Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal
Reexamination Certificate
2002-10-04
2004-09-28
Pham, Long (Department: 2814)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
C257S014000
Reexamination Certificate
active
06797533
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to quantum well devices and to a method of changing and/or controlling the effective bandgap energy in quantum well structures, particularly Indium Gallium Arsenide Phosphide (InGaAsP) devices or structures. More particularly, it is concerned with improving the characteristics of such devices, particularly their blue shift, through the use of a low temperature grown indium phosphide (LT-InP) cap layer.
DESCRIPTION OF RELATED ART
The semiconductor industry is currently interested in integrating various optoelectronic devices, such as lasers, modulators and detectors, within a single semiconductor structure. This initiative is motivated by the increasing demand for optoelectronic technology particularly in the optical telecommunications field.
Integrated optoelectronic devices are of great interest due to the optical alignment and optical coupling efficiency challenges associated with using discrete optoelectronic devices. Within an integrated optoelectronic device, each optical component is spatially self aligned as result of being fabricated within the same semiconductor structure. This inherently gives better transmission between the components of an integrated device, as compared to putting together discrete devices. However, in order to ensure that each of the separate optical components within the structure has its own independent characteristics, local modifications to the semiconductor quantum well structure of each component are usually necessary. Many known fabrication techniques for one component of an integrated structure tend to have the unwanted effect of distorting or affecting properties of neighboring components.
Quantum Well Intermixing (QWI) is a post-growth method of bandgap engineering known in the art, enabling controlled changes in the bandgap energy of selected regions of the quantum well structure. Quantum Well Intermixing uses a Rapid Thermal Annealing (RTA) process, also known in the art, to provide controlled diffusion of defects into the quantum well structure of an optoelectronic device. These defects are usually provided by a layer or layers of specially grown material that are grown above the quantum well structure. Under the influence of the RTA process, the defects diffuse down into the quantum well structure and introduce changes to the bandgap properties. QWI has attracted considerable interest in locally modifying the quantum well band structure of integrated optoelectronic devices, including tunable wavelength lasers, photodetectors, and modulators. It is believed to be capable of modifying one component with minimum impact on neighboring components.
Different thermally-driven quantum well intermixing techniques such as Ion-Implantation Disordering (IID), Impurity Free Defect Diffusion (IFDD), Photo-absorption Induced Disordering (PAID) and Impurity-induced Layer Disordering (IILD) have been utilized in order to modify the quantum well structure in selected regions.
In Ion Implantation Disordering (IID), high energy implanted ions may introduce lattice damage to the quantum well structure, resulting in reduced light output. The Impurity-Induced Layer Disordering (IILD) technique requires long anneal times and/or high anneal temperatures (>800° C.) for diffusing impurities into the quantum well region. This can cause undesirable changes in the characteristics of neighbouring components within an integrated optoelectronic devices. It also introduces unwanted impurities, causing undesirable changes to the properties of the quantum well structure. The Impurity Free Defect Diffusion (IFDD) technique is free of impurities, but control of the QWI process depends on the deposited cap layer being used, its deposition conditions and the subsequent thermal anneal treatment. If, for example, a silicon dioxide (SiO
2
) cap layer is used, the thermal anneal process requires the use of temperatures between 750-800° C. These anneal temperatures may cause an uncontrollable shift in device operating wavelength, such as the emission wavelength of laser devices. Also, the surface of the grown material may become unstable and therefore unsuitable for subsequent processing of components such as gratings. Furthermore, strain and damage may be introduced to the hetero-structure surface. Finally, Photo-absorption Induced Disordering (PAID) suffers from poor spatial resolution. Consequently, it is difficult to confine this effect to an intended component within an integrated device, without affecting adjacent components.
A QWI technique is described in the above-cited parent application. The technique will be disclosed here, with revisions to represent current knowledge.
FIG. 1
shows a typical InGaAsP multiple quantum well structure
10
of a laser device. The structure
10
is grown by GSMBE in sequential layers starting from a 5000 Å InP Buffer layer
22
which itself is formed on an n+InP substrate
11
. The next layer grown on top of the InP buffer layer
22
is an 800 Å 1.15Q guiding layer
18
a.
The following layers grown above the guiding layer
18
a
form a conventional quantum well active region
13
, which comprises three quantum well layers
16
and four 1.24Q quaternary layers
17
. The 1.24Q quaternary layers
17
provide barrier regions of higher bandgap energy between the quantum well layers
16
. Optical emissions are generated within this quantum well active region
13
. A second 1.15Q guiding layer
18
b
is grown on top of the last quaternary layer
17
. Optical emission generated in the active region
13
is mostly confined between the guiding layers
18
a,
18
b
in order to concentrate the optical output emission from the laser device. A 250 Å InP grating layer
14
is grown above the second 1.15Q guiding layer
18
b
and used in the process of etching a grating for a Distributed Feed-Back (DFB) laser A 50 Å InGaAsP layer
19
grown above the InP grating layer
14
is used as an etch stop layer for removal of the LT-InP layer
20
after patterning and removal of the remaining InP defect layer, and completeing the RTA process. This layer protects the underlying InP layer from being etched away during the removal of the InP defect layer
20
. The layers
14
,
19
and
20
are initially undoped but doped p-type at 5-6×10
17
upon growth over etched gratings. The InGaAsP layer
19
also protects the InP/1.15Q grating layers
14
and
18
b
from contamination prior to etching the grating. A 1000 Å InP defect layer
20
is grown above the InGaAsP layer
19
for the quantum well intermixing process. These defects have been postulated to be donor-like Phosphorus-antisites or acceptor-like Indium-Vacancies. Using standard photolithography, this layer is patterned and etched to define the areas to be modified by QWI During the RTA process, defects in the InP defect layer
20
diffuse into the quantum well region
13
. On completion of the photolithography/etching and the RTA process, the defect layer
20
, the etch stop layer
19
and the grating layer
14
are etched away. This etching process may be used to introduce a grating, if required, into the 1.15Q guiding layer
18
b.
Subsequently the device structure will be completed with additional growth of layers which are typically in sequence InP
14
, InGaAsP etch stop
19
, InP
21
and finally the 1000 Å InGaAs contact layer
15
. The InGaAs layer
15
is a contact layer for applying current to the device. Doping levels and type of these layers depends on the type of device being fabricated. The InGaAs contact layer and the layer of InP
21
is normally etched into a ridge structure for confining and guiding the applied device current from the InGaAs contact layer
15
to a narrow region of the quantum well active region
13
. The 1 &mgr;m InP layer
21
and the InGaAs contact layer
15
are grown after the RTA process, once the quantum well active region structure
13
has been grown. All layers with the exception of the InP defect layer
20
are typically used in optoelectronic device fabrication. Once
Gordon Brooke
Lee Alex S. W.
Letal Gregory J.
Robinson Bradley J.
Thompson David A.
Blank Rome LLP
Farahani Dana
McMaster University
Pham Long
LandOfFree
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