第26卷 第12期2005年12月
半 导 体 学 报
CHIN ESE J OURNAL OF SEMICONDUCTORS
Vol. 26 No. 12
Dec. ,2005
E ffect of Therm al Annealing on Characteristics
of Polycrystalline Silicon
Ren Bingyan 1, Gou Xianfang 1,2, Ma Lifen 1,2, Li Xudong 2, Xu Ying 2, and Wang Wenjing 2
(1Semiconductor Research I nstit ute , Hebei Uni versit y of Technolog y , Tianj in 300130, (2B ei j ing S olar Energ y Research I nstit ute , Bei j i 100083, China )
Abstract :Oxygen and carbon behaviors and minority 22(mc 2Si ) used for solar cells are investigated by FTIR and QSSPCD 750～1150℃in N 2and O 2ambient. For comparison ,the annealing of and carbon concentrations is also carried out under the same the and carbon concentrations of mc 2Si and CZ 2Si have a lesser are not generated ,and grain boundaries in mc 2Si do not affect car 2bon of 2Si increases in N 2and O 2ambient at 850,950,and 1150℃,and the lifetime of mc 2Si wafers O 2are higher than those annealed in N 2,which shows that a lot of impurities in mc 2Si at high temperature annealing diff use to grain boundaries ,greatly reducing recombination centers. Interstitial Si atoms filling vacancies or recombination centers increases lifetime. K ey w ords :polycrystalline silicon ; oxygen ; lifetime EEACC :2520C
C LC number :TN30411+2 Document code :A Article ID :025324177(2005) 1222294204
cells. In t his paper ,t he effect s of t hermal annealing
1 Introduction
Polycrystalline Si wafers have become preva 2lent in t he recent p hotovoltaic market. However , t hey need f urt her quality imp rovement for highly efficient ,low 2co st solar cells. First we must under 2stand t he behaviors of imp urities and defect s in t he polycrystalline Si wafers in more detail. Because t here are grain boundaries and more imp urities and defect s ,mc 2Si material has more complicated p hysi 2cal behavior in high temperat ure annealing t han mono 2crystalline silicon. Oxygen in mc 2Si is a very important imp urity t hat affect s t he elect rical and mechanical properties of silicon material during heat t reat ment s [1]. However ,t he formation of oxy 2gen precipitates , t he variety of minor carrier life 2times ,and t he influence of t he annealing ambient are less investigated for polycrystalline silico n solar
on oxygen behavior and carrier lifetimes for poly 2crystalline Si wafers are investigated.
2 Experiment
The polycrystalline Si wafers provided by Ba 2yer Solar Corporation in t his experiment were p 2
μm t hick. The interstitial type ,019Ω・cm ,and 285
oxygen and substit ute carbon concent rations of t he samples were 813×1017and 2×1017cm -3, respec 2tively. For comparison ,p 2type CZ 2Si samples wit h
〈100〉orientation , 1～3Ω・cm , a t hickness of
μm ,and almost t he same oxygen concent ration 330
were also st udied. The samples were cleaned wit h chemical solution ,and Si oxide was removed in an HF (10%) solution. Then t hey were subjected to heat t reat ment at 1260℃for 1h in N 2ambient so as to eliminate t he influence of t hermal history
before
Ζ2005Chinese Institute of Electronics
Received 10J une 2005
第12
期Ren Bingyan et al. : Effect of Thermal Annealing on Characteristics of Polycrystalline Silicon
2295
annealing [2]. A single step heat t reat ment was t hen carried out in t he temperat ure range of 750～1150℃for 4h in N 2and O 2ambient. Interstitial ox 2ygen concent rations and minority 2carrier lifetimes before and after annealing were determined by F T 2IR (Fourier transmission inf rared spect ro scopy ) and QSSPCD (quasi 2steady state p hotoconductance decay ) . Finally ,t he samples were etched by Sirtl or Wright solution and were examined wit h an optical microscope and SEM.
oxygen changes with annealing tem 2
3 R esults
3. 1 Change oxygen concentra 2
tions
under N 2and O 2surrounding in mc 2Si wafers
3. 2 Change of substitute carbon concentration
Figures 1and 2show t he profiles of t he inter 2stitial oxygen co ncent ration of CZ 2Si and mc 2Si during single step annealing in t he temperat ure range of 750～1150℃in N 2and O 2ambient. It can be seen t hat t he oxygen concent ration in CZ 2Si and mc 2Si slightly decreases (except for t he light in 2) . The change of oxygen crease in mc 2Si at 950℃
concentration is almo st uniform in N 2and O 2an 2nealing. This indicates t hat t here is almost no gen 2eration of oxygen p recipitates in CZ 2Si and mc 2Si wafers in N 2or O 2ambient. This has less influence on oxygen concent ration during single step annea 2
ling.
Carbon in CZ silicon has been reported to have a significant influence on oxygen precipitation dur 2ing heat t reat ment s. In addition , t here is a much higher carbon concent ration in mc 2Si t han in CZ silicon. Carbon can enhance t he nucleation of oxy 2
) and gen precipitates at low temperatures (
) [2]. Carbon con 2at higher temperat ures (>950℃
centration after annealing was determined by F T 2IR. As shown in Table 1,t he carbon concent ration does not affect t he amount of oxygen precipitate at higher temperat ures in single annealing ,and grain bo undaries in mc 2Si do not affect carbon behavior.
Table 1 Change of substitute carbon concentration of mc 2Si
T /℃
7502. 05
8501. 9
9502. 15
10502. 2
11501. 6
[C s ]/1017cm -3N 2ambient [C s ]/1017cm -3O 2ambient
2. 42. 62. 51.
82. 6
3. 3 Change of carrier lifetime
Figures 3and 4show change of carrier lifetime
Fig. 1 Interstitial oxygen changes with annealing tem 2perature under N 2and O 2surrounding in CZ 2Si
wafers
of silicon wafers during single step annealing in t he temperat ure range of 750～1150℃in N 2and O 2ambient. The lifetime of CZ 2Si wafers after annea 2ling decreases drastically wit h increasing annealing
22
96
半 导 体 学 报第26卷
temperat ure but has a recovery at 950℃, which p robably generates many defect s and new recombi 2nation centers ,and some interstitial Si atoms filling vacancies or recombination centers result in life 2time recovery at 950℃. Also ,t he lifetime of CZ 2Si wafers annealed in O 2ambient is lower t han t hose annealed in N 2. This p henomenon is probably due to t he fact t hat during annealing in O 2ambient ,in 2terstitial Si atoms are supplied from a growing SiO 2/Si interface so t hat excess interstitial Si at 2oms might recreate new recombination [3
The lifetime of mc 2Si wafers annealing at 1150℃increases 120%in O 2ambient and t he lifetime of mc 2Si wafers annealed in O 2ambient is higher t han t hose annealed in N 2. The reason for t he increase is p robably due to t he fact t hat a lot of imp urities in mc 2Si at high temperat ure annealing diff use to grain boundaries , greatly recombination centers. t he ot Si atoms result s in
Fig. 3 Ratio of minority carrier lifetime changes with annealing temperature under N 2and O 2surrounding in CZ 2Si
wafers
) (a ) and optical micrographs (200Fig. 5 SEM (500×
) (b ) of defects in the cleavage plane of mc 2Si after ×
annealing at 1050℃
4 Conclusion
Fig. 4 Ratio of minority carrier lifetime changes with annealing temperature under N 2and O 2surrounding in mc 2Si wafers
We conclude t hat t he oxygen concentration of mc 2Si and CZ 2Si had a slight decrease (except for a ) in N 2and O 2am 2light increase in mc 2Si at 950℃
bient during single 2step annealing , which means oxygen precipitates were generated. Lower carbon concent ration did not affect t he amount of o xygen p recipitates at
higher temperat ures , and grain
bo undaries in mc 2Si did not affect carbon behavior.
As shown in Fig. 4, t he lifetime of minority carriers in mc 2Si wafers has great increases at 850, 950,and 1150℃, respectively. The changes of life 2time in N 2and O 2ambient are almo st t he same.
第12期Ren Bingyan et
al. : Effect of Thermal Annealing on Characteristics of Polycrystalline Silicon
22
97
Bulk lifetime of mc 2Si increased wit h temperat ure in N 2ambient at 850,950,1150℃,and annealing in O 2showed better result s t han in N 2. On t he contra 2ry ,t he lifetime of CZ 2Si annealed in N 2or O 2de 2creased rapidly. The changes of lifetime and oxygen concentration in N 2and O 2annealing were almo st t he same. The reason for t he lifetime increase is p robably due to t he fact t hat a lot of imp urities in mc 2Si at high temperat ure annealing diff use to grain boundaries , greatly reducing recombination centers. On t he ot her hand , interstitial Si filling vacancies or result a lifetime increase. R eferences
[1] Yang D ,Li D S. Oxygen in Czochralski silicon used for solar
cells. Solar Energy Materials &Solar Cells ,2002,72:133
[2] Yang D , Moeller H J. Effect of heat treat ment on carbon in
multicrystalline silicon. Solar Energy &Solar Cells , 2002,72:542
[3]Mimura , S T. between t her 2
carrier lifetimes in p 2, FZ Energy Materials &Solar Cells , 热退火对多晶硅特性的影响
任丙彦1 勾宪芳1,2 马丽芬1,2 励旭东2 许 颖2 王文静2
(1河北工业大学半导体研究所, 天津 300130)
(2北京太阳能研究所, 北京 100083)
摘要:为研究热退火对太阳电池用多晶硅的影响, 在750～1150℃,N 2和O 2环境下分别对硅片进行热处理. 用傅里叶红外光谱仪和准稳态光电导衰减法测量退火前后的氧碳含量和少子寿命的变化. 为了比较, 对有相同氧碳含量的直拉硅片进行同样处理. 结果发现:在多晶和单晶片中氧碳含量下降很小, 意味着没有氧沉淀产生, 晶界对碳行为影响不大. 多晶硅片在N 2和O 2环境下,850、950和1150℃下退火, 少子寿命都有很大提高, 并且在O 2中退火比N 2中退火少子寿命上升得更多, 可能由于在高温退火时大量杂质扩散到晶界处, 减少了复合中心. 另外, 间隙硅原子填充了空位或复合中心从而导致寿命提高. 关键词:多晶硅; 氧; 寿命
EEACC :2520C
中图分类号:TN30411+2 文献标识码:A 文章编号:025324177(2005) 1222294204
2004206210收到Ζ2005中国电子学会
第26卷 第12期2005年12月
半 导 体 学 报
CHIN ESE J OURNAL OF SEMICONDUCTORS
Vol. 26 No. 12
Dec. ,2005
E ffect of Therm al Annealing on Characteristics
of Polycrystalline Silicon
Ren Bingyan 1, Gou Xianfang 1,2, Ma Lifen 1,2, Li Xudong 2, Xu Ying 2, and Wang Wenjing 2
(1Semiconductor Research I nstit ute , Hebei Uni versit y of Technolog y , Tianj in 300130, (2B ei j ing S olar Energ y Research I nstit ute , Bei j i 100083, China )
Abstract :Oxygen and carbon behaviors and minority 22(mc 2Si ) used for solar cells are investigated by FTIR and QSSPCD 750～1150℃in N 2and O 2ambient. For comparison ,the annealing of and carbon concentrations is also carried out under the same the and carbon concentrations of mc 2Si and CZ 2Si have a lesser are not generated ,and grain boundaries in mc 2Si do not affect car 2bon of 2Si increases in N 2and O 2ambient at 850,950,and 1150℃,and the lifetime of mc 2Si wafers O 2are higher than those annealed in N 2,which shows that a lot of impurities in mc 2Si at high temperature annealing diff use to grain boundaries ,greatly reducing recombination centers. Interstitial Si atoms filling vacancies or recombination centers increases lifetime. K ey w ords :polycrystalline silicon ; oxygen ; lifetime EEACC :2520C
C LC number :TN30411+2 Document code :A Article ID :025324177(2005) 1222294204
cells. In t his paper ,t he effect s of t hermal annealing
1 Introduction
Polycrystalline Si wafers have become preva 2lent in t he recent p hotovoltaic market. However , t hey need f urt her quality imp rovement for highly efficient ,low 2co st solar cells. First we must under 2stand t he behaviors of imp urities and defect s in t he polycrystalline Si wafers in more detail. Because t here are grain boundaries and more imp urities and defect s ,mc 2Si material has more complicated p hysi 2cal behavior in high temperat ure annealing t han mono 2crystalline silicon. Oxygen in mc 2Si is a very important imp urity t hat affect s t he elect rical and mechanical properties of silicon material during heat t reat ment s [1]. However ,t he formation of oxy 2gen precipitates , t he variety of minor carrier life 2times ,and t he influence of t he annealing ambient are less investigated for polycrystalline silico n solar
on oxygen behavior and carrier lifetimes for poly 2crystalline Si wafers are investigated.
2 Experiment
The polycrystalline Si wafers provided by Ba 2yer Solar Corporation in t his experiment were p 2
μm t hick. The interstitial type ,019Ω・cm ,and 285
oxygen and substit ute carbon concent rations of t he samples were 813×1017and 2×1017cm -3, respec 2tively. For comparison ,p 2type CZ 2Si samples wit h
〈100〉orientation , 1～3Ω・cm , a t hickness of
μm ,and almost t he same oxygen concent ration 330
were also st udied. The samples were cleaned wit h chemical solution ,and Si oxide was removed in an HF (10%) solution. Then t hey were subjected to heat t reat ment at 1260℃for 1h in N 2ambient so as to eliminate t he influence of t hermal history
before
Ζ2005Chinese Institute of Electronics
Received 10J une 2005
第12
期Ren Bingyan et al. : Effect of Thermal Annealing on Characteristics of Polycrystalline Silicon
2295
annealing [2]. A single step heat t reat ment was t hen carried out in t he temperat ure range of 750～1150℃for 4h in N 2and O 2ambient. Interstitial ox 2ygen concent rations and minority 2carrier lifetimes before and after annealing were determined by F T 2IR (Fourier transmission inf rared spect ro scopy ) and QSSPCD (quasi 2steady state p hotoconductance decay ) . Finally ,t he samples were etched by Sirtl or Wright solution and were examined wit h an optical microscope and SEM.
oxygen changes with annealing tem 2
3 R esults
3. 1 Change oxygen concentra 2
tions
under N 2and O 2surrounding in mc 2Si wafers
3. 2 Change of substitute carbon concentration
Figures 1and 2show t he profiles of t he inter 2stitial oxygen co ncent ration of CZ 2Si and mc 2Si during single step annealing in t he temperat ure range of 750～1150℃in N 2and O 2ambient. It can be seen t hat t he oxygen concent ration in CZ 2Si and mc 2Si slightly decreases (except for t he light in 2) . The change of oxygen crease in mc 2Si at 950℃
concentration is almo st uniform in N 2and O 2an 2nealing. This indicates t hat t here is almost no gen 2eration of oxygen p recipitates in CZ 2Si and mc 2Si wafers in N 2or O 2ambient. This has less influence on oxygen concent ration during single step annea 2
ling.
Carbon in CZ silicon has been reported to have a significant influence on oxygen precipitation dur 2ing heat t reat ment s. In addition , t here is a much higher carbon concent ration in mc 2Si t han in CZ silicon. Carbon can enhance t he nucleation of oxy 2
) and gen precipitates at low temperatures (
) [2]. Carbon con 2at higher temperat ures (>950℃
centration after annealing was determined by F T 2IR. As shown in Table 1,t he carbon concent ration does not affect t he amount of oxygen precipitate at higher temperat ures in single annealing ,and grain bo undaries in mc 2Si do not affect carbon behavior.
Table 1 Change of substitute carbon concentration of mc 2Si
T /℃
7502. 05
8501. 9
9502. 15
10502. 2
11501. 6
[C s ]/1017cm -3N 2ambient [C s ]/1017cm -3O 2ambient
2. 42. 62. 51.
82. 6
3. 3 Change of carrier lifetime
Figures 3and 4show change of carrier lifetime
Fig. 1 Interstitial oxygen changes with annealing tem 2perature under N 2and O 2surrounding in CZ 2Si
wafers
of silicon wafers during single step annealing in t he temperat ure range of 750～1150℃in N 2and O 2ambient. The lifetime of CZ 2Si wafers after annea 2ling decreases drastically wit h increasing annealing
22
96
半 导 体 学 报第26卷
temperat ure but has a recovery at 950℃, which p robably generates many defect s and new recombi 2nation centers ,and some interstitial Si atoms filling vacancies or recombination centers result in life 2time recovery at 950℃. Also ,t he lifetime of CZ 2Si wafers annealed in O 2ambient is lower t han t hose annealed in N 2. This p henomenon is probably due to t he fact t hat during annealing in O 2ambient ,in 2terstitial Si atoms are supplied from a growing SiO 2/Si interface so t hat excess interstitial Si at 2oms might recreate new recombination [3
The lifetime of mc 2Si wafers annealing at 1150℃increases 120%in O 2ambient and t he lifetime of mc 2Si wafers annealed in O 2ambient is higher t han t hose annealed in N 2. The reason for t he increase is p robably due to t he fact t hat a lot of imp urities in mc 2Si at high temperat ure annealing diff use to grain boundaries , greatly recombination centers. t he ot Si atoms result s in
Fig. 3 Ratio of minority carrier lifetime changes with annealing temperature under N 2and O 2surrounding in CZ 2Si
wafers
) (a ) and optical micrographs (200Fig. 5 SEM (500×
) (b ) of defects in the cleavage plane of mc 2Si after ×
annealing at 1050℃
4 Conclusion
Fig. 4 Ratio of minority carrier lifetime changes with annealing temperature under N 2and O 2surrounding in mc 2Si wafers
We conclude t hat t he oxygen concentration of mc 2Si and CZ 2Si had a slight decrease (except for a ) in N 2and O 2am 2light increase in mc 2Si at 950℃
bient during single 2step annealing , which means oxygen precipitates were generated. Lower carbon concent ration did not affect t he amount of o xygen p recipitates at
higher temperat ures , and grain
bo undaries in mc 2Si did not affect carbon behavior.
As shown in Fig. 4, t he lifetime of minority carriers in mc 2Si wafers has great increases at 850, 950,and 1150℃, respectively. The changes of life 2time in N 2and O 2ambient are almo st t he same.
第12期Ren Bingyan et
al. : Effect of Thermal Annealing on Characteristics of Polycrystalline Silicon
22
97
Bulk lifetime of mc 2Si increased wit h temperat ure in N 2ambient at 850,950,1150℃,and annealing in O 2showed better result s t han in N 2. On t he contra 2ry ,t he lifetime of CZ 2Si annealed in N 2or O 2de 2creased rapidly. The changes of lifetime and oxygen concentration in N 2and O 2annealing were almo st t he same. The reason for t he lifetime increase is p robably due to t he fact t hat a lot of imp urities in mc 2Si at high temperat ure annealing diff use to grain boundaries , greatly reducing recombination centers. On t he ot her hand , interstitial Si filling vacancies or result a lifetime increase. R eferences
[1] Yang D ,Li D S. Oxygen in Czochralski silicon used for solar
cells. Solar Energy Materials &Solar Cells ,2002,72:133
[2] Yang D , Moeller H J. Effect of heat treat ment on carbon in
multicrystalline silicon. Solar Energy &Solar Cells , 2002,72:542
[3]Mimura , S T. between t her 2
carrier lifetimes in p 2, FZ Energy Materials &Solar Cells , 热退火对多晶硅特性的影响
任丙彦1 勾宪芳1,2 马丽芬1,2 励旭东2 许 颖2 王文静2
(1河北工业大学半导体研究所, 天津 300130)
(2北京太阳能研究所, 北京 100083)
摘要:为研究热退火对太阳电池用多晶硅的影响, 在750～1150℃,N 2和O 2环境下分别对硅片进行热处理. 用傅里叶红外光谱仪和准稳态光电导衰减法测量退火前后的氧碳含量和少子寿命的变化. 为了比较, 对有相同氧碳含量的直拉硅片进行同样处理. 结果发现:在多晶和单晶片中氧碳含量下降很小, 意味着没有氧沉淀产生, 晶界对碳行为影响不大. 多晶硅片在N 2和O 2环境下,850、950和1150℃下退火, 少子寿命都有很大提高, 并且在O 2中退火比N 2中退火少子寿命上升得更多, 可能由于在高温退火时大量杂质扩散到晶界处, 减少了复合中心. 另外, 间隙硅原子填充了空位或复合中心从而导致寿命提高. 关键词:多晶硅; 氧; 寿命
EEACC :2520C
中图分类号:TN30411+2 文献标识码:A 文章编号:025324177(2005) 1222294204
2004206210收到Ζ2005中国电子学会