- Email: [email protected]
Electrical and structural properties of low temperature boron- and phosphorus-doped polycrystalline silicon thin films prepared by ECR-CVD
Applied Surface Science 142 Ž1999. 400–406
Electrical and structural properties of low temperature boron- and phosphorus-doped polycrystalline silicon thin films prepared by ECR-CVD H.L. Hsiao a , Y.Y. Shieh a , R.S. Lee a , R.Y. Wang a , K.C. Wang a , H.L. Hwang A.B. Yang b a
a,)
,
Department of Electrical Engineering, National Tsing Hua UniÕersity, Hsinchu 30043, Taiwan b Department of Physics, Tunghai UniÕersity, Taichung 40704, Taiwan
Abstract Boron-doped and phosphorus-doped polycrystalline silicon thin films were deposited on glass and SiO 2 substrates at a rather low temperature of 2508C by electron cyclotron resonance chemical vapor deposition ŽECR-CVD. using SiH 4rArrH 2rB 2 H 6 and SiH 4rArrH 2rPH 3 downstream plasma technique. The effects of in situ doping concentration and hydrogen dilution on the structural and electrical properties of heavily doped polycrystalline silicon thin films have been systematically investigated. The largest grain size of the heavily doped poly-Si films with ; 700 nm thickness Žgrowth rate ; 20 nmrmin. is approximately 500 nm, and the surface roughness is about 30 nm. With increasing the doping gas flow rate, the resistivity rapidly decreased and the minimum values of 0.025 V cm at wB 2 H 6 xrwSiH 4 x ; 9 = 10y2 for p-type poly-Si films and 0.036 V cm at wPH 3 xrwSiH 4 x ; 7 = 10y2 for n-type poly-Si films were achieved. The Hall mobility decrease as the carrier concentration increase can be explained by the impurity scattering increase with increasing the doping gas flow rate. X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated a change from - 110 ) to - 111 ) with slight decrease in the grain size. Furthermore, the grain shape of the deposited films was changed from elliptical to round. Similar to the undoped poly-Si thin films, larger hydrogen dilution ratio would increase the grain size, change the grain shape from round to elliptical. Otherwise, the Hall mobility simply decreased with increasing the hydrogen dilution ratio until 88% but it rapidly increased with further hydrogen dilution. On the other hand, the carrier concentration exhibited an entirely converse behavior with the hydrogen dilution ratio. The relationships between electrical properties and structure properties with regarding to the hydrogen dilution ratio and doping gas flow rate were attributed to the high surface coverage of atomic hydrogen species, doping precursors disturbance, solid solubility limitation and impurity scattering effect. q 1999 Elsevier Science B.V. All rights reserved. PACS: 73.61.C Keywords: Polycrystalline silicon; Boron-doped; Phosphorus-doped; Hall mobility; Resistivity
)
Corresponding author. Tel.: q886-3-5715131 ext. 4056; Fax: q886-3-5723927; E-mail: [email protected]
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 6 8 3 - 7
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
1. Introduction Polycrystalline silicon Žpoly-Si. is a very important material for the micro-electronics industry w1,2x. Thin films growth on glass substrates are of considerable interest owing to their wide applications to electronic devices such as solar cells, thin film transistors ŽTFTs. for display or photosensor devices. In this case, the poly-Si films must be prepared at a process temperature lower than the strain point of inexpensive glass substrates. As compared with amorphous silicon Ža-Si. TFT’s, poly-Si TFT’s provide higher field effect mobility and thus higher driving capability. Unfortunately, the conventional high quality thin film requires high temperature growth process. For device applications, it is essential to deposit poly-Si films at lower temperature but without compromising the material quality. In our previous studies w3–5x, we reported on the preparation and characterization of poly-Si films by electron cyclotron resonance downstream plasma chemical vapor deposition ŽECR-CVD. with the hydrogen dilution technique. Strongly - 110 ) and - 111 ) -oriented poly-Si films with a maximum grain size of 1 mm were obtained at a very low substrate temperature of 2508C. In order to apply the poly-Si films to device fabrication, the studies on n q and p q -type thin films are necessary. The in situ doping process is an attractive one because of its economic feasibility and no subsequent annealing process requirement. However, there has been rare study of in situ boron-doped and phosphorus-doped poly-Si thin films at such a low process temperature. In this paper, we investigate the effect of in situ heavy phosphine and diborane doping on the electronic and structural properties of the ECR-CVD deposited poly-Si thin films. Moreover, the relationships between the electrical properties, structural properties and hydrogen dilution ratio in phosphorus-doped and boron-doped poly-Si thin films are examined. The growth mechanism is also discussed.
401
tron cyclotron resonance downstream plasma chemical vapor deposition ŽECR-CVD. reactor Žbase pressure - 1 = 10y7 Torr. with hydrogen dilution technique. Gases used for the in situ doping with hydrogen dilution method were pure hydrogen gas Župstream., SiH 4rAr Ž1:19. and PH 3rH 2 ŽŽ1:99. or B 2 H 6rAr Ž1:99. mixed gases Ždownstream.. The deposition parameters for this study were 9–199 sccm for the H 2 gas flow rate, 20–100 sccm for the SiH 4rAr, 15–35 sccm for the PH 3rH 2 flow rate, 10–40 sccm for the B 2 H 6rAr flow rate, 2508C for the substrate temperature, 1200 W for the microwave power and 20 mTorr for the process pressure. Corning 7059 glass and thermally oxidized Si wafers were used as substrates for deposition of the poly-Si thin films.
3. Results and discussion 3.1. Doping effect Fig. 1 shows the resistivity change as a function of the phosphine or diborane doping ratio normalized to the silane flow rate ŽwPH 3 xr wSiH 4 x and wB 2 H 6 xrwSiH 4 x.. For this study, the hydrogen dilution ratio was 93% for p-type doping and 95% for n-type doping. The resistivity rapidly decreased with increasing the doping gas flow rate ratio. and the minimum values of 0.025 V P cm at wB 2 H 6 xrwSiH 4 x
2. Experimental procedures The boron-doped and phosphorus-doped polycrystalline silicon thin films were deposited in an elec-
Fig. 1. Resistivity of heavily doped poly-Si thin films as a function of wB 2 H 6 xrwSiH 4 x gas flow rate ratio Žp-type. or wPH 3 xrwSiH 4 x Žn-type. gas flow rate ratio.
402
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
; 9 = 10y2 for p-type poly-Si films and 0.036 V P cm at wPH 3 xrwSiH 4 x ; 7 = 10y2 for n-type poly-Si films were achieved. The dopant concentration measured from the SIMS data of the deposited poly-Si thin films was approximately 1 = 10 21 cmy3 for p-type samples and 3 = 10 20 cmy3 for n-type samples and they were almost the same for various gas phase doping ratios. In order to gain more information on this change in resistivity, Hall measurements were performed. Figs. 2 and 3 show the change of carrier Hall mobility and carrier concentration as a function of the doping gas flow rate ratio ŽwPH 3 xrwSiH 4 x for n-type doping and wB 2 H 6 xrwSiH 4 x for p-type doping.. It can be seen that the carrier mobility slightly decreased with increasing the gas phase doping ratio. On the other hand, carrier concentration exhibited an entirely converse behavior with doping. Therefore, we can attribute the decrease of resistivity with doping gas flow rate ratio to the change in carrier concentration. Furthermore, the X-ray diffraction intensity ŽFig. 4. of Ž111. and Ž220. peaks of heavily doped poly-Si thin films prepared under various doping ratios show that increasing the doping ratio will degrade the crystallinity and the grain size. The grain shape Žas shown in Fig. 5. of the deposited films was changed from elliptical to roundness. Considering the above experimental observations, the relationship between the electrical properties, structural properties and gas phase doping ratio can be explained as follows.
Fig. 2. Hall mobility of heavily doped poly-Si thin films as a function of wB 2 H 6 xrwSiH 4 x gas flow rate ratio Žp-type. or wPH 3 xrwSiH 4 x Žn-type. gas flow rate ratio.
Fig. 3. Carrier concentration of heavily doped poly-Si thin films as a function of wB 2 H 6 xrwSiH 4 x gas flow rate ratio Žp-type. or wPH 3 xrwSiH 4 x Žn-type. gas flow rate ratio.
The solid solubility of impurity atoms incorporated into single crystalline silicon wafers was estimated to be approximately 1 = 10 20 cmy3 for P atoms and 5 = 10 19 cmy3 for B atoms at 3008C by extrapolation from the high temperature data w6,7x. For poly-Si thin films, the disordered structure of grain boundary will accommodate more impurity atoms. That is, the excess impurity atoms will precipitate at the grain boundary. In our deposition, the amount of added impurity atoms is beyond the solid solubility of the poly-Si thin films. Therefore, the doping concentrations of the heavily doped poly-Si thin films were almost the same and the change in carrier concentration could have resulted from the structure variation. It is likely that the activation energy decreases with increasing the doping gas flow
Fig. 4. XRD intensity of boron-doped poly-Si thin films prepared under various wB 2 H 6 xrwSiH 4 x gas flow rate ratio.
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
403
Fig. 5. SEM images of boron-doped poly-Si thin films prepared under various wB 2 H 6 xrwSiH 4 x gas flow rate ratio Ža. 2% Žb. 6% Žc. 8% Žd. 9%.
rate ratio. Considering the variations of crystallinity and activation energy of the heavily doped poly-Si thin films w8,9x, we can find out that the higher the degree of crystalline fraction, the larger the activation energy. Therefore, the activation efficiency decreases and carrier concentration decreases with increasing the gas phase doping ratio. The decrease in Hall mobility as a result of the gas phase doping ratio increase can also be attributed to the structure degradation. 3.2. Hydrogen dilution effect Figs. 6–8 show changes of the resistivity, carrier concentration and Hall mobility with the hydrogen
dilution ratio. For this study, the gas phase doping ratio is 9% for p-type samples and 7% for n-type samples. It was found that the mobility of poly-Si thin films decreased from 1.79 to 0.8 cm2rV s for p-type samples and 1.67 to 0.9 cm2rV s for n-type samples as the hydrogen dilution ratio increased from 80% to 90% but it increased with further hydrogen dilution. On the other hand, the carrier concentration exhibited an entirely different behavior with the hydrogen dilution ratio. Fig. 9 shows the plane-view TEM dark field images of phosphorusdoped poly-Si thin films prepared under various hydrogen dilution conditions. With increasing the hydrogen flow rate ratio, the crystalline fraction was
404
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
Fig. 6. Resistivity of boron-doped Žp-type. and phosphorus-doped Žn-type. poly-Si thin films as a function of hydrogen dilution ratio ŽwH 2 xrSiH 4 4qwH 2 x...
enhanced and the grain shape changes from roundness to elliptical just like the case of undoped poly-Si deposition. From the Hall measurement data, we can find that the carrier concentration increased more rapidly below 90% hydrogen dilution ratio and dominated the decrease of resistivity. In this region, the relationship between resistivity, Hall mobility, carrier concentration and structure evolution is similar to the case of decreasing the doping ratio. Therefore, the change in electrical properties would also be attributed to the structure variation. We found that the change in carrier concentration was slower and the change in Hall mobility was more rapid with further hydrogen dilution, but the resistivity still decreased. It indi-
Fig. 7. Hall mobility of boron-doped Žp-type. and phosphorusdoped Žn-type. poly-Si thin films as a function of hydrogen dilution ratio ŽwH 2 xrSiH 4 4qwH 2 x...
Fig. 8. Carrier concentration of boron-doped Žp-type. and phosphorus-doped Žn-type. poly-Si thin films as function of hydrogen dilution ratios ŽwH 2 xrSiH 4 4qwH 2 x...
cates that the conductivity is dominated by the Hall mobility. This phenomenon is different to the doping effect case: the carrier concentration dominates the conductivity. To clarify this peculiar change, we must realize the role of hydrogen dilution. From our previous studies, we find that atomic hydrogen not only enhance the surface diffusion length, but also play the role of structure relaxation through the penetration into subsurface and weakbonds-breaking effect. That means more atomic hydrogen species will enhance the crystallinity and increase the efficiency of the doping atoms incorporation into the substitutional sites. Therefore, the carrier concentration increases with increasing the hydrogen dilution. But the activation efficiency would decrease as the crystallinity enhanced, just like the case of doping effect as discussed above. The competition of these two mechanisms would result in the peculiar change in carrier concentration. For the change in Hall mobility, there also exist two mechanisms, one is the impurity scattering effect, and another mechanism is the crystallinity. With increasing the atomic hydrogen species, the incorporation rate will be enhanced and the dopant concentration increases. That means the carrier concentration will also rapidly increase due to the increase of dopant concentration and doping efficiency. But the more dopant atoms in the films, impurity scattering increases and the Hall mobility decreases. With further increasing the hydrogen dilution, the dopant concentration almost saturates due to the solid solu-
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
405
Fig. 9. Plane-view TEM dark field images of phosphorus-doped poly-Si thin films prepared under various hydrogen dilution ratios Ža. 90% Žb. 95% Žc. 96% Žd. 97% Že. 98% Žf. 99%.
bility limitation. Therefore, the increase of activation energy causes the degradation of carrier concentration. And the enhancement of crystallinity increases the Hall mobility. 4. Growth mechanism Based on our previous studies on undoped poly-Si thin films and the results demonstrated above, we propose a grain formation model to illustrate the role of doping impurity atoms and hydrogen. Under a higher doping gas flow rate ratio condition, a large amount of doping atoms would accumulate on the growth surface before formation of the silicon nuclei, and this disturbs the subsequent film deposition and grain formation processes. As for the heavily doped poly-Si thin films, the growth rate increases monotonically with increasing the doping level. This is due to the fact that the impurity atoms tend to remove the surrounding hydrogen atoms on the growing surface that would enhance the rate of SiH n radicals to reach the surface w1x. Moreover, the surface mobility of radicals will be decreased through increasing the surface friction between the solid sur-
face and the SiH n radicals. Consequently, the doping atoms added will suppress the surface migration of SiH n radicals, and resulting in crystallinity decrease. The impurity atoms penetrate into the films will suppress the crystalline grain growth during deposition. The growth surface of thin films deposited with high hydrogen dilution conditions is covered by high-density atomic hydrogen and the dangling bonds are terminated with this atomic hydrogen. Thus, the SiH n precursors will search the sites without hydrogen termination. Their sticking coefficient will be very small and the effective diffusion length will be long. Moreover, the atomic hydrogen species also play the role of structure relaxation through the penetration into subsurface and weak-bonds-breaking effect. It also enhances the crystallinity of deposited poly-Si thin films and increase the efficiency of impurity atoms incorporation into the crystalline sites. 5. Conclusions The effects of in situ doping concentration and hydrogen dilution on the structural and electrical
406
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
properties of heavily doped polycrystalline silicon thin films have been systematically investigated. Doping impurity atoms added will suppress the surface migration of SiH n radicals, and resulting in crystallinity decrease. The structure variation with gas phase doping ratios will lead to the change of activation energy, Hall mobility and carrier concentration. The high-level hydrogen dilution will enhance the crystallinity and increase the efficiency of the impurity atom incorporation into the crystalline sites. The competition of crystallinity and impurity scattering effect will lead to the change of the carrier concentration and Hall mobility with the hydrogen dilution ratio.
Acknowledgements The financial support by the National Science Council of Republic of China ŽContract No. NSC 87-2215-E-007-031. are gratefully acknowledged.
References w1x Y. Miyata, M. Furuta, T. Yoshioka, T. Kawamura, Jpn. J. Appl. Phys. 32 Ž1993. 3375. w2x D. He, S. Ishihara, I. Shimizu, Jpn. J. Appl. Phys. 32 Ž1993. 3370. w3x H.L. Hwang, K.C. Wang, K.C. Hsu, R.Y. Wang, T.R. Yew, J.J. Loferski, Appl. Surf. Sci. 113r114 Ž1997. 741. w4x H.L. Hwang, K.C. Wang, K.C. Hsu, R.Y. Wang, T.R. Yew, J.J. Loferski, Prog. Photovolt. Res. Appl. 4 Ž1996. 165. w5x K.C. Wang, Studies on hydrogenated polycrystalline silicon thin films deposited by ECR-CVD and PE-CVD, Ph.D. Thesis in Dept. of Electrical Engineering Natl. Tsing Hua Univ., 1995. w6x H. Kakinuma, M. Mohri, T. Tsuruoka, Jpn. J. Appl. Phys. 31 Ž1992. 1932. w7x S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, 2nd edn., p. 69. w8x G. Baccarani, B. Ricco, G. Spadini, J. Appl. Phys 49 Ž1978. 5565. w9x J.Y.W. Seto, J. Appl. Phys 46 Ž1975. 5247.
Electrical and structural properties of low temperature boron- and phosphorus-doped polycrystalline silicon thin films prepared by ECR-CVD H.L. Hsiao a , Y.Y. Shieh a , R.S. Lee a , R.Y. Wang a , K.C. Wang a , H.L. Hwang A.B. Yang b a
a,)
,
Department of Electrical Engineering, National Tsing Hua UniÕersity, Hsinchu 30043, Taiwan b Department of Physics, Tunghai UniÕersity, Taichung 40704, Taiwan
Abstract Boron-doped and phosphorus-doped polycrystalline silicon thin films were deposited on glass and SiO 2 substrates at a rather low temperature of 2508C by electron cyclotron resonance chemical vapor deposition ŽECR-CVD. using SiH 4rArrH 2rB 2 H 6 and SiH 4rArrH 2rPH 3 downstream plasma technique. The effects of in situ doping concentration and hydrogen dilution on the structural and electrical properties of heavily doped polycrystalline silicon thin films have been systematically investigated. The largest grain size of the heavily doped poly-Si films with ; 700 nm thickness Žgrowth rate ; 20 nmrmin. is approximately 500 nm, and the surface roughness is about 30 nm. With increasing the doping gas flow rate, the resistivity rapidly decreased and the minimum values of 0.025 V cm at wB 2 H 6 xrwSiH 4 x ; 9 = 10y2 for p-type poly-Si films and 0.036 V cm at wPH 3 xrwSiH 4 x ; 7 = 10y2 for n-type poly-Si films were achieved. The Hall mobility decrease as the carrier concentration increase can be explained by the impurity scattering increase with increasing the doping gas flow rate. X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated a change from - 110 ) to - 111 ) with slight decrease in the grain size. Furthermore, the grain shape of the deposited films was changed from elliptical to round. Similar to the undoped poly-Si thin films, larger hydrogen dilution ratio would increase the grain size, change the grain shape from round to elliptical. Otherwise, the Hall mobility simply decreased with increasing the hydrogen dilution ratio until 88% but it rapidly increased with further hydrogen dilution. On the other hand, the carrier concentration exhibited an entirely converse behavior with the hydrogen dilution ratio. The relationships between electrical properties and structure properties with regarding to the hydrogen dilution ratio and doping gas flow rate were attributed to the high surface coverage of atomic hydrogen species, doping precursors disturbance, solid solubility limitation and impurity scattering effect. q 1999 Elsevier Science B.V. All rights reserved. PACS: 73.61.C Keywords: Polycrystalline silicon; Boron-doped; Phosphorus-doped; Hall mobility; Resistivity
)
Corresponding author. Tel.: q886-3-5715131 ext. 4056; Fax: q886-3-5723927; E-mail: [email protected]
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 6 8 3 - 7
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
1. Introduction Polycrystalline silicon Žpoly-Si. is a very important material for the micro-electronics industry w1,2x. Thin films growth on glass substrates are of considerable interest owing to their wide applications to electronic devices such as solar cells, thin film transistors ŽTFTs. for display or photosensor devices. In this case, the poly-Si films must be prepared at a process temperature lower than the strain point of inexpensive glass substrates. As compared with amorphous silicon Ža-Si. TFT’s, poly-Si TFT’s provide higher field effect mobility and thus higher driving capability. Unfortunately, the conventional high quality thin film requires high temperature growth process. For device applications, it is essential to deposit poly-Si films at lower temperature but without compromising the material quality. In our previous studies w3–5x, we reported on the preparation and characterization of poly-Si films by electron cyclotron resonance downstream plasma chemical vapor deposition ŽECR-CVD. with the hydrogen dilution technique. Strongly - 110 ) and - 111 ) -oriented poly-Si films with a maximum grain size of 1 mm were obtained at a very low substrate temperature of 2508C. In order to apply the poly-Si films to device fabrication, the studies on n q and p q -type thin films are necessary. The in situ doping process is an attractive one because of its economic feasibility and no subsequent annealing process requirement. However, there has been rare study of in situ boron-doped and phosphorus-doped poly-Si thin films at such a low process temperature. In this paper, we investigate the effect of in situ heavy phosphine and diborane doping on the electronic and structural properties of the ECR-CVD deposited poly-Si thin films. Moreover, the relationships between the electrical properties, structural properties and hydrogen dilution ratio in phosphorus-doped and boron-doped poly-Si thin films are examined. The growth mechanism is also discussed.
401
tron cyclotron resonance downstream plasma chemical vapor deposition ŽECR-CVD. reactor Žbase pressure - 1 = 10y7 Torr. with hydrogen dilution technique. Gases used for the in situ doping with hydrogen dilution method were pure hydrogen gas Župstream., SiH 4rAr Ž1:19. and PH 3rH 2 ŽŽ1:99. or B 2 H 6rAr Ž1:99. mixed gases Ždownstream.. The deposition parameters for this study were 9–199 sccm for the H 2 gas flow rate, 20–100 sccm for the SiH 4rAr, 15–35 sccm for the PH 3rH 2 flow rate, 10–40 sccm for the B 2 H 6rAr flow rate, 2508C for the substrate temperature, 1200 W for the microwave power and 20 mTorr for the process pressure. Corning 7059 glass and thermally oxidized Si wafers were used as substrates for deposition of the poly-Si thin films.
3. Results and discussion 3.1. Doping effect Fig. 1 shows the resistivity change as a function of the phosphine or diborane doping ratio normalized to the silane flow rate ŽwPH 3 xr wSiH 4 x and wB 2 H 6 xrwSiH 4 x.. For this study, the hydrogen dilution ratio was 93% for p-type doping and 95% for n-type doping. The resistivity rapidly decreased with increasing the doping gas flow rate ratio. and the minimum values of 0.025 V P cm at wB 2 H 6 xrwSiH 4 x
2. Experimental procedures The boron-doped and phosphorus-doped polycrystalline silicon thin films were deposited in an elec-
Fig. 1. Resistivity of heavily doped poly-Si thin films as a function of wB 2 H 6 xrwSiH 4 x gas flow rate ratio Žp-type. or wPH 3 xrwSiH 4 x Žn-type. gas flow rate ratio.
402
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
; 9 = 10y2 for p-type poly-Si films and 0.036 V P cm at wPH 3 xrwSiH 4 x ; 7 = 10y2 for n-type poly-Si films were achieved. The dopant concentration measured from the SIMS data of the deposited poly-Si thin films was approximately 1 = 10 21 cmy3 for p-type samples and 3 = 10 20 cmy3 for n-type samples and they were almost the same for various gas phase doping ratios. In order to gain more information on this change in resistivity, Hall measurements were performed. Figs. 2 and 3 show the change of carrier Hall mobility and carrier concentration as a function of the doping gas flow rate ratio ŽwPH 3 xrwSiH 4 x for n-type doping and wB 2 H 6 xrwSiH 4 x for p-type doping.. It can be seen that the carrier mobility slightly decreased with increasing the gas phase doping ratio. On the other hand, carrier concentration exhibited an entirely converse behavior with doping. Therefore, we can attribute the decrease of resistivity with doping gas flow rate ratio to the change in carrier concentration. Furthermore, the X-ray diffraction intensity ŽFig. 4. of Ž111. and Ž220. peaks of heavily doped poly-Si thin films prepared under various doping ratios show that increasing the doping ratio will degrade the crystallinity and the grain size. The grain shape Žas shown in Fig. 5. of the deposited films was changed from elliptical to roundness. Considering the above experimental observations, the relationship between the electrical properties, structural properties and gas phase doping ratio can be explained as follows.
Fig. 2. Hall mobility of heavily doped poly-Si thin films as a function of wB 2 H 6 xrwSiH 4 x gas flow rate ratio Žp-type. or wPH 3 xrwSiH 4 x Žn-type. gas flow rate ratio.
Fig. 3. Carrier concentration of heavily doped poly-Si thin films as a function of wB 2 H 6 xrwSiH 4 x gas flow rate ratio Žp-type. or wPH 3 xrwSiH 4 x Žn-type. gas flow rate ratio.
The solid solubility of impurity atoms incorporated into single crystalline silicon wafers was estimated to be approximately 1 = 10 20 cmy3 for P atoms and 5 = 10 19 cmy3 for B atoms at 3008C by extrapolation from the high temperature data w6,7x. For poly-Si thin films, the disordered structure of grain boundary will accommodate more impurity atoms. That is, the excess impurity atoms will precipitate at the grain boundary. In our deposition, the amount of added impurity atoms is beyond the solid solubility of the poly-Si thin films. Therefore, the doping concentrations of the heavily doped poly-Si thin films were almost the same and the change in carrier concentration could have resulted from the structure variation. It is likely that the activation energy decreases with increasing the doping gas flow
Fig. 4. XRD intensity of boron-doped poly-Si thin films prepared under various wB 2 H 6 xrwSiH 4 x gas flow rate ratio.
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
403
Fig. 5. SEM images of boron-doped poly-Si thin films prepared under various wB 2 H 6 xrwSiH 4 x gas flow rate ratio Ža. 2% Žb. 6% Žc. 8% Žd. 9%.
rate ratio. Considering the variations of crystallinity and activation energy of the heavily doped poly-Si thin films w8,9x, we can find out that the higher the degree of crystalline fraction, the larger the activation energy. Therefore, the activation efficiency decreases and carrier concentration decreases with increasing the gas phase doping ratio. The decrease in Hall mobility as a result of the gas phase doping ratio increase can also be attributed to the structure degradation. 3.2. Hydrogen dilution effect Figs. 6–8 show changes of the resistivity, carrier concentration and Hall mobility with the hydrogen
dilution ratio. For this study, the gas phase doping ratio is 9% for p-type samples and 7% for n-type samples. It was found that the mobility of poly-Si thin films decreased from 1.79 to 0.8 cm2rV s for p-type samples and 1.67 to 0.9 cm2rV s for n-type samples as the hydrogen dilution ratio increased from 80% to 90% but it increased with further hydrogen dilution. On the other hand, the carrier concentration exhibited an entirely different behavior with the hydrogen dilution ratio. Fig. 9 shows the plane-view TEM dark field images of phosphorusdoped poly-Si thin films prepared under various hydrogen dilution conditions. With increasing the hydrogen flow rate ratio, the crystalline fraction was
404
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
Fig. 6. Resistivity of boron-doped Žp-type. and phosphorus-doped Žn-type. poly-Si thin films as a function of hydrogen dilution ratio ŽwH 2 xrSiH 4 4qwH 2 x...
enhanced and the grain shape changes from roundness to elliptical just like the case of undoped poly-Si deposition. From the Hall measurement data, we can find that the carrier concentration increased more rapidly below 90% hydrogen dilution ratio and dominated the decrease of resistivity. In this region, the relationship between resistivity, Hall mobility, carrier concentration and structure evolution is similar to the case of decreasing the doping ratio. Therefore, the change in electrical properties would also be attributed to the structure variation. We found that the change in carrier concentration was slower and the change in Hall mobility was more rapid with further hydrogen dilution, but the resistivity still decreased. It indi-
Fig. 7. Hall mobility of boron-doped Žp-type. and phosphorusdoped Žn-type. poly-Si thin films as a function of hydrogen dilution ratio ŽwH 2 xrSiH 4 4qwH 2 x...
Fig. 8. Carrier concentration of boron-doped Žp-type. and phosphorus-doped Žn-type. poly-Si thin films as function of hydrogen dilution ratios ŽwH 2 xrSiH 4 4qwH 2 x...
cates that the conductivity is dominated by the Hall mobility. This phenomenon is different to the doping effect case: the carrier concentration dominates the conductivity. To clarify this peculiar change, we must realize the role of hydrogen dilution. From our previous studies, we find that atomic hydrogen not only enhance the surface diffusion length, but also play the role of structure relaxation through the penetration into subsurface and weakbonds-breaking effect. That means more atomic hydrogen species will enhance the crystallinity and increase the efficiency of the doping atoms incorporation into the substitutional sites. Therefore, the carrier concentration increases with increasing the hydrogen dilution. But the activation efficiency would decrease as the crystallinity enhanced, just like the case of doping effect as discussed above. The competition of these two mechanisms would result in the peculiar change in carrier concentration. For the change in Hall mobility, there also exist two mechanisms, one is the impurity scattering effect, and another mechanism is the crystallinity. With increasing the atomic hydrogen species, the incorporation rate will be enhanced and the dopant concentration increases. That means the carrier concentration will also rapidly increase due to the increase of dopant concentration and doping efficiency. But the more dopant atoms in the films, impurity scattering increases and the Hall mobility decreases. With further increasing the hydrogen dilution, the dopant concentration almost saturates due to the solid solu-
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
405
Fig. 9. Plane-view TEM dark field images of phosphorus-doped poly-Si thin films prepared under various hydrogen dilution ratios Ža. 90% Žb. 95% Žc. 96% Žd. 97% Že. 98% Žf. 99%.
bility limitation. Therefore, the increase of activation energy causes the degradation of carrier concentration. And the enhancement of crystallinity increases the Hall mobility. 4. Growth mechanism Based on our previous studies on undoped poly-Si thin films and the results demonstrated above, we propose a grain formation model to illustrate the role of doping impurity atoms and hydrogen. Under a higher doping gas flow rate ratio condition, a large amount of doping atoms would accumulate on the growth surface before formation of the silicon nuclei, and this disturbs the subsequent film deposition and grain formation processes. As for the heavily doped poly-Si thin films, the growth rate increases monotonically with increasing the doping level. This is due to the fact that the impurity atoms tend to remove the surrounding hydrogen atoms on the growing surface that would enhance the rate of SiH n radicals to reach the surface w1x. Moreover, the surface mobility of radicals will be decreased through increasing the surface friction between the solid sur-
face and the SiH n radicals. Consequently, the doping atoms added will suppress the surface migration of SiH n radicals, and resulting in crystallinity decrease. The impurity atoms penetrate into the films will suppress the crystalline grain growth during deposition. The growth surface of thin films deposited with high hydrogen dilution conditions is covered by high-density atomic hydrogen and the dangling bonds are terminated with this atomic hydrogen. Thus, the SiH n precursors will search the sites without hydrogen termination. Their sticking coefficient will be very small and the effective diffusion length will be long. Moreover, the atomic hydrogen species also play the role of structure relaxation through the penetration into subsurface and weak-bonds-breaking effect. It also enhances the crystallinity of deposited poly-Si thin films and increase the efficiency of impurity atoms incorporation into the crystalline sites. 5. Conclusions The effects of in situ doping concentration and hydrogen dilution on the structural and electrical
406
H.L. Hsiao et al.r Applied Surface Science 142 (1999) 400–406
properties of heavily doped polycrystalline silicon thin films have been systematically investigated. Doping impurity atoms added will suppress the surface migration of SiH n radicals, and resulting in crystallinity decrease. The structure variation with gas phase doping ratios will lead to the change of activation energy, Hall mobility and carrier concentration. The high-level hydrogen dilution will enhance the crystallinity and increase the efficiency of the impurity atom incorporation into the crystalline sites. The competition of crystallinity and impurity scattering effect will lead to the change of the carrier concentration and Hall mobility with the hydrogen dilution ratio.
Acknowledgements The financial support by the National Science Council of Republic of China ŽContract No. NSC 87-2215-E-007-031. are gratefully acknowledged.
References w1x Y. Miyata, M. Furuta, T. Yoshioka, T. Kawamura, Jpn. J. Appl. Phys. 32 Ž1993. 3375. w2x D. He, S. Ishihara, I. Shimizu, Jpn. J. Appl. Phys. 32 Ž1993. 3370. w3x H.L. Hwang, K.C. Wang, K.C. Hsu, R.Y. Wang, T.R. Yew, J.J. Loferski, Appl. Surf. Sci. 113r114 Ž1997. 741. w4x H.L. Hwang, K.C. Wang, K.C. Hsu, R.Y. Wang, T.R. Yew, J.J. Loferski, Prog. Photovolt. Res. Appl. 4 Ž1996. 165. w5x K.C. Wang, Studies on hydrogenated polycrystalline silicon thin films deposited by ECR-CVD and PE-CVD, Ph.D. Thesis in Dept. of Electrical Engineering Natl. Tsing Hua Univ., 1995. w6x H. Kakinuma, M. Mohri, T. Tsuruoka, Jpn. J. Appl. Phys. 31 Ž1992. 1932. w7x S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, 2nd edn., p. 69. w8x G. Baccarani, B. Ricco, G. Spadini, J. Appl. Phys 49 Ž1978. 5565. w9x J.Y.W. Seto, J. Appl. Phys 46 Ž1975. 5247.