Developments in the science and technology of
crystal growth have played a vital role in the
rapid technological progress in the latter part
of the 20th century.
Crystal growth finds applications
in areas as diverse as
- bulk growth of Si ingots (100m) that drive
the Pentium chips
- to epitaxial and non-epitaxial thin films
- to "nano"wires and quantum dots
(10-7 to 10-9m) which are hot research topics
currently, but expected to be of technological
importance in the future.
Professor Srinivasan Raghavan's
research interests encompass all the above areas
of crystal growth. He is currently concentrating
on growth of the group III-A(Al, Ga, In) nitrides
by chemical vapor deposition with particular emphasis
on the effects of stress on crystal growth, physical
properties and device performance.
Group III-A nitrides:
These nitrides by virtue of their band gap (~1-6
eV) are used in a number of opto-electronic applications
such as blue, white and ultraviolet light emitting
diodes, 400 nm laser diodes and could potentially
be used in photovoltaics or solar cells with efficiencies
>30%. They are also candidates for the next
generation high power-high frequency electronics
because of their large break down field strength,
high electron saturation velocity and high thermal
conductivity. (More) However, in spite of all
the impressive strides made in technology, much
of the basic science behind the growth of these
materials still remains poorly understood. This
is so because the poor stability of these nitrides
along with the lack of lattice matched substrates
makes growth relatively more complicated than
for example Si or GaAs based devices. Si and GaAs
devices are deposited on Si and GaAs substrates.
However, bulk nitride substrates are yet unavailable
because of difficulties in processing. Hence,
growth is currently done heterogeneously on sapphire
(Al2O3), SiC and Si substrates. The lack of wettability,
the lattice mismatch between the nitrides and
these substrates and the thermal expansion mismatch
all contribute to stresses and defects which affect
device properties and performance. Thus, growing
nitrides is a challenge.
Stresses and Defects:
Defects, such as grain boundaries, dislocations
and cracks are always detrimental to device performance.
In the case of nitride thin films, due to poor
wetting of the substrates by the film, growth
occurs by nucleation and coalescence of islands.
Due to small levels of misalignment between the
grains, the process of coalescence generates a
grain boundary and dislocations to accommodate
the misalignment. The process of coalescence also
gives rise to a tensile growth stress. If in addition
the epitaxial stresses and the thermal expansion
mismatch stress are tensile, then cooling to room
temperatures can cause cracking.
While defects are invariably
detrimental to device performance, stress could
also have benign effects. Strained (or stressed)
silicon in which electron mobilities are twice
as fast as the unstrained counter parts have been
used in Intel Pentium 4 chips (http://www.penstarsys.com/editor/tech/cpu/amd/str_sil/index.html).
In GaN/AlGaN high electron mobility transistors
for high power-high frequency devices in mobile
applications, piezo-electric (or stress induced)
polarization effects yield 2DEGs (2-dimensional
electron gases) with sheet carrier concentrations
in excess of 1013/cm2 close to an undoped GaN/AlGaN
interface, with 300 K mobilities greater than
1500 cm2/Vs. Piezo-electric fields have also been
shown to effect the optical properties in GaN/AlGaN
and GaN/InGaN quantum well structures.
Stress evolution and defect evolution
during crystal growth are inter-dependent. For
e.g. cracking occurs in response to a tensile
stress. Thus, the two need to be studied in tandem.
By understanding how these two are related during
crystal growth, devices can be grown with low
defect levels and the required levels of stress
(or strain), a process called strain engineering,
to yield the desirable properties.