Abstract:
A high efficiency, high density light-emitting diode array which provides improved light output efficiency and suppression of crosstalk between adjacent light-emitting elements without loss of reliability or reproducibility is disclosed. The array includes isolated light-emitting elements on a substrate. Each light-emitting element has a light-emitting layer between a pair of cladding layers with heterojunctions being formed between the light-emitting layer and the cladding layers. Each light-emitting element has a light-emitting surface and the light-emitting layer of each light-emitting element is of an area no greater than the area of the light-emitting surface.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a light-emitting diode array, and, more particularly, to a heterojunction light-emitting diode array for use as a printing light source in a optical printer and comprising a multiplicity of light-emitting diode closely arrayed on a single substrate. 
     BACKGROUND OF THE INVENTION 
     In recent years there is an increasing need for printers able to print faster and with a higher print density. Laser printers, which employ a laser light source, and LED (light-emitting diode) printers, which employ an LED array as the light source, are two examples of printers used in response to such needs. While a laser printer requires the use of a mechanical mechanism, such as a rotating polygonal mirror, for the scanning laser beam, with an LED printer it is only necessary for the light-emitting diodes (hereinafter also referred to as &#34;light-emitting elements&#34;) that make up the light-emitting diode array to be electrically controlled. The advantages of LED printers compared with laser printers are that as they do not have any mechanical moving parts, they are smaller, faster and more reliable. 
     Referring to FIG. 1, there is shown a cross-sectional view of two light-emitting elements 8 of a prior art AlGaAs-based homojunction LED array 9. The array 9 comprises a substrate 10 of p-type conductivity GaAs having on a surface 11 thereof a first layer 12 of p-type conductivity Al x  Ga 1-x  As. The first layer 12 is about 10 microns in thickness and is doped with Zn to a concentration of about 5×10 17  impurities/cm 3 . On a surface 24 of the first layer 12 is a second layer 14 of n-type conductivity Al y  Ga 1-y  As which is about 5 microns in thickness and is doped with Te to a concentration of about 8×10 17  impurities/cm 3 . For emitting light with a wavelength in the region of 720 nm., the aluminum composition in the first and second layers 12 and 14 is set at x=0.2 and y=0.5. Spaced grooves 15 extend through the second layer 14 and a portion of the first layer 12 to form mesa like light-emitting elements 8. 
     A separate contact layer 16 of n+ type conductivity GaAs is on a portion of the second layer 14 of each light-emitting element 8. The contact layers 16 are about 0.1 microns in thickness and are doped with Sn to a concentration of about 5×10 18  impurities/cm 3 . On each of the contact layer 16 is a separate electrode 18, and on a surface 13 of the substrate 10 is an electrode 20. An antireflection coating 22 of the SiN x  covers the light-emitting elements 8 and the bottom of the grooves 15. 
     The array 9 is made by depositing on the surface 11 of the substrate 10 in succession using liquid-phase epitaxy (LPE), the first layer 12, the second layer 14 and a contact layer 16. The electrode layer 18 is deposited on the contact layer 16 and the electrode layer 20 is deposited on the surface 13 of the substrate 10. Using photolithography and plasma etching, the electrode layer 18 is defined to leave portions of the electrode layer 18 only over the area which is to form the light-emitting elements 8. Then using a chemical etchant of NH 4  OH:H 2  O 2  =1:10, the contact layer 16 is removed except for the portions under the elecrodes 18. Using photolithography and a chemical etchant of H 2  SO 4  :H 2  O 2  :H 2  O=1:2:40, the grooves 15 are etched through the second layer 14 and about one micron into the first layer to form the light-emitting elements 8. Plasma CVD (chemical vapor deposition) is then used to form the antireflection SiN x  coating 22. This is followed by alloying to form the electrodes 18 and 20. 
     Structurally, this heterojunction light-emitting diode array 9 consists of discrete high-luminance LEDs 8 arranged into a single array. Using the n-type conductivity Al y  Ga 1-y  As second layer 14 that is transparent to the light emitted by the emission layer p-type conductivity Al x  Ga 1-x  As first layer 12 results in energy attenuation from internal absorption being avoided. Also, an epitaxial junction with excellent crystallinity is employed, and the improvement in injection efficiency provided by the heterojunction raises the overall external emission efficiency. 
     However, unlike the case with single, discrete LEDs, there are the following problems with LED arrays used for printer applications. First, optical crosstalk between elements has to be suppressed; and second, variation in characteristics from element to element has to be minimized. 
     For this, in the light-emitting diode array 9 shown in FIG. 1, the n-type Al y  Ga 1-y  As second layer 14 that forms a transparent window has to be completely removed between light-emitting elements 8. In addition, to reduce optical bleeding, the emission mesas have to be formed to a certain minimum depth into the p-type Al x  Ga 1-x  As emission first layer 12. 
     The diffusion length of minority carrier electrons injected into the p-type Al x  Ga 1-x  As first layer 12 decreases as the distance from the p-n junction increases, but is in the order of 10 microns. This means that at least about 10 microns of the p-type Al x  Ga 1-x  As first layer 12 has to be removed. However, it is difficult to accomplish this with adequate process uniformity and reproducibility. Thus, some degree of optical bleeding has been unavoidable in the resultant LED arrays in which the fabrication process took such factors into account. Furthermore, the p-type Al x  Ga 1-x  As first layer 12 that is within the diffusion length of electrons from the p-n junction 24 functions effectively as an emission layer. Thus, in order to optimize the emission efficiency, it is necessary to make the p-type Al x  Ga 1-x  As first layer 12 at least 10 microns thick. A problem is, however, that even if the emission efficiency is improved, owing to the high refractive index of the light-emitting portion, most of the light is lost through total reflection. This results in a very low external light output efficiency of no more than several percent. One cause may be the loss of high-intensity light in the emission portion directly beneath the electrode 18, owing to the fact that the electrode is formed directly on the upper part of the emission portion. 
     SUMMARY OF THE INVENTION 
     The present invention is, therefore, to provide a high efficiency, high density light-emitting diode array in which light output efficiency is improved and crosstalk between adjacent light-emitting elements can be suppressed without any loss of reliability or reproducibility. 
     In accordance with the present invention, this is attained by an array of light emitting elements arranged in spaced isolated relation on a surface of a substrate. Each light-emitting element has a light-emitting layer of a material which emits light when carriers are injected therein. The light-emitting layer is between cladding layers each of a material having a wider band gap than that of the material of the light-emitting layer to form a heterojunction with the light-emitting layer. An electrode is over one of the cladding layers and extends around a light-emitting surface, and the light-emitting layer is of an area so as to lie wholly within the light-emitting surface. 
     The invention will be better understood from the following more detailed description taken with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a prior art AlGaAs single heterojunction light-emitting diode array; 
     FIG. 2 is a top plan view of one form of a light-emitting diode array in accordance with the present invention; 
     FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2; 
     FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 2; 
     FIG. 5 is a top plan view of another form of a light-emitting diode array in accordance with the present invention; 
     FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 5; and 
     FIG. 7 is a cross-sectional view taken along line 7--7 of FIG. 5. 
     The drawings are not necessarily drawn to scale. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIGS. 2, 3 and 4, there is shown a top view (FIG. 2), a first cross-sectional view (FIG. 3) through a dashed line 3--3 of FIG. 1, and a second cross-sectional view (FIG. 4) through a dashed line 4--4 of FIG. 1 of a light-emitting diode array 28 having light-emitting elements 29. For simplicity, only two light-emitting elements 29 are shown. 
     As shown in FIGS. 3 and 4, the diode array 28 comprises a substrate 40 of n-type conductivity GaAs which is doped with Si to a concentration of about 1×10 18  impurities/cm 3 . The substrate 40 has opposed surfaces 41 and 43. On the surface 41 is a buffer layer 42 of n-type GaAs of a thickness of about 0.2 microns and which is doped with Se to a concentration of about 1×10 18  impurities/cm 3 . On the buffer layer 42 is a first cladding layer 44 of n-type conductivity Al x  Ga 1-x  As which is about 2.0 microns in thickness and is doped with Se to a concentration of about 5×10 18  impurities/cm 3 . A p-type conductivity light-emitting layer 46 of GaAs is on the first cladding layer 44. The light-emitting layer 46 is about 0.5 microns in thickness and is doped with Zn to a concentration of about 1×10 18  impurities/cm 3 . On the light-emitting layer 46 is a second cladding layer 48 of n-type conductivity Al x  Ga 1-x  As which is of a thickness of about 2.0 microns and is doped with Se to a concentration of about 5×10 18  impurities/cm 3 . On the second cladding layer 48 is a contact layer 50 of n-type conductivity GaAs which is about 0.05 microns in thickness and is doped with Se to a concentration of about 5×10 18  impurities/cm 3 . The cladding layers 44 and 48 are each of a material having a wider band gap than the material of the light-emitting layer 46. This forms a heterojunction between the light-emitting layer 46 and each of the cladding layers 44 and 48. 
     Trenches 38 extend through the contact layer 50, the second cladding layer 48 and the light-emitting layer 46 to form the light-emitting elements 29 which are connected to an electrode pad area 34 by a narrow feed through 36. In each light-emitting element 29, the contact layer 50 has an opening 51 therethrough to expose a light-emitting area 30. An electrode 32 is on each contact layer 50 of each light-emitting element 29 and extends over the feed through 36 to the electrode pad area 34. An electrode 54 is on the surface 43 of the substrate 40. 
     As is shown in FIG. 4, each of the trenches 38 extends under the second cladding layer 48 of its respective light-emitting element 39 a distance such that the light-emitting layer 46 of each light-emitting element 29 is no greater in area than the opening 51 in its respective contact layer 50. Thus, the light-emitting layer 46 of each light-emitting element 29 is no greater in area than the light-emitting area 30 of the light-emitting element 29. This provides a space 56 under the feed through 36. The space 56 and the rest of the trenches 38 are filled with a passivation layer 58. A p-type conductivity diffused region 48a extends through the second cladding layer 48 under each of the openings 51 in the contact layer 50 to the light-emitting layer 46. An antireflection layer 52 of SiN x  is over the light-emitting area 30 of the second cladding layer 48 and a portion of the electrode 32 surrounding the opening 51 in the contact layer 50. 
     To make the light-emitting diode array 28, metal organic chemical vapor deposition (MOCVD) is used to epitaxially deposit on the substrate surface 41 in succession the buffer layer 42, the first cladding layer 44, the light-emitting layer 46, the second cladding layer 48 and the contact layer 50. Plasma CVD is then used to deposit a SiN x  insulation layer 53 over the whole surface. Photolithography and plasma etching are used to etch away the portion of the SiN x  layer 2 which is over the light-emitting portion 30. Zinc is then diffused into the light-emitting portion 30 as far as the light-emitting layer 46. The electrodes 32 and 54 are then formed by vapor deposition. The unnecessary portions of the electrode 32 are then removed by photolithography and plasma etching. Using the electrode 32 as a mask and NH 4  OH:H 2  O 2  =1:10 chemical etching, the portions of the contact layer 50 which are not under the electrode 32 are then removed. 
     The antireflection SiN x  layer 52 is then formed over the whole surface. Photolithography and plasma etching are then used to remove the antireflection SiN x  layer 52 from the trench 38 portion around the light-emitting element 29 and feed through 36. H 2  SO 4  :H 2  O 2  :H 2  O=1:2:40 chemical etching is then used to remove the epitaxial layers of the trench 38 portion around the light-emitting element regions 29 down to the light-emitting layer 46. NH 4  OH:H 2  O 2  =1:10 chemical etching is then used to remove just the portions of the light-emitting layer 46 between the cladding layers 44 and 48, in the way indicated by the arrows E in FIGS. 3 and 4. This etching is carried out until the emission layer 46 is smaller than the internal diameter of the electrode 32. Etching is continued until there is no light-emitting layer 46 below the electrode feed through 36. This forms the space 56 below the feed through 36 shown in FIG. 4. 
     Photolithography and plasma etching are then used to form the electrode bonding pad 34 by removing the SiN x  layer from the electrode 32 pad portion. This is followed by the use of heat treatment to form ohmic contacts for the electrodes 32 and 50. Finally, polymide and oxygen reactive-ion etching is used to form a passivation layer 58 in the trenches 38 and along the side walls of the light-emitting layer 46. 
     In the light-emitting diode array 28, the thickness of the light-emitting layer 46, at 0.5 microns, can be made thinner than the light-emitting layer 12 of the prior art light-emitting diode array 9 shown in FIG. 1. If required, this thickness can be further decreased, to several hundred angstroms or less. The ability to decrease the thickness of the light-emitting layer 46 enables the carrier density in the emission region (light-emitting layer 46) to be raised, improving the internal efficiency. Also, making the layer 46 smaller than the internal diameter of the electrode 32 improves output efficiency, as the optical output is not hindered by the electrode 32. Contact resistance can be decreased by increasing the area of the electrode 32. Moreover, the second cladding layer 48 forms a bridge so that even after isolating the light-emitting portion 30, a conductive portion (feed through 36) can be formed on the flat portion between the electrode 32 and the electrode pad 34. This reduces problems of non-uniformity caused by a break in an electrode lead wire or the like. Also, crosstalk between adjacent elements can be suppressed by the provision of a light-absorbing wall 60, i.e., the portion of the light-emitting layer 46 not used as a light-emitting layer. 
     Referring now to FIGS. 5, 6 and 7, there is shown a top view (FIG. 5), a first cross-sectional view (FIG. 6) through a dashed line 6--6 of FIG. 5, and a second cross-sectional view (FIG. 7) through a dashed line 7--7 of FIG. 5 of a light-emitting diode array 128 in accordance with the present invention. For simplicity, again only two light-emitting elements 129 are shown. As shown in FIGS. 6 and 7, the light-emitting diode array 128 comprises a substrate 140 having opposed surfaces 141 and 143. The substrate 140 is of n-type conductivity GaAs which is doped with Si to a concentration of about 1×10 18  impurities/cm 3 . On the surface 141 of the substrate 140 is a buffer layer 142 which is about 0.2 microns in thickness and is of n-type conductivity GaAs doped with Se to a concentration of about 1×10 18  impurities/cm 3 . A first cladding layer 144 is on the buffer layer 142. The first cladding layer is of a thickness of about 2.0 microns and is of n-type conductivity Al x  Ga 1-x  As doped with Se to a concentration of about 5×10 18  impurities/cm 3 . On the first cladding layer 144 is a light-emitting layer 146 of p-type conductivity GaAs which is about 0.5 microns in thickness and doped with Zn to a concentration of about 1×10 18  impurities/cm 3 . On the light-emitting layer 146 is a second cladding layer 148 of p-type conductivity Al x  Ga 1-x  As. The second cladding layer 148 is about 2.0 microns in thickness and is doped with Zn to a concentration of about 5×10 18  impurities/cm 3 . A contact layer 150 is on the second cladding layer 148. The contact layer 150 is of p+ type conductivity GaAs which is about 0.05 microns in thickness and doped with Zn to a concentration of about 1×10 19  impurities/cm 3 . To block carriers and transmit light at the emission wavelength of the GaAs light-emitting layer 146, the aluminum content is set at x=0.3. Each of the cladding layers 144 and 148 are of a material having a wider band gap than the material of the light-emitting layer 146. This forms a heterojunction between the light-emitting layer 146 and each of the cladding layers 144 and 148. 
     A recess 149 is provided through the contact layer 150, the second cladding layer 148 and the light-emitting layer 146 to form the light-emitting elements 129 in mesa like form. The contact layer 150 of each of the light-emitting elements 129 has an opening 151 therethrough to provide a light-emitting area 130 for each light-emitting element 129. An electrode 132 is on the contact layer 150 of each light-emitting element 129 and an electrode 154 is on the surface 143 of the substrate 140. The light-emitting layer 146 of each light-emitting element 129 is of an area no greater than the opening 151 in its respective contact layer 150. A passivation layer 158 is around each of the light-emitting layers 146 and an antireflective layer 152 of SiN x  is over the light-emitting area 130 of the second cladding layer 148. 
     The light-emitting diode array 128 is made by depositing on the surface 141 of the substrate 140 in succession by MOCVD the buffer layer 142, the first cladding layer 144, the light-emitting layer 146, the second cladding layer 148 and the contact layer 150. Plasma CVD is then used to form an SiN x  layer 153 over the whole surface and photolithography and plasma etching are used to remove the SiN x  layer around the intended locations of the mesa formation that includes the light-emitting elements 129. H 2  SO 4  :H 2  O 2  :H 2  O=1:8:16 chemical etching is then used to remove the portions around the intended mesa formations down as far as the first cladding layer 144 to thereby form the light-emitting mesas 129. 
     The surface SiN x  layer is removed with a weak solution of hydrofluoric acid and plasma CVD is then used to form an SiN x  insulation layer 153 over the whole surface. Photolithography and plasma etching are then used again to remove the SiN x  layer from the light-emitting mesa 129. Electrodes 132 and 154 are then formed by vapor deposition. The unnecessary portions of the electrode 132 are then removed by photolithography and plasma etching. NH 4  OH:H 2  O 2  =1:10 chemical etching is then used to remove the contact layer 150 except for the portion under the electrode 132. The antireflection SiN x  layer 152 is then formed by plasma CVD and the electrodes 132 and 154 are formed by alloying. 
     Photolithography and plasma etching are used to remove the SiN x  antireflection layer around the mesa, and NH 4  OH:H 2  O 2  =1:10 chemical etching is then used to remove the light-emitting layer 146 in the way indicated by the arrows E in FIG. 6 until the diameter of the light-emitting layer 146 (shown in FIG. 6) is no greater than, and preferably smaller than, the internal diameter of the electrode 132. Finally, polymide and oxygen reactive-ion etching is used to form a passivation layer 158 on the side wall of the light-emitting layer 146. 
     In the light-emitting diode array 128, optical output is improved because, as shown in FIG. 6, the light-emitting portion 130 is not covered by the electrode 132. Also, with reference to FIG. 6, forming the light-emitting layer 146 so that in cross-section it is in the shape of an inverted mesa means that light from adjacent elements is reflected downwards by the mesa side faces, and with all light emerging horizontally from the interior being reflected, crosstalk between adjacent elements is suppressed. 
     Thus, there is provided by the present invention a light-emitting diode array which provides for greater output of the light generated by each light-emitting element. Also, crosstalk between adjacent light-emitting elements is suppressed. 
     It is to be appreciated and understood that the specific embodiments of the invention are merely illustrative of the general principles of the invention. Various modifications may be made consistent with the principles set forth. For example, although the light-emitting diode arrays have been described with reference to specific thicknesses and compositions of AlGaAs and GaAs, it is to be understood that the invention is not limited thereto but includes the use of semiconductor substances having different compositions and etched to different thicknesses. Still further, although MOCVD has been described as being used to deposit the various layers of the arrays, other epitaxial processes may be used such as GSMBE, MBE, CBE and LPCVD.