Patent Publication Number: US-2013247982-A1

Title: Solar cell

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2012-0029554 filed in the Korean Intellectual Property Office on Mar. 22, 2012, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Example embodiments provide a solar cell. 
     2. Description of the Related Art 
     Primary energy sources currently used for humankind are fossil fuels, e.g., coals and petroleum. However, fossil fuels are being exhausted and cause global warming and environmental pollution. Solar light, tidal power, wind power, and/or geothermal heat are being studied as an alternative energy source for replacing fossil fuel. 
     Among them, a solar cell technology of converting solar light into electricity utilizes a material that produces holes and electrons to generate currents upon receipt of light. However, the efficiency of the solar cell may be insufficient due to the hole-electron recombination at a surface of a layer or a film in the solar cell. 
     SUMMARY 
     According to example embodiments, a solar cell may include a PN junction including a semiconductor substrate of a first conductivity and an emitter of a second conductivity, a passivation layer on an exposed surface of the semiconductor substrate, a first electrode connected to the semiconductor substrate, and a second electrode connected to the emitter. The passivation layer may be configured to apply stress to the exposed surface of the substrate such that a mobility of minority charge carriers in the semiconductor substrate is decreased in a first direction perpendicular to a boundary surface of the semiconductor substrate and the passivation layer. 
     The first conductivity may be P-type, the second conductivity is N-type, and the passivation layer is configured to apply a tensile stress in the first direction to the exposed surface of the semiconductor substrate. The passivation layer may be configured to absorb a compressive stress in the first direction. The tensile stress in the first direction applied to the exposed surface of the semiconductor substrate by the passivation layer may have a value equal to or less than −800 MPa. 
     The passivation layer may include at least one of a thermal oxide, an oxide formed by plasma enhanced chemical vapor deposition (PECVD), a SiN layer formed by low pressure chemical vapor deposition (LPCVD), and a SiN layer formed by PECVD. 
     The first conductivity may be N-type, the second conductivity may be P-type, and the passivation layer may be configured to apply a compressive stress in the first direction to the exposed surface of the semiconductor substrate. The passivation layer may be configured to absorb a tensile stress in the first direction. The compressive stress in the first direction applied to the exposed surface of the semiconductor substrate by the passivation layer may have a value equal to or less than 4,000 MPa. 
     The passivation layer includes at least one of an Al 2 O 3  layer formed by atomic layer deposition (ALD) and a SiN layer formed by plasma enhanced chemical vapor deposition(PECVD). 
     The substrate may include single crystalline silicon, and the passivation layer may include at least one of an oxide, a nitride, amorphous silicon, ZnS and Mg F 2 . 
     An anti-reflection coating may be on an exposed surface of the emitter. The first electrode and the second electrode may be on opposite sides of the PN junction. The first electrode and the second electrode may be on a same side of the PN junction. 
     The anti-reflection coating may include at least one of MgF 2 , ZnS, SiN x , SiO 2 , and Al 2 O 3 . The first electrode and the second electrode may include at least one of a metal and a transparent conducting oxide (TOO). The metal may include one of Al, Ag, Au, and Cu. The thicknesses of the semiconductor substrate, the emitter, the passivation layer, and the anti-reflection coating may be about 1 um to about 500um, about 0.1um to about 10um, about 5 nm to about 500 nm, and about 5 nm to about 500 nm, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.  FIGS. 1-9  represent non-limiting, example embodiments as described herein. 
         FIG. 1  is a schematic sectional view of a solar cell according to example embodiments. 
         FIGS. 2 and 3  are perspective views schematically showing stress applied on a passivation layer of a solar cell according to example embodiments. 
         FIGS. 4 and 5  are perspective views schematically showing stress applied on a substrate and a passivation layer of a solar cell according to example embodiments. 
         FIG. 6  is a graph showing the efficiency of a solar cell including a P-type substrate as function of strength of a vertical stress on the substrate. 
         FIG. 7  is a graph showing the efficiency of a solar cell including an N-type substrate as function of strength of a vertical stress on the substrate. 
         FIGS. 8 and 9  are schematic sectional views of solar cells according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will be described more fully hereinafter with reference to the accompanying drawings. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope. In the drawings, parts having no relationship with the explanation are omitted for clarity, and the same or similar reference numerals designate the same or similar elements throughout the specification. 
     It will be understood that, although the terms “first”, “second”, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of example embodiments. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. 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”, “comprising”, “includes” and/or “including,” if used herein, 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. 
     Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, such as those defined in commonly-used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     A solar cell according to example embodiments is described in detail with reference to  FIGS. 1 to 5 .  FIG. 1  is a schematic sectional view of a solar cell according to example embodiments,  FIGS. 2 and 3  are perspective views schematically showing stress applied on a passivation layer of a solar cell according to example embodiments, and  FIGS. 4 and 5  are perspective views schematically showing stress applied on a substrate and a passivation layer of a solar cell according to example embodiments. 
     Referring to  FIG. 1 , a solar cell  100  according to example embodiments includes a semiconductor substrate  110 , an emitter  120 , a passivation layer  140 , an anti-reflection coating  150 , a substrate electrode  160 , and an emitter electrode  170 . 
     The semiconductor substrate  110  and the emitter  120  have opposite conductivities and are in contact with each other to form a PN junction  130 . For example, when the semiconductor substrate  110  is P-type, the emitter  120  is N-type. On the contrary, when the semiconductor substrate  110  is N-type, the emitter  120  is P-type. 
     The semiconductor substrate  110  may be a single crystalline silicon substrate, and the emitter  120  may be formed by implanting an impurity into the substrate  110  having a conductivity opposite a conductivity of the substrate  110 . 
     The passivation layer  140  contacts a substrate-side surface of the PN junction  130  and covers a surface of the substrate  110  to be protected. The passivation layer  140  is configured to suffer compressive or tensile stress in a direction (referred to as “a horizontal direction” hereinafter) substantially parallel to a boundary surface between the substrate  110  and the passivation layer  140 . 
     When a type of stress in the horizontal direction is applied to the passivation layer  140 , an opposite type of stress may be applied in a direction (referred to as “a vertical direction” hereinafter) substantially perpendicular to the boundary surface between the substrate  110  and the passivation layer. 
     Referring to  FIG. 2 , a tensile stress applied on the passivation layer  140  in the horizontal direction may cause a compressive stress applied on the passivation layer in the vertical direction, and, on the contrary, a compressive stress in the vertical direction may cause a tensile stress in the horizontal direction. Referring to  FIG. 3 , a compressive stress applied on the passivation layer  140  in the horizontal direction may cause a tensile stress applied on the passivation layer in the vertical direction, and, on the contrary, a tensile stress in the vertical direction may cause a compressive stress in the horizontal direction. Accordingly, the passivation layer  140  may be configured to suffer a tensile stress in the vertical direction, or a compressive stress in the horizontal direction. 
     The type of the stress on the passivation layer  140  may depend on the conductivity of the substrate  110 . For example, a vertical stress on the passivation layer  140  may be compressive for the substrate  110  with a P-type conductivity, while a vertical stress on the passivation layer  140  may be tensile for the substrate  110  with an N-type conductivity. 
     Examples of the passivation layer  140  may include oxides, e.g., SiO 2  and Al 2 O 3 , nitrides, e.g., SiN x , amorphous silicon, ZnS, MgF 2 , or combinations thereof. When the passivation layer  140  includes a nitride, ZnS, or MgF 2 , the passivation layer  140  may prevent or inhibit light reflection as well as protect the surface of the substrate  110 . 
     The anti-reflection coating  150  may be disposed on an emitter-side surface of the PN junction  130  and may prevent or inhibit the reflection of incident light to improve the efficiency of the solar cell  100 . The anti-reflection coating  150  may also cover and protect the emitter-side surface of the PN junction  130 . Examples of the anti-reflection coating  150  may include at least one of MgF 2 , ZnS, SiN x , SiO 2 , and Al 2 O 3 . The anti-reflection coating  150  may be omitted. 
     The substrate electrode  160  may be connected to the substrate  110  through a contact hole in the passivation layer  140 , and the emitter electrode  170  may be connected to the emitter  120  through a contact hole in the anti-reflection coating  150 . Materials for the electrodes  160  and  170  may include, for example, at least one of metals, e.g., Al, Ag, Au, and Cu or at least one of transparent conducting oxides (TCO). 
     The thicknesses of the substrate  110 , the emitter  120 , the passivation layer  140 , and the anti-reflection coating  150  may be about  1  um to about 500 um, about 0.1 um to about 10 um, about 5 nm to about 500 nm, and about 5 nm to about 500 nm, respectively. 
     When the solar cell  100  receives solar light, charge carriers, e.g., holes and electrons, may be produced in the PN junction  130  and move to the electrodes  160  and  170  such that currents flow outward through the electrodes  160  and  170 . 
     The charge carriers, e.g., majority charge carriers and/or minority charge carriers, produced in the substrate  110  may gather near the boundary surface between the substrate  110  and the passivation layer  140 . Therefore, the gathered charge carriers may recombine with each other near the boundary surface between the substrate  110  and the passivation layer  140 , and their recombination may decrease the efficiency of the solar cell  100 . The majority charge carriers are holes and the minority charge carriers are electrons when the substrate  110  has a P-type conductivity, and vice versa for the substrate  110  with an N-type conductivity. 
     According to example embodiments, the passivation layer  140 , which may suffer stress by itself, may apply vertical and horizontal stress on at least a portion of the substrate  110 , for example, a portion disposed near the boundary surface between the substrate  110  and the passivation layer  140 . The stress suffered by the substrate  110  may be a type opposite to a type of stress suffered by the passivation layer  140 . 
     As described above, the passivation layer  140  may be configured to suffer a tensile stress in the horizontal direction when the substrate  110  has a P-type conductivity, for example. Referring to  FIG. 4 , when a tensile stress in the horizontal direction is applied on the passivation layer  140 , a tensile stress in the vertical direction and a compressive stress in the horizontal direction may be applied on the substrate  110 . A vertical tensile stress applied to the substrate  110  with a P-type conductivity may cause decrease of electron mobility and increase of hole mobility in the vertical direction. The number of the minority charge carriers, e.g., the number of electrons that reach the boundary surface of the substrate  110  and the passivation layer  140 , may be reduced so that the hole-electron recombination may be decreased. 
     On the contrary, when the substrate  110  has an N-type conductivity, the passivation layer  140  may be configured to suffer a horizontal compressive stress. Referring to  FIG. 5 , the stresses applied to the substrate  110  with the N-type conductivity may be tensile in the horizontal direction and may be compressive in the vertical direction. In example embodiments, because hole mobility may be decreased and electron mobility may be increased in the vertical direction in the substrate  110  with the N-type conductivity, the number of the minority charge carriers, e.g., the number of holes that reach the boundary surface of the substrate  110  and the passivation layer  140 , and thus the hole-electron recombination, may be decreased. 
     As described above, example embodiments provide the passivation layer  140  that is configured to differentiate the type of stress depending on the conductivity of the substrate  110 . Accordingly, the number of the minority charge carriers that reach the boundary surface of the substrate  110  and the passivation layer  140  may be reduced, and thus, the recombination of the majority charge carriers and the minority charge carriers may be reduced to increase the efficiency of the solar cell  100 . 
     In order to obtain a desired type of stress applied to the passivation layer  140 , the size of the lattice of the passivation layer  140  may be adjusted. For example, when the lattice of the passivation layer  140  is larger than the lattice of the substrate  110 , a vertical stress applied to the substrate  110  by the passivation layer  140  may be compressive. On the contrary, the passivation layer  140  with the lattice smaller than the lattice of the substrate  110  may apply a tensile vertical stress to the substrate  110 . 
     When the passivation layer  140  is an oxide formed by thermal oxidation or plasma enhanced chemical vapor deposition (PECVD), a horizontal stress suffered by the passivation layer  140  may be a compressive stress of about 350 MPa to about 400 MPa. When the passivation layer  140  is a SiN layer formed by low pressure chemical vapor deposition (LPCVD), a horizontal stress suffered by the passivation layer  140  may be a compressive stress of about 700 MPa to about 1200 MPa. In contrast, when the passivation layer  140  is a SiN layer formed by PECVD, a horizontal stress suffered by the passivation layer  140  may have a strength of about −300 MPa to about 850 MPa, and may be tensile or compressive. When the passivation layer  140  is an Al 2 O 3  layer formed by atomic layer deposition (ALD), the stress suffered by the passivation layer  140  may be a tensile stress of about −300 MPa to about −1.3 GPa. 
     The strength of the stress suffered by the passivation layer  140 , or the strength of the stress applied to the substrate  110  by the passivation layer  140  may vary depending on a process condition. The strength of the stress may become as high as the deposition temperature is low. As the heat treatment is performed, the film thickness may become larger. When the passivation layer  140  is formed by depositing SiN using PECVD, the stress strength may vary depending on one of temperature and RF frequency. Thus, the passivation layer  140  with a desired type of stress may be obtained by adjusting one of temperature and RF frequency. 
     Characteristics of an experimental solar cell are described in detail with reference to  FIGS. 6 and 7 . 
       FIG. 6  is a graph showing the efficiency of a solar cell including a P-type substrate as function of strength of a vertical stress on the substrate, and  FIG. 7  is a graph showing the efficiency of a solar cell including an N-type substrate as function of strength of a vertical stress on the substrate. 
     In a solar cell  100  having a structure shown in  FIG. 1 , a stress that a passivation layer  140  applies to a surface of a substrate  110  was varied, and short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and efficiency (Eff) were calculated by using Synopsys Technology Computer-Aided Design (TCAD) computer simulation. 
     First, a result of a simulation for a solar cell  100  with a P-type substrate  110  and an N-type emitter  120  is shown in Table 1 and  FIG. 6 . 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Vertical Stress 
                 Horizontal Stress 
                   
                   
                   
                   
               
               
                 (MPa) 
                 (MPA) 
                 Jsc 
                 Voc 
                 FF 
                 Eff 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1000 
                 −500 
                 40.25 
                 0.629 
                 82.64 
                 20.93 
               
               
                 800 
                 −400 
                 40.37 
                 0.631 
                 82.65 
                 21.06 
               
               
                 600 
                 −300 
                 40.51 
                 0.633 
                 82.71 
                 21.21 
               
               
                 400 
                 −200 
                 40.67 
                 0.636 
                 82.75 
                 21.39 
               
               
                 200 
                 −100 
                 40.85 
                 0.639 
                 82.82 
                 21.61 
               
               
                 0 
                 0 
                 41.06 
                 0.643 
                 82.89  
                 21.88 
               
               
                 −200 
                 100 
                 41.31 
                 0.649 
                 83.01 
                 22.26 
               
               
                 −400 
                 200 
                 41.62 
                 0.660 
                 83.17 
                 22.83 
               
               
                 −600 
                 300 
                 42.01 
                 0.685 
                 83.49 
                 24.01 
               
               
                 −800 
                 400 
                 42.08 
                 0.699 
                 83.49 
                 24.55 
               
               
                 −1000 
                 500 
                 42.08 
                 0.699 
                 83.49  
                 24.55 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1 and  FIG. 6 , when the substrate  110  has a P-type conductivity, the solar cell  100  without stress on the substrate  110  shows an efficiency of about 21.88%. The efficiency Eff becomes higher as the vertical stress on the substrate  110  becomes more tensile. For example, the efficiency EFF for the stress equal to or smaller than about −800 MPa is about 24.55%, which is higher by about 2.67% compared with the case without stress. On the contrary, the efficiency Eff becomes lower as the vertical stress on the substrate  110  is more compressive. 
     A result of a simulation for a solar cell  100  with an N-type substrate  110  and a P-type emitter  120  is shown in Table 2 and  FIG. 7 . 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Vertical Stress  
                 Horizontal Stress 
                   
                   
                   
                   
               
               
                 (MPa) 
                 (MPa) 
                 Jsc 
                 Voc 
                 FF 
                 Eff 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 4000 
                 −2000 
                 40.76 
                 0.661 
                 82.81 
                 22.31 
               
               
                 2000 
                 −1000 
                 40.59 
                 0.659 
                 82.76 
                 22.12 
               
               
                 1000 
                 −500 
                 40.52 
                 0.657 
                 82.74 
                 22.04 
               
               
                 600 
                 −300 
                 40.49 
                 0.657 
                 82.73 
                 22.01 
               
               
                 200 
                 −100 
                 40.46 
                 0.657 
                 82.72 
                 21.97 
               
               
                 0 
                 0 
                 40.44 
                 0.656 
                 82.71 
                 21.96 
               
               
                 −200 
                 100 
                 40.43 
                 0.656 
                 82.71 
                 21.95 
               
               
                 −600 
                 300 
                 40.40 
                 0.656 
                 82.71 
                 21.92 
               
               
                 −1000 
                 500 
                 40.70 
                 0.648 
                 82.23 
                 21.68 
               
               
                 −2000 
                 1000 
                 40.64 
                 0.647 
                 82.21 
                 21.61 
               
               
                 −4000 
                 2000 
                 40.53 
                 0.645 
                 82.22 
                 21.5  
               
               
                   
               
            
           
         
       
     
     Referring to Table 2 and  FIG. 7 , when the substrate  110  has an N-type conductivity, the solar cell  100  without stress on the substrate  110  shows an efficiency of about 21.96%. The efficiency Eff becomes higher as the vertical stress on the substrate  110  becomes more compressive. For example, the efficiency EFF for the stress equal to or smaller than about 4000 MPa is about 22.31%. On the contrary, the efficiency Eff becomes lower as the vertical stress on the substrate  110  is more tensile. 
       FIGS. 8 and 9  are schematic sectional views of solar cells according to example embodiments. A solar cell  200  shown in  FIG. 8  includes a PN junction  230 , a passivation layer  240 , an anti-reflection coating  250 , a substrate electrode  260 , and an emitter electrode  270 . The detailed descriptions of each portion of the solar cell  200  may be omitted because the structure of the solar cell  200  is similar to the solar cell  100  shown in  FIG. 1 , and features of the solar cell  200  distinguished from the solar cell  100  shown in  FIG. 1  are mainly described. 
     The PN junction  230  includes a substrate  210  and an emitter  220  that have different conductivities, like  FIG. 1 . However, the emitter  220  is disposed under the substrate  210  according to example embodiments as illustrated in  FIG. 8  while the emitter  120  is disposed on the substrate  110  in  FIG. 1 . 
     The vertical positional relationship of the substrate  210  and the emitter  220  in the solar cell  200  shown in  FIG. 8 , which is the reverse of the vertical positional relationship of the substrate  110  and the emitter  120  in the solar cell  100  shown in  FIG. 1 , causes a reversed positional relationship between the passivation layer  240  and the anti-reflection coating  250  in the solar cell  200  shown in  FIG. 8  compared with the solar cell  100  shown in  FIG. 1 . In detail, the passivation layer  240  is disposed on a top surface of the PN junction  230 , and the anti-reflection coating  250  is disposed under a bottom surface of the PN junction  230  in example embodiments as shown in  FIG. 8 . 
     In addition, the substrate electrode  260  and the emitter electrode  270  in  FIG. 8  are disposed at the same side of the PN junction  230  while the substrate electrode  160  and the emitter electrode  170  are disposed opposite each other as shown in  FIG. 1 . In order to obtain the structure where the electrodes  260  and  270  are at the same side, both the substrate  210  and the emitter  220  may be exposed at a surface of the PN junction  230 . In detail, a portion of a bottom surface of the substrate  210  is covered by the emitter  220  and another portion of the bottom surface of the substrate  210  is not covered by the emitter  220  to be exposed in example embodiments as shown in  FIG. 8  while the emitter  120  covers an entire surface of the substrate  110  as shown in  FIG. 1 . 
     When the electrodes  260  and  270  are disposed under the PN junction  230  as shown in  FIG. 8 , the area exposed to incident light is relatively wide, and thus the efficiency may be relatively high. 
     A solar cell  300  shown in  FIG. 9  includes a PN junction  330 , a passivation layer  340 , an anti-reflection coating  350 , a substrate electrode  360 , and an emitter electrode  370 . The detailed descriptions of each portion of the solar cell  300  may be omitted since the structure of the solar cell  300  is similar to the solar cell  100  shown in  FIG. 1 , and features of the solar cell  300  distinguished from the solar cell  100  shown in  FIG. 1  are mainly described. 
     The PN junction  330  includes a substrate  310  and an emitter  320  that have different conductivities, and the emitter  320  is disposed on the substrate  310 , similar to example embodiments as shown in  FIG. 1 . However, both the substrate electrode  360  and the emitter electrode  370  in  FIG. 8  are disposed on the top surface of the PN junction  330  while the substrate electrode  160  and the emitter electrode  170  are disposed opposite each other as shown in  FIG. 1 , and both the substrate  310  and the emitter  320  are exposed at the top surface of the PN junction  330 . In detail, a portion of a top surface of the substrate  310  is covered by the emitter  320  and another portion of the top surface of the substrate  310  is not covered by the emitter  320  to be exposed in example embodiments as shown in  FIG. 9  while the emitter  120  covers an entire surface of the substrate  110  as shown in  FIG. 1 . 
     While this disclosure has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.