FIELD OF THE INVENTION 
     This invention relates to the heteroepitaxial deposition of high quality, epitaxial, crack-free, low defect density films onto a thermally and/or lattice mismatched substrate. More specifically, it relates to the heteroepitaxial deposition of thick layers of compound semiconductor materials for subsequent use as substrates for further deposition. More specifically, it relates to the deposition of thick films of Gallium Nitride (GaN) and other Group III Nitrides (AlN, InN) and their alloys, for use as substrate material for further growth of device structures. 
     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 or even thick layers of GaN. 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 he 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, and gallium arsenide. All heteroepitaxial substrates present challenges to the high-quality deposition of GaN because 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. 
     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 with GaN. For reasons unrelated to the scope of this invention, it 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. 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 several microns. Defect densities, however, predominantly in the form of dislocations, remain high (˜10 10 cm −2 ) resulting in diminished device quality. 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. 
     A simple solution to this problem is to increase the GaN buffer layer thickness in the hopes that the dislocation density will decrease with increasing distance from the substrate interface. Furthermore, a thick GaN buffer layer offers improved electrical and thermal properties, which aids in the design and processing of devices. These very thick GaN buffer layers have been called ‘pseudo-substrates’. The difficulty in growing a sufficiently thick GaN buffer layer on sapphire, to act as a pseudo-substrate for subsequent device-quality layer growth, arises from the effects of thermal mismatch. Typically the GaN is deposited onto the sapphire at a temperature of between 1000-1100° C.; as the sample cools to room temperature, the difference in thermal expansion (also 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 it cools, 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 results. 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. 
     Referring to the drawings, FIGS.  1 ( a )-( 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 FIG., sapphire substrate  101  is now under tensile stress and is bent concave with respect to the deposited film. If the stresses are great enough, cracks  103  may be present in the substrate. The epitaxial GaN  102  is under compressive 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. Step  203  calls for the deposition of the thick layer 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 in the thick layer and the substrate. 
     Therefore, a need exists in the art for arbitrarily thick, high quality, epitaxial, crack-free, low defect density films deposited onto thermally and/or lattice mismatched substrates and a method for depositing them. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method to allow the growth of an arbitrarily thick, crack-free layer of GaN or related III-V compound or alloy onto a heteroepitaxial substrate. Additionally, it is another object of this invention to provide a method for the growth of such a film with a top surface having significantly fewer defects than are present at the interface of the GaN film and the heteroepitaxial substrate. Additionally, it is another object of this invention to provide a method for the production of a freestanding substrate of GaN, AlN, InN or their alloys. 
     It is another object of the present invention to provide a method for the production of a defect-rich first heteroepitaxial layer, which is essentially stress-free, and a subsequent second homoepitaxial layer, atop the first layer, that is of a very high crystal quality. 
     It is another object of the present invention to provide a method for the production of a defect-rich first heteroepitaxial layer, which is essentially stress-free, and the formation, in-situ, of a subsequent second homoepitaxial layer, atop the first layer, that is of a very high crystal quality. 
     It is another object of the present invention to describe a method for the production of 1) a highly defect-rich, stress-relieved first layer, followed by 2) the deposition of a second layer atop the first layer; where 2) acts to cover, isolate, and eliminate the effects of the defects in layer  1 ). 
     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 )- 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 conventional thick heteroepitaxial growth of thick GaN. 
     FIGS.  3 ( a )- 3 ( c ) are cross sectional schematic views showing heteroepitaxial growth of thick and/or freestanding GaN according to a first embodiment of the invention. 
     FIG. 4 schematically shows the process steps for the heteroepitaxial growth of thick and/or freestanding GaN of the first embodiment of the invention. 
     FIGS.  5 ( a )- 5 ( d ) are cross sectional schematic views showing the heteroepitaxial growth of thick and/or freestanding GaN according to a second embodiment of the invention. 
     FIG. 6 schematically shows the process steps for the heteroepitaxial growth of thick and/or freestanding GaN in the second embodiment of the invention. 
     FIGS.  7 ( a )- 7 ( d ) are cross sectional schematic views showing the heteroepitaxial growth of thick and/or freestanding GaN according to a third embodiment of the invention. 
     FIG. 8 schematically shows the process steps for the heteroepitaxial growth of thick and/or freestanding GaN in the third embodiment of the invention. 
     FIGS.  9 ( a )- 9 ( c ) are cross sectional schematic views showing the heteroepitaxial growth of thick and/or freestanding GaN according to a fourth embodiment of the invention. 
     FIG. 10 schematically shows the process steps for the heteroepitaxial growth of thick and/or freestanding GaN in the fourth embodiment of the invention. 
     FIGS.  11 ( a )- 11 ( d ) are cross sectional schematic views showing the heteroepitaxial growth of thick and/or freestanding GaN according to a fifth embodiment of the invention. 
     FIG. 12 schematically shows the process steps for the heteroepitaxial growth of thick and/or freestanding GaN in the fifth embodiment of the invention. 
     FIGS.  13 ( a )- 13 ( d ) are cross sectional schematic views showing the heteroepitaxial growth of thick and/or freestanding GaN according to a sixth embodiment of the invention. 
     FIG. 14 schematically shows the process steps for the heteroepitaxial growth of thick and/or freestanding GaN in the sixth 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 layer of GaN on 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 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.). Although, for the purpose of example, the following embodiments are described with respect to GaN substrates, the invention is not limited to just GaN substrates. 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 
     FIGS.  3 ( a )- 3 ( c ) schematically depict the growth of a thick, crack-free layer of a group III nitride, e.g., GaN onto sapphire, according to the first embodiment of this invention. In FIG.  3 ( a ), a first, thick, defect-rich or very rough GaN layer  304  is grown by VPE or other related technique (e.g., HVPE, MOCVD, etc.) onto a substrate  301 . The substrate  301  may be made of sapphire, a III-V substrate, gallium arsenide, indium phosphide, gallium phosphide, zinc oxide, magnesium oxide, silicon, a silicon oxide, silicon carbide, lithium aluminate, lithium gallate, and/or lithium aluminum gallate. The GaN layer  304  may be grown, for example, by adjusting the growth parameters or material composition during growth to form crystalline facets  302 . Subsequently, a second high quality or very flat GaN layer  305  is grown onto the first GaN layer  304  by HVPE as shown in FIG.  3 ( b ). After the second growth, the wafer is cooled down to room temperature. Thermal mismatch-induced cracks do not form while cooling because the first GaN layer  304  alleviates the residual stress between the second epitaxial GaN layer  305  and the substrate  301 . The first layer  304  and second layer  305  form a heteroepitaxial layer  320 . Layer  320  is typically greater than about 35 μm thick. The substrate  301  may be removed by polishing, or some other technique, as shown in FIG.  3 ( c ). 
     FIG. 4 schematically shows the series of steps involved in a method for making a thick layer on a thermally and/or lattice mismatched substrate, according to the first embodiment of this invention. Step  401  calls for the provision of a prepared substrate. In Step  402 , the process parameters for the growth of the first layer are set are set. For the purpose of ease of reference, all process parameters and growth conditions are placed into a set, denoted as [C 1 ]. Process parameters [C 1 ] include all relevant parameters regarding the growth conditions for the deposition of the first layer, i.e. layer  302 . [C 1 ] includes process conditions such as reactor pressure, reactor geometry, reactor temperature(s), reactor temperature gradient(s), substrate temperature, gas flow rates, etc. used for the deposition of this thick layer. These differences may include, but are not limited to, changes in substrate or reactor temperatures, pressures, gas flow rates or ratios, and reactor geometry, for example. Step  403  calls for the growth of the defect-rich or rough first layer onto the prepared substrate. The thickness of this layer is preferentially in the range of 10-300 microns. The conditions [C 1 ] are such that the material deposited will have a high defect density or surface roughness, yet will remain essentially epitaxial in nature. Exemplary parameters [C 1 ] are typically as follows. The growth rate of layer  302  is typically between about 50 and 200 microns/hr, preferably about 100 microns/hr. The temperature of substrate  301  is typically between about 1000° C. and about 1070° C. preferably about 1030° C. A ratio of the flow of group V material to group III material, denoted herein as V/III ratio is typically between about 50 and about 250, preferably about 125. The reactor pressure is typically between about 0.9 atmospheres and 1.5 atmospheres, preferably slightly above 1 atmosphere (1 atmosphere is approximately equal to 14.7 lbs/in 2  at sea level). 
     Step  404  calls for the changing of growth conditions to [C 2 ], which may he different from [C 1 ] in terms of temperature, pressure, reactor geometry, flow rates, etc. Depending on the materials and type of deposition system in use, the parameters in set [C 1 ] used in the same reactor for the deposition of a defect-rich or rough layer, e.g. layer  304 , will differ from those used for the deposition of a second low defect density or flat layer, [C 2 ]. Step  405  calls for the deposition of the second, high quality, low defect or flat layer, e.g. layer  305  atop the first layer  302 , using the growth conditions [C 2 ]. Exemplary parameters [C 2 ] are typically as follows. A growth rate of layer  305  is typically between about 5 and 50 microns/hr, preferably about 25 microns/hr. The substrate temperature is typically between about 1000° C. and about 1070° C. preferably about 1030° C. The reactor pressure is typically between about 0.9 atmospheres and 1.5 atmospheres, preferably slightly above 1 atmosphere. The V/III ratio is typically between about 50 and about 250, preferably about 125. 
     This second layer covers the first, burying and isolating the defects, which do not propagate upward during growth. Alternatively, the second layer may be grown under conditions favoring lateral epitaxial growth, resulting in growth that smoothes and fills in surface irregularities in the first layer. The thickness of this second layer is preferentially in the range of 10-300 microns. In step  406 , the sample is cooled down to room temperature without the formation of cracks in the thick epitaxial layer or substrate; the residual stress is minimized due to the effect of the first layer. The epitaxial quality of the top surface of the film is very high, having been grown using high quality—high stress growth conditions. The deposited film has substantially reduced thermal stress, however, as it is relieved by the defects and/or roughness of the first layer, below. The result is a thick layer of low stress, crack-free material on a thermally and/or lattice mismatched substrate, such as GaN on sapphire. In some cases, as shown in step  407  in FIG. 4, the sapphire substrate is removed after cooling down, e.g. leaving heteroepitaxial layer  320  as a freestanding GaN substrate. 
     It is possible to vary the process described in the above embodiment without departing from the scope of the invention In an alternative process the growth rate may he kept constant and the temperature of substrate  301  may be raised from a rough growth regime to a smooth growth regime. Another alternative is a flux between the two “extremes”, i.e. reducing growth rate and increasing temperature slightly. 
     In the following embodiments of the present invention, the growth of a defect-rich or rough thick layer and subsequent growth of a high quality or smooth layer are common steps for the production of crack-free, high quality thick epitaxial layers. The following embodiments highlight additional process steps or variations on methods used to achieve this same goal. 
     SECOND EMBODIMENT 
     FIGS.  5 ( a )- 5 ( d ) schematically show the growth of a thick, crack-free layer of GaN onto sapphire according to the second embodiment of this invention. In FIG.  5 ( a ), a very thin buffer layer  506 , e.g. of GaN, may be grown on a substrate  501  by a vapor phase epitaxy technique such as metal organic chemical vapor deposition (MOCVD) or hydride vapor phase epitaxy (HVPE). The substrate may have features in common with substrate  301  of FIG.  3  and may be made of the same types of materials. Typically, the buffer thickness is on the order of 50 nm and is grown at a lower temperature than the subsequent HVPE growths. The buffer may also be of AlN, InN or an alloy of AlN, GaN and/or InN, ZnO, or MgO. Multiple layers of differing compositions may also be used instead of a single buffer layer. In such cases the buffer layer may be thicker than 50 nm, and may even consist of multiple MOCVD layers grown at different temperatures. For example, a low temperature buffer consisting of one or more layers, followed by the higher temperature growth of one or more additional layers, and possibly an additional low temperature layer, are within the scope of the present invention. The buffer may be used to help reduce interfacial stress caused by lattice and/or thermal mismatch, but it may also be used to assist in the nucleation and growth of subsequent GaN layers. Following the buffer growth is the deposition of a thick, defect-rich or very rough HVPE second layer  507 , as shown in FIG.  5 ( b ). Subsequently, a high quality or very flat HVPE GaN third layer  508  is grown onto the HVPE GaN second layer  507 , as shown in FIG.  5 ( c ). After. the third growth, the wafer is cooled down to room temperature. Thermal mismatch-induced cracks do not form while cooling because the second GaN layer  507  alleviates the residual stress between the third epitaxial GaN layer  508  and the sapphire substrate  501 . The sapphire substrate  1  may be removed by polishing, or some other technique, as shown in FIG.  5 ( d ). Layers  506 ,  507 , and  508  then form a free standing substrate  520 . 
     FIG. 6 schematically shows the series of steps involved in a method for making a thick layer on a thermally and/or lattice mismatched substrate, according to the second embodiment of this invention Step  601  calls for the provision of a prepared substrate. In Step  602 , the process parameters for the growth of the buffer layer (or layers) are set, hereby denoted as [C 1 ′]. As mentioned previously, [C 1 ′] contains all relevant parameters regarding the growth conditions for the deposition of the buffer layer (or layers). In the case of a buffer consisting of multiple layers, the process conditions set [C 1 ′] may consist of multiple unique sub-sets, one for each individual layer of the buffer structure. Exemplary parameters [C 1 ′] are typically as follows. Buffer layer  506  is typically to a thickness of between about 5 and 100 nm, preferably about 30 nm. A growth rate of buffer layer  506  is typically between about 0.1 and 1.0 microns/hr, preferably about 0.4 microns/hr. The substrate temperature is typically between about 400° C. and about 700° C. preferably about 500° C. The reactor pressure is typically between about 0.9 atmospheres and 1.5 atmospheres, preferably slightly above 1 atmosphere. The V/III is typically between about 4000 and about 7000, preferably about 5500. 
     Step  603  calls for the growth of the buffer layer (or layers) onto the prepared substrate, with a total thickness on the order of 50 nm. In Step  604  growth conditions are changed to [C 2 ′]. This may involve the use of a different growth technique than that used in Step  603 , and possibly even the use of a different growth system, which may require an additional set of steps for unloading and loading of the sample. Step  605  calls for the growth of a defect-rich or rough second layer onto the buffer layer. The thickness of this layer is preferentially in the range of 10-300 microns. The conditions [C 2 ′] are such that the material deposited will have a high defect density or surface roughness, yet will remain essentially epitaxial in nature. The process conditions [C 2 ′] are typically in the same ranges as those set forth in set [C 1 ] described above with respect to FIG.  4 . In Step  606  the growth conditions are set to [C 3 ], which may be different from [C 2 ′] in terms of temperature, pressure, reactor geometry, flow rates, etc. Step  607  calls for the deposition of the third, high quality, low defect or flat layer, e.g. layer  508  atop the second layer,  506  using the growth conditions [C 3 ]. The process conditions [C 3 ] are typically in the same ranges as those set forth in Set [C 2 ] described above with respect to FIG.  4 . This third layer covers the second, burying and isolating the defects, which do not propagate upward during growth. Alternatively, the third layer may be grown under conditions favoring lateral epitaxial growth, resulting in growth that smoothes and fills in surface irregularities in the second layer. The thickness of this third layer is preferentially in the range of 10-300 microns. 
     In step  608 , the sample is cooled down to room temperature without the formation of cracks in the thick epitaxial layer or substrate; the residual stress is minimized due to the effect of the second layer. The epitaxial quality of the top surface of the film is very high, having been grown using high quality high stress growth conditions. The deposited film has substantially reduced thermal stress, however, as it is relieved by the defects and/or roughness of the second layer, below. The result is a thick layer of low stress, crack-free material on a thermally and/or lattice mismatched substrate, such as GaN on sapphire. In some cases, as shown in step  609  in FIG. 6, the substrate  501  is removed after cooling down, to obtain a freestanding GaN substrate, e.g. substrate  520 . 
     THIRD EMBODIMENT 
     FIGS.  7 ( a )- 7 ( d ) schematically shows the growth of a thick, crack-free layer of GaN onto sapphire, according to the third embodiment of this invention. In FIG.  7 ( a ), a buffer layer structure  702  is formed on a substrate  701  of the type described above with respect to FIG.  3 . The buffer layer structure  702  generally comprises a lower GaN layer  709 , a patterned mask layer  710 , e.g., of silicon dioxide (SiO 2 ), and an epitaxial lateral overgrowth (ELOG) layer  711 . The mask material inhibits GaN growth; growth occurs only in unmasked areas. This serves to enhance lateral overgrowth of the ELOG layer  711 , as well as to partially decouple the substrate from this and subsequent epitaxial layers. Such decoupling reduces the residual thermal and/or lattice mismatch stresses in the epitaxial layers, and is achieved by reducing the effective contact area between substrate and epitaxy. 
     Typically the two GaN layers are grown by a vapor phase epitaxy technique, such as MOCVD or HVPE, and the total thickness of the buffer structure is on the order of one micron, although other growth processes and buffer structure thicknesses are within the scope of the present invention. The individual layers of this structure may also be of different compositions: for example layers  709  and  711  could each consist of GaN, AlN, InN, or an alloy of these constituents. The patterned mask layer  710  could consist of SiO 2 , silicon nitride (Si 3 N 4 ), polysilicon, a refractory metal, or any combination of these components. These are given merely as examples, and are not meant to limit the scope of this embodiment of the present invention; other suitable materials could be used to similar effect. Additionally, each individual layer may consist of a set of sub-layers. Layers  709 ,  710 , and/or  711  may each be composed of two or more layers of distinct composition, to further reduce the residual stress levels or to enhance lateral overgrowth. Typical thickness for the lower layer  709  is on the order of 1 micron; thickness of the patterned mask layer  710  is on the order of 500 nm; thickness of the ELOG layer  711  is on the order of 1-4 microns. Other layer thicknesses that serve to reduce the residual stress or improve the nucleation or lateral overgrowth of layer  711  are also within the scope of this embodiment of the present invention. 
     Following the growth of buffer layer structure  702  growth is the deposition of a thick, defect-rich or very rough HVPE layer  712 , as shown in FIG.  7 ( b ). Subsequently, a high quality or very flat HVPE GaN layer  713  is grown onto the third HVPE GaN layer  712 , as shown in FIG.  7 ( c ). After the growth of layer  713 , the wafer is cooled down to room temperature. Thermal mismatch-induced cracks do not form while cooling because of two complementary effects: the third GaN layer  712 , reduces the residual stress between the fourth epitaxial GaN layer  713  and the sapphire substrate  701 , as described in the first embodiment of the present invention. Additionally, the patterned mask layer  710  and lateral overgrowth layer  711  serve to mechanically decouple thick HVPE layers  712  and  713  from the sapphire substrate  701 . The substrate  701  and part of the buffer layer structure  702  may be removed by polishing, or some other technique, as shown in FIG.  7 ( d ). Typically the patterned layer and part of the lateral overgrowth layer  711  are removed. The remaining part of the ELOG layer  711  and layer  712 ,  713  form a free standing group III nitride substrate  720 . 
     FIG. 8 schematically shows the series of steps involved in a method for making a thick layer on a thermally and/or lattice mismatched substrate, according to the third embodiment of this invention. Step  801  calls for the provision of a prepared substrate. In Step  802 , the process parameters for the growth of the first buffer layer (or layers) are set, hereby denoted as [C 1 ″]. Process parameter set [C 1 ″] contains all relevant parameters regarding the growth conditions for the deposition of the first buffer layer (or layers), e.g. layer  709 . The process parameters [C 1 ″] are typically in the same ranges as those set forth in [C 1 ′] as described above with respect to FIG.  6 . Step  803  calls for the growth of the first buffer layer (or layers) onto the prepared substrate, with a total thickness on the order of 1 micron. In Step  804 , the sample is cooled and removed from the reactor. In Step  805 , a suitable mask material is deposited onto the sample, after which it is patterned using appropriate methods such as lithography and etching. In step  806  growth conditions are changed to [C 2 ″] for the growth of an ELOG layer (or layers), e.g. ELOG layer  711 . The process of reloading the sample into the growth reactor may also be included in parameter set [C 2 ″]. In Step  807  the ELOG layer (or layers) are grown for a typical thickness of 1-10 microns. The parameters [C 2 ″] are typically as follows. If the ELOG layer  711  is grown by HVPE, high growth rate, [C 2 ″] may comprise rough layer parameters such as parameters [C 1 ] described above with respect to FIG.  4 . If the ELOG layer  711  is grown by an MOCVD process parameter set [C 2 ″] may be as follows. The substrate temperature may be between about 970° C. and about 1070° C. The reactor pressure is typically less than 1 atmosphere, preferably about 0.1 atmosphere. The growth rate of ELOG layer  711  may be between about 0.5 microns/hr and about 5 microns/hr, preferably about 2 microns/hr. The V/III ratio is typically between about 500 and about 5000, preferably about 1500. The ELOG layer is grown through and over the masking layer below it, reducing the effective contact area between the epitaxial overgrowth and the sapphire substrate. The ELOG layer  711  reaches top of patterned layer  710  and then expands laterally. Lateral growth normally stops propagation of vertical defects from sapphire GaN interface. In the present invention the second buffer layer also promotes cracking at the interface between substrate  701  and buffer layer structure  702 . 
     In Step  808  the growth conditions are changed to [C 3 ′], which may involve the use of a different growth technique than that used in Step  806 , possibly even the use of a different growth system, which may require an additional set of steps for unloading and loading of the sample. Step  809  calls for the growth of a defect-rich or rough third layer, e.g., layer  712  onto the buffer layer. The thickness of this layer is preferentially in the range of 10-300 microns. The parameters in [C 3 ′] are typically in the same ranges as those described with respect to [C 1 ] of FIG.  4 . The conditions [C 3 ′] are such that the material deposited will have a high defect density or surface roughness, yet will remain essentially epitaxial in nature. 
     In Step  810  the growth conditions are set to [C 4 ], which may be different from [C 3 ′] in terms of temperature, pressure, reactor geometry, flow rates, etc. Step  811  calls for the deposition of the fourth, high quality, low defect or flat layer, e.g., layer  713 , atop the third layer, using the growth conditions [C 4 ]. The parameters in [C 4 ] are typically in the same ranges as those described with respect to [C 2 ] of FIG.  4 . This fourth layer covers the third, burying and isolating the defects, which do not propagate upward during growth. Alternatively, the fourth layer may be grown under conditions favoring lateral epitaxial growth, resulting in growth that smoothes and fills in surface irregularities in the third layer. The thickness of this fourth layer is preferentially in the range of 10-300 microns. In step  812 , the sample is cooled down to room temperature, without the formation of cracks in the thick epitaxial layer or substrate; the residual stress is minimized due both to the effects of the third layer, as described in the first embodiment of the present invention, as well as the effects of the masking and overgrowth layers, which reduce the contact between the substrate and the thick epitaxy. The epitaxial quality of the top surface of the film is very high, having been grown using high quality—high stress growth conditions, safely isolated from the effects of the sapphire substrate. The result is a thick layer of low stress, crack-free material on a thermally and/or lattice mismatched substrate, such as GaN on sapphire. In some cases, as shown in step  813  in FIG. 8, the sapphire substrate is removed after cooling down, to obtain a freestanding GaN substrate, e.g. freestanding substrate 
     FOURTH EMBODIMENT 
     FIGS.  9 ( a )- 9 ( c ) schematically show the growth of a thick, crack-free freestanding GaN substrate, according to the fourth embodiment of this invention. In FIG.  9 ( a ), a first, thick, defect-rich or very rough GaN layer  904  is grown by HVPE onto a sapphire substrate  901 . After this layer is grown, the sample is cooled to room temperature, removed from the reactor, and sapphire substrate  901  is removed by polishing or some other technique, as shown in FIG.  9 ( b ). Thermal mismatch-induced cracks do not form while cooling because the defect-rich or very rough GaN layer  904  is able to accommodate the stress without cracking. Subsequently, the now freestanding GaN layer is reloaded into the reactor and a second high quality or very flat GaN layer  905  is grown onto the first GaN layer  904  by HVPE as shown in FIG.  9 ( c ). This second layer  905  is grown homoepitaxially; freed of the thermal and lattice mismatch constraints imposed by the sapphire substrate  901 , the film does not crack or experience thermal stress upon cooling down. The result is a high quality freestanding GaN substrate  920 . 
     FIG. 10 schematically shows the series of steps involved in a method for making a thick a thick, crack-free freestanding GaN substrate, according to the fourth embodiment of this invention. Step  1001  calls for the provision of a prepared substrate. In Step  1002 , the process parameters for the growth of the first III-V layer, e.g. layer  904 , are set, hereby denoted as [C 1 ′″]. As mentioned previously, [C 1 ′″] contains all relevant parameters regarding the growth conditions for the deposition of the first layer. Step  1003  calls for the growth of the defect-rich or rough first layer onto the prepared substrate. The thickness of this layer is preferentially in the range of 20-300 microns. The conditions [C 1 ′″] are such that the material deposited will have a high defect density or surface roughness, yet will remain essentially epitaxial in nature. The parameters in [C 1 ′″] are typically in the same ranges as those described with respect to [C 1 ] of FIG.  4 . In step  1004 , the sample is cooled to room temperature without formation of cracks; the residual stress is minimized due to the effect of the defect-rich or rough first layer. Step  1005  calls for the removal of the sapphire substrate to obtain a rough or defect-rich freestanding GaN layer; while the freestanding layer is a single crystal, its defect density and/or roughness make it unsuitable to use as a GaN substrate for device applications. In Step  1006  the growth conditions are changed to [C 2 ′″], which includes a set of steps for reloading the sample into the reactor. Step  1007  calls for the deposition of the second, high quality, low defect or flat layer, e.g. layer  905  atop the first layer, using the growth conditions [C 2 ′″]. The parameters in [C 2 ′″] are typically in the same ranges as those described with respect to [C 2 ] of FIG.  4 . This second layer covers the first, burying and isolating the defects, which do not propagate upward during growth. Alternatively, the second layer may be grown under conditions favoring lateral epitaxial growth, resulting in growth that smoothes and fills in surface irregularities in the first layer. The thickness of this second layer is preferentially in the range of 10-300 microns. In step  1008 , the sample is cooled down to room temperature without the formation of cracks; there is no thermal mismatch without the presence of the sapphire substrate, which was removed in Step  1005  to form freestanding substrate  920   
     FIGS.  11 ( a )- 1 ( d ) schematically shows the growth of a thick, crack-free freestanding GaN substrate, according to the fifth embodiment of this invention. In FIG.  11 ( a ), a thin buffer layer  1106  of GaN, may be grown on a substrate  1101  by a vapor phase epitaxy technique such as MOCVD or HVPE. Typically, the buffer thickness is on the order of 50 nm and is grown at a lower temperature than the subsequent HVPE growths. The buffer may also be of AlN, InN or an alloy of AlN, GaN and/or InN, ZnO or MgO. Multiple layers of differing compositions may also be used instead of a single buffer layer. In such cases the buffer layer may be thicker than 50 nm, and may even consist of multiple MOCVD layers grown at different temperatures. For example, a low temperature buffer consisting of one or more layers, followed by the higher temperature growth of one or more additional layers, and even final low temperature layer, are within the scope of the present invention. The buffer may be used to help reduce interfacial stress caused by lattice and/or thermal mismatch, but it may also be used to assist in the nucleation and growth of subsequent GaN layers. Following the buffer growth is the deposition of a thick, defect-rich or very rough HVPE second layer  1107 , as shown in FIG.  11 ( b ). After this layer is grown, the sample is cooled to room temperature, removed from the reactor, and sapphire substrate  1101  is removed by polishing or some other technique, as shown in FIG.  11 ( c ). Thermal mismatch-induced cracks do not form while cooling because the defect-rich or very rough GaN second layer  1107  is able to accommodate the stress without cracking. Subsequently, the now freestanding GaN layer is reloaded into the reactor and a third high quality or very flat GaN layer  1108  is grown onto the second GaN layer  1107  by HVPE as shown in FIG.  11 ( d ). This third layer  1108  is grown homoepitaxially; freed of the thermal and lattice mismatch constraints imposed by the sapphire substrate  1101 , the film does not crack or experience thermal stress upon cooling down. The result is a high quality freestanding GaN substrate  1120 . 
     FIG. 12 schematically shows the series of steps involved in a method for making a thick, crack-free freestanding GaN substrate, according to the fourth embodiment of this invention. Step  1201  calls for the provision of a prepared substrate. In Step  1202 , the process parameters for the growth of the buffer layer (or layers) are set, hereby denoted as [C 1 ″″]. Parameter set [C 1 ″″] contains all relevant parameters regarding the growth conditions for the deposition of the buffer layer (or layers). In the case of a buffer consisting of multiple layers, the process conditions set [C 1 ″″] may consist of multiple unique sub-sets, one for each individual layer of the buffer structure. Step  1203  calls for the growth of the buffer layer (or layers), e.g. layer  1106 , onto the prepared substrate, with a total thickness on the order of 50 nm. The parameters in [C 1 ″″] are typically in the same ranges as those described with respect to [C 1 ′] of FIG.  6 . In Step  1204  growth conditions are changed to [C 2 ″″]. This may involve the use of a different growth technique than that used in Step  1203 , and possibly even the use of a different growth system, which may require an additional set of steps for unloading and loading of the sample. Step  1205  calls for the growth of a defect-rich or rough second layer, e.g. layer  1107  onto the buffer layer. The thickness of this layer is preferentially in the range of 20-300 microns. The parameters in [C 2 ″″] are typically in the same ranges as those described with respect to [C 1 ] of FIG.  4 . The conditions [C 2 ″″] are such that the material deposited will have a high defect density or surface roughness, yet will remain essentially epitaxial in nature. In step  1206 , the sample is cooled to room temperature without formation of cracks; the residual stress is minimized due to the effect of the defect-rich or rough second layer. Step  1207  calls for the removal of the sapphire substrate to obtain a rough or defect-rich freestanding GaN layer; while the freestanding layer is a single crystal, its defect density and/or roughness make it unsuitable to use as a GaN substrate for device applications. In Step  1208  the growth conditions are changed to [C 3 ″″], which includes a set of steps for reloading the sample into the reactor. Step  1209  calls for the deposition of the third, high quality, low defect or flat layer, e.g. layer  1108 , atop the second layer, using the growth conditions [C 3 ″″]. The parameters in [C 3 ″″] are typically in the same ranges as those described with respect to [C 2 ] of FIG.  4 . This third layer covers the second, burying and isolating the defects, which do not propagate upward during growth. Alternatively, the third layer may be grown under conditions favoring lateral epitaxial growth, resulting in growth that smoothes and fills in surface irregularities in the second layer. The thickness of this third layer is preferentially in the range of 10-300 microns. In step  1210 , the sample is cooled down to room temperature without the formation of cracks; there is no thermal mismatch without the presence of the sapphire substrate, which was removed in Step  1207  to form freestanding substrate  1120 . 
     SIXTH EMBODIMENT 
     FIGS.  13 ( a )- 13 ( d ) schematically show the growth of a thick, crack-free freestanding GaN substrate, according to the sixth embodiment of this invention. In FIG.  13 ( a ), a buffer layer structure is formed comprising a lower GaN layer  1309 , a patterned mask layer of silicon dioxide (SiO 2 )  1310 , and an upper GaN layer  1311 . The mask material inhibits GaN growth; growth occurs only in unmasked areas. This serves to enhance lateral overgrowth of the upper GaN layer  1311 , as well as to partially decouple the substrate from this and subsequent epitaxial layers. Such decoupling reduces the residual thermal and/or lattice mismatch stresses in the epitaxial layers, and is achieved by reducing the effective contact area between substrate and epitaxy. 
     Typically the two GaN layers are grown by a vapor phase epitaxy technique, e.g., MOCVD or HVPE, and the total thickness of the buffer structure is on the order of one micron, although other growth processes and buffer structure thicknesses are within the scope of the present invention. The individual layers of this structure may also be of different compositions. For example, layers  1309  and  1311  could each consist of GaN, AlN, InN, or an alloy of these constituents; layer  1310  could consist of SiO 2 , silicon nitride (Si 3 N 4 ), polysilicon, refractory metal, or any combination of these components. These are given merely as examples, and are not meant to limit the scope of this embodiment of the present invention; other suitable materials could be used to similar effect. Additionally, each individual layer may consist of a set of sub-layers: layers  1309 ,  1310 , and/or  1311  may each be composed of two or more layers of distinct composition, to further reduce the residual stress levels or to enhance lateral overgrowth. Typical thickness for the lower GaN layer  1309  is on the order of 1 micron; thickness of the patterned mask layer  1310  is on the order of 500 nm; thickness of the upper GaN layer  1311  is on the order of 1-10 microns. Other layer thicknesses that serve to reduce the residual stress or improve the nucleation or lateral overgrowth of layer  1311  are also within the scope of this embodiment of the present invention. 
     Following the buffer layer structure growth is the deposition of a third, thick, defect-rich or very rough HVPE layer  1312 , as shown in FIG.  13 ( b ). After this layer is grown, the sample is cooled to room temperature, removed from the reactor, and sapphire substrate  1  is removed by polishing or some other technique, as shown in FIG.  13 ( c ). Thermal mismatch-induced cracks do not form while cooling because of two complementary effects: the third GaN layer  1312 , accommodates the residual stress induced by the sapphire substrate  1 , as described in the fourth embodiment of the present invention. Additionally, the patterned mask layer  1310  and lateral overgrowth layer  1311  serve to decouple the thick HVPB layer  1312  from the substrate  1301 . Subsequently, the now freestanding GaN layer is reloaded into the reactor and a fourth high quality or very flat GaN layer  1313  is grown onto the third GaN layer  1312  by HVPE as shown in FIG.  13 ( d ). This fourth layer  1313  is grown homoepitaxially; freed of the thermal and lattice mismatch constraints imposed by the sapphire substrate  1301 , the film does not crack or experience thermal stress upon cooling down. The result is a high quality freestanding GaN substrate  1320 . 
     FIG. 14 schematically shows the series of steps involved in a method for making a thick a thick, crack-free freestanding GaN substrate, according to the sixth embodiment of this invention. Step  1401  calls for the provision of a prepared substrate. In Step  1402 , the process parameters for the growth of the first buffer layer (or layers) are set, hereby denoted as [C 1 ′″″]. Parameter set [C 1 ′″″] contains all relevant parameters regarding the growth conditions for the deposition of a buffer layer (or layers). The parameters in [C 1 ′″″] are typically in the same ranges as those described with respect to [C 1 ′] of FIG.  6 . Step  1403  calls for the growth of the buffer layer (or layers), e.g. layer  1309  onto the prepared substrate, with a total thickness on the order of 1 micron. In Step  1404 , the sample is cooled and removed from the reactor. In Step  1405 , a suitable mask material is deposited onto the sample, after which it is patterned using appropriate methods such as lithography and etching to produce a patterned layer such as layer  1310 . In step  1406  growth conditions are changed to [C 2 ′″″] for the growth of an ELOG layer (or layers), such as layer  1311 . The process of reloading the sample into the growth reactor may also be included in parameter set [C 2 ′″″]. In Step  1407  the ELOG layer (or layers) are grown for a typical thickness of 1-10 microns. The parameters in [C 2 ′″″] are typically in the same ranges as those described with respect to [C 2 ″] of FIG.  8 . The ELOG layer is grown through and over the masking layer below it, reducing the effective contact area between the epitaxial overgrowth and the sapphire substrate. In Step  1408  the growth conditions are changed to [C 3 ′″], which may involve the use of a different growth technique than that used in Step  1406 , possibly even the use of a different growth system, which may require an additional set of steps for unloading and loading of the sample. Step  1409  calls for the growth of a defect-rich or rough third layer, e.g. layer  1312  onto the buffer layer. The thickness of this layer is preferentially in the range of 20-300 microns. The parameters in [C 3 ′″] are typically in the same ranges as those described with respect to [C 1 ] of FIG.  4 . The conditions [C 3 ′″] are such that the material deposited will have a high defect density or surface roughness, yet will remain essentially epitaxial in nature. In step  1410 , the sample is cooled to room temperature without the formation of cracks; the residual stress is minimized due to the effects of the defect-rich or rough second layer as well as the decoupling between the epitaxial layer and the substrate induced by the mask and second layer growth in the buffer structure. Step  1411  calls for the removal of the sapphire substrate to obtain a rough or defect-rich freestanding GaN layer; while the freestanding layer is a single crystal, its defect density and/or roughness make it unsuitable to use as a GaN substrate for device applications. In Step  1412  the growth conditions are changed to [C 4 ], which includes a set of steps for reloading the sample into the reactor. Step  1413  calls for the deposition of the fourth, high quality, low defect or flat layer atop the third layer, using the growth conditions [C 4 ]. This fourth layer covers the third, burying and isolating the defects, which do not propagate upward during growth. Alternatively, the fourth layer may be grown under conditions favoring lateral epitaxial growth, resulting in growth that smoothes and fills in surface irregularities in the third layer. The thickness of this fourth layer is preferentially in the range of 10-300 microns. In step  1414 , the sample is cooled down to room temperature without the formation of cracks; there is no thermal mismatch without the presence of the sapphire substrate, which was removed in Step  1411  to form freestanding substrate  1320 . 
     The foregoing embodiments are exemplary, 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.
A method for the production of a crack-free epitiaxial film having a thickness greater than that which can be achieved by continuous epitaxial crystal growth. This epitaxial film can be used as is in a device, used as a substrate platform for further epitaxy, or separated from the initial substrate material and used as a free-standing substrate platform. The method utilizes a defect-rich initial layer that absorbs epitaxially derived stresses and another layer, which is not defect-rich, which planarizes the crystal growth front, if necessary and provides high quality epitaxial region near the surface.