Abstract:
A Vertical Cavity Surface Emitting Laser, VSEL. Prior-art VCSELs contain an optical cavity between two mirrors. Near one mirror are positioned current blockers which surround part of the cavity and prevent current from reaching regions of the cavity which are near edges of that mirror. If current reached those regions, lasing would occur there, and the light produced would be scattered by the nearby edges of the mirror. The current blockers reduce that scattering. However, the fabrication steps following those which create the current blockers are expensive. The invention eliminates the expensive steps by (1) placing a layer of gold atop the current blockers and the surrounded lasing region, (2) placing a dielectric layer of high index H atop the gold layer, and (3) placing a quarter-wave stack atop the high index dielectric layer, in the sequence L-H-L- . . . -H-L-H, wherein H represents a high index and L represents a low index. The gold delivers current to the cavity. Significantly, the stack (1) begins with an L adjacent the dielectric adjacent the gold and terminates with an H layer.

Description:
The invention concerns an approach to fabricating current blocking regions in a Vertical Cavity Surface Emitting Laser, VCSEL. The approach is simpler and less expensive than those used presently. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 is a simplified schematic of a homojunction laser  3 , and is not drawn to scale. A PN junction  4  is formed between a p-plus-type body  6  of gallium arsenide, GaAs, and an n-plus-type body  9  of gallium arsenide. Metal contacts  12  provide entry- and exit paths for current  15 , which is supplied by a voltage source V+. The laser produces laser output  18 , which travels in a plane parallel to the junction  4 . The laser will generally be situated in a resonant optical cavity, which is not shown. 
     FIG. 2 is a simplified schematic of a different type of laser, namely, the Vertical Cavity Surface Emitting Laser, VCSEL, labeled  30 , and is also not drawn to scale. The VCSEL  30  includes a top mirror  33  and a bottom mirror  34 . These mirrors are constructed of multiple layers L of dielectric material, each layer being ¼ wavelength thick. 
     Current  35 , indicated by the dashed arrow, flows from a metal contact  36 , through a p-type region  39 , through a gain region  42 , through an n-type region  43 , and to another metal contact  45 . The gain region  42  produces light, and multiple reflections of that light between the top mirror  33  and the bottom mirror  34  induce stimulated emission of laser light, which exits the device as indicated by ray  48 . 
     A significant feature of the VCSEL  30  is that the laser light travels perpendicular to the plane of the gain region  42 , that is, perpendicular to bottom mirror  34 . Gain region  42  is analogous to junction  4  in FIG. 1, in the sense that population inversion occurs in both the gain region  42  and the junction  4 . 
     In addition, in FIG. 2, the light which stimulates emission of photons within the gain region  42  bounces between the top mirror  33  and the bottom mirror  34 . However, stimulated emission only occurs within the gain region  42 . The thickness T of the gain region  42  is very small, of the order of a few hundred angstroms, and is much smaller than the corresponding distance Ti in Figure Thus, since stimulated emission in FIG. 2 only occurs along a relatively small thickness T, losses must be reduced to a minimum. One source of loss is scattering which would occur at the edge  50  of the top mirror  33 . To reduce this loss, current-blocking regions  53  are fabricated. They block current from flowing near the edge  50 . The absence of current means that photon generation is absent, so that stimulated emission is also absent, at that location. 
     Fabrication of the current-blocking region  53  is expensive, or at least complex. In one approach, ion implantation is used, wherein the p-type region  39  in FIG. 3 is bombarded by high-velocity ions, indicated by dashed arrows  54 . These ions bury themselves beneath the surface  55  and generate the current-blocking region  53  in FIG.  4 . Region  53  is generated because the ions  54  compensate the p-type dopants (not shown), effectively converting region  53  into an intrinsic semiconductor, which is low in conductivity, at least at room temperature. 
     However, this ion implantation technique requires strict process control in order to develop the proper profile  65  in plot  68  in FIG.  4 . Plot  68  indicates ion concentration, as a function of depth in the p-type layer  39 . Also, the overall process requires later annealing of the structure, after the implantation. 
     In another approach, current blocking region  53  is fabricated through lateral oxidation, wherein the oxidation is begun at regions  70  in FIG. 3, and invades the p-layer  39  as indicated by arrows  73 . However, the lateral oxidation process is difficult to control. 
     In a third approach, shown in FIG. 5, a p-type layer  80  in structure A, at the upper left of the Figure, is etched away to form the mesa  83  in structure B. Then, in structure C, the current blocking layer  53  is fabricated, by implantation or surface oxidation. (Intermediate steps required for generation of layer  53  are not indicated.) Next, the p-type layer is expanded in size through crystal regrowth into body  39 , as in Structure D. After that, known process steps are implemented to produce the final structure Z. 
     However, the processing steps required to convert structure C into structure D are expensive and complex. Specifically, the p-type layer  39  in structure D, as well as the gain region  42 , must all consist of a monocrystalline body of material. Adding a monocrystalline body to the p-layer  83  shown in structure C, to create structure D, is a complex process, as is crystal regrowth generally, which is the process used. 
     The Inventors have developed a process for producing the current blocking region  53  in FIG. 2, but in a simpler manner than described above. 
     Numerous textbooks exist on laser technology. A good simplified treatment is found in  Optoelectronics. An Introduction,  by Wilson and Hawkes, Third Edition (Prentice Hall, 1998, ISBN 0-13-103961-X). This book is hereby incorporated by reference, partly to show, in simplified terms, the present state of the art. 
     SUMMARY OF THE INVENTION 
     In one form of the invention, a film of gold is positioned across the optical gain path of a VCSEL. The gold film delivers electrical current into the semiconductor material within the gain path, and eliminates the need for a crystal re-growth step. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic of a prior-art homojunction laser. 
     FIG. 2 is a simplified schematic of a prior-art Vertical Cavity Surface Emitting Laser, VCSEL. 
     FIG. 3 illustrates ion implantation of layer  39  in FIG.  2 . 
     FIG. 4 illustrates the structure which results from the ion implantation of FIG. 3, and the spatial distribution of the ions. 
     FIG. 5 illustrates a sequence of processing steps which can be used to produce the structure of FIG.  2 . 
     FIGS. 6 and 7 illustrate two forms of the invention. 
     FIGS. 8 and 9 illustrate interfaces  150  at which a reflection coefficient is computed. The structures shown in FIGS. 8 and 9 are not admitted to be found in the prior art. 
     FIGS. 10,  11 ,  13 ,  14 ,  15 , and  16  illustrate plots of various reflection coefficients computed for the structures of FIGS. 8 and 9. 
     FIG. 12 illustrates a sequence of processing steps utilized by the invention. 
     FIG. 17 is a simplified view of a VCSEL. 
     FIG. 18 is a simplified view of one form of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 6 illustrates one form of the invention. A layer  100  of gold is shown, and is not drawn to scale. To attain the structure of FIG. 6, the layer of gold  100  can be deposited upon the intermediate structure C in FIG. 5, which is also shown as structure CC in FIG. 12, to produce structure DD in FIG.  12 . Then, known processing steps can be undertaken to produce structure zz. 
     This represents an advancement over the prior art because the steps of (1) fabricating intermediate structure C in FIG. 6, (2) depositing gold layer  100 , and then (3) producing the structure of FIG. 6 are simpler, less expensive, or both, than the prior-art processes of producing structure Z in FIG. 6, given the current state of technology. 
     In connection with FIG. 6, the Inventors have observed that, while gold is commonly thought to be absorptive of photons, it can nevertheless be used in a thin-layer form in FIG. 6 for a twofold purpose. One purpose is to act as part of, or cooperate with, a quarter-wave dielectric mirror stack, which is described later. A second purpose is to carry current  106  to the p-type layer  112 . These features will be explained in greater detail. However, first, the Inventors will point out a particular feature of the prior art. 
     In multi-layer dielectric mirrors in the prior art generally, such as top mirror  33  in structure Z in FIG. 5, the layer L 1  which is adjacent the p-layer  39  is given a low index of refraction (denoted “L” herein). The index is termed low because the mirror consists of a stack of alternating layers of high, and low, indices of refraction, as explained later in connection with FIGS. 8 and 9. The index of layer L 1  is low, compared with the high-index layers in the top mirror  33 . 
     Therefore, the prior-art feature of interest is that the index of refraction of the dielectric layer L 1  which is closest to the gain region  42  in FIG. 5 is generally low. The term “nearest” will be used to refer to this type of dielectric layer L 1 , because it is the layer nearest the gain region. 
     In contrast, under the invention, the nearest dielectric layer LL in FIG. 6 is given a high index of refraction, H. Given this background fact, this discussion will return to explaining the invention. 
     The mirrors in FIGS. 5 and 6 are simplified. Specifically, the top mirror  33  in structure Z in FIG. 5, as well as the top mirror  103  in FIG. 6, are shown as having only a few layers. However, the actual numbers of layers stands near twenty, as FIGS. 8 and 9 indicate. This discussion will explain the reflection coefficients obtained for the mirrors of FIGS. 8 and 9. 
     A primary difference between the mirrors of these two Figures is that FIG. 9 contains an extra layer  160 , located near the bottom and shown hatched. This layer is not present in FIG.  8 . Otherwise, both structures are similar, and share the following features. 
     One feature is that a semiconductor substrate  120  of aluminum gallium arsenide, AlGaAs, is used. 
     A second feature is that the topmost layer TOP of the 20-odd layers is of high, H, refractive index. Titanium dioxide was used, having a refractive index, N, of 2.0. 
     A third feature is that the topmost layer TOP is paired with a layer  152  beneath it, which is of low, L, index of refraction. Silicon dioxide was used, having a refractive index, N, of 1.45. 
     A fourth feature is that ten of these layer-pairs are present, labeled PAIR  1  through PAIR  10 . 
     A fifth feature is that a layer  155  of gold may be present. In some of the computations described below, the layer  155  is given zero thickness, meaning that the layer  155  of gold is absent for that computation. However, the layer  155  is illustrated in order to illustrate its position, when present. 
     The computations of the reflection coefficient of the mirrors of FIGS. 8 and 9, about to be described, presumed that plane-wave light  170  strikes the interface  150 . In effect, the interface  150  was presumed to be infinite in extent, and the light source (not shown) was presumed to be either (1) planar and infinite in extent or (2) a point source positioned infinitely far from the interface  150 . 
     FIG. 10 illustrates plots of the computations. The three plots in FIG. 10 drawn in solid lines correspond to FIG. 8, wherein the mirror-layer adjacent the gold layer  155  is of low index of refraction. That layer is the silicon dioxide layer within PAIR  1  in FIG. 8, having an index N of 1.45, as indicated. 
     The three plots in FIG. 10 drawn in dashed lines correspond to FIG. 9, wherein the layer  160  adjacent the gold layer  155  is of high index of refraction. As stated above, layer  160  is not present in FIG.  8 . That layer  160  is titanium dioxide, wherein N equals 2.0. 
     FIG. 11 is a magnified view of part of FIG. 10. A peak P 1  for FIG. 8 occurs in the dotted line corresponding to zero gold thickness. This peak represents the conventional wisdom that, at 850 nanometers, in the structure of FIG. 8, with no gold layer  155  present, the reflection coefficient attains a maximum at a wavelength of about 850 nanometers. The actual numerical value of the maximal reflection coefficient, at P 1 , is 0.998. (Numerical computations are not indicated.) 
     Point P 2  is also consistent with conventional wisdom: when a gold layer of 50 angstroms is added to FIG. 8, the peak reflection coefficient, near point P 2 , falls below 0.99. The reflection coefficient falls further for a thicker gold layer of 100 angstroms in FIG. 8 (point P 3 ). 
     Therefore, points P 1 , P 2 , and P 3  indicate that, as progressively thicker layers of gold are added in the form of layer  155  in FIG. 8, starting with zero thickness for point P 1 , the reflection coefficient progressively drops in value. 
     However, a reflection coefficient exceeding 0.99, that is, above 99 percent, is considered necessary in the top mirror  103  in FIG. 6 in order to attain a successful laser. Thus, points P 2  and P 3  represent reflection coefficients which are considered non-usable. Point P 1  is non-usable because no layer of gold is present, and such a layer is required for another purpose in the invention, which is described below. 
     In pursuit of a sufficiently high reflection coefficient, the Inventors have discovered that, if the structure of FIG. 8 is modified in certain ways, then the reflection coefficient can be driven above the required minimum of 0.99. Modifications are shown in FIG. 9, and include (1) addition of the gold layer  155  of a specific thickness, (2) addition of the high-index layer  160 , adjacent the gold, and (3) insertion of elements (1) and (2) in the positions indicated. 
     Restated, the Inventors begin with an ordinary mirror as in FIG. 8, with no gold layer  155  present. Then, as in FIG. 9, a high-index layer  160 , of one-quarter wave thickness, is added to the bottom of the mirror containing 10 layer-pairs, namely, PAIR  1 -PAIR  10 . Also, a layer of gold  155 , of proper thickness, is added adjacent the high-index layer  160 . 
     Under these conditions, if the layer of gold  155  is 100 angstroms thick, the reflection coefficient is found to lie near point P 10  in FIG.  11 . (The reader is reminded that the solid plots in FIG. 11 correspond to FIG. 9.) When computed numerically, the reflection coefficient in this case is 0.9935. With a gold layer of zero thickness, the reflection coefficient is slightly higher, at 0.994, near point P 11  in FIG.  11 . 
     Thus, the Inventors have discovered that the layer  155  of gold in FIG. 9 has an almost negligible effect on the reflection coefficient, if it is paired with the dielectric layer  160 . On the other hand, if dielectric layer  160  is absent, and a gold layer is merely added to the structure of FIG. 8, then the situation is that of the points P 2  and P 3  in FIG. 11, wherein the gold layer renders the mirror unusable. 
     Therefore, one form of the invention employs the structure of FIG. 9 as the top mirror of FIG.  6 . That structure includes (1) the ten pairs, PAIR  1 -PAIR  10 , (2) the high-index titanium dioxide layer  160 , and (3) the gold layer  155 , in that order. 
     As stated above, FIG. 6 is a simplified schematic: the four layers of FIG. 6 indicating the top mirror actually represent the 22 layers in FIG. 9, which contain (1) the ten pairs, (2) layer  160 , and (3) the gold layer  155 . 
     In FIG. 9, the gold layer  155  can be viewed as acting as part of the top mirror, or as acting as part of the gain region in FIG.  6 . These alternate views will be addressed later. 
     As stated above, the gold layer  100  in FIG. 6 provides another function, namely, it acts as a conductor for carrying current  106  from metal contact  109  to the p-type region  112 . 
     Therefore, in one form of the invention, a simplified process for fabricating the current blocking region  53  in FIG. 6 has been provided. Structures AA, BB, and CC in FIG. 12 are fabricated, using known techniques. For example, the p-layer  112  in structure AA is fabricated, and then etched away, to produce the p-type mesa  112  in structure BB. Then, the surrounding current-blocking regions  53  are fabricated, in structure CC. 
     Next, the gold layer  100  in structure DD is deposited. Following that, known fabrication steps, indicated by arrows  120 , produce structure ZZ. 
     Significantly, no crystal re-growth is undertaken, as occurs in reaching structure D of FIG.  5 . 
     The gold layer  100  provides a current path for current  106  in FIG.  6 . The gold layer also cooperates with the layers of FIG. 9, to provide a sufficiently high reflection coefficient to support stimulated emission. 
     Several refinements and additional embodiments will be discussed. 
     If the gold layer  155  in FIG. 8 is given a thickness of zero (that is, gold layer  155  is absent), then FIG. 8 illustrates a prior-art structure. The reflection coefficient at interface  150  is indicated by point P 1  in FIG.  11 . 
     The invention shifts that point P 1  to a point near point P 10  in FIG. 11, by utilizing the structure of FIG. 9, wherein the gold thickness is either 50 or 100 angstroms. However, P 10  in FIG. 11 is located at a wavelength which is different from 850 nanometers. The maximum reflectance, near P 10 , now occurs at a longer wavelength: P 10  lies to the right of point P 1  in FIG.  11 . The maximum reflectance point has been shifted toward the red end of the optical spectrum. 
     The Inventors have developed a stratagem for correcting this red-shift. FIG. 13 shows the results of the stratagem. The solid lines in FIG. 13 are magnified versions of parts of the corresponding lines of FIG.  11 . The dashed lines in FIG. 13 indicate how the solid lines in that Figure are shifted when the stratagem is implemented. Arrows A 1  and A 2  indicate the shifting. 
     The stratagem is to adjust the thickness of layer  160  in FIG. 9 by a correction factor. The correction factor is the quantity (1−t Au /3000 A), wherein T Au  is the thickness in Angstroms of the gold layer  155 . The correction factor is applied to layer  160 , to adjust its thickness. 
     Specifically, in computing the correction factor, one divides the thickness, in Angstroms, of the gold layer  155  by 3,000, to obtain a quotient. Then one subtracts that quotient from unity. The result is a correction factor which is multiplied by the thickness of the quarter-wave layer  160 . The result is the actual thickness of layer  160  which is to be used. That layer  160  in FIG. 9 is deposited upon the gold layer  155 . 
     As a numerical example, for a thickness of gold of 100 angstroms, the correction factor is (1−100/3,000), or 0.9667. One then multiplies a one-quarter-wave thickness of layer  160  in FIG. 9 by the correction factor, to obtain the actual thickness of layer  160  which is to be used. If the one-quarter-wave thickness is T, then the actual thickness used is 0.9667×T. 
     Of course, the one-quarter-wave original thickness of layer  160  is determined by the wavelength of light within layer  160 , not in air or vacuum. The wavelength of light in a medium equals the free-space wavelength divided by the index of refraction of that material. 
     Implementing the correction factor shifts the solid plots in FIG. 13 to the dashed positions, as indicated by arrows A 1  and A 2 . The peak reflectivity is now closer to 850 nanometers, or coincident with 850 nanometers, as indicated by arrow A 3 . 
     The Inventors point out that the thickness of the gold layer  155 , namely, 100 Angstroms, is a small fraction of the thickness of the quarter-wave layers. For example, FIG. 13 indicates a wavelength of 850 nanometers, which corresponds to 850×10 −9  meter. One hundred Angstroms corresponds to 100×10 −10  meter. Thus, the thickness of the gold layer  155  corresponds to 10/850 of one wavelength, or roughly one percent (ie, 1/85) of a wavelength. Similarly, the thickness of the gold layer  155  is roughly 3 percent (ie, 4/85) of a quarter wavelength. 
     Omit Omit 
     For present purposes, this 3 percent thickness will be ignored. 
     High-index layer  160  in FIG. 9 can be viewed as an extension of the resonant cavity, which is the region between the top mirror  103  in FIG.  6  and the bottom mirror  175 . It can be viewed as an extension of the resonant cavity because, traditionally, the cavity is viewed as terminating with a high-index material, and the mirror is viewed as beginning with a low-index material, namely layer LL in FIG. 6, which corresponds to the silicon dioxide layer in PAIR  1  in FIG.  9 . 
     Consequently, since high-index layer  160  is a nominal quarter-wavelength in thickness, and is added to the thickness of the resonant cavity, which is an even number of quarter wavelengths, the cavity has now become an odd number of wavelengths in length. This fact leads to two modifications to the invention will be explained, against the background of a computation for the prior-art structure of FIG.  2 . 
     FIG. 14 illustrates the reflection coefficient of the entire prior-art structure: top mirror  33 , bottom mirror  34 , and the resonant cavity between them. In FIG. 14, as the number of top pairs, N TOP  increases from zero to 7, the reflection coefficient drops. That is due to the fact that, as N TOP  increases, the top mirror  33  becomes a better reflector. As the reflectance becomes progressively better, a Fabry-Perot cavity is being generated, explaining the drop in reflectivity. When the reflectance of the top mirror equals that of the bottom mirror (the equality situation is not indicated), reflectance in FIG. 14 will equal zero: complete destructive interference will occur in the cavity. 
     FIG. 15 illustrates the analogous reflectance for one form of the invention, namely, the structure of FIG. 6 using the mirror of FIG.  9 . The length of the cavity has been changed: it is now is an odd multiple of quarter wavelengths, at 3 quarter-wavelengths in this example. That is, the distance between the top mirror  103  in FIG.  6  and the bottom mirror  175  is 3 quarter wavelengths. Top layer  160  in FIG. 9 provides an additional quarter-wavelength (nominal) to make the cavity an even number of quarter wavelengths, namely, four. 
     FIG. 15 indicates that, as the thickness of the gold layer increases, the wavelength at which minimal reflectance occurs increases. Another red shift has occurred. 
     FIG. 16 illustrates a second stratagem for countering this red shift: the cavity (the distance between the top mirror  103  in FIG.  6  and the bottom mirror  175 ) is made 0.725 wavelengths in length, or slightly less than 0.75 wavelengths, which is 3 quarter-wavelengths. The gold layer  155  in FIG. 9 is 100 Angstroms thick. Layer  160  is the thickness computed according to the first stratagem, namely, 0.9667×(one quarter wavelength). The minimum, point P 30  in FIG. 16, occurs at 850 nanometers. 
     The particular shapes and arrangements of the structures shown herein are dictated, in many cases, by the needs of the manufacturing processes used. At this point in the discussion, many of those structural aspects will be ignored, and some general principles will be discussed. 
     FIG. 17 illustrates a gain region  200 , the current blockers  205 , and mirrors  210  and  215 . Many of the components in, for example, FIG. 5 function to (1) hold the components of FIG. 17 in position and (2) deliver current to the gain region  200 . Those components have been eliminated from FIG.  17 . 
     FIG. 18 illustrates one form of the invention. The components of FIG. 17 appear in FIG. 18, with the addition of gold film  155  and dielectric layer  160 . The triplet of the top mirror  215 , dielectric layer  160 , and the gold layer  155  are preferably positioned so that gold layer  155  lies against current blockers  53 . 
     The current blockers  53  may be viewed as limiting the optical pathways taken by the photons reflected between the mirrors. Those pathways are limited to dashed box  220 . That is, the current blockers  53  surround part of the resonant cavity represented by dashed box  220 , and constrain the reflected light to travel through that region  222 . 
     The gold layer  155  delivers current to the mesa (illustrated as mesa  112  in FIG. 12) which occupies region  222  in FIG. 18, and which lies in the paths within dashed box  220 . 
     Alternate Embodiments 
     FIG. 7 illustrates an embodiment wherein the gold layer  300  is overlaid onto the p-layer mesa  305 . The top mirror  310  and the extra dielectric layer corresponding to layer  160  in FIG. 9 (layer  160  not shown in FIG. 7) are deposited onto the gold layer  300 . The layers composing top mirror  310  and layer  160  are conformal with the gold layer  300 . 
     The preceding discussion has been framed in the context of a film  155  constructed primarily of gold. However, other materials can be used, such as gold alloys. Other metals can be used, such as silver, copper, or any of the good conductors. Superconductors can be used, which are not necessarily classified as metals. Doped semiconductors of sufficiently high conductivity should not be ruled out, although they may form a PN junction with mesa  112 . The film  155  need not be of the single-crystal type. 
     Nomenclature 
     The term “quarter-wave dielectric layer” and similar terms are terms-of-art, and refer to a layer of dielectric material which is ¼ wavelength in thickness. The wavelength is measured within the layer, not in free space. 
     Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.