A Wavelength Converter Integrated with a Discretely Tunable Laser ...

5.2 Epitaxy 65
of the stack Q(1.25)/Q(1.55)/Q(1.25) with thicknesses of 190 nm/120 nm/190 nm. In
the top 50 nm of the film an p-doping of ¤¤ cm is applied in order to position
the p-n junction inside the active layer. Finally, on top of the film layer, the first 200 nm
of the p-type InP cladding (p=¤ cm ) is grown.
II) In order to define the active and passive regions, a 100 nm thick ¦ SiO masking layer is
deposited. On top of the masking layer, the active regions are defined in photo resist
using optical lithography. The resist pattern is transfered from the resist into the ¦ SiO
mask using reactive ion etching (RIE). The active regions are rectangular stripes. Stripes
can be as long as desired, but they must be
SiO ¦ masked regions can occur in the next re-growth step. Here, stripes with a width of
20 were used.
50 , because otherwise, growth on the
III) The InP cladding layer is etched away in a selective wetichemical etch in HCl. Then
the Q(1.25) film and Q(1.55) active layer are etched away in the passive regions using
a wet-chemical etch in hydrofluoric acid (buffered oxide etch ”BOE”). Since the etch
is not selective between Q(1.25) and Q(1.55) material, the etch must be reproducibily
slow. The etch is aimed slightly beyond the bottom (maximal 50 nm) of the active layer
to ensure that the active material is removed completely.
IV) The upper 360 nm of the film layer and the first 200 nm of the InP cladding of the passive
regions are re-grown. The doping profile of the re-grown material can be optimized for
passive waveguides or phase modulators. Re-grown material that was used for devices
in this thesis was not intentionally doped (n.i.d.), n=¨¤ cm . The total step
height of the re-growth step was 550 nm, a height up to which good butt-joint couplings
were demonstrated [91]. If good quality re-growth over larger step heights than this is
feasible, should be subject to future research. Larger steps are necessary for reducing
optical loss in the p-doped cladding.
V) In the third and last growth step, a 1300 nm thick highly p-doped InP top cladding
(p=¤¤ cm ) and 570 nm thick InGaAs contact layer (p=¤¤ cm ) are
grown.
Table 5.1 summarizes the compositions, layer thicknesses and doping levels of the active and
passive layer stacks. The definition of the active and passive regions is done with the 2” mask
that is shown in Fig. 5.2a. The mask contains four copies of a 0.5” mask (Fig. 5.2b), because the
processing following the epitaxy is done on quarters. Each 0.5” mask contains eight sections
with active stripes. The triangular shaped sections contain active stripes of increasing lengths:
from 20 to 100 in steps of 10 , from 100 to 1500 in steps of 10 , one of 750
and one of 1250 . The pitch of the stripes is 150 . The six rectangular shaped sections
contain 12 amplifiers of 500, 750 and 1000 at a 250 pitch. Between the amplifiers
in each section lies a 100 long active stripe, which can be used as power monitor or
(saturable) absorber. The short active stripes (20-100 ) can be used in absorption and buttjoint
loss characterization. The four bars along the edges of the wafers are used for the mask
See Appendix A for more details on the mask fabrication.