<s>
Chemical	B-Algorithm
beam	I-Algorithm
epitaxy	I-Algorithm
(	O
CBE	O
)	O
forms	O
an	O
important	O
class	O
of	O
deposition	O
techniques	O
for	O
semiconductor	B-Architecture
layer	O
systems	O
,	O
especially	O
III-V	O
semiconductor	B-Architecture
systems	O
.	O
</s>
<s>
This	O
form	O
of	O
epitaxial	O
growth	O
is	O
performed	O
in	O
an	O
ultrahigh	O
vacuum	B-General_Concept
system	O
.	O
</s>
<s>
The	O
term	O
CBE	O
is	O
often	O
used	O
interchangeably	O
with	O
metal-organic	B-Algorithm
molecular	I-Algorithm
beam	I-Algorithm
epitaxy	I-Algorithm
(	O
MOMBE	B-Algorithm
)	O
.	O
</s>
<s>
When	O
used	O
in	O
the	O
strictest	O
sense	O
,	O
CBE	O
refers	O
to	O
the	O
technique	O
in	O
which	O
both	O
components	O
are	O
obtained	O
from	O
gaseous	O
sources	O
,	O
while	O
MOMBE	B-Algorithm
refers	O
to	O
the	O
technique	O
in	O
which	O
the	O
group	O
III	O
component	O
is	O
obtained	O
from	O
a	O
gaseous	O
source	O
and	O
the	O
group	O
V	O
component	O
from	O
a	O
solid	O
source	O
.	O
</s>
<s>
Chemical	B-Algorithm
beam	I-Algorithm
epitaxy	I-Algorithm
was	O
first	O
demonstrated	O
by	O
W.T.	O
</s>
<s>
This	O
technique	O
was	O
then	O
described	O
as	O
a	O
hybrid	O
of	O
metal-organic	O
chemical	B-Algorithm
vapor	I-Algorithm
deposition	I-Algorithm
(	O
MOCVD	B-Algorithm
)	O
and	O
molecular	B-Algorithm
beam	I-Algorithm
epitaxy	I-Algorithm
(	O
MBE	O
)	O
that	O
exploited	O
the	O
advantages	O
of	O
both	O
the	O
techniques	O
.	O
</s>
<s>
Typical	O
pressure	O
in	O
the	O
gas	O
reactor	O
is	O
between	O
102	O
Torr	O
and	O
1	O
atm	O
for	O
MOCVD	B-Algorithm
.	O
</s>
<s>
Here	O
,	O
the	O
transport	O
of	O
gas	O
occurs	O
by	O
viscous	B-Application
flow	O
and	O
chemicals	O
reach	O
the	O
surface	O
by	O
diffusion	O
.	O
</s>
<s>
A	O
comparison	O
of	O
the	O
different	O
processes	O
in	O
the	O
growth	O
chamber	O
for	O
MOCVD	B-Algorithm
,	O
MBE	O
and	O
CBE	O
can	O
be	O
seen	O
in	O
figure	O
1	O
.	O
</s>
<s>
In	O
order	O
to	O
better	O
understand	O
the	O
growth	O
kinetics	O
associated	O
with	O
CBE	O
,	O
it	O
is	O
important	O
to	O
look	O
at	O
physical	O
and	O
chemical	O
processes	O
associated	O
with	O
MBE	O
and	O
MOCVD	B-Algorithm
as	O
well	O
.	O
</s>
<s>
It	O
is	O
worth	O
noting	O
that	O
no	O
chemical	O
reaction	O
is	O
involved	O
at	O
the	O
surface	O
since	O
the	O
atoms	O
are	O
generated	O
by	O
thermal	O
evaporation	B-Algorithm
from	O
solid	O
elemental	O
sources	O
.	O
</s>
<s>
In	O
MOCVD	B-Algorithm
,	O
group	O
III	O
alkyls	O
are	O
already	O
partially	O
dissociated	O
in	O
the	O
gas	O
stream	O
.	O
</s>
<s>
The	O
temperatures	O
are	O
typically	O
100-150	O
°C	O
lower	O
than	O
they	O
are	O
in	O
a	O
similar	O
MOCVD	B-Algorithm
or	O
MOVPE	B-Algorithm
.	O
</s>
<s>
There	O
is	O
also	O
no	O
boundary	O
layer	O
(	O
such	O
as	O
the	O
one	O
in	O
MOCVD	B-Algorithm
)	O
and	O
molecular	O
collisions	O
are	O
minimal	O
due	O
to	O
the	O
low	O
pressure	O
.	O
</s>
<s>
Selective	O
growth	O
through	O
dielectric	O
masking	O
is	O
readily	O
achieved	O
using	O
CBE	O
as	O
compared	O
to	O
its	O
parent	O
techniques	O
of	O
MBE	O
and	O
MOCVD	B-Algorithm
.	O
</s>
<s>
With	O
chemical	O
sources	O
,	O
the	O
reactions	O
associated	O
with	O
the	O
growth	O
rate	O
are	O
faster	O
on	O
the	O
semiconductor	B-Architecture
surface	O
than	O
on	O
the	O
dielectric	O
layer	O
.	O
</s>
<s>
This	O
makes	O
it	O
easier	O
to	O
perform	O
selective	O
epitaxy	O
using	O
CBE	O
and	O
at	O
lower	O
temperatures	O
,	O
compared	O
to	O
MOCVD	B-Algorithm
or	O
MOVPE	B-Algorithm
.	O
</s>
<s>
This	O
has	O
been	O
exploited	O
for	O
fabricating	O
high	O
quality	O
heterojunction	O
bipolar	O
transistors	B-Application
.	O
</s>
<s>
CBE	O
offers	O
many	O
other	O
advantages	O
over	O
its	O
parent	O
techniques	O
of	O
MOCVD	B-Algorithm
and	O
MBE	O
,	O
some	O
of	O
which	O
are	O
listed	O
below	O
:	O
</s>
<s>
Compatibility	O
with	O
other	O
high	O
vacuum	B-General_Concept
thin-film	O
processing	O
methods	O
,	O
such	O
as	O
metal	O
evaporation	B-Algorithm
and	O
ion	O
implantation	O
.	O
</s>
<s>
More	O
pumping	O
required	O
compared	O
to	O
MOCVD	B-Algorithm
.	O
</s>
