Abstract:
A photonic device includes a plurality of semi-conductor layers formed on a substrate. The semi-conductor layers include an active layer and a current controlling region in close proximity to the active layer. The current controlling region includes a relatively small electrically conductive section or aperture, and a relatively large non-conductive section. A plurality of channels in the device are arranged to bound the electrically conductive section within their perimeter. The channels are spaced close enough together, and close enough to the desired aperture region to permit a relatively brief oxidation process to precisely isolate the aperture. The remainder of the current controlling region, i.e. beyond the aperture is also altered to present a high resistance. The photonic device also includes a relatively large surface area electrical contact used to drive the device. The large size desirably lowers overall resistance.

Description:
FIELD OF INVENTION 
     The present invention relates generally to photonic devices. more specifically, the invention de-couples the traditional trade off between device resistance as a function of contact size and precision of the aperture. It is to be appreciated that the present invention is amenable to many applications such as, Vertical Cavity Surface Emitting Lasers (VCSEL&#39;s), resonant cavity LED&#39;s, photodetectors, edge emitting lasers, and LED&#39;s. 
     BACKGROUND OF THE INVENTION 
     There are many semiconductor devices that require within their structure an electrically conductive region bounded by an electrically insulating region. Examples of such devices include vertical cavity surface-emitting lasers (VCSELs), edge-emitting lasers, light-emitting diodes (LEDs) and photodetectors. In many of these devices, the electrically insulating region should preferably have a lower refractive index than the conductive region. Recently, there has been an increased interest in forming the electrically insulating region by laterally oxidizing at least one semiconductor layer. The oxidation process selectively converts the conducting semiconductor layer into a lower refractive index insulator. 
     A known technique to fabricate many photonic devices can be illustrated by a VCSEL formed by a lateral oxidation process, schematically illustrated in FIGS. 1 and 2. Under this approach, a laser structure comprising a plurality of layers is formed upon substrate  10 . These layers include an active layer  12  and an AlGaAs layer  14  with a high aluminum content. The AlGaAs layer  14  is placed either above or below the active layer of a laser structure. Then, the layered structure is masked and selectively etched to form a mesa structure  22  as illustrated in FIG.  2 . As a result of the etching, the AlGaAs layer  14  with a high aluminum content adjacent to the active layer  12  is exposed at the edges of the mesa structure  22 . To form the lasing emissive region or “aperture”, this AlGaAs layer is oxidized laterally from the edges towards the center of the mesa structure as represented by arrows A. Other layers in the structure remain essentially unoxidized since their aluminum content is lower. Consequently, their oxidation rates are also substantially lower. Therefore, only the AlGaAs layer with high aluminum content is being oxidized. The oxidized portions of the high aluminum content layer become electrically non-conductive as a result of the oxidation process. The remaining unoxidized region, which is conductive, in the AlGaAs layer forms the so-called “aperture”, a region which determines the current path in the laser structure, and thereby determines the region of laser emission. A VCSEL formed by such a technique is discussed in “Selectively Oxidized Vertical Cavity Surface Emitting Lasers With 50% Power Conversion Efficiency,” Electronics Letters, vol. 31, pp.208-209 (1995). 
     The most common lateral oxidation approach has several disadvantages, such as large mesa, large oxidation region, and poor control of the aperture size. A key disadvantage of this approach is the difficulty in controlling the amount of oxidation. Generally, the desired device aperture is on the order of one to ten microns (μm), which means that several tens of microns of lateral oxidation will typically be required in order to fabricate the device when oxidizing in from the sides of the much larger mesa, which must typically be 50 to 100 microns in diameter. Since the size of the resulting aperture is small relative to the extent of the lateral oxidation regions, the devices formed generally have severe variations in aperture size as a result of non-uniform oxidation rates from wafer to wafer and across a particular wafer. The oxidation rate of AlGaAs depends strongly on its aluminum composition. Any composition non-uniformity will be reflected by changes in the oxidation rate, which in turn creates uncertainty in the amount of oxidation. The process is also relatively temperature-sensitive. As the oxidation rate varies, it is difficult to ascertain the extent to which a laser structure will be oxidized, thereby decreasing reproducibility in device performance. In short, such a process often creates various manufacturability and yield problems. 
     Another disadvantage of a photonic device formed by a traditional lateral oxidation approach is that they often suffer from poor mechanical or structural integrity. It is well-known that the upward pressure applied during a packaging process may cause delamination of the entire mesa since the bonding of the oxide layer to the unoxidized GaAs or AlGaAs is generally weak. 
     Another known technique to fabricate VCSEL&#39;s, especially in highly compact arrays is known, and is described for example in U.S. Pat. No. 5,978,408 issued on Nov. 2, 1999 to Thornton. In such highly compact arrays, a number of laser apertures are packed closely together in various geometric shapes. Due to the closeness of the apertures however, relatively small metal contacts are used to bias the laser. Use of these smaller contacts has undesirably raised the resistance of these semi-conductor lasers, thus increasing the voltages required to drive them. 
     Accordingly, a need exists to separate the interdependence of the size of the electrical contact area and the accuracy with which a desired aperture size can be obtained. 
     The present invention contemplates a new and improved method and apparatus for forming photonic devices which overcome the above-referenced problems and others. 
     BRIEF SUMMARY OF INVENTION 
     The present invention provides a photonic structure, such as lasers, light emitting diodes or photo-detectors, having well-defined and well-controlled oxidized regions, which can be used to define the aperture of the structure. These oxidized regions are formed by the use of a multiplicity of channels arranged in a pattern in the structure. During the oxidation process, an AlGaAs layer with high aluminum content embedded in the semiconductor structure is oxidized radially outwards from each of these channels until overlap in the oxidized regions defines a central conductive aperture. The device aperture is the unoxidized region bounded by these oxidized regions centered about the channels. The AlGaAs layer with high aluminum content for forming the oxidized regions and the aperture is often referred to as an “oxidation layer.” 
     The present invention further provides a semi-conductor photonic structure having a large electrical contact. Larger contacts desirably tend to reduce the resistance of a device. The combination of the large contact size and the precisely placed and oxidized apertures effectively de-couple the usual tradeoff between device resistance and obtainable aperture precision. 
     In accordance with one aspect of the present invention, a photonic device includes a substrate, and a plurality of semi-conductor layers formed on the substrate. The semi-conductor layers include an active layer and a current controlling region near the active layer. The current controlling region includes an electrically insulating section and an electrically conductive section. The current controlling region is penetrated by a plurality of channels surrounding or bounding the electrically conductive section within their perimeter. An electrical contact is also provided on the laser such that at least part of the electrical contact lies outside the perimeter of the channels. 
     In accordance with another aspect of the present invention, the electrical contact includes an area greater than an area within the perimeter of the channels. 
     In accordance with another aspect of the present invention, the electrical contact includes an area at least two times larger than the area of the electrically conductive section. 
     In accordance with another aspect of the present invention, the current controlling region includes an oxidation layer. The electrically insulating section includes an oxidized portion of the oxidation layer. The electrically conductive section includes an unoxidized portion of the oxidation layer. 
     In accordance with another aspect of the present invention, the current controlling region further includes an electrically insulating ion implanted layer extending from a perimeter of the device or field implant region to at least an area adjacent to the oxidized portion of the oxidation layer. 
     In accordance with another embodiment of the present invention, a method of forming a photonic device includes forming a plurality of spaced channels in a surface of a semi-conductor layer bounding a desired aperture. The method further includes affixing an electrical contact to the semi-conductor layer, where the electrical contact has an area at least two times greater than an area of the desired aperture. 
     In accordance with another aspect of the present invention, the method further includes altering selected portions of at least one layer from a generally conductive state to a generally non-conductive state. 
     In accordance with another aspect of the present invention, the channels permit communication between a surface of the semi-conductor layer and a buried oxide layer. The altering step includes oxidizing a selected portion of the oxide layer. 
     In accordance with another aspect of the present invention, the altering step further includes implanting at least one layer adjoining the oxidized portion of the oxide layer with ions at a selected energy level. 
     In accordance with another aspect of the present invention, the electrical contact includes an interruption substantially corresponding to the desired aperture. When attached, the interruption in the contact aligns with the desired aperture. 
     An advantage of the present invention is that the dimension of the oxidized regions which define the lasing aperture is comparable to the dimension of the lasing aperture itself. Generally, the oxidation rates of AlGaAs depend upon material composition and process parameters, which exhibit significant non-uniformity. These non-uniformities have decreasing impact as the ratio of the size of the oxidized regions to the size of the final laser aperture is reduced. In other words, since the amount of oxidation required to form the lasing aperture is dramatically reduced, the aperture size is therefore much less sensitive to material or process variations. Hence, well-defined and well-controlled apertures can be achieved. 
     Another advantage of the present invention is that since the present invention does not need small total device size to improve the precision of the apertures, the size of the electrical contact can be increased to reduce the overall device resistance. 
     Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the invention, its preferred embodiments, the accompanying drawings, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not be construed as limiting the invention. 
     FIGS. 1 and 2 illustrate a prior art approach to the formation of a laser aperture in a VCSEL structure; 
     FIG. 3 illustrates a side sectional view of a semiconductor structure which is used in accordance with an embodiment of the present invention; 
     FIG. 4 is a top view of a portion of an exemplary mask applied to the semiconductor structure shown in FIG. 1 under the present invention; 
     FIG. 5 is a partial side sectional view of the semiconductor structure of FIG. 1 with a channel etched therein; 
     FIG. 6 is a simplified partial plan view of a portion of an oxidation layer, wherein the layers above it have been removed; 
     FIG. 7 is an enlarged, partial, cross-sectional view taken substantially along line  7 — 7  in FIG. 6; 
     FIG. 8 is an enlarged, partial, cross-sectional view taken substantially along line  8 — 8  in FIG. 6; 
     FIG. 9 is a view of a photonic device according to one embodiment of the present invention; 
     FIG. 10 is a cross-sectional view taken substantially along line  10 — 10  in FIG. 9; 
     FIG. 11 is a plan view of a photonic device according to another embodiment of the present invention; and, 
     FIG. 12 is a cross-sectional view taken substantially along line  12 — 12  in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIG. 3, a photonic semiconductor structure which is used to form an embodiment of the present invention includes a number of semiconductor layers, which can be used to form, for example, a vertical cavity surface emitting laser. As will be apparent, the layers are illustrated schematically only and bear no relationship to the relative thicknesses each to the other. As shown in FIG. 3, an n-type GaAs substrate  100  supports an n-type gas buffer layer  102  grown using an epitaxial deposition process, by liquid phase epitaxy (“LPE”), molecular beam epitaxy (“MBE”), or other known crystal growth processes. 
     Above the GaAs buffer layer  102  is a structure for forming a lower distributed Bragg reflector (“DBR”)  104  which provides a portion of the necessary internal reflection in a VCSEL structure. The lower DBR  104  is typically formed by multiple pairs of an AlGaAs layer with a high aluminum content and another AlGaAs layer with a low aluminum content as is conventional in the art. Artisians will appreciate that thickness of the pairs and materials used contribute to the reflectivity of the mirror in known ways. 
     In the illustrated embodiment of FIG. 3, since light is outcoupled through the top surface of the semiconductor sample, the reflectivity of the lower DBR  104  should be as close to 100% as possible in order to achieve high internal reflection. High internal reflection generally reduces the threshold current of a laser. 
     After the lower DBR structure  104  has been deposited, an AlGaAs cladding layer  106  is deposited. Above this AlGaAs cladding layer  106  is the active layer  108  of the structure which typically includes quantum wells separated by barriers. Depending upon the desired wavelength of the structure other active layer compositions, such as, pure GaAs or AlGaAs with a low aluminum content may be used to form the quantum well structures as is known. In a reverse biased structure, the active layer  108  can also function as a light collector, enabling the device to be used as a photodector. 
     Above the active layer  108  is an upper AlGaAs cladding layer  110 , which is structurally similar to the lower AlGaAs cladding layer  106  except for the polarity of its dopants. These two AlGaAs cladding layers,  106  and  110 , along with the active layer  108  generally form the optical channel in which the desired optical gain can be attained. The total thickness of layers  106 ,  108 , and  110  is adjusted to be equal to an integer multiple of the intended wavelength of laser operation. 
     Above the upper AlGaAs cladding layer  110  is an oxidation layer  112 , which is used to form the laser aperture. As will be further discussed below, the laser aperture controls the current flow and thus the lasing location in the active layer  108 . In this embodiment, this oxidation layer  112  is above the upper AlGaAs cladding layer  110 . However, those skilled in the art will recognize that placement of this oxidation layer  112  in another location, either further above or below the active layer  108 , will not impair functionality. Typically, this oxidation layer  112  constitutes the first layer of an upper DBR mirror and contains a p-type dopant. 
     After the oxidation layer  112  has been formed, the remainder of an upper DBR mirror  114  which contains p-type doping is deposited. The upper DBR mirror  114  is structurally similar to the lower DBR mirror  104  except for the polarity of its dopants. Additionally, the mirror layer closest to each side of the active region generally has a high aluminum content. As illustrated, this high aluminum content layer doubles as the oxidation layer  112 . In this embodiment, the reflectivity of the upper DBR  114  is typically 98% to 99% because light will be outcoupled through the surface of the semiconductor sample. 
     FIG. 4 is a top view of a portion of a mask which may be applied to the semiconductor structure shown in FIG. 3 under the present invention. First, as is conventional, a uniform layer of silicon nitride will be deposited over the entire semiconductor sample. Then, a photoresist layer  118  is deposited over the silicon nitride layer and is formed into the mask shown in FIG. 4 by a photolithographic process which removes photoresist material from four areas  120 . The areas  120  form a predetermined bounding pattern which will later be used to define the resulting aperture of a laser structure. It should be appreciated however, that while the areas  120  are illustrated as circular, they could be any shape selected for ease of etching, for example. Moreover, those skilled in the art will appreciate that the number of areas  120  merely affect the shape of the aperture and different numbers can be effectively utilized. 
     As illustrated in FIG. 5, the sample then undergoes an etching process during which channels  126  are formed in the semiconductor structure through the four exposed areas  120 . The etching is performed by a process such as reactive ion etching which provides for the formation of a deep depression with vertical sidewalls. The depth of each cylindrical channel  126  should extend at least into the oxidation layer  112 , as shown in FIG.  5 . After the formation of the channels  126  and the removal of any photoresist on the surface, the semiconductor sample undergoes an oxidation. The sample is typically oxidized with water vapor in a nitrogen environment at elevated temperatures, in excess of 350° C. During the oxidation process, the oxidation layer  112  is exposed to the ambient environment through each channel  126 , as indicated by arrows B. Thus, the oxidation layer  112 , which comprises of AlGaAs with a high aluminum content, is oxidized radially outward from each channel  126 , typically for an amount of time to allow the oxidized regions  124  surrounding each channel to approach one another and overlap, as can be seen in FIG.  6 . However, a small non-oxidized area between the oxidized regions may be permissible so long as electrical and optical fields are adequately confined. 
     During the oxidation process, other layers in the structure remain essentially unoxidized since their aluminum content is lower. The oxidation rate of AlGaAs increases with the aluminum content in a generally exponential manner at constant temperature. As is well known, the time duration of the oxidation process depends upon the aluminum content in the oxidation layer  112  and the oxidation temperature. A desirable, controllable oxidation duration would be a few tens of minutes. Therefore, the layer that is being oxidized is the AlGaAs which has a high aluminum content of close to 95%. The portion of the AlGaAs layer which remains unoxidized controls the current path through the active layer  108 . 
     FIG. 6 is a largely simplified top view of the oxidation layer  112  shown in FIG.5, after oxidation, with all the layers above it removed for illustration. The shaded region  122  represents the laser aperture in oxidation layer  112  which determines the region of laser emission by active layer  108 . It is formed by the oxidation process of the present invention. During the oxidation process, the oxidation fronts emanate through the oxidation layer from the pattern of channels  126 , and the aperture  122  is formed by the intersection of the boundaries of the oxidized regions  124 . The oxidation fronts emanating from the channels  126  are also generally similarly shaped, resulting in overlapping oxidized regions  124 . The overlapping regions  124  define a central unoxidized region or aperture  122 . After the oxidation process, an ion implantation process, which is next described, is used to complete the electrical isolation of the aperture  122 . 
     After the oxidation process, the areas  124  are non-conductive or insulating and the unoxidized portion or aperture  122  is conductive thus controlling the current path through the active layer  108 . Current flow through that portion of the active layer  108  which lies below the aperture  122  results in an injected density of p-type and n-type carriers, resulting in optical amplification. At sufficiently high current flow, this optical amplification, in combination with feedback from the DBR mirrors,  104  and  114 , will result in laser oscillation and emission from the active layer within the region defined by aperture  122 . 
     In addition to forming non-conductive areas  124  by oxidation, a process is needed to substantially increase the resistivity from a so-called isolation region  130  extending between a perimeter of the device or a top contact of the device inward to an area at least adjacent to areas  124 . The highly resistive isolation region  130  prevents currents from shunting through the non-oxidized parts of the oxidation layer  112 . In one embodiment, isolation region  130  is formed by using an ion implantation process. As is known in the art, ion implantation depth is dependent on energy levels and/or ions selected. Artisans can perform known calculations or employ commercially available software (“Athena” manufactured by Silvac International, among others) to estimate appropriate energies. 
     After the isolation process, metal contacts  132  and  134  are formed on the top surface and the bottom surface of the semiconductor structure, respectively, for biasing the laser, as illustrated in FIGS. 7-12. A typical material used for forming the contacts is a titanium/gold bilayer film. 
     FIG. 9 shows a top view of a semi-conductor structure formed in accordance with the present invention after a typical metallization process to form the top contact  132 . The top contact  132  is sized to cover the entire device, thus providing a large surface area and lowered resistance. In the illustrated device, an interruption or annular opening in the contact  132  allows light to be emitted from the device through the interruption. Those skilled in the art will appreciate that if the top contact is transparent, no interruption is required. As discussed above, the convergence of the plurality of oxidized regions  124  form the aperture  122  (shown in ghost). 
     With reference now to FIG. 10, the cross sectional view taken along lines  10 — 10  in FIG. 9 is shown. It can now be appreciated that aperture  122 , typically on the order of 4 microns in diameter, has a significantly smaller area than the top contact  132 . Further, currents generated between the contacts  132 ,  134  are funneled through the aperture  122  by the presence of the non-conducting regions formed by a combination of the isolation region  130  and the oxidized areas  124 . 
     Referring now to FIG. 11, a top view of an edge-emitting device such as a laser or LED formed in accordance with the present invention is shown. To recognize the similarity between components with similar functions, a convention of reference numeral usage is employed which denotes elements with similar function as the above-described device with a prime (′). 
     The edge-emitting device has stacked epitaxial layers similar to FIG.  3 . The layers, which typically are compound semiconductors containing In, Al, Ga, As, P, Sb, and N, function to confine charge carriers, transport charge carriers, emit light, and guide light. However, DBR mirrors  104  and  114  (FIGS. 3,  5 ) are not typically employed in edge-emitting devices. On the other hand, in one embodiment, the ends of the device are cleaved to form an edge exit for light. In the case of a laser, the cleaved facets form a mirror pair providing the optical feedback necessary to generate laser light. 
     The cross section taken along line  12 — 12  in FIG. 11 shown in FIG. 12 illustrates a plurality of channels  126 ′ providing access to at least one buried layer that can be selectively oxidized to form a lateral optical waveguide or aperture  122 ′. As above, the channels  126 ′ and the oxidized areas  124 ′ bound or define the aperture  122 ′. However, in edge-emitting devices, the boundary, like the direction of emitted light, is best viewed from the side or edge. A short lateral oxidation extent relative to the aperture size ensures accurate control of the final aperture size. In addition to the oxidized areas  124 ′, isolation region  130 ′ contributes to the electrical isolation of the aperture  122 ′. As known in the art, electrical isolation region  130 ′ can be formed, for example, by ion implantation of Hydrogen. 
     A typical metallization process has been used to form the top contact  132 ′. The bottom contact  134 ′ provides a return path for electrical current. Again, the top contact  132 ′ is sized to cover the entire device, thus providing a lowered resistance. 
     The composition, dopants, doping levels, and dimensions given above are exemplary only, and variations in these parameters are permissible. Additionally, other layers in addition to the ones shown in the figures may also be included. Variations in experimental conditions such as temperature and time are also permitted. Lastly, instead of GaAs and GaAIAs, other semiconductor materials such as GaAISb, InAIGaP, or other III-V alloys may also be used. 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.