Geometry Dependent Doping and Electronic Devices Produced Thereby

The invention relates generally to patterning of semiconductor devices. In particular, the invention relates to a method and the product thereof of providing lateral definition for current confinement or isolation in after-grown semiconductor layers.

All but the simplest electronic or opto-electronic components require some lateral definition of one or more semiconductor or insulating layers. Similarly, electronic or optoelectronic integrated circuits involve a number of separate electronic or optical components on the same chip which need to be laterally defined on the chip so as to form separate components. The usual method for patterning lateral features in a semiconductor wafer is photolithography. There are many variations of the photolithographic process but, with few exceptions, a lateral feature in the finished device needs to be defined in the photoresist during the photographic step. A separate etching step then transfers the feature from the photoresist into an already grown layer.

An electronic integrated circuit and particularly an opto-electronic integrated circuit require large numbers of layers. One standard measure of circuit complexity is the number of photographic masks or mask levels required to fabricate the circuit in a repetitive sequence of photolithographic steps. Each mask level usually requires a sequence of spinning on photoresist, photographic exposure, development of the photoresist to form a mask, etching the substrate through the mask, and removing the mask. Each mask level thus causes added expense in the fabrication process and additional equipment in a production environment.

A further problem arises with photolithographic definition of opto-electronic devices, for instance lasers. Typically for single-mode optical elements including lasers, the active lasing medium is photolithographically defined to have a lateral dimension close to the optical wavelength. Perhaps thereafter cladding material is grown adjacent the defined lasing medium although sometimes air/vacuum is used as the cladding. Nonetheless, there is an interface bordering the active medium which was exposed to ambient after the etching. Such an interface is likely to have a large number of electronic surface states which degrade the operation of the laser.

One novel approach in achieving lateral patterning has been disclosed by Kapon et al in a technical article entitled "Lateral patterning of semiconductor superlattice heterostructures by epitaxial growth on nonplanar substrates" appearing in Proceedings of SPIE, volume 944, 1988 at pages 80-91. This technique relies on the different growth rates on different crystal planes. They pattern a groove in a crystal substrate and thereafter grow multiple layers both in the groove and over the surrounding planar area. Because of the differential growth rates, layer thicknesses change in the vicinity of the groove. For very thin layers, quantum well effects cause the band gaps to be dependent on layer thicknesses. Thereby, lateral bandgap variations appear over the groove cross-section, which can be used for lateral light confinement. However, this effect requires very small dimensions associated with quantum wells. Furthermore, since the lateral current confinement is very poor, it requires extra steps, such as proton bombardment, to create the necessary confinement. The alignment becomes critical in such processing.

Jaeckel et al have recently disclosed in a technical article entitled "High-power fundamental mode AlGaAs quantum well channeled substrate laser grown by molecular beam epitaxy" appearing in Applied Physics Letters, volume 55, 1989 at pages 1059-1061 a method of growing a laser in a groove. A flat-bottomed groove is formed in a (100) substrate of GaAs and thereafter a graded-index separate confinement heterostructure laser is grown primarily of AlGaAs but with a GaAs quantum well active layer. The lower AlGaAs cladding layer is uniformly doped p-type. However, the upper AlGaAs cladding layer is doped with Si, which is amphoteric so as to produce n-type doping on the {100} plane exposed outside the groove and at the bottom of the groove and produce p-type doping on the {311} A planes exposed on the side of the grooves. Thereby, there is lateral current confinement in the upper cladding layer. This technique is of limited use however. It requires a unique combination of a dopant and substrate orientation. Furthermore, the p-type doping in the upper cladding layer extends only to the sides of the groove. Thereafter, this layer is again n-type. Therefore, the upper current electrode must be defined to within the width of the groove.

Accordingly, an object of the invention is to provide electronic devices for which the number of mask levels used in their fabrication is reduced.

It is another object of this invention to provide such electronic devices in which lateral interfaces can be formed without the use of photolithography.

It is still further object of this invention to provide such opto-electronic devices.

The method can be summarized as a method of growing laterally patterned structures in which a compound semiconductor substrate is formed with a groove having such an orientation that the doping concentration of subsequently formed epitaxial layers depends on the local crystalline orientation of the grooved substrate. Thereafter, an epitaxial structure is grown. In one embodiment, the structure includes at least one superlattice portion having alternating layers of a higher concentration of a first semiconductivity type and a lower concentration of a second semiconductivity type. Depending on the orientations and substrates, the superlattice doping is maintained over one area of the grooved substrate but diffuses to form a uniform first or second semiconductivity type over another area, the semiconductivity type being dependent upon orientations, layer thicknesses and dopant concentrations. The junctions of the maintained superlattice can block current that flows through the uniform conductivity region. In another embodiment, the epitaxial growth proceeds with both n-type and p-type dopants. However, dependent on the local crystalline orientation, the single growth produces n-type material in one area and p-type material in another.

FIG. 1 is a cross-section of a double heterostructure laser according to the invention and utilizing a superlattice current isolation region.

FIG. 2 is an electron micrograph of the laser of FIG. 1 taken on the same cross-section.

FIG. 3 is a cross-section of a lateral p-n structure built according to the invention.

FIG. 4 is a cross-section of another heterostructure laser according to the invention in which the laser is formed on a planar area.

FIG. 5 is a cross-section of an improvement to the heterostructure laser of FIG. 4.

A first embodiment of the invention, which is a double heterostructure laser, is illustrated in cross-section in FIG. 1. There are sequentially formed a lower cladding layer 10 and an active region 12. Thereafter, a superlattice structure 14 is grown which is divided naturally into an upper cladding region 16 and a surrounding current confining region 18. Except for a preprocessing step of photolithographically defining a groove 20 in a substrate 22, no lateral definition is required. Nonetheless, current is confined in the upper cladding region 16 overlying the groove 20 because of geometry dependent doping within the groove 20. This embodiment was experimentally verified as follows.

The substrate 22 was InP having a principal surface oriented in the [100] direction to within .+-.0.5.degree. . The substrate 22 was doped with S in order to be n.sup.+ and act as a current electrode for the laser. The groove 20 was photolithographically formed along the [011] direction using an SiO.sub.2 mask having an aperture of 1.5 .mu.m and then etching for 1 minute in H.sub.3 PO.sub.4 :HCl (3:1, by volume) to produce a V-shaped groove 20. Shorter etching times would have produced a flat-bottomed groove. It was observed that grooves oriented along the [011] direction had side walls close to the {211} A crystal planes. After etching, the SiO.sub.2 mask was removed with buffered HF. The grooved substrate 22 was degreased and etched in H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O (5:1:1, by volume) at room temperature prior to epitaxial growth. This last etch did not remove any measurable quantity of material and thus did not affect the shape of the groove 20.

The cleaned substrate 22 was then loaded into a horizontal OMCVD (organo-metallic chemical vapor deposition) chamber operating at 76 torr. The substrate was heated to the OMCVD growth temperature of 625.degree. in .about.2 minutes in an atmosphere consisting of hydrogen and phosphine (2.times.10.sup.-2 mole fraction). The lower cladding layer 10 of InP was grown to a thickness of 1 .mu.m and doped n to 1.times.10.sup.18 cm.sup.-3. The InP was grown from trimethylindium and phosphine and the n-type doping from S was produced by using hydrogen sulfide. In later steps, any p-type doping from Zn was produced by using diethylzinc. All doping concentrations and layer thicknesses were calibrated based on separate depositions upon planar (100) surfaces of InP and calibrating measurements on these planar layers.

The active layer 12 was epitaxially grown over the lower cladding layer 10 by OMCVD in order to serve as the active laser region. The active layer 12 was 0.2 .mu.m thick and was nominally undoped. It had a quaternary composition of InGaAsP that was lattice matched to InP on a (100) surface and had a photoluminescence peak wavelength at room temperature of 1.3 .mu.m.

Thereafter, the superlattice region 14 was grown by OMCVD in the form of alternating layers of n-type and p-type InP. The superlattice, as grown, consisted of 20 periods of an n-type InP layer 24 doped n=1.times.10.sup.18 cm.sup.-3 and a p-type InP layer 26 doped p=7.times.10.sup.17 cm.sup.-3. Both the n-type and p-type InP superlattice layers 24 and 26 had thicknesses of 55 nm. Finally, there was grown a p.sup.+ contact layer 28 of InGaAsP to a thickness of 0.2 .mu.m and having a concentration of p=1.times.10.sup.18 cm.sup.-3. The contact layer 28 was subjected to a shallow Zn diffusion to guarantee low contact resistance for the second and upper electrode of the laser. The p-contact was made through a 5 .mu.m wide opening in a SiO.sub.2 mask centered over the groove 20.

Although the n-type and p-type superlattice layers 24 and 26 were deposited continuously across the groove 20, the upper cladding region 16 above the groove 20 was uniformly p-type. FIG. 2 shows an SEM (scanning electron micrograph) cross-section of a structure very similar to that of FIG. 1 in which the cross-section was stained under illumination with 12 g of KOH, 9 g of KFe(CN).sub.6 and 75 cc of water so as to delineate the n-type layers from the p-type layers. It is clear that a uniform p-type region has been from the superlattice in the grooved area overlying the groove 20 but that the superlattice remains intact in the planar areas of the substrate away from the groove 20.

Several mechanisms appear to be responsible. In a separate experiment, S-doped layers and Zn-doped layers were grown on both a (100) InP substrate and a (111)A InP substrate. For doping levels of n=1.8.times.10.sup.18 cm.sup.-3 and p=7.times.10.sup.17 cm.sup.-3 on (100) substrates, the corresponding doping levels under identical growth conditions for (111)A substrates were n=1.8.times.10.sup.17 cm.sup.-3 and p=4.times.10.sup.18 cm.sup.-3. That is, the Zn incorporation is higher for (111)A substrates than for (100) substrates while the relative S incorporation rates are reversed. The layers are thin enough to expect significant diffusion of dopants from the neighboring layers. In particular, it is believed that the enhanced p-type dopant Zn in the p-type layers 26 on the sidewalls of the groove 20 vertically diffuses and overcompensates the depressed n-type doping of the n-type layers 24.

Similar differential doping measurements were performed for S and Zn on different planes of GaAs. The GaAs results very closely follow those for InP. Therefore, similar devices are achievable in InP and GaAs and other III-VI compounds of the zinc-blende crystal structure, all referred to as semiconductor compounds.

In the structure of the SEM micrograph, during OMCVD growth of the lower cladding layer 10, the original {211}A sidewalls in the groove 20 are replaced by planes close to {311}A and {111}A. It further shows that the quaternary layer 12 grows in the form of a cresent at the bottom of the then existing groove.

Whatever the mechanism, the upper cladding region 16 above the groove 20 is uniformly p-type while in the planar portions 18 the superlattice maintains its structure of multiple p-n junctions. Such p-n junctions present resistance to current flow and thus confine the current to the upper cladding region 16 overlying the groove 20. Thus, the lateral definition of either the contact layer 28 or its Zn diffusion is subject to very relaxed dimensional requirements. Further, the interface between the upper cladding region 16 and the current confining structure 18 is an epitaxial interface which should provide reduced number of surface states.

The sample formed with the structure of FIG. 1 was cleaved into a cavity having a length of 254 .mu.m. At 20.degree. C., the laser had a pulsed threshold current of around 100 mA. For CW operation, the threshold current increases to 120 mA but the efficiency was 0.170 mW/mA, comparable to that of a conventional buried heterostructure laser.

In the superlattice growth procedure described above, there is a maximum thickness of the n-type InP layer 24 that allows for complete conversion of that layer to p-type within the upper cladding region 16 above the groove 20. Even for a given set of growth conditions, the maximum thickness will depend on the relative n-type and p-type doping levels. Effective current blocking has been observed for InP superlattices 14 with complete conversion within the upper cladding region 16 for: (1) 100 nm thick layers of n=1.times.10.sup.18 cm.sup.-3 alternating with 100 nm thick layers of p=7.times.10.sup.17 cm.sup.-3 ; and (2) 140 nm thick layers of p=3.times.10.sup.17 cm.sup.-3 alternating with 85 nm thick layers of n=5.times.10.sup.17 cm.sup.-3.

A procedure similar to that used for the laser of FIG. 1 can be used to grow a lateral p-n junction. As illustrated in FIG. 3, the groove 20 is formed along the [011] direction in the (001) surface of the semiinsulating InP substrate 22. Thereafter, in a single growth a layer of InP is epitaxially grown by OMCVD above the substrate 22. The growth takes place with both p-type and n-type dopants (Zn and S) present in relative quantities such that an InP layer grown on a (001) surface is more n-type than p-type. There results a p-type InP layer 42 formed in the groove 20 and two n-type InP layers 44 and 46 formed on the {001} planes on the opposing sides of the groove 20. The three InP layers 42, 44 and 46 form two lateral p-n junctions or, alternatively, a lateral n-p-n transistor when respective contact layers are thereafter deposited on portions of the layers 42, 44 and 46. Multiple grooves easily provide an array of such junctions or transistors, which could be adapted to detector arrays, for example. Furthermore, if the groove 20 were oriented along the [011] direction, the positions of the n-type and p-type materials can be reversed from those of FIG. 3.

The laser embodiment of FIG. 1 demonstrates how a p/n superlattice is converted to a uniform p-type region in a [011] oriented groove. The co-doping procedure with a [011] groove described with reference to FIG. 3 can produce a uniform n-type region in that groove. The two effects can be combined to form a p/n superlattice with separate electrical connections to its p-type and n-type layers. A p/n superlattice is grown on a (100) surface on which had been previously formed two perpendicular grooves in the [011] and the [011] directions. All layers of the superlattice are co-doped with both n-type and p-type dopants. With the proper choice of thickness and doping levels, all the resulting p-type layers in the superlattice will be electrically connected to a uniform p-type conductor overlying the [011] groove (as in FIG. 1) while all the resulting n-type layers in the superlattice will be electrically connected to a uniform n-type conductor overlying the [011] groove. Typical doping levels, as measured on the (001) surface would be for the n-type layers of the superlattice n=1.times.10.sup.18 cm.sup.-3 and p=1.times.10.sup.17 cm.sup.-3 and for the p-type layers of the superlattice n=1.times.10.sup.17 cm.sup.-3 and p=1.times.10.sup.18 cm.sup.-3. Such a structure is useful for electrically contacting a superlattice modulator.

The structure of FIG. 3 can be used to form a semiconductor laser, illustrated in the cross-section of FIG. 4, with a vertical current path through a planar area and in which the current isolation is provided by two neighboring grooved areas. This structure is thus reversed from that of FIG. 1. A substrate 50 of p.sup.+ -type InP has a principal (100) surface in which are formed two grooves 52 and 54 extending along the [011] direction. The p.sup.+ substrate 50 serves as the lower current electrode. A lower cladding layer 56 of p-type InP is epitaxially grown over both the grooves 52 and 54 and the planar areas. Thereafter an active layer 58 is epitaxilly grown with no further lateral definition. The active layer 58 may be a quaternary III-V compound, such as InGaAsP, or a multi-quantum well based on this same compound family.

Thereafter, an upper cladding layer 60 of InP is epitaxially deposited with both S and Zn being simultaneously supplied as n-type and p-type dopants respectively. The S and Zn are supplied in relative amounts during the growth of the upper cladding layer 60 such that the growth on a (100) surface is n-type but that the growth on the sidewalls of the grooves 52 and 54 is p-type. Because of the orientation dependence of the S and Zn dopant incorporation rates, a central planar mesa region 62 between the grooves 52 and 54 and two outer planar regions 64 and 66 become n-type while two grooved regions 68 and 70 are grown p-type. Such relative doping levels have been achieved with OMCVD using dopant growth parameters which separately would achieve n=1.times.10.sup.18 cm.sup.-3 from S and p=1.times.10.sup.17 cm.sup.-3 from Zn on a InP (100) surface.

Finally, an n-contact layer 72 is deposited and defined over the mesa region 62 to serve as the upper current electrode. Because of the current confinement provided by the p-type grooved regions 68 and 70 for GaAs/AlGaAs lasers, the lateral registry of the n-contact layer 72 and the mesa region 62 is not crucial. However, in InP-based lasers, such confinement is not strong enough so such registry is then crucial.

Current is injected into the laser by the n-contact layer 72 and is guided through the n-type mesa region beneath the n-contact layer 72 by the surrounding p-type grooved regions 68 and 70. The laser light is generated in the vicinity of the active layer 58 and is guided out of the plane of the illustration by the upper and lower cladding layers 60 and 56. An advantage of the structure of FIG. 4 over that of FIG. 1 is that the laser is formed on a planar portion of the substrate rather than at the bottom of a groove. Thus, it is much easier to perform the quantum well growth and to etch gratings on the planar portion than within the groove. Such etched gratings are required for DFB (distributed feedback) and DBR (distributed Bragg reflector) lasers.

Current confinement could be further improved by inserting an n-type layer in the middle of the p-type grooved regions 68 and 70. As illustrated in cross-section in FIG. 5, the structure of the substrate 50, the lower cladding layer 56 and the active layer 58 is the same as that in FIG. 4. However, in place of the single upper cladding layer 60 there are formed thinner first and second upper cladding layers 80 and 82 of the same S and Zn doping as the single upper cladding layer 60 of FIG. 4. There is also formed between the first and second upper cladding layers 80 and 82 an n-type third upper cladding layer 84. This n-type layer 84 is grown with only S doping and is of sufficient thickness and dopant concentration to prevent the Zn from diffusing in areas of the grooves 52 and 54 and overcompensating the S.

Although several embodiments have been described, the invention is not limited thereto. The invention provides the possibility of building a heterojunction laser and other complex opto-electronic devices with a single series of growth steps uninterrupted by any photolithographic patterning. Thus, the cost of manufacturing these devices can be substantially reduced. Furthermore, since the lateral definition is achieved without etching or ion bombardment, surface states degrading the device performance are reduced.