Characterization of Local Electrical Properties of Poly-Si Thin Films
Exposed to Atomic Hydrogen by Conductive Atomic Force Microscopy
Emi Machida1, Yukiharu Uraoka1, Takashi Fuyuki1,
Manato Deki2, Mai Tani2 and Hiroshi Ikenoue2
1

Graduate School of Material Science, Nara Institute of Science and Technology
8916-5, Takayama, Ikoma City, Nara 630-0192, Japan
2
Advanced Course of Mechanical and Electrical Engineering, Kochi College of Technology
200-1, Monobe-otsu, Nankoku City, Kochi 783-8508, Japan
Keywords: poly-Si, local electrical properties, defect, atomic hydrogen, C-AFM
Abstract. In this study, we analyzed local electrical defects in polycrystalline silicon (poly-Si)
thin films. We observed local electrical properties of poly-Si thin films by conductive atomic
force microscopy (C-AFM). Moreover, we observed local electrical properties poly-Si thinfilms exposed to atomic hydrogen. Before the atomic hydrogen exposures, conductive regionsin grains disappeared with the repeated scanning of the cantilever, suggesting that the trappingsites in grains were charged by the injection of holes from the cantilever. On the other hand,
conductive regions in grain boundaries remained while the increased number of scans.
Therefore, it is thought that current flowed continuously without the hole trapping at theseelectrical defects in grain boundaries. After the atomic hydrogen exposures for 20 min,electrical defects were inactivated, and the conduction mechanism of device became similar tosemiconductor conduction mechanism behavior.
Introduction
Thin film transistors (TFTs) are widely used in switching devices for display [1~3], and
it is well known that their performance is significantly affected by field-effect mobility of thechannel material. In recent years, polycrystalline silicon (poly-Si) TFTs have attracted muchattention because their field-effect mobility (10~150 cm2/V·s) is greater than that of amorphoussilicon (a-Si) TFTs (<1 cm2/V·s). However, the poly-Si films have many electrical defects at theoxide interface and/or grain boundary. These electrical defects cause marked reduction in field
effect mobility or increase in threshold voltage [4~6]. Therefore, the investigation of the locationof electrical defects in poly-Si films is very important in order to improve the electricalproperties of poly-Si TFTs.
In this study, we observed local current images of poly-Si films by conductive atomic
force microscopy (C-AFM), and analyzed them. In the use of C-AFM, the density and theregions of formation of these electrical defects can be observed, because local current images
and surface topography can be simultaneously obtained by this method [7]. We observed local
current images and measured current-voltage (I-V) characteristics of the poly-Si films before
and after the atomic hydrogen exposures, and investigated their local electrical properties.
Experimental
The poly-Si thin films were formed by the crystallization of a-Si by XeCl excimer laser
annealing (λ = 308 nm). The a-Si film (50 nm) was deposited on a glass substrate by plasma
enhanced chemical vapor deposition (PECVD), and the structure of the sample is a-Si (50 nm)/
SiO2 (100 nm)/ SiN (50 nm)/ non-alkali glass substrate. Prior to laser annealing, the a-Si film

was annealed in a vacuum chamber at 400℃ for the dehydrogenation. The grain size of thepoly-Si thin films was estimated by scanning electron microscopy (SEM) image taken after
Secco etching. The microstructure of the poly-Si films was observed by cross-sectionaltransmission electron microscopy (TEM).
Before C-AFM measurements, native oxide on the poly-Si films was removed using a
HF solution (5%) at room temperature. C-AFM measurements were performed using Shimadzu
SPM-9600 with a platinum-coated cantilever at a constant sample voltage of -1.0 V or -2.0 V.
Surface topography and current images were simultaneously measured by C-AFM at room
temperature in air ambient. The sample holder, to which the sample voltage was applied, waselectrically connected to the poly-Si film surface by an electrode. Thus, the sample voltage wasapplied between the cantilever and the poly-Si film surface. The atomic hydrogen exposures ofpoly-Si films was performed for 5 min or 20 min (5 min x 4) by a hot-wire method [8]. Atomic
hydrogen was generated by the thermal separation of H2 gas due to contact with ahigh-temperature tungsten filament in a vacuum chamber. The poly-Si film surface was
exposed to the atomic hydrogen in a vacuum chamber to terminate the poly-Si surface. Thebase pressure of the chamber was 3.0 x 10-5 Pa, and the H2 gas pressure was 1.0 x 10-1 Pa. The
substrate temperature of poly-Si films was 300℃. Before and after the atomic hydrogen
exposures, native oxide on the poly-Si films was removed using a HF solution (5%) at room
temperature.
Results and discussion
Figure 1 shows SEM image of Secco etched poly-Si film formed by ELA. Grainboundaries can be clearly observed in SEM image. The average grain size was about 300 nm.
From AFM measurement of poly-Si film surface, the root mean square (rms) of surface
roughness was estimated 10.5 nm. Figure 2 shows cross-sectional TEM image of poly-Si film
formed by ELA. As shown in Fig.2, there is a prominent ridge at grain boundary.

Fig.1 SEM image of a poly-Si film.
Average grain size = 300 nm.

Fig. 2 Cross-sectional TEM image of apoly-Si film.

Figure 3 shows C-AFM images of (a) the surface topography and (b) the current imageof poly-Si thin film before the atomic hydrogen exposures taken at a sample voltage (Vsub) of-1.0 V. Surface topography and current images were simultaneously measured. As shown in
Fig. 3(b), conductive regions correspond to circled region by prominent ridge in Fig. 3(a). As
shown in Fig. 2, the poly-Si films crystallized by ELA have a ridge at grain boundaries. In
addition, as shown in Fig. 3(a), the size of circled region by prominent ridge is almost equal tograin size in Fig. 1. Therefore, we concluded that the conductive regions correspond to grainboundaries. Figure 4 shows C-AFM images of (a) the surface topography and (b) the current
image of the poly-Si thin film before the atomic hydrogen exposures taken at Vsub of -2.0 V. As
shown in Fig.4 (b), conductive regions covered whole surface of the poly-Si film.

(a)
(b)
Fig. 3 C-AFM images of (a) surface
topography and (b) corresponding current
image of a poly-Si film at a Vsub of -1.0 V.

(a)

(b)

(a)
(b)
Fig. 4 C-AFM images of (a) surface
topography and (b) corresponding current
image of a poly-Si film at a Vsub of -2.0 V.

(c)

(d)

(e)
(f)
Fig. 5 (a) ~ (d) Current images of an identical area of the poly-Si film taken at Vsub of -2.0 V.
(a) first, (b) second, (c) third, (d) fourth scans. (e), (f) Line profiles of surface topography andcorresponding current image of (e) first scan, (f) fourth scan.
Figure 5(a) ~ 5(d) show current images of the poly-Si films before the atomic
hydrogen exposures (Vsub = -2.0 V) and Fig. 5(e) ~ 5(f) show line profiles of surface
topography and corresponding current images. These current images are of the same area,
which underwent repeated scanning using a Pt-coated cantilever. Figure 5 shows (a) first, (b)second, (c) third and (d) fourth scanning image. The surface roughness of the poly-Si film had
little change owing to repeated scanning. As shown in Fig. 5(a) ~ (d), conductive regions in
grains significantly decrease in area with increasing number of scans. However, conductive
region in grain boundaries does not change while scanning. As shown in Fig. 5(e) and (f),current of about 30 pA flows continuously through grain boundaries. After changing themeasurement region to obtain a new first scanning image, we observed new conductive regions,similar to those shown in Fig. 5(a). From these results, we concluded that the decrease in thearea of the conductive regions does not occur due to the degradation of the cantilever. These
results indicate that many conductive regions correspond to low-resistance region. It is wellknown that there are many electrical defects in grain boundaries. Therefore, it can be thoughtthat these conductive regions correspond to electrical defects in grain boundaries. Conductive
regions in grains disappear with repeated scanning of the cantilever, and thus, it is thought thattrapping sites in grains were charged by the hole injection from cantilever. On the other hand, it
is well known that there are many electrical defects such as dangling-bond in grain boundary.
So, it is thought that current flowed continuously without the hole trapping at these electricaldefects in grain boundaries.

(a)
(b)
(c)
Fig. 6 Current images of the poly-Si films (Vsub = -2.0 V). (a) Before hydrogen termination. (b)After 5 min hydrogen termination. (c) After 20 min hydrogen termination.

(a)
(b)
(c)
(d)
Fig. 7 Current images of the poly-Si films (Vsub = -2.0 V). (a) and (b) Before the atomic
hydrogen exposures. (c) and (d) After the atomic hydrogen exposures for 20 min.(a) and (c)First scanning image. (b) and (d) Fourth scanning image.
Figure 6 shows current images of the poly-Si films before and after the atomic
hydrogen exposures. Figure 6(a) shows current image of non-treatment poly-Si film, (b) shows
current image of the atomic hydrogen exposures for 5 min poly-Si film and (c) shows current
image of the atomic hydrogen exposures for 20 min poly-Si film. As shown in Fig. 6,
conductive regions in the poly-Si film disappear with the atomic hydrogen exposures. Therefore
it can be concluded that electrical defects were inactivated by the atomic hydrogen exposures.
Figure 7 shows local current images of poly-Si films before and after the atomic hydrogen
exposures. To clarify change of conductive regions in grains, current range of local current
images was set to 6.0 pA. Figure 7(a) and (b) show current images of non-treatment poly-Si
film, and (c) and (d) show current images of the atomic hydrogen exposures for 20 min poly-Si
film. Figure 7(a) and (c) are first scanning image, and (b) and (d) are fourth scanning image. As
shown in Fig. 7(a) and (c), electrical defects in grains were also inactivated by the atomic
hydrogen exposures as in the case of grain boundaries. However, there is considerable
difference in change of conductive regions due to increase of number of scans. In Fig. 7(a) and(b), conductive regions in grains significantly decrease in area with increasing number of scans.
On the other hand, in Fig. 7(c) and (d) conductive regions in grains remained in spite ofincreasing in scanning number. From these results, it can be concluded that electrical defects ingrain are inactivated by the atomic hydrogen exposures, and the charge up in trapping sites does
not occur by the hole injection from the cantilever. In addition, as shown in Fig. 6(b), there islocally non-terminated region (circled region with dot-line in Fig. 6(b)) near grain boundary inthe case of the atomic hydrogen exposures for 5 min. Thus, it is thought that there arepartially-non-terminated regions such as point defect in poly-Si films.

(a)(b)
Fig. 8 I-V curves in grain region of poly-Si film. (a) Before hydrogen termination. (b) After 20
min hydrogen termination.

(a)(b)
Fig. 9 I-V curves at grain boundary of poly-Si film. (a) Before hydrogen termination. (b) After
20 min hydrogen termination.
Figure 8 shows current-voltage (I-V) curves in grain region of poly-Si films (a) beforeand (b) after the atomic hydrogen exposures. As shown in Fig. 8(a), the current edge was very
unstable and current does not flow until high gate voltage. On the other hand, after the atomic
hydrogen exposures, I-V curve indicates asymmetric rising and the edge was very stable, as
shown in Fig. 8(b). Moreover, width of the edge (1.0 eV) was almost equal to the band gap ofSi (1.1 eV) and thus, the conduction mechanism of device shown in Fig.8 is similar tosemiconductor conduction mechanism behavior. Figure 9 shows I-V curves at grain boundariesof the poly-Si films (a) before and (b) after the atomic hydrogen exposures. As shown in Fig.
9(a), the edge was nearly symmetrical and width of the edge (0.7 eV) was smaller than band
gap of Si. In addition, conductive mechanism of device of Fig. 9(a) is similar to metallic
conductive mechanism behavior. However, the I-V characteristic does not show complete
ohmic characteristics, so it is suggested that major conduction mechanism are hopping
conduction mediated by electrical defects in grain boundary[9, 10]. In Fig. 9(b), the current edgeof I-V curve is asymmetrical and width of the edge (0.9 eV) is close to band gap of Si.
Therefore, we concluded that electrical defects in grains and grain boundaries of the poly-Sifilm are inactivated by the atomic hydrogen exposures, and the conduction mechanism becomessimilar to semiconductor conduction mechanism behavior.
Summary
We analyzed local electrical properties in the poly-Si thin films formed by ELA. The
poly-Si thin films (50 nm) were crystallized of a-Si by XeCl excimer laser annealing. We
simultaneously observed surface topography and current images of the poly-Si thin films before
and after the atomic hydrogen exposures by conductive atomic force microscopy (C-AFM).
Before the atomic hydrogen exposures, conductive regions in grains disappeared with repeatedscanning of the cantilever. Thus, it is thought that trapping sites of grains were charged by thehole injection from cantilever. On the other hand, conductive regions in grain boundaries
unchanged while the increasing number of scanning. Therefore, it is thought that current flowed
continuously without the charge up of trapping sites in grain boundaries. In the case of after theatomic hydrogen exposures for 20 min, electrical defects were almost inactivated, and the
conduction mechanism of device became similar to semiconductor conduction mechanism
behavior.
Acknowledgments
The authors would like to acknowledge Shimadzu Corporation for technical support of
C-AFM (SPM-9600).

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