Optoelectronic semiconductor device

Abstract

An optoelectronic semiconductor component includes a layer stack based on a nitride compound semiconductor and has an n-type semiconductor region , a p-type semiconductor region and an active layer arranged between the n-type semiconductor region and the p-type semiconductor region. In order to form an electron barrier, the p-type semiconductor region includes a layer sequence having a plurality of p-doped layers composed of Al.sub.xIn.sub.yGa.sub.1xyN where 0<=x<=1, 0<=y<=1 and x+y<=1. The layer sequence includes a first p-doped layer having an aluminum proportion x1>=0.5 and a thickness of not more than 3 nm, and the first p-doped layer, at a side facing away from the active layer, is succeeded by at least a second p-doped layer having an aluminum proportion x2<x1 and a third p-doped layer having an aluminum proportion x3<x2.

Claims

1. An optoelectronic semiconductor device formed from a layer stack based on a nitride compound semiconductor, the layer stack comprising: an n-type semiconductor region; an active layer overlying the n-type semiconductor region; and a layer sequence comprising a plurality of p-doped layers of Al.sub.xIn.sub.yGa.sub.1xyN overlying the active layer, where 0x1, 0y1 and x+y1, the layer sequence including a first p-doped layer with an aluminum content x10.5 and a thickness of no more than 3 nm overlying the active layer, a second p-doped layer with an aluminum content x2<x1 overlying the first p-doped layer, and a third p-doped layer with an aluminum content x3<x2.

2. The optoelectronic semiconductor device according to claim 1, wherein the plurality of p-doped layers form an electron barrier.

3. The optoelectronic semiconductor device according to claim 1, wherein the first p-doped layer has an aluminum content x10.8.

4. The optoelectronic semiconductor device according to claim 1, wherein the second p-doped layer has an aluminum content x20.4.

5. The optoelectronic semiconductor device according to claim 4, wherein the second p-doped layer has an aluminum content x20.3.

6. The optoelectronic semiconductor device according to claim 1, wherein the second p-doped layer has an aluminum content gradient at least in a sub-region, wherein the aluminum content x2 decreases in a direction of the third p-doped layer.

7. The optoelectronic semiconductor device according to claim 1, wherein the second p-doped layer has a thickness of less than 20 nm.

8. The optoelectronic semiconductor device according to claim 1, wherein the third p-doped layer has an aluminum content x30.1.

9. The optoelectronic semiconductor device according to claim 1, further comprising a fourth p-doped layer between the first p-doped layer and the second p-doped layer, the fourth p-doped layer having a thickness of less than 4 nm.

10. The optoelectronic semiconductor device according to claim 9, wherein the fourth p-doped layer has an aluminum content x4, wherein x4<x2.

11. The optoelectronic semiconductor device according to claim 9, wherein the fourth p-doped layer has an aluminum content x4 having a gradient, wherein the aluminum content x4 in the fourth p-doped layer has a minimum, at which x4<x2.

12. The optoelectronic semiconductor device according to claim 1, wherein the first, second and third p-doped layers have an indium content, the indium content of each of the first, second and third p-doped layers being less than or equal to 0.1.

13. The optoelectronic semiconductor device according to claim 1, wherein the first, second and third p-doped layers have a dopant concentration of 110.sup.18 cm.sup.3 to 110.sup.20 cm.sup.3.

14. The optoelectronic semiconductor device according to claim 1, further comprising an interlayer arranged between the active layer and the first p-doped layer, wherein the interlayer has a dopant concentration of no more than 510.sup.17 cm.sup.3.

15. The optoelectronic semiconductor device according to claim 14, wherein the interlayer is undoped.

16. The optoelectronic semiconductor device according to claim 14, wherein the interlayer comprises Al.sub.xIn.sub.yGa.sub.1xyN with 0x0.02 and 0y0.1.

17. The optoelectronic semiconductor device according to claim 14, wherein the interlayer has an indium-content y<0.05.

18. An optoelectronic semiconductor device, comprising a layer stack based on a nitride compound semiconductor which comprises an n-type semiconductor region, a p-type semiconductor region and an active layer arranged between the n-type semiconductor region and the p-type semiconductor region; wherein to form an electron barrier, the p-type semiconductor region comprises a layer sequence with a plurality of p-doped layers of Al.sub.xIn.sub.yGa.sub.1xyN with 0x1, 0y1 and x+y1; wherein the layer sequence has a first p-doped layer with an aluminum content x10.5 and a thickness of no more than 3 nm; wherein the first p-doped layer is followed on a side remote from the active layer by at least one second p-doped layer with an aluminum content x2<x1 and one third p-doped layer with an aluminum content x3<x2; wherein a fourth p-doped layer is arranged between the first p-doped layer and the second p-doped layer, the fourth p-doped layer having a thickness of less than 4 nm; and wherein the fourth p-doped layer has an aluminum content x4 having a gradient, wherein the aluminum content x4 in the fourth p-doped layer has a minimum, at which x4<x1 and x4<x2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in greater detail below with reference to exemplary embodiments in conjunction with FIGS. 1 to 8, in which:

(2) FIG. 1 is a schematic representation of a cross-section through an optoelectronic semiconductor device according to a first exemplary embodiment;

(3) FIGS. 2 and 3 each show graphical representations of the profile of the aluminum content x and of the electronic band gap E.sub.g in one region of an optoelectronic semiconductor device according to the first exemplary embodiment;

(4) FIG. 4 shows a graphical representation of injection efficiency as a function of the current density at two optoelectronic semiconductor devices according to the first exemplary embodiment compared with a conventional optoelectronic semiconductor device;

(5) FIG. 5 shows a schematic representation of a cross-section through an optoelectronic semiconductor component according to a second exemplary embodiment; and

(6) FIGS. 6 to 8 each show graphical representations of the profile of the aluminum content x and of the electronic band gap E.sub.g in one region of an optoelectronic semiconductor device according to the second exemplary embodiment.

(7) In the figures identical or identically acting components are in each case provided with the same reference numerals. The components illustrated and the size ratios of the components to one another should not be regarded as to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(8) The optoelectronic device 10 according to a first exemplary embodiment and illustrated in FIG. 1 comprises a light-emitting semiconductor device, which may, for example, be an LED or a laser diode. The optoelectronic semiconductor device 10 comprises a layer stack applied to a substrate 5, with an n-type semiconductor region 6, a p-type semiconductor region 8 and an active layer 7 arranged between the n-type semiconductor region 6 and the p-type semiconductor region 8. The layer stack is in particular an epitaxial layer sequence, which was grown epitaxially onto the substrate 5. The optoelectronic semiconductor device 10 does not necessarily have to have a substrate 5, rather the latter may be detached after production of the epitaxial layer sequence.

(9) For electrical contacting, the optoelectronic semiconductor device 10, for example, comprises a first electrode layer 11 and a second electrode layer 12, wherein the first electrode layer 11 may, for example, be arranged on a back, remote from the layer stack, of the substrate 5 and the second electrode layer 12 may be arranged on an opposite surface of the layer stack from the substrate 5. Alternatively, there are also other possible ways of contacting the optoelectronic semiconductor device 10. For example, both electrode layers 11, 12 may be arranged on the same main face of the optoelectronic semiconductor device 10.

(10) The layer stack of the optoelectronic semiconductor device 10 is based on a nitride compound semiconductor, i.e., the semiconductor layers contained therein comprise III-nitride compound semiconductor materials, preferably Al.sub.xIn.sub.yGa.sub.1-x-yN, wherein 0x1, 0y1 and x+y1.

(11) The active layer 7 may, for example, have a quantum well structure or a multiple quantum well structure. The n-type semiconductor region 6 and the active layer 7 may in each case be formed from a plurality of sublayers, which are not illustrated individually to simplify the representation.

(12) The p-type semiconductor region 8 comprises a p-doped layer sequence 13, which serves as an electron barrier for electrons injected into the active layer 7. The p-type semiconductor region 8 may contain one or more further layers in addition to the layer sequence 13, for example, a p-type contact layer 14.

(13) The p-doped layers 1, 2, 3 of the layer sequence 13 each comprise a nitride compound semiconductor material with the composition Al.sub.xIn.sub.yGa.sub.1-x-yN with 0x1, 0y1 and x+y1. A first p-doped layer 1 closest to the active layer 7 has an aluminum content x10.5 and a thickness of no more than 3 nm. The first p-doped layer 1 is followed on a side remote from the active layer 7 by at least one second p-doped layer 2 with an aluminum content x2<x1 and one third p-doped layer 3 with an aluminum content x3<x2.

(14) Because the first p-doped layer 1 of the layer sequence 13 closest to the active layer 7 has a comparatively high aluminum content x10.5, the layer 1 has a comparatively large electronic band gap. The first p-doped layer 1 preferably has the largest electronic band gap within the layer stack of the optoelectronic semiconductor device 10. Due to the comparatively large electronic band gap, the first p-doped layer 1 forms an effective barrier for electrons propagating from the active layer 7 towards the second electrode layer 12, which in particular forms a p-electrode. Electrons are therefore advantageously confined in the active layer 7, whereby the number of radiative recombinations in the active layer 7 is increased, so improving the efficiency of the optoelectronic semiconductor device 10. Because the first p-doped layer 1 has a very large electronic band gap, while on the other hand being very thin, with a thickness of less than 3 nm, the injection of holes into the active layer 7 is not significantly reduced.

(15) The first p-doped layer 1 is followed in the layer sequence 13 by a second p-doped layer 2, which has an aluminum content x2 which is lower than the aluminum content x1 in the first p-doped layer 1. Furthermore, the second p-doped layer 2 is followed by a third p-doped layer 3, the aluminum content x3 of which is lower than the aluminum content x2 of the second p-doped layer 2. In other words, the aluminum content x in the layer sequence 13 decreases step-wise in a direction pointing from the active layer 7 to the p-electrode layer 12.

(16) Particularly preferably, the first p-doped layer 1 has an aluminum content x10.8. The second p-doped layer 2 preferably has an aluminum content x20.4, particularly preferably x20.3. The thickness of the second p-doped layer amounts preferably to less than 20 nm.

(17) The third p-doped layer 3 advantageously comprises an aluminum content x30.1. Through the step-wise decrease in aluminum content in the layer sequence 13, mechanical tensions caused by lattice mismatch and resultant piezoelectric fields, which could bring about recombinations of charge carriers outside the active zone and thus optical losses, are advantageously reduced.

(18) In the optoelectronic semiconductor device 10 the first p-doped layer 1 of the p-doped layer sequence 13 does not have to directly adjoin the active layer 7. Rather, an interlayer 9 may be arranged between the active layer 7 and the first p-doped layer 1, which interlayer is preferably undoped or has only a very low dopant concentration of less than 510.sup.17 cm.sup.3. The interlayer 9 may, for example, comprise p-doping with an above-mentioned low dopant concentration.

(19) In contrast, the p-doped layers 1, 2, 3 of the layer sequence 13 preferably comprise a dopant concentration of at least 110.sup.18 cm.sup.3, preferably between 110.sup.18 cm.sup.3 and 110.sup.20 cm.sup.3. The p-doped layers 1, 2, 3 may in particular have magnesium as p-dopant.

(20) FIGS. 2 and 3 are schematic representations along a spatial coordinate z extending in the growth direction of the layer stack of the profile of the aluminum content x, which corresponds qualitatively to the profile of the electronic band gap E.sub.g, in the interlayer 9 and the layers 1, 2, 3 of the p-doped layer sequence 13. The interlayer 9 has a very low aluminum content x9, for which x90.02 preferably applies. In the exemplary embodiment of FIG. 2, this is followed by the first p-doped layer 1 with an aluminum content x10.5, preferably x10.8, the second p-doped layer 2 with an aluminum content x20.4, preferably x20.3, and the third p-doped layer 3 with an aluminum content x30.1.

(21) The exemplary embodiment of FIG. 3 differs from the exemplary embodiment in FIG. 2 in that the aluminum content x2 in the second p-doped layer 2 is not constant, but rather has a gradient at least in places. Three variants x2.sub.a, x2.sub.b and x2.sub.c are shown schematically for the profile of the aluminum content x2 in the second p-doped layer 2. In variant x2.sub.a the aluminum content decreases continuously from the first p-doped layer 1 in a first region and is then constant in a following sub-region. In variant x2.sub.b the aluminum content in the second p-doped layer 2 decreases continuously from the value x1 in the first p-doped layer 1 to a lower value at the interface with the third p-doped layer 3. In variant x2.sub.c the aluminum content in the second p-doped layer 2 likewise decreases continuously from the first p-doped layer 1 towards the third p-doped layer 3, wherein the final value at the interface with the third p-doped layer 3 is however greater than in the variant x2.sub.b.

(22) FIG. 4 shows the injection efficiency IE as a function of the current density CD for an optoelectronic device according to the first exemplary embodiment with a first p-doped layer 1 of Al.sub.0.80In.sub.0.20N and a second p-doped layer 2 of Al.sub.0.30Ga.sub.0.70N (curve 15), for a further optoelectronic semiconductor device according to the first exemplary embodiment with a first p-doped layer 1 of Al.sub.0.99In.sub.0.01N and a second p-doped layer 2 of Al.sub.0.30Ga.sub.0.70N (curve 16) and for a conventional optoelectronic device with a simple electron barrier layer of Al.sub.0.30Ga.sub.0.70N (curve 17). It is clear that the injection efficiency in the case of the optoelectronic semiconductor devices according to the exemplary embodiment is improved over the conventional optoelectronic semiconductor device, especially for large current densities.

(23) FIG. 5 shows a second exemplary embodiment of the optoelectronic semiconductor device schematically in cross-section. The second exemplary embodiment differs from the first exemplary embodiment in that a fourth p-doped layer 4 is arranged between the first p-doped layer 1 and the second p-doped layer 2 of the layer sequence 13. The fourth p-doped layer 4 is preferably a very thin layer, like the first p-doped layer 1, and preferably has a thickness of no more than 4 nm. By means of the additional layer 4, the profile of the electronic band structure in the layer sequence 13 may be optimized still further to improve injection efficiency and to reduce piezoelectric fields.

(24) The second exemplary embodiment illustrated shown in FIG. 5 otherwise corresponds to the first exemplary embodiment shown in FIG. 1.

(25) Examples of profiles for the aluminum content x and the corresponding electronic band gap E.sub.g in the second exemplary embodiment are shown in FIGS. 6 to 8.

(26) In the exemplary embodiment of FIG. 6, the fourth p-doped layer 4 has an aluminum content x4 which is lower than the aluminum content x1 of the first p-doped layer and the aluminum content x2 of the second p-doped layer. It would alternatively however also be feasible for the fourth p-doped layer 4 to have an aluminum content x4 which is even greater than the aluminum content x1 in the first p-doped layer 1.

(27) In the exemplary embodiment of FIG. 6, the profile of the aluminum content x2 and of the electronic band gap in the second p-doped layer 2 and the third p-doped layer 3 corresponds to the example shown in FIG. 2.

(28) As in the exemplary embodiment of FIG. 3, the second p-doped layer 2 may also comprise an aluminum content x2 gradient in the embodiment with the additional fourth p-doped layer 4. Such an example is shown in FIG. 7. In this exemplary embodiment, as in the exemplary embodiment of FIG. 6, a fourth p-doped layer 4 with an aluminum content x4 is arranged between the first p-doped layer 1 and the second p-doped layer 2, wherein the profile of the aluminum content x2 in the second p-doped layer 2 and of the aluminum content x3 in the third p-doped layer 3 corresponds to the variants shown in FIG. 3.

(29) The exemplary embodiment of FIG. 8 differs from the exemplary embodiment shown in FIG. 6 in that the fourth p-doped layer 4 has an aluminum content x4 gradient. In particular, the aluminum content x4 within the fourth p-doped layer 4 falls, starting from the first p-doped layer 1, initially continuously down to a minimum, at which the aluminum content x4 is less than the aluminum content x2 in the second p-doped layer 2. After the minimum the aluminum content x4 in the fourth p-doped layer 4 rises continuously again to the value of the aluminum content x2 in the second p-doped layer 2.

(30) The invention is not restricted by the description given with reference to the exemplary embodiments. Rather, the invention encompasses any novel feature and any combination of features, including in particular any combination of features in the claims, even if this feature or this combination is not itself explicitly indicated in the claims or exemplary embodiments.