A semiconductor structure

Abstract

An epitaxial semiconductor structure comprising a silicon substrate in <100> orientation. On the substrate are sequentially a bixbyite oxide layer in <111> orientation, a rare earth nitride layer, an aluminium nitride layer, and a gallium nitride layer in <0001> orientation. Advantageously the semiconductor structure results in a better quality GaN layer.

Description

A semiconductor structureA semiconductor structure, particularly but not exclusively for a GaN-based device on a Si (100) substrate.It is known from EP4086939 to provide a layered (semiconductor) structure having a bixbyite oxide layer in <111> orientation, such as ScO, over a silicon (100) substrate with a metal containing layer such as GaN over the ScO layer. One problem with this structure is that surface morphology and crystal structure are both worse for GaN grown on ScO (111) which is grown on Si (100) compared to GaN grown on ScO (111) grown on Si (111).The present invention provides an epitaxial semiconductor structure comprising: a silicon substrate in <100> orientation; a bixbyite oxide layer in <111> orientation on the silicon substrate; a rare earth nitride layer on the bixbyite oxide layer; an aluminium nitride layer on the rare earth nitride layer; and a gallium nitride layer in <0001> orientation on the aluminium nitride layer.Advantageously the rare earth nitride layer nucleates better due to the <111> orientation of the bixbyite oxide layer. Advantageously the aluminium nitride layer seals the rare earth nitride layer. Advantageously the rare earth nitride layer isolates the aluminium nitride layer from the bixbyite oxide layer and prevents formation of aluminium oxide which is known to form a surface which cannot support successful growth of gallium nitride. Advantageously the aluminium nitride layer forms a good crystal quality template for growth of the gallium nitride layer.The aluminium nitride layer may comprise a first aluminium nitride sublayer and a second aluminium nitride sublayer. The first aluminium nitride sublayer may be grown at a lower temperature than the second aluminium nitride sublayer. Advantageously the first aluminium nitride sublayer seals the rare earth nitride layer and acts as a nucleation layer for the second aluminium nitride sublayer (i.e. it sets the crystal orientation and forms good bonds with the preceding layer). Advantageously the second aluminium nitride sublayer provide a high quality surface on which to grow gallium nitride because it has bulk-like properties.The first aluminium nitride sublayer may be greater than or equal to 5nm thick. Advantageously a thin layer is all that is required to seal the previous layer and set the crystal orientation. The second aluminium nitride sublayer may be greater than or equal to 15nm thick. Advantageously 15nm is sufficient for the layer to exhibit properties similar to bulk aluminium nitride and therefore to provide a high quality surface for GaN growth.The bixbyite oxide layer may comprise scandium oxide. Advantageously scandium oxide grows in <111> orientation on a silicon (100) substrate, given the appropriate controls of surface temperature, growth rate and oxygen concentration. Advantageously scandium oxide has a lower surface energy in <111> orientation than in <100> or <110> orientation.The gallium nitride layer may have a surface roughness less than lnm. The gallium nitride layer may have a surface roughness less than 0.5nm. Advantageously the surface roughness is kept low by the combination of layers below the gallium nitride layer. Advantageously such a low surface roughness enables the formation of high quality devices in and / or on the gallium nitride layer.The aluminium nitride layer may be in <0001> orientation. Advantageously this is the same orientation as the gallium nitride layer meaning that it is a good template for the gallium nitride growth.The rare earth nitride layer may be in <111> orientation. The scandium nitride layer may be in <111> orientation. Advantageously scandium nitride and other rare earth nitrides may follow the <111> orientation of the bixbyite oxide layer.There may be a second rare earth nitride layer between the bixbyite oxide layer and the rare earth nitride layer. The second rare earth nitride layer may comprise a different rare earth element to the rare earth nitride layer. Advantageously the second rare earth nitride can be chosen to have an intermediate lattice constant between the bixbyite oxide layer and the rare earth nitride layer. The second rare earth nitride layer may be a ternary (or multi-element) compound having two (or more) rare earth elements. The second rare earth nitride layer may have a graded composition through the thickness of the layer. Advantageously its lattice constant may be more closely matched with each of the adjacent layers without an abrupt interface between lattice constants.There may be a second bixbyite oxide layer adjacent to the bixbyite oxide layer. The second bixbyite oxide layer may have a device layer thereon. For example the second bixbyite oxide layer may be laterally adjacent to the bixbyite oxide layer. Advantageously electronics may be formed in or on the device layer on the second bixbyite oxide layer which drive the device formed in or on the gallium nitride layer.There may be an aluminium rare earth nitride layer between the aluminium nitride layer and the gallium nitride layer. The aluminium nitride layer may comprise aluminium scandium nitride. Advantageously this provides a stepped transition from the lattice constant of aluminium nitride to the lattice constant of gallium nitride and therefore helps to reduce defect density.The scandium content of the aluminium rare earth nitride layer may be graded from zero adjacent to the aluminium nitride layer. The scandium content of the aluminium rare earth nitride layer may be graded from zero adjacent to the aluminium nitride layer up to 18% adjacent to the gallium nitride layer. Advantageously grading the scandium content enables better lattice matching to both adjacent layers reduces strain in the semiconductor structure.The semiconductor structure may further comprise additional pairs of the rare earth nitride layer and the aluminium nitride layer. There may be between two and nine additional pairs of the rare earth nitride layer and the aluminium nitride layer. Thus there may be a total of between three and ten pairs of the rare earth nitride layer and the aluminium nitride layer. Advantageously process control is simpler for growing alternating thin layers of binary alloys than growing a thick tertiary alloy layer.The thickness of the rare earth nitride layer to the aluminium nitride layer in each pair may have ratio x:y. The thickness of the scandium nitride layer to the aluminium nitride layer in each pair may have ratio x:y. x may be smaller than y, for example x may be no more than a quarter of y. The thickness of the rare earth nitride layer to the aluminium nitride layer in each pair may have ratio 18:82. The thickness of the scandium nitride layer to the aluminium nitride layer in each pair may have ratio 18:82. Advantageously providing a pair of unequal thickness rare earth nitride and aluminium nitride layers creates the effect of a single layer of aluminium rare earth nitride having rare earth, for example scandium, content of 18% and aluminium content of 82%. Each pair of layers may have the same thickness ratio. Alternatively each pair of layer may have a different ratio, for example having a thicker aluminium nitride layer closer to the gallium nitride layer than to the bixbyite oxide layer. Advantageously this may have the same effect as a graded composition layer.The present invention also provides a semiconductor device comprising: a semiconductor structure as described; one or more device features formed in or on the gallium nitride layer; and / or one ormore device layers on the gallium nitride layer. The one or more device features may comprise etched trenches, deposited contacts or terminals, a mask for further deposition of layers, or any other features used for forming one or more device on (or partially in) the semiconductor structure. The one or more device layers may include a barrier, a contact, an active layer, a spacer, a channel or any other layer suitable or necessary for forming a device in or on GaN. Advantageously the device features and / or device layers are formed in or on high quality gallium nitride and thus themselves exhibit higher quality.The semiconductor device may comprise any one of a high electron mobility transistor (HEMT), an HBT, a switch, a light emitting device, an LED, a microLED (pLED), a photodiode.The present invention also provides a method to epitaxially grow a semiconductor structure comprising steps to: grow a bixbyite oxide layer in <111> orientation on a silicon substrate in <100> orientation; grow a rare earth nitride layer on the bixbyite oxide layer; grow an aluminium nitride layer on the rare earth nitride layer; and grow a gallium nitride layer in <0001> orientation on the aluminium nitride layer. Advantageously the step to grow the rare earth nitride layer on the bixbyite oxide layer blocks the oxide from forming aluminium oxide with the aluminium. Advantageously the step to grow the aluminium nitride layer seals the rare earth nitride layer within the same growth reactor. Advantageously the step to grow the aluminium nitride layer also forms a high quality layer with bulklike properties for the growth of the gallium nitride layer.The step to grow an aluminium nitride layer may comprise: a step to grow a first aluminium nitride sublayer; and a step to grow a second aluminium nitride sublayer. Advantageously the growth conditions of the two sublayers may be different in order to obtain different properties in the sublayers. The step to grow an aluminium nitride layer may comprise: a step to grow a first aluminium nitride sublayer at a lower growth temperature; and a step to grow a second aluminium nitride sublayer at a higher growth temperature. The higher temperature may be at least 200°C higher than the lower temperature. The higher temperature may be at least 300°C higher than the lower temperature. Advantageously the lower growth temperature seals the previously grown, rare earth nitride layer, and forms strong bonds with the rare earth nitride layer. It also acts as a nucleation layer for the second aluminium nitride sublayer because it sets the crystal orientation, reduces the defect density and reduces the effect of any twisted rare earth oxide, for example scandium oxide, domains which permeate through the rare earth nitride layer. Advantageously the higher growth temperature provides a layer with a high quality surface due to its bulk-like properties.The step to grow a bixbyite oxide layer may comprise growing a scandium oxide layer. Advantageously, given the appropriate growth conditions, a scandium oxide layer will grow in <111> orientation on a silicon (100) substrate.The step to grow a rare earth nitride layer may comprise growing a scandium nitride layer. Advantageously the same rare earth source may be provided on the reactor.The method may further comprise a step to grow a second rare earth nitride layer between the bixbyite oxide layer and the rare earth nitride layer. The rare earth element may be different to that in the rare earth nitride layer, for example the rare earth element in the second rare earth nitride layer may be erbium or gadolinium. Advantageously the second rare earth nitride layer may have an intermediate lattice constant and may transition the lattice constant from the bixbyite oxide layer to the rare earth nitride layer. The second rare earth nitride layer may be a ternary (or multi-element) compound having two (or more) rare earth elements. The second rare earth nitride layer may have a graded composition through the thickness of the layer. Advantageously its lattice constant may bemore closely matched with each of the adjacent layers without an abrupt interface between lattice constants.There may be a further step to grow an aluminium rare earth nitride layer between the aluminium nitride layer and the gallium nitride layer. Advantageously the addition of rare earth provides an intermediate lattice constant and therefore reduces the strain between layers. The aluminium rare earth nitride may have a graded composition in which the rare earth element gradually increases from zero. Advantageously the lattice constant is gradually increased from that of the aluminium nitride layer towards that of gallium nitride meaning that there is less strain from lattice mismatch and there are no abrupt interfaces between the layers.The method may further comprise steps to grow additional pairs of the rare earth nitride layer and the aluminium nitride layer. The method may further comprise steps to grow between two and nine additional pairs of the rare earth nitride layer and the aluminium nitride layer. Thus there may be a total of between three and ten pairs of the rare earth nitride layer and the aluminium nitride layer. Advantageously process control is simpler for growing alternating thin layers of binary alloys than growing a thick tertiary alloy layer.The method may be controlled to grow the rare earth nitride layer to have thickness x and the aluminium nitride layer to have thickness y so that each pair may have thickness ratio x:y. The method may be controlled to grow the scandium nitride layer to have thickness x and the aluminium nitride layer to have thickness y so that each pair may have thickness ratio x:y. x may be smaller than y, for example x may be no more than a quarter of y. The thickness of the rare earth nitride layer to the aluminium nitride layer in each pair may have ratio 18:82. The thickness of the scandium nitride layer to the aluminium nitride layer in each pair may have ratio 18:82. Advantageously growing a pair of unequal thickness rare earth nitride and aluminium nitride layers creates the effect of a single layer of aluminium rare earth nitride having rare earth, for example scandium, content of 18% and aluminium content of 82%. Each pair of layers may have the same thickness ratio. Alternatively each pair of layer may have a different ratio, for example having a thicker aluminium nitride layer closer to the gallium nitride layer than to the bixbyite oxide layer. Advantageously this may have the same effect as a graded composition layer.The method may further comprise steps to: etch through the layers to expose a portion of the substrate; grow a second bixbyite oxide on the exposed portion of the substrate; and grow a device layer on the second bixbyite oxide layer. Advantageously the second bixbyite oxide layer can be selected to have a different crystal orientation to the bixbyite oxide layer when grown on the silicon (100) substrate. It is therefore suitable for a different type of device. For example, electronics may be grown or deposited in the region over the second bixbyite oxide layer which drive a device grown or deposited in or over the gallium nitride layer.The method may further comprise steps to: mask a portion of a silicon substrate in <100> orientation; grow a bixbyite oxide layer in <111> orientation on the silicon substrate; grow a rare earth nitride layer on the bixbyite oxide layer; grow an aluminium nitride layer on the rare earth nitride layer; grow a gallium nitride layer in <0001> orientation on the aluminium nitride layer; remove the mask to expose the portion of the substrate and apply a mask to the grown layers; grow a second bixbyite oxide layer on the exposed portion of the substrate; and remove the mask. Advantageously the second bixbyite oxide layer can be selected to have a different crystal orientation to the bixbyite oxide layer when grown on the silicon (100) substrate. It is therefore suitable for a different type of device. For example, electronics may be grown or deposited in the region over the second bixbyiteoxide layer which drive a device grown or deposited in or over the gallium nitride layer. Advantageously the method avoids the need to etch away layers.The steps to grow the semiconductor structure may be performed in an epitaxial reactor. The steps to grow the semiconductor structure may be performed in a molecular beam epitaxy reactor. The steps to grow the semiconductor structure may be performed in a metal-organic chemical vapour deposition reactor. Advantageously method is independent of the platform in which it is grown and therefore the reactor can be chosen to suit other criteria, such as cost or availability.The present invention will be more fully described by way of example with reference to the accompanying drawings, in which:Figure 1 is a schematic cross-section of a semiconductor structure according to the present invention;Figure 2 is a schematic cross-section of a semiconductor structure according to the present invention;Figure 3 is a schematic cross-section of a semiconductor structure according to the present invention;Figure 4 is a schematic cross-section of a semiconductor structure according to the present invention;Figure 5 is a schematic cross-section of a semiconductor structure according to the present invention;Figure 6 is a schematic cross-section of a semiconductor structure according to the present invention;Figure 7 is a schematic cross-section of a semiconductor structure according to the present invention.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 metal-organic 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 thatgroup. Thus Alo ^GaAs 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 single-crystal 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 may include 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 (InjOa), vanadium oxide (V2O5), iron oxide (FejOa), manganese oxide (MnjOa) 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.The invention will now be described more particularly with reference to Figure 1 which illustrates an epitaxial semiconductor structure 10. The semiconductor structure 10 comprises a silicon substrate 12 in <100> orientation, a bixbyite oxide layer 14 in <111> orientation, a rare earth nitride layer 16, an aluminium nitride layer 18, and a metal-containing layer 20 in <0001> orientation.The substrate 12 is on-axis silicon, thus it has <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) also encompass a silicon substrate 12 which is miscut by up to 20° towards another crystallographic direction, for example towards the (111) plane.Si (100) is readily available in large volumes at relatively low cost because it is used for consumer electronics such as computer chips for processing, memory, or graphics. Si (100) is also widely used for the electronic circuits which drive photonic devices, such as complementary metal-oxide semiconductors (CMOS), because of its high charge carrier mobility (higher than in other orientations of Si). There is therefore an integration benefit to be gained by growing the photonic device on Si (100), with suitable layers in between, if sufficient crystal quality can be obtained.The silicon substrate 12 may be monolithic; that is comprising bulk single crystal silicon throughout. Alternatively the silicon substrate 12 may also comprise porous silicon for some or all of its thickness, for example it may form a sublayer. The substrate 12 may include an upper portion which is porous with a lower portion that is non-porous. Alternatively the substrate 12 may include one or more discrete, non-continuous portions that are porous with the remainder being non-porous (with bulk silicon 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 substrate 12, 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.Alternatively the substrate may comprise silicon-germanium (SiGe), for example Sio.gGeo.2, or silicon on insulator (SOI).A single crystal crystalline bixbyite oxide layer 14 is positioned, grown or deposited on the substrate 12. The crystalline bixbyite oxide layer 14 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 (ImOa), vanadium oxide (V2O5), iron oxide (FejOg), manganese oxide (MmCh) and ternary compounds of a rare earth, a metal and oxygen (RE-M-O).Preferably the crystalline bixbyite oxide layer 14 is epitaxially deposited. When a crystalline bixbyite oxide is deposited on a substrate 12, 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 12 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 crystalline bixbyite oxide layer 14 grows in

[0110] orientation which has lower surface energy than

[0100] orientation.The crystalline bixbyite oxide layer 14 comprises 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 mismatchto 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 matched to 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 Er2O3 is more than 5%.It has been observed that surface morphology and crystal structure are both worse for gallium nitride (GaN) grown on ScO (111) which is grown on Si (100) compared to GaN grown on ScO (111) grown on Si (111). This is because crystal defects related to ScO domains twist in-plane which restricts the surface mobility of adatoms. For example ScO may include twinned planes of the desired <111> orientation. The present inventors have identified that a two-part interlayer between the ScO layer 14 and the GaN layer 20 beneficially improves both crystal structure and surface morphology.The interlayer comprises a rare earth nitride layer 16 and an aluminium nitride layer 18. Thus the rare earth nitride layer 16 is epitaxially grown on or over the bixbyite oxide layer 14. The rare earth nitride layer 16 has rock salt structure, that is face-centred cubic NaCI structure, and preferably comprises scandium nitride. The scandium nitride layer 16 is formed by depositing scandium and nitrogen onto the bixbyite (scandium) oxide layer 14. The scandium nitride layer 16 may grow in <111> orientation on ScO (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 better nucleation of lll-N materials due to improved adatomic mobility which promotes growth of two-dimensional layers. However, the surface roughness of GaN grown on ScN (111) over ScO (111) over Si (100) is significantly worse than that seen without the ScN layer 16, for example 30% worse. It is believed that this is due to the growth kinetics of Ga on the defective (non-smooth) surface of the ScN.The second part of the interlayer is an aluminium nitride layer 18 epitaxially grown on or over the rare earth (scandium) nitride layer 16. The AIN layer 18 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. The surface roughness of GaN grown on AIN (0001) grown on ScO (111) over Si (100) is very poor, for example around twice that seen without the AIN layer 18. This is because the aluminium has very high affinity to the oxygen in the ScO layer 14. Once aluminium oxide forms the surface cannot support successful GaN growth.However, the combination of the ScN and AIN layers 16, 18 results in at least 100% improvement in surface roughness measurement compared to GaN grown directly on the scandium oxide layer 14 over Si (100) since the ScN layer 18 isolates the AIN layer 16 from the oxide and the AIN layer 18 seals the ScN and forms a good crystal quality template for GaN growth. It also provides a surface which is closely lattice matched to GaN since aGaN=3.186A which is less than 3% larger than aAiN=3.110A. Thus surface roughness of the GaN layer 20 is less than or equal to lnm. In some examples, the surface roughness of the GaN layer 20 is less than or equal to 0.5nm.The rare earth nitride layer 16 may alternatively comprise a ternary compound including two different rare earth elements. For example, the rare earth nitride layer 16 may comprise erbium gadolinium nitride (Ero.3Gdo.7N) or scandium gadolinium nitride (Sco.1Gdo.9N) which are each lattice coincident with SC2O3 since their lattice constants are equal to half that of SC2O3 (4.93A). The lattice constant of the rare earth nitride layer 16 can be graded or stepped through its thickness in order to decrease its lattice constant to be close to lattice matched with AIN, for example by increasing the proportion of Sc in ScGdN until the top of the rare earth nitride layer 16 is ScN with lattice constant 3.20A.The metal-containing layer 20 is a lll-V semiconductor, which is an alloy of one or more group III material with one or more group V material. Optionally the lll-V semiconductor may also include a rare earth, for example being a lll-RE-N material. The lll-V may be a binary or ternary alloy such as GaN, AIN, InGaN, AIGaN, lnN,AIScN, AIYN, GaScN, GaYN, AlYbN, GaYbN. Advantageously hexagonal lll-V materials such as GaN grow in Wurtzitic phase on a (111) sub-layer.The metal-containing layer 20 is a lll-N alloy in the

[0001] orientation. For example, the lll-N may be wurtzite GaN. GaN is widely used for electronic and photonic applications. For example, GaN is used in high electron mobility transistors (HEMTs) and for LED displays.Currently GaN devices such as HEMTs are connected to Si-based CMOS electronics by wire, for example in telecommunication or internet base stations. The wiring presents a potential failure mode. It may also limit the speed of operation. The present invention permits the growth of GaN devices over Si (100) substrates which can also host CMOS electronics. Since the CMOS electronics and GaN devices are adjacent the wiring is obviated which removes the associated failure modes and means that device performance is governed by the devices themselves and not by the limitations of the connections. Advantageously the ability to grow GaN (0001) means that the inherent charge is available, since the GaN is polar, which enables piezoelectric switching without doping. The GaN device may be configured to manage power, for example by performing step-down voltage conversion in a microprocessor.Similarly, for LED or pLED applications the GaN-based emitter, or an array of GaN-based emitters, can be grown on Si (100) according to the present epitaxial semiconductor structure 10. Each emitter corresponds to one pixel of a display. The emitter or emitters can be controlled by electronic control components or devices which are also grown on or mounted on Si (100). Thus the emitters and controls can be collocated, preferably adjacent, so that each pixel in an array can be individually addressed easily and directly. Advantageously the pixels can be lit and switched off quickly and accurately.For example, the semiconductor structure 10 may be grown on part of a Si (100) substrate 12 with electronic control components grown on another part of the Si (100) wafer as shown in Figure 2. The second part of the wafer may include a second bixbyite oxide layer 28 in

[0110] orientation. The second bixbyite oxide layer 28 may comprise scandium oxide in

[0111] orientation or erbium oxide (E^Os) which twists to

[0110] orientation when grown on Si (100). One or more device layer 30 can then be grown which is compatible with the orientation of the second bixbyite oxide layer 28. The one or more device layer 30 is suitable for the growth and / or deposition and / or bonding of electronic control components. For example, the layer 30 may comprise Mo

[0110] , the orientation which forms when grown on ScO (111), or Mo

[0112] , the orientation which forms on ErO (110). Epitaxially grown molybdenum (Mo) is suitable for use as a contact and may form part of an electronic control component.The semiconductor structure 10 illustrated in Figure 2 may be formed by growing the layers as described with respect to Figure 1 and then etching through the layers to expose a portion of the substrate 12. For example a mask may be applied to a portion of the surface of the GaN layer 20 to leave the rest of the surface exposed to the etchant. Once the portion of the substrate 12 is exposed (and cleaned as required) the second bixbyite layer 28 and then the device layer 30 can be grown on the substrate 12 to form adjacent structures on the same substrate 12.Alternatively the semiconductor structure 10 shown in Figure 2 may be formed by masking a portion of the substrate 12 before growing the layers described with respect to Figure 1. The mask can then be removed and a second mask be applied to the grown layers and patterned. Then the second bixbyite oxide layer 28 followed by the device layer 30 can be grown on the exposed portion of the substrate 12 to form adjacent structures on the same substrate 12.Figure 3 is the same as Figure 1 except for the addition of optional device features 22 and optional device layers 24. The optional device features 22 are formed on or in the gallium nitride layer 20 and may comprise, for example, etched trenches, deposited contacts or terminals, a mask for further deposition of layers, or any other features used for forming one or more device on (or partially in) the semiconductor structure 10. The device features 22 may include a through via to enable connection to a contact on the substrate 12. Such a contact may be grown as part of the semiconductor structure 10 or may be bonded or otherwise affixed subsequently. Additionally or alternatively there may be one or more optional device layers 24 grown or deposited on the gallium nitride layer 20. The device layer or layers 24 may include a barrier, a contact, an active layer, a spacer, a channel or any other layer suitable or necessary for forming a device in or on GaN.In Figure 4 a similar epitaxial semiconductor structure 10 is illustrated in which the aluminium nitride layer 18 is formed of a first aluminium nitride sublayer 18a and a second aluminium nitride sublayer 18b. The first AIN sublayer 18a is grown at a lower temperature than the second AIN sublayer 18b. For example the second AIN sublayer 18b may be grown at a temperature which is at least 200°C higher than the temperature at which the first AIN sublayer 18a is grown. For example the second AIN sublayer 18b may be grown at a temperature which is at least 300°C higher than the temperature at which the first AIN sublayer 18a is grown. For example the second AIN sublayer 18b may be grown at a temperature which is up to 350°C higher than the temperature at which the first AIN sublayer 18a is grown. Similarly the nitrogen flow rate, Al flux, plasma power and other parameters may be different between the two sublayers 18a, 18b, each of which may scale with or be independently adjusted from the growth temperature.By growing the first AIN sublayer 18a at a relatively low temperature it seals the ScN and acts as a nucleation layer for the second AIN sublayer 18b. Thus it sets the crystal orientation and forms good bonds with the preceding rare earth nitride (ScN) layer 16. The first AIN sublayer 18a also reduces the defect density and any effects of twisted ScO domains which permeate through the ScN layer 16. The second AIN sublayer 18b, on the other hand, is bulk-like because it is grown at a relatively high temperature. Thus it provides a high quality surface on which the GaN layer 20 is grown.The first AIN sublayer 18a may be greater than or equal to 5nm thick. By providing a layer that is at least 5nm thick it ensures that the crystal orientation is set so that the second AIN sublayer 18b will grow in a single crystal orientation. The second AIN sublayer 18b may be greater than or equal to 15nm thick. Growing this layer at least 15nm thick ensures that it exhibits bulk-like properties rather than thin-film properties.Figure 5 is similar to Figure 1 except that it includes an optional second rare earth nitride layer 26 between the bixbyite oxide layer 14 and the rare earth nitride layer 16. The optional rare earth nitride layer 26 may also be included in the arrangement having AIN sublayers 18a, 18b illustrated in Figure 4. The optional rare earth nitride layer 26 comprises a different rare earth element to that in the rare earth nitride layer 16 and has an intermediate lattice constant between those of the adjacent layers. For example, on the (111) surface scandium oxide has a lattice constant of 3.489A and ScN has a lattice constant of 3.20A. Erbium nitride, ErN, has a lattice constant of 3.42A on the (111) surface which sits between ScO and ScN. Alternatively the optional rare earth nitride layer 26 may comprise a ternary compound having two rare earth elements to give an intermediate lattice constant between ScO (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 ScO). Furthermore Sco.1Gdo.9N can be graded to ScN through the thickness of rare earth nitride layer 26.Figure 6 is similar to Figure 1 except that it includes a further, optional, aluminium rare earth nitride layer 28 between the aluminium nitride layer 18 and the gallium nitride layer 20. The optional aluminium rare earth nitride layer 28 may also be included in the arrangement having AIN sublayers 18a, 18b illustrated in Figure 4. The optional aluminium rare earth nitride layer 28 may comprise AIxSci-xN. When x=0.18 AIScN is lattice matched to GaN. The AIScN may have a graded composition with the Sc content increasing from 0 adjacent to the AIN layer 18 up to 18% adjacent to the GaN layer 20. Advantageously by providing an intermediate AIScN layer 28, especially a graded AIScN layer 28, the strain caused by lattice mismatch between layers is reduced or minimised.Figure 7 is similar to Figure 1 except that it includes further, optional, pairs 30 of a rare earth nitride (ScN) layer 16 and aluminium nitride layer 18 between the aluminium nitride layer 18 and the gallium nitride layer 20. The optional pairs 30 of layers may also be included in the arrangement having AIN sublayers 18a, 18b illustrated in Figure 4. The optional pairs 30 of layers may have unequal thicknesses, for example with a ratio 18:82 between the ScN layer 16 and the AIN layer 18. Such a thickness ratio means the layer pairs 30 act like an AIScN layer due to the straining of the crystal lattices in the layers. There may be multiple pairs 30 of ScN layer 16 and AIN layer 18. For example, between 3 and 10 layer pairs 30.The semiconductor structure 10 may be grown in a molecular beam epitaxy (MBE) reactor. Beneficially it is comparatively simple to add a scandium source to an MBE reactor. Beneficially an MBE reactor is able to handle the high temperatures required to grow lll-N materials including AIN and GaN. Furthermore an MBE reactor is also able to grow at relatively low temperatures making it suitable to grow the low temperature first AIN sublayer. Alternatively the semiconductor structure 10 may be grown in a metal-organic chemical vapour deposition (MOCVD) reactor. Beneficially an MOCVD reactor is commonly used for the growth of lll-N materials due to the high throughput and well-established processes.

Claims

Claims1. An epitaxial semiconductor structure (10) comprising:• a silicon substrate (12) in <100> orientation;• a bixbyite oxide layer (14) in <111> orientation on the silicon substrate (12);• a rare earth nitride layer (16) on the bixbyite oxide layer (14);• an aluminium nitride layer (18) on the rare earth nitride layer (16);• a gallium nitride layer (20) in <0001> orientation on the aluminium nitride layer (18).

2. A semiconductor structure (10) as claimed in claim 1 wherein the aluminium nitride layer (18) comprises a first aluminium nitride sublayer (18a) and a second aluminium nitride sublayer (18b).

3. A semiconductor structure (10) as claimed in claim 2 wherein the first aluminium nitride sublayer (18a) is greater than or equal to 5nm thick.

4. A semiconductor structure (10) as claimed in claim 2 or claim 3 wherein the second aluminium nitride sublayer (18b) is greater than or equal to 15nm thick.

5. A semiconductor structure (10) as claimed in any preceding claim wherein the bixbyite oxide layer (14) comprises scandium oxide.

6. A semiconductor structure (10) as claimed in any preceding claim wherein the rare earth nitride layer (16) comprises scandium nitride.

7. A semiconductor structure (10) as claimed in any preceding claim wherein the gallium nitride layer (20) has surface roughness less than lnm.

8. A semiconductor structure (10) as claimed in any preceding claim wherein the gallium nitride layer (20) has surface roughness less than 0.5nm.

9. A semiconductor structure (10) as claimed in any preceding claim wherein the aluminium nitride layer (18) is in <0001> orientation.

10. A semiconductor structure (10) as claimed in any preceding claim wherein the rare earth nitride layer (16) is in <111> orientation.

11. A semiconductor structure (10) as claimed in any preceding claim further comprising a second rare earth nitride layer (26) between the bixbyite oxide layer (14) and the rare earth nitride layer (16).

12. A semiconductor structure (10) as claimed in any preceding claim further comprising a second bixbyite oxide layer (28) adjacent to the bixbyite oxide layer (14) and having a device layer (30) thereon.

13. A semiconductor structure (10) as claimed in any preceding claim further comprising an aluminium rare earth nitride layer (28) between the aluminium nitride layer (18) and the gallium nitride layer (20).

14. A semiconductor structure (10) as claimed in claim 13 wherein the aluminium rare earth nitride layer (28) comprises aluminium scandium nitride.

15. A semiconductor structure (10) as claimed in claim 14 wherein the scandium content of the aluminium rare earth nitride layer (28) is graded from zero adjacent to the aluminium nitride layer (18) up to 18% adjacent to the gallium nitride layer (20).

16. A semiconductor structure (10) as claimed in any of claims I to 12 further comprising additional pairs of the rare earth nitride layer (16) and the aluminium nitride layer (18).

17. A semiconductor structure (10) as claimed in claim 16 wherein there are between two and nine additional pairs of the rare earth nitride layer (16) and the aluminium nitride layer (18).

18. A semiconductor structure (10) as claimed in claim 16 or claim 17 wherein the thickness of the rare earth nitride layer (16) to the aluminium nitride layer (18) has ratio 18:82.

19. A semiconductor device comprising:• a semiconductor structure (10) as claimed in any preceding claim;• one or more device features (22) formed in or on the gallium nitride layer (20); and / or• one or more device layers (24) on the gallium nitride layer (20).

20. A semiconductor device as claimed in claim 19 wherein the semiconductor device comprises any one of a high electron mobility transistor (HEMT), an HBT, a switch, a light emitting device, an LED, a pLED, a photodiode.

21. A method to epitaxially grow a semiconductor structure (10) comprising steps to:• grow a bixbyite oxide layer (14) in <111> orientation on a silicon substrate (12) in <100> orientation;• grow a rare earth nitride layer (16) on the bixbyite oxide layer (14);• grow an aluminium nitride layer (18) on the rare earth nitride layer (16); and• grow a gallium nitride layer (20) in <0001> orientation on the aluminium nitride layer (18).

22. A method as claimed in claim 21 wherein the step to grow an aluminium nitride layer (18) comprises:• a step to grow a first aluminium nitride sublayer (18a) at a lower growth temperature; and• a step to grow a second aluminium nitride sublayer (18b) at a higher growth temperature.

23. A method as claimed in claim 22 wherein the higher growth temperature is at least 200°C higher than the lower growth temperature.

24. A method as claimed in any of claims 21 to 23 wherein the step to grow a bixbyite oxide layer (14) comprises growing a scandium oxide layer.

25. A method as claimed in any of claims 21 to 24 wherein the step to grow a rare earth nitride layer (16) comprises growing a scandium nitride layer.

26. A method as claimed in any of claims 21 to 25 further comprising a step to grow a second rare earth nitride layer (26) between the bixbyite oxide layer (14) and the rare earth nitride layer (16).

27. A method as claimed in any of claims 21 to 26 further comprising a step to grow an aluminium rare earth nitride layer (28) between the aluminium nitride layer (18) and the gallium nitride layer (20).

28. A method as claimed in any of claims 21 to 26 further comprising steps to grow additional pairs of the rare earth nitride layer (16) and the aluminium nitride layer (18).

29. A method as claimed in claim 28 wherein the step to grow additional pairs of layers comprises growing between two and nine additional pairs of the aluminium rare earth nitride layer (16) and the aluminium nitride layer (18).

30. A method as claimed in any of claims 21 to 26 further comprising steps to:• etch through the layers to expose a portion of the substrate (12);• grow a second bixbyite oxide layer (28) on the exposed portion of the substrate (12); and• grow a device layer (30) on the second bixbyite oxide layer (28).

31. A method to epitaxially grow a semiconductor structure (10) comprising steps to:• mask a portion of a silicon substrate (12) in <100> orientation;• grow a bixbyite oxide layer (14) in <111> orientation on the silicon substrate (12);• grow a rare earth nitride layer (16) on the bixbyite oxide layer (14);• grow an aluminium nitride layer (18) on the rare earth nitride layer (16);• grow a gallium nitride layer (20) in <0001> orientation on the aluminium nitride layer (18);• remove the mask to expose the portion of the substrate (12) and apply a mask to the grown layers;• grow a second bixbyite oxide layer (28) on the exposed portion of the substrate (12);• grow a device layer (30) on the second bixbyite oxide layer (28); and• remove the mask.

32. A method as claimed in any of claims 21 to 31 wherein the steps to grow the semiconductor structure (10) are performed in an epitaxial reactor, a molecular beam epitaxy reactor, or a metal-organic chemical vapour deposition reactor.

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