Improved gels and several geometries were investigated for use in growing crystals. The use of lead sulfide test crystals proved workable, but it was impossible to obtain and maintain a sufficiently concentrated telluride ion solution to successfully grow lead telluride crystals. It appears that oxygen in the solution is capable of oxidizing the telluride ion up to tellurium metal. The method may still be successful, but only if precautions are taken to eliminate dissolved oxygen from the gels and aqueous solutions and to maintain a suitable concentration of telluride, Te(2)-(aq.)

Barber, Patrick G.

English

NASA Technical Reports Server

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FINAL PROJECT REPUHT 
"Growth of Lead T i n  T e l l u r i d e  C r y s t a l s  i n  Gels"  
NAG-1-498 
P a t r i c k  G. Barber 
Professor  o+ Chemistry 
Longwood Col 1 ege 
Farmvi 11 e ,  V i r g i  n i  a 23901 
1 .  
L 
(NASA-CR-181258) GBOCITH OF LEAD TIN 618 7-275 U 3 
IELLUBIDE CRYSTALS II GELS Pita1 Eroject 
Peport (Longwood Coll .)  15 F Avail: NTIS 
E C  A021ClP A01 C S C L  20L Unclas 
~ 3 1 7 6  0093 182 
31 May 1986 
https://ntrs.nasa.gov/search.jsp?R=19870018110 2020-03-20T09:32:32+00:00Z
i 
5 
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1. Purpose: 
Lead t i n  t e l l u r i d e  (LTT) i s  a compound semiconductor t h a t  
has p r o p e r t i e s  s u i t a b l e  f o r  use as an i n f r a r e d  de tec tor  i n  
wide area arrays. Another s i m i l a r  cornpound semiconductor, 
mercury cadmium t e l l u r i d e  (MCT) , is being used commercial ly 
f o r  t h a t  purpose. For s u i t a b l e  commercial u t i l i z a t i o n  0-1: 
such compound m a t e r i a l s  i t  i 5  important t h a t  they have 
un i fo rm compositions. I n  t h e  growth methods such as t h e  
Bridgman one, convect ion e x i s t s  i n  these mater ia ls ,  
and t h i s  leads t o  composi t ional  inhomogeneities. because MCT 
m u s t  be grown us ing h igh  vapor pressures o f  mercury, L T T  is a 
much sa fe r  subetance t o  use t o  study such f lows. L T T  thus 
f i n d 6  use a6 a model compound t o  i n v e s t i g a t e  the  processes 
t h a t  occur dur ing  c r y s t a l l i z a t i o n  of  such compound mater ia ls .  
I n  an e f f o r t  t o  suppress the convect ion t h a t  leads t o  
composi t ional  inhomogeneities, LTT has been grown i n  space on 
t h e  Space S h u t t l e  i n  October of 1985. 
A n  a l t e r n a t i v e  method t o  suppress convect ion i s  t o  grow 
t h e  c r y s t a l s  on e a r t h  i n  a medium t h a t  does no t  a l l ow  
macroscopic f lows. Such a mater ia l  i s  a g e l  and i s  t h e  
sub jec t  o f  t h i s  study and repor t .  
11. Experimental Methods: 
Growing c r y s t a l s  i n  g e l s  i s  a technique t h a t  has been 
used s ince  t h e  end o f  t h e  l a s t  century. A rev iew o f  t h i o  
technique has been g iven i n  a 1970 book by H. Henisch.' T h e  
lHenisch, Heinz K. , Crys ta l  G r o . w . t , h ~ , . . , l ~ , - _ ~ - ~ - l - ~ . ,  The Pennsylvania 
S ta te  U n i v e r s i t y  Press, Un ive rs i t y  Park, Pennsylvania, 
USA. 1970 (t271-00104-6). 
------------- 
advantage o f  utsing a ge l  a6 the growth medium .For c r y s t a l s  i s  
t h a t  microscopic +le% is n n e ~ i b ? ~  r-* thus sllowiny c r y s t a l  Cor-. 
mation, bu t  macroscopic, f l u i d  f low i s  re ta rded because o f  
t h e  increased v i s c o s i t y  o f  t h e  medium. 
T h i s  p r o j e c t  began w i t h  t h e  techniques t h a t  have been 
descr ibed i n  t h e  l i t e r a t u r e  f o r  growing cr-yskdls i n  ge ls .  
These methods were tes ted  f i r s t  t o  5ee how s u i t a b l e  they 
would be f o r  LTT  and second t o  see what mod i f i ca t i ons  i n  
m a t e r i a l s  and techniques would be needed. This  sec t i on  a+ 
t h e  r e p o r t  descr ibes the  development o f  methods t o  be used 
w i t h  LTT. 
There are  a number o f  g e l s  t h a t  would be candidates f o r  
use i n  t h i s  study. Even strawberry j a m  has been used t o  grow 
Crys ta l s ,  bu t  such a ge l  i s  ne i ther  reproduc ib le  nor 
genera l l y  accepted by t h e  s c i e n t i f i c  community. Three more 
genera l l y  accepted g e l s  were evaluated f o r  use i n  t h i s  
s tudy--ge lat in ,  agar, and s i l i c a .  The f i r s t  two, g e l a t i n  and 
agar, were not  used because they are  o f  b i o l o g i c a l  o r i g i n  and 
sub jec t  to some composi t ional  va r ia t i ans .  They tend t o  have 
v a r i a b l e  t r a c e  amounts of i m p u r i t i e s  depending upon sources 
and suppl iers .  CS more ser ious  proper ty  i s  the  a b i l i t y  0.f 
these t o  serve as a medium f o r  b i o l o g i c a l  growth. Although 
w i thout  n u t r i e n t s  requ i red  for  growth and w i t h  lead, t i n ,  and 
t e l l u r i d e  i ons  present,  i t  would be h i g h l y  u n l i k e l y  t h a t  
b a c t e r i a  would grow even on these gels.  With t ime a v a i l a b l e  
o n l y  t o  use one g e l  medium, the  s i l i c a  ge l  was selected. I t 
i s  a h i g h l y  re f ined,  inorgan ic  medium t h a t  forms a l oose ly  
cross- l inked polymer when i t  g e l s .  
The standard methods used t o  prepare  t h e  s i l i c a t e  ge l s  
were tested."  The standard method was s e n s i t i v e  tc:, 
"Henisch, i-bid,. p. 22. 
-------------- 
labora to ry  cond i t i ons  such as r e l a t i v e  humid i ty  and 
temperature. The g e l  t h a t  resu l ted  d i d  no t  have a 
rep roduc ib le  g e l a t i o n  t ime, and i t  was clo~tcly. Th is  
t rans lucen t  g e l  made i t  d i f f i c u l t  t o  observe c icys ta ls  as they 
formed and grew i n  t h e  gels. Consequently, a method wa5 
developed t o  c l a r i f y  t h e  gel .  This method invo lved t h e  use 
of  a c a t i o n  exchange r e s i n  t o  replace most o f  t he  sodium i o n s  
w i t h  potassium ions. Th is  "po l i sh ing"  procedure r e s u l t e d  i n  
g e l s  t h a t  were s i g n i f i c a n t l y  more transparent.  
T o  make t h e  g e l a t i o n  t imes more reproduc ib le ,  i t  was 
discovered t h a t  these t imes were rendered more reproduc ib le  
i f  a l l  s o l u t i o n s  were degassed by being prepared w i t h  b o i l e d  
and cooled d i s t i l l e d  water. The methods of c l a r i f y i n g  t h e  
g e l s  and us ing degassed water are explained i n  d e t a i l  i n  a 
p u b l i c a t i o n  t h a t  r e s u l t e d  from t h i s  p ro jec t . " .  
4 
=Barber, P a t r i c k  G. and Norman R. Simpson, "4 C l a r i f i e d  Gel 
f o r  C rys ta l  Growth. 'I Journal I 4 of -.... I Crys ta l  .. ..... - Growth- ...... 7-3 
( 1 9 8 5 ) ,  400-402. 
With a reproducib le ,  c l a r i f i e d  s i l i c a  ge l  a v a i l a b l e  +or- 
use, t h e  geometry o f  c r y s t a l  growth was s tud ied  next  50 t h a t  
a s u i t a b l e  one could be selected. Again t h e  c l a s s i c a l  
methods were t r i e d  f i r s t .  F i v e  geometries were 
invest igated-- the double bu lb,  t r s t t u b e ,  open tube, beaker, 
and p & r i  d i s h  cjem-~etries. .L inese a arc: i i i u v t r a t e d  i n  F igure  
1. 
1-1 
a. Double Bulb b. Testtube 
c. Beaker d .  P e t r i  Dish 
U 
e. Open tube 
F igure  1: Crys ta l  Growth Geometries 
5 
Geometriee b_, c-, d_,  and e - are b a s i c a l l y  t h e  same. One o.f 
t he  i ons  6Ub5equently used irr the c r y s t a l  is f i r B t  
incorporated i n t o  t h e  ge l .  This i s  s imply  dune by us ing  a 
s o l u t i o n  of  t h e  i o n  i n  p lace  o f  the  d i s t i l l e d  water i n  the  
r e c i p e  f o r  prepar ing t h e  gel .  The subsequent concentrat ion 
i s  most important,  so t h a t  care m u s t  be exercised i n  
p repar ing  t h e  aqueous concentrat ions t h a t  a re  then d i l u t e d .  
These geometries d i d  no t  have f l e x i b i l i t y ,  b u t  they were 
s imple and d i d  a l l ow  f o r  r a p i d  t e s t i n g  o f  several  s o l u t i o n  
concent ra t ions  or cond i t ions .  This wa5 e s p e c i a l l y  t r u e  a+ 
t h e  p e t r i  d i s h  method, which by us ing disposable dishes 
d i v i d e d  i n t o  t h r e e  sect ions,  allowed th ree  d i f f e r e n t  i o n  
concent ra t ions  i n  ge l5  t o  be tested a t  one t ime f o r  t h e  
e%ten t  and s i z e  o f  nuc leat ion.  
The geometries b_, 5, d, -.- and e_ were thus  used -for p r e l i -  
minary scans of  concentrat ions and cond i t ions .  Geometry a, 
t h e  double bu lb,  was selected f o r  ac tua l  growth cond i t ions ,  
s ince  i t  al lowed f o r  both greater f l e x i b i l i t y  and c o n t r o l  
over growth and s o l u t i o n  condi t ions.  Subsequently, advantage 
of t h i s  was taken i n  developing an e lect rochemical  procedure. 
To a i d  i n  t h e  development of  t he  methods and t o  t e s t  
them, use was made of  a simpler and 5ja.fer test c r y s t a l  
system. Lead s u l f i d e  c r y s t a l s  were used as the  t e s t  system. 
A t  t h e  t ime  t h a t  t h e  methods were ready t o  be tes ted  t h e  
P r i n c i p a l  I n v e s t i g a t o r  was i n v i t e d  t o  spend a year wor1::ing i a t  
NASA-Langley Research Center. Consequently, a student., 
Norman Simpson, was l e f t  a t  Longwood Col lege t o  conduct these 
t e s t s .  Although t h e  P I  was only  a t h r e e  h o u r ' s  d r i v e  away 
and although communications were made by telephone and 
occasionally in person on weekends, it was decided not to 
leave a student alone to undertake research using a 
6ubotance, telluride, with which he was unfamiliar. 'This war5 
especially true considering the potential toxic hazards from 
hydrogen telluride. Consequently, the student tested the 
gels, geometries, and methods using lead sulfids. He had 
worked with raulfidem before, and he WAS aware of the 
precautions that had to be taken. Using the clarified gel 
and mainly geometries d -- (Petri Dish) and e (Open  rube), he 
scanned series of concentrations 0 . F  lead and sulfide ions  
observing those that gave the most suitable crystal 
nucleation in the gel. His results are summarized in Figure 
2. 
-3 
Region u.f Suitable Nucle- 
ation i n  Gels. 
-7 
log CPb"'3 
-1 
Figure 2: Lead and Sulfide Ion Concentrations Leading to 
Nucleation in Gels. 
... " 
/ 
Upon the  r e t u r n  0 . F  t h e  PI, the concent ra t ion  data col-.  
l e c t e d  from t h e  p r e l i m i n a r y  s tud ies  u s i n g  lead s u l f i d e  were 
used by t h e  P I  and another student, James Coleman, t o  develop 
techniques us ing lead t e l l u r i d e  and geometry a (Double Eu lb) .  
111. Theore t ica l  Considerations: 
Because t h e  use o f  g e l s  i s  designed t o  reduce t h e  extent. 
of  convect ion,  t h e  poss ib le  r o l e s  t h a t  t h i s  f l u i d  f l o w  coul t i  
have i n  c r y s t a l  habit C a r m a t i c m  @are c c n s i d u r e d .  hlnI-B..- ,...,,, 
spher i ca l  p a r t i c l e s  w i l l  o r i e n t  i n  a f l u i d  f l o w  and t h i s  
o r i e n t a t i o n  w i l l  be governed by the moments o f  momenta of  
p a r t i c l e s .  Theee cons idera t ions  culminated i n  a NASA 
Con t rac to r ' s  Report #3907. 
Because such cons idera t ions  o f  o r i e n t a t i o n  i n  f l u i d  +lows 
are  impor tant  i n  o ther ,  commercially impor tant  c r y s t a l  growth 
system, t h e  t e c h n o l o g i c a l l y  s i g n i f i c a n t  consequences a+ t . h i ~ ;  
O r i e n t a t i o n  were summarized i n  a NASn Tech B r i e f .  Spec i f i - .  
c a l l y ,  t h e  h a b i t s  o f  t h e  c r y s t a l s  of  Gal l ium Arsenide grown 
i n  MOCVD systems may be se lected by us ing reagents having 
s u i t a b l e  in te ra tomic  spacings and molecular momenta. 
IV. Results: 
The double bulb technique was used in two groups of 
experiments. Because of  the difficulty of obtdining and 
k;eeping telluride ion (Te”-) in aqueous solutions in t h e  
presence of oxygen, the telluride ion was developed in- s-:-ti.~ 
in two ways. 
The lead ion solution was made and put in bulb A of the 
double bulb illustrated in Figure 3. The solution 
8mm opening 
covered with 
a testtube 
$ 24/4v JOl17t &&6 B 25QmL bulb A 
Figure 3: Details of the Double Bulb Apparatus 
that would develop the telluride ion was placed in bulb B .  
The first reagent used to develop telluride w a r  sodium 
tellurite, Na2Te03. This solution was reduced in two ways 
using hydrazine and sodium borohydride. The borohydride 
immediately reduced the telluride to a black precipitate of 
tellurium metal; and the hydrazine eolution did the same, but 
it did so more slowly taking several hours. To retard t h i s  
reduction process, a new method was developed to slowly 
release the reagents into the solutions by incorporating the 
reagents in gel filled tubes. When the hydrazine doped gel 
was squeezed out of the tube into the tellurite solution in 
bulb E, t h e  solution was slowly reduced to give an orange 
colored solution typical of polytelluride ion.4 Crystals 
developed in bulb Ec, and analysis b y  x-ray fluorescence and 
powder diffraction indicated that these crystals were 
crystals are attached in the appendix. 
applications a6 a safer way to carry chemicals into space. 
Consequently, a NASCI Tech Brief wa5 submitted for this. 
Even with the slow release of reagent from gel tubes, the 
tellurite solution would not be reduced to telluride. 
Consequently, an electrochemical method was developed to 
generate the telluride ion &-n,.,..~it.u. in b u l b  B. T h e  double 
bulb apparatus was modified a5 shown in Figure 4. 
..- 
Anode + r  7 Cathode 
Figure 4: Double Dulb Modified for Electrochemical F'ro- 
duction of Tel luride I-n-.S-ltg, 
The electrochemical preparation of telluride ion in situ -- -_ 
did prepare the desired ion. The oxygen in the aqueous 
solution and in the apparatus, however, caused the telluride 
to be oxidized back up to the black tellurium metal. In the 
aerobic environment it was not possible to keep the tellurium 
a5 the minus-two ion in sufficient concentration to enable 
crystals to grow. The following thermodynamics data 
substantiates this conclusion: 
The sulfide ion of the test solutions used to develop the 
i i lethsde doer not  S L I + + ~ W  thi5 tendency tu react  as readily 
with oxygen a5 shown by the following thermodynamic data: 
These data indicate that the eqi  ilibri 
E"' 
-0. 69 
( 3 .  141vol ts 
m .Favors the 
formation of H2!3(aq) ,  while in the tellurium case the metal 
is favored. Thus, it was not possible to develop and to 
maintain a sufficient telluride ion concentration to fo rce  
the growth of crystals of l e a d  telluride in the gel. 
V. Conclusion: 
Improved gels and several geometries were investigated 
for use in growing crystals. These worked ior lead sulfide 
test crystals, but it proved impossible to obtain and to 
maintain a sufficiently concentrated telluride ion solution 
to successfully grow lead telluride crystals. It appears as 
if oxygen in the solution is capable of oxidizing the tellu- 
ride ion up to tellurium metal. 
The method may still be  successfully but only i f  
precautions are taken to eliminate dissolved oxygen f r o m  t h e  
gels and aqueous solutions and to maintain a suitable 
concentration of tell ur i de, Te'>:-(aq) . 
VI. Papers: 
This project resulted in the following publications, NASA 
Tech Briefs, Patent Proposals, and presentations at 
scientific meetings: 
Jogrnal of Crystal Growth 73, 400-402 - Numerous reprint 
reqFests received mainly f rom East and West Europe. 
NASCI Rep. #3904 
Tech Brief and Patent Possibility Application - 
Gel Clarification Process LAR-15476-1-CU 
Tech Brief and Patent Possibility Application - Slow 
Release LAR-13607 
Tech Brief and Patent Possibility Application - Gel 
support for electrochemical crystal growth 
LAR-13608  
Tech Brief and Patent Possibility Application - 
Pre-crystal1 i ne aggregates for G a A s  and other 
substances LAH-13606 
12 
Present a t  i ons: 
1. MRS (Europe) ,  Strarbourg,  F R ,  May 1985 
2. American Chemical Society ,  N a t i o n a l  Meet ing,  NYC, 
March 1986 
A p p e n d i x  1 
C r y s t a l s  of  te l lurium d i o x i d e  grown i n  t h i s  project .  
t 
14 
Appendix 2 
Undergraduate students  a t  Longwood Col lege  who have 
worked on t h i s  P r o j e c t  w h i l e  supported by  NASA: 
Dawn Adams - s t i l l  an undergraduate 
James Coleman - 1986 graduate 
Judy P h i l l i p s  - U. of  F l o r i d a  graduate  5ChOOl 
Norman Simpson - employee NIH,  Washington, D.C. 
15 


Growth of lead tin telluride crystals in gels

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