Production of cesium-telluride for measurement of ultraviolet radiation intensit

Spicer, W. E.

English

NASA Technical Reports Server

I 
NASA QUARTERLY STATUS REPORT #io 
f o r  Period Ending 31 October 1970 
GRANT: NGR-05-020-066 
Principal  Invest igator  
W .  E. Spicer  
Stanford University 
Stanford, CA 94305 
https://ntrs.nasa.gov/search.jsp?R=19710003183 2020-03-12T00:44:57+00:00Z
I. INTRODUCTION 
A very l a r g e  e f f o r t  has  been expended t h i s  q u a r t e r  i n  developing 
techniques fo r  producing CsTe cells fo r  abso lu te  measurements of W radia- 
t i o n  i n t e n s i t y .  This work i s  covered i n  Sec t ion  11, W e  a r e  beginning t o  
o b t a i n  encouraging r e s u l t s .  
I n  Sec t ion .111  the r e s u l t s , f r o m  the  s i l v e r  h a l i d e s  i s  summarized. 
A f i n a l  r epor t  i s  being w r i t t e n  on t h i s  work. When t h i s  i s  f in i shed ,  work 
w i l l  begin on t h e  prepara t ion  of a r t i c l e s  f o r  publ ica t ion .  
- 1 -  
11. 
Our recent  
1 igh t i n t e n s i t y  
FURTHER DEVELOPMENT OF A Cs2Te STANDARD CELL, 
G ,  E. F i s h e r  
work a t  Stanford i n  developing s tandards of monochromatic 
i n  t h e  near  u l t r a v i o l e t  has  been devoted t o  a cel l  convenient 
f o r  astronomical research which i s  b l ind  t o  s c a t t e r e d  v i s i b l e  l i g h t  but  has  
a high s e n s i t i v i t y  i n  t h e  nea r  u l t r a v i o l e t  (3.5 e V  5 hv < 11.0 eV). C s  Te  
was chosen a s  t h e  photocathode ma te r i a l  f o r  i t s  threshold  nea r  4.0 e V  and 
a h@F2 window (10.8 e V  c u t o f f )  i s  used because i t  i s  l e s s  del iquescent  than 
LiF. The cells  conform t o  t h e  same geometry and small  s i z e  a s  t h e  successfu l  
LiF-Cs Sb cells of t h i s  labora tory .  
. 2  - 
3 
Several  c r i t e r i a  f o r  a u se fu l  s tandard were m e t  q u i t e  e a r l y  i n  ou r  
development. We rou t ine ly  produce ce l l s  with a sharp threshold and a response 
g r e a t e r  than 0.10 e l ec t rons / inc iden t  photon from 5.0 eV t o  10.7 e V  (Fig. 1 >. 
The low energy response (hv < 3.5 eV) observed i n  our  f i r s t  prototypes has  
been el iminated by s a t u r a t i n g  t h e  cathode with C s  vapor during processing 
a t  15OoC. 
on excess  C s  proposed by Ta f t  and Apker. A l s o  dark or leakage cu r ren t  i s  
less than 10 amp. and t h e  forward/reverse c u r r e n t  r a t i o  i s  t y p i c a l l y  300:l. 
A f t e r  t h e  e a r l y  successes,  t h e r e  have been seve ra l  d i f f i c u l t i e s  t h a t  
This  r e s u l t  r e f u t e s  t h e  explanat ion of l o w  energy response based 
1 
-13 
have been overcome more r ecen t ly  i n  o rde r  t o  o b t a i n  use fu l  cells ,  
problems have been mentioned previously (NASA Quar te r ly  S t a t u s  Report #9). 
I w i l l  d e t a i l  our  changes i n  usual  processing procedures t o  accommodate the  
MgF -Cs  Te cells  and t h e  motivat ions f o r  these changes. 
These 
2 2  
1. Brown s p o t s  found on t h e  i n s i d e  of t h e  window a f t e r  processing were 
found t o  be AgC1. 
sea l ing  area ( t inn ing) ,  and may s p l a t t e r  onto t h e  m i d d l e  of t h e  win2oii . 
0 . 7 0 0 - in .  m e t a 1 d i s c c o v e r i n g  t h e  c e n t e r  of t he  window 
When t h e  oven i s  a t  5OO0C t h e  AgCl i s  melted onto t h e  edge 
A 
- 2 -  
dur ing  t h i s  s t e p  i n  the  oven has  e l imina ted  
2. The loss i n  t ransmiss ion  above 6.5 
5OO0C seems t o  be due t o  a high temperature 
Oven chamber (2.5 cu. f t . )  i s  
argon t h a t  has  passed through 
water vapor. A l s o  both s i d e s  
with acetone and pol ished for 
each exposure t o  t h e  oven, 
now purged a t  
t hese  spots.  
e V  by MgF windows baked a t  
reac t ion  with water vapor. 
2 
a r a t e  of 10 cu. f t / h r .  w i t h  
a d r y  ice-acetone co ld  t r a p  to  e l imina te  
(if access ib l e )  of t h e  window a r e  cleaned 
t h r e e  minutes with 0.3~ alumina powder a f t e r  
Figure 2 shows t h e  t ransmission curve of ou r  most 
when f r e s h  (A), a f t e r  baking a t  500°C i n  a i r  f o r  t h r e e  
a f f ec t ed  MgF window 
hours  (B), and a f t e r  
2 
pol i sh ing  f o r  one minute on each s i d e  (C). The d e s i r a b i l i t y  of some polish- 
ing  i s  c l e a r .  The ex ten t  of success of our  c u r r e n t  technique can be  seen 
i n  Figure 3 which g ives  curves  before  and a f t e r  applying AgCl under our  
present  procedures. (Note: Acetone i s  p re fe rab le  t o  alcohol  f o r  degreasing 
MgF2. 
former has  no no t i ceab le  e f f e c t . )  
The l a t t e r  reduces  W transmission by a s  much a s  3-40/,, whereas the  
3 .  The problem of t i m e  s t a b i l i t y  of response seems t o  have been solved 
by heat-aging t h e  cel l  a f t e r  processing a t  a temperature above room temperature 
f o r  a t  l e a s t  h a l f  an hour t o  complete t h e  Cs-Te r eac t ion ,  Temperatures between 
100°C and 17OoC a r e  being evaluated f o r  any preference'between t h e i r  e f f e c t s .  
4. The major i ty  of  o u r  e f f o r t s  of t h e  p a s t  months have gone towards 
improving t h e  uniformity of response ac ross  t h e  0.725-in. diam. cathode of 
t h e  cell.  The major f a c t o r  i s  t o  o b t a i n  a uniform l a y e r  of evaporated t e l lu r ium 
on t h e  cathode. Our e a r l y  work showed t h a t  t h e  high vapor pressure  of Te was 
causing t h e  migrat ion of v i s i b l e  amounts of Te t o  su r faces  180° from t h e  
d i r e c t i o n  of evaporation. This  l e d  to  a magnet ical ly  removable s h i e l d  t o  
p r o t e c t  t h e  window during evaporat ions and t h e  narrowing of t h e  e x i t  of the  
- 3 -  
evaporat ion bucket t o  encourage a narrow angle  evaporation, T h i s  has  _ ,  
worked so w e l l  t h a t  high y i e l d s  have been found only i n  t h e  center of t h e  
cathode. W e  have r e c e n t l y  obtained our  most uniform respanse curve (Figure 4) 
by increas ing  t h e  bucket-cathode d i s t a n c e  from 0.250-in. t o  0.360-in. and 
arranging t h e  charge of indium t e l l u r i d e  a s  nea r ly  a s  w e  can t o  g ive  a wide 
angle  evaporation. Even with more t e l lu r ium on t h e  s i d e  walls, t h e  dark 
c u r r e n t  i s  s t i l l  less than 10 ampere. -13 
W e  hope t h a t  t h e  major d i f f i c u l t i e s  with t h e  hfgFe-Cs Te cells have been 2 
e f f e c t i v e l y  handled: Work cont inues t o  improve t h e  r e p r o d u c i b i l i t y  of t h e  
uniformity between cel ls  along with a high pho toe lec t r i c  y i e ld .  
REF% RENCES : 
1. E. T a f t  and L .  Apker, J. O p t .  SOC., - 43 81 (1953). 
-4- 
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111. THE ELECTRONIC STATES AND ELECTRON-LATTICE 
INTERACTION I N  THE SILVER HALIDES 
R. S .  Bauer 
A. In t roduct ion  
The experimental  p a r t  of t h i s  s tudy  h a s  been completed and t h e  r e s u l t s  
a r e  being w r i t t e n  up i n  t h e s i s  form a t  t h e  present  t i m e ,  This  should be 
completed and copies  a v a i l a b l e  i n  t h e  form of a report a t  t h e  end of t h e  
next  qua r t e r .  In  t h i s  report t h e  main r e s u l t s  of t hese  s t u d i e s  a r e  
summarized. 
B. Resul t s  
The major new f e a t u r e  of t h i s  work i s  t h e  discovery of a temperature 
dependence of c e r t a i n  s t r u c t u r e  i n  t h e  energy d i s t r i b u t i o n s  of photoemitted 
e l e c t r o n s  which i s  an order  of magnitude g r e a t e r  than  t h e  change i n  thermal 
energy, kBT. 
data  f o r  AgCl and AgBr a r e  summarized i n  Table I. 
The f e a t u r e s  of t h e  temperature dependent photoemission EDC 
Th i s  probably r ep resen t s  
the  f i r s t  observa t ion  of what may prove t o  be a whole new c l a s s  of e f f e c t s  
i n  s o l i d s  which s a t i s f y  t h e  genera l  c r i t e r i a  of:  (1) a Debye temperature i n  
the  region of temperature va r i a t ion ;  (2) e l e c t r o n i c  wavefunctions which a r e  
(a) f a i r l y  loca l i zed  on t h e  atomic s i t e s ,  and (b) of two compatible species 
i n  c l o s e  enough proximity i n  energy f o r  t h e  r e s u l t i n g  hybr id i za t ion  t o  be a 
s i g n i f i c a n t  po r t ion  of t he  mixed s t a t e ' s  energy; and (3) a l a r g e  enough 
amplitude of i o n i c  v i b r a t i o n  above t h e  Debye temperature t o  cause a s i g n i f i -  
can t  modulation of t h e  wavefunction hybr id iza t ion .  The temperature dependence 
of t h e  EDCs i s  bel ieved due t o  a very  s t rong  i n t e r a c t i o n  of t h e  e l e c t r o n s  
with t h e  l a t t i c e ,  very much l i k e  t h e  Jahn-Teller e f f e c t  but  many t i m e s  s t ronger .  
A model r e f e r r e d  to  a s  dynamic wavefunction hybr id iza t ion"  h a s  been proposed 
i n  a very genera l  sense to  desc r ibe  t h e  mdnner i n  which t h e  v i b r a t i o n s  of t h e  
11 
- 9 -  
i ons  may e f f e c t  t h e  ene rg ie s  of the  e l e c t r o n i c  states so s t rongly .  _Apelying 
t h i s  model i n  a very gene ra l  way, t h e  s i l v e r  s t a t e s  with almost pure 4d 
symmetry could be loca ted  by experiment with reasonable  c e r t a i n t y  f o r  t h e  
f i r s t  time; these  l o c a t i o n s  were 3.7, 3.3, and 4.4 e V  below t h e  h ighes t  
f i l l e d  s t a t e s  i n  AgBr, AgC1, and AgI respec t ive ly .  A l l  t h e  f e a t u r e s  of t h e  
e l e c t r o n i c  s t r u c t u r e  of a l l  t h r e e  h a l i d e s  which were experimental ly  loca ted  
by t h i s  study a r e  summarized i n  Table I1 along with suggested o r i g i n s  for 
the  s t r u c t u r e .  By using a v e r y  simple approximation of k = 0 o p t i c a l  phonons 
and r e s t r i c t i n g  t h e  ex ten t  of t he  i n t e r a c t i o n  t o  about one nea res t  neighbor 
d is tance ,  t he  proposed model was appl ied  t o  t h e  AgCl energy bands' and 
pressure  dependent o p t i c a l  absorp t ion  data  .' 
' 
These computations demonstrate 
i t s  p l a u s i b i l i t y  by p red ic t ing  t h e  p rope r  magni tudes.for  dens i ty  of s t a t e s  
peak broadenings and t h e i r  temperature dependences. Of course, i t  must be 
emphasized t h a t  t h e  physics  i s  r e a l l y  much more complex. A s  i n  t h e  case  of 
the  Jahn-Tel ler  e f f e c t ,  t h e  f rozen- l a t t i ce  approximation i s  not  v a l i d ,  and 
one must r e a l l y  cons ider  wavefunctions of t h e  e n t i r e  s o l i d  (e.g., v ibronic  
s t a t e s )  r a t h e r  than sepa ra t e  e l e c t r o n i c  and nuc lear  pa r t s .  This  f u r t h e r  
i m p l i e s  t h a t  t h e  e n t i r e  concept of an energy band s t r u c t u r e  i s  q u i t e  suspect .  
There is, therefore ,  much work t o  be done t o  t h e o r e t i c a l l y  account f o r  such 
many-body i n t e r a c t i o n s  accura te ly .  Doniach has  begun a Green's func t ion  
c a l c u l a t i o n  of t h e  problem which t akes  i n t o  account t h e  randomness of t h e  
ionic  motion.3 H i s  f i r s t  r e s u l t s  i n d i c a t e  t h a t  t h e  dynamic hybr id i za t ion  
p i c t u r e  i s  i n  genera l  a proper  r ep resen ta t ion  f o r  t h e  physical  phenomenon. 
B e s i d e s  t h i s  bas i c  conclusion and t h e  probable l o c a t i o n  of t h e  "pure" Ag 4d 
s t a t e s ,  t h e  o t h e r  i n t e r p r e t l v e  r e s u l t s  of t h i s  work must be considered only 
f i r s t  order a t tempts  a t  explaining many complex da ta  f e a t u r e s .  I t  i s  bel ieved 
t h a t  t h e  photoemission da ta  i t s e l f ,  which was c a r e f u l l y  and l abor ious ly  taken 
using newly developed techniques,  i s  q u i t e  accu ra t e ,  It should serve a s  a 
- 10 - 
good storehouse of information f o r  t h e  many f a s c i n a t i n g  ques t ions  which _ .  
remain t o  be answered. 
REFERENCES 
1. Reference R. S. Bauer, L a s t  -Quar te r ly .  
2. R. B.Aust, Phys. Rev. - 170, 784 (1968): 
3. S. Doniach, P r i v a t e  Communication (1970). 
- 11 - 
TABLE I 
Fina l  S t a t e  Energy* (eV) EDC S t r u c t u r e  
AgBr AgCl Cha r ac  t er TW~OOK 
Suggested 
I n t e r p r e t a t  ion 
<9.5 <10.2 Strong , 
3 h ighes t  Hybridized halogen 
Lowe st (1) Localized Ag 4d-derived 
N o  sharpening valence s t a t e s  
sharpen p -de r ived  valence s t a t e s  
t 
! 
>9.5 >lo. 2 
8.8 8.1 
I 
I 
Weak N o  sharpening Forbidden halogen p A g  5p' 
t ran  si. t ions? 
Enhanced Hybridized halogen d- 
s t rength  ; some derived conduc t ion . s t a t e s  
* 0 = E v  
new 
- 12  - 
TABLE I1 
Energy (ev) Suggested Or ig in  Degree of Mixing* 
>9.5 
8.8 
7.15 
0 
Ag ( 5 ~ )  ?
' B r  (4d) 
Vacuum Level 
Valence Band Maximum 
AgBr -0.85 B r  ( 4 ~ )  
-2.9 Br ( 4 ~ )  (+0.1) - 
-3.3 Bottom of B r  (4p) Band 
-3.7 
-5.0 
H 
H 
H 
H 
P 
H 
-6.0 Bottom of Valence Band 
>10 * 2 
8.1 
7.55 
AgCl 
(+O - * 1)  
0 
-0 B 
-2.65 
Ag ( 5 ~ )  ? 
C 1  (3d) 
Vacuum L e v e l  
Valence Band Maximum 
c1 (3P) 
c1 (3P) 
H 
H 
H 
H 
-3.0 Bottom of p Band 
-3 .3  Ag (4d) P 
8 .*io .o 
10.6->11.8eV 
r p S t a t e s  or  ~9 r S t a t e s  
7.8, 7.1 Ag (5~) or I (5d) 
6.6 . Vacuum Level 
AgI 0 r, Valence Band Maximum 
-0.45 I (5p), N o t  a t  I' 
-1.7 I. (5P) 
-4.4 - f 0.1 Ag (4d) 
(+O. - 2) 
H 
H 
H 
P 
* H = Hybridized, P = Pure 
- 13 - 


Cs2Te standard cell development and electronic characteristics of silver halides Quarterly status report, period ending 31 Oct. 1970

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