WO2001073859A1

WO2001073859A1 - Enhanced-output light emitting diode and method of making the same

Info

Publication number: WO2001073859A1
Application number: PCT/US2001/004576
Authority: WO
Inventors: Jun Li; Yu-Hwa Lo; Zuhua Zhu
Original assignee: Nova Crystals, Inc.
Priority date: 2000-03-24
Filing date: 2001-02-13
Publication date: 2001-10-04

Abstract

A light-emitting diode (LED) with on-chip substantially flat-topped dome-shaped structures and a method to make the same. The substantially flat-topped dome-shaped structures enable light generated within the semiconductor to be efficiently coupled to the outside world, resulting in a significant performance improvement over conventional LED structures. The disclosed method of fabrication is easy to implement and well suited for manufacturing.

Description

ENHANCED-OUTPUT LIGHT EMITTING DIODE AND METHOD OF MAKING THE SAME

Field of the Invention

The invention pertains to the field of light emitting diodes More particularly, the invention pertains to enhancing the output efficiency of light emitting diodes

Background of the Invention Light-emitting diodes (LEDs) are known as a means for generating an optical signal. High- brightness visible LEDs have found wide-ranging applications in displays, traffic lights, signs, automobile rear lights and many other illuminating devices. An LED generates an optical signal when voltage is applied to it. The voltage causes electrons and holes to be injected into hght- emissive material. The electrons and holes recombme m the light-emissive material in a process known as radiative recombination. This radiative recombination releases energy, which results in the emission of light. Today, indium aluminum gallium phosphide (InAlGaP) quantum wells produce the highest brightness red, orange, and yellow LEDs and indium gallium nitride (InGaN) quantum wells produce the brightest green and blue LEDs. White LEDs have been commercialized by using one or a combination of these mateπals. Besides selecting the πght matenals for specific colors, two major LED performance concerns are enhancement of internal quantum efficiency, which refers to the ratio of the carrier injection rate and the photon generation rate, and enhancement of external light coupling efficiency, defined as the ratio of the optical power emitted to the outside world and the optical power generated internally. If all other non-radiative earner recombination mechanisms are negligible or do not compete with the light-generating recombination, and if carrier leakage is negligible, internal quantum efficiency can approach 100 percent. State-of-the-art InAlGaP quantum wells can achieve nearly this result. In such a case, enhancing bπghtness depends upon the ability to boost the LED's external light coupling efficiency.

One of the best opportunities for boosting the external light coupling efficiency is through efficient side emissions. Common prior-art approaches to achieving this objective include using transparent substrates, growing thick window layers, and using mesa structures. The fundamental premise behind these approaches is that the side emission light beams withm the light cones defined by Snell's Law can have no or minimum intersections with any light absorptive mateπal before leaving the LED die. This technique is exemplified by the invention disclosed by Kish, et al., U.S. Patent No. 5,502,316, which uses a thick window layer (approximately 50 μm) to enhance external light coupling efficiency by extracting additional light from the edge Referring to Fig. 1, a conventional LED diode mesa 400 includes an absorptive substrate 100, an active region for light generation 200 and a top flat surface 500 Light is coupled out from the edge of an LED with these structures. Assuming the edge 300 of the mesa 400 is vertical, only the light beams withm a half cone 600 of 0 to approximately +17 degrees, as defined by Snell's Law, escape from the LED's edge. The beams within the corresponding half cone 601 of 0 to approximately -17 degrees are absorbed by the substrate 100 before escaping. For Lambertian sources (sources where the radiance is the same in all directions from the surface) such as surface- emitting LEDs, the intensity of the light exiting the LED is proportional to the cosme of the angle of incidence. Due to this cosme distribution, when the sidewalls 300 are vertical the power generated by the upper half-cone 600 from the edge 300 is only about 40 percent of the power generated by the emission from the top surface. This results m low output efficiency from the sides of the device.

The use of a rough top surface is one technique known in the art for reducing the reflectance from the top of the device. Especially when used in combination with a bottom reflector, such roughing of the top surface increases output because light beams that would otherwise be reflected are scattered out instead Attempts have not been made, however, to create roughness on the sidewalls only. This may be because, m the pπor art, creating roughness on vertical sidewalls requires additional process steps, and such vertical-sidewall roughness is a less effective means of enhancing output than top-surface roughness.

Due to the above-descπbed limitations of LED structures with vertical sidewalls, it has been suggested that substantially hemispherical-shaped LED structures might enhance output efficiency.

This idea, which was proposed at the early stages of LED development (W. N. Carr and G. E. Pittman, "One-Watt GaAs p-n Junction Infrared Source," 3 Applied Physics Letters No. 10, pp 173-175 (1963)), has been widely applied in LED packaging in the form of a substantially hemispheπcal-shaped epoxy lens covering the LED structure. Until now, however, there have been two inherent weaknesses in pπor-art substantially hemispherical-shaped LED technology. First, there has been no practical way to implement hemispheπcal structures at the LED semiconductor chip level. Second, the nature of the hemispheπcal shape requires that, for best results, the current be confined to a small area of the emissive mateπal in the central region under the hemisphere, as discussed below Referπng to Fig 2A, light beams 105 emanating upward from the central region 110 of the emissive mateπal 200 encounter the edge 115 of the substantially hemispherical structure 120 at an angle of incidence of approximately zero The light beams 105 thus escape the structure 120 no matter what their direction, as they are not subject to internal reflection. Referring to Fig. 2B, any portion 111 of the emissive material 200 substantially away from the central region 110 is directly under a significantly-sloping portion 119 of the substantially hemispherical structure 120 A light beam 118 emanating vertically upward from such area 111 of the emissive mateπal 200 encounters the portion 119 of the substantially hemispheπcal structure 120 immediately above it and is subject to total internal reflection because of the high angle of incidence. Referπng to Fig. 2C, the only light beams emitted from an area 111 of the emissive mateπal 200 substantially away from the central region 110 that can escape the substantially hemispherical structure 120 are those contained withm the horizontal Snell's Law cones 113 These horizontal cones 113 are similar to those 116 (Fig. 2D) existing in a conventional rectangular structure 121. In the substantially hemispheπcal structure 120, the horizontal cones 113 are slightly wider than those 116 existing in a rectangular structure 121, owing to the curvature surface 115. Hence, the substantially hemispheπcal structure

120 allows slightly more light to escape through the sides 103 than does the conventional rectangular structure 121.

The conventional rectangular structure 121, however, possesses a significant countervailing advantage over a substantially hemispheπcal structure 120. In particular, referπng to Fig. 2D, light emanating from a portion 112 of the emissive mateπal 200 significantly away from the central region 109 can escape through the top 108 of the structure 121 or through either of the sides 106; m the substantially hemispheπcal structure 120 (Fig. 2C), by contrast, such light can only escape through the sides 103. Due to the cosme distribution, the vertical cone 118 is more intense than the hoπzontal cones 113, 116. Hence, the advantage that a rectangular structure 121 possesses in this regard compensates for its disadvantage relative to side emissive light when compared to the substantially hemispherical structure 120 Accordingly, substantially hemispheπcal structures according to the prior art are not significantly advantageous over conventional rectangular structures.

Referring to Fig. 3, a conventional rectangular-shaped LED mesa structure 400 includes an active region 200 and a flat top surface 500 above an absorptive substrate 100. Light within the vertical cone 301 defined by Snell's Law having its center axis 801 perpendicular to the flat top surface 500 of the mesa 400 and delimited by beams 603, 604 escapes through the top 500 of the mesa 400. The center axis 802 of the horizontal half-cone 302 that can be coupled out from the side of the structure is perpendicular to the normal axis 801 of the active layer 200.

Summary of the Invention

The present invention discloses a light-emitting diode (LED) with on-chip substantially flat- topped dome-shaped structures and a method to make the same. The substantially flat-topped dome-shaped structures enable light generated withm the semiconductor to be efficiently coupled to the outside world, resulting in a significant performance improvement over conventional LED structures. The disclosed method of fabπcation is easy to implement and well suited for manufacturing.

An LED according to the invention includes an active region for light generation, first and second contacts for applying voltage across the active region, a substrate and a substantially flat- topped dome-shaped window layer that is transparent to the generated light. A reason for the invention's advantage over conventional LEDs with rectangular on-chip structures is that light can be efficiently coupled out of the sloping sidewalls of the substantially flat-topped dome structure. A reason for the invention's advantage over substantially hemispheπcal shaped structures suggested in the pπor art is that light emitted from a large "sheet" area of the emissive mateπal can be efficiently coupled out of both the top and the sides of the substantially flat-topped dome-shaped structure. Prototype LED devices according to the invention showed a performance improvement of up to 80 percent over conventional LEDs.

The disclosed method of fabrication entails applying a thin layer of photo resist mateπal and employing photolithography, as with conventional LED chips. The cπtical step, however, is heating the photo resist at a temperature that causes it to reflow and form into a dome shape. The subsequent etching and photo resist removal steps leave the LED chip structures shaped m the manner of a dome with a substantially flat top. This process is easy to implement and well suited for mass production.

Brief Description of the Drawings

Fig. 1 shows a simplified diagram of a conventional LED according to the prior art with an essentially rectangular, flat-top on-chip structure.

Fig. 2A shows an LED according to the pπor art, with a diagram of the light being emitted upward from a central region of the emissive material.

Fig. 2B shows an LED according to the pnor art, with a diagram of the light emanating vertically upward from an area of the emissive material substantially away from the central region.

Fig. 2C shows an LED according to the pπor art with a diagram of the light emitted from an area of the emissive material substantially away from the central region. Fig. 2D shows an LED having a rectangular configuration according to the pπor art, with a diagram of the light emanating upward from an area of the emissive mateπal substantially away from the central region.

Fig. 3 shows an LED according to the pπor art, with a diagram of the light emitted together with light-emission cones as defined by Snell's Law Fig. 4A shows an LED according to the present invention, having a substantially flat-topped dome-shaped mesa on a substrate.

Fig. 4B shows the LED of figure 4A, with a diagram of the light emitted, together with light-emission cones as defined by Snell's Law. Fig. 5 shows the LED of figure 4A, with a diagram of light emitted at several points.

Fig. 6 shows the LED of figure 4A in an embodiment having rough sloped sidewalls.

Fig. 7 shows a flowchart of a method of making an LED with a substantially flat-topped dome-shaped structure according to the present invention.

Figs 8A through 8D show an LED wafer with absorptive substrate and active layer, at vanous stages duπng the method of fig. 7.

Fig. 9 shows a top view of a portion of an LED according to the present invention.

Descπption of the Preferred Embodiment

This invention discloses a light emitting diode (LED) device with on-chip substantially flat- topped dome-shaped structures to enhance output efficiency and a method of making the same The novel device enables light generated within the semiconductor to be efficiently coupled to the outside world, resulting in a significant performance improvement over conventional LED devices. The invented method is easy to implement and well suited for manufacturing. LEDs made according to the invention have substantially flat-topped dome-shaped on-chip structures that bend side emission beams satisfying Snell's Law upwards with respect to the hoπzon, and also allow portion of the bottom half of the side beam cone to be coupled out. Therefore, the invention significantly improves LED output efficiency over the prior art.

Referπng to Fig. 4A, an LED device according to the present invention includes at least one substantially flat-topped dome-shaped mesa 450 on a substrate 150. The mesa 450 is preferably less than 100 μm m diameter. This diameter is substantially less than the typical, approximately 250- μm, diameter of LEDs according to the pπor art. The mesa includes an active region 200 and a transparent window having a body with a lower surface adjacent to the active region 200 and a substantially flat upper surface 550. The upper surface is smaller than the lower surface (smaller in diameter, if the body is round, or smaller m linear dimension, if it is rectangular or square or the hke), so that the sides of the body form sloped (I e., non-vertical) sidewalls 355.

The mesa 450 thus has a substantially flat-topped dome shape and does not contain any vertical edges For purposes of the present invention, it is preferable that the substantially flat- topped dome-shaped mesa have an x/y ratio of 2.0 or less, where x is the thickness (height) 94 of the window body 449 and y is the hoπzontal span 95 of the sloping sidewall 355. Expressed another way, it is preferable that the ratio between the adjacent sides of a right triangle having the ends of the hypotenuse at the points where the side wall intersects the upper and lower sides, with the side adjacent the active layer being the denominator, be 2.0 or less.

Referring to Fig. 4B, similar to the conventional structure shown in fig. 3, light within the vertical cone 351 defined by Snell's Law having a center axis 851 perpendicular to the flat top surface 550 of the mesa 450 and delimited by beams 653, 654 escapes through the top 550 of the mesa 450 The side-emission cone 352, however, is delimited by beams 655, 656 above the active layer and has a center axis 852 that forms an angle of less than 90° with the center axis 851 of the vertical cone 351 Unlike with conventional structures, this cone 352 is slanted upward rather than being horizontal, thus allowing light beams from the bottom half-cone 353 to escape the side surface 355 of the mesa 450.

Additionally, because of the sloping sidewalls 355, there are actually an infinite number of such cones that blend together to form an overall side-extraction cone that is substantially wider than the horizontal half-cone of light 302 extracted out of the side of the conventional rectangular structure depicted in Fig. 3. Additionally, pursuant to Lamber's law, the light beams in the slopmg- sidewall's side-extraction cone 355 have a higher intensity than those in the vertical-sidewall 's upper-half cone 302. The structure according to the present invention, therefore, significantly outperforms conventional LEDs having vertical sidewalls. An additional advantage of the substantially flat-topped dome-shaped mesa is that there is less light blockage between two light generating mesas positioned side-by-side than with conventional structures. This is because the edge emits light towards the top rather than horizontally.

Referring to Fig. 5, if the current is confined to the region 751 under the flat surface 550, the edge portions 752 of the emissive material 200 under the sloped sidewalls 355 do not contribute any light. If, however, the electπc current uniformly spreads throughout the entire mesa 450, some light 660 is generated by the regions 752 underneath the sidewalls 355. This light 660 emanates upwards and does not exit the top surface 550. Rather, it is internally reflected and ultimately absorbed by the absorptive substrate 150, thus resulting in a loss of bπghtness.

Creating rough sidewall surfaces, as shown in Fig. 6 where the sloped sidewalls 355 are formed with rough surfaces 651, alleviates this loss to some degree. The light 661 underneath the sidewalls 355 has a chance to be scattered out due to the sidewalls' roughness

To avoid these losses completely, however, the current must be confined to the portion 751 of the light-emissive mateπal 200 directly underneath the flat top 550 so the regions 752 directly under the sloped sidewalls 355 do not produce any light. Such confinement of the current ensures a side emission 658 that is substantially supeπor to that associated with existing structures, without reducing the top emission 657 which, due to the cosine distribution, is the most intense.

Further, as surface carrier recombination is most serious m the regions 752 near the boundary of the mesa 450, suppressing surface earner recombination is a significant benefit of the confinement of the current. Surface earner recombination not only reduces the internal quantum efficiency but also adversely affects the reliability of the device, particularly in an environment charactenzed by high temperature and high humidity. Indeed, it is well worth the effort of using current confinement solely to suppress surface recombination.

Referring to Fig 7, a process of making substantially flat-topped dome-shaped LED on-chip structures includes several steps. Assuming that the metal contacts and the wire bonding pads on the wafer have already been defined, m the first step 1, photo resist is applied to an LED wafer Preferably, the photo resist which is used is OCG 897 21ι from ARCH Chemicals of Norwalk, Connecticut. Preferable conditions for the photo resist application are: the photo resist is spun on the wafer at 4000 rpm for 30 seconds, and then pre-baked at 90°C for 60 seconds. The resulting photo resist thickness is approximately 2.3 μm. Next 2, photolithography is employed to define the photo resist to rectangular or circular patterns. In the next step 3, the photo resist is heated such that it reflows, i.e., its viscosity changes. Preferably, the photo resist is heated at 130°C for 10 minutes; if this is done, the height of the dome-shaped photo resist pattern is approximately 3.4 μm after the reflow. Next 4, dry mesa etching is utilized to transfer the rectangular or circular photo resist pattern onto the wafer as ordinary (rounded) dome shapes. Preferably, an ECR (Electron Cyclotron

Resonance) etcher with C12 and/or BC13 as the etching gases is used for the mesa etching for approximately 18 minutes at an etching rate of 0.7 μm/mm. Finally 5, the photo resist is removed. Preferably, photo resist removal is accomplished using 0₂ plasma and acetone soaking The resulting window layers 6 each have a thickness of approximately 10 μm and a substantially flat- topped dome-shaped top surface. LEDs produced according to the above process show up to 80% improvement m bπghtness over LEDs created according to conventional processes.

Referπng to Fig. 8A, the above-described process of making substantially flat-topped dome-shaped LED on-chip structures begins with an LED wafer 9 with a substrate 10, an active layer 20 and a window layer 30. After the step of employing photolithography (Fig. 7, step 2), the photo resist pattern 80 consists of rectangular shapes each having a width denominated as d 96 and a height denominated as h 97. A P metal contact 84 and an N metal contact on the back of the wafer (not shown) are attached to each LED die. A portion 83 of the photo resist extrudes due to the P metal contact 84. Preferably, the LED is composed with a bonding pad (not shown) and a number of light-generating satellites or dome-shaped mesas (see Fig. 9). The overall size or diameter of the satellites plus the bonding pad is approximately 250 μm, or about the same as a conventional LED. The bonding pad is for wire bonding so that current or voltage can be applied.

Referring to Fig 8B, after being patterned and then heated (Fig. 7, step 3), the photo resist

81 reflows and changes to ordinary (rounded) dome shapes each having a width of d 96 and a height of h] 98. If the photo resist 81 is burned, it is difficult to remove after dry etching. Hence, to avoid such burning, the photo resist 81 should be heated to a temperature withm a range of approximately 120°C - 200°C. Temperatures in this range cause the photo resist 81 to reflow without burning. The ideal temperature is the lowest temperature at which the photo resist 81 reflows. The photo resist 81 must be maintained at such temperature for approximately 5 to 10 minutes in order to assume its equilibrium shape.

The final shape after reflow depends upon the size (e.g., diameter) and thickness of the patterned photo resist. For example, if 2.3-μm thick photo resist is patterned to a size of 100 μm, the final shape after reflow is not that of a dome but that of two humps. If the same photo resist is patterned to a size of 50 μm, however, it is dome-shaped after reflow. The minimum ratio of thickness to pattern size which results in a dome-shaped final pattern depends upon the particular photo resist being used. Expeπmentation reveals that, for OCG897 21ι pattern of diameter 40 to 50 μm, reflow occurs at approximately 130°C; if the thickness of the photo resist, prior to heating, is greater than 1.7 μm, the pattern changes its shape from a rectangle to a dome for approximately 10 minutes. After that point, equilibrium is reached and the pattern retains its shape. Referring to Fig. 8C, after the step of dry mesa etching (Fig. 7, step 4), the mesa structures

31 are ordinary (rounded) dome-shaped. The top portion of each mesa structure 31 is covered by a residual layer of photo resist 81.

Referπng to Fig. 8D, after the step of photo resist removal (Fig. 7, step 5), the top surface

82 of each mesa 31 is substantially flat. The precise shape of the mesa 31 produced by this method depends upon the thickness of the photo resist 80 and the etching conditions With the wide range of choices for photo resists and etching conditions, it is easy to control the process to meet specific requirements for the values of d 96 and h 97.

The dry etching step 4 of the above-descπbed process often introduces roughness on the sidewalls of the mesas. As noted above, a light scattering effect from such sidewall roughness helps couple more light out from the semiconductor than if the sidewalls were smooth. Referπng to Fig

6, a roughing surface 651 usually results in reflectance from normal incidence of less than 20 percent, as compared to approximately 30 percent reflectance from a polished semiconductor surface. This is because when the sidewall surface 651 is rough, the light beam 661 is scattered out instead of being totally reflected. In combination with a bottom reflector, the rough surfaces are even more effective because some of the light beams take multiple bounces and eventually escape The use of rough sidewalls and bottom reflectors is especially helpful in the context of the present invention as more light has a chance to be scattered out. The reason is that more light has a chance to be scattered out because (a) the total rough area is greater with sloped sidewalls, and (b) the cosme law still applies even though Snell's Law does not.

Referring to Fig. 9, an LED according to the present invention preferably contains a metal bonding pad 900 approximately 120 μm in diameter, multiple dome-shaped light-generating satellites 901 each approximately 40 μm in diameter (only one is shown in Fig. 9), and metal bridges 902 connecting the bonding pad with the satellites. The metal bonding pad and bridges are insulated from the semiconductor they sit on by a thin layer of Sι02, so no current is allowed to flow into the areas covered by the metal bonding pad and bridges. The whole LED forms a 300μm- by-300μm square. This device provides 80% more light than a conventional 300μm-by-300μm LED fabricated on the same wafer and driven at the same current. Expeπments show that LEDs fabπcated using satellites with vertical sidewalls show only 40% improvement over conventional LED. The 80% improvement is only realized when the satellites are flat-topped-dome shaped, with diameters of 40-50 μm.

As previously noted, it is preferred that current confinement is introduced such that the regions 752 directly under the sloped sidewalls 355 do not produce any light. An approach that achieves complete current confinement includes use of a selective lateral oxidation process. In such approach, Al_xGaj _xAs layers with x > 90% (preferably x > 95%) are oxidized quickly at high temperatures (e.g., 400°C) in water vapor, and the oxidation rate increases rapidly as the Al concentration, x, increases from less than 90% to nearly 100%. When the AlGaAs layer is converted into AlOx by this wet oxidation process, it serves as a funnel to confine the current. In this case, a thm (< 700 Angstroms) Al_xGaι-_xAs layer with a well-defined x value must be added between the window layer and the active layer.

In the fabπcation process, a mesa structure is first etched to expose the AlGaAs layers. After leaving the sample at approximately 400°C m water vapor for several minutes, the AlGaAs layers are converted into AlOx layers from the sides of the mesa to form insulators. Those regions that are not oxidized remain to become high-quality AlGaAs layers that are transparent and current conductive. Expeπmentation shows that the lateral oxide can enter the mesa to a depth of 40 to 50 micrometers from each side before the oxidation process stops. This is a long enough distance to form structures having a typical mesa size of approximately 40 to 50 μm Experimentation shows that InAlGaP can also be oxidized laterally in a similar way to AlGaAs, although the process is slower and takes place at a higher temperature (typically around 550°C). This approach is more complex but can be accurately controlled to achieve the desired results.

Accordingly, it is to be understood that the embodiments of the invention herein descπbed are merely illustrative of the application of the pnnciples of the invention. Reference herein to details of the illustrated embodiments are not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention. It should be clear to those skilled in the art that further embodiments of the present invention may be made by those skilled in the art without departing from the teachings of the present invention.

What is claimed is:

Claims

1 A light emitting diode compπsmg- a) an active region for generating light from an electπc current; b) a window layer positioned above said active region, comprising a solid body transparent to the light generated by the active region, the solid body comprising:

(I) an inner side adjacent to the active region;

(n) an outer surface parallel to the inner side, the outer surface being smaller than the inner surface; (in) at least one side wall, the side wall sloping from the inner side to the outer surface; such that the body forms a domed shape, with the outer surface forming a flat top to the dome.

2. The light emitting diode of claim 1, in which the ratio between adjacent sides of a right triangle having the ends of the hypotenuse at the points where the side wall intersects the upper surface and lower sides, with the side of the tπangle adjacent the active region beinj the denominator, must be 2.0 or less.

3. The light emitting diode according to claim 1, wherein the side wall is rough

4. The light emitting diode according to claim 1 , further comprising a bottom reflector positioned underneath the active region.

5. A window layer for a light emitting diode having an active region for emitting light, comprising a solid transparent body comprising:

(l) an inner side adjacent to the active region;

(n) an outer surface parallel to the inner side, the outer surface being smaller than the inner surface; (in) at least one side wall, the side wall sloping from the inner side to the outer surface, such that the body forms a domed shape, with the outer surface forming a flat top to the dome

6. The window layer of claim 5, in which the ratio between adjacent sides of a right triangle having the ends of the hypotenuse at the points where the side wall intersects the upper surface and lower sides, with the side of the tπangle adjacent the active region being the denominator, must be 2 0 or less.

7. The window layer of claim 5, wherein the side wall is rough

8. A method of making a light emitting diode with a substantially flat-topped dome-shaped window comprising the steps of: a) providing an LED wafer with at least an active layer and a window layer; b) defining at least one LED die by applying a photo resist mateπal, wherein said photo resist mateπal is characteπzed by a pattern of shapes, thus yielding at least one LED die consisting of at least one light generating area; c) heating said photo resist material such that said pattern of shapes reflows and form into domes with a substantially round top surface with a top area of each shape covered by a residual portion of said photo resist material; and d) removing said residual portion of said photo resist mateπal.

9. The method according to claim 8, wherein the LED wafer in the step of providing further comprises the step of: adding a layer of ALGai _xAs having a thickness of less than or equal to 700 Angstroms between said window layer and said active layer, where x is a specified real number between 0 and 1.

10. The method according to claim 9, further comprising the step of laterally oxidizing said layer of Al_xGaι__xAs so as to introduce current confinement.

11. The method according to claim 8, in which the pattern comprises a shape selected from the group comprising rectangular, circular and oval.