Abstract:
The present invention creates oxide and air apertures in material systems, such as InP, that do not usually accommodate epitaxial incorporation of highly oxidizing materials, such as AlAs, of sufficient thickness to adequately provide optical as well as current aperturing. A composite structure of relatively slowly oxidizing layer or layers (e.g. AlInAs on InP) with a faster-oxidizing layer or layers (e.g. AlAs on InP) can be used to produce oxide and air apertures of various shapes and sizes, and to also increase the oxidation rate.

Description:
The present application claims priority to U.S. Provisional Application Ser. No. 60/233,013, filed Sep. 15, 2000, which is hereby incorporated by reference in its entirety into the present disclosure. 
    
    
     This invention was made with the support of the United States Government under Grant No. MDA972-98-1-0001, awarded by the Department of Defense (DARPA). The Government has certain rights in this invention under 35 U.S.C. §202. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed to apertures in semiconductors and more particularly to heterogeneous composite semiconductor structures for enhanced oxide and air aperture formation for semiconductor lasers and detectors and their method of manufacture. 
     2. General Background and State of the Art 
     Oxide and air apertures can enhance the performance of semiconductor lasers and detectors. However, such apertures are difficult to implement in material systems that do not accommodate epitaxial incorporation of highly oxidizing materials of sufficient thickness. For example, the oxide aperture technology has found wide use in devices made in the AlGaAs system, but not so for devices made on indium phosphide (InP) because lattice-matched materials on InP do not oxidize fast enough at temperatures not damaging to the material (except for AlAsSb which is unsuitable because it decomposes into metallic Sb as it oxidizes). Fast-oxidizing materials (e.g. AlAs) may be grown on InP in limited thickness, but such thin layers oxidize too slowly and are of a limited use as optical apertures due to the limited thickness. 
     The prior art shows several attempts at implementing apertures on InP. In one previous implementation (see Zhi-Jie Wang; Soo-Jin Chua; Zi-Ying Zhang; Fan Zhou; Jing-Yuan Zhang; Xiao-Jie Wang; Wei Wang; Hong-Liang Zhu. “Self-aligned current aperture in native oxidized AlInAs buried heterostructure InGaAsP/InP distributed feedback laser,” Applied Physics Letters, vol. 76, (no. 12), AIP, 20 Mar. 2000. p. 1492–4) AlInAs is used as the oxidation layer. AlInAs typically suffers from low oxidation rates, unless cladded by InP as it has been in this implementation. However, such oxidation layers suffer from rough oxide-semiconductor interface, and thus are unsuitable for optoelectronic device applications. 
     In another implementation (see Yen, J. C.; Blank, H.-R.; Mishra, U.K. “Lateral oxide current aperture for InP-based vertical electron current flow devices: Demonstration using RTD&#39;s,” Proceedings IEEE/Cornell Conference on Advanced Concepts in High Speed Semiconductor Devices and Circuits, Ithaca, N.Y., USA, 4–6 Aug. 1997) AlAsSb is used as the oxidation layer. AlAsSb is unsuitable for most optoelectronic and many electronic device applications because it leaves a layer of metallic Sb as it oxidizes. 
     In an additional implementation of the prior art (see Ohnoki, N.; Koyama, F.; Iga, K. “Superlattice AlAs/AlInAs-oxide current aperture for long wavelength InP-based vertical-cavity surface-emitting laser structure,” Applied Physics Letters, vol. 73, (no. 22), AIP, 30 Nov. 1998. p. 3262–4) thin layers of heavily-strained AlAs on InP are used. However, the layers described in this reference are extremely difficult to epitaxially grow without a significant amount of dislocations and thus, undesirable for most optoelectronic applications. 
     It is a goal of the present invention to create apertures in material systems, such as InP, that do not usually accommodate epitaxial incorporation of highly oxidizing materials, such as AlAs, of sufficient thickness to adequately provide optical as well as current aperturing. A further goal of this invention includes the creation of various types of apertures, including tapered apertures, of nearly arbitrary thickness and length. 
     INVENTION SUMMARY 
     The present invention creates oxide and air apertures in material systems, such as InP, that do not usually accommodate epitaxial incorporation of highly oxidizing materials, such as AlAs, of sufficient thickness to adequately provide optical as well as current aperturing. A composite structure of relatively slowly oxidizing layer or layers (e.g. AlInAs on InP) with a faster-oxidizing layer or layers (e.g. AlAs on InP) can be used to produce oxide and air apertures of various shapes and sizes, and to also increase the oxidation rate. 
     The semiconductor structure of the present invention is comprised of multiple layers of various composition, strain, and thickness, which oxidize together to form an oxide aperture or etch to form an air aperture. Because each individual layer oxidizes or etches at a different rate, the shape and size of the resulting oxide or air aperture can be independently controlled by controlling the individual layer composition and thickness. 
     The method for forming apertures in a semiconductor of the present invention comprises the steps of depositing multiple layers of various composition, strain, and thickness and oxidizing in an oxidizing ambient or etching in a selective etchant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following invention will become better understood with reference to the specification, appended claims, and accompanying drawings, where: 
         FIG. 1  is a diagrammatic representation, not to scale, of an example composite semiconductor structure of the present invention. 
         FIG. 2(A)  is a depiction of a SEM of the composite semiconductor structure of  FIG. 1 . 
         FIG. 2(B)  is a is a diagrammatic view of the SEM depiction of  FIG. 2 . 
         FIGS. 3(A) and 3(B)  illustrate an embodiment in which a faster-oxidizing layer is clad a the slower-oxidizing layer. 
         FIG. 4  illustrates an embodiment in which a faster-oxidizing layer is surrounded with slower-oxidizing layers. 
         FIG. 5  illustrates an embodiment in which a slower-oxidizing layer is surrounded with faster-oxidizing layers. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a diagrammatic representation, not to scale, of an example composite semiconductor structure  12  of the present invention. The composite semiconductor structure  12  includes a heterogeneous composite structure  14  from which an aperture is formed. The heterogeneous composite structure  14  is composed of yet another composite structure  16 , which is a combination of faster-oxidizing AlAs layers  20  and strain-reducing AlInAs layers  22  and a layer of more slowly-oxidizing material  18 . This combination  14  increases the overall oxidation rate and the oxide thickness. 
     In one embodiment the layer of more slowly-oxidizing material  18  is composed of 108 nm thick (approximately quarter wavelength) Al 0.47 In 0.53 As strained to +0.07%. The layer of faster-oxidizing heterogeneous semiconductor material  16  can be composed mainly of AlAs. In another embodiment, the layer of fast-oxidizing heterogeneous semiconductor material  16  is a composite of several materials. As illustrated in  FIG. 1 , the layer of faster-oxidizing heterogeneous semiconductor material  16  is formed from two 5 nm thick layers  20  of AlAs tensile strained to −3.6% adjacent to two layers of 1 nm thick layers  22  of Al 0.24 In 0.76 As strained to +1.6%. 
     The 1 nm thick layers  22  of Al 0.24 In 07.6 As increase the total amount of the layers of faster-oxidizing AlAs  20  that can be used in the structure  12 . They also serve to reduce the strain of the structure  12 . 
     In other embodiments the heterogeneous composite structure  14  is composed of multiple layers of the fast-oxidizing heterogeneous semiconductor material  16  placed adjacent to one or multiple layers of the more slowly-oxidizing material  18 . 
     The heterogeneous composite structure  14  can be formed upon an InP substrate  24 , buffer layer  26  and a layer  28  of 112 nm thick Al 0.09 Ga 0.37 In 0.54 As strained slightly compressively to +0.07%. On the opposite surface of heterogeneous composite structure  14  can be another layer  30  of 112 nm thick Al 0.09 Ga 0.37 In 0.54 As strained slightly compressively to +0.07%, so that the heterogeneous composite structure  14  is sandwiched between two layers  28 ,  30  of 112 nm thick (approximately quarter wavelength) Al 0.09 Ga 0.37 In 0.54 As strained slightly compressively to +0.07%. Adjacent the layer  30  is a layer  32  of 120 nm thick Al 0.47 In 0.53 As having a +0.07% strain followed by a layer  34  of 112 nm Al 0.10 Ga 0.37 In 0.53 As having 0% strain and a layer  36  of 120 nm thick Al 0.48 In 0.52 As lattice matched to InP and having 0% strain. Finally a layer  38  of 112 nm thick Al 0.10 Ga 0.37 In 0.53 As having 0% strain is shown. 
     The heterogeneous composite structure  14  is not limited to formation upon InP material systems. The heterogeneous composite structure  14  is useful for forming oxide and/or air apertures of sufficient thickness on other substrates and/or in other material systems in which apertures of sufficient thickness and/or length cannot be formed from a single oxidation layer or a single etch-out layer. 
     The layers forming the heterogeneous composite structure  14  can be oxidized to form an oxide aperture. Alternatively, rather than oxidizing the structure, or prior to oxidizing the structure, the semiconductor material of the structure  14  can be selectively chemically etched out to form an air aperture. Etching is chemically related to the oxidation process so that some etchants preferentially etch those materials that oxidize well. The structure  14  can also be oxidized and then the oxide selectively etched out to form an air aperture. The air apertures formed in such a manner have increased effectiveness both as optical apertures (due to increased index contrast) and as currents aperture (due to increased resistivity through or around the air gap). 
     The slowly oxidizing layer  18  is not as limited in thickness as are the fast-oxidizing layers  20 , and thus, the thickness of the oxide apertures formed from such heterogeneous composite structures may be arbitrarily controlled. Furthermore, both the shape and size of the oxide aperture may be tailored to maximize its effectiveness as an optical aperture (as well as a current aperture) by changing the compositions and thickness of individual layers and thereby controlling the oxidation rates of the individual layers. Such techniques can be used to form tapered oxide apertures, which have the advantage of being able to control the extent of the optical aperturing and that of the current aperturing independently. Prior to the present invention, such tapered apertures had not been implemented in any system other than AlGaAs on GaAs. 
       FIG. 2(   a ) is a depiction of a SEM of the composite semiconductor structure  12  corresponding to the diagrammatic representation of  FIG. 1  and  FIG. 2(   b ) is a diagrammatic view of the SEM depiction. A tapered oxide aperture  44  is shown within the heterogeneous composite structure  14  in  FIG. 2(   a ) and more clearly in  FIG. 2(   b ). The composite semiconductor structure  12  is shown having been etched through the structure to form a pillar perpendicular to the surface. In this illustrated embodiment, the sample is oxidized for 1 hour at 500 degrees C. in a steam ambient at atmospheric pressure. Other oxidation methods can be used as well, however. The oxidation proceeds much more rapidly through the faster-oxidizing heterogeneous semiconductor material  16  than it does through the more slowly-oxidizing material  18 , resulting in the tapering. The direction of the oxidation in both  FIGS. 2(   a ) and  2 ( b ) is indicated by the arrow  42 . The layer  32  of 120 nm thick Al 0.47 In 0.53 As and the layer  36  of 120 nm thick Al 0.48 In 0.52 As lattice matched to InP are also oxidized. The layer  36  is shown oxidized a distance  40 . The tapered oxide aperture  44  is oxidized to a distance of approximately 2 micrometers into the etched pillar, which is more than a factor of five greater than is possible with lattice-matched AlInAs  46  under the same conditions. 
     The thickness of the AlAs layers  20  may be modified to change the shape and oxidation rate of the composite aperture  14 . In addition, the AlInAs layers cladding the AlAs layers on both sides can be used to alter the shape, size and oxidation rate. Finally, the oxide may be etched out using a selective etchant such as KOH. 
       FIGS. 3(   a ) and  3 ( b ) illustrate an embodiment in which the faster-oxidizing layer  16 , such as AlAs, is clad with the slower-oxidizing layer  18 , such as AlInAs. As shown, this embodiment can increase the overall thickness of the aperture, increase the oxidation rate, while implementing an asymmetric tapered aperture  50 . The specific relative thicknesses serve only as an example and the usefulness of the invention is not limited to the thicknesses as illustrated. 
       FIG. 4  illustrates an embodiment in which the faster-oxidizing layer  16 , such as AlAs, is surrounded with the slower-oxidizing layers  18 , such as AlInAs. As shown, this embodiment can increase the overall thickness of the aperture, increase the oxidation rate, while implementing a symmetric tapered aperture  52 . Again, the specific relative thicknesses serve only as an example and the usefulness of the invention is not limited to the thicknesses as illustrated. 
       FIG. 5  illustrates an embodiment in which the slower-oxidizing layer  18 , such as AlInAs, is surrounded with the faster-oxidizing layers  16 , such as AlAs, as shown below. As shown, this embodiment has the potential to increase the overall thickness of the aperture, increase the oxidation rate, while not implementing a tapered aperture  54 . The slower-oxidizing layer is thus sandwiched between the two faster-oxidizing layers to form an symmetrically-untapered aperture. The specific relative thickness serve only as an example and the usefulness of the invention is not limited to the thickness as illustrated. 
     In other embodiments of those illustrated in  FIGS. 3–5 , the faster-oxidizing AlAs layers  16  can, rather than consisting of single layer AlAs, be replaced by two or more layers of AlAs separated by very thin layers of AlInAs. Thus, both the faster-oxidizing “layer”  16  as well as the slower-oxidizing layer  18  can be multiple layers. 
     The present invention can be used to aperture the semiconductor lasers described in the nonprovisional patent applications having the following attorney docket reference numerals which were all filed on Aug. 21, 2001 and all of which are hereby incorporated by reference in their entirety: 09/935,0099, 09/935,000 09/935,012, 09/934,789, 09/935,122, 09/935,352, 09/934,781, 09/935,279, and 09/934,791. 
     The present invention can be used with vertical-cavity surface-emitting semiconductor laser (VCSEL) for long wavelengths (i.e., 1.3 to 1.55 μm) to be used as optical sources for optical communication systems, optical interconnection, optical data-processing, or the like, in the field of optical data-communication or optical data-processing. 
     The present invention can be used to create an air or oxide aperture in the distributed Bragg reflectors, also known as DBRs or Bragg mirrors of the VCSEL, for example to confine the current to a desired area, reducing the device operating current. The optical mode is also confined, reducing optical loss and lowering the threshold current. The optical mode is confined by the aperture, resulting in reduced optical loss at the sidewall of the etched-pillar DBR, lower threshold current, lower threshold current density, larger differential quantum efficiency and higher output power. The tapering of the aperture can be very beneficial to a VCSEL design by providing an optical lens for focusing the light into the active region. 
     While the specification describes particular embodiments of the present invention, those of ordinary skill can devise variations of the present invention without departing from the inventive concept. For example the values for the strains of the layers can vary so long as there are alternating compressively and decompressively strained layers for reducing the overall strain of the structure. The exact relative quantities of the elements forming the materials of the layers can also vary. Also, the references to oxide apertures in the above description also apply to air apertures because etching is chemically related to the oxidation process and some etchants preferentially etch those materials that oxidize well. Also, the present invention can be used in many other applications other than VCSELs.