Abstract:
The present invention relates to the field of semiconductor manufacturing. More specifically, it relates to a method of forming islands of at least partially relaxed strained material on a target substrate including the steps of forming islands of the strained material over a side of a first substrate; bonding the first substrate, on the side including the islands of the strained material, to the target substrate; and after the step of bonding splitting the first substrate from the target substrate and at least partially relaxing the islands of the strained material by a first heat treatment.

Description:
BACKGROUND 
       [0001]    The present invention relates to the field of semiconductor manufacturing. More specifically, the present invention relates to a process for the realization of at least partially relaxed strained material. 
         [0002]    III-V materials have recently become the center of much research due to a vast range of applications in which they outperform the classic silicon wafers. For instance, III-V materials have excellent performances for optoelectronic, photovoltaic and power applications, such as photovoltaic cells, particularly multi-junction photovoltaic cells, laser diodes, LED, diodes and many more. 
         [0003]    Unlike silicon, however, realizing bulk wafers of III-V materials is very expensive or, in some cases, not possible. In those cases, one possible technique for obtaining bulk structures of III-V materials is to grow them epitaxially from a seed substrate. 
         [0004]    For instance, as shown in  FIG. 6 , it is possible to use a donor substrate with strained III-V epitaxial layers, one III-V layer (GaN) and an additional strained layer  6130  (InGaN) on which a compliant or low viscosity layer  6120  made of borophosphosilicate glass BPSG, that is, SiO 2  that contains for example 4.5% of boron and 2% of phosphorous and has a glass transition temperature about 800° C., is deposited, or another compliant material. The strained layer is transferred on a new intermediate substrate  611   0  via the borophosphosilicate layer, or another compliant material, by SmartCut™ processing resulting in structure  6100 . When carrying out a relaxation step for instance by increasing the temperature of the structure  6100 , the strained layer can be partially relaxed by flowing of the low viscosity layer, the flowing being a plastic deformation in contrast of the elastic relaxation of strained islands. 
         [0005]    Such a relaxation step S 61 , however, suffers from a problem known as buckling. More specifically, the resulting at least partially relaxed layer of InGaN  6231  has an undulated shape, due to the strain of the lattice being released at least partially in direction D 1 . Furthermore, in structure  6200 , the layer  6221  may also be buckled. 
         [0006]    The above mentioned buckling problems can be reduced by a manufacturing process such as the one illustrated in  FIG. 7  and known from patent document EP2151852A1. As can be seen in  FIG. 7 , a structure  7100  includes a support substrate, e.g. of Si, SiC, Ge or sapphire,  7110 , a compliant or relaxing layer e.g. BPSG, an SiO 2  compound comprising B (BSG) or P (BPG),  7120  and islands of strained material, e.g. InGaN,  7130 . The structure  7100  is then subjected to a relaxation step S 71  by heating the structure  7100  and flowing of the compliant layer. The resulting structure  7200  includes islands  7231  of at least partially relaxed strained material. The at least partial relaxation of the islands  7231  results in an elongated compliant layer  7221 , at least at the interface with the islands  7231 . 
         [0007]    During a further deposition step S 72 , a buried layer  7340  is deposited on the islands  7231  of at least partially relaxed material. On top of the buried layer  7340 , a second support substrate  7350  is subsequently bonded to bury layer  7340 . The resulting structure  7300  therefore comprises islands  7231  of at least partially relaxed material which are connected to both the first support substrate  7110  via layer  7221  and the second support substrate  7350  via layer  7340 . During a subsequent transfer step S 73 , the structure  7400  including the islands  7231  of at least partially relaxed material, the buried layer  7340  and the second support substrate  7350  is detached from structure  7300 . The detachment can be achieved, for instance, by laser lift off that comprises irradiation of an absorbing layer between the substrate and the islands  7231  with a laser. 
         [0008]    Alternatively, the detachment could be achieved by removing both the support substrate  7110  and the elongated seed layer  7221 , for instance, by Chemical Mechanical Polishing (CMP) or also by implantation ions in the seed layer  7120  and by subsequently heating structure  7300 . 
         [0009]    At this point, an epitaxial growth of at least partially relaxed structures  7560 , by using the islands  7231  as seeds, can be carried out in a step S 74 . Also, by controlling the amount of relaxation during step S 71 , the physical characteristics such as lattice parameter, low dislocation density, degree of relaxation, of the at least partially relaxed structures  7560  can be controlled. 
         [0010]    Such a process, however, still has some drawbacks. It requires a high number of steps and the buckling phenomenon can still occur depending of the size of the islands. 
         [0011]    Moreover, when the strain of the strained material is high, cracking and delamination of the strained InGaN can be observed. The strain may depend on the lattice mismatch between the seed layer and the islands and the thickness of the islands. The higher the thickness, the higher is the strain. In the case of an InGaN island and GaN seed layer, the higher the amount of Indium in InGaN the higher the lattice mismatch and strain. 
         [0012]    There therefor is a need to improve these prior art processes by reducing the amount of steps and by further reducing the buckling phenomenon. The present invention now satisfies this need. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention now provides a method of forming islands of at least partially relaxed strained material on a target substrate by forming islands of the strained material over a face of a first substrate; bonding the face of the first substrate to a target substrate; and detaching, by splitting, the first substrate from the target substrate and at least partially relaxing the islands of the strained material by a first heat treatment. 
         [0014]    By carrying out the inventive method, it is possible to achieve islands of at least partially relaxed material which are connected to a transfer substrate and the buckling phenomenon can be further reduced thanks to the presence of both the first substrate and the target substrate on both the sides of the islands. 
         [0015]    In some embodiments, the method can further comprise forming a layer of a gripping material or a layer of relaxing material over the islands of the strained material, with the layer of gripping material provided for improving adhesion between the islands and the layer of relaxing material. 
         [0016]    Thanks to the formation of the layer of a relaxing material, bonding between the two substrates can be facilitated. In addition, the target substrate can include over its surface a layer of a bonding material, so that bonding can occur between the layer of relaxing material and the layer of a bonding material. 
         [0017]    In some embodiments, the relaxing material and the bonding material can be BPSG or any material that flows at a glass transition temperature comprised between 600 and 1000° C. 
         [0018]    In some embodiments, the bonding layer can comprise an absorbing layer of electromagnetic irradiation such as Si x N y :H. 
         [0019]    In some embodiments, the step of at least partially relaxing the islands can include the steps of heating the bonded first substrate and target substrate at about the glass transition temperature of the relaxing layer, preferably 600° C.-1000° C., during few hours, e.g., 2 to 6 hours. 
         [0020]    In some embodiments, the splitting step can include an elastic relaxation. 
         [0021]    In some embodiments, after the splitting step, the surface topology of the islands of at least partially relaxed material can be kept under 10 nm RMS and preferably under 6 nm as measured by Zygo metrology over a field of approximately 140 micrometers by 100 micrometers 
         [0022]    In some embodiments, the method can further comprise the step of forming a seed layer, on the face of the first substrate, before forming the islands so that the islands of the strained material can be formed on the seed layer. 
         [0023]    In some embodiments, the seed layer can be any of GaN, any semi-conductor materials, silicon, III-V alloys, III-N materials, binary, ternary, quaternary alloys as AlGaN or Al n GaN. 
         [0024]    In some embodiments, the method can further comprise implanting ionic species in a region within the seed layer on the first substrate, or in the first substrate, to form a weakened layer prior to bonding the first substrate to the target substrate. By implanting ions in a region within the layer of the at least partially relaxed material, it is possible, in addition to at least partially relaxing the islands, to detach and transfer the islands from the first substrate to the target substrate, by weakening of the layer of the at least partially relaxed material Thanks to the fact that the relaxing and transferring are achieved in a single step, the number of necessary steps can be reduced. 
         [0025]    In some embodiments, during the relaxing, the heat treatment can be carried out such that the weakened layer is detached. 
         [0026]    By this method, relaxing of the islands and weakening of the ion implanted region can be achieved during the same heat treatment. Moreover, by performing this method, the islands can be at least partially relaxed while hindering the occurrence of buckling and without causing delamination. This is because the target substrate acts as a support and a stiffening substrate at the same time and this promotes lateral relaxation without buckling. 
         [0027]    In some embodiments, during the relaxing, the splitting and the relaxation can occur simultaneously. 
         [0028]    In some embodiments, the islands of at least partially relaxed strained material bonded to the target substrate can be detached together and physically moved away from the first substrate. 
         [0029]    In some embodiments, the strained material and the seed layer that can be detached together from the first substrate can have a bilayer thickness in the range of 150-500 nm. 
         [0030]    In some embodiments, the forming of the islands can include forming trenches in the first substrate or in the seed layer following the shape of the islands of strained material. For this embodiment, the weakened layer can be formed below the trenches. 
         [0031]    In some embodiments, during the first heat treatment a heating temperature can be changed at a rate in the range of 1° C./min to 50° C./min, and preferably in the range of 1° C./min to 20° C./min. 
         [0032]    In some embodiments, the method can further include applying a second heat treatment, at about the glass transition temperature of the relaxing layer after physically moving away the at least partially relaxed strained material from the first substrate 
         [0033]    In some embodiments, the method can further comprise thinning the seed layer before or after applying the second heat treatment. By carrying out the thinning before the second heat treatment, the relaxation of the islands can be facilitated by the reduced amount of the stiffening seed material. 
         [0034]    In some embodiments, the thinning and second heating can be repeated until the islands are substantially relaxed. 
         [0035]    In some embodiments, the layer of a relaxing material or the layer of a bonding material can be partially or completely patterned following the islands of strained material before thinning or applying the second heating to help the complete relaxation 
         [0036]    In some embodiments, the strained material can be strained InGaN preferably including indium between 3-20%, more preferably between 5-15%, or it can be strained SiGe or strained III-N material. 
         [0037]    In some embodiments, either of the first substrate and the target substrate can be any of sapphire, silicon, SiC, or Ge, preferably with the first and the target substrate being identical or having a similar coefficient of thermal expansion. A similar coefficient of thermal expansion helps to avoid different expansion of the substrates during heat treatment that would weaken the structure. The identical material for the substrates provides no difference in expansion. 
         [0038]    In some embodiments, the size of the islands can be in the range of about 300 nm by 300 nm, to a few millimeters by few millimeters and could have any shape. 
         [0039]    In some embodiments, the islands of at least partially relaxed material can be bonded to a final substrate and the target substrate can be detached by laser lift off. 
         [0040]    For example any of the first substrate, intermediate substrate and the target substrate may be in sapphire that is transparent to the electromagnetic irradiation of an Ar/F laser providing light of 193 nm. If the process flow uses a silicon substrate or SiC substrate, etching or polishing of the substrate may be used to remove it or the substrate may not be recycled and can be destroyed. 
         [0041]    A second transfer of the relaxed InGaN by a second bonding on a third substrate and a Laser lift off process can permit to re-growth by epitaxy the relaxed III-V on the Ga face. 
         [0042]    In some embodiments, the islands at least partially relaxed material can be bonded to the final substrate by a non-compliant oxide layer with a glass transition temperature above 1000-1200° C. 
         [0043]    The present invention can further relate to a working layer of relaxed III-V, II-VI or IV material which can be epitaxially grown on the islands of at least partially relaxed strained material obtained by the method of any of the previous embodiments. Thus, the structure comprises a working layer of relaxed III-V, II-VI or IV material on islands of at least partially relaxed strained material on a substrate. 
         [0044]    The present invention can further relate to an optoelectronic or photovoltaic device such as a Led, Laser, solar cell, formed in or on the working layer of the structure according to the previous embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    The accompanying drawings are incorporated into and form a part of a specification to illustrate several embodiments of the present invention. These drawings together with the description serve to explain the features, advantages and principles of the invention. The drawings are only for the purpose of illustrating preferred and alternative examples of how the invention can be made and used, and are not to be construed as limiting the invention to only the illustrated and described embodiments. Further features and advantage will become apparent from the following and more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like reference prefer to like elements and wherein: 
           [0046]      FIGS. 1-5  are schematic drawings illustrating a process of forming islands of at least partially relaxed strained material according to the present invention; 
           [0047]      FIG. 6  is a schematic drawing illustrating a process of forming a layer of at least partially relaxed strained material according to the state of the art; and 
           [0048]      FIG. 7  is a schematic drawing illustrating a process of forming islands of at least partially relaxed strained material according to the state of the art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0049]      FIGS. 1-5  illustrate detailed steps of a process of forming islands of at least partially relaxed strained material according to an embodiment of the present invention. 
         [0050]    As can be seen in  FIG. 1 , a structure  1100  includes a first support substrate  1110 , a seed layer  1120  and a strained material layer  1130 . The support substrate  1110  could be sapphire (Al 2 O 3 ), or silicon, SiC, or Ge. The seed layer  1120  is a monocrystalline layer and could be GaN. The strained material layer  1130  is a crystalline layer, preferably a monocrystalline material, and could be InGaN, with a high Indium concentration in InGaN if the aim is to obtain a high lattice parameter after the relaxation, for epitaxy of relaxed InGaN material to form working layer on top of it, preferably higher than 7-8% up to 15%. The thickness of the seed layer  1120  could be in the range of at least 2 μm in order to decrease the dislocation density on the surface. The thickness of the strained material layer  1130  could be in the range of 50 nm to 300 nm depending of the In content or percentage of the InGaN alloy. 
         [0051]    The atomic lattices of the seed layer  1120  and the strained material layer  1130  are different when the materials of the layers  1120  and  1130  are in their natural-unstressed state, that is, with nominal lattice parameter. When the layer  1130  is grown epitaxially on top of layer  1120 , with a pseudomorphic growth, however, this results in the crystalline material of layer  1130  being strained. For example, if layer  1120  is GaN and layer  1110  is sapphire; GaN is grown on sapphire that has not the same lattice parameter of GaN so that GaN layer is strained. Furthermore, for instance, if layer  1130  is InGaN, it may keep the lattice parameter of GaN seed layer that kept the lattice parameter of sapphire substrate  1110 . As InGaN has more lattice mismatch with respect to sapphire than GaN, the InGaN layer may be more strained than the GaN layer. 
         [0052]    Alternatively, the structure  1100  could comprise further layers between the support substrate  1110  and the seed layer  1120  or between the seed layer  1120  and the strained material layer  1130 . 
         [0053]    During a subsequent patterning step S 11 , one or more holes or trenches  1240  are then realized in the structure  1100  e.g., using lithography or laser ablation, thereby obtaining structure  1200 . As can be seen in  FIG. 1 , the hole  1240  cuts through the strained material layer  1130  and at least within in the seed layer  1120 , thereby realizing at least two islands  1231  of strained material. The depth  1241  of the hole  1240  along direction D 1  is at least longer than the thickness of the strained material layer  1130 . More preferably, the depth is longer than the depth of the weakened zone formed by subsequent implantation in the seed layer  1120 . The width  1242  of the hole  1240  along direction D 2  could be in the range 5 μm to 50 μm for example, preferably between 5-10 μm. The smaller the width  1242  along direction D 2 , the easier the later step of transferring of the islands of strained material  1231 . Typically, a width  1242  of the hole  1240  in the order of 8 μm can be achieved. 
         [0054]    As illustrated in  FIG. 2 , one or more layer deposition steps S 21  are carried out to deposit at least one of a layer of a first bonding or gripping material  2151 , a layer of a second bonding or relaxing material  2150 , and a layer of a dielectric material  2153  or any oxide layer and possibly the same than the gripping material or relaxing material. Depending on the way the deposition steps are carried out, two resulting structures could be obtained: structure  2100 A and structure  2100 B. 
         [0055]    In the first variant, structure  2100 A is obtained by filling the hole  1240 , via a step of depositing a layer of a dielectric material  2153  on structure  1200 , the dielectric material  2153  having a thickness corresponding to at least the depth  1241  of the hole  1240 , and a  30  step of performing a CMP so as to remove excess dielectric material  2153  from the islands  1231  of strained material, thereby leaving the dielectric material only inside the hole  1240 . Following such a filling step, a layer of a gripping material  2151  and subsequently a relaxing material layer  2150  is deposited, the two layers  2151  and  2150  having a combined thickness  2154  along direction D 1 . 
         [0056]    In the second variant, structure  2100 B is obtained by depositing a gripping material  2151  and subsequently depositing a layer of a relaxing material  2150  having a combined thickness  2154  along direction D 1 . In  FIG. 2 , the combined thickness  2154  is shorter than the depth  1241  of the hole  1240 . The two layers of material  2150 ,  2151  would thereby result in islands  2152 B of materials  2150 ,  2151  such as represented in structure  2100 B. 
         [0057]    The dielectric material  2153  could be any material which can be polished and planarized by CMP and may be e.g., silicon oxide. A preferred relaxing material  2150  in this embodiment is BPSG, but it could also be any compliant layer for the relaxation step. Generally speaking, it could be any material that flows between a glass transition temperature 600 and 1000° C., for instance, a BPSG layer that has a content of 4.5% boron and 2% phosphorous has a glass transition temperature of about 850° C. More preferably the material does not release any contaminating particles during thermal treatment. 
         [0058]    The usage of such material may enable a better bonding with the target substrate  3170 , like described later, in particular in case the same material  3190  is also provided on the target substrate  3170 . The thickness of the layer of the relaxing material  2150  along direction D 1  is typically in the range of 300 nm to more than 1 μm, preferably has a value of about 500 nm. 
         [0059]    The gripping material  2151  in this embodiment is SiO 2 , but could also be any of material that allows a strong adhesion between III-N material and BPSG. The SiO 2  may be deposited by PECVD (Plasma-enhanced chemical vapor deposition) and a thermal treatment may be applied in order to degassing and make denser the layer to provide a strongest adhesion effect. The use of SiO 2  allows for a better adherence between the strained material layer  1130 , or the islands  1231 , and the gripping material  2151 . The thickness of the layer of the gripping material  2151  along direction D 1  is less than the one of the relaxing material  2150  and could be in the range of 10 nm to 100 nm, preferably a value of 50 nm 
         [0060]    In a third variant not shown in  FIG. 2 , only the relaxing material  2150  could be deposited, instead of the layer of gripping material  2151  and the relaxing material  2150 . Still alternatively, more than two materials could be deposited with each material contributing to the various bonding or relaxing properties of the substrate. 
         [0061]    The deposition step S 21  of the gripping layer  2151  and relaxing material  2150  could be carried out by any suitable technique like LPCVD (Low Pressure Chemical Vapor Deposition), PECVD or any other techniques. 
         [0062]    Following the deposition of the relaxing material  2150 , a heating step could be carried out for any of structures  2100 A and  2100 B in order to densify the materials  2150  and  2151  at the same time. The heating step could be carried out by inserting any of structures  2100 A and  2100 B in a heating chamber at a temperature higher than the deposition temperature and below the flowing temperature during a time in the range of 1 hour to 4 hours, preferably one hour. This annealing depends on the properties of the materials in the structure  2100 A and  2100 B. 
         [0063]    An ion implantation step S 22  could then be performed on any of structures  2100 A and  2100 B resulting respectively into structures  2200 A and  2200 B. The implantation step implants ionic species, e.g. hydrogen and/or helium ions, inside the seed layer  1120  to form a predetermined weakened area  2260 ,  2261 that results in the detachment or splitting of the first substrate from the target substrate after bonding. 
         [0064]    As can be seen in  FIG. 2 , different implantation regions  2260  and  2261  could be achieved, having different depths along direction D 1 , depending on the implantation energy. For instance, a shallower depth  2260  is achieved with implantation energies in the range of 60 KeV to 90 KeV. A deeper depth  2261  could be achieved with implantation energies in the range of 100 KeV to 130 KeV. 
         [0065]    By controlling the depth of the implantation region within the seed layer  1120 , the thickness  2262  or  2263  respectively of the detached seed layer  1120  and the strained material layer  1130  or islands  1231  up to the implantation region  2260  or  2261 , can be controlled. Typically, the thickness  2262 ,  2263  is in a range of 100 nm to 500 nm. This implantation may be done through the  2150  relaxing layer. The thickness  2262  and  2263  begins from bottom of the strained InGaN  1130  layer to the top of the arrows  2262  and  2263  on  FIG. 2 . This layer will be transferred with the strained III-V layer and GaN  1120  layer that provides a part of stiffening effect present in the process flow. The deeper the implantation, the less the strained material layer is damaged by the implantation. For a deeper implantation, however, there may be a higher strain in the bonded substrate composed by layers  1120 ,  1130  and the target substrate, which may result in delamination. 
         [0066]    Alternatively, the depth of the implantation region and the thickness of layers  2150 ,  2151 ,  1130  and  1120  could be controlled so as to achieve an implantation region within the support substrate  1110 . 
         [0067]    Concerning structure  2100 B, due to the presence of hole  2140 B, the ion implantation step S 22  results into implantation regions  2260 A and  2261 A as well as  2260 B and  2261 B. Implantation regions  2260 A and  2261 A substantially correspond to implantation regions  2260  and  2261 . Due to the presence of hole  2140 B, however, the ions which are implanted through the hole will reach a deeper level in structure  2200 B. More specifically, they will result into implantation regions  2260 B and  2261 B. 
         [0068]    Subsequently a bonding step S 31  is carried out on any of structures  2200 A and  2200 B by bonding structure  2200 A to structure  3100 A or structure  2200 B to structure  3100 B respectively, as illustrated in  FIG. 3A . 
         [0069]    As can be seen in  FIG. 3A , structure  3100 A or  3100 B includes a target substrate  3170 , a detachment layer or absorbing layer  3180  and a bonding layer  3190 . The target substrate  3170  in this embodiment is sapphire, but could also be any substrates if the coefficient of thermal expansion is similar to the first substrate, in which case it may be a transparent substrate if one laser lift off is required in the process flow. Optionally, the first and the target substrate are identical or have similar coefficient of thermal expansion to avoid any different thermal expansion and any delaminating of layer during thermal treatment. The detachment layer  3180  in this embodiment is Si x N y :H, but could also be any one of detachment layers and target substrate matching the wavelength of the Laser lift equipment. The transparent sapphire target substrate can at a later stage removed by a laser lift off process during which laser light enters via the transparent target substrate  3170  and be absorbed by the layer  3180 . The thickness of the SiN layer  3180  could be in the range of 50 nm to 300 nm, preferably 200 nm. 
         [0070]    The bonding layer  3190  in this embodiment is BPSG, but could also be any compliant material layer. The usage of such material is advantageous in proving a better bonding and relaxing effect with the relaxing layer  2150  of structures  2100 A,  2100 B. The thickness of the bonding layer  3190  along direction D 1  could be in the range of 500 nm to 3 μm, preferably 1 μm. 
         [0071]    The bonding step S 31  could include a CMP step consisting in performing a CMP on both the bonding layer  3190  and the layer of relaxing material  2150  in order to achieve a roughness of the surface of both layers  2150  and  3190  in the range of 5 Angstrom RMS maximum, preferably a value of less than 5 Angstrom RMS, measured by an Atomic Force Microscope (AFM) in a field in the range of 5×5 square micrometers. The choice of such values for the roughness improves bonding. 
         [0072]    Subsequently, the structure  3100 A is brought in contact with structure  3200 A or the structure  3100 B is brought in contact with structure  3200 B to initiate bonding. The resulting bonded structures are illustrated in  FIG. 3B . 
         [0073]    Structure  3300 B is substantially similar to structure  3300 A, except for the area  2140 B between two islands. 
         [0074]    Subsequently, through a splitting and relaxation step, S 41  structures  4100 A and  4100 B are obtained from structures  3300 A and  3300 B respectively, as illustrated in  FIG. 4 . 
         [0075]    The splitting and relaxation step might be realized by inserting structure  3300 A or structure  3300 B into a heating chamber and carrying out the following heating steps: 
         [0076]    (i) starting at room temperature; then 
         [0077]    (ii) heating up to a temperature in a range of 50° C. to 250° C., and holding the temperature for a duration in a range of 30 min to few hours; then 
         [0078]    (iii) heating up to a temperature in a range of 280° C. to 700° C., and holding the temperature for a duration in a range of few min to more than 20 hours depending on the implant conditions; then 
         [0079]    (iv) heating up to a temperature in a range of 400° C. to 900° C. depending on the compliant layer properties, and holding the temperature for a duration in a range of 2 to 6 hours. 
         [0080]    From one heating step to the next the temperature changes are realized at a rate in the range of 1° C./min to 50° C./min and more preferably, at 1° C./min to 20° C./min, for all the ramp up and ramp down or cooling. 
         [0081]    The effect of the splitting and relaxation step S 41  can be seen in structures  4100 A or  4100 B of  FIG. 4 . As can be seen in  FIG. 4 , structure  4100 A or  4100 B results from the detachment of the support substrate  1110  and a part of the seed layer  1120  from structures  3300 A and  3300 B respectively. The part of the seed layer  1120  detached consists in the part of the seed layer  1120  which is located between the ion implantation region  2260 ,  2261  and the support substrate  1110 . 
         [0082]    Moreover, the splitting and relaxation step S 41  causes the islands  1231  of strained material to at least partially relax thank to the flowing of the relaxing layer  2150  and bonding layer  3190 , thereby forming islands  4132  of at least partially relaxed strained material. This can be seen in  FIG. 4 , wherein the length  4102  along direction D 2  of the strained seed layer  4120  is longer than the original length  3101  of any of structures  3300 A and  3300 B along direction D 2 . 
         [0083]    The advantage of performing both the splitting and the relaxation in a single heating step is that the number of steps can be reduced. Moreover, by maintaining a transferred part of the seed layer  1120  ( 4121 ) and the first substrate  1110  in structure  3300 A and  3300 B, the relaxation of islands  1231  of strained material may be achieved without buckling, and in an elastic way so that no new crystal defects such as dislocations are formed, no cracks are formed in the island materials, and no delaminating occurs. Indeed, the presence of the first substrate  1110  on top of the structure during relaxing step provides a great stiffening effect that allows to avoid buckling and to maintain the stability of the composite structure. 
         [0084]    Furthermore, if the splitting and relaxation step occur simultaneously, the handling and risk of damage from possible scratches with the chuck are significantly reduced. 
         [0085]    Concerning structure  4100 B, the same effects are observed, except that structure  4100 B further includes a hole  4191 B deriving from hole  2140 B between the islands. 
         [0086]    Subsequently and optionally, an additional relaxation and thinning step S 42  could be carried out on structures  4100 A or  4100 B thereby resulting in structures  4200 A and  4200 B. The compliant layers  4191  and  4151  on the target substrate may also be at least patterned following the borders of existing islands to help the relaxation if needed. 
         [0087]    The relaxation and thinning step S 42  might be realized by inserting any of structures  4100 A and  4100 B into a heating chamber and heating the chamber at a range of 800° C. to 1000° C., during a period in a range of 1 hour to 8 hours. Moreover, before, during or after the heating process, the strained seed layer  4121  could be thinned, resulting in a reduced thickness  4204  compared to the thickness  4103  of structures  4100 A or  4100 B. 
         [0088]    This has the effect of flowing of the compliant BPSG layer  4151  and compliant layer  4191 , so that at least part of the compressive strain in the islands  4132  of at least partially relaxed strained material can relax and the lattice parameter is enlarged. This can be seen in  FIG. 4 , wherein, for illustration purposes, the length  4203  along direction D 2  of the at least relaxed islands  4233  is longer than the length  4102  of the less relaxed islands  4232 . 
         [0089]    The same effects are achieved for structure  4200 B except that structure  4200 B further includes a hole  4191 B deriving from hole  2140 B. 
         [0090]    According to further variants, the relaxation and thinning step S 42  could also be carried out more than once, in order to progressively further relax the islands  4132  of at least partially relaxed strained material while still preventing buckling due to the presence of layer  4222 . 
         [0091]    Subsequently, as can be seen in  FIG. 5 , a further relaxation and thinning step S 51  is carried out on structures  4200 A and  4200 B thereby resulting in structures  5100 A and  5100 B. The relaxation and thinning step S 51  might be realized by inserting any of structures  4200 A and  4200 B into a heating chamber and heating the chamber at a range of 800° C. to 1000° C., during a period in a range of 1 to 8 hours. 
         [0092]    Moreover, before, during or after the heating process, the layer  4222  could be thinned eventually resulting in the layer  4222  to be totally removed. This has the effect of carrying at least partially the strain of the islands  4132  of at least partially relaxed strained material to the layer of strained gripping material, and/or to the strained bonding layer  4292 , and/or layer  4252 , resulting into a layer of strained gripping material, and/or a strained bonding layer  5193  and/or layer  5153  respectively. This can be seen in  FIG. 5 , wherein the length  5104  along direction D 2  of the islands  5134  of at least partially relaxed strained material of structures  5100 A and  5100 B is longer than the length  4203  of the strained seed layer  4222  and of the islands  4233  of at least partially relaxed strained material of structures  4200 A and  4200 B. 
         [0093]    The same effects are achieved on structure  5100 B except that it further includes a hole  4191 B deriving from hole  2140 B. 
         [0094]    The relaxed islands may be transferred onto a final substrate such as sapphire to recover the right polarity and to have a bonding layer that has a high viscosity between island and final substrate, the III-element facing Ga. The bonding may be a direct bonding or performed with a high viscosity layers, or non-compliant layer or that has a glass transition temperature above 1000° C., such as Si02 so that no flowing occurs during thermal treatment of epitaxy. The second transfer can be done by laser lift off if an absorbing Si x N y :H layer may be provided between the target substrate and the islands. Any of the layers  3180 ,  5193 ,  5153 ,  2151  and  5134  and the new bonding layers may be transferred on the final substrate. 
         [0095]    During the growth step S 52  structures  5000  consisting of at least partially relaxed strained material, are epitaxially grown by using the islands  5134  of at least partially relaxed strained material as seeds. The material of structures  5000  could be the same as the material of islands  5134 . Alternatively, it could be a different material having a compatible atomic lattice structure, such as III-N materials, II-VI or IV materials. Preferably, the structures  5000  are grown when the islands of material are transferred on the final substrate with a high viscosity layer. Then, any of the  3180 ,  3193 ,  5153  and  2151  layers may be totally removed. This last step could include a CMP and/or etching steps. 
         [0096]    Once the  5134  layer of the top of the final structure, the InGaN re-growth could be achieved by epitaxy on the relaxed strained material. If the relaxed strained material is non polar, the re-growth can be achieved by epitaxy directly on the  5134  layer showed in the step S 52 . The same effects are achieved on structure  5200 B except that structure  5200 B further includes a hole  4191 B deriving from hole  2140 B. 
         [0097]    The amount of relaxation of structures  5000 , and correspondingly, islands  5134  of at least partially relaxed strained material can be precisely controlled by controlling the number of times the relaxation and thinning step S 42  is carried out. Preferably, the islands are substantially relaxed at the end of the process flow such that the structure  5000  is also substantially relaxed. As the seed layer  4222  is only removed towards the end of the process, the occurrence of buckling can be prevented thanks to the remaining substrate  1110  during the relaxation step. 
         [0098]    The process according to the embodiment and the variants of the present invention described above thus allows the number of steps to be reduced and while at the same time a relaxation of the strained III-V material is achieved wherein buckling can essentially be prevented. Furthermore, the present invention reduces the cracking and the delamination observed in the prior art, thanks to the presence of the first substrate during heating and relaxation.