Quantum well structure

Abstract

A quantum well structure, preferably configured to emit red light, comprising: a first barrier layer 310a, preferably GaN; a first quantum well layer 320a, preferably InGaN; a first spacer layer 330a,

Description

Technical field The present application relates to a quantum well structure. The present application also relates to a semiconductor device, an electronic device and a method of forming a multiple quantum well structure. Background Light emitting diodes (LEDs) have found use in many applications. Their improved attributes over incandescent light sources in areas such as efficiency, size and brightness have contributed to their widespread use. Increasingly, there is a desire to shrink the size of LEDs even further and form a new class of LEDs known as MicroLEDs. MicroLEDs typically comprise a diameter of 100 pm or less. However, in some examples, microLEDs can comprise a diameter of 10 pm or less. There is great interest in microLEDs because their smaller size, compared to conventional LEDs, leads to a greater number of emitters per pixel with increased brightness, contrast, colour gamut and efficiency. These attributes are particularly desirable for display systems in smaller battery-powered electronic devices including smartwatches or other wearable technology, as well as virtual reality (VR) or augmented reality (AR) headsets or smart glasses. Conceptually, microLEDs displays are formed in a similar manner to conventional LED displays. Initially, semiconductor layers, typically lll-V semiconductor layers, are epitaxially formed on a substrate to form a semiconductor wafer. The semiconductor layers include a quantum well structure, which dictates the colour emission wavelength. The semiconductor layers are then patterned and etched to form the microLED structure and contacts. The semiconductor wafer thus comprises many individual microLEDs arranged on the wafer. The wafer is subsequently bonded to driving circuitry, commonly formed using Si components. The driving circuitry can control induvial microLEDs to light up to thereby control the display. Display systems using microLEDs involve the use of red, blue and green emitting microLEDs. Displays using larger LEDs involve the use of GaN-based materials to form the blue and green emitting LEDs and AllnGaP-based materials to form the red emitting LEDs. It would be desirable to achieve red emitting LEDs using GaN-based materials, as this would enable the ability to produce red, blue and green emitting microLEDs in a common material system. However, reliably achieving red emission in microLED structures in the GaN-material system has proved challenging. Summary It is an object of the disclosure to obviate or eliminate at least some of the abovedescribed disadvantages associated with existing techniques. According to a first aspect there is provided a quantum well structure comprising: a first barrier layer; a first quantum well layer over the first barrier layer; a first spacer layer over the first quantum well layer; and a second quantum well layer over the first spacer layer. The first spacer layer comprises a larger bandgap than the first barrier layer; and the first spacer layer comprises a thinner thickness than the first barrier layer. In some examples, the quantum well structure according is configured to emit a first light comprising a wavelength between 610 nm to 650 nm. In some examples, a thickness of the first quantum well layer is greater than a thickness of the first spacer layer. In some examples, the thickness of the first spacer layer is 1.5 nm or less. In some examples, the thickness of the first quantum well layer and the second quantum well layer is 3 nm or less. In some examples, the quantum well structure comprises 11 l-N semiconductor material. In some examples, the first quantum well layer and the second quantum well layer comprises InxGal-xN, where 0.2 <x <0.3. In some examples, x = 0.25. In some examples, the first quantum well layer and the second quantum well layer comprise GaN. In some examples, the first spacer layer comprises AlyGa1-yN, where 0.5 <y <1. In some examples, 0.75 <y <1. In some examples, the first quantum well layer comprises a first sub-layer and a second sub-layer. In some examples, the first sub-layer comprises InGaN and the second sublayer comprises GaN. In some examples, the second quantum well layer comprises a third sub-layer and a fourth sub-layer. In some examples, the third sub-layer comprises GaN and the fourth sub-layer comprises InGaN. In some examples, the quantum well structure is configured to emit a second light comprising a different wavelength to the first light. In some examples, the first barrier layer comprises GaN. In some examples, the first barrier layer comprises a cladding layer. In some examples, the spacer layer comprises a smaller lattice constant than the first barrier layer; and the first quantum well layer and the second quantum well layer comprise a larger lattice constant than the first barrier layer. According to a second aspect there is provided a semiconductor device comprising the quantum well structure according to the first aspect. According to a third aspect there is provided an electronic device comprising the semiconductor device according to the second aspect. According to a fourth aspect there is provided a semiconductor structure comprising: a first barrier layer; a quantum well layer over the first barrier layer; a second barrier layer directly on the first barrier layer; and a cladding layer directly on the second barrier layer. The second barrier layer comprises a larger bandgap than the first barrier layer. In some examples, the second barrier layer comprises a thinner thickness than the quantum well layer. In some examples, the first barrier layer comprises a second cladding layer. According to a fifth aspect there is provided a method of forming a quantum well structure comprising: forming a first barrier layer; forming a first quantum well layer over the first barrier layer; forming a first spacer layer over the first quantum well layer; and forming a second quantum well layer over the first spacer layer. The first spacer layer comprises a larger bandgap than the first barrier layer; and the first spacer layer comprises a thinner thickness than the first barrier layer. Brief description of the drawings For a better understanding of the techniques, and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is an example of a semiconductor structure; Figure 2 is an example of a quantum well structure; Figure 3 is an example of a quantum well structure; Figure 4 is an example of a quantum well structure; Figure 5 is an example of a quantum well structure; Figure 6 is an example of a semiconductor structure; Figure 7 is an example of a semiconductor structure; Figure 8 is a flowchart illustrating process steps in a method. Detailed Description Epitaxy or epitaxial means crystalline growth of material, usually via high temperature deposition. Epitaxy can be effected in a molecular beam epitaxy (MBE) tool in which layers are grown on a heated substrate in an ultra-high vacuum environment. Elemental sources are heated in a furnace and directed towards the substrate without carrier gases. The elemental constituents react at the substrate surface to create a deposited layer. Each layer is allowed to reach its lowest energy state before the next layer is grown so that bonds are formed between the layers. Epitaxy can also be performed in a metalorganic vapour phase epitaxy (MOVPE) tool, also known as a metal-organic chemical vapour deposition (MOCVD) tool. Compound metal-organic and hydride sources are flowed over a heated surface using a carrier gas, typically hydrogen. Epitaxial deposition occurs at much higher pressure than in an MBE tool. The compound constituents are cracked in the gas phase and then reacted at the surface to grow layers of desired composition. Deposition means the depositing of a layer on another layer or substrate. It encompasses epitaxy, chemical vapour deposition (CVD), powder bed deposition and other known techniques to deposit material in a layer. A compound material comprising one or more materials from group III of the periodic table with one or more materials from group V is known as a lll-V material. The compounds have a 1:1 combination of group III and group V regardless of the number of elements from each group. Subscripts in chemical symbols of compounds refer to the proportion of that element within that group. Thus Alo.25Gao.75As means the group III part comprises 25% Al, and thus 75% Ga, whilst the group V part comprises 100% As. Crystalline means a material or layer with a single crystal orientation. In epitaxial growth or deposition subsequent layers with the same or similar lattice constant follow the registry of the previous crystalline layer and therefore grow with the same crystal orientation. In-plane is used herein to mean parallel to the surface of the substrate; out-of-plane is used to mean perpendicular to the surface of the substrate. Substrate means a planar wafer on which subsequent layers may be deposited or grown. A substrate may be formed of a single element or a compound material, and may be doped or undoped. For example, common substrates include silicon (Si), gallium arsenide (GaAs), silicon germanium (SiGe), silicon germanium tin (SiGeSn), indium phosphide (InP), and gallium antimonide (GaSb). A substrate may be on-axis, that is where the growth surface aligns with a crystal plane. For example it has <100> crystal orientation. References herein to a substrate in a given orientation also encompass a substrate which is miscut by up to 10° towards another crystallographic direction, for example a (100) substrate miscut towards the (111) plane. Vertical or out of plane means in the growth direction; lateral or in-plane means parallel to the substrate surface and perpendicular to the growth direction. Doping means that a layer or material contains a small impurity concentration of another element (dopant) which donates (donor) or extracts (acceptor) charge carriers from the parent material and therefore alters the conductivity. Charge carriers may be electrons or holes. A doped material with extra electrons is called n-type whilst a doped material with extra holes (fewer electrons) is called p-type. Where a device is described it should be understood that it will typically be formed on a circular substrate wafer of 4” (100mm), 6” (150mm), 8” (200mm), 12” (300mm) or greater diameter. After growth, deposition, bonding and other fabrication steps the devices are separated by dicing the wafer and layers into devices (chips) of appropriate dimensions. Typically tens, hundreds or thousands of devices are cut from a single wafer. Throughout the present disclosure corresponding elements in the Figures are labelled with corresponding reference numerals. To provide additional context to the examples according to the present disclosure, there now follows a further discussion of the drawbacks, which conventional examples suffer from. Figure 1 illustrates an example of a semiconductor structure 101 for forming a microLED. The semiconductor structure 101 includes a substrate 100. The substrate 100 may comprise a growth substrate to enable the epitaxial formation of semiconductor layers thereon. Semiconductor structure 101 further comprises a first cladding layer 200, an active region 300, a second cladding layer 400 and a cap layer 500 stacked on the substrate 100. In some examples, the first cladding layer 200, the active region 300, the second cladding layer 400 and the cap layer 500 may comprise a microLED layer stack for forming a microLED. In some examples, the microLED layer stack may thus comprise device layers for a microLED. In some examples, the microLED stack may be epitaxially grown on the substrate 100, such as by MOCVD or MBE. The active region 300 may comprise one or more light emitting layers comprising structures such as quantum wells, quantum dots, or a combination thereof. The active region 300 may further comprise one or more barrier layers separating the light emitting layers. The cladding layers 200, 400 are disposed either side of the active region 300. The cladding layers 200, 400 are configured to form a p-n junction either side of the active region 300, such that carriers can recombine in the active region 300 for light emission. The cladding layers 200, 400 may be oppositely doped. For example, the first cladding layer 200 may be doped n-type and the second cladding layer 400 may be doped p-type. In other examples, the first cladding layer 200 may be p-type and the second cladding layer 400 may be n-type. Semiconductor structure 101 further comprises cap layer 500. Cap layer 500 is highly doped in order to form an ohmic contact with contacts fabricated on the cap layer 500. In some examples the semiconductor structure 101 may further comprise one or more mirrors. For example, the semiconductor structure 101 may comprise a first mirror situated between the cladding layer 200 and the substrate 100 to reflect light and increase the microLED brightness. In some examples, the semiconductor structure may comprise a second mirror situated between the cladding layer 400 and the cap layer 500. The first and second mirrors may thus form a resonant cavity, which may be used for forming a resonant cavity microLED. In such examples, the mirror may be formed of semiconductor material such as those for forming a distributed Bragg reflector (DBR). In some examples, contacts may be fabricated on to the semiconductor structure 101 to form the microLED. For example, a first contact may be applied to the substrate 100 and a second contact may be applied to the cap layer 500. The contacts inject carriers into the active region 300, where they recombine to result in the emission of light from the microLED. As described above, semiconductor structure 101 may comprise lll-N semiconductor material. For example, the cladding layer 200, active region 300, cladding layer 400 and cap layer 500 may comprise lll-N semiconductor material. The substrate 100 may thus comprise a material to permit the formation of lll-N epitaxial material thereon. For example, the substrate 100 may comprise Si, SiC or sapphire. As further described above, lll-N semiconductor material has been used for producing blue and green emitting microLEDs. It is desirable to additionally achieve red emission in the lll-N material system. However, the reliable fabrication of red-emitting lll-N microLEDs is challenging. One of the main challenges that has restricted the utilization of red-emitting lll-N microLEDs is the quantum confined Stark effect. The active region 300 commonly includes a multiple quantum well structure comprising a plurality of quantum well structures stacked on one another. The multiple quantum well structure typically comprises quantum well layers separated by barrier layers. The quantum well layers and barrier layers are typically polar-orientated. The interfaces between the polar-orientated quantum well layers and barrier layers comprise a difference in spontaneous and strain-induced polarization, which results in the build-up of charges at the interfaces. The charges result in the build-up of an electric field in the quantum well layers and barrier layers. The electric field in the quantum well layers results in the spatial separation of electrons and holes, which is a phenomenon known as the quantum-confined Stark effect. The quantum-confined Stark effect reduces quantum efficiency and increases non-radiative recombination. The reduced quantum efficiency and increased non-radiative recombination is also more problematic for lll-N quantum well layers that are designed for longer wavelength emission, for example, for red-emission. The increased quantum-confined Stark effect for red-emitting InGaN quantum well layers results in poor carrier confinement, which degrades the quantum efficiency and further results in increased current density. Additionally, lll-N quantum well layers designed to emit longer wavelengths, such as for red-emission, typically comprise InGaN with a greater concentration of In, compared to InGaN quantum well layers that are designed to emit shorter wavelengths, such as for blue-emission or green-emission. The barrier layers typically comprise GaN. The greater the concentration of In in the InGaN quantum well layers, the greater the strain that is induced between the InGaN quantum well layers and the GaN barrier layers. The increased strain results in a greater polarization difference creating a more pronounced quantum-confined Stark effect. Furthermore, epitaxially forming InGaN quantum well layers with a greater In concentration typically involves low growth temperatures. Lower growth temperatures typically result in InGaN quantum well layers with a greater impurity concentration, which also increases non-radiative recombination. Examples according to the present disclosure provide a quantum well structure that aims to at least partially mitigate the above-described problems. Examples according to the present disclosure provide a quantum well structure that comprises a first barrier layer; a first quantum well layer over the first barrier layer; a first spacer layer over the first quantum well layer; and a second quantum well layer over the first spacer layer. The first spacer layer comprises a larger bandgap than the first barrier layer and a thinner thickness than the first barrier layer. In examples, according to the present disclosure the increased bandgap and the reduced thickness of the spacer layer helps to improve quantum efficiency and reduce non-radiative recombination. In some examples, the larger bandgap and thin thickness of the spacer layer can result in a band bending effect, which can promote radiative recombination in the quantum wells. Additionally, in some examples, the larger bandgap of the spacer layer may increase the confinement of holes in the valence band of the first quantum well layer and / or second quantum well. The increased confinement of holes may further improve the radiative recombination of the quantum well structure. Furthermore, in some examples, the spacer layer may help to strain balance the multiple quantum well structure. For example, the larger bandgap of the spacer layer may also result in the spacer layer comprising a smaller lattice constant than the barrier layer, whereas the first and second quantum well layers may comprise larger lattice constant than the barrier layer. This can result in strain balancing in the quantum well structure thereby reducing the quantum confined Stark effect. In some examples, it is hypothesised that the thin thickness of the spacer layer may result in carriers from one of the first quantum well or the second quantum well transitioning through the spacer layer and recombining with carriers in the other of the first quantum well or the second quantum well. For example, by appropriately configuring the spacer layer thickness and bandgap, a favourable recombination may occur between the top of the valance band in first quantum well or the second quantum well and the bottom of the conduction band in the other of the first quantum well or the second quantum well. In some examples, the bandgap and thickness of the spacer layer may therefore determine the recombination bandgap and thus the emission wavelength of the multiple quantum well structure. In some examples, however, the radiative transition of electrons and holes may not occur through the spacer layer. In some examples, recombination may thus occur between electrons and holes confined in the first quantum well layer and between electrons and holes confined in the second quantum well layer. Figure 2 is an example of a quantum well structure 301a. In some examples, active region 300 may comprise the quantum well structure 301a. First quantum well structure 301a comprises a barrier layer 310a, first quantum well layer 320a, spacer layer 330a and a second quantum well layer 340a. As described above, in some examples, the spacer layer 330a comprises a larger bandgap than the barrier layer 310a. Additionally, the spacer layer 330a further comprises a thinner thickness than the barrier layer 310a. In some examples, such a configuration of the spacer layer 330a and barrier layer 310a may improve quantum efficiency and reduce non-radiative recombination. In some examples, the barrier layer 310a comprises GaN, the first quantum well layer 320a comprises InGaN, the spacer layer 330a comprises AIGaN and the second quantum well layer 340a comprises InGaN. In such examples, the spacer layer 330a may thus comprise a larger bandgap than the barrier layer 310a. Furthermore, the spacer layer 330a may thus comprise a larger bandgap than the first quantum well layer 320a and the second quantum well layer 340a. The spacer layer 330a further comprises a thin thickness, for example, of 1.5 nm or less. The barrier layer 310a may comprise a thickness of about 10 nm. In such examples, the spacer layer 330a may thus provide a band bending effect to improve the quantum efficiency of the quantum well structure 301a. In some examples, the large bandgap and thin thickness of the spacer layer may enable carriers to transition across the spacer layer 330a from the first quantum well layer 320a and recombine with carriers in the second quantum well layer 340a. In some examples, the transition process may involve carrier tunnelling. The thickness and the bandgap of the spacer layer 330a may thus result in a spatial and energy separation that may set the emission wavelength of the first quantum well structure 301a. In some examples, the spacer layer 330a and / or the barrier layer 310a may comprise InAIN, InGaAIN, ScAIN or BAIN. In some examples, the spacer layer 330a and / or the barrier layer 310a may be doped n-type or p-type. In some examples, the composition of the spacer layer 330a and / or the barrier layer 310a may be graded in a continuous or stepped manner. In such examples, the alloy, doping and / or composition grading of the spacer layer 330a and / or the barrier layer 310a may result in band bending or strain compensation effects to improve the quantum efficiency of the quantum well structure 301a. In some examples, the first quantum well layer 320a and the second quantum well layer 340a may comprise lnxGai.xN, where 0.2 <x <0.3, for example x = 0.25. In some examples, the spacer layer 330a may comprise AlyGai.yN, where 0.5 <y <1. In such examples, this configuration of the first quantum well layer 320a and the second quantum well layer 340a and the spacer layer 330a may result in a band bending effects to result in emission in the visible part of the electromagnetic spectrum. In some examples, the first quantum well layer 320a and the second quantum well layer 340a may comprise InGaAIN or InN. In some examples, the first quantum well layer 320a and the second quantum well layer 340a may be doped n-type or p-type. In some examples, increasing the concentration of Al in the AlyGai.yN, where 0.5 <y <1 spacer layer 330a may increase the emission wavelength of the quantum well structure 301a. In some examples, the emission wavelength of the quantum well structure 301a may lie within the red part of the visible spectrum. In some examples, the wavelength emission may lie within the range 610 nm to 650 nm. Furthermore, the spacer layer 330a comprises a smaller lattice constant than the barrier layer 310a. The first quantum well layer 320a and the second quantum well layer 340a additionally comprise a larger lattice constant than the barrier layer 310a. In some examples, the combination of lattice constants of the barrier layer 310a, first quantum well layer 320a, spacer layer 330a and second quantum well layer 340a may thus provide a strain compensation effect, which may help combat the quantum confined Stark effect and further improve the quantum efficiency and reduce non-radiative recombination of the first quantum well structure 301a. In some examples, the spacer layer 330a may thus comprise a thinner thickness than the barrier layer 310a, and the spacer layer 330a may further comprise a thinner thickness than the first quantum well layer 320a and the second quantum well layer 340a. In some examples, first quantum well layer 320a and the second quantum well layer 340a may comprise a thinner thickness than the barrier layer 310a. In such examples, the thicknesses and lattice constants of the layers of the first quantum well structure 301a may thus assist in providing a strain compensation effect to the first quantum well structure 301a to reduce the effects of the quantum confined Stark effect. In some examples, the barrier layer 310a may comprise a thickness of about 10 nm. In some examples, the first quantum well layer 320a may comprise a thickness of 3 nm or less. In some examples, the spacer layer 330a may comprise a thickness of 1.5 nm or less, for example 1 nm. In some examples, the second quantum well layer 340a may comprise a thickness of about 3 nm or less. In some examples, the first quantum well layer 320a and the second quantum well layer 340a may comprise GaN. In some examples, the effect of the spacer layer 330a may enable the formation of quantum well layers 320a, 340a, which do not comprise In. The formation of In-free quantum well layers 320a, 340a may permit the formation of the quantum well layers 320a, 340a at higher temperatures, which may reduce the presence of defects in the quantum well layers 320a, 340a and thereby reduce non-radiative recombination. In such examples, the spacer layer 330a may comprise AIN. In some examples, the thickness of the first quantum well layer 320a and the second quantum well layer 340a may be varied to adjust the emission wavelength of the quantum well structure 301a. For example, the thickness of the quantum well layers 320a, 320b may vary from 5 nm to 15 nm and may comprise, for example, about 10 nm. In such examples, the quantum well structure 301a may emit light in the visible part of the electromagnetic spectrum. Figure 3 illustrates a multiple quantum well (MQW) structure comprising a plurality of quantum well structures 301a, 301b stacked on one another. In some examples, the active region 300 may comprise the MQW structure. The first quantum well structure 301a comprises the quantum well structure 301a as described above. The MQW structure further comprises a second quantum well structure 301b. The MQW structure thus comprises two quantum well structures. However, in other examples the active region 300 may comprise any suitable number of quantum well structures. Second quantum well structure 301b comprises a barrier layer 310b, first quantum well layer 320b, spacer layer 330b and a second quantum well layer 340b. The features and functionality described above for the layers of the first quantum well structure 301a may equally apply to the corresponding layers of the second quantum well structure 301b. As illustrated in Figure 3, the barrier layer 310b of the second quantum well structure 301b is stacked on an upper surface of the second quantum well layer 340a of the first quantum well structure. Barrier layer 310b comprises a thicker thickness than the spacer layer 330a, which may confine carriers to the second quantum well layer 340a of the first quantum well structure 301a, and to the first quantum well layer 320b of the second quantum well structure 301b disposed either side of the barrier layer 310b. As such, carriers from the second quantum well layer 340a of first quantum well structure 301a may not transition through the barrier layer 310b of the second quantum well structure 301b to recombine with carriers in the first quantum well 320b of the second quantum well structure 301b. For examples, the carriers may have a negligible probability of tunnelling through the barrier layer 310b. In one example, the barrier layers 310a, 310b may comprise GaN with a thickness of about 10 nm. In some examples, recombination may therefore be confined between the first and second quantum well layers 320a, 340a of the first quantum well structure 301a. Similarly, the materials and thicknesses of the layers of the second quantum well structure 301b may be configured such that recombination may be confined between the first quantum well layer 320a and the second quantum well layer 340b of the second quantum well structure 301b, but not with quantum well layers outside of the second quantum well structure 301b. Figure 4 is another example of a first quantum well structure 301a. The first quantum well structure 301a comprises a first quantum well layer 320a comprising a first sub-layer 322a and a second sub-layer 324a, over the first sub-layer 322a. In some examples, dividing the first quantum well layer 320a into two sub-layers 322a, 324a, may enable first quantum well structure 301a to emit two wavelengths of light. For example, first sub-layer 322a may comprise a first material and the second sub-layer 324a may comprise a second material different to the first material. In one example, the first sub-layer 322a comprises InGaN and the second sub-layer 324a comprises GaN. In such examples, the band structure of the first sub-layer 322a and the second sublayer 324a is different. In particular, the bandgap defined between, for example, the top of the valance band in the second quantum well layer 340a and the bottom of the conduction band in the first sub-layer 322a may be different to the top of the valance band in the second quantum well layer 340a and the bottom of the conduction band in the second sub-layer 324a. As such, the inclusion of the first sub-layer 322a and the second sub-layer 324a in the first quantum well layer 320a may permit two locations for the recombination of carriers through the spacer layer 330a between the second quantum well layer 340a and each of the first and second sub-layers 322a, 324a of the first quantum well layer 320a. The two recombination paths have different bandgaps and, as such, result in the emission of two different wavelengths. Figure 5 illustrates another example of a first quantum well structure 301a. The first quantum well structure 301a comprises a second quantum well layer 340a comprising a third sub-layer 342a and a fourth sub-layer 324a, over the third sub-layer 342a. In a similar manner to that described above, dividing the second quantum well layer 340a into two sub-layers 342a, 344a, may enable first quantum well structure 301a to emit two wavelengths of light. In a similar manner to that described above, the third sub-layer 342a may comprise a first material and the fourth sub-layer 344a may comprise a second material different to the first material. In one example, the third sub-layer 342a comprises GaN and the fourth sub-layer 324a comprises InGaN. The band structure of the third sub-layer 342a and the fourth sub-layer 324a may thus be different. In a similar manner to that described above, the inclusion of the third sub-layer 342a and the fourth sub-layer 344a in the second quantum well layer 340a may permit two locations for the recombination of carriers through the spacer layer 330a between the first quantum well layer 320a and each of the third and fourth sub-layers 342a, 344a of the second quantum well layer 340a. The two recombination paths have different bandgaps and, as such, result in the emission of two different wavelengths. In some examples, the quantum well structure 301a illustrated in Figure 4 and Figure 5 may thus be configured to emit two wavelengths. In some examples, the dominant recombination path may be dependent on the voltage and / or current density applied to the quantum well structure 301a. For example, one recombination path may be dominant at a first bias voltage and / or a first applied current density, and a second recombination path may be dominant at a second bias voltage and / or a second applied current density. Furthermore, it will be appreciated that whilst the discussion in relation to Figures 4 and 5 provide examples of a quantum well structure capable of emitting two different wavelengths of light, in other examples, a quantum well structure according to examples of the present disclosure may be configured to emit more than two wavelengths of light. Figure 6 is an example of a semiconductor structure 600. Semiconductor structure comprises elements in common with semiconductor structure 100 described above. In some examples, semiconductor structure 600 may thus comprise an epitaxial semiconductor stack for forming a LED or microLED. Semiconductor structure 600 comprises an active region 300 comprising first quantum well layer 320a, spacer layer 330a and a second quantum well layer 340a. As illustrated in Figure 6, the first semiconductor layer 320a is over the first cladding layer. In some examples, the cladding layer 200 may perform a similar function to the barrier layer 310a described above. As such, a quantum well structure according to examples of the present disclosure may comprise a barrier layer comprising the cladding layer 200. In such examples, the spacer layer 330a may thus comprise a larger bandgap and a thinner thickness than the first cladding layer 200. In such examples, the configuration of the spacer layer 330a relative to the cladding layer 200 may improve the quantum efficiency and reduce non-radiative recombination for the active layer 300. In one example, the first cladding layer comprises n-type (AI)GaN. In a similar manner, second cladding layer 400 is over the second quantum well layer 340a. In some examples, second cladding layer 400 may thus perform a similar function to a barrier layer in a similar manner to the first cladding layer 200. In some examples, the second cladding layer 400 may comprise p-type (AI)GaN. In such examples, the first cladding layer 200 and second cladding layer 400 may provide a barrier function that may confine carriers to recombine in the quantum well layers 320a, 340a of the active region 300. Figure 7 is an example of a semiconductor structure 700 Semiconductor structure comprises elements in common with semiconductor structure 100 described above. In some examples, semiconductor structure 700 may thus comprise an epitaxial semiconductor stack for forming a LED or microLED. Semiconductor structure 700 comprises an active region 300 over the first cladding layer 200. Active region 300 comprises a first quantum well layer 320a and barrier layer 730a. In some examples, the barrier layer 730a may be directly on the quantum well layer 320a. Semiconductor structure 700 further comprises a second cladding layer 400. In some examples, second cladding layer 400 may be directly on the barrier layer 730a. Semiconductor structure 700 thus illustrates a semiconductor structure with a single quantum well active region 300. Semiconductor structure 700 further comprises a barrier layer 730a acting as an interfacial layer between the quantum well layer 320a and the second cladding layer 400. In some examples, the barrier layer 730a comprise a large bandgap and a thin thickness. For example, the barrier layer 730a may comprise AI(Ga)N and thickness of 1.5 nm or less, for example 1 nm. In some examples, the quantum well layer 320a may comprise InGaN with a thickness of 3 nm or less. The barrier layer 730a may thus comprise a larger bandgap and thinner thickness than the quantum well layer 730a. In such examples, the barrier layer 730a may thus be configured to result in band bending effects, which may result in the quantum well layer 320a emitting light at longer wavelengths in the red part of the visible spectrum. Furthermore, the barrier layer 730a may comprise a lattice constant that is smaller than the lattice constant of first and second cladding layers 200, 400, and the quantum well layer 320a may comprise a lattice constant that is larger than the lattice constant of the first and second cladding layers 200, 400. In such examples, the barrier layer 730a may thus provide strain compensation effects in the semiconductor structure 700. Furthermore, as the active region 300 comprises a single quantum well layer 730a, when fabricated into a LED or microLED structure, the quantum well layer 730a may emit light with a low pump density, compared to an active region with a multiple quantum well structure. In some examples, the barrier layer 730a may comprise AlyGai.yN, where 0.5 <y <1. In some examples, 0.75 <y <1. In some examples, the barrier layer 730a may comprise a thickness of 1.5 nm or less, for example 1 nm. In some examples, the quantum well layer may comprise lnxGai.xN, where 0.2 <x <0.3, for example, x = 0.25. In some examples, a second barrier layer may be inserted between the first cladding layer 200 and the quantum well layer 320a. In some examples, the second barrier layer may comprise substantially corresponding features to the barrier layer 730a. The second barrier layer may thus additionally provide band bending and / or strain compensation effects. Furthermore, in some examples, the second barrier layer may shield the quantum well layer 320a from dopant diffusion from the first cladding layer 200. In some examples, the barrier layer 730a may comprise a graded layer. In some examples, the barrier layer 730a may grade from a high Al % composition at the surface of the barrier layer 730a adjacent the quantum well layer 320a to a low Al % composition, or GaN composition at the surface of the barrier layer 730a adjacent the second barrier layer 400. In such examples, the barrier layer 730a with a high Al % near the quantum well layer 320a may provide the band bending a strain compensation effect described above. However, the grade towards a low Al % composition or a GaN composition at a surface near the cladding layer 400 may increase the conductivity in layers above the barrier layer 730a. For example, decreasing the Al % composition, decreases the bandgap of the material, which in turn increases conductivity. Additionally, the grade towards a lower Al % composition can increase the conductivity due to a distributed polarization doping effect. In some examples, the barrier layer 730a may comprise a composition of AlyGai.yN, where 0.75 <y <1 at the surface of the barrier layer 730a adjacent to the quantum well layer 320a and a composition of AlyGai.yN, where 0 <y <0.25 at the surface of the barrier layer 320a adjacent to the cladding layer 400. In some examples, the cladding layer 400 may comprise a graded composition. In some examples, the cladding layer 400 may comprise a graded composition in addition to or instead of the barrier layer 730a. In such examples, the grading of the cladding layer may improve the conductivity of the cladding layer, in a similar manner to the barrier layer 730a described above. In some examples, the cladding layer 400 may comprise a composition of AlyGai.yN, where 0.75 <y <1 at the surface of the cladding layer 400 adjacent to the barrier layer 720a and a composition of AlyGai.yN, where 0 <y <0.25 at the surface of the cladding layer 400 adjacent to the cap layer 500. Figure 8 is a flowchart illustrating a method 800 of forming a quantum well structure. The method 800 comprises, in a first step 810, forming a first barrier layer. The method 800 further comprises, in a second step 820, forming a first quantum well layer over the first barrier layer. The method 800 further comprises, in a third step 830, forming a first spacer layer over the first quantum well layer. The method 800 further comprises, in a fourth step 840, forming a second quantum well layer over the first spacer layer. The first spacer layer comprises a larger bandgap than the first barrier layer and the first spacer layer comprises a thinner thickness than the first barrier layer. The present disclosure further provides a semiconductor device comprising a quantum well structure according to examples of the present disclosure. In some examples the semiconductor device may comprise a photonic semiconductor device. In some examples the photonic semiconductor device may comprise one of: a light emitting diode (LED) or a microLED (pLED). The present disclosure further provides an electronic device comprising a semiconductor device according to examples of the present disclosure. In some examples the electronic device may comprise an electronic device for user operation. In some examples the electronic device may comprise a communication device such as a mobile telephone, smartphone or similar. In some examples the electronic device may comprise handheld computing device, such as a tablet or similar. In some examples the electronic device may comprise a visual display device, such as a television, a monitor or similar. In some examples the electronic device may comprise a wearable device, such as a smartwatch, smart glasses, or similar. In some examples the electronic device may comprise a gaming device such as a games console, or similar. In some examples the electronic device may comprise a comprise a headset such as a virtual reality (VR) headset, an augmented reality (AR) headset, a mixed reality headset, or similar. In some examples the electronic device may comprise an appliance such as a household appliance, for 5 example a refrigerator or a washing machine, or similar. It should be noted that the above-mentioned embodiments illustrate rather than limit the idea, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does 10 not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope. 15

Claims

1. A lll-N quantum well structure (301a) comprising:a first barrier layer 310a);5 a first quantum well layer (320a) over the first barrier layer (310a);a first spacer layer (330a) over the first quantum well layer (320a); anda second quantum well layer (340a) directly on the first spacer layer (330a); wherein the first spacer layer (330a) comprises a larger bandgap than the first barrier layer (310a); and10 wherein the first spacer layer (330a) comprises a thinner thickness than the first barrier layer (310a); andwherein the first quantum well layer (320a) and the second quantum well layer (340a) comprises lnxGai.xN, where 0.2 <x <0.3.15 2. The quantum well structure (301a) according to claim 1 configured to emit a firstlight comprising a wavelength between 610 nm to 650 nm.

3. The quantum well structure (301a) according to any preceding claim wherein a thickness of the first quantum well layer (320a) is greater than a thickness of the 20 first spacer layer (330a).

4. The quantum well structure (301a) according to any preceding claim wherein the thickness of the first spacer layer (330a) is 1.5 nm or less.25 5. The quantum well structure (301a) according to any preceding claim wherein thethickness of the first quantum well layer (320a) and the second quantum well layer (340a) is 3 nm or less.

6. The quantum well structure (301a) according to any preceding claim comprising 30 lll-N semiconductor material.

7. The quantum well structure (301a) according to claim 1 wherein x = 0.25.

8. The quantum well structure (301a) according to of claims 1-6 wherein the first 35 quantum well layer (320a) and the second quantum well layer (340a) compriseGaN.11 09 259. The quantum well structure (301a) according to any preceding claim wherein the first spacer layer (330a) comprises AlyGai.yN, where 0.5 <y <1.

10. The quantum well structure (301a) according to claim 9 wherein 0.75 <y <1.

511. The quantum well structure (301a) according to any preceding claim wherein the first quantum well layer (320a) comprises a first sub-layer (322a) and a second sub-layer (324a).10 12. The quantum well structure (301a) according to claim 11, wherein the first sublayer (322a) comprises InGaN and the second sub-layer (324a) comprises GaN.

13. The quantum well structure (301a) according to any preceding claim wherein the second quantum well layer (340a) comprises a third sub-layer (342a) and a fourth 15 sub-layer (344a).

14. The quantum well structure (301a) according to claim 13, wherein the third sublayer (342a) comprises GaN and the fourth sub-layer (344a) comprises InGaN.20 15. The quantum well structure (301a) according to any of claims 11-14, whendependent on claim 2, configured to emit a second light comprising a different wavelength to the first light.

16. The quantum well structure (301a) according to any preceding claim wherein the25 first barrier layer (310a) comprises GaN.

17. The quantum well layer (301a) according to any preceding claim wherein the first barrier layer (310a) comprises a cladding layer.30 18. The quantum well structure (301a) according to any preceding claim wherein thespacer layer (330a) comprises a smaller lattice constant than the first barrier layer (310a); and the first quantum well layer (320a) and the second quantum well layer (340a) comprise a larger lattice constant than the first barrier layer (310a).35 19. A semiconductor device comprising the quantum well structure (301a) accordingto any preceding claim.

20. An electronic device comprising the semiconductor device according to claim 19.10forming a first barrier layer (310a);forming a first quantum well layer (320a) over the first barrier layer (310a);forming a first spacer layer (330a) over the first quantum well layer (320a); andforming a second quantum well layer (340a) over the first spacer layer (330a);wherein the first spacer layer (330a) comprises a larger bandgap than the first barrier layer (310a); andwherein the first spacer layer (330a) comprises a thinner thickness than the first barrier layer (310a); andwherein the first quantum well layer (320a) and the second quantum well layer (340a) comprises lnxGai.xN, where 0.2 <x <0.3.11 09 25s

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