Abstract:
A cavity-less vertical semiconductor optical amplifier is provided which includes an active region of an intrinsic bulk semiconductor material sandwiched between p- and n-layers of semiconductor materials in which a vertical gain channel of a predetermined confined cross-sectional configuration is formed to constitute an amplification region of the optical amplifier. The amplification region is sandwiched between layers of p- and n-doped layers of linearly graded semiconductor material supplying holes and electrons to the active region upon switching “ON” of the optical amplifier. Several factors contribute to substantial amplification of an optical signal at a relatively low injection current which include a relatively long active region allowing sufficient single pass gain as well as a strictly confined cross-sectional configuration of the vertical gain channel which reduces the active volume of the amplification region resulting in substantially high gain at a relatively low current. Flattening of the conduction band and valence band profiles allows easy access of the holes and electrons into the active region. The cavity-less vertical semiconductor optical amplifier of the present invention is intended for multidimensional architectural structures for high speed communication.

Description:
REFERENCE TO RELATED APPLICATION 
     This patent application is based upon U.S. Provisional Patent Application Serial No. 60/140,388, filed on Jun. 22, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor optical amplifiers and more particularly, to vertically arranged semiconductor structures for amplification of an optical signal where resonant cavities are avoided, which are free of the disadvantages imposed by prior art cavity amplifying structures. 
     Particularly, the present invention relates to a cavity-less vertical semiconductor optical amplifier which has a broad transmission bandwidth and uses a relatively low injection current for amplification of an optical signal. 
     Further, the present invention relates to a cavity-less vertical semiconductor optical amplifier based on Periodic Table groups III-V or II-VI material system which includes a relatively thick layer of intrinsic semiconductor material sandwiched between p-doped and n-doped layers of semiconductor materials for amplification of an input optical signal within a strictly confined vertical gain channel extending through a created p-i-n structure. In this manner, a directed flow of p- and n-type carriers is obtained in the active region within the confined cross-section of the vertical gain channel. Additionally, substantial carrier concentration, as well as high current density is achieved within the active region which contributes to amplification of an input optical signal. 
     Further, the present invention relates to a cavity-less vertical semiconductor optical amplifier which may be arranged into multidimensional architectural structures to form an optical cross-bar switch, spatial light modulator with gain, optical boundary detection device, optical OR gate, and other optical mechanisms. 
     Still further, the present invention relates to a technological process of manufacturing a cavity-less semiconductor optical amplifier. 
     BACKGROUND OF THE INVENTION 
     Optical communication has emerged as one of the most powerful driving forces in present digital systems. The increasing demand for wider bandwidth and the quest for speedier transmission have led to extensive deployment of optical fiber networks for data and voice communications and the concept of parallel optical architecture. As a result, vertical cavity surface emitting lasers (VCSEL) are being actively researched and are used in this type of parallel optical architecture. 
     PRIOR ART 
     Vertical cavity surface emitting lasers are explicitedly described in U.S. Pat. Nos. 6,061,381, 6,064,683, 6,067,307, 6,069,905, and 6,069,908. Generally, VCSELs include a resonant cavity formed between top and bottom distributed Bragg reflectors. The resonant cavity contains an active region composed of a bulk semiconductor layer or one or more quantum well layers which are interleaved with barrier layers. On opposite sides of the active regions are mirror stacks (Bragg reflectors) which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength of interest thereby forming the mirrors for the laser cavity. Generally there are opposite conductivity type regions on opposite sides of the active region and the laser is turned on and off by passing the current through the active region. 
     Problems have arisen in that these prior art devices operate typically in either a transmission mode or a reflection mode. Such prior art devices suffer from a narrow gain band width due to the physics of the vertical cavity surface emitting laser using distributed Bragg reflector mirrors. In order to compensate for the small single pass gain in a typical cavity multiple quantum well gain region, multiple recirculation of the light beam within the resonant cavity is needed. Thus a high Q cavity is needed which is obtained by growing thick distributed Bragg reflectors mirrors on both sides of the cavity. These mirrors are wavelength selective which means that they reflect the light recirculating within the resonant cavity over a very narrow band of wavelength. 
     By reducing the number of Bragg reflector mirrors on both sides of the cavity it is possible to slightly broaden the bandwidth of the vertical cavity surface emitting lasers however, this approach leads to formation of a “clumsy” and low gain amplifier which still fails to provide operation in sufficient bandwidth spectrum. 
     As shown in FIG. 1, a transmission spectrum for a vertical cavity surface emitting laser amplifier having twenty periods of Al 0.7  Ga 0.3 , As/Al 0.1  Ga 0.9 As mirrors on both sides of the resonant cavity has a very narrow bandwidth at 850 nm of the bandwidth spectrum. These narrow bandwidths of gain of typical vertical cavity surface emitting lasers are not sufficient for spatially-parallel optical communication applications. 
     It is thus desirable to have a vertical optical amplifier capable of providing much broader bandwidths gain than a typical vertical cavity surface emitting laser, and which would be capable of being turned on and off in a nanosecond time scale. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a vertical semiconductor optical amplifier having the gain bandwidth which is several hundred times higher than that of typical vertical cavity surface emitting lasers, and which would be well suited for multi-dimensional interconnects for optical signal processing in a nanosecond time scale. Additionally, it is an object to provide an optical amplifier which may be used for large parallel interconnect, two dimensional optical signal processing and for implementing a free-space cross connect switch. 
     It is a further object of the present invention to provide a cavity-less vertical semiconductor optical amplifier, the physics of which does not rely on a resonant cavity sandwiched between distributed Bragg reflector mirrors for amplification of the optical signal. 
     It is still another object of the present invention to provide a cavity-less vertical semiconductor optical amplifier in which amplification of the signal is achieved in a vertical gain channel of a predetermined confined cross-sectional configuration which includes a p-i-n structure having an active regions of a thick layer of an intrinsic semiconductor material sandwiched between p-doped and n-doped layers of semiconductor materials. This results in a flow of p- and n-type carriers into the active region being confined within the very narrow cross-sectional configuration of the gain channel and results in formation of a substantial carrier concentration, as well as current density within the active region, thus contributing to amplification of an input optical signal. 
     It is an object of the present invention to provide a cavity-less semiconductor optical amplifier using a thick active region of an intrinsic semiconductor material in combination with confining the injected current over very narrow volume within the thick active region, i.e., to provide an amplification structure having a substantial length for high single pass gain in conjunction with a smaller active volume to lessening the injected current needed to obtain gain of the signal. In this manner, the optimization of performance and operational parameters of the cavity-less semiconductor optical amplifier is obtained. 
     It is another object of the present invention to provide a method of manufacturing a cavity-less semiconductor optical amplifier which has a wide bandwidth gain and optimized performance and operational parameters. 
     According to the teaching of the present invention, a cavity-less semiconductor optical amplifier comprises a vertical gain channel of a predetermined confined cross-sectional configuration which includes a p-i-n structure including an active region (thick layer of an intrinsic semiconductor material) sandwiched between a layer of a p-doped semiconductor material on one surface of the active region and a layer of an n-doped semiconductor material on an opposite surface of the active region. 
     A pair of partially oxidized layers sandwiches the p-i-n structure therebetween. Each of these partially oxidized layers has a current injection path formed therein and arranged in vertically aligned relationship each to the other, thus defining the confined cross-sectional configuration of the gain channel. 
     The optical amplifier of the present invention also includes a top structural arrangement spaced from the p-i-n structure through one of the partially oxidized layers. Preferably the top structural arrangement includes a p-doped linearly graded layer made of p-type semiconductor material disposed in intimate contact with the top partially oxidized layer, a cap layer formed of p-type semiconductor material positioned on the top of the p-doped linearly graded layer, an anti-reflection area formed in the center of the top of the cap layer, and a contact surface formed on the top of the cap layer and surrounding the anti-reflection area thereon. 
     The cavity-less semiconductor optical amplifier further includes a bottom structural arrangement spaced from the p-i-n structure through another partially oxidized layer. Preferably the bottom structural arrangement includes a shoulder region layer formed of n-doped semiconductor material disposed in intimate contact with another partially oxidized layer, an etch-stop layer formed of a semiconductor material positioned on the bottom of the shoulder region layer, a n-doped linearly graded layer formed of an n-type semiconductor material positioned on the bottom of the etch-stopped layer, and a substrate supporting the n-doped linearly graded layer thereon. An aperture is etched within the substrate and the n-doped linearly graded layer to form an output window for the output optical signal. An anti-reflection area is formed at the bottom of the etch-stop layer within peripherals of the output window, and another contact surface is formed on the bottom of the substrate. 
     The thickness of the active region is preferably in the order of 1.0-1.2 micron which is generally thick enough to provide a high single pass gain for the light. 
     The whole structure may be built on the basis of either III-V or II-VI material systems. 
     The thicknesses of each layer within the cavity-less vertical semiconductor optical amplifier of the present invention as well as specifics of the linearly graded layers are chosen to provide ease of access of the p- and n-type carriers into the active region in order to form a high concentration of the carriers within the laterally confined vertical gain channel. The cross-sectional diameter of the gain channel is preferred to be not greater than 5 microns. The dimensions of the current injection path within the partially oxidized layers are obtained by controlled selective etching of the partially oxidized layers from peripherals thereof towards the center at controlled conditions and during predetermined time periods. 
     The cavity-less vertical semiconductor optical amplifier as previously described may be arranged in multi-dimensional architectural structures to form optical cross-bar switches, spatial light modulator with gain, optical boundary detection apparatuses, optoelectronic OR gates, as well as a number of other optical elements. 
     The present invention includes a method of manufacturing a cavity-less semiconductor optical amplifier which includes the steps of: 
     growing a linearly graded n-doped layer of AlGaAs of 0.1 micron thickness with concentration of Al gradually changing along the thickness of the layer on the surface of n-GaAs substrate; 
     depositing an etch-stop AlGaAs layer with approximately 80% content of Al on the top of the linearly graded n-doped layer; 
     forming on the top of the etch-stop layer an n-doped AlGaAs shoulder region of 0.2 micron thickness with approximately 70% Al content; 
     growing an n-doped AlAs layer of approximately 0.1 micron thickness on the top of the n-doped AlGaAs shoulder region layer; 
     positioning an n-doped AlGaAs shoulder layer on the top of the n-doped AlAs layer; 
     forming a bulk intrinsic GaAs active region of 1.0-1.2 micron thickness on the top of the n-doped shoulder layer; 
     growing a p-doped AlGaAs shoulder layer on the top of the active region; 
     forming a p-doped AlAs layer of approximately 0.1 micron thickness on the top of the p-doped shoulder layer; 
     positioning a p-doped linearly graded AlGaAs layer of approximately 0.1 micron thickness with concentration of Al gradually changing along the thickness thereof; 
     depositing a p-doped GaAs cap layer of approximately 200 Angstrom thickness on the top of the p-doped linearly graded layer; 
     etching the layers formed above the n-doped AlGaAs shoulder region layer from the sides thereof, thus forming a cylindrical mesa and exposing from the sides thereof said p-doped and n-doped AlAs layers; 
     selectively oxidizing the p-doped and n-doped AlAs layers from the peripherals towards the center, thereby defining an injected current path at the centers of the p-doped and n-doped AlAs layers; 
     etching through the substrate since the substrate is not transparent and the n-doped linearly graded AlGaAs layer to form an output window below the cylindrical mesa; 
     passivating the cylindrical mesa at the sides thereof; 
     forming anti-reflection areas on the top of the cap layer and at the bottom of the etch-stop layer within the boundaries of the output window; and 
     forming contacts on the top of said cap layer, top of the passivation structure and the bottom of the substrate. 
     The present invention is a cavity-less vertical semiconductor optical amplifier which, due to its specific structure and the use of a unique approach to the physics of the amplification of an optical signal is free of the disadvantages of prior art vertical cavity surface emitting lasers. The subject amplifier provides a much higher bandwidth gain than that of vertical cavity surface emitting lasers, and due to its two dimensional geometry is well suited for large parallel interconnect, two dimensional optical signal processing in a nanosecond time scale, and for implementing a free-space cross-connect switch or a spatial modulator with gain as well as other multi-dimensional architectural structures. 
     These and other novel features and advantages of this invention will be fully understood from the following detailed description of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical representation of transmitted spectrum of a conventional vertical cavity surface emitting laser amplifier of the prior art; 
     FIG. 2 is a cross-sectional schematic of the cavity-less vertical semiconductor optical amplifier structure of the present invention; 
     FIG. 3 is a graphical representation of calculated conduction band and valence band profiles for the cavity-less semiconductor optical amplifier structure of the present invention; 
     FIG. 4 is a graphical representation of a calculated carrier concentration profile in the cavity-less vertical semiconductor optical amplifier structure of the present invention; 
     FIG. 5 is a diagram showing a gain spectrum of bulk GaAs active region at room temperature calculated for different injected carrier concentrations; 
     FIGS. 6A and 6B are schematic representations of operational principles of a 4×4 all optical crossbar switch using the cavity-less semiconductor optical amplifier of the present invention; 
     FIGS. 7A and 7B show diagrams illustrating a method of boundary detection using the architecture similar to that of FIGS. 6A and 6B; and, 
     FIG. 8 is a schematic representation of implementation of an optical OR gate using the cavity-less semiconductor optical amplifiers of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, a cavity-less semiconductor optical amplifier  10  is shown which includes active region  11  having a thick layer (the thickness being in the order of 1.0-1.2 micron) formed of bulk intrinsic GaAs semiconductor material. The active region  11  is sandwiched between a p-doped AlGaAs shoulder layer  12  on the top of the active region  11  and an n-doped AlGaAs shoulder layer  13  on the bottom of the active region  11 . Together, the active region  11  and the p- and n-doped AlGaAs shoulder layers  12  and  13  form a p-i-n structure which mainly contributes to the amplification of a signal introduced into the amplifier  10 . The shoulder layers  12  and  13  have approximately a 70% Al concentration. The layers are provided on both surfaces of the active region  11  in order to form a heterojunction on the boundaries between the p-doped shoulder layer  12  and active region  11  as well as between the n-doped shoulder layer  13  and the active region  11 . By doping the shoulder layers  12  and  13  with Al, a larger band gap is obtained so that the intrinsic semiconductor (in this case GaAs) of the active region  11  having a narrower band gap is sandwiched between two semiconductor layers having a larger band gap. This provides for carrier confinement within the active region  11 . 
     When the amplifier  10  is turned ON by a properly directed injection current, the p-type current composed mainly of holes, flows in the direction from the shoulder  12  to the active region  11 , and n-type current composed mainly of electrons flows from the shoulder layer  13  to the active region  11 . The electrons and holes pile up in the active region  11  and are confined therein by heterojuctions created at the interfaces between the shoulder layer  12  and active region  11  as well as between the shoulder layer  13  and the active region  11 . 
     The concentration of the electron and holes within the active region  11  increases to the point where it is sufficient to obtain gain of a optical signal within the amplifier  10 . Light travel within the active region  11  in a direction from the top thereof (shoulder layer  12 ) towards the bottom of the active region  11  (shoulder layer  13 ). The light covers a length equal to the thickness of the active region  11  which is large enough (1.0-1.2 micron) to provide optimal opportunities for the a single pass gain of the signal. 
     During the advancement of the light within the active region, the gain process takes place due to spontaneous and stimulated emission via the recombination of electrons and holes within the active region  11 . This is a process known to those skilled in the art and includes generation of photons which are stimulated by the photons of the input light signal which enters the optical amplifier  10  (stimulated emission). 
     Additionally, during passage of the light signal through the active region, the carriers (the holes and electrons) recombine, thus producing a photon at each recombination. The recombination of the holes and the electrons may be either spontaneous or stimulated, i.e. caused by the photons of the input light signal. Due to the high concentration of the carriers of both signs in the active region  11 , the quantity of emitted photons is sufficiently high and increases due to stimulated emission caused by the photons of the input light signal as well as by secondary emitted (recombination) photons. The aforementioned process takes place throughout the entire thickness of the active region  11  which is chosen to be lengthy enough to provide a substantial single pass gain. 
     However, in the structure of the optical amplifier  10  of the present invention gain is not only obtained by the large thickness of the active region  11  and confinement of the carriers within the active region  11  by heterojunctions created between the shoulder layer  12  and the active region  11  and the shoulder layer  13  and the active region  11 . Gain is also provided by a unique design of the structure of the present invention so that a vertical gain channel of a predetermined confined cross-sectional configuration is formed in order to limit a volume of the active region which is activated by injected current (while maintaining the length of the active region  11  lengthy enough to obtain a substantial single pass gain). 
     In order to confine an amplification region in the lateral direction, a pair of partially oxidized layers  15  and  16  respectively formed of p-doped and n-doped AlAs semiconductor material of 0.1 micron thickness each, are positioned on the top of the p-doped AlGaAs shoulder layer  12  and on the bottom of the n-doped AlGaAs shoulder layer  13 . 
     These two layers  15  and  16  are symmetrical with relation to the p-i-n structure  14  and are partially oxidized (as will be described in detailed further herein) to define unoxidized regions  17  and  18  surrounded by oxidized (Al x O y ) areas  19  and  20 , as best shown in FIG.  2 . The diameter of each unoxidized region  17  and  18  (which serves as a current injection path formed within the top and bottom partially oxidized layers  15  and  16 ) is preferably approximately 5 microns. As can be seen, the current injection paths  17  and  18  are vertically aligned each with the other in order to define a vertical gain channel through the p-i-n structure  14 . 
     By confining the lateral configuration of the vertical gain channel to the 5 micron diameter, the structure of the present invention enjoys a limited volume of the amplification region and sufficient length of the vertical gain channel to obtain high gain. In this manner, due to lateral confinement, a relatively low current is needed to be injected into the optical amplifier  10  to obtain needed gain as opposed to a high current which would be needed to activate the active region  11  if a confined laterally vertical gain channel is not formed. 
     As will be described further herein, an injection current of 10 mA is sufficient to obtain a high gain in the cavity-less semiconductor optical amplifier  10  of the present invention. 
     On the top of the partially oxidized layer  15 , a p-doped linearly graded layer  21  formed of AlGaAs is located having 0.1 micron thickness with gradually changing Al concentration over the width of the layer  21 . The material transforms from GaAs at the top surface  22  thereof to AlAs at the bottom surface  23  which is in intimate contact with the partially oxidized layer  15 , i.e., the content of aluminum changes over the 0.1 microns thickness of the layer  21  from 0% (at the surface  22  or the layer  21 ) to 100% (at the surface  23  of the layer  21 ). The function of the p-doped linearly graded AlGaAs layer  21  is to contribute p-carriers, i.e., holes, into the overall flow of the carriers into the active region. The gradual change of the aluminum concentration over the thickness of the layer  21  allows a flattening of the valence band profile at the p-side of the amplifier  10  to ease the holes flow into the active region  11  as will be further discussed with regard to FIG.  3 . 
     On the top of the p-doped linearly graded layer  21 , a p-doped gallium arsenide (GaAs) cap layer  24  is formed having a thickness of approximately 200 Angstrom. The function of the cap layer  24  is to make the contact with a metal layer  25  through which the amplifier  10  is activated-deactivated by controlling the current injected into the optical amplifier  10 . The cap layer  24  is made extremely thin (200 Angstrom) in order to decrease (or eliminate) absorption of the incoming light signal within the cap layer  24  and thus to allow passage of substantially the entire incoming light to the bulk active region  11 . 
     Below the partially oxidized n-doped aluminum arsenide layer  16 , an n-doped aluminum gallium arsenide (n-AlGaAs) shoulder region  26  is formed having a thickness of 0.2 microns with approximately 70% concentration of aluminum therein. The shoulder region layer  26  serves as a source of n-carriers, i.e., electrons, mostly contributing into the flow of the n-carriers into the bulk active region  11 . 
     Beneath the n-doped AlGaAs shoulder region layer  26 , an etch-stop layer  27  is formed which is formed of AlGaAs having approximately 80% concentration of aluminum therein. The composition of the etch-stop layer  27  is selected to provide termination of an etch process (to be discussed in detail in further paragraphs.) once the etching chemical (for example, acid) approaches the etch-stop layer  27 . 
     Below the etch-stop layer  27 , an n-doped linearly graded AlGaAs layer  28  is formed in which a content of aluminum changes from 80% at the surface  29  of the layer  28  to 0% at the surface  30  thereof. Thus, the composition of the n-doped AlGaAs linearly graded layer  28  changes from GaAs at the surface  30  to AlAs at the surface  29 . Along with the n-doped AlGaAs shoulder region layer  26 , the n-doped AlGaAs linearly graded layer  28  serves as a source of electrons contributing in the overall flow of the carriers into the active region  11 . 
     Similar to the p-type linearly graded AlGaAs layer  21 , the grading in the layer  28  smooths the carriers pass into the active region. As will be further discussed in detail with respect to the FIG. 3, since the conduction band profile of the n-side of the amplifier  10  is substantially flattened by the linear grading of the layer  28  the flow of the electrons is facilitated into the active region  11  from the layer  28 . 
     The multilayer structure of the amplifier  10  is supported on a n-GaAs substrate  31  of approximately 150 micron thickness. Due to the high width of the substrate and because of the similar composition of the material of the substrate with respect to the composition of the material of the active region  11 , the output light may be substantially absorbed within the material of the substrate  11 . In order to eliminate or substantially decrease such an absorption, and to allow substantially the entire amplified output light to exit the optical amplifier  10 , an output aperture  32  is formed through the substrate  31  and the n-doped AlGaAs linearly graded layer  28 . In this manner, the output light avoids traveling through the substrate  31  where it otherwise would be absorbed. 
     The output aperture  32  is formed by etching through the substrate  31  and the layer  28  (as will be discussed in detail in further paragraphs). As shown in FIG. 2, the output aperture is positioned in vertical alignment with the vertical gain channel where amplification of the optical signal  33  takes place to provide the amplified output signal  34  with the shortest passage from the optical amplifier  10  through the output aperture  32 . 
     Anti-reflection coating is deposited on the top of the cap layer  24 , (forming an anti-reflection area  35 ) and on the bottom of the etch-stop layer  27  within the boundaries of the output aperture  32 , (forming an anti-reflection area  36 ). 
     The bottom of the substrate  31  left after the formation of the output aperture  32  is covered with metal layer  37 , thus forming a n-contact for the optical amplifier  10  which in conjunction with the p-contact  25  serves as an activation-deactivation mechanism for the amplifier  10 . 
     The multilayered structure of the optical amplifier  10  of the present invention is formed by using standard technological processes such as liquid-phase, local, molecular-beam, solid-phase, vapor-phase epitaxy, film deposition, ion-beam deposition, dry and wet etching, oxidation, passivation, metal deposition, etc. These processes are not discussed herein in detail as they are readily known to those skilled in the art and do not present a novel subject matter of the invention in question. 
     The overall technological process of the creation of the cavity-less semiconductor optical amplifier however is unique and includes the following technological steps: 
     (a) Forming a linearly graded n-doped layer of AlGaAs of 0.1 micron thickness  28  with gradually changing concentrations of aluminum on the n-doped GaAs substrate along the width of the layer  28 ; 
     (b) Depositing on the top of the linearly graded n-doped layer  28 , an etch-stop AlGaAs layer with 80% content of aluminum; 
     (c) Forming on the top of the etch-stop layer  27  an n-doped AlGaAs shoulder region layer  26  of 0.2 micron thickness with a 70% content of aluminum; 
     (d) Forming on the top of the n-doped shoulder region layer  26  a n-doped AlAs layer  16  of 0.1 micron thickness; 
     (e) Forming on the top of the n-doped AlAs layer  16  a n-doped AlGaAs shoulder layer  13 ; 
     (f) Forming a bulk intrinsic GaAs active region  11  of 1.0-1.2 micron thickness on the top of the n-doped AlGaAs shoulder layer  16 ; 
     (g) Forming a p-doped AlGaAs shoulder layer  12  on the top of the active region  11 ; 
     (h) Forming a p-doped AlAs layer of 0.1 micron thickness of the top of the shoulder layer  12 ; 
     (i) Forming a p-doped linearly graded AlGaAs layer  21  of 0.1 micron thickness with concentration of aluminum gradually changing along the width thereof; 
     (j) Forming above said layer  21  a p-doped GaAs cap layer of 200 Å thickness; 
     (k) Dry etching the layers  16 ,  13 ,  11 ,  12 ,  15 ,  21 , and  24 , in sequence, from the sides towards the center thereof to form a cylindrical mesa  38  erecting above the n-doped AlGaAs layer  26 ; 
     (1) Wet oxidizing the mesa  38  at approximately 450 centigrade in N 2  gas flow saturated in water vapor to selectively oxidize the layers  15  and  16 . The oxidation of these layers  15 ,  16  advances from the peripherals of the cylindrical mesa  38  towards the center thereof which define current apertures  17  and  18  (also referred to herein as current injection paths) on both sides of the p-i-n structure  14 ; 
     (m) Wet etching the substrate  31  and the n-doped AlGaAs linearly graded layer  28  to form the output aperture  32  extending completely through the substrate  31  and the layer  28 . The etching stops at the etch-stop layer  27  due to 80% content of aluminum therein; 
     (n) Passivating the cylindrical mesa  38  so that passivation structure  39  is formed which is substantially the dielectric material covering the top surface of the layer  26  surrounding the cylindrical mesa  38 ; 
     (o) Forming annular contacts  25  and  37  on the top cap layer  24  and on the bottom of the substrate  31 ; and 
     (p) Covering the top of the cap layer  24  and the etch-stop layer  29  within the boundaries of output aperture  32  with anti-reflection coating to prevent reflection of the input optical signal  33  and output optical signal  34  on the interface between air and semiconductor material. 
     The structure of the optical amplifier  10  of the present invention is not limited to the AlGaAs based material system since it is equally valid for both III-V and II-VI material systems. For example, using bulk InGaAsP active region lattice matched to InP, similar cavity-less semiconductor optical amplifier can be designed that operates at 1.3 micron or 1.55 micron wavelength and other wavelengths of significance. 
     In an alternative implementation, instead of thick bulk active region  11 , periodic gain structures or an amplifying structure with more than one active layer may be used. The active region  11  may also contain several p-n regions in series each of which would contain an undoped multi-quantum well region. In this case, the layer separating two groups of quantum wells is a highly doped tunneling junction driven in a backward or reverse direction. Under forward bias, the tunnel junction breaks down and all the quantum well gain regions operate in series. 
     In system lattice matched to InP, selective oxidation of InAlAsP or AlSbAs can be used in the same manner as selective oxidation described in previous paragraphs. Alternatively, ion implantation to obtain current constriction or some lattice disordering techniques can be used. 
     Several computer simulations with the proposed cavity-less vertical amplifier structure  10  have been carried out. As can be seen from FIG. 3, showing the calculated band profile for the GaAs based structure  10 , the conduction band in the n-side and the valence band in the p-side are substantially flattened to allow electrons and holes to easily flow into the active region  11 . As discussed in previous paragraphs, the flattened profile for the conduction band and the valence band is obtained by careful selection of the width of the layers of the structure  10  along with linearly grading the concentration of aluminum over the thickness of the p-doped and n-doped linearly graded layers  21  and  28 . 
     With regard to FIG. 4, showing a simulated carrier density profile in the amplifier  10  for an injection current density of 4.8×10 4  Amp/cm 2 , a carrier concentration of 4E18 cm −3  is obtained in the active region  11 . Since the current injected into the active region  11  is laterally confined within small apertures  17  and  18 , defined by selectively oxidizing the layers  15  and  16  on both sides of the active region  11 , a very high current density is achieved in the vertical gain region for relatively low injection current. For a 5 micron diameter current injection passage  17  and  18 , a current density of 4.8×10 4  Amp/cm 2  in the active region  11  corresponds to an injection current of 10 mA. 
     With regard to FIG. 5, representing the gain spectrum of bulk GaAs at room temperature calculated for different injected carrier concentrations, it is seen that for an injected carrier density of 4.0E18 cm −3 , peak gain is obtained for a broad spectrum of wavelengths which is larger than 1,000 cm −1 . This corresponds to a single pass gain of more than unity for a 1.2 micron thick active region  11 . 
     The optical amplifier  10  is a perfect candidate for multidimensional structures, such as N×N cross bar switch, optical boundary detection architecture, OR gates, etc. 
     One of the applications of the optical amplifier  10  of the present invention is to implement a free-space all optical crossbar switching element using a N×N cavity-less vertical amplifier array along with a vertical cavity surface emitting lasers (VCSEL), MSM or other detector array and custom made lens array or defractive element. Such a switch architecture is shown in FIGS. 6A and 6B. Although the architecture shown is for N=4, it can be easily extended to any N. 
     As shown in FIG. 6, four input channels  40  drive four vertical cavity surface emitting lasers  41  using CMOS driver chip (not shown in the drawing). A vertical cylindrical lens array  42  (or any other suitably designed optical element including diffractive optics) fans light from each of the four vertical cavity surface emitting lasers  41  across a row of the cavity-less vertical amplifier array  43 . A second array of horizontal cylindrical lens (or a defractive optical element with similar functionality)  44  focus the light from each column of the array  43  of the cavity-less vertical optical amplifiers  10  onto a detector in a detector row  45 . 
     Thus, as best shown in FIG. 6B, if the i th  input channel  40  is to be routed to the j th  output channel, the i th  VCSEL  41  in the row of  4  VCSELs will be driven. Light output of this i th  VCSEL will be distributed to the entire i th  row of the array  43  of the cavity-less vertical optical amplifiers  10  by the cylindrical lens array  42 . Only the (i,j) th  element of the array  43  will be turned “ON” for transmission. The transmitted light out the i th  cavity-less vertical optical amplifier  10  will be collected by the second cylindrical lens array  44  and since it is coming out of the j th  column of the array  43 , the light will be focused on the j th  detector. Thus, any of the four input channel  40  may be routed into any of the four output channels  46  in a non-blocking fashion. 
     Similar architecture described in the previous paragraphs can be applied to optical boundary detection for a monochrome image, the technique of which is shown in FIGS. 7A and 7B. It is well known, that boundary detection can be accomplished by looking at the neighborhood of each pixel of the image. The known neighborhood operator is the matrix:        M   =     [         0       1       0           1       1       1           0       1       0         ]                            
     For the boundary elements, at least one of the four neighboring pixels will be different from the original pixel. Thus, the matrix M has to be used as a mask, and one has to look for one different pixel out of the five detected pixels. This mask can be easily implemented using the cavity-less vertical-amplifier array of the present invention. The VSCEL array  41  of FIGS. 6A and 6B, is used as the source image, and the MSM detector array  45  is used to detect the image masked by the neighborhood operator implemented by the cavity-less vertical amplifiers (CLVA). 
     The M-matrix pattern is scanned electronically in the CLVA array  43 . Thus, at every clock cycle, only 5 neighboring pixels pass through the CLVA array  43  and are detected by the MSM-detector array  45 . The detector array output can be easily analyzed with simple logic circuits to look for at least one pixel different from the rest. If all the 5 pixels are similar (all ‘0’s or all ‘1’s), then that particular pixel is not at the boundary; otherwise, the original pixel is located at the boundary of the image. 
     It is possible to implement an optical OR gate to increase the processing speed or to use standard electrical logic gates for doing the same operation. It was previously noted that if the pixel is not in the boundary, then all the five pixels passing through the CLVA array will be either ‘0’ or ‘1’. Looking at both the images and the inverted image shown in FIGS. 7A and 7B a detection of all ‘0’s in either of the images for a particular pixel rules out that the particular pixel is at the boundary. If this condition is not satisfied, the pixel will be at the boundary. 
     Thus an optoelectronic ‘OR’ function is implemented for all the pixels and if the output is ‘0’, all the pixels passing through the CLVA are zero. FIG. 8 shows the optoelectronic OR gate implemented with the CLVA  10  of the present invention for optical processing with very fast boundary-detection schemes for monochrome images, as opposed to electronic Matrix operation that involves large processing circuit and is slower. In this particular scheme, part of the processing is done optically, improving the performance of the border detection circuit. 
     As it is clear from the above disclosure, the structure of the novel cavity-less vertical semiconductor optical amplifier increases bandwidth gain, allows nanosecond time scale communication, and is well suited for large parallel interconnect and multidimensional optical signal processing. 
     Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modifications other than those discussed above may be resorted to without departing from the spirit or scope of the invention. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular application of elements may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended claims.