 360 380

Δ L ( μm )