Surface emitting laser using two wafer bonded mirrors

The present invention provides 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.

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 .mu.m and 1.5
 .mu.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 .mu.m to 1.6 .mu.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 .mu.m to 1.5 .mu.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 .mu.m and 1.6 .mu.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.sub.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 .mu.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 .mu.m, it is probably not capable of CW high power operation at 1.3
 .mu.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 .mu.m and 1.5 .mu.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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to FIG. 3, the present invention provides a vertical cavity
 surface emitting laser 300 comprised of a first mirror region 302, a
 second mirror region 304 and an active region 306. The active region 306
 has a first major surface 308 and the second major surface 310. Unlike
 previous VCSELs, the present invention fuses both the first major surface
 308 and the second major surface 310 of the active region to the first and
 second mirror regions 302, 304. Specifically, the first major surface 308
 of the active region 306 is fused to the first major surface 312 of the
 first mirror region 302 and the second major surface 310 of the active
 region 306 is fused to the first major surfaces 314 of the second mirror
 region 304.
 Although any number of materials may be used for formation of the mirror
 regions 302, 304, the mirror regions 302, 304 are preferably made of a
 semiconductor material having a high reflectivity in the desired
 wavelength range and good thermal and electrical conductivity. Similarly,
 the material used for formation of the active region 306 is preferably a
 semiconductor material having high optical gain, a low transparency
 threshold current density, and high reliability in the desired wavelength
 range. In the preferred embodiment, the material from which the first
 mirror region 302 is formed is lattice-matched to a substrate crystal with
 a different lattice parameter than the substrate crystal to which the
 active region 306 is lattice-matched. The material chosen for formation of
 the mirror regions 302, 304 or the active region 306 may be any material
 which allows high quality epitaxial growth.
 Although a number of materials may be used, in the preferred embodiment the
 first mirror region 302 is comprised of a plurality of alternating pairs
 of n-doped GaAs/AlAs (gallium arsenide/aluminum arsenide)
 quarter-wavelength thick layers epitaxially grown on a gallium arsenide
 substrate 316. Typically, the n-mirror region 302 consists of 20 to 30
 periods of alternating n-type GaAs/AlAs quarter-wave layers. The interface
 between the layers may be graded in alloy composition or using an
 AlAs/GaAs/Al.sub.0.3 Ga.sub.0.7 As variable duty cycle short period
 superlattice ("SPSL"). The grading reduces any heterojunction band
 discontinuity at the GaAs interface.
 The active region 306 includes a gain region 317 and typically also
 includes first and second cladding layers 318, 320 which sandwich the gain
 region 317. The gain region 317 is typically constructed of one or more
 quantum wells of InGaAsP. Alternative materials such as InAlGaAs,
 InGaAsSb,GaAs, InGaAs, InGaP, HgCdTe, PbSnTe, ZnSSe, GaAlAs, and InGaAlP
 may be used for construction of the gain region. In the embodiment shown
 in FIG. 3, the first cladding layer 318 is n-doped and the second cladding
 layer 320 is p-doped. In the preferred embodiment the cladding regions are
 comprised of InP although alternative materials such as any of the
 materials listed above, Si or SiGe may be used. The first cladding layer
 318 has a first major surface 321 and a second major surface 322, the
 first major surface 321 of the first cladding layer 318 being fused to the
 first major surface 312 of the first mirror region 302 and the second
 major surface 322 of the first cladding layer 318 positioned next to the
 gain region 317. The second cladding layer 320 has a first major surface
 323 and a second major surface 325, the first major surface 325 of the
 second cladding layer 320 being fused to the first major surface 314 of
 the second mirror region 304 and the second major surface 325 of the
 second cladding layer 320 positioned next to the gain region 317.
 Similar to the first mirror region 302, the second mirror region 304 is
 typically comprised of alternating quarter-wave layers of GaAs/AlAs.
 However, the second mirror region should have a conductivity type opposite
 to that of the first mirror region, in this case p-doped. The second
 mirror region 304 may also contain proton isolation regions 324 for more
 efficient current channeling and electrode 326 is formed on the top
 surface (the second major surface) of the second mirror region. An
 electrode 328 is also formed on the bottom side of the gallium arsenide
 substrate 316.
 The method of making the optoelectronic device according to the present
 invention includes the steps of: forming a first region 302 on a first
 substrate 316, 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 306 on the first major surface
 of a second substrate 332, 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 304 on the first major surface of a
 third substrate 336, the third region being comprised of a third material
 having a lattice parameter different than the second lattice parameter;
 fusing the first region 302 to the second region 306 and removing the
 second substrate 332; and fusing the second region 306 to the third region
 304 and removing the third substrate 336. Typically, the lattice-matched
 materials have a less than 0.1% difference to the substrate to which they
 are matched. Typically, the lattice parameter of the second region which
 has a lattice parameter different than the lattice parameter of the first
 and third materials has a difference of between 1 to 5% as compared to the
 lattice parameter of the first and third regions.
 In the case of a vertical cavity surface emitting laser the first and third
 regions 302, 304 are highly reflective mirror regions and the second
 region 306 is an active region. Specifically, the method of making the
 vertical cavity surface emitting laser 300, includes the steps of: forming
 the first mirror region 302 on a substrate 316, the first mirror region
 302 being comprised of a first material; forming an active region 306 on
 the first major surface 312 of the first mirror region 302, the active
 region 306 comprised of a second material different from the first
 material; forming a second mirror region 304; fusing the first mirror
 region 302 to the active region 306; and fusing the second mirror region
 304 to the active region 306.
 A step in forming the VCSEL 300 is forming a first mirror region 302 on a
 first surface 330 of a substrate 316. In the preferred embodiment the
 first mirror region 302 is a Bragg mirror comprised of alternating pairs
 of n-doped GaAs/AlAs quarter-wave layers epitaxially grown on a substrate
 316. The interface between the quarter-wave layers may be graded. The
 grading smooths any heterojunction band transition at the GaAs/AlAs
 interface. The doping level is typically 1.times.10.sup.18 /cm.sup.3 in
 uniform regions and approximately 3.times.10.sup.18 /cm.sup.3 in the
 graded regions. Reflectivity of the first mirror region 302 is
 approximately 98.9%. For simplicity, only a few of the alternating pairs
 of layers are shown in FIGS. 3 and 4. Further, the VCSEL structures shown
 in FIGS. 3 and 4 are not to scale.
 A further step in forming of a VCSEL 300 is formation of the gain region
 306. The gain region 306 is formed from a second material lattice matched
 to a substrate different than the first material from which the first
 mirror region 302 is formed. In the preferred embodiment, the second
 substrate material is indium phosphide (InP). The gain region 306 is
 typically comprised of a n-cladding layer 318, a light generation region
 317, and a p-cladding layer 320.
 Referring to FIGS. 4A-4C shows a series of cross-sectional view of the
 method steps required for formation of the VCSEL. In the preferred
 embodiment, the gain region 306 is formed on a substrate 332 after the
 formation of an etch stop layer 334. The etch stop layer 334 is positioned
 between the gain region 306 and the substrate 332. The etch stop layer
 334, allows the indium phosphide substrate 332 to be removed in a
 subsequent step without removing the gain region 306.
 After formation of the etch stop layer 334 on the substrate 332, a
 p-cladding layer 320 is formed on the etch stop layer 334. The p-cladding
 layer 320 is typically comprised of indium phosphide having a thickness of
 approximately 500 nanometers and p-doped to a concentration of
 approximately 1.times.10.sup.18 atoms/cm.sup.3.
 After growth of the p-cladding layer 320, a light generation region 317 is
 formed. The light generation region 317 includes at least one layer
 typically comprised of InGaAsP and grown to a thickness between 10
 angstroms and 0.2 microns. The layer is typically doped to a concentration
 of 2.times.10.sup.17 atoms/cm.sup.3 and has an emission wavelength near
 1.3 or 1.55 microns. Next, a n-cladding layer 318 is grown on the top
 surface of the light generation region 317. The n-cladding layer 318 is
 typically comprised of indium phosphide having a thickness of less than
 four emission wavelengths of the cladding material and a dopant
 concentration of approximately 1.times.10.sup.18 atoms/cm.sup.3.
 After formation of the first mirror region 302 and the gain region 306, the
 first mirror region 302 is fused to the gain region 306. FIGS. 4A-C show a
 cross-sectional view of the preferred method steps required for formation
 of the VCSEL shown in FIG. 3. FIG. 4A shows the resultant VCSEL structure
 after the steps of forming of the first mirror region 302 on a substrate
 316 and formation of the gain region 306 on a substrate 332, but before
 the step of fusing the gain region 306 to the first mirror region 302.
 In the embodiment shown in FIG. 4A and 4B, the n-cladding layer 318 of the
 active region 306 is fused to the n-type first mirror region 302 before
 fusion of the p-type cladding layer 320 to the p-type second mirror region
 304. Alternatively, the p-type cladding layer 320 may be fused to the
 p-type second mirror region 304 before fusion of the n-type cladding
 region 318 to the n-type mirror region 302. In this alternative process,
 the n-type cladding layer 318 is formed on the surface of the etch stop
 layer 332, followed by formation of the light generation region 317,
 followed by formation of the p-cladding layer 320. This exposes the
 p-cladding layer 320 for fusing.
 Fusing is defined as forming covalent bonds between two dissimilar
 materials. Fusing the GaAs/AlAs first mirror region 302 and the InP gain
 region 306 into a single hybrid solid is performed using a technique
 called wafer fusion (or wafer bonding). In the present invention, wafer
 fusion is performed by aligning the first major surface 321 of the first
 mirror region 302 to the first major surface 308 of the gain region 306,
 and placing the two surfaces 321, 308 in physical contact at a temperature
 of approximately 650.degree. C. for approximately 30 minutes. The fusion
 temperature (in this case approximately 650.degree. C.) is the temperature
 at which mass transport of atoms starts to occur for either the first
 mirror material or the gain region. In the preferred embodiment, the first
 mirror region 302 and the gain region 306 are placed in a hydrogen ambient
 to help to remove oxide at the prepared surfaces.
 After the two structures 302 and 306 have been fused together, the indium
 phosphide substrate 332 upon which the gain region 306 has been formed is
 removed. The indium phosphide substrate 332 is removed by exposing the
 substrate 332 to a wet chemical etchant such as HCl:H.sub.2 O which reacts
 with the indium phosphide substrate until reaching the InGaAs(P) etch stop
 layer 334. Next, the etch stop layer 334 is selectively removed exposing
 the active region 304. Typically, this active region surface is InP which
 is preferred over the etch stop material for the bonding process.
 Some additional advantages to the double bonding process concern the
 residual stress profile and the ease of fabrication. Since the bonding
 process occurs at elevated temperature, about 650 degrees Celsius, there
 is a residual stress in the structure at room temperature. This is the
 result of the different thermal expansion coefficients of the two
 materials joined in the bonding process. This residual stress can be a
 problem if it intersects an exposed surface of the material as any small
 crack can be quickly propagated through the material causing wafer
 breakage. If the first and third materials are the same or at least have
 the closely matched thermal expansion coefficients, then the residual
 stress is confined to the interior of the structure making it less
 susceptible to breakage. If the first and third materials are GaAs or
 materials lattice-matched to GaAs, then the processes developed earlier
 for device fabrication of GaAs devices will also work on this
 double-bonded structure. For the purposes of contact formation and current
 isolation, no new processing technology needs to be developed.
 A further step in the formation of the vertical cavity surface emitting
 laser 300 is the formation of a second mirror region 304. Although not
 required, in the preferred embodiment the second mirror region 304 is
 comprised of the same material as the first mirror region 302, in this
 case gallium arsenide. The second mirror region 304 is formed on the
 surface of a gallium arsenide substrate 336 in a manner similar to the
 formation of the first mirror region 302. However, because the gallium
 arsenide substrate 336 will be removed in a subsequent step, an etch stop
 layer 338 is formed between the substrate 336 and the second mirror region
 304. Further, the second mirror region 304 is p-doped so that it has a
 conductivity type opposite to that of the first mirror region 302.
 Similar to the first mirror region 302, the second mirror region 304 is
 typically comprised of alternating GaAs/AlAs quarter-wave epitaxially
 grown layers. Typically the mirror is p-type and number of alternating
 quarter-wave pairs is in the range of 15 to 30. The doping and grading of
 the alternating layers is typically in the same ranges as the n-doped
 GaAs/AlAs alternating layers in the n-doped first mirror region 302.
 Since the overall length of the resonant cavity formed between the two
 mirror regions 302 and 304 must be precisely controlled, the mirror region
 304 may be designed to accommodate any inaccuracies in the growth of
 either the first mirror region 302 or the active region 306. This is
 accomplished by measuring the reflectance spectrum of the fused active
 region 306 and mirror region 302 with the active region substrate 332
 removed. The structure of regions 302 and 306 can be deduced from this
 measurement and any inaccuracies adjusted for by changing the thickness of
 key layers in the second mirror region 304. The ability to make such an
 adjustment improves the probability of successfully achieving the proper
 cavity length.
 After formation of the second mirror region 304 and formation of the gain
 region 306, the second mirror region 304 is fused to the gain region 306.
 Typically, the second mirror region 304 is fused to the gain region 306
 after fusion of the gain region 306 to the first mirror region 302.
 Referring to FIG. 4B shows the VCSEL structure after the step of fusing
 the first mirror region 302 to the gain region 306 and removing the
 substrate 332 but before the step of fusing the second mirror region 304
 to the gain region. Before fusion, the second mirror region 304 and the
 gain region 306 are positioned so that the first major surface 314 of the
 second mirror region 304 faces the second major surface 310 of the gain
 region 306.
 Fusion of the second mirror region is similar to the previously described
 fusion of the first mirror region to the gain region. Typically wafer
 fusion occurs by aligning the second mirror region 304 and gain region 306
 and then placing them into physical contact for approximately 650.degree.
 C. for 30 minutes in a hydrogen ambient. After fusion of the second mirror
 region 304 to the gain region 306, the gallium arsenide substrate 336 is
 removed. In this case, since the substrate 336 is removed so that only the
 p-doped second mirror region 304 remains, it is immaterial whether the
 substrate 336 upon which the second mirror region 336 is formed is n-type
 or p-type.
 Typically the gallium arsenide substrate 336 is removed by exposing the
 substrate 336 to a wet chemical etchant such as NH.sub.4 OH:H.sub.2
 O.sub.2 :H.sub.2 O. The etchant removes the substrate and stops on an etch
 stop layer 338, typically comprised of aluminum arsenide. The aluminum
 arsenide layer is then selectively removed exposing the second major
 surface 340 of the second mirror region 304. FIG. 4C shows the resultant
 VCSEL structure after the steps of (1) fusing the first mirror region 302
 to the gain region 306 and (2) fusing the second mirror region 304 to the
 gain region 306.
 The removal of the substrate 336 is typically followed by formation of
 proton or mesa isolation regions and electrode or contact 326, 328
 formation using techniques well known in the art. The VCSEL embodiments
 shown in FIG. 3A and 3C include proton isolation regions 324 formed after
 formation of the mirror region 304. Typically, the proton isolation
 regions 324 are formed by forming a mask on the surface of mirror region
 304 and implanting H+ ions to form proton isolation regions 304. In
 contrast, the mesa isolation regions as seen in FIGS. 3B and 3D are formed
 by etching. FIG. 3B shows a bottom emitting VCSEL according to an
 alternative embodiment of the present invention. The mesa isolation
 regions are formed by forming a mask on the surface of mirror region 304
 and etching a predetermined distance into the mirror region 304 and/or the
 active region 306.
 After either the formation of the proton isolation region 324 or the
 formation of mesa isolation regions, contacts are formed using techniques
 well known in the art. Contact placement is dependent in part on whether
 the VCSEL is a top emitting laser or a bottom emitting laser. FIGS. 3A and
 3B show bottom emitting VCSELs while FIGS. 3C and 3D show a top emitting
 VCSELs. Typically, electrodes 326 and 328 are formed of gold or gold
 alloys.
 It is understood that the above description is intended to be illustrative
 and not restrictive. By way of example, the step of fusing the gain region
 to the second mirror region may occur before the step of fusing the gain
 region to the first mirror region. Further, the number of Bragg mirrors in
 the mirror regions, the dopant concentration levels and the materials used
 in the first mirror region, the second mirror region and the gain region
 may vary dependent upon the desired laser characteristics. The process of
 dual wafer bonding is applicable to other light emitting devices including
 but not limited to resonant cavity LEDs, surface normal tunable detectors,
 and resonant photodiodes. The invention should therefore not be determined
 with reference to the above description, but instead should be determined
 with reference to the appended claims, along with the full scope of
 equivalent to which such claims are entitled.