LED that has bounding silicon-doped regions on either side of a strain release layer

Abstract

A strain release layer adjoining the active layer in a blue LED is bounded on the bottom by a first relatively-highly silicon-doped region and is also bounded on the top by a second relatively-highly silicon-doped region. The second relatively-highly silicon-doped region is a sublayer of the active layer of the LED. The first relatively-highly silicon-doped region is a sublayer of the N-type layer of the LED. The first relatively-highly silicon-doped region is also separated from the remainder of the N-type layer by an intervening sublayer that is only lightly doped with silicon. The silicon doping profile promotes current spreading and high output power (lumens/watt). The LED has a low reverse leakage current and a high ESD breakdown voltage. The strain release layer has a concentration of indium that is between 510.sup.19 atoms/cm.sup.3 and 510.sup.20 atoms/cm.sup.3, and the first and second relatively-highly silicon-doped regions have silicon concentrations that exceed 110.sup.18 atoms/cm.sup.3.

Claims

1. A light emitting device comprising: an n-side layer including a first n-type sublayer having a first silicon concentration; a strain release layer formed on the first n-type sublayer of the n-side layer and having a second silicon concentration; an active layer formed on the strain release layer and including a plurality of quantum well layers and a plurality of quantum barrier layers; an interface region between the strain release layer and the active layer, the interface region having a third silicon concentration; and a p-type layer formed on the active layer, wherein the first silicon concentration of the first n-type sublayer of the n-side layer is higher than the second silicon concentration of the strain release layer, the third silicon concentration of the interface region is higher than the second silicon concentration of the strain release layer, and the third silicon concentration of the interface region is higher than a silicon concentration of the plurality of quantum well layers and the plurality of quantum barrier layers.

2. The light emitting device of claim 1, wherein the n-side layer further includes a second n-type sublayer having a fourth silicon concentration and a third n-type sublayer having a fifth silicon concentration, the second n-type sublayer disposed between the first n-type sublayer and the third n-type sublayer, the first n-type sublayer disposed between the second n-type sublayer and the strain release layer, and the fourth silicon concentration is lower than the first silicon concentration and the fifth silicon concentration.

3. The light emitting device of claim 2, wherein the fifth silicon concentration is higher than the first silicon concentration.

4. The light emitting device of claim 3, wherein the fifth silicon concentration is higher than the first silicon concentration and the third silicon concentration.

5. The light emitting device of claim 2, wherein the second n-type sublayer of the n-side layer is more than fifty nanometers thick.

6. The light emitting device of claim 1, wherein the third silicon concentration of the interface region is higher than the first silicon concentration of the first n-type sublayer of the n-side layer.

7. The light emitting device of claim 1, wherein the third silicon concentration is higher than 110.sup.18 atoms/cm.sup.3, and the active layer has a silicon concentration lower than 110.sup.18 atoms/cm.sup.3.

8. The light emitting device of claim 1, wherein the strain release layer comprising a plurality of periods, each period comprising a first layer of an indium gallium nitride and a second layer of indium gallium nitride, the first layer of indium gallium nitride having a first indium concentration, the second layer of indium gallium nitride having a second indium concentration different than the first indium concentration.

9. The light emitting device of claim 8, wherein the first layer of each period of the strain release layer is less than five nanometers thick, and the second layer of each period of the strain release layer is less than five nanometers thick.

10. The light emitting device of claim 1, wherein the interface region includes a barrier layer.

11. The light emitting device of claim 10, wherein the barrier layer is a gallium nitride (GaN) layer.

12. The light emitting device of claim 1, wherein the interface region is formed directly on the strain release layer.

13. The light emitting device of claim 1, wherein the strain release layer is formed directly on the first n-type sublayer of the n-side layer.

14. The light emitting device of claim 1, wherein the interface region includes a peak of silicon concentration.

15. The light emitting device of claim 1, wherein the strain release layer includes a peak of silicon concentration.

16. A light emitting device comprising: an n-side layer including a first n-type sublayer having a first silicon concentration, a second n-type sublayer having a second silicon concentration lower than the first silicon concentration, and a third n-type sublayer having a third silicon concentration higher than the first silicon concentration, the second n-type sublayer disposed between the first n-type sublayer and the third n-type sublayer; a strain release layer formed on the first n-type sublayer and having a fourth silicon concentration, the first n-type sublayer disposed between the second n-type sublayer and the strain release layer; an active layer formed on the strain release layer and including a plurality of quantum well layers and a plurality of quantum barrier layers; an interface region between the strain release layer and the active layer, the interface region having a fifth silicon concentration; and a p-type layer formed on the active layer, wherein the first silicon concentration of the first n-type sublayer is higher than the fourth silicon concentration of the strain release layer, the fifth silicon concentration of the interface region is higher than the fourth silicon concentration of the strain release layer and the first silicon concentration of the first n-type sublayer, and the fifth silicon concentration of the interface region is higher than a silicon concentration of the plurality of quantum well layers and the plurality of quantum barrier layers.

17. The light emitting device of claim 16, wherein the third silicon concentration of the third n-type sublayer is higher than the fifth silicon concentration of the interface region.

18. The light emitting device of claim 16, wherein the second n-type sublayer of the n-side layer is more than fifty nanometers thick.

19. The light emitting device of claim 16, wherein the fifth silicon concentration is higher than 110.sup.18 atoms/cm.sup.3, and the active layer has a silicon concentration lower than 110.sup.18 atoms/cm.sup.3.

20. The light emitting device of claim 16, wherein the strain release layer comprising a plurality of periods, each period comprising a first layer of an indium gallium nitride and a second layer of indium gallium nitride, the first layer of indium gallium nitride having a first indium concentration, the second layer of indium gallium nitride having a second indium concentration different than the first indium concentration.

21. The light emitting device of claim 20, wherein the first layer of each period of the strain release layer is less than five nanometers thick, and the second layer of each period of the strain release layer is less than five nanometers thick.

22. The light emitting device of claim 16, wherein the interface region includes a barrier layer.

23. The light emitting device of claim 21, wherein the barrier layer is a gallium nitride (GaN) layer.

24. The light emitting device of claim 16, wherein the interface region is formed directly on the strain release layer.

25. The light emitting device of claim 16, wherein the strain release layer is formed directly on the first n-type sublayer of the n-side layer.

26. The light emitting device of claim 16, wherein the interface region includes a peak of silicon concentration.

27. The light emitting device of claim 16, wherein the strain release layer includes a peak of silicon concentration.

28. A light emitting device comprising: an n-side layer including a first n-type sublayer having a first silicon concentration; a strain release layer formed on the first n-type sublayer of the n-side layer and having a second silicon concentration; an active layer formed on the strain release layer and including a plurality of quantum well layers and a plurality of quantum barrier layers; and a p-type layer formed on the active layer, wherein a peak of silicon concentration is located at an interface region between the strain release layer and the active layer, the first silicon concentration of the first n-type sublayer is higher than the second silicon concentration of the strain release layer, the peak of silicon concentration located at the interface region is higher than the second silicon concentration of the strain release layer, and the peak of silicon concentration located at the interface region is higher than a silicon concentration of the plurality of quantum well layers and the plurality of quantum barrier layers.

29. The light emitting device of claim 28, wherein the n-side layer further includes a second n-type sublayer having a third silicon concentration and a third n-type sublayer having a fourth silicon concentration, the second n-type sublayer disposed between the first n-type sublayer and the third n-type sublayer, the first n-type sublayer disposed between the second n-type sublayer and the strain release layer, and the third silicon concentration is lower than the first silicon concentration and the fourth silicon concentration.

30. The light emitting device of claim 29, wherein the fourth silicon concentration is higher than the first silicon concentration.

31. The light emitting device of claim 30, wherein the fourth silicon concentration is higher than the first silicon concentration and the peak of silicon concentration located at the interface region.

32. The light emitting device of claim 29, wherein the second n-type sublayer of the n-side layer is more than fifty nanometers thick.

33. The light emitting device of claim 28, wherein the peak of silicon concentration located at the interface region is higher than the first silicon concentration of the first n-type sublayer of the n-side layer.

34. The light emitting device of claim 28, wherein the peak of silicon concentration located at the interface region is higher than 110.sup.18 atoms/cm.sup.3, and the active layer has a silicon concentration lower than 110.sup.18 atoms/cm.sup.3.

35. The light emitting device of claim 28, wherein the strain release layer comprising a plurality of periods, each period comprising a first layer of an indium gallium nitride and a second layer of indium gallium nitride, the first layer of indium gallium nitride having a first indium concentration, the second layer of indium gallium nitride having a second indium concentration different than the first indium concentration.

36. The light emitting device of claim 35, wherein the first layer of each period of the strain release layer is less than five nanometers thick, and the second layer of each period of the strain release layer is less than five nanometers thick.

37. The light emitting device of claim 28, wherein the strain release layer is formed directly on the first n-type sublayer of the n-side layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.

(2) FIG. 1 is a cross-sectional side view of blue Light Emitting Diode (LED) device in accordance with one novel aspect.

(3) FIG. 2 is a table that sets forth the thicknesses and constituent materials of the various layers of the LED device of FIG. 1.

(4) FIG. 3 is a diagram that sets forth a silicon doping profile through the various layers of the LED device of FIG. 1.

(5) FIG. 4 is a table that compares testing results of the LED structure of FIGS. 1-3 with testing results of other LED structures that do not have the same silicon doping profile but otherwise are similar to the LED structure of FIGS. 1-3.

(6) FIG. 5 is a chart of SIMS (Secondary Ion Mass Spectroscopy) concentration profile data for various constituent elements of an LED device.

(7) FIG. 6 is a flowchart of a method of manufacture in accordance with one novel aspect.

DETAILED DESCRIPTION

(8) Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.

(9) Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. In the description and claims below, when a first layer is referred to as being disposed over a second layer, it is to be understood that the first layer can be directly on the second layer, or an intervening layer or layers may be present between the first and second layers. The terms such as over, under, upper, lower, top, bottom, upward, downward, vertically, and laterally are used herein to describe relative orientations between different parts of the blue LED device being described, and it is to be understood that the overall blue LED device being described can actually be oriented in any way in three-dimensional space.

(10) FIG. 1 is a cross-sectional side view of blue light emitting diode (LED) device 1 in accordance with one aspect. LED device 1 includes a P-type layer 2, a Multiple Quantum Well (MQW) active layer 3, a strain release layer 4, and N-type layer 5, a buffer layer 6, and a substrate 7, stacked upon each other as illustrated. Substrate 7 may be sapphire or silicon. There may be additional intervening layers disposed between the buffer layer 6 and the N-type layer 5 such as a template layer of undoped GaN. A first electrode 8, which includes a metal portion 9 and a transparent conductor portion 10, makes electrical contact with the top of the P-layer 2. In one embodiment, the transparent conductor portion 10 is made of indium tin oxide (ITO). A second electrode 11 of metal makes electrical contact with the N-type layer 5.

(11) FIG. 2 is a table that sets forth more detail about the composition and thicknesses of the various layers of the LED device of FIG. 1.

(12) FIG. 3 is a diagram that sets forth a silicon doping profile through the various layers of the LED device 1 of FIG. 1. N-type layer 5 in one embodiment includes about five periods, where each period includes a relatively thick layer of n-GaN (for example, 900 nm thick) and a relatively thin layer of n-AlGaN:Si (for example, 25 nm thick). There is one of these relatively thick n-GaN layers at the bottom of the N-type layer 5, and there is one of these relatively thick n-GaN layers at the top of the N-type layer 5. The relatively thick n-GaN layer at the top of the N-type layer 5 includes a first sublayer NSL1, a second sublayer NSL2 and a third sublayer NSL3 as illustrated in FIG. 3. FIG. 3 shows that the third sublayer NSL3 is about thirty nanometers thick. The bottom of sublayer NSL1 is above the level 12 of a trench upon which n-electrode 11 is disposed. The silicon dopant concentration in NSL1 is higher than 110.sup.19 atoms/cm.sup.3 (for example, 210.sup.19 atoms/cm.sup.3). The silicon dopant concentration in NSL2 is lower than 110.sup.18 atoms/cm.sup.3 (for example, 210.sup.17 atoms/cm.sup.3). The silicon dopant concentration in NSL3 is higher than 110.sup.18 atoms/cm.sup.3 (for example, 310.sup.18 atoms/cm.sup.3).

(13) Strain release layer 4 in one example includes thirty periods, where each period includes a 2 nm thick layer of In.sub.xGa.sub.1-xN, 0<x<0.12, and where each period also includes a 2 nm thick layer of In.sub.yGa.sub.1-yN, 0<y<0.12, where xy. Strain release layer 4 is particularly important where substrate 7 is silicon as opposed to sapphire. The N-type layer 5 of n-GaN exhibits a high level of stress where substrate 7 is silicon because of the lattice mismatch between crystalline silicon and GaN. Strain release layer 4 helps to relieve some of the strain on the GaN and InGaN lattices. Strain release layer 4 is considered to involve a first portion SRLP1 and a second portion SRLP2. SRLP1 and SRLP2 are disposed with respect to one another as illustrated in FIG. 3. Second portion SRLP2 in one example is the thickness of three of the 4.0 nm thick In.sub.xGa.sub.1-xN/In.sub.yGa.sub.1-yN periods. In the example of FIG. 3, the silicon concentration in both portions SRLP1 and SRLP2 is below 510.sup.16 atoms/cm.sup.3 (for example, as close to zero atoms/cm.sup.3 as practical).

(14) Active layer 3 in one example includes ten periods, where each period includes a 3 nm thick quantum well layer of InGaN and a 10 nm thick quantum barrier layer of GaN. Active layer 3 is considered to involve a first sublayer ALSL1 and a second sublayer ALSL2. ALSL1 and ALSL2 are disposed with respect to one another as illustrated in FIG. 3. FIG. 3 shows that first sublayer ALSL1 is about twenty-six nanometers thick. First sublayer ALSL1 has a silicon concentration more than 110.sup.18 atoms/cm.sup.3 (for example, 510.sup.18 atoms/cm.sup.3), and the second sublayer ALSL2 has a silicon concentration less than 110.sup.18 atoms/cm.sup.3 (for example, 510.sup.18 atoms/cm.sup.3).

(15) P-type layer 2 is doped with magnesium to a concentration of more than 110.sup.19 atoms/cm3 (for example, 210.sup.19 atoms/cm.sup.3). The silicon concentration in the P-type layer is below 510.sup.16 atoms/cm.sup.3 (for example, as close to zero atoms/cm.sup.3 as practical). P-type layer 2 has a concentration of indium of approximately 410.sup.18 atoms/cm.sup.3.

(16) Inspection of line 13 of the silicon concentration profile in FIG. 3 reveals that two peaks 14 and 15 are present. Peak 14 corresponds to a bounding silicon-doped region (a layer having a silicon concentration greater than 110.sup.18 atoms/cm.sup.3) disposed directly underneath the strain release layer 4. The silicon-doped region is a first relatively-highly silicon-doped region that bounds the strain release layer. Peak 15 corresponds to a bounding silicon-doped region disposed directly on top of the strain release layer 4 (a layer having a silicon concentration greater than 110.sup.18 atoms/cm.sup.3). The silicon-doped region is a first relatively-highly silicon-doped region that bounds the strain release layer. The first and second relatively-highly silicon-doped regions (layers) sandwich the strain release layer from above and below and improve the performance of the LED device, at least in one specific embodiment.

(17) In addition, relatively highly silicon-doped sublayer NSL3 in combination with relatively lightly silicon-doped sublayer NSL2 improve current spreading, and thereby increase LED output power (lumens/watt). Sublayer NSL3 is relatively conductive as compared to sublayer NSL2, so there is an increased amount of lateral current flow in sublayer NSL3 as compared to an example of an LED in which sublayers NSL2 and NSL3 are of the same conductivity.

(18) FIG. 4 is a table that illustrates experimental results. Many LED device structures were fabricated, where the LED devices were substantially identical but for the LED devices having different silicon doping profiles in their active and strain release layers. Experiment #1 involved a strain release layer that was undoped with silicon. Experiment #2 involved a strain release layer that was uniformly doped throughout its thickness with a light concentration of silicon. Experiment #3 involved a LED where a thin portion of the active layer adjoining the strain release layer had a relatively high silicon concentration. This structure is referred to as having one spike at the interface between the active layer and the strain release layer. Experiment #4 involved a LED where a thin portion of the N-type layer adjoining the strain release layer had a relatively high silicon concentration. This structure is referred to as having one spike at the interface between the strain release layer and the N-type layer. Experiment #5 involved an LED device such as the LED device described above in connection with FIGS. 1-3, where a thin portion of the active layer adjoining the strain release layer had a relatively high silicon concentration, and where a thin portion of the N-type layer adjoining the strain release layer had a relatively high silicon concentration. Of all the samples tested, the LED device of experiment #5 had the lowest reverse leakage current (about 0.1 microamperes), the highest output power (140 lumens/watt), and the highest ESD breakdown voltage (about 4000 volts). The individual rows of the table of FIG. 4 do not list actual measured data for just one tested LED, but rather each row represents the noticed trend in the testing a many such LED devices.

(19) FIG. 5 is a chart of SIMS (Secondary Ion Mass Spectroscopy) concentration profile data for various constituent element of an LED device. Lines 100-104 indicate the concentrations of the elements silicon, indium, aluminum, magnesium, and hydrogen, respectively. Depth on the horizontal axis is measured relative to the upper surface of the P-type layer of the LED device. Although the indium concentration 101 in the noted region of the strain release layer appears to be quite uniform throughout the strain release layer, the actual indium concentration varies as set forth in the table of FIG. 2. The uniform appearance of line 101 in FIG. 6 in the region of the strain release layer is due to resolution limitations of the SIMS testing equipment.

(20) Peak 105 corresponds to sublayer NSL3 in the N-type layer. Peak 106 corresponds to sublayer ALSL1 in the active layer. Note that peak 106 is not entirely within the active layer, but rather peak 106 is seen straddling the interface between the active layer and the strain release layer. The SIMS data of FIG. 5 is for an LED structure where the second portion SRLP2 of the strain release layer is doped with silicon at a concentration of 510.sup.18 atoms/cm.sup.3 in the same way that sublayer ALSL1 of the active layer is doped. Accordingly, in some embodiments the bounding silicon-doped regions may extend into the strain release layer, from the top, from the bottom, or from the top and bottom, such that the first few (for example, three) periods of the strain release layer are doped with silicon at levels greater than 110.sup.18 atoms/cm.sup.3.

(21) The high concentration of indium in strain release layer 4 better relieves the strain on the GaN and InGaN lattices caused by the lattice mismatch with silicon. The lattice mismatch between GaN and crystalline silicon causes a large tensile strain on the GaN layer at the interface with the silicon. Indium is added to the GaN in order to form the larger lattice constant of InGaN. In the embodiment of LED device 1 represented by the concentrations of FIG. 5, the indium concentration throughout strain release layer 4 is relatively constant. In other embodiments, the indium concentration in strain release layer 4 begins lower after sublayer NSL3 and gradually increases towards sublayer ALSL1.

(22) In embodiments where the indium concentration is maintained relatively constant throughout strain release layer 4, the optimum range of the indium concentration is between 510.sup.19 atoms/cm.sup.3 and 510.sup.20 atoms/cm.sup.3. The concentration of indium in strain release layer 4 in the embodiment of FIG. 5 remains constant at about and 410.sup.20 atoms/cm.sup.3. Where the indium concentration in strain release layer 4 exceeded 510.sup.20 atoms/cm.sup.3, the performance of active layer 3 deteriorated. The optimum performance for LED device 1 over crystalline silicon was achieved with the aforementioned range of indium concentration in strain release layer 4 bounded by two regions NSL3 and ALSL1 of high silicon concentration, wherein the silicon dopant concentration in strain release layer 4 was below 510.sup.16 atoms/cm.sup.3. The silicon dopant concentration of sublayer NSL3 was higher than 110.sup.18 atoms/cm.sup.3, and the silicon dopant concentration of first sublayer ALSL1 was more than 110.sup.18 atoms/cm.sup.3.

(23) FIG. 6 is a flowchart of a method of manufacture 200 in accordance with one novel aspect. A relatively-lightly silicon-doped region of GaN is formed (step 201). In one example, this region is sublayer NSL2 of the N-type layer 5 of FIGS. 1-3. A first relatively-highly silicon-doped region of GaN is formed (step 202) on the relatively-lightly silicon-doped region. In one example, this region is sublayer NSL3 of the N-type layer 5 of FIGS. 1-3. A strain release layer is formed (step 203) on the first relatively-highly silicon-doped region. In one example, this strain release layer is strain release layer 4 of FIGS. 1-3 and it has a silicon concentration less than 510.sup.16 atoms/cm.sup.3. Strain release layer 4 has an indium concentration between 510.sup.19 atoms/cm.sup.3 and 510.sup.20 atoms/cm.sup.3.

(24) A second relatively-highly silicon-doped region is then formed (step 204) on the strain release layer. In one example, the second relatively-highly silicon-doped region is sublayer ALSL1 of the active layer 3 of FIGS. 1-3. The relatively-lightly silicon-doped region formed in step 201 and the relatively-highly silicon-doped region formed in step 202 are parts of an N-type layer of an LED. The second relatively-highly silicon-doped region is a sublayer of an active layer of the LED. The various layers of the epitaxial stack of the LED device of FIG. 1 are formed by conventional MOCVD (Metal-Organic Chemical Vapor Deposition) processes using conventional source gases and techniques used in the manufacture of GaN LEDs.

(25) Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.