Abstract:
A method for layer transfer using a boron-doped silicon germanium (SiGe) layer includes forming a boron-doped SiGe layer on a bulk silicon substrate; forming an upper silicon (Si) layer over the boron-doped SiGe layer; hydrogenating the boron-doped SiGe layer; bonding the upper Si layer to an alternate substrate; and propagating a fracture at an interface between the boron-doped SiGe layer and the bulk silicon substrate. A system for layer transfer using a boron-doped silicon germanium (SiGe) layer includes a bulk silicon substrate; a boron-doped SiGe layer formed on the bulk silicon substrate, such that the boron-doped SiGe layer is located underneath an upper silicon (Si) layer, wherein the boron-doped SiGe layer is configured to propagate a fracture at an interface between the boron-doped SiGe layer and the bulk silicon substrate after hydrogenation of the boron-doped SiGe layer; and an alternate substrate bonded to the upper Si layer.

Description:
BACKGROUND 
     This disclosure relates generally to the field of layer transfer of a thin layer of silicon. 
     As the demand for alternative forms of energy increases, higher demands are placed on the conversion efficiency of photovoltaic cells such as solar cells. Simultaneously, the cost of such cells is expected to decrease. For silicon (Si) based solar cell technology, the trade-off between cell fabrication cost and conversion efficiency is directly related to the quality of the silicon used to fabricate the solar cell. Single-crystal (SC) silicon cells have relatively high efficiency, but also high cost. To form a single-crystal silicon-based solar cell, a layer of thin (&lt;50 micrometers, or μm) silicon may be formed on an alternate substrate such as glass, ceramic or metal by a process referred to as layer transfer. However, successful layer transfer of a single-crystal layer of silicon may require the use of relatively expensive equipment and techniques. 
     SUMMARY 
     An exemplary embodiment of a method for layer transfer using a boron-doped silicon germanium (SiGe) layer includes forming a boron-doped SiGe layer on a bulk silicon substrate; forming an upper silicon (Si) layer over the boron-doped SiGe layer; hydrogenating the boron-doped SiGe layer; bonding the upper Si layer to an alternate substrate; and propagating a fracture at an interface between the boron-doped SiGe layer and the bulk silicon substrate. 
     An exemplary embodiment of a system for layer transfer using a boron-doped silicon germanium (SiGe) layer includes a bulk silicon substrate; a boron-doped SiGe layer formed on the bulk silicon substrate, such that the boron-doped SiGe layer is located underneath an upper silicon (Si) layer, wherein the boron-doped SiGe layer is configured to propagate a fracture at an interface between the boron-doped SiGe layer and the bulk silicon substrate after hydrogenation of the boron-doped SiGe layer; and an alternate substrate bonded to the upper Si layer. 
     Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  illustrates an embodiment of a method for layer transfer using a boron-doped SiGe layer. 
         FIG. 2  illustrates an embodiment a boron-doped SiGe layer formed on a bulk silicon substrate. 
         FIG. 3  illustrates an embodiment of a boron-doped SiGe layer formed on a bulk silicon substrate after bonding an alternate substrate to an upper silicon layer. 
         FIG. 4  illustrates an embodiment of a fracture propagated at an interface between the boron-doped SiGe layer and the bulk silicon substrate. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of systems and methods for layer transfer using a boron-doped silicon germanium (SiGe) layer are provided, with exemplary embodiments being discussed below in detail. Addition of boron (B) to the SiGe layer allows a thin upper layer of silicon, which may comprise a single-crystal layer of silicon, to be transferred from a bulk silicon substrate onto an alternate substrate, such as glass, ceramic, plastic, or metal, using relatively low-cost processing steps and equipment. The boron-doped SiGe layer is hydrogenated to facilitate mechanical fracture propagation in the doped SiGe layer. The boron doping of the SiGe layer significantly enhances the efficiency with which hydrogen is trapped at the interface between the doped SiGe and the bulk silicon substrate; this increased hydrogen trapping efficiency allows for successful fracture propagation at the interface and layer transfer of the single-crystal silicon to an alternate substrate. 
     The boron concentration in the boron-doped SiGe layer may be greater than about 10^19 B/cm 3 . The boron-doped SiGe may be separated from the bulk silicon at the SiGe/Bulk silicon interface by fracturing along the interface. The boron-doped SiGe layer may be fully strained and grown using any appropriate method, including but not limited to rapid thermal chemical vapor deposition (RTCVD) epitaxial growth, molecular beam epitaxy (MBE), or clean evaporation (or any physical vapor deposition, or PVD) followed by solid-phase epitaxial growth (SPE). 
       FIG. 1  illustrates an embodiment of a method  100  of layer transfer using a boron-doped SiGe layer.  FIG. 1  is discussed with reference to  FIGS. 2-4 . In block  101 , as shown in wafer  200  of  FIG. 2 , a boron-doped SiGe layer  202  is formed in a bulk silicon substrate  201  below an upper silicon layer  203 . Upper silicon layer  203  may be less than about 50 μm thick in some embodiments, and may comprise a single crystal layer in some embodiments. Upper silicon layer  203  may further comprise additional layers, materials, and/or devices, including but not limited to p-n diode structures in some embodiments. Interface  204  separates SiGe layer  202  and bulk silicon substrate  201 . 
     The doping of the SiGe layer  202  may be achieved during the formation of the SiGe layer  202  using gaseous B-containing sources such as diborane, using solid and/or liquid sources such as B-doped Si or Ge during evaporation, or using B-metal in a Knudsen cell during MBE. Doping may also be achieved after a SiGe layer is formed using ion implantation techniques or B diffusion techniques (such as spin-on B-doped glass and annealing). The Ge content of the SiGe layer  202  may be between 5 and 50 atomic percent, and between 10 and 40 atomic percent in some embodiments. The thickness of the SiGe layer  202  may be between 1 and 1000 nm, and between 5 to 100 nm in some embodiments. The Ge content of the boron-doped SiGe layer  202  may not be constant throughout the SiGe layer in some embodiments. 
     The boron concentration in the doped SiGe layer  202  may be greater than about 10^19 B/cm 3 . The boron concentration may not be constant throughout the boron-doped SiGe layer  202  in some embodiments; it may be linearly graded, or spiked at an interface to encourage fracture propagation at the interface. The B-doped SiGe layer  202  may contain carbon (C) to control the amount or distribution of strain within the SiGe layer in some embodiments; the C content may be at or below 3%. 
     Formation of the doped SiGe layer  202  may be preceded by formation of an optional buffer layer in some embodiments. The buffer layer, which may be disposed between the bulk Si  201  and the doped SiGe layer  202 , may comprise Si, carbon-doped Si (Si:C), or undoped SiGe. The optional buffer layer allows the recovery of the bulk Si  201  after layer transfer of the upper silicon layer  203  through use of controlled selective etching techniques, allowing for Si substrate reuse. 
     In block  102 , boron-doped SiGe layer  301  is hydrogenated. Hydrogenation may be performed by exposing the wafer  200  to atomic hydrogen at a temperature of greater than 100° C. The atomic hydrogen may comprise direct current (DC) or radio frequency (RF) plasma, or energetic ions. In some embodiments, hydrogenation may be performed by exposure to 1000 W of RF hydrogen plasma for 30 minutes at 250° C. The hydrogen is trapped at interface  204 . The boron in doped SiGe layer  202  increases the amount of hydrogen trapped at interface  204  during hydrogenation of boron-doped SiGe layer  202 . The hydrogen may also be incorporated into the SiGe layer  202  through electrochemical means, which may use catalytic surface layers such as platinum (Pt) in some embodiments. 
     In block  103 , as shown in  FIG. 3 , upper silicon layer  203  is bonded to an alternate substrate  301 . Bonding may be performed by any appropriate method, including anodic, hydrophilic, or thermocompression bonding. Alternate substrate  301  may comprise any appropriate material, including but not limited to glass, ceramic, metal or plastic, or a combination thereof. The alternate substrate  301  may be chosen to have a coefficient of thermal expansion (CTE) that is the similar to the CTE of silicon, or may be selected such that the CTE of the alternate substrate  301  results in a predetermined residual strain in alternate substrate  301  after bonding and cooling. 
     In block  104 , as shown in  FIG. 4 , the bulk silicon substrate  201  is separated from the upper silicon layer  203  by propagating a fracture at interface  204 , resulting in break  401 . The hydrogen trapped at interface  204  promotes fracture propagation at interface  204  by weaking the local atomic bonding. An asymmetric concentration of Ge in boron-doped SiGe layer  202  may be used to restrict fracture to interface  204 ; or conversely, the Ge content (and optionally the B concentration) may be retrograded to promote fracture at the interface between SiGe layer  202  and upper silicon layer  203 . Fracture and separation may be performed by applying an external force, by an intrinsic force due to residual strain resulting from varying CTEs between the bulk silicon  201  and alternate substrate  301 , or by additional hydrogen loading, using plasma or acid. After upper silicon layer  203  is separated from bulk silicon substrate  201  by break  401 , upper silicon layer  203  may be used to form a high-efficiency solar cell. 
     In an exemplary embodiment, a SiGe layer having 24 atomic percent concentration of Ge that is grown to a thickness of about 20 nm on a bulk Si wafer using a single-wafer reduced pressure rapid thermal chemical vapor deposition (RTCVD) system is covered with a 70 nm thick Si capping layer that is grown on the SiGe layer. The SiGe layer may be doped during growth using B at a concentration of about 2×10^20 B/cm^3. For comparison purposes, a structure comprising the above-described doped SiGe layer and a second structure comprising an undoped SiGe layer of the same dimensions are exposed to RF hydrogen plasma at 250° C. for a period of 30 minutes, and both structures are then anodically bonded to glass substrates at a bonding temperature of about 350° C. The structure comprising the B doped SiGe layer demonstrates relatively large area transfer of the SiGe and upper Si layer upon initiating fracture between the glass substrate and the layer-containing Si substrate, whereas the structure comprising the undoped SiGe layer does not successfully fracture in a manner that allowing large-area layer transfer along the SiGe interface. Addition of raised Ge content, longer H exposure, or different fracture strategies also failed to produce large area layer transfer when the SiGe was undoped. 
     The technical effects and benefits of exemplary embodiments include relatively low-cost layer transfer of a single-crystal silicon layer onto a substrate. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.