applied surface sc ience

ELSEVIER Applied Surface Science 79/80 (1994) 250-254

Precursor lifetime estimation by ultraviolet laser modulation of a hydrogenated amorphous silicon growth surface

Atsushi Suzuki *, Gautam Ganguly, Akihisa Matsuda Nonequilibrium Materials Section, Electrotechnical Laboratory. 1-1-4 Umezono. Tsukuba. lbaraki 305, Japan

(Received 13 October 1993; accepted for publication 17 November 19931

Abstract

The reaction lifetime of the film precursors on a no-plasma surface of hydrogenated amorphous silicon (a-Si: H) has been evaluated by periodic deposition and ultraviolet (UV) laser irradiation. The reaction lifetime of the precursors on the no-plasma surface was estimated to be less than 1 s from the change of photoconductivity in the resulting films as a function of the delay time between deposition and UV laser irradiation. A comparison of the lifetimes of the precursors on the plasma-exposed and no-plasma surface of a-Si : H leads to the conclusion that the reactivities of the precursors are comparable on both surfaces. However, UV laser irradiation encrgizes the precursors which results in an enhancement of the dangling-bond-termination reaction rate.

1. Introduct ion

The reac t ion l i fe t ime of film p recur so r s is an i m p o r t a n t fac tor for the surface reac t ions in the growth p rocess of h y d r o g e n a t e d a m o r p h o u s sili- con (a-Si : H) which is used as the active ma te r i a l in solar cells, thin film t ransis tors , etc. The film qual i ty of a - S i : H is d o m i n a t e d by the densi ty of dangl ing bonds which are a defec t that works as a r e combina t i on cen t e r for p h o t o - c r e a t e d e l e c t r o n - hole pairs, A c c o r d i n g to the surface diffusion mode l [1], the dens i ty of dang l ing -bonds in the resul t ing a - S i : H film is d e t e r m i n e d by a ba lance be tween t h e ra tes of dangl ing bond c rea t ion and

* Corresponding author. Fax: (+ 811 298 58 5425.

t e rmina t ion reac t ions by the film p recur so r s (ex, S i l l 3 radical ) on the film growth surface in p l a s m a - e n h a n c e d c h e m i c a l - v a p o r d e p o s i t i o n (PECVD) . The p recur so r s which cannot t e rmi- na te dangl ing bonds may be c o n s u m e d by the abs t rac t ion reac t ion with hydrogen on the growth surface. Namely , the l i fe t ime of p recursors is d e t e r m i n e d by the ra te of abs t rac t ion reac t ion and d a n g l i n g - b o n d - t e r m i n a t i o n react ion. This means that the l i fe t ime of the p recursors is influ- enced by the ra t io of these two reac t ions on the growth surface. The re fo re , m e a s u r e m e n t of the l i fe t ime of the film p recur so r s u n d e r d i f ferent depos i t i on condi t ions is necessary to quant i fy the ra tes of surface react ions , which are requ i red to d e t e r m i n e growth condi t ions to ob ta in the high- qual i ty a -Si : H films.

In-si tu u l t raviole t (UV) laser i r rad ia t ion of the film growth surface dur ing P E C V D (in-situ UV-

0169-4332/94/$07.00 ~') 1994 Elsevier Science B.V. All rights reserved SSDI 0l 69-4332(94)00059-A

A. Suzuki et al. /Applied Surface Science 79 / 80 (1994) 250-254 251

PECVD technique) results in an effect equivalent to an increase of the substrate temperature [2], which enhances the dangling-bond-termination reaction by energizing film precursors. As a con- sequence, the in-situ UV-PECVD technique im- proves the photoconductivity from 10 -9 to 10 -5 S / c m in a-Si:H films prepared at low substrate temperatures [3]. Utilizing the effect of improve- ment of the photoconductivity by in-situ UV- PECVD, we have estimated the reaction lifetime of film precursors on the surface during the plasma deposition period (plasma-exposed sur- face) [4]. We separated the in-situ UV-PECVD method into the plasma deposition period and the UV laser irradiation period by time. The UV laser irradiation was carried out during the inter- val between successive deposition periods by in- termittent PECVD (post-deposition irradiation). The post-deposition irradiation procedure re- suited that the photoconductivity was mostly im- proved at the shortest deposition period and de- creased with increase the one cycle deposition period. These results can be explained by the existence of the film precursors which can con- tribute the enhancement of dangling-bond-termi- nation reaction through energizing by UV laser irradiation on the plasma-exposed surface even after the plasma is turned off. Namely, each film precursor has a lifetime on the plasma-exposed surface without UV laser irradiation (rg). ~-g can be estimated to be 10- ~ s from the change of the photoconductivity against the one cycle deposi- tion period in the post-deposition irradiation pro- cedure.

Since the growth surface is exposed to a plasma, energetic species like ions, excited molecules and electrons from the plasma may affect the surface reactions. Therefore, the effect of the existence of a plasma for the surface reac- tions needs to be experimentally examined.

In this work, we have evaluated the lifetime of film precursors on the surface without plasma (no-plasma surface) of a-Si:H by the modifica- tion of the post-deposition irradiation procedure outlined above. The reactivity of the growth sur- face and the effect of UV laser irradiation are discussed by comparison of the lifetime on the plasma-exposed and no-plasma surface.

2. Experimental

We have tried two kinds of the modified post- deposition irradiation procedures. Sequences of one cycle in the post-deposition irradiation and the modified procedures are shown in Fig. 1. As reported previously [4], the photoconductivity in the resulting a-Si:H can be improved up to 2 × 10 6 S / c m for a equal deposition and irradiation period of 5 s using the post-deposition irradiation procedure shown in Fig. la. In this study, the a-Si:H film is deposited by a helium (100 sccm)- diluted silane (5 sccm) intermittent plasma for 5 s corresponding to the monolayer growth time. Other experimental conditions for PECVD and a UV excimer laser were identical to those de-

(a) Post-deposition irradiation

KrF laser irradiation

Plasma deposition

5S 5S

D

Time

(b) Delayed irradiation

, 5 s # 5sD_ KrF laser irradiation I ~ ~ ,

Plasma deposition Time

(c) Irradiation-time variation

KrF laser irradiation

5s

Plasma deposition Time

Fig. 1. Schematic diagrams of one cycle during post-deposi- tion irradiation procedure (a), the delayed irradiation proce- dure (b) and the irradiation-time variation procedure (c). The delay time (t D) and UV laser irradiation time (t L) were varied while keeping other parameters fixed in (b) and (c), respectively.

252 A. Suzuki et al. /Applied Surf:ace Science 79 /80 (1994) 250 254

scribed elsewhere [2-4]. A glass substrate was used for measurement of the photoconductivity. The substrate tempera ture was set at room tem- perature (RT).

In the first modified procedure (Fig. lb), a delay time (t D) is inserted between the deposi- tion period and the irradiation period in the post-deposition irradiation procedure (delayed ir- radiation). The deposited a -S i : H layer at the deposition period is irradiated by a UV laser for 5 s after a delay time, t D. From the change of the photoconductivity when changing t D, the reaction lifetime of the film precursors on the no-plasma surface of a -S i :H without UV laser irradiation (%) will be evaluated.

In the second modified procedure (Fig. lc), we have varied the UV laser irradiation time (t c) in the post-deposition irradiation procedure (irradi- ation-time variation), t c was varied from 0 to 30 s with t D = 0.

Each sequence is controlled by the apparatus whose diagram is shown in Fig. 2. The intermit- tent plasma is generated by a sequential signal generator (Dainichi Shinkuu Co.). The delay time t D is provided by a pu l se / func t ion generator (lwatsu Co., SG-4511) which is triggered by a sequential signal generator after each 5 s deposi- tion. The pu l se / func t ion generator also switches the KrF excimer laser (248 nm) for periodic laser

Radio Trigger frequency =

( 13.56 MHz) generator

[ Radio frequency power amplifier

Hcliuln-dilutcd Jsilanc plasma

48 nm ] 10 mJ/cm 2 ' J 100 ttz

PECVD chamber

Sequential signal generator

Trigger

Pulse / function generator

~ Trigger

KrF excimer laser

Fig. 2. Block diagram of the apparatus used in this study.

E ¢,.)

>,

>

"0

0

0

e- rr

1 0 .6(

10 .7

b

, , , ~ . . . . i . . . . i . . . . i . . . . i . . . .

1 2 3 4 5

Delay time (s)

Fig. 3. AMI (ll)O m W / c m 2) photoconductivity for films ob- tained using the delayed irradiation procedure as a function of delay time (tD).

irradiation of t L S. The KrF laser is operated at a repetition rate of 100 Hz and a fluence of 10 m J / c m 2.

Using these procedures, each cycle is rel~eated until the total film thickness reaches 5000 A. The photoconductivity in the resulting a -S i :H films was measured by the AM1 light source with an illumination intensity of 100 m W / c m 2.

3. Results and discussion

The room tempera tu re photoconductivity (AM1, 100 m W / c m 2) in the as-deposited a-Si: H films prepared by the delayed irradiation proce- dure is plotted as a function of t D in Fig. 3. The photoconductivity decreases rapidly up to 1 s and saturates to the value of 2 × 10 -v S / c m at t D > 1 s. The photoconductivity of 2 × l(1-7 S / c m is comparable to the value in a -S i :H prepared by continuous deposition at RT and post-deposition UV laser irradiation for the same time as the deposition time [4]. It should be noted that the photoconductivity of 2 × 10 -7 S / c m is compara- ble to the value for films prepared at a substrate temperature of 100°C by a conventional PECVD without UV laser irradiation, but further experi- ments will be required to identify the causes of improvement up to 2 × 10 7 S / c m . Therefore, we will restrict the discussion to the change of photoconductivity against t D in the region where the photoconductivity is higher than 2 X 10 7 S / cm.

A. Suzuki et al. /Applied Surface Science 79/80 (1994) 250-254 253

The improvement of the photoconductivity above 2 × 10 -7 S / c m is due to the suppression of the dangling bond density in the resulting a -S i :H caused by the energized precursors [3]. During the plasma deposition, the number of the precursors on the plasma-exposed surface is de- termined by the balance between the supply from the p lasma and the consumpt ion by the dangling-bond-termination and the hydrogen-ab- straction reactions. Since the precursors can sur- vive on the plasma-exposed surface without UV laser irradiation for Zg, some part of the precur- sors are still active at the end of the deposition period. At t t ) = 0 s, all surviving precursors can contribute to the enhancement of the dangling- bond-termination reaction when energized by the UV laser irradiation. However, when t D > 0 s, the number of precursors decreases with t o be- cause no further supply arrives from the plasma during t D. The precursors are consumed by the reactions with hydrogen or dangling bond within their lifetime on the no-plasma surface without UV laser irradiation (%). This leads to a de- crease of the number of the precursors which can contribute to the enhancement of the dangling- bond-termination reaction rate at the start of UV laser irradiation, after t o. As a consequence, the improvement of photoconductivity in the result- ing a-Si : H films prepared by the delayed irradia- tion becomes smaller for t o > 0 s.

From the t o dependence of the photoconduc- tivity in the region below 1 s, ~0 can be evaluated to be less than 1 s on assuming a first-order reaction process of the precursors. We defined % as the lifetime of film precursors on the no-plasma surface without UV laser irradiation. In other words, % means a quenching time of the film precursors on the growth surface after a plasma is turned off. The result shows that the lifetimes of film precursors (~-g) are not so long even on the no-plasma surface.

Fig. 4 shows the photoconductivity in the a- S i : H films prepared by the irradiation-time vari- ation procedure plotted against t L. At t L = 0 (no UV laser irradiation case), the photoconductivity in the resulting a -S i :H is the same as that for a continuous PECVD without UV laser irradiation at RT.

E 10 "5 O

10"6

1 0 "> ;3

"0 ,r = 0 10 .8 0 0

0.,t ..c 1 O.

' ' ' 1 . . . . I . . . . I . . . . ~ . . . . i . . . . ~ ' ' '

No UV laser irradiation , I . . . . I , , , , I , , , , I . . . . I . . . . I . . ,

5 10 1 5 20 2 5 3 0

Laser irradiation time (s) 35

Fig. 4. AM1 (100 mW/cm 2) photoconductivity for films ob- tained from the irradiation-time variation procedure plotted against UV laser irradiation time (tL).

The photoconductivity starts to improve with UV laser irradiation. As explained before, con- sidering that film precursors have a lifetime on the plasma-exposed surface (~-g), the surviving precursors can enhance the dangling-bond- termination reaction rate if they are energized by UV laser irradiation and therefore, improve the photoconductivity. The result seems to show that the number of precursors which can terminate dangling bonds increases with t L. However, this might be explained by the increase of the temper- ature at the start of the each cycle of deposition. When the substrate temperature increases, the number of contributing precursors for the termi- nation reaction increases significantly. In the pre- sent irradiation-time variation procedure, there is no interval between the UV laser irradiation and the next 5 s deposition. Therefore, the heat gen- erated by the UV laser irradiation may still be stored on the surface at the start of the next deposition due to the low thermal conductivity (0.01 W / c m . deg) of the glass substrate. Though the heat is reduced during the next deposition period, this raised temperature at the initial stage of the deposition period may enhance the dan- gling-bond-termination reaction rate. If an inter- val time is inserted between the UV laser irradia- tion period and the next deposition period, the surface temperature would decrease back to RT at the start of the deposition period. Such other modified procedures will clarify the effect of tem- perature rise for the increase of the photocon- ductivity up to t e = 5 s.

254 A. Suzuki et al. /Applied Surface Science 79 /80 (1994) 250-254

The photoconductivity in the a-Si: H films pre- pared by the irradiation-time variation procedure saturates at a value of about 2 x 10 ~ S / c m for t L > 5 s. This value is less than that obtained with continuous in-situ UV-PECVD because the num- ber of the precursors at the start of UV laser irradiation on the growth surface is small com- pared to those that arrive on the surface during the entire period in the case of in-situ UV- PECVD. Therefore, the saturation of the photo- conductivity up to 2 × 10 6 S / c m is explained by the limitation of the number of the film precur- sors at the start of the UV laser irradiation.

no-plasma surface of a -Si :H is estimated to bc below 1 s without UV laser irradiation at RT. Therefore, it is found that the lifetime of the film precursors is not so long even when no energy is supplied from the plasma. UV laser irradiation energizes the film precursor on the no-plasma surface, which contributes the improvement of the photoconductivity.

Acknowledgement

The authors would like to thank Dr. N. Hata for helpful discussions.

4. Conclusions

In conclusion, variation of the photoconductiv- ity in a-Si : H films as functions of the delay time and post-deposition irradiation-time has been in- terpreted in terms of the termination reaction enhanced by the precursors on the film growth surface. The lifetime of film precursors on the

References

[1] G. Ganguly and A. Matsuda, Phys. Rev. B 47 (1993) 3661. [2] A. Suzuki, N. Hata and A. Matsuda, J. Non-Crys. Solids

164 166 (1993) 51. [3] A. Suzuki, Y. Toyoshima, P.J. McElheny and A. Matsuda,

Jpn. J. Appl. Phys. 30 (1991) L790. [4] A. Suzuki, G. Ganguly and A. Matsuda, Appl. Phys. gett.

63 (1993) 2806.