Abstract:
The present invention discloses a light-emitting semiconductor device, includes: a first electrode that is made of a high reflective metal; a second electrode; a tunnel junction layer coupling to the first electrode through a first ohmic contact and generating a tunnel current by applying a reverse bias voltage between the first electrode and the second electrode; a light-emitting layer provided between the tunnel junction layer and the second electrode.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This Application claims the benefit of priority and is a Continuation application of the prior International Patent Application No. PCT/JP2007/060549, with an international filing date of Nov. 14, 2006, which designated the United States, and is related to the Japanese Patent Application No. 2006-142853, filed May 23, 2007, the entire disclosures of all applications are expressly incorporated by reference in their entirety herein. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a light-emitting semiconductor device. 
     (2) Description of the Related Art 
     Light-emitting diodes disclosed in, for example, Patent documents JP2000-182542 and U.S. Pat. No. 6,526,082 have a tunnel junction of GaN, and an anode placed in ohmic contact on the tunnel junction. 
     The tunnel junction enables light emission at a low voltage as well as formation of an anode and a cathode made of the same material. Therefore those electrodes can be efficiently manufactured. Electrodes mentioned in JP2000-182542 and capable of satisfactorily coming into ohmic contact with the GaN tunnel junction are made of AuGe/Ni. 
     In the light-emitting diode, the electrode is used as a reflecting surface to guide light in a light-emitting direction and to emit the light reflected by the electrode efficiently. Although light can be emitted at a high efficiency when the electrode has a higher reflectivity, the AuGe/Ni cannot form an electrode having a high reflectivity. Thus, the light-emitting diode provided with electrodes of AuGe/Ni cannot emit light at a high efficiency. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention discloses a light-emitting semiconductor device, includes: a first electrode that is made of a high reflective metal; a second electrode; a tunnel junction layer coupling to the first electrode through a first ohmic contact and generating a tunnel current by applying a reverse bias voltage between the first electrode and the second electrode; a light-emitting layer provided between the tunnel junction layer and the second electrode. 
     These and other features, aspects, and advantages of invention will be apparent to those skilled in the art from the following detailed description of preferred non-limiting exemplary embodiments, taken together with the drawings and the claims that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       It is to be understood that the drawings are to be used for the purposes of exemplary illustration only and the drawings are to be used not as a definition of the limits of the invention. Throughout the disclosure, the word “exemplary” is used exclusively to mean “serving as an example, instance, or illustration.” Any embodiment described as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
         FIG. 1  is an exemplary typical side elevation of a daylight light-emitting diode. 
         FIG. 2  is an exemplary typical side elevation of an electrode. 
         FIG. 3  is an exemplary typical side elevation of a daylight light-emitting diode. 
         FIG. 4  is an exemplary typical side elevation of a daylight light-emitting diode. 
         FIG. 5  is an exemplary typical side elevation of a daylight light-emitting diode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of presently preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be constructed and or utilized. 
     Preferred embodiments of the present invention will be described in the following order. 
     (1) First Embodiment 
     (2) Second Embodiment 
     (3) Third Embodiment 
     (4) Conclusion 
     (1) First Embodiment 
       FIG. 1  is a typical view of a daylight light emitting diode, namely, a light-emitting semiconductor device in a first embodiment according to the present invention. Referring to  FIG. 1 , a daylight light-emitting diode  1  includes a fluorescent substrate  10 , a fluorescent layer  1 , a buffer layer  12 , an n-type contact layer  13 , a multiple-quantum-well active layer  14 , an electron blocking layer  15 , a p-type connecting layer  16 , a tunnel-junction layer  17 , an anode  18 , and a cathode  19 . The anode  18  and the cathode  19  are a first electrode and a second electrode, respectively, of the present invention. A substantially plate-shaped fluorescent substrate  10  forming the lowermost layer is a 6H-type SiC single crystal. The fluorescent substrate  10  contains B (boron), namely, an acceptor impurity, and N (nitrogen), namely, a donor impurity. The fluorescent substrate  10  has a B atom and an N atom concentration in the range of about 10 17  to about 10 19  cm −3 . 
     The fluorescent layer  11  is formed on the fluorescent substrate  10 . The fluorescent layer  11 , similarly to the fluorescent substrate  10 , is a 6H-type SiC single crystal and contains N, namely, a donor impurity, and Al (aluminum), namely, an acceptor impurity. The fluorescent layer  11  has a B atom and an Al atom concentration in the range of about 10 17  to about 10 19  cm −3 . The buffer layer  12  overlies the fluorescent layer  11  and is formed of AlGaN. The n-type contact layer  13  overlies the buffer layer  12  and is formed of n-GaN. The multiple-quantum-well active layer  14  overlies the n-type contact layer  13  and is formed of GaInN/GaN in multiple-quantum-well construction. The electron blocking layer  15  overlies the multiple-quantum-well active layer  14  and is formed of p-AlGaN. The multiple-quantum-well active layer  14  and the electron blocking layer  15  are light-emitting layers in the present invention. The p-type connecting layer  16  overlies the electron blocking layer  15  and is formed of p-GaN. 
     The tunnel junction layer  17  overlies the p-type connecting layer  16  and consists of a p-type semiconductor layer  17   a  and an n-type semiconductor layer  17   b . The p-type semiconductor layer  17   a  overlies the p-type connecting layer  16  and is formed of p + -GaN. The p-type semiconductor layer  17   a  contains Mg, namely, an acceptor impurity, in a concentration not lower than 10 19  cm −3 . Thus, the p-type semiconductor layer  17   a  has a high carrier concentration. The n-type semiconductor layer  17   b  overlies the p-type semiconductor layer  17   a , is formed of n + -GaInN, and contains Si, namely, a donor impurity, in a concentration not lower than 10 19  cm −3 . Thus, the n-type semiconductor layer  17   b  has a high carrier concentration. The n-type semiconductor layer  17   b  contains InN in a molar fraction of 25% and has a thickness of 20 nm. The anode  18  overlies the n-type semiconductor layer  17   b  of the tunnel-junction layer  17 . The cathode  19  is formed on the n-type contact layer  13 . The semiconductor layers  10  to  17  of the daylight light-emitting diode  1 , excluding the fluorescent substrate  10  and the fluorescent layer  11 , are formed of nitride semiconductors. 
       FIG. 2  typically shows the anode  18 . Since the cathode  19  is similar in construction to the anode  18 , the illustration and description thereof will be omitted. Referring to  FIG. 2 , the anode  18  consists of a Ag layer  18   a  of Ag, two protective layers  18   b   1  and  18   b   2  of Ti, and a Au layer  18   c  of Au (gold). First, the protective layer  18   b   1  is formed on the n-type semiconductor layer  17   b  and the Ag layer  18   a  is formed on the protective layer  18   b   1 . Then, the protective layer  18   b   2  and the Au layer  18   c  are formed in that order on the Ag layer  18   a . For example, the protective layer  18   b   1  is 2 nm in thickness, the Ag layer  18   a  is 100 nm in thickness, the protective layer  18   b   2  is 20 nm in thickness and the Au layer  18   c  is 300 nm in thickness. The Ag layer  18   a  does not necessarily need to be formed of only Ag. For example Cu and Pd may be added in contents of several percent to the Ag layer  18   a  to ensure the thermal and the chemical stability of the Ag layer  18   a . The thermal and the chemical stability of the Ag layer  18   a  can be ensured by adding Bi in several percent to the Ag layer  18   a.    
     A method of fabricating the daylight light-emitting diode  1  will be described. The fluorescent substrate  10  is formed by growing a single crystal of 6H-type SiC by a sublimation process while the crystal is being doped properly with B and N. Then, the fluorescent layer  11  is deposited on the fluorescent substrate  10  by step flow growing a single crystal of 6H-type SiC by a close sublimation process while the crystal is being properly doped with Al and N. Then, layers of AlGaN, GaN, GaInN/GaN, AlGaN, GaN and GaInN, namely, base materials, are grown sequentially by epitaxial grow by a close sublimation process while the layers are being doped properly with impurities specified respectively for the layers to superpose sequentially the buffer layer  12 , the n-type contact layer  13 , the multiple-quantum-well active layer  14 , the electron blocking layer  15 , the p-type connecting layer  17 , and the tunnel junction layer  17  in that order. The crystal growth for the n-type semiconductor layer  17   b  containing InN in a molar fraction of 25% can be achieved at a comparatively low temperature. 
     After the semiconductor layers  10  to  17  have been thus formed, an upper part of the n-type contact layer  13 , and parts of the multiple-quantum-well active layer  14 , the electron blocking layer  15 , the p-type connecting layer  16  and the tunnel junction layer  17  are removed by etching to expose a part of the upper surface of the n-type contact layer  13 . Then, the anode  18  and the cathode  19  are formed on the n-type semiconductor layer  17   b  of the tunnel junction layer  17  and the exposed part of the n-type contact layer  13 , respectively. Each of the anode  18  and the cathode  19  is formed by sequentially depositing Ti, Ag, Ti and Au layers in that order by, for example, an evaporation process. Needless to say, the anode  18  and the cathode  19  can be formed by a wet deposition process. 
     The daylight light-emitting diode  1  emits light when a forward bias voltage is applied across the anode  18  and the cathode  19  formed as described above. When the forward bias voltage is applied across the anode  18  and the cathode  19 , a reverse bias voltage is applied to the tunnel junction layer  17  consisting of the p-type semiconductor layer  17   a  and the n-type semiconductor layer  17   b  forming a p-n junction. Consequently, a depletion layer is formed in the vicinity of the interface between the n-type semiconductor layer  17   b  and the protective layer  18   b   1  in contact with the n-type semiconductor layer  17   b . Since the n-type semiconductor layer contains Si, namely, a donor impurity, in a high concentration not lower than 10 19  cm −3 , the depletion layer has a small thickness on the order of 10 nm smaller than the thickness of the n-type semiconductor layer  17   b.    
     The protective layer  18   b   1  of Ti can be brought substantially into ohmic contact with the n-type semiconductor layer  17   b  by tunnel effect. Since the n-type semiconductor layer  17   b  contains InN in a molar fraction of 25%, the n-type semiconductor layer  17   b  has a small bandgap on the order of 2.1 eV and a small strain owing to stress relaxation by low-temperature growth. Therefore, a tunnel current can be surely produced in the tunnel-junction layer  17  by a low voltage. Since Ti and Ag forming the anode  18  are metals having a small work function and the cathode  19  has a low resistance. Therefore, a tunnel current can be produced by a low voltage. Since the ohmic contact surface has a moderate resistance in the direction of the thickness, the tunnel junction layer  17  has a uniform current density with respect to the direction of the surface. Since a forward bias voltage is applied to the interface between the n-type contact layer  13  of n-GaN and the cathode  19 , a current flows through the n-type contact layer  13  and the cathode  19 . 
     A current can be made to flow from the anode  18  to the cathode  19  by applying the forward bias voltage across the anode  18  and the cathode  19 . Thus, a current flows through the multiple-quantum-well active layer  14  and the electron blocking layer  15  formed between the anode  18  and the cathode  19  and, consequently, the multiple-quantum-well active layer  14  can be made to emit light. The multiple-quantum-well active layer  14  emits, for example, near ultraviolet radiation of 385 nm in wavelength. The near ultraviolet radiation radiated from the multiple-quantum-well active layer  14  travels through the fluorescent substrate  10  and the fluorescent layer  11  doped with the acceptor impurity and the donor impurity, respectively, and is absorbed by the fluorescent substrate  10  and the fluorescent layer  11 . Donor electrons excited by the near ultraviolet radiation radiated from the multiple-quantum-well active layer  14  and acceptor holes recombine in the fluorescent substrate  10  and the fluorescent layer  11  to emit fluorescent light. 
     Both the base materials of the fluorescent substrate  10  and the fluorescent layer  11  are 6H-type SiC single crystals and basically have the same bandgap. However, the acceptor impurities added respectively to the fluorescent substrate  10  and the fluorescent layer  11  are different. That is, the fluorescent substrate  10  and the fluorescent layer  11  have different depths of acceptor level, respectively. Therefore, the fluorescent substrate  10  and the fluorescent layer  11  emit fluorescent light having a wavelength spectrum including wavelengths in different wavelength bands, respectively. The fluorescent substrate  10  containing boron as an acceptor impurity emits fluorescent light having a wide wavelength spectrum including wavelengths between green and red. The fluorescent  11  containing aluminum as an acceptor impurity emits fluorescent light having a wide wavelength spectrum including wavelengths between blue and green. The florescent light rays of those wavelengths are synthesized to produce white light satisfactory in color rendering. The white light is emitted outside through the lower surface of the fluorescent substrate  10  and is used for displaying and illumination. 
     The near ultraviolet radiation produced in the multiple-quantum-well active layer  14  travels downward or upward in the daylight light-emitting diode  1 . The near ultraviolet radiation traveled downward in the multiple-quantum-well active layer  14  reaches the fluorescent layer  11  and the fluorescent substrate  10  and causes the fluorescent layer  11  and the fluorescent substrate  10  to emit fluorescent light. On the other hand, the near ultraviolet radiation that has traveled upward in the multiple-quantum-well active layer  14  and reached the anode  18  is completely reflected downward from the anode  18  and causes the fluorescent layer  11  and the fluorescent substrate  10  emit fluorescent light. Since the anode  18  has the Ag layer  18   a  having a very high reflectivity, the near ultraviolet radiation that has reached the anode  18  can be reflected downward scarcely causing loss. Thus the near ultraviolet radiation produced by the multiple-quantum-well active layer  14  can be efficiently converted into fluorescent light. Interfaces between the adjacent ones of the layers respectively having different refractive indices of the daylight light-emitting diode  1  reflect the light and part of the light reaches the cathode  19  in some cases. The cathode  19  made of the same material as the anode  18  can surely reflect the light reached thereto downward. 
     The Ag layer  18   a  is sandwiched between the protective layers  18   b   1  and  18   b   2 . Thus the Ag layer  18   a  having a high reflectivity is coated with a Ti layer. The Ti layer is not porous like the Au layer and has packed construction. Therefore, oxygen that has permeated the Au layer  18   c  can be stopped by the protective layer  18   b   2  to prevent the oxygen from reaching the Ag layer  18   a . Thus, the oxidation of the Ag layer  18  can be prevented and the reduction of the reflectivity of the Ag layer  18   a  with time can be prevented. Consequently, the daylight light-emitting diode  1  can maintain high light-emitting efficiency for a long time. Since the protective layer  18   b   1  on which light falls has a thickness of 2 nm, light permeates the protective layer  18   b   1  and can be reflected by the Au layer  18   c . Since the Ag layer  18   a  has a thickness of 100 nm, light cannot permeate the Ag layer  18   a  and can be reflected at a high reflectivity. 
     Although the first embodiment employs the substrate of 6H-type SiC, a substrate of SiC of other polytype, such as a 4H-type, a 3C-type or a 15R-type, may be used. The use of a single-crystal substrate of, for example, sapphire is effective in achieving high efficiency and low voltage. The material of the anode  18  and the cathode  19  may be a material having a small work function and a high reflectivity other than Ag, such as Al. Although the first embodiment achieves light emission at a high efficiency by bonding the electron blocking layer  15  to the multiple-quantum-well active layer  14 , other light-emitting structure, such as a double heterostructure, may be used. 
     (2) Second Embodiment 
       FIG. 3  is a typical view of a daylight light emitting diode  2  in a second embodiment according to the present invention. Referring to  FIG. 3 , a daylight light-emitting diode  2  includes a fluorescent substrate  110 , a fluorescent layer  111 , a buffer layer  112 , an n-type contact layer  113 , a multiple-quantum-well active layer  114 , an electron blocking layer  115 , a p-type connecting layer  116 , a tunnel-junction layer  117 , an anode  118 , and a cathode  119 . The tunnel junction layer  117  consists of a p-type semiconductor layer  117   a  and an n-type semiconductor layer  117   b . As shown in  FIG. 3 , the daylight light-emitting diode  2  in the second embodiment is substantially similar in construction to the daylight light-emitting diode  1  in the first embodiment, except that the daylight light-emitting diode  2  has a tunnel junction layer  117  having an n-type semiconductor layer  117   b  different from that of the daylight light-emitting diode  1 . The n-type semiconductor layer  117   b  of the second embodiment has a multiple-quantum-well structure of n+-GaInN/GaN. The n-type semiconductor layer  17   b  contains Si as a donor impurity and Mg as an acceptor impurity. Whereas the Mg concentration is distributed substantially uniformly in the n-type semiconductor layer  117   b , the Si concentration is higher than the Mg concentration by 1019 cm-3 or above only in the vicinity of the boundary surfaces of the n-type semiconductor layer  117   b . Thus, the n-type semiconductor layer  117   b  has a high carrier concentration in the vicinity of its boundary surfaces. 
     Since the n-type semiconductor layer  117   b  has a high carrier concentration in its boundary surfaces and is in ohmic contact with an anode  118 , a tunnel current can be produced, similarly to the first embodiment, in a tunnel junction layer  117  by applying a voltage across the anode  118  and a cathode  119 . Thus a multiple-quantum-well active layer  114  formed between the anode  118  and the cathode  119  can emit near ultraviolet radiation. 
     Near ultraviolet radiation radiated downward from the multiple-quantum-well active layer  114  directly reaches the fluorescent substrate  110  and the fluorescent layer  111  to make the fluorescent substrate  110  and the fluorescent layer  11  produce fluorescent light. Near ultraviolet radiation radiated upward from the multiple-quantum-well active layer  114  reaches the n-type semiconductor layer  117  having multiple-quantum-well structure and containing Si, namely, a donor impurity, and Mg, namely, an acceptor impurity A narrow bandgap is formed in the n-type semiconductor layer  117   b  by forming the multiple-quantum-well structure of GaInN containing InN in a large molar fraction. Since Mg, namely, an acceptor impurity, has a high acceptor level, transition energy needed by the recombination of donor electrons excited by the near ultraviolet radiation and acceptor holes in the n-type semiconductor layer  117   b  can be reduced. Thus, the n-type semiconductor layer  117   b  can emit red fluorescent light having a long wavelength. Since the n-type semiconductor layer  117   b  has a multiple-quantum-well structure, the near ultraviolet radiation can be efficiently converted into fluorescent light by the confining effect of the donor electrons and the acceptor holes. Part around a boundary surface of the n-type semiconductor layer  117   b  forming the tunnel junction has a high carrier concentration, and the rest of the n-type semiconductor layer  117   b  has an impurity concentration suitable for the conversion of the near ultraviolet radiation to fluorescent light to achieve a high conversion efficiency. Therefore, the thickness of the n-type semiconductor layer  117   b  may be small. 
     The fluorescent substrate  110  produces fluorescent light having a wide wavelength spectrum including wavelengths between green and red. The n-type semiconductor layer  117   b  having a bandgap smaller than that of the fluorescent substrate  110  can produce reddish fluorescent light having long wavelengths. Thus, white light satisfactory in color rendering can be emitted by synthesizing the florescent light produced by the fluorescent substrate  110 , the fluorescent light layer  111 , and the n-type semiconductor layer  117   b . Warm color illumination using light of a warm color having a color temperature on the order of 2800 K can be achieved y using the white light. The anode  118  and the cathode  119  of the second embodiment have construction similar to that shown in  FIG. 2  of the anode  118  and the cathode  119  of the first embodiment. Therefore, the anode  118  and the cathode  119  can reflect light of those wavelength at a high reflectivity. Consequently, light can be emitted at a high efficiency from the fluorescent substrate  110 . The n-type semiconductor layer  117   b  of the second embodiment can be in ohmic contact with the anode  118  and can produce fluorescent light of long wavelengths. Therefore, separate layers respectively for ohmic contact and long-wavelength light production do not need to be formed. Thus, the daylight light-emitting diode  2  can be manufactured at low material and manufacturing costs. The n-type semiconductor layer  117   b  corresponds to the fluorescent layer of the present invention. 
     (3) Third Embodiment 
       FIG. 4  is a typical view of a daylight light emitting diode  3  in a third embodiment according to the present invention. Referring to  FIG. 4 , a daylight light-emitting diode  3  includes a fluorescent substrate  210 , a buffer layer  212 , an n-type contact layer  213 , a multiple-quantum-well active layer  214 , an electron blocking layer  215 , a p-type connecting layer  216 , a tunnel-junction layer  217 , an anode  218 , and a cathode  219 . The tunnel junction layer  217  consists of a p-type semiconductor layer  217   a  and an n-type semiconductor layer  217   b . As shown in  FIG. 4 , the daylight light-emitting diode  2  in the third embodiment is substantially similar in construction to the daylight light-emitting diode  3  in the second embodiment, except that the daylight light-emitting diode  3  does not have any layer corresponding to the fluorescent layer  111 . In the third embodiment, a buffer layer  212  is formed on a fluorescent substrate  210 . Since the fluorescent layer  111  is omitted, the daylight light-emitting diode  3  in the third embodiment, as compared with the daylight light-emitting diode  2  in the second embodiment, is apt to emit an insufficient amount of light rays having long wavelengths. The amount of long-wavelength components of fluorescent light produced by an n-type semiconductor layer  217   b  can be increased by adjusting the composition of GaInN/GaN forming the n-type semiconductor layer  217   b . Thus, a deficiency in long-wave components due to the omission of the fluorescent layer  111  can be supplemented and well-balanced white light can be produced. More concretely, transition energy at which donor-acceptor pairs recombine can be suppressed by increasing the InN molar fraction of the n-type semiconductor layer  217   b  or by deepening the acceptor level to increase long-wavelength fluorescent light. 
     It is effective in supplementing the long-wave components of white light to insert a fluorescent layer having multiple-quantum-well structure of GaInN/GaN. The fluorescent layer does not necessarily contribute to ohmic contact between a semiconductor layer and an electrode. The color temperature of the white light can be adjusted to a color temperature of a warm color by inserting the fluorescent layer having multiple-quantum-well structure of GaInN/GaN into an optional position.  FIG. 5  typically shows a daylight light-emitting diode  4  having a p-electrode and an n-electrode to p-type and n-type contact layers, respectively. The daylight light-emitting diode  4  has an n-contact layer  313  of n-GaN and a p-contact layer  317  of p-GaN. An n-electrode  319  is joined to the n-contact layer  313  and a p-electrode  318  is joined to the p-contact layer  317 . The daylight light-emitting diode  4  has a fluorescent substrate  310 , a buffer layer  312 , a multiple-quantum-well active layer  314 , and an electron blocking layer  315 , which are the same in construction as those of the second embodiment. 
     The fluorescent layer  316  having multiple-quantum-well structure of GaInN/GaN is sandwiched between the electron blocking layer  315  and the p-contact layer  317 . The fluorescent layer  316  is similar in construction to the n-type semiconductor layer  117   b  of the second embodiment. The fluorescent layer  316  contains Si, namely, a donor impurity, and Mg, namely, a donor impurity. The Si and the Mg concentration are determined such that the fluorescent layer  316  is a p-type fluorescent layer. The fluorescent layer  316  having multiple-quantum-well structure of GaInN/GaN can produce long-wavelength fluorescent light, and the daylight light-emitting diode  4  can emit white light tinged with a warm color. 
     (4) Conclusion 
     The anode  18  has the Ag layer  18   a  of Ag having a high reflectivity. The Ag layer  18   a  is sandwiched between the protective layers  18   b   1  and  18   b   2  each of Ti. The protective layer  18   b   1  is in ohmic contact with the tunnel-junction layer  17 . The anode  18  formed of Ti/Ag having a small work function has a low resistance, and tunnel current flows through the tunnel-junction layer  17 . The anode  18  and the cathode  19  can be formed of the same material, and can be formed of Ag having a high reflectivity. Consequently, the daylight light-emitting diode can emit light at a high light-emitting efficiency. 
     Although the invention has been described in considerable detail in language specific to structural features or method acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as preferred forms of implementing the claimed invention. Therefore, while exemplary illustrative embodiments of the invention have been described, numerous variations and alternative embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention. 
     It is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     It should further be noted that throughout the entire disclosure, the labels such as left, right, front, back, top, bottom, forward, reverse, clockwise, counter clockwise, up, down, or other similar terms such as upper, lower, aft, fore, vertical, horizontal, proximal, distal, etc. have been used for convenience purposes only and are not intended to imply any particular fixed direction or orientation. Instead, they are used to reflect relative locations and/or directions/orientations between various portions of an object. 
     In addition, reference to “first,” “second,” “third,” and etc. members throughout the disclosure (and in particular, claims) is not used to show a serial or numerical limitation but instead is used to distinguish or identify the various members of the group.