Abstract:
A germanium-containing layer is deposited on a single crystalline bulk silicon substrate in an ambient including a level of oxygen partial pressure sufficient to incorporate 1%-50% of oxygen in atomic concentration. The thickness of the germanium-containing layer is preferably limited to maintain some degree of epitaxial alignment with the underlying silicon substrate. Optionally, a graded germanium-containing layer can be grown on, or replace, the germanium-containing layer. An at least partially crystalline silicon layer is subsequently deposited on the germanium-containing layer. A handle substrate is bonded to the at least partially crystalline silicon layer. The assembly of the bulk silicon substrate, the germanium-containing layer, the at least partially crystalline silicon layer, and the handle substrate is cleaved within the germanium-containing layer to provide a composite substrate including the handle substrate and the at least partially crystalline silicon layer. Any remaining germanium-containing layer on the composite substrate is removed.

Description:
BACKGROUND 
     The present disclosure relates to a method of forming a semiconductor structure, and more particularly to a method of forming a semiconductor structure including a transferred semiconductor layer, and structures for effecting the same and formed by the same. 
     A substrate including a thin silicon layer can be formed by employing a hydrogen-containing cleavage layer. For example, hydrogen ions (protons) can be implanted into a bulk silicon substrate to form a hydrogen-containing layer at a constant depth from a top surface of the bulk silicon substrate. A handle substrate is bonded to the top surface of the bulk silicon substrate, and the bulk silicon substrate is subsequently cleaved at the hydrogen-containing layer so that a thin silicon layer above the hydrogen-containing layer is “transferred” to the handle substrate to form a new substrate, which is an assembly of the handle substrate and the transferred thin silicon layer. The remaining portion of the bulk substrate is planarized by chemical mechanical planarization and re-used to provide another thin silicon layer for another layer transfer process until the thickness of the bulk substrate becomes too thin to be employed for layer transfer purposes. 
     The method of forming a substrate including a thin silicon layer employing hydrogen implantation is subject to many limitations. First, a hydrogen-containing layer must be formed through hydrogen implantation. Because of inherent depth distribution of the implanted hydrogen ions, a high dose of hydrogen ions must be implanted into the bulk silicon substrate to be able to induce cleavage at the hydrogen-containing layer. Because the vertical distribution range of the hydrogen ions increases with increasing depth of implantation, higher dose of hydrogen ions is needed as the depth of the hydrogen-containing layer increases. 
     Further, due to the propensity of bulk silicon substrates to cleave along major crystallographic planes, cleavage along only some crystallographic orientations of a silicon crystal produces clean cleavage planes with atomic planarity, while cleavage along other crystallographic orientations can produce cleavage planes that include facets and/or rough surfaces that need to be planarized, for example, by chemical mechanical planarization. 
     Yet further, the bulk substrate after cleavage needs to be planarized before re-usage. In addition, any modification to the dopant concentration in the transferred layer requires additional processes that include implantation or plasma treatment and dopant activation by a high temperature anneal. 
     Thus, a process of forming a transferred silicon layer without employing hydrogen ion implantation is desired. 
     BRIEF SUMMARY 
     A germanium-containing layer is deposited on a single crystalline bulk silicon substrate in an ambient including a level of oxygen partial pressure sufficient to incorporate 1%-50% of oxygen in atomic concentration. The thickness of the germanium-containing layer may be limited to facilitate some degree of epitaxial alignment with the underlying silicon substrate. Optionally, a graded germanium-containing layer including a graded silicon-germanium alloy can be grown on, or replace, the germanium-containing layer. An at least partially crystalline silicon layer is subsequently deposited on the germanium-containing layer. A handle substrate is bonded to the at least partially crystalline silicon layer. The assembly of the bulk silicon substrate, the germanium-containing layer, the at least partially crystalline silicon layer, and the handle substrate is cleaved within the germanium-containing layer to provide a composite substrate including the handle substrate and the at least partially crystalline silicon layer. Any remaining portion of the germanium-containing layer on the composite substrate is removed. 
     According to an aspect of the present disclosure, a method of forming a semiconductor structure includes: growing a germanium-containing layer on a single crystalline silicon substrate; growing an at least partially crystalline silicon layer on the germanium-containing layer; bonding a handle substrate to the at least partially crystalline silicon layer; and cleaving an assembly of the handle substrate and the at least partially crystalline silicon layer off the single crystalline silicon substrate along a plane in the germanium-containing layer. 
     According to another aspect of the present disclosure, a semiconductor structure including a material stack, which includes: a single crystalline silicon substrate; a germanium-containing layer contacting the single crystalline silicon substrate; an at least partially crystalline silicon layer located on the germanium-containing layer; and a handle substrate bonded to the at least partially crystalline silicon layer. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1A-1F  are vertical cross-sectional views of a first exemplary semiconductor structure according to a first embodiment of the present disclosure. 
         FIG. 2A-2F  are vertical cross-sectional views of a second exemplary semiconductor structure according to a second embodiment of the present disclosure. 
         FIG. 3A-3F  are vertical cross-sectional views of a third exemplary semiconductor structure according to a third embodiment of the present disclosure. 
         FIG. 4A-4F  are vertical cross-sectional views of a fourth exemplary semiconductor structure according to a fourth embodiment of the present disclosure. 
         FIG. 5  is a graph of an X-ray diffraction data from a first sample according to the first embodiment of the present disclosure as a function of 2θ. 
         FIG. 6  is a graph of an X-ray diffraction data from a second sample according to the second embodiment of the present disclosure as a function of 2θ. 
         FIG. 7  is a graph of an X-ray diffraction data from a third sample including a polysilicon film as a function of 2θ. 
         FIG. 8  is a graph illustrating data from a secondary ion mass spectroscopy (SIMS) run on the second sample according to the second embodiment of the present disclosure. 
         FIG. 9  is a scanning electron micrograph (SEM) of a broken portion of the first sample along a vertical cleavage plane without cleaving along the oxygen-containing germanium layer. 
         FIG. 10  is a scanning electron micrograph (SEM) of a portion of the first sample, which was cleaved along the oxygen-containing germanium layer and includes the bulk silicon substrate and a portion of the oxygen-containing germanium layer. 
     
    
    
     DETAILED DESCRIPTION 
     As stated above, the present disclosure relates to a method of forming a semiconductor structure including a transferred semiconductor layer, and structures for effecting the same and formed by the same, which are now described in detail with accompanying figures. Throughout the drawings, the same reference numerals or letters are used to designate like or equivalent elements. The drawings are not necessarily drawn to scale. 
     As used herein, a “textured crystalline layer” is a polycrystalline layer including grains, in which a predominant portion of the grains have the same set of crystallographic orientations. A “predominant portion” of an element refers to more than 50% in volume of the element. Likewise, as used herein, a “polycrystalline layer” is a more general term that includes both textured crystalline layers and crystalline layers including grains with a mix of crystallographic orientations with no single dominant orientation. 
     As used herein, an “at least partially crystalline layer” is a layer that is either a single crystalline layer, a textured crystalline layer, or polycrystalline layer. 
     As used herein, an “at least partially crystalline silicon layer” is an at least partially crystalline layer including intrinsic silicon or doped silicon. 
     Referring to  FIG. 1A , a first exemplary semiconductor structure according to a first embodiment of the present disclosure includes a single crystalline semiconductor substrate  10 . In one embodiment, the single crystalline semiconductor substrate  10  consists of intrinsic silicon or doped silicon. In one embodiment, the single crystalline silicon substrate  10  is a bulk silicon. In this embodiment, the single crystalline silicon substrate  10  is thick enough to allow mechanical handling of the single crystalline silicon substrate  10  without breakage, and can have a thickness from 200 microns to 2,000 microns. In another embodiment, the single crystalline silicon substrate  10  can be attached to another substrate that facilitates handling of the single crystalline silicon substrate  10 . For example, the single crystalline silicon substrate  10  can be attached to an insulating substrate or a conductive substrate. In this embodiment, the single crystalline silicon substrate  10  can have a thickness from 5 microns to 2,000 microns. 
     The surface normal of a planar top surface of the single crystalline silicon substrate  10  can have any crystallographic orientation. In one embodiment, the surface normal of the planar top surface of the single crystalline silicon substrate  10  can have a “major crystallographic orientation,” which is defined herein as an orientation having a set of Miller indices in which each Miller index in the set of Miller indices has an absolute value that does not exceed 6. In another embodiment, the surface normal of the planar top surface of the single crystalline silicon substrate  10  can have a “non-major crystallographic orientation,” which is defined herein as an orientation having a set of Miller indices in which at least one Miller index in the set of Miller indices has an absolute value that exceeds 6. Thus, the orientation of the surface normal of a planar top surface of the single crystalline silicon substrate  10  is not limited in any way. Non-conventional surface orientations having a “high Miller index,” i.e., a Miller index having an absolute value that exceeds 6, can be provided on the single crystalline silicon substrate  10  by angled polishing on a conventional single crystalline silicon substrate having “low Miller indices,” i.e., Miller indices having absolute values that do not exceed 6. 
     Referring to  FIG. 1B , a germanium-containing layer  20  is grown directly on the top surface of the single crystalline silicon substrate  10 , preferably with at least some degree of epitaxial alignment. The degree of epitaxial alignment between the germanium-containing layer  20  and the single crystalline silicon substrate  10  may be complete or incomplete, depending on embodiments. In one embodiment, the germanium-containing layer  20  is a single crystalline layer having an epitaxial alignment with the single crystalline silicon substrate  10  at an atomic level. In another embodiment, the germanium-containing layer  20  is a textured crystalline layer in which grains are predominantly oriented in a direction providing epitaxial alignment with the single crystalline structure of the single crystalline silicon substrate  10  at an atomic level. In this embodiment, a predominant portion of the grains of the germanium-containing layer  20  has the same set of crystallographic orientations as the single crystalline silicon substrate  10 . In yet another embodiment, germanium-containing layer  20  may be polycrystalline. 
     The germanium-containing layer  20  is grown in an ambient having an oxygen partial pressure at a level that incorporates oxygen into the germanium-containing layer  20  at an atomic concentration between 1% and 50%, and typically between 2% and 20%. The oxygen partial pressure can be provided by residual gases in a high vacuum environment having a base pressure of 10 −6  Torr to 100 mTorr, and typically from 10 −5  Ton and 10 mTorr. Alternately, the oxygen partial pressure can be provided by supplying an oxygen-containing gas such as oxygen, ozone, or carbon dioxide in an ultrahigh vacuum environment having a base pressure less than 10 −6  Torr. Yet alternately or in addition, the germanium-containing layer  20  can be deposited in an environment having a low oxygen partial pressure such that the germanium-containing layer  20  has an atomic concentration of oxygen less than 1% as deposited. In this case, the germanium-containing layer  20  can be exposed to an oxygen-containing ambient to allow adsorption of oxygen and subsequent incorporation of oxygen into the germanium-containing layer  20  at an atomic concentration between 1% and 50%, and typically between 2% and 20%. 
     In one embodiment, the germanium-containing layer  20  can have a substantially constant germanium concentration at an atomic concentration from 30% to 99%. The germanium concentration is “substantially constant” because statistical variations in germanium concentration is inherently present due to the statistical nature of composition of the germanium-containing layer  20 . In one case, the germanium-containing layer  20  can include germanium and oxygen, and the sum of the atomic concentration of germanium and the atomic concentration of oxygen is greater than 99%. The germanium-containing layer  20  may consist essentially of germanium and oxygen, and the sum of the atomic concentration of germanium and the atomic concentration of oxygen is greater than 99%. In another case, the germanium-containing layer  20  can include germanium, silicon, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, and the atomic concentration of oxygen is greater than 99%. The germanium-containing layer  20  may consist essentially of germanium, silicon, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, and the atomic concentration of oxygen is greater than 99%. In yet another case, the germanium-containing layer  20  can include germanium, silicon, at least another atom, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, the atomic concentration of the at least another atom, and the atomic concentration of oxygen is greater than 99%. The germanium-containing layer  20  may consist essentially of germanium, silicon, at least another atom, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, the atomic concentration of the at least another atom, and the atomic concentration of oxygen is greater than 99%. The at least another atom can be carbon, a p-type dopant such as boron, gallium, or indium, an n-type dopant such as phosphorus, arsenic, or antimony, any other impurity atoms such as nitrogen, fluorine, hydrogen, or argon, or a combination thereof. 
     In another embodiment, the germanium-containing layer  20  can include silicon, germanium, and oxygen. The atomic concentration of germanium decreases in the germanium-containing layer  20  with the distance from the single crystalline silicon substrate  10 . Thus, the atomic concentration of germanium in the germanium-containing layer  20  has variable values, which can be in a range from 0% and 99%. In this case, the atomic concentration of germanium in the graded germanium-containing layer  20  has a maximum value that is at least 50%, which occurs at or near the interface with the single crystalline silicon substrate  10 . In one case, the germanium-containing layer  20  can include germanium, silicon, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, and the atomic concentration of oxygen is greater than 99% in each location within the germanium-containing layer  20 . The germanium-containing layer  20  may consist essentially of germanium, silicon, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, and the atomic concentration of oxygen is greater than 99% in each location within the germanium-containing layer  20 . In another case, the germanium-containing layer  20  can include germanium, silicon, at least another atom, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, the atomic concentration of the at least another atom, and the atomic concentration of oxygen is greater than 99% in each location within the germanium-containing layer  20 . The germanium-containing layer  20  may consist essentially of germanium, silicon, at least another atom, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, the atomic concentration of the at least another atom, and the atomic concentration of oxygen is greater than 99% in each location within the germanium-containing layer  20 . The at least another atom can be carbon, a p-type dopant such as boron, gallium, or indium, an n-type dopant such as phosphorus, arsenic, or antimony, any other impurity atoms such as nitrogen, fluorine, hydrogen, or argon, or a combination thereof. 
     In one embodiment, germanium-containing layer  20  is at least partially epitaxial. The thickness of the germanium-containing layer  20  is maintained not to exceed the critical thickness at which the epitaxial alignment between the single crystalline silicon substrate  10  and the germanium-containing layer  20  is destroyed through stress relaxation. The oxygen content of germanium-containing layer  20  is also kept low (e.g., 1-3%) to help preserve epitaxy. In another embodiment, the thickness of the germanium-containing layer  20  may be less than, the same as, or exceed the critical thickness at which the epitaxial alignment between the single crystalline silicon substrate  10  and the germanium-containing layer  20  is destroyed through stress relaxation. If the thickness of the germanium-containing layer  20  exceeds the critical thickness, the germanium-containing layer  20  may develop dislocations therein. 
     If the thickness of the germanium-containing layer  20  does not exceed the critical thickness, the thickness of the germanium-containing layer  20  is between 5 nm and 80 nm, and preferably between 10 nm and 60 nm, although lesser and greater thicknesses can also be employed depending on the concentration of germanium provided that at least some epitaxial alignment between the germanium-containing layer  20  and the single crystalline silicon substrate  10  is maintained. 
     The germanium-containing layer  20  can be deposited by chemical vapor deposition (CVD), vacuum evaporation, or atomic layer deposition (ALD). The deposition temperature is set at a temperature that provides sufficient surface diffusion to germanium atoms and silicon atoms, if silicon is incorporated in the germanium-containing layer  20 , and any other atoms, if any other atoms are incorporated into the germanium-containing layer  20 . For example, the deposition temperature can be 450° C. to 900° C., and typically from 500° C. to 700° C. The pressure of the deposition chamber can vary depending on the deposition process employed. In general, chemical vapor deposition processes employ deposition conditions including a total pressure from 0.1 Torr to 10 Torr, and typically from 0.2 Torr to 5 Torr. A predominant portion of the total pressure is the partial pressure of a carrier gas. If vacuum evaporation or atomic layer deposition is employed, the deposition pressure is typically from 10 −6  Torr to 10 −3  Torr, depending on the base pressure of the deposition system and whether oxygen gas is flowed into the deposition chamber in addition to residual oxygen gases inherently present in any vacuum chamber having a finite (non-zero) base pressure. In case atomic layer deposition is employed, at least one reactant gas and oxygen gas can be alternately flowed into a deposition chamber with optional adjustments to the temperature of the single crystalline silicon substrate  10  to control the amount of oxygen incorporated into the germanium-containing layer  20 . 
     In case chemical vapor deposition is employed, the single crystalline silicon substrate  10  is placed in a vacuum environment, of which the base pressure can vary as discussed above. Low pressure chemical vapor deposition (LPCVD) process or plasma enhanced chemical vapor deposition (PECVD) may be employed. Energy to decompose one or more reactant gases is provided by thermal energy, whereas energy to decompose one or more reactant gases is provided by plasma energy. A germanium-containing reactant gas, which includes at least one atom of germanium, is flowed into the deposition chamber. Exemplary germanium-containing reactant gases include GeH 4 , GeH 2 Cl 2 , GeCl 4 , and Ge 2 H 6 . If silicon is incorporated into the germanium-containing layer  20 , a silicon-containing reactant gas including at least one atom of silicon, e.g., SiH 4 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , and Si 2 H 6 , can be flowed into the deposition chamber. Atomic layer deposition can employ the same reactants and/or dopants as chemical vapor deposition. 
     If vacuum evaporation is employed, germanium and/or silicon can be evaporated from an evaporation source, which can be an electron beam source or an effusion cell. Typically, the evaporation source is heated at or near the melting temperature of the source material, i.e., the melting temperature of germanium or the melting temperature of silicon. Oxygen can be provided by background level residual oxygen in a vacuum system having a base pressure greater than 10 −6  Torr. Alternatively or in addition, oxygen gas can be continually or intermittently provided into the deposition chamber from an oxygen source such as a mass flow controller connected to an oxygen tank. Alternatively or in addition, oxygen can be provided to the top surface of the germanium-containing layer  20  and incorporated therein by diffusion. 
     Optionally, the material stack including the single crystalline silicon substrate  10  and the germanium-containing layer  20  may be maintained at an elevated temperature for a period of time to enhance the degree of epitaxial alignment between the single crystalline silicon substrate  10  and the germanium-containing layer  20  and/or to repair crystalline defects in the germanium-containing layer  20 . Because silicon and germanium have the same crystal structures, a crystalline germanium-containing layer  20  would be expected to have a crystallographic orientation epitaxially related to that of the single crystalline silicon substrate  10 . 
     Referring to  FIG. 1C , an at least partially crystalline silicon layer  30  having at least some degree of crystallinity is grown directly on the top surface of the germanium-containing layer  20 . In one embodiment, the at least partially crystalline silicon layer  30  is a single crystalline layer having a complete epitaxial alignment with the germanium-containing layer  20  at an atomic level. In another embodiment, the at least partially crystalline silicon layer  30  is a textured crystalline layer in which grains are predominantly oriented in a direction providing epitaxial alignment with the single crystalline structure of the germanium-containing layer  20  at an atomic level. In this embodiment, a predominant portion of the grains of the at least partially crystalline silicon layer  30  has the same set of crystallographic orientations as the germanium-containing layer  20 . In yet another embodiment, the at least partially crystalline silicon layer  30  is a textured crystalline layer in which grains are predominantly oriented in a direction providing epitaxial alignment with a textured crystalline structure of the germanium-containing layer  20  at an atomic level. In this embodiment, a predominant portion of the grains of the at least partially crystalline silicon layer  30  has the same set of crystallographic orientations as the germanium-containing layer  20 . 
     The thickness of the at least partially crystalline silicon layer  30  is selected to apply sufficient stress to the germanium-containing layer  20  to cause formation of a plurality of cavities  27  within the germanium-containing layer  20  by the end of deposition of the at least partially crystalline silicon layer  30 . The thickness of the at least partially crystalline silicon layer  30  needed to generate cavities  27  within the germanium-containing layer  20  depends on the thickness of the germanium-containing layer  20  and the germanium content and the oxygen content in the germanium-containing layer  20 . In general, the thickness of the at least partially crystalline silicon layer  30  is at least equal to the thickness of the germanium-containing layer  20 , and is typically greater than twice the thickness of the germanium-containing layer  20 . In case the germanium-containing layer  20  includes silicon in addition to germanium and oxygen, the thickness of the at least partially crystalline silicon layer  30  can be greater than three times the thickness of the germanium-containing layer  20 . Typically, the at least partially crystalline silicon layer  30  has a thickness that is greater than 100 nm. For example, the thickness of the at least partially crystalline silicon layer  30  can be from 100 nm to 500 nm, although lesser and greater thicknesses can also be employed. 
     The plurality of cavities  27  is formed during the epitaxial growth of the at least partially crystalline silicon layer  30 , i.e., before the completion of deposition of the silicon material of the at least partially crystalline silicon layer  30 . The lateral dimensions of the plurality of cavities  27  is on the same order of magnitude as the thickness of the germanium-containing layer  20 . Typically, each cavity in the plurality of cavities has a maximum lateral dimension less than 200 nm. 
     The at least partially crystalline silicon layer  30  is grown in a vacuum environment in which oxygen partial pressure is minimal or in an ambient in which oxygen partial pressure is minimized. Any oxygen incorporated in the at least partially crystalline silicon layer  30  is maintained below 5% in atomic concentration, and preferably below 2% in atomic concentration, and most preferably as low as possible. 
     The at least partially crystalline silicon layer  30  can be deposited by chemical vapor deposition (CVD) or vacuum evaporation. The deposition temperature is set at a temperature that provides sufficient surface diffusion to silicon atoms. For example, the deposition temperature can be from 500° C. to 1,100° C., and typically from 500° C. to 700° C. The pressure of the deposition chamber can vary depending on the deposition process employed. In general, chemical vapor deposition processes employ deposition conditions including a total pressure from 0.1 Torr to 10 Torr, and typically from 0.2 Torr to 5 Torr. A predominant portion of the total pressure is the partial pressure of a carrier gas such as hydrogen gas. If vacuum evaporation or atomic layer deposition is employed, the deposition pressure is typically from 10 −6  Torr to 10 −3  Torr. 
     In case chemical vapor deposition is employed, the stack of the single crystalline silicon substrate  10  and the germanium-containing layer  20  is placed in a vacuum environment such that the top surface of the germanium-containing layer  20  is exposed. Low pressure chemical vapor deposition (LPCVD) process or plasma enhanced chemical vapor deposition (PECVD) may be employed. A silicon-containing reactant gas including at least one atom of silicon, e.g., SiH 4 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , and Si 2 H 6 , is flowed into the deposition chamber. The at least partially crystalline silicon layer  30  can be doped in-situ with p-type dopants or n-type dopants by concurrently flowing dopant gases such as B 2 H 6 , PH 3 , AsH 3 , SbH 3 , or a combination thereof. If vacuum evaporation is employed, silicon can be evaporated from an evaporation source, which can be an electron beam source or an effusion cell. 
     Optionally, the material stack including the single crystalline silicon substrate  10 , the germanium-containing layer  20 , and the at least partially crystalline silicon layer  30  may be maintained at an elevated temperature for a period of time to enhance the degree of epitaxial alignment between the germanium-containing layer  20  and the at least partially crystalline silicon layer  30  and/or to repair crystalline defects in the at least partially crystalline silicon layer  30 . 
     Referring to  FIG. 1D , a handle substrate  40  is attached to the top surface of the at least partially crystalline silicon layer  30 , for example, by bonding. The handle substrate  40  can include a dielectric material layer, a conductive material layer, a polycrystalline semiconductor material layer, a single crystalline semiconductor material layer, or a combination thereof. In non-limiting exemplary cases, the handle substrate  40  can be a glass substrate or a metal substrate. Any bonding method known in the art may be employed including anodic bonding, in which an electrical bias voltage is applied across the interface between the at least partially crystalline silicon layer  30  and the handle substrate  40 . Typically, the handle substrate  40  is thick enough to allow mechanical handling without significant risk of breakage. For example, the handle substrate  40  can have a thickness from 200 microns to 5 mm, and typically from 500 microns to 1 mm, although lesser and greater thicknesses can also be employed. 
     Referring to  FIG. 1E , the first exemplary semiconductor structure is cleaved along a plane, which is herein referred to as a “cleavage plane,” located within the germanium-containing layer  20 . An upper assembly of the handle substrate  40  and the at least partially crystalline silicon layer  30  and an upper portion of the germanium-containing layer  20 , which is herein referred to as an upper epitaxial germanium-containing portion  27 B, is cleaved off a lower assembly including the single crystalline silicon substrate  10  and a lower portion of the germanium-containing layer  20 , which is herein referred to as a lower epitaxial germanium-containing portion  27 A, along the cleavage plane. 
     Treatment of the first exemplary semiconductor structure by any chemical treatment, thermal treatment, or ion implantation is not necessary because the germanium-containing layer  20  is under stress induced by the lattice mismatch with the single crystalline silicon substrate  10  and the at least partially crystalline silicon layer  30 . Thus, mechanical shear stress applied to the first exemplary semiconductor structure (i.e., twisting the upper assembly ( 40 ,  30 ,  20 B) relative to the lower assembly ( 10 ,  20 A)) or mechanical tensile stress applied the first exemplary semiconductor structure (i.e., pulling the upper assembly ( 40 ,  30 ,  20 B) away from the lower assembly ( 10 ,  20 A)) can separate the upper assembly ( 40 ,  30 ,  20 B) from the lower assembly ( 10 ,  20 A). 
     Referring to  FIG. 1F , the upper germanium-containing portion  27 B is removed selective to the at least partially crystalline silicon layer  30  by an etch or planarization. The etch can be an isotropic etch such as a wet etch employing hydrogen peroxide that removes germanium or a silicon-germanium alloy with a high atomic percentage of germanium (i.e., at least 30% of germanium in atomic concentration), or a dry etch such as a reactive ion etch that is selective to silicon. Alternately or in addition, chemical mechanical planarization can be employed to remove the upper germanium-containing portion  27 B. The upper assembly at this point includes a stack of the handle substrate  40  and the at least partially crystalline silicon layer  30 . 
     Referring to  FIG. 2A , a second exemplary semiconductor structure according to a second embodiment of the present disclosure includes a single crystalline silicon substrate  10 , which is the same as the single crystalline silicon substrate  10  of the first embodiment. 
     Referring to  FIG. 2B , a stack of a germanium-containing layer  20  and a graded germanium-containing layer  22  are grown on the single crystalline at least partially crystalline silicon layer  10 . Each of the germanium-containing layer  20  and the graded germanium-containing layer  22  can be deposited by chemical vapor deposition, vacuum evaporation, or atomic layer deposition as described in the first embodiment. 
     The germanium-containing layer  20  includes oxygen at an atomic concentration between 1% and 50%, and typically between 2% and 20%. The germanium-containing layer  20  has a substantially constant germanium concentration at an atomic concentration from 30% to 99%. In one case, the germanium-containing layer  20  can include germanium and oxygen, and the sum of the atomic concentration of germanium and the atomic concentration of oxygen is greater than 99%. The germanium-containing layer  20  may consist essentially of germanium and oxygen, and the sum of the atomic concentration of germanium and the atomic concentration of oxygen is greater than 99%. In another case, the germanium-containing layer  20  can include germanium, silicon, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, and the atomic concentration of oxygen is greater than 99%. The germanium-containing layer  20  may consist essentially of germanium, silicon, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, and the atomic concentration of oxygen is greater than 99%. In yet another case, the germanium-containing layer  20  can include germanium, silicon, at least another atom, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, the atomic concentration of the at least another atom, and the atomic concentration of oxygen is greater than 99%. The germanium-containing layer  20  may consist essentially of germanium, silicon, at least another atom, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, the atomic concentration of the at least another atom, and the atomic concentration of oxygen is greater than 99%. The at least another atom can be carbon, a p-type dopant such as boron, gallium, or indium, an n-type dopant such as phosphorus, arsenic, or antimony, any other impurity atoms such as nitrogen, fluorine, hydrogen, or argon, or a combination thereof. 
     The graded germanium-containing layer  22  includes silicon, germanium, and oxygen. The atomic concentration of germanium decreases in the graded germanium-containing layer  22  with the distance from the germanium-containing layer  20 . Thus, the atomic concentration of germanium in the graded germanium-containing layer  22  has variable values, which can be in a range from 0% and 99%. The atomic concentration of germanium in the graded germanium-containing layer  22  has a maximum value that is at least 50%, which occurs at or near the interface with the germanium-containing layer  20 . In one case, the graded germanium-containing layer  22  can include germanium, silicon, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, and the atomic concentration of oxygen is greater than 99% in each location within the graded germanium-containing layer  22 . The graded germanium-containing layer  22  may consist essentially of germanium, silicon, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, and the atomic concentration of oxygen is greater than 99% in each location within the graded germanium-containing layer  22 . In another case, the graded germanium-containing layer  22  can include germanium, silicon, at least another atom, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, the atomic concentration of the at least another atom, and the atomic concentration of oxygen is greater than 99% in each location within the graded epitaxial germanium-containing layer  22 . The graded germanium-containing layer  22  may consist essentially of germanium, silicon, at least another atom, and oxygen, and the sum of the atomic concentration of germanium, the atomic concentration of silicon, the atomic concentration of the at least another atom, and the atomic concentration of oxygen is greater than 99% in each location within the graded germanium-containing layer  22 . The at least another atom can be carbon, a p-type dopant such as boron, gallium, or indium, an n-type dopant such as phosphorus, arsenic, or antimony, any other impurity atoms such as nitrogen, fluorine, hydrogen, or argon, or a combination thereof. 
     In one embodiment, the germanium-containing layer  20  and the graded germanium-containing layer  22  are at least partially epitaxially aligned with silicon substrate  10 . The combined thickness of the stack of the germanium-containing layer  20  and the graded germanium-containing layer  22  is maintained not to exceed the critical thickness at which the epitaxial alignment between the single crystalline silicon substrate  10  and the germanium-containing layer  20  is destroyed through stress relaxation. In general, the thickness of the stack of the germanium-containing layer  20  and the graded germanium-containing layer  22  is between 5 nm and 100 nm, and preferably between 10 nm and 80 nm, although lesser and greater thicknesses can also be employed depending on the concentration levels of germanium in the stack of the germanium-containing layer  20  and the graded germanium-containing layer  22 , provided that the at least partial epitaxial alignment between the germanium-containing layer  20 , the graded germanium-containing layer  22 , and the single crystalline silicon substrate  10  is maintained. 
     Optionally, the material stack including the single crystalline silicon substrate  10  and the stack of the germanium-containing layer  20  and the graded germanium-containing layer  22  may be maintained at an elevated temperature for a period of time to enhance the degree of epitaxial alignment between the single crystalline silicon substrate  10  and the stack of the germanium-containing layer  20  and the graded germanium-containing layer  22  and/or to cure crystalline defects in the stack of the germanium-containing layer  20  and the graded germanium-containing layer  22 . 
     Referring to  FIGS. 2C ,  2 D,  2 E, and  2 F, the processing steps of  FIGS. 1C ,  1 D,  1 E, and  1 F are performed as in the first embodiment. At the processing step of  FIG. 2C , the thickness of the at least partially crystalline silicon layer  30  is selected to apply sufficient stress to the germanium-containing layer  20  to cause formation of a plurality of cavities  27  within the germanium-containing layer  20  by the end of deposition of the at least partially crystalline silicon layer  30 . Depending on the relative germanium concentrations in the germanium-containing layer  20  and in the graded germanium-containing layer  22  and the relative thicknesses of the germanium-containing layer  20  and in the graded germanium-containing layer  22 , the plurality of cavities  27  may be formed solely within the germanium-containing layer  20  or across the germanium-containing layer  20  and the graded germanium-containing layer  22 . 
     The plurality of cavities  27  is formed during the growth of the at least partially crystalline silicon layer  30 , i.e., before the completion of deposition of the silicon material of the at least partially crystalline silicon layer  30 . The lateral dimensions of the plurality of cavities  27  is on the same order of magnitude as the combined thickness of the germanium-containing layer  20  and the graded germanium-containing layer  22 . Typically, each cavity in the plurality of cavities has a maximum lateral dimension less than 200 nm. While the cleavage plane in  FIG. 2E  is illustrated as passing only through the germanium-containing layer  20 , it is understood that the cleavage plane may pass through the graded germanium-containing layer  22  in some of the cases where the plurality of cavities  27  is formed across the germanium-containing layer  20  and the graded germanium-containing layer  22 . Remnants of the germanium-containing layer  20  and the graded germanium-containing layer  22  are removed at the processing steps of  FIG. 2F  employing the same methods as in the first embodiment. 
     Referring to  FIGS. 3A ,  3 B, and  3 C, a third exemplary semiconductor structure according to a third embodiment of the present disclosure is the same as the first exemplary semiconductor structure as shown in  FIGS. 1A ,  1 B, and  1 C, respectively, and can be formed by employing the same processing steps. 
     Referring to  FIG. 3D , a bonding material layer  42  may be employed to bond the at least partially crystalline silicon layer  30  to the handle substrate  40 . The bonding material layer  42  can be a semiconductor oxide layer form on the handle substrate  10  if the handle substrate includes a semiconductor material that can be converted into a semiconductor oxide such as silicon oxide or germanium oxide. Alternately, the bonding material layer  42  can be an adhesive layer that includes resin, polymer, epoxy, or any other material that can be employed as an adhesive. 
     Referring to  FIGS. 3E and 3F , the processing steps of  FIGS. 1E and 1F  are performed as in the first embodiment. 
     Referring to  FIGS. 4A ,  4 B, and  4 C, a fourth exemplary semiconductor structure according to a fourth embodiment of the present disclosure is the same as the second exemplary semiconductor structure as shown in  FIGS. 2A ,  2 B, and  2 C, respectively, and can be formed by employing the same processing steps. 
     Referring to  FIGS. 4C ,  4 D, and  4 F, the fourth exemplary semiconductor structure according to the fourth embodiment of the present disclosure can be formed employing the same processing steps as shown in  FIGS. 3A ,  3 B, and  3 C, respectively, and can be formed by employing the same processing steps. 
     Referring to  FIG. 5 , a graph of an X-ray diffraction data from a first sample according to the first embodiment of the present disclosure is illustrated as a function of 2θ. Specifically, the first sample includes a stack of a handle substrate  10  and a at least partially crystalline silicon layer  30  as illustrated in  FIG. 1F . The handle substrate  10  had an amorphous material (and thereby not contributing any sharp peak to the X-ray diffraction data), and the at least partially crystalline silicon layer  30  had a thickness of 300 nm. To manufacture the first sample, a germanium-containing layer  20  (see  FIGS. 1B-1D ) consisting essentially of germanium and oxygen and having a thickness of 30 nm was employed. The single peak at  20  of approximately 69 degrees indicates that the at least partially crystalline silicon layer  30  within the first sample is a single crystalline silicon layer having a (100) surface orientation or a textured crystalline layer in which a predominant portion of grains is aligned along a (100) surface orientation and has a high quality of crystallinity. 
     Referring to  FIG. 6 , a graph of an X-ray diffraction data from a second sample according to the second embodiment of the present disclosure is illustrated as a function of 2θ. Specifically, the second sample includes a stack of a handle substrate  10  and an at least partially crystalline silicon layer  30  as illustrated in  FIG. 1F . The handle substrate  10  had an amorphous material (and thereby not contributing any sharp peak to the X-ray diffraction data), and the at least partially crystalline silicon layer  30  had a thickness of 120 nm. To manufacture the second sample, a stack of an germanium-containing layer  20  and a graded germanium-containing layer  22  (See  FIGS. 2B-2D ) was employed. The germanium-containing layer  20  consisted essentially of germanium and oxygen and had a thickness of 30 nm. The graded germanium-containing layer  22  consisted essentially of germanium, silicon, and oxygen, and had a thickness of 50 nm. The single peak at  20  of approximately 69 degrees suggests that the at least partially crystalline silicon layer  30  within the second sample is a single crystalline silicon layer having a (100) surface orientation or a textured crystalline layer in which a predominant portion of grains is aligned along a (100) surface orientation and has an even higher quality of crystallinity than the first sample of  FIG. 5 . 
     For illustrative purposes, an X-ray diffraction data from a third sample including a polysilicon film as a function of 2θ is shown in  FIG. 7 . The polysilicon film of the third sample provided multiple peaks corresponding to various crystallographic orientations in the grains of the polysilicon film. 
     Referring to  FIG. 8 , a graph illustrates data from a secondary ion mass spectroscopy (SIMS) run on the second sample of  FIG. 6  prior to cleavage, i.e., the second sample according to the second embodiment of the present disclosure at the step of  FIG. 2C . The atomic concentrations of germanium, silicon, and oxygen are shown by three curves labeled “Ge,” “Si,” and “O,” respectively. The graph in  FIG. 8  is divided into portions corresponding to the various structural elements in the first exemplary semiconductor structure of  FIG. 2C , and each portion is labeled with the corresponding reference numeral in the first exemplary semiconductor structure of  FIG. 2C . 
     Referring to  FIG. 9 , a scanning electron micrograph (SEM) is shown of a broken portion of the first sample along a vertical cleavage plane at a step corresponding to  FIG. 1C , i.e., prior to attaching a handle substrate  10  or cleaving along the oxygen-containing germanium layer  20 . This SEM shows, from bottom to top, a vertical surface of a single crystalline silicon substrate  10  as cleaved for SEM preparation, a vertical surface of an oxygen-containing germanium layer  20  as cleaved for SEM preparation, and a vertical surface of an at least partially crystalline silicon layer  30  as cleaved for SEM preparation, and a top surface of the at least partially crystalline silicon layer  30 . A plurality of cavities is shown in the oxygen-containing germanium layer  20  in dark color. 
     Referring to  FIG. 10 , a scanning electron micrograph (SEM) is shown of a portion of the first sample, which was cleaved along a plane in the oxygen-containing germanium layer  20  at a step corresponding to  FIG. 1E . This portion of the first sample includes a bulk silicon substrate  10  and a portion of the oxygen-containing germanium layer  22 , i.e., a lower germanium-containing portion  27 A. This SEM shows, from bottom to top, a vertical surface of the single crystalline silicon substrate  10  as cleaved for SEM preparation, a vertical surface of the lower germanium-containing portion  27 A as cleaved for SEM preparation, and a top surface of the lower germanium-containing portion  27 A. A plurality of cavities is shown on the top surface of the lower germanium-containing portion  27 A in dark color. 
     While the present disclosure has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details can be made without departing from the spirit and scope of the present disclosure. It is therefore intended that the present disclosure not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.