Light emitting device with multi-layered quantum well

Abstract

The invention concerns a light emitting device, in particular μLED, comprising a semiconductor layer stack of at least a first layer of a first conductivity type, a second layer of a second conductivity type as well as an active region arranged between the first and the second layer. The active region comprises a central region as well as a side region laterally surrounding the central region. In addition, the active region comprises at least one quantum well arranged between a first and a second barrier layer, wherein the first and the second barrier layer each comprise at least a first and a second sublayer, with the second sublayers each being adjacent to one of the at least one quantum well. The first sublayers at least in the central region comprise a smaller average bandgap than the second sublayers the at least one quantum well at least in the central region comprises a smaller average bandgap than the second sublayers and the at least one quantum well at least in the central region comprises a smaller average bandgap than the first sublayers.

Description

[0001] LIGHT EMITTING DEVICE WITH MULTI-LAYERED QUANTUM WELL

[0002] The present invention concerns a light emitting device (LED ) , in particular pLED, with multi layered quantum well ( s ) as well as a method for manufacturing the same .

[0003] BACKGROUND pLEDs are optoelectronic devices that comprise a lateral dimension in the range from a few pm to about 40 pm. Such devices provide a variety of different applications , including but not limited to displays .

[0004] In order to increase the external quantum efficiency ( EQE ) of for example pLEDs , or more general light emitting nanostructures , one can engineer the geometry, shape and surroundings (passivation, reflective mirror, ...) of the pLEDs in a way that the light extraction efficiency (LEE ) in a specific solid angle is maximized . Apart from the LEE the internal quantum efficiency ( IQE ) is as important , where a main loss factor originates from non-radiative recombination of charge carriers within the pLED .

[0005] For example , plasma etching is a common and standard process for pixel etching ( etching through the active region and physical separation of individual pixels ) , but introduces plasma damage to the surface and underlying layers dependent on the specific process and parameters used . Plasma damage in this case means the creation of defect centres in the crystal lattice caused by Ion bombardment . Especially for small pixels such as for the case of pLEDs with a high surf ace-to-volume ratio , in particular a high ratio of the exposed active region surface on the pixel ' s sidewall vs . the total active region volume , this is a maj or challenge . Defects created on the surface and below of the active region - which is exposed during and after pixel etching by means of plasma etching - act as non-radiative recombination (NRR) channels which in combination with NRR occurring anyhow for some material systems at the outer surfaces of the pLED dramatically reduce the IQE of the pLED leading to a bad overall performance . To reduce non-radiative recombination at the edges of a pLED, a possible approach is to remove plasma damage in a subsequent so-called defect etching step, where defects are reduced or even removed while not creating new defects. Therefore, usually wet chemistry is used, mostly aqueous KOH. A further approach to reduce non-radiative recombination at the edges of a pLED is to keep the charge carriers away from the edges which comprise the non-radiative recombination centres. This can for example be done by quantum well intermixing the active regions in areas along the edges of the pLED by means of which a respective dopant is diffused into the vicinity of the active region causing the bandgap of the active region along the side surfaces to enlarge due to intermixing processes. Therefore, barrier layer (s) are used being introduced adjacent to the quantum well(s) of the active region of a pLED, which for a high intermixing potential, comprise an Aluminium (Al) to Gallium (Ga) ratio that is different from that in the quantum well(s) , with a higher value of Al content in the barrier layer (s) and thus a higher In content in the quantum well(s) . The intermixing now equilibrates the Al to Ga ratio of the barrier layer (s) and the quantum well(s) in the areas along the edges of the pLED thereby increasing the bandgap in these areas due to a shift of the Al content. By this charge carriers can be kept away from the edges which comprise the non-radiative recombination centres .

[0006] However, a disadvantage of high-Al-containing barrier layer (s) in the active region is a high bandgap of the barrier layer (s) leading to a decreased overall carrier mobility. This makes higher operating voltages necessary for the pLEDs or results in less brightness of the pLEDs. Therefore, to date a certain compromise has to be made between the intermixing potential of the barrier layer (s) and the quantum well(s) vs. a high bandgap of the barrier layer (s) which limits the overall brightness of the pLED.

[0007] It is thus an object of the present application to provide an LED which overcomes at least some of aforementioned aspects, as well as to provide a method for manufacturing such an LED.

[0008] SUMMARY OF THE INVENTION This and other objects are addressed by the subject matter of the independent claims . Features and further aspects of the proposed principles are outlined in the dependent claims .

[0009] The idea of the inventors is to separate the quantum well(s) into several sublayers with different tasks, which allows to overcome aforementioned conflict and allows high intermixing potential as well as a carrier high mobility at the same time.

[0010] Typically a uniform material is used for the quantum well(s) of the active region of a pLED, where for a high intermixing potential, the Al to Ga ratio is chosen to be different in the quantum well(s) and the barrier layers, with a high value of the In content in the quantum well(s) . However, due to a limited diffusion length of the group-III- atoms during quantum well intermixing, the In contained in the quantum well(s) close to the interface between the quantum well(s) and the barrier layers contributes more to the quantum well intermixing than in a region therebetween. The quantum well(s) as known to date is therefore divided into a set of sublayers with different compositions in a way, that the sublayers of the quantum well(s) which are close to the neighbouring barrier layers comprise a higher compressive strain due to for example a higher In content, than the quantum well(s) as an average .

[0011] The compressive strain due to for example a higher In content and the thickness of the sublayers at the interface is thereby chosen to be as high as possible without leading to strain-induced damage or defects of the structural and optical properties of the pLED. By doing this the majority of compressive strain in the quantum well(s) is located close to the barrier layers and the intermixing, which only affects a few monolayers of the quantum well(s) which are close to the barrier layers, is enhanced.

[0012] At the same time the central sublayer (s) of the quantum well(s) can detached from an optimization with regard to quantum well intermixing be composed to control the emission wavelength of the pLED. This in combination allows to maintain a high intermixing potential and gives another degree of freedom for bandgap engineering of the active region of the pLED . At the same time , the overall carrier mobility of the pLED remains high as the Al content of the barrier layers can be reduced when improving the intermixing potential by means of increasing the compressive strain in the interface sublayers of the quantum well ( s ) .

[0013] According to a first aspect , a light emitting device comprising a semiconductor layer stack is provided . The semiconductor layer stack comprises at least a first layer of a first conductivity type , a second layer of a second conductivity type as well as an active region arranged between the first and the second layer .

[0014] The active region of the semiconductor layer stack comprises a central region as well as a side region laterally surrounding the central region . In addition, the active region of the semiconductor layer stack comprises at least one quantum well arranged between a first and a second barrier layer . The active region can however also comprise a multi quantum well structure comprising several quantum wells as well as several barrier layers , wherein each a quantum well is arranged between the two barrier layers .

[0015] The at least one quantum well comprises at least a first and a second quantum well sublayer as well as a third quantum well sublayer arranged between the first and the second quantum well sublayer . The first and the second quantum well sublayer at least in the central region comprise a larger compressive strain than the third quantum well sublayer, in particular at least 5 % larger, for example a strain of 100 ppm caused by for example a corresponding In content . In addition, the at least one quantum well at least in the central region comprises a smaller average bandgap than the barrier layers , in particular at least 10 % smaller . In particular the at least one quantum well at least in the central region comprises a smaller average bandgap than the barrier layers , by for example at least 100 meV smaller .

[0016] By means of the term "average" bandgap, the bandgap of the respective layer or sublayer with respect to its thickness is to be understood . The bandgap of a layer or sublayer can for example ramp up from a first side of a layer or sublayer to a second side of a layer or sublayer and can for example not be constant throughout the thickness of the layer or sublayer . The average bandgap is in such a case to be understood as the mean bandgap of the respective layer or sublayer with respect to its thickness .

[0017] By means of dividing the at least one quantum well into sublayers it is to be understood that the at least one quantum well can be divided in actually separated sublayers with different compositions and different compressive strains . However , it can also be that the sublayers of the at least one quantum well are of substantially the same composition with only for example the compressive strain of the at least one quantum well varying from a first side of the quantum well to a second side of the quantum well . Hence the at least one quantum well can vary with regard to an element concentration from a first side of the quantum well to a second side of the quantum well resulting in an increasing or decreasing compressive strain from a first side of the quantum well to a second side of the quantum well , in particular in an area between the first and the second side of the quantum well . Still , it is to be understood that the at least one quantum well can, at least in a virtual way, be divided into at least a first , a second and a third quantum well sublayer with at least the third quantum well sublayer having a different compressive strain than the first and second quantum well sublayer . For example , the at least one quantum well can at least virtually be divided into sublayers with at least the first and second sublayer having a substantially similar slope of the compressive strain increase or decrease , or the at least one quantum well can at least virtually be divided into sublayers resulting of sequentially performed growth steps of substantially the same material with only a different composition of elements which however in the final product can be difficult to later be differentiated .

[0018] According to some aspects , the active region comprises at least two quantum wells arranged between the first and the second barrier layer , wherein at least one third barrier layer is arranged between the at least two quantum wells . Such a structure with more than one quantum well can in particular be called a multi quantum well structure . The quantum wells can thereby be substantially equal in size and / or composition, can however also vary between each other . The at least two quantum wells can for example comprise a substantially equal average bandgap and can in particular be configured to emit light of a substantially equal wavelength .

[0019] According to some aspects , the first and the second quantum well sublayer at least in the central region comprise a substantially equal average bandgap, however the average bandgap can also be different , as long as the compressive strain of the first and second quantum well sublayer is larger than that of the third quantum well sublayer at least in the central region . The first and the second quantum well sublayer can for example be substantially equal in size and / or composition, can however also vary between each other .

[0020] According to some aspects , the first and the second quantum well sublayer at least in the central region comprise a smaller average bandgap than the third quantum well sublayer . The first , second and third quantum well sublayers can for example be substantially equal in size and / or composition, can however also vary between each other .

[0021] According to some aspects , the first layer and / or the second layer comprise a smaller average bandgap than the first and second barrier layer at least in the central region, in particular at least 5 % smaller . In particular the semiconductor layers adj acent to the active region can comprise a smaller average bandgap than the first and second barrier layer at least in the central region . However, it is also possible that the first layer and / or the second layer comprise a larger average bandgap than the first and second barrier layer at least in the central region, in particular at least 5 % larger . In particular the semiconductor layers adj acent to the active region can comprise a larger average bandgap than the first and second barrier layer at least in the central region .

[0022] According to some aspects , the first and second quantum well sublayer comprise a thickness of less than 10 nm, in particular less than 5 nm, or less than 3 nm, or less than 2 nm . Such a thickness is j ust enough to provide a sufficient thickness with a high compressive strain due to for example a high In content to allow an efficient quantum well intermixing with the adj acent barrier layers . In combination with for example barrier layers having a lower Al content , the overall charge carrier mobility can be kept high while still providing a sufficient quantum well intermixing potential with the adj acent first and second quantum well sublayers .

[0023] According to some aspects , the third quantum well sublayer comprise a thickness of less than 15 nm, in particular less than 10 nm or 5 nm down to for example 2 nm.

[0024] According to some aspects , the at least one quantum well comprise a thickness of less than 15 nm, in particular less than 10 nm or 5 nm down to for example 2 nm.

[0025] According to some aspects , each of the central region and the side region comprises respective portions of the first and second barrier layer as well as the quantum well ( s ) . In other words , the active region can be separated into a central region and a side region laterally surrounding the central region, with the first and second barrier layer as well as the quantum well ( s ) being separated in a respective way . The separation into regions may however not necessarily be understood as a geometrical separation in terms of separate components but may for example be understood as regions of one and the same body . In some embodiments these regions can for example comprise different properties but may also be more or less similar to each other in terms of properties .

[0026] According to some aspects , the side region, and in particular quantum well portion ( s ) of the side region, comprises a quantum well intermixing resulting in an enlarged bandgap of the quantum well portion ( s ) of the side region compared to respective quantum well portion ( s ) of the central region . In particular by means of a local quantum well intermixing , the side region of the active region can be treated by controllable diffusion of a dopant into the active region to create a higher bandgap material at the sidewalls compared to the central region which minimizes carrier leakage and is a way to fabricate more efficient pLEDs .

[0027] According to some aspects , the side region, and in particular quantum well portion ( s ) of the side region, comprises a dotation with a dopant of the second conductivity type , in particular Zn . In particular quantum well portion ( s ) of the side region and barrier layer portions of the side region as well as portions of the first and / or second layer adj acent to the side region can comprise a dotation with the dopant . Such a dotation can in particular result from a quantum well intermixing process by diffusing the dopant into the side region of the active region .

[0028] According to some aspects , the semiconductor layer stack is of an InGaAlP or InAlGaAs material system and in particular Al and In containing semiconductor material system. For example , the semiconductor layer stack can be of a material system comprising Indium ( In ) and Aluminium (Al ) and Gallium (Ga ) and Arsenide (As ) and / or Phosphide ( P ) .

[0029] According to some aspects , the first and second quantum well sublayer at least in the central region comprise a higher In content than the third quantum well sublayer, in particular at least 5 % higher . The higher In content can besides others cause a higher compressive strain of the first and second quantum well sublayer compared to the third quantum well sublayer and / or the higher In content can besides others cause a smaller average bandgap of the first and second quantum well sublayer compared to the third quantum well sublayer . The Element In is to be understood as exemplary here and the first and second quantum well sublayer at least in the central region can comprise a higher content of another material than the third quantum well sublayer that increases the compressive strain of the first and second quantum well sublayer with regard to the third quantum well sublayer . Such elements can for example be Group II I elements .

[0030] According to some aspects , the first and the second barrier layer each comprise at least a first and a second barrier sublayer, with the second barrier sublayers each being adj acent to one of the at least one quantum well . The first barrier sublayers at least in the central region comprise a smaller average bandgap than the second barrier sublayers , in particular at least 5 % or at least 10 meV smaller, the at least one quantum well at least in the central region comprises a smaller average bandgap than the second barrier sublayers , in particular at least 10 % or at least 20 meV smaller, and the at least one quantum well at least in the central region comprises a smaller average bandgap than the first barrier sublayers , in particular at least 5 % or at least 10 meV smaller .

[0031] A further idea of the inventors is to separate the barrier layer ( s ) into several sublayers with different tas ks , which allows a high intermixing potential as well as a high carrier mobility at the same time . The barrier layers are therefore divided into a set of sublayers with different compositions in a way, that the sublayers of the barrier layers which are close to the quantum well ( s ) contain a higher aluminium content and the other barrier sublayers contain less aluminium than the whole barrier layers as an average . By doing this the maj ority of the Al contained in the barrier layers is located close to the quantum well ( s ) and the intermixing, which only affects a few monolayers adj acent to the quantum well ( s ) , dominated by the sublayers of the barrier layer ( s ) which are close to the quantum well ( s ) , is enhanced . At the same time , the overall carrier mobility of the pLED remains high as the mean bandgap of the barrier layers is affected by the mean Al content of the barrier layers . In particular the sublayers of the barrier layers , which are close to the quantum well ( s ) with a high Al content , can comprise such a small thickness such that even if having a larger bandgap, the carrier mobility is not significantly influenced by it . By means of the proposed structure , a higher intermixing of quantum well ( s ) is achievable at a lower mean Al content in the barrier layers and thus a smaller mean band gap of the barrier layers .

[0032] According to some aspects , the active region comprises at least two quantum wells arranged between the first and the second barrier layer , wherein at least one third barrier layer is arranged between the at least two quantum wells . According to some aspects , the at least one third barrier layer comprises a third barrier sublayer arranged between two second barrier sublayers , with the second barrier sublayers each being adj acent to one of the at least two quantum wells .

[0033] According to some aspects , the at least two quantum wells at least in the central region comprise a smaller average bandgap than the third barrier sublayer ( s ) , in particular at least 5 % smaller . In case of several third barrier layers and thus third barrier sublayers , these can at least in the central region comprise a substantially equal average bandgap . For example , the third barrier sublayers can at least in the central region be substantially equal in size and / or composition, can however also vary between each other .

[0034] The third barrier sublayer ( s ) can for example at least in the central region comprise an average bandgap substantially equal to the average bandgap of the first barrier sublayers , however the average bandgap of the third barrier sublayer ( s ) and the first barrier sublayers can also be different , as long as both the average bandgap of the third barrier sublayer ( s ) and the first barrier sublayers is larger than that of the quantum well ( s ) at least in the central region .

[0035] According to some aspects , the first barrier sublayers at least in the central region comprise a substantially equal average bandgap . The first barrier sublayers of the first , second and third barrier layers can for example be substantially equal in size and / or composition, can however also vary between each other .

[0036] According to some aspects , the second barrier sublayers at least in the central region comprise a substantially equal average bandgap . The second barrier sublayers of the first , second and third barrier layers can for example be substantially equal in size and / or composition, can however also vary between each other and for example comprise a different average bandgap .

[0037] According to some aspects , the first layer and / or the second layer comprise a smaller average bandgap than the first barrier sublayers at least in the central region, in particular at least 5 % smaller . In particular the semiconductor layers adj acent to the active region can comprise a smaller average bandgap than the first barrier sublayers at least in the central region . However , it is also possible that the first layer and / or the second layer comprise a larger average bandgap than the first barrier sublayers at least in the central region, in particular at least 5 % larger . In particular the semiconductor layers adj acent to the active region can comprise a larger average bandgap than the first barrier sublayers at least in the central region .

[0038] According to some aspects , the first barrier sublayers comprise at least in the central region a lower Al content than the second barrier sublayers , in particular at least 5 % lower , the at least one quantum well comprises at least in the central region a lower Al content than the second barrier sublayers , in particular at least 10 % lower , and the at least one quantum well comprises at least in the central region a lower Al content than the first barrier sublayers , in particular at least 5 % lower . In other words , the second barrier sublayers can at least in the central region comprise a higher Al content than the first barrier sublayers , in particular at least 5 % higher , the second barrier sublayers can at least in the central region comprise a higher Al content than the at least one quantum well , in particular at least 10 % higher, and the first barrier sublayers can at least in the central region comprise a higher Al content than the at least one quantum well , in particular at least 5 % higher .

[0039] The aforementioned contents of In in case of the quantum well sublayers and Al in case of the barrier sublayers can for example extend laterally throughout the whole layer ( s ) and quantum well ( s ) , can however in the final light emitting device also be present in the given ratios only in the central region . This can for example be as a quantum well intermixing in the side region may change the In and / or Al ratios in the side region .

[0040] Hence according to some aspects , first and the second quantum well sublayer portions of the central region comprise a larger compressive strain / larger In content than third quantum well sublayer portions of the central region, in particular at least 5 % larger .

[0041] According to some aspects , the quantum well ( s ) and in particular at least the first and second quantum well sublayer is / are -at least at some point of manufacture of the light emitting device and optionally also in the final device- compression strained, in particular with regard to the growth substrate on which the semiconductor layer stack is grown . This can lead to the quantum well ( s ) and in particular at least the first and second quantum well sublayer being compression strained compared to the first barrier layer and / or the second barrier layer . The quantum well ( s ) and in particular at least the first and second quantum well sublayer can for example be compression strained by reducing the Al content of the quantum well ( s ) and in particular at least the first and second quantum well sublayer compared to the Al content of the barrier layers , or by increasing the Al content of the barrier layers compared to the Al content of the quantum well ( s ) and in particular at least the first and second quantum well sublayer .

[0042] According to some aspects , first barrier sublayer portions of the central region comprise a lower Al content than second barrier sublayer portions of the central region, in particular at least 5 % lower , quantum well portion ( s ) of the central region comprise a lower Al content than second barrier sublayer portions of the central region, in particular at least 10 % lower, and quantum well portion ( s ) of the central region comprise a lower Al content than first barrier sublayer portions of the central region, in particular at least 5 % lower . In other words , second barrier sublayer portions of the central region can comprise a higher Al content than first barrier sublayer portions of the central region, in particular at least 5 % higher , second barrier sublayer portions of the central region can comprise a higher Al content than quantum well portion ( s ) of the central region, in particular at least 10 % higher, and first barrier sublayer portions of the central region can comprise a higher Al content than quantum well portion ( s ) of the central region, in particular at least 5 % higher . According to some aspects , the first barrier layer and / or the second barrier layer and / or the third barrier layer ( s ) are tensile strained layers , in particular with regard to the growth substrate on which the semiconductor layer stack is grown . This can lead to the first barrier layer and / or the second barrier layer and / or the third barrier layer ( s ) being tensile strained layers compared to the quantum well ( s ) . The first barrier layer and / or the second barrier layer and / or the third barrier layer ( s ) can for example be tensile strained by reducing the In content of the layer ( s ) compared to the In content of the quantum well ( s ) , or by increasing the In content of the quantum well ( s ) compared to the In content of the barrier layer ( s ) .

[0043] According to some aspects , the first conductivity type is a n-type and the second conductivity type is a p-type . The first layer can thus be a n-type semiconductor layer and the second layer can be a p-type semiconductor layer .

[0044] According to some aspects , the light emitting device further comprises a first contact layer arranged on the first layer and / or a second contact layer arranged on the second layer . In particular the first contact layer can be electrically conductive n-type contact layer, whereas the second contact layer can be electrically conductive p-type contact layer . The first and second contact layer can thereby of the same material system as the semiconductor layer stack but can also be contact layers comprising a metal and / or a transparent conductive oxide (TCO ) such as for example indium tin oxide ( ITO ) . By means of the first and second contact layer a first and a second potential can be applied to the light emitting device to operate the light emitting device in a desired manner .

[0045] The light emitting device can in particular be a small light emitting component / element such as a small LED or pLED . A pLED can in particular be a very small LED with edge lengths down to 40 pm, down to 10 pm, down to 5 pm or even less . Such small LEDs can be free of a growth substrate and require a special handling and processing to improve their IQE and light outcoupling efficiency . According to a further aspect , a method for manufacturing a light emitting device is provided . The method can in particular be a method for manufacturing a light emitting device according to at least some of aforementioned aspects . Hence all aspects already described for the light emitting device can in the same way be applied to the method for manufacturing the same .

[0046] The method comprises at least the following step :

[0047] Providing a semiconductor layer stack of at least a first layer of a first conductivity type , a second layer of a second conductivity type as well as an active region arranged between the first and the second layer , wherein the active region comprises a central region and a side region laterally surrounding the central region, wherein the active region comprises at least one quantum well arranged between a first and a second barrier layer , wherein the at least one quantum well comprises at least a first and a second quantum well sublayer as well as a third quantum well sublayer arranged between the first and the second quantum well sublayer, wherein the first and the second quantum well sublayer at least in the central region comprise a larger compressive strain than the third quantum well sublayer , in particular at least 5 % larger , for example a strain of 100 ppm caused by for example a corresponding In content , and wherein the at least one quantum well at least in the central region comprises a smaller average bandgap than the barrier layers , in particular at least 10 % or at least 100 meV smaller .

[0048] According to some aspects , the method further comprises a step of diffusing a dopant of the second conductivity type into the side region of the active region . The diffusion of the dopant of the second conductivity type into the side region of the active region can then cause a quantum well intermixing in the side region .

[0049] According to some aspects , the step of providing the semiconductor layer stack comprises providing the first and second quantum well sublayers with a higher In content than the third quantum well sublayer . According to some aspects , the step of providing the semiconductor layer stack comprises providing the first and / or second barrier layer with a higher Al content than the quantum well ( s ) and in particular the first and second quantum well sublayer, in particular at least 5 % higher . As a result , the first and / or second barrier layer at the same time comprises a lower Ga / In content than the quantum well ( s ) and in particular the first and second quantum well sublayer . The step of diffusing the dopant of the second conductivity type into the semiconductor layer stack may then cause an equilibration of the Al- Ga / In concentrations within the barrier layers and the quantum well ( s ) and in particular the first and second quantum well sublayer . In particular by means of the quantum well intermixing the Al content in the quantum well ( s ) and in particular the first and second quantum well sublayer is increased ( decreased in the barrier layer ( s ) ) due to Al atoms of the barrier layer ( s ) (with a higher Al concentration and thus a lower Ga concentration ) intermix with Ga / In atoms of the quantum well (with a higher Ga / In concentration and thus a lower Al concentration ) . This equilibration of the Al-Ga / In concentrations within the barrier layers in turn enlarges the bandgap within the quantum well ( s ) and in particular the first and second quantum well sublayer in the side region and keeps charge carriers away from the side region .

[0050] According to some aspects , the step of providing the semiconductor layer stack comprises providing the first and the second barrier layer with each comprising a first and a second barrier sublayer , with the second barrier sublayers each being adj acent to one of the at least one quantum well , wherein the first barrier sublayers at least in the central region comprise a smaller average bandgap than the second barrier sublayers , in particular at least 5 % or at least 10 meV smaller , wherein the at least one quantum well at least in the central region comprises a smaller average bandgap than the second barrier sublayers , in particular at least 10 % or at least 20 meV smaller , and wherein the at least one quantum well at least in the central region comprises a smaller average bandgap than the first barrier sublayers , in particular at least 5 % or at least 10 meV smaller .

[0051] SHORT DESCRIPTION OF THE DRAWINGS Further aspects and embodiments in accordance with the proposed principle will become apparent in relation to the various embodiments and examples described in detail in connection with the accompanying drawings in which

[0052] Fig . 1 shows a cross section of a light emitting device in accordance with some aspects of the proposed principle ;

[0053] Fig . 2 shows a cross section of further embodiment of a light emitting device in accordance with some aspects of the proposed principle ;

[0054] Fig . 3 shows a cross section of further embodiment of a light emitting device in accordance with some aspects of the proposed principle ;

[0055] Fig . 4 shows a diagram of the bandgap of the layers of an embodiment of a light emitting device with regard to its cross section in accordance with some aspects of the proposed principle ;

[0056] Fig . 5 shows a diagram of the bandgap of the layers of another embodiment of a light emitting device with regard to its cross section in accordance with some aspects of the proposed principle ; and

[0057] Fig . 6 shows a diagram of the bandgap of the layers of another embodiment of a light emitting device with regard to its cross section in accordance with some aspects of the proposed principle .

[0058] DETAILED DESCRIPTION

[0059] The following embodiments and examples disclose various aspects and their combinations according to the proposed principle . The embodiments and examples are not always to scale . Likewise , different elements can be displayed enlarged or reduced in size to emphasize individual aspects . It goes without saying that the individual aspects of the embodiments and examples shown in the figures can be combined with each other without further ado , without this contradicting the principle according to the invention . Some aspects show a regular structure or form. It should be noted that in practice slight differences and deviations from the ideal form may occur without , however, contradicting the inventive idea .

[0060] In addition, the individual figures and aspects are not necessarily shown in the correct size , nor do the proportions between individual elements have to be essentially correct . Some aspects are highlighted by showing them enlarged . However , terms such as "above" , "over" , "below" , "under" "larger" , "smaller" and the like are correctly represented with regard to the elements in the figures . So it is possible to deduce such relations between the elements based on the figures .

[0061] Figure 1 shows a cross section of a light emitting device 1 in accordance with some aspects of the proposed principle . The light emitting device 1 comprises a semiconductor layer stack 2 of a first layer 3 of a first conductivity type , a second layer 4 of a second conductivity type as well as an active region 5 arranged between the first and the second layer 3 , 4 .

[0062] In addition, the light emitting device 1 further comprises a first contact layer 9 arranged on the first layer 3 and a second contact layer 10 arranged on the second layer 4 . The first layer 3 is in particular an n-Type layer whereas the second layer 4 is a p-type layer . Hence the first contact layer 9 is an electrically conductive n-type contact layer, whereas the second contact layer 10 is an electrically conductive p-type contact layer . The first and second contact layer can be of the same material system as the semiconductor layer stack 2 but can also be contact layers comprising a metal and / or a transparent conductive oxide (TCO ) such as for example indium tin oxide ( ITO ) . By means of the first and second contact layer 9 , 10 a first and a second potential can be applied to the light emitting device 1 to operate the light emitting device 1 in a desired manner . The active region 5 comprises a central region I la and a side region 11b laterally surrounding the central region Ila . Hence the active region 5 can in a lateral direction be divided in a central region Ila and a side region 11b . In addition, the active region 5 comprises a quantum well 6a arranged between a first and a second barrier layer 7a, 7b . Hence the active region 5 can in a vertical direction perpendicular to the lateral direction be divided into a quantum well 6a arranged between a first and a second barrier layer 7a, 7b . The quantum well 6a is separated into a first and a second quantum well sublayer 8a , 8b as well as a third quantum well sublayer 8 c arranged between the first and the second quantum well sublayer 8a , 8b .

[0063] The layers of the semiconductor layer stack 2 are made of the same material system but comprise different compound concentrations . This is the first and a second barrier layer 7a, 7b comprise a higher Al content than the quantum well 6a which is therefore compression strained . Within the quantum well 6a, the first and second quantum well sublayers 8b comprise a higher In content than the third quantum well sublayer 8c . Hence within the quantum well 6a, the first and second quantum well sublayers 8b comprise a higher compression strain than the third quantum well sublayer 8 c .

[0064] To in addition increase the IQE of the light emitting device 1 , the side region 11b of the active region 5 can comprise a quantum well intermixing to increase the bandgap of the quantum well portion in the side region such that charge carriers are hindered to move into the direction of the side region . This can in particular be suitable as the side region may not only due to singulation processes for singulation of the light emitting device 1 comprise non-radiative recombination centres . By this radiative recombination within the central region can be improved .

[0065] Due to such quantum well intermixing , the mentioned ratios of the Al in the barrier layers as well as the mentioned ratio of In in the first and second quantum well sublayer can be substantially limited only to the central region Ila, since the quantum well intermixing can change the ratios of the Al and In contents in the side region 11b . Figure 2 shows a cross section of a further embodiment of a light emitting device 1 in accordance with some aspects of the proposed principle . The light emitting device 1 and in particular the active region 5 of the light emitting device 1 in contrast to the embodiment of figure 1 comprises a multi quantum well structure .

[0066] In particular , the active region 5 comprises a central region I la and a side region 11b laterally surrounding the central region Ila . Hence the active region 5 can in a lateral direction be divided in a central region I la and a side region 11b . In addition, the active region 5 comprises a first and a second quantum well 6a, 6b, as well as a third barrier layer 7 c arranged between the first and second quantum well 6a, 6b . Hence the active region 5 can in a vertical direction perpendicular to the lateral direction be divided into a first and a second quantum well 6a, 6b arranged between a first and a second barrier layer 7a , 7b with a third barrier layer 7c arranged between the quantum wells 6a , 6b . The quantum wells 6a , 6b are each separated into a first and a second quantum well sublayer 8a, 8b as well as a third quantum well sublayer 8c arranged between the first and the second quantum well sublayer 8a, 8b . The quantum well sublayers 8a, 8b , 8 c are thereby configured and formed as the quantum well sublayers as already described with regard to figure 1 .

[0067] In the embodiment shown only two quantum wells 6a, 6b are drawn . It is however indicated by means of the dots and to be understood that the light emitting device can also comprise more than two quantum wells that can be added in the respective shown order . Hence the second quantum well 6b would be followed again by a third barrier layer 7 c and a third quantum well 6c and so on . In addition, in the embodiments shown, the barrier layers and quantum wells / quantum well sublayers are drawn symmetrically to each other as well as equal in size . This is however only for illustrative purposes and the size and so on can vary in any possible way .

[0068] Figure 3 shows a cross section of a further embodiment of a light emitting device 1 in accordance with some aspects of the proposed principle . The light emitting device 1 and in particular the barrier layers 7a, 7b of the light emitting device 1 in contrast to the embodiment of figure 1 are each separated into a first and a second barrier sublayer 12a , 12b, wherein the second barrier sublayers 12b are each arranged adj acent , in particular directly adj acent in the shown case , to the quantum well 6a .

[0069] Within the first and second barrier layer 7a, 7b , the second barrier sublayers 12b comprise a higher Al content than the first barrier sublayers 8a . In addition, at least the second barrier sublayers 12b comprise a thickness of less than 10 nm and in particular of less than 5 nm . By means of this , the first barrier sublayers 12a can comprise a much less Al content than the second barrier sublayers 12b while still keeping the Al content within the whole barrier layer in the vicinity of the quantum well 6a high enough to enable an efficient quantum well intermixing in side regions of the active region 5 . The reduced thickness of the second barrier sublayers 12b with an increased Al content at the same time allows quantum tunnelling effects through the second barrier sublayers 12b, such that the overall charge carrier mobility within the active region 5 is not or not significantly influenced and the light emitting device 1 can be operated at lower voltages .

[0070] Figure 4 shows a diagram of the average bandgap of the layers of an embodiment of a light emitting device 1 with regard to its cross section in accordance with some aspects of the proposed principle . In particular figure 4 shows a light emitting device 1 as shown in figure 2 with two quantum wells 6a , 6b but without the first and second contact layer .

[0071] In the diagram in the upper part of the figure the respective bandgap for the layer / sublayers of the semiconductor layer stack 2 , as shown in the lower part , is drawn . The first and the second quantum well sublayer 8a , 8b of the quantum wells 6a, 6b comprise the smallest bandgap and are together with the bandgap of the third quantum well sublayer 8 c tuned to emit light of a respective wavelength when operating the light emitting device . The quantum wells 6a, 6b comprise a thickness of at least 15 nm and are arranged between the barrier layers 7a, 7b , 7 c with the highest bandgap , in particular due to a very high Al content .

[0072] In addition, in the embodiments shown, the barrier layers , quantum wells and quantum well sublayers are drawn symmetrically to each other as well as equal in size and bandgap . This is however only for illustrative purposes and the size and bandgap can vary . For example , the third barrier layer 7 c can comprise a different bandgap that is larger or smaller than the one drawn and in particular larger or smaller than the bandgap of the first and second barrier layer 7a, 7b , as long as it larger than that of the quantum wells 6a, 6b .

[0073] Figure 5 shows a diagram of the average bandgap of the layers of another embodiment of a light emitting device 1 with regard to its cross section in accordance with some aspects of the proposed principle . In particular figure 5 shows a light emitting device 1 as shown in figure 2 with two quantum wells 6a , 6b but without the first and second contact layer .

[0074] In the diagram in the upper part of the figure the respective bandgap for the layer / sublayers of the semiconductor layer stack 2 and in particular of the active region within the central region, as shown in the lower part , is drawn . The first and the second quantum well sublayer 8a, 8b of the quantum wells 6a, 6b comprise the smallest bandgap and are together with the bandgap of the third quantum well sublayer 8 c tuned to emit light of a respective wavelength when operating the light emitting device . The division of the quantum wells 6a, 6b into the quantum well sublayers 8a , 8b , 8 c can in particular be a virtual division, as the layers only differ in their ratio of elements , in particular it' s In content throughout its thickness but may by besides this form a continuous layer throughout the thickness of a quantum well .

[0075] It is thus to be understood that the quantum wells 6a , 6b can compared to the embodiment shown be separated into further (virtual ) sublayers , wherein the further sublayers may differ in that they comprise different average bandgaps . As shown, the bandgaps are graded towards the barrier layers , e . g . exponentially or quadratically increasing towards the barrier layers , in particular similar to a polynomial function of fourth order . However , a continuous increase towards the barrier layers and a funnel-shaped graph between the quantum wells is also possible .

[0076] Figure 6 shows a diagram of the average bandgap of the layers of another embodiment of a light emitting device 1 with regard to its cross section in accordance with some aspects of the proposed principle . In particular figure 6 shows a light emitting device 1 as shown in figure 3 but with two quantum wells 6a, 6b and without the first and second contact layer . In the diagram in the upper part of the figure the respective bandgap for the layer / sublayers of the semiconductor layer stack 2 and in particular of the active region within the central region, as shown in the lower part , is drawn .

[0077] The barrier layers 7a , 7b of the light emitting device 1 are each separated into a first and a second barrier sublayer 12a, 12b , wherein the second barrier sublayers 12b are each arranged adj acent , in particular directly adj acent in the shown case , to the quantum well 6a . Hence the quantum wells 6a , 6b are covered from both sides from each a thin second barrier sublayer 12b comprising a high Al content with a third barrier sublayer 12 c being arranged between the quantum wells 6a, 6b with a comparable low Al content . By this the Al content within the whole barrier layers 7a , 7b , 7c can be kept low while in the vicinity of the quantum wells 6a, 6b keeping it high enough to enable an efficient quantum well intermixing in side regions of the active region 5 . The reduced thickness of the second barrier sublayers 12b at the same time allows quantum tunnelling effects through the second barrier sublayers 12b, such that the overall charge carrier mobility within the active region 5 is not or not significantly influenced and the light emitting device 1 can be operated at lower voltages . LIST OF REFERENCES

[0078] 1 light emitting device

[0079] 2 semiconductor layer stack 3 first layer

[0080] 4 second layer

[0081] 5 active region

[0082] 6a, 6b quantum well

[0083] 7a, 7b, 7 c barrier layer 8a, 8b, 8 c quantum well sublayer

[0084] 9 first contact layer

[0085] 10 second contact layer

[0086] Ila central region

[0087] 11b side region 12a , 12b, 12c barrier sublayer

Claims

CLAIMS1. Light emitting device (1) , in particular pLED, comprising a semiconductor layer stack (2) of at least a first layer (3) of a first conductivity type, a second layer (4) of a second conductivity type as well as an active region (5) arranged between the first and the second layer (3, 4) ; wherein the active region (5) comprises a central region (Ila) and a side region (11b) laterally surrounding the central region (Ila) ; wherein the active region (5) comprises at least one quantum well (6a) arranged between a first and a second barrier layer (7a, 7b) ; wherein the at least one quantum well (6a) comprises at least a first and a second quantum well sublayer (8a, 8b) as well as a third quantum well sublayer (8c) arranged between the first and the second quantum well sublayer (8a, 8b) ; wherein the first and the second quantum well sublayer (8a, 8b) at least in the central region (Ila) comprise a larger compressive strain than the third quantum well sublayer (8c) , in particular at least 5 % larger; and wherein the at least one quantum well (6a) at least in the central region (Ila) comprises a smaller average bandgap than the barrier layers (7a, 7b) , in particular at least 10 % smaller.

2. Light emitting device according to claim 1, wherein the active region (5) comprises at least two quantum wells (6a, 6b) arranged between the first and the second barrier layer (7a, 7b) and at least one third barrier layer (7c) arranged between the at least two quantum wells (6a, 6b) .

3. Light emitting device according to claim 2, wherein the at least two quantum wells (6a, 6b) at least in the central region (Ila) comprise a substantially equal average bandgap .

4. Light emitting device according to any one of the preceding claims,wherein the first and the second quantum well sublayer (8a, 8b) at least in the central region (Ila) comprise a substantially equal average bandgap.

5. Light emitting device according to any one of the preceding claims, wherein the first and the second quantum well sublayer (8a, 8b) at least in the central region (Ila) comprise a smaller average bandgap than the third quantum well sublayer (8c) .

6. Light emitting device according to any one of the preceding claims, wherein the first and the second quantum well sublayer (8a, 8b) comprise a thickness of less than 5 nm, in particular less than 3 nm or 2 nm.

7. Light emitting device according to any one of the preceding claims, wherein the third quantum well sublayer (8c) comprises a thickness of less than 7 nm, in particular less than 5 nm or 3 nm.

8. Light emitting device according to any one of the preceding claims, wherein the semiconductor layer stack (2) is of an InGaAlP or InAlGaAs material system.

9. Light emitting device according to any one of the preceding claims, wherein the first and second quantum well sublayer (8a, 8b) at least in the central region (Ila) comprise a higher In content than the third quantum well sublayer (8c) , in particular at least 5 % higher.

10. Light emitting device according to any one of the preceding claims, wherein the side region (11b) , and in particular a quantum well portion of the side region (11b) , comprises a dotation with a dopant of the second conductivity type, in particular Zn.

11. Light emitting device according to claim 10, wherein the side region (11b) , and in particular a quantum well portion of the side region (11b) , comprises a quantum well intermixing .

12. Light emitting device according to any one of the preceding claims, wherein the first and the second barrier layer (7a, 7b) each comprise at least a first and a second barrier sublayer (12a, 12b) , with the second barrier sublayers (12b) each being adjacent to one of the at least one quantum well (6a) ; wherein the first barrier sublayers (12a) at least in the central region (Ila) comprise a smaller average bandgap than the second barrier sublayers (12b) , in particular at least 5 % smaller; wherein the at least one quantum well (6a) at least in the central region (Ila) comprises a smaller average bandgap than the second barrier sublayers (12b) , in particular at least 10 % smaller; and wherein the at least one quantum well (6a) at least in the central region (Ila) comprises a smaller average bandgap than the first barrier sublayers (12a) , in particular at least 5 % smaller.

13. Light emitting device according to claim 12, wherein the first barrier sublayers (12a) at least in the central region (Ila) comprise a lower Al content than the second barrier sublayers (12b) , in particular at least 5 % lower; wherein the first and second quantum well sublayer (8a, 8b) at least in the central region (Ila) comprises a lower Al content than the second barrier sublayers (12b) , in particular at least 10 % lower; and wherein the first and second quantum well sublayer (8a, 8b) at least in the central region (Ila) comprises a lower Al content than the first barrier sublayers (12a) , in particular at least 5 % lower.

14. Light emitting device according to any one of the preceding claims, further comprising a first contact layer (9) arranged on the first layer (3) and / or a second contact layer (10) arranged on the second layer ( 4 ) .

15. Method for manufacturing a light emitting device (1) , in particular pLED, comprising the step:Providing a semiconductor layer stack (2) of at least a first layerconductivity type as well as an active region (5) arranged between the first and the second layer (3, 4) ; wherein the active region (5) comprises a central region (Ila) and a side region (11b) laterally surrounding the central region (Ila) ; wherein the active region (5) comprises at least one quantum well (6a) arranged between a first and a second barrier layer (7a, 7b) ; wherein the at least one quantum well (6a) comprises at least a first and a second quantum well sublayer (8a, 8b) as well as a third quantum well sublayer (8c) arranged between the first and the second quantum well sublayer (8a, 8b) ; wherein the first and the second quantum well sublayer (8a, 8b) at least in the central region (Ila) comprise a larger compressive strain than the third quantum well sublayer (8c) , in particular at least 5 % larger; and wherein the at least one quantum well (6a) at least in the central region (Ila) comprises a smaller average bandgap than the barrier layers (7a, 7b) , in particular at least 10 % smaller.

16. The method according to claim 15, further comprising a step of diffusing a dopant of the second conductivity type into the side region (11b) of the active region (5) , wherein the dopant is in particular Zn.

17. The method according to claim 15 or 16, further comprising a step of quantum well intermixing the side region (11b) of the active region (5) .

18. The method according to any one of claims 15 to 17, wherein the first and the second barrier layer (7a, 7b) each comprise at least a first and a second barrier sublayer (12a, 12b) , with the second barrier sublayers (12b) each being adjacent to one of the at least one quantum well (6a) ; wherein the first barrier sublayers (12a) at least in the central region (Ila) comprise a smaller average bandgap than the second barrier sublayers (12b) , in particular at least 5 % smaller;wherein the at least one quantum well (6a) at least in the central region (Ila) comprises a smaller average bandgap than the second barrier sublayers (12b) , in particular at least 10 % smaller; and wherein the at least one quantum well (6a) at least in the central region (Ila) comprises a smaller average bandgap than the first barrier sublayers (12a) , in particular at least 5 % smaller.

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