Page 174 - Electrical Properties of Materials

156 Semiconductors

8.11.5 Molecular beam epitaxy
This is probably the best and most versatile method. Each material (various
semiconductors which make up the desired compound plus the dopants) sits
in a little box of its own in which it is heated by an oven to a temperature
usually above its melting point, all in ultra-high vacuum. The atoms evaporated
from the surface of the heated material are ready to move to the substrate,
provided they can get out of the box, that is the shutter is open. The duration for
which each shutter is open and the temperature of the oven will then determine
the flux of each element toward the substrate, and thus the composition of the
growth material. The growth can be monitored by electron scattering from the
surface.
Altogether this is a very precise method capable of excellent composition
control, once all the sources have been experimentally calibrated. It was used
to explore ternary and quaternary compounds such as GaAlAs but gave way
to cheaper liquid or vapour phase epitaxy once these techniques were estab-
lished. A further use has been for compounds such as many of the II–VIs, for
example ZnS, ZnSe, and CdS where it proved difficult to get both n- and p-
types, usually p was the stumbling block. Whilst the p-type impurities could
be established in the crystal lattice, the process of chemical equilibrium on
the heated substrate, essential for a good crystalline quality caused the hole
centres to be neutralized by electrons. MBE can avoid thermodynamic equilib-
rium and maintain the p centres active, provided temperature and the ambient
atmosphere are controlled.
Growing InGaN has been proved to be successful too after overcoming the
difficulty of a nitrogen source. Initially NH 3 was used, but decomposition is
only moderately efficient at temperatures below 800 C, so copious streams of
◦
NH 3 are needed in what is ideally a high-vacuum system. Nitrogen plasma
sources have evolved based on either an RF discharge at 13.56 MHz or an
electron cyclotron resonance breakdown at 2.45 GHz. This allows higher qual-
–1
ity growth at about 1 μmh . MOCVD is preferred for commercial production,
but MBE has produced higher quality with GaN, for example an electron mo-
–1 –1
2
bility of 1200 cm V s has been reported for an epilayer grown by MBE
on a MOCVD template. The usually quoted mobility for ‘typical’ GaN is
about half this. The combined use of two techniques to produce better material
properties is perhaps the way ahead for InGaN.

8.11.6 Metal–organic chemical vapour deposition
This process is more suited to be adopted by manufacturers. As the name im-
plies, it is a chemical process. The required elements are introduced into the
growth tube as compounds bound to organic substances, and they are then
chemically released to be deposited upon the substrate. One of the advantages
of the technique is that the otherwise poisonous gases are relatively harmless
in organic compounds. Another advantage is that the reaction can take place
over substrates that have a large area and, in addition, the process is quite fast.
Its disadvantage is that it is done at about atmospheric pressure which is in
general detrimental to accurate deposition and, among others, it prevents the
use of an electron beam to monitor thickness.