Optoelectronic semiconductor chip and method for producing same

An optoelectronic semiconductor chip (10) is specified, comprising a p-type semiconductor region (4), an n-type semiconductor region (6), and an active layer arranged between the p-type semiconductor region (4) and the n-type semiconductor region (6), said active layer being designed as a multiple quantum well structure (5), wherein the multiple quantum well structure (5) comprises quantum well layers (53) and barrier layers (51), wherein the barrier layers (51) are doped, and wherein undoped intermediate layers (52, 54) are arranged between the quantum well layers (53) and the barrier layers (51). Furthermore, a method for producing the optoelectronic semiconductor chip (10) is specified.

The invention relates to an optoelectronic semiconductor chip and to a method for producing same.

CROSS-REFERENCE OF RELATED APPLICATIONS

This patent application is the U.S. National Phase under 35 U.S.C. § 371 of International Patent Application No. PCT/EP2016/070173, filed on Aug. 26, 2016, which in turn claims the priority of German patent application 10 2015 114 478.6, filed on Aug. 31, 2015, the disclosure content of which is hereby incorporated by reference.

The generation of radiation in the active layer of an optoelectronic semiconductor chip is based on the radiative recombination of electrons and holes with the emission of a photon. Non-radiative recombinations of electrons and holes are a possible loss mechanism in radiation-emitting semiconductor chips. To increase the efficiency of an optoelectronic semiconductor chip, it is therefore advantageous to reduce the proportion of non-radiative recombinations.

The invention is based on the object of providing an optoelectronic semiconductor chip in which non-radiative recombinations of electrons and holes are reduced. Furthermore, a method for producing the optoelectronic semiconductor chips is to be provided.

These objects are achieved by an optoelectronic semiconductor chip and a method according to the independent claims. Advantageous embodiments and developments of the invention are subject matter of the dependent claims.

The optoelectronic semiconductor chip according to at least one embodiment comprises a p-type semiconductor region, an n-type semiconductor region and an active layer arranged between the p-type semiconductor region and the n-type semiconductor region, said active layer being in the form of a multi-quantum-well structure.

According to at least one embodiment, the multi-quantum-well structure has quantum-well layers and barrier layers. The barrier layers have a larger band gap than the quantum-well layers. The multi-quantum-well structure is in particular a periodic layer sequence, in which each period has a quantum-well layer and a barrier layer. In the quantum-well structure described here, the barrier layers are advantageously doped and, between the barrier layers and the quantum-well layers, undoped intermediate layers are arranged.

The doping of the barrier layers of the multi-quantum-well structure is advantageous in order to enable short switching times to be achieved in the optoelectronic semiconductor chip. The barrier layers are e.g. p-doped, the dopant being for example carbon.

It has been shown that doping of the barrier layers is advantageous for achieving short switching times. On the other hand, however, it has been found that non-radiative recombinations can occur to an increased degree at the interface between the doped barrier layers and the quantum-well layers if the barrier layers directly adjoin the quantum-well layers. This is based in particular on the fact that the electrons can come into contact with the ionized dopant atoms at the interface between the quantum-well layers and the barrier layers. More precisely, an overlap of the electron wave function with the wave function of the dopant atoms in directly mutually adjacent barrier layers and quantum-well layers would be non-zero, which would result in non-radiative recombinations and thus a loss of brightness.

In the optoelectronic semiconductor chip described here, the overlap between the electron wave function and the wave function of the ionized dopant atoms of the barrier layers is advantageously reduced by means of undoped intermediate layers being arranged between the doped barrier layers and the quantum-well layers. Advantageously, the doped barrier layers and the quantum-well layers thus do not adjoin one another directly but are spaced apart from one another by the undoped intermediate layers. In this way, losses of brightness due to non-radiative recombinations are reduced and the efficiency of the optoelectronic semiconductor chip is thus increased.

According to at least one advantageous embodiment, the doped barrier layers each adjoin an undoped intermediate layer on both sides. In other words, viewed in the growth direction of the multi-quantum-well structure, an undoped intermediate layer is arranged both above and below each barrier layer. The multi-quantum-well structure can in particular be a periodic layer sequence in which each period has a first undoped intermediate layer, a doped barrier layer, a second undoped intermediate layer and a quantum-well layer. In particular, each period of the multi-quantum-well layers can consist of precisely the above-mentioned four layers. The multi-quantum-well structure in this embodiment advantageously has no interfaces between a doped barrier layer and a quantum-well layer. Losses due to non-radiative recombinations are reduced particularly effectively in this way.

According to an advantageous embodiment, the undoped intermediate layers are between 1 nm and 10 nm thick. A thickness of at least 1 nm is advantageous to reduce the overlap between the electron wave function and the wave function of the dopant atoms in the barrier layers effectively.

Furthermore, it is advantageous if the undoped intermediate layers are comparatively thin, since too great a thickness of the undoped intermediate layers could have a negative effect on the switching times of the optoelectronic semiconductor chip. Particularly preferably, the undoped intermediate layers are less than 3 nm thick. Furthermore, it is advantageous if the undoped intermediate layers are thinner than the doped barrier layers.

According to a preferred embodiment, the multi-quantum-well structure is based on an arsenide compound semiconductor. Preferably, the quantum-well layers comprise InxAlyGa1-x-yAs with 0≤x≤1, 0≤y≤1 and x+y≤1. The barrier layers and/or the undoped intermediate layers preferably comprise AlmGa1-mAsnP1-nwith 0≤m≤1 and 0≤n≤1.

The band gaps of the barrier layers, quantum-well layers and intermediate layers can be adjusted in particular by the material composition. An increase in the band gap can be achieved in particular by increasing the aluminum content y and/or reducing the indium content x. Preferably, the barrier layers have a greater aluminum content than the quantum-well layers. In particular, the quantum-well layers can be free from aluminum. The quantum-well layers preferably have a greater indium content than the barrier layers, the barrier layers preferably being free from indium.

According to at least one advantageous embodiment, the band gap of the undoped intermediate layers substantially corresponds to the band gap of the doped barrier layers. Preferably, a difference between the electronic band gaps of the doped barrier layers and the undoped intermediate layers is no more than 0.1 eV.

The undoped intermediate layers therefore have substantially the same electrooptical properties as the barrier layers. In particular, the undoped intermediate layers have a greater band gap than the quantum-well layers and in this way, like the barrier layers, they cause a confinement of charge carriers in the quantum-well layers.

According to at least one advantageous embodiment, the barrier layers have substantially the same material composition as the undoped intermediate layers, apart from a dopant. “Substantially the same material composition” here is intended to mean in particular that the contents of the elements of the semiconductor materials in the barrier layers and the intermediate layers differ from one another by no more than 10%, particularly preferably no more than 5%. The barrier layers preferably comprise Alm1Ga1-m1Asn1P1-n1with 0≤m1≤1 and 0≤n1≤1. The undoped intermediate layers preferably comprise Alm2Ga1-m2Asn2P1-n2with 0≤m2≤1 and 0≤n2≤1. The following preferably applies here: |m1−m2|≤0.1, particularly preferably |m1−m2|≤0.05. Furthermore, the following preferably applies: |n1−n2|≤0.1, particularly preferably |n1−n2|≤0.05.

In the intermediate layers, n2=1 preferably applies, i.e. the intermediate layers preferably comprise a ternary semiconductor material with the composition Alm2Ga1-m2As with 0≤m2≤1. The phosphorus content of the intermediate layers 1−n2 is thus preferably equal to zero. A ternary compound semiconductor material is advantageously easier to produce than a quaternary compound semiconductor material. If the barrier layers have a phosphorus content 1−n1>0, the aluminum content m2 in the intermediate layers is preferably greater than the aluminum content m1 of the intermediate layers. Preferably, therefore, the following applies: 1−n1<0, n2=1 and m2>m1. As a result of the higher aluminum content of the intermediate layers, the band gap is advantageously increased to counteract a reduction in the band gap compared to the barrier layers due to the lack of a phosphorus content.

The optoelectronic semiconductor chip is preferably a light-emitting diode emitting in the infrared range of the spectrum. The optoelectronic semiconductor chip can have an emission wavelength of e.g. between 750 nm and 1000 nm.

The method for producing the optoelectronic semiconductor chip according to at least one embodiment comprises an epitaxial growth of a semiconductor layer sequence having a p-type semiconductor region, an n-type semiconductor region and an active layer arranged between the p-type semiconductor region and the n-type semiconductor region, which active layer is in the form of a multi-quantum-well structure. The multi-quantum-well structure comprises quantum-well layers and barrier layers, the barrier layers being doped. Between the quantum-well layers and the barrier layers, an undoped intermediate layer is arranged in each case. The epitaxial growth preferably takes place by means of MOVPE.

In the method, the undoped intermediate layers are advantageously grown at a higher growth temperature than the doped barrier layers. The higher growth temperature during the growth of the undoped intermediate layers compared to the growth temperature during the growth of the barrier layers has the advantage that the unintentional incorporation of impurities into the undoped intermediate layers is reduced. Unintentionally incorporated impurities could otherwise act as non-radiative recombination centers. By avoiding non-radiative recombinations at impurities, the efficiency of the radiation generation in the optoelectronic semiconductor chip is advantageously increased further. Preferably, the growth temperature during the growth of the undoped intermediate layers is at least 650° C.

The doped barrier layers are preferably grown at a growth temperature of less than 600° C. The lower growth temperature during the growth of the barrier layers has the advantage that the incorporation of dopant atoms into the barrier layers is favored. By reducing the barrier layers, an optoelectronic semiconductor chip with comparatively short switching times can be achieved. The barrier layers can be doped e.g. with C.

Further advantageous embodiments of the method can be taken from the description of the optoelectronic component and vice versa.

The invention is explained in more detail below with the aid of exemplary embodiments in association withFIGS. 1 to 3.

The components illustrated and the size ratios to one another of the components should not be regarded as being to scale.

The optoelectronic semiconductor chip10according to one exemplary embodiment illustrated inFIG. 1is an LED chip comprising a p-type semiconductor region4, an n-type semiconductor region6and an active layer5capable of emitting radiation, which is arranged between the p-type semiconductor region4and the n-type semiconductor region6. The LED chip10is preferably an LED chip emitting in the infrared range of the spectrum.

In the exemplary embodiment of the optoelectronic semiconductor chip10, the chip is a so-called thin-film semiconductor chip, from which a growth substrate originally used for the epitaxial growth of the semiconductor layers4,5,6has been removed and instead, the semiconductor layer sequence has been connected by means of a connecting layer2, in particular a solder layer, to a carrier substrate1which is different from the growth substrate.

In a thin-film LED chip10of this type, the p-type semiconductor region4generally faces towards the carrier substrate1. Between the p-type semiconductor region4and the carrier substrate1, a mirror layer3is advantageously arranged, which advantageously deflects radiation emitted towards the carrier substrate1towards a radiation exit surface9of the optoelectronic semiconductor chip. The mirror layer3is e.g. a metal layer, which contains Ag, Al or Au.

For the electrical contacting of the optoelectronic semiconductor chip10, e.g. a first contact layer7can be provided on a rear side of the carrier substrate1and a second contact layer8on a subregion of the radiation exit surface9.

The p-type semiconductor region4and the n-type semiconductor region6can each be composed of multiple sublayers and do not necessarily have to consist exclusively of p-doped layers or n-doped layers but can also comprise e.g. one or more undoped layers.

As an alternative to the exemplary embodiment illustrated, the optoelectronic semiconductor chip10could also have an opposite polarity, i.e. the n-type semiconductor region6could face towards a substrate and the p-type semiconductor region4towards a radiation exit surface9of the optoelectronic semiconductor chip (not illustrated). This is generally the case in optoelectronic semiconductor chips in which the growth substrate used for the epitaxial growth of the semiconductor layers is not removed, since the n-type semiconductor region is generally grown first on the growth substrate.

The active layer of the optoelectronic semiconductor chip10provided to emit radiation is in the form of a multi-quantum-well structure5. The multi-quantum-well structure5comprises a plurality of alternately arranged quantum-well layers53and barrier layers51. The quantum-well layers53have a band gap EQWund the barrier layers53have a band gap EB>EQW. The multi-quantum-well structure5is in particular a periodic layer sequence comprising a number N of periods, wherein N is preferably between 2 and 50. For example, the multi-quantum-well structure can comprise twelve periods.

The barrier layers51in the multi-quantum-well structure5are doped. The dopant concentration in the barrier layers51is advantageously at least 1*1018cm−3, preferably at least 1*1019cm−3, e.g. for instance 3*1019cm−3. The doping of the barrier layers51has the advantage that comparatively short switching times can be achieved in the optoelectronic semiconductor chip.

Between the quantum-well layers53and the barrier layers51, undoped intermediate layers52,54are advantageously arranged. A period of the quantum-well structure can consist of e.g. a doped barrier layer51, a first undoped intermediate layer52, a quantum-well layer53and a second undoped intermediate layer54, wherein each quantum-well layer53adjoins an undoped intermediate layer52,54on both sides. The quantum-well layer53therefore advantageously has no interface with a doped barrier layer51. This has the advantage that electrons in the quantum-well layers53do not come into direct contact with the ionized dopant atoms of the barrier layers51. More precisely, an overlap of the electron wave function with the wave function of the ionized dopant atoms in the barrier layers51is reduced. In this way, non-radiative recombinations of electrons are reduced and the efficiency of the optoelectronic semiconductor chip10is thus increased.

The undoped intermediate layers52,54preferably have a thickness of at least 1 nm and no more than 10 nm, particularly preferably no more than 3 nm. The undoped intermediate layers52,54are preferably thinner than the barrier layers51and/or the quantum-well layers53. The short switching times that are made possible by the doping of the barrier layers are preferably not substantially affected by the undoped intermediate layers, which are thin compared to the barrier layers.

In the exemplary embodiment illustrated inFIG. 1, for example, the barrier layer51can have a thickness of 8.4 nm, the first undoped intermediate layer52a thickness of 1.4 nm, the quantum-well layer53a thickness of 4.4 nm and the second undoped intermediate layer54a thickness of 1.4 nm.

During the production of the multi-quantum-well structure5, the barrier layers51are preferably grown at a lower growth temperature than the undoped intermediate layers52,54and the quantum-well layers53. The growth of the barrier layers51takes place at a growth temperature of preferably less than 600° C., e.g. at about 575° C. The undoped intermediate layers52,54and the quantum-well layers53are preferably grown at a growth temperature of more than 650° C., e.g. at about 665° C. As a result of the higher growth temperature during the growth of the undoped intermediate layers52,54and the quantum-well layers53, the incorporation of foreign atoms (impurities) is advantageously kept low. Since impurities can form centers for non-radiative recombinations, non-radiative recombinations are further reduced by a reduction of impurities and thus the efficiency of the optoelectronic semiconductor chip is increased further.

The band gaps of the semiconductor materials of the quantum-well layers53, the barrier layers51and the undoped intermediate layers52,54can in particular be adjusted by varying the aluminum content and/or the indium content in the semiconductor material. For example, the quantum-well layers and barrier layers can comprise semiconductor materials with the composition InxAlyGa1-x-yAs or InxAlyGa1-x-yAszP1-zwith 0≤x≤1, 0≤y≤1, x+y≤1 and 0≤z≤1. In these types of semiconductors, the band gap increases with increasing aluminum content y and decreases with increasing indium content x. In the exemplary embodiment ofFIG. 1, for example, the quantum-well layers53comprise Ga0.92In0.08As, the barrier layers51comprise Al0.23Ga0.77As0.94P0.06and the undoped intermediate layers52,54comprise Al0.28Ga0.72As.

The barrier layers51and the undoped intermediate layers52,54have substantially the same material composition. The barrier layers51preferably comprise Alm1Ga1-m1Asn1P1-n1with 0≤m1≤1 and 0≤n1≤1. A dopant of the barrier layers51, such as for example C, can be ignored here since the concentration of the dopant is typically orders of magnitude lower than that of the other material components. For example, the barrier layers can have a dopant concentration of about 2*1019cm−3.

The undoped intermediate layers52,54preferably comprise Alm2Ga1-m2Asn2P1-n2with 0≤m2≤1 and 0≤n2≤1. Preferably, n2=1, i.e. the undoped intermediate layers have no phosphorus content. The following preferably applies here: |m1−m2|≤0.1, particularly preferably |m1−m2|≤0.05. Furthermore, the following preferably applies: |n1−n2|≤0.1, particularly preferably |n1−n2|≤0.05. Since the material compositions of the barrier layers51and the undoped intermediate layers52,54do not differ substantially from one another, the electronic band gap EBof the barrier layers also does not differ substantially from the electronic band gap EIL, of the undoped intermediate layers. The following preferably applies: |EB−EIL|≤0.1 eV, particularly preferably |EB−EIL|≤0.05 eV. The energetic properties of the undoped intermediate layers52,54therefore correspond substantially to the barrier layers51.

FIG. 2shows a bar diagram, which shows the brightness of the emitted radiation Ie(in arbitrary units) for exemplary embodiments of optoelectronic semiconductor chips with various layer thicknesses dILof the undoped intermediate layers. The bar labelled “0 nm” relates to an exemplary embodiment which is not according to the invention, in which no undoped intermediate layers are arranged between the quantum-well layers and the barrier layers. The bar labelled “ref” relates to a further exemplary embodiment which is not according to the invention, in which the barrier layers are undoped and no undoped intermediate layers are arranged between the quantum-well layers and the barrier layers.

It is shown that, for the layer thicknesses cited, the brightness of the emitted radiation increases with the thickness of the undoped intermediate layers. This can be attributed in particular to the reduction in non-radiative recombinations of charge carriers.

InFIG. 3, a further bar diagram is shown which shows the rise time trise(left-hand bar) and the fall time tfall(right-hand bar) when the optoelectronic semiconductor chips are operated in pulsed mode with a current strength of 1 A as a function of the layer thickness dILof the undoped intermediate layers. It is shown that the switching times triseand tfallincrease with increasing layer thickness of the undoped intermediate layers. The increase in switching times is only very low with low layer thicknesses, however, so that comparatively short switching times can be achieved despite the undoped intermediate layers. Furthermore,FIG. 3shows that the switching times are significantly greater in the comparative example labelled “ref”, in which the barrier layers are undoped.

The description with the aid of the exemplary embodiments does not limit the invention thereto. Rather, the invention comprises any new feature and any combination of features, which in particular includes any combination of features in the patent claims, even if this feature or this combination is not itself explicitly stated in the patent claims or exemplary embodiments.

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