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Analysis and fabrication of MIS solar cells with majority carrier conduction characteristics Au, Ho Piu 1978

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ANALYSIS  AND F A B R I C A T I O N OF M I S S O L A R  WITH M A J O R I T Y  CELLS  C A R R I E R CONDUCTION C H A R A C T E R I S T I C S  by  P I U [AU  HO B.Sc.  (Physics), M.Sc.  A  Chinese  •( P h y s i c s ) ,  THESIS THE  University University  o f Hong Kong, of  Toronto,  SUBMITTED I N P A R T I A L FULFILMENT REQUIREMENTS  FOR  THE DEGREE  M A S T E R OF A P P L I E D  OF  SCIENCE  IN THE F A C U L T Y in  OF  GRADUATE  STUDIES  the Department of  Electrical  We  accept  this  the  THE  ..Engineering  thesis  required  UNIVERSITY  standard  OF B R I T I S H C O L U M B I A  December (c)  as conforming  1978  Ho P i u A u  to  1969  1974  OF  In presenting this thesis in p a r t i a l  fu1fiIment of the requirements for  an advanced degree at the University of B r i t i s h Columbia, I agree that the Library  shall make it freely available for  reference and study.  I further agree that permission for extensive copying of  this'thesis  for scholarly purposes may be granted by the Head of my Department or by his representatives.  It  is understood that copying o r publication  of this thesis for financial gain shall not be allowed without my writ ten pe rm i ss ion .  Department of  Eiec.TgicA-»-  EMCn/QeeBit^C^  The University of B r i t i s h Columbia  2075 Wesbrook Place Vancouver, Canada V6T 1W5  Date  "^C  2o  (0)>§>  ABSTRACT  A inversion  simple model  layer  generated.  The  theoretical  In  have  a  that  have  mode. an  and  As  been the  proposed.  I-V  very  reported present  to  model  explanation  with  solar  non-uniform these  cell  terms  the  similar be  to  in  of  the  data were proposed  in  of  state  charge  analysis forward  the  effects  has  been  results bias  in  p r a c t i c a l MIS  of  in the  solar  dark, cells  a minority  carrier  conduction  solely majority  carrier  conduction,  operation obtained theory  i i  the  under  that  operating  considers  incorporating  surface  characteristics which,  Experimental  agreement  a MIS  i n c l u s i o n of  two s l o p e n a t u r e  alternative  good  charge  for  was  of  these  devices  from Al/SiO obtained.  /pSi  can solar  be cells  and  TABLE  OF  CONTENTS  PAGE  ABSTRACT TABLE  i i  OF  CONTENTS  LIST  OF  FIGURES  LIST  OF  TABLES  i i i v v i i :  ACKNOWLEGEMENTS  CHAPTER  v i i i  1  INTRODUCTION  „  1  CHAPTER 2  S I L I C O N MIS SOLAR C E L L THEORIES  ~4  2.1  Review  of  the Theory  of Minority  2.2  Classical  Thermionic  Emission  2.3  Development the  2.3.1  Effect  The Case Energy  2.3.2  of  of  the Thermionic Surface  States  Theory  f o r MIS  Emission  Theory  to  of  Surface  and N e g l e c t i n g  the Presence  of  Inversion  Presence  a Non-Uniform of  Distribution  an I n v e r s i o n  .  .  Layer  .  of .  :  4 12  include  i n an MIS S t r u c t u r e  Distribution  of  Diodes  Cells  a Uniform  The Case the  of  C a r r i e r MIS T u n n e l  14  States Layer  Surface  with Charge  States  .  14  with 18  CHAPTER 3  EXPERIMENTAL 3.1  Sample  3.2  The I - V  PROCEDURES Preparation  35 .  .<  Characteristic Testing  35 \ Station  i i i  41  PAGE  CHAPTER 4  RESULTS  48  4.1  Dark  I-V  Curves  4.2  Light  4.3  Stability  48  Characteristics of  I-V  48  Characteristics  53  CHAPTER 5  DISCUSSION  62  5.1  Comparison  5.2  Surface  5.3  Stability  of  Theoretical  States of  Present  t h e MIS  and P r a c t i c a l Dark  i n t h e MIS C e l l s  Cells  I-V  Characteristics  .  62 65 66  CHAPTER 6  CONCLUSION  67.  References  68  APPENDIX  69  1  iv  FIGURES  PAGE  1.  Band  Diagram  f o r P-Type  2.  Double  3.  Sensitivity  Curve  (1)  4.  Sensitivity  Curve  (2)  20 '  5.  Sensitivity  Curve  (3)  21  6.  Sensitivity  Curve  (4)  22  7.  Band Diagram  8.  Sensitivity  Curve  (5)  29  9.  Sensitivity  Curve  (6)  30  Exponential  at  States  MIS C e l l  I-V  Curve .  .  5  From Green  Et  A l  [6]  8  .  the Surface  19  of  P-Type  Semiconductor  from  Profile  SZE  [7]  .  23  10.  Surface  11.  Theoretical  12.  Diode  13.  Mask  14.  I-V  15.  Booster  16.  Dark  17.  Light  18.  Experimental  19.  Light  I-V  20.  Light  Bias  21.  Stability  Curve  (1)  22.  Stability  Curve  (2)  55  23.  Stability  Curve  (3)  56  24.  Stability  Curve  (4)  57  Curves  Factor  n -  31  for Various  Voltage  Surface  State  V plot  42  Station  43  Circuit  I-V  45  Measuring C i r c u i t  and L i g h t  Bias  Dark  Curves I-V  .32 33  for Evaporation Test  Profiles  I-V  I-V .  46  Measuring C i r c u i t  Curves  .  .  .  .  .  .  47 48  .  50  Curves  51  .  .  54  v  PAGE  25.  S t a b i l i t y Curve (5)  58  26.  S t a b i l i t y Curve (6)  59R  27.  S t a b i l i t y Curve (7)  60  28.  S t a b i l i t y Curve (8)  61  29.  Comparison of T h e o r e t i c a l and E x p e r i m e n t a l I-V Curves  63  vi  TABLES  PAGE  1.1  Comparison o f T h e o r e t i c a l and E x p e r i m e n t a l n V a l u e s  34  2.1  Test Sample. Parameters  35  3.1  C h a r a c t e r i s t c s o f T e s t S l i c e s used to I n v e s t i g a t e Boron D i f f u s i o n  4.1  Summary o f E x p e r i m e n t a l Diode Data Shown i n F i g . 18  5.  Summary of E x p e r i m e n t a l Diode Data Shown i n F i g . 19 and 20  vii  .  39 52  . . . .  :  52  ACKNOWLEGEMENTS  The a u t h o r w i s h e s t o e x p r e s s h i s g r a t i t u d e t o h i s r e s e a r c h s u p e r v i s o r Dr. D.L. P u l f r e y f o r h i s guidance and p a t i e n c e throughout the c o u r s e o f t h i s work.  The h e l p o f Mr. H. Hogenboom i s much a p p r e c i a t e d .  Thanks a r e a l s o e x p r e s s e d t o Dr. L. Young and M e s s r s . V. Drobny, H.Y. and D. Smith f o r t h e i r many p e r t i n e n t comments and d i s c u s s i o n s .  viii  Tsoi,  CHAPTER  1  INTRODUCTION  The for  major  large-scale  Schottky  not  terrestrial  barrier  overcoming  this  use high  solar  very  poor  thermionic  to using  quality  cells  photovoltage  emission  they  cells  response.  This  stems  current  i n a Schottky  current  than  structures.  Recent  experimental  i n t e r f a c i a l oxide  layer  can increase  metal  the photovoltaic  solar  intimate  deficiency  from  h a s shown  that  and  form the usual  leads  to  a  heterojunction  the presence  and semiconductor conversion  contact  junction  that  and need  i n the  the fact  barrier  cells. for  to fabricate  i n many h o m o j u n c t i o n  work"[l]  between  of  conversion  one c a n d i d a t e s  however,  a serious  energy  cost  simple  dark dark  cell  are very  exhibit  higher  barrier  i s the high  In p r a c t i c e ,  considerably  thin  solar  seem t o b e t h e number  substrates. solar  photovoltaic  applications  problem because  metal/semiconductor of  handicap  i n the  efficiency  of  of  a  Schottky such  a  cell. In  t h e MIS s t r u c t u r e  reduced by e i t h e r height,  the thermionic  increasing the effective  decreasing the probability  encouraging carriers,  interface  states  or reducing  with  t h e number  emission dark  metal/semiconductor  of majority a large  current  carrier  capture  of majority  barrier  tunnelling,  cross-section for  carriers  can be  at  the  majority  semiconductor  surface. In  several  S i MIS s t r u c t u r e s  the dark  currentlhas  sufficiently  reduced  that  voltages  appraching  very  best  homojunction  open  diodes  circuit have been  obtained,  i . e . V  been those  of  > 6 0 0 mV  the [2,3].  2.  A  feature  of  exponential'forward  I  where  I  from  the  diodes  ^ »  these  b i a s In  I-V  =  single  exponential led  to  characteristic  are  behaving  indeed  true  of  s i m p l i c i t y and  tional  p-n  then  the  this  junction,  characteristic  is  belief  carrier  (thermionic are  termed  minority Chapter of  minority  diodes  exponential  can a l s o be  dominated  by  current),  provided  ductor  i t  I-V  interface  of  in  the  i s  chapter  2,  following  related  to  Schottky  p-n  prospect  MIS  technology  is  thought  to  the  induced  bulk  be  to  be  i s  described at  postulated  by  the  a brief barrier  to  view  current to  the  layer-  and  these  are  As  the  current such  diodes  [4-6].  this  into  i s  conven-  opposed  suppressed,  and  and  good  emission surface  account.  review diodes.  of  to  describe  present  solar  considering  of  this  as  carrier  totally  If  in  depletion  majority  former  photovoltaics  semiconductor.  usual  different  barrier  [2].  related  presented  action  by  markedly  this  dark  been  curve  described  diodes  for  bias  diodes  double  Schottky  exhibiting  forward  the  a  can be  for  The  in  thermionic  taken  the  the  d e s c r i b e d by  classical  expected  has  [6] is  possess  characteristic is  diodes  exciting  considered  theory  diodes  However  2.  i s  a  induced junction  diodes  and  carrier  detailed  carrier  double  currents  emission)  A  as  current  i n j e c t i o n - d i f f u s i o n ..current  Such  that  technology. MIS  they  eV  relationship  cheapness  i n . these  that  exp  02  ~ 2.  a most  recombination-generation  minority  I  ^ 1 and  N  IQ-^J ^  is  c h a r a c t e r i s t i c which  +  and has  the  diodes  author  diode  current  states  at  new  classical  that  dark  the  current  of  of features  efficiency  (majority the  operation  beginning  conversion  the  This  the  the  to  in  MIS  be  carrier  insulator/semicon-  theory  is  thermionic  developed emission  in theory  3.  In (see and  chapter double  data  on  the 3)  present and  stability  conjunction with of  surface  states  silicon/insulator described very In  diodes. for  in  the  with  future  I-V  thus  conclusion i s  work.  be  be  Taking  the  a  were  good  that  band  gap  taken  a_  presented  in  as  chapter  the  presents  the  data  of  silicon  at  used  a double  of  in  the  is appears  exponential  minority  along with  some  distribution  distribution  evidence 6,  of  procedure  state  existence  priori  4 also  efficiency  experimental  of  The  surface the  the  fabricated  conversion;  description  formulated.  resultant  confirming  cannot  diodes  exhibit  allows  across  to  The  devices. model  energy  5.  to  4)  MIS  c h a r a c t e r i s t i c s . .Chapter  these  interface  relationship The  In of  Aj/SiO^/pSi  (chapter  proposed  chapter  reasonable,  I-V  shown  exponential  the  study  carrier  suggestions  4.  CHAPTER 2  SILICON MIS  SOLAR CELL THEORIES  As d i s c u s s e d i n the p r e c e e d i n g maintains  t h a t e f f i c i e n t MIS  S i s o l a r c e l l s e x h i b i t i n g double e x p o n e n t i a l ,  forward b i a s , . d a r k , I n I-V curves may solar c e l l s .  c h a p t e r , one s c h o o l of thought  be d e s c r i b e d as m i n o r i t y c a r r i e r  However i t i s t h e . p r e s e n t  diodes w i t h such f e a t u r e s may  author's  i n t e n t i o n to i n d i c a t e that  a l s o be d e s c r i b e d as m a j o r i t y c a r r i e r  where the c u r r e n t i s dominated by t h e r m i o n i c e m i s s i o n performance i s s t r o n g l y a f f e c t e d by s u r f a c e s t a t e s . c a r r i e r t u n n e l s o l a r c e l l s i s . . b r i e f l y reviewed  tunnel  and The  devices,  device  theory of m i n o r i t y  i n t h i s c h a p t e r and then the  more c l a s s i c a l t h e r m i o n i c e m i s s i o n t h e o r y i s p r e s e n t e d  and then developed  t o i n c l u d e the e f f e c t of s u r f a c e s t a t e s w h i c h i s r e q u i r e d to d e s c r i b e experimental  2.1  the  data.  Review of the t h e o r y of m i n o r i t y c a r r i e r MIS  M i n o r i t y c a r r i e r MIS p-n j u n c t i o n d e v i c e s as regards  t u n n e l diodes  tunnel  diodes  are i d e n t i c a l to  conventional  t h e i r o p e r a t i o n a t low b i a s , e x c e p t f o r the  l o c a t i o n o f the d e p l e t i o n r e g i o n .  I n t h i s k i n d o f MIS  d i o d e the  contact  m e t a l work f u n c t i o n and the dopant s p e c i e s i n the s e m i c o n d u c t o r a r e chosen t o ensure t h a t the dominant component of the d i o d e c u r r e n t t u n n e l s between the m e t a l and the m i n o r i t y c a r r i e r energy band i n the s e m i c o n d u c t o r . illustration,  As  an  l e t us choose a p- type s u b s t r a t e .  F i g u r e 1 shows t h a t near z e r o b i a s , the s u r f a c e o f the p-type s i l i c o n i s i n v e r t e d i f the m e t a l c o n t a c t has a low enough v a l u e of work f u n c t i o n <j>m, (e.g. aluminum).  The  system f e r m i l e v e l i s n e a r e r i n energy  to the c o n d u c t i o n band edge t h e n to the v a l e n c e - b a n d edge  and:correspondingly  5.  METAL  INSULATOR  Fig.l.  P - TYPE  Band Diagram f o r P-Type MIS  SEMICONDUCTOR  Cell  6.  there i s a much l a r g e r p r o b a b i l i t y t h a t a s t a t e  o f g i v e n energy above the  c o n d u c t i o n band edge i s o c c u p i e d by an e l e c t r o n then a c o r r e s p o n d i n g s t a t e i n the v a l e n c e band by a h o l e .  T h e r e f o r e i t i s much e a s i e r f o r an e l e c t r o n  to t u n n e l through ,the o x i d e and then occupy a vacancy i n the c o n d u c t i o n band than i t i s f o r a h o l e from the semiconductor to t u n n e l through the oxide i n t o the m e t a l .  These statements are embodied i n the t u n n e l i n g  [6].  integrals  dE(r 2TT h  v T  1  ,2. 2 TT h  c o n d u c t i o n band  ds  a  s  •  m  e  _ T 1  J  s  d E ! ( f ' - f ')  , T. J •'valence band  where  - f j ^  '  (1)  ds e  (2)  f' = 1 - f J ._ i s the c u r r e n t from the semiconductor >s.,m s ,m cT c o n d u c t i o n band to the m e t a l and J i s the c u r r e n t from the semiconductor vT :i  m  v a l e n c e band to the m e t a l ; f' , f ' are the p r o b a b i l i t i e s of occupancy m  of  g  s t a t e s o f energy E i n the m e t a l and i n the semiconductor.  D e f i n i n g the  "shadow" of a c o n s t a n t energy s u r f a c e t o be i t s p r o j e c t i o n i n wave number space onto a p l a n e p a r a l l e l to the b a r r i e r ,  the I n t e g r a ] i s over the  o v e r l a p of shadows of the m e t a l and the semiconductor c o n s t a n t energy s u r f a c e s f o r energy E. ^b n  " h  n i s g i v e n by  2  2 ^  CP . - P. ) Ti l  S  m  Jx  dx  (3)  where x i s the d i r e c t i o n  a  p e r p e n d i c u l a r to the b a r r i e r , x  and x, are the c l a s s i c a l t u r n i n g p o i n t s , QL  D  and P„,. and P. are the t r a n s v e r s e and t o t a l momentum .of the t u n n e l i n g Ti l 6  p a r t i c l e s i n the i n s u l a t o r  region.  F o r the above r e a s o n s , i n an A l / S i C ^ / S i MIS  tunnel diode,  e l e c t r o n s dominate the c a r r i e r s which t r a n s p o r t c u r r e n t from the conductor to the m e t a l .  semi-  S i n c e the o x i d e between the m e t a l and the  7.  semiconductor i s v e r y can  d i s t u r b the MIS  using Poisson's  t h i n , the t u n n e l l i n g c u r r e n t i s so l a r g e such t h a t i t  diode from thermal e q u i l i b r i u m .  equation.,  the e l e c t r o n and  the c u r r e n t d e n s i t y e x p r e s s i o n s and  hole  Green et a l . [6]  c o n t i n u i t y equations  i n terms of d r i f t and  diffusion  r e c o m b i n a t i o n formulae i n the b u l k semiconductor based on  R e a d - H a l l recombination s t a t i s t i c s , nonequilibrium  MIS  havev  thickness.  r a p i d l y with  At h i g h  can t u n n e l between the m e t a l and  approximately e x p o n e n t i a l l y with  components  Shbckley-  forward b i a s  decreasing i n s u l a t o r  In t h i s r e g i o n , the diode c u r r e n t i s l i m i t e d by  which p a r t i c l e s  and  generated a s e t of curves f o r the  diode as shown i n f i g u r e 2.  (>0.3V), the c u r r e n t f l o w i n c r e a s e s  by  the r a t e of  semiconductor, which v a r i e s  the i n s u l a t o r t h i c k n e s s .  However, at  s m a l l forward b i a s and moderate r e v e r s e b i a s , the diode c u r r e n t i s virtually  independent of the i n s u l a t o r t h i c k n e s s  f o r thicknesses  less!'than  o  28A.  This difference  a r i s e s because under r e v e r s e  the diode c u r r e n t i s l i m i t e d by than by  transport  and  s m a l l forward b i a s ,  through the semiconductor  rather  t u n n e l l i n g through the i n s u l a t o r . I t s h o u l d be r e a l i z e d that in. the m i n o r i t y c a r r i e r diodes under  discussion  the semiconductor s u r f a c e i s i n v e r t e d near zero b i a s .  c o n d i t i o n remains unchanged f o r s m a l l forward b i a s .  This  I t means t h a t ,  on  moving from the i n t e r f a c e i n t o the semiconductor the • _ i n v e r s i o n region; i s f o l l o w e d by Therefore  a d e p l e t i o n r e g i o n and  according  f i n a l l y a space change n e u t r a l r e g i o n .  to conventional  p-n  diode theory, we  have the sum  the d i f f u s i o n c u r r e n t i n the space charge n e u t r a l r e g i o n of the the g e n e r a t i o n - r e c o m b i n a t i o n  current  associated with  Moreover J _ can be s p l i t i n t o two  devicesand  depletion region.  must n o t i c e t h a t the d i f f u s i v e m i n o r i t y c a r r i e r s i n the b u l k are e l e c t r o n s .  of  portions J  One  semiconductor T  = Jp . M  J  M  r  8.  Fig.2.  Computed I-V c h a r a c t e r i s t i c s f o r " n o n - e q u i l i b r i u m " MIS d i o d e s for various insulator thicknesses. (From Green e t a l . [ 6 ] )  c>j  = 3.2eV, p = 2ft cm.  9.  where  J  CM  2TT J.  MC  conduction  h  conduction the  where  (4)  E  s  f  band  conventional  ds  ,  '  e  _ n  J  (5)  assumptions  CM  is  "  MC  J  the  =  J  d  +  of  junction  diode  theory  diffusive  J"rg i s  the  As  oxide  the  r g  J  (6)  component.  recombination-generation becomes  thin,  both  eventually equation  each becomes at  (6),  least  regions,  and  than  J — , or  either  f  CM  f  they  distribution f  m  =  and  so  the  small  that  and  MC terms  on  forward  their  increase rapidly  the  right  bias.  difference  In is  hand  these  much  side  of  bias  smaller  i . e .  <7> are  s  the  energy  distributions  band  respectively.  conduction are  f..  from  described  adjust  than  current.  i d e n t i c a l and  are  given  by  of-electrons' in Under the  thermal  Fermi  the  metal  equilibrium  Mrac  function  where departure  reverse  CM  the  ~~ J M C  semiconductor  conditions  larger  J„_  of  MC  and  m  at  must  CM J  much  component  J„,, a n d  '  be  _ n  obtains  J  and  e  band  d  2. 2ir h  Employing one  ds  2.  in  E„ r  =  {1 +  is  thermal terms  the  exp  [(E  system  -  fermi  equilibrium, of  this  Epl/ki.]}"1  the  level.  ( g )  At  electron  distribution  law by  least energy using  for  moderate  distributions quasi-fermi  can  levels.  F o r the m e t a l  i s r e p l a c e d by E ^ ,  the semiconductor E from equations  i s replaced E  p  (4),  (5),  (7)  and  p  the m e t a l q u a s i - f e r m i l e v e l , w h i l e s  n  for  the e l e c t r o n q u a s i - f e r m i l e v e l .  (8) we  get  f  - f  or E ^  = s n  E F m  ,  Then  showing  that f o r a t h i n i n s u l a t o r , the^.semiconductor e l e c t r o n q u a s i - f e r m i l e v e l i s e f f e c t i v e l y pinned  to the m e t a l f e r m i l e v e l under r e v e r s e and  small  forward  bias. The  injection-diffusion  components can be d e s c r i b e d  J  df  =  J  dfo  rg  =  respectively.  ex  J  2kT  As  (9)  1] ( 1 Q )  f o r c o n v e n t i o n a l p-n  of the forward As  no  1 ]  [ exp (TTTT^ -  rgo  magnitude g r e a t e r than  diode  current  eV  J  nature  recombination-generation  by  qV [ P<kT> "  3 1 1 ( 1  and  [4]. b i a s curve  diodes,  J g r  Q  i-  s  (6)  f  T h i s e x p l a i n s the double f o r MIS  e  w  orders  exponential  increases r a p i d l y .  direction,  Eventually  The  f e r m i l e v e l s become unpinned and  tunnel l i m i t e d .  the  and  l o n g e r l a r g e compared to the term on the r i g h t hand s i d e of  (6).  of  diodes.  the diode b i a s i s i n c r e a s e d i n the forward  c u r r e n t g i v e n by  a  are  equation  the c u r r e n t flow becomes  T h i s phenomenon of p i n n i n g i s d e s c r i b e d i n more d e t a i l  below. A v o l t a g e V a p p l i e d to the diode v o l t a g e drop a c r o s s the i n s u l a t o r and surface p o t e n t i a l .  i s absorbed as a change i n the  a change i n i j / t h e  I f the i n s u l a t o r i s t h i c k and  semiconductor  the p i n n i n g of the  fermi  l e v e l as d e s c r i b e d above does not  occur, the s i t u a t i o n i s i d e n t i c a l to t h a t  of a c o n v e n t i o n a l MIS  A p o s i t i v e v o l t a g e V would cause ij^ to  change by  capacitor.  an amount l e s s then V r e d u c i n g  1  the e l e c t r o n c o n c e n t r a t i o n i n the  s u r f a c e r e g i o n which i n t u r n reduces the e l e c t r i c f i e l d at  the  semiconductor in  voltage  surface.  across  if  has  established  for  application  of  V  due  to  decrease, be  the  changed  surface would not  of  $s .  It the  V.  A  as  in  a i t  band  level  in  effectively  quasi-fermi  i s  effect  tends  to  an  for  the  inversion  by  reverse doped  and  clamp  becomes  at  large  in the  a  the  the  and  small  the  bias.  conventional current  consequently  ip  in  the of  the  level  V This  charage  is  not  decrease  continuing  follows  the  must  insulator.  respect  Thus  at  some  does by  . It  change to  the  fact,  level^as  is  Thus  region  with  the  to  the  that the  the  metal  voltage  diode  bias  i s  its  value  is  related  minority  in  the  zero  bias  discussion,  this  means  corresponds  p-n  junction  is  eventually  diode. limited  to  the  As  the  by  This  semi-  under  interface  to  carrier  conditions.  concentration  under  This  the  semiconductor  forward  carrier to  of  semiconductor-insulator  forward  fermi  l e v e l would  absorbed  slightly.  behavior  reverse  diodes  potential.  layer.  and  region  In  regime.  level  minority  and  across  pinned  limited  surface  quasi-fermi  capacitor.  reduction  concentration  field.  surface  a  concentration  been  electron  depletion  carrier  diode  already  effectively  interface  minority  voltage  fermi  the  electron  as  metal  fermi  edge  surface  in  the  electron  increase  summary,  metal  moderate  regions  the  in  under  layer  also  semiconductor  conductor-insulator Thus,  shown  in  to the  band  in  the  the  to  the  level  when  can be  edge  diodes the  partly  a change  pinned  the  V has  MIS  the  of  in  conventional  absorbed  pinning  in  concentration  Therefore, the  than  that  If  decrease  a reduction  s i n c e more  is  absorbed  as  conduction  a reduction  concluded  argument  conduction fermi  more  electron  as  bias.  the  be  partly  insulator  of  implies  is  thin this  than  and  quasi-fermi  potential  absorbed  bias  above  electron  possible  pinned, with  again  be  is  by  therefore  insulator  However, been  the  the  V would  under  conditions there  is  both  differently forward  tunnelling  bias through  the i n s u l a t o r r a t h e r than by d i f f u s i o n through t h e semiconductor inversion layer disappears.  The u s u a l t h e r m i o n i c e m i s s i o n c u r r e n t  ( m a j o r i t y c a r r i e r ) i s c o m p l e t e l y suppressed carriers  2.2  at the i n t e r f a c e  and t h e  as t h e r e a r e no m a j o r i t y  t o communicate w i t h t h e m e t a l [ 1 ] .  C l a s s i c a l Thermionic Emission  Theory f o r MIS C e l l s  The t h e r m i o n i c e m i s s i o n t h e o r y i s d e r i v e d from t h e assumptions t h a t (1) t h e b a r r i e r h e i g h t o f ^  i s much l a r g e r  than k T ( 2 ) e l e c t r o n  c o l l i s i o n s ^ w i t h i n the d e p l e t i o n . r e g i o n a r e n e g l e c t e d and (3) t h e e f f e c t o f image f o r c e s i s a l s o n e g l e c t e d .  Because o f t h e above assumptions t h e shape  o f the b a r r i e r p r o f i l e i s i m m a t e r i a l and t h e c u r r e n t f l o w depends s o l e l y on the b a r r i e r h e i g h t .  The c u r r e n t d e n s i t y J from t h e semiconductor s -> m  &  J  to  m e t a l i s then g i v e n by t h e s t a n d a r d t h e r m i o n i c e m i s s i o n e q u a t i o n [ 7 ] *X-f 0 ,3/2 qp(m*r -> m = — — — „ ,„ (2irkT)  f  / Z  J  /•oo  - o o  /-co  , y , dv. dv_ -°° . " -°° " v  3 / 2  J  J  J  v  exp  J  ox 2  m*(v  2  + v  x  2 + v ) z  y  "]  J  2kT  dv  " x  oo  =  q P  (  , m* 2^kT  A )  m  .00  m  v  x  *  3 C  2 v  V  exp (( ^|-)  dv  x  Jv  "ox ox 2  =  q p  e x p  W  (  " -2kT"  }  (10)  where x i s t h e d i r e c t i o n p e r p e n d i c u l a r t o the b a r r i e r , p i s t h e h o l e c o n c e n t r a t i o n and m* i s t h e h o l e e f f e c t i v e mass.  The v e l o c i t y J  v  i s the ox  minimum v e l o c i t y r e q u i r e d i n t h e x - d i r e c t i o n t o surmount t h e b a r r i e r and i s g i v e n by t h e r e l a t i o n m  i *  v 2  ox  = q (V  M  - V ) s  13.  where insulator  if  ^£>  v  V =  +  1  V  the  pv o t e n t i a l  built  in  an  The  hole  voltage  x  i s  semiconductor  zero.  The  surface  at  the  relationship  (12) 0  i s  the  bias  p  (  /_ _E F_ _  p  v N )_ .  E  change  of  potential  drop  across  the  V.  concentration  e  the  ,, s  external  = M Nv  of  s  h o l d where  under  n p  i s  s  the  V.  must insulator  V  i s  given  by  _ ( ,— 2 T_T )m * k T , 3 / 2 e x p . ( ,-  0  q—V A)  2  (13)  Substitution  of  eqns.  2  =  A*T2  qtf)  exp *  where  A*  (13)  and  B  into  (10)  yields  qv^  q^  (-  (11)  exp  -M kT'  v  ( & nkTy  (14)  2  =  a  n  (  in  i s  the  diode  ideality  factor,  h given  by n  V~  =  Since the  the  semiconductor  flowing  into  It  therefore  the  must  the  m e t a l when  corresponding  (15)  barrier  remains  the  semiconductor be  equal  thermal current  J m+s  height  =  -  A*  to  same i s the  for  the  during thus  Tl 2  exp  is  (-  unaffected  current  T  ;  J jg  i e .  from  T  from  bias, by  flowing  obtained  k  moving  external  equilibrium prevails, density  holes  the  the  from  the  (14)  by  metal  into  current  applied  when V =  Eq.  the  voltage.  semiconductor 0.  to  The  setting  V  =  0  (  1  6  )  The  total  current  J  We s h a l l  = J  p  make  following  The  of  where  eqns.  of  the  States  case of  In given  the  sum o f  in  the  and  (16)  (14)  (  and  (14)  an MIS  later  (17)  in  the  development  of  i  y  )  the  Qgc  = -  is  the  acceptor  after  s  the E  distribution of  simple,  of  inversion  surface layer  development  concentration we  and W t h e  E  -j^3x  =  as  depletion  s  e E s  boundary  yields  =  qN.x A  n  +  c  condition  at  =0  the  Effect  of  with  energy  and  semiconductor  <* N A  +  x = W  potential £SV  states  get  ,  yields  e E  = qN  = qNA  is  V =  0  at  x  (*y- - Wx) x  =  0  £ g  +  c'  = w  V. s  = qNA  (x A  2 -  Include  charge  the  S  the  to  space  charge ( 1 8 )  integration,  e  Theory  qN^W  equation =  Emission  Structure  presence  first,  , 2 9x  Therefore  eqns.  1]  Thermionic  by  3%  Applying  -  a uniform  this  From P o i s s o n ' s  which  g i v e n by  [ e x p ( ^ )  T  neglecting  is  is  section.  Surface  2.3.1  s  use  Development  2.3  density  W 2  2  W)  region  width.  Therefore  in  Q The  charge  thermal  = "  S C  in  Q  the  where level are  acceptors  having fermi  at  q D  M  B  is  the  the  surface.  of  fixed  virtually  E  =  F  to s  call  ;  W  the i t  charge  distributed  fix  =  q  N  and  fix  where  6 is  charge.  The  the  is  in  by (E_ Fss  and  are  the  the  -  v  and  the  of  we  as  the  at  and  surface,  semiconductor  quasi-fermi get,  states  level.  following  Jfche  (19).  - V  the  given  1  s  *  (21)  E  - <  , 3  8  - v >  interfacial layer  (  is  assumed  to  2  2  )  be  by  (22a)  5  insulator  effect  of  thickness  fixed  indistinguishable from  charge  change  height.  For  the  silicon  located  near  the  semiconductor-insulator  in barrier  the  the  in  and N p ^ x in  the  the  the  volume  insulator  metal  to  system,  height  of  a positive  O'leV  charge  insulator  interface  [10].  density causes  However,  as  of an  a rule  density  region  16  reduction  energy  charge  level  surface  state  dark  , _ m (20)  above w h i c h  fermi  fermi  $ ) o  defined  condition  surface  in  E  is  o  donors  metal  bias  <> j  -  relevant  t n e  = - ^ s s « B - * o - «  fixed  ss  height  f u l f i l  preceeding equation  G  q D  fr  Is  s  voltage  V  given  states  between  We m a y forward  is  barrier  the  Any  Q  <j>, ) o  i s necessary  "sc' - - "  uniformly  states  interface  surface.  argument  (19)  /  (<t> .B  ss  possible positions  Q  \  and  Under same  A  q N  interface  (jv  below which  neutrality  <2£s  = -  ss  equilibrium  10  is  barrier —2 m  effective of  thumb,  16.  the  barrier  height  of  a MIS  cell  i s approximately  a s much  as the open  circuit  2 voltage the  o f MIS c e l l  experimental The  at  work  1 0 0 mw/cm presented  electric  ,E:(x)  field  [ q N  - 1  F  .  i n chapter  across  turned  +  out  to be  the case  i n  4.  the oxide  (Qss  X -  x  This  [8].  layer  i s  Qs,)]  (  2  3  )  l From G a u s s ' s interface  '  law applied  the  semiconductor/insulator  (x = 0 ) we h a v e  - iV°> - s V £  where  to  e  E (0)  =Qss  0 )  i s obtained  G  from  (24) the following  equation  evaluated  at x = 0  qNA ;  E (x) = - - i S  s  e  Therefore  qN '  A, the potential o  A  d x ".= •  across  —— q N , . e  i'  (25) the oxide  ,2  1  E . dx = -  =  ( x - t»T) s  £  ^  (Q e.  -6  Similarly,  A'  f° J  '  E.  dx,=  l  under  1  -  V. i (19),  bias  A  —  + Q  sc  =  —  e.  [ - Hq ^  i s  (S  2  1  = A' (20),  -  A  = ~ e  '  '  (Q + Q ) ^ sc ss (27)  1  [(Q'  i  s  s  -  Q s  s  )  +  (Q  ss T ) .  conveniently  <b "B  Fss  -  iB  where  -  -  (E_ • Fss  expressed  c  -  Q  s  c  )]  (28)  E )) v  E  ) v  +  l  i  ( 2 e q N . ) 2 {V, * -(V, s A bt bt  ranges °  f r o m +V. l  to -V  -  .  -  V  s  ) } 2  (29)  [10] and can be s  as  (E:,, - E ) ) Fss v  a +  s  ( 2 2 ) we g e t  (21),  (fc - ( E _  Where  )  ss  j..  V. i  equilibrium),  (26)  q N . . - — + — f i x 2 £.  e.  -S  From  thermal  1  the p o t e n t i a l  =  (at  j.  § - +  -i  i s  B = 1  = a qn V . l  and  B q V  s  0 <  . ^ (30)  a  < 1  •'•  V  q  i =F7 ^ s s  a ( V  1  i  +  +  V  q 2 D  V  ss s  +  ( V  ^ V V H t *  V  - bt" s)"  }  (31) Here; a  on how  can be conveniently used as the parameter which depends a = 1 implies that the effective  , changes xvith applied voltages.  Fs-s  fermi surface follows the metal fermi surface and a = 0 implies that the effective fermi surface follows the semiconductor fermi surface . If-  5  •  V. = — x e . x  (.qD  = — - e.  (qD  After  D  ss i s i n  V. ss x  ss  (1  -  eV  —1  cm—2 we get  qD a(V. + V ) • ss x s  a)V  arranging  s  - -q'D  ss  terms,  +  (2e  aV. + x  (2e  this  qN ) s^ A  s  J..  2  qn N . ) 2 " A  i  {V, bt  2  {V, } bt  -  (V, _ bt  V  -  (V, . bt  V  -  i  s  )2}  )*}  s  yields  x (1  +  r  a ^-qD )n e. • ss x  r  = 1 + —— e. i  D  r  % V  Y = 1 + a- —  Let  e. x  A = 1 + — e. x  and  B = —— i  Therefore  ^  Y  Finally  (2E q N . )  + ^ ss e. x  a quadratic  A, 2V2  +, (. y 2 V„ 2 -  =  n  A  qD  ,  ^D  n  A  ^  ,"  1  D  C  „  - (V  bt  -  ,  n  ss  ss  (2esqNA)i  +  - ~v  i  v  "  ?[V  bt  equation  for  A  <  v  b t - ^  (  = - r  n emerges  „Y „ T , 2V „ v 2 +, (/ B 2 B TT V b t 2i„ V)n  v  b  t  - h ?  namely  „2 .A y V„ 2 +  2ABVb1_2V)n>=  0 (30.)  The  solution  -(2ABV, n  :  •.  to  (31)  *V + B V  -  2  i s  2AYV2)  +  A***\t  V  ^  +  '  ^AyV  ^  2  ) -4A 2  2  ? V  (  Y  2  V  -2YBV ^0  2  b  : 2(  Y  2  V  -  2  2YBV  b t  i  V)  (31) In was  found  estimating n  that  D  to  s h o u l d be  ss  f i t  the  around  experimental  10  13  data  - 1 - 2 cm ,  eV  (see  <jk, a r o u n d (B  chapter 0'8  4),  it  eV,  o  6 -  20 A  and  generated 6 and  performed  the on  are  In  are  given  to  certainly  be  therefore  and  in  view  inversion  Fig.  3-6.  apparent  from  lines  with,  a double  surface  TI  for  to  include  more  were  a non-uniform  presence  of  an i n v e r s i o n  the  as  a  first  surface  general zero  in  step  in  potential,  situation the  bulk  the  the  a non-uniform of  see  <> | ,  a  and  The surface model,  beyond  state the  slope.  postulated  this  theory  were  the  single was  were  Dgs>  Calculations  curves  set,  used,  this  curves  c a l c u l a t o r and  £n I - V  [7].  I - V  parameters  characteristic i t  s h o u l d be  nature  In  above.  parameter  practice  considered,  of  of  quoted  that  a given  in  complete  also  3-6  density  refined  ranges  various  59 p r o g r a m m a b l e  exponential  state  test  values  Figs.  obtain  the  for  the  case  The  defined  of  to  of  As between  ( 3 1 ) a n d (17)  expected  layer  The  a sensitivity  Instrument  in  obtain  was  [7].  is  a variable  2.3.2  As  neighbourhood  straight  order  that  0'23.  a Texas  It O'lV  -  using equations  a in  results  a  would  just  described  distribution  effects  of  an  below.  distribution  of  surface  states  with  the  layer  this  more  space  detailed  charge  i s  depicted  of  the  in  and Fig.  semiconductor  analysis,  the  electric  field  7,  the  where  and  relations are  derived  potential  i s measured  with  is  Fig.3.  S e n s i t i v i t y c u r v e (1) f o r the model of s e c t i o n 2.3.1.  Fig.4.  S e n s i t i v i t y c u r v e (2)  f o r the model of s e c t i o n 2.3.1.  Fig.5.  S e n s i t i v i t y curve (3) f o r t h e model o f s e c t i o n 2.3.1.  Fig.6.  Sensitivity curve (4) for the model of section 2.3.1.  23.  Fig.7.  Band Diagram a t t h e S u r f a c e o f P-Type S e m i c o n d u c t o r from SZE  [7]  respect  to  the i n t r i n s i c  a n d i|| , i s  if; = $  s  concentrations  called  Fermi  l e v e l E^.  the surface  as f u n c t i o n s  of  At  the semiconductor  potential.  are g i v e n  by  The e l e c t r o n the following  surface  and  hole  relationships  [7]. p '  =  Pp  = P  n  7),  a  respectively the  n  s  t  ^  equilibrium densities  i e  of  the semiconductor  p o  potential  In  \j> a s a f u n c t i o n  At  the  surface  of  equation  distance  can be o b t a i n e d  by  -  _, P. ( x )  2  E  s e  g  i s the p e r m i t t i v i t y density  given  are the density  must +  D  general  p  .  •  =  charge  N  =  and holes  (36)  i n the bulk  neutrality  a n d B'  electrons  i n  exp(-B'^s)  where  and N  of  ( a s shown  = p  '2  r. ox  space  downward  (35)  t h e one d i m e n s i o n a l P o i s s o n 1  i s bent  exp(B*4.s)  6 j>  Now,  e  the band  (34)  ='npo  The  total  r  when  e x p (-8 ifi)  are  Ps  using  a  Q  i n the bulk  densities n  exp(-qip/kT) = P  p o  Pp  (33)  •po  J IJ i s p o s i t i v e d  exp(g  n  po  where Fig.  •exp(q>/kT) =  n  of  NA  ionized  = n  Therefore po  - p  .-, po-.  v  v  the semiconductor  = q(N^+ -  donors far  + p^ -  and acceptors  from  the surface,  and p(x) n^)  i s  where  respectively. charge  P,(x) = 0 a n d IJJ=0 a n d w e h a v e  • :po •  f o r any v a l u e  p  by p(x)  the semiconductor,  exist. -  of  of  of  w  Y /  ij; w e h a v e  from  equations(33)  and  (34)  (37)  the  The  resultant  Polsson's  f | =  Integration  [p-  _3_.  6x^  e  of  F  s  giving  E  the  2  =  ( e  po  Eq.(38)  Sx  thus  equation  v  "6>  from  2kT}2  6xy  (  J^o 2  be  1)  solved  -  bulk  n  [  the  is  ;<e '* B  pov  P p o  the  ( e -  e  >  e l e c t r i c f i e l d E.  [ ( e - P ^  +  therefore  1)]  -  (38)  towards  e  )  £  -  the  r e l a t i o n between  (  to  surface  -  l ) - P  and  yields  p  o  ( e ^  -  potential  l)]d*  as  B ^ - D + ' . ^ C e ^ - B * - ! ) ]  s  po (39)  Let  2kTe  s / 5 qp. po  po  (40)  and  F(B>,  £  [ ( e ^ ' *  +  B'¥  -  1)  + ^  (  B ' * _  e  g  >  _  >  o  po  po  (41) where  L^  electric  is  c a l l e d the  field  =  ±  2 k T  6x  determine  ,  for  Thus  holes,  the  sign  P *po  for  \p < 0 a n d  electric field  ±  j o  qL„ H D  positive the  length  becomes  E = - f  with  e x t r i n s i c Debye  ^ q L  D  at  the  F ( 3 ' V ^ S  P  po  (42)  negative  surface,  we  sign let  for  ij; > 0 .  ip = i p  g  and  To thus  (43)  26.  Similarly, to  produce  by  this  Gauss'  field  law  the  space  charge  sc  s  unit  area  required  is 2e k T  x  per  s  n  qL^ D  v  '  y  p r  po (44)  As  i t  becomes  completely  g e n e r a l way,  experimental this  point  cumbersome  to  some n u m e r i c a l  A£/SiOx/pSi  devices  proceed with parameters  described  in  the  treatment  relevant  chapter  to  4 are  in  a  the  inserted  at  namely n . l  e  s  , , = 1* 6 x  i r  =  1*5  10  =  1-04  T = giving  at  x  1*50  room  Q  .10 10  cm  15  x  10  x  10  cm  -3 -3  -12  [13]  (taken from sample)  the  resistivity  of  chosen  farad/cm  300°K  =  As  rather  ^  cm  temperature  =  f— qLn  sc  = p^o  3  [(e  '  +  V b t :  it  follows  B'V^  -  1)  +  from  eqn.(44)  1*138  x  10  10  that  (ep,t>t_g'v, bt  and  i , (45a)  f  7F Q*  sc  = -  [(e-  B  b  '  ( V  b  Notice square  brackets  about  charge.  c  kT  - f q  bt  "  -  V  -^'(V,,  is  -1)]*  bt  5% o f  that in  the  Therefore  t  )  (V. bt  (45b),  this  +  t  e'(V  K  -  V  s  )-l)+1.38  x  lO^V'  C V  bt" s V  (45b) -  which  V  s  )  is  = 0-6V  corresponds  term which error  )  -1)]*  s  if  first  V  the to  corresponds  present  in  second the to  term  inside  inversion the  previous  the  layer  depletion  treatments  charge, layer  [10]  which  27.  have  neglected  the  The are  given  effect  surface  =  ss  -  inversion  charge,  layer  charge.  for  the  case  |  D  dE  of  zero  and p o s i t i v e  D  dE  ss  J (J)0  = -q  T B  I  ss  cj>o  * B -q3V +, q a V .  ^/qa(V.+Vs)-qV  g  Q  The  ' s s  -  "  J' ' <j>0  q  difference  i s  S S  s s  D  -  Q  ss  where  •  d E  "  <> j  =  +  qa (V. i  +  V  +  1  V  =  s  D  )  ss  =  dE  -q  *-<b o  D  ss  dE"  -qV  s  -q  D  ss  V.  '  q  thus d> T B  Q'  bias  by  " ^ s s  q  the  states  respectively  Q  of  ss  n  s  dE  (46)  V  - I  V s  n (1 - -^")V,  =  n  > 1 is  the  diode  ideality  factor  (47) The  required  follows  from  equation  equation  linking n  and V  turns  out  to  be  non  linear  and  (28) i . e . V  0 = _ lH=ii n  V  + (1-138  q L  +i - [-q e. l  x  D  ^  10" )  x  "  3'(V  10  n  q a V _ q  V  2e  kT  dE .+ — f — ss qLD  (e6'Vbt  -  3'v^-l)]  t(e  B  '  V  b  t  +  B'/V,  bt  -1)  2  D  (eB'<Vbt  n  }  -  b t  -  J)-!)] ] 1  (48)  s  28.  where  V, bt  =  <j>„ B  equations  were  solved  UBC  zero  1  files  among  unequal As  space  the  inclusion  of  values  data  the  = 0*23  n  IBM  points the  370  D  .  in  I-V  layer  The  in  the  present  case.  The  above  at  UBC  and  employing  0INT4?  for  d i g i t a l computer equations  (program  computer  in  inversion  of  V  solving nonlinear  difference the  and  n  using  a check on  comparing  uniform  for  V  shown  and  in  computation  appendix and  for  integrating  1).  the  sake  c h a r a c t e r i s t i c s o c c a s i o n e d by term  results  several  are  curves  shown  in  were  fig.  of the  generated  8 and  9 and  using  should  be  s s compared w i t h  figure  Notice that be  in  the  by  to  of  arrive The  at  single  the  with  holes  the  i s  profiles from  a profile  are  given  linear  the  holes  S^e  that I-V in  [7]  however  to  of  would curves  Fig.11. no  voltage  graphically  and  in  extract  justifying  the  fit  was  10.  the  In  longer these two  inversion  have  to  than  This  potential  solved  The  I-V  to  can  layer,  the  therefore  overcome  cases,  see  the  a  the  higher  various  profile  data  In  "douple  shown were  n  surface  i s  12.  n values  a  It  in  made  section  distinctly  fact  Fig.  (see  above  show  representative  description  the  modifications  curves  obtains.  for  starting  experimental  corresponding The  given  subsequent  the  smaller  current.  (48)  Fig.  is  considered. the  a  equation in  was  for  conduction  shown  relationship  function  so  f u l l  layer  current  higher  the  the  the  i n c l u s i o n of  to  taken  t a b l e 1,  that  depletion  contribute  varying  see  the  potential,  smaller because  t h e o r e c t i c a l In  profiles  only  given  is  states  F i g . 1 0 was  a  current  Finally, surface  for  noting  barrier  corresponding barrier  that  c a s e when  explained  potential  3-6.  to  4*1).  state that  a  'rapidly is  for  exponential"  possible each  curve,  curve.  29.  Fig.8.  S e n s i t i v i t y curve (5) f o r the model o f s e c t i o n 2.3.2 of u n i f o r m d i s t r i b u t i o n of s u r f a c e  states  f o r the case  Fig.9.  _ l  I  I  ./  z  -3  I  1— 'S  1—— '6  S e n s i t i v i t y c u r v e (6) f o r the model o f s e c t i o n 2.3.2 u n i f o r m d i s t r i b u t i o n of s u r f a c e  states  V  f o r the case of  Fig.11.  T h e o r e t i c a l Curves f o r V a r i o u s S u r f a c e S t a t e P r o f i l e s  33.  * b c  J  PROPUB PROP t LB PtoPiLB  *Z #  3  * 3  (MODIPIBD)  EXPERIMENTAL  H. pxof=iLE> * ±  A  I  I  I  * Fig.12.  I  I  *  «  1 ^\  £  I  »5  Diode factor (n) vs. voltage plot using data of TABLE 1  —  \/  34.  TABLE  1:  C O M P A R I S O N OF  V  PROFILE  THEORETICAL  #1  AND E X P E R I M E N T A L  #2.  #3  n  VALUES  #3 m o d i f i e d  exp  0.5  1.845  1.938  1.873  1.777  0.10  1.799  1.906  1.849  1.753  1.756  0.15  1.759  1.869  1.892  1.801  1.753  0.20  1.723  1.839  1.824  1.753  1.753  0.25  1.684  1.811  1.772  1.716  1.745  0.30  1.652  1.786  1.731  1.687  1.745  0.35  1.621  1.767  1.703  1.666  1.722  0.40  1.592  1.748  1.672  1.637  1.67  0.45  1.564  1.727  1.636  1.606  1.625  0.50  1.540  1.698  1.610  1.585  1.585  = 1.90  n =  =  n  REPRESENTATIVE  n  ,  n  2  Each 15"  x 12"  points. 150  = 1.80  n  = 1.50  n  of  2  the surface  diagram. There were  points  ±  f o r each  1.70  state  An e l e c t r o n i c 10 p o i n t s profile.  2  1.85 =1.62  profiles  digitizer  per inch.  n  2  = 1.75  n  = 1.60  n  was o r i g i n a l l y was used  Therefore  drawn  to generate  altogether  =1.75 =  2  1.62  on a data  there  were  CHAPTER 3  EXPERIMENTAL  3.1  Sample  Preparation  The slices  respectively.  nominal  order  and r e s i s t i v i t y  determined  of  The c i r c u l a r  resistivity  to be able  to  6 < 100 > o r i e n t a t i o n  using a micrometer  The b o o k k e e p i n g ' i n d e c i e s  P33,P34,P35,P36. of  [9]  thickness  was f i r s t  PROCEDURES  slices  of  these  were  of  2 - 8 ohm c m , a n d w e r e  grow  oxides  of  and a four  point  six slices  1.990"to  p-type probe  were  2.010"  thickness  P31,P32,  diameter  p o l i s h e d on the front  the required  S i  f o r MIS  side.  In  cells  o  (< 3 0 A ) , The  the s i l i c o n  surface  cleaning procedure  TABLE  2:  (i)  TEST  used i s d e s c r i b e d  below.  310  ym  7.43 Q-cm  P32  291  ym  6.26 Q-cm  P33  315 y m  7.57  ft-cm  P34  313 ym  7.50  ft-cm  P35  325 y m  7.35  fi-cm  P36  305 y m  7.42  fi-cm  were  loaded  The volume  with  the beaker.  into  400 m l . o f of water  60 m  l . of  a teflon  RESISTIVITY  cleaning basket.  clean D.I.  was reduced  NH.0H  and  water to  then  to  the water.  completely  Keeping  immersed  into  heated  to  60 m l .  of  were  H 0„ o  the  some  then  2 2  the temperature the solution  were  Two  360 m l . b y p o u r i n g  4 added  oxidation.  P31  boiling  out of  to  THICKNESS  filled  i t  cleaned prior  SAMPLE  500 m l . b e a k e r s  of  be thoroughly  SAMPLE PARAMETERS  The w a f e r s  point.  must  at  75 -  80°C,  f o r 10 m i n u t e s .  the slices This  were  procedure  was  designed  solvating  to  remove  action  of  hydrogen  peroxide.  I  II  metals  (ii)  The  and  The  organic  ammonium The  such  Cu,  of  Ni,  slices  was  A nalgene  beaker  slices  were  in  (iv)  The  basket  was  this  of  Co  the  serves  D.I.  by  both  oxidizing to  the  action  complex  water  ml.  450  30  again  attacked  some  of  group  Cd.  with for  are  powerful  also  and  filled  was  which  rinsed in  solution  slices  and  hydroxide  Ag,  (iii)  kept  hydroxide  ammonium  as  basket  contaminants  of  for  DI  minutes,  10  water  and  ml.HF.  50  seconds.  rinsed  in  D.I.  water  for  10  minutes. (v)  When  the  water  in  this  beaker  of  ^02  were  75 - 80°C, for  forming  added  the  second beaker  reduced to  the  slices  to  The  slices  were  again  rinsed in  (vii)  The  slices  were  blown  dry  s i d e was  employed was (i)  fabrication  made as  to  the  the  temperature  completely designed replating  the  D.I.  to  ohmic  in  for  +  the  the  10  ml.  60 at  solution heavy solution  minutes, gas.  diffusion  contact.  water  ions.  nitrogen  a p  and  remove  from  resulting water  HGl  hot  Keeping  cells,  an  the  of  a stream of  solar  f a c i l i t a t e making  follows  oxidation:-  of  in  boil,  ml.  was  with  to  60  displacement  complexes  began  ml.;  immersed  procedure  prevent  soluble  360  water.  were  This  to  (vi)  Before back  and  the  was  minutes.  10  metals by  in  The  in  the  procedure  [9]. The  target  of  the  thickness  for  the  masking  oxidation  o  was  6000A.  The  furnace  set  so  the  centre  gas  flow  that  rates  used  temperature zone  were  p r o f i l e was  temperature  was  1100  established ±  5°C.  and  The  1  N  2  litre/min  1.6  litre/min  1.0  litre/min  50  and  the  which  gas  has  cycle  cc/min  was  5 -  5 -  2Hrs  the  following  5 minutes initiate  the  passivate  heat  for  up  for  the  thin  first  was  to  was  used  for The  states  and  "plowed the  of  due  side  before  the  whole  after  batch  chromic by  the  acid wax  to  oxide  against  layer  of  oxide  and  to  0^ + HCil  to  enhance  contamination  for used  to  was  rapid  any  Hclipitting  polished  10  the  to  HC1  clean  the  silicon  "wet"  and  to  oxidation The  from  side)  of  into  of  10  fkom the  After the  the  a  two  the  HF  that  to system.  the  usage  5 minutes  and  was  reduce  of of  may  each. the  trichlorethylene  of  then  procedure,  to  cool  skillfully  for  15  waxed  minutes.  dry  in  nitrogen,  boiling  After dirt  have  allowed  and b l o w i n g  beakers  surface  oxidation  solution  water  slice  to  above  slice  75%  minutes the  and  next  S1O2,  furnace  each  i n D.I. into  was  HC1.  densify  out  dipped  by  the  p o s s i b l e phosphorous  minutes  clean  removed  used  dipped  periods to  purge  of  by.sodium.  oxidation.  pulled  b a t c h was  and not  thin  usage  sequentially  was  a  The  was  (the  trichlomathylene  grow  5% H G l y  rinsing for  was  and  reasons  slices  One  the  0^  redistribute  down.  Then,  to  ©2  final„N2.step to  30  meaning::  from  S1O2  safety  under"  batch  +  keep  5 -  cycle.  5 minutes  2 hrs HC1  -  blowing  w h i c h was  treatment.  dry, contributed  After  rinsing  in  D.I.  water,  cleaning  procedure  carrying  out  the  The  p  predeposition  p  the  Boron  below With  carried  back  +  The mentioned  side  the  then  beginning  put  of  through  section  the  3.1,  complete  prior  to  diffusion.  c o n s i s t e d of was  two  the  parts,  drive-in  the  first  part  was  the  step.  for  loading  that  the  following  the  slice  doping  was  time was  predoped  for  prolonged  c h a r a c t e r i s t i c s , a 10  -  5 -  to 15  the  cycle  15-20  minutes.  cycle  was  out:temperature  1090  (source):-  ^2^2  Source  temperature  :-  flows  :-  Coarse N£  Besides half  :-  Dopant  Gas  junction  at  was  Predeposition  boat  the  batch  second part  except  Furnace  (four  whole  described  diffusion  +  and  (a)  the  slices)  depth  were  s  m  e  t  n  5°C y l  alcohol plus  some  HC1  15°C or  carrier ^  2 litres/min,  Fine  or  source  c.c./min  main b a t c h loaded  measuring.  tabulated, in .table  The  the  60  ^  ±  of  into  The  slices the  four  boat  test-pieces  for  c h a r a c t e r i s t i c s of  sheet the  of  N  type  resistance, test  slices  and were  as  3.  c y c l e s were  1 0 - 5 - 1 5  Explanation  10  minutes  of  coarse  5  minutes  of  souce  15  minutes  of  course  Function  N,. +  coarse N,  Heat N,.  up  Doping purge  part part  dopant  diffusion  of of  cycle the  plus  cycle slight  TABLE  CHARACTERISTICS  OF  TEST  SAMPLE  THICKNESS  n-type  Vim  SLICES  3  U S E D TO  INVESTIGATE  RESISTIVITY  RESISTIVITY  BEFORE DOPING  Bl  300  0.0131  B2  303  0.0131  BORON  JUNCTION  AFTER ft-cm  DOPING  Not  DEPTH  Q-cm  ym  measured  0.0025  DIFFUSION  8.4  Not  measured  (p-type)  B3  303  0.0129  B4  301  0.0128  Not  8.8  measured  0.0026  Not  (p-type)  (b)  Drive  i n  The  step.:-  diffusion  conditions  were:-  Furnace  temperature:-  1090 ± 1 ° C  Gases  0£  1*5  litre/min  N_  1*5  litre/min  measured  Cycle 2i  hr  2i  hr  for  dry  oxygen,.^  -  i  hr  hr.  nitrogen  (purge  and  surface  state  reduction). After and  cooled  these  down  procedures,  then  stored  As. r e q u i r e d immersed away.  in  a  After  quarters  by  slices was  into  and  the  specially  for  15  for  removed minutes  dry,  final  the  the  normal  solar  cell  under  7  then  x  10  deposited  ^ mm H g  designed planetary  to  vacuum  the  strip was  were  the  on  unpolished  a Veeco  fixture.  The  thickness  monitor.  container  cut  to  into  the  and  oxide four  storage  each quarter handling  slice  quarter  page,35 side  vacuum  t h i c k n e s s was  furnance  protecting  returned  for  the  boxes.  storage  fabrication  the  from  carefully  mentioned  in  out  diameter  suitable  as on  1"  from  slice  slices  cleaning procedure was  pulled  plastic  s p e c i a l l y designed basket  Gold  slice  were  s c r i b e r and  needed a  performed.  quarter  solution  r i n s i n g and b l o w i n g  When  loaded  HF  b a t c h was  Fluroware  the wafers  a diamond  container. was  75%  in  the  of  the  system using  determined  to  a  be  o  2000A,  using  an>inficon  The procedure The  as  next the  sample was  oxygen  flow  thickness changed After  of  to  this  allowed  of  to  appearance successful.  step  very  thin  for  lm  alloy  a  furnace  times  of  i n t e r f a c i a l oxide (1*0  down i n  the  gold  in  the  gold  i n t e r f a c i a l oxide  1*0  l m ^)  sample was  cool  to  into  nitrogen  of  was  inserted  the  the  321  ;  and  took  at  to  minutes,  the  end  stream.  a mottled  also  495°C  The  a l l o y i n g was  nitrogen on  2-10  was  required.  withdrawn  the  held  f i l m and was grown and  The  character  the  this  subject  for  furnace  previously if  critical  on  to  the  20  time. an  the  environment  continued of  at  depending gas  a  was  then  minutes.  tube  and  bright  a l l o y i n g had  been  The this  was  mask  (shown  four  circular  next  processing step  a c h i e v e d by  thermal  was  the  evaporation  barrier metal  of  aluminum - -_ ^  in  Fig.  13)  in  a vacuum  of  deposition  through  -  a  and  metal  8 :x 1 0  mmHs.  The mask  each  a very  thin  had  2 holes  with  area 0.08;cm  and  transparent  o  aluminum  film  (  -  100A  )  was  d e p o s i t e d on  the  o  Slice to  at  a rate  optimize  The  first  of  the  lOA/sec.  trade-off  mask was  This  between  r e p l a c e d by  low  evaporation  transparency  rate  and  a s e c o n d mask w h i c h  and  sheet  thickness  resistance  defined  the  was  [8].  area  for  a  o  contact the  stripe  contact  3.2  The  the  each  I-V  order  to  measure  a special  aluminum e n c l o s u r e was fitting  lidr,  characteristics. removed lamps The  JEA a  and  the  (colour  450B  the  painted  the  operational 5 mA  (which was  irradiation)  to  s u p p l i e d to  on  the  used  for  large  This  enclosure the  circuit  the  inside  and  c o u l d be  to  permit  the  taking  to  from  f i l t e r e d by (AMI and  -4000B  to  ELH  a wide  was  from  EG the  also  could only the  by  dark  I-V  of  l i d  hot  was halogen  mirror.  intensity)  used  to  cell.  was As  the  about  under at  a  calibrate  deliver  operation  by  measured by  which  photodiode  allow  sealed  tungsten  band  a n d G) test  The  the  i n t e n s i t y was  alongside  supply  four  sunlight  the  instument  was b u i l t  14.  simulated sunlight  photosenser  enough  characteristics  see F i g .  light  lamp  ( m o d e l HAV  inside  a booster  under  and i l l u m i n a t e d I-V built,  lOOmWcm  meter.  measurement  not  black  = 3200°K)  adjusted  amplifer  dark  s t a t i o n was  i l l u m i n a t e d by  temperature  o p t i c a l power  in  1000A was  Station  the  measurements  sample  power  both  testing  commercial photosensor  mounted  aluminium thickness of  also painted black,  For  i r r a d i a n c e was  varying  An  Characteristic Testing  diodes  a tight  diode.  fingers.  In of  to  1  100 Sun  2 mW/cm  42  I'XStm  \t—  Fig.13.  1-xS cm  Masks f o r Aluminum B a r r i e r M e t a l and C o n t a c t F i n g e r E v a p o r a t i o n s  43.  AtfntteJ frit** I T  V  Fig.14.  I-V T e s t S t a t i o n  J  44.  "intensity, cell,  the  sensor moved  so  vacuum  sockets the  current  The  drop  the  is  in  to  electrometer  current  is  measuring  the  resistance current  was  good  with  sensor  the  supported contact  was  and  gold  plated  the  back  side,  enclosure  avoid  the  Fig.l7| The  load  voltage  point  across  current  for is  the  from  was  c a l c u l a t e d and was  the  diode.  A  ohm r e s i s t a n c e w a s  1  reason  nearly  for  short  resistance  The  choosing circuit  shorted  block,  the  BNC  dark  connected  taken  into  a  low  to  cell  after  potential account  measuring  added  was  voltmeter  The  for  a  coaxial  the  the  the  contact  measuring  is  was  using  test  the  value  c o n d i t i o n s when out.  top  diode.  such  mounted  the  that  circuit  commercial  connect  diode  the  solar  o c c u p i e d by  brass and  to  the  was  Standard  served  circuit  important  drawing  across  The  using cell  a  the  The  set  previously  of  equipment.  experimental  test  position  on  to  each  wall  ensure the  intensity  the  assumed  of  a micropositioner.  current. to  testing  to  electrometer  in  sunlight  Fig.16.  voltage  the  fixed  measure to  shown the  cell  measuring  used  the  the  to  on w h i c h  w a f e r was  on  shown  across  test  probe  mounted outside  base  ensure  a gold  obtaining  the  the  to  Prior  simulated  test  chuck  w h i c h was the  then  that  made v i a  Fig.15.  desired  and  sensor.  to  see  the  circuit of  measuring  for light for  .15.  Booster C i r c u i t f o r Reference P h o t o c e l l  46.  SAM  Ej Vi  Fig.16.  Dark I-V M e a s u r i n g C i r c u i t  EL EC TRO  METER  VOLTMETER  plS  LIGHT  I-V  MEASURING  CIRCUIT  M  : MICROVOLT  V t  LIGHT B I A S  VOL  METER.  TMETER  I-V M E A S U R I N G  CIRCUIT  SAMPLE  M  :  V :  Fig.17.  Ml  CRO  VOLTMETER  VOLTMETER  L i g h t and L i g h t B i a s I-V M e a s u r i n g C i r c u i t  48.  CHAPTER 4  RESULTS  4.1  Dark  I - V Curves  The cells  shown  1.4  to  shown  limiting  =  ^ 1  effects  e  However,  p  4 ..  that  for  4.2  the "5 minute"  high  bias  e  x  p  5 5 0 mV,  beyond  bend  seems  down  thick  the solar  toward  oxide  minutes  from  T h e IQ-^> •'"02'  cell with  the voltage  thinner  to be c o n t r a d i c t o r y  that  different curves  tunnel  (5 m i n .  bv "allowing  Again-this  ranged  resistance or  should have  i n these  10,5,2,0  equation  oxide, c e l l s  been w i d e l y  'diodes.'  curves  i n the  series  cell with  400mV  of  eV n^k?  and the curves  then  times  respectively.  the terms  02  the solar  effect  formation  solar  f o r t h e two " c e l l s  was n o t v e r y  oxidation oxide to  smaller  the surface  axis.  a  recent  dark.current).  state [6],  conspicuous  The .except  could be explained by the surface  effect.  Characteristics  The with  Beyond  could be explained  exponential  Light  X  the t h i c k e r  d i s t r i b u t i o n s - may h a v e "double  +  current  time)  (in which  this  eV n^kf  dark  oxidation  t h e o r y [12]  x  c h a r a c t e r i s t i c s o f MIS  factors, f o r these  correspond"to  dominate  effect  has larger  (2 m i n s  which  i n Table  curious  time)  The diode  1 . 9 f o r low and r e l a t i v e l y  1  The  current-voltage  i n t e r f a c i a l oxide  i n F i g . 18.  and n^ v a l u e s  are  forward  f o r devices with  are  n^  dark  I-V  and w i t h o u t  characteristics of electrical  bias  the 4 diodes  a r e shown  under  a F i g . 19  illumination  and Fig.,-20  both  respectively,.'  Fig.18.  E x p e r i m e n t a l Dark I-V Curves  51.  Fig.20.  E x p e r i m e n t a l L i g h t B i a s I-V  Curves  52.  TABLE 4 SUMMARY OF EXPERIMENTAL DIODE DATA SHOWN IN FIG.  10~9  18  10"9  REF#  OT(min)  I  P33(4)b  0  10.21  1.69  1.84  1.41  P33(3)a  2  6.19  1.69  1.88  1.51  P33(l)a  5  3.76  0.76  1.80  1.41  P33(2)b  10  7.56  7.56  1.64  1.64  Q1  x  AMP  I  Q  x  2  Hi  TABEE 5 SUMMARY OF EXPERIMENTAL DIODE DATA SHOWN IN FIGS. 1 9 AND 2 0  REF#  OT(mln)  Voc (VOLTAGE)  I C sc  ^ cm  2  )  FF_  P33(4)b  0  0.476  15.3  0.72  5%  P33(3)a  2  0.480  15.0  0.74  5.2%  P33(l)a  5  0.486  15.5  0.69  5%  P33(2)b  10  0.462  11.8  0.25  1.2%  OT = OXIDATION TIME  53.  One is  very  factor  may  sensitive is  (around  very  0.72)  observe to  the  poor for  that  oxidation  0.2).  the  the  photovoltaic time.  However  diodes  For  very  performance  the  good  u l t i l i z i n g 5,2,0  "10  f i l l  of  the  minute" factors  minutes  diodes  diode, were  oxidation  the  f i l l  observed  times.  The  2 efficiencies  of  the  latter  cells  was  around  5% u n d e r  100  mW/cm  illumination  2 with  a  short  rather Al  the not  vary  this  process  4.3  in  After  days this  of  can be  can  mA/cm  are  c o a t i n g was  extraction  is  at  such  (see  likely  explained  occur  insignificant.  I-V  to  used.  the  in  due  Table  One  "5,2,0  terms  a "rate  of  that  Thus  a l l  the  obtained  and  changes can  the  The  fact  that  the  devices  following. resistance  current'from  only  the  also-observe  may  minutes"  the  available  5).  occur  the  with  Characteristics  completed  solar  and  the  I-V  time  the  nitrogen  the  samples were  was  taken  in  AR  17  characteristics for  This  devices  current  and no  light  much.  is  for  Stability  12  circuit  around  illumination.  The for  in  these  semiconductor change  short  density  not*optimal  very  for  current  of  difference  Tunneling via  values  t h i c k n e s s was  that did  low  circuit  79  days  F i g s . " 21-28 .  were  stored  c h a r a c t e r i s t i c s were  maintained after  cells  gas in  supply  the  was  covered  p r o c e s s i n g of  the  in  a nitrogen  determined allowed  to  enclosure. solar  cells.  at  environment  various  become A  gas  intervals.  exhausted  further Results  but  measurement are  shown  £ 3 5 ( 3 ) 0 .  DARK FORWARD  1-V  (2mm.o ) M  54.  AKEA  Fig.22.  scot,****  Experimental S t a b i l i t y Data (2)  uJ  I  58.  P 5 3 Cl) ex. DARK FOR WART> Xr.V (S rnit>. O ) x  P33 AKBA  Fig.26.  0)*  UGrHT *O-O6CM  Experimental S t a b i l i t y Data (6)  X-V  (5  *»*,. O*)  X  to  j>33  (2)  h_ P*Rk  I - V  FORWARD  60.  ('° *»'»• o ) x  LNX Amf>  -ZO  I  ,  1  1  1  1  *  a./  0.2  o-%  O'V  o-S  Fig.27.  Experimental S t a b i l i t y Data  (7)  r  1  6'6  °'7  V  CHAPTER 5  DISCUSSION  5.1  Comparison  As density were  purpose the  P33(l)  (most  theoretical solar  curve  a t t e m p t w a s made  for  surface  of  were  to  over  generate  state  together 3  curve  beyond  a r e (1)  was around  10  -1 eV  5 0 mV -  carrier  the  from  diode  It  The  can be  the  5 5 0 mV.  No  5 5 0 mV b e c a u s e w i l l  be  the e f f e c t i v e  -2 cm near  For  of  fits  limiting effects  o c c u p a t i o n was t h e m a j o r i t y  density  those  (modified)  a theoretical  curves  profiles  the results  i n Fig.29.  of  theoretical  These  was 5 m i n u t e s .  range  and t u n n e l  state  in Fig.11.  data,  time  surface [7].  namely  formation  13 surface  curves  the bias  resistance  these  state  well  I-V  to profile  of  i n ref.  taken,  are brought  corresponding very  5 5 0 mV s e r i e s  features  cell  Characteristics  profiles  given In  I-V  and experimental  the i n t e r f a c i a l oxide  the curve  The  2.3.2 four  on t h e c u r v e  and p r a c t i c a l data  experimental  after  based  efficient)  for which  that  and P r a c t i c a l Dark  10 a n d t h e r e s u l t i n g  comparing  theoretical seen  generated  in Fig.  of  best  Theoretical  described i n section  were  shown  of  fermi  fermi  level;  the centre  dominant.  of  level  (2)  the  the  band  o  gap; was  (3)  the oxide  0.82eV.  The r e a s o n s (1)  of  the metal  interfacial metal that  Actually  layer  the metal.  difficult,  for choosing the choice  and the semiconductor  i s extremely of  t h i c k n e s s was assumed  [14].  If  the effective  these  values  was dependent  then  fermi  l e v e l would  Fermi  between  hand tend  the b a r r i e r  fermi  i f to  l e v e l between  interface  tunneling  were  the  of  states  l e v e l would  follow  height  below:-  on the p r o p e r t i e s  the effective  on t h e o t h e r  (4)  are discussed  the effective  the communication  efficient, However,  of  to be 20A;  that  the and  follow extremely  semiconductor  63.  Fig.29.  C o m p a r i s o n o f T h e o r e t i c a l and E x p e r i m e n t a l D a r k I-V D a t a  Fermi  level.  thesis, I-V  In  (see  curve,  experimental  sensitivity  Fig.  interfacial  the  18),  states  there  is  and m e t a l  conductor  and  of  (corresponding  curves  the  curve  work  (4)  which  F i g . 6^ a n d  evidence is  not  that  so  x  =  1,  been  the  This  see  done  experimental  is  as  that  because  Fig.6),  to  support  dark  communication  efficient  interfacial states. to  has  is  far  forward between  between  the  the  slope  from  the  the  of  semithe  set  experimental o  one.  Although'  better  be  insulator  communication  conductor. to  the  It  was  nearer  to  exists  is  between  therefore  the  layer  extremely  the  concluded  semiconductor  thin  interface that  fermi  the  level  states  reasonable  one  The  surface  state  and  follows  from  density ref.[3]  of  than  10  and  and  effective  13 (2)  (assumed  to  ev  the  fermi  the  -1  to  be  20A),  semilevel  metal  has  fermi  level.  -2 cm  is  quite  a  ref.[7], o  (3)  The  the  theoretical  the  current  reason  results  obtained  in  for  of the  choosing  an  oxide  thickness  ref.[6],  (see  figure  2).  present  work  agrees  well with  of  i . e .  20A  the the  stems  from  magnitude  of  theoretical  o  curves  for  unique.  20A  Further  concerned  can be (4)  the also  diodes.  dark,I-V agrees  experimental  curve  the  work  unequivocally  The  with  Obviously  choosing  has  to  be  height  c o u l d be  back  the  I  a previous  done  the  above  before  figure  a l l  the  is  not  parameters  determined.  barrier to  of  In  axis  suggestion  easily  (see  [8]  found  Fig.18).  which  states  by  extrapolating  However that  0.82  the  eV  barrier 2  height  is  approximately  i n the:.present  case  <{>  twice  the  open  =  2 x  0-4V  (see  excellent  fit  obtained  circuit  voltage  at  100  mm/cm  i . e .  Fig.20).  D The results  from  the  present  Al/SiO  modified  /pSi  diodes  thermionic can be  between  the  emission  described  experimental  theory  data  indicates  as m a j o r i t y  carrier  that  and the  diodes.  the  65.  The  importance  thus  brought  suitably the  of  out  surface in  current  c o u l d be  dominate  5.2  near  the  affecting  work.  states  so  and  that  the  However,  the  I-V  i t  obtaining  oxide/insulator  suppressed  States  Surface  For  these  in  characteristics  s h o u l d be  that  by  significant inversion  of  interface  minority  the  (neutral  Present  states  i n MIS  can be  or n e g a t i v e l y  acceptor-like  majority  carrier  negative  charge  exerts  positive  to  dark  broadly  states  this  on  or  classified  as b e i n g  either  donor-like  (neutral  or  there  is  a  layer  is  MIS  metal,  means  the  dark  opposed  Therefore  interface  p-type  insulator-semiconductor  the  This  fact,  current  the  a shielding effect  charge  reduced. like  would  Cell  charged)  interface  at  semiconductor. Due  carrier  current  (if  F„  the  states  solar  to  thus  that  a lowering  current the  an  [10].  blocks  lowering  is  light  efficiency  have  cell  which  the  the of  surface  the  current,  is  This  effect  the  reverse  applies  negative  field  from  As  circuit  the  of  increased.  light  for  o  may  means  the occur. the  voltage  that  is  acceptor-  performance  donor-like  of  surface  states. The observe  that  deliberate  light the  of  thickness  open  oxidation  fabricated with value  bias  an  c h a r a c t e r i s t i c s are  circuit step  is  oxidation the  voltage  of  the  smaller  than  step  2-5  the  thickness  of  of  2 minutes  oxidation  of  oxide  in  solar  cell  that min.  which  time.  shown  Also  seems  This  is  Fig.20.  One  prepared  resulting  from  there  is  to  correspond  in  good  charged)  of  height  open  reduced. on  <J> )  potential  barrier  the  >  This  electric  correspondingly  adverse The  interface.  acceptor-  positively  Fss  J  charge  noted  is  [2].  Surface  like  the* p r e s e n t  passivating  semiconductor  states  with  the an to  agreement  may no  diodes  optimal the with  the  66.  Viktorovitch  etal's  thickness  interfacial  layer  voltage  f i l l  •open  of  circuit  interesting 10  minutes  and  features  of  oxidation  current  axis  certain  extent  of  photocurrent  5.3  result  the in  The  of  the  in  of  instability  a basic  were  not  this  work  encapsulated  reflection  coating. that  were  metal  interfacial  indicate  that  air  reaching  from  degradation voltage, at  low  either SiO  to  in  be  the  decreased  bias)is from  leading  any  layer].  two  for  bias  of  the  an  the  solar point  of  cell  the  increases  being  with  about  saturated,  oxide  optimal  properties  current  becomes  is  very  inflexion  AR  It  with  an the  the  increase  in  and  thin  encapsulant of  under  be  for  a  i . e .  too  some  not  even  that  encapsulant  data  do,  thick.  illumination,  MIS as  It  these by  would  of :the  cells an  serve  or  migration  of  oxygen  the  thickness  of  the  latter.  to  that  high  This  through  be  exclude  forward  ( p r a c t i c a l l y no  resistance.  to  barrier  appears at  anti-  have  however,  current  cells  indicative  satisfactorily  device.  reduced  the  covered  f i l m nature  series  of  interpreted  used must  the as  in  emphasized  experimental  increase in Al  is  and were  coating  junction  factor  observed  should not  c h a r a c t e r i s t i c s such  of  an  i t  account  cells.  The  any  active  f i l l  is  before  fashion,  suitable  caused by  an  on  photovoltaic  observed  reverse  22-29)  these  useful  the  the  performance  compatible  oxidation to  in  (Figs.  in  are  there  Cells  in  [A  developed and  occurs  degradation  fabricated  (2)  the  that  Furthermore  there  quadrant  MIS  concludes  regards  (1)  curve. 3rd  as  curves  time:-  the  which  factor.-  these  suppression  Stability  [11],  changes  could  the  Al  arise to  the  CHAPTER 6  CONCLUSION  An minority dark  the  explanation  diodes  has  theory  space  modified  dark  Poisson's  charge  the  metal  of  I/S  to  level  and  current-voltage  experimental  sets  critical  influence  has  observed.  formation  step  been  and  majority  In the  suitable The  state  l i e  at  new  the  carrier  Fermi  in  of  proposal  The  model  surface is  based of  excellent  the  on  effective  The  states  at  the  semiconductor  intermediate level.  so-called  thermionic  conditions  level  in  proposed  classical  charge. a  conduction  profile  appropriate  c h a r a c t e r i s t i c s are  of of  MI'S the  The  which  the  established.  need  solar oxide  best  utilized  Some p r e l i m i n a r y obtained  to  by  of  fermi  level  between  the  theoretical  agreement  with  data.  Four  been  surface  mode  proposed.  a  under  assumed  the  the  interface.  equation  is  of  explained  include  and n o n - u n i f o r m  interface  Fermi  been  c h a r a c t e r i s t i c s are  insulator/semiconductor  solution  at  c a r r i e r MIS  current  emission  alternative  for  cells  have  thickness  performance a 2 minute  data  on  the  encapsulating  on was  been the  fabricated  and  performance  of  achieved with  oxidation  stability the  of  an  the these  cells  insulator  time. MIS  cells  basic Al/SiOx/Si  has  been  structure  has  68.  REFERENCES  1.  D.L.  Pulfrey,  2.  M.A. Green, Conf.,  3.  A.E.  p.  IEEE  R . B . Godfrey  Dev. ED-25,  and L.W. Davles,  W.A. Anderson  Conf.,  p.308  Pulfrey,  Solid  4.  D.L.  5.  R.  6.  M.A. Green,  Singh  Elec.  1308 (1978).  Proc.  12th IEEE  Photo.  Spec.  896 (1976).  Delahoy,  Energy  Trans.  and J . F.D.  and J . K . K i m , Proc.  1st C.E.C.  Photo.  Solar  (1977). State  Shewchun, King  Electron. Appl.  and J .  2 0 , 455 (1977).  Phys.  Lett.  Shewchun,  2 8 , 512  Solid  State  (1976).  Electron.  17, .551  (1974). 7.  S.M. Sze, "Physics  of Semiconductor  Devices",  Chpt.8,  J .  Wiley:  (1969). 8.  E.J.  9.  H.  Charlson  Hogenboom,  10.  D.L.  11.  P.  Pulfrey,  and J . C . L i e n , Spec. IEEE  Viktorovitch,  Electron,  G.  (accepted  12.  H.C.  Card,  13.  J.C.  Irvin,  14.  S . J . Fonash,-J.  SSEE, Trans.  J.Appl.  private Elec.  Kamarinos,  Phys.  4 6 , 3982  communication.  Dev. ED-23, P.  Even  587  a n d E.  State  Electron,  Bell  Syst.  Tech.  Appl.  Phys.  J .  2 0 , 971 41,  4 6 , 1286  (1977).  388 (1962). (1975).  (1976).  fabre,  for publication).  Solid  (1975).  Solid  State  New Y o r k  APPENDIX 1 Program f o r s o l v i n g n o n - l i n e a r equation and  (48) i n p 27 using UBC f i l e s Zero 1  QINT4P  NlCHiei* TZHM!H  4L  —TTP1  JvSTE" FORTHIU t « h ! i ) (  :  05-11-73  090! o o o ? 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V »0PTI0NS I N EFFECT* NAME » MAIN , LINECNT «' 60 ~' • STATISTICS* SOURCE STATEMENTS • l^PROGPA* SIZE * 10«J •STATISTICS* WO DIAGNOSTICS GENERATED '• " _____ wo MnrtiinN ^ A i N '• : ~ ~ r~ ' : • :  :  R  1  !  -  ;  70.  r  lltMIGAN  Tl^MINiL  oiot "ooo?"  SYSTE"  FUPTOjK  Gf<Ut36)  FUNCTION FNC*L) '"'I"PLICIT Fr*L*«fA-H.O-Z) DIVISION v(S<M.IvflS0),IY(lSO).»X(lSJ5.VTfl5O),v:^fI5OJ CQViif- V . F l » r r ' - T i , t L ° ' . I « . I Y . J _ ' E A I . . U XX ,Y».» XFTS.JLIT.XLI'T I.. ATF A 1 , A ' E A 2 , t P ? » 3 ,"S*'E A j l , f l ^ I»=! _C C 1 N V E » T I X , T Y I N T O X X . Y Y TO F I T A C T U A L S C A L E C ~ T 6 I J N C A T F TO G t T I P S F n t T w o I N T E G R A L C C » L L OlNTii=> T w n T T » E S P J S S X L T o U P P E R L I " I T of I N T E G R A L C w P I T E 0OW>i F U N C T I O N • • DO 2 1 « = 1 . 1 5 0 XX(i<)-Ix(i<l/liJoOo'o _ YI CK)= IYfK)/lfl00.0  0003  K  0001  t  ooos  000k  0007 0008 OOCO ~001O~ POtt 001?  M  2Y"YY(i<)3l0.n*«YI«t<Kl 1  XFIIsF Ip.lCI, ! IRIrI>iTfXFjR)»i C  PASS  0013 00 10  X L TP llPOF'R L f f l T O F I N T E G R A L X L I T = ( F l H . f J J.ALPHA-V C J ) / X | _ ) * 1 O O . O IS2=I>iT(XLtT)»t inj =I H 2 i l " " •—' " ~ - ~ XLITL = FtnAT(IH?-M lSF«t=STMT<i»CXx.Yv,ISO,I*.IPl)' V  '  -Ootv  '•  0016 00)7 001S 001<> 0020  A^riT=~TN'TiP-?Ty7vr'.T5^7':"ATra?T » B E « 3 = «TVTi:P{x<. v V , 1 5 0 . 1 3 ? , IS J ) »«EA31=(XLTT-XlIT|).AREAS " C R A V E T O H I I L T I P L T E P BY 0 " '" C W R I T E DOW* F U N C T I O N »PTsFIB.0.?3 C LO= 1 . 5 5 = 5 " C CONSTANT LEAO 3.»0-R 0 = t'.».12n-|o . - ,, , c j | •- - — "  0021 002? 002J002a 0025  -  4  B  4  =  n  4  =  A ° E A 2 ? = .':.ABFA2 »BEAJ^=r:«.ipFAT)  "FSPT-~5«'.4ll0iV(rTTrS'8t*T  002*  lvRTi-T8.6'nn«vBT.),oo) T E B « 2 = " F < P r - 3 ' ' . « > l n O « ( V B T - v t J ) / X t ) ) * 3 8 . 4 ! B ( l » C - ' f T - V C J ) / X L ) - 1 . 0 0 » 1.13 " 2 8 0 - 1 C • ( . • ) £ X P f 3 « . 6 1 0 0 « ( v H T - V ( J ) / X L ) ) - 3 S . » 1 0 f i * ( V 3 7 - V f J J / X L 1-1 . 0 0 ) FNsfOFLT«/j.uO.t3i»(AHEAH-«aEA52-A=!EAJ3*3,6O-.a«03J^T£TESf!i)-J 65^o.0S0»'fTFs-2))-txL-1.00)*V(J)/»L • .  0027  ,  ,  002*  Prnw  -012*  0030 E N D •OPTIONS I N EFFECT* IP,EBCDIC.SOURCE,NOLIST.NODFCK,LOSO,NO»AP —*0PTTO\S IN'EFFECT* NAVC r F N , LI'JECNT = 60 •STATISTICS* SOURCE STATE»ENTS = 30.PROGRAM SIZE a . •STATISTICS* NODIAGNOSTICS GENERATED N l t R O t i ^ S I N CM NO  STATEMENTS  N»»E  FLAGGED  NUMBER  I N T*E ABOVE  u  CF EBPOPS/ APNtMGS  M«TN FN  FXFCUTtON TERMINATED  0 0 ' I T j I2't32  COMPILATIONS,  SFVERITY 0 0  Ts'.33»  RC=0  S.31  3296  

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