Semiconductor structure and method for forming a semiconductor structure

Abstract

The present disclosure relates to a semiconductor structure that comprises: a substrate comprising Si with a (100) crystal orientation; a first transition layer comprising a rare earth oxide over the substrate, wherein the first transition layer comprises a (111) crystal orientation; a first semiconductor layer comprising a first III-N semiconductor material over the first transition layer; and an active region over the first semiconductor layer comprising quantum dots, wherein the quantum dots comprise a second III-N semiconductor material.

Description

SEMICONDUCTOR STRUCTURE AND METHOD FOR FORMING A SEMICONDUCTOR STRUCTURETechnical fieldThe present application relates to a semiconductor structure. The present application also relates to a semiconductor device, an electronic device and a method.BackgroundIt is becoming increasingly popular to form semiconductor devices from lll-N semiconductor materials. In particular, GaN possesses attractive photonic properties that can be used to form photonic semiconductor devices, such as, photodetectors, lasers, light-emitting diodes (LEDs) and microLEDs (pLEDs). Such devices include an active region for generating or absorbing light comprising a lll-N material such as GaN. The active region may comprise quantum wells, quantum dots, quantum dashes or a combination thereof.It is desirable to incorporate lll-N materials on to a Si substrate. Such a structure allows the formation of a semiconductor device comprising lll-N materials, but can also take advantage of the well-established fabrication techniques associated with Si substrates.One technique to produce lll-N materials on Si substrates is epitaxial growth. Epitaxial growth can produce lll-N materials with sufficient crystallographic quality to allow the reliable formation of photonic semiconductor devices.It is difficult, however, to epitaxially grow lll-N materials on Si substrates. For example, GaN possesses a large lattice constant and thermal mismatch to Si. Therefore, in many examples, a buffer layer is included between the GaN layers and the Si substrate, which provides a suitable transition to reduce the lattice constant and thermal mismatch between GaN and Si.Much of the work related to growing GaN on Si substrates has been performed on Si with a (111) crystal orientation. GaN layers grown on a Si(111) substrate typically orientate in the (0001) crystal orientation because of the favourable surface compatibilityof GaN (0001) and Si(111). GaN (0001) and Si(111) both have a six-fold symmetry leading to the favourable formation of high crystal quality GaN(0001) on Si(111 ).It is desirable, however, to grow lll-N materials, such as, GaN on Si substrates with a (100) crystal orientation. Si(100) is the most widely available Si substrate and available in large diameters. The large diameter enables an increased number of lll-N-based semiconductor devices to be fabricated compared to a smaller diameter substrate.Furthermore, the growth of lll-N materials on Si(100) offers the opportunity to integrate lll-N photonic semiconductor devices with Si complementary metal-oxide semiconductor (CMOS) devices. The Si CMOS devices drive the lll-N photonic devices. Conventionally, lll-N photonic semiconductor devices are bonded with Si CMOS devices. Integrating the two devices together would enable the formation of smaller circuits including the devices, as well as increasing throughput and reducing manufacturing costs.The growth of lll-N materials, such as GaN on Si(100), however, has not been commercially utilised because GaN does not generally form with high quality crystallinity and low strain on Si(100). Furthermore, it is difficult to form GaN on Si(100) with the desirable GaN(0001) crystal orientation because of the lack of a six fold symmetry in the surface of Si(100). Thus growing GaN on Si(100) is challenging and can lead to poor crystal quality and even polycrystalline GaN layers.Furthermore, the reduced crystal quality may not enable the formation of high quality quantum structures for generating or absorbing light. In particular, forming high quality quantum structures, such as quantum wells (QW) or quantum dots (QDs), which have a peak photoluminescence at longer wavelengths within the red part of the visible spectrum is challenging. The surface on which the quantum structures are grown can play a large part in the quality of the quantum structures. As such, growing high quality quantum structures with a peak photoluminescence within the red part of the visible spectrum in a lll-N material system on Si(100) is challenging because of the poor crystal quality that can be exhibited in the lll-N layers under the quantum structures.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 semiconductor structure that comprises: a substrate comprising Si with a (100) crystal orientation; a first transition layer comprising a bixbyite oxide over the substrate, wherein the first transition layer comprises a (111) crystal orientation; a first semiconductor layer comprising a first lll-N semiconductor material over the first transition layer; and an active region over the first semiconductor layer comprising quantum dots, wherein the quantum dots comprise a second lll-N semiconductor material.According to a second aspect there is provided a semiconductor device comprising the semiconductor 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 method for forming a semiconductor structure that comprises forming a first transition layer comprising a bixbyite oxide over a substrate comprising Si(100), wherein the first transition layer comprises a (111) crystal orientation; forming a first semiconductor layer comprising a first lll-N semiconductor material over the first transition layer; and forming a first active region comprising quantum dots over the first semiconductor layer wherein the quantum dots comprise a second lll-N semiconductor material.Brief description of the drawingsFor 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 another example of a semiconductor structure;Figures 3a-b are graphs showing example photoluminescence results;Figure 4 is another example of a semiconductor structure;Figure 5 is another example of a semiconductor structure;Figure 6 is another example of a semiconductor structure;Figure 7 is another example of a semiconductor structure;Figure 8 is a graph showing example X-ray diffraction (XRD) results;Figure 9 is an example of a semiconductor device;Figure 10 is an example of an atomic force microscope (AFM) image.Detailed DescriptionEpitaxy 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 an 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 theproportion 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.Throughout this disclosure, as will be understood by the skilled reader, crystal orientation <100> means the face of a cubic crystal structure and encompasses

[0100] ,

[0010] and

[0001] orientations using the Miller indices. Similarly <0001 > encompasses

[0001] and [000-1] except if the material polarity is critical. Integer multiples of any one or more of the indices are equivalent to the unitary version of the index. For example, (222) is equivalent to, the same as, (111).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 20° 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.Lattice matched means that two crystalline layers have the same, or similar, lattice spacing and so the second layer will tend to grow isomorphically on the first layer. Lattice constant is the unstrained lattice spacing of the crystalline unit cell. Lattice coincident means that a crystalline layer has a lattice constant which is, or is close to, an integer multiple of the previous layer so that the atoms can be in registry with the previous layer. Lattice mismatch is where the lattice constants of two adjacent layers are neither lattice matched nor lattice coincident. Such mismatch introduces elastic strain into the structure, particularly the second layer, as the second layer adopts the in-plane lattice spacing of the first layer. The strain is compressive where the second layer has a larger lattice constant and tensile where the second layer has a smaller lattice constant.Where the strain is too great the structure relaxes to minimise energy through defect generation, typically dislocations, known as slip, or additional interstitial bonds, each of which allows the layer to revert towards its lattice constant. The strain may be too great due to a large lattice mismatch or due to an accumulation of small mismatches over many layers. A relaxed layer is known as metamorphic, incoherent, incommensurate or relaxed, which terms are also commonly interchangeable.A pseudomorphic system is one in which a single-crystal thin layer overlies a singlecrystal substrate and where the layer and substrate have similar crystal structures and nearly identical lattice constants. In a pseudomorphic structure the in-plane lattice spacing of the thin layer adopts the in-plane lattice constant of the substrate and is therefore elastically strained, either compressively where the layer has a larger lattice spacing than the substrate or tensilely where the layer has a smaller lattice spacing than the substrate. A pseudomorphic structure is not constrained in the out-of-plane direction and so the lattice spacing of the thin layer in this direction may change to accommodate the strain generated by the mismatch between lattice spacing. The thin layer may alternatively be described as “coherent”, “commensurate”, “strained” or “unrelaxed”, which terms are often used interchangeably. In a pseudomorphic structure all the layers adopt the lattice spacing of the substrate in their respective in-plane lattice spacing.A layer may be monolithic, that is comprising bulk material throughout. Alternatively it may be porous for some or all of its thickness. A porous layer includes air or vacuum pores, with the porosity defined as the proportion of the area which is occupied by the pores rather than the bulk material. The porosity can vary through the thickness of the layer. For example, the layer may be porous in one or more sublayer. The layer mayinclude an upper portion which is porous with a lower portion that is non-porous. Alternatively the layer may include one or more discrete, non-continuous portions (domains) that are porous with the remainder being non-porous (with bulk material properties). The portions may be non-continuous within the plane of a sublayer and / or through the thickness of the layer (horizontally and / or vertically in the sense of the growth direction). The portions may be distributed in a regular array or irregular pattern across the layer, and / or through it. The porosity may be constant or variable within the porous regions. Where the porosity is variable it may be linearly varied through the thickness, or may be varied according to a different function such as quadratic, logarithmic or a step function.A porous layer means that pores have been formed through bulk material so that voids are intentionally introduced. Porosity is expressed in percentages which refers to the volume of bulk material which has been removed so 25% porosity means that the 25% of the equivalent volume of bulk material is voided.A fully depleted porous layer means a layer in which there are no charge carriers.A crystalline bixbyite oxide layer may be a rare earth oxide layer. The rare earth elements are scandium (Sc), yttrium (Y) and all of the lanthanoid series which is lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). The bixbyite oxides are bixbyite in crystal structure. Other bixbyite oxides include indium oxide (ln2O3), vanadium oxide (V2O5), iron oxide (Fe2O3), manganese oxide (Mn2O3) and ternary compounds of a rare earth, a metal and oxygen (RE-M-O).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.Examples according to the present disclosure provide a semiconductor structure that comprises a Si(100) substrate and a first semiconductor layer comprising a lll-N semiconductor material over the Si(100) substrate. In some examples, the lll-Nsemiconductor material may comprise GaN. Between the Si(100) substrate and the first semiconductor layer is a first transition layer comprising a bixbyite oxide, which comprises a (111) crystal orientation. As will be described in more detail below, the transition layer comprises a bixbyite oxide, for which it is energetically favourable to form on the Si(100) substrate with a (111) crystal orientation. The (111) surface of the bixbyite oxide layer provides a suitable surface on which lll-N semiconductor materials, such as, GaN may form with a single crystal morphology. In some examples, the GaN layer may comprise a crystal orientation of (0001), which is favourable for many photonic and electronic applications.Whilst the lll-N semiconductor layer may form with the desirable (0001) crystal orientation, it has been observed that threading dislocations that are present in the bixbyite oxide layer may also be present in the lll-N semiconductor layer. The treading dislocations can be detrimental to the formation of some structures for light emission such as quantum wells (QWs). Examples according to the present disclosure thus further present a semiconductor structure that comprises a first active region over the first semiconductor layer comprising quantum dots (QDs) comprising a lll-N semiconductor material. As the QDs form as discrete structures, they are more tolerant of the threading dislocations in the lll-N semiconductor layer. As will be described in more detail below, the composition of the QDs may be tuned to comprise a peak photoluminescence wavelength within the visible spectrum. In particular, the peak photoluminescence wavelength may be within the red part of the visible light spectrum.The semiconductor structure according to examples of the present disclosure may thus enable the formation of a lll-N semiconductor material for photonic applications on Si(100). The semiconductor structure may thus further enable the integration of lll-N photonic semiconductor devices with CMOS Si semiconductor devices. The semiconductor structure may further enable the formation of red-emitting QDs on a Si(100) substrate.Figure 1 is an example of a semiconductor structure 100. Semiconductor structure 100 comprises a substrate 110 that comprises Si(100). The substrate 110 is on-axis silicon, thus it has a <100> crystal orientation. As will be understood by the skilled reader, crystal orientation <100> means the face of a cubic crystal structure and encompasses

[0100] ,

[0010] and

[0001] orientations using the Miller indices. References herein to Si (100) alsoencompass a silicon substrate 110 which is miscut by up to 20° towards another crystallographic direction, for example towards the (111) plane.Semiconductor structure 100 further comprises a transition layer 120 over the substrate 110. Transition layer 120 comprises a bixbyite oxide material. The bixbyite oxide material may be a rare earth oxide layer. The rare earth elements are scandium (Sc), yttrium (Y) and all of the lanthanoid series which is lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). The bixbyite oxides are bixbyite in crystal structure. Other bixbyite oxides include indium oxide (ln2O3), vanadium oxide (V2O5), iron oxide (Fe2O3), manganese oxide (Mn2O3) and ternary compounds of a rare earth, a metal and oxygen (RE-M-O).Preferably the bixbyite oxide material is epitaxially deposited. When a crystalline bixbyite oxide is deposited on a substrate 110, or another layer, at sufficient surface temperature, low enough oxygen concentration and slow enough growth rate it does not match the crystal orientation of the previous layer, depending on the orientation of the substrate 110 or previous layer. Instead it grows in a different orientation, a process called “epi Twist™” by the inventors. When growing most crystalline bixbyite oxides on Si (100) the bixbyite oxide material grows in

[0110] orientation which has lower surface energy than

[0100] orientation.The bixbyite oxide material may comprise scandium oxide, SC2O3, which grows in

[0111] orientation on Si (100). Scandium oxide, which is a rare earth oxide but not in the lanthanoid series of elements, surprisingly behaves differently to other crystalline bixbyite oxides, particularly rare earth oxides, in that its lowest energy orientation is

[0111] and so it grows in this orientation on Si (100). The preferred crystal orientation of a material is dependent on its surface energy and the lattice mismatch to the layer below. The arrangement of the atoms within the crystal is also a factor since this affects the spacing between atoms on different surfaces, and whether a surface has only oxygen atoms, only metal atoms or a combination of both. Sc is a smaller atom than the other rare earth elements and the lattice spacing of SC2O3 is smaller than for other crystalline bixbyite oxides. Therefore SC2O3 (111) exhibits a greater lattice mismatch to Si (100): around -9%. For this reason the surface energy becomes the dominant factor to define crystal orientation whereas in crystalline bixbyite oxides which are better lattice matchedto Si (100), for example up to ±2%, the lattice match is the dominant factor even though the surface energy in (110) orientation may be twice that in (111) orientation.By increasing the lattice constant of the substrate, for example using Sio.4Geo.6 (lattice constant 5.57A) instead of Si (lattice constant 5.43A), additional rare-earth oxides will tend to grow with <111> orientation perpendicular to the substrate 12. For example, lattice mismatch between Sio.4Geo.6 and E^Oa is more than 5%.The (111) crystal orientation of the first transition layer 120 may provide a suitable surface for the formation of lll-N semiconductor material thereon. In particular, the (111) crystal orientation may provide a surface for the formation of lll-N semiconductor material with a crystal orientation of (0001), which is desirable for many photonic and electronic applications.Semiconductor structure 100 thus further comprises a semiconductor layer 130 over the transition layer 120. Semiconductor layer 130 comprises a lll-N semiconductor material, such as, GaN. As described above, lll-N semiconductor material may form on the (111) transition layer 120 with a single crystal morphology. In some examples, the lll-N semiconductor material semiconductor layer 130 may form on the transition layer 120 with a (0001) crystal orientation. The (0001) crystal orientation may provide a useful orientation which permits the formation of quantum dots for generating light.Semiconductor structure 100 further comprises an active region 140 over the semiconductor layer 130. The active region 140 comprises QDs 142, which comprise III- N semiconductor material. QDs 142 may be formed due to strain and lattice mismatch between the semiconductor layer 130 and the material of the active region 140. Thus QDs 142 may comprise a different lll-N semiconductor material to the semiconductor layer 130. For example, QDs 142 may comprise InGaN and semiconductor layer 130 may comprise GaN. The strain and lattice mismatch between the InGaN material and the GaN material may thus lead to the formation of QDs 142.In one example, the QDs 142 may comprise an average lateral diameter, in the X-Y plane, of about 20 nm to about 25 nm. In one example, the QDs may comprise an average height in the Z plane of about 3.5 nm. Referring briefly to Figure 10, an atomic force microscope (AFM) image 1000 is presented illustrating QDs 142, which comprise InGaN. The QDs 142 comprise an average lateral diameter 1042, in the X-Y plane, ofabout 20 nm to about 25 nm. In other examples, the QDs 142 may comprise an average lateral diameter, in the X-Y plane, of about 10 nm to about 35 nm. In other examples, the QDs may comprise an average height in the Z plane of about 3 nm to about 10 nm.Referring again to Figure 1 , in some examples, the composition of the QDs 142 may be tuned to comprise a peak photoluminescence wavelength within a region of the visible spectrum. For example, the QDs 142 may comprise AlxGai.xN or lnxGai.xN, the composition of which may be adjusted to achieve a desired peak photoluminescence wavelength.In one example, the QDs 142 may comprise InGaN. In one example, the InGaN QDs 142 may comprise a lateral diameter, in the X-Y plane, of about 20 nm to about 25 nm and a height of about 3.5 nm in the Z plane. In such examples, the composition of In may be adjusted to achieve the desired wavelength. For example, an In composition in the range of 28%-42% may result in the QDs 142 having a peak photoluminescence wavelength in the red part of the visible spectrum, which may range from about 620 nm to about 750 nm. QDs may be preferred to other quantum structures to tune the peak photoluminescence wavelength to a region within the red part of the visible spectrum. The addition of a greater concentration of In results in a red-shift in the peak photoluminescence wavelength of many quantum structures. However, the addition of In in greater concentrations can cause InN segregation in other quantum structures. For example, it has been observed that quantum wells (QWs) cannot handle an increased In concentration, as InN segregation begins to detrimentally affect other qualities of the QW structures. It is possible to achieve the colour red in InGaN QWs with a reduced In concentration by increasing the QW layer thickness. However, increasing the QW layer thickness can cause additional challenges and can lead to the formation of defects within the QW.In another example, however, the In composition of the InGaN QDs 142 may be adjusted to comprise a peak photoluminescence wavelength within the green or blue parts of the visible light spectrum. For example, an In composition in the range of 14%-20% may result in the QDs 142 having a peak photoluminescence wavelength in the green part of the visible spectrum, which may range from about 495 nm to about 570 nm. In another example, an In composition in the range of 5%-12% may result in the QDs 142 having a peak photoluminescence wavelength in the blue part of the visible spectrum, which may range from about 450 nm to about 495 nm. The ability to form red, green and blueemitting QD structures may make the semiconductor structure 100 attractive for use in pLED applications.In one example, the InGaN QDs may be grown on the semiconductor layer 130 in the Stranski-Krastanov mode. In one example, the formation of the InGaN QDs may be formed using MBE by evaporating In and Ga from effusion cells and using N2-plasma as the activated nitrogen source. In one example, the formation of the QDs make take place at a temperature of about 600 °C.Figure 2 is another example of a semiconductor structure 200. Semiconductor structure 200 comprises common elements to semiconductor structure 100 described above. Said common elements are labelled with corresponding reference numerals and may comprise all of the above-described features and functionality.Semiconductor structure 200 comprises an active region 140 comprising a plurality of active layers 140a-e on semiconductor layer 130. The plurality of active layers 140a-e each comprise a plurality of QDs 142a-e. The plurality of active layers 140a-e may thus form a multiple QD (MQD) active region for generating light. The plurality of QDs 142a- e may each comprise a lll-N semiconductor material. In a similar manner to that described above, the composition of the QDs 142a-e may thus be tuned such that the plurality of QDs 142a-e comprise a peak photoluminescence wavelength at a desired point across the visible spectrum.Active region 140 further comprises a plurality of barrier layers 150a-e between the plurality of active layers 140a-e and the plurality of QDs 142a-e. The barrier layers 150a- e may further thus form part of the MQD active region 140. In some examples, each of the barrier layers 150a-e may comprise a (0001) crystal orientation. The barrier layers may further comprise a lll-N semiconductor material. The lll-N semiconductor material (0001) barrier layers 150a-e may provide a suitable surface for the formation of QDs 142a-e thereon.In one example, the plurality of QDs 142a-e may comprise InGaN and the barrier layers 150a-e may comprise GaN (0001). A MQD active region 140 comprising such InGaN QDs 142a-e and GaN (0001) barrier layers 150a-e may be used for outputting peak photoluminescence wavelength in a desired part of the visible spectrum. In one example,such a MQD active region 140 may output light in the red part of the visible light spectrum, which may range from about 620 nm to about 750 nm.In another example, barrier layers 150a-e may be replaced by a plurality of QW layers, separating the plurality of QD layers. In such examples, the QDs 142a-e may thus be arranged in a QW, which in some examples may be referred to a dot in a well (DWELL) arrangement.Figures 3a-b are examples of photoluminescence results 300a-b obtained from semiconductor structure 200 described above comprising a MQD active region comprising a plurality of InGaN QD layers and a plurality of GaN barrier layers.Referring to Figure 3a, results 300a illustrate a first photoluminescence result 302a with a peak wavelength of about 620 nm. The first photoluminescence result 302a was obtained where the InGaN QDs of the MQD active region comprised a In concentration of about 28%.Results 300a further illustrate a second photoluminescence result 304a with a peak wavelength of about 640 nm. The second photoluminescence result 304a was obtained where the InGaN QDs of the MQD active region comprised a In concentration of about 32%.Results 300a further illustrate a third photoluminescence result 306a with a peak wavelength of about 740 nm. The third photoluminescence result 306a was obtained where the InGaN QDs of the MQD active region comprised a In concentration of about 42%.Thus, the peak photoluminescence wavelength of the MQD active region comprising InGaN QDs can be tuned to output light within the wavelength range of 620-740 nm by adjusting the In concentration. The wavelength range of 620-740 nm covers the red part of the visible light spectrum. The increase in the In concentration can thus result in a redshift of the peak photoluminescence wavelength.It will therefore be appreciated that the composition of the InGaN QDs can be adjusted to red or blue shift the peak photoluminescence wavelength across the visible light spectrum. For example, the In concentration could be adjusted to achieve theappropriate shift in wavelength. However, one skilled in the art would be aware of other ways in which the composition and / or structure of MQD active region could be adjusted in order to output light with a desired wavelength across the visible spectrum.Referring to Figure 3b, results 300b illustrate another photoluminescence result 302b with a peak wavelength of about 620 nm. The photoluminescence result 302b was obtained where the InGaN QDs of the MQD active region comprised a In concentration of about 28%. Photoluminescence result 302b exhibits a narrow full width half maximum (FWHM) 303b of about 40 nm.Figure 4 is another example of a semiconductor structure 400. Semiconductor structure 400 comprises common elements to semiconductor structures 100, 200 described above. Said common elements are labelled with corresponding reference numerals and may comprise all of the above-described features and functionality.Semiconductor structure 400 further comprises a second transition layer 410 between the first transition layer 120 and the semiconductor layer 130. The second transition layer 410 comprises a metal nitride. In some examples, the second transition layer may comprise one of ScN, I nN , AIN or MoN.In some examples, the bixbyite oxide of first transition layer 120 may comprise SC2O3. On Si(100), SC2O3 forms with a (111) crystal orientation. In some examples, the semiconductor layer 130 may comprise GaN. The successful epitaxial growth of GaN occurs when the Ga adatom mobility on the deposited surface is high enough to allow Ga atoms to move across the surface thereby supporting 2D GaN growth. In some examples, however, it has been observed that the growth of GaN on SC2O3 (111) on Si(100) may tend towards 3D due to the low surface mobility, which Sc2O3(111) can possess when grown on Si(100). The low surface mobility is due to domain defects related to in-plane twisting present in the SC2O3 layer. For example, SC2O3 may include twinned planes of the desired <111> orientation.Second transition layer 410 may thus act as an interlayer between lll-N semiconductor layer 130 and first transition layer 120 to improve the surface mobility for the epitaxial growth of the group III atoms, such as, Ga atoms, compared to the bixbyite oxide material of the first transition layer 120. For example, where the semiconductor layer 130 comprise GaN and first transition layer comprises SC2O3, the second transition layer 410may comprise a rare-earth nitride, such as ScN. Rare-earth nitrides, such as ScN, have a rock salt structure, that is face-centred cubic NaCI structure. ScN may grow in <111> orientation on SC2O3 (111) although it has rock salt structure in which the (111) surface energy is higher than that of (110) or (100) orientations. Since the (111) is the highest energy surface this leads to improved nucleation of lll-N materials due to improved adatomic mobility which promotes growth of two-dimensional layers. Thus, second transition layer 140 comprising a rare-earth nitride, such as ScN may provide improved surface mobility for Ga atoms to improve the crystal quality of the grown GaN material. As such, the second transition layer 410 may improve the epitaxial growth and crystal properties of the semiconductor layer 130 and thus, in turn, the epitaxial growth and crystal properties of the active region 140.In some examples, the second transition layer 410 may be grown as a single layer. In other examples the second transition layer 410 may comprise two layers. The second transition layer 410 may comprise a thin nucleation layer followed by the growth of a bulk layer of the same metal nitride material, for example, ScN.In some examples, the semiconductor structure 400 may comprise a rare earth nitride layer between the bixbyite oxide layer 120 and the second transition layer 410. In examples where the second transition layer 410 comprises a rare earth nitride, such as ScN, the rare earth nitride layer may comprise a different rare earth nitride to that of the second transition layer 410, which has an intermediate lattice constant between those of the adjacent layers. For example, on the (111) surface SC2O3 has a lattice constant of 3.489A and ScN has a lattice constant of 3.20A. ErN has a lattice constant of 3.42A on the (111) surface which sits between SC2O3 and ScN. Alternatively, the rare earth nitride layer may comprise a ternary compound having two rare earth elements to give an intermediate lattice constant between SC2O3 (3.489A) and ScN (3.20A) such as Sco.1Gdo.9N or Ero.33Gdo.67N (which each have a lattice constant approximately half that of SC2O3). Furthermore Sco.1Gdo.9N can be graded to ScN through the thickness of rare the earth nitride layer.Figure 5 is another example of a semiconductor structure 500. Semiconductor structure 500 comprises common elements to semiconductor structures 100, 200, 400 described above. Said common elements are labelled with corresponding reference numerals and may comprise all of the above-described features and functionality.Semiconductor structure 500 further comprises a third transition layer 510 between the second transition layer 410 and the first semiconductor layer 130. The third transition layer 510 comprises a metal nitride, such as AIN. Third transition layer 510 may provide an improved surface for the growth of the first semiconductor layer 130.In some examples, third transition layer 510 may comprise AIN and semiconductor layer 130 may comprise GaN. As described above, the growth of some bixbyite oxide materials, such as SC2O3 on Si(100) can introduce domain defects into the bixbyite oxide material. These domain defects can detrimentally impact the GaN material grown above the bixbyite oxide material of first transition layer 120. For example, the domain defects can reduce the surface mobility of the Ga adatoms, which can inhibit 2D growth of the GaN material. Third transition layer 510 may thus reduce the defect density and provide an improved surface for the growth of semiconductor layer 130. Third transition layer 510 can thus act as an additional buffer, along with buffer properties provided by second transition layer 410, to reduce the defect density and provide an improved surface for the growth of semiconductor layer 130. Third transition layer 510 may thus improve the crystal quality and surface roughness of the semiconductor layer 130. Thus, in turn, third transition layer 510 can provide an improved surface for the growth thereon of the active region 140.In some examples, the third transition layer 510 may comprise a (0001) orientation. For example, the third transition layer 510 may comprise a lll-N material, such as, AIN and the second transition layer 510 may comprise ScN. AIN may grow in <0001 > orientation on ScN (111) because that is the orientation in which lll-N materials, such as AIN, tend to nucleate and grow on the (111) planes of rare earth nitride materials such as ScN. AIN provides a surface which is closely lattice matched to GaN since acaN=3.186A which is less than 3% larger than aAiN=3.110A. In such examples, the AIN third transition layer 510 may comprise an improved surface for the formation of GaN thereon and improve the surface roughness of the GaN semiconductor layer 130. For example, semiconductor layer 130 comprising GaN may comprise a surface roughness of less than or equal to 1 nm. In some examples, the surface roughness of the semiconductor layer 130 comprising GaN may be less than or equal to 0.5 nm.In some examples, third transition layer 510 may comprise two sub-layers. For example, third transition layer 510 may comprise a first transition sub-layer and a second transition sub-layer over the first transition sub-layer. The first transition sub-layer and a secondtransition sub-layer may each comprise AIN. The first transition sub-layer may be grown at a low temperature and act as a diffusion barrier on the second transition layer 510, to prevent oxygen diffusion to the semiconductor layer 130 from the bixbyite oxide layer 120. Furthermore, the first transition sub-layer may set the crystal orientation and form good bonds with the preceding second transition layer 410. However, because the first transition sub-layer is grown at a low temperature, first transition sub-layer may comprise relatively poor crystal quality, which may not enable the formation of a semiconductor layer 130 with a high crystal quality. Second transition sub-layer may thus be grown over the first transition sub-layer at a higher temperature. The higher temperature may result in the second transition sub-layer comprising bulk-like properties and a relatively high crystal quality, which is greater than the first transition sub-layer. For example, across an x-ray diffraction (XRD) measurement the peak associated with the second transition sublayer may comprise a full width half maximum (FWHM) of less than 0.5°. In some examples, across an XRD measurement the peak associated with the first transition sublayer may comprise a FWHM of greater than 0.5°. As such, the second transition sublayer may improve the crystal quality of the semiconductor layer 130 and thus, in turn, the active region 140.The first transition sub-layer may be greater than or equal to 5nm thick. By providing a layer that is at least 5 nm thick it ensures that the crystal orientation is set so that the second transition sub-layer will grow in a single crystal orientation. The second transition sub-layer may be greater than or equal to 15 nm thick. Growing this layer at least 15 nm thick ensures that it exhibits bulk-like properties rather than thin-film properties.In some examples, the second transition sub-layer may be grown at a temperature which is at least 200°C higher than the temperature at which the first transition sub-layer is grown. In some examples, the second transition sub-layer may be grown at a temperature which is at least 300°C higher than the temperature at which the first transition sub-layer is grown. In some examples, the second transition sub-layer may be grown at a temperature which is up to 350°C higher than the temperature at which the first transition sub-layer is grown. Similarly, the nitrogen flow rate, Al flux, plasma power and other parameters may be different between the two transition sub-layers, each of which may scale with or be independently adjusted from the growth temperature.Figure 6 is another example of a semiconductor structure 600. Semiconductor structure 600 comprises common elements to semiconductor structures 100, 200, 400, 500described above. Said common elements are labelled with corresponding reference numerals and may comprise all of the above-described features and functionality.Semiconductor structure 600 further comprises a fourth transition layer 610 between the third transition layer 510 and the semiconductor layer 130. The fourth transition layer 610 comprises a metal nitride. In one example, the metal nitride comprises AIScN. In some examples, the fourth transition layer 610 may comprise a (0001) crystal orientation.In some examples, the fourth transition layer 610 may be lattice matched to the semiconductor layer 130. For example, the semiconductor layer may comprise GaN and the fourth transition layer 610 may comprise AIScN. AIScN comprising a composition of 18% Sc may be lattice matched to GaN. As such, the growth of GaN on AIScN may result in the semiconductor layer 130 comprising improved strain and crystal quality, which in turn, provides increased control for the growth of the QDs 142.Figure 7 is another example of a semiconductor structure 700. Semiconductor structure 700 comprises common elements to semiconductor structures 100, 200, 400, 500, 600 described above. Said common elements are labelled with corresponding reference numerals and may comprise all of the above-described features and functionality.Semiconductor structure 700 comprises second transition layer 410 and third transition layer 510, as similarly described above in relation to semiconductor structures 500, 600. Semiconductor structure 700 further comprises a plurality of transition layer pairs 710a- d, where a first pair 710a of the plurality comprises the second transition layer 410 and the third transition layer 510.In some examples, one or more of the plurality of other transition layer pairs 710b-d may correspond to the first transition layer pair 710a, such that one or more of the other transition layer pairs 710b-d may comprise the same material pairs as the first transition layer pair 710a. In some examples, each of the plurality of other transition layer pairs 710a-d may comprise corresponding materials. In such examples, the pair formed by materials of the second transition layer 410 and the third transition layer 510 may be repeated n times to form the plurality of transition layer pairs 710a-d. Although four transition layer pairs 710a-d are illustrated in semiconductor structure 700, it will be appreciated that any suitable number of transition layer pairs 710a-d may be presentbetween the first transition layer 120 and the semiconductor layer 130. In one example, the plurality of transition layer pairs may comprise n pairs, where 3 < n < 10.In some examples, the plurality of transition layer pairs 710a-d may be lattice matched to the semiconductor layer 130. In such examples, each pair of the plurality of transition layer pairs 710a-d may comprise materials and a layer thickness ratio resulting in the plurality of transition layer pairs 710a-d being lattice matched to the semiconductor layer 130. For example, the semiconductor layer 130 may comprise GaN and each pair of the plurality of transition layer pairs 710a-d may comprise a ScN layer and an AIN layer. In such examples, each pair of the plurality of transition layer pairs 710a-d may comprise a layer thickness ratio of 18:82 for ScN:AIN. In such examples, the plurality of transition layer pairs 710a-d may be lattice matched to the semiconductor layer 130, which can in turn lead to improved strain in the semiconductor layer 130. Furthermore, the improved strain in the semiconductor layer 130 can result in improved control of the growth for the QDs 142.As described above, in some examples, one or more pairs of the plurality of transition layer pairs 710a-d may comprise different material pairs. In such examples, the thickness ratio of the plurality of transition layer pairs 710a-d may be adjusted between each of the plurality of transition layer pairs 710a-d in order to lattice match the plurality of transition layer pairs 710a-d to the semiconductor layer 130.Figure 8 are X-ray diffraction (XRD) results 800 of three semiconductor structures comprising a similar structure to semiconductor structure 500 described above. The three semiconductor structures each comprise a Si(100) substrate, a SC2O3 layer over the substrate, a AIN layer over the SC2O3 layer and a MQD active region over the AIN layer. The In concentration of the QDs of the MQD active regions was varied across the three semiconductor structures.XRD results 800 show a first peak 802 associated with a substrate comprising Si(100). XRD results 800 further comprise a second peak 804 associated with a first transition layer comprising SC2O3. XRD results 800 further comprise a third peak 806 associated with a second transition layer comprising AIN. The results 800 thus show that each of the three structures comprise a Si(100) substrate, SC2O3 layer and a AIN layer.XRD results 800 further comprises a plurality of peaks 808a-c associated with different MQD active regions comprising InGaN QDs of varying In concentrations. Fourth peak 808a is associated with a first semiconductor structure comprising an active region comprising InGaN QDs comprising a In concentration of 42%. Fifth peak 808b is associated with a second semiconductor structure comprising an active region comprising InGaN QDs comprising a In concentration of 32%. Sixth peak 808c is associated with a third semiconductor structure comprising an active region comprising InGaN QDs comprising a In concentration of 28%.A semiconductor structure according to examples of the present disclosure comprising a Si(100) substrate, a first transition layer and a second transition layer can provide a suitable template for the formation of InGaN QDs thereon. Furthermore, the composition of the InGaN QDs can be varied and may still be grown on the template provided by the Si(100) substrate, the first transition layer and the second transition layer.Figure 9 is an example of a semiconductor device 900. Semiconductor device 900 comprises corresponding elements to semiconductor structures 100, 200, 400, 500, 600, 700 described above. Said common elements are labelled with corresponding reference numerals and may comprise all of the above-described features and functionality.Semiconductor device 900 comprises a photonic semiconductor device 902 formed on substrate 110 comprising Si(100). Photonic semiconductor device 902 comprises active region 940 for generating light. In some examples, active region 940 may comprise a MQD structure comprising a plurality of InGaN QD layers separated by GaN barrier layers. Photonic semiconductor device 902 further comprises capping layer 950.Photonic semiconductor device 902 further comprises a first contact 960 connected to the capping layer 960. Photonic semiconductor device 902 further comprises a second contact 970 connected to the semiconductor layer 130. In one example, the capping layer 950 may be doped p-type and the semiconductor layer 130 may be doped n-type. As such, an electrical current may be provided to the photonic device 902 for generating light.Semiconductor device 900 further comprises a CMOS Si device 904. As illustrated in Figure 9, CMOS Si device is fabricated on substrate 110 comprising Si(100) alongside photonic semiconductor device 902. As such, photonic semiconductor device 902 andCMOS Si device 904 can be integrated together on the same Si(100) substrate. Conventional techniques to combine the operation of photonic semiconductor devices with CMOS Si devices involve the use of bonding. Examples according to the present disclosure enable the integration of a photonic semiconductor device 902 with a CMOS Si device 904, which greatly improves the efficiency for fabricating these devices together and reduces the footprint of the final semiconductor device 900, compared to conventional techniques.The present disclosure further provides a semiconductor device comprising a semiconductor 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 micro LED (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 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 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.

Claims

CLAIMS1 . A semiconductor structure comprising: a substrate comprising Si with a (100) crystal orientation; a first transition layer comprising a bixbyite oxide over the substrate, wherein the first transition layer comprises a (111) crystal orientation; a first semiconductor layer comprising a first lll-N semiconductor material over the first transition layer; and an active region over the first semiconductor layer comprising quantum dots, wherein the quantum dots comprise a second lll-N semiconductor material.

2. The semiconductor structure according to claim 1 wherein the active region comprises a peak photoluminescence wavelength of between 620 nm to 750 nm.

3. The semiconductor structure according to claim 1 or 2 wherein the first lll-N semiconductor material comprises GaN.

4. The semiconductor structure according to any preceding claim wherein the first semiconductor layer comprises a (0001) crystal orientation.

5. The semiconductor structure according to any preceding claim wherein the bixbyite oxide comprises a rare earth oxide.

6. The semiconductor structure according to claim 5 wherein the rare earth oxide comprises Sc.

7. The semiconductor structure according to any preceding claim wherein the second lll-N semiconductor material comprises InGaN.

8. The semiconductor structure according to claim 7 wherein the second lll-N semiconductor material comprises a In concentration between 28% and 42%.

9. The semiconductor structure according to any preceding claim wherein the active region comprises a first active layer comprising the quantum dots; and wherein the active region further comprises one or more active layers comprising quantum dots over the first active layer.

10. The semiconductor structure according to claim 9 wherein the quantum dots of the one or more active layers comprise a peak photoluminescence of between 620 nm to 750 nm.

11. The semiconductor structure according to claim 9 or 10 further comprising a second semiconductor layer comprising a third lll-N semiconductor material between the first active layer and the one or more active layers.

12. The semiconductor structure according to claim 11 wherein the third lll-N semiconductor material comprises GaN.

13. The semiconductor structure according to any preceding claim further comprising a second transition layer between the first transition layer and the first semiconductor layer, wherein the second transition layer comprises a first metal nitride.

14. The semiconductor structure according to claim 13 wherein the first metal nitride comprises Sc.

15. The semiconductor structure according claims 13 or 14 further comprising a third transition layer between the second transition layer and the first semiconductor layer, wherein the third transition layer comprises a second metal nitride.

16. The semiconductor structure according to claim 15 wherein the second metal nitride comprises Al.

17. The semiconductor structure according to claim 15 or 16 wherein the third transition layer comprises a first transition sub-layer and a second transition sublayer over the first transition sub-layer.

18. The semiconductor structure according to any of claims 15-17 further comprising a plurality of transition layer pairs, wherein a first pair of the plurality comprises the second transition layer and the third transition layer; and wherein the plurality of the transition layer pairs are lattice matched to the first semiconductor layer.

19. The semiconductor structure according to claim 18 wherein at least two pairs of the plurality of the transition layer pairs comprise the first metal nitride and the second metal nitride.

20. The semiconductor structure according to any of claims 15-17 further comprising a fourth transition layer between the third transition layer and the first semiconductor layer, wherein the fourth transition layer is lattice matched to the first semiconductor layer.

21. The semiconductor structure according to claim 20 wherein the fourth transition layer comprises a third metal nitride.

22. The semiconductor structure according to claim 21 wherein the third metal nitride comprises AIScN.

23. A semiconductor device comprising the semiconductor structure according to any preceding claim.

24. An electronic device comprising the semiconductor device according to claim 23.

25. A method for forming a semiconductor structure comprising: forming a first transition layer comprising a bixbyite oxide over a substrate comprising Si(100), wherein the first transition layer comprises a (111) crystal orientation; forming a first semiconductor layer comprising a first lll-N semiconductor material over the first transition layer; and forming a first active region comprising quantum dots over the first semiconductor layer wherein the quantum dots comprise a second lll-N semiconductor material.

Images