A surface potential model is proposed to consistently explain the known dopant gas effects on silicon chemical vapor deposition. This model predicts that the effects of the same dopant gases on the diamond deposition rate using methane and carbon tetrachloride should be opposite and similar to those of silane, respectively. Available data are in agreement with this prediction

Chang, C. A.

English

Chang, Chin-An.

eScholarship - University of California

DOPANT GAS EFFECT ON SILICON CHEMICAL VAPOR DEPOSITIONS. A SURFACE POTENTIAL MODEL

NASA Technical Reports Server

N76 12503  
Dopant Gas Erec t  on Silicon Chemical Vapor Depositions 
A Surface Potential Model 
rpr 
Chin-An Chang 
Inorganic Materials Research Diwisioa, Lawrence Berkeley W o r a t o q  
Univeryity af Califor Aa, Berkeley, California 94.. 20 
A surface potential model is proposed to consistently explain the known 
dopant gas effects on silicon chemical vapor deposition. This model predicts 
that the effects of the same dopant gases on the diamond deposition rate using 
methane and carbon tetrachloride should be opposite and similar to those of 
silane, respectively. Available data is in agreement with this prediction. 
* Present address: IBM Thomas J. Watson RFsearch Center, 
Yorktown Heights, New York, 13598 
630 
https://ntrs.nasa.gov/search.jsp?R=19760005415 2020-03-22T17:56:35+00:00Z
Chemical vapor deposition (CVD) has been widely used for thin filrn 
depositions. This technique is especially applicable to thin film silicon solar 
cells. Deposition rate of a few microns per minute can be easily obtained, 
and p-n junction can be made by mixing silicon chemical vapors (CV), such as 
silane and silicon tetrachloride, with depant gases, like diborane and phosphine. 
In generai CVD of silicon is carried out at a substrate temperature around 
1OGO to 1200"CD and single crystal silicon wafer is used for an epitaxial growth 
of thin films. However, for an economic thin film solar cell, aon-crystalline, 
and oftentimes non-silicon substrates, a re  required. Furthermore, inter- 
action between the substrate chosen and the silicon thin film deposited should 
be kept minimal. For example, at a substrate temperature around 1200"c, 
silicon thin films deposited on graphite show a significant diffusion of silicon 
1 and carbon and the formation of silicon carbide . Much less diffusion is noted, 
however, when the substrate temperature is below 800°C . 
temperature interaction between the silicon films deposited and other types of 
substrate has also been reported . Preferably, one should use the lowest 
possible substrate temperatures to minimize such interactions and diffusions. 
2 Similar high 
3 
A t  low substrate temperatures, however, other problems arise. First, 
4 silicon film deposited is polycrystalline with small grain sizes . Second, 
much lower deposition rate than that a t  high substrate temperature is obtained 
using the CVD technique . Small grain size means a shortening of lifetime for 
the charge carriers due to trapping by the grain boundaries ; low deposition 
rate makes CVD a non-economic technique for depositions. 
former problem the author has developed a technique to increase *Ae silicon 
crystallinity a t  low substrate temperatures . An enhancement of two to {our 
orders of magnitude is obtained for the silicon films deposited on quartz and 
graphite at 600°C substrate temperature', '. Our next goal is to increase the 
deposition rate of low temperature CVD. As part of our effort toward this 
goal we describe in t'lis paper a conceptual model which could lead to a better 
understanding of the existing information on the variation of CVD deposition 
rates. 
effects on the deposition rates of silicon chemical vapors and correctly pre- 
dicts for those on carbon chemical vapors. 
of this model to other studies are  also discussed. 
5 
6 
To solve the 
7 
. .  
This model is shown to consistently explain the known dopant gas 
The implications and application: 
63 1 
It has been known that silicon deposition rate using silane, SiH4, and 
silicon tetrachloride, Sic1 , is increased by diborane, B2H6, and decreased 
8 by phosphine, PH3, and arsine, AsH3 . Existing theories considering active 
si te blocking8b and strong bonding between the C V  molecules and dopant gas 
molecules8c have not been advanced enough to explain the known effects i n  a 
consistent way, nor can they be used to predict the dopant gas effect on other 
C V  molecules. 
dopant gas for silicon, gives an opposite effect on the deposition rate of silicon 
chemical vapors from those by phosphine and arsine both of which being n-type 
dopant gases. 
structuro of the dopant atoms with the observed effects cited. 
involved in the C V D  process is first analyzed, in order to see how this property 
can be incorporated into the deposition process. 
4 
In our approach we make use of the fact that diborane, a p-type 
This implies a possible correlation between the electronic 
The mechanism 
Chemical vapor deposition can be viewc-l as a two-step process: adsorp- 
tion of the C V  molecule on the substrate surface followed by its thermal decom- 
position. As an example, deposition of silane follows 
SiH4 (gas) - SiH4 (adsorbed) heat si t Z H ~ .  
The thermal decomposition part involves a transfer of thermal energy from the 
substrate to the C V  molecule needed for its decomposition. 
temperature, the maximal amount of thermal energy which can be acquired by 
the C V  molecule is constant. The efficiency of this energy transfer, however, 
depends on the residence times of the C V  molecule on the substrate surface. 
In other words, for an effective decomposition of the C V  molecule to take place 
on the surface, a sufficient amount of thermal energy necessary for such decom- 
position should be transferred to the C V  molecule before it desorbs from the 
surface. Therefore, a t  a fixed substrate temperature, decomposition of the 
C V  molecule will be determined by the adsorption efficiency of this molecule 
on the substrate surface. 
Farrow who found that silane adsorption is the rate limiting step for the 
deccmposition of this molecule8b. Accordingly, a t  a given substrate tempera- 
ture, any factors which enhance the adsorption rate of the C V  molecule should 
also enhance its decomposition and therefore i ts  deposition rate. 
At a fixed substrate 
This argument is in agreement with the work of 
Our problem 
632 
is thus simplified to that of the effect of dopant gas on the adsorption of silicon 
chemical vapor molecules. 
+ From the chemical point of view, silane has the partially ionic Si 
9 10 bonds . Being tetrahedral, such a molecule possesses no net dipole moment . 
However, the ionic character mentioned would make silane a molecule with 
four negatively charged hydrogen atoms surrounding a positively charged 
silicon atom in the center. 
with positive surface potential and be repelled from one with negative surface 
potei,t.al. In other words, making the surface potential more positive would 
'.  . 
-H- 
Such a molecule would be attracted by a surface 
: . lzne better and enhance i ts  adsorption rate. The same argument applies 
t -*licc.i tetrachloride which has Si -C1- ionic bonds, similar to those in 
.me. 
Next, we study the effect of dopant gas on the surface potential of a 
substrate. 
boron, arsenic and phosphorus atoms, respectively. These atoms then adsorb 
on the surface and beconre doped into the silicon film deposited. As mentioned 
earlier, the observed deposition rate dependence of silane and silicon tetrachlo- 
ride showed possible correlation with the electronic structure of the dopant 
atoms. Furthermore, during silicon CVD, there is always a silicon surface 
with freshly adsorbed silicon and dopant atoms1'. Our problem is therefore 
similar to the study of the effect of dopant atoms on the . ce potential of a 
silicon surface. Supporting this approach is the experimental observation that 
when the substrate is covered with a monolayer of boron atoms, the depcsition 
rate of silane is the highest among the depositions using the silane-diborane 
mirtures . 
Diborane, arsine and phosphine thermally decompose to release 
8C 
First, w e  define a reference surface to be a silicon surface with only 
adsorbed silicun atoms. 
used. A boron adso. bed silicon surface can be seen to be different from the 
reference one. 
silicon, should, relative to the reference surface, lower the local electron 
density on the surface sltizon atoms around th? adsorption site. 
increase the electron affinity of the silicon surface onto .vhich boron is 
This is the case when pure silicon chemical vapor is 
Boron, being a p-type dopant and electron deficient relative to 
This would 
633 
adsorbed". The effect on the surface potential is seen from the relation 
defining this property . I2  
= (EV - Ei) t 
Here EF, E 
of the valence band, respectively, S stands for surface, @and 9 are  photo- 
electric threshold and work function, respectively. 
x is the electron affinity, and EG, the band gap between the bottom of the 
conduction band and the top of the valence band. An increase in electron 
affinity, on the surface is seen to make the surface potential more positive than 
the reference surface defined above13. Such a surface would then a t s a c t  
molecules like silane and silicon tetrachloride. 
tion and therefore the deposition rate of these molecules. 
phosphorus and 
relative to s i l ic  Jn, would make the surface potential more negative thar, the 
reference one. 
ride should be Lowered when they a re  mixed with phosphine and arsine. 
observed dopa?-,t gas effect on the deposition rates of silicon chemical vapors is 
thus satisfactorily explained. 
E 1' v a re  the energies for the Fermi ::vel, intrinsic level and top 
Furthermore, @ =  X t EG, 
This should enhance the adsorp- 
On the other hand, 
rsenic, which a re  n-type dopants anel electron-excessive 
, Accordingly, the deposition rate of silane and silicon tetrachlo- 
The 
These results are summarized in Table I. 
An immediate test of the proposed model is to choose some CV mole- 
cule of different bonding character from that of SiH4 and SiClq. 
dopant gas effect on the deposition rate of this chosen CV molecule should then 
be expected. One such molecule is methane, CH4. Methane is also tetrahedral 
but has the C- -H type of ionic bonds. 
tively charged in methane, opposite to that for SilaIte. 
on the deposition rate of methane should then be just the opposite of that for 
silane. 
CC14, however, the dopant gas effect should be similar to that for silane. 
This is because of the C -C1- ionic bonding character of this molecule. 
2redicted dopant pas effect for methane and carbon tetrachloride a re  also 
1is;er' 
t. . . I ' - 1  =sition shoulci be used. Diamond film has the similar 
st, 
Different 
t The hydrogen atoms are partially posi- 
The dopant gas effect 
When one goes to another carbon C V  molecule, carbon tetrachloride, 
t The 
- cle 1. For a meaningful test of o;lr model to carbon CVD, data on 
*I f i lm ,  and differs from the layer structure of a graphite 
T'hc t-t.. i.tcr2ture data on the dopant g a s  effect an diamond 
634 
Table I. Predicted and observed dopant gas effects on the deposition rates of * 
silicon and carbon chemical vapors 
cc l4  cH4 SiH4 SiC14 
Bonding 
c+ - c1- + si+ - H- si+ - ~ 1 -  C- - H 
Characters 
B2H6 
PH3 
AsH3 
I I D I 
Ia Ia Db 
D D I D 
Da 
D D I D 
Da 
L I and D indicate an increase and decrease, respectively, in deposition 
rate. 
second row for the observed effects. 
The f i r s t  row for each dopant gas is fo r  the predicted effects, the 
a. Ref. 9. 
b. Ref. 14. 
14 deposition concerns the methane-diborane system . The results clearly show 
from that observed using only methane. This is just the effect predicted by our 
model. 
Although limited by the existing information for  a full test of our model, 
agreement with the available data is encouraging. It is therefore worthwhile 
to point out some useful works, both experimental and theoretical, that a r e  
necessary for a better understanding of the CVD process. 
for the lackinr information listed in Table I should be carr ied out for a complete 
(1) Experiments 
635 
tes t  of our model. 
dopant atom effect on the surface potential. 
applying a bias voltage to the substrate and observe its effect on the deposition 
rate of pure CV molecules. 
surface and tested for its effect on the deposition rate  of each CV molecule. 
(3) Surface properties for silicon and diamond should be studied in the presence 
of adsorbed boron, arsenic and phosphorus atoms. This includes surface pro- 
perties such a s  the surface states and work function. Such study would allow 
a direct  understanding of the adsorbed dopant atom effect on the surface pro- 
perties and is essential to our final understanding of the problem discussed in 
this paper. 
dopant atom and another adsorbed CV molecule is also very important. Such 
interactions could be long ranged and involve coupling with the surface . 
Calculations of the interactions described in this paper could be very difficult 
a t  the mornent16 but certainly a r e  necessary for  a better understanding of the 
adsorption mechanism. 
(2) Our model descrives the dopant gas effect through the 
This can therefore be tested by 
Similarly, an eiectric field can be applied to the 
(4) Theoretical calculation on the interactions between an adsorbed 
15 
Once the suggested experiments a r e  shown to support our model proposed 
here, :t will then be possible to increase the deposition rate of CVD a t  low 
temperatures. 
described in this paper one should be able to increase the deposition :ate beyond 
the current limit. 
and dopant gas that allows the maximal increase in deposition rate. 
from Table I a r e  SiH4-B,H6, CH4-PH3. CC14-BZH6, etc. 
limiting factor will then be the amount of thermal energy available a t  low 
temperatures. 
thermal energy needed and the minimal interaction and diffusion allowed between 
the deposited thin film and the substrate chosen. 
also be applicable to the CVD of other types of thin films, such as those for 
Si3N4# Sic ,  W, etc. 
By adjusting the surface potential according to the principles 
One can also choose the right combination of CV molecule 
Examples 
Th? remaining " 
One needs then to find a compromise between the maximal 
Al l  these principles should 
Finally, since ocr model is concerned mainly with the eifect of one 
adso, bed species on the adsorption of another mo;ecule, this work should also 
be u s e h l  to surface catalysis studies. In surface catalysis molecular adsorp- 
tion plays an essential role before dec.>mposition or chemical reaction takes 
636 
place. By varying the surface potential as described in this paper one should 
observe changes of the chemical reaction rates  between adsorbed molecules. 
This should allow a better understanding and control of the chemical reactions 
under study. 
should provide impo: ;ant information on the formation and strength of the 
chemisorptive bonds between the adsorbed molecule and the surface. Again, 
more  experimental and theoretical work along this line a re  needed to further 
advance our idea to surface catalysis. 
In addition, wrying the magnitude and sign of the surface potential 
Ac know ledge ment 
This work was supported by the U. S. Energy Research and Development 
Administration. 
References 
1 .  
2. 
3. 
4. 
5. 
6. 
7.  
8. 
9. 
10. 
C. -A. Chang and W. J. Siekhaus, J. Appl. Phys. in press.  
C. -A. Chang, unpublished results. 
T. L. Chu, NSF Report, NSF/RANN/SE. GI-38981 /PR/73/4,  1974. 
(a) C.-A. Chang, unpublished results. (b) A. Emmanual and H. M.- 
Pollock, J. Electrochem. SOC., 120, 1586 (1973). 
(a) H. C. Theuerer, J. Electrochem. SOC. , 108 649 (1961); F. C. 
Everstegn and G. H. Put, ibid, 120, 106 (1973), L. H. Hall and K. M. 
Koliwad, ibid, - 120 1438 (1973). -(b) R. G. Frieser ,  ibid, - 115, 401 
(1968). (c) Ref. 4b. 
T. I. Kamins, J. Appl. Phys., - 42, 4357 (1971); Y Maturkura, Japanese 
J. Appl. Phys .2 ,  91 (1363). 
C-A. Chang, W. J. Siekhaus, T. Kaminska and D. T. HUO, Appl. 
Phys. Lett., 26, 178 (1975); C. -A. Chang and W. J. Siekhaus, to be 
published. 
(a) Ref. 5a. (b) Ria-Choudhury and P. L. Hower, J. Electrochem. 
SOC. -* 120 1761 (1973). (c) R .  F. C. Farrow, ibid, -* 121 899 (1974). 
F. A. Cotton and G. Wilkinson, - Advanced Inorganic Chemistry, Second 
Ed. , John Wiley and Scns, New York, (1966), p. 466. 
A. L. McCellan, Tables of Experimental Dipole Moments, W. H. 
Freeman and Company, San Francisco (1963). 
637 
11 . This is true for the silicon f i h s  deposited on silicon wafers, and also true 
btyond the first layer  of silicon film on non-silicon substrates. 
12. F. G. Allen and G. W. Gobeli, Phys. Rev. -' 127 150 (1962). 
13. W. H. Brattain and J. Bardenn, Bell System Tech. J. 32, 1 (l'f53); 
H. C. Montogmery and W. L. Brown, Phys. Rev. -@ 103 '9a5 (1956). 
D. J. Poferl, N. C. Gardner and J. C. Angus, J. Appl. Phys., 44, 1428 14. 
(1973). 
15. 
16. 
T. B. Grimley, Proc. Phys. SOC. - 90, 751 (1967). 
3 .  R. Schrieffer, private communication. 
63 8 


Dopant gas effect on silicon chemical vapor depositions: A surface potential model

Abstract

Similar works

Full text

Available Versions

eScholarship - University of California

NASA Technical Reports Server