Method for a reflective digitally tunable laser

A method for forming a reflective digitally tunable laser using selective area epitaxy is disclosed. The laser comprises passive waveguides and a plurality of optical amplifiers. The waveguides and optical amplifiers are formed by depositing multiple quantum wells having a suitable bandgap. According to the method, the multiple quantum wells forming both the passive waveguides and the optical amplifiers are deposited simultaneously using a dielectric mask. The mask comprises dual, rectangularly-shaped strips of dielectric material, spaced to form a gap. The multiple quantum wells grown in the gap are suitable for use as optical amplifiers, and those grown outside of the gap are suitable for use as passive waveguides.

CROSS REFERENCE TO RELATED APPLICATION 
This invention is related to application Ser. No. 08/152,603 [Kaminow 45-6] 
of Ivan Kaminow and Martin Zirngibl, entitled "Reflective Digitally 
Tunable Laser". 
FIELD OF THE INVENTION 
This invention relates to an epitaxial growth method for forming a laser, 
and more particularly to using selective area epitaxy to form a reflective 
digitally tunable laser. 
BACKGROUND OF THE INVENTION 
Photonic integrated circuits are typically comprised of a plurality of 
photonic devices, located on a semiconductor substrate, that are in 
optical communication with one another. Most methods for creating photonic 
integrated circuits involve forming one photonic device at a time. This is 
due to an inability to regionally vary the bandgap of the quantum well 
(QW) material being deposited in a given epitaxial growth. 
In the methods noted above, the epitaxial layers required to form a first 
type of photonic device, such as a laser, are grown over the whole 
substrate. The temperature, pressure and source materials used for the 
growth are selected so that the quantum well (QW) material that is 
deposited has the requisite characteristics, i.e., band gap, to function 
as the desired device. The layers are then masked at the region where the 
first photonic device is desired. Subsequently, the layers in unprotected 
regions are etched away where other devices, such as modulators or 
waveguides, are desired. After etching, layers corresponding to a second 
type of photonic device are grown on the substrate in the etched regions. 
Growth conditions are adjusted for the second growth so that the QW 
material exhibits the appropriate band gap. If a third type of photonic 
device is desired, the layers are again masked and etched, conditions are 
adjusted and a third series of epitaxial layers are grown in the etched 
region. 
Methods that utilize successive growths as described above may collectively 
be referred to as "etch and regrow" methods. Devices grown from the etch 
and regrow method frequently exhibit poor optical interface quality, which 
can result in internal reflections and coupling losses. 
Selective area epitaxy (SAE) is an epitaxial growth method that minimizes 
the poor optical interface problems associated with the etch and regrow 
method. Using SAE, the bandgap of QW material can be varied in the same 
plane with a single growth. Thus, layers defining various photonic devices 
can be grown simultaneously. See Joyner et al., "Extremely Large Band Gap 
Shifts for MQW Structures by Selective Epitaxy on SiO.sub.2 Masked 
Substrates," IEEE Phot. Tech. Lett., Vol. 4, No. 9 (September 1992) at 
1006-09 and Caneau et al., "Selective Organometallic Vapor Phase Epitaxy 
of Ga and In Compounds: A Comparison of TMIn and TEGa versus TMIn and 
TMGa," J. Crystal Growth, Vol. 132 (1993) at 364-70. 
In the SAE method, dielectric masks, such as SiO.sub.x or SiN.sub.x, are 
deposited on a substrate. Such masks typically comprise two strips of 
dielectric material, spaced to form a gap. Source material for forming the 
epitaxial layers, such as indium (In), gallium (Ga), arsenic (As), and 
phosphorus (P), is typically delivered via metalorganic vapor phase 
epitaxial (MOVPE) methods. 
Source material arriving from the vapor phase will grow epitaxially in 
regions where the mask is open, i.e, the substrate is uncovered. Source 
material landing on the mask itself will not readily nucleate. Given the 
proper temperature and mask width, most of the source material that lands 
on the mask reenters the vapor phase and diffuses, due to the local 
concentration gradient, to find an unmasked region. 
Compared to a completely unmasked substrate, the QW growth that occurs in 
the gap for both InGaAs and InGaAsP epilayers will be thicker, and richer 
in indium. This effect is due to the relative diffusion coefficients of In 
and Ga under typical MOVPE growth conditions. As the QW layers thicken, 
changes occur in the quantum confined Stark effect resulting in longer 
wavelength (lower energy band gap) QW material. Increasing indium content 
also results in longer wavelength QW material. Thus, from both the 
quantum-size effect and change in alloy composition, the QWs in the gap 
are shifted to lower energy band gaps than regions far from the mask. By 
varying the ratio of the mask width to the gap (width), the composition, 
and hence the bandgap, of QW material can be varied. 
SUMMARY OF THE INVENTION 
A method is disclosed for forming a reflective digitally tunable laser 
wherein the QW material for forming the passive waveguides as well as the 
optically active regions are deposited in a single QW growth step using 
SAE. The method uses masks comprising dual, rectangularly-shaped strips of 
dielectric material, spaced to form a gap. Mask and gap size is varied so 
that the QW material deposited in the gap has a bandgap suitable for use 
as an optical amplifier, while QW material deposited at other regions has 
a bandgap suitable for use as waveguiding material.

EXAMPLE 
An embodiment of the laser 1 was formed according to the present invention. 
The entire device was fabricated in the six primary steps described above. 
The source materials were deposited using MOVPE at 100 torr and 
615.degree. C. Ethyldimethylindium (10.degree. C.), trimethylgallium 
(-11.degree. C.), 100 percent arsine and 100 percent phosphine were used 
as the source materials. The substrate was 2 inch &lt;100&gt; oriented indium 
phosphide (InP), heavily S doped at about 10.sup.18 /cm.sup.3. 
First, a 4000 angstrom thick guiding layer of InGaAsP having a bandgap 
wavelength varying from 1.1 to 1.35 micrometers was deposited on the 
substrate. Next, a layer of undoped InP about 150 angstroms thick was 
deposited on the guiding layer. 
In step 2, twelve SiO.sub.2 masks were deposited on the undoped InP layer 
at the desired location for the optical amplifiers. The SiO.sub.2 was 
deposited to a thickness of about 3000 angstroms and then wet etched 
through baked photoresist masks using buffered HF to created the desired 
mask configuration. Each mask comprised a pair of SiO.sub.2 rectangles, 
each 40 micrometers in width and spaced from one another by 15 
micrometers. 
The MQW stacks were grown using SAE. The InGaAsP QW layers in the optically 
active region were 95 angstroms thick. The InGaAsP barrier layers were 173 
angstroms thick. The InGaAsP QW layers in the passive waveguiding region 
were 50 angstroms thick and the InGaAsP barrier layers were 65 angstroms 
thick. Five QW layers were grown in both the optically active regions and 
the passive waveguiding regions. 
After depositing the MQW stacks, the masks were etched away using HF. 
In the third step, a 150 angstrom thick layer of lightly n-type (about 
5.times.10.sup.15 /cm.sup.3) InP was deposited on the MQW stacks. A 300 
angstrom thick layer of InGaAsP having a bandgap wavelength of about 1.35 
micrometers was then deposited on the InP layer. A layer of lightly Zn 
doped InP, about 6000 angstroms thick, was deposited on the InGaAsP layer. 
Next, a 300 angstroms thick cap layer of InGaAsP was deposited on the InP 
layer. 
Next, the MQW stacks in the passive waveguiding regions were uncovered and 
patterned into 2 to 4 micrometer wide ribs. After the ribs 46 are 
patterned, the optical amplifier regions were protected with SiO.sub.2. 
In the fourth growth, a 3000 angstrom layer of undoped InP was deposited. 
The passive waveguiding regions were then protected with SiO.sub.2 and 
photoresist. Next, the optical amplifier regions were patterned into mesas 
using a wet chemical HBr etch. The waveguides were then uncovered. 
In the fifth step, a Fe doped InP current blocking layer two microns thick 
was deposited over all exposed surfaces. Next, a 1000 angstrom layer of 
lightly n-doped InP was deposited. The strips of SiO.sub.2, and the cap 
layer of InGaAsP were removed in preparation for the last growth. 
In the sixth and final growth, a layer of heavily Zn doped InP 
(2.times.10.sup.18 /cm.sup.3) was deposited over all surfaces, followed by 
a layer of more heavily Zn doped InGaAsP (5.times.10.sup.18 /cm.sup.3). 
Gold contacts were deposited over each optical amplifier and the wafer was 
cleaved at two locations to define the laser cavity. 
It should be understood that the embodiments described herein are 
illustrative of the principles of this invention and that various 
modifications may occur to, and be implemented by those skilled in the art 
without departing from the scope and spirit of the invention. For example, 
the quantum layers for other devices, such as modulators, could be 
deposited in the same QW growth step in which the optical amplifiers and 
passive waveguides are deposited. Thus, the method of the present 
invention for forming a reflective digitally tunable laser can be used to 
form a variety of photonic integrated circuits.