High-power laser pulses were used to replace the conventional high temperature furnace processing for the p-n junction formation step in high speed, low cost solar cell fabrication. Three different approaches to junction formation were tested: (1) laser annealing of ion-implanted Si in which laser radiation is used to remove the radiation damage and to recover the electrical activity in the implanted layer; (2) a process in which a thin film of dopant is first deposited on the substrate and then incorporated into the near-surface region by laser-induced diffusion; and (3) a process in which a heavily doped amorphous silicon layer is deposited on the Si substrate and epitaxially regrown from the melted substrate layer by laser radiation. All three methods were found to provide suitable candidates for high efficiency Si solar cells

Narayan, J.

Westbrook, R. D.

White, C. W.

Wood, R. F.

Young, R. T.

English

NASA Technical Reports Server

LASER TECHNIQUES IN PHOTOVOLTAIC RESEARCH* 
R .  T. Young, R .  F.  Wood, R .  D . Westbrook, J .  Narayan, and C . W. White 
Sol id State Division, Oak Ridge National Laboratory 
SUMMARY 
High-power laser  pulses are being used t o  replace the conventional high 
temperature furnace processing for  the p-n junction formation s tep i n  h i g h -  
speed, low-cost solar  cell  fabrication. Three different  approaches to junction 
formation have been used: (1 ) laser  anneal ing of ion-implanted Si in which 
laser  radiation i s  used to remove the radiation damage and to recover the 
electr ical  ac t iv i ty  in the implanted layer; ( 2 )  a process in which a thin film 
of dopant i s  f i r s t  deposited on the substrate and then incorporated into the 
near-surface region by laser-induced diffusion; and (3) a process in which a 
heavily doped anlorphous s i l icon layer i s  deposited on the Si substrate and 
epitaxial l y  regrown from the me1 ted substrate 1 ayer by 1 aser radiation. We 
have demonstrated that  a l l  three methods provide sui table  candidates for  high 
efficiency Si solar  ce l l s .  
INTRODUCTION 
Approximately two years ago a t  the photovoltaic conference in Luxembourg, 
we described the use of high-power laser  pulses to anneal the l a t t i c e  damage 
in ion-implanted Si solar c e l l s  ( re f .  1 ) .  We demonstrated that  pulsed laser  
annealing i s  superior to  conventional thermal annealing for  the removal of 
l a t t i c e  damage, fo r  the recovery of e lectr ical  ac t iv i ty ,  and for  the preserva- 
tion of the minority car r ie r  1 ifetime in the substrate,  and we showed that  
th i s  superiority resulted in improved solar cel l  performance. Pulsed laser  
anneal ing i s  now understood (ref .  2 )  t o  take place by a mechanism involving 
the ultrarapid melting of the en t i re  damaged layer and subsequent liquid-phase 
epitaxial regrowth from the underlying perfect substrate. The dopant profiles 
a re  usually broadened considerably by pulsed laser  annealing and th is  can be 
readily explained quantitatively by dopant diffusion in the molten s t a t e .  An 
understanding of the importance of near-surface me1 ting by laser  heating led 
to the developnient ( refs .  3-6) of two other laser-assisted junction formation 
techniques, i  .e.,  laser-induced diffusion of dopant films and laser-induced 
recrystal l izat ion of doped amorphous films to  form epitaxial junctions. These 
techniques are  important because they are  much simpler and more easi ly  auto- 
mated processes than e i ther  thermal diffusion or ion implantation. This i s  
'~esearch sponsored by the Division of Materials Sciences, U .  S. Department 
of Energy under contract W-7405-eng-26 with the Union Carbide Corporation. 
https://ntrs.nasa.gov/search.jsp?R=19790024484 2020-03-21T21:02:25+00:00Z
p a r t i c u l a r l y  advantageous f o r  s o l a r  c e l l s  which r e q u i r e  high-volume, low-cost 
f a b r i c a t i o n  processes t o  become economical ly  compe t i t i ve  i n  t e r r e s t i a l  a p p l i -  
ca t ions .  I n  t h i s  paper we rev iew and compare the  th ree  l ase r -ass i s ted  junc- 
t i o n  fo rmat ion  techniques and d iscuss t h e i r  a p p l i c a t i o n s  t o  s o l a r  c e l l  f a b r i c a -  
t i o n .  Throughout t h i s  work, t h e  l a s e r  anneal ing was performed i n  a i r  w i t h  t he  
Q-switched ou tpu t  o f  a ruby  l a s e r  ( A  = 0.694 p, T = 15-60 nsec). A f t e r  t he  
samples were exposed t o  t he  l a s e r  pulses, t h e  p e r f e c t i o n  o f  t he  e m i t t e r  reg ions 
was s tud ied  by t ransmiss ion  e l e c t r o n  microscopy (TEM) , and t h e i r  e l e c t r i c a l  
parameters were determined by van de r  Pauw measurements. The dopant p r o f i l e s  
were obta ined by secondary i o n  mass spectrometry (SIMS) o r  anodic o x i d a t i o n  
and s t r i p p i n g  technique. The r e s u l t i n g  j u n c t i o n  c h a r a c t e r i s t i c s  were s tud ied  
by I - V  and C - V  measurements on mesa d iode s t ruc tu res .  F i n a l l y  s o l a r  c e l l s  
were f a b r i c a t e d  and t e s t e d  under AM1 i l l u m i n a t i o n .  
JUNCTION FORMATION BY I O N  IMPLANTATION AND LASER ANNEALING 
I n  t h i s  method, energe t ic  dopant ions a re  a l lowed t o  impinge on a S i  sub- 
s t r a t e  and l a s e r  r a d i a t i o n  i s  used t o  remove the  r a d i a t i o n  damage and t o  b r i n g  
the  dopant ions i n t o  e l e c t r i c a l l y  a c t i v e  s u b s t i t u t i o n a l  s i t e s .  The r e s u l t i n g  
dopant p r o f i l e  and j u n c t i o n  depth a re  determined by t h e  as-implanted p r o f i l e ,  
which i s  c o n t r o l l e d  by the  s topp ing  power f o r  t he  dopant p a r t i c l e s  i n  s i l i c o n  
and t h e  imp lan ta t i on  energy, and by t h e  du ra t i on  of t he  laser- induced sur face  
m e l t i n g  which i s  c o n t r o l l e d  by the  l a s e r  parameters such as energy d e n s i t y  and 
pulse d u r a t i o n  t ime. The i n f l u e n c e  o f  the  l a s e r  energy dens i t y  on the  dopant 
p r o f i l e  behavior  i s  g iven  i n  f i g u r e  1 which shows r e s u l t s  f o r  samples implanted 
w i t h  l l B  (35 keV, 1 x 1016 c r 2 )  and subsequent ly annealed w i t h  l a s e r  pulses 
i n  t he  energy d e n s i t y  range o f  0.64 J/cm2 t o  3.1 J/cm2. A t  a l a s e r  energy 
dens i t y  %.6 J/cm2, t h e  boron p r o f i l e  i s  i n d i s t i n g u i s h a b l e  from t h a t  o f  t he  as- 
implanted specimen. Evidence t h a t  t h e  anneal ing was incomplete i n  t h i s  sample 
was obta ined f rom van der  Pauw measurements and f rom TEM which showed t h a t  
o n l y  30% o f  t he  expected e l e c t r i c a l  a c t i v i t y  had been obtained, and t h a t  
s i g n i f i c a n t  damage i n  t h e  form o f  d i s l o c a t i o n  loops remained. Complete anneal- 
i n g  was achieved w i t h  energy d e n s i t i e s  o f  1.1 J/cm2 and grea ter ;  t he  p r o f i l e s  
were then almost f l a t  topped i n  t h e  sur face  r e g i o n  w i t h  t h e  pene t ra t i on  depth 
i nc reas ing  as the  energy dens i t y  was increased. 'F igure 2 g ives t h e  comparison 
o f  dopant p r o f i l e s  obta ined f rom samples implanted w i t h  boron a t  5 keV and 
35 keV and annealed w i t h  l a s e r  pulses o f  1.7 J/cm2. These r e s u l t s  i n d i c a t e  
t h a t  i n  o rder  t o  o b t a i n  a d e f e c t - f r e e  shal low j u n c t i o n  by the  ion- imp lan ta t ion ,  
laser-anneal i n g  technique, t h e  imp1 a n t a t i o n  energy should be kept  low and the  
l a s e r  energy j u s t  h igh  enough t o  m e l t  t he  sur face  l a y e r  which conta ins t h e  
damaged region.  So la r  c e l l s  w i t h  convers ion e f f i c i e n c i e s  o f  14-15% can be 
r o u t i n e l y  f a b r i c a t e d  by t h i s  technique. Parameters o f  a t y p i c a l  c e l l  made 
f rom S i  implanted w i t h  7 5 ~ s +  (5 keV, 2 x 1015 c r 2 )  and annealed w i t h  one 
pulse o f  a ruby l a s e r  a re  Is, = 33.6 ma/cm2, Voc = 565 mV and FF = 0.76 and 
a r e s u l t i n g  conversion e f f i c i e n c y  o f  14.5%. The c e l l s  were f a b r i c a t e d  w i t h  
s i n g l e - l a y e r  Ta205 a n t i r e f l e c t i o n  coat ings  and no back sur face  f i e l d s .  
JUNCTION FORMATION BY LASER-INDUCED DIFFUSION 
In t h i s  technique, th in  (50-100 f i )  films of dopants such as  boron and 
aluminum a re  f i r s t  vacuum-deposited on the subs t ra te  and then driven in to  the  
near-s-urface region by laser-induced surface melting and diffusion.  Trans- 
mission electron microscopy and e l ec t r i c a l  measurements show t h a t  the dopants 
a r e  dissolved in the  s i l i con  l a t t i c e  and e l e c t r i c a l l y  activated.  Resultant 
emitter  regions w i t h  appropriate sheet  r e s i s t i v i t i e s  (30-100 n/U) f o r  so la r  
ce l l  appl icat ions  can be obtained and the dopant prof i les  a re  nearly ideal f o r  
h i g h  ef f ic iency ce l l s .  Figure 3 gives the boron concentration prof i l e  a f t e r  
l a s e r  i r rad ia t ion  w i t h  pulses of 1.5 J/cm2. This p rof i l e  has a high dopant 
concentration near the  surface region and an intermediate region decreasing 
toward the  junction. The high potential  ba r r i e r  near the  surface will  r e f l e c t  
the minority c a r r i e r s  from the  surface and minimize surface recombination 
losses. The r e su l t s  of I-V and reversed-biased C-V measurements on Al-depos- 
i t ed ,  laser-annealed mesa diodes a r e  given in f igure  4. The near uni t  value 
of A ,  t he  small reverse-bias current  and the  good agreement between the  meas- 
ured and expected junction potent ia ls  suggest t h a t  a defect-free junction was 
obtained. Solar c e l l s  were fabr icated and tes ted and the e f f ic ienc ies  were 
found to  be comparable t o  o r  s l i g h t l y  lower than those of the ion-implanted 
laser-annealed ce l l s .  The e f fec t s  on ce l l  performance of impurities in the 
dopant source and of surface contamination during deposition a r e  not c lea r  a t  
the present time. 
FORMATION OF EPITAXIAL p-n JUNCTION BY PULSED-LASER RADIATION 
In a more recent study,  we have demonstrated t ha t  l a s e r  radiat ion can be 
used f o r  epi taxia l  regrowth of a doped amorphous s i l i con  layer  deposited on a 
s i l i con  subst ra te .  A s ing le  pulse from a ruby l a s e r  w i t h  an energy density 
of 1.8 J/cm2 was su f f i c i en t  t o  recrystal  1 i z e ,  w i t h  almost perfect  crys ta l  1 i n -  
i t y ,  an As-doped amorphous s i l i con  layer  deposited on a (100) subst ra te  to  a 
depth of about 3000 a. Junction charac te r i s t i cs  determined by I-V and C-V 
measurements showed t h a t  a perfect  junction w i t h  a low leakage current  was 
achieved. Use of t h i s  technique f o r  junction formation provides many advan- 
tages normally a t t r ibu ted  t o  ion implantation, such as r e l a t i ve ly  easy con- 
t ro l  of dopant concentration and junction depth. Figure 5 shows dopant pro- 
f i l e s  measured by Hall e f f ec t  measurements in combination w i t h  gnodic oxida- 
t ion and s t r ipping techniques f o r  the  cases of 1000 a and 2000 A deposited 
layers annealed w i t h  pulses of 1.55 J/cm2 and 1.78 J/cm2, respectively. The 
dopants have spread deeper in to  the  subs t ra te  i n  a manner very s imilar  t o  
dopant red i s t r ibu t ion  i n  As-implanted laser-anneal ed samples ( re f .  2) .  The 
so l id  l i n e  i n  the  f igures  a r e  calculated prof i les  using techniques described 
i n  reference 2. The l iqu id  phase di f fus ion of As was calculated from ideal 
as-deposi ted p rof i l es  (dotted 1 ines )  without considering the possible compl i -  
cations due t o  t h i n  i n t e r f ac i a l  oxygen layers between the  subs t ra te  and de- 
posited layers.  The agreement between the  experimental and calculated pro- 
f i l e s  i s  very good. From these r e su l t s ,  i t  appears t h a t  junction parameters 
can ea s i l y  be controlled w i t h  t h i s  method by a var ia t ion of the  dopant con- 
centra t ion i n  the  evaporant source, the  thickness of the  deposited l ayer ,  and 
the energy density of the  l a s e r  radia t ion.  Solar  c e l l s  have not ye t  been 
fabricated w i t h  this  technique. However, from the  r e su l t s  described above, 
we an t i c i pa t e  t ha t  i t  should provide another a l t e rna t i ve  method f o r  high 
eff ic iency so l a r  ce l l  fabr ica t ion.  Extensive s tud ies  of the  e f f ec t s  of sub- 
s t r a t e  o r ien ta t ion  on recrystal  1 i za t ion ,  i n t e r f ac i a l  problems such as the  
presence of Si02 o r  hydrocarbons, and the  influence of the  various dopant 
impurities and residual gas atoms (such as H 2 ,  N 2 ,  and 02) on the  regrowth 
behavior a r e  current ly  underway. 
CONCLUSIONS 
In conclusion, high-quality p-n junctions can be formed by the  use of 
l a s e r  pulses t o  anneal radia t ion damage, induce di f fus ion,  a l loying,  and re- 
growth of films deposited on s i l i con  subst ra tes .  Due t o  the  s impl ic i ty  and 
low cost  of t h i s  type of processing, we expect t h a t  the  methods discussed 
here, a f t e r  f u r t he r  development, wil l  be extremely useful i n  the  1 arge-vol ume 
fabr icat ion of high-efficiency so l a r  c e l l s .  
REFERENCES 
1. Young, R. T . ;  White, C.  W.; Clark, G. J . ;  Narayan, J.;  and Chr i s t i e ,  W. H.: 
Laser Annealed Boron Implanted Si l icon Solar  Cells .  Photovoltaic Solar  
Energy Conference, Proceedings of the  International  Conference, Luxembourg, 
September 27-30, 1977, Rei del , Boston, 1978, p. '861 . 
2. Wang, J .  C . ;  Wood, R.  F.;  Pronko, P. P . :  Theoretical Analysis of Thermal 
and Mass Transport in Ion Implanted, Laser Annealed Sil icon.  Appl. Phys. 
Let t . ,  Vol. 33, 1978, p. 455. 
3. Fa i r f i e l d ,  J. M e  and Schwuttke, G. H . :  S i l icon Diodes Made by Laser 
I r radia t ion.  Solid S t a t e  Electron., Vol. 11, 1968, p. 1175. 
4. Harper, F. I .  and Cohen, M .  I .  : Propert ies of Si Diodes Prepared by Alloy- 
ing A1 Into n-Type Si with Heat Pulses From a Nd:YAG Laser. Solid S t a t e  
Electron.,  Vol. 13, 1970, p.  1103. 
5. Narayan, J . ;  Young, R. T . ;  Wood, R. F. ;  and Chr i s t i e ,  W. H . :  p-n Junction 
Formation in  Boron-Deposi ted Si 1 icon by Laser-Induced Diffusion. Appl . 
Phys. Le t t . ,  Vol . 33, 1978, p. 338. 
6. Young, R. T. ;  Narayan, J . ;  Westbrook, R. D . ;  and Wood, R. F.: Transmission 
Electron Microscopy and Elect r ica l  Propert ies Measurements of Laser Doped 
Si l icon and GaAs. Proceedings of Laser-Solid In teract ions  and Laser 
Processing Symposium, Boston, Massachusetts, Nov. 28 - Dec. 1 ,  1978, p. 579. 
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Figure 3. Depth p r o f i l e s  f o r  boron-deposited, laser-annealed S i .  
ORNL-DWG 79-11899 
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BlAS VOLTAGE (V) 
F igure 4. Dark I-V and reverse-biased C-V measurement on Al-deposited 
laser-annealed S i  mesa diodes. 
ORNL-DWG 79- 4 1478 
2 I I 
ARSENIC DOPED Si ON (4OO)Si 
Figure 5. Comparis n of experimental and ca lcula ted  dopant p ro f i l e s  
on 1000 1 and 2000 a deposited l aye r s  a f t e r  l a s e r  i r r ad i a t ion .  


Laser Techniques in Photovoltaic Research

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