Buried hetero-structure InP-based opto-electronic device with native oxidized current blocking layer

A buried hetero-structure with native oxidized current blocking layer for InP-based opto-electronic devices comprises a InP semiconductor substrate, a buffer layer, a ridge mesa containing lower confinement layer, active layer and upper grating confinement layer, a first InP cladding layer and a native oxidized Al-bearing layer as current blocking layers at both lateral edges, a second InP cladding layer, contact layer, contact metal, and the second ridge mesa covered with insulating layer. This method is to facilitate the processing of conventional buried hetero-structure InP-based opto-electronic device and improve the performance under high temperature and high current operation.

FIELD OF THE INVENTION
 The present invention relates to buried hetero-structure InP-based
 opto-electronic devices and more specifically to an improved method of
 manufacturing such devices.
 BACKGROUND OF THE INVENTION
 It is well-known that InP-based opto-electronic devices, such as
 distributed feedback (DFB) laser, distributed bragg reflector (DBR) laser,
 Fabry-Perot (FP) laser, waveguide, optical switch, amplifier, modulator
 and integrated devices etc., play an important role in optical fiber
 communication systems. Optical fiber communication systems require high
 performance lasers which are characterized by low threshold, high linear
 power output, stable fundamental mode, high modulation response, high
 temperature operation, etc.
 There are two common types InP based lasers used for opto-communication
 systems. The first type is weak index-guided laser, which requires that
 the thickness of at least one layer be laterally non-uniform. An example
 is ridge waveguide (RWG) laser, where the cladding layer is etched into
 ridge type, and the active layer laterally extends beyond ridge. the
 optical confinement is formed by the index step (.about.10.sup.-2) by both
 ridge waveguide geometry effective index step and the carrier induced
 index change. But the carriers are still not confined laterally. These
 type of lasers can be easily fabricated, for the FP lasers only require
 one epitaxy growth for DFB lasers it only require two types of lasers. But
 due to the intrinsic mechanism of weak index guided laser, in some
 functions, such as stable fundamental mode controlling, they are not good
 enough.
 The second type of laser is the strongly index-guided laser where the
 active layer is surrounded by a larger band gap, low index material (the
 index step is about 0.2) which forms a carrier and optical confinement,
 such as buried hetero-structure (BH) with reverse P-N junction lasers, The
 InGaAsP active strip is buried with a larger band gap InP material, and so
 the regrown material block current flows through reverse-biased junctions.
 So this type lasers combines all the good features of current, carrier,
 and photon confinements, which could satisfy the most demanding
 application in the fiber optical communication systems.
 However, the conventional fabrication process of such BH InP-based
 opto-electronic devices is very complicated. Without loss of generality,
 we will focus on the DFB laser for the following introduction because BH
 DFB laser is the most important device and is also the most difficult
 device to fabricate. Other BH InP-based devices can be similarly
 fabricated without grating. FIG. 1 illustrates a typical buried
 hetero-structure DFB laser processing procedure, which comprises the
 following steps: (1) FIG. 1a: first growth of buffer layer, cladding
 layers and active layer; (2) FIG. 1b: fabrication of grating and (3) FIG.
 1c: regrowth of cladding layer; (4) FIG. 1d: deposition of dielectric film
 such as SiO.sub.2 or Si.sub.3 N.sub.4 film and wet etching above layers
 into specific strip rib; (5) FIG. 1e: selective regrowth of p-n current
 block layer; (6) FIG. 1f: removal of dielectric film and regrowth of the
 cladding and contact layer; (7) FIG. 1g: wet etching to form two grooves
 parallel to the first rib to reduce the device capacitance; (8) FIG. 1h:
 deposition of dielectric layers and etching out a window for the contact
 layer and finally, evaporation of contact metals for both n and p
 metallisation.
 The complexity of the regrowth, selective regrowth, cleaning, deposition
 and removal of dielectric film, and wet etching of the stripe mesa,
 greatly increases the cost of BH DFB lasers and reduces the yield.
 Moreover, as can be seen in FIG. 2(a), for this type of BH laser, there are
 unavoidable leakage currents both from I to II and from III to IV for the
 pnpn type thyristor. From the equivalent circuit FIG. 2(b), the anode
 current I.sub.A until the thyristor break over could be:
EQU I.sub.A
 =[Io+(.alpha..sub.2.cndot.M.sub.n.box-solid.I.sub.g)]/[1-(.alpha..sub.
 1.cndot.M.sub.p +.alpha..sub.2.cndot.M.sub.n)] (1)
 Where
 I.sub.o : a leakage current of the junction J.sub.2
 I.sub.g : gate current
 .alpha..sub.1 : common based current gain of transistor T.sub.r1
 .alpha..sub.2 : common based current gain of transistor T.sub.r2
 M.sub.n : avalanche amplification factor for the electrons in the depletion
 layer of junction J.sub.2
 M.sub.p : avalanche amplification factor for the holes in the depletion
 layer of junction J.sub.2.
 As can be seen from the equation (1), the break-over conditions of the
 thyristor are represented by the following equation (2).
EQU .alpha..sub.1.cndot.M.sub.p +.alpha..sub.2.cndot.M.sub.n =1 (2)
 The .alpha..sub.1 and .alpha..sub.2 of the equation (2) are drastically
 increased normally when the anode current I.sub.A increases or the
 temperature of the diode rises.
 Further, the M.sub.n and M.sub.p generally have the dependency as
 represented by the following equation (3). In the equation (3), in the
 case of V&lt;&lt;V.sub.a1 M=1 is satisfied, but as the V approaches V.sub.a, the
 M is drastically increased.
EQU M=1/[1-(V/V.sub.a).sup.2 ] (3)
 Because of such behaviors of .alpha. and the M together with the
 relationship of the equation (1), increases in the gate current and the
 applied voltage V.sub.a, and temperature rise cause an increase in the
 anode current of the thyristor. Further, since a positive feedback system
 of increasing the .alpha..sub.1, and .alpha..sub.2 due to the increase in
 the anode current is formed, the thyristor feasibly break over. So the
 leakage current under high temperature and high output power operation is
 unavoidable for the p-InP/n-InP reversed biased junction BH laser.
 To address the above mentioned problems, we propose to replace InP p-n
 reverse junction block layer in conventional buried heterostructure (BH)
 InGaAsP/InP lasers by Al-bearing compound native oxide layer. This type of
 laser device not only keeps good features of the conventional BH laser by
 more facilitated processing method, but also the high insulation
 characteristic of native oxide will avoid the leakage current from p-n-p-n
 InP thyristor like junction dependence on the temperature and high power
 operation, so it will improve the high temperature and high power
 performance of the conventional BH InGaAsP laser.
 There has been an interest in the recent years to apply Al-bearing compound
 native oxide layer to opto-electronic devices because it is an insulator
 layer and has a low refractive index (n.about.1.6). The native oxide of
 Al-bearing compounds provides both electrical and optical confinement and,
 moreover, simplifies processing of the lasers. The principle of native
 oxidation is to expose heated Al-bearing materials in water vapor
 saturated gaseous ambient to form an anhydrous oxide of Aluminum which is
 very stable and does not degrade under normal operating conditions. This
 thickness of this type of oxide layer is the same or less than the
 thickness of the as-grown Al-bearing materials and it does not cause
 disruption or induce strain. Moreover, the refractive index is less than
 2, and therefore the oxidized layer will facilitate electron block and
 optical confinement.
 For the surface-emitting lasers, AlAs oxide has been successfully used for
 DBR structures and for current constriction to achieve low threshold
 devices. For edge emitting lasers, the native oxide of AlGaAs has been
 utilized to fabricate stripe-geometry lasers and index-guided buried ridge
 waveguide lasers. This technology is widely used for GaAs-based
 optoelectronic devices containing Al-bearing materials such as AlAs,
 AlGaAs, AllnP etc.. See U.S. Pat. Nos. 5,262,360 by N. Holonyak, Jr and J.
 Dadallesasse (AlGaAs native oxide), 5,550,081 by N. Holonyak, Jr., S. A.
 Maranowski., method of semiconductor device by oxidizing Aluminum-bearing
 III-V semiconductor in water vapor environment), S. A. Maranowski, A. R.
 Sugg, E. I. Chen, and N. Holonyak, Jr., Appl. Phys.Lett. 63, 1660 (1993),
 Y. Cheng, P. D. Dapkus, M. H. MacDouugal, and G. M. Yang, IEEE Photonics
 Technol, Lett. 8, 176 (1996), J. J. Wierer, S. A. Maranowski, N. Holonyak,
 Jr., P. W. Evans, and E. I. Chen, Appi, Phys. Lett. 8, 176 (1996), D. L.
 Huffaker, D. G. Deppe, Kumar, and T. J. Rogers, Appi. Phys. Lett. 65. 97
 (1994), K. L. lear, K. D. Choquette, R. P. Schneider, Jr., S. Kiloyne, and
 K. M. Geib, Electron. Lett. 31, 208 (1995), B. J. Thibeault, E. R.
 Hegblom, P. D. Floyd, R. L. Naone, Y. Akulova, and L. A. Coldren, IEEE
 Photonics Technol. Lett.8, 593 (1996), P. D. Floyd, B. J. Thibeault, E. R.
 Hegblom, J. Ko, L. A. Coldren, and J. L. Merz, IEEE Photonics Technol,
 Lett. 8, 590 (1996).
 So far, advances in the Al-bearing native oxidation in InP-based devices
 have been much slower. The native oxide of InAIAs, lattice-matched to InP,
 has been employed in the long wavelength InP/InGaAsP gain-guided or weak
 index guided buried ridge waveguide lasers, and as a gate insulator in a
 InAlAs/InGaAs MOSFET. There has also been a report using native oxide of
 AlAsSb, lattice-matched to InP with Al mole fraction of 1.0.
 Here, we propose to apply Al-bearing oxide to InP-based BH lasers to
 facilitate conventional InP based BH laser processing and prevent leakage
 current for the conventional BH laser.
 SUMMARY OF THE INVENTION
 Therefore, it is the object of the present invention to provide a
 cost-effective method of producing InP-based devices and improve the
 current blocking characteristics to suppress the leakage current under
 high temperature and high output power.
 In this invention, the applicants propose to grow Al-bearing material as
 cladding layer on InP based laser structure instead of the growth of
 reverse p-n junction as current blocking layers, and to utilize native
 oxidation technique to form a current blocking layer in the lateral part
 of the Al-bearing cladding layer, whereas the center non-oxidized region
 allows current passage. Therefore, this technique can avoid the second
 regrowth, selective regrowth, deposition and removal of dielectric film
 and several cleaning procedures proceeding each step. It also obviates the
 rigid requirement of the special shape of the ridge rib to prevent
 SiO.sub.2 polycrystal deposition. It only requires two regrowth steps, so
 the fabrication of BH DFB laser could be greatly facilitated by utilizing
 native oxidation.
 The Al-bearing layer non-planer growth on the active strip mesa will form a
 lateral growth step just around active mesa sides. Due to the fact that
 InAlAs oxidant diffusion rate is the rate limiting mechanism, the InAlAs
 oxide laterally travels and terminates at the corner of the Al-bearing
 non-planar growth step. It can easily achieve good current blocking
 performance as selective regrowth n-type current block layer in the
 conventional fabricating method.
 In the conventional method of wet oxidation of InAlAs single layer, the
 oxidation rate is usually much slower, normally about 2.about.3 .mu.m per
 hour at 500.degree. C. In the present invention, we use mixed InAlAs
 layers with high Al composition, which contain higher Al-composition to
 accelerate oxidation.
 Moreover, the high insulating characteristic of Al-bearing oxide will
 suppress the current leakage for conventional pnpn thyristor during high
 temperature or high current operation.

DETAILED DESCRIPTION OF THE INVENTION
 In order to better describe the present invention, a brief description of
 the processing method for the conventional BH DFB laser is given, as is
 illustrated in FIGS. 1a through 1h. The procedure comprises of the
 following: (1) FIG. 1a: A n-doped InP buffer layer 2, lower confining
 layer 3, active layer 4 and upper confining layer 5 are grown on InP
 substrate 1; (2) FIG. 1b: Fabrication of the grating on the surface of
 layer 5; (3) FIG. 1c: Regrowth a p-InP cladding layer 6 on the grating
 layer 5; (4) FIG. 1d: Deposition of dielectric layer 7 on the regrowth
 layer 6 and the wet etching of layers 7, 6, 5, 4, 3, and part of layer 2
 into a 2 .mu.m wide ridge rib with the ridge direction normal to the
 grooves of the grating. The shape of the rib is strictly controlled to
 avoid polycrystal deposition on the dielectric film cap layer 7 during the
 next selective regrowth. (5) FIG. 1e: p-InP layer 8 and n-InP layer 9 are
 selectively grown on both sides of the rib without depositing on the,
 dielectric film 7, so that they will automatically form a lateral current
 blocking layer beside the active layer; (6) FIG. 1f: Regrowth of p-InP
 cladding layer 10 and p.sup.+ -InGaAs contact layer 11 on the cladding
 layer 6 after removal of the film 7; (7) FIG. 1g: To increase the
 modulation bandwidth, it is required to reduce the capacitance of the
 device by etching two parallel grooves 12 along either side of the first
 stripe ridge; (8) FIG. 1h: Deposition of the dielectric film 13 and with a
 etched window for the p-metal contact 14 to contact layer 13. Contact
 metal 14 is just deposited on one side of the groove with specific shape
 for high speed modulation. The thinned down substrate is deposited with
 n-type contact metal 15.
 The present invention is described in FIGS. 3a through 3f. The process
 comprises the following steps: (1) FIG. 3a: A n-typed InP buffer layer 17,
 lower confining layer 18, active layer 19 and upper confining layer 20 are
 grown on the substrate 16; (2) FIG. 3b: Fabrication of the grating on the
 surface of layer 20; (3) FIG. 3c: Wet etching or reactive ion etching
 (RIE) of layer 17, 18, 19, 20 into &lt;2 .mu.m wide ridge rib with the ridge
 direction normal to the grating grooves, the height of the rib is equal to
 the thickness of the following growth layer 21, the strip mesa is along
 the [110] crystallographic direction, the shape of mesa could be straight
 or reverse-mesa; (4) FIG. 3d: Regrowth p-InP cladding layer 21, p-type
 Al-bearing material 22, p-InP cladding layer 23 and p.sup.+ -InGaAs
 contact layer 24 on the above said ridge; (5) FIG. 3e: Deposit SiO.sub.2
 or Si.sub.3 N.sub.4 film on the wafer as a following etching mask and long
 term oxidation protection mask (if oxidation time is less than 1 hour,
 this step could be omitted), patterning align two parallel grooves 25
 along either side of the ridge rib, which kept on the middle, wet etching,
 and utilizing the native oxidation technique to form current blocking
 layer along the lateral side of the Al-bearing layer 22 while the central
 part remains non-oxidized for current passage; (6) FIG. 3f: Deposition of
 the dielectric film 26 and with a etched window for the P-metal contact 27
 to contact layer 24, contact metal 27 is just deposited on one side of the
 groove with specific shape for high speed modulation. The thinned down
 substrate is deposited with n-type contact metal 28.
 In the above-mentioned two structures, the layers before fabrication of the
 grating are the same. The InP substrate 1 and 16 is n-doped
 (.about.10.sup.18 cm.sup.-3); the n-InP buffer layer 2 and 17
 (.about.8.times.10.sup.18 cm.sup.-3) is about 1 .mu.m thick; the undoped
 lower confinement separate layer 3 and 18 and the undoped upper
 confinement layer 5 and 20 are about 800.about.1500 .ANG. thick. Their
 energy band gap should be larger enough to form a electronic barrier. The
 active layer 4 or 19 could be a bulk structure, a quantum well structure
 or a strained or strain balanced quantum well structure. The confinement
 layer and active layer could be InGaAsP or AlGalnAs quaternary material
 system, which satisfies the requirement of a laser, such as electrical and
 optical confinement.
 For the conventional structure, p-InP cladding layer 6
 (.about.5.times.10.sup.17 cm.sup.-3) is about 1 .mu.m thick; the current
 blocking InP layer 8 ( p-type doped with .about.2.times.10.sup.18
 cm.sup.-3) and layer 9 (n-doped .about.2.times.10.sup.18 cm.sup.-3) are
 both about 0.5.about.1.5 .mu.m thick; the p-InP cladding layer 10
 (.about.5.times.10.sup.17 cm.sup.-3) is about 1.5.about.2.0 .mu.m thick;
 The p.sup.+ -InGaAs contact layer (10.sup.18.about.10.sup.19 cm.sup.-3) is
 less than 1.0 .mu.m.
 For the present invention, however, the thickness p-InP cladding layer 21
 (.about.5.times.10.sup.17 cm.sup.-3) is from 0 .mu.m to the height of
 active mesa(composed with the layers 17, 18, 19, 20). Preferably it's
 thickness should be the same as the height of the active mesa.
 The p-doped Al-bearing material 22 (.about.5.times.10.sup.17 cm.sup.-3)
 preferably be Al.sub.x Ga.sub.y In.sub.1-x-y As bulk, Al.sub.x Ga.sub.y
 In.sub.1-x-y As/Al.sub.x Ga.sub.yIn.sub.l-x-y As mixed multiple layers or
 AlAsSb, which could be oxidized without resulting in defect and device
 degradation, normally x&gt;0.4 and 0&lt;y&lt;1. The designed mixed layers
 containing high Al composition is to accelerate the oxidation process.
 Along the side part of the active mesa, the bottom of Al-bearing layers
 should be lower and just surpassed over the top of the active mesa, and
 the top of the Al-bearing layers should surpass the top of the active
 mesa. The total thickness of Al-bearing layers is in an range of 20 nm to
 1 .mu.m, The non-planer regrowth of layer 22 forms a growth step 29 around
 active mesa, the step will form an oxidation barrier, i.e making oxide 22'
 travel and terminate at step 29. This is because the InAlAs oxidant
 diffusion rate is the rate limited mechanism. This mechanism also reduces
 the rigid requirement for the photolithography alignment in the double
 groves etching, and make the oxidation reproducible comparing with planer
 lateral oxidation.
 The p-InP cladding layer 23 (.about.2.times.10.sup.18 cm.sup.-3) is 2 .mu.m
 or less in thickness; and the p.sup.+ -InGaAs contact layer
 (10.sup.18.about.10.sup.19 cm.sup.-3) is about &lt;1.0 .mu.m.
 The above-mentioned epitaxial layers are grown under low pressure using the
 MOVPE technique. The dielectric films 7, 13 and 27 are deposited by a
 plasma CVD method, and they could be SiO.sub.2 film or Si.sub.3 N.sub.4
 films. The contact metals 14 or 27 are p-ohmic contact which could be
 Au--Pt--Ti, while contact metals 15 and 28 are n-ohmic contact which could
 be Au--Ge--Ni, which can be thermally evaporated or sputter-deposited onto
 the contact epitaxial layer. The contact metal 14 and 27 could be
 patterned by using conventional photoresist lift-off technique.
 The grating formed on the layer 5 and 20 is fabricated by holographic
 photoresist and (CH.sub.4 /H.sub.2) reactive ion etching (RIE) technique.
 The ridge rib and grooves are formed by wet etching, and the etching
 solution could be Br:HBr:H.sub.2 O (1:8:25). The dielectric film, such a
 SiO.sub.2 is etched either by RIE or wet etching.
 The oxidation is carried out by heating the wafer in a water-containing
 environment up to a temperature of about 350 to 550.degree. C. Such a
 moist environment may be generated by flowing a gas, such as nitrogen
 through water heated to about 80-90.degree. C. The flow of the water vapor
 is continued for about half an hour to 10 hours.
 Thus, it will be understood that the present invention makes it possible to
 facilitate the fabrication of conventional buried hetero-structure
 distributed feedback laser.
 Without the fabrication of grating, we can similarly fabricate other
 InP-based opto-electronic devices, such as BH FP laser, optical amplifier,
 switch, modulator and integrated devices using this native oxidization BH
 technology.
 The present invention may be embodied in other specific forms without
 departing from the spirit or essential characteristics thereof. The
 presently disclosed embodiments are, therefore, to be considered in all
 respects as illustrative and not restrictive, the scope of the invention
 being indicated by the appended claims and all changes which come within
 the meaning and range of equivalency of the claims are, therefore, to be
 embraced therein.