CA1194971A - Semiconductor laser with conductive current mask - Google Patents
Semiconductor laser with conductive current maskInfo
- Publication number
- CA1194971A CA1194971A CA000410492A CA410492A CA1194971A CA 1194971 A CA1194971 A CA 1194971A CA 000410492 A CA000410492 A CA 000410492A CA 410492 A CA410492 A CA 410492A CA 1194971 A CA1194971 A CA 1194971A
- Authority
- CA
- Canada
- Prior art keywords
- semiconductor laser
- fabricated
- layer
- active region
- current mask
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
- H01S5/1228—DFB lasers with a complex coupled grating, e.g. gain or loss coupling
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
- H01S5/042—Electrical excitation ; Circuits therefor
- H01S5/0425—Electrodes, e.g. characterised by the structure
- H01S5/04254—Electrodes, e.g. characterised by the structure characterised by the shape
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
- H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
- H01S5/06203—Transistor-type lasers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
- H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
- H01S5/06209—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes in single-section lasers
- H01S5/0622—Controlling the frequency of the radiation
Abstract
SEMICONDUCTOR LASER WITH
CONDUCTIVE CURRENT MASK
Abstract A semiconductor laser is disclosed wherein a grid-like conductive current mask is fabricated between the active region of the laser and one of its electrodes. In one embodiment, the conductive current mask is fabricated in tile bottom regions of a corrugated pattern that is created along the length of the semiconductor laser. In a second embodiment the conductive current mask is totally embedded within a lightly doped layer that is grown proximate to the active region. The grid structure provides a novel means for regulating the light output of the laser.
CONDUCTIVE CURRENT MASK
Abstract A semiconductor laser is disclosed wherein a grid-like conductive current mask is fabricated between the active region of the laser and one of its electrodes. In one embodiment, the conductive current mask is fabricated in tile bottom regions of a corrugated pattern that is created along the length of the semiconductor laser. In a second embodiment the conductive current mask is totally embedded within a lightly doped layer that is grown proximate to the active region. The grid structure provides a novel means for regulating the light output of the laser.
Description
~L9~7~
~rlIC~N~CT~E~ LAS~ ~IT~
~Uc~Cl'IV~ ~U~ ' MASK
lechnical ~leld 'l'ilis invention relates to semiconductor lasers and, rnore particularly, to semiconductor lasers having a current mask~
~ac~ground of tile Invention One tYpe of laser whic~l does not require end i0 mirrors is the distributed feedback lasers described in u. S. Patent ~,760,292 iss~ed Se~tel~ber 18, 1~73 to ~1. W~
~ogelnik et al. 1n Lhe distributed feedback laser the gain mediurn or tlle index of refraction is Inodulated at periodic intervals in order to provide reflections along the entire lengt~l of the gain rnedium. One form of distributed feecibac~ laser tha~ is pLoposed in the ~ogelnik et al ~atent is a semiconductor laser in which a current mask is ~ositioned witnin the laser structure in order. to permit purnpillg or energizing curr~nt to pass ~hrough the active re~ion of the ldser only at periodic intervals along the lenytil of the laser. Such a laser is snowrl in ~IG. 5 of the Kogelrlik et al patent. The net efi-ect of thè current mask is to p~oduce a ~eriodic gain vari.ation ~itnin the semiconducror lase~.
~5 A new ty~e of tnree terminal device was disclosed at the lnterrlatiollal L1ectron Devices ~.leeting in ~ashington, D. C., 1979 by scientists f.rom the Massachusetts Lnstitute of Technology, Lincolrl Laboratory, Lexincjton, MA. This device lS shown and described on ~U ~a~e 130 of Electronics, ~ecember 6, 1~7~ As shown in E1ectrorlics, the device is a permeable base transistor wllose ~se consists of a layer of tungsten that has been patterned into a 3200 A ~eriod grating layer which layer is epitaxially embedded in a sinc31e crystal of n-ty~e gallium arserlide. This cransistor wherein the base is a grid-like metallic structure is said to provide l~lany advantages over .. ~
~ ~ ~9~7~
gallium arsenide field effect transistorsO This trans-istor~ however, is not a laser, and the present invention relates to the recognition of the utility, and the means for incorporating, the new technology ;n laser devicesO
Summary of the Invention In accordance with an aspect of the invention there is provided a semiconductor laser having an active region and at least two electrodes between which a potential can be established to cause a pump current to flow through said a~tive region, characterized in that said semiconductor laser includes a gating electrode fabricated from a conductive material and disposed between one of said two electrodes and said active region.
In accordance with the present invention/ a grid-like conductive current mask is fabricated between the active region in a semiconductor laser and one of the electrodes between which the pump current flows. Changing the potential that is provided to this conductive current mask can regulate the light outpu, from the semiconductor laser and also provide a means for adjusting the output waveleng-th in those lasers where the conductive current mask is sufficiently close to the acti~e region to cause it to function as a distributed feedback laser. For maximum regulation, the conductive current mask is placed in a lightly doped buffer layer so that maximum modulation o~ the depletion depth around the current is obtained.
Doping o this buffer layer must be suEficiently great, however, so that sufficient current to operate the laser is available.
In accordance with one embodiment of the present invention a semiconductor laser structure is etched in o~der to establish a pattern of grooves~ or notches over the active region along the length of the :Laser structure. A
conductive terminal is deposited over lhe top parts of the notches that are not affected by the etchingl and a '7~L
~ 2a -conductive terminal is deposited over the entire bottom of the semiconductor laser structure thereby forming the two terminals throuyh which the pump current flows. A third conductive layer is deposited at the bottoms of the grooves or notches to form the gate electrode wh:ich controls the pump current.
In accordance with the second embodiment a clad ding region is epitaxially grown over the active region, and growth is terminated so that a conductive .~ .
L?~
metc~ c grid can ~e deposited over the claddincJ layer.
axia1 Jrowtil is then resuMe~ anu a top buffer layer of low col~ductivity is gr~wn over tne grid structure followed by a cap and con~act lciyer. In this second embodiment, the yr1d stIucture is totally embedded wichin the claddiny and ~op bufLer layers, an~ contact to the yrid structure is ma~e by yrooves t~at ar~ etc~e~ along th~ length of the ~ellliconduc~or la~r.
~rief ~esCri~tlon of tlle Drawlng ~r~. 1 is a piCtOLl al drawin~ of a section of a sellliconductor laser constructed in accordance with the esent inven~ion;
~ . 2 i~ a cross sec~ional diagralil of ~he device illustrated in ~I~. l;
~ . 3 and 4 are top and end views, respectively, of a second embodilllellt constructed in accordance witn the pr~sent lr~vention; and F1~. ~ is a cross sectional diagram of the device illustrated in ~IG~ 3 an~ 4.
~0 ~etailed Description ~ re embodinlerlt of the preserlt invelltion is l~lustrated in the pictorial view shown in ~I~. 1. A cross sectiorlal view desiynated by the numerdL 2 in ~ . l is s~lown ln ~ . The features and layers of this device 2S are excly~erdtec in order to illusLrate ~he construction.
lhe devic~ clS shown is constructed, Lor exampl~l by yro~iny an e~i~axial n-~y~e bufL~r layer 11 of indium phosphide on an n+ substrate 10 of indium phosptlide. rhis is then followed by an epitaxial yrowt.l of a quaternary layer 12 tlaving a com~Osition whicll will lase at the choserl waveleny~tl. For th~ mat~rials system used in this device usirl~J indiul,l, yalliunl, arsenic and phosphorous, a ~avelenyth of 1.3~m would be typical. The active ~uaternary lay~r 12 lS then covered ~y a cladding layer 13 3~ of p-ty~e indlum phospSIide whics~ is grown relatively thick (about ~ m) for th~- cshoserl wavelength of 1.3~m. The cla~dislg layer is therl followed by the yrowth of a contact 9~7~
lay~r 14 ~f hlgh conductivity ~-cy~e in~iutn phos~hide or quaternary material. The doping of cladding layer 13 should be suf~iciently liyht so tnat tne grid structure to be describe~ hereirlafter will be able to ~inch off a chdnnel wid~n of about ~/~ where A is the period of ttle grid s~ructure. tlowever, the doping must be sufficiently niyll to carry tne necessary currr~nt to o~erate the laser.
rIhe entlre to~ strùcture of the e~itaxially grown device is then covered Wittl a silicorl nitride layer 15.
1~ ~ilicon nitride layer 1~ lS then coated wi~h a photoresist, and the photoresist is exposed to ~roduce a ~acterll whictl will perrllit etchin-; of ~he layers in the lnderlted areas of t~le device illustrated by the pictorial view in ~I~. 1. Thi~ etching is initia~ed by a reactive ion etcn using C~3 or a CF4+02 mixture in a ~lasma etch yeolretry to cut through the si1icon nitride layer. This step is then Lollowed by a reactive ion etch using ~ure Cl2 in ofder to cut away all of contact layer 14 and ap~roxilliately l~ m of the indium phospilide claddiny layer 13 in the areas being etcn~d. ~n ~r~er ~o alleviate problems ttlat nligilt re~ult when usillg a ~hotoresist, one could also use a titanium an~ alulllinum mask over the silicon rlitride and C12 + ~ for a redctive iO-I etch of the indium ~hos ptl i de cap layer.
At this ~oint in time, ~he device will begin to resemb1e trle one illustrated in k`IG. i. A 5 SeCOtld immersion of ttle device in concentrated hydrocllloric acid is used to SllloOtil the lnP and sligtltly undercut the silicon nitride layer in order to ~roduce an overhallg of the ~0 silicon nltride layer of a~rvximately 0.2~m âs illustrated ~n rl~. 1. Thi~ overhal-ly is ~esirable in order to prevent the material wnic~ de~osited in the followiny step from adhering tu the sidewalls of the ~osts in the corrugated ~atterll ~ro~uced duritlg tne reactive ion etch.
~S Alternatively, if the cor~egation ~eriod ~ is sufficiently lar~Je, tne etchillg may be wholly accom~listled with ~ICl alvne. 1t is im,oortant that the mask be oriented as shown in ~IG. 1 so that tne side walls of the corrugated ~attern be (01 ï ) or ( 011 ) crystal ~larles after ilCl etcs~ing.
n!aterial that will form a yood blockislg contact ~ics~ tlle claddin{J materidl is then deposited over the top J surLacc of the device in order to creal:e a gating électrode 16 which exists not only in the bottom-most regions of tsle notches that are create(i but also extends out over portlon ~o of the claddin~3 region which is exposed at the fcoslt edge o~ the devic~ illustrated in LI~. 1. In lU this colltext, a ~lockillg contact is defined as one whicn will cause a de~letion of tsle surrounding material when Lhe a~propriate bias lS appli~d. This action can be ~rovided by ~everse biased ~-n, ~ch~t~y barrier, or metal-insulator-semiconductor junctions. In the embodiment lllustrdt~a in ~IG. 1 and 2, wh~re the claddiny layer is p-type InP, LAe ga~ing elect~o~e can be fabricated from a ~old-~er~ianium-alloy. ~irlng at a suitably elevated temperatule may be used to improve the blocking quality of the gate. ~he gold-~erManium that ls cleposited over the mask covering the rldges is then removed by using the a~ro~riate resist stripLver to remove the photoresist mas~, or by u~ing sodium hydroxide to remove a titanium+ aluminum maskO
A secosld resist pat~ern is then Lormed over the device to credte win~ows in the to~ posts of tsle corrugated pattern. A plasma etch can be used to cut through the silicon nitride isl these winciows and a gold-~inc layer 17 can cherl be depos1ted isl the windows and fired if necéssary n order to produce a jood ohmic contact with the sligh ~U conductivicy contacL layer 14. A third mas~ ~,attern using a ~hotosesist mdterial is tshen utilized to create a chromium gold layer 1~ which no~ only mak~es contact with the wlsl~owed layers 17 but also extellds out over the rear portion of tlle d~vi~e shown in ~I~. 1 in order to provide ~5 an extended contact to the anodec of tne diode wslich has been coslstructed.
rllne devices on a single ~afer can then be se~aLat~d either by sawing or by using a bromine-lDetrlallol soluLiorl. It sho~ld be apparent to ttlose skilled in the art that end mirLors can then be formecl by cleaving or ~y a ~ui-ca~le etchiny procedure in those cases wtlere Lhe devic~
mar be ~art of a more cornplicated inteclrated optics structure.
lf the spacing d is rnade sufliciently small (~ ~ ) and ~ period is chosen to satisfy the ~ra~y 1~ condicion, distri~uted feed~ack action will occur~ 'l`hen the output surfdces would not ~ave to ~e mirrors, and they coul~ be AR coat~d to prevent unwanted reflection. The device shown in FIG~. 1 and ~ is utilized by applyirlg a poSitiVe potential to anode contact 1~ and a negative poten~ial to the cathode contact 1~ which is a gold~inc contact that has been deposited over the entire ~ottom surface of subst~ate 1~. A~plication of a negative potential to the gating electroae 16 then creates de~letion reyions of the type shown and designated in ~IG. 2 as regions 21. 'l'hese aepletiOn regions can be utilized to ~inch off current tha~ is flowing from anode 1~ before this current passes through the active layer 12 in a marlner illustrated ~y tne current lines ~2 sho~rl in ~1~. 2. ~lence gating el~c~rode lS not only a current mask but is a ~5 current mask ttlat carl change its degree of effectiveress ueuendirly on the potential chdt is ap~lied.
~ secolld em~odiment of t~le present invention can be constructed to provide a device of the type illustrated in FI~ , 4 and 5. FIGS. 3 and 4 are top and side views ~0 of the device, aTld E'l~. 5 is a cross sectional view of the device tnrouyh a plane designated ~y ttle nuT~ral 5 in ~lG. 3. This ellibodil~lent is constructed by first growing a p-type buffer layer 41 of indlum phos~tlide on p+
substrate 40 of inaium pho~p~l1de. 'l'his growtll is followed by an undoped active ~uacerrlary lay~r 42 and an n-type cladding layer 43 of indium ~hos~tlide. These layers are identical to those ~hicrl are yrown ~or a typical ~ouble 97~
e~erostLuccure laseL fabricdted on a p-ty~e substrate except tne n+ cont~c~ layer normally grown on a double heterostructure is left off. For distributed feedback action, claddirlg layer 43 is grown to a thickness (d) that is sufficiently small to permit the evanescent Eield of the ~ptical mode to reach its top surface.
l~le yrowth of layers 41, 42 and 43 illustratively involves the pre~aration of a (10~) caamium or zinc doped substrate wlth a ~romirle-inethailol ~ollsh-etch. The suDstrate is inserted into the liquid phase epitaxial system and ~leated to ap~roximately 650 degrees C and permi~te~ to cool at a rate of about 0.~ degrees C ~er ~inute, while melts suitable for growirl~ the firs~ set of layers shown in FI~. 5 are alternatel~ slid into position on t~le su~strate.
After the cladding layer 43 nas ~een grown, th device is removed from tne liquid phase epitaxial system an~ the cladding layer 43 is immediately coated with a photoresist and exposed with the desired yrating period, . Lf distributed reedback operation is desired, the period, ~ , is chosen to provide ~rayg reflection at the laser wavelength.
A~ this point, a tungsten or conductive carbon gating electrode 48 may be evaporated over the photoresist ~attern. rf desired, shallow grooves may first be formed by ~lasma etciling in order to allow tne tungsten or carbon gating electrode 48 to be recessed into th~ surface of the n-ty~e cladding layer 43. This recessing can ~rovide a ~lanar surf dCe for the growtns which are ~o follow.
After solvent cleaniny, tne entire substrate is then placed into the liquid phase epitaxial system and a lightly doped n-ty~e top buffer layer 44 of indium ~hos~rlide is grohn over the gating electrode 48. 'rhis to~
~u~fer layer 44 lS followed by an n-ty~e cap layer 45 of il~dium phos~ e and a neavily do~ed ~uaternary contact layer 46. Contact is made LO the gating electrode 4~ by etching cllannels along the entire length of the device as illustrdted in FI~ and 4. A ca~tlc,de electrode 47 of gold ar)d tin is tnen deposited over the entire len~th of the devlce, and a yold-zinc anode electrode 49 is formed over the entire bottolll surface of substrate 40.
S ~y choosing tle electrical bias applied to gating ~lectro~es 16 and 4~ in either of t~le described embodiments and s~lectirlcJ various layer thicknesses and dopings during growth, severdl modes of o~eration are ~jossible for both er,lbodimellts. If ttle ~eparation ~ ~etween the yatincJ
electrode 16 or 48 and Lne active 1ayer is large, tnat is, d lS nlUCII greater tharl tne opeLating wavelength, there will be no distributed feed~ack dction and ~he devices will function only a~ moduldted iabLy-~erut lasers with end mirrors required. The gating electrode unde~ these circumstdnces can be utilized to r,lodulate the level of the lasec out~ut. ~ligh n,odulation efficiency and s~eed will be ~rovidea tnrough the transistor action of the gatirl-J
electrode.
lf, however, tne ~ating electrode is brought to within d waveleflglh of the active ~uaternary layer and the ~erlod A chosen to ~Deet the ~ragg criteria, distributed ~eedback action will be added by the resulting spatially ~erlodic gais~. In the latter case all of tne reflections re~uired can be provideu by the gating electrode, and the
~rlIC~N~CT~E~ LAS~ ~IT~
~Uc~Cl'IV~ ~U~ ' MASK
lechnical ~leld 'l'ilis invention relates to semiconductor lasers and, rnore particularly, to semiconductor lasers having a current mask~
~ac~ground of tile Invention One tYpe of laser whic~l does not require end i0 mirrors is the distributed feedback lasers described in u. S. Patent ~,760,292 iss~ed Se~tel~ber 18, 1~73 to ~1. W~
~ogelnik et al. 1n Lhe distributed feedback laser the gain mediurn or tlle index of refraction is Inodulated at periodic intervals in order to provide reflections along the entire lengt~l of the gain rnedium. One form of distributed feecibac~ laser tha~ is pLoposed in the ~ogelnik et al ~atent is a semiconductor laser in which a current mask is ~ositioned witnin the laser structure in order. to permit purnpillg or energizing curr~nt to pass ~hrough the active re~ion of the ldser only at periodic intervals along the lenytil of the laser. Such a laser is snowrl in ~IG. 5 of the Kogelrlik et al patent. The net efi-ect of thè current mask is to p~oduce a ~eriodic gain vari.ation ~itnin the semiconducror lase~.
~5 A new ty~e of tnree terminal device was disclosed at the lnterrlatiollal L1ectron Devices ~.leeting in ~ashington, D. C., 1979 by scientists f.rom the Massachusetts Lnstitute of Technology, Lincolrl Laboratory, Lexincjton, MA. This device lS shown and described on ~U ~a~e 130 of Electronics, ~ecember 6, 1~7~ As shown in E1ectrorlics, the device is a permeable base transistor wllose ~se consists of a layer of tungsten that has been patterned into a 3200 A ~eriod grating layer which layer is epitaxially embedded in a sinc31e crystal of n-ty~e gallium arserlide. This cransistor wherein the base is a grid-like metallic structure is said to provide l~lany advantages over .. ~
~ ~ ~9~7~
gallium arsenide field effect transistorsO This trans-istor~ however, is not a laser, and the present invention relates to the recognition of the utility, and the means for incorporating, the new technology ;n laser devicesO
Summary of the Invention In accordance with an aspect of the invention there is provided a semiconductor laser having an active region and at least two electrodes between which a potential can be established to cause a pump current to flow through said a~tive region, characterized in that said semiconductor laser includes a gating electrode fabricated from a conductive material and disposed between one of said two electrodes and said active region.
In accordance with the present invention/ a grid-like conductive current mask is fabricated between the active region in a semiconductor laser and one of the electrodes between which the pump current flows. Changing the potential that is provided to this conductive current mask can regulate the light outpu, from the semiconductor laser and also provide a means for adjusting the output waveleng-th in those lasers where the conductive current mask is sufficiently close to the acti~e region to cause it to function as a distributed feedback laser. For maximum regulation, the conductive current mask is placed in a lightly doped buffer layer so that maximum modulation o~ the depletion depth around the current is obtained.
Doping o this buffer layer must be suEficiently great, however, so that sufficient current to operate the laser is available.
In accordance with one embodiment of the present invention a semiconductor laser structure is etched in o~der to establish a pattern of grooves~ or notches over the active region along the length of the :Laser structure. A
conductive terminal is deposited over lhe top parts of the notches that are not affected by the etchingl and a '7~L
~ 2a -conductive terminal is deposited over the entire bottom of the semiconductor laser structure thereby forming the two terminals throuyh which the pump current flows. A third conductive layer is deposited at the bottoms of the grooves or notches to form the gate electrode wh:ich controls the pump current.
In accordance with the second embodiment a clad ding region is epitaxially grown over the active region, and growth is terminated so that a conductive .~ .
L?~
metc~ c grid can ~e deposited over the claddincJ layer.
axia1 Jrowtil is then resuMe~ anu a top buffer layer of low col~ductivity is gr~wn over tne grid structure followed by a cap and con~act lciyer. In this second embodiment, the yr1d stIucture is totally embedded wichin the claddiny and ~op bufLer layers, an~ contact to the yrid structure is ma~e by yrooves t~at ar~ etc~e~ along th~ length of the ~ellliconduc~or la~r.
~rief ~esCri~tlon of tlle Drawlng ~r~. 1 is a piCtOLl al drawin~ of a section of a sellliconductor laser constructed in accordance with the esent inven~ion;
~ . 2 i~ a cross sec~ional diagralil of ~he device illustrated in ~I~. l;
~ . 3 and 4 are top and end views, respectively, of a second embodilllellt constructed in accordance witn the pr~sent lr~vention; and F1~. ~ is a cross sectional diagram of the device illustrated in ~IG~ 3 an~ 4.
~0 ~etailed Description ~ re embodinlerlt of the preserlt invelltion is l~lustrated in the pictorial view shown in ~I~. 1. A cross sectiorlal view desiynated by the numerdL 2 in ~ . l is s~lown ln ~ . The features and layers of this device 2S are excly~erdtec in order to illusLrate ~he construction.
lhe devic~ clS shown is constructed, Lor exampl~l by yro~iny an e~i~axial n-~y~e bufL~r layer 11 of indium phosphide on an n+ substrate 10 of indium phosptlide. rhis is then followed by an epitaxial yrowt.l of a quaternary layer 12 tlaving a com~Osition whicll will lase at the choserl waveleny~tl. For th~ mat~rials system used in this device usirl~J indiul,l, yalliunl, arsenic and phosphorous, a ~avelenyth of 1.3~m would be typical. The active ~uaternary lay~r 12 lS then covered ~y a cladding layer 13 3~ of p-ty~e indlum phospSIide whics~ is grown relatively thick (about ~ m) for th~- cshoserl wavelength of 1.3~m. The cla~dislg layer is therl followed by the yrowth of a contact 9~7~
lay~r 14 ~f hlgh conductivity ~-cy~e in~iutn phos~hide or quaternary material. The doping of cladding layer 13 should be suf~iciently liyht so tnat tne grid structure to be describe~ hereirlafter will be able to ~inch off a chdnnel wid~n of about ~/~ where A is the period of ttle grid s~ructure. tlowever, the doping must be sufficiently niyll to carry tne necessary currr~nt to o~erate the laser.
rIhe entlre to~ strùcture of the e~itaxially grown device is then covered Wittl a silicorl nitride layer 15.
1~ ~ilicon nitride layer 1~ lS then coated wi~h a photoresist, and the photoresist is exposed to ~roduce a ~acterll whictl will perrllit etchin-; of ~he layers in the lnderlted areas of t~le device illustrated by the pictorial view in ~I~. 1. Thi~ etching is initia~ed by a reactive ion etcn using C~3 or a CF4+02 mixture in a ~lasma etch yeolretry to cut through the si1icon nitride layer. This step is then Lollowed by a reactive ion etch using ~ure Cl2 in ofder to cut away all of contact layer 14 and ap~roxilliately l~ m of the indium phospilide claddiny layer 13 in the areas being etcn~d. ~n ~r~er ~o alleviate problems ttlat nligilt re~ult when usillg a ~hotoresist, one could also use a titanium an~ alulllinum mask over the silicon rlitride and C12 + ~ for a redctive iO-I etch of the indium ~hos ptl i de cap layer.
At this ~oint in time, ~he device will begin to resemb1e trle one illustrated in k`IG. i. A 5 SeCOtld immersion of ttle device in concentrated hydrocllloric acid is used to SllloOtil the lnP and sligtltly undercut the silicon nitride layer in order to ~roduce an overhallg of the ~0 silicon nltride layer of a~rvximately 0.2~m âs illustrated ~n rl~. 1. Thi~ overhal-ly is ~esirable in order to prevent the material wnic~ de~osited in the followiny step from adhering tu the sidewalls of the ~osts in the corrugated ~atterll ~ro~uced duritlg tne reactive ion etch.
~S Alternatively, if the cor~egation ~eriod ~ is sufficiently lar~Je, tne etchillg may be wholly accom~listled with ~ICl alvne. 1t is im,oortant that the mask be oriented as shown in ~IG. 1 so that tne side walls of the corrugated ~attern be (01 ï ) or ( 011 ) crystal ~larles after ilCl etcs~ing.
n!aterial that will form a yood blockislg contact ~ics~ tlle claddin{J materidl is then deposited over the top J surLacc of the device in order to creal:e a gating électrode 16 which exists not only in the bottom-most regions of tsle notches that are create(i but also extends out over portlon ~o of the claddin~3 region which is exposed at the fcoslt edge o~ the devic~ illustrated in LI~. 1. In lU this colltext, a ~lockillg contact is defined as one whicn will cause a de~letion of tsle surrounding material when Lhe a~propriate bias lS appli~d. This action can be ~rovided by ~everse biased ~-n, ~ch~t~y barrier, or metal-insulator-semiconductor junctions. In the embodiment lllustrdt~a in ~IG. 1 and 2, wh~re the claddiny layer is p-type InP, LAe ga~ing elect~o~e can be fabricated from a ~old-~er~ianium-alloy. ~irlng at a suitably elevated temperatule may be used to improve the blocking quality of the gate. ~he gold-~erManium that ls cleposited over the mask covering the rldges is then removed by using the a~ro~riate resist stripLver to remove the photoresist mas~, or by u~ing sodium hydroxide to remove a titanium+ aluminum maskO
A secosld resist pat~ern is then Lormed over the device to credte win~ows in the to~ posts of tsle corrugated pattern. A plasma etch can be used to cut through the silicon nitride isl these winciows and a gold-~inc layer 17 can cherl be depos1ted isl the windows and fired if necéssary n order to produce a jood ohmic contact with the sligh ~U conductivicy contacL layer 14. A third mas~ ~,attern using a ~hotosesist mdterial is tshen utilized to create a chromium gold layer 1~ which no~ only mak~es contact with the wlsl~owed layers 17 but also extellds out over the rear portion of tlle d~vi~e shown in ~I~. 1 in order to provide ~5 an extended contact to the anodec of tne diode wslich has been coslstructed.
rllne devices on a single ~afer can then be se~aLat~d either by sawing or by using a bromine-lDetrlallol soluLiorl. It sho~ld be apparent to ttlose skilled in the art that end mirLors can then be formecl by cleaving or ~y a ~ui-ca~le etchiny procedure in those cases wtlere Lhe devic~
mar be ~art of a more cornplicated inteclrated optics structure.
lf the spacing d is rnade sufliciently small (~ ~ ) and ~ period is chosen to satisfy the ~ra~y 1~ condicion, distri~uted feed~ack action will occur~ 'l`hen the output surfdces would not ~ave to ~e mirrors, and they coul~ be AR coat~d to prevent unwanted reflection. The device shown in FIG~. 1 and ~ is utilized by applyirlg a poSitiVe potential to anode contact 1~ and a negative poten~ial to the cathode contact 1~ which is a gold~inc contact that has been deposited over the entire ~ottom surface of subst~ate 1~. A~plication of a negative potential to the gating electroae 16 then creates de~letion reyions of the type shown and designated in ~IG. 2 as regions 21. 'l'hese aepletiOn regions can be utilized to ~inch off current tha~ is flowing from anode 1~ before this current passes through the active layer 12 in a marlner illustrated ~y tne current lines ~2 sho~rl in ~1~. 2. ~lence gating el~c~rode lS not only a current mask but is a ~5 current mask ttlat carl change its degree of effectiveress ueuendirly on the potential chdt is ap~lied.
~ secolld em~odiment of t~le present invention can be constructed to provide a device of the type illustrated in FI~ , 4 and 5. FIGS. 3 and 4 are top and side views ~0 of the device, aTld E'l~. 5 is a cross sectional view of the device tnrouyh a plane designated ~y ttle nuT~ral 5 in ~lG. 3. This ellibodil~lent is constructed by first growing a p-type buffer layer 41 of indlum phos~tlide on p+
substrate 40 of inaium pho~p~l1de. 'l'his growtll is followed by an undoped active ~uacerrlary lay~r 42 and an n-type cladding layer 43 of indium ~hos~tlide. These layers are identical to those ~hicrl are yrown ~or a typical ~ouble 97~
e~erostLuccure laseL fabricdted on a p-ty~e substrate except tne n+ cont~c~ layer normally grown on a double heterostructure is left off. For distributed feedback action, claddirlg layer 43 is grown to a thickness (d) that is sufficiently small to permit the evanescent Eield of the ~ptical mode to reach its top surface.
l~le yrowth of layers 41, 42 and 43 illustratively involves the pre~aration of a (10~) caamium or zinc doped substrate wlth a ~romirle-inethailol ~ollsh-etch. The suDstrate is inserted into the liquid phase epitaxial system and ~leated to ap~roximately 650 degrees C and permi~te~ to cool at a rate of about 0.~ degrees C ~er ~inute, while melts suitable for growirl~ the firs~ set of layers shown in FI~. 5 are alternatel~ slid into position on t~le su~strate.
After the cladding layer 43 nas ~een grown, th device is removed from tne liquid phase epitaxial system an~ the cladding layer 43 is immediately coated with a photoresist and exposed with the desired yrating period, . Lf distributed reedback operation is desired, the period, ~ , is chosen to provide ~rayg reflection at the laser wavelength.
A~ this point, a tungsten or conductive carbon gating electrode 48 may be evaporated over the photoresist ~attern. rf desired, shallow grooves may first be formed by ~lasma etciling in order to allow tne tungsten or carbon gating electrode 48 to be recessed into th~ surface of the n-ty~e cladding layer 43. This recessing can ~rovide a ~lanar surf dCe for the growtns which are ~o follow.
After solvent cleaniny, tne entire substrate is then placed into the liquid phase epitaxial system and a lightly doped n-ty~e top buffer layer 44 of indium ~hos~rlide is grohn over the gating electrode 48. 'rhis to~
~u~fer layer 44 lS followed by an n-ty~e cap layer 45 of il~dium phos~ e and a neavily do~ed ~uaternary contact layer 46. Contact is made LO the gating electrode 4~ by etching cllannels along the entire length of the device as illustrdted in FI~ and 4. A ca~tlc,de electrode 47 of gold ar)d tin is tnen deposited over the entire len~th of the devlce, and a yold-zinc anode electrode 49 is formed over the entire bottolll surface of substrate 40.
S ~y choosing tle electrical bias applied to gating ~lectro~es 16 and 4~ in either of t~le described embodiments and s~lectirlcJ various layer thicknesses and dopings during growth, severdl modes of o~eration are ~jossible for both er,lbodimellts. If ttle ~eparation ~ ~etween the yatincJ
electrode 16 or 48 and Lne active 1ayer is large, tnat is, d lS nlUCII greater tharl tne opeLating wavelength, there will be no distributed feed~ack dction and ~he devices will function only a~ moduldted iabLy-~erut lasers with end mirrors required. The gating electrode unde~ these circumstdnces can be utilized to r,lodulate the level of the lasec out~ut. ~ligh n,odulation efficiency and s~eed will be ~rovidea tnrough the transistor action of the gatirl-J
electrode.
lf, however, tne ~ating electrode is brought to within d waveleflglh of the active ~uaternary layer and the ~erlod A chosen to ~Deet the ~ragg criteria, distributed ~eedback action will be added by the resulting spatially ~erlodic gais~. In the latter case all of tne reflections re~uired can be provideu by the gating electrode, and the
2~ end mirrors are no longer necessary, thereby perlllittirlg the device to be incor~orated int~ a complex integraced optics ~tructure. Wavelengtfl control is ex~ected to be posslble while ~ne laser ~Lovides a constarlt out~ut by simultaneously varying ~he anode bias and the voltage ap~lied to the gatlng electrode. Agairl, high modulation efLici~ncy dnd s~ee~ is reali~ed by modulatin~ only the ga~e electrode.
~ na has been described l~ereinabove are two illustrative embodiments of tne present lnvention. rhe first describes a p on n configuration while the second describes an n on ~ configuration. Iwo more ernbodiments are possible ~y intefcflangirly n for p and vice-versa in (~
bo~n cas~s, providea only ~ndt tn~ associated contactillcJ
l~laterials dre also chanc~ed ln accordance with the layer to whlch they are appliecl.
~ na has been described l~ereinabove are two illustrative embodiments of tne present lnvention. rhe first describes a p on n configuration while the second describes an n on ~ configuration. Iwo more ernbodiments are possible ~y intefcflangirly n for p and vice-versa in (~
bo~n cas~s, providea only ~ndt tn~ associated contactillcJ
l~laterials dre also chanc~ed ln accordance with the layer to whlch they are appliecl.
Claims (13)
1. A semiconductor laser having an active region and at least two electrodes between which a potential can be established to cause a pump current to flow through said active region, CHARACTERIZED IN THAT said semiconductor laser includes a gating electrode fabricated from a conductive material and disposed between one of said two electrodes and said active region.
2. A semiconductor laser as defined in claim 1 wherein said gating electrode is fabricated as a single layered structure positioned in a lightly doped layer proximate to said active region.
3. A semiconductor laser as defined in claim 1 wherein said gating electrode is fabricated in the bottom regions of a corrugated structure created along the length of said semiconductor laser.
4. A semiconductor laser as defined in claim 1 wherein said corrugated structure has a period, A , which is chosen to provide Bragg reflection.
5, A semiconductor laser comprising a substrate of III-V semiconductor material having a first electrode fabricated on one of its surfaces and a plurality of epitaxial layers grown on an opposite surface, one of said plurality of layers including an active region which generates light in response to a pumping current, a second electrode structure fabricated on said plurality of epitaxial layers such that a potential can be applied between said first and second electrodes to develop a pumping current which flows through said active region CHARACTERIZED IN THAT
said semiconductor laser further includes a current mask fabricated of conductive material and positioned between said second electrode and said active region.
said semiconductor laser further includes a current mask fabricated of conductive material and positioned between said second electrode and said active region.
6. A semiconductor laser as defined in claim wherein said plurality of epitaxial layers includes a lightly doped layer proximate to said active region and said conductive current mask is disposed within said lightly doped epitaxial layer.
7. A semiconductor laser as defined in claim 5 wherein said plurality of epitaxial layers is notched to create a corrugated pattern, and said conductive current mask is fabricated in the lower regions of said corrugated pattern.
8. A semiconductor laser as defined in claim 7 wherein said corrugated pattern is fabricated to have a period, A , which causes the conductive current mask to provide Bragg reflection.
9. A semiconductor laser comprising a substrate of one type conductivity having a first electrode means fabricated on one surface and a first epitaxial layer of said one type conductivity grown on an opposite surface of said substrate, an undoped active layer epitaxially grown on said first epitaxial layer, a plurality of cladding layers of opposite conductivity grown on said active layer, and a second electrode means fabricated on the top surface of said cladding layers, said first and second electrode means and said active region being oriented such that a potential which is applied between said first and second electrode means causes a pumping current to flow through said active layer CHARACTERIZED IN THAT
said plurality of cladding layers includes a grid-like conductive current mask to which a potential can be applied to regulate the flow of said pumping current.
said plurality of cladding layers includes a grid-like conductive current mask to which a potential can be applied to regulate the flow of said pumping current.
10. A semiconductor laser as defined in claim 9 wherein said plurality of cladding layers includes a lightly doped layer, and said grid-like conductive current mask is disposed thin said lightly doped cladding layer.
11. A semiconductor laser as defined in claim 9 wherein a notched pattern is created in said plurality of cladding layers along tile length of said active layer, said second electrode means is positioned at the topmost regions of said notched pattern and said grid-like conductive current mask is positioned at the bottom regions of said notched pattern.
12. A semiconductor laser as defined in claim 11 wherein said notched pattern is fabricated with a period, A , such that said grid-like conductive current mask provides Bragg reflection.
13. A semiconductor laser as defined in claim 11 wherein said grid-like conductive current mask is fabricated from materials that form a blocking contact with the surrounding cladding layer.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/306,287 US4445218A (en) | 1981-09-28 | 1981-09-28 | Semiconductor laser with conductive current mask |
| US306,287 | 1994-09-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1194971A true CA1194971A (en) | 1985-10-08 |
Family
ID=23184626
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000410492A Expired CA1194971A (en) | 1981-09-28 | 1982-08-31 | Semiconductor laser with conductive current mask |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US4445218A (en) |
| EP (1) | EP0089980B1 (en) |
| JP (1) | JPS58501567A (en) |
| CA (1) | CA1194971A (en) |
| DE (1) | DE3280027D1 (en) |
| GB (1) | GB2106706B (en) |
| IT (1) | IT1152641B (en) |
| WO (1) | WO1983001155A1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2525033B1 (en) * | 1982-04-08 | 1986-01-17 | Bouadma Noureddine | SEMICONDUCTOR LASER HAVING SEVERAL INDEPENDENT WAVELENGTHS AND ITS MANUFACTURING METHOD |
| US4679199A (en) * | 1985-09-23 | 1987-07-07 | Gte Laboratories Incorporated | High power InGaAsP/InP semiconductor laser with low-doped active layer and very low series resistance |
| JP2539368B2 (en) * | 1985-12-20 | 1996-10-02 | 株式会社日立製作所 | Semiconductor laser device |
| JPS62257783A (en) * | 1986-04-30 | 1987-11-10 | Sharp Corp | Semiconductor laser element |
| FR2598862B1 (en) * | 1986-05-16 | 1994-04-08 | Bouley Jean Claude | SEMICONDUCTOR LASER WITH DISTRIBUTED REACTION AND CONTINUOUSLY TUNABLE WAVELENGTH. |
| DE3713045A1 (en) * | 1987-04-16 | 1988-10-27 | Siemens Ag | Method for producing a laser diode having a buried active layer and lateral current limiting, and a laser diode having a buried active layer and lateral current limiting |
| US5224115A (en) * | 1991-07-17 | 1993-06-29 | The United States Of America As Represented By The Secretary Of The Air Force | Distributed feedback laser implemented using an active lateral grating |
| JP2705409B2 (en) * | 1991-11-21 | 1998-01-28 | 三菱電機株式会社 | Semiconductor distributed feedback laser device |
| US5338394A (en) * | 1992-05-01 | 1994-08-16 | Alliedsignal Inc. | Method for etching indium based III-V compound semiconductors |
| JP3204474B2 (en) * | 1993-03-01 | 2001-09-04 | キヤノン株式会社 | Gain-coupled distributed feedback semiconductor laser and its fabrication method |
| US5509093A (en) * | 1993-10-13 | 1996-04-16 | Micron Optics, Inc. | Temperature compensated fiber fabry-perot filters |
| FR2713350B1 (en) * | 1993-12-06 | 1995-12-29 | Franck Delorme | Optical component with a plurality of bragg gratings and method for manufacturing this component. |
| FR2715251B1 (en) * | 1994-01-20 | 1996-04-05 | Christophe Kazmierski | Semiconductor structure with virtual diffraction grating. |
| FR2716303B1 (en) * | 1994-02-11 | 1996-04-05 | Franck Delorme | Wavelength tunable distributed Bragg reflector laser with selectively activated virtual diffraction gratings. |
| KR0146713B1 (en) * | 1994-09-30 | 1998-11-02 | 양승택 | Fabrication method of surface emission micro-laser |
| US6291839B1 (en) * | 1998-09-11 | 2001-09-18 | Lulileds Lighting, U.S. Llc | Light emitting device having a finely-patterned reflective contact |
| US20030219917A1 (en) * | 1998-12-21 | 2003-11-27 | Johnson Ralph H. | System and method using migration enhanced epitaxy for flattening active layers and the mechanical stabilization of quantum wells associated with vertical cavity surface emitting lasers |
| US7435660B2 (en) * | 1998-12-21 | 2008-10-14 | Finisar Corporation | Migration enhanced epitaxy fabrication of active regions having quantum wells |
| US7286585B2 (en) * | 1998-12-21 | 2007-10-23 | Finisar Corporation | Low temperature grown layers with migration enhanced epitaxy adjacent to an InGaAsN(Sb) based active region |
| US7167495B2 (en) * | 1998-12-21 | 2007-01-23 | Finisar Corporation | Use of GaAs extended barrier layers between active regions containing nitrogen and AlGaAs confining layers |
| US7058112B2 (en) | 2001-12-27 | 2006-06-06 | Finisar Corporation | Indium free vertical cavity surface emitting laser |
| US6922426B2 (en) | 2001-12-20 | 2005-07-26 | Finisar Corporation | Vertical cavity surface emitting laser including indium in the active region |
| US7408964B2 (en) | 2001-12-20 | 2008-08-05 | Finisar Corporation | Vertical cavity surface emitting laser including indium and nitrogen in the active region |
| US7095770B2 (en) | 2001-12-20 | 2006-08-22 | Finisar Corporation | Vertical cavity surface emitting laser including indium, antimony and nitrogen in the active region |
| US7257143B2 (en) * | 1998-12-21 | 2007-08-14 | Finisar Corporation | Multicomponent barrier layers in quantum well active regions to enhance confinement and speed |
| US6975660B2 (en) | 2001-12-27 | 2005-12-13 | Finisar Corporation | Vertical cavity surface emitting laser including indium and antimony in the active region |
| US7295586B2 (en) * | 2002-02-21 | 2007-11-13 | Finisar Corporation | Carbon doped GaAsSb suitable for use in tunnel junctions of long-wavelength VCSELs |
| US6822995B2 (en) * | 2002-02-21 | 2004-11-23 | Finisar Corporation | GaAs/AI(Ga)As distributed bragg reflector on InP |
| US7860137B2 (en) * | 2004-10-01 | 2010-12-28 | Finisar Corporation | Vertical cavity surface emitting laser with undoped top mirror |
| CN101432936B (en) * | 2004-10-01 | 2011-02-02 | 菲尼萨公司 | Vertical cavity surface emitting laser having multiple top-side contacts |
| CA2533225C (en) | 2006-01-19 | 2016-03-22 | Technologies Ltrim Inc. | A tunable semiconductor component provided with a current barrier |
| JP5865860B2 (en) * | 2013-03-25 | 2016-02-17 | 株式会社東芝 | Semiconductor device |
| DE102020108941B4 (en) * | 2020-03-31 | 2022-05-25 | Ferdinand-Braun-Institut gGmbH, Leibniz- Institut für Höchstfrequenztechnik | Diode laser with reduced beam divergence |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2420517A1 (en) * | 1974-04-27 | 1975-11-06 | Licentia Gmbh | Opto-electronic semiconductor element - has substrate with deposited semiconductor layers of opposite conductivity with substrate dependent pn-junction |
| JPS571912B2 (en) * | 1974-05-31 | 1982-01-13 | ||
| US4023993A (en) * | 1974-08-22 | 1977-05-17 | Xerox Corporation | Method of making an electrically pumped solid-state distributed feedback laser |
| US3959808A (en) * | 1974-09-19 | 1976-05-25 | Northern Electric Company Limited | Variable stripe width semiconductor laser |
| US4152711A (en) * | 1976-04-01 | 1979-05-01 | Mitsubishi Denki Kabuchiki Kaisha | Semiconductor controlled luminescent device |
| JPS52147087A (en) * | 1976-06-01 | 1977-12-07 | Mitsubishi Electric Corp | Semiconductor light emitting display device |
| US4169997A (en) * | 1977-05-06 | 1979-10-02 | Bell Telephone Laboratories, Incorporated | Lateral current confinement in junction lasers |
| JPS606553B2 (en) * | 1977-08-30 | 1985-02-19 | 富士通株式会社 | semiconductor light emitting device |
| US4190813A (en) * | 1977-12-28 | 1980-02-26 | Bell Telephone Laboratories, Incorporated | Strip buried heterostructure laser |
| US4236122A (en) * | 1978-04-26 | 1980-11-25 | Bell Telephone Laboratories, Incorporated | Mesa devices fabricated on channeled substrates |
| JPS56104488A (en) * | 1980-01-23 | 1981-08-20 | Hitachi Ltd | Semiconductor laser element |
-
1981
- 1981-09-28 US US06/306,287 patent/US4445218A/en not_active Expired - Lifetime
-
1982
- 1982-08-30 JP JP57502933A patent/JPS58501567A/en active Pending
- 1982-08-30 WO PCT/US1982/001171 patent/WO1983001155A1/en active IP Right Grant
- 1982-08-30 DE DE8282902872T patent/DE3280027D1/en not_active Expired
- 1982-08-30 EP EP82902872A patent/EP0089980B1/en not_active Expired
- 1982-08-31 CA CA000410492A patent/CA1194971A/en not_active Expired
- 1982-09-23 GB GB08227184A patent/GB2106706B/en not_active Expired
- 1982-09-24 IT IT23442/82A patent/IT1152641B/en active
Also Published As
| Publication number | Publication date |
|---|---|
| EP0089980A1 (en) | 1983-10-05 |
| IT8223442A0 (en) | 1982-09-24 |
| US4445218A (en) | 1984-04-24 |
| WO1983001155A1 (en) | 1983-03-31 |
| EP0089980A4 (en) | 1986-05-14 |
| GB2106706A (en) | 1983-04-13 |
| GB2106706B (en) | 1985-07-10 |
| JPS58501567A (en) | 1983-09-16 |
| IT1152641B (en) | 1987-01-07 |
| EP0089980B1 (en) | 1989-11-15 |
| DE3280027D1 (en) | 1989-12-21 |
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