CA1181669A - Method of preferentially etching optically flat mirror facets in ingaasp/inp heterostructures - Google Patents

Abstract

METHOD OF PREFERENTIALLY ETCHING OPTICALLY FLAT
MIRROR FACETS IN InGaAsP/InP HETEROSTRUCTURES

Abstract of the Disclosure Highly reproducible, optically flat mirror facets are created by etching a predetermined area of the InGaAsP/InP heterostructure system to expose a crystallographic surface throughout the entire heterostructure system. Contact of the exposed surface with HCl causes a preferred crystallographic plane to be exposed as an optically flat mirror face.

Description

I~ETHOD OE` PREE'EI~ENTTALLY ETCHI~IG OPTICALLY FLAT
MI R~OR FACETS I~ InGaAsP/InP HETEROSTKUCTURES

Technical Field This invention relates to a method of chemical etching an optically flat facet on a preferred crystallographic plane of a multilayer InGaAsP/InP device.
Back~round of the Invention In general, an optoelectronic device such as a laser is fabricated along a preferred crystallographic direction. Mirror facets for such a device are formed on a plane perpendicular to the preferred direction and sidewalls of the device are formed on planes parallel to that direction. Also~ it is desirable for at least the mirror facets to exhibit the characteristic of optical flatness.
Optically flat mirror facets are created by manual cleaving or by etching. Though manual cleaving does produce high quality mirror facets, this technique has a low yield.
Etching methods encompass both wet and dry chemical etching. Wet chemical etching techniques gen~r~lly cause mask undercutting thereby not producing the desired flatnessr Examples of wet chemical etching techniques are given in the following references: K. Iga et al., "GaInAsP/InP DH Lasers with a Chemically Etched Facet," IEEE Journal of Quantum Electronics, QE-16, p. 1044 (1980), (a solution o HCl: CH3COOH: d202 = (1:2:1));
P. D. Wright et al., "InGaAsP Double Heterostructure Lasers tl = 1-3~m~ with Etched Reflectors," Applied Physics Letters, Vol. 36, p. 518 (1980), (a solution of Br: CH30H);
and S. Arai et al., "New 1.6 ~m Wavelength GaInAsP/InP
Buried Heterostructure Lasers," Electronics Letters, Vol. 16, p. 349 (1980), (a sequential process of ~r: CH30H
followed by 4HCl-H20).
~' Dry chemical etching techniques include reactive ion etchingl reactive-ion beam etching and plasma etching.
For separate descrip-tions of each of the above, see R. E.
Howard et al~ eactive-Ion Etching of III-V Compoundsr"
Topical Meetin~ on Integrated and Guided Wave Optics Digest (IEEE: New York 1980) WA-2; M.A. Bosch et al., "Reactive-Ion Beam Etching of InP with C12," Applied Physics Letters, Vol~ 38, p. 264 (1980); and Ro H. Burton et al., "Plasma Separation of InGaAsP/InP Light-Emittiny Diodes,"
Ap~lied Physics Letters, Vol. 37, p. 411 (1980~.
Reactive ion etching avoids some of the problems of the wet chemical etching methods and is useful in making grooves in a heterostructure system. This type of etching is effectively a single step process which results in facets which are approximately planar but "overcut". That is, the facets which form the groove slope toward each other from the top of the groove to the bottom. Although these facets are reproducible, they lack the optically flat mirror quality necessary for certain applications.
Similarly, the other dry etching techniques create facets satisfactory for use as waveguide sidewalls and die separ-ations but lack the optically flat mirror quality necessary for optoelectronic and integrated optics devices.
Summary_of the Invention According to the invention there is provided a method for etching a multilayer se~iconductor heterostruc-ture body having alternating layers of InGaAsP and InP, characterized by etching a given surface of the semicon-ductor body to expose a first crystallographic surface through the alternating layers of the semiconductor body, selectively etching the exposed surface to expose InGaAsP
portions of a preferred crystallographic plane substan-tially perpendicular to the given surface~ and etching the exposed InP layers with HCl to expose the InP portions of the preferred crystallographic plane to comple~e an ~1 t;~9 - 2a -optically flat mirror facet of the semiconductor heterostructure body.
Other aspects of this invention are claimed in our copending Canadian patent application Serial No. 405,781 filed on June 23, 1982 of which the present application is a division, and in other divisions thereof.
Brief Description of the Drawings FIG. 1 shows a portion of a multilayer semiconductor heterostructure body having a stripe-mask thereon;

6~i FIGS. 2 and 3 show structural changes of the semiconductor body in FIG. 1 after successive steps in a first exemplary etching method embodying the invention;
FIG~. 4, 5, 6 and 7 show structural changes of the semiconductor ~ody in FIG. 1 after each of four sequential steps in a second exemplary etching method embodying the invention;
FIGS~ 8, 9, 10 and 11 illustrate structural changes of the semiconductor body in FIG. 1 after each of four sequential steps in a third exemplary etching method embodyiny the invention;
FXG. 12 shows a portion of a multilayer semiconductor heterostructure body having a stripe-mask thereon in a direc~ion different from that in FIG. l; and FIGSo 13, 14 and 15 show structural changes of the semiconductor body of FIG~ 12 after each of three successive steps in a fourth exemplary etching method embodyin~ the invention.
Detailed Description _ Optoelectronic and integrated optics devices are grown in certain desirable crystallographic directions.
For III-V semiconductor heterostructure lasers and the like composed of InGaAsP/InP on a (100) substrate, the desirable direction for the laser axis is <011>. Hence, it is necessary to create optically flat, mirror quality facets on the (oll) crystallograpnic plane, because this plane is perpendicular (vertical) to the <011> direction and the (100) plane of the heterostructure heterostructure substrate.
EIG. 1 shows a multilayer InGaAsP/InP
heterostructure body having mask 1 on the (100) crystallographic plane. Also included in FIG. 1, as well as all remaining figures, is a set of basis lattice vectors indicating the three-dimensional orientation of the semiconductor body.
Tne semiconductor heterostructure of FIG. 1 comprises mask layer 1, p+-type cap layer 2, p-type upper 6~

clad~ing cladding layer 3, n-type or undoped active layer 4, n-ty~e lower cladding layer 5, and n-type substrate 6. The conductivity type for each layer can be reversed so that-each p-layer becomes an n-layer and each n-layer becomes a p-layer. For the example described herein, cap layer 2 is approximately 3000-5000 angstroms thick, cladding layers 3 and 5 are a?proximately 1.5-2~m thick, active layer 4 is approximately 1000-3000 angstroms thickr and substrate 6 is approximately 75-100 ~m thick.
Semiconductor materials for the heterostructure are chosen from the group of lII-V compounds. In uarticular, a binary III-V compound, InP, is employed for cladding layers 3 and 5 and for substrate 6. A quaternary III-V compound, Inl_yGayAsxPl_x, is utili~ed for cap layer 2 and active layer 4, wherein the alloy composition ratios x and y are chosen to produce a particular wavelength or energy bandgap and lattice constant for the heterostruct~re. For a description of techniques for choosing x and y, see R. Moon et al, "Bandgap and Lattice Constant of GaInAsP as a Function of Alloy Composition", J.
Electron. Materials, Vol. 3, p. 635 (1974). In the description which follows, exemplary composition ratios, x = 0.52 and y = 0.22, are selected to produce a wavelength of 1.3 ~m 10.95eV). It is important to note that the 2S inventive method is equally applicable when these ratios are varied to produce wavelengths in the range of 1.1 ~m to 1.7 ~m. For concentration ratios producing wavelengths above 1.5 ~m, it is necessary to grow a quaternary antimeltback layer between layers 3 and 4 during liquid phase e~itaxial growth of the heterostructure. The presence of such an antimeltback layer requires the inventive method to be modified only slightly, in terms of etching exposure times, to provide acceptable results.
A mask layer is deposited on the (100) plane of the semiconductor body by any suitable deposition process such as chemical vapor deposition or the like. An exemplary mask layer is chemically composed of silicon-nitride. Mask 1 is formed by ~hotolithography and dryetching of the silicon nitride to l~ave edges which are substantially smooth. Striped regions in mask 1 leave surface areas such as surface 10 completely exposed, as opposed to being covered by mask 1. The stripe in mask 1 is aligned with the <011> direction of the semiconductor heterostructure ~ody. Although this type of stripe mask produces a groove in the semiconductor body, other masks such as the one shown in FIGo 12 can be utilized to produce a single wall, i.e., for effectively slicing away an unmasked portion of the semiconductor body.
FIG. 2 illustrates the structural changes in the semiconductor body of FIG. 1 after processing that body with a wet chemical etchant. A wet chemical etchant suitable for creating the structural change shown in FIG. 2 in a single step is HCl Hl~03 = (1 ), where 1< <5 and, preferably, is equal to 3~ The etching process is anisotropic and is substantially self-stop~ing when the (01l) plane is reached in each of the various layers of the semiconductor body. This plane is perpendicular to the (100) plane. The proportion of HCl and HN03 is critical to ensuring that no step discontinuities appear at the interface of the heterojunction and surface 20 exposed by the HCl:HN03 etchant. By experimentation, it has been found that, for less HN03 than an a.mount dictated by an opti~um proportion, ~uaternary layer 4 is etched more slowly than binary layer 3. This gives surface 20 the appearance of being stepped outward toward the etched groove such that layers 4 and 5 protrude into the groove be~ond the ex~osed edge of layer 3. If the amount of HN03 exceeds the optimurn proportion, the opposite result appears because quaternary layer 4 etches rriore quickly than layer 3. So, surface 20 appears to be stepped inward from the etched groove and layer 3 protrudes into the groove beyond tlle exposed edges of layers 4 and 5. Optimization of the value of ~ermits the etchant to react with both the binary layers (layers 3, 5, and 6) and quaternary layers (layers 2 and 4) at approximately the same rate.
Hence, surface 20, which is exposed by this optimized etchant, is substantially planar through at least layers 2, 3, and 4.
In practice, optimization is performed by using a small sample of the semiconductor body to be etched. The sa~ple is then subjected to the etchant while the value of ~ is adjusted until the optimum value is found~ Certain factors influence the selec~ion of a value ~or a such as the alloy composition ratios x and y, the thickness of each semiconductor layer in the heterostructure, the age and strength or diluteness of the etchan~ component chemicals, and tlle temperature of the etchant.
Assuming that the value of ~ is optimized for the wet chemical etchant, HCl: a~O3, the exposed crystallographic surface 20 is substantially perpendicular to the (100) plane.
In one exa~ple from experimental practice, the semiconductor heterostructure body defined above is immersed and agitated in a chemical bath of ~C1:3HNO3 for approximately 30 seconds at 22 degrees Centigrade. After this immersion, the etching process is halted by rinsing the ~C1:3HN03 from the se~iconductor body with deionized water and surface 20 is exposed. However, surface 20 has a rouyhened appearance exhibiting irregular characteristics such as high spots and striations generally along the ~100>
direction, and a polishing step is necessary to remove these irregularities from exposed crystallographic surface 20.
3U FIG. 3 illustrates the structural changes which aupear after t`ne semiconductor body of FIG. 2 is polished with a chemical etchant. In this instance, polishing entails contacting exposed surface 20 (FIG. 2) with HCl for a time sufficient to expose a preferred crystallographic plane of the semiconductor body. HCl is both material selective and orientationally preferential (anisotropic) as an etchant. As before, the semiconductor body of FIG. 2 is immersed in a bath of HCl and agitated. The polishing proces~ is halted by rinsing the etched semiconductor body in deionized water. In one example, concentrated HCl is utilized in the bath at 22 degrees Centigrade with an immersion or etching time of approxiamtely 3 seconds. For more dilute concentrations of HCl, the etching time must be adjusted and increased accordingly.
For the example shown in FIG. 3, the crystallographic plane preferentially exposed by the HCl etchant is (011) plane, denoted as surface 21, which is perpendicular to the (loO) plane. Surface 21 is an op~ically flat mirror facet. Althouyh HCl preferentially exposes the (011) crystallographic plane of only the InP
layers, i.e., layers 3 and 5, and does not etch the quaternary layers, layers 2 and ~, the amount of etching (polishin~) is so small as not to impair the substantially coplanar relationship of the groove walls. If desired, the value of a can be selected to cause a slight undercutting of the layers 2 and 4 during the first etching step, whereupon the polishing of the layers 3 and 5 moves the side walls thereof into coplanar relationship with the side walls o layers 2 and 4. Generally, however, such fine adjustment is not necessary.
At the lower pOrtion of a trough or groove in layers 5 and 6 created by the etching process, crystallographic plane (111) denoted as surface 22) is also ex~osed as a polished facet. Surface 22 is generically reerred to as a (111)~ crystallographic plane which includes planes (111, (111), (111), and (111). The suffix 30 'B' means that the particular plane includes only phosphorous atoms which are chemically reactive and, therefore, capable of being removed by a chemical etchantO
Similarly, a (lll~A crystallographic plane, which will be discussed balow, includes planes (111), (111), ~111), and (111). The suffix 'A' means that the particular plane includes only indium atoms which appear to be substantially inert and resist removal by chemical etching.

FIGS. 4, 5, 6, and 7 sho~ structural changes which appear after the semiconductor heterostruc~ure body of E`IG. 1 is sub~ected to the etchants in a sequential etching process. The method shown in FIGS. 4 through 7 is called sequential etching ~ecause each layer of the multilayer structure directly under exposed surface 10 (FIG. 1) is etched away in sequence. That isr the portion of cap layer 2 directly under surface 10 is etched a-~ay with a wet or dry chemical etchant to expose surface 12 on cladding layer 3. Preferably~ each etchant used is material selective, i.e., an etchant which attacks either layers 2 and 4 or layers 3 and 5, but not both. An advantage of this is that it provides greater control of the process. Eor example, variations in the per~ormance of one etchin~ step has little or no affect uyon the performance of the next step.
Several wet chemical etchants have been shown to be effective for selectively etching quaternary layers such as layers 2 and 4O Examples of several selective etchants include: a solution of H2SO4:H2O2:H2O = (10:1:1) as descriDed in R. J. Nelson et al., "~igh-Output Power in InGaAsP/InP (~ = 1.3 ~m~ Strip-Buried Heterostructure Lasers," Applied Physics Letters, Vol. 36, p. 35~ (1980);
or AB etchant, wherein the A solution is (4O.OID1.
H2O + 0.3g.Ag NO3 t 40.0ml. HF) and the B solution is (40.0g~ CrO3 + 40.0ml. H2O) and A:B=(l:l) as described in G. ~. ~lsen et al., "Universal Stain/Etchant for Interfaces in III-V Compounds," Journal of Applied Physics, Vol. 45, No. 11, p. 5112 (1974); or a solution of KO~I:K3Fe(CN)6:H2O.
Etching time for the quaternary layers varies according to thickness oE the quaternary layer r temperature, and alloy composition ratios, x and y, for the quaternary layers.
For a 3000 arlgstroms ~hickness of layer 2 (~ = 1.3~m) and a tempel-ature of 22 degrees Centigrade, the following approximate &tching times produce the results shown in FIGS. 4 and 6: AB etchant for approximately 15 seconds, and KOH:K3E`e(CN~6: H2O etch for approximately 8 seconds. This ~tchiny step is halted by rinsing the etched semiconductor body in deionized water.
FIGo S shows the structural change of the semiconductor body in FIG. 4 after etching in an InP
selective etch. ~or this etching step, HCl is a suitable etchant to cut away the portion of layer 3 under surface 12 (E`IG. 4~ thereby exposing surface 13 on quaternary layer 4. This etchant stops reacting automatically at surface 13. For an InP layer thickness of approximately 1.5 ~m, an exemplary etching time period for concentrated HCl is approximately 45 seconds to produce the results shown both in FIG. 5 and FIG. 7. After this etching step as shown in FIG 5, it is irnportant to note that the etched, exposed walls of layer 3 exhibit crys~allographic smoothness.
FIG. 6 illustrates the structural change apparent in the semiconductor heterostructure body, after the body shown in FIGo 5 is contacted with a wet or dry chemical etchant to selectively etch quaternary layer 4 directly under surface 13 for a time period sufficient to expose surface 14 on layer 5. Also, crystallographic surface 20 is exposed at a preselected ~e.g., perpendicular) slope to the surface containing mask 1 and surface 10 (EIG~ 1). The etching procedure and the etchants employed at this step have been described above in relation to FIG~ 4~
FIG. 7 shows the completion of all structural changes caused by the sequential etching process. Again, ar, InP selective etchant, HCl, is contacted with exposed surfaces of the semiconductor body create an optically flat mirror facet at surface 21. In particular, surface 14 and crystallographic surface 20 are brought into contact, via immersion and agitation as described above~ with a solution of HCl for a time period sufficient to expose a preferred crystallographic plane as the optically flat mirror facet.
Yor this example, the etching time period in a bath of concentrated HCl required to produce exyosure of the (011) cr~stallogra~hic plane at surface 21 is approximately 20 !

seconds. As stated above, surface 22 is also exposed through the InP material comprising layer 5 and substrate 6. surface 22 is a (lll)B crystallographic plane~
Although, as aforedescribed, either wet or dry etching can be used for the layers 2 and 4, dry etching i5 generally better because it avoids any undercutting problems associated with wet etching.
FIGS. 8, 9, 10, and 11 show anotller process for etching the semiconductor heterostructure of FIG. 1 In FIG. 8, the semiconductor body is shown to have a groove directly under surface 10 (FIG. 1), i.e., the unmasked stripe region between adjacent sections of mask 1.
~his groove is created by either the wet or dry chemical etchants described in relation to FIG. 2. FIG. 8 shows the result using a dry etching process, the walls of the grooves thus slightly converging towards one another as is characteristic of generally known dry etching procedures.
Also, no undercutting of the mask 1 occurs.
FIG. 9 illustrates the structural changes to layers 3 and 5 and to substrate 6 after the semiconductor body of FIG. 8 is immersed in an InP selective polishing etchant such as HCl. The etched surfaces of layers 3 and 5 and substrate 6 are depicted as being crystallographically smooth. Immersion and agitation in this step are required for only a short time period, for example, 3 seconds. This immersion is followed by a rinse in deionized water to halt tlle etchin~ process.
The results shown in FIG. 9 are exaggerated for clarity of presentation. Layers 4 and 5 protrude only slightly into the groove because of the inclination of the sidewalls of the groove. If it is necessary to remove this slight protrusion, the remaining steys shown in FIGS. 10 and 11 are available to create a perfectly flat surface.
FIG. 10 shows structural changes to quaternary layers 2 and 4 after yrocessing the semiconductor body of FIG. 9 with a quaternary material selective etch such as the wet or dry cllemical etchants described above in relation to FIG. 4. For the wet chemical etci~ants described above, the etching times to complete this step at room temperatur~ are shorter, by a factor of approximately one-quarter to one-third, than the etching times given in relation to the step s~lown in FIG. 4. It is critical that the etching be controlled to align the exposed surfaces of layers 2 and 4 on substantially the same plane.
A final polishing step for the semiconductor body of GIG. 10 is shown in FIG. 11. After this step, crystallographic plane (01I) is preferentially exposed by HCl at surface 21. surface 21 is an optically flat mirror facet because of the alignment of layers 3, 4, and 5. For concentrated HCl, this polishing etch step is necessary for only a short time period such as 3 seconds.
It is also possible to omit the first HCl etching step (the result ~eing shown in FIG. 9) and proceed directly to the layers 2 and 4 etching step (FIG. 10). The final polishing step (FIG. 11) corrects any roughness left by omission of t~le FIG. 9 step. Also, the step illustrated in FIG. 10, of sliyhtly etching back the layer 4 prior to the final pclishing step, can be done in the aforedescribed process illustrated by FIGS. 1-3. That is, such layer 4 etching step can be performed to remove any ledge of layer 4 protruding into the groove after the first etching step, as well as for slightly recessing the layer 4 edge to insure its coplanar relationship with layers 3 and 5 after the final polishing step.
The semiconductor body shown in FIG. 12 is at an orientation disulaced 9o degrees from that sho~n in FIG. 1.
Mirror facets are created on crystallographic plane (011) in this orientation and are useful in producing ring lasers and sidewalls parallel to the laser axis for heterostructure lasers. The (100) surface of layer 2 has mask 1 partially disposed thereon with stripes in the <011>
direction. Layer 7 is quaternary layer similar in com~osition and thickness to layers ~ and 4.

After the semiconductor body of FIG. 12 has reacted with a chemical etchant such as a Br:CH30~l solution (1~, for 30 seconds) or HCl:HN03 = ~ ) (as described earlier), for 30 secondsr crystallographic surface 30 appears as shown in FIG. 13. surface 30 is an oblique surface cutting through each layer of the semiconductor heterostructure near a (lll)A crystallographic plane which is inert as mentioned above in relation to FIG. 3.
FIG. 14 shows the structural changes which result in quaternary layers 4 and 7 after the semiconductor heterostructure of FIG. 13 is contacted by a selective quaternary etchant such as one described above in relation to FIG. 4. It is important that the thickness and composition of layers 4 and 7 be chosen appropriately so that the selective quaternary etchant causes surfaces 31 to be substantially coplanar. For the exemplary se~iconductor body and layer thicknesses described above, etching time periods approximately seven or eight times longer than those defined for the quaternary layer selective wet chemical etchants described in relation to FIG. 4 above.
The results of a final polish etching step on the semiconductor heterostructure body of FIG~ 14 are shown in FIG. 15. The polish etchant is HCl which is material selective (InP) and orientationally preferential (crystallographic plane (011)). Because of the hollow cavities left by the removal of quaternary layers 4 and 7, the HCl is able to etch layers 3 and 5 from underneath through the (111)B plane and exposing, thereby, crystallo~raphic plane (011) at surface 32 on layers 3 and 5. By proper experimentation with layer and composition thickness and etching times, it is possible to have surfaces 31 and 32 of layers 3 and 5, and 2, 4, and 7 , respectively, coplanar in crystallographic plane (011) as an optically flat mirror facet. For concentrated HCl, the etching time period is determined to be approximately 20 seconds.

Claims (6)

Claims:

1. A method for etching a multilayer semiconductor heterostructure body having alternating layers of InGaAsP
and InP, CHARACTERIZED BY
etching a given surface of the semiconductor body to expose a first crystallographic surface through the alternating layers of the semiconductor body, selectively etching the exposed surface to expose InGaAsP portions of a preferred crystallographic plane substantially perpendicular to the given surface, and etching the exposed InP layers with HCl to expose the InP portions of the preferred crystallographic plane to complete an optically flat mirror facet of the semi-conductor heterostructure body.

2. The method as defined in claim 1 wherein the first etching step is performed using a dilute solution of Br:CH30H.

3. The method as defined in claim 1 wherein the first etching step is performed using HCl:HN03 = (1.alpha.)for a in the range 1 <.alpha.< 5.

4. The method as defined in claim 2 wherein the second etching step is performed using AB etchant.

5. The method as defined in claim 2 wherein the second etching step is performed using H2S04:H202:H20 = (10:1:1).

6. The method as defined in claim 2 wherein the second etching step is performed using a solution of KOH:K3Fe(CN)6:
H20 .