Abstract:
An edge viewing semiconductor photodetector may be provided. Light may be transmitted through an optical fiber conduit comprising a core region surrounded by a cladding region. The light may be received at the edge viewing semiconductor photodetector having an active area. The active area may be substantially contained within a first plane. The edge viewing semiconductor photodetector may further have conducting contact pads connected to the active area. The contact pads may be substantially contained within plural planes. The first plane may have its normal direction substantially inclined with respect to a normal direction of the plural planes. The first plane may further have its normal direction substantially inclined with respect to a direction of the received light incident to the active area. Next, a signal may be received from the pads. The signal may correspond to the transmitted light.

Description:
RELATED APPLICATION 
     Under provisions of 35 U.S.C. § 119(e), Applicant claims the benefit of U.S. provisional application No. 60/655,623, Filed Feb. 23, 2005, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Photodetectors (PDs) are physical devices that are used to measure light&#39;s presence by converting energy contained in a light quantum (i.e. a photon) into an electrical form that is easier to measure, amplify, and quantify. The energy conversion process may be indirect in which many photons over a period of time generate a detectable heat amount. The heat is then converted to an electrical signal by a thermocouple junction or a resistive bolometer or other thermodynamic processes. The energy conversion process may, in contrast, be direct in that one light quantum or photon causes temporary matter ionization by removing an electron from a chemically bound state to a free vacuum state. The damaged bond may or may not migrate among neighboring atoms, but may eventually be restored to a full bonding state, although there are exceptions such as f-centers in halide salts. 
     If the PD is a vacuum photomultiplier tube and an incident photon has sufficient energy, an electron may be ejected from the anode surface and may be collected by an accelerating potential in vacuum. The accelerated electron may collide with other anode surfaces to create an electron avalanche which may be registered electrically as a quantity of charge. The net directional movement of many such charge quanta gives rise to an electric current that can be interpreted as information. 
     If the PD is a semiconductor device, the ionization process is said to be internal and both the ionized electron and the damaged bond are mobile and can move by scattering from one atom to another in a well ordered crystal lattice. Because the electron and damaged bonding state necessarily have opposite electrical charge, they drift in opposite directions in an electric field. Each contributes to the electrical current generated by an incident stream of ionizing photons. The damaged bond carries a positive charge and is referred to as a “hole”. The electron carries a negative charge. A “hole” current as well as an electron current can be characterized. 
     PDs available today for the rapid transport of information are the semiconductor type. Semiconductor PDs work on the quantum energy conversion principle described above, with variations designed to improve amplification, light detection efficiency, and fast response to a burst of photons. For example, an inhomogeneous semiconductor junction suffices to effectively separate and collect photo-generated electron-hole current, but is not optimized. An avalanche photodiode (APD) offers greater sensitivity because, the initial ionization charge created by a sufficiently energetic incident photon is amplified by using an electric field acceleration and charge amplification process within the semiconductor that is similar to that occurring in a vacuum photomultiplier tube discussed above. Resonance cavity enhanced (RCE) PDs utilize an enhanced back-side reflection structure to record as much light as possible. A metal-semiconductor junction is an inhomogeneous semiconductor junction that is generally referred to as a “Schottky” junction and is also effective in collecting photo-ionization current that is generated in the semiconductor substrate. The metal-semiconductor-metal (MSM) PD works on the principle of the Schottky junction and is designed primarily for speed. 
     The P-I-N structure is the basic semiconductor junction structure prevalent today in optical communication and is a good compromise between high speed and good detection efficiency. PDs that have the P-I-N structure are called P-I-N PDs. In P-I-N PDs, the semiconductor ionization and electron-hole generation process occurs in a first chemically pure or intrinsic-type semiconductor layer. A second semiconductor layer is purposely contaminated with atoms that come to equilibrium in the same or similar semiconductor crystal lattice by releasing a spare, mobile electron that is shared by all the atoms. This spare atom is said to occupy states in the conduction band. This second semiconductor layer is called the “n-type” layer. A third semiconductor layer is purposely contaminated with atoms that come to equilibrium in the same or similar semiconductor crystal lattice by trapping electrons from lattice atoms in order to form stable bond. The resulting unpaired bond has a positive charge that is shared by all the atoms and is said to occupy states in the valence band. The third semiconductor layer is called the p-type layer. 
     Due to the periodic nature of the crystal potential, crystalline semiconductors, metals, and insulators are characterized by bands of states that are distinguished only by small increments of energy and momentum. In addition, crystalline potentials promote the appearance of regions of energy in which stable states are forbidden. This energy distribution is in sharp contrast to the discrete nature of energy states in isolated atoms. Bonding states occupy the valence band and un-bound electrons occupy the conduction band. Separating the two bands is a band-gap in which no stable states exist. In metals, electronic states overlap energetically with the conduction band and the metal is conductive. In semiconductors, when electronic states overlap with the conduction band, the semiconductor is n-type. When hole states, or shared unpaired bonds, overlap energetically with the valence band, the semiconductor is p-type. When the semiconductor is intrinsic or uncontaminated, there are few mobile electrons and holes, only those that are thermodynamically generated by the sample temperature, and the semiconductor is a poor conductor. Crystalline insulators can be characterized as having an energetically high conduction band. 
     Waveguide-type P-I-N designs are represented in the publication by Vincent Magnin, et al., in the Journal of Lightwave Technology, Vol. 20, p. 477 (2002). These have rapid response when the intrinsic-type region is very narrow, typically ½ to 1 μm in thickness. Unfortunately, this means that the semiconductor waveguide used to channel light into the detection i-type region is the same thickness and unable to collect sufficient light from an optical fiber or polymer waveguide whose core dimensions are typically 9 μm to 50 μm. In this situation, most of the light is lost. This design type has little sensitivity at high speed. If a much thicker semiconductor waveguide is used to channel more light into the detection i-type region, the waveguide-type P-I-N design will have increased sensitivity, but will have a much slower temporal response. This is because the electrons and holes that are photo generated in the intrinsic or undoped-type layer, having a finite field drift velocity, will take a longer time to travel to the p-type and n-type sides and be recorded as a current. 
     Refraction-type P-I-N designs are represented in the publication by Hideki Fukano et al., in the Journal of Lightwave Technology, Vol. 15, p. 894 (1997). These designs rely on using an oblique light entry facet and Snell&#39;s law to guide light to a P-I-N photo-ionization and charge collection region that is located on a different surface. The oblique surface makes a substantial angle with the travel direction for the incoming light and with the planar surfaces containing the P-I-N layer structure and electrical contact pads. Generally, the incident light emanates from a semiconductor laser, optical fiber, or optical waveguide and is usually diverging. Consequently, the oblique surface has to be in close proximity to the area containing the P-I-N layers, referred to as the active area. If the active area is small, as it must be in order to minimize capacitance and promote rapid temporal response, then much of the light incident on the refraction surface will miss the active area and will not be registered, reducing the sensitivity of the refraction-type. If the active area is enlarged in order to collect more of the refracted light, then the speed of the PD is lowered. 
     SUMMARY 
     An edge viewing PD may be provided. This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter. Nor is this Summary intended to be used to limit the scope of the claimed subject matter. 
     In accordance with one embodiment, a system for providing light detection comprises a contiguous semiconductor volume; a plurality of surface area segments on the contiguous semiconductor volume, at least one of the plurality of surface area segments being optically active and at least one of the plurality of surface area segments being adjacent to the at least one of the plurality of surface area segments; at least one intersection angle between the at least one of the optically active plurality of surface area segments and the at least one of the adjacent plurality of surface area segments respectively, none of the at least one intersection angle being substantially equal to one of the following: 0 degrees, 90 degrees, and 180 degrees; and at least one electrically conductive line connected to an electrically active area on the at least one of the optically active plurality of surface area segments and extending to the at least one of the plurality of surface area segments being adjacent to the at least one of the plurality of surface area segments. 
     According to another embodiment, a method for measuring light comprises transmitting light through a polymer waveguide conduit comprising a core region surrounded by a cladding region, receiving the light at a semiconductor photodetector having an active area, the active area being substantially contained within a first plane, the semiconductor photodetector further having conducting contact pads connected to the active area, the contact pads being substantially contained within plural planes, the first plane having its normal direction substantially inclined with respect to a normal direction of the plural planes, the first plane further having its normal direction substantially inclined with respect to a direction of the received light incident to the active area, and receiving a signal from the pads, the signal corresponding to the transmitted light. 
     According to yet another embodiment, a method for measuring light comprises transmitting light through an optical fiber conduit comprising a core region surrounded by a cladding region, a “V”-shaped or “U”-shaped groove for directing and stabilizing the optical fiber, receiving the light at a semiconductor photodetector having an active area, the active area being substantially contained within a first plane, the semiconductor photodetector further having conducting contact pads connected to the active area, the contact pads being substantially contained within plural planes, the first plane having its normal direction substantially inclined with respect to a normal direction of the plural planes, the first plane further having its normal direction substantially inclined with respect to a direction of the received light incident to the active area, and receiving a signal from the pads, the signal corresponding to the transmitted light. 
     In accordance with yet another embodiment, a photodetector comprises a heteropolar semiconductor, an active area, at least one polar plane on the heteropolar semiconductor wherein the active area residing on the at least one polar plane of the heteropolar semiconductor, and electrical contact pads making electrical contacts to the active area. 
     Both the foregoing general description and the following detailed description provide examples and are explanatory only. Accordingly, the foregoing general description and the following detailed description should not be considered to be restrictive. Further, features or variations may be provided in addition to those set forth herein. For example, embodiments may be directed to various feature combinations and sub-combinations described in the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate various embodiments of the present invention. In the drawings: 
         FIG. 1  is a cross-section view of a P-I-N layer structure for generating a photo-ionization current and a light entry facet formed in a plane that is parallel to the planes of electrical contact pads and to a plane of the P-I-N layer structure; 
         FIG. 2  is a cross-section view of a P-I-N layer structure for generating a photo-ionization current wherein a layer structure additionally serves to guide light to a current detection region from a light entry facet that is formed in a plane that is substantially perpendicular to planes of the electrical contact pads and the P-I-N layer structure; 
         FIG. 3  is a cross-section of a P-I-N layer structure for generating a photo-ionization current and an oblique light entry facet formed in a plane that is oblique to planes of the electrical contact pads and the P-I-N layer structure; 
         FIG. 4A  is a cross-section view of an EVPD structure having a light entry facet and P-I-N layer structure that are formed in a plane that is oblique to a plane of the electrical contact pads; 
         FIG. 4B  shows a top view of the same EVPD structure of  FIG. 4A ; 
         FIG. 5A  is a cross section view of an EVPD structure having a light entry facet and P-I-N layer structure that are formed in a plane that is oblique to a plane of the electrical contact pads; 
         FIG. 5B  shows a top view of the same EVPD structure of  FIG. 5A ; 
         FIG. 6  shows a direct end-coupling of an EVPD structure to a polymer waveguide; 
         FIG. 7  shows a direct end-coupling of an EVPD structure to an optical fiber by a “V-groove” formed directly on, or attached to the, PD; 
         FIG. 8  shows an orientation of a semiconductor substrate for the fabrication of an EVPD structure; 
         FIG. 9  shows a first process stage for the fabrication of an EVPD structure; 
         FIG. 10A  shows a second process stage for the fabrication of an EVPD structure; 
         FIG. 10B  shows a top view of the same structure of  FIG. 10A ; 
         FIG. 11  shows a third process stage for the fabrication of an EVPD structure; and 
         FIG. 12  shows a fourth process stage for the fabrication of an EVPD structure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description refers to the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the following description to refer to the same or similar elements. While embodiments of the invention may be described, modifications, adaptations, and other implementations are possible. For example, substitutions, additions, or modifications may be made to the elements illustrated in the drawings, and the methods described herein may be modified by substituting, reordering, or adding stages to the disclosed methods. Accordingly, the following detailed description does not limit the invention. Instead, the proper scope of the invention is defined by the appended claims. 
     An edge viewing PD may be provided. Consistent with embodiments of the present invention, a method for detecting light that may be propagating in a plane that may be parallel to plane(s) containing a PD&#39;s electrical contact pads is provided. The light entry area on the light entry and detection facet, or the facet containing the P-I-N layer structure, can be constructed to be any desired dimension and may be oriented at a substantial angle with respect to the plane(s) containing the electrical contact pads. 
     Consistent with embodiments of the invention, a new edge viewing PD (EVPD) class in which a light entry and detection facet containing a P-I-N layer structure may be formed on a “mesa” structure sidewall formed, for example, in a semiconductor substrate. The mesa structure may be defined by an anisotropic liquid etching process and may form an atomic surface that may be suitable for hetero-junction epitaxial layer growth by metallo-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) processes. The sidewall geometry may be particularly useful for embedding the EVPD, for example, in a polymer lightwave circuit and for more convenient alignment to optical fibers. In optoelectronic packaging for the telecommunication industry, the EVPD, for example, may be fitted with a “V-groove” on a lower mesa to facilitate direct optical fiber coupling without using additional components such as mirrors and lenses. 
     Accordingly, embodiments of the invention may provide, for example, a method for greatly increasing the sensitivity and temporal response for EVPDs. Furthermore, embodiments of the invention may provide a method for the rapid and precise EVPD alignment with respect to polymer optical waveguides in lightwave circuits. In addition, embodiments of the invention may provide a method for the simplified optical packaging and alignment between optical fibers and PDs. Moreover, embodiments of the invention may provide a method for decreasing feedback noise that may be caused by back reflected light from a PD into a laser waveguide. 
       FIG. 1  is a cross-section view of a P-I-N layer structure  100  for generating a photo-ionization current and a light entry facet formed in a plane that is parallel to planes of electrical contact pads and to a plane of P-I-N layer structure  100 . As shown in  FIG. 1 , a first layer  13  may be a p-type layer and a second layer  15  may be an n-type layer. These layers may be used to generate a static electric field in an intrinsic layer  14  in order to accelerate and efficiently collect the electron (e) and hole (h) charge currents generated by the photo ionization process that occurs mostly in intrinsic layer  14  due to incident sufficient energy photons. The incident light may be represented by a group of four arrows  19  incident on an entry facet  18 . As the P-I-N layers are very thin, these are normally formed on a substrate  16  of chemical doping type similar to second layer  15 . An anti-reflection layer  12  may be formed on first layer  13  and intrinsic layer  14  that may also be used to adjust the spectral detection width by adjusting the semiconductor band-gap with appropriate semiconductor alloying. Metallic electrical contacts  11  and  17  may be used to connect to external instrumentation to apply an external accelerating electric field and to detect the ionization current in a number of ways. One process for measuring the photo-current, I eh , is shown in  FIG. 1  that uses a resistor R across which is measured an electrical potential, V. 
     Because P-I-N PDs may be used in many applications, numerous attempts have been made to adapt their light detection efficiency for specific applications. Thus, while structure  100  in  FIG. 1  has a “top-viewing” light entry facet, structure  100  may be easily adaptable to a “bottom-viewing” light entry facet configuration. In both cases, a light plane entry facet may be parallel to a plane of a P-I-N layer structure and to an electrical contacts plane. 
       FIG. 2  is a cross-section view depicting a P-I-N layer structure  200  for generating a photo-ionization current wherein a layer structure additionally serves to guide light to a current detection region from a light entry facet. The light entry facet may be formed in a plane that is substantially perpendicular to planes of the electrical contact pads and the P-I-N layer structure. As sown in  FIG. 2 , the light entry facet may be planar and perpendicular to the planes of the P-I-N structure and electrical contacts. The semiconductor layer structure may be similar to that in structure  100  wherein a layer  22  may be a p-type layer, a layer  24  may be an intrinsic layer, a layer  25  may be the n-type layer, and a layer  26  is a substrate. Metal electrical contacts are  23  and  27  and the photo-current may be registered as described above with respect to  FIG. 1 . An antireflection coating  21  may be formed on a side light entry facet  28  and light, represented by four arrows  29  may be guided to an electrical current collection region under metal contact  23  by a waveguide structure defined by layers  22 ,  24 , and  25  (and possibly additional layers.) Light may be absorbed and may generate mobile electrons and holes through its path in layer  24 . This type of PD may be called a “waveguide P-I-N PD.” A waveguide P-I-N PD may be designed to be used in cases where light may be propagating in a plane that may be parallel to the plane of the P-I-N layer structure and it may not be convenient to re-orient or otherwise guide light to an entry facet of a top-viewing or bottom-viewing PD. 
       FIG. 3  is a cross-section of a P-I-N layer structure  300  for generating a photo-ionization current and having an oblique light entry facet formed in a plane that is oblique to planes of the electrical contact pads and the P-I-N layer structure. As shown in  FIG. 3 , structure  300  may comprise an EVPD. Again, a goal may be to detect light that is propagating in a plane that is parallel to the plane of the P-I-N layer structure when it is not convenient to re-orient or otherwise guide light to an entry facet of a top-viewing or bottom-viewing PD. As shown in  FIG. 3 , light may be guided to an intrinsic layer photo-ionization region and electrical charge collection region by a refracting entry facet  38  with an anti-reflection coating  37 . An incident light path may be represented by four arrows  39 . The P-I-N photo-ionization and electrical charge detection structure is again similar to that shown in  FIG. 1 . Layers  32 ,  33  and  34  may be the p-type layer, the intrinsic-layer, and the n-type layer respectively. Top and bottom metal contacts are  31  and  36  respectively. Substrate  35  may be the same type as layer  34 . 
       FIG. 4A  is a cross-section view of an EVPD structure  400  having a light entry facet and P-I-N layer structure that may be formed in a plane that may be oblique to a plane of the electrical contact pads.  FIG. 4B  shows a top view of EVPD structure  400  of  FIG. 4A .  FIGS. 4A and 4B  illustrates a P-I-N layer structure for generating a photo-ionization current and a light entry facet, both that may be formed on a sidewall of a purposely fabricated mesa structure. The mesa wall may be formed by anisotropic chemical etching in a liquid solution and may be oblique to the planes of the electrical contact pads. Thus, with embodiments of the present invention, the plane of the P-I-N layer structure may be parallel to the plane of the light entry facet. For example, and in contrast to conventional systems, both planes may be inclined to the direction of incident light and inclined to the plane of the electrical contact pads. Thus a layer  45  may be a p-type layer, a layer  44  may be an intrinsic-type layer, and a layer  40  may be an n-type layer. A layer  46  may be an antireflection layer. A substrate  43  may also be n-type. The P-I-N layers continuously extend from a portion of a top mesa  48 , a mesa wall  41 , and a lower mesa  49 . Light, as represented by four arrows  47 , may be incident on a light entry facet  401  on mesa wall  41 . Most of the photo-ionization may occur in intrinsic layer  44 . As shown in  FIG. 4B , top electrical contact pad  42  makes electrical contact with the p-type layer all around a perimeter on the top mesa  48 , mesa wall  41 , and lower mesa  49  except for an open segment  402  that may be placed anywhere along that periphery. Bottom n-type contact is  403 . An angle θ of mesa wall  41  may be determined by the liquid anisotropic etching conditions and may be generally in the range of approximately 54.7 degrees but is not limited to this value. An external electrical circuit for detecting the ionization current I eh  may be similar to that shown in  FIG. 1 . 
       FIG. 5A  is a cross section view of an EVPD structure  500  having a light entry facet and P-I-N layer structure that may be formed in a plane that may be oblique to a plane of the electrical contact pads.  FIG. 5B  shows a top view of EVPD structure  500  of  FIG. 5A . Consistent with embodiments of the invention, both electrical contacts  54  and  51  may be formed on a top mesa  50 . Layers  59 ,  58 , and  57  may be p-type, i-type, and n-type semiconductor layers, respectively, that may compose the P-I-N structure of this embodiment. The aforementioned P-I-N layers may continuously extend from a portion of top mesa  50 , a mesa wall  56 , and a lower mesa  502 . An electrical contact pad  51  may make metallic contact all around the periphery of p-type layer  59  on top mesa  50 , mesa side wall  56 , and a mesa bottom  502 . An insulating gap  503  may be formed in contact metallurgy  51 . Similarly, an electrical contact pad  54  may make metallic contact all around the perimeter of n-type layer  57  on top mesa  50 , mesa side wall  56 , and mesa bottom  502 . An insulating gap  501  may be formed in contact metallurgy  54 . An insulating layer  507  may ensure that the two top electrical contact pads are isolated from one another. Light, as represented by four arrows  505 , may be incident on a light entry facet  506  on mesa wall  56  with most of the photo-ionization occurring in intrinsic layer  58 . An antireflection layer  53  may minimize reflection losses. Angle may θ carry the same meaning as in  FIG. 4A . 
       FIG. 6  shows a direct end-coupling of an EVPD structure  65  to a polymer waveguide. For example, embodiments of the present invention may include a method of use in which a method for direct coupling an EVPD to polymer waveguides.  FIG. 6  shows a cross section  600  consistent with embodiments of the invention for coupling EVPD  65  to a polymer waveguide. EVPD  65  may be electrically connected to a substrate electrical circuit  64  by a bottom electrical contact  403 . A buffer layer  63  may be formed on substrate  64 . A lower cladding layer  62 , a waveguide core  61 , and a top cladding layer  60  may be formed on buffer layer  63 . Light that may be guided by core  61  may be incident on oblique light entry facet  401 . Consistent with embodiments of the invention, light that my be reflected from EVPD  65 &#39;s surface may be reflected away from optical waveguide core  61  and may not find its way back through waveguide core  61  to a laser source. Rather, the reflected light may enter a laser waveguide cavity and increase laser instability and cause noise. Furthermore, consistent with embodiments of the inventions, there may be no need for light steering mirrors and light collimating or focusing lenses. 
       FIG. 7  shows a direct end-coupling of an EVPD structure  78  to an optical fiber by, for example, a “V-shaped” or “U-shaped” groove or cavity formed directly or indirectly on a PD. The “V-shape” and “U-shape” are examples and other shapes may be used. The “V” or “U” groove may be fabricated on a different substrate to which the PD is bonded. The perspective  700  in  FIG. 7  is consistent with embodiments of the invention for coupling EVPD  78  to an optical fiber  76 . EVPD  78  may be electrically connected to a substrate  77  via a bottom contact  74 . A top electrical contact  71  may be formed on a top mesa  70 . Contact  71  may make electrical contact with a periphery of a p-layer as it may extend onto a mesa wall  72  around a light entry surface area  73  just above a P-I-N photo-ionization area. A cavity  75  (e.g. “V” or “U” shaped) may be formed on a lower mesa  79  or EVPD  78  may be lithographically aligned and attached to a substrate having pre-fabricated cavities. An optical fiber  76  may be aligned in place by, for example, “V” or “U” shaped cavity  75 . A fiber core  701  may be aimed directly at light entry surface area  73 . Consistent with embodiments of the present invention, light that may be reflected from EVPD  78 &#39;s surface may be reflected away from optical fiber core  701 . Consequently, this light may not find its way back through the fiber to a laser source where the reflected light may increase laser instability and cause noise. Consistent with embodiments of the present invention, there may be no need for light steering mirrors and light collimating or focusing lenses. 
       FIG. 8  shows an orientation of a semiconductor substrate for the fabrication of an EVPD structure. A method for constructing an EVPD consistent with embodiments of the present invention may begin by orienting a principal crystal directions of a crystalline semiconductor substrate material wafer with respect to a lithographic mask. As shown in  FIG. 8 , a semiconductor material may be shaped in the form of a wafer. The semiconductor material may comprise, but is not limited to, gallium arsenide (GaAs), indium phosphide (InP), and Silicon, for example. All three of the aforementioned semiconductors may have a cubic crystal structure. A wafer  81  may be constructed so that the principal cubic crystal directions may be defined by knowledge of the surface orientation and a flat edge  80 . In this case, one may start with a wafer whose surface is in the [100]-type crystal direction and the flat may be aligned in a [110]-type crystal direction. A lithographic mask having rectangular features may be aligned with one side of a features parallel to the [110] direction. 
       FIG. 9 . shows a first process stage for the fabrication of an EVPD structure consistent with embodiments of the present invention. For example, a relative orientation of one rectangular lithographic feature with respect to the major cubic directions is shown in  FIG. 9 . A hard mask  91  may be formed on a semiconductor substrate  93  that may be an n-type semiconductor. A rectangular opening  92  may be formed in the hard mask. The semiconductor substrate with the hard mask and the rectangular opening may be exposed to a liquid that may cause the semiconductor to be etched so that the rate of material removal may be dependent on the crystal plane that may be exposed to the etchant solution. The etching, for example, may proceed more rapidly in a [110]-type direction, followed by a [100]-type direction, then followed by a [111]-type direction as discussed and demonstrated in many books and articles, in particular, in the publication by Sadao Adachi and Kunishige Oe, entitled “Chemical etching characteristics of (001) GaAs” which appears in the Journal the Electrochemical Society (USA): Solid State Science and Technology, Volume 130, No. 12, pp. 2427-2435, published in 1983. 
       FIG. 10A  shows a second process stage for the fabrication of an EVPD structure.  FIG. 10B  shows a top view of the same structure of  FIG. 1A . A result of this second process stage may be shown in the cross section shown in  FIG. 10A  and the top view shown in  FIG. 10B . A rectangular mesa opening  1004  may be formed having a top mesa  1005 , a bottom mesa  1006 , and mesa walls, one of which is indicated by as mesa wall  1002 . A surface normal to the mesa walls may be parallel to a [111]-type direction and may intersect a [100]-type direction at an angle, for example, of approximately 54.7 degrees. A mesa wall thus etched in heteropolar semiconductors may be a polar plane and can support a diapole layer near the surface. The planar dimensions of bottom mesa  1006  may be substantially of a rectangular mask opening. The depth of the mesa structure may be determined by the length of time that the semiconductor substrate  93  may be exposed to the etching solution, temperature, and the composition of the etchant solution. The mesa etching process may be followed by mask removal and a epitaxial growth of a P-I-N structure. 
       FIG. 11  shows a third process stage for the fabrication of an EVPD structure. The cross section shown in  FIG. 11  may depict the cross section of a mesa structure in a n-type material  93 . On the mesa structures are then grown an n-type layer  1101 , an intrinsic layer  1102 , and a p-type layer  1103 . In particular, the layer sequence may also be formed on mesa walls  1002 . A gap  1104  in the layer sequence may be formed on the lower mesa  1006 . 
       FIG. 12  illustrates a fourth process stage for the fabrication of an EVPD structure.  FIG. 12  shows a cross section view  1200  of the structure when the fabrication sequence is completed. An antireflection coating layer  1206  may be added to minimize reflection losses, and a top electrical contact  1205  and a bottom electrical contact  1207  may be added in order that the device be conveniently used. The final stage may be to cleave the bottom mesa to produce individual or linear arrays of EVPDs. 
     Consistent with embodiments of the inventions, an active area of a semiconductor PD may comprise an area within which incident quanta of light, having sufficient energy, ionize a portion of the semiconductor material by an internal ionization process and creates mobile electron-hole pairs which are then collected and measured by appropriate conducting electrodes. Furthermore, a polar plane in a heteropolar semiconductor may comprise a plane that may be populated, for example, by one atomic specie comprising the heteropolar semiconductor crystal lattice. 
     Embodiments of the present invention, for example, are described above with reference to block diagrams and/or operational illustrations of methods, systems, and computer program products according to embodiments of the invention. The functions/acts noted in the blocks may occur out of the order as show in any flowchart. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     While certain embodiments of the invention have been described, other embodiments may exist. Also, while the specification includes examples, the invention&#39;s scope is indicated by the following claims. Furthermore, while the specification has been described in language specific to structural features and/or methodological acts, the claims are not limited to the features or acts described above. Rather, the specific features and acts described above are disclosed as example for embodiments of the invention.