Method of manufacturing light emitting diodes and light emitting diode

Abstract

In an embodiment a light emitting diode includes an n-type n-layer, a p-type p-layer and an intermediate active zone configured to generate ultraviolet radiation, a p-type semiconductor contact layer having a varying thickness and a plurality of thickness maxima directly located on the p-layer and an ohmic-conductive electrode layer directly located on the semiconductor contact layer, wherein the n-layer and the active zone are each of AlGaN and the p-layer is of AlGaN or InGaN, wherein the semiconductor contact layer is a highly doped GaN layer, and wherein the thickness maxima have an area concentration of at least 10.sup.4 cm.sup.−2.

Claims

1. A light emitting diode comprising: an n-type n-layer, a p-type p-layer and an intermediate active zone configured to generate ultraviolet radiation; a p-type semiconductor contact layer having a varying thickness and a plurality of thickness maxima directly located on the p-layer; and an ohmic-conductive electrode layer directly located on the semiconductor contact layer, wherein the n-layer and the active zone are each of AlGaN and the p-layer is of AlGaN or InGaN, wherein the semiconductor contact layer is a doped GaN layer and has a doping concentration of at least 10.sup.19 cm.sup.−3, wherein the electrode layer comprises a side facing the semiconductor contact layer, the side being planar, wherein the thickness maxima have an area concentration of at least 10.sup.4 cm.sup.−2, wherein the thickness maxima are formed by V-defects filled with a material of the semiconductor contact layer, and wherein the V-defects have an opening angle between 30° and 90°, inclusive.

2. The light emitting diode according to claim 1, wherein the semiconductor contact layer comprises contact islands.

3. The light emitting diode according to claim 2, further comprising a masking layer arranged on a side of the p-layer facing away from the active zone, wherein the masking layer only partially covers the p-layer and has a plurality of openings, wherein the semiconductor contact layer is arranged in the openings of the masking layer, and wherein the contact islands forming the semiconductor contact layer directly adjoin the p-layer in the openings of the masking layer.

4. The light emitting diode according to claim 2, further comprising an intermediate layer arranged between the contact islands, which have an Mg dopant concentration between 10.sup.19 cm.sup.−3 and 10.sup.23 cm.sup.−3, inclusive, wherein the intermediate layer directly adjoins the p-layer and terminates flush with the contact islands on a side of the intermediate layer that faces the electrode layer.

5. The light emitting diode according to claim 4, wherein the intermediate layer and the contact islands directly adjoin the electrode layer.

6. The light emitting diode according to claim 5, wherein the intermediate layer and the contact islands terminate flush at a side facing the electrode layer.

7. The light emitting diode according to claim 1, wherein a side of the active zone facing away from the p-layer are planar.

8. The light emitting diode according to claim 1, wherein the semiconductor contact layer is formed by a plurality of contact islands and is not a continuous layer.

9. The light emitting diode according to claim 8, wherein a degree of coverage of the p-layer by the semiconductor contact layer is between 0.5% and 10%, inclusive, and wherein adjacent thickness maxima, which are equal to the contact islands, have, seen in top view, an average distance from one another between 1 μm and 30 μm, inclusive.

10. The light emitting diode according to claim 1, wherein the p-layer comprises an opening layer in or at which the defects open to the V-defects, and wherein the opening layer is of AlInGaN or InGaN and contains indium and all remaining regions of the p-layer are of AlGaN.

11. The light emitting diode according to claim 10, wherein the opening layer is located on a side of the p-layer facing the active zone and a distance between the active zone and the opening layer is at most 30 nm.

12. The light emitting diode according to claim 11, wherein exactly one contact island is present per opening of a masking layer and adjacent contact islands are not interconnected by the material of the semiconductor contact layer itself so that the masking layer is only partially covered by the material of the semiconductor contact layer.

13. The light emitting diode according to claim 1, wherein the electrode layer comprises a transparent conductive oxide directly on the semiconductor contact layer, or the electrode layer consists of at least one transparent conductive oxide.

14. The light emitting diode according to claim 1, wherein the ultraviolet radiation has a wavelength of maximum intensity between 205 nm and 260 nm, inclusive.

15. A light emitting diode comprising: an n-type n-layer, a p-type p-layer and an intermediate active zone configured to generate ultraviolet radiation; a p-type semiconductor contact layer having a varying thickness and a plurality of thickness maxima directly located on the p-layer, wherein the contact layer comprises contact islands; an intermediate layer arranged between the contact islands; and an ohmic-conductive electrode layer directly located on the semiconductor contact layer, wherein the n-layer and the active zone are each of AlGaN and the p-layer is of AlGaN or InGaN, wherein the semiconductor contact layer is a doped GaN layer and has a doping concentration of at least 10.sup.19 cm.sup.−3, wherein the intermediate layer is of undoped AlGaN, and wherein the thickness maxima have an area concentration of at least 10.sup.4 cm.sup.−2.

16. The light emitting diode according to claim 15, wherein the intermediate layer and the contact islands directly adjoin the electrode layer.

17. The light emitting diode according to claim 16, wherein the intermediate layer and the contact islands terminate flush at a side facing the electrode layer.

18. The light emitting diode according to claim 15, wherein a side of the active zone facing away from the p-layer are planar.

19. The light emitting diode according to claim 15, wherein the electrode layer comprises a transparent conductive oxide directly on the semiconductor contact layer, or the electrode layer consists of at least one transparent conductive oxide.

20. The light emitting diode according to claim 15, wherein the ultraviolet radiation has a wavelength of maximum intensity between 205 nm and 260 nm, inclusive.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, a method described here and a light emitting diode described here are explained in more detail with reference to the drawing on the basis of exemplary embodiments. Same reference signs indicate the same elements in the individual figures. However, there are no references to scale shown, rather individual elements may be exaggeratedly large for a better understanding.

(2) In the Figures:

(3) FIGS. 1A and 1B show schematic sections of variations of light emitting diodes;

(4) FIGS. 2A, 3A, 5A, 5B, 6A, 7A and 7C show schematic sectional views of exemplary embodiments of method steps of a method;

(5) FIGS. 2B, 3B, 6B and 7B show schematic top views of exemplary embodiments of method steps of a method; and

(6) FIG. 4 shows a schematic cross-sectional view of an exemplary embodiment of a light emitting diode.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(7) In FIG. 1 a modification of a light emitting diode is shown. The light emitting diode is configured for emission of ultraviolet radiation R. On a growth substrate 51, the light emitting diode comprises a semiconductor layer sequence 2, see FIG. 1A. The semiconductor layer sequence 2 comprises a buffer layer 21, an n-conductive n-layer 22, an active zone 23, a p-conductive p-layer 24 and a continuous, areal p-conductive semiconductor contact layer 25. An ohmic-conductive electrode layer 3 is located on the semiconductor contact layer 25.

(8) Radiation R generated in the active zone 23 is usually reflected several times in the semiconductor layer sequence 2 before the radiation R is coupled out from a roughening 26, see FIG. 1B. Since the semiconductor contact layer 25 absorbs the radiation R, comparatively large absorption losses occur at the areal semiconductor contact layer 25. This reduces efficiency. A wavelength of the radiation R is below 360 nm, corresponding to the absorption edge of GaN.

(9) In FIGS. 2 and 3 the method steps for the production of light emitting diodes described here are illustrated, figure parts A showing a sectional view and figure parts B showing a schematic top view.

(10) According to FIG. 2A, a growth substrate 51 is used, such as aluminum nitride. Buffer layer 21, for example, is made of aluminum nitride or AlGaN and is optionally n-doped. The n-layer 22 is an n-doped AlGaN layer. The active zone 23 is based on AlGaN and contains non-drawn quantum well layers as well as few drawn barrier layers. The active zone 23 is followed by the p-layer 24 over the entire surface, which is made of p-doped AlGaN, with an aluminum content, for example, of between 10% and 90% inclusive.

(11) V-Defects 41 is opened at a border region of p-layer 24 to the active zone 23. Seen in cross-section the V-defects 41 are triangular, seen in top view they are hexagonal, see FIG. 2B. The V-defects 41 show in particular the shape of inverted, mostly regular hexagonal pyramids. In order to achieve opening of the V-defects 41 on previously existing, non-drawn dislocations, the p-layer 24 is particularly preferred grown at a combination of comparatively low temperatures, in particular T≤1000° C., comparatively high pressure, in particular p≥200 mbar, comparatively high hydrogen content in the gas phase and/or comparatively weakly doped with magnesium, for example, with a dopant concentration of ≤2×10.sup.19 cm.sup.−3. An opening angle of the V-defects 41, seen in cross-section, is approximately 60°.

(12) FIG. 3 illustrates that the V-defects 41 are filled with a material for the semiconductor contact layer 25, wherein reference numeral 4 denotes a thickness maxima. The semiconductor contact layer 25 is of p-GaN with a dopant concentration of preferably at least 10.sup.19 cm.sup.−3 to 10.sup.21 cm.sup.−3, whereby magnesium is used as the dopant. The semiconductor contact layer 25 is preferably deposited at a combination of comparatively high temperatures, in particular T≥900° C., and comparatively highly doped with magnesium, for example, with a dopant concentration of ≥2*10.sup.19 cm.sup.−3.

(13) The semiconductor contact layer 25 in the V-defects 41, for example, extends at least 50% or 75% and/or at most 90% or 95% through the p-layer 24. Deviating from the representation in FIGS. 2 and 3, the V-defects can also begin in the n-layer 22 or in the active zone 23 (not shown). A thickness of p-layer 24, for example, is, as is preferred in all other exemplary embodiments, at least 50 nm or 100 nm or 200 nm and/or at most 500 nm or 300 nm. Seen from above, the semiconductor contact layer 25 only covers a small part of the surface of the p-layer 24.

(14) Optionally, a sublayer 43 of the semiconductor contact layer 25 can be produced over the entire surface of the filled V-defects 41 and on the p-layer 24. This sublayer 43 forms an improved contact with the electrode layer 3, which is subsequently applied, e.g., of aluminum. The sublayer 43 has only a small thickness, for example, between 5 nm and 15 nm, so that the absorption of UV radiation in the sublayer 43 is weak.

(15) The method steps of FIGS. 2 and 3 are preferably carried out in a wafer composite. Steps such as applying a carrier substrate, detaching the growth substrate 51 or singulation into individual light emitting diodes, and applying electrical contact structures are each not shown to simplify the representation.

(16) In FIG. 4 an exemplary embodiment of the LED 10 is shown. The carrier substrate 52 is used for electrical contacting via electrode layer 3. Current flows dominantly from electrode layer 3 into semiconductor contact layer 25 and from this into p-layer 24 and over this into the active zone 23. Reference numeral 4 denotes a thickness maxima. The n-side 22, for example, can be electrically connected via an electrical contact surface 55, such as a bond pad. As in all other exemplary embodiments, the growth substrate 51 is optionally removed and a roughening 26 is provided to improve the light extraction.

(17) Due to the triangular cross-sectional area of the filled V-defects 41, a large ratio of the outer surface of the semiconductor contact layer 25 to its volume is realized. Thus, on the one hand an efficient current injection into the p-side 24 can be achieved, and on the other hand, due to the small volume, only little absorption of UV radiation takes place in the semiconductor contact layer 25. By exploiting defects such as dislocations and the targeted opening of the V-defects, small structures and small mean distances between adjacent thickness maxima 4 can be achieved. Since the V-defects 41 are completely filled, a smooth surface can be achieved on one side of the p-layer 24 and the semiconductor contact layer 25 facing away from the active zone 23, whereby an increased reflectivity can be achieved on the electrode layer 3, which is preferably a mirror.

(18) In FIG. 5, the semiconductor layer sequence 2 additionally has an opening layer 44. The opening layer 44 is, for example, made of AlInGaN, with a comparatively low indium content. This makes it possible to open the V-defects 41 in or at the opening layer 44 in a targeted manner.

(19) According to FIG. 5A, the opening layer 44 is located at the boundary between the active zone 23 and the p-layer 24, whereas according to FIG. 5B, the opening layer 44 is located within the p-layer 24, for example, with a distance to the active zone 23 of about 10 nm.

(20) In FIGS. 6 and 7 a further example of a manufacturing method for the production of light emitting diodes is shown. Deviating from FIG. 2, no V-defects are generated in the p-layer. The prevention of the opening of V-defects is achieved in particular by growing the p-layer 24 at comparatively high temperatures, especially at 1200° C.+/−50° C., in contrast to FIG. 2. A masking layer 45 is applied locally to the p-layer 24, for example, of silicon nitride and with an average thickness of only less than one monolayer up to several monolayers. The masking layer 45 contains a large number of openings which can be self-organized and in which the p-layer 24 remains free, see FIG. 6B.

(21) According to FIG. 7A, the material for the semiconductor contact layer 25 is deposited starting from the openings in the masking layer 45, so that thickness maxima 4 are formed in the form of individual, non-contiguous contact islands 42, see the top view in FIG. 7B.

(22) Optionally, an intermediate layer 46 can be produced before the electrode layer 3 is applied, which can be done by sputtering or vapor deposition, for example. The intermediate layer 46 serves to planarize the thickness maxima 4. For example, the intermediate layer 46 is made of AlGaN, as is the p-layer 24. In this case, the intermediate layer 46 can be doped or undoped. Alternatively, a dielectric material such as silicon dioxide is deposited in order to achieve increased reflectivity in the interaction of the electrode layer 3.

(23) According to FIG. 7A, the intermediate layer 46 is optionally deposited in such a way that the contact islands 42 are at least partially covered by the material of the intermediate layer 46. FIG. 7C illustrates that after generating the intermediate layer 46, it is partially removed again, so that the intermediate layer 46 together with the contact islands 42 form a smooth side facing away from the active zone 23 on which the electrode layer 3 is directly applied. Thus, the contact islands 42 are located directly on the electrode layer 3 on the side facing away from the active zone 23.

(24) The masking layer 45 is completely covered by the intermediate layer 46 together with the contact islands 42. The contact islands 42 at one edge of the openings partially cover the masking layer 45.

(25) The intermediate layer 46 can be removed mechanically and/or chemically. In contrast to the illustration in FIGS. 7A and 7C, the intermediate layer 46 may also be composed of several sublayers, in particular to achieve increased reflectivity at the intermediate layer 46 and at the electrode layer 3.

(26) The proportion of the surface of the p-layer directly covered by the contact islands 42 can be adjusted by such a masking layer 45, e.g., by the growth time of the masking layer 45. Instead of a self-organized masking layer 45, structuring can alternatively be carried out using a stamping method or lithographic methods.

(27) As an alternative to a metallic, reflecting electrode layer 3, an electrode layer transparent to the generated radiation R can be used in each case, in particular made of transparent conductive oxides, TCOs for short. For example, Ga2O.sub.3, ITO or a Sr—Cu oxide, individually or in combination, can be used.

(28) Creating the intermediate layer 46 is optional. If the intermediate layer 46 is omitted, electrode layer 3 is applied directly to the masking layer 45 and the contact islands 42 and has a comparatively rough, structured side facing the active zone 23. This is particularly possible if the electrode layer 3 is formed from a TCO.

(29) Light emitting diodes 10 described here are used, for example, for gas sensors to detect certain gas absorption lines. For example, a wavelength of maximum intensity of the generated radiation R lies between 217 nm and 230 nm. An emitted radiant power of the radiation R, for example, is approximately 1 mW.

(30) Unless otherwise indicated, the components shown in the figures follow each other directly in the order indicated. Layers not touching each other in the figures are spaced from each other. As far as lines are drawn parallel to each other, corresponding surfaces are also parallel to each other. Also, unless otherwise indicated, the relative thickness ratios, length ratios and positions of the drawn components to each other are correctly reproduced in the figures.

(31) The invention described here is not limited by the description given by way of the exemplary embodiments. Rather, the invention includes each new feature as well as each combination of features, which in particular includes each combination of features in the patent claims, even if that feature or combination itself is not explicitly stated in the patent claims or exemplary embodiments.