Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
1996-11-15
2001-08-21
Kunemund, Robert (Department: 1765)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S246000, C438S287000, C438S485000
Reexamination Certificate
active
06277696
ABSTRACT:
BACKGROUND OF THE INVENTION
Vertical cavity surface emitting lasers capable of emitting long wavelengths are of interest in optical communication systems. In particular, emission of light having wavelengths near 1.3 &mgr;m and 1.5 &mgr;m has wide applications in fiber optic communications. Unfortunately, for a given wavelength, materials ideal for formation of the gain region of a vertical cavity surface emitting laser (VCSEL) are not always ideally suited for formation of the mirror regions of the VCSEL. For example, for light emission in the 1.2 &mgr;m to 1.6 &mgr;m wavelength range, the material which can be grown lattice matched to indium phosphide (InP) is ideal for gain region formation. However, material which is lattice matched to indium phosphide is undesirable for VCSEL mirror formation since it does not provide high reflectivity in the 1.2 &mgr;m to 1.5 &mgr;m wavelength range. Similarly, while the material lattice-matched to GaAs substrates makes highly reflective mirrors, it is not a good material choice for VCSEL gain region formation in the 1.3 &mgr;m and 1.6 &mgr;m wavelength range.
The reference “Continuous Wave GaInAsP/InP Surface Emitting Lasers with a Thermally Conductive MgO/Si Mirror”, T. Baba, et al, Jpn. J, Appl. Phys., Vol. 33 (1994), pp. 1905-1909, describes a VCSEL which uses different materials for the gain region and mirror region formation.
FIG. 1
shows an etched well VCSEL
100
such as is described in Baba, et al. The etched well VCSEL
100
shown in
FIG. 1
is comprised of a gain region
102
formed on an indium phosphide substrate
104
, an n-side mirror region
112
comprised of six pairs of SiO
2
/Si layers, and a p-side mirror region
110
comprised of 8.5 pairs of (MgO/Si) layers. The mirror regions
110
,
112
are formed by depositing dielectric films on the active region
102
and the indium phosphide substrate
104
, respectively. Although, the dielectric mirror regions
110
and
112
provide high reflectivity which could not be accomplished by using semiconductor layers lattice-matched to an indium phosphide substrate, the dielectric mirrors
110
,
112
provide poor thermal and no electrical conduction. Poor electrical and thermal conductivity of the mirror regions results in overheating of the VCSEL, negatively impacting device performance characteristics.
Alternatively, different materials for manufacture of the gain region and mirror regions of a VCSEL may be integrated by fusing a second mirror region material to a first material used for gain region formation. The gain region being previously deposited on the first mirror region using deposition techniques well known in the art. One example of such a structure is shown in the article by Babic', et al., “Optically Pumped all-epitaxial wafer-fused 1.52 &mgr;m vertical cavity lasers,” Electronic Letters, Apr. 28, 1994, Vol. 30, No. 9. Although the semiconductor mirror regions of the VCSEL structure described in Babic', et al. offer improved thermal and electrical conductivity compared to the insulating dielectric mirrors
110
,
112
of Baba, et al., the Babic' laser is difficult to manufacture. Although the Babic' laser design is useable for operating at 1.5 &mgr;m, it is probably not capable of CW high power operation at 1.3 &mgr;m.
Another example of fusing a first mirror region comprised of a first material to a second material for forming the gain region comprised of a different material is described in the reference “Low Threshold Wafer Fused Long Wavelength Vertical Cavity Lasers,” by Dudley, et al., Applied Physics Letters, Vol. 64, No. 12, 1463-5, Mar. 21, 1994.
FIG. 2
shows a single fused VCSEL
200
as described by Dudley, et al. The VCSEL described in Dudley, et al. combines a semiconductor mirror region
212
with an alternating semiconductor/dielectric mirror region
210
. Although the semiconductor mirror
212
offers improved thermal and electrical conductivity compared to the insulating dielectric mirror
112
shown in
FIG. 1
, the VCSEL
200
shown in
FIG. 2
still has poor thermal and electrical conductivity through dielectric mirror
210
. Further, the laser shown in
FIG. 2
injects current at the edge of the device. Injecting current at the device edge instead of through the center of the device causes an increase the heat generated, decreasing laser performance. The laser performance is also decreased due to the poor overlap of the carrier profile and the optical mode profile. This could cause the laser to operate in multiple transverse modes which is a problem for communication systems and for stable fiber optic coupling.
An example of using wafer bonding techniques for LED formation is shown in U.S. Pat. No. 5,376,580. Referring to FIG. 8 of U.S. Pat. No. 5,376,580, for example, shows wafer bonding a first growth substrate
30
and a second substrate
48
to epitaxial layers
32
-
38
. Wafer bonding for LED formation is typically used to bond a substrate material that is optically transparent to a LED active region formed of a different material.
A top or bottom emitting VCSEL in the 1.3 &mgr;m and 1.5 &mgr;m wavelength range which provides a high gain, high reflectivity, good thermal conductivity and good electrical conduction through both mirrors is needed.
SUMMARY OF THE INVENTION
The present invention provides an optoelectronic device, specifically a vertical cavity surface emitting laser having high gain and high reflectivity in the desired wavelength range and good thermal and electrical conductivity. The laser structure is comprised of a first mirror region, a second mirror region, and an active region positioned between the first and second mirror regions. Unlike, prior VCSELs, the active region is fused to both the first mirror region and the second mirror region. This allows the laser designer to optimize laser performance for the desired wavelength range by allowing the choice of different materials for the first mirror region, the second mirror region, and the active region.
In the preferred embodiment the VCSEL structure has electrodes formed on the top surface of the second mirror region and the bottom surface of the substrate. The first electrode, being placed in close proximity to the gain region, can more easily withdraw heat generated in the laser. Efficient heat removal is beneficial since many of the characteristics of the VCSEL deteriorate with increased temperature.
The method of making the optoelectronic device according to the present invention includes the steps of: forming a first region on a first substrate, the first region being comprised of a first material lattice-matched to the first substrate, the first material having a first lattice parameter; forming a second region on the first major surface of a second substrate, the second region being comprised of a second material lattice-matched to the second substrate, the second material having a second lattice parameter different than the first lattice parameter; forming a third region on the first major surface of a third substrate, the third region being comprised of a third material having a lattice parameter different than the second lattice parameter; fusing the first region to the second region and removing the second substrate; and fusing the second region to the third region and removing the third substrate. In the case of a vertical cavity surface emitting laser the first and third regions are highly reflective mirror regions and the second region is an active region.
REFERENCES:
patent: 5013681 (1991-05-01), Godbey et al.
patent: 5376579 (1994-12-01), Annamalai
patent: 5376580 (1994-12-01), Kish et al.
patent: 5395788 (1995-03-01), Abe et al.
patent: 5449659 (1995-09-01), Garrison et al.
patent: 5455202 (1995-10-01), Mahloy et al.
James J. Dudley, Wafer Fused Vertical Cavity Lasers, Aug. 1994, University of California, Santa Barbara, complete document, cover page +pp. ii-176.
T. Baba, et al., “Continuous Wave GalnAsP/InP Surface Emitting Lasers with a Thermally Conductive MgO/Si Mirror”, Jpn. J, Appl. Phys., vol. 33 (1994), pp. 1905-1
Babic Dubravko I.
Bowers John E.
Carey Kent W.
Dudley James J.
Yang Long
Hewlett--Packard Company
Kunemund Robert
Lee Denise A.
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