Abstract:
A method for the production of crack-free Group III-Nitride layers is disclosed. The method proceeds by growing a crack-free first layer of Group III-Nitride on a starting substrate. A partial to complete loss of coherency is then achieved between a lattice of the first layer and a lattice of the starting substrate. A second layer is grown to form a composite layer that includes the first layer and the second layer such that the first layer is between the second layer and the substrate. The starting substrate may then be completely separated from the composite layer to produce the freestanding crack-free Group III-Nitride layer.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for the production of a high quality free-standing layer of Gallium Nitride or similar material by heteroepitaxial deposition and subsequent removal from a transparent substrate. 
     BACKGROUND 
     Gallium Nitride (GaN) has been recognized as having great potential as a technological material. For example, GaN is used in the manufacture of blue light emitting diodes, semiconductor lasers, and other opto-electronic devices, as well as in the fabrication of high-temperature electronics devices. 
     One of the greatest challenges for the large-scale production of GaN-based devices is the lack of a suitable native GaN substrate. GaN is not found in nature; it cannot be melted and pulled from a boule like silicon, gallium arsenide, sapphire, etc., because at reasonable pressures its theoretical melting temperature exceeds its dissociation temperature. However, the fabrication of very high crystal quality, thin layers of GaN, and its related alloys, for use in electronic devices, requires that they be deposited homoepitaxially onto an existing GaN surface. Such high quality device layers cannot be directly grown heteroepitaxially, for reasons that are outside the scope of this invention. 
     The techniques currently in use for the fabrication of high quality GaN and related layers involve the heteroepitaxial deposition of a GaN device layer onto a suitable but non-ideal substrate. Currently such substrates include (but are not limited to) materials such as sapphire, silicon, silicon carbide, gallium arsenide, lithium gallate, lithium aluminate, and lithium aluminum gallate. All heteroepitaxial substrates present challenges to the high-quality deposition of GaN, in the form of lattice and thermal mismatch. Lattice mismatch is caused by the difference in interatomic spacing of atoms in dissimilar crystals. Thermal mismatch is caused by differences in the coefficient of thermal expansion (CTE) between joined dissimilar materials, as the temperature is raised or lowered. 
     For the purpose of clarity, heteroepitaxial growth is defined herein as a process whereby the atomic lattices of two dissimilar materials are intimately joined together by atomic bonds across their common interface. When the cross-linking bonds are made in a regular and orderly array displaying long-range order, the interface is said to be coherent. When the cross-linking bonds are broken, bent, twisted, or otherwise distorted such that there is no long-range order, the interface is said to have lost coherency. Coherent interfaces are much stronger than incoherent interfaces, due to the greater number of cross-linking bonds between the materials. The loss of coherency may be partial; if only a percentage of cross-linking bonds are broken or distorted in an interface, the interface is partially coherent. The percentage (by area) of broken or distorted bonds represents the level of incoherency or loss of coherency for that interface. 
     The most commonly used heteroepitaxial substrate for GaN deposition is sapphire (Al 2 O 3 ), which has both a large thermal mismatch and a large lattice mismatch compared to GaN. In addition, the sapphire substrate is not electrically conductive, and has poor thermal conductivity, limiting its heat sinking capabilities, further reducing device performance and complicating device processing. For reasons unrelated to the scope of this invention, sapphire otherwise possesses superior properties as a hetero-substrate. However, the large lattice mismatch results in films that have very high defect densities, specifically in the form of dislocations, which are especially undesirable from a device fabrication point of view. (The formation of dislocations at regular intervals along the interface does not affect its coherency, as defined for the purposes of this application, for the dislocations themselves exhibit a type of long-range order in their distribution.) As with other epitaxial crystal growth processes, it is necessary to grow a buffer layer of GaN on the sapphire surface prior to the formation of device-quality layers. The buffer layer will vary, depending on device tolerance to dislocations, whether or not special growth techniques (such as growth through a mask pattern, use of low temperature buffer layers, etc.) are employed, as well as other factors. Typically, this GaN buffer thickness is less than one micron to tens of microns thick. Defect densities, however, predominantly in the form of dislocations, remain high (˜10 10  cm −2 ) resulting in diminished device quality. In addition to the conventional buffer layer, a low temperature GaN buffer layer is nearly always used. This layer is the first layer deposited on the sapphire. The buffer layer is initially amorphous and typically is 30-50 nm thick; it is recrystallized at the growth temperature. 
     Besides dislocations and lattice mismatch problems, thermal mismatch is also a consideration. Typically the GaN is deposited onto sapphire at a temperature of between 1000-1100° C.; as the sample cools to room temperature, the difference in thermal expansion (contraction) rates gives rise to high levels of stress at the interface between the two materials. Sapphire has a higher coefficient of thermal expansion (CTE) than does GaN. As the sapphire substrate and GaN layer cool, the mismatch at the interface puts the GaN under compression and the sapphire under tension. Up to a point, the amount of stress is directly related to the thickness of the deposited GaN, such that the thicker the film, the greater the stress. Above a film thickness of approximately 10 microns, the stress levels exceed the fracture limits of the GaN, and cracking and peeling of the film may result. Cracks in this layer are much less desirable than high dislocation densities, and should be avoided because of the risk of their catastrophic propagation into the device layer during subsequent processing steps. 
     One method to prevent such thermal stress-related problems involves separating the sapphire substrate from the deposited film. This may be done by physically removing the substrate (lapping and polishing), or by focusing a very high-intensity light source (such as from a laser) from the substrate side of the sample. The light source emits photons having an emission energy that is not absorbed by the sapphire. This second technique utilizes the difference in absorption between the two materials: GaN has a room temperature electron bandgap of approximately 3.45 eV, whereas sapphire has a bandgap of 9.9 eV. Photons with an energy greater than approximately 3.45 eV and less than 9.9 eV (corresponding to vacuum wavelengths less than 359 nm but greater than 125 nm) are able to pass through the back side of a sapphire wafer, where they are absorbed in various amounts, depending on energy, by the GaN at the interface. Once absorbed, the photons are converted to heat, which locally disrupts the Ga—N bonds. If the incident radiation is intense enough, large-scale local disruption results in a complete loss of coherency between the lattice of the sapphire substrate and the GaN. At lower radiation levels, the loss of coherency may only be partial and incomplete, resulting in a film that is still attached to the sapphire substrate, but is no longer completely bonded to it. 
     Both aforementioned techniques have limitations. A free-standing film must be sufficiently thick to have the required mechanical strength necessary for subsequent device processing. Typically, this requires a minimum thickness on the order of 50-100 microns. Deposition of a crack-free film with this thickness onto sapphire is feasible if done carefully, however thermal stresses will cause severe bowing in the wafer as it cools to room temperature. Conventional lapping and polishing processes are not effective at removing a concave substrate; alternatively, use of a laser to remove the GaN from the sapphire can create unstable localized regions of stress in the partially-removed film, leading to layer fracture during the lift-off process. 
     Referring to the drawings, FIGS.  1 ( a )- 1 ( b ) schematically illustrate the prior art when deposition of a thick layer of GaN onto sapphire is desired. In FIG.  1 ( a ), sapphire substrate  101  has a thick (greater than 10 microns) film of GaN  102  deposited onto it, at the growth temperature, which may be in the range of 1000-1100° C. The actual method of deposition is not relevant to this invention. Because the film of GaN nucleates onto the substrate at this temperature, there is no thermal stress present. FIG.  1 ( b ) shows the effects of the large temperature change as the sample cools to room temperature. In this figure, sapphire substrate  101  is now under compressive stress and is bent concave with respect to the deposited film. If the stresses are great enough, cracks  103  may form in the substrate. The epitaxial GaN  102  is under tensile stress, and is cracked, and may also peel away from or otherwise degrade the interface with substrate  101 . 
     FIG. 2 schematically represents a series of steps involved in the conventional method for making a thick layer on a thermally and/or lattice mismatched substrate. Step  201  calls for the provision of a prepared substrate. This prepared substrate may be, for example, plain sapphire, chemically cleaned prior to use. Step  202  is the setting of process parameters and growth conditions for the growth of the thick, flat, high quality layer. Typically these conditions are growth temperature, growth rate, flow rates for precursor compounds, and relative ratios of gas flows in the reactor. Step  203  calls for the deposition of the thick layer  102  onto the prepared substrate. The thickness of this layer is preferentially in the range of 10-400 microns. In step  204 , the sample is cooled down to room temperature where it is removed, intact, from the reactor. The wafer is bowed due to the residual stress caused by the thermal mismatch between the epitaxial layer and the substrate. This stress also leads to the formation of many cracks  103  in the thick layer and the substrate. 
     FIGS.  3 ( a )- 3 ( d ) schematically illustrate the prior art technique of using laser lift-off (LLO) to release a deposited GaN film from the sapphire substrate. In FIG.  3 ( a ), sapphire substrate  301  has had a film of GaN  302  deposited onto it, at the growth temperature, and has subsequently been cooled to room temperature. Film  302  is deposited in such a manner that cracks do not form during the cooling down stage. In FIG.  3 ( b ), laser beam  303  impinges upon the back side of the sapphire substrate. The laser is of an energy such that its photons are strongly absorbed by the GaN layer, while passing through the sapphire largely unabsorbed. Typically the energy range of such photons is above 3.45 eV, corresponding to a wavelength (in vacuum) of less than 359 nm but greater than 125 nm. The source of these photons is typically a pulsed ultraviolet laser, such as a tripled YAG or excimer laser; however the characteristics that are important for this process are not laser-specific. Any highly intense light source that can be focused down to a spot will suffice. Because the beam impinges from the sapphire side of the wafer, the GaN at the sapphire/GaN interface  304  absorbs the photons very strongly, resulting in localized heating. This localized heating is sufficient to disrupt the Ga—N bonds, breaking the strained but coherent interface between the lattice of the substrate  301  and the film  302 . Typically, the laser beam is swept across the backside of the wafer to gradually release the epitaxial film from the substrate. If the beam is sufficiently intense, all bonds will be broken, isolating the two lattices. A less intense beam may be used to partially disrupt the interface, breaking as few as 5% of the bonds, if such an effect is desired. In FIG.  3 ( c ) the process has continued. If the laser beam is too intense or not swept properly, localized hot areas can develop where the pressure from liberated nitrogen gas beneath the epitaxial film can build up and cause a rupture in the surface of the film,  305 . Additionally, residual thermal stresses in the as-yet unreleased areas can cause cracks  306  to develop in the film, especially as the stress profile changes during the debonding process. Both of these effects are undesirable and must be avoided, typically by careful modulation of the impinging laser power and scan rate, choosing a laser with a short pulse length, and/or using a beam homogenizer to form an illuminated spot with uniform intensity, among other techniques. Even with such precautions, cracking of the released epitaxial film may still occur, preventing the lift-off and removal of a whole layer to be used as a free-standing substrate. 
     FIG. 4 schematically represents a series of steps involved in the conventional method for laser lift-off of a GaN film from a sapphire substrate. In step  401  a prepared substrate is provided. This prepared substrate may be, for example, plain sapphire, chemically cleaned prior to use. In step  402 , the substrate has a layer of GaN  302  deposited onto it at an elevated growth temperature. In step  403  the substrate with GaN epitaxy is allowed to cool to the ambient temperature and is unloaded from the growth apparatus. In step  404  the grown wafer is placed into the LLO apparatus, which typically consists of a laser, laser power regulator, a wafer holder, and a beam steering mechanism to allow the beam  303  to impinge over the entire backside surface of the wafer. The beam then impinges over the backside of the wafer, gradually debonding the epitaxial film from the sapphire. In step  405 , the debonding is complete, the debonded epitaxial film is removed from the sapphire by heating the wafer above 30° C. (the melting point of gallium metal) and the layers are gently pulled apart. Often, the debonded layer is cleaned in an acid solution to dissolve any remaining gallium from its backside surface. Although free-standing epitaxial GaN films may be produced by LLO, the high stress between the sapphire substrate and the GaN layer often leads to cracking, fractures and other failures in the GaN layer. Thus the yield of usable free-standing epitaxial GaN films is often unacceptably low. 
     Because of the problems encountered with growing thick layers of GaN on sapphire, and of the problems encountered in attempting to remove GaN from the sapphire substrate using a conventional LLO technique, a need exists for a method for the laser lift-off and removal of GaN films from sapphire substrates for the creation of high quality free-standing substrates. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for the production of crack-free Group III-Nitride layers. The method proceeds by growing a crack-free first layer of Group III-Nitride on a starting substrate. A partial to complete loss of coherency is then achieved between a lattice of the first layer and a lattice of the starting substrate. A second layer is grown to form a composite layer that includes the first layer and the second layer such that the first layer is between the second layer and the substrate. 
     The present invention also provides a method for the production of arbitrarily thick, crack-free, freestanding layers of GaN or similar material for subsequent use as substrates. This method proceeds by growing a crack-free first layer of Group III-Nitride on a starting substrate. A partial to complete loss of coherency between a lattice of the first layer and a lattice of the starting substrate is then achieved. A second layer is grown to form a composite layer that includes the first layer and the second layer, and where the first layer is between the second layer and the substrate. The starting substrate is then completely separated from the composite layer to produce the freestanding substrate. In both methods, an intense light source may be used to partially disrupt the interface between this layer and the underlying starting substrate, making said interface partially incoherent. 
     In both methods a crack-free second layer may be grown on top of a crack-free first layer that has a partially incoherent interface with respect to the underlying starting substrate. 
     Furthermore, a crack-free second layer may be grown on top of a crack-free first layer (which has a partially incoherent interface with respect to the underlying starting substrate), in-situ, without necessitating a further cooling-down step. 
     These and other objects, advantages, and features of the invention will be set forth in part in the description which follows, and in part will become apparent to those having ordinary skill in the art upon examination of the following, or may be realized and attained as particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a  and  1   b  are cross sectional schematic views showing conventional (prior art) heteroepitaxial growth of thick GaN on sapphire. 
     FIG. 2 schematically shows the process steps for the conventional thick heteroepitaxial growth of GaN. 
     FIGS. 3 a-c  are cross-sectional schematic views showing a conventional (prior art) technique for the laser lift-off of a GaN film from a sapphire substrate. 
     FIG. 4 schematically shows the process steps for the laser lift-off of a GaN film from a sapphire substrate, using the prior art. 
     FIGS. 5 a-e  are cross-sectional schematic views illustrating the process for the production of a freestanding GaN substrate according to the first embodiment of the invention 
     FIG. 6 schematically shows the process steps for the fabrication of a freestanding GaN substrate according to the first embodiment of the invention 
     FIGS. 7 a-e  are cross-sectional schematic views illustrating the process for the production of a freestanding GaN substrate according to the second embodiment of the invention 
     FIG. 8 schematically shows the process steps for the fabrication of freestanding GaN substrate according to the second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For purposes of illustration, the present invention will be described primarily in relation to the fabrication of a thick freestanding layer of GaN grown on and subsequently removed from a sapphire substrate, using a suitable growth technique such as hydride vapor phase epitaxy (HVPE). It should be understood, however, that the present invention is applicable to the deposition of other materials including GaN, AlN, InN and/or their alloys, and/or onto substrates other than sapphire, and/or using other deposition techniques (such as metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), sputtering, evaporation, etc.). For the purpose of providing an example, the following embodiments are described with respect to fabrication of GaN substrates. The invention is not limited to just GaN substrates, in fact, it is intended to be utilized with other III-V materials. Those skilled in the art will recognize that the process is equally applicable to producing substrates of other group III nitrides and other III-V compounds. 
     First Embodiment of the Invention 
     FIGS. 5 a-e  schematically depict the method for fabricating a crack-free freestanding GaN layer according to the first embodiment of the invention. In FIG. 5 a ) a starting substrate  501  has a crack-free first layer of GaN  502  deposited by means of HVPE or other suitable method (such as MOCVD, MBE, etc.) The starting substrate  501  may be sapphire, but it may also be of any other material that is transparent to the region of the ultraviolet spectrum where the energy exceeds the bandgap of the desired III-V freestanding substrate material. The starting substrate may also be specially prepared prior to the deposition of the first layer. Such preparation may include special cleaning procedures or surface treatments, and/or the application of a low temperature buffer layer or layers, and/or the use of a patterned growth mask that allows growth only on selected areas of the starting substrate. 
     First layer  502  is deposited in such a way as to avoid crack formation. This may be accomplished by depositing a sufficiently thin layer (preferably between 0.1 and 10 μm) such that accumulated thermal stresses on cooling down will not exceed the physical limits of GaN. Alternatively, a thicker layer (up to 100 μm) may be deposited if the resulting film has lower potential for thermal stress accumulation, due to its higher defect density or increased surface roughness. It is primarily important, however, that first layer  502  cannot crack during deposition or during cooling down to a subsequent processing temperature. First layer  502  may also be composed of a plurality of layers of varying thickness and composition, as needed. 
     In FIG. 5 b ), the starting substrate  501  and first layer  502  have been removed from the growth reactor. First layer  502  is under some thermal stress due to the thermal mismatch between the materials, but cracks have not formed in the film. A laser beam  503  impinges upon the backside of starting substrate  501 . Starting substrate  501  does not absorb these photons, whereas the GaN at the interface  504  does. Typically the photons are generated by a pulsed ultraviolet laser, two examples of which are a XeCl excimer laser (wavelength 345 nm) or a tripled YAG laser (wavelength 355 nm). Both wavelengths are strongly absorbed by GaN, which has a room temperature absorption edge of 359 nm, corresponding to an electron bandgap of 3.45 eV. 
     To avoid thermal mismatch effects, it is sometimes helpful to heat the starting substrate  501  and first layer  502  to an elevated temperature prior to application of the laser beam. This elevated temperature may be as high as, or higher than, the actual growth temperature used during the deposition process (typically 1000°-1100° C.). The heating effectively reduces the magnitude of the thermal strain, reducing the risk of crack formation caused by non-uniform stress fields induced during the laser process. In such cases of heating the starting substrate  501  and first layer  502  above 600° C., it may be necessary to supply a non-inert nitrogen bearing atmosphere (such as ammonia, NH 3 ) to prevent the surface of first layer  502  from suffering the effects of thermal decomposition. 
     Photons  503  are strongly absorbed at the interface  504 , where they disrupt the Ga—N bonds, leading to a loss of coherency  505  between the lattice of the starting substrate  501  and the first layer  502 . Depending on factors such as laser pulse energy, peak power, pulse duration, spot size, beam scan rate, etc. the desired loss of coherency can be adjusted from partial (fewer than 5% of bonds broken) to complete (100% of bonds broken.) The loss of coherency between the lattice of the starting substrate  501  and the lattice of the first layer  502  relieves the stress between the starting substrate  501  and the first layer  502 . Although the coherency may be lost between the two lattices, the first layer  502  and starting substrate  501  are not yet physically separated. 
     In FIG. 5 c ), the starting substrate  501  with the first layer  502  is loaded again into the growth system for the deposition of the second layer  506 . The thickness of the second layer  506  layer may be set arbitrarily; for use as a substrate typically the thickness of layer  506  is between 50 and 500 μm. The deposition technique and conditions for depositing the second layer  506  may be the same as, or different from the conditions used for the deposition of layer  502 . Together, layers  502  and  506  merge to form a composite layer  507 . 
     In FIG. 5 d ) the starting substrate  501  and composite layer  507  is again cooled and unloaded from the growth system. Although the thickness of composite layer  507  is sufficiently large to induce catastrophic thermal stress cracks, the partially disrupted interface  505  effectively limits or eliminates the transmission of stress between the dissimilar materials. Thus, cracks do not form. Laser beam  503  is again applied to the backside of the starting substrate  501  to effect the complete disruption of the Ga—N bonds at the interface, allowing the release of the composite GaN layer  507  from the starting substrate. Alternatively, as shown in FIG. 5 e ), if the initial level of Ga—N bond disruption caused during the first laser step was sufficiently high (&gt;99%), the accumulated strain during the cooling process will concentrate the stresses on the remaining bonds, causing the composite layer  507  to spontaneously shear away from the starting substrate  501 . In such a case, no second laser step will be necessary as the GaN substrate will spring free of the starting substrate of its own accord. Also within the scope of this invention are alternative techniques for removing the substrate from the composite layer, such as lapping and polishing, etc. 
     FIG. 6 schematically shows the series of steps involved in the method for producing a thick freestanding layer from GaN on a sapphire substrate, according to the first embodiment of the invention. Step  601  calls for the provision of a prepared substrate. In (optional) step  602 , the prepared substrate has a mask pattern applied to its surface. The mask is intended to prevent growth except in the opened areas of the mask, in order to improve crystal quality or aid in the later separation. The mask may be of any material which inhibits growth on its surface and is compatible with the growth process; typically such masks are made of silicon oxide, silicon nitride, or silicon oxynitride. 
     The starting substrate is loaded into the growth system in step  603 . In step  604 , an optional low temperature buffer layer is set down, prior to step  605 , where the initial layer of GaN is deposited onto the sapphire. This layer may consist of a single layer deposited at one temperature, or of a plurality of layers of different compositions, deposited at different temperatures. In step  606 , the wafer, i.e. the starting substrate with the initial GaN layer, is cooled to ambient temperature and unloaded. In optional step  607 , the wafer may be patterned with a mask, similar to that which may have been applied in optional step  602 , or consisting of a different type of pattern, if desired. The purpose of the mask layer is to improve the crystal quality of material grown through and over it and/or to aid in the later removal of the film from the substrate. 
     In step  608 , the wafer is affixed to the LLO apparatus for the partial to complete disruption of the Ga—N bonds linking the starting substrate  501  to the first layer  502 . Typically, this can be done using different methods, as described herein. 
     In one alternative step  608 -A the laser pulse intensity, pulse width, and scan rate may be modulated such that each spot induces a uniform but incomplete loss of coherency between the lattice of the GaN layer and the lattice of the sapphire substrate. The entire wafer may be uniformly illuminated, and experiences a uniform loss of coherency of between 5% and greater than 99% between the two lattices. 
     Alternatively, in step  608 -B, the pulse intensity, width, spot size, etc. may be set to cause total disruption of the coherency at the interface of the lattice of the GaN and the lattice of the sapphire. Each illuminated spot has total loss of coherency associated with it; however the beam is swept in such a manner that the entire surface is not illuminated uniformly. Some areas of the substrate are not exposed, and have total coherency maintained, whereas others are made completely incoherent by exposure. By choosing parameters such as spot pitch distance, the ratio of area made incoherent to the total area of the wafer can be adjusted from 5% up to greater than 99%. 
     Alternatively, in step  608 -C, the laser spot may be rastered across the backside of the wafer in a pattern, such as a spiral, square, diagonal, etc. The effect is to disrupt the coherency between the lattice of the GaN and the sapphire in a systematic fashion, reducing or eliminating the thermal stresses in a geometrically controlled way to avoid cracking. 
     Regardless of which approach is followed for step  608 , the laser that is used is typically a tripled YAG or excimer laser, with a spot size of 50 μm to 500 μm, a pulse width of 3 to 50 nanoseconds, and a total fluence of between 300 mJ and 500 mJ per pulse. As the first layer is under thermal stress, it is often advantageous to use an auxiliary heating mechanism such as a hot plate to keep the wafer at an intermediate to high temperature during this process. For example, if the layer is grown at 1000° C., heating the wafer to 500° C. during the laser process reduces the thermal stress approximately by half, reducing the film&#39;s tendency to crack. 
     The wafer is loaded into the growth system in step  609 , and the growth of the second GaN layer  506  is done in step  610 . Thickness of this layer is preferably between 50 and 500 microns, more preferably 300 microns. The growth conditions for this layer may be the same as those used for the first layer, or they may differ in terms of growth rate, gas flows, partial pressures of precursor gases, composition of material deposited, temperature, etc. Layers  502  and  506  merge to form composite layer  507 . In step  611  the wafer is cooled once again and removed from the growth system. Although the total combined thickness of composite layer  507  on the starting wafer is considerable, the partially-to-completely isolated lattices of the starting substrate  501  and first layer  502  do not transmit stresses effectively, preventing crack formation. 
     In step  612  the composite layer  507  is removed from the starting substrate  501 . There are different methods by which this may be accomplished, as described herein. 
     In a first alternative step  612 -A, the wafer may be affixed again into the LLO apparatus. This time, the laser is used to completely disrupt 100% of the bonds at the interface, allowing for the straightforward physical removal of the composite layer by sliding it off the sapphire wafer. 
     Alternatively, in step  612 -B, the few remaining bonds that were left from the first laser step  608  may serve to concentrate the now-intensified thermal stress induced by the thicker second layer  506 . As the wafer is cooled to the ambient temperature, the concentrated stress exceeds the physical limits of the GaN at the interface, causing the composite layer to spontaneously shear away from the sapphire substrate. 
     Or, alternatively, in step  612 -C, the composite layer  507  is separated from the sapphire starting substrate  501  by methods such as lapping or polishing the backside of the sapphire away. As the coherency of the interface was already significantly reduced in the first laser step  608 , the wafer does not experience the severe bowing that otherwise would be evident on such a wafer with a thick layer deposited onto it. 
     Regardless of which alternative method is used in step  612 , the end result is a freestanding, crack-free GaN substrate  508  including the composite layer  507 . 
     Second Embodiment of the Invention 
     FIGS. 7 a-e  schematically depict a method for fabricating a crack-free freestanding GaN layer according to the second embodiment of the invention. In FIG. 7 a ) a starting substrate  701  is loaded into a growth system  702 . The growth system  702  may be, for instance, a HVPE system. Substrate  701  is placed onto a susceptor  703 , which holds the substrate in position during the growth process. Susceptor  703  may be fashioned with a slit or window  704  on its underside, which is designed to allow for the free transmission of a laser beam through the susceptor onto the underside of substrate  701 . 
     In the growth system  702 , substrate  701  has a crack-free first layer of GaN  705  deposited by means of HVPE or other suitable method (such as MOCVD, MBE, etc.) The starting substrate  701  may be sapphire, but it may also be of any other material that is transparent to the region of the ultraviolet spectrum where the energy exceeds the bandgap of the desired III-V freestanding substrate material. The starting substrate may also be specially prepared prior to the deposition of the first layer. Such preparation may include special cleaning procedures or surface treatments, and/or the application of a low temperature buffer layer or layers, and/or the use of a patterned growth mask that allows growth only on selected areas of the starting substrate. 
     First layer  705  is deposited in such a way as to avoid crack formation. This may be accomplished by depositing a sufficiently thin layer (preferably between 0.1 and 10 μm) such that accumulated thermal stresses on cooling down will not exceed the physical limits of GaN. Alternatively, a thicker layer (up to 100 μm) may be deposited if the resulting film has lower potential for thermal stress accumulation, due to its higher defect density or increased surface roughness. It is primarily important, however, that first layer  705  cannot crack during deposition or during cooling down to a subsequent processing temperature. First layer  705  may also be composed of a plurality of layers of varying thickness and composition, as needed. 
     In FIG. 7 b ), a laser beam  706  impinges in-situ upon the backside of starting wafer  701 , coming through the slit or window  704  in susceptor  703 . Starting wafer  701  does not absorb the laser light, whereas the GaN at the interface  707  between the starting substrate and the first layer, does. Laser light  706  is strongly absorbed at the interface  707 , where it disrupts the Ga—N bonds, leading to a loss of coherency  708  between the lattice of the starting substrate  701  and the first layer  705 . Depending on factors such as laser pulse energy, peak power, pulse duration, spot size, beam scan rate, etc. the desired loss of coherency can be adjusted from partial (fewer than 5% of bonds broken) to complete (100% of bonds broken.) 
     This laser process is performed in-situ in the growth reactor, the starting substrate  701  is not unloaded during the procedure. The wafer may be kept at or above the growth temperature (typically between 1000-1100° for GaN, lower for indium gallium nitride-based alloys) to eliminate thermal mismatch effects during the process. Alternatively, the wafer may be cooled to an intermediate temperature below the growth temperature for the procedure. In the case that the procedure occurs at a temperature above 600° C., it may be necessary to supply a non-inert nitrogen bearing atmosphere (such as ammonia NH 3 ) to prevent the surface of first layer  705  from thermally-induced decomposition. In other cases, however, it may be desirable to perform the laser procedure at lower temperatures. 
     In FIG. 7 c ), the second layer  709  is grown on top of the first layer  705 . The thickness of this layer may be set arbitrarily; for use as a substrate, the thickness of layer  709  is typically between 50 and 500 μm. The deposition conditions for depositing layer  709  may be the same as, or different from the conditions used for the deposition of layer  705 . Second layer  709  may also be grown substantially simultaneously with the laser process, without interruption between the steps. Together, layers  705  and  709  merge to form a composite layer  710 . 
     In FIG. 7 d ) the starting substrate  701  and composite layer  710  are subjected to an in-situ laser process. A laser beam  706 ′ is applied to the backside of the starting substrate  701  to effect the complete disruption of the Ga—N bonds at the interface, allowing the release of the composite layer  710  from the starting substrate. The laser beam  706 ′ in this step may be different from the laser beam  706  in the earlier laser step shown in FIG. 7 b.    
     If the initial level of Ga— (or In— or Al—)N bond disruption caused during the first laser step was sufficiently high (&gt;99%), this step will be unnecessary, as the accumulated strain during the cooling process will concentrate the stresses on the remaining bonds, causing the composite layer to spontaneously shear away from the starting substrate, Alternatively, it is possible to perform the second laser process ex-situ, out of the reactor, if it is so desired, or to use an alternative method to separate the substrate from the composite layer, such as a lapping and polishing technique. Regardless of the method, the next result, as shown in FIG. 7 e ) is the complete crack-free separation of the composite layer  710  from the starting substrate  701  to form a freestanding, crack-free GaN substrate  711 . 
     FIG. 8 schematically shows the series of steps involved in the method for producing a thick freestanding layer of GaN on a sapphire substrate, according to the second embodiment of the invention. Step  801  calls for the provision of a prepared substrate. In (optional) step  802 , the prepared substrate has a mask pattern applied to its surface. The mask is intended to prevent growth except in the opened areas of the mask, in order to improve crystal quality or aid in the later separation. The mask may be of any material which inhibits growth on its surface and is compatible with the growth process; typically such masks are made of silicon oxide, silicon nitride, or silicon oxynitride. 
     The substrate is loaded into the growth system in step  803 . In step  804 , an optional low temperature buffer layer is set down, prior to step  805 , where the initial layer of GaN is deposited onto the sapphire. This layer may consist of a single layer deposited at one temperature, or of a plurality of layers of different compositions, deposited at different temperatures. 
     In step  806  the backside of the substrate is illuminated with the laser beam  706 , in situ, through the slit or window  704  in susceptor  703  for the partial to completed disruption of the Ga—N bonds linking starting substrate  701  to the first layer  705 . Typically, this can be done using different methods, as described herein. 
     In one alternative step  806 -A the laser pulse intensity, pulse width, and/or scan rate are modulated such that each spot induces a uniform but incomplete loss of coherency between the lattice of the GaN layer and the lattice of the sapphire substrate. The entire wafer is uniformly illuminated, and experiences a uniform loss of coherency of between 5% and greater than 99% between the two lattices. 
     In method  806 -B, the pulse intensity, width, spot size, etc. are set to cause total disruption of the coherency at the interface of the lattice of the GaN and the lattice of the sapphire starting substrate  701 . Each illuminated spot has total loss of coherency associated with it; however the beam is swept in such a manner that the entire surface is not illuminated uniformly. Some areas of the substrate are not exposed, and have total coherency maintained, whereas others are made completely incoherent by exposure. By choosing parameters such as spot pitch distance, the ratio of area made incoherent to the total area of the wafer can be adjusted from 5% up to greater than 99%. 
     Alternatively, in step  806 -C, the laser spot may be rastered across the backside of the starting substrate in a pattern, such as a spiral, square, diagonal, etc. The effect is to disrupt the coherency between the lattice of the GaN and the sapphire starting substrate in a systematic fashion, reducing or eliminating the thermal stresses in a geometrically controlled way to avoid cracking. 
     Regardless of which method is used in step  806 , laser beam  706  is typically from a tripled YAG or excimer laser, with a spot size of 50 μm to 500 μm, and a total fluence of between 300 mJ and 500 mJ per pulse. As this process is performed in-situ, it is possible to keep the wafer at an elevated temperature (up to or above the growth temperature, typically 1000-1100° C.) to eliminate the effects of thermal mismatch. If this is done at a temperature above 600° C., it will be necessary to have a non-inert nitrogen-bearing atmosphere (such as ammonia, NH 3 ) present to prevent the surface of the first layer from suffering the effects of thermal decomposition. 
     In step  807 , the second layer  709  is deposited on top of the first layer  705 . The thickness of the second layer  709  is preferably between 50 and 500 microns, more preferably 300 microns. The growth conditions for this layer may be the same as those used for the first layer  705 , or they may differ in terms of growth rate, gas flows, partial pressures of precursor gases, temperature, etc. It is also within the scope of this invention to have step  807  occur concurrently with step  806 , i.e. the interface disruption may occur at the same time as layer  709  is being deposited. Together, layers  705  and  709  merge to form a composite layer  710 . 
     In (optional) step  808 , the laser is again applied, in-situ, to the backside of the starting substrate  701 . This time the laser is used to completely disrupt the bonds, allowing the composite layer to be removed from the starting substrate. Under certain circumstances, described herein, this step may be omitted in lieu of other steps  810 -B,  810 -C, or  810 -D, below. 
     In step  809  the wafer is cooled once again and removed from the growth system. Although the total combined thickness of GaN on the starting wafer is considerable, the partially-to-completely isolated lattices of the starting substrate  701  and first layer  705  do not transmit stresses effectively, preventing crack formation. 
     In step  810  the composite layer  710  is removed from the starting substrate  701 . There are different methods by which this may be accomplished, as described herein. 
     In method  810 -A, which assumes that optional laser step  808  was done, the composite layer can be physically lifted or dragged free of the sapphire substrate. 
     In one alternative step  810 -B, the cooled wafer may be affixed into an ex-situ LLO apparatus. This time, the laser is used to completely disrupt 100% of the bonds at the interface, allowing for the straightforward physical removal of the composite layer by sliding it off the sapphire wafer. 
     Alternatively, in step  810 -C, the few remaining bonds that were left from the first laser step  806  may serve to concentrate the now-intensified thermal stress induced by the thicker composite layer  710 . As the starting substrate  701  and composite layer  710  cool to ambient temperature, the concentrated stress exceeds the physical limits of the GaN at the interface, causing the composite layer to spontaneously shear away from the sapphire substrate. 
     In method  810 -D, the composite layer  710  is separated from the sapphire by methods such as lapping or polishing the backside of the sapphire away. As the coherency of the interface  707  was already significantly reduced in the first laser step  806 , the wafer does not experience the severe bowing that otherwise would be evident on such a wafer with a thick layer deposited onto it. 
     Regardless of which method is used in step  810 , the end result is a freestanding, crack-free GaN substrate  711 . 
     The foregoing embodiments are set forth for the purpose of example, and should not be construed as limiting the present invention. The present teaching may be applied to other types of apparatuses and methods. The description of the present invention is intended to be illustrative and not limiting the scope of the appended claims. Alternatives, modifications, and variations on this method will be apparent to those skilled in the art.