CA1181668A - 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 facet.

Description

METHOV OF PREFERENTIALLY ETCHING OPTICALLY FLAT
MIR~OR E'ACE~S IN InGaAsP/InP HETEROST~UCTllRES

Technical Field _ . . ... ...............
This invention relates to a method of chemical etching an optically flat facet Oll a preferred crystallographic plane of a multilayer InGaAsP/InP device.
Backqround of the Invention , In general, an optoelectronic device such as a laser is fabricated along a preferred crystallographic direction. Mirror facets for uch a device are formed on a plane perpendicular to the preferred directlon 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. Thou~h manual cleaving does produce high quality mirror facets, this technique has a low yield.
Etchin~ methods encompass both wet and dry chemical etching. Wet chemical etching techni~ues generally cause mask undercutting thereby no~ producing the desired flatnessO Examples of wet chemical etching techniques are given in the following references: K Iga et al., "GaInAsP/InP DH Lasers witll a Chemically Etched Facet," IEEE Journal of Quantum Electronics, QE-16, p. 1044 (1980), (a solution of HCl: CH3COOH: H202 = (1:2:1));
P. D. Wright et al., "InGaAsP Double Heterostructure Lasers (A = 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 Br: C~130H
followed by 4~1Cl-H20).

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Dry chemical etching techniques include reactive-ion etching, reactive-ion beam etching and plasma etching.
For separate descriptions of each of the above, see R. E.
Howard et al., "Reactive-Ion Etching of III-V Compounds,"
Topical Meetin~ on Inte~rated and Guided Wave Optics Di~est ~ . _ (IEEE: New York 1980) WA-2; M.A. Bosch et al., "Reactive-Ion Beam Etching of InP with C12," Applied ~y~
Letters, Vol. 38, p. 264 tl980); and R. H. Burton et al., "Plasma Separation of InGaAsP/InP Light-Emitting Diodes,"
Applied Physics Letters, Vol. 37, p. 411 (1~80~.
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 ~or optoelectronic and integrated optics devices.
Summarx~ the Invention According to the invention there is provided a method for etching a multilayer semiconductor heterostruc-ture body having al~ernating layers of InGa~sP and InP, characteri2ed by etching a given surface of the semicon-ductor body with a material selective chemical etchant to expose a portion of a crystallographic surface of an InGaAsP layer and an abutting InP layer, etching the exposed InP layer with HCl to expose a preferred crystal-lographic plane thereof and an abutting InGaAsP layer, the preferred plane being substantially coplanar with the exposed portion of the InGaAsP crystallographic surface, - 2a - -and iterating the aforesaid processing steps to provide an optically flat mirror facet across four of said alternat-ing layers.
Other aspects of this invention are claimed in our copending Canadian patent application Serial No~ 405,781 filed on June 23, 1982 o~ which the present application is a division, and in other divisions thereof.
Brief Description of the Drawin~s FIG. 1 shows a portion of a multilayer semiconductor heterostructure body having a stripe-mask thereon;

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FIGS~ 2 and 3 show structural changes of the semiconductor body in FIG. 1 after successive steps in a first exemplary etching metl~od 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;
~ IGS. 8, 9, 10 and 11 illustrate structural changes of the semiconductQr body in FIG. 1 after each of four se~uential steps in a third exemplary etching method embodying the invention;
FIG. 12 shows a portion of a multilayer semiconductor heterostructure body ~aving a stripe-mask thereon in a direction different from that in FIG. l; and FIGS. 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 embodying the invention~
Detailed Description -Optoelectronic and integrated optics devices are grown in certain desirable crystallographic directions.
For III-V semiconductor heteros~ructure 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 (011) crystallograpnic plane, because this plane is ~erpendicular (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 fiyures, is a set of basis lattice vectors indicating tlle three-dimensional orientation oE the semiconductor body.
Tne semiconductor heterostructure of FIG. 1 comprises mask layer 1, p+-type caL~ layer 2, p-type upper cladding 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 eac~
n-layer becomes a p-layer. For the example described herein, cap layer 2 is a~proximately 3000~5000 angstroms thick, cladding layers 3 and 5 are a~proximately 1.5-2~m thick, active layer ~ is approximately 1000-3000 angstroms thick, and substrate 6 is approximately 75-100 ~m thick.
Semiconductor materials for the heterostructure are chosen from the group of III-V compQunds. In uarticular, a binary III-V compound, InP, is employed for cladding layers 3 and 5 and or substrate ~. A quaternary III-V compound, Inl_yGayAsxPl x~ is utilized 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 heterostructure. 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 (On95eV). It is important to note that the inventive method is equally applicable when these ratios are varied to pronuce 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 ~hase epitaxial growtl~ of the heterostructure. The presence of such an antimeltback layer requires the inventive metllod 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. ~n exemplary mask layer is chemically composed of silicon-nitride. Mask 1 is formed by photolitllography and dryetching of the silicon nitride to have edges which are substantially smooth. ~triped regions in mask 1 leave surface areas such as surface 10 completely exposed, as opposed to being cov~red by mask 1. The stripe in mask 1 is aligned with the <011> direction of the semiconductor heterostructure body. Although this type of stripe mask produces a groove in the semiconductor body, other masks such as the one shown in EIG. 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 tile structural changes in the semiconductor body of FIG. 1 after processing that body with a wet chemical etcharlt. A wet chemical etchant suitable for creating the structural change shown in FIG. 2 in a single step is HCl:~N03 = (1:~), where 1< a<5 and, preferably, ~ is equal to 3. The etching process is anisotropic and is substantially self-stopping when the (011) 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:H~03 etchant. By experimentation, it has been found that, for less HN03 than an amount dictated by an optimum proportion, ~uaternary layer 4 is etched more slowly than binary layer 3. ThiS gives surface 20 the appearance of being steyped 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 HNO3 exceeds the optimurn proportion, the opposite result appears because quaternary layer 4 etches more quickly than layer 3. So, surface 2~ appears to be stepped inward from the etched groove and layer 3 protrudes into the groove beyond the exposed edges of layers g 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 e~posed by this optimized etchant, is substantially planar through at least layers 2, 3, and g.
In practice, optimization is performed by using a small sample of the semiconductor body to be etched. The sample is then subjected to the etchant while the value of ~ is adjusted until the optimum value is found. Certain factors influence the selection of a value for ~ 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 etchant component chemicals and the temperature of the etchant.
Assuming that the value of ~ is optimized for the wet chemical etchant, HCl: ~NO3, the exposed crystallographic surface 20 is substantially perpendicular to ~he (100) plane.
In one example from experimental practice, the semiconductor heterostructure body defined above is immersed and agitated in a chemical bath of ~Cl:3HNO3 for approximately 30 seconds at 22 degrees Centigrade. After this immersion, the etching process is halted by rinsing the ~C1:3HNO3 from the semiconductor body with deionized water and surface 20 is exposed. However, surface 20 has a roughened appearance exhibiting irregular characteristics such as high spots and striations generally along the <100>
direction~ and a polishing step is necessary to relnove these irregularities from exposed crystallographic surface 20.
FIG. 3 illustrates the structural change~ which appear after the semiconductor body of FIG. 2 is polished with a chemical etchant. In this instance, polishing entails contacting exposed surface 20 ~FIG. 2) witl- HCl for a time ~ufficient 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 selniconductor body of FIG. 2 is 6~

imln~rsed in a bath of HCl and ayitated. The polishing process is halted by rinsing the etched semiconductor body in deionized water. In one example, concentrated HCl is utilized in the batt~ at 22 deyrees Centi~rade with an immersion or etching time oE 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 per~endicular to the (100) plane. Surface 21 is an optically flat mirror facet. Although HCl preferentially exposes the (011) crystallographic plane of only the XnP
layers, i.e., layers 3 and 5, and does not etch the quaternary layers, layers 2 and ~, the a.nount of etching (polishing~ is so small as not to impair the substantially coplanar relationship of the groove walls. If desired, the value of 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 tlle side walls thereof into coplanar relationship with the side walls of 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 referred to as a (lll)B crystallographic plane which includes planes (111, (111), (111), and (111). The suffix '~' means that the particular plane includes only phosphorous atoms which are chemically reactive and, therefore, capable of bein~ removed by a chemical etchant.
Similarly, a (lll)A crystallographic plane, which will be discussed below, 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 reinoval by chemical etching.

FIGS. 4, 5, 6, and 7 sho~ structural changes which appear after the semiconductor heterostructure body of E`IG. 1 is subjected to the etchants in a sequential etching process. The method shown in EIG~. 4 through 7 is called seguential etchiny because each layer of the multilayer structure directly under exposed surface 10 (FIG. 1) is etched away in sequence~ That is, the portion of cap layer 2 directly under surface 10 is etched away 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 laylers 2 and 4 or layers 3 an~ 5, but not both. An advantage of this is that it provides greater control of the process. Eor example, variations in the performance 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 4. Examples of several selective etchants include: a solution of H2SO4:H2O2:H2O = (10:1:1) as described in R. J. Nelson et al., "High-Output Power in InGaAsP/InP (A = 1~3 ~m) Strip-Buried Heterostructure Lasers," Applied Physics Letters, Vol. 36, p. 353 ~1980);
or AB etchant, wherein the A solution is (40.0ml.
H2O ~ 0.3~.Ag NO3 ~ 40.0ml. HF) and the B solution is (40.0g. Cro3 ~ 40.0ml. H2O) and A:B=(l:l~ as described in G. ~. Olsen 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 KOH:K3Fe(CN)5:H2O.
Etching time for the quaternary layers varies according to thickness of the quaternary layer, temperature, and alloy colDpoSition ratios, x and y, for the quaternary layers.
For a 3000 arlystroms thickness of layer 2 (A = 1.3~in) and a temperature of 22 degrees Centigrade, the following approximate etching times produce the results shown in FIGS. 4 and 6: AB etchant for approximately 15 seconds, and KO~:K3Fe~CNj6 M2O etch for approximately 8 seconds. This etching step is halted by rinsing the etched semiconductor body in deionized water.
FIG. 5 shows the structural change of the semiconductor body in ~IG. 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 (EIG. 4), thereby exposing surface 13 on quaternary layer 4. This etchant stops reacting automatically at surface 13. For an InP layer thickness of apuroximately 0 105 ~ m, an exemplary etching time period for concentrated HCl is approximately 45 seconds to produce the results shown both in FIGo 5 and FIGo 7~ After this etching step as shown in FI~ 5, it is important to note that the etched, exposed walls of layer 3 exhibit crystallographic smoothness.
FIG. 6 illustrates the structural change apparent in the semiconductor l)eterostructure body, after the body shown in E~IG. 5 is contacted with a wet or dry chemical etchant to selectively etch ~uaternary 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, ~r. InP selective etchant t 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 suficient to expose a preferred crystallographic ~lane as the optically flat mirror facet.
~or this example, the e~ching time period in a bath of concentrated HCl required to produce exposure of the (011) crystallogra~hic plane at surface 21 is approximately 20 seconds. As stated above, surface 22 is also exposed through the InP material com~rising layer 5 and substrate 6. Surface 22 is a (lll)B crystallographic l~lane.
Although, as aforedescribed, either wet or dry etching can be used ~or the layers 2 and g, dry etching is ~enerally better because it avoids any undercutting problems associated with wet etching.
FIGS. ~, 9, 10, and 11 show another process for et~hing the semiconductor heterostructure of FIG. 1.
In EIG. 8, the semiconductor body is shown to have a groove directly under surface 10 (FIG. 1), i.e., the unmasked stripe region ~etween adjacent sections of mask 1.
This 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.
~`IG. 9 illustrates the st.uctural 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 ~5 smooth. Immersion and agitation in this step are required for only a short time yeriod, for example, 3 seconds. This immersion is followed by a rinse in deionized water to halt the etching 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 prorusion, the remaining steps shown in FIGS. 10 and 11 are available to create a perfectly flat surface.
FIG. 10 shows structural changes to quaternary layers 2 and g after ~rocessin~ the semiconductor body of FIG. 9 with a quaternary material selective etch such as the wet or dry chemical etchants described above in relation to ~IG. 4. For the wet chemical e~ci~ants described above, the etching times to complete this step at room temyerat~r~ are shorter, by a factor of approximately one-quarter to one-third, than the etching times given in relation to the step shown in ~IG. 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 ~IG. 10 is shown in FIG. 11. After this step, crystallographic plane (oll) is preerentially 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 tthe result being shown in FIG. 9) and proceed directly to the layers 2 and 4 etching step (FIG. 10). The final polishing step (~IG. 11) corrects any roughness left by omission of the FIG. 9 step. Also, the step illustrated in ~IG. 10, of slightly etching back the layer 4 prior to the final polishing 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 led~e 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 disylaced 9o degrees from that shown 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. T~le (100~ surface of layer 2 has mask 1 partially disposed thereon with stripes in the <Oli>
direction. Layer 7 is quaternary layer similar in com~osition and thickness to layers 2 and 4.

~ 12 -A~ter the semiconductor body of FIG. 12 has reacted with a chemical etchant such as a Br:CH3O~I solution (1%, for 30 seconds) Or HCl:HNO3 = (1:~) (as described earlier), for 30 seconds, crystallographic surface 30 appears as shown in FIG. 13. surface 30 is an oblique surface cutting through each layer o 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 semiconductor body and layer thicknesses described above, etching time periods approximately seven or eight times longer than those defined for the quaternary layer selective wet 2Q chemical etchants descri~ed in relation to FIG. 4 above.
The results of a final polish etching step on the sem7 conductor 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 (lll)B plane and exposing, thereby, crystallographic plane (011) at surface 32 on layers 3 and S. By yroper 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 (4)

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 with a material selective chemical etchant to expose a portion of a crystallographic surface of an InGaAsP layer and an abutting InP layer, etching the exposed InP layer with HCl to expose a preferred crystallographic plane thereof and an abutting InGaAsP layer, the preferred plane being substantially coplanar with the exposed portion of the InGaAsP crystal-lographic surface, and iterating the aforesaid processing steps to provide an optically flat mirror facet across four of said alter-nating layers.

2. The method as defined in claim 1 wherein the chemical etchant is AB etchant.

3. The method as defined in claim 1 wherein the chemical etchant is H2S04:H202:H202 = (10:1:1).

4. The method as defined in claim 1 wherein the chemical etchant is a solution of KOH:K3Fe(CN)6:H2O.