Integrated Optics - Applied Optics Fundamentals and Device Applications - page 235

these growth chambers are completely isolated and contain their own indi-
vidual material sources, one can grow different material systems simultane-
ously. MBE systems are much more complex than MOCVD and thus more
susceptible to down time.
Material purity and defect density are two very important parameters that
need to be considered when growing devices. In general, MBE demonstrated
better control of background carrier concentration. MBE and MOCVD can
achieve background carrier concentrations of less than 10 15 cm −3 in AlGaAs.
It is easier to achieve these low levels in GaAs epitaxial growth than it is in
the ternary AlGaAs. One indication of the effect of background carrier con-
centration is to grow a multiple quantum well structure using both MOCVD
and MBE and to subject the devices to high temperatures (∼700°C) and mea-
sure the interdiffusion of the layers. This was performed by Hutcheson [26]
and the results showed the MOCVD layers to have about 10 times more inter-
diffusion than the MBE structure. For a large number of devices (including
lasers and passive waveguides) low background carrier concentration is not
required. In fact, laser structures require carrier concentrations of 10 18 cm −3 .
High speed modulators and switches, however, do require background car-
rier concentrations of 10 15 cm −3 or less. This is driven by the need to have
high resistivity material when applying the voltages required to induce
index changes via the electro-optic effect. When designing the material
requirements for an optical circuit which includes modulators and switches
one must specify the breakdown voltage to be much larger than required
because all of the device processing after material growth tends to reduce
the breakdown voltage by a factor of three to five.
Surface defect density has always been a problem for MBE growth while
MOCVD has been able to overcome this particular problem. When com-
paring defect densities between MOCVD and MBE one must be cautious
to compare only the relevant results. MOCVD can routinely produce defect
densities of 10-50 cm −2 while, for optoelectronic devices, MBE has defect
densities of 200-500 cm −2 . Although MBE material growers have achieved
defect densities of less than 100 cm −2 , this was accomplished on MODFET
and HEMT electronic structures. These structures are grown at low tem-
perature (350°C) and are not suitable for optical devices. This is due to the
traps that exist in the bandgap and cause excessive optical losses (>100 dB
cm −1 ). To get rid of these traps in the bandgap the layers must be grown at a
higher temperature. In MBE, the higher the growth temperature, the larger
the defect density. When integrating the large numbers of devices on a single
substrate, yield is directly related to defect density. This may cause a severe
limitation for the MBE technique.
MOCVD demonstrates better routine control of Al concentration and
thickness than MBE. This means that if a design specified a certain Al con-
centration and thickness, MOCVD would usually come closer to meeting the
specifications; however, MBE will in general have better uniformity over a
full 3-in.-diameter substrate than MOCVD. Growth of AlGaAs on GaAs by