Semiconductor light-emitting diodes

Abstract: A high-density radiation-emitting semiconductor array (10) comprises disordered regions (24) extending through a confinement layer (18), an active layer (16) and partially into another confinement layer (14) to define light-emitting regions (28) between the disordered regions. Individual contacts on one confinement layer (20) aligned with each emitting area inject current through the layers to a contact (34) on a substrate (12), causing emission of radiation from the active layer through an exposed surface of the other confinement layer. One or both confinement layer can be replaced with a distributed Bragg reflector to form an enhanced array light-emitting diode (48) or a laser array (76). A refractive semiconductor layer (134), fresnel lenses (164) or a micro lens array (178) can be used to modify the manner in which the radiation is emitted from an exposed surface of the array. The array may be linear (10) or two-dimensional ( 190).

Description: This invention relates to a semiconductor light-emitting diode structure and, more particularly, to a high-density surface-emitting semiconductor structure with independently-addressable individual light-emitting elements. The ability to fabricate closely-spaced, or high-density, independently-addressable laser and light-emitting diode (LED) sources is important for many applications, such as optical disk technology, laser printing and scanning, optical interconnection and fiber optic communications. Individually, semiconductor light-emitting diodes and semiconductor lasers are low-power output devices. Arrays of semiconductor light-emitting diodes or semiconductor lasers can be used to increase the power output and to simplify optical system design. To provide and maintain good optical alignment of the light-emitting elements of the array with one another and to minimize the assembly involved, arrays have been fabricated so that the light-emitting elements are in a single semiconductor substrate. One problem with such arrays is maintaining the electrical and optical isolation between the individual light-emitting elements. Another problem is increasing the density of the light-emitting elements in the substrate by closely spacing the elements together while still maintaining the isolation, avoiding heat dissipation problems, and providing precise alignment of the elements. Another problem is making each individual light-emitting element in the array independently addressable. As the light-emitting elements are spaced closer together in higher densities, it is progressively more difficult to cause each element to emit light separately, individually and independently. Known laser and LED sources have been edge emitters, in which the radiation is emitted from the edge of a monolithic structure of semiconductor layers. An alternative construction is surface emitters, where the radiation is emitted from the surface of the monolithic structure of semiconductor layers. US-A-4,870,652 discloses an array of edge emitting lasers. The independently addressable laser array comprises a plurality of semiconductor layers with at least two lasing elements disposed in separate optical cavities, the layers including an active region for light generation and propagation under lasing conditions. Impurity induced disordering through the active layer produce disordered regions laterally adjacent and between optical cavities to optically isolate the lasing elements. A barrier is formed between lasing elements which electrically isolates the individual lasing elements. Surface-emitting semiconductor light sources have several advantages over edge emitters. The light-emitting surface of a surface emitter is larger than that of an edge emitter. The power needed to cause emission of light is less for a surface emitter than an edge emitter. Surface-emitting LEDs are more efficient than edge emitting LEDs because they have that larger emitting area. Fabrication of surface emitting lasers can be less complex than fabrication of edge emitting lasers since the cleaving and mirror passivation needed for edge emitters are eliminated. If the individual contacts that make the surface emitters of the array independently addressable are on the same side of the semiconductor as the light-emitting surface, the lateral spacing between the surface-emitting light sources is restricted, and consequently the density of the array is limited. The diameter of the contact will be larger than the diameter of the emitting surface. Annular contact shapes are also more difficult to contact while maintaining a clear aperture for the emitting surface. For direct wire bonds the contacts may require an auxiliary pad. Same surface contacts and emitting surfaces also may increase the amount of heat generated and may cause dissipation problems. The individual contacts that make the surface emitters of the array independently addressable should also be directly aligned with the light-emission optical cavity. Alignment is advantageous because it minimizes the electrical resistance, the current spreading to each emitter, and the size of the electrode. It also places the heat sink as close to the emitting area as possible. Minimizing current spreading helps electrically isolate the individual elements. Electronics Letters, Vol.26, No.13, pp.940-942 discloses a two-dimensional, vertical-cavity, surface-emitting laser having, as confinement layers, a p-distributed Bragg reflector (DBR) and a n-DBR on opposite sides of a spacer-covered active layer. A proton implant partially through the p-DBR confinement layer provides the disordered regions. The n-DBR confinement layer is provided on an n-substrate of GaAs. A contact of n-metal is produced on the n-substrate (bottom) and multiple contacts of p-metal are produced on the outer or top surface of the non-disordered regions of the p-DBR layer. The lasers emit through the n-substrate, as shown in Figure 1 and as specifically stated in the paragraph describing Figure 1. It is an object of this invention, therefore, to provide a high-density array of light source surface emitters on a single substrate. In accordance with the present invention, a high-density semiconductor light-emitting array comprises: a first semiconductor confinement layer deposited on a substrate, said first confinement layer and said substrate having a first conductivity type; an active semiconductor layer deposited on the first confinement layer, to provide light generation and propagation, the material of the active layer having a smaller bandgap than that of the first confinement layer; a second semiconductor confinement layer deposited on the active layer, of a second conductivity type, which is opposite to that of the first confinement layer; a semiconductor contact layer deposited on the second confinement layer, said contact layer and the second confinement layer having the same conductivity type; disordered regions extending through the contact layer, the second confinement layer and the active layer, and at least partially into the first confinement layer, the disordered regions having a conductivity type opposite to that of the second confinement layer; at least one trough formed in the substrate, exposing a surface of the first confinement layer; at least one contact formed on surfaces of the substrate adjacent said trough; light-emitting zones formed between the disordered regions, and comprising the non-disordered second confinement layer, the non-disordered active layer and the non-disordered first confinement layer, the non-disordered first confinement layer containing in the surface exposed by the trough emitter surface areas for the emission of light; contacts formed on the contact layer, the contacts being aligned with and covering each of the light-emitting zones such that current injected between a selected one of the contacts and the contact on the substrate will cause radiation emission from one of the light-emitting zones through a corresponding one of said emitter surface areas. The second confinement layer can be replaced with multiple layers making a highly reflective mirror known as a distributed Bragg reflector (DBR) to form an enhanced LED array. Both confinement layers can be replaced with DBRs to form an optical cavity, instead of a light-emitting area, and a laser array rather than a LED array. Similarly, either or both DBRs can be replaced with a confinement layer and a high reflectivity coating. A transparent refractive layer can be deposited between the substrate and the first confinement layer to diverge the emitted light. Alternatively, the output surface of the first confinement layer, the first DBR or the refractive layer can be etched to form a Fresnel lens for collimating or focussing the laser output. A micro lens array can be positioned adjacent to the surface emitting areas. The semiconductor laser/light-emitting diode arrays can also be formed into two-dimensional arrays. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a side view of a high-density, independently-addressable, surface emitting, semiconductor LED array formed according to this invention; Figure 2 is a schematic illustration of the bottom view of the LED array of Figure 1; Figure 3 is a side view of an enhanced, high-density, independently-addressable, surface emitting, semiconductor LED array with a DBR formed according to this invention; Figure 4 is a side view of a high-density, independently-addressable, surface emitting, semiconductor laser array formed according to this invention; Figure 5 is a side view of an alternative embodiment of an array formed according to this invention; Figure 6 is a side view of another embodiment of this invention; Figure 7 is a bottom view of an other embodiment of this invention with individual Fresnel lenses; Figure 8 is a bottom view of an embodiment of this invention with a micro lens array; Figure 9 is a bottom view of a two-dimensional laser array formed according to this invention. Reference is now made to Figure 1, wherein there is illustrated a high-density, independently-addressable, surface emitting semiconductor LED array 10 of this invention. LED array 10 comprises a substrate 12 of n-GaAs upon which is epitaxially deposited a first confinement layer 14 of n-Al x Ga 1-x As; an active layer 16 of nondoped GaAs for providing light wave generation and propagation, a second confinement layer 18 of p-Al y Ga 1-y As where x = or ≠ y, and a contact layer 20 of p-GaAs. The bandgap of the semiconductor material of the active layer 16 is less than that of the first confinement layer 14 so that the first confinement layer is transparent to the light generated in the active layer. The first confinement layer also has a doping level as low as possible without introducing unnecessary resistance so that the first confinement layer is transparent to the light generated in the active layer. The substrate, on the other hand, should be heavily doped to increase electrical conductivity. Since the first confinement layer 14 and the substrate 12 are both n-type conductivity, that side of the LED array is referred to as the n-side of the LED array. Similarly, since the second confinement layer 18 and the contact layer 20 are both p-type conductivity, that side of the LED array is referred to as the p-side of the LED array. The active layer may be nondoped, p-type doped or n-type doped; GaAs, Al z Ga 1-z As or (Al z Ga 1-z ) 0.5 In 0.5 P; or a relatively thin conventional double heterostructure (DH) active layer; or a single quantum well, such as GaAs or Al z Ga 1-z As where z is very small and z<x and y; or a multiple quantum well superlattice, such as alternating layers of GaAs and Al z Ga 1-z As where z<x and y or alternating layers of Al w Ga 1-w As and Al B Ga 1-B As where w<B<x or y (w for well and B for barrier). Also any of the aforementioned active layers can be deposited between two semiconductor confinement layers of Al m Ga 1-m As and Al n Ga 1-n As, where m may or may not equal n, but with bandgaps intermediate the bandgaps of the active layer and the first and second confinement layers, in a separate confinement structure. As is known, the epitaxial growth of LED array 10 may be carried out by molecular beam epitaxy (MBE) or metallo-organic chemical vapor deposition (MOCVD). The substrate 12 may be about 100 µm thick. Confinement layers 14 and 18 may have a thickness in the range of 0. 1 to 1 µm. Active layer 16 may be a thin conventional layer, having a thickness of 50 nm to 2 µm or may be comprised of a superlattice structure of quantum wells which may be 3 to 50 nm thick. The contact layer is typically 0. 1 to 0.2 µm thick. There are alternative conventional techniques and diffusion/implant species for carrying out the desired disordering or the elemental implant/annealing technique. Discussion hereafter will be confined to impurity-induced disordering. However, other techniques and elemental diffusions or implants are equally applicable. Upon completion of the epitaxial growth, a Si 3 N 4 mask is formed on the top surface 22 of the contact layer 20 with openings exposing regions of the semiconductor structure to impurity-induced disordering. The mask protects the unexposed regions which will form and shape the light-emitting areas, which can be circular, elliptical, square, rectangular, trapezoidal, triangular or of any desired shape or size. The light-emitting areas are established by first selectively diffusing a high concentration n-impurity dopant, such as silicon, into the regions of the semiconductor structure exposed through the mask. Other n-impurity dopant elements would include Ge and Sn. A silicon layer is deposited in the opening in the Si 3 N 4 mask and then capped with an additional layer of Si 3 N 4 . The diffusion of silicon is accomplished at a temperature of approximately 800° C and is maintained for a sufficiently long period, e.g. seven to eight hours, for the silicon to penetrate the contact layer 20, the second confinement layer 18 and the active layer 16, and partially penetrate the first confinement layer 14. The diffusion of silicon through and into the active layer 16, the contact layer 20 and the confinement layers 14 and 18 causes an intermixing of Ga and Al in the active layer 16, the contact layer 20 and the confinement layers 14 and 18, thereby forming a n-impurity induced disordered region 24. Upon completion of the impurity-induced disordering steps, electrically-insulating regions 26 are formed in the disordered regions 24 by proton implantation through the top surface 22 to provide a level of electrical isolation of the light-emitting areas, which allows independent addressability of each individual light-emitting area. Between the disordered regions 24 in the semiconductor LED array are the light-emitting areas 28 consisting of the non-disordered sections of the second confinement layer 18, the active layer 16 and the first confinement layer 14. The disordered regions, optically and electrically, isolate and separate the light-emitting areas. The light-emitting areas are shaped by the confinement layers in the vertical direction and the shape of the disordered regions in the horizontal direction. Standard chemical etching or other techniques are employed to form metal contacts 30 of Cr-Au or Ti-Pt-Au on the top surface 22 of the non-disordered contact layer 20 aligned with each optical cavity 28. Each contact 30 contacts an optical cavity. The contacts 30 are referred to as p-contacts, since they are on the p-side of the LED array. The contacts are typically circular or annular in shape for ease in forming high density arrays, although the contacts may also be elliptical, half-moon or other shapes. The individual contact can be equal in size or smaller than the active layer in the aligned light-emitter area, and can be shaped to match the active layer or the light emitter area. The contact is shaped by shaping the hole in a metallization mask. The shape of the contact is important so that the contact has the largest area possible, for reducing electrical resistance without limiting the density of the emitters, and to match the bonding pad for effective heat dissipation. The non-disordered contact layer 20 beneath each contact 30 provides low contact resistance to the other semiconductor layers, and reduces contact heating. The bottom surface 32 of the substrate 12 is also metallized with Au/Ge to form a substrate contact 34. The substrate contact is referenced to ground and referred to as the n-contact, since it is on the n-side of the LED array. Current is injected between the p-contact 30 and the n-contact 34 to forward-bias the p-n junction of p-confinement layer 18 and n-confinement layer 14 to cause the active layer 16 to emit light 36 from the light-emitting area 28. The current is injected substantially parallel to the light-emitting area, through the p-contact 30, the p-contact layer 20, the p-confinement layer 18, the active layer 16 of the individual light-emitting area, and then spreads in the n-confinement layer 14 into the substrate 12 and out the n-contact 34. The ground or n-contact is common to all the light-emitting areas. However, each light-emitting area or LED element contains a p-n junction that is separately biased through its p-contact from all the others. Since each p-contact is positively biased with respect to ground, current flows only from each p-contact to ground. The electrically isolating regions and the disordered regions prevent any single p-contact from causing adjacent light-emitting areas to emit light. Flow between different p-contacts does not occur, because any small potential difference between the addressed p-contact and a neighboring p-contact corresponds to a reverse voltage on the neighboring p-contact. The light 36 is emitted through the emitter surface area 38, substantially perpendicular to the bottom surface 32 of the substrate 12 and aligned with the p-contact 30 and the light-emitting area 28, hence LED array 10 is a surface emitting LED. The p-contact 30 aligned with the light-emitting area 28 prevents light from being emitted through the p-side of the LED array. The shape of the emitter surface area, and therefore the cross-sectional shape of the resulting beam of emitted radiation is determined by the shape of the light-emitting areas, which can be circular, elliptical, square, rectangular, trapezoidal, triangular or any desired shape or size. The emitted light itself can be either continuous wave or pulse. Typically, the LED array 10 has an operating current of about 10 mA with an output power of about 30 µW per individual light-emitting element. Figure 2 is the bottom view of the LED array 10 of Figure 1, showing the bottom surface 32 of the substrate with the stripe n-contacts 34 and the array of circular emitter surface areas 38 of the light-emitting areas where the light is emitted through the surface between the n-contacts. The thickness of the substrate 12 may impede emission of the light from the active layer of the LED array. To avoid this a stripe of the substrate is removed. Prior to the formation of the n-contacts, a trough 40 or stripe is etched in the surface 32 of the substrate 12, by standard chemical etching or other techniques, to expose the surface 42 of the first confinement layer 14 to expose the emitter surface areas 38 of the LED optical cavities 28 therebeneath to facilitate light emission. The exposed surface of the first confinement layer may also be coated with an anti-reflective coating to facilitate light emission. The processing of the LED array 10 is done from both the p-side and the n-side of the LED array. The active layer is shallower in the LED array and closer to the light-emitting surface. Similarly, the light emitted from the active layer does not pass through a heavily-doped contact layer, unlike when the individual contacts and the light-emitting surface areas are on the same side of the light-emitting semiconductor. The diameters of the emitter surface areas 38 are typically 2 µm or less. The spacing 44 from the center 46 of one individual LED element to the center 46 of the next LED element, as measured from the emitter surface areas 38, is 3 µm or less, which provides for a high density of LEDs in the array. The impurity-induced disordering provides accurate and closer center-to-center spacing of the individual LED elements. Since the individually-addressable contacts, the p-contacts, are not on the same surface as the light-emitting areas, the lateral spacing of the light-emitting elements is slightly closer. Also, the light emitter geometry can be staggered or offset if the p-contacts and the light-emitter areas are also staggered, rather than the straight line of p-contacts and LED emitter surface areas shown in Figure 2. A staggered or offset geometry, of course, provides a higher density of LED elements. In Figure 3, enhanced LED array 48 is of identical structure to the LED array 10 of Figure 1, except that the p-confinement layer 18 of Figure 1 has been replaced by a distributed Bragg reflector (DBR) 50. Thus, the LED array 48 comprises a substrate 52 of n-GaAs upon which is epitaxially deposited a confinement layer 54 of n-Al x Ga 1-x As; an active layer 56 of nondoped GaAs for providing light wave generation and propagation, a p-DBR 50 of alternating layers of p-Al y Ga 1-y As and p-Al z Ga 1-z As where z ≠ y, and a contact layer 58 of p-GaAs. N-impurity-induced disordered regions 60 are formed in the enhanced LED array 48 extending through portions of the contact layer 58, the p-DBR 50, the active layer 56 and into the n-confinement layer 54. Electrical isolating regions 62 are formed on the top surface of the disordered regions. Between the disordered regions 60 in the enhanced LED array 48 are the enhanced LED light-emitting areas 64 consisting of the non-disordered sections of the p-DBR 50, the active layer 56 and the n-confinement layer 54. The p-DBR is constructed to have a reflectivity as close to unity as possible. P-contacts 66 are formed on the p-contact layer 58, aligned with each enhanced LED light-emitting area 64 and a n-contact 68 is formed on the substrate 52, except in a trough formed in the substrate (not shown). The light 70 is emitted through the emitter surface area 72 on the bottom surface of the n-confinement layer, substantially perpendicular to the bottom surface 74 of the substrate 52. The p-DBR reflects the light being emitted by the optical cavity toward the p-side of the LED array back through the surface area on the n-side of the LED array, thus enhancing the amount of light emission of the LED array. The exposed surface of the confinement layer and the emitter surface area in the trough may also be coated with an anti-reflective coating to facilitate light emission. Typically, the enhanced LED array 48 has an operating current of about 10 mA with an output power of about 60 µW. Alternatively, the active layer 56 can be in a separate confinement structure to keep the carriers away from the DBR. Also alternatively, the p-confinement layer 18 of Figure 1 may be coated with a highly-reflective metal which serves as an electrically-conductive, optical reflector to form an enhanced LED array. In Figure 4, laser array 76 is of identical structure to the array 48 of Figure 3, except that the n-confinement layer 54 of Figure 3 has been replaced by a DBR 78 and that active layer 56 of Figure 3 has been replaced by active layer 80 which provides lightwave generation and propagation under lasing conditions. Thus the laser array 76 comprises a substrate 82 of n-GaAs upon which is epitaxially deposited a n-DBR 78 of alternating layers of n-Al x Ga 1-x As and n-Al y Ga 1-y As where x ≠ y, an active layer 80 of nondoped GaAs for providing light wave generation and propagation under lasing conditions, a p-DBR 84 of alternating layers of p-Al A Ga 1-A As and p-Al B Ga 1-B As where A ≠ B, and a contact layer 86 of p-GaAs. N-impurity induced disordered regions 88 are formed in the laser array 76 extending through portions of the contact layer 86, the p-DBR 84, the active layer 80 and the n-DBR 78. Electrical isolating regions 90 are formed on the top surface of the disordered regions. Between the disordered regions 88 in the laser array 76 are the laser optical cavity regions 92 consisting of the non-disordered sections of the p-DBR 84, the active layer 80 and the n-DBR 78. The p-DBR is constructed to have a reflectivity as close to unity as possible, while the n-DBR is constructed with a reflectivity close to unity but less than that of the p-DBR. P-contacts 94 are formed on the p-contact layer 86, aligned with each laser optical cavity region 92, and a n-contact 96 is formed on the substrate 82 except in a trough formed in the substrate (not shown). The light 98 is emitted through the emitter surface area 100 on the bottom surface of the n-DBR in the trough (not shown) in the substrate, substantially perpendicular to the bottom surface 102 of the substrate 82. An anti-reflective coating is not required to coat the trough. Since the light is from a laser optical cavity, it is, of course, coherent and can be either continuous wave or pulse. Typically, the laser array 76 has a threshold current of 1 to 5 mA and an operating current of about 10 mA with an output power of about 1 to 3 mW. Threshold currents of less than 1 mA are also possible for the laser array. Alternatively, the active layer 80 can be in a separate confinement structure to keep the carriers away from the DBR. Also alternatively, the p-confinement layer 18 of Figure 1, which was replaced by a distributed Bragg reflector (DBR) 84 in Figure 4, may be coated with a highly-reflective metal which serves as an electrically-conductive, optical reflector to form a laser array. In Figure 5, the laser array 104 is of identical structure to the laser array 76 of Figure 4, except that the n-DBR 78 has been replaced with a n-confinement layer 106 and a high reflectivity coating 108. Thus the laser array 104 comprises a substrate 110 of n-GaAs, a n-confinement layer 106 of n-Al x Ga 1-x As, an active layer 112 of nondoped GaAs, a p-DBR 114 of alternating layers of p-Al y Ga 1-y As and p-Al z Ga 1-z As where z ≠ y, a contact layer 116, n-impurity induced disordered regions 118, electrical isolating regions 120, laser optical cavity 122, p-contacts 124, and stripe n-contact 126 formed on the substrate 110, along with a trough in the substrate (not shown). After the trough is etched in a substrate, a high reflectivity coating 108 is deposited by evaporation on the exposed surface of the n-confinement layer 106. The coating covers the surface emitter areas 128 and forms one end of the laser optical cavity 122. The high reflectivity coating is typically a multilayer stack of materials such as silicon and aluminum oxide, as is known. Alternatively, the active layer 112 can be in a separate confinement structure to keep the carriers away from the DBR. The p-confinement layer 18 of Figure 1 which was replaced by a distributed Bragg reflector (DBR) 114 in Figure 5, may instead be coated with a highly-reflective metal to serve as an electrically-conductive optical reflector. In Figure 6, the laser array 130 is of identical structure to the laser array 76 of Figure 4, except that a refractive layer 132 is deposited between the substrate 134 and the n-DBR 136. Thus, the laser array 130 comprises a substrate 134 of n-GaAs upon which is deposited a refractive layer 132 of Al v Ga 1-v As, a n-DBR 136 of alternating layers of n-Al x Ga 1-x As and n-Al y Ga 1-y As where x ≠ y, an active layer 138 of nondoped GaAs, a p-DBR 140 of alternating layers of p-Al x Ga 1-x As and p-Al y Ga 1-y As where x ≠ y, and a contact layer 142 of p-GaAs. The laser array further comprises n-impurity induced disordered regions 144 and electrical isolating regions 146. The laser optical cavity 148 consists of the non-disordered sections of the p-DBR 140, active layer 138 and n-DBR 136. The emitter surface areas 150 are the interface surface 152 between the n-DBR 136 and the refractive layer 132. P-contacts 154 are formed on the p-contact layer 142, and a stripe n-contact 156 is formed on the substrate 134. A trough (not shown) is etched in the substrate 134 to expose the surface 158 of the refractive layer 132. The refractive layer is transparent to light emitted from the laser optical cavity through the emitter surface areas, but the refractive layer does diverge the light 160 emitted from the laser array, increasing the output spot size. The refractive layer should be thick enough to give the appropriate expansion of the spot size. A 5 to 10 µm thick refractive layer, for example, should double the size of the spot. Increasing the thickness of the refractive layer 132 increases the refractive power, diverges the angle of the emitted light further and increases the output spot size. Selection of an appropriate thickness for the refractive layer will result in partially-overlapping output spots of emitted light between the adjacent laser optical cavities of the adjacent laser elements. Fabricating the surface emitter area in an elliptical shape, for example, will further facilitate overlapping the diverging emitted light in the short axis direction of the ellipse. Overlapping in the long axis direction is pertinent to printing, where controlled overlap from line to line may be desirable. One advantage to the refractive layer is that no phase coherence is introduced between adjacent laser emitters. Consequently, the overlapping beams can be used optically without any optical interference effects. Although the light emitted from the LED array and the enhanced LED array is not coherent, the refractive layer of AlGaAs can still be used to diverge the light emitted from the array, although it would not be as effective as with light emitted from a laser, since the radiation pattern from an LED is lambertian. As shown in Figure 7, a laser array 162 is fabricated in the identical fashion to laser array 76 of Figure 4, except that fresnel lenses 164 are formed on the exposed n-DBR 166 over the emitter surface areas 168. Thus, the laser array 162 has stripe n-contacts 170 alternating with troughs 172 of the exposed surfaces 166 of the n-DBR with its emitter surface areas 168. A fresnel lens 164 is formed for each individual laser element by etching, by ion milling, reactive ion etching or other means. Each lens can be shaped to magnify the spot size of the emitted light, change the direction of the emitted light or otherwise alter the emitted light optically. The lateral shape of the lens can be formed to match the transverse shape of the optical cavity. Again, although the light emitted from the LED array and the enhanced LED array is not coherent, the fresnel lenses can still be used to collimate, diverge, focus or otherwise modify the light emitted from the array by forming the lenses in the n-confinement layer over the emitter surface areas, although it would not be as effective as with light emitted from a laser since the radiation pattern from a LED is lambertian. Fresnel lenses can also be formed in the refractive layer of Figure 6 over the emitter surface areas. As shown in Figure 8, a laser array 174 is fabricated in identical fashion to laser array 76 of Figure 4 except that an array 176 of micro lenses 178 is positioned flush in the trough 180 of the exposed surface 182 of the n-DBR 184. Each micro lens 178 is aligned with an emitter surface area 186. The laser array 174 has stripe n-contacts 188 alternating with the troughs 180 and the micro lens array 176 positioned within the trough. Each micro lens 178 in the array is formed for each individual laser element by known means. Each micro lens can be shaped to magnify the spot size of the emitted light, change the direction of the emitted light or otherwise modify the emitted light optically. The lateral shape of the micro lens can be formed to match the transverse shape of the optical cavity. Alternatively, the micro lens array can be positioned adjacent to the emitter surface areas of the LED array to magnify the size and spacing of the light-emitting areas. By way of illustration, a 3 µm spaced array over a 30 mm distance with an optical magnification of 7 will result in 48 spots per mm over a pagewidth of 212 mm. Similarly, a 4.5 µm spacing of an array will yield 32 spots per mm for a pagewidth. Again, although the light emitted from the LED array and the enhanced LED array is not coherent, the micro lens array can still be used to collimate, diverge, focus or otherwise modify the light emitted from the LED or enhanced LED array by positioning the micro lens array over the emitter surface areas. A micro lens array can also be used with the refractive layer of Figure 6 over the emitter surface areas. The driver circuitry for the p-contacts and the necessary bonding pads can be fabricated on a silicon chip (not shown) placed flush against the top surface of the LED or laser array. The silicon chip need not be optically transparent since the light emission will be from the other surface of the LED or laser array. Alternatively, electronic driver circuitry can be fabricated on the semiconductor LED or laser array or separate and adjacent to the array or on or in the substrate of the semiconductor array. Since the p-contact is directly below the active region, and the light is emitted from the surface of the n-side of the array, the full cross-sectional area of the array is used for heat dissipation. In Figure 9, a two-dimensional array 190 is fabricated in the identical fashion to laser array 76 of Figure 4 except that linear arrays are formed adjacent to each other. Thus, the two-dimensional array 190 has stripe n-contacts 192 alternating with troughs 194 of the exposed surfaces 196 of n-DBRs with its elliptical emitter surface areas 198. The spacing 200 between the center 202 of one individual laser element in one linear array row to the center 202 of one corresponding individual laser element in the next linear array row, as measured from the emitter surface areas 198, is typically 9 to 18 µm which provides for a high density of laser elements in the two-dimensional array. Two-dimensional arrays of surface emitters can be used to expose several lines simultaneously or sequential interlacing of the printed lines. In combination with a refractive layer, two-dimensional arrays can provide optimum overlap of scanning lines. The two-dimensional array is also not restricted to linear rows and columns of surface emitters but the surface emitter layout can be staggered or offset. The two-dimensional laser array is by way of illustration. The LED and enhanced LED arrays can also be fabricated into a two-dimensional array. Refractive layers, fresnel lens and micro lens arrays, as previously, discussed can also be used with a two-dimensional array. The above embodiments of this invention have all been fabricated with a substrate which, to facilitate light emission, has a trough etched in the substrate to the confinement layer or DBR therebeneath to expose the surface emitters.

Claims: A high-density semiconductor light-emitting array (10) comprising: a first semiconductor confinement layer (14) deposited on a substrate (12), said first confinement layer and said substrate having a first conductivity type; an active semiconductor layer (16) deposited on the first confinement layer, to provide light generation and propagation, the material of the active layer having a smaller bandgap than that of the first confinement layer; a second semiconductor confinement layer (18) deposited on the active layer, of a second conductivity type, which is opposite to that of the first confinement layer; a semiconductor contact layer (20) deposited on the second confinement layer, said contact layer and the second confinement layer having the same conductivity type; disordered regions (24) extending through the contact layer, the second confinement layer and the active layer, and at least partially into the first confinement layer, the disordered regions having a conductivity type opposite to that of the second confinement layer; at least one trough (40) formed in the substrate (12), exposing a surface (42) of the first confinement layer; at least one contact (34) formed on surfaces (32) of the substrate adjacent said trough; light-emitting zones (28) formed between the disordered regions, and comprising the non-disordered second confinement layer, the non-disordered active layer and the nondisordered first confinement layer, the non-disordered first confinement layer containing in the surface (42) exposed by the trough emitter surface areas (38) for the emission of light; and contacts (30) formed on the contact layer, the contacts (30) being aligned with and covering each of the light-emitting zones such that current injected between a selected one of the contacts (30) and the contact (34) on the substrate will cause radiation emission from one of the light-emitting zones through a corresponding one of said emitter surface areas. An array as claimed in claim 1, in which the second confinement layer takes the form of a multiplicity of layers of semiconductor material forming a distributed Bragg reflector (114). The array of any preceding claim, wherein the first confinement layer and the disordered regions are of n-type conductivity, and wherein the second confinement layer is of p-type conductivity. The array of any preceding claim, wherein an electrically insulating region (26) is formed on each of the disordered regions. The array of any preceding claim, further comprising: a refractive semiconductor layer (132) deposited on the substrate (134), the first semiconductor confinement layer (136) being deposited on the refractive layer, and the trough formed in the substrate exposing the surface of the refractive layer through which radiation is emitted. The array of any preceding claim, further comprising an anti-reflective coating (108) on the exposed surface of said first confinement layer. The array of any preceding claim, further comprising fresnel lenses (164) etched over the light-emitting areas on the exposed surface of the first confinement layer. The array of any of claims 1 to 6, further comprising a micro lens array (178) positioned over the light-emitting areas on the exposed surface of the first confinement layer. An array as claimed in any preceding claim, wherein the light-emitting areas form a two-dimensional array. An array as claimed in any preceding claim, in which the first confinement layer takes the form of a multiplicity of layers of semiconductor material forming a distributed Bragg reflector.

Inventors: PAOLI THOMAS L, PAOLI, THOMAS L.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1991
Main classification: H01L27
IPC codes: H01S5, H01L27, B41J2